1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
8 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
9 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU project C and C++ compiler
33 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
39 [@option{-o} @var{outfile}] @var{infile}@dots{}
41 Only the most useful options are listed here; see below for the
42 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
45 gpl(7), gfdl(7), fsf-funding(7),
46 cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{as},
48 @file{ld}, @file{binutils} and @file{gdb}.
51 For instructions on reporting bugs, see
52 @w{@uref{http://gcc.gnu.org/bugs.html}}.
55 See the Info entry for @command{gcc}, or
56 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
57 for contributors to GCC@.
62 @chapter GCC Command Options
63 @cindex GCC command options
64 @cindex command options
65 @cindex options, GCC command
67 @c man begin DESCRIPTION
68 When you invoke GCC, it normally does preprocessing, compilation,
69 assembly and linking. The ``overall options'' allow you to stop this
70 process at an intermediate stage. For example, the @option{-c} option
71 says not to run the linker. Then the output consists of object files
72 output by the assembler.
74 Other options are passed on to one stage of processing. Some options
75 control the preprocessor and others the compiler itself. Yet other
76 options control the assembler and linker; most of these are not
77 documented here, since you rarely need to use any of them.
79 @cindex C compilation options
80 Most of the command line options that you can use with GCC are useful
81 for C programs; when an option is only useful with another language
82 (usually C++), the explanation says so explicitly. If the description
83 for a particular option does not mention a source language, you can use
84 that option with all supported languages.
86 @cindex C++ compilation options
87 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
88 options for compiling C++ programs.
90 @cindex grouping options
91 @cindex options, grouping
92 The @command{gcc} program accepts options and file names as operands. Many
93 options have multi-letter names; therefore multiple single-letter options
94 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
97 @cindex order of options
98 @cindex options, order
99 You can mix options and other arguments. For the most part, the order
100 you use doesn't matter. Order does matter when you use several options
101 of the same kind; for example, if you specify @option{-L} more than once,
102 the directories are searched in the order specified.
104 Many options have long names starting with @samp{-f} or with
105 @samp{-W}---for example, @option{-fforce-mem},
106 @option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
107 these have both positive and negative forms; the negative form of
108 @option{-ffoo} would be @option{-fno-foo}. This manual documents
109 only one of these two forms, whichever one is not the default.
113 @xref{Option Index}, for an index to GCC's options.
116 * Option Summary:: Brief list of all options, without explanations.
117 * Overall Options:: Controlling the kind of output:
118 an executable, object files, assembler files,
119 or preprocessed source.
120 * Invoking G++:: Compiling C++ programs.
121 * C Dialect Options:: Controlling the variant of C language compiled.
122 * C++ Dialect Options:: Variations on C++.
123 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
125 * Language Independent Options:: Controlling how diagnostics should be
127 * Warning Options:: How picky should the compiler be?
128 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
129 * Optimize Options:: How much optimization?
130 * Preprocessor Options:: Controlling header files and macro definitions.
131 Also, getting dependency information for Make.
132 * Assembler Options:: Passing options to the assembler.
133 * Link Options:: Specifying libraries and so on.
134 * Directory Options:: Where to find header files and libraries.
135 Where to find the compiler executable files.
136 * Spec Files:: How to pass switches to sub-processes.
137 * Target Options:: Running a cross-compiler, or an old version of GCC.
138 * Submodel Options:: Specifying minor hardware or convention variations,
139 such as 68010 vs 68020.
140 * Code Gen Options:: Specifying conventions for function calls, data layout
142 * Environment Variables:: Env vars that affect GCC.
143 * Precompiled Headers:: Compiling a header once, and using it many times.
144 * Running Protoize:: Automatically adding or removing function prototypes.
150 @section Option Summary
152 Here is a summary of all the options, grouped by type. Explanations are
153 in the following sections.
156 @item Overall Options
157 @xref{Overall Options,,Options Controlling the Kind of Output}.
158 @gccoptlist{-c -S -E -o @var{file} -combine -pipe -pass-exit-codes @gol
159 -x @var{language} -v -### --help --target-help --version}
161 @item C Language Options
162 @xref{C Dialect Options,,Options Controlling C Dialect}.
163 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
164 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
165 -fhosted -ffreestanding -fms-extensions @gol
166 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
167 -fallow-single-precision -fcond-mismatch @gol
168 -fsigned-bitfields -fsigned-char @gol
169 -funsigned-bitfields -funsigned-char}
171 @item C++ Language Options
172 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
173 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
174 -fconserve-space -fno-const-strings @gol
175 -fno-elide-constructors @gol
176 -fno-enforce-eh-specs @gol
177 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
178 -fno-implicit-templates @gol
179 -fno-implicit-inline-templates @gol
180 -fno-implement-inlines -fms-extensions @gol
181 -fno-nonansi-builtins -fno-operator-names @gol
182 -fno-optional-diags -fpermissive @gol
183 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
184 -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
185 -fno-default-inline -fvisibility-inlines-hidden @gol
186 -Wabi -Wctor-dtor-privacy @gol
187 -Wnon-virtual-dtor -Wreorder @gol
188 -Weffc++ -Wno-deprecated @gol
189 -Wno-non-template-friend -Wold-style-cast @gol
190 -Woverloaded-virtual -Wno-pmf-conversions @gol
193 @item Objective-C and Objective-C++ Language Options
194 @xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
195 Objective-C and Objective-C++ Dialects}.
197 -fconstant-string-class=@var{class-name} @gol
198 -fgnu-runtime -fnext-runtime @gol
199 -fno-nil-receivers @gol
200 -fobjc-exceptions @gol
201 -freplace-objc-classes @gol
204 -Wno-protocol -Wselector -Wundeclared-selector}
206 @item Language Independent Options
207 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
208 @gccoptlist{-fmessage-length=@var{n} @gol
209 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}}
211 @item Warning Options
212 @xref{Warning Options,,Options to Request or Suppress Warnings}.
213 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
214 -w -Wextra -Wall -Waggregate-return @gol
215 -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
216 -Wconversion -Wno-deprecated-declarations @gol
217 -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels @gol
218 -Werror -Werror-implicit-function-declaration @gol
219 -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
220 -Wno-format-extra-args -Wformat-nonliteral @gol
221 -Wformat-security -Wformat-y2k @gol
222 -Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
223 -Wimport -Wno-import -Winit-self -Winline @gol
224 -Wno-invalid-offsetof -Winvalid-pch @gol
225 -Wlarger-than-@var{len} -Wlong-long @gol
226 -Wmain -Wmissing-braces -Wmissing-field-initializers @gol
227 -Wmissing-format-attribute -Wmissing-include-dirs @gol
228 -Wmissing-noreturn @gol
229 -Wno-multichar -Wnonnull -Wpacked -Wpadded @gol
230 -Wparentheses -Wpointer-arith -Wredundant-decls @gol
231 -Wreturn-type -Wsequence-point -Wshadow @gol
232 -Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 @gol
233 -Wswitch -Wswitch-default -Wswitch-enum @gol
234 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
235 -Wunknown-pragmas -Wunreachable-code @gol
236 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
237 -Wunused-value -Wunused-variable -Wwrite-strings @gol
240 @item C-only Warning Options
241 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
242 -Wmissing-prototypes -Wnested-externs -Wold-style-definition @gol
243 -Wstrict-prototypes -Wtraditional @gol
244 -Wdeclaration-after-statement -Wno-pointer-sign}
246 @item Debugging Options
247 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
248 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
249 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
250 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
251 -fdump-ipa-all -fdump-ipa-cgraph @gol
253 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
254 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
255 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
256 -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias @gol
258 -fdump-tree-ssa@r{[}-@var{n}@r{]} -fdump-tree-pre@r{[}-@var{n}@r{]} @gol
259 -fdump-tree-ccp@r{[}-@var{n}@r{]} -fdump-tree-dce@r{[}-@var{n}@r{]} @gol
260 -fdump-tree-gimple@r{[}-raw@r{]} -fdump-tree-mudflap@r{[}-@var{n}@r{]} @gol
261 -fdump-tree-dom@r{[}-@var{n}@r{]} @gol
262 -fdump-tree-dse@r{[}-@var{n}@r{]} @gol
263 -fdump-tree-phiopt@r{[}-@var{n}@r{]} @gol
264 -fdump-tree-forwprop@r{[}-@var{n}@r{]} @gol
265 -fdump-tree-copyrename@r{[}-@var{n}@r{]} @gol
266 -fdump-tree-nrv -fdump-tree-vect @gol
267 -fdump-tree-sink @gol
268 -fdump-tree-sra@r{[}-@var{n}@r{]} @gol
269 -fdump-tree-salias @gol
270 -fdump-tree-fre@r{[}-@var{n}@r{]} @gol
271 -ftree-vectorizer-verbose=@var{n} @gol
272 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
273 -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -ftree-based-profiling @gol
274 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
275 -ftest-coverage -ftime-report -fvar-tracking @gol
276 -g -g@var{level} -gcoff -gdwarf-2 @gol
277 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
278 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
279 -print-multi-directory -print-multi-lib @gol
280 -print-prog-name=@var{program} -print-search-dirs -Q @gol
283 @item Optimization Options
284 @xref{Optimize Options,,Options that Control Optimization}.
285 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
286 -falign-labels=@var{n} -falign-loops=@var{n} @gol
287 -fbounds-check -fmudflap -fmudflapth -fmudflapir @gol
288 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize @gol
289 -fbranch-target-load-optimize2 -fbtr-bb-exclusive @gol
290 -fcaller-saves -fcprop-registers -fcse-follow-jumps @gol
291 -fcse-skip-blocks -fcx-limited-range -fdata-sections @gol
292 -fdelayed-branch -fdelete-null-pointer-checks @gol
293 -fexpensive-optimizations -ffast-math -ffloat-store @gol
294 -fforce-addr -fforce-mem -ffunction-sections @gol
295 -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload @gol
296 -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 @gol
297 -finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
298 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
299 -fmodulo-sched -fno-branch-count-reg @gol
300 -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants @gol
301 -fno-function-cse -fno-guess-branch-probability @gol
302 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
303 -funsafe-math-optimizations -ffinite-math-only @gol
304 -fno-trapping-math -fno-zero-initialized-in-bss @gol
305 -fomit-frame-pointer -foptimize-register-move @gol
306 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
307 -fprofile-generate -fprofile-use @gol
308 -fregmove -frename-registers @gol
309 -freorder-blocks -freorder-blocks-and-partition -freorder-functions @gol
310 -frerun-cse-after-loop -frerun-loop-opt @gol
311 -frounding-math -fschedule-insns -fschedule-insns2 @gol
312 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
313 -fsched-spec-load-dangerous @gol
314 -fsched-stalled-insns=@var{n} -sched-stalled-insns-dep=@var{n} @gol
315 -fsched2-use-superblocks @gol
316 -fsched2-use-traces -freschedule-modulo-scheduled-loops @gol
317 -fsignaling-nans -fsingle-precision-constant -fspeculative-prefetching @gol
318 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
319 -funroll-all-loops -funroll-loops -fpeel-loops @gol
320 -fsplit-ivs-in-unroller -funswitch-loops @gol
321 -fvariable-expansion-in-unroller @gol
322 -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize @gol
323 -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts @gol
324 -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink @gol
325 -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize @gol
326 -ftree-salias -fweb @gol
327 --param @var{name}=@var{value}
328 -O -O0 -O1 -O2 -O3 -Os}
330 @item Preprocessor Options
331 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
332 @gccoptlist{-A@var{question}=@var{answer} @gol
333 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
334 -C -dD -dI -dM -dN @gol
335 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
336 -idirafter @var{dir} @gol
337 -include @var{file} -imacros @var{file} @gol
338 -iprefix @var{file} -iwithprefix @var{dir} @gol
339 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
340 -M -MM -MF -MG -MP -MQ -MT -nostdinc @gol
341 -P -fworking-directory -remap @gol
342 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
343 -Xpreprocessor @var{option}}
345 @item Assembler Option
346 @xref{Assembler Options,,Passing Options to the Assembler}.
347 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
350 @xref{Link Options,,Options for Linking}.
351 @gccoptlist{@var{object-file-name} -l@var{library} @gol
352 -nostartfiles -nodefaultlibs -nostdlib -pie @gol
353 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
354 -Wl,@var{option} -Xlinker @var{option} @gol
357 @item Directory Options
358 @xref{Directory Options,,Options for Directory Search}.
359 @gccoptlist{-B@var{prefix} -I@var{dir} -iquote@var{dir} -L@var{dir} -specs=@var{file} -I-}
362 @c I wrote this xref this way to avoid overfull hbox. -- rms
363 @xref{Target Options}.
364 @gccoptlist{-V @var{version} -b @var{machine}}
366 @item Machine Dependent Options
367 @xref{Submodel Options,,Hardware Models and Configurations}.
368 @c This list is ordered alphanumerically by subsection name.
369 @c Try and put the significant identifier (CPU or system) first,
370 @c so users have a clue at guessing where the ones they want will be.
373 @gccoptlist{-EB -EL @gol
374 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
375 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
378 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
379 -mabi=@var{name} @gol
380 -mapcs-stack-check -mno-apcs-stack-check @gol
381 -mapcs-float -mno-apcs-float @gol
382 -mapcs-reentrant -mno-apcs-reentrant @gol
383 -msched-prolog -mno-sched-prolog @gol
384 -mlittle-endian -mbig-endian -mwords-little-endian @gol
385 -mfloat-abi=@var{name} -msoft-float -mhard-float -mfpe @gol
386 -mthumb-interwork -mno-thumb-interwork @gol
387 -mcpu=@var{name} -march=@var{name} -mfpu=@var{name} @gol
388 -mstructure-size-boundary=@var{n} @gol
389 -mabort-on-noreturn @gol
390 -mlong-calls -mno-long-calls @gol
391 -msingle-pic-base -mno-single-pic-base @gol
392 -mpic-register=@var{reg} @gol
393 -mnop-fun-dllimport @gol
394 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
395 -mpoke-function-name @gol
397 -mtpcs-frame -mtpcs-leaf-frame @gol
398 -mcaller-super-interworking -mcallee-super-interworking}
401 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
402 -mcall-prologues -mno-tablejump -mtiny-stack -mint8}
404 @emph{Blackfin Options}
405 @gccoptlist{-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer -mcsync @gol
406 -mno-csync -mlow-64k -mno-low64k -mid-shared-library @gol
407 -mno-id-shared-library -mshared-library-id=@var{n} @gol}
410 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
411 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
412 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
413 -mstack-align -mdata-align -mconst-align @gol
414 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
415 -melf -maout -melinux -mlinux -sim -sim2 @gol
416 -mmul-bug-workaround -mno-mul-bug-workaround}
418 @emph{Darwin Options}
419 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
420 -arch_only -bind_at_load -bundle -bundle_loader @gol
421 -client_name -compatibility_version -current_version @gol
423 -dependency-file -dylib_file -dylinker_install_name @gol
424 -dynamic -dynamiclib -exported_symbols_list @gol
425 -filelist -flat_namespace -force_cpusubtype_ALL @gol
426 -force_flat_namespace -headerpad_max_install_names @gol
427 -image_base -init -install_name -keep_private_externs @gol
428 -multi_module -multiply_defined -multiply_defined_unused @gol
429 -noall_load -no_dead_strip_inits_and_terms @gol
430 -nofixprebinding -nomultidefs -noprebind -noseglinkedit @gol
431 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
432 -private_bundle -read_only_relocs -sectalign @gol
433 -sectobjectsymbols -whyload -seg1addr @gol
434 -sectcreate -sectobjectsymbols -sectorder @gol
435 -segaddr -segs_read_only_addr -segs_read_write_addr @gol
436 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
437 -segprot -segs_read_only_addr -segs_read_write_addr @gol
438 -single_module -static -sub_library -sub_umbrella @gol
439 -twolevel_namespace -umbrella -undefined @gol
440 -unexported_symbols_list -weak_reference_mismatches @gol
441 -whatsloaded -F -gused -gfull -mone-byte-bool}
443 @emph{DEC Alpha Options}
444 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
445 -mieee -mieee-with-inexact -mieee-conformant @gol
446 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
447 -mtrap-precision=@var{mode} -mbuild-constants @gol
448 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
449 -mbwx -mmax -mfix -mcix @gol
450 -mfloat-vax -mfloat-ieee @gol
451 -mexplicit-relocs -msmall-data -mlarge-data @gol
452 -msmall-text -mlarge-text @gol
453 -mmemory-latency=@var{time}}
455 @emph{DEC Alpha/VMS Options}
456 @gccoptlist{-mvms-return-codes}
459 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
460 -mhard-float -msoft-float @gol
461 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
462 -mdouble -mno-double @gol
463 -mmedia -mno-media -mmuladd -mno-muladd @gol
464 -mfdpic -minline-plt -mgprel-ro -multilib-library-pic @gol
465 -mlinked-fp -mlong-calls -malign-labels @gol
466 -mlibrary-pic -macc-4 -macc-8 @gol
467 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
468 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
469 -mvliw-branch -mno-vliw-branch @gol
470 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
471 -mno-nested-cond-exec -mtomcat-stats @gol
475 @emph{H8/300 Options}
476 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
479 @gccoptlist{-march=@var{architecture-type} @gol
480 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
481 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
482 -mfixed-range=@var{register-range} @gol
483 -mjump-in-delay -mlinker-opt -mlong-calls @gol
484 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
485 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
486 -mno-jump-in-delay -mno-long-load-store @gol
487 -mno-portable-runtime -mno-soft-float @gol
488 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
489 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
490 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
491 -munix=@var{unix-std} -nolibdld -static -threads}
493 @emph{i386 and x86-64 Options}
494 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
495 -mfpmath=@var{unit} @gol
496 -masm=@var{dialect} -mno-fancy-math-387 @gol
497 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
498 -mno-wide-multiply -mrtd -malign-double @gol
499 -mpreferred-stack-boundary=@var{num} @gol
500 -mmmx -msse -msse2 -msse3 -m3dnow @gol
501 -mthreads -mno-align-stringops -minline-all-stringops @gol
502 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
503 -m96bit-long-double -mregparm=@var{num} -momit-leaf-frame-pointer @gol
504 -mno-red-zone -mno-tls-direct-seg-refs @gol
505 -mcmodel=@var{code-model} @gol
509 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
510 -mvolatile-asm-stop -mregister-names -mno-sdata @gol
511 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
512 -minline-float-divide-max-throughput @gol
513 -minline-int-divide-min-latency @gol
514 -minline-int-divide-max-throughput @gol
515 -minline-sqrt-min-latency -minline-sqrt-max-throughput @gol
516 -mno-dwarf2-asm -mearly-stop-bits @gol
517 -mfixed-range=@var{register-range} -mtls-size=@var{tls-size} @gol
518 -mtune=@var{cpu-type} -mt -pthread -milp32 -mlp64}
520 @emph{M32R/D Options}
521 @gccoptlist{-m32r2 -m32rx -m32r @gol
523 -malign-loops -mno-align-loops @gol
524 -missue-rate=@var{number} @gol
525 -mbranch-cost=@var{number} @gol
526 -mmodel=@var{code-size-model-type} @gol
527 -msdata=@var{sdata-type} @gol
528 -mno-flush-func -mflush-func=@var{name} @gol
529 -mno-flush-trap -mflush-trap=@var{number} @gol
532 @emph{M680x0 Options}
533 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
534 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
535 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
536 -malign-int -mstrict-align -msep-data -mno-sep-data @gol
537 -mshared-library-id=n -mid-shared-library -mno-id-shared-library}
539 @emph{M68hc1x Options}
540 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
541 -mauto-incdec -minmax -mlong-calls -mshort @gol
542 -msoft-reg-count=@var{count}}
545 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
546 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
547 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
548 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
549 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
552 @gccoptlist{-EL -EB -march=@var{arch} -mtune=@var{arch} @gol
553 -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
554 -mips16 -mno-mips16 -mabi=@var{abi} -mabicalls -mno-abicalls @gol
555 -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 @gol
556 -mhard-float -msoft-float -msingle-float -mdouble-float @gol
557 -mpaired-single -mips3d @gol
558 -mint64 -mlong64 -mlong32 -msym32 -mno-sym32 @gol
559 -G@var{num} -membedded-data -mno-embedded-data @gol
560 -muninit-const-in-rodata -mno-uninit-const-in-rodata @gol
561 -msplit-addresses -mno-split-addresses @gol
562 -mexplicit-relocs -mno-explicit-relocs @gol
563 -mcheck-zero-division -mno-check-zero-division @gol
564 -mdivide-traps -mdivide-breaks @gol
565 -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls @gol
566 -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp @gol
567 -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 @gol
568 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 @gol
569 -mfix-sb1 -mno-fix-sb1 @gol
570 -mflush-func=@var{func} -mno-flush-func @gol
571 -mbranch-likely -mno-branch-likely @gol
572 -mfp-exceptions -mno-fp-exceptions @gol
573 -mvr4130-align -mno-vr4130-align}
576 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
577 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
578 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
579 -mno-base-addresses -msingle-exit -mno-single-exit}
581 @emph{MN10300 Options}
582 @gccoptlist{-mmult-bug -mno-mult-bug @gol
583 -mam33 -mno-am33 @gol
584 -mam33-2 -mno-am33-2 @gol
588 @gccoptlist{-m32032 -m32332 -m32532 -m32081 -m32381 @gol
589 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd @gol
590 -mregparam -mnoregparam -msb -mnosb @gol
591 -mbitfield -mnobitfield -mhimem -mnohimem}
593 @emph{PDP-11 Options}
594 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
595 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
596 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
597 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
598 -mbranch-expensive -mbranch-cheap @gol
599 -msplit -mno-split -munix-asm -mdec-asm}
601 @emph{PowerPC Options}
602 See RS/6000 and PowerPC Options.
604 @emph{RS/6000 and PowerPC Options}
605 @gccoptlist{-mcpu=@var{cpu-type} @gol
606 -mtune=@var{cpu-type} @gol
607 -mpower -mno-power -mpower2 -mno-power2 @gol
608 -mpowerpc -mpowerpc64 -mno-powerpc @gol
609 -maltivec -mno-altivec @gol
610 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
611 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
612 -mnew-mnemonics -mold-mnemonics @gol
613 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
614 -m64 -m32 -mxl-compat -mno-xl-compat -mpe @gol
615 -malign-power -malign-natural @gol
616 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
617 -mstring -mno-string -mupdate -mno-update @gol
618 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
619 -mstrict-align -mno-strict-align -mrelocatable @gol
620 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
621 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
622 -mdynamic-no-pic @gol
623 -mprioritize-restricted-insns=@var{priority} @gol
624 -msched-costly-dep=@var{dependence_type} @gol
625 -minsert-sched-nops=@var{scheme} @gol
626 -mcall-sysv -mcall-netbsd @gol
627 -maix-struct-return -msvr4-struct-return @gol
628 -mabi=altivec -mabi=no-altivec @gol
629 -mabi=spe -mabi=no-spe @gol
630 -misel=yes -misel=no @gol
631 -mspe=yes -mspe=no @gol
632 -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double @gol
633 -mprototype -mno-prototype @gol
634 -msim -mmvme -mads -myellowknife -memb -msdata @gol
635 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
637 @emph{S/390 and zSeries Options}
638 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
639 -mhard-float -msoft-float -mbackchain -mno-backchain @gol
640 -mpacked-stack -mno-packed-stack @gol
641 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
642 -m64 -m31 -mdebug -mno-debug -mesa -mzarch @gol
643 -mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd @gol
644 -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard}
647 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
648 -m4-nofpu -m4-single-only -m4-single -m4 @gol
649 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al @gol
650 -m5-64media -m5-64media-nofpu @gol
651 -m5-32media -m5-32media-nofpu @gol
652 -m5-compact -m5-compact-nofpu @gol
653 -mb -ml -mdalign -mrelax @gol
654 -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave @gol
655 -mieee -misize -mpadstruct -mspace @gol
656 -mprefergot -musermode}
659 @gccoptlist{-mcpu=@var{cpu-type} @gol
660 -mtune=@var{cpu-type} @gol
661 -mcmodel=@var{code-model} @gol
662 -m32 -m64 -mapp-regs -mno-app-regs @gol
663 -mfaster-structs -mno-faster-structs @gol
664 -mfpu -mno-fpu -mhard-float -msoft-float @gol
665 -mhard-quad-float -msoft-quad-float @gol
666 -mimpure-text -mno-impure-text -mlittle-endian @gol
667 -mstack-bias -mno-stack-bias @gol
668 -munaligned-doubles -mno-unaligned-doubles @gol
669 -mv8plus -mno-v8plus -mvis -mno-vis
672 @emph{System V Options}
673 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
675 @emph{TMS320C3x/C4x Options}
676 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
677 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
678 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
679 -mparallel-insns -mparallel-mpy -mpreserve-float}
682 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
683 -mprolog-function -mno-prolog-function -mspace @gol
684 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
685 -mapp-regs -mno-app-regs @gol
686 -mdisable-callt -mno-disable-callt @gol
692 @gccoptlist{-mg -mgnu -munix}
694 @emph{x86-64 Options}
695 See i386 and x86-64 Options.
697 @emph{Xstormy16 Options}
700 @emph{Xtensa Options}
701 @gccoptlist{-mconst16 -mno-const16 @gol
702 -mfused-madd -mno-fused-madd @gol
703 -mtext-section-literals -mno-text-section-literals @gol
704 -mtarget-align -mno-target-align @gol
705 -mlongcalls -mno-longcalls}
707 @emph{zSeries Options}
708 See S/390 and zSeries Options.
710 @item Code Generation Options
711 @xref{Code Gen Options,,Options for Code Generation Conventions}.
712 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
713 -ffixed-@var{reg} -fexceptions @gol
714 -fnon-call-exceptions -funwind-tables @gol
715 -fasynchronous-unwind-tables @gol
716 -finhibit-size-directive -finstrument-functions @gol
717 -fno-common -fno-ident @gol
718 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
719 -freg-struct-return -fshared-data -fshort-enums @gol
720 -fshort-double -fshort-wchar @gol
721 -fverbose-asm -fpack-struct[=@var{n}] -fstack-check @gol
722 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
723 -fargument-alias -fargument-noalias @gol
724 -fargument-noalias-global -fleading-underscore @gol
725 -ftls-model=@var{model} @gol
726 -ftrapv -fwrapv -fbounds-check @gol
731 * Overall Options:: Controlling the kind of output:
732 an executable, object files, assembler files,
733 or preprocessed source.
734 * C Dialect Options:: Controlling the variant of C language compiled.
735 * C++ Dialect Options:: Variations on C++.
736 * Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
738 * Language Independent Options:: Controlling how diagnostics should be
740 * Warning Options:: How picky should the compiler be?
741 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
742 * Optimize Options:: How much optimization?
743 * Preprocessor Options:: Controlling header files and macro definitions.
744 Also, getting dependency information for Make.
745 * Assembler Options:: Passing options to the assembler.
746 * Link Options:: Specifying libraries and so on.
747 * Directory Options:: Where to find header files and libraries.
748 Where to find the compiler executable files.
749 * Spec Files:: How to pass switches to sub-processes.
750 * Target Options:: Running a cross-compiler, or an old version of GCC.
753 @node Overall Options
754 @section Options Controlling the Kind of Output
756 Compilation can involve up to four stages: preprocessing, compilation
757 proper, assembly and linking, always in that order. GCC is capable of
758 preprocessing and compiling several files either into several
759 assembler input files, or into one assembler input file; then each
760 assembler input file produces an object file, and linking combines all
761 the object files (those newly compiled, and those specified as input)
762 into an executable file.
764 @cindex file name suffix
765 For any given input file, the file name suffix determines what kind of
770 C source code which must be preprocessed.
773 C source code which should not be preprocessed.
776 C++ source code which should not be preprocessed.
779 Objective-C source code. Note that you must link with the @file{libobjc}
780 library to make an Objective-C program work.
783 Objective-C source code which should not be preprocessed.
787 Objective-C++ source code. Note that you must link with the @file{libobjc}
788 library to make an Objective-C++ program work. Note that @samp{.M} refers
789 to a literal capital M@.
792 Objective-C++ source code which should not be preprocessed.
795 C, C++, Objective-C or Objective-C++ header file to be turned into a
800 @itemx @var{file}.cxx
801 @itemx @var{file}.cpp
802 @itemx @var{file}.CPP
803 @itemx @var{file}.c++
805 C++ source code which must be preprocessed. Note that in @samp{.cxx},
806 the last two letters must both be literally @samp{x}. Likewise,
807 @samp{.C} refers to a literal capital C@.
811 C++ header file to be turned into a precompiled header.
814 @itemx @var{file}.for
815 @itemx @var{file}.FOR
816 Fortran source code which should not be preprocessed.
819 @itemx @var{file}.fpp
820 @itemx @var{file}.FPP
821 Fortran source code which must be preprocessed (with the traditional
825 Fortran source code which must be preprocessed with a RATFOR
826 preprocessor (not included with GCC)@.
829 @itemx @var{file}.f95
830 Fortran 90/95 source code which should not be preprocessed.
832 @c FIXME: Descriptions of Java file types.
839 Ada source code file which contains a library unit declaration (a
840 declaration of a package, subprogram, or generic, or a generic
841 instantiation), or a library unit renaming declaration (a package,
842 generic, or subprogram renaming declaration). Such files are also
845 @itemx @var{file}.adb
846 Ada source code file containing a library unit body (a subprogram or
847 package body). Such files are also called @dfn{bodies}.
849 @c GCC also knows about some suffixes for languages not yet included:
858 Assembler code which must be preprocessed.
861 An object file to be fed straight into linking.
862 Any file name with no recognized suffix is treated this way.
866 You can specify the input language explicitly with the @option{-x} option:
869 @item -x @var{language}
870 Specify explicitly the @var{language} for the following input files
871 (rather than letting the compiler choose a default based on the file
872 name suffix). This option applies to all following input files until
873 the next @option{-x} option. Possible values for @var{language} are:
875 c c-header c-cpp-output
876 c++ c++-header c++-cpp-output
877 objective-c objective-c-header objective-c-cpp-output
878 objective-c++ objective-c++-header objective-c++-cpp-output
879 assembler assembler-with-cpp
881 f77 f77-cpp-input ratfor
888 Turn off any specification of a language, so that subsequent files are
889 handled according to their file name suffixes (as they are if @option{-x}
890 has not been used at all).
892 @item -pass-exit-codes
893 @opindex pass-exit-codes
894 Normally the @command{gcc} program will exit with the code of 1 if any
895 phase of the compiler returns a non-success return code. If you specify
896 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
897 numerically highest error produced by any phase that returned an error
901 If you only want some of the stages of compilation, you can use
902 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
903 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
904 @command{gcc} is to stop. Note that some combinations (for example,
905 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
910 Compile or assemble the source files, but do not link. The linking
911 stage simply is not done. The ultimate output is in the form of an
912 object file for each source file.
914 By default, the object file name for a source file is made by replacing
915 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
917 Unrecognized input files, not requiring compilation or assembly, are
922 Stop after the stage of compilation proper; do not assemble. The output
923 is in the form of an assembler code file for each non-assembler input
926 By default, the assembler file name for a source file is made by
927 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
929 Input files that don't require compilation are ignored.
933 Stop after the preprocessing stage; do not run the compiler proper. The
934 output is in the form of preprocessed source code, which is sent to the
937 Input files which don't require preprocessing are ignored.
939 @cindex output file option
942 Place output in file @var{file}. This applies regardless to whatever
943 sort of output is being produced, whether it be an executable file,
944 an object file, an assembler file or preprocessed C code.
946 If @option{-o} is not specified, the default is to put an executable
947 file in @file{a.out}, the object file for
948 @file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
949 assembler file in @file{@var{source}.s}, a precompiled header file in
950 @file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
955 Print (on standard error output) the commands executed to run the stages
956 of compilation. Also print the version number of the compiler driver
957 program and of the preprocessor and the compiler proper.
961 Like @option{-v} except the commands are not executed and all command
962 arguments are quoted. This is useful for shell scripts to capture the
963 driver-generated command lines.
967 Use pipes rather than temporary files for communication between the
968 various stages of compilation. This fails to work on some systems where
969 the assembler is unable to read from a pipe; but the GNU assembler has
974 If you are compiling multiple source files, this option tells the driver
975 to pass all the source files to the compiler at once (for those
976 languages for which the compiler can handle this). This will allow
977 intermodule analysis (IMA) to be performed by the compiler. Currently the only
978 language for which this is supported is C@. If you pass source files for
979 multiple languages to the driver, using this option, the driver will invoke
980 the compiler(s) that support IMA once each, passing each compiler all the
981 source files appropriate for it. For those languages that do not support
982 IMA this option will be ignored, and the compiler will be invoked once for
983 each source file in that language. If you use this option in conjunction
984 with @option{-save-temps}, the compiler will generate multiple
986 (one for each source file), but only one (combined) @file{.o} or
991 Print (on the standard output) a description of the command line options
992 understood by @command{gcc}. If the @option{-v} option is also specified
993 then @option{--help} will also be passed on to the various processes
994 invoked by @command{gcc}, so that they can display the command line options
995 they accept. If the @option{-Wextra} option is also specified then command
996 line options which have no documentation associated with them will also
1000 @opindex target-help
1001 Print (on the standard output) a description of target specific command
1002 line options for each tool.
1006 Display the version number and copyrights of the invoked GCC@.
1010 @section Compiling C++ Programs
1012 @cindex suffixes for C++ source
1013 @cindex C++ source file suffixes
1014 C++ source files conventionally use one of the suffixes @samp{.C},
1015 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
1016 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
1017 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
1018 files with these names and compiles them as C++ programs even if you
1019 call the compiler the same way as for compiling C programs (usually
1020 with the name @command{gcc}).
1024 However, C++ programs often require class libraries as well as a
1025 compiler that understands the C++ language---and under some
1026 circumstances, you might want to compile programs or header files from
1027 standard input, or otherwise without a suffix that flags them as C++
1028 programs. You might also like to precompile a C header file with a
1029 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
1030 program that calls GCC with the default language set to C++, and
1031 automatically specifies linking against the C++ library. On many
1032 systems, @command{g++} is also installed with the name @command{c++}.
1034 @cindex invoking @command{g++}
1035 When you compile C++ programs, you may specify many of the same
1036 command-line options that you use for compiling programs in any
1037 language; or command-line options meaningful for C and related
1038 languages; or options that are meaningful only for C++ programs.
1039 @xref{C Dialect Options,,Options Controlling C Dialect}, for
1040 explanations of options for languages related to C@.
1041 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
1042 explanations of options that are meaningful only for C++ programs.
1044 @node C Dialect Options
1045 @section Options Controlling C Dialect
1046 @cindex dialect options
1047 @cindex language dialect options
1048 @cindex options, dialect
1050 The following options control the dialect of C (or languages derived
1051 from C, such as C++, Objective-C and Objective-C++) that the compiler
1055 @cindex ANSI support
1059 In C mode, support all ISO C90 programs. In C++ mode,
1060 remove GNU extensions that conflict with ISO C++.
1062 This turns off certain features of GCC that are incompatible with ISO
1063 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
1064 such as the @code{asm} and @code{typeof} keywords, and
1065 predefined macros such as @code{unix} and @code{vax} that identify the
1066 type of system you are using. It also enables the undesirable and
1067 rarely used ISO trigraph feature. For the C compiler,
1068 it disables recognition of C++ style @samp{//} comments as well as
1069 the @code{inline} keyword.
1071 The alternate keywords @code{__asm__}, @code{__extension__},
1072 @code{__inline__} and @code{__typeof__} continue to work despite
1073 @option{-ansi}. You would not want to use them in an ISO C program, of
1074 course, but it is useful to put them in header files that might be included
1075 in compilations done with @option{-ansi}. Alternate predefined macros
1076 such as @code{__unix__} and @code{__vax__} are also available, with or
1077 without @option{-ansi}.
1079 The @option{-ansi} option does not cause non-ISO programs to be
1080 rejected gratuitously. For that, @option{-pedantic} is required in
1081 addition to @option{-ansi}. @xref{Warning Options}.
1083 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1084 option is used. Some header files may notice this macro and refrain
1085 from declaring certain functions or defining certain macros that the
1086 ISO standard doesn't call for; this is to avoid interfering with any
1087 programs that might use these names for other things.
1089 Functions which would normally be built in but do not have semantics
1090 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1091 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1092 built-in functions provided by GCC}, for details of the functions
1097 Determine the language standard. This option is currently only
1098 supported when compiling C or C++. A value for this option must be
1099 provided; possible values are
1104 ISO C90 (same as @option{-ansi}).
1106 @item iso9899:199409
1107 ISO C90 as modified in amendment 1.
1113 ISO C99. Note that this standard is not yet fully supported; see
1114 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1115 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1118 Default, ISO C90 plus GNU extensions (including some C99 features).
1122 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1123 this will become the default. The name @samp{gnu9x} is deprecated.
1126 The 1998 ISO C++ standard plus amendments.
1129 The same as @option{-std=c++98} plus GNU extensions. This is the
1130 default for C++ code.
1133 Even when this option is not specified, you can still use some of the
1134 features of newer standards in so far as they do not conflict with
1135 previous C standards. For example, you may use @code{__restrict__} even
1136 when @option{-std=c99} is not specified.
1138 The @option{-std} options specifying some version of ISO C have the same
1139 effects as @option{-ansi}, except that features that were not in ISO C90
1140 but are in the specified version (for example, @samp{//} comments and
1141 the @code{inline} keyword in ISO C99) are not disabled.
1143 @xref{Standards,,Language Standards Supported by GCC}, for details of
1144 these standard versions.
1146 @item -aux-info @var{filename}
1148 Output to the given filename prototyped declarations for all functions
1149 declared and/or defined in a translation unit, including those in header
1150 files. This option is silently ignored in any language other than C@.
1152 Besides declarations, the file indicates, in comments, the origin of
1153 each declaration (source file and line), whether the declaration was
1154 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1155 @samp{O} for old, respectively, in the first character after the line
1156 number and the colon), and whether it came from a declaration or a
1157 definition (@samp{C} or @samp{F}, respectively, in the following
1158 character). In the case of function definitions, a K&R-style list of
1159 arguments followed by their declarations is also provided, inside
1160 comments, after the declaration.
1164 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1165 keyword, so that code can use these words as identifiers. You can use
1166 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1167 instead. @option{-ansi} implies @option{-fno-asm}.
1169 In C++, this switch only affects the @code{typeof} keyword, since
1170 @code{asm} and @code{inline} are standard keywords. You may want to
1171 use the @option{-fno-gnu-keywords} flag instead, which has the same
1172 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1173 switch only affects the @code{asm} and @code{typeof} keywords, since
1174 @code{inline} is a standard keyword in ISO C99.
1177 @itemx -fno-builtin-@var{function}
1178 @opindex fno-builtin
1179 @cindex built-in functions
1180 Don't recognize built-in functions that do not begin with
1181 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1182 functions provided by GCC}, for details of the functions affected,
1183 including those which are not built-in functions when @option{-ansi} or
1184 @option{-std} options for strict ISO C conformance are used because they
1185 do not have an ISO standard meaning.
1187 GCC normally generates special code to handle certain built-in functions
1188 more efficiently; for instance, calls to @code{alloca} may become single
1189 instructions that adjust the stack directly, and calls to @code{memcpy}
1190 may become inline copy loops. The resulting code is often both smaller
1191 and faster, but since the function calls no longer appear as such, you
1192 cannot set a breakpoint on those calls, nor can you change the behavior
1193 of the functions by linking with a different library. In addition,
1194 when a function is recognized as a built-in function, GCC may use
1195 information about that function to warn about problems with calls to
1196 that function, or to generate more efficient code, even if the
1197 resulting code still contains calls to that function. For example,
1198 warnings are given with @option{-Wformat} for bad calls to
1199 @code{printf}, when @code{printf} is built in, and @code{strlen} is
1200 known not to modify global memory.
1202 With the @option{-fno-builtin-@var{function}} option
1203 only the built-in function @var{function} is
1204 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1205 function is named this is not built-in in this version of GCC, this
1206 option is ignored. There is no corresponding
1207 @option{-fbuiltin-@var{function}} option; if you wish to enable
1208 built-in functions selectively when using @option{-fno-builtin} or
1209 @option{-ffreestanding}, you may define macros such as:
1212 #define abs(n) __builtin_abs ((n))
1213 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1218 @cindex hosted environment
1220 Assert that compilation takes place in a hosted environment. This implies
1221 @option{-fbuiltin}. A hosted environment is one in which the
1222 entire standard library is available, and in which @code{main} has a return
1223 type of @code{int}. Examples are nearly everything except a kernel.
1224 This is equivalent to @option{-fno-freestanding}.
1226 @item -ffreestanding
1227 @opindex ffreestanding
1228 @cindex hosted environment
1230 Assert that compilation takes place in a freestanding environment. This
1231 implies @option{-fno-builtin}. A freestanding environment
1232 is one in which the standard library may not exist, and program startup may
1233 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1234 This is equivalent to @option{-fno-hosted}.
1236 @xref{Standards,,Language Standards Supported by GCC}, for details of
1237 freestanding and hosted environments.
1239 @item -fms-extensions
1240 @opindex fms-extensions
1241 Accept some non-standard constructs used in Microsoft header files.
1243 Some cases of unnamed fields in structures and unions are only
1244 accepted with this option. @xref{Unnamed Fields,,Unnamed struct/union
1245 fields within structs/unions}, for details.
1249 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1250 options for strict ISO C conformance) implies @option{-trigraphs}.
1252 @item -no-integrated-cpp
1253 @opindex no-integrated-cpp
1254 Performs a compilation in two passes: preprocessing and compiling. This
1255 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1256 @option{-B} option. The user supplied compilation step can then add in
1257 an additional preprocessing step after normal preprocessing but before
1258 compiling. The default is to use the integrated cpp (internal cpp)
1260 The semantics of this option will change if "cc1", "cc1plus", and
1261 "cc1obj" are merged.
1263 @cindex traditional C language
1264 @cindex C language, traditional
1266 @itemx -traditional-cpp
1267 @opindex traditional-cpp
1268 @opindex traditional
1269 Formerly, these options caused GCC to attempt to emulate a pre-standard
1270 C compiler. They are now only supported with the @option{-E} switch.
1271 The preprocessor continues to support a pre-standard mode. See the GNU
1272 CPP manual for details.
1274 @item -fcond-mismatch
1275 @opindex fcond-mismatch
1276 Allow conditional expressions with mismatched types in the second and
1277 third arguments. The value of such an expression is void. This option
1278 is not supported for C++.
1280 @item -funsigned-char
1281 @opindex funsigned-char
1282 Let the type @code{char} be unsigned, like @code{unsigned char}.
1284 Each kind of machine has a default for what @code{char} should
1285 be. It is either like @code{unsigned char} by default or like
1286 @code{signed char} by default.
1288 Ideally, a portable program should always use @code{signed char} or
1289 @code{unsigned char} when it depends on the signedness of an object.
1290 But many programs have been written to use plain @code{char} and
1291 expect it to be signed, or expect it to be unsigned, depending on the
1292 machines they were written for. This option, and its inverse, let you
1293 make such a program work with the opposite default.
1295 The type @code{char} is always a distinct type from each of
1296 @code{signed char} or @code{unsigned char}, even though its behavior
1297 is always just like one of those two.
1300 @opindex fsigned-char
1301 Let the type @code{char} be signed, like @code{signed char}.
1303 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1304 the negative form of @option{-funsigned-char}. Likewise, the option
1305 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1307 @item -fsigned-bitfields
1308 @itemx -funsigned-bitfields
1309 @itemx -fno-signed-bitfields
1310 @itemx -fno-unsigned-bitfields
1311 @opindex fsigned-bitfields
1312 @opindex funsigned-bitfields
1313 @opindex fno-signed-bitfields
1314 @opindex fno-unsigned-bitfields
1315 These options control whether a bit-field is signed or unsigned, when the
1316 declaration does not use either @code{signed} or @code{unsigned}. By
1317 default, such a bit-field is signed, because this is consistent: the
1318 basic integer types such as @code{int} are signed types.
1321 @node C++ Dialect Options
1322 @section Options Controlling C++ Dialect
1324 @cindex compiler options, C++
1325 @cindex C++ options, command line
1326 @cindex options, C++
1327 This section describes the command-line options that are only meaningful
1328 for C++ programs; but you can also use most of the GNU compiler options
1329 regardless of what language your program is in. For example, you
1330 might compile a file @code{firstClass.C} like this:
1333 g++ -g -frepo -O -c firstClass.C
1337 In this example, only @option{-frepo} is an option meant
1338 only for C++ programs; you can use the other options with any
1339 language supported by GCC@.
1341 Here is a list of options that are @emph{only} for compiling C++ programs:
1345 @item -fabi-version=@var{n}
1346 @opindex fabi-version
1347 Use version @var{n} of the C++ ABI@. Version 2 is the version of the
1348 C++ ABI that first appeared in G++ 3.4. Version 1 is the version of
1349 the C++ ABI that first appeared in G++ 3.2. Version 0 will always be
1350 the version that conforms most closely to the C++ ABI specification.
1351 Therefore, the ABI obtained using version 0 will change as ABI bugs
1354 The default is version 2.
1356 @item -fno-access-control
1357 @opindex fno-access-control
1358 Turn off all access checking. This switch is mainly useful for working
1359 around bugs in the access control code.
1363 Check that the pointer returned by @code{operator new} is non-null
1364 before attempting to modify the storage allocated. This check is
1365 normally unnecessary because the C++ standard specifies that
1366 @code{operator new} will only return @code{0} if it is declared
1367 @samp{throw()}, in which case the compiler will always check the
1368 return value even without this option. In all other cases, when
1369 @code{operator new} has a non-empty exception specification, memory
1370 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1371 @samp{new (nothrow)}.
1373 @item -fconserve-space
1374 @opindex fconserve-space
1375 Put uninitialized or runtime-initialized global variables into the
1376 common segment, as C does. This saves space in the executable at the
1377 cost of not diagnosing duplicate definitions. If you compile with this
1378 flag and your program mysteriously crashes after @code{main()} has
1379 completed, you may have an object that is being destroyed twice because
1380 two definitions were merged.
1382 This option is no longer useful on most targets, now that support has
1383 been added for putting variables into BSS without making them common.
1385 @item -fno-const-strings
1386 @opindex fno-const-strings
1387 Give string constants type @code{char *} instead of type @code{const
1388 char *}. By default, G++ uses type @code{const char *} as required by
1389 the standard. Even if you use @option{-fno-const-strings}, you cannot
1390 actually modify the value of a string constant.
1392 This option might be removed in a future release of G++. For maximum
1393 portability, you should structure your code so that it works with
1394 string constants that have type @code{const char *}.
1396 @item -fno-elide-constructors
1397 @opindex fno-elide-constructors
1398 The C++ standard allows an implementation to omit creating a temporary
1399 which is only used to initialize another object of the same type.
1400 Specifying this option disables that optimization, and forces G++ to
1401 call the copy constructor in all cases.
1403 @item -fno-enforce-eh-specs
1404 @opindex fno-enforce-eh-specs
1405 Don't check for violation of exception specifications at runtime. This
1406 option violates the C++ standard, but may be useful for reducing code
1407 size in production builds, much like defining @samp{NDEBUG}. The compiler
1408 will still optimize based on the exception specifications.
1411 @itemx -fno-for-scope
1413 @opindex fno-for-scope
1414 If @option{-ffor-scope} is specified, the scope of variables declared in
1415 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1416 as specified by the C++ standard.
1417 If @option{-fno-for-scope} is specified, the scope of variables declared in
1418 a @i{for-init-statement} extends to the end of the enclosing scope,
1419 as was the case in old versions of G++, and other (traditional)
1420 implementations of C++.
1422 The default if neither flag is given to follow the standard,
1423 but to allow and give a warning for old-style code that would
1424 otherwise be invalid, or have different behavior.
1426 @item -fno-gnu-keywords
1427 @opindex fno-gnu-keywords
1428 Do not recognize @code{typeof} as a keyword, so that code can use this
1429 word as an identifier. You can use the keyword @code{__typeof__} instead.
1430 @option{-ansi} implies @option{-fno-gnu-keywords}.
1432 @item -fno-implicit-templates
1433 @opindex fno-implicit-templates
1434 Never emit code for non-inline templates which are instantiated
1435 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1436 @xref{Template Instantiation}, for more information.
1438 @item -fno-implicit-inline-templates
1439 @opindex fno-implicit-inline-templates
1440 Don't emit code for implicit instantiations of inline templates, either.
1441 The default is to handle inlines differently so that compiles with and
1442 without optimization will need the same set of explicit instantiations.
1444 @item -fno-implement-inlines
1445 @opindex fno-implement-inlines
1446 To save space, do not emit out-of-line copies of inline functions
1447 controlled by @samp{#pragma implementation}. This will cause linker
1448 errors if these functions are not inlined everywhere they are called.
1450 @item -fms-extensions
1451 @opindex fms-extensions
1452 Disable pedantic warnings about constructs used in MFC, such as implicit
1453 int and getting a pointer to member function via non-standard syntax.
1455 @item -fno-nonansi-builtins
1456 @opindex fno-nonansi-builtins
1457 Disable built-in declarations of functions that are not mandated by
1458 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1459 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1461 @item -fno-operator-names
1462 @opindex fno-operator-names
1463 Do not treat the operator name keywords @code{and}, @code{bitand},
1464 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1465 synonyms as keywords.
1467 @item -fno-optional-diags
1468 @opindex fno-optional-diags
1469 Disable diagnostics that the standard says a compiler does not need to
1470 issue. Currently, the only such diagnostic issued by G++ is the one for
1471 a name having multiple meanings within a class.
1474 @opindex fpermissive
1475 Downgrade some diagnostics about nonconformant code from errors to
1476 warnings. Thus, using @option{-fpermissive} will allow some
1477 nonconforming code to compile.
1481 Enable automatic template instantiation at link time. This option also
1482 implies @option{-fno-implicit-templates}. @xref{Template
1483 Instantiation}, for more information.
1487 Disable generation of information about every class with virtual
1488 functions for use by the C++ runtime type identification features
1489 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1490 of the language, you can save some space by using this flag. Note that
1491 exception handling uses the same information, but it will generate it as
1496 Emit statistics about front-end processing at the end of the compilation.
1497 This information is generally only useful to the G++ development team.
1499 @item -ftemplate-depth-@var{n}
1500 @opindex ftemplate-depth
1501 Set the maximum instantiation depth for template classes to @var{n}.
1502 A limit on the template instantiation depth is needed to detect
1503 endless recursions during template class instantiation. ANSI/ISO C++
1504 conforming programs must not rely on a maximum depth greater than 17.
1506 @item -fno-threadsafe-statics
1507 @opindex fno-threadsafe-statics
1508 Do not emit the extra code to use the routines specified in the C++
1509 ABI for thread-safe initialization of local statics. You can use this
1510 option to reduce code size slightly in code that doesn't need to be
1513 @item -fuse-cxa-atexit
1514 @opindex fuse-cxa-atexit
1515 Register destructors for objects with static storage duration with the
1516 @code{__cxa_atexit} function rather than the @code{atexit} function.
1517 This option is required for fully standards-compliant handling of static
1518 destructors, but will only work if your C library supports
1519 @code{__cxa_atexit}.
1521 @item -fvisibility-inlines-hidden
1522 @opindex fvisibility-inlines-hidden
1523 Causes all inlined methods to be marked with
1524 @code{__attribute__ ((visibility ("hidden")))} so that they do not
1525 appear in the export table of a DSO and do not require a PLT indirection
1526 when used within the DSO@. Enabling this option can have a dramatic effect
1527 on load and link times of a DSO as it massively reduces the size of the
1528 dynamic export table when the library makes heavy use of templates. While
1529 it can cause bloating through duplication of code within each DSO where
1530 it is used, often the wastage is less than the considerable space occupied
1531 by a long symbol name in the export table which is typical when using
1532 templates and namespaces. For even more savings, combine with the
1533 @option{-fvisibility=hidden} switch.
1537 Do not use weak symbol support, even if it is provided by the linker.
1538 By default, G++ will use weak symbols if they are available. This
1539 option exists only for testing, and should not be used by end-users;
1540 it will result in inferior code and has no benefits. This option may
1541 be removed in a future release of G++.
1545 Do not search for header files in the standard directories specific to
1546 C++, but do still search the other standard directories. (This option
1547 is used when building the C++ library.)
1550 In addition, these optimization, warning, and code generation options
1551 have meanings only for C++ programs:
1554 @item -fno-default-inline
1555 @opindex fno-default-inline
1556 Do not assume @samp{inline} for functions defined inside a class scope.
1557 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1558 functions will have linkage like inline functions; they just won't be
1561 @item -Wabi @r{(C++ only)}
1563 Warn when G++ generates code that is probably not compatible with the
1564 vendor-neutral C++ ABI@. Although an effort has been made to warn about
1565 all such cases, there are probably some cases that are not warned about,
1566 even though G++ is generating incompatible code. There may also be
1567 cases where warnings are emitted even though the code that is generated
1570 You should rewrite your code to avoid these warnings if you are
1571 concerned about the fact that code generated by G++ may not be binary
1572 compatible with code generated by other compilers.
1574 The known incompatibilities at this point include:
1579 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1580 pack data into the same byte as a base class. For example:
1583 struct A @{ virtual void f(); int f1 : 1; @};
1584 struct B : public A @{ int f2 : 1; @};
1588 In this case, G++ will place @code{B::f2} into the same byte
1589 as@code{A::f1}; other compilers will not. You can avoid this problem
1590 by explicitly padding @code{A} so that its size is a multiple of the
1591 byte size on your platform; that will cause G++ and other compilers to
1592 layout @code{B} identically.
1595 Incorrect handling of tail-padding for virtual bases. G++ does not use
1596 tail padding when laying out virtual bases. For example:
1599 struct A @{ virtual void f(); char c1; @};
1600 struct B @{ B(); char c2; @};
1601 struct C : public A, public virtual B @{@};
1605 In this case, G++ will not place @code{B} into the tail-padding for
1606 @code{A}; other compilers will. You can avoid this problem by
1607 explicitly padding @code{A} so that its size is a multiple of its
1608 alignment (ignoring virtual base classes); that will cause G++ and other
1609 compilers to layout @code{C} identically.
1612 Incorrect handling of bit-fields with declared widths greater than that
1613 of their underlying types, when the bit-fields appear in a union. For
1617 union U @{ int i : 4096; @};
1621 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1622 union too small by the number of bits in an @code{int}.
1625 Empty classes can be placed at incorrect offsets. For example:
1635 struct C : public B, public A @{@};
1639 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1640 it should be placed at offset zero. G++ mistakenly believes that the
1641 @code{A} data member of @code{B} is already at offset zero.
1644 Names of template functions whose types involve @code{typename} or
1645 template template parameters can be mangled incorrectly.
1648 template <typename Q>
1649 void f(typename Q::X) @{@}
1651 template <template <typename> class Q>
1652 void f(typename Q<int>::X) @{@}
1656 Instantiations of these templates may be mangled incorrectly.
1660 @item -Wctor-dtor-privacy @r{(C++ only)}
1661 @opindex Wctor-dtor-privacy
1662 Warn when a class seems unusable because all the constructors or
1663 destructors in that class are private, and it has neither friends nor
1664 public static member functions.
1666 @item -Wnon-virtual-dtor @r{(C++ only)}
1667 @opindex Wnon-virtual-dtor
1668 Warn when a class appears to be polymorphic, thereby requiring a virtual
1669 destructor, yet it declares a non-virtual one.
1670 This warning is enabled by @option{-Wall}.
1672 @item -Wreorder @r{(C++ only)}
1674 @cindex reordering, warning
1675 @cindex warning for reordering of member initializers
1676 Warn when the order of member initializers given in the code does not
1677 match the order in which they must be executed. For instance:
1683 A(): j (0), i (1) @{ @}
1687 The compiler will rearrange the member initializers for @samp{i}
1688 and @samp{j} to match the declaration order of the members, emitting
1689 a warning to that effect. This warning is enabled by @option{-Wall}.
1692 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1695 @item -Weffc++ @r{(C++ only)}
1697 Warn about violations of the following style guidelines from Scott Meyers'
1698 @cite{Effective C++} book:
1702 Item 11: Define a copy constructor and an assignment operator for classes
1703 with dynamically allocated memory.
1706 Item 12: Prefer initialization to assignment in constructors.
1709 Item 14: Make destructors virtual in base classes.
1712 Item 15: Have @code{operator=} return a reference to @code{*this}.
1715 Item 23: Don't try to return a reference when you must return an object.
1719 Also warn about violations of the following style guidelines from
1720 Scott Meyers' @cite{More Effective C++} book:
1724 Item 6: Distinguish between prefix and postfix forms of increment and
1725 decrement operators.
1728 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1732 When selecting this option, be aware that the standard library
1733 headers do not obey all of these guidelines; use @samp{grep -v}
1734 to filter out those warnings.
1736 @item -Wno-deprecated @r{(C++ only)}
1737 @opindex Wno-deprecated
1738 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1740 @item -Wno-non-template-friend @r{(C++ only)}
1741 @opindex Wno-non-template-friend
1742 Disable warnings when non-templatized friend functions are declared
1743 within a template. Since the advent of explicit template specification
1744 support in G++, if the name of the friend is an unqualified-id (i.e.,
1745 @samp{friend foo(int)}), the C++ language specification demands that the
1746 friend declare or define an ordinary, nontemplate function. (Section
1747 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1748 could be interpreted as a particular specialization of a templatized
1749 function. Because this non-conforming behavior is no longer the default
1750 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1751 check existing code for potential trouble spots and is on by default.
1752 This new compiler behavior can be turned off with
1753 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1754 but disables the helpful warning.
1756 @item -Wold-style-cast @r{(C++ only)}
1757 @opindex Wold-style-cast
1758 Warn if an old-style (C-style) cast to a non-void type is used within
1759 a C++ program. The new-style casts (@samp{static_cast},
1760 @samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
1761 unintended effects and much easier to search for.
1763 @item -Woverloaded-virtual @r{(C++ only)}
1764 @opindex Woverloaded-virtual
1765 @cindex overloaded virtual fn, warning
1766 @cindex warning for overloaded virtual fn
1767 Warn when a function declaration hides virtual functions from a
1768 base class. For example, in:
1775 struct B: public A @{
1780 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1788 will fail to compile.
1790 @item -Wno-pmf-conversions @r{(C++ only)}
1791 @opindex Wno-pmf-conversions
1792 Disable the diagnostic for converting a bound pointer to member function
1795 @item -Wsign-promo @r{(C++ only)}
1796 @opindex Wsign-promo
1797 Warn when overload resolution chooses a promotion from unsigned or
1798 enumerated type to a signed type, over a conversion to an unsigned type of
1799 the same size. Previous versions of G++ would try to preserve
1800 unsignedness, but the standard mandates the current behavior.
1805 A& operator = (int);
1815 In this example, G++ will synthesize a default @samp{A& operator =
1816 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1819 @node Objective-C and Objective-C++ Dialect Options
1820 @section Options Controlling Objective-C and Objective-C++ Dialects
1822 @cindex compiler options, Objective-C and Objective-C++
1823 @cindex Objective-C and Objective-C++ options, command line
1824 @cindex options, Objective-C and Objective-C++
1825 (NOTE: This manual does not describe the Objective-C and Objective-C++
1826 languages themselves. See @xref{Standards,,Language Standards
1827 Supported by GCC}, for references.)
1829 This section describes the command-line options that are only meaningful
1830 for Objective-C and Objective-C++ programs, but you can also use most of
1831 the language-independent GNU compiler options.
1832 For example, you might compile a file @code{some_class.m} like this:
1835 gcc -g -fgnu-runtime -O -c some_class.m
1839 In this example, @option{-fgnu-runtime} is an option meant only for
1840 Objective-C and Objective-C++ programs; you can use the other options with
1841 any language supported by GCC@.
1843 Note that since Objective-C is an extension of the C language, Objective-C
1844 compilations may also use options specific to the C front-end (e.g.,
1845 @option{-Wtraditional}). Similarly, Objective-C++ compilations may use
1846 C++-specific options (e.g., @option{-Wabi}).
1848 Here is a list of options that are @emph{only} for compiling Objective-C
1849 and Objective-C++ programs:
1852 @item -fconstant-string-class=@var{class-name}
1853 @opindex fconstant-string-class
1854 Use @var{class-name} as the name of the class to instantiate for each
1855 literal string specified with the syntax @code{@@"@dots{}"}. The default
1856 class name is @code{NXConstantString} if the GNU runtime is being used, and
1857 @code{NSConstantString} if the NeXT runtime is being used (see below). The
1858 @option{-fconstant-cfstrings} option, if also present, will override the
1859 @option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
1860 to be laid out as constant CoreFoundation strings.
1863 @opindex fgnu-runtime
1864 Generate object code compatible with the standard GNU Objective-C
1865 runtime. This is the default for most types of systems.
1867 @item -fnext-runtime
1868 @opindex fnext-runtime
1869 Generate output compatible with the NeXT runtime. This is the default
1870 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1871 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1874 @item -fno-nil-receivers
1875 @opindex fno-nil-receivers
1876 Assume that all Objective-C message dispatches (e.g.,
1877 @code{[receiver message:arg]}) in this translation unit ensure that the receiver
1878 is not @code{nil}. This allows for more efficient entry points in the runtime
1879 to be used. Currently, this option is only available in conjunction with
1880 the NeXT runtime on Mac OS X 10.3 and later.
1882 @item -fobjc-exceptions
1883 @opindex fobjc-exceptions
1884 Enable syntactic support for structured exception handling in Objective-C,
1885 similar to what is offered by C++ and Java. Currently, this option is only
1886 available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
1894 @@catch (AnObjCClass *exc) @{
1901 @@catch (AnotherClass *exc) @{
1904 @@catch (id allOthers) @{
1914 The @code{@@throw} statement may appear anywhere in an Objective-C or
1915 Objective-C++ program; when used inside of a @code{@@catch} block, the
1916 @code{@@throw} may appear without an argument (as shown above), in which case
1917 the object caught by the @code{@@catch} will be rethrown.
1919 Note that only (pointers to) Objective-C objects may be thrown and
1920 caught using this scheme. When an object is thrown, it will be caught
1921 by the nearest @code{@@catch} clause capable of handling objects of that type,
1922 analogously to how @code{catch} blocks work in C++ and Java. A
1923 @code{@@catch(id @dots{})} clause (as shown above) may also be provided to catch
1924 any and all Objective-C exceptions not caught by previous @code{@@catch}
1927 The @code{@@finally} clause, if present, will be executed upon exit from the
1928 immediately preceding @code{@@try @dots{} @@catch} section. This will happen
1929 regardless of whether any exceptions are thrown, caught or rethrown
1930 inside the @code{@@try @dots{} @@catch} section, analogously to the behavior
1931 of the @code{finally} clause in Java.
1933 There are several caveats to using the new exception mechanism:
1937 Although currently designed to be binary compatible with @code{NS_HANDLER}-style
1938 idioms provided by the @code{NSException} class, the new
1939 exceptions can only be used on Mac OS X 10.3 (Panther) and later
1940 systems, due to additional functionality needed in the (NeXT) Objective-C
1944 As mentioned above, the new exceptions do not support handling
1945 types other than Objective-C objects. Furthermore, when used from
1946 Objective-C++, the Objective-C exception model does not interoperate with C++
1947 exceptions at this time. This means you cannot @code{@@throw} an exception
1948 from Objective-C and @code{catch} it in C++, or vice versa
1949 (i.e., @code{throw @dots{} @@catch}).
1952 The @option{-fobjc-exceptions} switch also enables the use of synchronization
1953 blocks for thread-safe execution:
1956 @@synchronized (ObjCClass *guard) @{
1961 Upon entering the @code{@@synchronized} block, a thread of execution shall
1962 first check whether a lock has been placed on the corresponding @code{guard}
1963 object by another thread. If it has, the current thread shall wait until
1964 the other thread relinquishes its lock. Once @code{guard} becomes available,
1965 the current thread will place its own lock on it, execute the code contained in
1966 the @code{@@synchronized} block, and finally relinquish the lock (thereby
1967 making @code{guard} available to other threads).
1969 Unlike Java, Objective-C does not allow for entire methods to be marked
1970 @code{@@synchronized}. Note that throwing exceptions out of
1971 @code{@@synchronized} blocks is allowed, and will cause the guarding object
1972 to be unlocked properly.
1974 @item -freplace-objc-classes
1975 @opindex freplace-objc-classes
1976 Emit a special marker instructing @command{ld(1)} not to statically link in
1977 the resulting object file, and allow @command{dyld(1)} to load it in at
1978 run time instead. This is used in conjunction with the Fix-and-Continue
1979 debugging mode, where the object file in question may be recompiled and
1980 dynamically reloaded in the course of program execution, without the need
1981 to restart the program itself. Currently, Fix-and-Continue functionality
1982 is only available in conjunction with the NeXT runtime on Mac OS X 10.3
1987 When compiling for the NeXT runtime, the compiler ordinarily replaces calls
1988 to @code{objc_getClass("@dots{}")} (when the name of the class is known at
1989 compile time) with static class references that get initialized at load time,
1990 which improves run-time performance. Specifying the @option{-fzero-link} flag
1991 suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
1992 to be retained. This is useful in Zero-Link debugging mode, since it allows
1993 for individual class implementations to be modified during program execution.
1997 Dump interface declarations for all classes seen in the source file to a
1998 file named @file{@var{sourcename}.decl}.
2001 @opindex Wno-protocol
2002 If a class is declared to implement a protocol, a warning is issued for
2003 every method in the protocol that is not implemented by the class. The
2004 default behavior is to issue a warning for every method not explicitly
2005 implemented in the class, even if a method implementation is inherited
2006 from the superclass. If you use the @option{-Wno-protocol} option, then
2007 methods inherited from the superclass are considered to be implemented,
2008 and no warning is issued for them.
2012 Warn if multiple methods of different types for the same selector are
2013 found during compilation. The check is performed on the list of methods
2014 in the final stage of compilation. Additionally, a check is performed
2015 for each selector appearing in a @code{@@selector(@dots{})}
2016 expression, and a corresponding method for that selector has been found
2017 during compilation. Because these checks scan the method table only at
2018 the end of compilation, these warnings are not produced if the final
2019 stage of compilation is not reached, for example because an error is
2020 found during compilation, or because the @option{-fsyntax-only} option is
2023 @item -Wundeclared-selector
2024 @opindex Wundeclared-selector
2025 Warn if a @code{@@selector(@dots{})} expression referring to an
2026 undeclared selector is found. A selector is considered undeclared if no
2027 method with that name has been declared before the
2028 @code{@@selector(@dots{})} expression, either explicitly in an
2029 @code{@@interface} or @code{@@protocol} declaration, or implicitly in
2030 an @code{@@implementation} section. This option always performs its
2031 checks as soon as a @code{@@selector(@dots{})} expression is found,
2032 while @option{-Wselector} only performs its checks in the final stage of
2033 compilation. This also enforces the coding style convention
2034 that methods and selectors must be declared before being used.
2036 @item -print-objc-runtime-info
2037 @opindex print-objc-runtime-info
2038 Generate C header describing the largest structure that is passed by
2043 @node Language Independent Options
2044 @section Options to Control Diagnostic Messages Formatting
2045 @cindex options to control diagnostics formatting
2046 @cindex diagnostic messages
2047 @cindex message formatting
2049 Traditionally, diagnostic messages have been formatted irrespective of
2050 the output device's aspect (e.g.@: its width, @dots{}). The options described
2051 below can be used to control the diagnostic messages formatting
2052 algorithm, e.g.@: how many characters per line, how often source location
2053 information should be reported. Right now, only the C++ front end can
2054 honor these options. However it is expected, in the near future, that
2055 the remaining front ends would be able to digest them correctly.
2058 @item -fmessage-length=@var{n}
2059 @opindex fmessage-length
2060 Try to format error messages so that they fit on lines of about @var{n}
2061 characters. The default is 72 characters for @command{g++} and 0 for the rest of
2062 the front ends supported by GCC@. If @var{n} is zero, then no
2063 line-wrapping will be done; each error message will appear on a single
2066 @opindex fdiagnostics-show-location
2067 @item -fdiagnostics-show-location=once
2068 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
2069 reporter to emit @emph{once} source location information; that is, in
2070 case the message is too long to fit on a single physical line and has to
2071 be wrapped, the source location won't be emitted (as prefix) again,
2072 over and over, in subsequent continuation lines. This is the default
2075 @item -fdiagnostics-show-location=every-line
2076 Only meaningful in line-wrapping mode. Instructs the diagnostic
2077 messages reporter to emit the same source location information (as
2078 prefix) for physical lines that result from the process of breaking
2079 a message which is too long to fit on a single line.
2083 @node Warning Options
2084 @section Options to Request or Suppress Warnings
2085 @cindex options to control warnings
2086 @cindex warning messages
2087 @cindex messages, warning
2088 @cindex suppressing warnings
2090 Warnings are diagnostic messages that report constructions which
2091 are not inherently erroneous but which are risky or suggest there
2092 may have been an error.
2094 You can request many specific warnings with options beginning @samp{-W},
2095 for example @option{-Wimplicit} to request warnings on implicit
2096 declarations. Each of these specific warning options also has a
2097 negative form beginning @samp{-Wno-} to turn off warnings;
2098 for example, @option{-Wno-implicit}. This manual lists only one of the
2099 two forms, whichever is not the default.
2101 The following options control the amount and kinds of warnings produced
2102 by GCC; for further, language-specific options also refer to
2103 @ref{C++ Dialect Options} and @ref{Objective-C and Objective-C++ Dialect
2107 @cindex syntax checking
2109 @opindex fsyntax-only
2110 Check the code for syntax errors, but don't do anything beyond that.
2114 Issue all the warnings demanded by strict ISO C and ISO C++;
2115 reject all programs that use forbidden extensions, and some other
2116 programs that do not follow ISO C and ISO C++. For ISO C, follows the
2117 version of the ISO C standard specified by any @option{-std} option used.
2119 Valid ISO C and ISO C++ programs should compile properly with or without
2120 this option (though a rare few will require @option{-ansi} or a
2121 @option{-std} option specifying the required version of ISO C)@. However,
2122 without this option, certain GNU extensions and traditional C and C++
2123 features are supported as well. With this option, they are rejected.
2125 @option{-pedantic} does not cause warning messages for use of the
2126 alternate keywords whose names begin and end with @samp{__}. Pedantic
2127 warnings are also disabled in the expression that follows
2128 @code{__extension__}. However, only system header files should use
2129 these escape routes; application programs should avoid them.
2130 @xref{Alternate Keywords}.
2132 Some users try to use @option{-pedantic} to check programs for strict ISO
2133 C conformance. They soon find that it does not do quite what they want:
2134 it finds some non-ISO practices, but not all---only those for which
2135 ISO C @emph{requires} a diagnostic, and some others for which
2136 diagnostics have been added.
2138 A feature to report any failure to conform to ISO C might be useful in
2139 some instances, but would require considerable additional work and would
2140 be quite different from @option{-pedantic}. We don't have plans to
2141 support such a feature in the near future.
2143 Where the standard specified with @option{-std} represents a GNU
2144 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
2145 corresponding @dfn{base standard}, the version of ISO C on which the GNU
2146 extended dialect is based. Warnings from @option{-pedantic} are given
2147 where they are required by the base standard. (It would not make sense
2148 for such warnings to be given only for features not in the specified GNU
2149 C dialect, since by definition the GNU dialects of C include all
2150 features the compiler supports with the given option, and there would be
2151 nothing to warn about.)
2153 @item -pedantic-errors
2154 @opindex pedantic-errors
2155 Like @option{-pedantic}, except that errors are produced rather than
2160 Inhibit all warning messages.
2164 Inhibit warning messages about the use of @samp{#import}.
2166 @item -Wchar-subscripts
2167 @opindex Wchar-subscripts
2168 Warn if an array subscript has type @code{char}. This is a common cause
2169 of error, as programmers often forget that this type is signed on some
2171 This warning is enabled by @option{-Wall}.
2175 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
2176 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
2177 This warning is enabled by @option{-Wall}.
2179 @item -Wfatal-errors
2180 @opindex Wfatal-errors
2181 This option causes the compiler to abort compilation on the first error
2182 occurred rather than trying to keep going and printing further error
2187 @opindex ffreestanding
2188 @opindex fno-builtin
2189 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
2190 the arguments supplied have types appropriate to the format string
2191 specified, and that the conversions specified in the format string make
2192 sense. This includes standard functions, and others specified by format
2193 attributes (@pxref{Function Attributes}), in the @code{printf},
2194 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
2195 not in the C standard) families (or other target-specific families).
2196 Which functions are checked without format attributes having been
2197 specified depends on the standard version selected, and such checks of
2198 functions without the attribute specified are disabled by
2199 @option{-ffreestanding} or @option{-fno-builtin}.
2201 The formats are checked against the format features supported by GNU
2202 libc version 2.2. These include all ISO C90 and C99 features, as well
2203 as features from the Single Unix Specification and some BSD and GNU
2204 extensions. Other library implementations may not support all these
2205 features; GCC does not support warning about features that go beyond a
2206 particular library's limitations. However, if @option{-pedantic} is used
2207 with @option{-Wformat}, warnings will be given about format features not
2208 in the selected standard version (but not for @code{strfmon} formats,
2209 since those are not in any version of the C standard). @xref{C Dialect
2210 Options,,Options Controlling C Dialect}.
2212 Since @option{-Wformat} also checks for null format arguments for
2213 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2215 @option{-Wformat} is included in @option{-Wall}. For more control over some
2216 aspects of format checking, the options @option{-Wformat-y2k},
2217 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2218 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2219 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2222 @opindex Wformat-y2k
2223 If @option{-Wformat} is specified, also warn about @code{strftime}
2224 formats which may yield only a two-digit year.
2226 @item -Wno-format-extra-args
2227 @opindex Wno-format-extra-args
2228 If @option{-Wformat} is specified, do not warn about excess arguments to a
2229 @code{printf} or @code{scanf} format function. The C standard specifies
2230 that such arguments are ignored.
2232 Where the unused arguments lie between used arguments that are
2233 specified with @samp{$} operand number specifications, normally
2234 warnings are still given, since the implementation could not know what
2235 type to pass to @code{va_arg} to skip the unused arguments. However,
2236 in the case of @code{scanf} formats, this option will suppress the
2237 warning if the unused arguments are all pointers, since the Single
2238 Unix Specification says that such unused arguments are allowed.
2240 @item -Wno-format-zero-length
2241 @opindex Wno-format-zero-length
2242 If @option{-Wformat} is specified, do not warn about zero-length formats.
2243 The C standard specifies that zero-length formats are allowed.
2245 @item -Wformat-nonliteral
2246 @opindex Wformat-nonliteral
2247 If @option{-Wformat} is specified, also warn if the format string is not a
2248 string literal and so cannot be checked, unless the format function
2249 takes its format arguments as a @code{va_list}.
2251 @item -Wformat-security
2252 @opindex Wformat-security
2253 If @option{-Wformat} is specified, also warn about uses of format
2254 functions that represent possible security problems. At present, this
2255 warns about calls to @code{printf} and @code{scanf} functions where the
2256 format string is not a string literal and there are no format arguments,
2257 as in @code{printf (foo);}. This may be a security hole if the format
2258 string came from untrusted input and contains @samp{%n}. (This is
2259 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2260 in future warnings may be added to @option{-Wformat-security} that are not
2261 included in @option{-Wformat-nonliteral}.)
2265 Enable @option{-Wformat} plus format checks not included in
2266 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2267 -Wformat-nonliteral -Wformat-security -Wformat-y2k}.
2271 Warn about passing a null pointer for arguments marked as
2272 requiring a non-null value by the @code{nonnull} function attribute.
2274 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2275 can be disabled with the @option{-Wno-nonnull} option.
2277 @item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
2279 Warn about uninitialized variables which are initialized with themselves.
2280 Note this option can only be used with the @option{-Wuninitialized} option,
2281 which in turn only works with @option{-O1} and above.
2283 For example, GCC will warn about @code{i} being uninitialized in the
2284 following snippet only when @option{-Winit-self} has been specified:
2295 @item -Wimplicit-int
2296 @opindex Wimplicit-int
2297 Warn when a declaration does not specify a type.
2298 This warning is enabled by @option{-Wall}.
2300 @item -Wimplicit-function-declaration
2301 @itemx -Werror-implicit-function-declaration
2302 @opindex Wimplicit-function-declaration
2303 @opindex Werror-implicit-function-declaration
2304 Give a warning (or error) whenever a function is used before being
2305 declared. The form @option{-Wno-error-implicit-function-declaration}
2307 This warning is enabled by @option{-Wall} (as a warning, not an error).
2311 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2312 This warning is enabled by @option{-Wall}.
2316 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2317 function with external linkage, returning int, taking either zero
2318 arguments, two, or three arguments of appropriate types.
2319 This warning is enabled by @option{-Wall}.
2321 @item -Wmissing-braces
2322 @opindex Wmissing-braces
2323 Warn if an aggregate or union initializer is not fully bracketed. In
2324 the following example, the initializer for @samp{a} is not fully
2325 bracketed, but that for @samp{b} is fully bracketed.
2328 int a[2][2] = @{ 0, 1, 2, 3 @};
2329 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2332 This warning is enabled by @option{-Wall}.
2334 @item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
2335 @opindex Wmissing-include-dirs
2336 Warn if a user-supplied include directory does not exist.
2339 @opindex Wparentheses
2340 Warn if parentheses are omitted in certain contexts, such
2341 as when there is an assignment in a context where a truth value
2342 is expected, or when operators are nested whose precedence people
2343 often get confused about. Only the warning for an assignment used as
2344 a truth value is supported when compiling C++; the other warnings are
2345 only supported when compiling C@.
2347 Also warn if a comparison like @samp{x<=y<=z} appears; this is
2348 equivalent to @samp{(x<=y ? 1 : 0) <= z}, which is a different
2349 interpretation from that of ordinary mathematical notation.
2351 Also warn about constructions where there may be confusion to which
2352 @code{if} statement an @code{else} branch belongs. Here is an example of
2367 In C, every @code{else} branch belongs to the innermost possible @code{if}
2368 statement, which in this example is @code{if (b)}. This is often not
2369 what the programmer expected, as illustrated in the above example by
2370 indentation the programmer chose. When there is the potential for this
2371 confusion, GCC will issue a warning when this flag is specified.
2372 To eliminate the warning, add explicit braces around the innermost
2373 @code{if} statement so there is no way the @code{else} could belong to
2374 the enclosing @code{if}. The resulting code would look like this:
2390 This warning is enabled by @option{-Wall}.
2392 @item -Wsequence-point
2393 @opindex Wsequence-point
2394 Warn about code that may have undefined semantics because of violations
2395 of sequence point rules in the C standard.
2397 The C standard defines the order in which expressions in a C program are
2398 evaluated in terms of @dfn{sequence points}, which represent a partial
2399 ordering between the execution of parts of the program: those executed
2400 before the sequence point, and those executed after it. These occur
2401 after the evaluation of a full expression (one which is not part of a
2402 larger expression), after the evaluation of the first operand of a
2403 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2404 function is called (but after the evaluation of its arguments and the
2405 expression denoting the called function), and in certain other places.
2406 Other than as expressed by the sequence point rules, the order of
2407 evaluation of subexpressions of an expression is not specified. All
2408 these rules describe only a partial order rather than a total order,
2409 since, for example, if two functions are called within one expression
2410 with no sequence point between them, the order in which the functions
2411 are called is not specified. However, the standards committee have
2412 ruled that function calls do not overlap.
2414 It is not specified when between sequence points modifications to the
2415 values of objects take effect. Programs whose behavior depends on this
2416 have undefined behavior; the C standard specifies that ``Between the
2417 previous and next sequence point an object shall have its stored value
2418 modified at most once by the evaluation of an expression. Furthermore,
2419 the prior value shall be read only to determine the value to be
2420 stored.''. If a program breaks these rules, the results on any
2421 particular implementation are entirely unpredictable.
2423 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2424 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2425 diagnosed by this option, and it may give an occasional false positive
2426 result, but in general it has been found fairly effective at detecting
2427 this sort of problem in programs.
2429 The present implementation of this option only works for C programs. A
2430 future implementation may also work for C++ programs.
2432 The C standard is worded confusingly, therefore there is some debate
2433 over the precise meaning of the sequence point rules in subtle cases.
2434 Links to discussions of the problem, including proposed formal
2435 definitions, may be found on the GCC readings page, at
2436 @w{@uref{http://gcc.gnu.org/readings.html}}.
2438 This warning is enabled by @option{-Wall}.
2441 @opindex Wreturn-type
2442 Warn whenever a function is defined with a return-type that defaults to
2443 @code{int}. Also warn about any @code{return} statement with no
2444 return-value in a function whose return-type is not @code{void}.
2446 For C, also warn if the return type of a function has a type qualifier
2447 such as @code{const}. Such a type qualifier has no effect, since the
2448 value returned by a function is not an lvalue. ISO C prohibits
2449 qualified @code{void} return types on function definitions, so such
2450 return types always receive a warning even without this option.
2452 For C++, a function without return type always produces a diagnostic
2453 message, even when @option{-Wno-return-type} is specified. The only
2454 exceptions are @samp{main} and functions defined in system headers.
2456 This warning is enabled by @option{-Wall}.
2460 Warn whenever a @code{switch} statement has an index of enumerated type
2461 and lacks a @code{case} for one or more of the named codes of that
2462 enumeration. (The presence of a @code{default} label prevents this
2463 warning.) @code{case} labels outside the enumeration range also
2464 provoke warnings when this option is used.
2465 This warning is enabled by @option{-Wall}.
2467 @item -Wswitch-default
2468 @opindex Wswitch-switch
2469 Warn whenever a @code{switch} statement does not have a @code{default}
2473 @opindex Wswitch-enum
2474 Warn whenever a @code{switch} statement has an index of enumerated type
2475 and lacks a @code{case} for one or more of the named codes of that
2476 enumeration. @code{case} labels outside the enumeration range also
2477 provoke warnings when this option is used.
2481 Warn if any trigraphs are encountered that might change the meaning of
2482 the program (trigraphs within comments are not warned about).
2483 This warning is enabled by @option{-Wall}.
2485 @item -Wunused-function
2486 @opindex Wunused-function
2487 Warn whenever a static function is declared but not defined or a
2488 non\-inline static function is unused.
2489 This warning is enabled by @option{-Wall}.
2491 @item -Wunused-label
2492 @opindex Wunused-label
2493 Warn whenever a label is declared but not used.
2494 This warning is enabled by @option{-Wall}.
2496 To suppress this warning use the @samp{unused} attribute
2497 (@pxref{Variable Attributes}).
2499 @item -Wunused-parameter
2500 @opindex Wunused-parameter
2501 Warn whenever a function parameter is unused aside from its declaration.
2503 To suppress this warning use the @samp{unused} attribute
2504 (@pxref{Variable Attributes}).
2506 @item -Wunused-variable
2507 @opindex Wunused-variable
2508 Warn whenever a local variable or non-constant static variable is unused
2509 aside from its declaration
2510 This warning is enabled by @option{-Wall}.
2512 To suppress this warning use the @samp{unused} attribute
2513 (@pxref{Variable Attributes}).
2515 @item -Wunused-value
2516 @opindex Wunused-value
2517 Warn whenever a statement computes a result that is explicitly not used.
2518 This warning is enabled by @option{-Wall}.
2520 To suppress this warning cast the expression to @samp{void}.
2524 All the above @option{-Wunused} options combined.
2526 In order to get a warning about an unused function parameter, you must
2527 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2528 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2530 @item -Wuninitialized
2531 @opindex Wuninitialized
2532 Warn if an automatic variable is used without first being initialized or
2533 if a variable may be clobbered by a @code{setjmp} call.
2535 These warnings are possible only in optimizing compilation,
2536 because they require data flow information that is computed only
2537 when optimizing. If you don't specify @option{-O}, you simply won't
2540 If you want to warn about code which uses the uninitialized value of the
2541 variable in its own initializer, use the @option{-Winit-self} option.
2543 These warnings occur only for variables that are candidates for
2544 register allocation. Therefore, they do not occur for a variable that
2545 is declared @code{volatile}, or whose address is taken, or whose size
2546 is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
2547 structures, unions or arrays, even when they are in registers.
2549 Note that there may be no warning about a variable that is used only
2550 to compute a value that itself is never used, because such
2551 computations may be deleted by data flow analysis before the warnings
2554 These warnings are made optional because GCC is not smart
2555 enough to see all the reasons why the code might be correct
2556 despite appearing to have an error. Here is one example of how
2577 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2578 always initialized, but GCC doesn't know this. Here is
2579 another common case:
2584 if (change_y) save_y = y, y = new_y;
2586 if (change_y) y = save_y;
2591 This has no bug because @code{save_y} is used only if it is set.
2593 @cindex @code{longjmp} warnings
2594 This option also warns when a non-volatile automatic variable might be
2595 changed by a call to @code{longjmp}. These warnings as well are possible
2596 only in optimizing compilation.
2598 The compiler sees only the calls to @code{setjmp}. It cannot know
2599 where @code{longjmp} will be called; in fact, a signal handler could
2600 call it at any point in the code. As a result, you may get a warning
2601 even when there is in fact no problem because @code{longjmp} cannot
2602 in fact be called at the place which would cause a problem.
2604 Some spurious warnings can be avoided if you declare all the functions
2605 you use that never return as @code{noreturn}. @xref{Function
2608 This warning is enabled by @option{-Wall}.
2610 @item -Wunknown-pragmas
2611 @opindex Wunknown-pragmas
2612 @cindex warning for unknown pragmas
2613 @cindex unknown pragmas, warning
2614 @cindex pragmas, warning of unknown
2615 Warn when a #pragma directive is encountered which is not understood by
2616 GCC@. If this command line option is used, warnings will even be issued
2617 for unknown pragmas in system header files. This is not the case if
2618 the warnings were only enabled by the @option{-Wall} command line option.
2620 @item -Wstrict-aliasing
2621 @opindex Wstrict-aliasing
2622 This option is only active when @option{-fstrict-aliasing} is active.
2623 It warns about code which might break the strict aliasing rules that the
2624 compiler is using for optimization. The warning does not catch all
2625 cases, but does attempt to catch the more common pitfalls. It is
2626 included in @option{-Wall}.
2628 @item -Wstrict-aliasing=2
2629 @opindex Wstrict-aliasing=2
2630 This option is only active when @option{-fstrict-aliasing} is active.
2631 It warns about all code which might break the strict aliasing rules that the
2632 compiler is using for optimization. This warning catches all cases, but
2633 it will also give a warning for some ambiguous cases that are safe.
2637 All of the above @samp{-W} options combined. This enables all the
2638 warnings about constructions that some users consider questionable, and
2639 that are easy to avoid (or modify to prevent the warning), even in
2640 conjunction with macros. This also enables some language-specific
2641 warnings described in @ref{C++ Dialect Options} and
2642 @ref{Objective-C and Objective-C++ Dialect Options}.
2645 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2646 Some of them warn about constructions that users generally do not
2647 consider questionable, but which occasionally you might wish to check
2648 for; others warn about constructions that are necessary or hard to avoid
2649 in some cases, and there is no simple way to modify the code to suppress
2656 (This option used to be called @option{-W}. The older name is still
2657 supported, but the newer name is more descriptive.) Print extra warning
2658 messages for these events:
2662 A function can return either with or without a value. (Falling
2663 off the end of the function body is considered returning without
2664 a value.) For example, this function would evoke such a
2678 An expression-statement or the left-hand side of a comma expression
2679 contains no side effects.
2680 To suppress the warning, cast the unused expression to void.
2681 For example, an expression such as @samp{x[i,j]} will cause a warning,
2682 but @samp{x[(void)i,j]} will not.
2685 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2688 Storage-class specifiers like @code{static} are not the first things in
2689 a declaration. According to the C Standard, this usage is obsolescent.
2692 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2696 A comparison between signed and unsigned values could produce an
2697 incorrect result when the signed value is converted to unsigned.
2698 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2701 An aggregate has an initializer which does not initialize all members.
2702 This warning can be independently controlled by
2703 @option{-Wmissing-field-initializers}.
2706 A function parameter is declared without a type specifier in K&R-style
2714 An empty body occurs in an @samp{if} or @samp{else} statement.
2717 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2718 @samp{>}, or @samp{>=}.
2721 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2724 Any of several floating-point events that often indicate errors, such as
2725 overflow, underflow, loss of precision, etc.
2727 @item @r{(C++ only)}
2728 An enumerator and a non-enumerator both appear in a conditional expression.
2730 @item @r{(C++ only)}
2731 A non-static reference or non-static @samp{const} member appears in a
2732 class without constructors.
2734 @item @r{(C++ only)}
2735 Ambiguous virtual bases.
2737 @item @r{(C++ only)}
2738 Subscripting an array which has been declared @samp{register}.
2740 @item @r{(C++ only)}
2741 Taking the address of a variable which has been declared @samp{register}.
2743 @item @r{(C++ only)}
2744 A base class is not initialized in a derived class' copy constructor.
2747 @item -Wno-div-by-zero
2748 @opindex Wno-div-by-zero
2749 @opindex Wdiv-by-zero
2750 Do not warn about compile-time integer division by zero. Floating point
2751 division by zero is not warned about, as it can be a legitimate way of
2752 obtaining infinities and NaNs.
2754 @item -Wsystem-headers
2755 @opindex Wsystem-headers
2756 @cindex warnings from system headers
2757 @cindex system headers, warnings from
2758 Print warning messages for constructs found in system header files.
2759 Warnings from system headers are normally suppressed, on the assumption
2760 that they usually do not indicate real problems and would only make the
2761 compiler output harder to read. Using this command line option tells
2762 GCC to emit warnings from system headers as if they occurred in user
2763 code. However, note that using @option{-Wall} in conjunction with this
2764 option will @emph{not} warn about unknown pragmas in system
2765 headers---for that, @option{-Wunknown-pragmas} must also be used.
2768 @opindex Wfloat-equal
2769 Warn if floating point values are used in equality comparisons.
2771 The idea behind this is that sometimes it is convenient (for the
2772 programmer) to consider floating-point values as approximations to
2773 infinitely precise real numbers. If you are doing this, then you need
2774 to compute (by analyzing the code, or in some other way) the maximum or
2775 likely maximum error that the computation introduces, and allow for it
2776 when performing comparisons (and when producing output, but that's a
2777 different problem). In particular, instead of testing for equality, you
2778 would check to see whether the two values have ranges that overlap; and
2779 this is done with the relational operators, so equality comparisons are
2782 @item -Wtraditional @r{(C only)}
2783 @opindex Wtraditional
2784 Warn about certain constructs that behave differently in traditional and
2785 ISO C@. Also warn about ISO C constructs that have no traditional C
2786 equivalent, and/or problematic constructs which should be avoided.
2790 Macro parameters that appear within string literals in the macro body.
2791 In traditional C macro replacement takes place within string literals,
2792 but does not in ISO C@.
2795 In traditional C, some preprocessor directives did not exist.
2796 Traditional preprocessors would only consider a line to be a directive
2797 if the @samp{#} appeared in column 1 on the line. Therefore
2798 @option{-Wtraditional} warns about directives that traditional C
2799 understands but would ignore because the @samp{#} does not appear as the
2800 first character on the line. It also suggests you hide directives like
2801 @samp{#pragma} not understood by traditional C by indenting them. Some
2802 traditional implementations would not recognize @samp{#elif}, so it
2803 suggests avoiding it altogether.
2806 A function-like macro that appears without arguments.
2809 The unary plus operator.
2812 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2813 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2814 constants.) Note, these suffixes appear in macros defined in the system
2815 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2816 Use of these macros in user code might normally lead to spurious
2817 warnings, however GCC's integrated preprocessor has enough context to
2818 avoid warning in these cases.
2821 A function declared external in one block and then used after the end of
2825 A @code{switch} statement has an operand of type @code{long}.
2828 A non-@code{static} function declaration follows a @code{static} one.
2829 This construct is not accepted by some traditional C compilers.
2832 The ISO type of an integer constant has a different width or
2833 signedness from its traditional type. This warning is only issued if
2834 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2835 typically represent bit patterns, are not warned about.
2838 Usage of ISO string concatenation is detected.
2841 Initialization of automatic aggregates.
2844 Identifier conflicts with labels. Traditional C lacks a separate
2845 namespace for labels.
2848 Initialization of unions. If the initializer is zero, the warning is
2849 omitted. This is done under the assumption that the zero initializer in
2850 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2851 initializer warnings and relies on default initialization to zero in the
2855 Conversions by prototypes between fixed/floating point values and vice
2856 versa. The absence of these prototypes when compiling with traditional
2857 C would cause serious problems. This is a subset of the possible
2858 conversion warnings, for the full set use @option{-Wconversion}.
2861 Use of ISO C style function definitions. This warning intentionally is
2862 @emph{not} issued for prototype declarations or variadic functions
2863 because these ISO C features will appear in your code when using
2864 libiberty's traditional C compatibility macros, @code{PARAMS} and
2865 @code{VPARAMS}. This warning is also bypassed for nested functions
2866 because that feature is already a GCC extension and thus not relevant to
2867 traditional C compatibility.
2870 @item -Wdeclaration-after-statement @r{(C only)}
2871 @opindex Wdeclaration-after-statement
2872 Warn when a declaration is found after a statement in a block. This
2873 construct, known from C++, was introduced with ISO C99 and is by default
2874 allowed in GCC@. It is not supported by ISO C90 and was not supported by
2875 GCC versions before GCC 3.0. @xref{Mixed Declarations}.
2879 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
2881 @item -Wno-endif-labels
2882 @opindex Wno-endif-labels
2883 @opindex Wendif-labels
2884 Do not warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
2888 Warn whenever a local variable shadows another local variable, parameter or
2889 global variable or whenever a built-in function is shadowed.
2891 @item -Wlarger-than-@var{len}
2892 @opindex Wlarger-than
2893 Warn whenever an object of larger than @var{len} bytes is defined.
2895 @item -Wpointer-arith
2896 @opindex Wpointer-arith
2897 Warn about anything that depends on the ``size of'' a function type or
2898 of @code{void}. GNU C assigns these types a size of 1, for
2899 convenience in calculations with @code{void *} pointers and pointers
2902 @item -Wbad-function-cast @r{(C only)}
2903 @opindex Wbad-function-cast
2904 Warn whenever a function call is cast to a non-matching type.
2905 For example, warn if @code{int malloc()} is cast to @code{anything *}.
2909 Warn whenever a pointer is cast so as to remove a type qualifier from
2910 the target type. For example, warn if a @code{const char *} is cast
2911 to an ordinary @code{char *}.
2914 @opindex Wcast-align
2915 Warn whenever a pointer is cast such that the required alignment of the
2916 target is increased. For example, warn if a @code{char *} is cast to
2917 an @code{int *} on machines where integers can only be accessed at
2918 two- or four-byte boundaries.
2920 @item -Wwrite-strings
2921 @opindex Wwrite-strings
2922 When compiling C, give string constants the type @code{const
2923 char[@var{length}]} so that
2924 copying the address of one into a non-@code{const} @code{char *}
2925 pointer will get a warning; when compiling C++, warn about the
2926 deprecated conversion from string constants to @code{char *}.
2927 These warnings will help you find at
2928 compile time code that can try to write into a string constant, but
2929 only if you have been very careful about using @code{const} in
2930 declarations and prototypes. Otherwise, it will just be a nuisance;
2931 this is why we did not make @option{-Wall} request these warnings.
2934 @opindex Wconversion
2935 Warn if a prototype causes a type conversion that is different from what
2936 would happen to the same argument in the absence of a prototype. This
2937 includes conversions of fixed point to floating and vice versa, and
2938 conversions changing the width or signedness of a fixed point argument
2939 except when the same as the default promotion.
2941 Also, warn if a negative integer constant expression is implicitly
2942 converted to an unsigned type. For example, warn about the assignment
2943 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
2944 casts like @code{(unsigned) -1}.
2946 @item -Wsign-compare
2947 @opindex Wsign-compare
2948 @cindex warning for comparison of signed and unsigned values
2949 @cindex comparison of signed and unsigned values, warning
2950 @cindex signed and unsigned values, comparison warning
2951 Warn when a comparison between signed and unsigned values could produce
2952 an incorrect result when the signed value is converted to unsigned.
2953 This warning is also enabled by @option{-Wextra}; to get the other warnings
2954 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
2956 @item -Waggregate-return
2957 @opindex Waggregate-return
2958 Warn if any functions that return structures or unions are defined or
2959 called. (In languages where you can return an array, this also elicits
2962 @item -Wstrict-prototypes @r{(C only)}
2963 @opindex Wstrict-prototypes
2964 Warn if a function is declared or defined without specifying the
2965 argument types. (An old-style function definition is permitted without
2966 a warning if preceded by a declaration which specifies the argument
2969 @item -Wold-style-definition @r{(C only)}
2970 @opindex Wold-style-definition
2971 Warn if an old-style function definition is used. A warning is given
2972 even if there is a previous prototype.
2974 @item -Wmissing-prototypes @r{(C only)}
2975 @opindex Wmissing-prototypes
2976 Warn if a global function is defined without a previous prototype
2977 declaration. This warning is issued even if the definition itself
2978 provides a prototype. The aim is to detect global functions that fail
2979 to be declared in header files.
2981 @item -Wmissing-declarations @r{(C only)}
2982 @opindex Wmissing-declarations
2983 Warn if a global function is defined without a previous declaration.
2984 Do so even if the definition itself provides a prototype.
2985 Use this option to detect global functions that are not declared in
2988 @item -Wmissing-field-initializers
2989 @opindex Wmissing-field-initializers
2992 Warn if a structure's initializer has some fields missing. For
2993 example, the following code would cause such a warning, because
2994 @code{x.h} is implicitly zero:
2997 struct s @{ int f, g, h; @};
2998 struct s x = @{ 3, 4 @};
3001 This option does not warn about designated initializers, so the following
3002 modification would not trigger a warning:
3005 struct s @{ int f, g, h; @};
3006 struct s x = @{ .f = 3, .g = 4 @};
3009 This warning is included in @option{-Wextra}. To get other @option{-Wextra}
3010 warnings without this one, use @samp{-Wextra -Wno-missing-field-initializers}.
3012 @item -Wmissing-noreturn
3013 @opindex Wmissing-noreturn
3014 Warn about functions which might be candidates for attribute @code{noreturn}.
3015 Note these are only possible candidates, not absolute ones. Care should
3016 be taken to manually verify functions actually do not ever return before
3017 adding the @code{noreturn} attribute, otherwise subtle code generation
3018 bugs could be introduced. You will not get a warning for @code{main} in
3019 hosted C environments.
3021 @item -Wmissing-format-attribute
3022 @opindex Wmissing-format-attribute
3024 If @option{-Wformat} is enabled, also warn about functions which might be
3025 candidates for @code{format} attributes. Note these are only possible
3026 candidates, not absolute ones. GCC will guess that @code{format}
3027 attributes might be appropriate for any function that calls a function
3028 like @code{vprintf} or @code{vscanf}, but this might not always be the
3029 case, and some functions for which @code{format} attributes are
3030 appropriate may not be detected. This option has no effect unless
3031 @option{-Wformat} is enabled (possibly by @option{-Wall}).
3033 @item -Wno-multichar
3034 @opindex Wno-multichar
3036 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
3037 Usually they indicate a typo in the user's code, as they have
3038 implementation-defined values, and should not be used in portable code.
3040 @item -Wnormalized=<none|id|nfc|nfkc>
3041 @opindex Wnormalized
3044 @cindex character set, input normalization
3045 In ISO C and ISO C++, two identifiers are different if they are
3046 different sequences of characters. However, sometimes when characters
3047 outside the basic ASCII character set are used, you can have two
3048 different character sequences that look the same. To avoid confusion,
3049 the ISO 10646 standard sets out some @dfn{normalization rules} which
3050 when applied ensure that two sequences that look the same are turned into
3051 the same sequence. GCC can warn you if you are using identifiers which
3052 have not been normalized; this option controls that warning.
3054 There are four levels of warning that GCC supports. The default is
3055 @option{-Wnormalized=nfc}, which warns about any identifier which is
3056 not in the ISO 10646 ``C'' normalized form, @dfn{NFC}. NFC is the
3057 recommended form for most uses.
3059 Unfortunately, there are some characters which ISO C and ISO C++ allow
3060 in identifiers that when turned into NFC aren't allowable as
3061 identifiers. That is, there's no way to use these symbols in portable
3062 ISO C or C++ and have all your identifiers in NFC.
3063 @option{-Wnormalized=id} suppresses the warning for these characters.
3064 It is hoped that future versions of the standards involved will correct
3065 this, which is why this option is not the default.
3067 You can switch the warning off for all characters by writing
3068 @option{-Wnormalized=none}. You would only want to do this if you
3069 were using some other normalization scheme (like ``D''), because
3070 otherwise you can easily create bugs that are literally impossible to see.
3072 Some characters in ISO 10646 have distinct meanings but look identical
3073 in some fonts or display methodologies, especially once formatting has
3074 been applied. For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
3075 LETTER N'', will display just like a regular @code{n} which has been
3076 placed in a superscript. ISO 10646 defines the @dfn{NFKC}
3077 normalisation scheme to convert all these into a standard form as
3078 well, and GCC will warn if your code is not in NFKC if you use
3079 @option{-Wnormalized=nfkc}. This warning is comparable to warning
3080 about every identifier that contains the letter O because it might be
3081 confused with the digit 0, and so is not the default, but may be
3082 useful as a local coding convention if the programming environment is
3083 unable to be fixed to display these characters distinctly.
3085 @item -Wno-deprecated-declarations
3086 @opindex Wno-deprecated-declarations
3087 Do not warn about uses of functions, variables, and types marked as
3088 deprecated by using the @code{deprecated} attribute.
3089 (@pxref{Function Attributes}, @pxref{Variable Attributes},
3090 @pxref{Type Attributes}.)
3094 Warn if a structure is given the packed attribute, but the packed
3095 attribute has no effect on the layout or size of the structure.
3096 Such structures may be mis-aligned for little benefit. For
3097 instance, in this code, the variable @code{f.x} in @code{struct bar}
3098 will be misaligned even though @code{struct bar} does not itself
3099 have the packed attribute:
3106 @} __attribute__((packed));
3116 Warn if padding is included in a structure, either to align an element
3117 of the structure or to align the whole structure. Sometimes when this
3118 happens it is possible to rearrange the fields of the structure to
3119 reduce the padding and so make the structure smaller.
3121 @item -Wredundant-decls
3122 @opindex Wredundant-decls
3123 Warn if anything is declared more than once in the same scope, even in
3124 cases where multiple declaration is valid and changes nothing.
3126 @item -Wnested-externs @r{(C only)}
3127 @opindex Wnested-externs
3128 Warn if an @code{extern} declaration is encountered within a function.
3130 @item -Wunreachable-code
3131 @opindex Wunreachable-code
3132 Warn if the compiler detects that code will never be executed.
3134 This option is intended to warn when the compiler detects that at
3135 least a whole line of source code will never be executed, because
3136 some condition is never satisfied or because it is after a
3137 procedure that never returns.
3139 It is possible for this option to produce a warning even though there
3140 are circumstances under which part of the affected line can be executed,
3141 so care should be taken when removing apparently-unreachable code.
3143 For instance, when a function is inlined, a warning may mean that the
3144 line is unreachable in only one inlined copy of the function.
3146 This option is not made part of @option{-Wall} because in a debugging
3147 version of a program there is often substantial code which checks
3148 correct functioning of the program and is, hopefully, unreachable
3149 because the program does work. Another common use of unreachable
3150 code is to provide behavior which is selectable at compile-time.
3154 Warn if a function can not be inlined and it was declared as inline.
3155 Even with this option, the compiler will not warn about failures to
3156 inline functions declared in system headers.
3158 The compiler uses a variety of heuristics to determine whether or not
3159 to inline a function. For example, the compiler takes into account
3160 the size of the function being inlined and the the amount of inlining
3161 that has already been done in the current function. Therefore,
3162 seemingly insignificant changes in the source program can cause the
3163 warnings produced by @option{-Winline} to appear or disappear.
3165 @item -Wno-invalid-offsetof @r{(C++ only)}
3166 @opindex Wno-invalid-offsetof
3167 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
3168 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
3169 to a non-POD type is undefined. In existing C++ implementations,
3170 however, @samp{offsetof} typically gives meaningful results even when
3171 applied to certain kinds of non-POD types. (Such as a simple
3172 @samp{struct} that fails to be a POD type only by virtue of having a
3173 constructor.) This flag is for users who are aware that they are
3174 writing nonportable code and who have deliberately chosen to ignore the
3177 The restrictions on @samp{offsetof} may be relaxed in a future version
3178 of the C++ standard.
3181 @opindex Winvalid-pch
3182 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
3183 the search path but can't be used.
3187 @opindex Wno-long-long
3188 Warn if @samp{long long} type is used. This is default. To inhibit
3189 the warning messages, use @option{-Wno-long-long}. Flags
3190 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
3191 only when @option{-pedantic} flag is used.
3193 @item -Wvariadic-macros
3194 @opindex Wvariadic-macros
3195 @opindex Wno-variadic-macros
3196 Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU
3197 alternate syntax when in pedantic ISO C99 mode. This is default.
3198 To inhibit the warning messages, use @option{-Wno-variadic-macros}.
3200 @item -Wdisabled-optimization
3201 @opindex Wdisabled-optimization
3202 Warn if a requested optimization pass is disabled. This warning does
3203 not generally indicate that there is anything wrong with your code; it
3204 merely indicates that GCC's optimizers were unable to handle the code
3205 effectively. Often, the problem is that your code is too big or too
3206 complex; GCC will refuse to optimize programs when the optimization
3207 itself is likely to take inordinate amounts of time.
3209 @item -Wno-pointer-sign
3210 @opindex Wno-pointer-sign
3211 Don't warn for pointer argument passing or assignment with different signedness.
3212 Only useful in the negative form since this warning is enabled by default.
3213 This option is only supported for C and Objective-C@.
3217 Make all warnings into errors.
3220 @node Debugging Options
3221 @section Options for Debugging Your Program or GCC
3222 @cindex options, debugging
3223 @cindex debugging information options
3225 GCC has various special options that are used for debugging
3226 either your program or GCC:
3231 Produce debugging information in the operating system's native format
3232 (stabs, COFF, XCOFF, or DWARF 2)@. GDB can work with this debugging
3235 On most systems that use stabs format, @option{-g} enables use of extra
3236 debugging information that only GDB can use; this extra information
3237 makes debugging work better in GDB but will probably make other debuggers
3239 refuse to read the program. If you want to control for certain whether
3240 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
3241 @option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).
3243 GCC allows you to use @option{-g} with
3244 @option{-O}. The shortcuts taken by optimized code may occasionally
3245 produce surprising results: some variables you declared may not exist
3246 at all; flow of control may briefly move where you did not expect it;
3247 some statements may not be executed because they compute constant
3248 results or their values were already at hand; some statements may
3249 execute in different places because they were moved out of loops.
3251 Nevertheless it proves possible to debug optimized output. This makes
3252 it reasonable to use the optimizer for programs that might have bugs.
3254 The following options are useful when GCC is generated with the
3255 capability for more than one debugging format.
3259 Produce debugging information for use by GDB@. This means to use the
3260 most expressive format available (DWARF 2, stabs, or the native format
3261 if neither of those are supported), including GDB extensions if at all
3266 Produce debugging information in stabs format (if that is supported),
3267 without GDB extensions. This is the format used by DBX on most BSD
3268 systems. On MIPS, Alpha and System V Release 4 systems this option
3269 produces stabs debugging output which is not understood by DBX or SDB@.
3270 On System V Release 4 systems this option requires the GNU assembler.
3272 @item -feliminate-unused-debug-symbols
3273 @opindex feliminate-unused-debug-symbols
3274 Produce debugging information in stabs format (if that is supported),
3275 for only symbols that are actually used.
3279 Produce debugging information in stabs format (if that is supported),
3280 using GNU extensions understood only by the GNU debugger (GDB)@. The
3281 use of these extensions is likely to make other debuggers crash or
3282 refuse to read the program.
3286 Produce debugging information in COFF format (if that is supported).
3287 This is the format used by SDB on most System V systems prior to
3292 Produce debugging information in XCOFF format (if that is supported).
3293 This is the format used by the DBX debugger on IBM RS/6000 systems.
3297 Produce debugging information in XCOFF format (if that is supported),
3298 using GNU extensions understood only by the GNU debugger (GDB)@. The
3299 use of these extensions is likely to make other debuggers crash or
3300 refuse to read the program, and may cause assemblers other than the GNU
3301 assembler (GAS) to fail with an error.
3305 Produce debugging information in DWARF version 2 format (if that is
3306 supported). This is the format used by DBX on IRIX 6. With this
3307 option, GCC uses features of DWARF version 3 when they are useful;
3308 version 3 is upward compatible with version 2, but may still cause
3309 problems for older debuggers.
3313 Produce debugging information in VMS debug format (if that is
3314 supported). This is the format used by DEBUG on VMS systems.
3317 @itemx -ggdb@var{level}
3318 @itemx -gstabs@var{level}
3319 @itemx -gcoff@var{level}
3320 @itemx -gxcoff@var{level}
3321 @itemx -gvms@var{level}
3322 Request debugging information and also use @var{level} to specify how
3323 much information. The default level is 2.
3325 Level 1 produces minimal information, enough for making backtraces in
3326 parts of the program that you don't plan to debug. This includes
3327 descriptions of functions and external variables, but no information
3328 about local variables and no line numbers.
3330 Level 3 includes extra information, such as all the macro definitions
3331 present in the program. Some debuggers support macro expansion when
3332 you use @option{-g3}.
3334 @option{-gdwarf-2} does not accept a concatenated debug level, because
3335 GCC used to support an option @option{-gdwarf} that meant to generate
3336 debug information in version 1 of the DWARF format (which is very
3337 different from version 2), and it would have been too confusing. That
3338 debug format is long obsolete, but the option cannot be changed now.
3339 Instead use an additional @option{-g@var{level}} option to change the
3340 debug level for DWARF2.
3342 @item -feliminate-dwarf2-dups
3343 @opindex feliminate-dwarf2-dups
3344 Compress DWARF2 debugging information by eliminating duplicated
3345 information about each symbol. This option only makes sense when
3346 generating DWARF2 debugging information with @option{-gdwarf-2}.
3348 @cindex @command{prof}
3351 Generate extra code to write profile information suitable for the
3352 analysis program @command{prof}. You must use this option when compiling
3353 the source files you want data about, and you must also use it when
3356 @cindex @command{gprof}
3359 Generate extra code to write profile information suitable for the
3360 analysis program @command{gprof}. You must use this option when compiling
3361 the source files you want data about, and you must also use it when
3366 Makes the compiler print out each function name as it is compiled, and
3367 print some statistics about each pass when it finishes.
3370 @opindex ftime-report
3371 Makes the compiler print some statistics about the time consumed by each
3372 pass when it finishes.
3375 @opindex fmem-report
3376 Makes the compiler print some statistics about permanent memory
3377 allocation when it finishes.
3379 @item -fprofile-arcs
3380 @opindex fprofile-arcs
3381 Add code so that program flow @dfn{arcs} are instrumented. During
3382 execution the program records how many times each branch and call is
3383 executed and how many times it is taken or returns. When the compiled
3384 program exits it saves this data to a file called
3385 @file{@var{auxname}.gcda} for each source file. The data may be used for
3386 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3387 test coverage analysis (@option{-ftest-coverage}). Each object file's
3388 @var{auxname} is generated from the name of the output file, if
3389 explicitly specified and it is not the final executable, otherwise it is
3390 the basename of the source file. In both cases any suffix is removed
3391 (e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
3392 @file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
3394 @cindex @command{gcov}
3398 This option is used to compile and link code instrumented for coverage
3399 analysis. The option is a synonym for @option{-fprofile-arcs}
3400 @option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
3401 linking). See the documentation for those options for more details.
3406 Compile the source files with @option{-fprofile-arcs} plus optimization
3407 and code generation options. For test coverage analysis, use the
3408 additional @option{-ftest-coverage} option. You do not need to profile
3409 every source file in a program.
3412 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3413 (the latter implies the former).
3416 Run the program on a representative workload to generate the arc profile
3417 information. This may be repeated any number of times. You can run
3418 concurrent instances of your program, and provided that the file system
3419 supports locking, the data files will be correctly updated. Also
3420 @code{fork} calls are detected and correctly handled (double counting
3424 For profile-directed optimizations, compile the source files again with
3425 the same optimization and code generation options plus
3426 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3427 Control Optimization}).
3430 For test coverage analysis, use @command{gcov} to produce human readable
3431 information from the @file{.gcno} and @file{.gcda} files. Refer to the
3432 @command{gcov} documentation for further information.
3436 With @option{-fprofile-arcs}, for each function of your program GCC
3437 creates a program flow graph, then finds a spanning tree for the graph.
3438 Only arcs that are not on the spanning tree have to be instrumented: the
3439 compiler adds code to count the number of times that these arcs are
3440 executed. When an arc is the only exit or only entrance to a block, the
3441 instrumentation code can be added to the block; otherwise, a new basic
3442 block must be created to hold the instrumentation code.
3444 @item -ftree-based-profiling
3445 @opindex ftree-based-profiling
3446 This option is used in addition to @option{-fprofile-arcs} or
3447 @option{-fbranch-probabilities} to control whether those optimizations
3448 are performed on a tree-based or rtl-based internal representation.
3449 If you use this option when compiling with @option{-fprofile-arcs},
3450 you must also use it when compiling later with @option{-fbranch-probabilities}.
3451 Currently the tree-based optimization is in an early stage of
3452 development, and this option is recommended only for those people
3453 working on improving it.
3456 @item -ftest-coverage
3457 @opindex ftest-coverage
3458 Produce a notes file that the @command{gcov} code-coverage utility
3459 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3460 show program coverage. Each source file's note file is called
3461 @file{@var{auxname}.gcno}. Refer to the @option{-fprofile-arcs} option
3462 above for a description of @var{auxname} and instructions on how to
3463 generate test coverage data. Coverage data will match the source files
3464 more closely, if you do not optimize.
3466 @item -d@var{letters}
3467 @item -fdump-rtl-@var{pass}
3469 Says to make debugging dumps during compilation at times specified by
3470 @var{letters}. This is used for debugging the RTL-based passes of the
3471 compiler. The file names for most of the dumps are made by appending a
3472 pass number and a word to the @var{dumpname}. @var{dumpname} is generated
3473 from the name of the output file, if explicitly specified and it is not
3474 an executable, otherwise it is the basename of the source file.
3476 Most debug dumps can be enabled either passing a letter to the @option{-d}
3477 option, or with a long @option{-fdump-rtl} switch; here are the possible
3478 letters for use in @var{letters} and @var{pass}, and their meanings:
3483 Annotate the assembler output with miscellaneous debugging information.
3486 @itemx -fdump-rtl-bp
3488 @opindex fdump-rtl-bp
3489 Dump after computing branch probabilities, to @file{@var{file}.09.bp}.
3492 @itemx -fdump-rtl-bbro
3494 @opindex fdump-rtl-bbro
3495 Dump after block reordering, to @file{@var{file}.30.bbro}.
3498 @itemx -fdump-rtl-combine
3500 @opindex fdump-rtl-combine
3501 Dump after instruction combination, to the file @file{@var{file}.17.combine}.
3504 @itemx -fdump-rtl-ce1
3505 @itemx -fdump-rtl-ce2
3507 @opindex fdump-rtl-ce1
3508 @opindex fdump-rtl-ce2
3509 @option{-dC} and @option{-fdump-rtl-ce1} enable dumping after the
3510 first if conversion, to the file @file{@var{file}.11.ce1}. @option{-dC}
3511 and @option{-fdump-rtl-ce2} enable dumping after the second if
3512 conversion, to the file @file{@var{file}.18.ce2}.
3515 @itemx -fdump-rtl-btl
3516 @itemx -fdump-rtl-dbr
3518 @opindex fdump-rtl-btl
3519 @opindex fdump-rtl-dbr
3520 @option{-dd} and @option{-fdump-rtl-btl} enable dumping after branch
3521 target load optimization, to to @file{@var{file}.31.btl}. @option{-dd}
3522 and @option{-fdump-rtl-dbr} enable dumping after delayed branch
3523 scheduling, to @file{@var{file}.36.dbr}.
3527 Dump all macro definitions, at the end of preprocessing, in addition to
3531 @itemx -fdump-rtl-ce3
3533 @opindex fdump-rtl-ce3
3534 Dump after the third if conversion, to @file{@var{file}.28.ce3}.
3537 @itemx -fdump-rtl-cfg
3538 @itemx -fdump-rtl-life
3540 @opindex fdump-rtl-cfg
3541 @opindex fdump-rtl-life
3542 @option{-df} and @option{-fdump-rtl-cfg} enable dumping after control
3543 and data flow analysis, to @file{@var{file}.08.cfg}. @option{-df}
3544 and @option{-fdump-rtl-cfg} enable dumping dump after life analysis,
3545 to @file{@var{file}.16.life}.
3548 @itemx -fdump-rtl-greg
3550 @opindex fdump-rtl-greg
3551 Dump after global register allocation, to @file{@var{file}.23.greg}.
3554 @itemx -fdump-rtl-gcse
3555 @itemx -fdump-rtl-bypass
3557 @opindex fdump-rtl-gcse
3558 @opindex fdump-rtl-bypass
3559 @option{-dG} and @option{-fdump-rtl-gcse} enable dumping after GCSE, to
3560 @file{@var{file}.05.gcse}. @option{-dG} and @option{-fdump-rtl-bypass}
3561 enable dumping after jump bypassing and control flow optimizations, to
3562 @file{@var{file}.07.bypass}.
3565 @itemx -fdump-rtl-eh
3567 @opindex fdump-rtl-eh
3568 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3571 @itemx -fdump-rtl-sibling
3573 @opindex fdump-rtl-sibling
3574 Dump after sibling call optimizations, to @file{@var{file}.01.sibling}.
3577 @itemx -fdump-rtl-jump
3579 @opindex fdump-rtl-jump
3580 Dump after the first jump optimization, to @file{@var{file}.03.jump}.
3583 @itemx -fdump-rtl-stack
3585 @opindex fdump-rtl-stack
3586 Dump after conversion from registers to stack, to @file{@var{file}.33.stack}.
3589 @itemx -fdump-rtl-lreg
3591 @opindex fdump-rtl-lreg
3592 Dump after local register allocation, to @file{@var{file}.22.lreg}.
3595 @itemx -fdump-rtl-loop
3596 @itemx -fdump-rtl-loop2
3598 @opindex fdump-rtl-loop
3599 @opindex fdump-rtl-loop2
3600 @option{-dL} and @option{-fdump-rtl-loop} enable dumping after the first
3601 loop optimization pass, to @file{@var{file}.06.loop}. @option{-dL} and
3602 @option{-fdump-rtl-loop2} enable dumping after the second pass, to
3603 @file{@var{file}.13.loop2}.
3606 @itemx -fdump-rtl-sms
3608 @opindex fdump-rtl-sms
3609 Dump after modulo scheduling, to @file{@var{file}.20.sms}.
3612 @itemx -fdump-rtl-mach
3614 @opindex fdump-rtl-mach
3615 Dump after performing the machine dependent reorganization pass, to
3616 @file{@var{file}.35.mach}.
3619 @itemx -fdump-rtl-rnreg
3621 @opindex fdump-rtl-rnreg
3622 Dump after register renumbering, to @file{@var{file}.29.rnreg}.
3625 @itemx -fdump-rtl-regmove
3627 @opindex fdump-rtl-regmove
3628 Dump after the register move pass, to @file{@var{file}.19.regmove}.
3631 @itemx -fdump-rtl-postreload
3633 @opindex fdump-rtl-postreload
3634 Dump after post-reload optimizations, to @file{@var{file}.24.postreload}.
3637 @itemx -fdump-rtl-expand
3639 @opindex fdump-rtl-expand
3640 Dump after RTL generation, to @file{@var{file}.00.expand}.
3643 @itemx -fdump-rtl-sched2
3645 @opindex fdump-rtl-sched2
3646 Dump after the second scheduling pass, to @file{@var{file}.32.sched2}.
3649 @itemx -fdump-rtl-cse
3651 @opindex fdump-rtl-cse
3652 Dump after CSE (including the jump optimization that sometimes follows
3653 CSE), to @file{@var{file}.04.cse}.
3656 @itemx -fdump-rtl-sched
3658 @opindex fdump-rtl-sched
3659 Dump after the first scheduling pass, to @file{@var{file}.21.sched}.
3662 @itemx -fdump-rtl-cse2
3664 @opindex fdump-rtl-cse2
3665 Dump after the second CSE pass (including the jump optimization that
3666 sometimes follows CSE), to @file{@var{file}.15.cse2}.
3669 @itemx -fdump-rtl-tracer
3671 @opindex fdump-rtl-tracer
3672 Dump after running tracer, to @file{@var{file}.12.tracer}.
3675 @itemx -fdump-rtl-vpt
3676 @itemx -fdump-rtl-vartrack
3678 @opindex fdump-rtl-vpt
3679 @opindex fdump-rtl-vartrack
3680 @option{-dV} and @option{-fdump-rtl-vpt} enable dumping after the value
3681 profile transformations, to @file{@var{file}.10.vpt}. @option{-dV}
3682 and @option{-fdump-rtl-vartrack} enable dumping after variable tracking,
3683 to @file{@var{file}.34.vartrack}.
3686 @itemx -fdump-rtl-flow2
3688 @opindex fdump-rtl-flow2
3689 Dump after the second flow pass, to @file{@var{file}.26.flow2}.
3692 @itemx -fdump-rtl-peephole2
3694 @opindex fdump-rtl-peephole2
3695 Dump after the peephole pass, to @file{@var{file}.27.peephole2}.
3698 @itemx -fdump-rtl-web
3700 @opindex fdump-rtl-web
3701 Dump after live range splitting, to @file{@var{file}.14.web}.
3704 @itemx -fdump-rtl-all
3706 @opindex fdump-rtl-all
3707 Produce all the dumps listed above.
3711 Produce a core dump whenever an error occurs.
3715 Print statistics on memory usage, at the end of the run, to
3720 Annotate the assembler output with a comment indicating which
3721 pattern and alternative was used. The length of each instruction is
3726 Dump the RTL in the assembler output as a comment before each instruction.
3727 Also turns on @option{-dp} annotation.
3731 For each of the other indicated dump files (either with @option{-d} or
3732 @option{-fdump-rtl-@var{pass}}), dump a representation of the control flow
3733 graph suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3737 Just generate RTL for a function instead of compiling it. Usually used
3738 with @samp{r} (@option{-fdump-rtl-expand}).
3742 Dump debugging information during parsing, to standard error.
3745 @item -fdump-unnumbered
3746 @opindex fdump-unnumbered
3747 When doing debugging dumps (see @option{-d} option above), suppress instruction
3748 numbers and line number note output. This makes it more feasible to
3749 use diff on debugging dumps for compiler invocations with different
3750 options, in particular with and without @option{-g}.
3752 @item -fdump-translation-unit @r{(C and C++ only)}
3753 @itemx -fdump-translation-unit-@var{options} @r{(C and C++ only)}
3754 @opindex fdump-translation-unit
3755 Dump a representation of the tree structure for the entire translation
3756 unit to a file. The file name is made by appending @file{.tu} to the
3757 source file name. If the @samp{-@var{options}} form is used, @var{options}
3758 controls the details of the dump as described for the
3759 @option{-fdump-tree} options.
3761 @item -fdump-class-hierarchy @r{(C++ only)}
3762 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3763 @opindex fdump-class-hierarchy
3764 Dump a representation of each class's hierarchy and virtual function
3765 table layout to a file. The file name is made by appending @file{.class}
3766 to the source file name. If the @samp{-@var{options}} form is used,
3767 @var{options} controls the details of the dump as described for the
3768 @option{-fdump-tree} options.
3770 @item -fdump-ipa-@var{switch}
3772 Control the dumping at various stages of inter-procedural analysis
3773 language tree to a file. The file name is generated by appending a switch
3774 specific suffix to the source file name. The following dumps are possible:
3778 Enables all inter-procedural analysis dumps; currently the only produced
3779 dump is the @samp{cgraph} dump.
3782 Dumps information about call-graph optimization, unused function removal,
3783 and inlining decisions.
3786 @item -fdump-tree-@var{switch} @r{(C and C++ only)}
3787 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C and C++ only)}
3789 Control the dumping at various stages of processing the intermediate
3790 language tree to a file. The file name is generated by appending a switch
3791 specific suffix to the source file name. If the @samp{-@var{options}}
3792 form is used, @var{options} is a list of @samp{-} separated options that
3793 control the details of the dump. Not all options are applicable to all
3794 dumps, those which are not meaningful will be ignored. The following
3795 options are available
3799 Print the address of each node. Usually this is not meaningful as it
3800 changes according to the environment and source file. Its primary use
3801 is for tying up a dump file with a debug environment.
3803 Inhibit dumping of members of a scope or body of a function merely
3804 because that scope has been reached. Only dump such items when they
3805 are directly reachable by some other path. When dumping pretty-printed
3806 trees, this option inhibits dumping the bodies of control structures.
3808 Print a raw representation of the tree. By default, trees are
3809 pretty-printed into a C-like representation.
3811 Enable more detailed dumps (not honored by every dump option).
3813 Enable dumping various statistics about the pass (not honored by every dump
3816 Enable showing basic block boundaries (disabled in raw dumps).
3818 Enable showing virtual operands for every statement.
3820 Enable showing line numbers for statements.
3822 Enable showing the unique ID (@code{DECL_UID}) for each variable.
3824 Turn on all options, except @option{raw}, @option{slim} and @option{lineno}.
3827 The following tree dumps are possible:
3831 Dump before any tree based optimization, to @file{@var{file}.original}.
3834 Dump after all tree based optimization, to @file{@var{file}.optimized}.
3837 Dump after function inlining, to @file{@var{file}.inlined}.
3840 @opindex fdump-tree-gimple
3841 Dump each function before and after the gimplification pass to a file. The
3842 file name is made by appending @file{.gimple} to the source file name.
3845 @opindex fdump-tree-cfg
3846 Dump the control flow graph of each function to a file. The file name is
3847 made by appending @file{.cfg} to the source file name.
3850 @opindex fdump-tree-vcg
3851 Dump the control flow graph of each function to a file in VCG format. The
3852 file name is made by appending @file{.vcg} to the source file name. Note
3853 that if the file contains more than one function, the generated file cannot
3854 be used directly by VCG@. You will need to cut and paste each function's
3855 graph into its own separate file first.
3858 @opindex fdump-tree-ch
3859 Dump each function after copying loop headers. The file name is made by
3860 appending @file{.ch} to the source file name.
3863 @opindex fdump-tree-ssa
3864 Dump SSA related information to a file. The file name is made by appending
3865 @file{.ssa} to the source file name.
3868 @opindex fdump-tree-salias
3869 Dump structure aliasing variable information to a file. This file name
3870 is made by appending @file{.salias} to the source file name.
3873 @opindex fdump-tree-alias
3874 Dump aliasing information for each function. The file name is made by
3875 appending @file{.alias} to the source file name.
3878 @opindex fdump-tree-ccp
3879 Dump each function after CCP@. The file name is made by appending
3880 @file{.ccp} to the source file name.
3883 @opindex fdump-tree-pre
3884 Dump trees after partial redundancy elimination. The file name is made
3885 by appending @file{.pre} to the source file name.
3888 @opindex fdump-tree-fre
3889 Dump trees after full redundancy elimination. The file name is made
3890 by appending @file{.fre} to the source file name.
3893 @opindex fdump-tree-dce
3894 Dump each function after dead code elimination. The file name is made by
3895 appending @file{.dce} to the source file name.
3898 @opindex fdump-tree-mudflap
3899 Dump each function after adding mudflap instrumentation. The file name is
3900 made by appending @file{.mudflap} to the source file name.
3903 @opindex fdump-tree-sra
3904 Dump each function after performing scalar replacement of aggregates. The
3905 file name is made by appending @file{.sra} to the source file name.
3908 @opindex fdump-tree-sink
3909 Dump each function after performing code sinking. The file name is made
3910 by appending @file{.sink} to the source file name.
3913 @opindex fdump-tree-dom
3914 Dump each function after applying dominator tree optimizations. The file
3915 name is made by appending @file{.dom} to the source file name.
3918 @opindex fdump-tree-dse
3919 Dump each function after applying dead store elimination. The file
3920 name is made by appending @file{.dse} to the source file name.
3923 @opindex fdump-tree-phiopt
3924 Dump each function after optimizing PHI nodes into straightline code. The file
3925 name is made by appending @file{.phiopt} to the source file name.
3928 @opindex fdump-tree-forwprop
3929 Dump each function after forward propagating single use variables. The file
3930 name is made by appending @file{.forwprop} to the source file name.
3933 @opindex fdump-tree-copyrename
3934 Dump each function after applying the copy rename optimization. The file
3935 name is made by appending @file{.copyrename} to the source file name.
3938 @opindex fdump-tree-nrv
3939 Dump each function after applying the named return value optimization on
3940 generic trees. The file name is made by appending @file{.nrv} to the source
3944 @opindex fdump-tree-vect
3945 Dump each function after applying vectorization of loops. The file name is
3946 made by appending @file{.vect} to the source file name.
3949 @opindex fdump-tree-all
3950 Enable all the available tree dumps with the flags provided in this option.
3953 @item -ftree-vectorizer-verbose=@var{n}
3954 @opindex ftree-vectorizer-verbose
3955 This option controls the amount of debugging output the vectorizer prints.
3956 This information is written to standard error, unless @option{-fdump-tree-all}
3957 or @option{-fdump-tree-vect} is specified, in which case it is output to the
3958 usual dump listing file, @file{.vect}.
3960 @item -frandom-seed=@var{string}
3961 @opindex frandom-string
3962 This option provides a seed that GCC uses when it would otherwise use
3963 random numbers. It is used to generate certain symbol names
3964 that have to be different in every compiled file. It is also used to
3965 place unique stamps in coverage data files and the object files that
3966 produce them. You can use the @option{-frandom-seed} option to produce
3967 reproducibly identical object files.
3969 The @var{string} should be different for every file you compile.
3971 @item -fsched-verbose=@var{n}
3972 @opindex fsched-verbose
3973 On targets that use instruction scheduling, this option controls the
3974 amount of debugging output the scheduler prints. This information is
3975 written to standard error, unless @option{-dS} or @option{-dR} is
3976 specified, in which case it is output to the usual dump
3977 listing file, @file{.sched} or @file{.sched2} respectively. However
3978 for @var{n} greater than nine, the output is always printed to standard
3981 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
3982 same information as @option{-dRS}. For @var{n} greater than one, it
3983 also output basic block probabilities, detailed ready list information
3984 and unit/insn info. For @var{n} greater than two, it includes RTL
3985 at abort point, control-flow and regions info. And for @var{n} over
3986 four, @option{-fsched-verbose} also includes dependence info.
3990 Store the usual ``temporary'' intermediate files permanently; place them
3991 in the current directory and name them based on the source file. Thus,
3992 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
3993 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
3994 preprocessed @file{foo.i} output file even though the compiler now
3995 normally uses an integrated preprocessor.
3997 When used in combination with the @option{-x} command line option,
3998 @option{-save-temps} is sensible enough to avoid over writing an
3999 input source file with the same extension as an intermediate file.
4000 The corresponding intermediate file may be obtained by renaming the
4001 source file before using @option{-save-temps}.
4005 Report the CPU time taken by each subprocess in the compilation
4006 sequence. For C source files, this is the compiler proper and assembler
4007 (plus the linker if linking is done). The output looks like this:
4014 The first number on each line is the ``user time'', that is time spent
4015 executing the program itself. The second number is ``system time'',
4016 time spent executing operating system routines on behalf of the program.
4017 Both numbers are in seconds.
4019 @item -fvar-tracking
4020 @opindex fvar-tracking
4021 Run variable tracking pass. It computes where variables are stored at each
4022 position in code. Better debugging information is then generated
4023 (if the debugging information format supports this information).
4025 It is enabled by default when compiling with optimization (@option{-Os},
4026 @option{-O}, @option{-O2}, ...), debugging information (@option{-g}) and
4027 the debug info format supports it.
4029 @item -print-file-name=@var{library}
4030 @opindex print-file-name
4031 Print the full absolute name of the library file @var{library} that
4032 would be used when linking---and don't do anything else. With this
4033 option, GCC does not compile or link anything; it just prints the
4036 @item -print-multi-directory
4037 @opindex print-multi-directory
4038 Print the directory name corresponding to the multilib selected by any
4039 other switches present in the command line. This directory is supposed
4040 to exist in @env{GCC_EXEC_PREFIX}.
4042 @item -print-multi-lib
4043 @opindex print-multi-lib
4044 Print the mapping from multilib directory names to compiler switches
4045 that enable them. The directory name is separated from the switches by
4046 @samp{;}, and each switch starts with an @samp{@@} instead of the
4047 @samp{-}, without spaces between multiple switches. This is supposed to
4048 ease shell-processing.
4050 @item -print-prog-name=@var{program}
4051 @opindex print-prog-name
4052 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
4054 @item -print-libgcc-file-name
4055 @opindex print-libgcc-file-name
4056 Same as @option{-print-file-name=libgcc.a}.
4058 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
4059 but you do want to link with @file{libgcc.a}. You can do
4062 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
4065 @item -print-search-dirs
4066 @opindex print-search-dirs
4067 Print the name of the configured installation directory and a list of
4068 program and library directories @command{gcc} will search---and don't do anything else.
4070 This is useful when @command{gcc} prints the error message
4071 @samp{installation problem, cannot exec cpp0: No such file or directory}.
4072 To resolve this you either need to put @file{cpp0} and the other compiler
4073 components where @command{gcc} expects to find them, or you can set the environment
4074 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
4075 Don't forget the trailing @samp{/}.
4076 @xref{Environment Variables}.
4079 @opindex dumpmachine
4080 Print the compiler's target machine (for example,
4081 @samp{i686-pc-linux-gnu})---and don't do anything else.
4084 @opindex dumpversion
4085 Print the compiler version (for example, @samp{3.0})---and don't do
4090 Print the compiler's built-in specs---and don't do anything else. (This
4091 is used when GCC itself is being built.) @xref{Spec Files}.
4093 @item -feliminate-unused-debug-types
4094 @opindex feliminate-unused-debug-types
4095 Normally, when producing DWARF2 output, GCC will emit debugging
4096 information for all types declared in a compilation
4097 unit, regardless of whether or not they are actually used
4098 in that compilation unit. Sometimes this is useful, such as
4099 if, in the debugger, you want to cast a value to a type that is
4100 not actually used in your program (but is declared). More often,
4101 however, this results in a significant amount of wasted space.
4102 With this option, GCC will avoid producing debug symbol output
4103 for types that are nowhere used in the source file being compiled.
4106 @node Optimize Options
4107 @section Options That Control Optimization
4108 @cindex optimize options
4109 @cindex options, optimization
4111 These options control various sorts of optimizations.
4113 Without any optimization option, the compiler's goal is to reduce the
4114 cost of compilation and to make debugging produce the expected
4115 results. Statements are independent: if you stop the program with a
4116 breakpoint between statements, you can then assign a new value to any
4117 variable or change the program counter to any other statement in the
4118 function and get exactly the results you would expect from the source
4121 Turning on optimization flags makes the compiler attempt to improve
4122 the performance and/or code size at the expense of compilation time
4123 and possibly the ability to debug the program.
4125 The compiler performs optimization based on the knowledge it has of
4126 the program. Optimization levels @option{-O2} and above, in
4127 particular, enable @emph{unit-at-a-time} mode, which allows the
4128 compiler to consider information gained from later functions in
4129 the file when compiling a function. Compiling multiple files at
4130 once to a single output file in @emph{unit-at-a-time} mode allows
4131 the compiler to use information gained from all of the files when
4132 compiling each of them.
4134 Not all optimizations are controlled directly by a flag. Only
4135 optimizations that have a flag are listed.
4142 Optimize. Optimizing compilation takes somewhat more time, and a lot
4143 more memory for a large function.
4145 With @option{-O}, the compiler tries to reduce code size and execution
4146 time, without performing any optimizations that take a great deal of
4149 @option{-O} turns on the following optimization flags:
4150 @gccoptlist{-fdefer-pop @gol
4151 -fdelayed-branch @gol
4152 -fguess-branch-probability @gol
4153 -fcprop-registers @gol
4154 -floop-optimize @gol
4155 -fif-conversion @gol
4156 -fif-conversion2 @gol
4164 -ftree-copyrename @gol
4169 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
4170 where doing so does not interfere with debugging.
4174 Optimize even more. GCC performs nearly all supported optimizations
4175 that do not involve a space-speed tradeoff. The compiler does not
4176 perform loop unrolling or function inlining when you specify @option{-O2}.
4177 As compared to @option{-O}, this option increases both compilation time
4178 and the performance of the generated code.
4180 @option{-O2} turns on all optimization flags specified by @option{-O}. It
4181 also turns on the following optimization flags:
4182 @gccoptlist{-fthread-jumps @gol
4184 -foptimize-sibling-calls @gol
4185 -fcse-follow-jumps -fcse-skip-blocks @gol
4186 -fgcse -fgcse-lm @gol
4187 -fexpensive-optimizations @gol
4188 -fstrength-reduce @gol
4189 -frerun-cse-after-loop -frerun-loop-opt @gol
4193 -fschedule-insns -fschedule-insns2 @gol
4194 -fsched-interblock -fsched-spec @gol
4196 -fstrict-aliasing @gol
4197 -fdelete-null-pointer-checks @gol
4198 -freorder-blocks -freorder-functions @gol
4199 -funit-at-a-time @gol
4200 -falign-functions -falign-jumps @gol
4201 -falign-loops -falign-labels @gol
4204 Please note the warning under @option{-fgcse} about
4205 invoking @option{-O2} on programs that use computed gotos.
4209 Optimize yet more. @option{-O3} turns on all optimizations specified by
4210 @option{-O2} and also turns on the @option{-finline-functions},
4211 @option{-funswitch-loops} and @option{-fgcse-after-reload} options.
4215 Do not optimize. This is the default.
4219 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
4220 do not typically increase code size. It also performs further
4221 optimizations designed to reduce code size.
4223 @option{-Os} disables the following optimization flags:
4224 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
4225 -falign-labels -freorder-blocks -freorder-blocks-and-partition -fprefetch-loop-arrays}
4227 If you use multiple @option{-O} options, with or without level numbers,
4228 the last such option is the one that is effective.
4231 Options of the form @option{-f@var{flag}} specify machine-independent
4232 flags. Most flags have both positive and negative forms; the negative
4233 form of @option{-ffoo} would be @option{-fno-foo}. In the table
4234 below, only one of the forms is listed---the one you typically will
4235 use. You can figure out the other form by either removing @samp{no-}
4238 The following options control specific optimizations. They are either
4239 activated by @option{-O} options or are related to ones that are. You
4240 can use the following flags in the rare cases when ``fine-tuning'' of
4241 optimizations to be performed is desired.
4244 @item -fno-default-inline
4245 @opindex fno-default-inline
4246 Do not make member functions inline by default merely because they are
4247 defined inside the class scope (C++ only). Otherwise, when you specify
4248 @w{@option{-O}}, member functions defined inside class scope are compiled
4249 inline by default; i.e., you don't need to add @samp{inline} in front of
4250 the member function name.
4252 @item -fno-defer-pop
4253 @opindex fno-defer-pop
4254 Always pop the arguments to each function call as soon as that function
4255 returns. For machines which must pop arguments after a function call,
4256 the compiler normally lets arguments accumulate on the stack for several
4257 function calls and pops them all at once.
4259 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4263 Force memory operands to be copied into registers before doing
4264 arithmetic on them. This produces better code by making all memory
4265 references potential common subexpressions. When they are not common
4266 subexpressions, instruction combination should eliminate the separate
4269 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4272 @opindex fforce-addr
4273 Force memory address constants to be copied into registers before
4274 doing arithmetic on them. This may produce better code just as
4275 @option{-fforce-mem} may.
4277 @item -fomit-frame-pointer
4278 @opindex fomit-frame-pointer
4279 Don't keep the frame pointer in a register for functions that
4280 don't need one. This avoids the instructions to save, set up and
4281 restore frame pointers; it also makes an extra register available
4282 in many functions. @strong{It also makes debugging impossible on
4285 On some machines, such as the VAX, this flag has no effect, because
4286 the standard calling sequence automatically handles the frame pointer
4287 and nothing is saved by pretending it doesn't exist. The
4288 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
4289 whether a target machine supports this flag. @xref{Registers,,Register
4290 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
4292 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4294 @item -foptimize-sibling-calls
4295 @opindex foptimize-sibling-calls
4296 Optimize sibling and tail recursive calls.
4298 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4302 Don't pay attention to the @code{inline} keyword. Normally this option
4303 is used to keep the compiler from expanding any functions inline.
4304 Note that if you are not optimizing, no functions can be expanded inline.
4306 @item -finline-functions
4307 @opindex finline-functions
4308 Integrate all simple functions into their callers. The compiler
4309 heuristically decides which functions are simple enough to be worth
4310 integrating in this way.
4312 If all calls to a given function are integrated, and the function is
4313 declared @code{static}, then the function is normally not output as
4314 assembler code in its own right.
4316 Enabled at level @option{-O3}.
4318 @item -finline-limit=@var{n}
4319 @opindex finline-limit
4320 By default, GCC limits the size of functions that can be inlined. This flag
4321 allows the control of this limit for functions that are explicitly marked as
4322 inline (i.e., marked with the inline keyword or defined within the class
4323 definition in c++). @var{n} is the size of functions that can be inlined in
4324 number of pseudo instructions (not counting parameter handling). The default
4325 value of @var{n} is 600.
4326 Increasing this value can result in more inlined code at
4327 the cost of compilation time and memory consumption. Decreasing usually makes
4328 the compilation faster and less code will be inlined (which presumably
4329 means slower programs). This option is particularly useful for programs that
4330 use inlining heavily such as those based on recursive templates with C++.
4332 Inlining is actually controlled by a number of parameters, which may be
4333 specified individually by using @option{--param @var{name}=@var{value}}.
4334 The @option{-finline-limit=@var{n}} option sets some of these parameters
4338 @item max-inline-insns-single
4339 is set to @var{n}/2.
4340 @item max-inline-insns-auto
4341 is set to @var{n}/2.
4342 @item min-inline-insns
4343 is set to 130 or @var{n}/4, whichever is smaller.
4344 @item max-inline-insns-rtl
4348 See below for a documentation of the individual
4349 parameters controlling inlining.
4351 @emph{Note:} pseudo instruction represents, in this particular context, an
4352 abstract measurement of function's size. In no way, it represents a count
4353 of assembly instructions and as such its exact meaning might change from one
4354 release to an another.
4356 @item -fkeep-inline-functions
4357 @opindex fkeep-inline-functions
4358 In C, emit @code{static} functions that are declared @code{inline}
4359 into the object file, even if the function has been inlined into all
4360 of its callers. This switch does not affect functions using the
4361 @code{extern inline} extension in GNU C@. In C++, emit any and all
4362 inline functions into the object file.
4364 @item -fkeep-static-consts
4365 @opindex fkeep-static-consts
4366 Emit variables declared @code{static const} when optimization isn't turned
4367 on, even if the variables aren't referenced.
4369 GCC enables this option by default. If you want to force the compiler to
4370 check if the variable was referenced, regardless of whether or not
4371 optimization is turned on, use the @option{-fno-keep-static-consts} option.
4373 @item -fmerge-constants
4374 Attempt to merge identical constants (string constants and floating point
4375 constants) across compilation units.
4377 This option is the default for optimized compilation if the assembler and
4378 linker support it. Use @option{-fno-merge-constants} to inhibit this
4381 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4383 @item -fmerge-all-constants
4384 Attempt to merge identical constants and identical variables.
4386 This option implies @option{-fmerge-constants}. In addition to
4387 @option{-fmerge-constants} this considers e.g.@: even constant initialized
4388 arrays or initialized constant variables with integral or floating point
4389 types. Languages like C or C++ require each non-automatic variable to
4390 have distinct location, so using this option will result in non-conforming
4393 @item -fmodulo-sched
4394 @opindex fmodulo-sched
4395 Perform swing modulo scheduling immediately before the first scheduling
4396 pass. This pass looks at innermost loops and reorders their
4397 instructions by overlapping different iterations.
4399 @item -fno-branch-count-reg
4400 @opindex fno-branch-count-reg
4401 Do not use ``decrement and branch'' instructions on a count register,
4402 but instead generate a sequence of instructions that decrement a
4403 register, compare it against zero, then branch based upon the result.
4404 This option is only meaningful on architectures that support such
4405 instructions, which include x86, PowerPC, IA-64 and S/390.
4407 The default is @option{-fbranch-count-reg}, enabled when
4408 @option{-fstrength-reduce} is enabled.
4410 @item -fno-function-cse
4411 @opindex fno-function-cse
4412 Do not put function addresses in registers; make each instruction that
4413 calls a constant function contain the function's address explicitly.
4415 This option results in less efficient code, but some strange hacks
4416 that alter the assembler output may be confused by the optimizations
4417 performed when this option is not used.
4419 The default is @option{-ffunction-cse}
4421 @item -fno-zero-initialized-in-bss
4422 @opindex fno-zero-initialized-in-bss
4423 If the target supports a BSS section, GCC by default puts variables that
4424 are initialized to zero into BSS@. This can save space in the resulting
4427 This option turns off this behavior because some programs explicitly
4428 rely on variables going to the data section. E.g., so that the
4429 resulting executable can find the beginning of that section and/or make
4430 assumptions based on that.
4432 The default is @option{-fzero-initialized-in-bss}.
4434 @item -fbounds-check
4435 @opindex fbounds-check
4436 For front-ends that support it, generate additional code to check that
4437 indices used to access arrays are within the declared range. This is
4438 currently only supported by the Java and Fortran front-ends, where
4439 this option defaults to true and false respectively.
4441 @item -fmudflap -fmudflapth -fmudflapir
4445 @cindex bounds checking
4447 For front-ends that support it (C and C++), instrument all risky
4448 pointer/array dereferencing operations, some standard library
4449 string/heap functions, and some other associated constructs with
4450 range/validity tests. Modules so instrumented should be immune to
4451 buffer overflows, invalid heap use, and some other classes of C/C++
4452 programming errors. The instrumentation relies on a separate runtime
4453 library (@file{libmudflap}), which will be linked into a program if
4454 @option{-fmudflap} is given at link time. Run-time behavior of the
4455 instrumented program is controlled by the @env{MUDFLAP_OPTIONS}
4456 environment variable. See @code{env MUDFLAP_OPTIONS=-help a.out}
4459 Use @option{-fmudflapth} instead of @option{-fmudflap} to compile and to
4460 link if your program is multi-threaded. Use @option{-fmudflapir}, in
4461 addition to @option{-fmudflap} or @option{-fmudflapth}, if
4462 instrumentation should ignore pointer reads. This produces less
4463 instrumentation (and therefore faster execution) and still provides
4464 some protection against outright memory corrupting writes, but allows
4465 erroneously read data to propagate within a program.
4467 @item -fstrength-reduce
4468 @opindex fstrength-reduce
4469 Perform the optimizations of loop strength reduction and
4470 elimination of iteration variables.
4472 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4474 @item -fthread-jumps
4475 @opindex fthread-jumps
4476 Perform optimizations where we check to see if a jump branches to a
4477 location where another comparison subsumed by the first is found. If
4478 so, the first branch is redirected to either the destination of the
4479 second branch or a point immediately following it, depending on whether
4480 the condition is known to be true or false.
4482 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4484 @item -fcse-follow-jumps
4485 @opindex fcse-follow-jumps
4486 In common subexpression elimination, scan through jump instructions
4487 when the target of the jump is not reached by any other path. For
4488 example, when CSE encounters an @code{if} statement with an
4489 @code{else} clause, CSE will follow the jump when the condition
4492 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4494 @item -fcse-skip-blocks
4495 @opindex fcse-skip-blocks
4496 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
4497 follow jumps which conditionally skip over blocks. When CSE
4498 encounters a simple @code{if} statement with no else clause,
4499 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
4500 body of the @code{if}.
4502 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4504 @item -frerun-cse-after-loop
4505 @opindex frerun-cse-after-loop
4506 Re-run common subexpression elimination after loop optimizations has been
4509 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4511 @item -frerun-loop-opt
4512 @opindex frerun-loop-opt
4513 Run the loop optimizer twice.
4515 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4519 Perform a global common subexpression elimination pass.
4520 This pass also performs global constant and copy propagation.
4522 @emph{Note:} When compiling a program using computed gotos, a GCC
4523 extension, you may get better runtime performance if you disable
4524 the global common subexpression elimination pass by adding
4525 @option{-fno-gcse} to the command line.
4527 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4531 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
4532 attempt to move loads which are only killed by stores into themselves. This
4533 allows a loop containing a load/store sequence to be changed to a load outside
4534 the loop, and a copy/store within the loop.
4536 Enabled by default when gcse is enabled.
4540 When @option{-fgcse-sm} is enabled, a store motion pass is run after
4541 global common subexpression elimination. This pass will attempt to move
4542 stores out of loops. When used in conjunction with @option{-fgcse-lm},
4543 loops containing a load/store sequence can be changed to a load before
4544 the loop and a store after the loop.
4546 Not enabled at any optimization level.
4550 When @option{-fgcse-las} is enabled, the global common subexpression
4551 elimination pass eliminates redundant loads that come after stores to the
4552 same memory location (both partial and full redundancies).
4554 Not enabled at any optimization level.
4556 @item -fgcse-after-reload
4557 @opindex fgcse-after-reload
4558 When @option{-fgcse-after-reload} is enabled, a redundant load elimination
4559 pass is performed after reload. The purpose of this pass is to cleanup
4562 @item -floop-optimize
4563 @opindex floop-optimize
4564 Perform loop optimizations: move constant expressions out of loops, simplify
4565 exit test conditions and optionally do strength-reduction as well.
4567 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4569 @item -floop-optimize2
4570 @opindex floop-optimize2
4571 Perform loop optimizations using the new loop optimizer. The optimizations
4572 (loop unrolling, peeling and unswitching, loop invariant motion) are enabled
4575 @item -fcrossjumping
4576 @opindex crossjumping
4577 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
4578 resulting code may or may not perform better than without cross-jumping.
4580 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4582 @item -fif-conversion
4583 @opindex if-conversion
4584 Attempt to transform conditional jumps into branch-less equivalents. This
4585 include use of conditional moves, min, max, set flags and abs instructions, and
4586 some tricks doable by standard arithmetics. The use of conditional execution
4587 on chips where it is available is controlled by @code{if-conversion2}.
4589 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4591 @item -fif-conversion2
4592 @opindex if-conversion2
4593 Use conditional execution (where available) to transform conditional jumps into
4594 branch-less equivalents.
4596 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4598 @item -fdelete-null-pointer-checks
4599 @opindex fdelete-null-pointer-checks
4600 Use global dataflow analysis to identify and eliminate useless checks
4601 for null pointers. The compiler assumes that dereferencing a null
4602 pointer would have halted the program. If a pointer is checked after
4603 it has already been dereferenced, it cannot be null.
4605 In some environments, this assumption is not true, and programs can
4606 safely dereference null pointers. Use
4607 @option{-fno-delete-null-pointer-checks} to disable this optimization
4608 for programs which depend on that behavior.
4610 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4612 @item -fexpensive-optimizations
4613 @opindex fexpensive-optimizations
4614 Perform a number of minor optimizations that are relatively expensive.
4616 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4618 @item -foptimize-register-move
4620 @opindex foptimize-register-move
4622 Attempt to reassign register numbers in move instructions and as
4623 operands of other simple instructions in order to maximize the amount of
4624 register tying. This is especially helpful on machines with two-operand
4627 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
4630 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4632 @item -fdelayed-branch
4633 @opindex fdelayed-branch
4634 If supported for the target machine, attempt to reorder instructions
4635 to exploit instruction slots available after delayed branch
4638 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4640 @item -fschedule-insns
4641 @opindex fschedule-insns
4642 If supported for the target machine, attempt to reorder instructions to
4643 eliminate execution stalls due to required data being unavailable. This
4644 helps machines that have slow floating point or memory load instructions
4645 by allowing other instructions to be issued until the result of the load
4646 or floating point instruction is required.
4648 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4650 @item -fschedule-insns2
4651 @opindex fschedule-insns2
4652 Similar to @option{-fschedule-insns}, but requests an additional pass of
4653 instruction scheduling after register allocation has been done. This is
4654 especially useful on machines with a relatively small number of
4655 registers and where memory load instructions take more than one cycle.
4657 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4659 @item -fno-sched-interblock
4660 @opindex fno-sched-interblock
4661 Don't schedule instructions across basic blocks. This is normally
4662 enabled by default when scheduling before register allocation, i.e.@:
4663 with @option{-fschedule-insns} or at @option{-O2} or higher.
4665 @item -fno-sched-spec
4666 @opindex fno-sched-spec
4667 Don't allow speculative motion of non-load instructions. This is normally
4668 enabled by default when scheduling before register allocation, i.e.@:
4669 with @option{-fschedule-insns} or at @option{-O2} or higher.
4671 @item -fsched-spec-load
4672 @opindex fsched-spec-load
4673 Allow speculative motion of some load instructions. This only makes
4674 sense when scheduling before register allocation, i.e.@: with
4675 @option{-fschedule-insns} or at @option{-O2} or higher.
4677 @item -fsched-spec-load-dangerous
4678 @opindex fsched-spec-load-dangerous
4679 Allow speculative motion of more load instructions. This only makes
4680 sense when scheduling before register allocation, i.e.@: with
4681 @option{-fschedule-insns} or at @option{-O2} or higher.
4683 @item -fsched-stalled-insns=@var{n}
4684 @opindex fsched-stalled-insns
4685 Define how many insns (if any) can be moved prematurely from the queue
4686 of stalled insns into the ready list, during the second scheduling pass.
4688 @item -fsched-stalled-insns-dep=@var{n}
4689 @opindex fsched-stalled-insns-dep
4690 Define how many insn groups (cycles) will be examined for a dependency
4691 on a stalled insn that is candidate for premature removal from the queue
4692 of stalled insns. Has an effect only during the second scheduling pass,
4693 and only if @option{-fsched-stalled-insns} is used and its value is not zero.
4695 @item -fsched2-use-superblocks
4696 @opindex fsched2-use-superblocks
4697 When scheduling after register allocation, do use superblock scheduling
4698 algorithm. Superblock scheduling allows motion across basic block boundaries
4699 resulting on faster schedules. This option is experimental, as not all machine
4700 descriptions used by GCC model the CPU closely enough to avoid unreliable
4701 results from the algorithm.
4703 This only makes sense when scheduling after register allocation, i.e.@: with
4704 @option{-fschedule-insns2} or at @option{-O2} or higher.
4706 @item -fsched2-use-traces
4707 @opindex fsched2-use-traces
4708 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
4709 allocation and additionally perform code duplication in order to increase the
4710 size of superblocks using tracer pass. See @option{-ftracer} for details on
4713 This mode should produce faster but significantly longer programs. Also
4714 without @option{-fbranch-probabilities} the traces constructed may not
4715 match the reality and hurt the performance. This only makes
4716 sense when scheduling after register allocation, i.e.@: with
4717 @option{-fschedule-insns2} or at @option{-O2} or higher.
4719 @item -freschedule-modulo-scheduled-loops
4720 @opindex fscheduling-in-modulo-scheduled-loops
4721 The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled
4722 we may want to prevent the later scheduling passes from changing its schedule, we use this
4723 option to control that.
4725 @item -fcaller-saves
4726 @opindex fcaller-saves
4727 Enable values to be allocated in registers that will be clobbered by
4728 function calls, by emitting extra instructions to save and restore the
4729 registers around such calls. Such allocation is done only when it
4730 seems to result in better code than would otherwise be produced.
4732 This option is always enabled by default on certain machines, usually
4733 those which have no call-preserved registers to use instead.
4735 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4738 Perform Partial Redundancy Elimination (PRE) on trees. This flag is
4739 enabled by default at @option{-O2} and @option{-O3}.
4742 Perform Full Redundancy Elimination (FRE) on trees. The difference
4743 between FRE and PRE is that FRE only considers expressions
4744 that are computed on all paths leading to the redundant computation.
4745 This analysis faster than PRE, though it exposes fewer redundancies.
4746 This flag is enabled by default at @option{-O} and higher.
4749 Perform structural alias analysis on trees. This flag
4750 is enabled by default at @option{-O} and higher.
4753 Perform forward store motion on trees. This flag is
4754 enabled by default at @option{-O} and higher.
4757 Perform sparse conditional constant propagation (CCP) on trees. This flag
4758 is enabled by default at @option{-O} and higher.
4761 Perform dead code elimination (DCE) on trees. This flag is enabled by
4762 default at @option{-O} and higher.
4764 @item -ftree-dominator-opts
4765 Perform dead code elimination (DCE) on trees. This flag is enabled by
4766 default at @option{-O} and higher.
4769 Perform loop header copying on trees. This is beneficial since it increases
4770 effectiveness of code motion optimizations. It also saves one jump. This flag
4771 is enabled by default at @option{-O} and higher. It is not enabled
4772 for @option{-Os}, since it usually increases code size.
4774 @item -ftree-loop-optimize
4775 Perform loop optimizations on trees. This flag is enabled by default
4776 at @option{-O} and higher.
4778 @item -ftree-loop-linear
4779 Perform linear loop transformations on tree. This flag can improve cache
4780 performance and allow further loop optimizations to take place.
4782 @item -ftree-loop-im
4783 Perform loop invariant motion on trees. This pass moves only invariants that
4784 would be hard to handle on rtl level (function calls, operations that expand to
4785 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves
4786 operands of conditions that are invariant out of the loop, so that we can use
4787 just trivial invariantness analysis in loop unswitching. The pass also includes
4790 @item -ftree-loop-ivcanon
4791 Create a canonical counter for number of iterations in the loop for that
4792 determining number of iterations requires complicated analysis. Later
4793 optimizations then may determine the number easily. Useful especially
4794 in connection with unrolling.
4797 Perform induction variable optimizations (strength reduction, induction
4798 variable merging and induction variable elimination) on trees.
4801 Perform scalar replacement of aggregates. This pass replaces structure
4802 references with scalars to prevent committing structures to memory too
4803 early. This flag is enabled by default at @option{-O} and higher.
4805 @item -ftree-copyrename
4806 Perform copy renaming on trees. This pass attempts to rename compiler
4807 temporaries to other variables at copy locations, usually resulting in
4808 variable names which more closely resemble the original variables. This flag
4809 is enabled by default at @option{-O} and higher.
4812 Perform temporary expression replacement during the SSA->normal phase. Single
4813 use/single def temporaries are replaced at their use location with their
4814 defining expression. This results in non-GIMPLE code, but gives the expanders
4815 much more complex trees to work on resulting in better RTL generation. This is
4816 enabled by default at @option{-O} and higher.
4819 Perform live range splitting during the SSA->normal phase. Distinct live
4820 ranges of a variable are split into unique variables, allowing for better
4821 optimization later. This is enabled by default at @option{-O} and higher.
4823 @item -ftree-vectorize
4824 Perform loop vectorization on trees.
4828 Perform tail duplication to enlarge superblock size. This transformation
4829 simplifies the control flow of the function allowing other optimizations to do
4832 @item -funroll-loops
4833 @opindex funroll-loops
4834 Unroll loops whose number of iterations can be determined at compile
4835 time or upon entry to the loop. @option{-funroll-loops} implies both
4836 @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
4837 option makes code larger, and may or may not make it run faster.
4839 @item -funroll-all-loops
4840 @opindex funroll-all-loops
4841 Unroll all loops, even if their number of iterations is uncertain when
4842 the loop is entered. This usually makes programs run more slowly.
4843 @option{-funroll-all-loops} implies the same options as
4844 @option{-funroll-loops},
4846 @item -fsplit-ivs-in-unroller
4847 @opindex -fsplit-ivs-in-unroller
4848 Enables expressing of values of induction variables in later iterations
4849 of the unrolled loop using the value in the first iteration. This breaks
4850 long dependency chains, thus improving efficiency of the scheduling passes
4851 (for best results, @option{-fweb} should be used as well).
4853 Combination of @option{-fweb} and CSE is often sufficient to obtain the
4854 same effect. However in cases the loop body is more complicated than
4855 a single basic block, this is not reliable. It also does not work at all
4856 on some of the architectures due to restrictions in the CSE pass.
4858 This optimization is enabled by default.
4860 @item -fvariable-expansion-in-unroller
4861 @opindex -fvariable-expansion-in-unroller
4862 With this option, the compiler will create multiple copies of some
4863 local variables when unrolling a loop which can result in superior code.
4865 @item -fprefetch-loop-arrays
4866 @opindex fprefetch-loop-arrays
4867 If supported by the target machine, generate instructions to prefetch
4868 memory to improve the performance of loops that access large arrays.
4870 These options may generate better or worse code; results are highly
4871 dependent on the structure of loops within the source code.
4874 @itemx -fno-peephole2
4875 @opindex fno-peephole
4876 @opindex fno-peephole2
4877 Disable any machine-specific peephole optimizations. The difference
4878 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
4879 are implemented in the compiler; some targets use one, some use the
4880 other, a few use both.
4882 @option{-fpeephole} is enabled by default.
4883 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4885 @item -fno-guess-branch-probability
4886 @opindex fno-guess-branch-probability
4887 Do not guess branch probabilities using heuristics.
4889 GCC will use heuristics to guess branch probabilities if they are
4890 not provided by profiling feedback (@option{-fprofile-arcs}). These
4891 heuristics are based on the control flow graph. If some branch probabilities
4892 are specified by @samp{__builtin_expect}, then the heuristics will be
4893 used to guess branch probabilities for the rest of the control flow graph,
4894 taking the @samp{__builtin_expect} info into account. The interactions
4895 between the heuristics and @samp{__builtin_expect} can be complex, and in
4896 some cases, it may be useful to disable the heuristics so that the effects
4897 of @samp{__builtin_expect} are easier to understand.
4899 The default is @option{-fguess-branch-probability} at levels
4900 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4902 @item -freorder-blocks
4903 @opindex freorder-blocks
4904 Reorder basic blocks in the compiled function in order to reduce number of
4905 taken branches and improve code locality.
4907 Enabled at levels @option{-O2}, @option{-O3}.
4909 @item -freorder-blocks-and-partition
4910 @opindex freorder-blocks-and-partition
4911 In addition to reordering basic blocks in the compiled function, in order
4912 to reduce number of taken branches, partitions hot and cold basic blocks
4913 into separate sections of the assembly and .o files, to improve
4914 paging and cache locality performance.
4916 This optimization is automatically turned off in the presence of
4917 exception handling, for linkonce sections, for functions with a user-defined
4918 section attribute and on any architecture that does not support named
4921 @item -freorder-functions
4922 @opindex freorder-functions
4923 Reorder functions in the object file in order to
4924 improve code locality. This is implemented by using special
4925 subsections @code{.text.hot} for most frequently executed functions and
4926 @code{.text.unlikely} for unlikely executed functions. Reordering is done by
4927 the linker so object file format must support named sections and linker must
4928 place them in a reasonable way.
4930 Also profile feedback must be available in to make this option effective. See
4931 @option{-fprofile-arcs} for details.
4933 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4935 @item -fstrict-aliasing
4936 @opindex fstrict-aliasing
4937 Allows the compiler to assume the strictest aliasing rules applicable to
4938 the language being compiled. For C (and C++), this activates
4939 optimizations based on the type of expressions. In particular, an
4940 object of one type is assumed never to reside at the same address as an
4941 object of a different type, unless the types are almost the same. For
4942 example, an @code{unsigned int} can alias an @code{int}, but not a
4943 @code{void*} or a @code{double}. A character type may alias any other
4946 Pay special attention to code like this:
4959 The practice of reading from a different union member than the one most
4960 recently written to (called ``type-punning'') is common. Even with
4961 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
4962 is accessed through the union type. So, the code above will work as
4963 expected. However, this code might not:
4974 Every language that wishes to perform language-specific alias analysis
4975 should define a function that computes, given an @code{tree}
4976 node, an alias set for the node. Nodes in different alias sets are not
4977 allowed to alias. For an example, see the C front-end function
4978 @code{c_get_alias_set}.
4980 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4982 @item -falign-functions
4983 @itemx -falign-functions=@var{n}
4984 @opindex falign-functions
4985 Align the start of functions to the next power-of-two greater than
4986 @var{n}, skipping up to @var{n} bytes. For instance,
4987 @option{-falign-functions=32} aligns functions to the next 32-byte
4988 boundary, but @option{-falign-functions=24} would align to the next
4989 32-byte boundary only if this can be done by skipping 23 bytes or less.
4991 @option{-fno-align-functions} and @option{-falign-functions=1} are
4992 equivalent and mean that functions will not be aligned.
4994 Some assemblers only support this flag when @var{n} is a power of two;
4995 in that case, it is rounded up.
4997 If @var{n} is not specified or is zero, use a machine-dependent default.
4999 Enabled at levels @option{-O2}, @option{-O3}.
5001 @item -falign-labels
5002 @itemx -falign-labels=@var{n}
5003 @opindex falign-labels
5004 Align all branch targets to a power-of-two boundary, skipping up to
5005 @var{n} bytes like @option{-falign-functions}. This option can easily
5006 make code slower, because it must insert dummy operations for when the
5007 branch target is reached in the usual flow of the code.
5009 @option{-fno-align-labels} and @option{-falign-labels=1} are
5010 equivalent and mean that labels will not be aligned.
5012 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
5013 are greater than this value, then their values are used instead.
5015 If @var{n} is not specified or is zero, use a machine-dependent default
5016 which is very likely to be @samp{1}, meaning no alignment.
5018 Enabled at levels @option{-O2}, @option{-O3}.
5021 @itemx -falign-loops=@var{n}
5022 @opindex falign-loops
5023 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
5024 like @option{-falign-functions}. The hope is that the loop will be
5025 executed many times, which will make up for any execution of the dummy
5028 @option{-fno-align-loops} and @option{-falign-loops=1} are
5029 equivalent and mean that loops will not be aligned.
5031 If @var{n} is not specified or is zero, use a machine-dependent default.
5033 Enabled at levels @option{-O2}, @option{-O3}.
5036 @itemx -falign-jumps=@var{n}
5037 @opindex falign-jumps
5038 Align branch targets to a power-of-two boundary, for branch targets
5039 where the targets can only be reached by jumping, skipping up to @var{n}
5040 bytes like @option{-falign-functions}. In this case, no dummy operations
5043 @option{-fno-align-jumps} and @option{-falign-jumps=1} are
5044 equivalent and mean that loops will not be aligned.
5046 If @var{n} is not specified or is zero, use a machine-dependent default.
5048 Enabled at levels @option{-O2}, @option{-O3}.
5050 @item -funit-at-a-time
5051 @opindex funit-at-a-time
5052 Parse the whole compilation unit before starting to produce code.
5053 This allows some extra optimizations to take place but consumes
5054 more memory (in general). There are some compatibility issues
5055 with @emph{unit-at-at-time} mode:
5058 enabling @emph{unit-at-a-time} mode may change the order
5059 in which functions, variables, and top-level @code{asm} statements
5060 are emitted, and will likely break code relying on some particular
5061 ordering. The majority of such top-level @code{asm} statements,
5062 though, can be replaced by @code{section} attributes.
5065 @emph{unit-at-a-time} mode removes unreferenced static variables
5066 and functions are removed. This may result in undefined references
5067 when an @code{asm} statement refers directly to variables or functions
5068 that are otherwise unused. In that case either the variable/function
5069 shall be listed as an operand of the @code{asm} statement operand or,
5070 in the case of top-level @code{asm} statements the attribute @code{used}
5071 shall be used on the declaration.
5074 Static functions now can use non-standard passing conventions that
5075 may break @code{asm} statements calling functions directly. Again,
5076 attribute @code{used} will prevent this behavior.
5079 As a temporary workaround, @option{-fno-unit-at-a-time} can be used,
5080 but this scheme may not be supported by future releases of GCC@.
5082 Enabled at levels @option{-O2}, @option{-O3}.
5086 Constructs webs as commonly used for register allocation purposes and assign
5087 each web individual pseudo register. This allows the register allocation pass
5088 to operate on pseudos directly, but also strengthens several other optimization
5089 passes, such as CSE, loop optimizer and trivial dead code remover. It can,
5090 however, make debugging impossible, since variables will no longer stay in a
5093 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os},
5094 on targets where the default format for debugging information supports
5097 @item -fno-cprop-registers
5098 @opindex fno-cprop-registers
5099 After register allocation and post-register allocation instruction splitting,
5100 we perform a copy-propagation pass to try to reduce scheduling dependencies
5101 and occasionally eliminate the copy.
5103 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
5105 @item -fprofile-generate
5106 @opindex fprofile-generate
5108 Enable options usually used for instrumenting application to produce
5109 profile useful for later recompilation with profile feedback based
5110 optimization. You must use @option{-fprofile-generate} both when
5111 compiling and when linking your program.
5113 The following options are enabled: @code{-fprofile-arcs}, @code{-fprofile-values}, @code{-fvpt}.
5116 @opindex fprofile-use
5117 Enable profile feedback directed optimizations, and optimizations
5118 generally profitable only with profile feedback available.
5120 The following options are enabled: @code{-fbranch-probabilities},
5121 @code{-fvpt}, @code{-funroll-loops}, @code{-fpeel-loops}, @code{-ftracer}.
5125 The following options control compiler behavior regarding floating
5126 point arithmetic. These options trade off between speed and
5127 correctness. All must be specifically enabled.
5131 @opindex ffloat-store
5132 Do not store floating point variables in registers, and inhibit other
5133 options that might change whether a floating point value is taken from a
5136 @cindex floating point precision
5137 This option prevents undesirable excess precision on machines such as
5138 the 68000 where the floating registers (of the 68881) keep more
5139 precision than a @code{double} is supposed to have. Similarly for the
5140 x86 architecture. For most programs, the excess precision does only
5141 good, but a few programs rely on the precise definition of IEEE floating
5142 point. Use @option{-ffloat-store} for such programs, after modifying
5143 them to store all pertinent intermediate computations into variables.
5147 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
5148 @option{-fno-trapping-math}, @option{-ffinite-math-only},
5149 @option{-fno-rounding-math}, @option{-fno-signaling-nans}
5150 and @option{fcx-limited-range}.
5152 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
5154 This option should never be turned on by any @option{-O} option since
5155 it can result in incorrect output for programs which depend on
5156 an exact implementation of IEEE or ISO rules/specifications for
5159 @item -fno-math-errno
5160 @opindex fno-math-errno
5161 Do not set ERRNO after calling math functions that are executed
5162 with a single instruction, e.g., sqrt. A program that relies on
5163 IEEE exceptions for math error handling may want to use this flag
5164 for speed while maintaining IEEE arithmetic compatibility.
5166 This option should never be turned on by any @option{-O} option since
5167 it can result in incorrect output for programs which depend on
5168 an exact implementation of IEEE or ISO rules/specifications for
5171 The default is @option{-fmath-errno}.
5173 @item -funsafe-math-optimizations
5174 @opindex funsafe-math-optimizations
5175 Allow optimizations for floating-point arithmetic that (a) assume
5176 that arguments and results are valid and (b) may violate IEEE or
5177 ANSI standards. When used at link-time, it may include libraries
5178 or startup files that change the default FPU control word or other
5179 similar optimizations.
5181 This option should never be turned on by any @option{-O} option since
5182 it can result in incorrect output for programs which depend on
5183 an exact implementation of IEEE or ISO rules/specifications for
5186 The default is @option{-fno-unsafe-math-optimizations}.
5188 @item -ffinite-math-only
5189 @opindex ffinite-math-only
5190 Allow optimizations for floating-point arithmetic that assume
5191 that arguments and results are not NaNs or +-Infs.
5193 This option should never be turned on by any @option{-O} option since
5194 it can result in incorrect output for programs which depend on
5195 an exact implementation of IEEE or ISO rules/specifications.
5197 The default is @option{-fno-finite-math-only}.
5199 @item -fno-trapping-math
5200 @opindex fno-trapping-math
5201 Compile code assuming that floating-point operations cannot generate
5202 user-visible traps. These traps include division by zero, overflow,
5203 underflow, inexact result and invalid operation. This option implies
5204 @option{-fno-signaling-nans}. Setting this option may allow faster
5205 code if one relies on ``non-stop'' IEEE arithmetic, for example.
5207 This option should never be turned on by any @option{-O} option since
5208 it can result in incorrect output for programs which depend on
5209 an exact implementation of IEEE or ISO rules/specifications for
5212 The default is @option{-ftrapping-math}.
5214 @item -frounding-math
5215 @opindex frounding-math
5216 Disable transformations and optimizations that assume default floating
5217 point rounding behavior. This is round-to-zero for all floating point
5218 to integer conversions, and round-to-nearest for all other arithmetic
5219 truncations. This option should be specified for programs that change
5220 the FP rounding mode dynamically, or that may be executed with a
5221 non-default rounding mode. This option disables constant folding of
5222 floating point expressions at compile-time (which may be affected by
5223 rounding mode) and arithmetic transformations that are unsafe in the
5224 presence of sign-dependent rounding modes.
5226 The default is @option{-fno-rounding-math}.
5228 This option is experimental and does not currently guarantee to
5229 disable all GCC optimizations that are affected by rounding mode.
5230 Future versions of GCC may provide finer control of this setting
5231 using C99's @code{FENV_ACCESS} pragma. This command line option
5232 will be used to specify the default state for @code{FENV_ACCESS}.
5234 @item -fsignaling-nans
5235 @opindex fsignaling-nans
5236 Compile code assuming that IEEE signaling NaNs may generate user-visible
5237 traps during floating-point operations. Setting this option disables
5238 optimizations that may change the number of exceptions visible with
5239 signaling NaNs. This option implies @option{-ftrapping-math}.
5241 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
5244 The default is @option{-fno-signaling-nans}.
5246 This option is experimental and does not currently guarantee to
5247 disable all GCC optimizations that affect signaling NaN behavior.
5249 @item -fsingle-precision-constant
5250 @opindex fsingle-precision-constant
5251 Treat floating point constant as single precision constant instead of
5252 implicitly converting it to double precision constant.
5254 @item -fcx-limited-range
5255 @itemx -fno-cx-limited-range
5256 @opindex fcx-limited-range
5257 @opindex fno-cx-limited-range
5258 When enabled, this option states that a range reduction step is not
5259 needed when performing complex division. The default is
5260 @option{-fno-cx-limited-range}, but is enabled by @option{-ffast-math}.
5262 This option controls the default setting of the ISO C99
5263 @code{CX_LIMITED_RANGE} pragma. Nevertheless, the option applies to
5268 The following options control optimizations that may improve
5269 performance, but are not enabled by any @option{-O} options. This
5270 section includes experimental options that may produce broken code.
5273 @item -fbranch-probabilities
5274 @opindex fbranch-probabilities
5275 After running a program compiled with @option{-fprofile-arcs}
5276 (@pxref{Debugging Options,, Options for Debugging Your Program or
5277 @command{gcc}}), you can compile it a second time using
5278 @option{-fbranch-probabilities}, to improve optimizations based on
5279 the number of times each branch was taken. When the program
5280 compiled with @option{-fprofile-arcs} exits it saves arc execution
5281 counts to a file called @file{@var{sourcename}.gcda} for each source
5282 file The information in this data file is very dependent on the
5283 structure of the generated code, so you must use the same source code
5284 and the same optimization options for both compilations.
5286 With @option{-fbranch-probabilities}, GCC puts a
5287 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
5288 These can be used to improve optimization. Currently, they are only
5289 used in one place: in @file{reorg.c}, instead of guessing which path a
5290 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
5291 exactly determine which path is taken more often.
5293 @item -fprofile-values
5294 @opindex fprofile-values
5295 If combined with @option{-fprofile-arcs}, it adds code so that some
5296 data about values of expressions in the program is gathered.
5298 With @option{-fbranch-probabilities}, it reads back the data gathered
5299 from profiling values of expressions and adds @samp{REG_VALUE_PROFILE}
5300 notes to instructions for their later usage in optimizations.
5302 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5306 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5307 a code to gather information about values of expressions.
5309 With @option{-fbranch-probabilities}, it reads back the data gathered
5310 and actually performs the optimizations based on them.
5311 Currently the optimizations include specialization of division operation
5312 using the knowledge about the value of the denominator.
5314 @item -fspeculative-prefetching
5315 @opindex fspeculative-prefetching
5316 If combined with @option{-fprofile-arcs}, it instructs the compiler to add
5317 a code to gather information about addresses of memory references in the
5320 With @option{-fbranch-probabilities}, it reads back the data gathered
5321 and issues prefetch instructions according to them. In addition to the opportunities
5322 noticed by @option{-fprefetch-loop-arrays}, it also notices more complicated
5323 memory access patterns---for example accesses to the data stored in linked
5324 list whose elements are usually allocated sequentially.
5326 In order to prevent issuing double prefetches, usage of
5327 @option{-fspeculative-prefetching} implies @option{-fno-prefetch-loop-arrays}.
5329 Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.
5331 @item -frename-registers
5332 @opindex frename-registers
5333 Attempt to avoid false dependencies in scheduled code by making use
5334 of registers left over after register allocation. This optimization
5335 will most benefit processors with lots of registers. Depending on the
5336 debug information format adopted by the target, however, it can
5337 make debugging impossible, since variables will no longer stay in
5338 a ``home register''.
5340 Not enabled by default at any level because it has known bugs.
5344 Perform tail duplication to enlarge superblock size. This transformation
5345 simplifies the control flow of the function allowing other optimizations to do
5348 Enabled with @option{-fprofile-use}.
5350 @item -funroll-loops
5351 @opindex funroll-loops
5352 Unroll loops whose number of iterations can be determined at compile time or
5353 upon entry to the loop. @option{-funroll-loops} implies
5354 @option{-frerun-cse-after-loop}. It also turns on complete loop peeling
5355 (i.e.@: complete removal of loops with small constant number of iterations).
5356 This option makes code larger, and may or may not make it run faster.
5358 Enabled with @option{-fprofile-use}.
5360 @item -funroll-all-loops
5361 @opindex funroll-all-loops
5362 Unroll all loops, even if their number of iterations is uncertain when
5363 the loop is entered. This usually makes programs run more slowly.
5364 @option{-funroll-all-loops} implies the same options as
5365 @option{-funroll-loops}.
5368 @opindex fpeel-loops
5369 Peels the loops for that there is enough information that they do not
5370 roll much (from profile feedback). It also turns on complete loop peeling
5371 (i.e.@: complete removal of loops with small constant number of iterations).
5373 Enabled with @option{-fprofile-use}.
5375 @item -fmove-loop-invariants
5376 @opindex fmove-loop-invariants
5377 Enables the loop invariant motion pass in the new loop optimizer. Enabled
5378 at level @option{-O1}
5380 @item -funswitch-loops
5381 @opindex funswitch-loops
5382 Move branches with loop invariant conditions out of the loop, with duplicates
5383 of the loop on both branches (modified according to result of the condition).
5385 @item -fprefetch-loop-arrays
5386 @opindex fprefetch-loop-arrays
5387 If supported by the target machine, generate instructions to prefetch
5388 memory to improve the performance of loops that access large arrays.
5390 Disabled at level @option{-Os}.
5392 @item -ffunction-sections
5393 @itemx -fdata-sections
5394 @opindex ffunction-sections
5395 @opindex fdata-sections
5396 Place each function or data item into its own section in the output
5397 file if the target supports arbitrary sections. The name of the
5398 function or the name of the data item determines the section's name
5401 Use these options on systems where the linker can perform optimizations
5402 to improve locality of reference in the instruction space. Most systems
5403 using the ELF object format and SPARC processors running Solaris 2 have
5404 linkers with such optimizations. AIX may have these optimizations in
5407 Only use these options when there are significant benefits from doing
5408 so. When you specify these options, the assembler and linker will
5409 create larger object and executable files and will also be slower.
5410 You will not be able to use @code{gprof} on all systems if you
5411 specify this option and you may have problems with debugging if
5412 you specify both this option and @option{-g}.
5414 @item -fbranch-target-load-optimize
5415 @opindex fbranch-target-load-optimize
5416 Perform branch target register load optimization before prologue / epilogue
5418 The use of target registers can typically be exposed only during reload,
5419 thus hoisting loads out of loops and doing inter-block scheduling needs
5420 a separate optimization pass.
5422 @item -fbranch-target-load-optimize2
5423 @opindex fbranch-target-load-optimize2
5424 Perform branch target register load optimization after prologue / epilogue
5427 @item -fbtr-bb-exclusive
5428 @opindex fbtr-bb-exclusive
5429 When performing branch target register load optimization, don't reuse
5430 branch target registers in within any basic block.
5432 @item --param @var{name}=@var{value}
5434 In some places, GCC uses various constants to control the amount of
5435 optimization that is done. For example, GCC will not inline functions
5436 that contain more that a certain number of instructions. You can
5437 control some of these constants on the command-line using the
5438 @option{--param} option.
5440 The names of specific parameters, and the meaning of the values, are
5441 tied to the internals of the compiler, and are subject to change
5442 without notice in future releases.
5444 In each case, the @var{value} is an integer. The allowable choices for
5445 @var{name} are given in the following table:
5448 @item sra-max-structure-size
5449 The maximum structure size, in bytes, at which the scalar replacement
5450 of aggregates (SRA) optimization will perform block copies. The
5451 default value, 0, implies that GCC will select the most appropriate
5454 @item sra-field-structure-ratio
5455 The threshold ratio (as a percentage) between instantiated fields and
5456 the complete structure size. We say that if the ratio of the number
5457 of bytes in instantiated fields to the number of bytes in the complete
5458 structure exceeds this parameter, then block copies are not used. The
5461 @item max-crossjump-edges
5462 The maximum number of incoming edges to consider for crossjumping.
5463 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
5464 the number of edges incoming to each block. Increasing values mean
5465 more aggressive optimization, making the compile time increase with
5466 probably small improvement in executable size.
5468 @item min-crossjump-insns
5469 The minimum number of instructions which must be matched at the end
5470 of two blocks before crossjumping will be performed on them. This
5471 value is ignored in the case where all instructions in the block being
5472 crossjumped from are matched. The default value is 5.
5474 @item max-goto-duplication-insns
5475 The maximum number of instructions to duplicate to a block that jumps
5476 to a computed goto. To avoid @math{O(N^2)} behavior in a number of
5477 passes, GCC factors computed gotos early in the compilation process,
5478 and unfactors them as late as possible. Only computed jumps at the
5479 end of a basic blocks with no more than max-goto-duplication-insns are
5480 unfactored. The default value is 8.
5482 @item max-delay-slot-insn-search
5483 The maximum number of instructions to consider when looking for an
5484 instruction to fill a delay slot. If more than this arbitrary number of
5485 instructions is searched, the time savings from filling the delay slot
5486 will be minimal so stop searching. Increasing values mean more
5487 aggressive optimization, making the compile time increase with probably
5488 small improvement in executable run time.
5490 @item max-delay-slot-live-search
5491 When trying to fill delay slots, the maximum number of instructions to
5492 consider when searching for a block with valid live register
5493 information. Increasing this arbitrarily chosen value means more
5494 aggressive optimization, increasing the compile time. This parameter
5495 should be removed when the delay slot code is rewritten to maintain the
5498 @item max-gcse-memory
5499 The approximate maximum amount of memory that will be allocated in
5500 order to perform the global common subexpression elimination
5501 optimization. If more memory than specified is required, the
5502 optimization will not be done.
5504 @item max-gcse-passes
5505 The maximum number of passes of GCSE to run. The default is 1.
5507 @item max-pending-list-length
5508 The maximum number of pending dependencies scheduling will allow
5509 before flushing the current state and starting over. Large functions
5510 with few branches or calls can create excessively large lists which
5511 needlessly consume memory and resources.
5513 @item max-inline-insns-single
5514 Several parameters control the tree inliner used in gcc.
5515 This number sets the maximum number of instructions (counted in GCC's
5516 internal representation) in a single function that the tree inliner
5517 will consider for inlining. This only affects functions declared
5518 inline and methods implemented in a class declaration (C++).
5519 The default value is 450.
5521 @item max-inline-insns-auto
5522 When you use @option{-finline-functions} (included in @option{-O3}),
5523 a lot of functions that would otherwise not be considered for inlining
5524 by the compiler will be investigated. To those functions, a different
5525 (more restrictive) limit compared to functions declared inline can
5527 The default value is 90.
5529 @item large-function-insns
5530 The limit specifying really large functions. For functions larger than this
5531 limit after inlining inlining is constrained by
5532 @option{--param large-function-growth}. This parameter is useful primarily
5533 to avoid extreme compilation time caused by non-linear algorithms used by the
5535 This parameter is ignored when @option{-funit-at-a-time} is not used.
5536 The default value is 2700.
5538 @item large-function-growth
5539 Specifies maximal growth of large function caused by inlining in percents.
5540 This parameter is ignored when @option{-funit-at-a-time} is not used.
5541 The default value is 100 which limits large function growth to 2.0 times
5544 @item inline-unit-growth
5545 Specifies maximal overall growth of the compilation unit caused by inlining.
5546 This parameter is ignored when @option{-funit-at-a-time} is not used.
5547 The default value is 50 which limits unit growth to 1.5 times the original
5550 @item max-inline-insns-recursive
5551 @itemx max-inline-insns-recursive-auto
5552 Specifies maximum number of instructions out-of-line copy of self recursive inline
5553 function can grow into by performing recursive inlining.
5555 For functions declared inline @option{--param max-inline-insns-recursive} is
5556 taken into acount. For function not declared inline, recursive inlining
5557 happens only when @option{-finline-functions} (included in @option{-O3}) is
5558 enabled and @option{--param max-inline-insns-recursive-auto} is used. The
5559 default value is 450.
5561 @item max-inline-recursive-depth
5562 @itemx max-inline-recursive-depth-auto
5563 Specifies maximum recursion depth used by the recursive inlining.
5565 For functions declared inline @option{--param max-inline-recursive-depth} is
5566 taken into acount. For function not declared inline, recursive inlining
5567 happens only when @option{-finline-functions} (included in @option{-O3}) is
5568 enabled and @option{--param max-inline-recursive-depth-auto} is used. The
5569 default value is 450.
5571 @item inline-call-cost
5572 Specify cost of call instruction relative to simple arithmetics operations
5573 (having cost of 1). Increasing this cost disqualifies inlining of non-leaf
5574 functions and at the same time increases size of leaf function that is believed to
5575 reduce function size by being inlined. In effect it increases amount of
5576 inlining for code having large abstraction penalty (many functions that just
5577 pass the arguments to other functions) and decrease inlining for code with low
5578 abstraction penalty. The default value is 16.
5580 @item max-unrolled-insns
5581 The maximum number of instructions that a loop should have if that loop
5582 is unrolled, and if the loop is unrolled, it determines how many times
5583 the loop code is unrolled.
5585 @item max-average-unrolled-insns
5586 The maximum number of instructions biased by probabilities of their execution
5587 that a loop should have if that loop is unrolled, and if the loop is unrolled,
5588 it determines how many times the loop code is unrolled.
5590 @item max-unroll-times
5591 The maximum number of unrollings of a single loop.
5593 @item max-peeled-insns
5594 The maximum number of instructions that a loop should have if that loop
5595 is peeled, and if the loop is peeled, it determines how many times
5596 the loop code is peeled.
5598 @item max-peel-times
5599 The maximum number of peelings of a single loop.
5601 @item max-completely-peeled-insns
5602 The maximum number of insns of a completely peeled loop.
5604 @item max-completely-peel-times
5605 The maximum number of iterations of a loop to be suitable for complete peeling.
5607 @item max-unswitch-insns
5608 The maximum number of insns of an unswitched loop.
5610 @item max-unswitch-level
5611 The maximum number of branches unswitched in a single loop.
5614 The minimum cost of an expensive expression in the loop invariant motion.
5616 @item iv-consider-all-candidates-bound
5617 Bound on number of candidates for induction variables below that
5618 all candidates are considered for each use in induction variable
5619 optimizations. Only the most relevant candidates are considered
5620 if there are more candidates, to avoid quadratic time complexity.
5622 @item iv-max-considered-uses
5623 The induction variable optimizations give up on loops that contain more
5624 induction variable uses.
5626 @item iv-always-prune-cand-set-bound
5627 If number of candidates in the set is smaller than this value,
5628 we always try to remove unnecessary ivs from the set during its
5629 optimization when a new iv is added to the set.
5631 @item max-iterations-to-track
5633 The maximum number of iterations of a loop the brute force algorithm
5634 for analysis of # of iterations of the loop tries to evaluate.
5636 @item hot-bb-count-fraction
5637 Select fraction of the maximal count of repetitions of basic block in program
5638 given basic block needs to have to be considered hot.
5640 @item hot-bb-frequency-fraction
5641 Select fraction of the maximal frequency of executions of basic block in
5642 function given basic block needs to have to be considered hot
5644 @item tracer-dynamic-coverage
5645 @itemx tracer-dynamic-coverage-feedback
5647 This value is used to limit superblock formation once the given percentage of
5648 executed instructions is covered. This limits unnecessary code size
5651 The @option{tracer-dynamic-coverage-feedback} is used only when profile
5652 feedback is available. The real profiles (as opposed to statically estimated
5653 ones) are much less balanced allowing the threshold to be larger value.
5655 @item tracer-max-code-growth
5656 Stop tail duplication once code growth has reached given percentage. This is
5657 rather hokey argument, as most of the duplicates will be eliminated later in
5658 cross jumping, so it may be set to much higher values than is the desired code
5661 @item tracer-min-branch-ratio
5663 Stop reverse growth when the reverse probability of best edge is less than this
5664 threshold (in percent).
5666 @item tracer-min-branch-ratio
5667 @itemx tracer-min-branch-ratio-feedback
5669 Stop forward growth if the best edge do have probability lower than this
5672 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
5673 compilation for profile feedback and one for compilation without. The value
5674 for compilation with profile feedback needs to be more conservative (higher) in
5675 order to make tracer effective.
5677 @item max-cse-path-length
5679 Maximum number of basic blocks on path that cse considers. The default is 10.
5681 @item global-var-threshold
5683 Counts the number of function calls (@var{n}) and the number of
5684 call-clobbered variables (@var{v}). If @var{n}x@var{v} is larger than this limit, a
5685 single artificial variable will be created to represent all the
5686 call-clobbered variables at function call sites. This artificial
5687 variable will then be made to alias every call-clobbered variable.
5688 (done as @code{int * size_t} on the host machine; beware overflow).
5690 @item max-aliased-vops
5692 Maximum number of virtual operands allowed to represent aliases
5693 before triggering the alias grouping heuristic. Alias grouping
5694 reduces compile times and memory consumption needed for aliasing at
5695 the expense of precision loss in alias information.
5697 @item ggc-min-expand
5699 GCC uses a garbage collector to manage its own memory allocation. This
5700 parameter specifies the minimum percentage by which the garbage
5701 collector's heap should be allowed to expand between collections.
5702 Tuning this may improve compilation speed; it has no effect on code
5705 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
5706 RAM >= 1GB@. If @code{getrlimit} is available, the notion of "RAM" is
5707 the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}. If
5708 GCC is not able to calculate RAM on a particular platform, the lower
5709 bound of 30% is used. Setting this parameter and
5710 @option{ggc-min-heapsize} to zero causes a full collection to occur at
5711 every opportunity. This is extremely slow, but can be useful for
5714 @item ggc-min-heapsize
5716 Minimum size of the garbage collector's heap before it begins bothering
5717 to collect garbage. The first collection occurs after the heap expands
5718 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
5719 tuning this may improve compilation speed, and has no effect on code
5722 The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which
5723 tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
5724 with a lower bound of 4096 (four megabytes) and an upper bound of
5725 131072 (128 megabytes). If GCC is not able to calculate RAM on a
5726 particular platform, the lower bound is used. Setting this parameter
5727 very large effectively disables garbage collection. Setting this
5728 parameter and @option{ggc-min-expand} to zero causes a full collection
5729 to occur at every opportunity.
5731 @item max-reload-search-insns
5732 The maximum number of instruction reload should look backward for equivalent
5733 register. Increasing values mean more aggressive optimization, making the
5734 compile time increase with probably slightly better performance. The default
5737 @item max-cselib-memory-location
5738 The maximum number of memory locations cselib should take into acount.
5739 Increasing values mean more aggressive optimization, making the compile time
5740 increase with probably slightly better performance. The default value is 500.
5742 @item reorder-blocks-duplicate
5743 @itemx reorder-blocks-duplicate-feedback
5745 Used by basic block reordering pass to decide whether to use unconditional
5746 branch or duplicate the code on its destination. Code is duplicated when its
5747 estimated size is smaller than this value multiplied by the estimated size of
5748 unconditional jump in the hot spots of the program.
5750 The @option{reorder-block-duplicate-feedback} is used only when profile
5751 feedback is available and may be set to higher values than
5752 @option{reorder-block-duplicate} since information about the hot spots is more
5755 @item max-sched-region-blocks
5756 The maximum number of blocks in a region to be considered for
5757 interblock scheduling. The default value is 10.
5759 @item max-sched-region-insns
5760 The maximum number of insns in a region to be considered for
5761 interblock scheduling. The default value is 100.
5763 @item max-last-value-rtl
5765 The maximum size measured as number of RTLs that can be recorded in an expression
5766 in combiner for a pseudo register as last known value of that register. The default
5769 @item integer-share-limit
5770 Small integer constants can use a shared data structure, reducing the
5771 compiler's memory usage and increasing its speed. This sets the maximum
5772 value of a shared integer constant's. The default value is 256.
5777 @node Preprocessor Options
5778 @section Options Controlling the Preprocessor
5779 @cindex preprocessor options
5780 @cindex options, preprocessor
5782 These options control the C preprocessor, which is run on each C source
5783 file before actual compilation.
5785 If you use the @option{-E} option, nothing is done except preprocessing.
5786 Some of these options make sense only together with @option{-E} because
5787 they cause the preprocessor output to be unsuitable for actual
5792 You can use @option{-Wp,@var{option}} to bypass the compiler driver
5793 and pass @var{option} directly through to the preprocessor. If
5794 @var{option} contains commas, it is split into multiple options at the
5795 commas. However, many options are modified, translated or interpreted
5796 by the compiler driver before being passed to the preprocessor, and
5797 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
5798 interface is undocumented and subject to change, so whenever possible
5799 you should avoid using @option{-Wp} and let the driver handle the
5802 @item -Xpreprocessor @var{option}
5803 @opindex preprocessor
5804 Pass @var{option} as an option to the preprocessor. You can use this to
5805 supply system-specific preprocessor options which GCC does not know how to
5808 If you want to pass an option that takes an argument, you must use
5809 @option{-Xpreprocessor} twice, once for the option and once for the argument.
5812 @include cppopts.texi
5814 @node Assembler Options
5815 @section Passing Options to the Assembler
5817 @c prevent bad page break with this line
5818 You can pass options to the assembler.
5821 @item -Wa,@var{option}
5823 Pass @var{option} as an option to the assembler. If @var{option}
5824 contains commas, it is split into multiple options at the commas.
5826 @item -Xassembler @var{option}
5828 Pass @var{option} as an option to the assembler. You can use this to
5829 supply system-specific assembler options which GCC does not know how to
5832 If you want to pass an option that takes an argument, you must use
5833 @option{-Xassembler} twice, once for the option and once for the argument.
5838 @section Options for Linking
5839 @cindex link options
5840 @cindex options, linking
5842 These options come into play when the compiler links object files into
5843 an executable output file. They are meaningless if the compiler is
5844 not doing a link step.
5848 @item @var{object-file-name}
5849 A file name that does not end in a special recognized suffix is
5850 considered to name an object file or library. (Object files are
5851 distinguished from libraries by the linker according to the file
5852 contents.) If linking is done, these object files are used as input
5861 If any of these options is used, then the linker is not run, and
5862 object file names should not be used as arguments. @xref{Overall
5866 @item -l@var{library}
5867 @itemx -l @var{library}
5869 Search the library named @var{library} when linking. (The second
5870 alternative with the library as a separate argument is only for
5871 POSIX compliance and is not recommended.)
5873 It makes a difference where in the command you write this option; the
5874 linker searches and processes libraries and object files in the order they
5875 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
5876 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
5877 to functions in @samp{z}, those functions may not be loaded.
5879 The linker searches a standard list of directories for the library,
5880 which is actually a file named @file{lib@var{library}.a}. The linker
5881 then uses this file as if it had been specified precisely by name.
5883 The directories searched include several standard system directories
5884 plus any that you specify with @option{-L}.
5886 Normally the files found this way are library files---archive files
5887 whose members are object files. The linker handles an archive file by
5888 scanning through it for members which define symbols that have so far
5889 been referenced but not defined. But if the file that is found is an
5890 ordinary object file, it is linked in the usual fashion. The only
5891 difference between using an @option{-l} option and specifying a file name
5892 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
5893 and searches several directories.
5897 You need this special case of the @option{-l} option in order to
5898 link an Objective-C or Objective-C++ program.
5901 @opindex nostartfiles
5902 Do not use the standard system startup files when linking.
5903 The standard system libraries are used normally, unless @option{-nostdlib}
5904 or @option{-nodefaultlibs} is used.
5906 @item -nodefaultlibs
5907 @opindex nodefaultlibs
5908 Do not use the standard system libraries when linking.
5909 Only the libraries you specify will be passed to the linker.
5910 The standard startup files are used normally, unless @option{-nostartfiles}
5911 is used. The compiler may generate calls to @code{memcmp},
5912 @code{memset}, @code{memcpy} and @code{memmove}.
5913 These entries are usually resolved by entries in
5914 libc. These entry points should be supplied through some other
5915 mechanism when this option is specified.
5919 Do not use the standard system startup files or libraries when linking.
5920 No startup files and only the libraries you specify will be passed to
5921 the linker. The compiler may generate calls to @code{memcmp}, @code{memset},
5922 @code{memcpy} and @code{memmove}.
5923 These entries are usually resolved by entries in
5924 libc. These entry points should be supplied through some other
5925 mechanism when this option is specified.
5927 @cindex @option{-lgcc}, use with @option{-nostdlib}
5928 @cindex @option{-nostdlib} and unresolved references
5929 @cindex unresolved references and @option{-nostdlib}
5930 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
5931 @cindex @option{-nodefaultlibs} and unresolved references
5932 @cindex unresolved references and @option{-nodefaultlibs}
5933 One of the standard libraries bypassed by @option{-nostdlib} and
5934 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
5935 that GCC uses to overcome shortcomings of particular machines, or special
5936 needs for some languages.
5937 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
5938 Collection (GCC) Internals},
5939 for more discussion of @file{libgcc.a}.)
5940 In most cases, you need @file{libgcc.a} even when you want to avoid
5941 other standard libraries. In other words, when you specify @option{-nostdlib}
5942 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
5943 This ensures that you have no unresolved references to internal GCC
5944 library subroutines. (For example, @samp{__main}, used to ensure C++
5945 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
5946 GNU Compiler Collection (GCC) Internals}.)
5950 Produce a position independent executable on targets which support it.
5951 For predictable results, you must also specify the same set of options
5952 that were used to generate code (@option{-fpie}, @option{-fPIE},
5953 or model suboptions) when you specify this option.
5957 Remove all symbol table and relocation information from the executable.
5961 On systems that support dynamic linking, this prevents linking with the shared
5962 libraries. On other systems, this option has no effect.
5966 Produce a shared object which can then be linked with other objects to
5967 form an executable. Not all systems support this option. For predictable
5968 results, you must also specify the same set of options that were used to
5969 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
5970 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
5971 needs to build supplementary stub code for constructors to work. On
5972 multi-libbed systems, @samp{gcc -shared} must select the correct support
5973 libraries to link against. Failing to supply the correct flags may lead
5974 to subtle defects. Supplying them in cases where they are not necessary
5977 @item -shared-libgcc
5978 @itemx -static-libgcc
5979 @opindex shared-libgcc
5980 @opindex static-libgcc
5981 On systems that provide @file{libgcc} as a shared library, these options
5982 force the use of either the shared or static version respectively.
5983 If no shared version of @file{libgcc} was built when the compiler was
5984 configured, these options have no effect.
5986 There are several situations in which an application should use the
5987 shared @file{libgcc} instead of the static version. The most common
5988 of these is when the application wishes to throw and catch exceptions
5989 across different shared libraries. In that case, each of the libraries
5990 as well as the application itself should use the shared @file{libgcc}.
5992 Therefore, the G++ and GCJ drivers automatically add
5993 @option{-shared-libgcc} whenever you build a shared library or a main
5994 executable, because C++ and Java programs typically use exceptions, so
5995 this is the right thing to do.
5997 If, instead, you use the GCC driver to create shared libraries, you may
5998 find that they will not always be linked with the shared @file{libgcc}.
5999 If GCC finds, at its configuration time, that you have a non-GNU linker
6000 or a GNU linker that does not support option @option{--eh-frame-hdr},
6001 it will link the shared version of @file{libgcc} into shared libraries
6002 by default. Otherwise, it will take advantage of the linker and optimize
6003 away the linking with the shared version of @file{libgcc}, linking with
6004 the static version of libgcc by default. This allows exceptions to
6005 propagate through such shared libraries, without incurring relocation
6006 costs at library load time.
6008 However, if a library or main executable is supposed to throw or catch
6009 exceptions, you must link it using the G++ or GCJ driver, as appropriate
6010 for the languages used in the program, or using the option
6011 @option{-shared-libgcc}, such that it is linked with the shared
6016 Bind references to global symbols when building a shared object. Warn
6017 about any unresolved references (unless overridden by the link editor
6018 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
6021 @item -Xlinker @var{option}
6023 Pass @var{option} as an option to the linker. You can use this to
6024 supply system-specific linker options which GCC does not know how to
6027 If you want to pass an option that takes an argument, you must use
6028 @option{-Xlinker} twice, once for the option and once for the argument.
6029 For example, to pass @option{-assert definitions}, you must write
6030 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
6031 @option{-Xlinker "-assert definitions"}, because this passes the entire
6032 string as a single argument, which is not what the linker expects.
6034 @item -Wl,@var{option}
6036 Pass @var{option} as an option to the linker. If @var{option} contains
6037 commas, it is split into multiple options at the commas.
6039 @item -u @var{symbol}
6041 Pretend the symbol @var{symbol} is undefined, to force linking of
6042 library modules to define it. You can use @option{-u} multiple times with
6043 different symbols to force loading of additional library modules.
6046 @node Directory Options
6047 @section Options for Directory Search
6048 @cindex directory options
6049 @cindex options, directory search
6052 These options specify directories to search for header files, for
6053 libraries and for parts of the compiler:
6058 Add the directory @var{dir} to the head of the list of directories to be
6059 searched for header files. This can be used to override a system header
6060 file, substituting your own version, since these directories are
6061 searched before the system header file directories. However, you should
6062 not use this option to add directories that contain vendor-supplied
6063 system header files (use @option{-isystem} for that). If you use more than
6064 one @option{-I} option, the directories are scanned in left-to-right
6065 order; the standard system directories come after.
6067 If a standard system include directory, or a directory specified with
6068 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
6069 option will be ignored. The directory will still be searched but as a
6070 system directory at its normal position in the system include chain.
6071 This is to ensure that GCC's procedure to fix buggy system headers and
6072 the ordering for the include_next directive are not inadvertently changed.
6073 If you really need to change the search order for system directories,
6074 use the @option{-nostdinc} and/or @option{-isystem} options.
6076 @item -iquote@var{dir}
6078 Add the directory @var{dir} to the head of the list of directories to
6079 be searched for header files only for the case of @samp{#include
6080 "@var{file}"}; they are not searched for @samp{#include <@var{file}>},
6081 otherwise just like @option{-I}.
6085 Add directory @var{dir} to the list of directories to be searched
6088 @item -B@var{prefix}
6090 This option specifies where to find the executables, libraries,
6091 include files, and data files of the compiler itself.
6093 The compiler driver program runs one or more of the subprograms
6094 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
6095 @var{prefix} as a prefix for each program it tries to run, both with and
6096 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
6098 For each subprogram to be run, the compiler driver first tries the
6099 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
6100 was not specified, the driver tries two standard prefixes, which are
6101 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc/}. If neither of
6102 those results in a file name that is found, the unmodified program
6103 name is searched for using the directories specified in your
6104 @env{PATH} environment variable.
6106 The compiler will check to see if the path provided by the @option{-B}
6107 refers to a directory, and if necessary it will add a directory
6108 separator character at the end of the path.
6110 @option{-B} prefixes that effectively specify directory names also apply
6111 to libraries in the linker, because the compiler translates these
6112 options into @option{-L} options for the linker. They also apply to
6113 includes files in the preprocessor, because the compiler translates these
6114 options into @option{-isystem} options for the preprocessor. In this case,
6115 the compiler appends @samp{include} to the prefix.
6117 The run-time support file @file{libgcc.a} can also be searched for using
6118 the @option{-B} prefix, if needed. If it is not found there, the two
6119 standard prefixes above are tried, and that is all. The file is left
6120 out of the link if it is not found by those means.
6122 Another way to specify a prefix much like the @option{-B} prefix is to use
6123 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
6126 As a special kludge, if the path provided by @option{-B} is
6127 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
6128 9, then it will be replaced by @file{[dir/]include}. This is to help
6129 with boot-strapping the compiler.
6131 @item -specs=@var{file}
6133 Process @var{file} after the compiler reads in the standard @file{specs}
6134 file, in order to override the defaults that the @file{gcc} driver
6135 program uses when determining what switches to pass to @file{cc1},
6136 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
6137 @option{-specs=@var{file}} can be specified on the command line, and they
6138 are processed in order, from left to right.
6142 This option has been deprecated. Please use @option{-iquote} instead for
6143 @option{-I} directories before the @option{-I-} and remove the @option{-I-}.
6144 Any directories you specify with @option{-I} options before the @option{-I-}
6145 option are searched only for the case of @samp{#include "@var{file}"};
6146 they are not searched for @samp{#include <@var{file}>}.
6148 If additional directories are specified with @option{-I} options after
6149 the @option{-I-}, these directories are searched for all @samp{#include}
6150 directives. (Ordinarily @emph{all} @option{-I} directories are used
6153 In addition, the @option{-I-} option inhibits the use of the current
6154 directory (where the current input file came from) as the first search
6155 directory for @samp{#include "@var{file}"}. There is no way to
6156 override this effect of @option{-I-}. With @option{-I.} you can specify
6157 searching the directory which was current when the compiler was
6158 invoked. That is not exactly the same as what the preprocessor does
6159 by default, but it is often satisfactory.
6161 @option{-I-} does not inhibit the use of the standard system directories
6162 for header files. Thus, @option{-I-} and @option{-nostdinc} are
6169 @section Specifying subprocesses and the switches to pass to them
6172 @command{gcc} is a driver program. It performs its job by invoking a
6173 sequence of other programs to do the work of compiling, assembling and
6174 linking. GCC interprets its command-line parameters and uses these to
6175 deduce which programs it should invoke, and which command-line options
6176 it ought to place on their command lines. This behavior is controlled
6177 by @dfn{spec strings}. In most cases there is one spec string for each
6178 program that GCC can invoke, but a few programs have multiple spec
6179 strings to control their behavior. The spec strings built into GCC can
6180 be overridden by using the @option{-specs=} command-line switch to specify
6183 @dfn{Spec files} are plaintext files that are used to construct spec
6184 strings. They consist of a sequence of directives separated by blank
6185 lines. The type of directive is determined by the first non-whitespace
6186 character on the line and it can be one of the following:
6189 @item %@var{command}
6190 Issues a @var{command} to the spec file processor. The commands that can
6194 @item %include <@var{file}>
6196 Search for @var{file} and insert its text at the current point in the
6199 @item %include_noerr <@var{file}>
6200 @cindex %include_noerr
6201 Just like @samp{%include}, but do not generate an error message if the include
6202 file cannot be found.
6204 @item %rename @var{old_name} @var{new_name}
6206 Rename the spec string @var{old_name} to @var{new_name}.
6210 @item *[@var{spec_name}]:
6211 This tells the compiler to create, override or delete the named spec
6212 string. All lines after this directive up to the next directive or
6213 blank line are considered to be the text for the spec string. If this
6214 results in an empty string then the spec will be deleted. (Or, if the
6215 spec did not exist, then nothing will happened.) Otherwise, if the spec
6216 does not currently exist a new spec will be created. If the spec does
6217 exist then its contents will be overridden by the text of this
6218 directive, unless the first character of that text is the @samp{+}
6219 character, in which case the text will be appended to the spec.
6221 @item [@var{suffix}]:
6222 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
6223 and up to the next directive or blank line are considered to make up the
6224 spec string for the indicated suffix. When the compiler encounters an
6225 input file with the named suffix, it will processes the spec string in
6226 order to work out how to compile that file. For example:
6233 This says that any input file whose name ends in @samp{.ZZ} should be
6234 passed to the program @samp{z-compile}, which should be invoked with the
6235 command-line switch @option{-input} and with the result of performing the
6236 @samp{%i} substitution. (See below.)
6238 As an alternative to providing a spec string, the text that follows a
6239 suffix directive can be one of the following:
6242 @item @@@var{language}
6243 This says that the suffix is an alias for a known @var{language}. This is
6244 similar to using the @option{-x} command-line switch to GCC to specify a
6245 language explicitly. For example:
6252 Says that .ZZ files are, in fact, C++ source files.
6255 This causes an error messages saying:
6258 @var{name} compiler not installed on this system.
6262 GCC already has an extensive list of suffixes built into it.
6263 This directive will add an entry to the end of the list of suffixes, but
6264 since the list is searched from the end backwards, it is effectively
6265 possible to override earlier entries using this technique.
6269 GCC has the following spec strings built into it. Spec files can
6270 override these strings or create their own. Note that individual
6271 targets can also add their own spec strings to this list.
6274 asm Options to pass to the assembler
6275 asm_final Options to pass to the assembler post-processor
6276 cpp Options to pass to the C preprocessor
6277 cc1 Options to pass to the C compiler
6278 cc1plus Options to pass to the C++ compiler
6279 endfile Object files to include at the end of the link
6280 link Options to pass to the linker
6281 lib Libraries to include on the command line to the linker
6282 libgcc Decides which GCC support library to pass to the linker
6283 linker Sets the name of the linker
6284 predefines Defines to be passed to the C preprocessor
6285 signed_char Defines to pass to CPP to say whether @code{char} is signed
6287 startfile Object files to include at the start of the link
6290 Here is a small example of a spec file:
6296 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
6299 This example renames the spec called @samp{lib} to @samp{old_lib} and
6300 then overrides the previous definition of @samp{lib} with a new one.
6301 The new definition adds in some extra command-line options before
6302 including the text of the old definition.
6304 @dfn{Spec strings} are a list of command-line options to be passed to their
6305 corresponding program. In addition, the spec strings can contain
6306 @samp{%}-prefixed sequences to substitute variable text or to
6307 conditionally insert text into the command line. Using these constructs
6308 it is possible to generate quite complex command lines.
6310 Here is a table of all defined @samp{%}-sequences for spec
6311 strings. Note that spaces are not generated automatically around the
6312 results of expanding these sequences. Therefore you can concatenate them
6313 together or combine them with constant text in a single argument.
6317 Substitute one @samp{%} into the program name or argument.
6320 Substitute the name of the input file being processed.
6323 Substitute the basename of the input file being processed.
6324 This is the substring up to (and not including) the last period
6325 and not including the directory.
6328 This is the same as @samp{%b}, but include the file suffix (text after
6332 Marks the argument containing or following the @samp{%d} as a
6333 temporary file name, so that that file will be deleted if GCC exits
6334 successfully. Unlike @samp{%g}, this contributes no text to the
6337 @item %g@var{suffix}
6338 Substitute a file name that has suffix @var{suffix} and is chosen
6339 once per compilation, and mark the argument in the same way as
6340 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
6341 name is now chosen in a way that is hard to predict even when previously
6342 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
6343 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
6344 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
6345 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
6346 was simply substituted with a file name chosen once per compilation,
6347 without regard to any appended suffix (which was therefore treated
6348 just like ordinary text), making such attacks more likely to succeed.
6350 @item %u@var{suffix}
6351 Like @samp{%g}, but generates a new temporary file name even if
6352 @samp{%u@var{suffix}} was already seen.
6354 @item %U@var{suffix}
6355 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
6356 new one if there is no such last file name. In the absence of any
6357 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
6358 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
6359 would involve the generation of two distinct file names, one
6360 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
6361 simply substituted with a file name chosen for the previous @samp{%u},
6362 without regard to any appended suffix.
6364 @item %j@var{suffix}
6365 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
6366 writable, and if save-temps is off; otherwise, substitute the name
6367 of a temporary file, just like @samp{%u}. This temporary file is not
6368 meant for communication between processes, but rather as a junk
6371 @item %|@var{suffix}
6372 @itemx %m@var{suffix}
6373 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
6374 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
6375 all. These are the two most common ways to instruct a program that it
6376 should read from standard input or write to standard output. If you
6377 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
6378 construct: see for example @file{f/lang-specs.h}.
6380 @item %.@var{SUFFIX}
6381 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
6382 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
6383 terminated by the next space or %.
6386 Marks the argument containing or following the @samp{%w} as the
6387 designated output file of this compilation. This puts the argument
6388 into the sequence of arguments that @samp{%o} will substitute later.
6391 Substitutes the names of all the output files, with spaces
6392 automatically placed around them. You should write spaces
6393 around the @samp{%o} as well or the results are undefined.
6394 @samp{%o} is for use in the specs for running the linker.
6395 Input files whose names have no recognized suffix are not compiled
6396 at all, but they are included among the output files, so they will
6400 Substitutes the suffix for object files. Note that this is
6401 handled specially when it immediately follows @samp{%g, %u, or %U},
6402 because of the need for those to form complete file names. The
6403 handling is such that @samp{%O} is treated exactly as if it had already
6404 been substituted, except that @samp{%g, %u, and %U} do not currently
6405 support additional @var{suffix} characters following @samp{%O} as they would
6406 following, for example, @samp{.o}.
6409 Substitutes the standard macro predefinitions for the
6410 current target machine. Use this when running @code{cpp}.
6413 Like @samp{%p}, but puts @samp{__} before and after the name of each
6414 predefined macro, except for macros that start with @samp{__} or with
6415 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
6419 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
6420 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
6421 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
6425 Current argument is the name of a library or startup file of some sort.
6426 Search for that file in a standard list of directories and substitute
6427 the full name found.
6430 Print @var{str} as an error message. @var{str} is terminated by a newline.
6431 Use this when inconsistent options are detected.
6434 Substitute the contents of spec string @var{name} at this point.
6437 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
6439 @item %x@{@var{option}@}
6440 Accumulate an option for @samp{%X}.
6443 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
6447 Output the accumulated assembler options specified by @option{-Wa}.
6450 Output the accumulated preprocessor options specified by @option{-Wp}.
6453 Process the @code{asm} spec. This is used to compute the
6454 switches to be passed to the assembler.
6457 Process the @code{asm_final} spec. This is a spec string for
6458 passing switches to an assembler post-processor, if such a program is
6462 Process the @code{link} spec. This is the spec for computing the
6463 command line passed to the linker. Typically it will make use of the
6464 @samp{%L %G %S %D and %E} sequences.
6467 Dump out a @option{-L} option for each directory that GCC believes might
6468 contain startup files. If the target supports multilibs then the
6469 current multilib directory will be prepended to each of these paths.
6472 Process the @code{lib} spec. This is a spec string for deciding which
6473 libraries should be included on the command line to the linker.
6476 Process the @code{libgcc} spec. This is a spec string for deciding
6477 which GCC support library should be included on the command line to the linker.
6480 Process the @code{startfile} spec. This is a spec for deciding which
6481 object files should be the first ones passed to the linker. Typically
6482 this might be a file named @file{crt0.o}.
6485 Process the @code{endfile} spec. This is a spec string that specifies
6486 the last object files that will be passed to the linker.
6489 Process the @code{cpp} spec. This is used to construct the arguments
6490 to be passed to the C preprocessor.
6493 Process the @code{cc1} spec. This is used to construct the options to be
6494 passed to the actual C compiler (@samp{cc1}).
6497 Process the @code{cc1plus} spec. This is used to construct the options to be
6498 passed to the actual C++ compiler (@samp{cc1plus}).
6501 Substitute the variable part of a matched option. See below.
6502 Note that each comma in the substituted string is replaced by
6506 Remove all occurrences of @code{-S} from the command line. Note---this
6507 command is position dependent. @samp{%} commands in the spec string
6508 before this one will see @code{-S}, @samp{%} commands in the spec string
6509 after this one will not.
6511 @item %:@var{function}(@var{args})
6512 Call the named function @var{function}, passing it @var{args}.
6513 @var{args} is first processed as a nested spec string, then split
6514 into an argument vector in the usual fashion. The function returns
6515 a string which is processed as if it had appeared literally as part
6516 of the current spec.
6518 The following built-in spec functions are provided:
6521 @item @code{if-exists}
6522 The @code{if-exists} spec function takes one argument, an absolute
6523 pathname to a file. If the file exists, @code{if-exists} returns the
6524 pathname. Here is a small example of its usage:
6528 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
6531 @item @code{if-exists-else}
6532 The @code{if-exists-else} spec function is similar to the @code{if-exists}
6533 spec function, except that it takes two arguments. The first argument is
6534 an absolute pathname to a file. If the file exists, @code{if-exists-else}
6535 returns the pathname. If it does not exist, it returns the second argument.
6536 This way, @code{if-exists-else} can be used to select one file or another,
6537 based on the existence of the first. Here is a small example of its usage:
6541 crt0%O%s %:if-exists(crti%O%s) \
6542 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
6545 @item @code{replace-outfile}
6546 The @code{replace-outfile} spec function takes two arguments. It looks for the
6547 first argument in the outfiles array and replaces it with the second argument. Here
6548 is a small example of its usage:
6551 %@{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)@}
6557 Substitutes the @code{-S} switch, if that switch was given to GCC@.
6558 If that switch was not specified, this substitutes nothing. Note that
6559 the leading dash is omitted when specifying this option, and it is
6560 automatically inserted if the substitution is performed. Thus the spec
6561 string @samp{%@{foo@}} would match the command-line option @option{-foo}
6562 and would output the command line option @option{-foo}.
6564 @item %W@{@code{S}@}
6565 Like %@{@code{S}@} but mark last argument supplied within as a file to be
6568 @item %@{@code{S}*@}
6569 Substitutes all the switches specified to GCC whose names start
6570 with @code{-S}, but which also take an argument. This is used for
6571 switches like @option{-o}, @option{-D}, @option{-I}, etc.
6572 GCC considers @option{-o foo} as being
6573 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
6574 text, including the space. Thus two arguments would be generated.
6576 @item %@{@code{S}*&@code{T}*@}
6577 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
6578 (the order of @code{S} and @code{T} in the spec is not significant).
6579 There can be any number of ampersand-separated variables; for each the
6580 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
6582 @item %@{@code{S}:@code{X}@}
6583 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
6585 @item %@{!@code{S}:@code{X}@}
6586 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
6588 @item %@{@code{S}*:@code{X}@}
6589 Substitutes @code{X} if one or more switches whose names start with
6590 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
6591 once, no matter how many such switches appeared. However, if @code{%*}
6592 appears somewhere in @code{X}, then @code{X} will be substituted once
6593 for each matching switch, with the @code{%*} replaced by the part of
6594 that switch that matched the @code{*}.
6596 @item %@{.@code{S}:@code{X}@}
6597 Substitutes @code{X}, if processing a file with suffix @code{S}.
6599 @item %@{!.@code{S}:@code{X}@}
6600 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
6602 @item %@{@code{S}|@code{P}:@code{X}@}
6603 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
6604 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
6605 although they have a stronger binding than the @samp{|}. If @code{%*}
6606 appears in @code{X}, all of the alternatives must be starred, and only
6607 the first matching alternative is substituted.
6609 For example, a spec string like this:
6612 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
6615 will output the following command-line options from the following input
6616 command-line options:
6621 -d fred.c -foo -baz -boggle
6622 -d jim.d -bar -baz -boggle
6625 @item %@{S:X; T:Y; :D@}
6627 If @code{S} was given to GCC, substitutes @code{X}; else if @code{T} was
6628 given to GCC, substitutes @code{Y}; else substitutes @code{D}. There can
6629 be as many clauses as you need. This may be combined with @code{.},
6630 @code{!}, @code{|}, and @code{*} as needed.
6635 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
6636 construct may contain other nested @samp{%} constructs or spaces, or
6637 even newlines. They are processed as usual, as described above.
6638 Trailing white space in @code{X} is ignored. White space may also
6639 appear anywhere on the left side of the colon in these constructs,
6640 except between @code{.} or @code{*} and the corresponding word.
6642 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
6643 handled specifically in these constructs. If another value of
6644 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
6645 @option{-W} switch is found later in the command line, the earlier
6646 switch value is ignored, except with @{@code{S}*@} where @code{S} is
6647 just one letter, which passes all matching options.
6649 The character @samp{|} at the beginning of the predicate text is used to
6650 indicate that a command should be piped to the following command, but
6651 only if @option{-pipe} is specified.
6653 It is built into GCC which switches take arguments and which do not.
6654 (You might think it would be useful to generalize this to allow each
6655 compiler's spec to say which switches take arguments. But this cannot
6656 be done in a consistent fashion. GCC cannot even decide which input
6657 files have been specified without knowing which switches take arguments,
6658 and it must know which input files to compile in order to tell which
6661 GCC also knows implicitly that arguments starting in @option{-l} are to be
6662 treated as compiler output files, and passed to the linker in their
6663 proper position among the other output files.
6665 @c man begin OPTIONS
6667 @node Target Options
6668 @section Specifying Target Machine and Compiler Version
6669 @cindex target options
6670 @cindex cross compiling
6671 @cindex specifying machine version
6672 @cindex specifying compiler version and target machine
6673 @cindex compiler version, specifying
6674 @cindex target machine, specifying
6676 The usual way to run GCC is to run the executable called @file{gcc}, or
6677 @file{<machine>-gcc} when cross-compiling, or
6678 @file{<machine>-gcc-<version>} to run a version other than the one that
6679 was installed last. Sometimes this is inconvenient, so GCC provides
6680 options that will switch to another cross-compiler or version.
6683 @item -b @var{machine}
6685 The argument @var{machine} specifies the target machine for compilation.
6687 The value to use for @var{machine} is the same as was specified as the
6688 machine type when configuring GCC as a cross-compiler. For
6689 example, if a cross-compiler was configured with @samp{configure
6690 i386v}, meaning to compile for an 80386 running System V, then you
6691 would specify @option{-b i386v} to run that cross compiler.
6693 @item -V @var{version}
6695 The argument @var{version} specifies which version of GCC to run.
6696 This is useful when multiple versions are installed. For example,
6697 @var{version} might be @samp{2.0}, meaning to run GCC version 2.0.
6700 The @option{-V} and @option{-b} options work by running the
6701 @file{<machine>-gcc-<version>} executable, so there's no real reason to
6702 use them if you can just run that directly.
6704 @node Submodel Options
6705 @section Hardware Models and Configurations
6706 @cindex submodel options
6707 @cindex specifying hardware config
6708 @cindex hardware models and configurations, specifying
6709 @cindex machine dependent options
6711 Earlier we discussed the standard option @option{-b} which chooses among
6712 different installed compilers for completely different target
6713 machines, such as VAX vs.@: 68000 vs.@: 80386.
6715 In addition, each of these target machine types can have its own
6716 special options, starting with @samp{-m}, to choose among various
6717 hardware models or configurations---for example, 68010 vs 68020,
6718 floating coprocessor or none. A single installed version of the
6719 compiler can compile for any model or configuration, according to the
6722 Some configurations of the compiler also support additional special
6723 options, usually for compatibility with other compilers on the same
6726 These options are defined by the macro @code{TARGET_SWITCHES} in the
6727 machine description. The default for the options is also defined by
6728 that macro, which enables you to change the defaults.
6730 @c This list is ordered alphanumerically by subsection name.
6731 @c It should be the same order and spelling as these options are listed
6732 @c in Machine Dependent Options
6738 * Blackfin Options::
6741 * DEC Alpha Options::
6742 * DEC Alpha/VMS Options::
6746 * i386 and x86-64 Options::
6758 * RS/6000 and PowerPC Options::
6759 * S/390 and zSeries Options::
6762 * System V Options::
6763 * TMS320C3x/C4x Options::
6767 * Xstormy16 Options::
6773 @subsection ARC Options
6776 These options are defined for ARC implementations:
6781 Compile code for little endian mode. This is the default.
6785 Compile code for big endian mode.
6788 @opindex mmangle-cpu
6789 Prepend the name of the cpu to all public symbol names.
6790 In multiple-processor systems, there are many ARC variants with different
6791 instruction and register set characteristics. This flag prevents code
6792 compiled for one cpu to be linked with code compiled for another.
6793 No facility exists for handling variants that are ``almost identical''.
6794 This is an all or nothing option.
6796 @item -mcpu=@var{cpu}
6798 Compile code for ARC variant @var{cpu}.
6799 Which variants are supported depend on the configuration.
6800 All variants support @option{-mcpu=base}, this is the default.
6802 @item -mtext=@var{text-section}
6803 @itemx -mdata=@var{data-section}
6804 @itemx -mrodata=@var{readonly-data-section}
6808 Put functions, data, and readonly data in @var{text-section},
6809 @var{data-section}, and @var{readonly-data-section} respectively
6810 by default. This can be overridden with the @code{section} attribute.
6811 @xref{Variable Attributes}.
6816 @subsection ARM Options
6819 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
6823 @item -mabi=@var{name}
6825 Generate code for the specified ABI@. Permissible values are: @samp{apcs-gnu},
6826 @samp{atpcs}, @samp{aapcs} and @samp{iwmmxt}.
6829 @opindex mapcs-frame
6830 Generate a stack frame that is compliant with the ARM Procedure Call
6831 Standard for all functions, even if this is not strictly necessary for
6832 correct execution of the code. Specifying @option{-fomit-frame-pointer}
6833 with this option will cause the stack frames not to be generated for
6834 leaf functions. The default is @option{-mno-apcs-frame}.
6838 This is a synonym for @option{-mapcs-frame}.
6841 @c not currently implemented
6842 @item -mapcs-stack-check
6843 @opindex mapcs-stack-check
6844 Generate code to check the amount of stack space available upon entry to
6845 every function (that actually uses some stack space). If there is
6846 insufficient space available then either the function
6847 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
6848 called, depending upon the amount of stack space required. The run time
6849 system is required to provide these functions. The default is
6850 @option{-mno-apcs-stack-check}, since this produces smaller code.
6852 @c not currently implemented
6854 @opindex mapcs-float
6855 Pass floating point arguments using the float point registers. This is
6856 one of the variants of the APCS@. This option is recommended if the
6857 target hardware has a floating point unit or if a lot of floating point
6858 arithmetic is going to be performed by the code. The default is
6859 @option{-mno-apcs-float}, since integer only code is slightly increased in
6860 size if @option{-mapcs-float} is used.
6862 @c not currently implemented
6863 @item -mapcs-reentrant
6864 @opindex mapcs-reentrant
6865 Generate reentrant, position independent code. The default is
6866 @option{-mno-apcs-reentrant}.
6869 @item -mthumb-interwork
6870 @opindex mthumb-interwork
6871 Generate code which supports calling between the ARM and Thumb
6872 instruction sets. Without this option the two instruction sets cannot
6873 be reliably used inside one program. The default is
6874 @option{-mno-thumb-interwork}, since slightly larger code is generated
6875 when @option{-mthumb-interwork} is specified.
6877 @item -mno-sched-prolog
6878 @opindex mno-sched-prolog
6879 Prevent the reordering of instructions in the function prolog, or the
6880 merging of those instruction with the instructions in the function's
6881 body. This means that all functions will start with a recognizable set
6882 of instructions (or in fact one of a choice from a small set of
6883 different function prologues), and this information can be used to
6884 locate the start if functions inside an executable piece of code. The
6885 default is @option{-msched-prolog}.
6888 @opindex mhard-float
6889 Generate output containing floating point instructions. This is the
6893 @opindex msoft-float
6894 Generate output containing library calls for floating point.
6895 @strong{Warning:} the requisite libraries are not available for all ARM
6896 targets. Normally the facilities of the machine's usual C compiler are
6897 used, but this cannot be done directly in cross-compilation. You must make
6898 your own arrangements to provide suitable library functions for
6901 @option{-msoft-float} changes the calling convention in the output file;
6902 therefore, it is only useful if you compile @emph{all} of a program with
6903 this option. In particular, you need to compile @file{libgcc.a}, the
6904 library that comes with GCC, with @option{-msoft-float} in order for
6907 @item -mfloat-abi=@var{name}
6909 Specifies which ABI to use for floating point values. Permissible values
6910 are: @samp{soft}, @samp{softfp} and @samp{hard}.
6912 @samp{soft} and @samp{hard} are equivalent to @option{-msoft-float}
6913 and @option{-mhard-float} respectively. @samp{softfp} allows the generation
6914 of floating point instructions, but still uses the soft-float calling
6917 @item -mlittle-endian
6918 @opindex mlittle-endian
6919 Generate code for a processor running in little-endian mode. This is
6920 the default for all standard configurations.
6923 @opindex mbig-endian
6924 Generate code for a processor running in big-endian mode; the default is
6925 to compile code for a little-endian processor.
6927 @item -mwords-little-endian
6928 @opindex mwords-little-endian
6929 This option only applies when generating code for big-endian processors.
6930 Generate code for a little-endian word order but a big-endian byte
6931 order. That is, a byte order of the form @samp{32107654}. Note: this
6932 option should only be used if you require compatibility with code for
6933 big-endian ARM processors generated by versions of the compiler prior to
6936 @item -mcpu=@var{name}
6938 This specifies the name of the target ARM processor. GCC uses this name
6939 to determine what kind of instructions it can emit when generating
6940 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
6941 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
6942 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
6943 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
6944 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
6945 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm7tdmi-s},
6946 @samp{arm8}, @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
6947 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
6948 @samp{arm920t}, @samp{arm922t}, @samp{arm946e-s}, @samp{arm966e-s},
6949 @samp{arm968e-s}, @samp{arm926ej-s}, @samp{arm940t}, @samp{arm9tdmi},
6950 @samp{arm10tdmi}, @samp{arm1020t}, @samp{arm1026ej-s},
6951 @samp{arm10e}, @samp{arm1020e}, @samp{arm1022e},
6952 @samp{arm1136j-s}, @samp{arm1136jf-s}, @samp{mpcore}, @samp{mpcorenovfp},
6953 @samp{arm1176jz-s}, @samp{arm1176jzf-s}, @samp{xscale}, @samp{iwmmxt},
6956 @itemx -mtune=@var{name}
6958 This option is very similar to the @option{-mcpu=} option, except that
6959 instead of specifying the actual target processor type, and hence
6960 restricting which instructions can be used, it specifies that GCC should
6961 tune the performance of the code as if the target were of the type
6962 specified in this option, but still choosing the instructions that it
6963 will generate based on the cpu specified by a @option{-mcpu=} option.
6964 For some ARM implementations better performance can be obtained by using
6967 @item -march=@var{name}
6969 This specifies the name of the target ARM architecture. GCC uses this
6970 name to determine what kind of instructions it can emit when generating
6971 assembly code. This option can be used in conjunction with or instead
6972 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
6973 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
6974 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{armv6}, @samp{armv6j},
6975 @samp{iwmmxt}, @samp{ep9312}.
6977 @item -mfpu=@var{name}
6978 @itemx -mfpe=@var{number}
6979 @itemx -mfp=@var{number}
6983 This specifies what floating point hardware (or hardware emulation) is
6984 available on the target. Permissible names are: @samp{fpa}, @samp{fpe2},
6985 @samp{fpe3}, @samp{maverick}, @samp{vfp}. @option{-mfp} and @option{-mfpe}
6986 are synonyms for @option{-mfpu}=@samp{fpe}@var{number}, for compatibility
6987 with older versions of GCC@.
6989 If @option{-msoft-float} is specified this specifies the format of
6990 floating point values.
6992 @item -mstructure-size-boundary=@var{n}
6993 @opindex mstructure-size-boundary
6994 The size of all structures and unions will be rounded up to a multiple
6995 of the number of bits set by this option. Permissible values are 8, 32
6996 and 64. The default value varies for different toolchains. For the COFF
6997 targeted toolchain the default value is 8. A value of 64 is only allowed
6998 if the underlying ABI supports it.
7000 Specifying the larger number can produce faster, more efficient code, but
7001 can also increase the size of the program. Different values are potentially
7002 incompatible. Code compiled with one value cannot necessarily expect to
7003 work with code or libraries compiled with another value, if they exchange
7004 information using structures or unions.
7006 @item -mabort-on-noreturn
7007 @opindex mabort-on-noreturn
7008 Generate a call to the function @code{abort} at the end of a
7009 @code{noreturn} function. It will be executed if the function tries to
7013 @itemx -mno-long-calls
7014 @opindex mlong-calls
7015 @opindex mno-long-calls
7016 Tells the compiler to perform function calls by first loading the
7017 address of the function into a register and then performing a subroutine
7018 call on this register. This switch is needed if the target function
7019 will lie outside of the 64 megabyte addressing range of the offset based
7020 version of subroutine call instruction.
7022 Even if this switch is enabled, not all function calls will be turned
7023 into long calls. The heuristic is that static functions, functions
7024 which have the @samp{short-call} attribute, functions that are inside
7025 the scope of a @samp{#pragma no_long_calls} directive and functions whose
7026 definitions have already been compiled within the current compilation
7027 unit, will not be turned into long calls. The exception to this rule is
7028 that weak function definitions, functions with the @samp{long-call}
7029 attribute or the @samp{section} attribute, and functions that are within
7030 the scope of a @samp{#pragma long_calls} directive, will always be
7031 turned into long calls.
7033 This feature is not enabled by default. Specifying
7034 @option{-mno-long-calls} will restore the default behavior, as will
7035 placing the function calls within the scope of a @samp{#pragma
7036 long_calls_off} directive. Note these switches have no effect on how
7037 the compiler generates code to handle function calls via function
7040 @item -mnop-fun-dllimport
7041 @opindex mnop-fun-dllimport
7042 Disable support for the @code{dllimport} attribute.
7044 @item -msingle-pic-base
7045 @opindex msingle-pic-base
7046 Treat the register used for PIC addressing as read-only, rather than
7047 loading it in the prologue for each function. The run-time system is
7048 responsible for initializing this register with an appropriate value
7049 before execution begins.
7051 @item -mpic-register=@var{reg}
7052 @opindex mpic-register
7053 Specify the register to be used for PIC addressing. The default is R10
7054 unless stack-checking is enabled, when R9 is used.
7056 @item -mcirrus-fix-invalid-insns
7057 @opindex mcirrus-fix-invalid-insns
7058 @opindex mno-cirrus-fix-invalid-insns
7059 Insert NOPs into the instruction stream to in order to work around
7060 problems with invalid Maverick instruction combinations. This option
7061 is only valid if the @option{-mcpu=ep9312} option has been used to
7062 enable generation of instructions for the Cirrus Maverick floating
7063 point co-processor. This option is not enabled by default, since the
7064 problem is only present in older Maverick implementations. The default
7065 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
7068 @item -mpoke-function-name
7069 @opindex mpoke-function-name
7070 Write the name of each function into the text section, directly
7071 preceding the function prologue. The generated code is similar to this:
7075 .ascii "arm_poke_function_name", 0
7078 .word 0xff000000 + (t1 - t0)
7079 arm_poke_function_name
7081 stmfd sp!, @{fp, ip, lr, pc@}
7085 When performing a stack backtrace, code can inspect the value of
7086 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
7087 location @code{pc - 12} and the top 8 bits are set, then we know that
7088 there is a function name embedded immediately preceding this location
7089 and has length @code{((pc[-3]) & 0xff000000)}.
7093 Generate code for the 16-bit Thumb instruction set. The default is to
7094 use the 32-bit ARM instruction set.
7097 @opindex mtpcs-frame
7098 Generate a stack frame that is compliant with the Thumb Procedure Call
7099 Standard for all non-leaf functions. (A leaf function is one that does
7100 not call any other functions.) The default is @option{-mno-tpcs-frame}.
7102 @item -mtpcs-leaf-frame
7103 @opindex mtpcs-leaf-frame
7104 Generate a stack frame that is compliant with the Thumb Procedure Call
7105 Standard for all leaf functions. (A leaf function is one that does
7106 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
7108 @item -mcallee-super-interworking
7109 @opindex mcallee-super-interworking
7110 Gives all externally visible functions in the file being compiled an ARM
7111 instruction set header which switches to Thumb mode before executing the
7112 rest of the function. This allows these functions to be called from
7113 non-interworking code.
7115 @item -mcaller-super-interworking
7116 @opindex mcaller-super-interworking
7117 Allows calls via function pointers (including virtual functions) to
7118 execute correctly regardless of whether the target code has been
7119 compiled for interworking or not. There is a small overhead in the cost
7120 of executing a function pointer if this option is enabled.
7125 @subsection AVR Options
7128 These options are defined for AVR implementations:
7131 @item -mmcu=@var{mcu}
7133 Specify ATMEL AVR instruction set or MCU type.
7135 Instruction set avr1 is for the minimal AVR core, not supported by the C
7136 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
7137 attiny11, attiny12, attiny15, attiny28).
7139 Instruction set avr2 (default) is for the classic AVR core with up to
7140 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
7141 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
7142 at90c8534, at90s8535).
7144 Instruction set avr3 is for the classic AVR core with up to 128K program
7145 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
7147 Instruction set avr4 is for the enhanced AVR core with up to 8K program
7148 memory space (MCU types: atmega8, atmega83, atmega85).
7150 Instruction set avr5 is for the enhanced AVR core with up to 128K program
7151 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
7152 atmega64, atmega128, at43usb355, at94k).
7156 Output instruction sizes to the asm file.
7158 @item -minit-stack=@var{N}
7159 @opindex minit-stack
7160 Specify the initial stack address, which may be a symbol or numeric value,
7161 @samp{__stack} is the default.
7163 @item -mno-interrupts
7164 @opindex mno-interrupts
7165 Generated code is not compatible with hardware interrupts.
7166 Code size will be smaller.
7168 @item -mcall-prologues
7169 @opindex mcall-prologues
7170 Functions prologues/epilogues expanded as call to appropriate
7171 subroutines. Code size will be smaller.
7173 @item -mno-tablejump
7174 @opindex mno-tablejump
7175 Do not generate tablejump insns which sometimes increase code size.
7178 @opindex mtiny-stack
7179 Change only the low 8 bits of the stack pointer.
7183 Assume int to be 8 bit integer. This affects the sizes of all types: A
7184 char will be 1 byte, an int will be 1 byte, an long will be 2 bytes
7185 and long long will be 4 bytes. Please note that this option does not
7186 comply to the C standards, but it will provide you with smaller code
7190 @node Blackfin Options
7191 @subsection Blackfin Options
7192 @cindex Blackfin Options
7195 @item -momit-leaf-frame-pointer
7196 @opindex momit-leaf-frame-pointer
7197 Don't keep the frame pointer in a register for leaf functions. This
7198 avoids the instructions to save, set up and restore frame pointers and
7199 makes an extra register available in leaf functions. The option
7200 @option{-fomit-frame-pointer} removes the frame pointer for all functions
7201 which might make debugging harder.
7205 When enabled, the compiler will ensure that the generated code does not
7206 contain speculative loads after jump instructions. This option is enabled
7211 Don't generate extra code to prevent speculative loads from occurring.
7215 When enabled, the compiler is free to take advantage of the knowledge that
7216 the entire program fits into the low 64k of memory.
7219 @opindex mno-low-64k
7220 Assume that the program is arbitrarily large. This is the default.
7222 @item -mid-shared-library
7223 @opindex mid-shared-library
7224 Generate code that supports shared libraries via the library ID method.
7225 This allows for execute in place and shared libraries in an environment
7226 without virtual memory management. This option implies @option{-fPIC}.
7228 @item -mno-id-shared-library
7229 @opindex mno-id-shared-library
7230 Generate code that doesn't assume ID based shared libraries are being used.
7231 This is the default.
7233 @item -mshared-library-id=n
7234 @opindex mshared-library-id
7235 Specified the identification number of the ID based shared library being
7236 compiled. Specifying a value of 0 will generate more compact code, specifying
7237 other values will force the allocation of that number to the current
7238 library but is no more space or time efficient than omitting this option.
7242 @subsection CRIS Options
7243 @cindex CRIS Options
7245 These options are defined specifically for the CRIS ports.
7248 @item -march=@var{architecture-type}
7249 @itemx -mcpu=@var{architecture-type}
7252 Generate code for the specified architecture. The choices for
7253 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
7254 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX@.
7255 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
7258 @item -mtune=@var{architecture-type}
7260 Tune to @var{architecture-type} everything applicable about the generated
7261 code, except for the ABI and the set of available instructions. The
7262 choices for @var{architecture-type} are the same as for
7263 @option{-march=@var{architecture-type}}.
7265 @item -mmax-stack-frame=@var{n}
7266 @opindex mmax-stack-frame
7267 Warn when the stack frame of a function exceeds @var{n} bytes.
7269 @item -melinux-stacksize=@var{n}
7270 @opindex melinux-stacksize
7271 Only available with the @samp{cris-axis-aout} target. Arranges for
7272 indications in the program to the kernel loader that the stack of the
7273 program should be set to @var{n} bytes.
7279 The options @option{-metrax4} and @option{-metrax100} are synonyms for
7280 @option{-march=v3} and @option{-march=v8} respectively.
7282 @item -mmul-bug-workaround
7283 @itemx -mno-mul-bug-workaround
7284 @opindex mmul-bug-workaround
7285 @opindex mno-mul-bug-workaround
7286 Work around a bug in the @code{muls} and @code{mulu} instructions for CPU
7287 models where it applies. This option is active by default.
7291 Enable CRIS-specific verbose debug-related information in the assembly
7292 code. This option also has the effect to turn off the @samp{#NO_APP}
7293 formatted-code indicator to the assembler at the beginning of the
7298 Do not use condition-code results from previous instruction; always emit
7299 compare and test instructions before use of condition codes.
7301 @item -mno-side-effects
7302 @opindex mno-side-effects
7303 Do not emit instructions with side-effects in addressing modes other than
7307 @itemx -mno-stack-align
7309 @itemx -mno-data-align
7310 @itemx -mconst-align
7311 @itemx -mno-const-align
7312 @opindex mstack-align
7313 @opindex mno-stack-align
7314 @opindex mdata-align
7315 @opindex mno-data-align
7316 @opindex mconst-align
7317 @opindex mno-const-align
7318 These options (no-options) arranges (eliminate arrangements) for the
7319 stack-frame, individual data and constants to be aligned for the maximum
7320 single data access size for the chosen CPU model. The default is to
7321 arrange for 32-bit alignment. ABI details such as structure layout are
7322 not affected by these options.
7330 Similar to the stack- data- and const-align options above, these options
7331 arrange for stack-frame, writable data and constants to all be 32-bit,
7332 16-bit or 8-bit aligned. The default is 32-bit alignment.
7334 @item -mno-prologue-epilogue
7335 @itemx -mprologue-epilogue
7336 @opindex mno-prologue-epilogue
7337 @opindex mprologue-epilogue
7338 With @option{-mno-prologue-epilogue}, the normal function prologue and
7339 epilogue that sets up the stack-frame are omitted and no return
7340 instructions or return sequences are generated in the code. Use this
7341 option only together with visual inspection of the compiled code: no
7342 warnings or errors are generated when call-saved registers must be saved,
7343 or storage for local variable needs to be allocated.
7349 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
7350 instruction sequences that load addresses for functions from the PLT part
7351 of the GOT rather than (traditional on other architectures) calls to the
7352 PLT@. The default is @option{-mgotplt}.
7356 Legacy no-op option only recognized with the cris-axis-aout target.
7360 Legacy no-op option only recognized with the cris-axis-elf and
7361 cris-axis-linux-gnu targets.
7365 Only recognized with the cris-axis-aout target, where it selects a
7366 GNU/linux-like multilib, include files and instruction set for
7371 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
7375 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
7376 to link with input-output functions from a simulator library. Code,
7377 initialized data and zero-initialized data are allocated consecutively.
7381 Like @option{-sim}, but pass linker options to locate initialized data at
7382 0x40000000 and zero-initialized data at 0x80000000.
7385 @node Darwin Options
7386 @subsection Darwin Options
7387 @cindex Darwin options
7389 These options are defined for all architectures running the Darwin operating
7392 FSF GCC on Darwin does not create ``fat'' object files; it will create
7393 an object file for the single architecture that it was built to
7394 target. Apple's GCC on Darwin does create ``fat'' files if multiple
7395 @option{-arch} options are used; it does so by running the compiler or
7396 linker multiple times and joining the results together with
7399 The subtype of the file created (like @samp{ppc7400} or @samp{ppc970} or
7400 @samp{i686}) is determined by the flags that specify the ISA
7401 that GCC is targetting, like @option{-mcpu} or @option{-march}. The
7402 @option{-force_cpusubtype_ALL} option can be used to override this.
7404 The Darwin tools vary in their behavior when presented with an ISA
7405 mismatch. The assembler, @file{as}, will only permit instructions to
7406 be used that are valid for the subtype of the file it is generating,
7407 so you cannot put 64-bit instructions in an @samp{ppc750} object file.
7408 The linker for shared libraries, @file{/usr/bin/libtool}, will fail
7409 and print an error if asked to create a shared library with a less
7410 restrictive subtype than its input files (for instance, trying to put
7411 a @samp{ppc970} object file in a @samp{ppc7400} library). The linker
7412 for executables, @file{ld}, will quietly give the executable the most
7413 restrictive subtype of any of its input files.
7418 Add the framework directory @var{dir} to the head of the list of
7419 directories to be searched for header files. These directories are
7420 interleaved with those specified by @option{-I} options and are
7421 scanned in a left-to-right order.
7423 A framework directory is a directory with frameworks in it. A
7424 framework is a directory with a @samp{"Headers"} and/or
7425 @samp{"PrivateHeaders"} directory contained directly in it that ends
7426 in @samp{".framework"}. The name of a framework is the name of this
7427 directory excluding the @samp{".framework"}. Headers associated with
7428 the framework are found in one of those two directories, with
7429 @samp{"Headers"} being searched first. A subframework is a framework
7430 directory that is in a framework's @samp{"Frameworks"} directory.
7431 Includes of subframework headers can only appear in a header of a
7432 framework that contains the subframework, or in a sibling subframework
7433 header. Two subframeworks are siblings if they occur in the same
7434 framework. A subframework should not have the same name as a
7435 framework, a warning will be issued if this is violated. Currently a
7436 subframework cannot have subframeworks, in the future, the mechanism
7437 may be extended to support this. The standard frameworks can be found
7438 in @samp{"/System/Library/Frameworks"} and
7439 @samp{"/Library/Frameworks"}. An example include looks like
7440 @code{#include <Framework/header.h>}, where @samp{Framework} denotes
7441 the name of the framework and header.h is found in the
7442 @samp{"PrivateHeaders"} or @samp{"Headers"} directory.
7446 Emit debugging information for symbols that are used. For STABS
7447 debugging format, this enables @option{-feliminate-unused-debug-symbols}.
7448 This is by default ON@.
7452 Emit debugging information for all symbols and types.
7454 @item -mone-byte-bool
7455 @opindex -mone-byte-bool
7456 Override the defaults for @samp{bool} so that @samp{sizeof(bool)==1}.
7457 By default @samp{sizeof(bool)} is @samp{4} when compiling for
7458 Darwin/PowerPC and @samp{1} when compiling for Darwin/x86, so this
7459 option has no effect on x86.
7461 @strong{Warning:} The @option{-mone-byte-bool} switch causes GCC
7462 to generate code that is not binary compatible with code generated
7463 without that switch. Using this switch may require recompiling all
7464 other modules in a program, including system libraries. Use this
7465 switch to conform to a non-default data model.
7467 @item -mfix-and-continue
7468 @itemx -ffix-and-continue
7469 @itemx -findirect-data
7470 @opindex mfix-and-continue
7471 @opindex ffix-and-continue
7472 @opindex findirect-data
7473 Generate code suitable for fast turn around development. Needed to
7474 enable gdb to dynamically load @code{.o} files into already running
7475 programs. @option{-findirect-data} and @option{-ffix-and-continue}
7476 are provided for backwards compatibility.
7480 Loads all members of static archive libraries.
7481 See man ld(1) for more information.
7483 @item -arch_errors_fatal
7484 @opindex arch_errors_fatal
7485 Cause the errors having to do with files that have the wrong architecture
7489 @opindex bind_at_load
7490 Causes the output file to be marked such that the dynamic linker will
7491 bind all undefined references when the file is loaded or launched.
7495 Produce a Mach-o bundle format file.
7496 See man ld(1) for more information.
7498 @item -bundle_loader @var{executable}
7499 @opindex bundle_loader
7500 This option specifies the @var{executable} that will be loading the build
7501 output file being linked. See man ld(1) for more information.
7504 @opindex -dynamiclib
7505 When passed this option, GCC will produce a dynamic library instead of
7506 an executable when linking, using the Darwin @file{libtool} command.
7508 @item -force_cpusubtype_ALL
7509 @opindex -force_cpusubtype_ALL
7510 This causes GCC's output file to have the @var{ALL} subtype, instead of
7511 one controlled by the @option{-mcpu} or @option{-march} option.
7513 @item -allowable_client @var{client_name}
7515 @itemx -compatibility_version
7516 @itemx -current_version
7518 @itemx -dependency-file
7520 @itemx -dylinker_install_name
7522 @itemx -exported_symbols_list
7524 @itemx -flat_namespace
7525 @itemx -force_flat_namespace
7526 @itemx -headerpad_max_install_names
7529 @itemx -install_name
7530 @itemx -keep_private_externs
7531 @itemx -multi_module
7532 @itemx -multiply_defined
7533 @itemx -multiply_defined_unused
7535 @itemx -no_dead_strip_inits_and_terms
7536 @itemx -nofixprebinding
7539 @itemx -noseglinkedit
7540 @itemx -pagezero_size
7542 @itemx -prebind_all_twolevel_modules
7543 @itemx -private_bundle
7544 @itemx -read_only_relocs
7546 @itemx -sectobjectsymbols
7550 @itemx -sectobjectsymbols
7553 @itemx -segs_read_only_addr
7554 @itemx -segs_read_write_addr
7555 @itemx -seg_addr_table
7556 @itemx -seg_addr_table_filename
7559 @itemx -segs_read_only_addr
7560 @itemx -segs_read_write_addr
7561 @itemx -single_module
7564 @itemx -sub_umbrella
7565 @itemx -twolevel_namespace
7568 @itemx -unexported_symbols_list
7569 @itemx -weak_reference_mismatches
7572 @opindex allowable_client
7573 @opindex client_name
7574 @opindex compatibility_version
7575 @opindex current_version
7577 @opindex dependency-file
7579 @opindex dylinker_install_name
7581 @opindex exported_symbols_list
7583 @opindex flat_namespace
7584 @opindex force_flat_namespace
7585 @opindex headerpad_max_install_names
7588 @opindex install_name
7589 @opindex keep_private_externs
7590 @opindex multi_module
7591 @opindex multiply_defined
7592 @opindex multiply_defined_unused
7594 @opindex no_dead_strip_inits_and_terms
7595 @opindex nofixprebinding
7596 @opindex nomultidefs
7598 @opindex noseglinkedit
7599 @opindex pagezero_size
7601 @opindex prebind_all_twolevel_modules
7602 @opindex private_bundle
7603 @opindex read_only_relocs
7605 @opindex sectobjectsymbols
7609 @opindex sectobjectsymbols
7612 @opindex segs_read_only_addr
7613 @opindex segs_read_write_addr
7614 @opindex seg_addr_table
7615 @opindex seg_addr_table_filename
7616 @opindex seglinkedit
7618 @opindex segs_read_only_addr
7619 @opindex segs_read_write_addr
7620 @opindex single_module
7622 @opindex sub_library
7623 @opindex sub_umbrella
7624 @opindex twolevel_namespace
7627 @opindex unexported_symbols_list
7628 @opindex weak_reference_mismatches
7629 @opindex whatsloaded
7631 These options are passed to the Darwin linker. The Darwin linker man page
7632 describes them in detail.
7635 @node DEC Alpha Options
7636 @subsection DEC Alpha Options
7638 These @samp{-m} options are defined for the DEC Alpha implementations:
7641 @item -mno-soft-float
7643 @opindex mno-soft-float
7644 @opindex msoft-float
7645 Use (do not use) the hardware floating-point instructions for
7646 floating-point operations. When @option{-msoft-float} is specified,
7647 functions in @file{libgcc.a} will be used to perform floating-point
7648 operations. Unless they are replaced by routines that emulate the
7649 floating-point operations, or compiled in such a way as to call such
7650 emulations routines, these routines will issue floating-point
7651 operations. If you are compiling for an Alpha without floating-point
7652 operations, you must ensure that the library is built so as not to call
7655 Note that Alpha implementations without floating-point operations are
7656 required to have floating-point registers.
7661 @opindex mno-fp-regs
7662 Generate code that uses (does not use) the floating-point register set.
7663 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
7664 register set is not used, floating point operands are passed in integer
7665 registers as if they were integers and floating-point results are passed
7666 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
7667 so any function with a floating-point argument or return value called by code
7668 compiled with @option{-mno-fp-regs} must also be compiled with that
7671 A typical use of this option is building a kernel that does not use,
7672 and hence need not save and restore, any floating-point registers.
7676 The Alpha architecture implements floating-point hardware optimized for
7677 maximum performance. It is mostly compliant with the IEEE floating
7678 point standard. However, for full compliance, software assistance is
7679 required. This option generates code fully IEEE compliant code
7680 @emph{except} that the @var{inexact-flag} is not maintained (see below).
7681 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
7682 defined during compilation. The resulting code is less efficient but is
7683 able to correctly support denormalized numbers and exceptional IEEE
7684 values such as not-a-number and plus/minus infinity. Other Alpha
7685 compilers call this option @option{-ieee_with_no_inexact}.
7687 @item -mieee-with-inexact
7688 @opindex mieee-with-inexact
7689 This is like @option{-mieee} except the generated code also maintains
7690 the IEEE @var{inexact-flag}. Turning on this option causes the
7691 generated code to implement fully-compliant IEEE math. In addition to
7692 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
7693 macro. On some Alpha implementations the resulting code may execute
7694 significantly slower than the code generated by default. Since there is
7695 very little code that depends on the @var{inexact-flag}, you should
7696 normally not specify this option. Other Alpha compilers call this
7697 option @option{-ieee_with_inexact}.
7699 @item -mfp-trap-mode=@var{trap-mode}
7700 @opindex mfp-trap-mode
7701 This option controls what floating-point related traps are enabled.
7702 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
7703 The trap mode can be set to one of four values:
7707 This is the default (normal) setting. The only traps that are enabled
7708 are the ones that cannot be disabled in software (e.g., division by zero
7712 In addition to the traps enabled by @samp{n}, underflow traps are enabled
7716 Like @samp{su}, but the instructions are marked to be safe for software
7717 completion (see Alpha architecture manual for details).
7720 Like @samp{su}, but inexact traps are enabled as well.
7723 @item -mfp-rounding-mode=@var{rounding-mode}
7724 @opindex mfp-rounding-mode
7725 Selects the IEEE rounding mode. Other Alpha compilers call this option
7726 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
7731 Normal IEEE rounding mode. Floating point numbers are rounded towards
7732 the nearest machine number or towards the even machine number in case
7736 Round towards minus infinity.
7739 Chopped rounding mode. Floating point numbers are rounded towards zero.
7742 Dynamic rounding mode. A field in the floating point control register
7743 (@var{fpcr}, see Alpha architecture reference manual) controls the
7744 rounding mode in effect. The C library initializes this register for
7745 rounding towards plus infinity. Thus, unless your program modifies the
7746 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
7749 @item -mtrap-precision=@var{trap-precision}
7750 @opindex mtrap-precision
7751 In the Alpha architecture, floating point traps are imprecise. This
7752 means without software assistance it is impossible to recover from a
7753 floating trap and program execution normally needs to be terminated.
7754 GCC can generate code that can assist operating system trap handlers
7755 in determining the exact location that caused a floating point trap.
7756 Depending on the requirements of an application, different levels of
7757 precisions can be selected:
7761 Program precision. This option is the default and means a trap handler
7762 can only identify which program caused a floating point exception.
7765 Function precision. The trap handler can determine the function that
7766 caused a floating point exception.
7769 Instruction precision. The trap handler can determine the exact
7770 instruction that caused a floating point exception.
7773 Other Alpha compilers provide the equivalent options called
7774 @option{-scope_safe} and @option{-resumption_safe}.
7776 @item -mieee-conformant
7777 @opindex mieee-conformant
7778 This option marks the generated code as IEEE conformant. You must not
7779 use this option unless you also specify @option{-mtrap-precision=i} and either
7780 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
7781 is to emit the line @samp{.eflag 48} in the function prologue of the
7782 generated assembly file. Under DEC Unix, this has the effect that
7783 IEEE-conformant math library routines will be linked in.
7785 @item -mbuild-constants
7786 @opindex mbuild-constants
7787 Normally GCC examines a 32- or 64-bit integer constant to
7788 see if it can construct it from smaller constants in two or three
7789 instructions. If it cannot, it will output the constant as a literal and
7790 generate code to load it from the data segment at runtime.
7792 Use this option to require GCC to construct @emph{all} integer constants
7793 using code, even if it takes more instructions (the maximum is six).
7795 You would typically use this option to build a shared library dynamic
7796 loader. Itself a shared library, it must relocate itself in memory
7797 before it can find the variables and constants in its own data segment.
7803 Select whether to generate code to be assembled by the vendor-supplied
7804 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
7822 Indicate whether GCC should generate code to use the optional BWX,
7823 CIX, FIX and MAX instruction sets. The default is to use the instruction
7824 sets supported by the CPU type specified via @option{-mcpu=} option or that
7825 of the CPU on which GCC was built if none was specified.
7830 @opindex mfloat-ieee
7831 Generate code that uses (does not use) VAX F and G floating point
7832 arithmetic instead of IEEE single and double precision.
7834 @item -mexplicit-relocs
7835 @itemx -mno-explicit-relocs
7836 @opindex mexplicit-relocs
7837 @opindex mno-explicit-relocs
7838 Older Alpha assemblers provided no way to generate symbol relocations
7839 except via assembler macros. Use of these macros does not allow
7840 optimal instruction scheduling. GNU binutils as of version 2.12
7841 supports a new syntax that allows the compiler to explicitly mark
7842 which relocations should apply to which instructions. This option
7843 is mostly useful for debugging, as GCC detects the capabilities of
7844 the assembler when it is built and sets the default accordingly.
7848 @opindex msmall-data
7849 @opindex mlarge-data
7850 When @option{-mexplicit-relocs} is in effect, static data is
7851 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
7852 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
7853 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
7854 16-bit relocations off of the @code{$gp} register. This limits the
7855 size of the small data area to 64KB, but allows the variables to be
7856 directly accessed via a single instruction.
7858 The default is @option{-mlarge-data}. With this option the data area
7859 is limited to just below 2GB@. Programs that require more than 2GB of
7860 data must use @code{malloc} or @code{mmap} to allocate the data in the
7861 heap instead of in the program's data segment.
7863 When generating code for shared libraries, @option{-fpic} implies
7864 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
7868 @opindex msmall-text
7869 @opindex mlarge-text
7870 When @option{-msmall-text} is used, the compiler assumes that the
7871 code of the entire program (or shared library) fits in 4MB, and is
7872 thus reachable with a branch instruction. When @option{-msmall-data}
7873 is used, the compiler can assume that all local symbols share the
7874 same @code{$gp} value, and thus reduce the number of instructions
7875 required for a function call from 4 to 1.
7877 The default is @option{-mlarge-text}.
7879 @item -mcpu=@var{cpu_type}
7881 Set the instruction set and instruction scheduling parameters for
7882 machine type @var{cpu_type}. You can specify either the @samp{EV}
7883 style name or the corresponding chip number. GCC supports scheduling
7884 parameters for the EV4, EV5 and EV6 family of processors and will
7885 choose the default values for the instruction set from the processor
7886 you specify. If you do not specify a processor type, GCC will default
7887 to the processor on which the compiler was built.
7889 Supported values for @var{cpu_type} are
7895 Schedules as an EV4 and has no instruction set extensions.
7899 Schedules as an EV5 and has no instruction set extensions.
7903 Schedules as an EV5 and supports the BWX extension.
7908 Schedules as an EV5 and supports the BWX and MAX extensions.
7912 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
7916 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
7919 @item -mtune=@var{cpu_type}
7921 Set only the instruction scheduling parameters for machine type
7922 @var{cpu_type}. The instruction set is not changed.
7924 @item -mmemory-latency=@var{time}
7925 @opindex mmemory-latency
7926 Sets the latency the scheduler should assume for typical memory
7927 references as seen by the application. This number is highly
7928 dependent on the memory access patterns used by the application
7929 and the size of the external cache on the machine.
7931 Valid options for @var{time} are
7935 A decimal number representing clock cycles.
7941 The compiler contains estimates of the number of clock cycles for
7942 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
7943 (also called Dcache, Scache, and Bcache), as well as to main memory.
7944 Note that L3 is only valid for EV5.
7949 @node DEC Alpha/VMS Options
7950 @subsection DEC Alpha/VMS Options
7952 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
7955 @item -mvms-return-codes
7956 @opindex mvms-return-codes
7957 Return VMS condition codes from main. The default is to return POSIX
7958 style condition (e.g.@ error) codes.
7962 @subsection FRV Options
7969 Only use the first 32 general purpose registers.
7974 Use all 64 general purpose registers.
7979 Use only the first 32 floating point registers.
7984 Use all 64 floating point registers
7987 @opindex mhard-float
7989 Use hardware instructions for floating point operations.
7992 @opindex msoft-float
7994 Use library routines for floating point operations.
7999 Dynamically allocate condition code registers.
8004 Do not try to dynamically allocate condition code registers, only
8005 use @code{icc0} and @code{fcc0}.
8010 Change ABI to use double word insns.
8015 Do not use double word instructions.
8020 Use floating point double instructions.
8025 Do not use floating point double instructions.
8030 Use media instructions.
8035 Do not use media instructions.
8040 Use multiply and add/subtract instructions.
8045 Do not use multiply and add/subtract instructions.
8050 Select the FDPIC ABI, that uses function descriptors to represent
8051 pointers to functions. Without any PIC/PIE-related options, it
8052 implies @option{-fPIE}. With @option{-fpic} or @option{-fpie}, it
8053 assumes GOT entries and small data are within a 12-bit range from the
8054 GOT base address; with @option{-fPIC} or @option{-fPIE}, GOT offsets
8055 are computed with 32 bits.
8058 @opindex minline-plt
8060 Enable inlining of PLT entries in function calls to functions that are
8061 not known to bind locally. It has no effect without @option{-mfdpic}.
8062 It's enabled by default if optimizing for speed and compiling for
8063 shared libraries (i.e., @option{-fPIC} or @option{-fpic}), or when an
8064 optimization option such as @option{-O3} or above is present in the
8070 Assume a large TLS segment when generating thread-local code.
8075 Do not assume a large TLS segment when generating thread-local code.
8080 Enable the use of @code{GPREL} relocations in the FDPIC ABI for data
8081 that is known to be in read-only sections. It's enabled by default,
8082 except for @option{-fpic} or @option{-fpie}: even though it may help
8083 make the global offset table smaller, it trades 1 instruction for 4.
8084 With @option{-fPIC} or @option{-fPIE}, it trades 3 instructions for 4,
8085 one of which may be shared by multiple symbols, and it avoids the need
8086 for a GOT entry for the referenced symbol, so it's more likely to be a
8087 win. If it is not, @option{-mno-gprel-ro} can be used to disable it.
8089 @item -multilib-library-pic
8090 @opindex multilib-library-pic
8092 Link with the (library, not FD) pic libraries. It's implied by
8093 @option{-mlibrary-pic}, as well as by @option{-fPIC} and
8094 @option{-fpic} without @option{-mfdpic}. You should never have to use
8100 Follow the EABI requirement of always creating a frame pointer whenever
8101 a stack frame is allocated. This option is enabled by default and can
8102 be disabled with @option{-mno-linked-fp}.
8105 @opindex mlong-calls
8107 Use indirect addressing to call functions outside the current
8108 compilation unit. This allows the functions to be placed anywhere
8109 within the 32-bit address space.
8111 @item -malign-labels
8112 @opindex malign-labels
8114 Try to align labels to an 8-byte boundary by inserting nops into the
8115 previous packet. This option only has an effect when VLIW packing
8116 is enabled. It doesn't create new packets; it merely adds nops to
8120 @opindex mlibrary-pic
8122 Generate position-independent EABI code.
8127 Use only the first four media accumulator registers.
8132 Use all eight media accumulator registers.
8137 Pack VLIW instructions.
8142 Do not pack VLIW instructions.
8147 Do not mark ABI switches in e_flags.
8152 Enable the use of conditional-move instructions (default).
8154 This switch is mainly for debugging the compiler and will likely be removed
8155 in a future version.
8157 @item -mno-cond-move
8158 @opindex mno-cond-move
8160 Disable the use of conditional-move instructions.
8162 This switch is mainly for debugging the compiler and will likely be removed
8163 in a future version.
8168 Enable the use of conditional set instructions (default).
8170 This switch is mainly for debugging the compiler and will likely be removed
8171 in a future version.
8176 Disable the use of conditional set instructions.
8178 This switch is mainly for debugging the compiler and will likely be removed
8179 in a future version.
8184 Enable the use of conditional execution (default).
8186 This switch is mainly for debugging the compiler and will likely be removed
8187 in a future version.
8189 @item -mno-cond-exec
8190 @opindex mno-cond-exec
8192 Disable the use of conditional execution.
8194 This switch is mainly for debugging the compiler and will likely be removed
8195 in a future version.
8198 @opindex mvliw-branch
8200 Run a pass to pack branches into VLIW instructions (default).
8202 This switch is mainly for debugging the compiler and will likely be removed
8203 in a future version.
8205 @item -mno-vliw-branch
8206 @opindex mno-vliw-branch
8208 Do not run a pass to pack branches into VLIW instructions.
8210 This switch is mainly for debugging the compiler and will likely be removed
8211 in a future version.
8213 @item -mmulti-cond-exec
8214 @opindex mmulti-cond-exec
8216 Enable optimization of @code{&&} and @code{||} in conditional execution
8219 This switch is mainly for debugging the compiler and will likely be removed
8220 in a future version.
8222 @item -mno-multi-cond-exec
8223 @opindex mno-multi-cond-exec
8225 Disable optimization of @code{&&} and @code{||} in conditional execution.
8227 This switch is mainly for debugging the compiler and will likely be removed
8228 in a future version.
8230 @item -mnested-cond-exec
8231 @opindex mnested-cond-exec
8233 Enable nested conditional execution optimizations (default).
8235 This switch is mainly for debugging the compiler and will likely be removed
8236 in a future version.
8238 @item -mno-nested-cond-exec
8239 @opindex mno-nested-cond-exec
8241 Disable nested conditional execution optimizations.
8243 This switch is mainly for debugging the compiler and will likely be removed
8244 in a future version.
8246 @item -mtomcat-stats
8247 @opindex mtomcat-stats
8249 Cause gas to print out tomcat statistics.
8251 @item -mcpu=@var{cpu}
8254 Select the processor type for which to generate code. Possible values are
8255 @samp{frv}, @samp{fr550}, @samp{tomcat}, @samp{fr500}, @samp{fr450},
8256 @samp{fr405}, @samp{fr400}, @samp{fr300} and @samp{simple}.
8260 @node H8/300 Options
8261 @subsection H8/300 Options
8263 These @samp{-m} options are defined for the H8/300 implementations:
8268 Shorten some address references at link time, when possible; uses the
8269 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
8270 ld, Using ld}, for a fuller description.
8274 Generate code for the H8/300H@.
8278 Generate code for the H8S@.
8282 Generate code for the H8S and H8/300H in the normal mode. This switch
8283 must be used either with @option{-mh} or @option{-ms}.
8287 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
8291 Make @code{int} data 32 bits by default.
8295 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
8296 The default for the H8/300H and H8S is to align longs and floats on 4
8298 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
8299 This option has no effect on the H8/300.
8303 @subsection HPPA Options
8304 @cindex HPPA Options
8306 These @samp{-m} options are defined for the HPPA family of computers:
8309 @item -march=@var{architecture-type}
8311 Generate code for the specified architecture. The choices for
8312 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8313 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8314 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8315 architecture option for your machine. Code compiled for lower numbered
8316 architectures will run on higher numbered architectures, but not the
8320 @itemx -mpa-risc-1-1
8321 @itemx -mpa-risc-2-0
8322 @opindex mpa-risc-1-0
8323 @opindex mpa-risc-1-1
8324 @opindex mpa-risc-2-0
8325 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8328 @opindex mbig-switch
8329 Generate code suitable for big switch tables. Use this option only if
8330 the assembler/linker complain about out of range branches within a switch
8333 @item -mjump-in-delay
8334 @opindex mjump-in-delay
8335 Fill delay slots of function calls with unconditional jump instructions
8336 by modifying the return pointer for the function call to be the target
8337 of the conditional jump.
8339 @item -mdisable-fpregs
8340 @opindex mdisable-fpregs
8341 Prevent floating point registers from being used in any manner. This is
8342 necessary for compiling kernels which perform lazy context switching of
8343 floating point registers. If you use this option and attempt to perform
8344 floating point operations, the compiler will abort.
8346 @item -mdisable-indexing
8347 @opindex mdisable-indexing
8348 Prevent the compiler from using indexing address modes. This avoids some
8349 rather obscure problems when compiling MIG generated code under MACH@.
8351 @item -mno-space-regs
8352 @opindex mno-space-regs
8353 Generate code that assumes the target has no space registers. This allows
8354 GCC to generate faster indirect calls and use unscaled index address modes.
8356 Such code is suitable for level 0 PA systems and kernels.
8358 @item -mfast-indirect-calls
8359 @opindex mfast-indirect-calls
8360 Generate code that assumes calls never cross space boundaries. This
8361 allows GCC to emit code which performs faster indirect calls.
8363 This option will not work in the presence of shared libraries or nested
8366 @item -mfixed-range=@var{register-range}
8367 @opindex mfixed-range
8368 Generate code treating the given register range as fixed registers.
8369 A fixed register is one that the register allocator can not use. This is
8370 useful when compiling kernel code. A register range is specified as
8371 two registers separated by a dash. Multiple register ranges can be
8372 specified separated by a comma.
8374 @item -mlong-load-store
8375 @opindex mlong-load-store
8376 Generate 3-instruction load and store sequences as sometimes required by
8377 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8380 @item -mportable-runtime
8381 @opindex mportable-runtime
8382 Use the portable calling conventions proposed by HP for ELF systems.
8386 Enable the use of assembler directives only GAS understands.
8388 @item -mschedule=@var{cpu-type}
8390 Schedule code according to the constraints for the machine type
8391 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
8392 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
8393 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
8394 proper scheduling option for your machine. The default scheduling is
8398 @opindex mlinker-opt
8399 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
8400 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
8401 linkers in which they give bogus error messages when linking some programs.
8404 @opindex msoft-float
8405 Generate output containing library calls for floating point.
8406 @strong{Warning:} the requisite libraries are not available for all HPPA
8407 targets. Normally the facilities of the machine's usual C compiler are
8408 used, but this cannot be done directly in cross-compilation. You must make
8409 your own arrangements to provide suitable library functions for
8410 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
8411 does provide software floating point support.
8413 @option{-msoft-float} changes the calling convention in the output file;
8414 therefore, it is only useful if you compile @emph{all} of a program with
8415 this option. In particular, you need to compile @file{libgcc.a}, the
8416 library that comes with GCC, with @option{-msoft-float} in order for
8421 Generate the predefine, @code{_SIO}, for server IO@. The default is
8422 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
8423 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO@. These
8424 options are available under HP-UX and HI-UX@.
8428 Use GNU ld specific options. This passes @option{-shared} to ld when
8429 building a shared library. It is the default when GCC is configured,
8430 explicitly or implicitly, with the GNU linker. This option does not
8431 have any affect on which ld is called, it only changes what parameters
8432 are passed to that ld. The ld that is called is determined by the
8433 @option{--with-ld} configure option, GCC's program search path, and
8434 finally by the user's @env{PATH}. The linker used by GCC can be printed
8435 using @samp{which `gcc -print-prog-name=ld`}.
8439 Use HP ld specific options. This passes @option{-b} to ld when building
8440 a shared library and passes @option{+Accept TypeMismatch} to ld on all
8441 links. It is the default when GCC is configured, explicitly or
8442 implicitly, with the HP linker. This option does not have any affect on
8443 which ld is called, it only changes what parameters are passed to that
8444 ld. The ld that is called is determined by the @option{--with-ld}
8445 configure option, GCC's program search path, and finally by the user's
8446 @env{PATH}. The linker used by GCC can be printed using @samp{which
8447 `gcc -print-prog-name=ld`}.
8450 @opindex mno-long-calls
8451 Generate code that uses long call sequences. This ensures that a call
8452 is always able to reach linker generated stubs. The default is to generate
8453 long calls only when the distance from the call site to the beginning
8454 of the function or translation unit, as the case may be, exceeds a
8455 predefined limit set by the branch type being used. The limits for
8456 normal calls are 7,600,000 and 240,000 bytes, respectively for the
8457 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
8460 Distances are measured from the beginning of functions when using the
8461 @option{-ffunction-sections} option, or when using the @option{-mgas}
8462 and @option{-mno-portable-runtime} options together under HP-UX with
8465 It is normally not desirable to use this option as it will degrade
8466 performance. However, it may be useful in large applications,
8467 particularly when partial linking is used to build the application.
8469 The types of long calls used depends on the capabilities of the
8470 assembler and linker, and the type of code being generated. The
8471 impact on systems that support long absolute calls, and long pic
8472 symbol-difference or pc-relative calls should be relatively small.
8473 However, an indirect call is used on 32-bit ELF systems in pic code
8474 and it is quite long.
8476 @item -munix=@var{unix-std}
8478 Generate compiler predefines and select a startfile for the specified
8479 UNIX standard. The choices for @var{unix-std} are @samp{93}, @samp{95}
8480 and @samp{98}. @samp{93} is supported on all HP-UX versions. @samp{95}
8481 is available on HP-UX 10.10 and later. @samp{98} is available on HP-UX
8482 11.11 and later. The default values are @samp{93} for HP-UX 10.00,
8483 @samp{95} for HP-UX 10.10 though to 11.00, and @samp{98} for HP-UX 11.11
8486 @option{-munix=93} provides the same predefines as GCC 3.3 and 3.4.
8487 @option{-munix=95} provides additional predefines for @code{XOPEN_UNIX}
8488 and @code{_XOPEN_SOURCE_EXTENDED}, and the startfile @file{unix95.o}.
8489 @option{-munix=98} provides additional predefines for @code{_XOPEN_UNIX},
8490 @code{_XOPEN_SOURCE_EXTENDED}, @code{_INCLUDE__STDC_A1_SOURCE} and
8491 @code{_INCLUDE_XOPEN_SOURCE_500}, and the startfile @file{unix98.o}.
8493 It is @emph{important} to note that this option changes the interfaces
8494 for various library routines. It also affects the operational behavior
8495 of the C library. Thus, @emph{extreme} care is needed in using this
8498 Library code that is intended to operate with more than one UNIX
8499 standard must test, set and restore the variable @var{__xpg4_extended_mask}
8500 as appropriate. Most GNU software doesn't provide this capability.
8504 Suppress the generation of link options to search libdld.sl when the
8505 @option{-static} option is specified on HP-UX 10 and later.
8509 The HP-UX implementation of setlocale in libc has a dependency on
8510 libdld.sl. There isn't an archive version of libdld.sl. Thus,
8511 when the @option{-static} option is specified, special link options
8512 are needed to resolve this dependency.
8514 On HP-UX 10 and later, the GCC driver adds the necessary options to
8515 link with libdld.sl when the @option{-static} option is specified.
8516 This causes the resulting binary to be dynamic. On the 64-bit port,
8517 the linkers generate dynamic binaries by default in any case. The
8518 @option{-nolibdld} option can be used to prevent the GCC driver from
8519 adding these link options.
8523 Add support for multithreading with the @dfn{dce thread} library
8524 under HP-UX@. This option sets flags for both the preprocessor and
8528 @node i386 and x86-64 Options
8529 @subsection Intel 386 and AMD x86-64 Options
8530 @cindex i386 Options
8531 @cindex x86-64 Options
8532 @cindex Intel 386 Options
8533 @cindex AMD x86-64 Options
8535 These @samp{-m} options are defined for the i386 and x86-64 family of
8539 @item -mtune=@var{cpu-type}
8541 Tune to @var{cpu-type} everything applicable about the generated code, except
8542 for the ABI and the set of available instructions. The choices for
8546 Original Intel's i386 CPU@.
8548 Intel's i486 CPU@. (No scheduling is implemented for this chip.)
8550 Intel Pentium CPU with no MMX support.
8552 Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
8553 @item i686, pentiumpro
8554 Intel PentiumPro CPU@.
8556 Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.
8557 @item pentium3, pentium3m
8558 Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set
8561 Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set
8562 support. Used by Centrino notebooks.
8563 @item pentium4, pentium4m
8564 Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.
8566 Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction
8569 Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE,
8570 SSE2 and SSE3 instruction set support.
8572 AMD K6 CPU with MMX instruction set support.
8574 Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.
8575 @item athlon, athlon-tbird
8576 AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions
8578 @item athlon-4, athlon-xp, athlon-mp
8579 Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE
8580 instruction set support.
8581 @item k8, opteron, athlon64, athlon-fx
8582 AMD K8 core based CPUs with x86-64 instruction set support. (This supersets
8583 MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)
8585 IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction
8588 IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW!
8589 instruction set support.
8591 Via C3 CPU with MMX and 3dNOW! instruction set support. (No scheduling is
8592 implemented for this chip.)
8594 Via C3-2 CPU with MMX and SSE instruction set support. (No scheduling is
8595 implemented for this chip.)
8598 While picking a specific @var{cpu-type} will schedule things appropriately
8599 for that particular chip, the compiler will not generate any code that
8600 does not run on the i386 without the @option{-march=@var{cpu-type}} option
8603 @item -march=@var{cpu-type}
8605 Generate instructions for the machine type @var{cpu-type}. The choices
8606 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
8607 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
8609 @item -mcpu=@var{cpu-type}
8611 A deprecated synonym for @option{-mtune}.
8620 @opindex mpentiumpro
8621 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
8622 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
8623 These synonyms are deprecated.
8625 @item -mfpmath=@var{unit}
8627 Generate floating point arithmetics for selected unit @var{unit}. The choices
8632 Use the standard 387 floating point coprocessor present majority of chips and
8633 emulated otherwise. Code compiled with this option will run almost everywhere.
8634 The temporary results are computed in 80bit precision instead of precision
8635 specified by the type resulting in slightly different results compared to most
8636 of other chips. See @option{-ffloat-store} for more detailed description.
8638 This is the default choice for i386 compiler.
8641 Use scalar floating point instructions present in the SSE instruction set.
8642 This instruction set is supported by Pentium3 and newer chips, in the AMD line
8643 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
8644 instruction set supports only single precision arithmetics, thus the double and
8645 extended precision arithmetics is still done using 387. Later version, present
8646 only in Pentium4 and the future AMD x86-64 chips supports double precision
8649 For the i386 compiler, you need to use @option{-march=@var{cpu-type}}, @option{-msse}
8650 or @option{-msse2} switches to enable SSE extensions and make this option
8651 effective. For the x86-64 compiler, these extensions are enabled by default.
8653 The resulting code should be considerably faster in the majority of cases and avoid
8654 the numerical instability problems of 387 code, but may break some existing
8655 code that expects temporaries to be 80bit.
8657 This is the default choice for the x86-64 compiler.
8660 Attempt to utilize both instruction sets at once. This effectively double the
8661 amount of available registers and on chips with separate execution units for
8662 387 and SSE the execution resources too. Use this option with care, as it is
8663 still experimental, because the GCC register allocator does not model separate
8664 functional units well resulting in instable performance.
8667 @item -masm=@var{dialect}
8668 @opindex masm=@var{dialect}
8669 Output asm instructions using selected @var{dialect}. Supported choices are
8670 @samp{intel} or @samp{att} (the default one).
8675 @opindex mno-ieee-fp
8676 Control whether or not the compiler uses IEEE floating point
8677 comparisons. These handle correctly the case where the result of a
8678 comparison is unordered.
8681 @opindex msoft-float
8682 Generate output containing library calls for floating point.
8683 @strong{Warning:} the requisite libraries are not part of GCC@.
8684 Normally the facilities of the machine's usual C compiler are used, but
8685 this can't be done directly in cross-compilation. You must make your
8686 own arrangements to provide suitable library functions for
8689 On machines where a function returns floating point results in the 80387
8690 register stack, some floating point opcodes may be emitted even if
8691 @option{-msoft-float} is used.
8693 @item -mno-fp-ret-in-387
8694 @opindex mno-fp-ret-in-387
8695 Do not use the FPU registers for return values of functions.
8697 The usual calling convention has functions return values of types
8698 @code{float} and @code{double} in an FPU register, even if there
8699 is no FPU@. The idea is that the operating system should emulate
8702 The option @option{-mno-fp-ret-in-387} causes such values to be returned
8703 in ordinary CPU registers instead.
8705 @item -mno-fancy-math-387
8706 @opindex mno-fancy-math-387
8707 Some 387 emulators do not support the @code{sin}, @code{cos} and
8708 @code{sqrt} instructions for the 387. Specify this option to avoid
8709 generating those instructions. This option is the default on FreeBSD,
8710 OpenBSD and NetBSD@. This option is overridden when @option{-march}
8711 indicates that the target cpu will always have an FPU and so the
8712 instruction will not need emulation. As of revision 2.6.1, these
8713 instructions are not generated unless you also use the
8714 @option{-funsafe-math-optimizations} switch.
8716 @item -malign-double
8717 @itemx -mno-align-double
8718 @opindex malign-double
8719 @opindex mno-align-double
8720 Control whether GCC aligns @code{double}, @code{long double}, and
8721 @code{long long} variables on a two word boundary or a one word
8722 boundary. Aligning @code{double} variables on a two word boundary will
8723 produce code that runs somewhat faster on a @samp{Pentium} at the
8724 expense of more memory.
8726 @strong{Warning:} if you use the @option{-malign-double} switch,
8727 structures containing the above types will be aligned differently than
8728 the published application binary interface specifications for the 386
8729 and will not be binary compatible with structures in code compiled
8730 without that switch.
8732 @item -m96bit-long-double
8733 @itemx -m128bit-long-double
8734 @opindex m96bit-long-double
8735 @opindex m128bit-long-double
8736 These switches control the size of @code{long double} type. The i386
8737 application binary interface specifies the size to be 96 bits,
8738 so @option{-m96bit-long-double} is the default in 32 bit mode.
8740 Modern architectures (Pentium and newer) would prefer @code{long double}
8741 to be aligned to an 8 or 16 byte boundary. In arrays or structures
8742 conforming to the ABI, this would not be possible. So specifying a
8743 @option{-m128bit-long-double} will align @code{long double}
8744 to a 16 byte boundary by padding the @code{long double} with an additional
8747 In the x86-64 compiler, @option{-m128bit-long-double} is the default choice as
8748 its ABI specifies that @code{long double} is to be aligned on 16 byte boundary.
8750 Notice that neither of these options enable any extra precision over the x87
8751 standard of 80 bits for a @code{long double}.
8753 @strong{Warning:} if you override the default value for your target ABI, the
8754 structures and arrays containing @code{long double} variables will change
8755 their size as well as function calling convention for function taking
8756 @code{long double} will be modified. Hence they will not be binary
8757 compatible with arrays or structures in code compiled without that switch.
8761 @itemx -mno-svr3-shlib
8762 @opindex msvr3-shlib
8763 @opindex mno-svr3-shlib
8764 Control whether GCC places uninitialized local variables into the
8765 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
8766 into @code{bss}. These options are meaningful only on System V Release 3.
8770 Use a different function-calling convention, in which functions that
8771 take a fixed number of arguments return with the @code{ret} @var{num}
8772 instruction, which pops their arguments while returning. This saves one
8773 instruction in the caller since there is no need to pop the arguments
8776 You can specify that an individual function is called with this calling
8777 sequence with the function attribute @samp{stdcall}. You can also
8778 override the @option{-mrtd} option by using the function attribute
8779 @samp{cdecl}. @xref{Function Attributes}.
8781 @strong{Warning:} this calling convention is incompatible with the one
8782 normally used on Unix, so you cannot use it if you need to call
8783 libraries compiled with the Unix compiler.
8785 Also, you must provide function prototypes for all functions that
8786 take variable numbers of arguments (including @code{printf});
8787 otherwise incorrect code will be generated for calls to those
8790 In addition, seriously incorrect code will result if you call a
8791 function with too many arguments. (Normally, extra arguments are
8792 harmlessly ignored.)
8794 @item -mregparm=@var{num}
8796 Control how many registers are used to pass integer arguments. By
8797 default, no registers are used to pass arguments, and at most 3
8798 registers can be used. You can control this behavior for a specific
8799 function by using the function attribute @samp{regparm}.
8800 @xref{Function Attributes}.
8802 @strong{Warning:} if you use this switch, and
8803 @var{num} is nonzero, then you must build all modules with the same
8804 value, including any libraries. This includes the system libraries and
8807 @item -mpreferred-stack-boundary=@var{num}
8808 @opindex mpreferred-stack-boundary
8809 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
8810 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
8811 the default is 4 (16 bytes or 128 bits), except when optimizing for code
8812 size (@option{-Os}), in which case the default is the minimum correct
8813 alignment (4 bytes for x86, and 8 bytes for x86-64).
8815 On Pentium and PentiumPro, @code{double} and @code{long double} values
8816 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
8817 suffer significant run time performance penalties. On Pentium III, the
8818 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
8819 penalties if it is not 16 byte aligned.
8821 To ensure proper alignment of this values on the stack, the stack boundary
8822 must be as aligned as that required by any value stored on the stack.
8823 Further, every function must be generated such that it keeps the stack
8824 aligned. Thus calling a function compiled with a higher preferred
8825 stack boundary from a function compiled with a lower preferred stack
8826 boundary will most likely misalign the stack. It is recommended that
8827 libraries that use callbacks always use the default setting.
8829 This extra alignment does consume extra stack space, and generally
8830 increases code size. Code that is sensitive to stack space usage, such
8831 as embedded systems and operating system kernels, may want to reduce the
8832 preferred alignment to @option{-mpreferred-stack-boundary=2}.
8850 These switches enable or disable the use of built-in functions that allow
8851 direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the
8854 @xref{X86 Built-in Functions}, for details of the functions enabled
8855 and disabled by these switches.
8857 To have SSE/SSE2 instructions generated automatically from floating-point
8858 code, see @option{-mfpmath=sse}.
8861 @itemx -mno-push-args
8863 @opindex mno-push-args
8864 Use PUSH operations to store outgoing parameters. This method is shorter
8865 and usually equally fast as method using SUB/MOV operations and is enabled
8866 by default. In some cases disabling it may improve performance because of
8867 improved scheduling and reduced dependencies.
8869 @item -maccumulate-outgoing-args
8870 @opindex maccumulate-outgoing-args
8871 If enabled, the maximum amount of space required for outgoing arguments will be
8872 computed in the function prologue. This is faster on most modern CPUs
8873 because of reduced dependencies, improved scheduling and reduced stack usage
8874 when preferred stack boundary is not equal to 2. The drawback is a notable
8875 increase in code size. This switch implies @option{-mno-push-args}.
8879 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
8880 on thread-safe exception handling must compile and link all code with the
8881 @option{-mthreads} option. When compiling, @option{-mthreads} defines
8882 @option{-D_MT}; when linking, it links in a special thread helper library
8883 @option{-lmingwthrd} which cleans up per thread exception handling data.
8885 @item -mno-align-stringops
8886 @opindex mno-align-stringops
8887 Do not align destination of inlined string operations. This switch reduces
8888 code size and improves performance in case the destination is already aligned,
8889 but GCC doesn't know about it.
8891 @item -minline-all-stringops
8892 @opindex minline-all-stringops
8893 By default GCC inlines string operations only when destination is known to be
8894 aligned at least to 4 byte boundary. This enables more inlining, increase code
8895 size, but may improve performance of code that depends on fast memcpy, strlen
8896 and memset for short lengths.
8898 @item -momit-leaf-frame-pointer
8899 @opindex momit-leaf-frame-pointer
8900 Don't keep the frame pointer in a register for leaf functions. This
8901 avoids the instructions to save, set up and restore frame pointers and
8902 makes an extra register available in leaf functions. The option
8903 @option{-fomit-frame-pointer} removes the frame pointer for all functions
8904 which might make debugging harder.
8906 @item -mtls-direct-seg-refs
8907 @itemx -mno-tls-direct-seg-refs
8908 @opindex mtls-direct-seg-refs
8909 Controls whether TLS variables may be accessed with offsets from the
8910 TLS segment register (@code{%gs} for 32-bit, @code{%fs} for 64-bit),
8911 or whether the thread base pointer must be added. Whether or not this
8912 is legal depends on the operating system, and whether it maps the
8913 segment to cover the entire TLS area.
8915 For systems that use GNU libc, the default is on.
8918 These @samp{-m} switches are supported in addition to the above
8919 on AMD x86-64 processors in 64-bit environments.
8926 Generate code for a 32-bit or 64-bit environment.
8927 The 32-bit environment sets int, long and pointer to 32 bits and
8928 generates code that runs on any i386 system.
8929 The 64-bit environment sets int to 32 bits and long and pointer
8930 to 64 bits and generates code for AMD's x86-64 architecture.
8933 @opindex no-red-zone
8934 Do not use a so called red zone for x86-64 code. The red zone is mandated
8935 by the x86-64 ABI, it is a 128-byte area beyond the location of the
8936 stack pointer that will not be modified by signal or interrupt handlers
8937 and therefore can be used for temporary data without adjusting the stack
8938 pointer. The flag @option{-mno-red-zone} disables this red zone.
8940 @item -mcmodel=small
8941 @opindex mcmodel=small
8942 Generate code for the small code model: the program and its symbols must
8943 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
8944 Programs can be statically or dynamically linked. This is the default
8947 @item -mcmodel=kernel
8948 @opindex mcmodel=kernel
8949 Generate code for the kernel code model. The kernel runs in the
8950 negative 2 GB of the address space.
8951 This model has to be used for Linux kernel code.
8953 @item -mcmodel=medium
8954 @opindex mcmodel=medium
8955 Generate code for the medium model: The program is linked in the lower 2
8956 GB of the address space but symbols can be located anywhere in the
8957 address space. Programs can be statically or dynamically linked, but
8958 building of shared libraries are not supported with the medium model.
8960 @item -mcmodel=large
8961 @opindex mcmodel=large
8962 Generate code for the large model: This model makes no assumptions
8963 about addresses and sizes of sections. Currently GCC does not implement
8968 @subsection IA-64 Options
8969 @cindex IA-64 Options
8971 These are the @samp{-m} options defined for the Intel IA-64 architecture.
8975 @opindex mbig-endian
8976 Generate code for a big endian target. This is the default for HP-UX@.
8978 @item -mlittle-endian
8979 @opindex mlittle-endian
8980 Generate code for a little endian target. This is the default for AIX5
8987 Generate (or don't) code for the GNU assembler. This is the default.
8988 @c Also, this is the default if the configure option @option{--with-gnu-as}
8995 Generate (or don't) code for the GNU linker. This is the default.
8996 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9001 Generate code that does not use a global pointer register. The result
9002 is not position independent code, and violates the IA-64 ABI@.
9004 @item -mvolatile-asm-stop
9005 @itemx -mno-volatile-asm-stop
9006 @opindex mvolatile-asm-stop
9007 @opindex mno-volatile-asm-stop
9008 Generate (or don't) a stop bit immediately before and after volatile asm
9011 @item -mregister-names
9012 @itemx -mno-register-names
9013 @opindex mregister-names
9014 @opindex mno-register-names
9015 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9016 the stacked registers. This may make assembler output more readable.
9022 Disable (or enable) optimizations that use the small data section. This may
9023 be useful for working around optimizer bugs.
9026 @opindex mconstant-gp
9027 Generate code that uses a single constant global pointer value. This is
9028 useful when compiling kernel code.
9032 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9033 This is useful when compiling firmware code.
9035 @item -minline-float-divide-min-latency
9036 @opindex minline-float-divide-min-latency
9037 Generate code for inline divides of floating point values
9038 using the minimum latency algorithm.
9040 @item -minline-float-divide-max-throughput
9041 @opindex minline-float-divide-max-throughput
9042 Generate code for inline divides of floating point values
9043 using the maximum throughput algorithm.
9045 @item -minline-int-divide-min-latency
9046 @opindex minline-int-divide-min-latency
9047 Generate code for inline divides of integer values
9048 using the minimum latency algorithm.
9050 @item -minline-int-divide-max-throughput
9051 @opindex minline-int-divide-max-throughput
9052 Generate code for inline divides of integer values
9053 using the maximum throughput algorithm.
9055 @item -minline-sqrt-min-latency
9056 @opindex minline-sqrt-min-latency
9057 Generate code for inline square roots
9058 using the minimum latency algorithm.
9060 @item -minline-sqrt-max-throughput
9061 @opindex minline-sqrt-max-throughput
9062 Generate code for inline square roots
9063 using the maximum throughput algorithm.
9065 @item -mno-dwarf2-asm
9067 @opindex mno-dwarf2-asm
9068 @opindex mdwarf2-asm
9069 Don't (or do) generate assembler code for the DWARF2 line number debugging
9070 info. This may be useful when not using the GNU assembler.
9072 @item -mearly-stop-bits
9073 @itemx -mno-early-stop-bits
9074 @opindex mearly-stop-bits
9075 @opindex mno-early-stop-bits
9076 Allow stop bits to be placed earlier than immediately preceding the
9077 instruction that triggered the stop bit. This can improve instruction
9078 scheduling, but does not always do so.
9080 @item -mfixed-range=@var{register-range}
9081 @opindex mfixed-range
9082 Generate code treating the given register range as fixed registers.
9083 A fixed register is one that the register allocator can not use. This is
9084 useful when compiling kernel code. A register range is specified as
9085 two registers separated by a dash. Multiple register ranges can be
9086 specified separated by a comma.
9088 @item -mtls-size=@var{tls-size}
9090 Specify bit size of immediate TLS offsets. Valid values are 14, 22, and
9093 @item -mtune-arch=@var{cpu-type}
9095 Tune the instruction scheduling for a particular CPU, Valid values are
9096 itanium, itanium1, merced, itanium2, and mckinley.
9102 Add support for multithreading using the POSIX threads library. This
9103 option sets flags for both the preprocessor and linker. It does
9104 not affect the thread safety of object code produced by the compiler or
9105 that of libraries supplied with it. These are HP-UX specific flags.
9111 Generate code for a 32-bit or 64-bit environment.
9112 The 32-bit environment sets int, long and pointer to 32 bits.
9113 The 64-bit environment sets int to 32 bits and long and pointer
9114 to 64 bits. These are HP-UX specific flags.
9118 @node M32R/D Options
9119 @subsection M32R/D Options
9120 @cindex M32R/D options
9122 These @option{-m} options are defined for Renesas M32R/D architectures:
9127 Generate code for the M32R/2@.
9131 Generate code for the M32R/X@.
9135 Generate code for the M32R@. This is the default.
9138 @opindex mmodel=small
9139 Assume all objects live in the lower 16MB of memory (so that their addresses
9140 can be loaded with the @code{ld24} instruction), and assume all subroutines
9141 are reachable with the @code{bl} instruction.
9142 This is the default.
9144 The addressability of a particular object can be set with the
9145 @code{model} attribute.
9147 @item -mmodel=medium
9148 @opindex mmodel=medium
9149 Assume objects may be anywhere in the 32-bit address space (the compiler
9150 will generate @code{seth/add3} instructions to load their addresses), and
9151 assume all subroutines are reachable with the @code{bl} instruction.
9154 @opindex mmodel=large
9155 Assume objects may be anywhere in the 32-bit address space (the compiler
9156 will generate @code{seth/add3} instructions to load their addresses), and
9157 assume subroutines may not be reachable with the @code{bl} instruction
9158 (the compiler will generate the much slower @code{seth/add3/jl}
9159 instruction sequence).
9162 @opindex msdata=none
9163 Disable use of the small data area. Variables will be put into
9164 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
9165 @code{section} attribute has been specified).
9166 This is the default.
9168 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
9169 Objects may be explicitly put in the small data area with the
9170 @code{section} attribute using one of these sections.
9173 @opindex msdata=sdata
9174 Put small global and static data in the small data area, but do not
9175 generate special code to reference them.
9179 Put small global and static data in the small data area, and generate
9180 special instructions to reference them.
9184 @cindex smaller data references
9185 Put global and static objects less than or equal to @var{num} bytes
9186 into the small data or bss sections instead of the normal data or bss
9187 sections. The default value of @var{num} is 8.
9188 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
9189 for this option to have any effect.
9191 All modules should be compiled with the same @option{-G @var{num}} value.
9192 Compiling with different values of @var{num} may or may not work; if it
9193 doesn't the linker will give an error message---incorrect code will not be
9198 Makes the M32R specific code in the compiler display some statistics
9199 that might help in debugging programs.
9202 @opindex malign-loops
9203 Align all loops to a 32-byte boundary.
9205 @item -mno-align-loops
9206 @opindex mno-align-loops
9207 Do not enforce a 32-byte alignment for loops. This is the default.
9209 @item -missue-rate=@var{number}
9210 @opindex missue-rate=@var{number}
9211 Issue @var{number} instructions per cycle. @var{number} can only be 1
9214 @item -mbranch-cost=@var{number}
9215 @opindex mbranch-cost=@var{number}
9216 @var{number} can only be 1 or 2. If it is 1 then branches will be
9217 preferred over conditional code, if it is 2, then the opposite will
9220 @item -mflush-trap=@var{number}
9221 @opindex mflush-trap=@var{number}
9222 Specifies the trap number to use to flush the cache. The default is
9223 12. Valid numbers are between 0 and 15 inclusive.
9225 @item -mno-flush-trap
9226 @opindex mno-flush-trap
9227 Specifies that the cache cannot be flushed by using a trap.
9229 @item -mflush-func=@var{name}
9230 @opindex mflush-func=@var{name}
9231 Specifies the name of the operating system function to call to flush
9232 the cache. The default is @emph{_flush_cache}, but a function call
9233 will only be used if a trap is not available.
9235 @item -mno-flush-func
9236 @opindex mno-flush-func
9237 Indicates that there is no OS function for flushing the cache.
9241 @node M680x0 Options
9242 @subsection M680x0 Options
9243 @cindex M680x0 options
9245 These are the @samp{-m} options defined for the 68000 series. The default
9246 values for these options depends on which style of 68000 was selected when
9247 the compiler was configured; the defaults for the most common choices are
9255 Generate output for a 68000. This is the default
9256 when the compiler is configured for 68000-based systems.
9258 Use this option for microcontrollers with a 68000 or EC000 core,
9259 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
9265 Generate output for a 68020. This is the default
9266 when the compiler is configured for 68020-based systems.
9270 Generate output containing 68881 instructions for floating point.
9271 This is the default for most 68020 systems unless @option{--nfp} was
9272 specified when the compiler was configured.
9276 Generate output for a 68030. This is the default when the compiler is
9277 configured for 68030-based systems.
9281 Generate output for a 68040. This is the default when the compiler is
9282 configured for 68040-based systems.
9284 This option inhibits the use of 68881/68882 instructions that have to be
9285 emulated by software on the 68040. Use this option if your 68040 does not
9286 have code to emulate those instructions.
9290 Generate output for a 68060. This is the default when the compiler is
9291 configured for 68060-based systems.
9293 This option inhibits the use of 68020 and 68881/68882 instructions that
9294 have to be emulated by software on the 68060. Use this option if your 68060
9295 does not have code to emulate those instructions.
9299 Generate output for a CPU32. This is the default
9300 when the compiler is configured for CPU32-based systems.
9302 Use this option for microcontrollers with a
9303 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
9304 68336, 68340, 68341, 68349 and 68360.
9308 Generate output for a 520X ``coldfire'' family cpu. This is the default
9309 when the compiler is configured for 520X-based systems.
9311 Use this option for microcontroller with a 5200 core, including
9312 the MCF5202, MCF5203, MCF5204 and MCF5202.
9317 Generate output for a 68040, without using any of the new instructions.
9318 This results in code which can run relatively efficiently on either a
9319 68020/68881 or a 68030 or a 68040. The generated code does use the
9320 68881 instructions that are emulated on the 68040.
9324 Generate output for a 68060, without using any of the new instructions.
9325 This results in code which can run relatively efficiently on either a
9326 68020/68881 or a 68030 or a 68040. The generated code does use the
9327 68881 instructions that are emulated on the 68060.
9330 @opindex msoft-float
9331 Generate output containing library calls for floating point.
9332 @strong{Warning:} the requisite libraries are not available for all m68k
9333 targets. Normally the facilities of the machine's usual C compiler are
9334 used, but this can't be done directly in cross-compilation. You must
9335 make your own arrangements to provide suitable library functions for
9336 cross-compilation. The embedded targets @samp{m68k-*-aout} and
9337 @samp{m68k-*-coff} do provide software floating point support.
9341 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9342 Additionally, parameters passed on the stack are also aligned to a
9343 16-bit boundary even on targets whose API mandates promotion to 32-bit.
9346 @opindex mnobitfield
9347 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
9348 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
9352 Do use the bit-field instructions. The @option{-m68020} option implies
9353 @option{-mbitfield}. This is the default if you use a configuration
9354 designed for a 68020.
9358 Use a different function-calling convention, in which functions
9359 that take a fixed number of arguments return with the @code{rtd}
9360 instruction, which pops their arguments while returning. This
9361 saves one instruction in the caller since there is no need to pop
9362 the arguments there.
9364 This calling convention is incompatible with the one normally
9365 used on Unix, so you cannot use it if you need to call libraries
9366 compiled with the Unix compiler.
9368 Also, you must provide function prototypes for all functions that
9369 take variable numbers of arguments (including @code{printf});
9370 otherwise incorrect code will be generated for calls to those
9373 In addition, seriously incorrect code will result if you call a
9374 function with too many arguments. (Normally, extra arguments are
9375 harmlessly ignored.)
9377 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
9378 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
9381 @itemx -mno-align-int
9383 @opindex mno-align-int
9384 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
9385 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
9386 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
9387 Aligning variables on 32-bit boundaries produces code that runs somewhat
9388 faster on processors with 32-bit busses at the expense of more memory.
9390 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
9391 align structures containing the above types differently than
9392 most published application binary interface specifications for the m68k.
9396 Use the pc-relative addressing mode of the 68000 directly, instead of
9397 using a global offset table. At present, this option implies @option{-fpic},
9398 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
9399 not presently supported with @option{-mpcrel}, though this could be supported for
9400 68020 and higher processors.
9402 @item -mno-strict-align
9403 @itemx -mstrict-align
9404 @opindex mno-strict-align
9405 @opindex mstrict-align
9406 Do not (do) assume that unaligned memory references will be handled by
9410 Generate code that allows the data segment to be located in a different
9411 area of memory from the text segment. This allows for execute in place in
9412 an environment without virtual memory management. This option implies
9416 Generate code that assumes that the data segment follows the text segment.
9417 This is the default.
9419 @item -mid-shared-library
9420 Generate code that supports shared libraries via the library ID method.
9421 This allows for execute in place and shared libraries in an environment
9422 without virtual memory management. This option implies @option{-fPIC}.
9424 @item -mno-id-shared-library
9425 Generate code that doesn't assume ID based shared libraries are being used.
9426 This is the default.
9428 @item -mshared-library-id=n
9429 Specified the identification number of the ID based shared library being
9430 compiled. Specifying a value of 0 will generate more compact code, specifying
9431 other values will force the allocation of that number to the current
9432 library but is no more space or time efficient than omitting this option.
9436 @node M68hc1x Options
9437 @subsection M68hc1x Options
9438 @cindex M68hc1x options
9440 These are the @samp{-m} options defined for the 68hc11 and 68hc12
9441 microcontrollers. The default values for these options depends on
9442 which style of microcontroller was selected when the compiler was configured;
9443 the defaults for the most common choices are given below.
9450 Generate output for a 68HC11. This is the default
9451 when the compiler is configured for 68HC11-based systems.
9457 Generate output for a 68HC12. This is the default
9458 when the compiler is configured for 68HC12-based systems.
9464 Generate output for a 68HCS12.
9467 @opindex mauto-incdec
9468 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
9475 Enable the use of 68HC12 min and max instructions.
9478 @itemx -mno-long-calls
9479 @opindex mlong-calls
9480 @opindex mno-long-calls
9481 Treat all calls as being far away (near). If calls are assumed to be
9482 far away, the compiler will use the @code{call} instruction to
9483 call a function and the @code{rtc} instruction for returning.
9487 Consider type @code{int} to be 16 bits wide, like @code{short int}.
9489 @item -msoft-reg-count=@var{count}
9490 @opindex msoft-reg-count
9491 Specify the number of pseudo-soft registers which are used for the
9492 code generation. The maximum number is 32. Using more pseudo-soft
9493 register may or may not result in better code depending on the program.
9494 The default is 4 for 68HC11 and 2 for 68HC12.
9499 @subsection MCore Options
9500 @cindex MCore options
9502 These are the @samp{-m} options defined for the Motorola M*Core
9510 @opindex mno-hardlit
9511 Inline constants into the code stream if it can be done in two
9512 instructions or less.
9518 Use the divide instruction. (Enabled by default).
9520 @item -mrelax-immediate
9521 @itemx -mno-relax-immediate
9522 @opindex mrelax-immediate
9523 @opindex mno-relax-immediate
9524 Allow arbitrary sized immediates in bit operations.
9526 @item -mwide-bitfields
9527 @itemx -mno-wide-bitfields
9528 @opindex mwide-bitfields
9529 @opindex mno-wide-bitfields
9530 Always treat bit-fields as int-sized.
9532 @item -m4byte-functions
9533 @itemx -mno-4byte-functions
9534 @opindex m4byte-functions
9535 @opindex mno-4byte-functions
9536 Force all functions to be aligned to a four byte boundary.
9538 @item -mcallgraph-data
9539 @itemx -mno-callgraph-data
9540 @opindex mcallgraph-data
9541 @opindex mno-callgraph-data
9542 Emit callgraph information.
9545 @itemx -mno-slow-bytes
9546 @opindex mslow-bytes
9547 @opindex mno-slow-bytes
9548 Prefer word access when reading byte quantities.
9550 @item -mlittle-endian
9552 @opindex mlittle-endian
9553 @opindex mbig-endian
9554 Generate code for a little endian target.
9560 Generate code for the 210 processor.
9564 @subsection MIPS Options
9565 @cindex MIPS options
9571 Generate big-endian code.
9575 Generate little-endian code. This is the default for @samp{mips*el-*-*}
9578 @item -march=@var{arch}
9580 Generate code that will run on @var{arch}, which can be the name of a
9581 generic MIPS ISA, or the name of a particular processor.
9583 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
9584 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
9585 The processor names are:
9586 @samp{4kc}, @samp{4kp}, @samp{5kc}, @samp{20kc},
9588 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
9589 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000}, @samp{rm7000},
9593 @samp{vr4100}, @samp{vr4111}, @samp{vr4120}, @samp{vr4130}, @samp{vr4300},
9594 @samp{vr5000}, @samp{vr5400} and @samp{vr5500}.
9595 The special value @samp{from-abi} selects the
9596 most compatible architecture for the selected ABI (that is,
9597 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
9599 In processor names, a final @samp{000} can be abbreviated as @samp{k}
9600 (for example, @samp{-march=r2k}). Prefixes are optional, and
9601 @samp{vr} may be written @samp{r}.
9603 GCC defines two macros based on the value of this option. The first
9604 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
9605 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
9606 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
9607 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
9608 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
9610 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
9611 above. In other words, it will have the full prefix and will not
9612 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
9613 the macro names the resolved architecture (either @samp{"mips1"} or
9614 @samp{"mips3"}). It names the default architecture when no
9615 @option{-march} option is given.
9617 @item -mtune=@var{arch}
9619 Optimize for @var{arch}. Among other things, this option controls
9620 the way instructions are scheduled, and the perceived cost of arithmetic
9621 operations. The list of @var{arch} values is the same as for
9624 When this option is not used, GCC will optimize for the processor
9625 specified by @option{-march}. By using @option{-march} and
9626 @option{-mtune} together, it is possible to generate code that will
9627 run on a family of processors, but optimize the code for one
9628 particular member of that family.
9630 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
9631 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
9632 @samp{-march} ones described above.
9636 Equivalent to @samp{-march=mips1}.
9640 Equivalent to @samp{-march=mips2}.
9644 Equivalent to @samp{-march=mips3}.
9648 Equivalent to @samp{-march=mips4}.
9652 Equivalent to @samp{-march=mips32}.
9656 Equivalent to @samp{-march=mips32r2}.
9660 Equivalent to @samp{-march=mips64}.
9666 Use (do not use) the MIPS16 ISA@.
9678 Generate code for the given ABI@.
9680 Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
9681 generates 64-bit code when you select a 64-bit architecture, but you
9682 can use @option{-mgp32} to get 32-bit code instead.
9684 For information about the O64 ABI, see
9685 @w{@uref{http://gcc.gnu.org/projects/mipso64-abi.html}}.
9688 @itemx -mno-abicalls
9690 @opindex mno-abicalls
9691 Generate (do not generate) SVR4-style position-independent code.
9692 @option{-mabicalls} is the default for SVR4-based systems.
9698 Lift (do not lift) the usual restrictions on the size of the global
9701 GCC normally uses a single instruction to load values from the GOT@.
9702 While this is relatively efficient, it will only work if the GOT
9703 is smaller than about 64k. Anything larger will cause the linker
9704 to report an error such as:
9706 @cindex relocation truncated to fit (MIPS)
9708 relocation truncated to fit: R_MIPS_GOT16 foobar
9711 If this happens, you should recompile your code with @option{-mxgot}.
9712 It should then work with very large GOTs, although it will also be
9713 less efficient, since it will take three instructions to fetch the
9714 value of a global symbol.
9716 Note that some linkers can create multiple GOTs. If you have such a
9717 linker, you should only need to use @option{-mxgot} when a single object
9718 file accesses more than 64k's worth of GOT entries. Very few do.
9720 These options have no effect unless GCC is generating position
9725 Assume that general-purpose registers are 32 bits wide.
9729 Assume that general-purpose registers are 64 bits wide.
9733 Assume that floating-point registers are 32 bits wide.
9737 Assume that floating-point registers are 64 bits wide.
9740 @opindex mhard-float
9741 Use floating-point coprocessor instructions.
9744 @opindex msoft-float
9745 Do not use floating-point coprocessor instructions. Implement
9746 floating-point calculations using library calls instead.
9748 @item -msingle-float
9749 @opindex msingle-float
9750 Assume that the floating-point coprocessor only supports single-precision
9753 @itemx -mdouble-float
9754 @opindex mdouble-float
9755 Assume that the floating-point coprocessor supports double-precision
9756 operations. This is the default.
9758 @itemx -mpaired-single
9759 @itemx -mno-paired-single
9760 @opindex mpaired-single
9761 @opindex mno-paired-single
9762 Use (do not use) paired-single floating-point instructions.
9763 @xref{MIPS Paired-Single Support}. This option can only be used
9764 when generating 64-bit code and requires hardware floating-point
9765 support to be enabled.
9771 Use (do not use) the MIPS-3D ASE@. @xref{MIPS-3D Built-in Functions}.
9772 The option @option{-mips3d} implies @option{-mpaired-single}.
9776 Force @code{int} and @code{long} types to be 64 bits wide. See
9777 @option{-mlong32} for an explanation of the default and the way
9778 that the pointer size is determined.
9780 This option has been deprecated and will be removed in a future release.
9784 Force @code{long} types to be 64 bits wide. See @option{-mlong32} for
9785 an explanation of the default and the way that the pointer size is
9790 Force @code{long}, @code{int}, and pointer types to be 32 bits wide.
9792 The default size of @code{int}s, @code{long}s and pointers depends on
9793 the ABI@. All the supported ABIs use 32-bit @code{int}s. The n64 ABI
9794 uses 64-bit @code{long}s, as does the 64-bit EABI; the others use
9795 32-bit @code{long}s. Pointers are the same size as @code{long}s,
9796 or the same size as integer registers, whichever is smaller.
9802 Assume (do not assume) that all symbols have 32-bit values, regardless
9803 of the selected ABI@. This option is useful in combination with
9804 @option{-mabi=64} and @option{-mno-abicalls} because it allows GCC
9805 to generate shorter and faster references to symbolic addresses.
9809 @cindex smaller data references (MIPS)
9810 @cindex gp-relative references (MIPS)
9811 Put global and static items less than or equal to @var{num} bytes into
9812 the small data or bss section instead of the normal data or bss section.
9813 This allows the data to be accessed using a single instruction.
9815 All modules should be compiled with the same @option{-G @var{num}}
9818 @item -membedded-data
9819 @itemx -mno-embedded-data
9820 @opindex membedded-data
9821 @opindex mno-embedded-data
9822 Allocate variables to the read-only data section first if possible, then
9823 next in the small data section if possible, otherwise in data. This gives
9824 slightly slower code than the default, but reduces the amount of RAM required
9825 when executing, and thus may be preferred for some embedded systems.
9827 @item -muninit-const-in-rodata
9828 @itemx -mno-uninit-const-in-rodata
9829 @opindex muninit-const-in-rodata
9830 @opindex mno-uninit-const-in-rodata
9831 Put uninitialized @code{const} variables in the read-only data section.
9832 This option is only meaningful in conjunction with @option{-membedded-data}.
9834 @item -msplit-addresses
9835 @itemx -mno-split-addresses
9836 @opindex msplit-addresses
9837 @opindex mno-split-addresses
9838 Enable (disable) use of the @code{%hi()} and @code{%lo()} assembler
9839 relocation operators. This option has been superseded by
9840 @option{-mexplicit-relocs} but is retained for backwards compatibility.
9842 @item -mexplicit-relocs
9843 @itemx -mno-explicit-relocs
9844 @opindex mexplicit-relocs
9845 @opindex mno-explicit-relocs
9846 Use (do not use) assembler relocation operators when dealing with symbolic
9847 addresses. The alternative, selected by @option{-mno-explicit-relocs},
9848 is to use assembler macros instead.
9850 @option{-mexplicit-relocs} is the default if GCC was configured
9851 to use an assembler that supports relocation operators.
9853 @item -mcheck-zero-division
9854 @itemx -mno-check-zero-division
9855 @opindex mcheck-zero-division
9856 @opindex mno-check-zero-division
9857 Trap (do not trap) on integer division by zero. The default is
9858 @option{-mcheck-zero-division}.
9860 @item -mdivide-traps
9861 @itemx -mdivide-breaks
9862 @opindex mdivide-traps
9863 @opindex mdivide-breaks
9864 MIPS systems check for division by zero by generating either a
9865 conditional trap or a break instruction. Using traps results in
9866 smaller code, but is only supported on MIPS II and later. Also, some
9867 versions of the Linux kernel have a bug that prevents trap from
9868 generating the proper signal (@code{SIGFPE}). Use @option{-mdivide-traps} to
9869 allow conditional traps on architectures that support them and
9870 @option{-mdivide-breaks} to force the use of breaks.
9872 The default is usually @option{-mdivide-traps}, but this can be
9873 overridden at configure time using @option{--with-divide=breaks}.
9874 Divide-by-zero checks can be completely disabled using
9875 @option{-mno-check-zero-division}.
9881 Force (do not force) the use of @code{memcpy()} for non-trivial block
9882 moves. The default is @option{-mno-memcpy}, which allows GCC to inline
9883 most constant-sized copies.
9886 @itemx -mno-long-calls
9887 @opindex mlong-calls
9888 @opindex mno-long-calls
9889 Disable (do not disable) use of the @code{jal} instruction. Calling
9890 functions using @code{jal} is more efficient but requires the caller
9891 and callee to be in the same 256 megabyte segment.
9893 This option has no effect on abicalls code. The default is
9894 @option{-mno-long-calls}.
9900 Enable (disable) use of the @code{mad}, @code{madu} and @code{mul}
9901 instructions, as provided by the R4650 ISA@.
9904 @itemx -mno-fused-madd
9905 @opindex mfused-madd
9906 @opindex mno-fused-madd
9907 Enable (disable) use of the floating point multiply-accumulate
9908 instructions, when they are available. The default is
9909 @option{-mfused-madd}.
9911 When multiply-accumulate instructions are used, the intermediate
9912 product is calculated to infinite precision and is not subject to
9913 the FCSR Flush to Zero bit. This may be undesirable in some
9918 Tell the MIPS assembler to not run its preprocessor over user
9919 assembler files (with a @samp{.s} suffix) when assembling them.
9922 @itemx -mno-fix-r4000
9924 @opindex mno-fix-r4000
9925 Work around certain R4000 CPU errata:
9928 A double-word or a variable shift may give an incorrect result if executed
9929 immediately after starting an integer division.
9931 A double-word or a variable shift may give an incorrect result if executed
9932 while an integer multiplication is in progress.
9934 An integer division may give an incorrect result if started in a delay slot
9935 of a taken branch or a jump.
9939 @itemx -mno-fix-r4400
9941 @opindex mno-fix-r4400
9942 Work around certain R4400 CPU errata:
9945 A double-word or a variable shift may give an incorrect result if executed
9946 immediately after starting an integer division.
9950 @itemx -mno-fix-vr4120
9951 @opindex mfix-vr4120
9952 Work around certain VR4120 errata:
9955 @code{dmultu} does not always produce the correct result.
9957 @code{div} and @code{ddiv} do not always produce the correct result if one
9958 of the operands is negative.
9960 The workarounds for the division errata rely on special functions in
9961 @file{libgcc.a}. At present, these functions are only provided by
9962 the @code{mips64vr*-elf} configurations.
9964 Other VR4120 errata require a nop to be inserted between certain pairs of
9965 instructions. These errata are handled by the assembler, not by GCC itself.
9968 @opindex mfix-vr4130
9969 Work around the VR4130 @code{mflo}/@code{mfhi} errata. The
9970 workarounds are implemented by the assembler rather than by GCC,
9971 although GCC will avoid using @code{mflo} and @code{mfhi} if the
9972 VR4130 @code{macc}, @code{macchi}, @code{dmacc} and @code{dmacchi}
9973 instructions are available instead.
9978 Work around certain SB-1 CPU core errata.
9979 (This flag currently works around the SB-1 revision 2
9980 ``F1'' and ``F2'' floating point errata.)
9982 @item -mflush-func=@var{func}
9983 @itemx -mno-flush-func
9984 @opindex mflush-func
9985 Specifies the function to call to flush the I and D caches, or to not
9986 call any such function. If called, the function must take the same
9987 arguments as the common @code{_flush_func()}, that is, the address of the
9988 memory range for which the cache is being flushed, the size of the
9989 memory range, and the number 3 (to flush both caches). The default
9990 depends on the target GCC was configured for, but commonly is either
9991 @samp{_flush_func} or @samp{__cpu_flush}.
9993 @item -mbranch-likely
9994 @itemx -mno-branch-likely
9995 @opindex mbranch-likely
9996 @opindex mno-branch-likely
9997 Enable or disable use of Branch Likely instructions, regardless of the
9998 default for the selected architecture. By default, Branch Likely
9999 instructions may be generated if they are supported by the selected
10000 architecture. An exception is for the MIPS32 and MIPS64 architectures
10001 and processors which implement those architectures; for those, Branch
10002 Likely instructions will not be generated by default because the MIPS32
10003 and MIPS64 architectures specifically deprecate their use.
10005 @item -mfp-exceptions
10006 @itemx -mno-fp-exceptions
10007 @opindex mfp-exceptions
10008 Specifies whether FP exceptions are enabled. This affects how we schedule
10009 FP instructions for some processors. The default is that FP exceptions are
10012 For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
10013 64-bit code, then we can use both FP pipes. Otherwise, we can only use one
10016 @item -mvr4130-align
10017 @itemx -mno-vr4130-align
10018 @opindex mvr4130-align
10019 The VR4130 pipeline is two-way superscalar, but can only issue two
10020 instructions together if the first one is 8-byte aligned. When this
10021 option is enabled, GCC will align pairs of instructions that it
10022 thinks should execute in parallel.
10024 This option only has an effect when optimizing for the VR4130.
10025 It normally makes code faster, but at the expense of making it bigger.
10026 It is enabled by default at optimization level @option{-O3}.
10030 @subsection MMIX Options
10031 @cindex MMIX Options
10033 These options are defined for the MMIX:
10037 @itemx -mno-libfuncs
10039 @opindex mno-libfuncs
10040 Specify that intrinsic library functions are being compiled, passing all
10041 values in registers, no matter the size.
10044 @itemx -mno-epsilon
10046 @opindex mno-epsilon
10047 Generate floating-point comparison instructions that compare with respect
10048 to the @code{rE} epsilon register.
10050 @item -mabi=mmixware
10052 @opindex mabi-mmixware
10054 Generate code that passes function parameters and return values that (in
10055 the called function) are seen as registers @code{$0} and up, as opposed to
10056 the GNU ABI which uses global registers @code{$231} and up.
10058 @item -mzero-extend
10059 @itemx -mno-zero-extend
10060 @opindex mzero-extend
10061 @opindex mno-zero-extend
10062 When reading data from memory in sizes shorter than 64 bits, use (do not
10063 use) zero-extending load instructions by default, rather than
10064 sign-extending ones.
10067 @itemx -mno-knuthdiv
10069 @opindex mno-knuthdiv
10070 Make the result of a division yielding a remainder have the same sign as
10071 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10072 remainder follows the sign of the dividend. Both methods are
10073 arithmetically valid, the latter being almost exclusively used.
10075 @item -mtoplevel-symbols
10076 @itemx -mno-toplevel-symbols
10077 @opindex mtoplevel-symbols
10078 @opindex mno-toplevel-symbols
10079 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10080 code can be used with the @code{PREFIX} assembly directive.
10084 Generate an executable in the ELF format, rather than the default
10085 @samp{mmo} format used by the @command{mmix} simulator.
10087 @item -mbranch-predict
10088 @itemx -mno-branch-predict
10089 @opindex mbranch-predict
10090 @opindex mno-branch-predict
10091 Use (do not use) the probable-branch instructions, when static branch
10092 prediction indicates a probable branch.
10094 @item -mbase-addresses
10095 @itemx -mno-base-addresses
10096 @opindex mbase-addresses
10097 @opindex mno-base-addresses
10098 Generate (do not generate) code that uses @emph{base addresses}. Using a
10099 base address automatically generates a request (handled by the assembler
10100 and the linker) for a constant to be set up in a global register. The
10101 register is used for one or more base address requests within the range 0
10102 to 255 from the value held in the register. The generally leads to short
10103 and fast code, but the number of different data items that can be
10104 addressed is limited. This means that a program that uses lots of static
10105 data may require @option{-mno-base-addresses}.
10107 @item -msingle-exit
10108 @itemx -mno-single-exit
10109 @opindex msingle-exit
10110 @opindex mno-single-exit
10111 Force (do not force) generated code to have a single exit point in each
10115 @node MN10300 Options
10116 @subsection MN10300 Options
10117 @cindex MN10300 options
10119 These @option{-m} options are defined for Matsushita MN10300 architectures:
10124 Generate code to avoid bugs in the multiply instructions for the MN10300
10125 processors. This is the default.
10127 @item -mno-mult-bug
10128 @opindex mno-mult-bug
10129 Do not generate code to avoid bugs in the multiply instructions for the
10130 MN10300 processors.
10134 Generate code which uses features specific to the AM33 processor.
10138 Do not generate code which uses features specific to the AM33 processor. This
10143 Do not link in the C run-time initialization object file.
10147 Indicate to the linker that it should perform a relaxation optimization pass
10148 to shorten branches, calls and absolute memory addresses. This option only
10149 has an effect when used on the command line for the final link step.
10151 This option makes symbolic debugging impossible.
10154 @node NS32K Options
10155 @subsection NS32K Options
10156 @cindex NS32K options
10158 These are the @samp{-m} options defined for the 32000 series. The default
10159 values for these options depends on which style of 32000 was selected when
10160 the compiler was configured; the defaults for the most common choices are
10168 Generate output for a 32032. This is the default
10169 when the compiler is configured for 32032 and 32016 based systems.
10175 Generate output for a 32332. This is the default
10176 when the compiler is configured for 32332-based systems.
10182 Generate output for a 32532. This is the default
10183 when the compiler is configured for 32532-based systems.
10187 Generate output containing 32081 instructions for floating point.
10188 This is the default for all systems.
10192 Generate output containing 32381 instructions for floating point. This
10193 also implies @option{-m32081}. The 32381 is only compatible with the 32332
10194 and 32532 cpus. This is the default for the pc532-netbsd configuration.
10197 @opindex mmulti-add
10198 Try and generate multiply-add floating point instructions @code{polyF}
10199 and @code{dotF}. This option is only available if the @option{-m32381}
10200 option is in effect. Using these instructions requires changes to
10201 register allocation which generally has a negative impact on
10202 performance. This option should only be enabled when compiling code
10203 particularly likely to make heavy use of multiply-add instructions.
10205 @item -mnomulti-add
10206 @opindex mnomulti-add
10207 Do not try and generate multiply-add floating point instructions
10208 @code{polyF} and @code{dotF}. This is the default on all platforms.
10211 @opindex msoft-float
10212 Generate output containing library calls for floating point.
10213 @strong{Warning:} the requisite libraries may not be available.
10215 @item -mieee-compare
10216 @itemx -mno-ieee-compare
10217 @opindex mieee-compare
10218 @opindex mno-ieee-compare
10219 Control whether or not the compiler uses IEEE floating point
10220 comparisons. These handle correctly the case where the result of a
10221 comparison is unordered.
10222 @strong{Warning:} the requisite kernel support may not be available.
10225 @opindex mnobitfield
10226 Do not use the bit-field instructions. On some machines it is faster to
10227 use shifting and masking operations. This is the default for the pc532.
10231 Do use the bit-field instructions. This is the default for all platforms
10236 Use a different function-calling convention, in which functions
10237 that take a fixed number of arguments return pop their
10238 arguments on return with the @code{ret} instruction.
10240 This calling convention is incompatible with the one normally
10241 used on Unix, so you cannot use it if you need to call libraries
10242 compiled with the Unix compiler.
10244 Also, you must provide function prototypes for all functions that
10245 take variable numbers of arguments (including @code{printf});
10246 otherwise incorrect code will be generated for calls to those
10249 In addition, seriously incorrect code will result if you call a
10250 function with too many arguments. (Normally, extra arguments are
10251 harmlessly ignored.)
10253 This option takes its name from the 680x0 @code{rtd} instruction.
10258 Use a different function-calling convention where the first two arguments
10259 are passed in registers.
10261 This calling convention is incompatible with the one normally
10262 used on Unix, so you cannot use it if you need to call libraries
10263 compiled with the Unix compiler.
10266 @opindex mnoregparam
10267 Do not pass any arguments in registers. This is the default for all
10272 It is OK to use the sb as an index register which is always loaded with
10273 zero. This is the default for the pc532-netbsd target.
10277 The sb register is not available for use or has not been initialized to
10278 zero by the run time system. This is the default for all targets except
10279 the pc532-netbsd. It is also implied whenever @option{-mhimem} or
10280 @option{-fpic} is set.
10284 Many ns32000 series addressing modes use displacements of up to 512MB@.
10285 If an address is above 512MB then displacements from zero can not be used.
10286 This option causes code to be generated which can be loaded above 512MB@.
10287 This may be useful for operating systems or ROM code.
10291 Assume code will be loaded in the first 512MB of virtual address space.
10292 This is the default for all platforms.
10296 @node PDP-11 Options
10297 @subsection PDP-11 Options
10298 @cindex PDP-11 Options
10300 These options are defined for the PDP-11:
10305 Use hardware FPP floating point. This is the default. (FIS floating
10306 point on the PDP-11/40 is not supported.)
10309 @opindex msoft-float
10310 Do not use hardware floating point.
10314 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
10318 Return floating-point results in memory. This is the default.
10322 Generate code for a PDP-11/40.
10326 Generate code for a PDP-11/45. This is the default.
10330 Generate code for a PDP-11/10.
10332 @item -mbcopy-builtin
10333 @opindex bcopy-builtin
10334 Use inline @code{movmemhi} patterns for copying memory. This is the
10339 Do not use inline @code{movmemhi} patterns for copying memory.
10345 Use 16-bit @code{int}. This is the default.
10351 Use 32-bit @code{int}.
10354 @itemx -mno-float32
10356 @opindex mno-float32
10357 Use 64-bit @code{float}. This is the default.
10360 @itemx -mno-float64
10362 @opindex mno-float64
10363 Use 32-bit @code{float}.
10367 Use @code{abshi2} pattern. This is the default.
10371 Do not use @code{abshi2} pattern.
10373 @item -mbranch-expensive
10374 @opindex mbranch-expensive
10375 Pretend that branches are expensive. This is for experimenting with
10376 code generation only.
10378 @item -mbranch-cheap
10379 @opindex mbranch-cheap
10380 Do not pretend that branches are expensive. This is the default.
10384 Generate code for a system with split I&D@.
10388 Generate code for a system without split I&D@. This is the default.
10392 Use Unix assembler syntax. This is the default when configured for
10393 @samp{pdp11-*-bsd}.
10397 Use DEC assembler syntax. This is the default when configured for any
10398 PDP-11 target other than @samp{pdp11-*-bsd}.
10401 @node PowerPC Options
10402 @subsection PowerPC Options
10403 @cindex PowerPC options
10405 These are listed under @xref{RS/6000 and PowerPC Options}.
10407 @node RS/6000 and PowerPC Options
10408 @subsection IBM RS/6000 and PowerPC Options
10409 @cindex RS/6000 and PowerPC Options
10410 @cindex IBM RS/6000 and PowerPC Options
10412 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
10419 @itemx -mno-powerpc
10420 @itemx -mpowerpc-gpopt
10421 @itemx -mno-powerpc-gpopt
10422 @itemx -mpowerpc-gfxopt
10423 @itemx -mno-powerpc-gfxopt
10425 @itemx -mno-powerpc64
10429 @opindex mno-power2
10431 @opindex mno-powerpc
10432 @opindex mpowerpc-gpopt
10433 @opindex mno-powerpc-gpopt
10434 @opindex mpowerpc-gfxopt
10435 @opindex mno-powerpc-gfxopt
10436 @opindex mpowerpc64
10437 @opindex mno-powerpc64
10438 GCC supports two related instruction set architectures for the
10439 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
10440 instructions supported by the @samp{rios} chip set used in the original
10441 RS/6000 systems and the @dfn{PowerPC} instruction set is the
10442 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
10443 the IBM 4xx microprocessors.
10445 Neither architecture is a subset of the other. However there is a
10446 large common subset of instructions supported by both. An MQ
10447 register is included in processors supporting the POWER architecture.
10449 You use these options to specify which instructions are available on the
10450 processor you are using. The default value of these options is
10451 determined when configuring GCC@. Specifying the
10452 @option{-mcpu=@var{cpu_type}} overrides the specification of these
10453 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
10454 rather than the options listed above.
10456 The @option{-mpower} option allows GCC to generate instructions that
10457 are found only in the POWER architecture and to use the MQ register.
10458 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
10459 to generate instructions that are present in the POWER2 architecture but
10460 not the original POWER architecture.
10462 The @option{-mpowerpc} option allows GCC to generate instructions that
10463 are found only in the 32-bit subset of the PowerPC architecture.
10464 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
10465 GCC to use the optional PowerPC architecture instructions in the
10466 General Purpose group, including floating-point square root. Specifying
10467 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
10468 use the optional PowerPC architecture instructions in the Graphics
10469 group, including floating-point select.
10471 The @option{-mpowerpc64} option allows GCC to generate the additional
10472 64-bit instructions that are found in the full PowerPC64 architecture
10473 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
10474 @option{-mno-powerpc64}.
10476 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
10477 will use only the instructions in the common subset of both
10478 architectures plus some special AIX common-mode calls, and will not use
10479 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
10480 permits GCC to use any instruction from either architecture and to
10481 allow use of the MQ register; specify this for the Motorola MPC601.
10483 @item -mnew-mnemonics
10484 @itemx -mold-mnemonics
10485 @opindex mnew-mnemonics
10486 @opindex mold-mnemonics
10487 Select which mnemonics to use in the generated assembler code. With
10488 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
10489 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
10490 assembler mnemonics defined for the POWER architecture. Instructions
10491 defined in only one architecture have only one mnemonic; GCC uses that
10492 mnemonic irrespective of which of these options is specified.
10494 GCC defaults to the mnemonics appropriate for the architecture in
10495 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
10496 value of these option. Unless you are building a cross-compiler, you
10497 should normally not specify either @option{-mnew-mnemonics} or
10498 @option{-mold-mnemonics}, but should instead accept the default.
10500 @item -mcpu=@var{cpu_type}
10502 Set architecture type, register usage, choice of mnemonics, and
10503 instruction scheduling parameters for machine type @var{cpu_type}.
10504 Supported values for @var{cpu_type} are @samp{401}, @samp{403},
10505 @samp{405}, @samp{405fp}, @samp{440}, @samp{440fp}, @samp{505},
10506 @samp{601}, @samp{602}, @samp{603}, @samp{603e}, @samp{604},
10507 @samp{604e}, @samp{620}, @samp{630}, @samp{740}, @samp{7400},
10508 @samp{7450}, @samp{750}, @samp{801}, @samp{821}, @samp{823},
10509 @samp{860}, @samp{970}, @samp{8540}, @samp{common}, @samp{ec603e}, @samp{G3},
10510 @samp{G4}, @samp{G5}, @samp{power}, @samp{power2}, @samp{power3},
10511 @samp{power4}, @samp{power5}, @samp{powerpc}, @samp{powerpc64},
10512 @samp{rios}, @samp{rios1}, @samp{rios2}, @samp{rsc}, and @samp{rs64a}.
10514 @option{-mcpu=common} selects a completely generic processor. Code
10515 generated under this option will run on any POWER or PowerPC processor.
10516 GCC will use only the instructions in the common subset of both
10517 architectures, and will not use the MQ register. GCC assumes a generic
10518 processor model for scheduling purposes.
10520 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
10521 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
10522 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
10523 types, with an appropriate, generic processor model assumed for
10524 scheduling purposes.
10526 The other options specify a specific processor. Code generated under
10527 those options will run best on that processor, and may not run at all on
10530 The @option{-mcpu} options automatically enable or disable the
10531 following options: @option{-maltivec}, @option{-mhard-float},
10532 @option{-mmfcrf}, @option{-mmultiple}, @option{-mnew-mnemonics},
10533 @option{-mpower}, @option{-mpower2}, @option{-mpowerpc64},
10534 @option{-mpowerpc-gpopt}, @option{-mpowerpc-gfxopt},
10535 @option{-mstring}. The particular options set for any particular CPU
10536 will vary between compiler versions, depending on what setting seems
10537 to produce optimal code for that CPU; it doesn't necessarily reflect
10538 the actual hardware's capabilities. If you wish to set an individual
10539 option to a particular value, you may specify it after the
10540 @option{-mcpu} option, like @samp{-mcpu=970 -mno-altivec}.
10542 On AIX, the @option{-maltivec} and @option{-mpowerpc64} options are
10543 not enabled or disabled by the @option{-mcpu} option at present, since
10544 AIX does not have full support for these options. You may still
10545 enable or disable them individually if you're sure it'll work in your
10548 @item -mtune=@var{cpu_type}
10550 Set the instruction scheduling parameters for machine type
10551 @var{cpu_type}, but do not set the architecture type, register usage, or
10552 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
10553 values for @var{cpu_type} are used for @option{-mtune} as for
10554 @option{-mcpu}. If both are specified, the code generated will use the
10555 architecture, registers, and mnemonics set by @option{-mcpu}, but the
10556 scheduling parameters set by @option{-mtune}.
10559 @itemx -mno-altivec
10561 @opindex mno-altivec
10562 Generate code that uses (does not use) AltiVec instructions, and also
10563 enable the use of built-in functions that allow more direct access to
10564 the AltiVec instruction set. You may also need to set
10565 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
10570 Extend the current ABI with SPE ABI extensions. This does not change
10571 the default ABI, instead it adds the SPE ABI extensions to the current
10575 @opindex mabi=no-spe
10576 Disable Booke SPE ABI extensions for the current ABI@.
10578 @item -misel=@var{yes/no}
10581 This switch enables or disables the generation of ISEL instructions.
10583 @item -mspe=@var{yes/no}
10586 This switch enables or disables the generation of SPE simd
10589 @item -mfloat-gprs=@var{yes/single/double/no}
10590 @itemx -mfloat-gprs
10591 @opindex mfloat-gprs
10592 This switch enables or disables the generation of floating point
10593 operations on the general purpose registers for architectures that
10596 The argument @var{yes} or @var{single} enables the use of
10597 single-precision floating point operations.
10599 The argument @var{double} enables the use of single and
10600 double-precision floating point operations.
10602 The argument @var{no} disables floating point operations on the
10603 general purpose registers.
10605 This option is currently only available on the MPC854x.
10611 Generate code for 32-bit or 64-bit environments of Darwin and SVR4
10612 targets (including GNU/Linux). The 32-bit environment sets int, long
10613 and pointer to 32 bits and generates code that runs on any PowerPC
10614 variant. The 64-bit environment sets int to 32 bits and long and
10615 pointer to 64 bits, and generates code for PowerPC64, as for
10616 @option{-mpowerpc64}.
10619 @itemx -mno-fp-in-toc
10620 @itemx -mno-sum-in-toc
10621 @itemx -mminimal-toc
10623 @opindex mno-fp-in-toc
10624 @opindex mno-sum-in-toc
10625 @opindex mminimal-toc
10626 Modify generation of the TOC (Table Of Contents), which is created for
10627 every executable file. The @option{-mfull-toc} option is selected by
10628 default. In that case, GCC will allocate at least one TOC entry for
10629 each unique non-automatic variable reference in your program. GCC
10630 will also place floating-point constants in the TOC@. However, only
10631 16,384 entries are available in the TOC@.
10633 If you receive a linker error message that saying you have overflowed
10634 the available TOC space, you can reduce the amount of TOC space used
10635 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
10636 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
10637 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
10638 generate code to calculate the sum of an address and a constant at
10639 run-time instead of putting that sum into the TOC@. You may specify one
10640 or both of these options. Each causes GCC to produce very slightly
10641 slower and larger code at the expense of conserving TOC space.
10643 If you still run out of space in the TOC even when you specify both of
10644 these options, specify @option{-mminimal-toc} instead. This option causes
10645 GCC to make only one TOC entry for every file. When you specify this
10646 option, GCC will produce code that is slower and larger but which
10647 uses extremely little TOC space. You may wish to use this option
10648 only on files that contain less frequently executed code.
10654 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
10655 @code{long} type, and the infrastructure needed to support them.
10656 Specifying @option{-maix64} implies @option{-mpowerpc64} and
10657 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
10658 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
10661 @itemx -mno-xl-compat
10662 @opindex mxl-compat
10663 @opindex mno-xl-compat
10664 Produce code that conforms more closely to IBM XLC semantics when using
10665 AIX-compatible ABI. Pass floating-point arguments to prototyped
10666 functions beyond the register save area (RSA) on the stack in addition
10667 to argument FPRs. Do not assume that most significant double in 128
10668 bit long double value is properly rounded when comparing values.
10670 The AIX calling convention was extended but not initially documented to
10671 handle an obscure K&R C case of calling a function that takes the
10672 address of its arguments with fewer arguments than declared. AIX XL
10673 compilers access floating point arguments which do not fit in the
10674 RSA from the stack when a subroutine is compiled without
10675 optimization. Because always storing floating-point arguments on the
10676 stack is inefficient and rarely needed, this option is not enabled by
10677 default and only is necessary when calling subroutines compiled by AIX
10678 XL compilers without optimization.
10682 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
10683 application written to use message passing with special startup code to
10684 enable the application to run. The system must have PE installed in the
10685 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
10686 must be overridden with the @option{-specs=} option to specify the
10687 appropriate directory location. The Parallel Environment does not
10688 support threads, so the @option{-mpe} option and the @option{-pthread}
10689 option are incompatible.
10691 @item -malign-natural
10692 @itemx -malign-power
10693 @opindex malign-natural
10694 @opindex malign-power
10695 On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
10696 @option{-malign-natural} overrides the ABI-defined alignment of larger
10697 types, such as floating-point doubles, on their natural size-based boundary.
10698 The option @option{-malign-power} instructs GCC to follow the ABI-specified
10699 alignment rules. GCC defaults to the standard alignment defined in the ABI@.
10701 On 64-bit Darwin, natural alignment is the default, and @option{-malign-power}
10705 @itemx -mhard-float
10706 @opindex msoft-float
10707 @opindex mhard-float
10708 Generate code that does not use (uses) the floating-point register set.
10709 Software floating point emulation is provided if you use the
10710 @option{-msoft-float} option, and pass the option to GCC when linking.
10713 @itemx -mno-multiple
10715 @opindex mno-multiple
10716 Generate code that uses (does not use) the load multiple word
10717 instructions and the store multiple word instructions. These
10718 instructions are generated by default on POWER systems, and not
10719 generated on PowerPC systems. Do not use @option{-mmultiple} on little
10720 endian PowerPC systems, since those instructions do not work when the
10721 processor is in little endian mode. The exceptions are PPC740 and
10722 PPC750 which permit the instructions usage in little endian mode.
10727 @opindex mno-string
10728 Generate code that uses (does not use) the load string instructions
10729 and the store string word instructions to save multiple registers and
10730 do small block moves. These instructions are generated by default on
10731 POWER systems, and not generated on PowerPC systems. Do not use
10732 @option{-mstring} on little endian PowerPC systems, since those
10733 instructions do not work when the processor is in little endian mode.
10734 The exceptions are PPC740 and PPC750 which permit the instructions
10735 usage in little endian mode.
10740 @opindex mno-update
10741 Generate code that uses (does not use) the load or store instructions
10742 that update the base register to the address of the calculated memory
10743 location. These instructions are generated by default. If you use
10744 @option{-mno-update}, there is a small window between the time that the
10745 stack pointer is updated and the address of the previous frame is
10746 stored, which means code that walks the stack frame across interrupts or
10747 signals may get corrupted data.
10750 @itemx -mno-fused-madd
10751 @opindex mfused-madd
10752 @opindex mno-fused-madd
10753 Generate code that uses (does not use) the floating point multiply and
10754 accumulate instructions. These instructions are generated by default if
10755 hardware floating is used.
10757 @item -mno-bit-align
10759 @opindex mno-bit-align
10760 @opindex mbit-align
10761 On System V.4 and embedded PowerPC systems do not (do) force structures
10762 and unions that contain bit-fields to be aligned to the base type of the
10765 For example, by default a structure containing nothing but 8
10766 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
10767 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
10768 the structure would be aligned to a 1 byte boundary and be one byte in
10771 @item -mno-strict-align
10772 @itemx -mstrict-align
10773 @opindex mno-strict-align
10774 @opindex mstrict-align
10775 On System V.4 and embedded PowerPC systems do not (do) assume that
10776 unaligned memory references will be handled by the system.
10778 @item -mrelocatable
10779 @itemx -mno-relocatable
10780 @opindex mrelocatable
10781 @opindex mno-relocatable
10782 On embedded PowerPC systems generate code that allows (does not allow)
10783 the program to be relocated to a different address at runtime. If you
10784 use @option{-mrelocatable} on any module, all objects linked together must
10785 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
10787 @item -mrelocatable-lib
10788 @itemx -mno-relocatable-lib
10789 @opindex mrelocatable-lib
10790 @opindex mno-relocatable-lib
10791 On embedded PowerPC systems generate code that allows (does not allow)
10792 the program to be relocated to a different address at runtime. Modules
10793 compiled with @option{-mrelocatable-lib} can be linked with either modules
10794 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
10795 with modules compiled with the @option{-mrelocatable} options.
10801 On System V.4 and embedded PowerPC systems do not (do) assume that
10802 register 2 contains a pointer to a global area pointing to the addresses
10803 used in the program.
10806 @itemx -mlittle-endian
10808 @opindex mlittle-endian
10809 On System V.4 and embedded PowerPC systems compile code for the
10810 processor in little endian mode. The @option{-mlittle-endian} option is
10811 the same as @option{-mlittle}.
10814 @itemx -mbig-endian
10816 @opindex mbig-endian
10817 On System V.4 and embedded PowerPC systems compile code for the
10818 processor in big endian mode. The @option{-mbig-endian} option is
10819 the same as @option{-mbig}.
10821 @item -mdynamic-no-pic
10822 @opindex mdynamic-no-pic
10823 On Darwin and Mac OS X systems, compile code so that it is not
10824 relocatable, but that its external references are relocatable. The
10825 resulting code is suitable for applications, but not shared
10828 @item -mprioritize-restricted-insns=@var{priority}
10829 @opindex mprioritize-restricted-insns
10830 This option controls the priority that is assigned to
10831 dispatch-slot restricted instructions during the second scheduling
10832 pass. The argument @var{priority} takes the value @var{0/1/2} to assign
10833 @var{no/highest/second-highest} priority to dispatch slot restricted
10836 @item -msched-costly-dep=@var{dependence_type}
10837 @opindex msched-costly-dep
10838 This option controls which dependences are considered costly
10839 by the target during instruction scheduling. The argument
10840 @var{dependence_type} takes one of the following values:
10841 @var{no}: no dependence is costly,
10842 @var{all}: all dependences are costly,
10843 @var{true_store_to_load}: a true dependence from store to load is costly,
10844 @var{store_to_load}: any dependence from store to load is costly,
10845 @var{number}: any dependence which latency >= @var{number} is costly.
10847 @item -minsert-sched-nops=@var{scheme}
10848 @opindex minsert-sched-nops
10849 This option controls which nop insertion scheme will be used during
10850 the second scheduling pass. The argument @var{scheme} takes one of the
10852 @var{no}: Don't insert nops.
10853 @var{pad}: Pad with nops any dispatch group which has vacant issue slots,
10854 according to the scheduler's grouping.
10855 @var{regroup_exact}: Insert nops to force costly dependent insns into
10856 separate groups. Insert exactly as many nops as needed to force an insn
10857 to a new group, according to the estimated processor grouping.
10858 @var{number}: Insert nops to force costly dependent insns into
10859 separate groups. Insert @var{number} nops to force an insn to a new group.
10862 @opindex mcall-sysv
10863 On System V.4 and embedded PowerPC systems compile code using calling
10864 conventions that adheres to the March 1995 draft of the System V
10865 Application Binary Interface, PowerPC processor supplement. This is the
10866 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
10868 @item -mcall-sysv-eabi
10869 @opindex mcall-sysv-eabi
10870 Specify both @option{-mcall-sysv} and @option{-meabi} options.
10872 @item -mcall-sysv-noeabi
10873 @opindex mcall-sysv-noeabi
10874 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
10876 @item -mcall-solaris
10877 @opindex mcall-solaris
10878 On System V.4 and embedded PowerPC systems compile code for the Solaris
10882 @opindex mcall-linux
10883 On System V.4 and embedded PowerPC systems compile code for the
10884 Linux-based GNU system.
10888 On System V.4 and embedded PowerPC systems compile code for the
10889 Hurd-based GNU system.
10891 @item -mcall-netbsd
10892 @opindex mcall-netbsd
10893 On System V.4 and embedded PowerPC systems compile code for the
10894 NetBSD operating system.
10896 @item -maix-struct-return
10897 @opindex maix-struct-return
10898 Return all structures in memory (as specified by the AIX ABI)@.
10900 @item -msvr4-struct-return
10901 @opindex msvr4-struct-return
10902 Return structures smaller than 8 bytes in registers (as specified by the
10905 @item -mabi=altivec
10906 @opindex mabi=altivec
10907 Extend the current ABI with AltiVec ABI extensions. This does not
10908 change the default ABI, instead it adds the AltiVec ABI extensions to
10911 @item -mabi=no-altivec
10912 @opindex mabi=no-altivec
10913 Disable AltiVec ABI extensions for the current ABI@.
10916 @itemx -mno-prototype
10917 @opindex mprototype
10918 @opindex mno-prototype
10919 On System V.4 and embedded PowerPC systems assume that all calls to
10920 variable argument functions are properly prototyped. Otherwise, the
10921 compiler must insert an instruction before every non prototyped call to
10922 set or clear bit 6 of the condition code register (@var{CR}) to
10923 indicate whether floating point values were passed in the floating point
10924 registers in case the function takes a variable arguments. With
10925 @option{-mprototype}, only calls to prototyped variable argument functions
10926 will set or clear the bit.
10930 On embedded PowerPC systems, assume that the startup module is called
10931 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
10932 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
10937 On embedded PowerPC systems, assume that the startup module is called
10938 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
10943 On embedded PowerPC systems, assume that the startup module is called
10944 @file{crt0.o} and the standard C libraries are @file{libads.a} and
10947 @item -myellowknife
10948 @opindex myellowknife
10949 On embedded PowerPC systems, assume that the startup module is called
10950 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
10955 On System V.4 and embedded PowerPC systems, specify that you are
10956 compiling for a VxWorks system.
10960 Specify that you are compiling for the WindISS simulation environment.
10964 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
10965 header to indicate that @samp{eabi} extended relocations are used.
10971 On System V.4 and embedded PowerPC systems do (do not) adhere to the
10972 Embedded Applications Binary Interface (eabi) which is a set of
10973 modifications to the System V.4 specifications. Selecting @option{-meabi}
10974 means that the stack is aligned to an 8 byte boundary, a function
10975 @code{__eabi} is called to from @code{main} to set up the eabi
10976 environment, and the @option{-msdata} option can use both @code{r2} and
10977 @code{r13} to point to two separate small data areas. Selecting
10978 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
10979 do not call an initialization function from @code{main}, and the
10980 @option{-msdata} option will only use @code{r13} to point to a single
10981 small data area. The @option{-meabi} option is on by default if you
10982 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
10985 @opindex msdata=eabi
10986 On System V.4 and embedded PowerPC systems, put small initialized
10987 @code{const} global and static data in the @samp{.sdata2} section, which
10988 is pointed to by register @code{r2}. Put small initialized
10989 non-@code{const} global and static data in the @samp{.sdata} section,
10990 which is pointed to by register @code{r13}. Put small uninitialized
10991 global and static data in the @samp{.sbss} section, which is adjacent to
10992 the @samp{.sdata} section. The @option{-msdata=eabi} option is
10993 incompatible with the @option{-mrelocatable} option. The
10994 @option{-msdata=eabi} option also sets the @option{-memb} option.
10997 @opindex msdata=sysv
10998 On System V.4 and embedded PowerPC systems, put small global and static
10999 data in the @samp{.sdata} section, which is pointed to by register
11000 @code{r13}. Put small uninitialized global and static data in the
11001 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
11002 The @option{-msdata=sysv} option is incompatible with the
11003 @option{-mrelocatable} option.
11005 @item -msdata=default
11007 @opindex msdata=default
11009 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
11010 compile code the same as @option{-msdata=eabi}, otherwise compile code the
11011 same as @option{-msdata=sysv}.
11014 @opindex msdata-data
11015 On System V.4 and embedded PowerPC systems, put small global and static
11016 data in the @samp{.sdata} section. Put small uninitialized global and
11017 static data in the @samp{.sbss} section. Do not use register @code{r13}
11018 to address small data however. This is the default behavior unless
11019 other @option{-msdata} options are used.
11023 @opindex msdata=none
11025 On embedded PowerPC systems, put all initialized global and static data
11026 in the @samp{.data} section, and all uninitialized data in the
11027 @samp{.bss} section.
11031 @cindex smaller data references (PowerPC)
11032 @cindex .sdata/.sdata2 references (PowerPC)
11033 On embedded PowerPC systems, put global and static items less than or
11034 equal to @var{num} bytes into the small data or bss sections instead of
11035 the normal data or bss section. By default, @var{num} is 8. The
11036 @option{-G @var{num}} switch is also passed to the linker.
11037 All modules should be compiled with the same @option{-G @var{num}} value.
11040 @itemx -mno-regnames
11042 @opindex mno-regnames
11043 On System V.4 and embedded PowerPC systems do (do not) emit register
11044 names in the assembly language output using symbolic forms.
11047 @itemx -mno-longcall
11049 @opindex mno-longcall
11050 Default to making all function calls indirectly, using a register, so
11051 that functions which reside further than 32 megabytes (33,554,432
11052 bytes) from the current location can be called. This setting can be
11053 overridden by the @code{shortcall} function attribute, or by
11054 @code{#pragma longcall(0)}.
11056 Some linkers are capable of detecting out-of-range calls and generating
11057 glue code on the fly. On these systems, long calls are unnecessary and
11058 generate slower code. As of this writing, the AIX linker can do this,
11059 as can the GNU linker for PowerPC/64. It is planned to add this feature
11060 to the GNU linker for 32-bit PowerPC systems as well.
11062 On Darwin/PPC systems, @code{#pragma longcall} will generate ``jbsr
11063 callee, L42'', plus a ``branch island'' (glue code). The two target
11064 addresses represent the callee and the ``branch island''. The
11065 Darwin/PPC linker will prefer the first address and generate a ``bl
11066 callee'' if the PPC ``bl'' instruction will reach the callee directly;
11067 otherwise, the linker will generate ``bl L42'' to call the ``branch
11068 island''. The ``branch island'' is appended to the body of the
11069 calling function; it computes the full 32-bit address of the callee
11072 On Mach-O (Darwin) systems, this option directs the compiler emit to
11073 the glue for every direct call, and the Darwin linker decides whether
11074 to use or discard it.
11076 In the future, we may cause GCC to ignore all longcall specifications
11077 when the linker is known to generate glue.
11081 Adds support for multithreading with the @dfn{pthreads} library.
11082 This option sets flags for both the preprocessor and linker.
11086 @node S/390 and zSeries Options
11087 @subsection S/390 and zSeries Options
11088 @cindex S/390 and zSeries Options
11090 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
11094 @itemx -msoft-float
11095 @opindex mhard-float
11096 @opindex msoft-float
11097 Use (do not use) the hardware floating-point instructions and registers
11098 for floating-point operations. When @option{-msoft-float} is specified,
11099 functions in @file{libgcc.a} will be used to perform floating-point
11100 operations. When @option{-mhard-float} is specified, the compiler
11101 generates IEEE floating-point instructions. This is the default.
11104 @itemx -mno-backchain
11105 @opindex mbackchain
11106 @opindex mno-backchain
11107 Store (do not store) the address of the caller's frame as backchain pointer
11108 into the callee's stack frame.
11109 A backchain may be needed to allow debugging using tools that do not understand
11110 DWARF-2 call frame information.
11111 When @option{-mno-packed-stack} is in effect, the backchain pointer is stored
11112 at the bottom of the stack frame; when @option{-mpacked-stack} is in effect,
11113 the backchain is placed into the topmost word of the 96/160 byte register
11116 In general, code compiled with @option{-mbackchain} is call-compatible with
11117 code compiled with @option{-mmo-backchain}; however, use of the backchain
11118 for debugging purposes usually requires that the whole binary is built with
11119 @option{-mbackchain}. Note that the combination of @option{-mbackchain},
11120 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11121 to build a linux kernel use @option{-msoft-float}.
11123 The default is to not maintain the backchain.
11125 @item -mpacked-stack
11126 @item -mno-packed-stack
11127 @opindex mpacked-stack
11128 @opindex mno-packed-stack
11129 Use (do not use) the packed stack layout. When @option{-mno-packed-stack} is
11130 specified, the compiler uses the all fields of the 96/160 byte register save
11131 area only for their default purpose; unused fields still take up stack space.
11132 When @option{-mpacked-stack} is specified, register save slots are densely
11133 packed at the top of the register save area; unused space is reused for other
11134 purposes, allowing for more efficient use of the available stack space.
11135 However, when @option{-mbackchain} is also in effect, the topmost word of
11136 the save area is always used to store the backchain, and the return address
11137 register is always saved two words below the backchain.
11139 As long as the stack frame backchain is not used, code generated with
11140 @option{-mpacked-stack} is call-compatible with code generated with
11141 @option{-mno-packed-stack}. Note that some non-FSF releases of GCC 2.95 for
11142 S/390 or zSeries generated code that uses the stack frame backchain at run
11143 time, not just for debugging purposes. Such code is not call-compatible
11144 with code compiled with @option{-mpacked-stack}. Also, note that the
11145 combination of @option{-mbackchain},
11146 @option{-mpacked-stack} and @option{-mhard-float} is not supported. In order
11147 to build a linux kernel use @option{-msoft-float}.
11149 The default is to not use the packed stack layout.
11152 @itemx -mno-small-exec
11153 @opindex msmall-exec
11154 @opindex mno-small-exec
11155 Generate (or do not generate) code using the @code{bras} instruction
11156 to do subroutine calls.
11157 This only works reliably if the total executable size does not
11158 exceed 64k. The default is to use the @code{basr} instruction instead,
11159 which does not have this limitation.
11165 When @option{-m31} is specified, generate code compliant to the
11166 GNU/Linux for S/390 ABI@. When @option{-m64} is specified, generate
11167 code compliant to the GNU/Linux for zSeries ABI@. This allows GCC in
11168 particular to generate 64-bit instructions. For the @samp{s390}
11169 targets, the default is @option{-m31}, while the @samp{s390x}
11170 targets default to @option{-m64}.
11176 When @option{-mzarch} is specified, generate code using the
11177 instructions available on z/Architecture.
11178 When @option{-mesa} is specified, generate code using the
11179 instructions available on ESA/390. Note that @option{-mesa} is
11180 not possible with @option{-m64}.
11181 When generating code compliant to the GNU/Linux for S/390 ABI,
11182 the default is @option{-mesa}. When generating code compliant
11183 to the GNU/Linux for zSeries ABI, the default is @option{-mzarch}.
11189 Generate (or do not generate) code using the @code{mvcle} instruction
11190 to perform block moves. When @option{-mno-mvcle} is specified,
11191 use a @code{mvc} loop instead. This is the default.
11197 Print (or do not print) additional debug information when compiling.
11198 The default is to not print debug information.
11200 @item -march=@var{cpu-type}
11202 Generate code that will run on @var{cpu-type}, which is the name of a system
11203 representing a certain processor type. Possible values for
11204 @var{cpu-type} are @samp{g5}, @samp{g6}, @samp{z900}, and @samp{z990}.
11205 When generating code using the instructions available on z/Architecture,
11206 the default is @option{-march=z900}. Otherwise, the default is
11207 @option{-march=g5}.
11209 @item -mtune=@var{cpu-type}
11211 Tune to @var{cpu-type} everything applicable about the generated code,
11212 except for the ABI and the set of available instructions.
11213 The list of @var{cpu-type} values is the same as for @option{-march}.
11214 The default is the value used for @option{-march}.
11217 @itemx -mno-tpf-trace
11218 @opindex mtpf-trace
11219 @opindex mno-tpf-trace
11220 Generate code that adds (does not add) in TPF OS specific branches to trace
11221 routines in the operating system. This option is off by default, even
11222 when compiling for the TPF OS@.
11225 @itemx -mno-fused-madd
11226 @opindex mfused-madd
11227 @opindex mno-fused-madd
11228 Generate code that uses (does not use) the floating point multiply and
11229 accumulate instructions. These instructions are generated by default if
11230 hardware floating point is used.
11232 @item -mwarn-framesize=@var{framesize}
11233 @opindex mwarn-framesize
11234 Emit a warning if the current function exceeds the given frame size. Because
11235 this is a compile time check it doesn't need to be a real problem when the program
11236 runs. It is intended to identify functions which most probably cause
11237 a stack overflow. It is useful to be used in an environment with limited stack
11238 size e.g.@: the linux kernel.
11240 @item -mwarn-dynamicstack
11241 @opindex mwarn-dynamicstack
11242 Emit a warning if the function calls alloca or uses dynamically
11243 sized arrays. This is generally a bad idea with a limited stack size.
11245 @item -mstack-guard=@var{stack-guard}
11246 @item -mstack-size=@var{stack-size}
11247 @opindex mstack-guard
11248 @opindex mstack-size
11249 These arguments always have to be used in conjunction. If they are present the s390
11250 back end emits additional instructions in the function prologue which trigger a trap
11251 if the stack size is @var{stack-guard} bytes above the @var{stack-size}
11252 (remember that the stack on s390 grows downward). These options are intended to
11253 be used to help debugging stack overflow problems. The additionally emitted code
11254 cause only little overhead and hence can also be used in production like systems
11255 without greater performance degradation. The given values have to be exact
11256 powers of 2 and @var{stack-size} has to be greater than @var{stack-guard}.
11257 In order to be efficient the extra code makes the assumption that the stack starts
11258 at an address aligned to the value given by @var{stack-size}.
11262 @subsection SH Options
11264 These @samp{-m} options are defined for the SH implementations:
11269 Generate code for the SH1.
11273 Generate code for the SH2.
11276 Generate code for the SH2e.
11280 Generate code for the SH3.
11284 Generate code for the SH3e.
11288 Generate code for the SH4 without a floating-point unit.
11290 @item -m4-single-only
11291 @opindex m4-single-only
11292 Generate code for the SH4 with a floating-point unit that only
11293 supports single-precision arithmetic.
11297 Generate code for the SH4 assuming the floating-point unit is in
11298 single-precision mode by default.
11302 Generate code for the SH4.
11306 Generate code for the SH4al-dsp, or for a SH4a in such a way that the
11307 floating-point unit is not used.
11309 @item -m4a-single-only
11310 @opindex m4a-single-only
11311 Generate code for the SH4a, in such a way that no double-precision
11312 floating point operations are used.
11315 @opindex m4a-single
11316 Generate code for the SH4a assuming the floating-point unit is in
11317 single-precision mode by default.
11321 Generate code for the SH4a.
11325 Same as @option{-m4a-nofpu}, except that it implicitly passes
11326 @option{-dsp} to the assembler. GCC doesn't generate any DSP
11327 instructions at the moment.
11331 Compile code for the processor in big endian mode.
11335 Compile code for the processor in little endian mode.
11339 Align doubles at 64-bit boundaries. Note that this changes the calling
11340 conventions, and thus some functions from the standard C library will
11341 not work unless you recompile it first with @option{-mdalign}.
11345 Shorten some address references at link time, when possible; uses the
11346 linker option @option{-relax}.
11350 Use 32-bit offsets in @code{switch} tables. The default is to use
11355 Enable the use of the instruction @code{fmovd}.
11359 Comply with the calling conventions defined by Renesas.
11363 Comply with the calling conventions defined by Renesas.
11367 Comply with the calling conventions defined for GCC before the Renesas
11368 conventions were available. This option is the default for all
11369 targets of the SH toolchain except for @samp{sh-symbianelf}.
11372 @opindex mnomacsave
11373 Mark the @code{MAC} register as call-clobbered, even if
11374 @option{-mhitachi} is given.
11378 Increase IEEE-compliance of floating-point code.
11382 Dump instruction size and location in the assembly code.
11385 @opindex mpadstruct
11386 This option is deprecated. It pads structures to multiple of 4 bytes,
11387 which is incompatible with the SH ABI@.
11391 Optimize for space instead of speed. Implied by @option{-Os}.
11394 @opindex mprefergot
11395 When generating position-independent code, emit function calls using
11396 the Global Offset Table instead of the Procedure Linkage Table.
11400 Generate a library function call to invalidate instruction cache
11401 entries, after fixing up a trampoline. This library function call
11402 doesn't assume it can write to the whole memory address space. This
11403 is the default when the target is @code{sh-*-linux*}.
11406 @node SPARC Options
11407 @subsection SPARC Options
11408 @cindex SPARC options
11410 These @samp{-m} options are supported on the SPARC:
11413 @item -mno-app-regs
11415 @opindex mno-app-regs
11417 Specify @option{-mapp-regs} to generate output using the global registers
11418 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
11421 To be fully SVR4 ABI compliant at the cost of some performance loss,
11422 specify @option{-mno-app-regs}. You should compile libraries and system
11423 software with this option.
11426 @itemx -mhard-float
11428 @opindex mhard-float
11429 Generate output containing floating point instructions. This is the
11433 @itemx -msoft-float
11435 @opindex msoft-float
11436 Generate output containing library calls for floating point.
11437 @strong{Warning:} the requisite libraries are not available for all SPARC
11438 targets. Normally the facilities of the machine's usual C compiler are
11439 used, but this cannot be done directly in cross-compilation. You must make
11440 your own arrangements to provide suitable library functions for
11441 cross-compilation. The embedded targets @samp{sparc-*-aout} and
11442 @samp{sparclite-*-*} do provide software floating point support.
11444 @option{-msoft-float} changes the calling convention in the output file;
11445 therefore, it is only useful if you compile @emph{all} of a program with
11446 this option. In particular, you need to compile @file{libgcc.a}, the
11447 library that comes with GCC, with @option{-msoft-float} in order for
11450 @item -mhard-quad-float
11451 @opindex mhard-quad-float
11452 Generate output containing quad-word (long double) floating point
11455 @item -msoft-quad-float
11456 @opindex msoft-quad-float
11457 Generate output containing library calls for quad-word (long double)
11458 floating point instructions. The functions called are those specified
11459 in the SPARC ABI@. This is the default.
11461 As of this writing, there are no SPARC implementations that have hardware
11462 support for the quad-word floating point instructions. They all invoke
11463 a trap handler for one of these instructions, and then the trap handler
11464 emulates the effect of the instruction. Because of the trap handler overhead,
11465 this is much slower than calling the ABI library routines. Thus the
11466 @option{-msoft-quad-float} option is the default.
11468 @item -mno-unaligned-doubles
11469 @itemx -munaligned-doubles
11470 @opindex mno-unaligned-doubles
11471 @opindex munaligned-doubles
11472 Assume that doubles have 8 byte alignment. This is the default.
11474 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
11475 alignment only if they are contained in another type, or if they have an
11476 absolute address. Otherwise, it assumes they have 4 byte alignment.
11477 Specifying this option avoids some rare compatibility problems with code
11478 generated by other compilers. It is not the default because it results
11479 in a performance loss, especially for floating point code.
11481 @item -mno-faster-structs
11482 @itemx -mfaster-structs
11483 @opindex mno-faster-structs
11484 @opindex mfaster-structs
11485 With @option{-mfaster-structs}, the compiler assumes that structures
11486 should have 8 byte alignment. This enables the use of pairs of
11487 @code{ldd} and @code{std} instructions for copies in structure
11488 assignment, in place of twice as many @code{ld} and @code{st} pairs.
11489 However, the use of this changed alignment directly violates the SPARC
11490 ABI@. Thus, it's intended only for use on targets where the developer
11491 acknowledges that their resulting code will not be directly in line with
11492 the rules of the ABI@.
11494 @item -mimpure-text
11495 @opindex mimpure-text
11496 @option{-mimpure-text}, used in addition to @option{-shared}, tells
11497 the compiler to not pass @option{-z text} to the linker when linking a
11498 shared object. Using this option, you can link position-dependent
11499 code into a shared object.
11501 @option{-mimpure-text} suppresses the ``relocations remain against
11502 allocatable but non-writable sections'' linker error message.
11503 However, the necessary relocations will trigger copy-on-write, and the
11504 shared object is not actually shared across processes. Instead of
11505 using @option{-mimpure-text}, you should compile all source code with
11506 @option{-fpic} or @option{-fPIC}.
11508 This option is only available on SunOS and Solaris.
11510 @item -mcpu=@var{cpu_type}
11512 Set the instruction set, register set, and instruction scheduling parameters
11513 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
11514 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
11515 @samp{f930}, @samp{f934}, @samp{hypersparc}, @samp{sparclite86x},
11516 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
11517 @samp{ultrasparc3}.
11519 Default instruction scheduling parameters are used for values that select
11520 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
11521 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
11523 Here is a list of each supported architecture and their supported
11528 v8: supersparc, hypersparc
11529 sparclite: f930, f934, sparclite86x
11531 v9: ultrasparc, ultrasparc3
11534 By default (unless configured otherwise), GCC generates code for the V7
11535 variant of the SPARC architecture. With @option{-mcpu=cypress}, the compiler
11536 additionally optimizes it for the Cypress CY7C602 chip, as used in the
11537 SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
11538 SPARCStation 1, 2, IPX etc.
11540 With @option{-mcpu=v8}, GCC generates code for the V8 variant of the SPARC
11541 architecture. The only difference from V7 code is that the compiler emits
11542 the integer multiply and integer divide instructions which exist in SPARC-V8
11543 but not in SPARC-V7. With @option{-mcpu=supersparc}, the compiler additionally
11544 optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and
11547 With @option{-mcpu=sparclite}, GCC generates code for the SPARClite variant of
11548 the SPARC architecture. This adds the integer multiply, integer divide step
11549 and scan (@code{ffs}) instructions which exist in SPARClite but not in SPARC-V7.
11550 With @option{-mcpu=f930}, the compiler additionally optimizes it for the
11551 Fujitsu MB86930 chip, which is the original SPARClite, with no FPU@. With
11552 @option{-mcpu=f934}, the compiler additionally optimizes it for the Fujitsu
11553 MB86934 chip, which is the more recent SPARClite with FPU@.
11555 With @option{-mcpu=sparclet}, GCC generates code for the SPARClet variant of
11556 the SPARC architecture. This adds the integer multiply, multiply/accumulate,
11557 integer divide step and scan (@code{ffs}) instructions which exist in SPARClet
11558 but not in SPARC-V7. With @option{-mcpu=tsc701}, the compiler additionally
11559 optimizes it for the TEMIC SPARClet chip.
11561 With @option{-mcpu=v9}, GCC generates code for the V9 variant of the SPARC
11562 architecture. This adds 64-bit integer and floating-point move instructions,
11563 3 additional floating-point condition code registers and conditional move
11564 instructions. With @option{-mcpu=ultrasparc}, the compiler additionally
11565 optimizes it for the Sun UltraSPARC I/II chips. With
11566 @option{-mcpu=ultrasparc3}, the compiler additionally optimizes it for the
11567 Sun UltraSPARC III chip.
11569 @item -mtune=@var{cpu_type}
11571 Set the instruction scheduling parameters for machine type
11572 @var{cpu_type}, but do not set the instruction set or register set that the
11573 option @option{-mcpu=@var{cpu_type}} would.
11575 The same values for @option{-mcpu=@var{cpu_type}} can be used for
11576 @option{-mtune=@var{cpu_type}}, but the only useful values are those
11577 that select a particular cpu implementation. Those are @samp{cypress},
11578 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
11579 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
11580 @samp{ultrasparc3}.
11585 @opindex mno-v8plus
11586 With @option{-mv8plus}, GCC generates code for the SPARC-V8+ ABI@. The
11587 difference from the V8 ABI is that the global and out registers are
11588 considered 64-bit wide. This is enabled by default on Solaris in 32-bit
11589 mode for all SPARC-V9 processors.
11595 With @option{-mvis}, GCC generates code that takes advantage of the UltraSPARC
11596 Visual Instruction Set extensions. The default is @option{-mno-vis}.
11599 These @samp{-m} options are supported in addition to the above
11600 on SPARC-V9 processors in 64-bit environments:
11603 @item -mlittle-endian
11604 @opindex mlittle-endian
11605 Generate code for a processor running in little-endian mode. It is only
11606 available for a few configurations and most notably not on Solaris and Linux.
11612 Generate code for a 32-bit or 64-bit environment.
11613 The 32-bit environment sets int, long and pointer to 32 bits.
11614 The 64-bit environment sets int to 32 bits and long and pointer
11617 @item -mcmodel=medlow
11618 @opindex mcmodel=medlow
11619 Generate code for the Medium/Low code model: 64-bit addresses, programs
11620 must be linked in the low 32 bits of memory. Programs can be statically
11621 or dynamically linked.
11623 @item -mcmodel=medmid
11624 @opindex mcmodel=medmid
11625 Generate code for the Medium/Middle code model: 64-bit addresses, programs
11626 must be linked in the low 44 bits of memory, the text and data segments must
11627 be less than 2GB in size and the data segment must be located within 2GB of
11630 @item -mcmodel=medany
11631 @opindex mcmodel=medany
11632 Generate code for the Medium/Anywhere code model: 64-bit addresses, programs
11633 may be linked anywhere in memory, the text and data segments must be less
11634 than 2GB in size and the data segment must be located within 2GB of the
11637 @item -mcmodel=embmedany
11638 @opindex mcmodel=embmedany
11639 Generate code for the Medium/Anywhere code model for embedded systems:
11640 64-bit addresses, the text and data segments must be less than 2GB in
11641 size, both starting anywhere in memory (determined at link time). The
11642 global register %g4 points to the base of the data segment. Programs
11643 are statically linked and PIC is not supported.
11646 @itemx -mno-stack-bias
11647 @opindex mstack-bias
11648 @opindex mno-stack-bias
11649 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
11650 frame pointer if present, are offset by @minus{}2047 which must be added back
11651 when making stack frame references. This is the default in 64-bit mode.
11652 Otherwise, assume no such offset is present.
11655 These switches are supported in addition to the above on Solaris:
11660 Add support for multithreading using the Solaris threads library. This
11661 option sets flags for both the preprocessor and linker. This option does
11662 not affect the thread safety of object code produced by the compiler or
11663 that of libraries supplied with it.
11667 Add support for multithreading using the POSIX threads library. This
11668 option sets flags for both the preprocessor and linker. This option does
11669 not affect the thread safety of object code produced by the compiler or
11670 that of libraries supplied with it.
11673 @node System V Options
11674 @subsection Options for System V
11676 These additional options are available on System V Release 4 for
11677 compatibility with other compilers on those systems:
11682 Create a shared object.
11683 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
11687 Identify the versions of each tool used by the compiler, in a
11688 @code{.ident} assembler directive in the output.
11692 Refrain from adding @code{.ident} directives to the output file (this is
11695 @item -YP,@var{dirs}
11697 Search the directories @var{dirs}, and no others, for libraries
11698 specified with @option{-l}.
11700 @item -Ym,@var{dir}
11702 Look in the directory @var{dir} to find the M4 preprocessor.
11703 The assembler uses this option.
11704 @c This is supposed to go with a -Yd for predefined M4 macro files, but
11705 @c the generic assembler that comes with Solaris takes just -Ym.
11708 @node TMS320C3x/C4x Options
11709 @subsection TMS320C3x/C4x Options
11710 @cindex TMS320C3x/C4x Options
11712 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
11716 @item -mcpu=@var{cpu_type}
11718 Set the instruction set, register set, and instruction scheduling
11719 parameters for machine type @var{cpu_type}. Supported values for
11720 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
11721 @samp{c44}. The default is @samp{c40} to generate code for the
11726 @itemx -msmall-memory
11728 @opindex mbig-memory
11730 @opindex msmall-memory
11732 Generates code for the big or small memory model. The small memory
11733 model assumed that all data fits into one 64K word page. At run-time
11734 the data page (DP) register must be set to point to the 64K page
11735 containing the .bss and .data program sections. The big memory model is
11736 the default and requires reloading of the DP register for every direct
11743 Allow (disallow) allocation of general integer operands into the block
11744 count register BK@.
11750 Enable (disable) generation of code using decrement and branch,
11751 DBcond(D), instructions. This is enabled by default for the C4x. To be
11752 on the safe side, this is disabled for the C3x, since the maximum
11753 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
11754 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
11755 that it can utilize the decrement and branch instruction, but will give
11756 up if there is more than one memory reference in the loop. Thus a loop
11757 where the loop counter is decremented can generate slightly more
11758 efficient code, in cases where the RPTB instruction cannot be utilized.
11760 @item -mdp-isr-reload
11762 @opindex mdp-isr-reload
11764 Force the DP register to be saved on entry to an interrupt service
11765 routine (ISR), reloaded to point to the data section, and restored on
11766 exit from the ISR@. This should not be required unless someone has
11767 violated the small memory model by modifying the DP register, say within
11774 For the C3x use the 24-bit MPYI instruction for integer multiplies
11775 instead of a library call to guarantee 32-bit results. Note that if one
11776 of the operands is a constant, then the multiplication will be performed
11777 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
11778 then squaring operations are performed inline instead of a library call.
11781 @itemx -mno-fast-fix
11783 @opindex mno-fast-fix
11784 The C3x/C4x FIX instruction to convert a floating point value to an
11785 integer value chooses the nearest integer less than or equal to the
11786 floating point value rather than to the nearest integer. Thus if the
11787 floating point number is negative, the result will be incorrectly
11788 truncated an additional code is necessary to detect and correct this
11789 case. This option can be used to disable generation of the additional
11790 code required to correct the result.
11796 Enable (disable) generation of repeat block sequences using the RPTB
11797 instruction for zero overhead looping. The RPTB construct is only used
11798 for innermost loops that do not call functions or jump across the loop
11799 boundaries. There is no advantage having nested RPTB loops due to the
11800 overhead required to save and restore the RC, RS, and RE registers.
11801 This is enabled by default with @option{-O2}.
11803 @item -mrpts=@var{count}
11807 Enable (disable) the use of the single instruction repeat instruction
11808 RPTS@. If a repeat block contains a single instruction, and the loop
11809 count can be guaranteed to be less than the value @var{count}, GCC will
11810 emit a RPTS instruction instead of a RPTB@. If no value is specified,
11811 then a RPTS will be emitted even if the loop count cannot be determined
11812 at compile time. Note that the repeated instruction following RPTS does
11813 not have to be reloaded from memory each iteration, thus freeing up the
11814 CPU buses for operands. However, since interrupts are blocked by this
11815 instruction, it is disabled by default.
11817 @item -mloop-unsigned
11818 @itemx -mno-loop-unsigned
11819 @opindex mloop-unsigned
11820 @opindex mno-loop-unsigned
11821 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
11822 is @math{2^{31} + 1} since these instructions test if the iteration count is
11823 negative to terminate the loop. If the iteration count is unsigned
11824 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
11825 exceeded. This switch allows an unsigned iteration count.
11829 Try to emit an assembler syntax that the TI assembler (asm30) is happy
11830 with. This also enforces compatibility with the API employed by the TI
11831 C3x C compiler. For example, long doubles are passed as structures
11832 rather than in floating point registers.
11838 Generate code that uses registers (stack) for passing arguments to functions.
11839 By default, arguments are passed in registers where possible rather
11840 than by pushing arguments on to the stack.
11842 @item -mparallel-insns
11843 @itemx -mno-parallel-insns
11844 @opindex mparallel-insns
11845 @opindex mno-parallel-insns
11846 Allow the generation of parallel instructions. This is enabled by
11847 default with @option{-O2}.
11849 @item -mparallel-mpy
11850 @itemx -mno-parallel-mpy
11851 @opindex mparallel-mpy
11852 @opindex mno-parallel-mpy
11853 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
11854 provided @option{-mparallel-insns} is also specified. These instructions have
11855 tight register constraints which can pessimize the code generation
11856 of large functions.
11861 @subsection V850 Options
11862 @cindex V850 Options
11864 These @samp{-m} options are defined for V850 implementations:
11868 @itemx -mno-long-calls
11869 @opindex mlong-calls
11870 @opindex mno-long-calls
11871 Treat all calls as being far away (near). If calls are assumed to be
11872 far away, the compiler will always load the functions address up into a
11873 register, and call indirect through the pointer.
11879 Do not optimize (do optimize) basic blocks that use the same index
11880 pointer 4 or more times to copy pointer into the @code{ep} register, and
11881 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
11882 option is on by default if you optimize.
11884 @item -mno-prolog-function
11885 @itemx -mprolog-function
11886 @opindex mno-prolog-function
11887 @opindex mprolog-function
11888 Do not use (do use) external functions to save and restore registers
11889 at the prologue and epilogue of a function. The external functions
11890 are slower, but use less code space if more than one function saves
11891 the same number of registers. The @option{-mprolog-function} option
11892 is on by default if you optimize.
11896 Try to make the code as small as possible. At present, this just turns
11897 on the @option{-mep} and @option{-mprolog-function} options.
11899 @item -mtda=@var{n}
11901 Put static or global variables whose size is @var{n} bytes or less into
11902 the tiny data area that register @code{ep} points to. The tiny data
11903 area can hold up to 256 bytes in total (128 bytes for byte references).
11905 @item -msda=@var{n}
11907 Put static or global variables whose size is @var{n} bytes or less into
11908 the small data area that register @code{gp} points to. The small data
11909 area can hold up to 64 kilobytes.
11911 @item -mzda=@var{n}
11913 Put static or global variables whose size is @var{n} bytes or less into
11914 the first 32 kilobytes of memory.
11918 Specify that the target processor is the V850.
11921 @opindex mbig-switch
11922 Generate code suitable for big switch tables. Use this option only if
11923 the assembler/linker complain about out of range branches within a switch
11928 This option will cause r2 and r5 to be used in the code generated by
11929 the compiler. This setting is the default.
11931 @item -mno-app-regs
11932 @opindex mno-app-regs
11933 This option will cause r2 and r5 to be treated as fixed registers.
11937 Specify that the target processor is the V850E1. The preprocessor
11938 constants @samp{__v850e1__} and @samp{__v850e__} will be defined if
11939 this option is used.
11943 Specify that the target processor is the V850E@. The preprocessor
11944 constant @samp{__v850e__} will be defined if this option is used.
11946 If neither @option{-mv850} nor @option{-mv850e} nor @option{-mv850e1}
11947 are defined then a default target processor will be chosen and the
11948 relevant @samp{__v850*__} preprocessor constant will be defined.
11950 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
11951 defined, regardless of which processor variant is the target.
11953 @item -mdisable-callt
11954 @opindex mdisable-callt
11955 This option will suppress generation of the CALLT instruction for the
11956 v850e and v850e1 flavors of the v850 architecture. The default is
11957 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
11962 @subsection VAX Options
11963 @cindex VAX options
11965 These @samp{-m} options are defined for the VAX:
11970 Do not output certain jump instructions (@code{aobleq} and so on)
11971 that the Unix assembler for the VAX cannot handle across long
11976 Do output those jump instructions, on the assumption that you
11977 will assemble with the GNU assembler.
11981 Output code for g-format floating point numbers instead of d-format.
11984 @node x86-64 Options
11985 @subsection x86-64 Options
11986 @cindex x86-64 options
11988 These are listed under @xref{i386 and x86-64 Options}.
11990 @node Xstormy16 Options
11991 @subsection Xstormy16 Options
11992 @cindex Xstormy16 Options
11994 These options are defined for Xstormy16:
11999 Choose startup files and linker script suitable for the simulator.
12002 @node Xtensa Options
12003 @subsection Xtensa Options
12004 @cindex Xtensa Options
12006 These options are supported for Xtensa targets:
12010 @itemx -mno-const16
12012 @opindex mno-const16
12013 Enable or disable use of @code{CONST16} instructions for loading
12014 constant values. The @code{CONST16} instruction is currently not a
12015 standard option from Tensilica. When enabled, @code{CONST16}
12016 instructions are always used in place of the standard @code{L32R}
12017 instructions. The use of @code{CONST16} is enabled by default only if
12018 the @code{L32R} instruction is not available.
12021 @itemx -mno-fused-madd
12022 @opindex mfused-madd
12023 @opindex mno-fused-madd
12024 Enable or disable use of fused multiply/add and multiply/subtract
12025 instructions in the floating-point option. This has no effect if the
12026 floating-point option is not also enabled. Disabling fused multiply/add
12027 and multiply/subtract instructions forces the compiler to use separate
12028 instructions for the multiply and add/subtract operations. This may be
12029 desirable in some cases where strict IEEE 754-compliant results are
12030 required: the fused multiply add/subtract instructions do not round the
12031 intermediate result, thereby producing results with @emph{more} bits of
12032 precision than specified by the IEEE standard. Disabling fused multiply
12033 add/subtract instructions also ensures that the program output is not
12034 sensitive to the compiler's ability to combine multiply and add/subtract
12037 @item -mtext-section-literals
12038 @itemx -mno-text-section-literals
12039 @opindex mtext-section-literals
12040 @opindex mno-text-section-literals
12041 Control the treatment of literal pools. The default is
12042 @option{-mno-text-section-literals}, which places literals in a separate
12043 section in the output file. This allows the literal pool to be placed
12044 in a data RAM/ROM, and it also allows the linker to combine literal
12045 pools from separate object files to remove redundant literals and
12046 improve code size. With @option{-mtext-section-literals}, the literals
12047 are interspersed in the text section in order to keep them as close as
12048 possible to their references. This may be necessary for large assembly
12051 @item -mtarget-align
12052 @itemx -mno-target-align
12053 @opindex mtarget-align
12054 @opindex mno-target-align
12055 When this option is enabled, GCC instructs the assembler to
12056 automatically align instructions to reduce branch penalties at the
12057 expense of some code density. The assembler attempts to widen density
12058 instructions to align branch targets and the instructions following call
12059 instructions. If there are not enough preceding safe density
12060 instructions to align a target, no widening will be performed. The
12061 default is @option{-mtarget-align}. These options do not affect the
12062 treatment of auto-aligned instructions like @code{LOOP}, which the
12063 assembler will always align, either by widening density instructions or
12064 by inserting no-op instructions.
12067 @itemx -mno-longcalls
12068 @opindex mlongcalls
12069 @opindex mno-longcalls
12070 When this option is enabled, GCC instructs the assembler to translate
12071 direct calls to indirect calls unless it can determine that the target
12072 of a direct call is in the range allowed by the call instruction. This
12073 translation typically occurs for calls to functions in other source
12074 files. Specifically, the assembler translates a direct @code{CALL}
12075 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
12076 The default is @option{-mno-longcalls}. This option should be used in
12077 programs where the call target can potentially be out of range. This
12078 option is implemented in the assembler, not the compiler, so the
12079 assembly code generated by GCC will still show direct call
12080 instructions---look at the disassembled object code to see the actual
12081 instructions. Note that the assembler will use an indirect call for
12082 every cross-file call, not just those that really will be out of range.
12085 @node zSeries Options
12086 @subsection zSeries Options
12087 @cindex zSeries options
12089 These are listed under @xref{S/390 and zSeries Options}.
12091 @node Code Gen Options
12092 @section Options for Code Generation Conventions
12093 @cindex code generation conventions
12094 @cindex options, code generation
12095 @cindex run-time options
12097 These machine-independent options control the interface conventions
12098 used in code generation.
12100 Most of them have both positive and negative forms; the negative form
12101 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
12102 one of the forms is listed---the one which is not the default. You
12103 can figure out the other form by either removing @samp{no-} or adding
12107 @item -fbounds-check
12108 @opindex fbounds-check
12109 For front-ends that support it, generate additional code to check that
12110 indices used to access arrays are within the declared range. This is
12111 currently only supported by the Java and Fortran 77 front-ends, where
12112 this option defaults to true and false respectively.
12116 This option generates traps for signed overflow on addition, subtraction,
12117 multiplication operations.
12121 This option instructs the compiler to assume that signed arithmetic
12122 overflow of addition, subtraction and multiplication wraps around
12123 using twos-complement representation. This flag enables some optimizations
12124 and disables other. This option is enabled by default for the Java
12125 front-end, as required by the Java language specification.
12128 @opindex fexceptions
12129 Enable exception handling. Generates extra code needed to propagate
12130 exceptions. For some targets, this implies GCC will generate frame
12131 unwind information for all functions, which can produce significant data
12132 size overhead, although it does not affect execution. If you do not
12133 specify this option, GCC will enable it by default for languages like
12134 C++ which normally require exception handling, and disable it for
12135 languages like C that do not normally require it. However, you may need
12136 to enable this option when compiling C code that needs to interoperate
12137 properly with exception handlers written in C++. You may also wish to
12138 disable this option if you are compiling older C++ programs that don't
12139 use exception handling.
12141 @item -fnon-call-exceptions
12142 @opindex fnon-call-exceptions
12143 Generate code that allows trapping instructions to throw exceptions.
12144 Note that this requires platform-specific runtime support that does
12145 not exist everywhere. Moreover, it only allows @emph{trapping}
12146 instructions to throw exceptions, i.e.@: memory references or floating
12147 point instructions. It does not allow exceptions to be thrown from
12148 arbitrary signal handlers such as @code{SIGALRM}.
12150 @item -funwind-tables
12151 @opindex funwind-tables
12152 Similar to @option{-fexceptions}, except that it will just generate any needed
12153 static data, but will not affect the generated code in any other way.
12154 You will normally not enable this option; instead, a language processor
12155 that needs this handling would enable it on your behalf.
12157 @item -fasynchronous-unwind-tables
12158 @opindex fasynchronous-unwind-tables
12159 Generate unwind table in dwarf2 format, if supported by target machine. The
12160 table is exact at each instruction boundary, so it can be used for stack
12161 unwinding from asynchronous events (such as debugger or garbage collector).
12163 @item -fpcc-struct-return
12164 @opindex fpcc-struct-return
12165 Return ``short'' @code{struct} and @code{union} values in memory like
12166 longer ones, rather than in registers. This convention is less
12167 efficient, but it has the advantage of allowing intercallability between
12168 GCC-compiled files and files compiled with other compilers, particularly
12169 the Portable C Compiler (pcc).
12171 The precise convention for returning structures in memory depends
12172 on the target configuration macros.
12174 Short structures and unions are those whose size and alignment match
12175 that of some integer type.
12177 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
12178 switch is not binary compatible with code compiled with the
12179 @option{-freg-struct-return} switch.
12180 Use it to conform to a non-default application binary interface.
12182 @item -freg-struct-return
12183 @opindex freg-struct-return
12184 Return @code{struct} and @code{union} values in registers when possible.
12185 This is more efficient for small structures than
12186 @option{-fpcc-struct-return}.
12188 If you specify neither @option{-fpcc-struct-return} nor
12189 @option{-freg-struct-return}, GCC defaults to whichever convention is
12190 standard for the target. If there is no standard convention, GCC
12191 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
12192 the principal compiler. In those cases, we can choose the standard, and
12193 we chose the more efficient register return alternative.
12195 @strong{Warning:} code compiled with the @option{-freg-struct-return}
12196 switch is not binary compatible with code compiled with the
12197 @option{-fpcc-struct-return} switch.
12198 Use it to conform to a non-default application binary interface.
12200 @item -fshort-enums
12201 @opindex fshort-enums
12202 Allocate to an @code{enum} type only as many bytes as it needs for the
12203 declared range of possible values. Specifically, the @code{enum} type
12204 will be equivalent to the smallest integer type which has enough room.
12206 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
12207 code that is not binary compatible with code generated without that switch.
12208 Use it to conform to a non-default application binary interface.
12210 @item -fshort-double
12211 @opindex fshort-double
12212 Use the same size for @code{double} as for @code{float}.
12214 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
12215 code that is not binary compatible with code generated without that switch.
12216 Use it to conform to a non-default application binary interface.
12218 @item -fshort-wchar
12219 @opindex fshort-wchar
12220 Override the underlying type for @samp{wchar_t} to be @samp{short
12221 unsigned int} instead of the default for the target. This option is
12222 useful for building programs to run under WINE@.
12224 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
12225 code that is not binary compatible with code generated without that switch.
12226 Use it to conform to a non-default application binary interface.
12228 @item -fshared-data
12229 @opindex fshared-data
12230 Requests that the data and non-@code{const} variables of this
12231 compilation be shared data rather than private data. The distinction
12232 makes sense only on certain operating systems, where shared data is
12233 shared between processes running the same program, while private data
12234 exists in one copy per process.
12237 @opindex fno-common
12238 In C, allocate even uninitialized global variables in the data section of the
12239 object file, rather than generating them as common blocks. This has the
12240 effect that if the same variable is declared (without @code{extern}) in
12241 two different compilations, you will get an error when you link them.
12242 The only reason this might be useful is if you wish to verify that the
12243 program will work on other systems which always work this way.
12247 Ignore the @samp{#ident} directive.
12249 @item -finhibit-size-directive
12250 @opindex finhibit-size-directive
12251 Don't output a @code{.size} assembler directive, or anything else that
12252 would cause trouble if the function is split in the middle, and the
12253 two halves are placed at locations far apart in memory. This option is
12254 used when compiling @file{crtstuff.c}; you should not need to use it
12257 @item -fverbose-asm
12258 @opindex fverbose-asm
12259 Put extra commentary information in the generated assembly code to
12260 make it more readable. This option is generally only of use to those
12261 who actually need to read the generated assembly code (perhaps while
12262 debugging the compiler itself).
12264 @option{-fno-verbose-asm}, the default, causes the
12265 extra information to be omitted and is useful when comparing two assembler
12270 @cindex global offset table
12272 Generate position-independent code (PIC) suitable for use in a shared
12273 library, if supported for the target machine. Such code accesses all
12274 constant addresses through a global offset table (GOT)@. The dynamic
12275 loader resolves the GOT entries when the program starts (the dynamic
12276 loader is not part of GCC; it is part of the operating system). If
12277 the GOT size for the linked executable exceeds a machine-specific
12278 maximum size, you get an error message from the linker indicating that
12279 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
12280 instead. (These maximums are 8k on the SPARC and 32k
12281 on the m68k and RS/6000. The 386 has no such limit.)
12283 Position-independent code requires special support, and therefore works
12284 only on certain machines. For the 386, GCC supports PIC for System V
12285 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
12286 position-independent.
12290 If supported for the target machine, emit position-independent code,
12291 suitable for dynamic linking and avoiding any limit on the size of the
12292 global offset table. This option makes a difference on the m68k,
12293 PowerPC and SPARC@.
12295 Position-independent code requires special support, and therefore works
12296 only on certain machines.
12302 These options are similar to @option{-fpic} and @option{-fPIC}, but
12303 generated position independent code can be only linked into executables.
12304 Usually these options are used when @option{-pie} GCC option will be
12305 used during linking.
12307 @item -ffixed-@var{reg}
12309 Treat the register named @var{reg} as a fixed register; generated code
12310 should never refer to it (except perhaps as a stack pointer, frame
12311 pointer or in some other fixed role).
12313 @var{reg} must be the name of a register. The register names accepted
12314 are machine-specific and are defined in the @code{REGISTER_NAMES}
12315 macro in the machine description macro file.
12317 This flag does not have a negative form, because it specifies a
12320 @item -fcall-used-@var{reg}
12321 @opindex fcall-used
12322 Treat the register named @var{reg} as an allocable register that is
12323 clobbered by function calls. It may be allocated for temporaries or
12324 variables that do not live across a call. Functions compiled this way
12325 will not save and restore the register @var{reg}.
12327 It is an error to used this flag with the frame pointer or stack pointer.
12328 Use of this flag for other registers that have fixed pervasive roles in
12329 the machine's execution model will produce disastrous results.
12331 This flag does not have a negative form, because it specifies a
12334 @item -fcall-saved-@var{reg}
12335 @opindex fcall-saved
12336 Treat the register named @var{reg} as an allocable register saved by
12337 functions. It may be allocated even for temporaries or variables that
12338 live across a call. Functions compiled this way will save and restore
12339 the register @var{reg} if they use it.
12341 It is an error to used this flag with the frame pointer or stack pointer.
12342 Use of this flag for other registers that have fixed pervasive roles in
12343 the machine's execution model will produce disastrous results.
12345 A different sort of disaster will result from the use of this flag for
12346 a register in which function values may be returned.
12348 This flag does not have a negative form, because it specifies a
12351 @item -fpack-struct[=@var{n}]
12352 @opindex fpack-struct
12353 Without a value specified, pack all structure members together without
12354 holes. When a value is specified (which must be a small power of two), pack
12355 structure members according to this value, representing the maximum
12356 alignment (that is, objects with default alignment requirements larger than
12357 this will be output potentially unaligned at the next fitting location.
12359 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
12360 code that is not binary compatible with code generated without that switch.
12361 Additionally, it makes the code suboptimal.
12362 Use it to conform to a non-default application binary interface.
12364 @item -finstrument-functions
12365 @opindex finstrument-functions
12366 Generate instrumentation calls for entry and exit to functions. Just
12367 after function entry and just before function exit, the following
12368 profiling functions will be called with the address of the current
12369 function and its call site. (On some platforms,
12370 @code{__builtin_return_address} does not work beyond the current
12371 function, so the call site information may not be available to the
12372 profiling functions otherwise.)
12375 void __cyg_profile_func_enter (void *this_fn,
12377 void __cyg_profile_func_exit (void *this_fn,
12381 The first argument is the address of the start of the current function,
12382 which may be looked up exactly in the symbol table.
12384 This instrumentation is also done for functions expanded inline in other
12385 functions. The profiling calls will indicate where, conceptually, the
12386 inline function is entered and exited. This means that addressable
12387 versions of such functions must be available. If all your uses of a
12388 function are expanded inline, this may mean an additional expansion of
12389 code size. If you use @samp{extern inline} in your C code, an
12390 addressable version of such functions must be provided. (This is
12391 normally the case anyways, but if you get lucky and the optimizer always
12392 expands the functions inline, you might have gotten away without
12393 providing static copies.)
12395 A function may be given the attribute @code{no_instrument_function}, in
12396 which case this instrumentation will not be done. This can be used, for
12397 example, for the profiling functions listed above, high-priority
12398 interrupt routines, and any functions from which the profiling functions
12399 cannot safely be called (perhaps signal handlers, if the profiling
12400 routines generate output or allocate memory).
12402 @item -fstack-check
12403 @opindex fstack-check
12404 Generate code to verify that you do not go beyond the boundary of the
12405 stack. You should specify this flag if you are running in an
12406 environment with multiple threads, but only rarely need to specify it in
12407 a single-threaded environment since stack overflow is automatically
12408 detected on nearly all systems if there is only one stack.
12410 Note that this switch does not actually cause checking to be done; the
12411 operating system must do that. The switch causes generation of code
12412 to ensure that the operating system sees the stack being extended.
12414 @item -fstack-limit-register=@var{reg}
12415 @itemx -fstack-limit-symbol=@var{sym}
12416 @itemx -fno-stack-limit
12417 @opindex fstack-limit-register
12418 @opindex fstack-limit-symbol
12419 @opindex fno-stack-limit
12420 Generate code to ensure that the stack does not grow beyond a certain value,
12421 either the value of a register or the address of a symbol. If the stack
12422 would grow beyond the value, a signal is raised. For most targets,
12423 the signal is raised before the stack overruns the boundary, so
12424 it is possible to catch the signal without taking special precautions.
12426 For instance, if the stack starts at absolute address @samp{0x80000000}
12427 and grows downwards, you can use the flags
12428 @option{-fstack-limit-symbol=__stack_limit} and
12429 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
12430 of 128KB@. Note that this may only work with the GNU linker.
12432 @cindex aliasing of parameters
12433 @cindex parameters, aliased
12434 @item -fargument-alias
12435 @itemx -fargument-noalias
12436 @itemx -fargument-noalias-global
12437 @opindex fargument-alias
12438 @opindex fargument-noalias
12439 @opindex fargument-noalias-global
12440 Specify the possible relationships among parameters and between
12441 parameters and global data.
12443 @option{-fargument-alias} specifies that arguments (parameters) may
12444 alias each other and may alias global storage.@*
12445 @option{-fargument-noalias} specifies that arguments do not alias
12446 each other, but may alias global storage.@*
12447 @option{-fargument-noalias-global} specifies that arguments do not
12448 alias each other and do not alias global storage.
12450 Each language will automatically use whatever option is required by
12451 the language standard. You should not need to use these options yourself.
12453 @item -fleading-underscore
12454 @opindex fleading-underscore
12455 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
12456 change the way C symbols are represented in the object file. One use
12457 is to help link with legacy assembly code.
12459 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
12460 generate code that is not binary compatible with code generated without that
12461 switch. Use it to conform to a non-default application binary interface.
12462 Not all targets provide complete support for this switch.
12464 @item -ftls-model=@var{model}
12465 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
12466 The @var{model} argument should be one of @code{global-dynamic},
12467 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
12469 The default without @option{-fpic} is @code{initial-exec}; with
12470 @option{-fpic} the default is @code{global-dynamic}.
12472 @item -fvisibility=@var{default|internal|hidden|protected}
12473 @opindex fvisibility
12474 Set the default ELF image symbol visibility to the specified option---all
12475 symbols will be marked with this unless overridden within the code.
12476 Using this feature can very substantially improve linking and
12477 load times of shared object libraries, produce more optimized
12478 code, provide near-perfect API export and prevent symbol clashes.
12479 It is @strong{strongly} recommended that you use this in any shared objects
12482 Despite the nomenclature, @code{default} always means public ie;
12483 available to be linked against from outside the shared object.
12484 @code{protected} and @code{internal} are pretty useless in real-world
12485 usage so the only other commonly used option will be @code{hidden}.
12486 The default if @option{-fvisibility} isn't specified is
12487 @code{default}, i.e., make every
12488 symbol public---this causes the same behavior as previous versions of
12491 A good explanation of the benefits offered by ensuring ELF
12492 symbols have the correct visibility is given by ``How To Write
12493 Shared Libraries'' by Ulrich Drepper (which can be found at
12494 @w{@uref{http://people.redhat.com/~drepper/}})---however a superior
12495 solution made possible by this option to marking things hidden when
12496 the default is public is to make the default hidden and mark things
12497 public. This is the norm with DLL's on Windows and with @option{-fvisibility=hidden}
12498 and @code{__attribute__ ((visibility("default")))} instead of
12499 @code{__declspec(dllexport)} you get almost identical semantics with
12500 identical syntax. This is a great boon to those working with
12501 cross-platform projects.
12503 For those adding visibility support to existing code, you may find
12504 @samp{#pragma GCC visibility} of use. This works by you enclosing
12505 the declarations you wish to set visibility for with (for example)
12506 @samp{#pragma GCC visibility push(hidden)} and
12507 @samp{#pragma GCC visibility pop}. These can be nested up to sixteen
12508 times. Bear in mind that symbol visibility should be viewed @strong{as
12509 part of the API interface contract} and thus all new code should
12510 always specify visibility when it is not the default ie; declarations
12511 only for use within the local DSO should @strong{always} be marked explicitly
12512 as hidden as so to avoid PLT indirection overheads---making this
12513 abundantly clear also aids readability and self-documentation of the code.
12514 Note that due to ISO C++ specification requirements, operator new and
12515 operator delete must always be of default visibility.
12517 An overview of these techniques, their benefits and how to use them
12518 is at @w{@uref{http://gcc.gnu.org/wiki/Visibility}}.
12524 @node Environment Variables
12525 @section Environment Variables Affecting GCC
12526 @cindex environment variables
12528 @c man begin ENVIRONMENT
12529 This section describes several environment variables that affect how GCC
12530 operates. Some of them work by specifying directories or prefixes to use
12531 when searching for various kinds of files. Some are used to specify other
12532 aspects of the compilation environment.
12534 Note that you can also specify places to search using options such as
12535 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
12536 take precedence over places specified using environment variables, which
12537 in turn take precedence over those specified by the configuration of GCC@.
12538 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
12539 GNU Compiler Collection (GCC) Internals}.
12544 @c @itemx LC_COLLATE
12546 @c @itemx LC_MONETARY
12547 @c @itemx LC_NUMERIC
12552 @c @findex LC_COLLATE
12553 @findex LC_MESSAGES
12554 @c @findex LC_MONETARY
12555 @c @findex LC_NUMERIC
12559 These environment variables control the way that GCC uses
12560 localization information that allow GCC to work with different
12561 national conventions. GCC inspects the locale categories
12562 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
12563 so. These locale categories can be set to any value supported by your
12564 installation. A typical value is @samp{en_GB.UTF-8} for English in the United
12565 Kingdom encoded in UTF-8.
12567 The @env{LC_CTYPE} environment variable specifies character
12568 classification. GCC uses it to determine the character boundaries in
12569 a string; this is needed for some multibyte encodings that contain quote
12570 and escape characters that would otherwise be interpreted as a string
12573 The @env{LC_MESSAGES} environment variable specifies the language to
12574 use in diagnostic messages.
12576 If the @env{LC_ALL} environment variable is set, it overrides the value
12577 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
12578 and @env{LC_MESSAGES} default to the value of the @env{LANG}
12579 environment variable. If none of these variables are set, GCC
12580 defaults to traditional C English behavior.
12584 If @env{TMPDIR} is set, it specifies the directory to use for temporary
12585 files. GCC uses temporary files to hold the output of one stage of
12586 compilation which is to be used as input to the next stage: for example,
12587 the output of the preprocessor, which is the input to the compiler
12590 @item GCC_EXEC_PREFIX
12591 @findex GCC_EXEC_PREFIX
12592 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
12593 names of the subprograms executed by the compiler. No slash is added
12594 when this prefix is combined with the name of a subprogram, but you can
12595 specify a prefix that ends with a slash if you wish.
12597 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
12598 an appropriate prefix to use based on the pathname it was invoked with.
12600 If GCC cannot find the subprogram using the specified prefix, it
12601 tries looking in the usual places for the subprogram.
12603 The default value of @env{GCC_EXEC_PREFIX} is
12604 @file{@var{prefix}/lib/gcc/} where @var{prefix} is the value
12605 of @code{prefix} when you ran the @file{configure} script.
12607 Other prefixes specified with @option{-B} take precedence over this prefix.
12609 This prefix is also used for finding files such as @file{crt0.o} that are
12612 In addition, the prefix is used in an unusual way in finding the
12613 directories to search for header files. For each of the standard
12614 directories whose name normally begins with @samp{/usr/local/lib/gcc}
12615 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
12616 replacing that beginning with the specified prefix to produce an
12617 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
12618 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
12619 These alternate directories are searched first; the standard directories
12622 @item COMPILER_PATH
12623 @findex COMPILER_PATH
12624 The value of @env{COMPILER_PATH} is a colon-separated list of
12625 directories, much like @env{PATH}. GCC tries the directories thus
12626 specified when searching for subprograms, if it can't find the
12627 subprograms using @env{GCC_EXEC_PREFIX}.
12630 @findex LIBRARY_PATH
12631 The value of @env{LIBRARY_PATH} is a colon-separated list of
12632 directories, much like @env{PATH}. When configured as a native compiler,
12633 GCC tries the directories thus specified when searching for special
12634 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
12635 using GCC also uses these directories when searching for ordinary
12636 libraries for the @option{-l} option (but directories specified with
12637 @option{-L} come first).
12641 @cindex locale definition
12642 This variable is used to pass locale information to the compiler. One way in
12643 which this information is used is to determine the character set to be used
12644 when character literals, string literals and comments are parsed in C and C++.
12645 When the compiler is configured to allow multibyte characters,
12646 the following values for @env{LANG} are recognized:
12650 Recognize JIS characters.
12652 Recognize SJIS characters.
12654 Recognize EUCJP characters.
12657 If @env{LANG} is not defined, or if it has some other value, then the
12658 compiler will use mblen and mbtowc as defined by the default locale to
12659 recognize and translate multibyte characters.
12663 Some additional environments variables affect the behavior of the
12666 @include cppenv.texi
12670 @node Precompiled Headers
12671 @section Using Precompiled Headers
12672 @cindex precompiled headers
12673 @cindex speed of compilation
12675 Often large projects have many header files that are included in every
12676 source file. The time the compiler takes to process these header files
12677 over and over again can account for nearly all of the time required to
12678 build the project. To make builds faster, GCC allows users to
12679 `precompile' a header file; then, if builds can use the precompiled
12680 header file they will be much faster.
12682 @strong{Caution:} There are a few known situations where GCC will
12683 crash when trying to use a precompiled header. If you have trouble
12684 with a precompiled header, you should remove the precompiled header
12685 and compile without it. In addition, please use GCC's on-line
12686 defect-tracking system to report any problems you encounter with
12687 precompiled headers. @xref{Bugs}.
12689 To create a precompiled header file, simply compile it as you would any
12690 other file, if necessary using the @option{-x} option to make the driver
12691 treat it as a C or C++ header file. You will probably want to use a
12692 tool like @command{make} to keep the precompiled header up-to-date when
12693 the headers it contains change.
12695 A precompiled header file will be searched for when @code{#include} is
12696 seen in the compilation. As it searches for the included file
12697 (@pxref{Search Path,,Search Path,cpp,The C Preprocessor}) the
12698 compiler looks for a precompiled header in each directory just before it
12699 looks for the include file in that directory. The name searched for is
12700 the name specified in the @code{#include} with @samp{.gch} appended. If
12701 the precompiled header file can't be used, it is ignored.
12703 For instance, if you have @code{#include "all.h"}, and you have
12704 @file{all.h.gch} in the same directory as @file{all.h}, then the
12705 precompiled header file will be used if possible, and the original
12706 header will be used otherwise.
12708 Alternatively, you might decide to put the precompiled header file in a
12709 directory and use @option{-I} to ensure that directory is searched
12710 before (or instead of) the directory containing the original header.
12711 Then, if you want to check that the precompiled header file is always
12712 used, you can put a file of the same name as the original header in this
12713 directory containing an @code{#error} command.
12715 This also works with @option{-include}. So yet another way to use
12716 precompiled headers, good for projects not designed with precompiled
12717 header files in mind, is to simply take most of the header files used by
12718 a project, include them from another header file, precompile that header
12719 file, and @option{-include} the precompiled header. If the header files
12720 have guards against multiple inclusion, they will be skipped because
12721 they've already been included (in the precompiled header).
12723 If you need to precompile the same header file for different
12724 languages, targets, or compiler options, you can instead make a
12725 @emph{directory} named like @file{all.h.gch}, and put each precompiled
12726 header in the directory, perhaps using @option{-o}. It doesn't matter
12727 what you call the files in the directory, every precompiled header in
12728 the directory will be considered. The first precompiled header
12729 encountered in the directory that is valid for this compilation will
12730 be used; they're searched in no particular order.
12732 There are many other possibilities, limited only by your imagination,
12733 good sense, and the constraints of your build system.
12735 A precompiled header file can be used only when these conditions apply:
12739 Only one precompiled header can be used in a particular compilation.
12742 A precompiled header can't be used once the first C token is seen. You
12743 can have preprocessor directives before a precompiled header; you can
12744 even include a precompiled header from inside another header, so long as
12745 there are no C tokens before the @code{#include}.
12748 The precompiled header file must be produced for the same language as
12749 the current compilation. You can't use a C precompiled header for a C++
12753 The precompiled header file must be produced by the same compiler
12754 version and configuration as the current compilation is using.
12755 The easiest way to guarantee this is to use the same compiler binary
12756 for creating and using precompiled headers.
12759 Any macros defined before the precompiled header is included must
12760 either be defined in the same way as when the precompiled header was
12761 generated, or must not affect the precompiled header, which usually
12762 means that the they don't appear in the precompiled header at all.
12764 The @option{-D} option is one way to define a macro before a
12765 precompiled header is included; using a @code{#define} can also do it.
12766 There are also some options that define macros implicitly, like
12767 @option{-O} and @option{-Wdeprecated}; the same rule applies to macros
12770 @item If debugging information is output when using the precompiled
12771 header, using @option{-g} or similar, the same kind of debugging information
12772 must have been output when building the precompiled header. However,
12773 a precompiled header built using @option{-g} can be used in a compilation
12774 when no debugging information is being output.
12776 @item The same @option{-m} options must generally be used when building
12777 and using the precompiled header. @xref{Submodel Options},
12778 for any cases where this rule is relaxed.
12780 @item Each of the following options must be the same when building and using
12781 the precompiled header:
12783 @gccoptlist{-fexceptions -funit-at-a-time}
12786 Some other command-line options starting with @option{-f},
12787 @option{-p}, or @option{-O} must be defined in the same way as when
12788 the precompiled header was generated. At present, it's not clear
12789 which options are safe to change and which are not; the safest choice
12790 is to use exactly the same options when generating and using the
12791 precompiled header. The following are known to be safe:
12793 @gccoptlist{-fpreprocessed -pedantic-errors}
12797 For all of these except the last, the compiler will automatically
12798 ignore the precompiled header if the conditions aren't met. If you
12799 find an option combination that doesn't work and doesn't cause the
12800 precompiled header to be ignored, please consider filing a bug report,
12803 If you do use differing options when generating and using the
12804 precompiled header, the actual behavior will be a mixture of the
12805 behavior for the options. For instance, if you use @option{-g} to
12806 generate the precompiled header but not when using it, you may or may
12807 not get debugging information for routines in the precompiled header.
12809 @node Running Protoize
12810 @section Running Protoize
12812 The program @code{protoize} is an optional part of GCC@. You can use
12813 it to add prototypes to a program, thus converting the program to ISO
12814 C in one respect. The companion program @code{unprotoize} does the
12815 reverse: it removes argument types from any prototypes that are found.
12817 When you run these programs, you must specify a set of source files as
12818 command line arguments. The conversion programs start out by compiling
12819 these files to see what functions they define. The information gathered
12820 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
12822 After scanning comes actual conversion. The specified files are all
12823 eligible to be converted; any files they include (whether sources or
12824 just headers) are eligible as well.
12826 But not all the eligible files are converted. By default,
12827 @code{protoize} and @code{unprotoize} convert only source and header
12828 files in the current directory. You can specify additional directories
12829 whose files should be converted with the @option{-d @var{directory}}
12830 option. You can also specify particular files to exclude with the
12831 @option{-x @var{file}} option. A file is converted if it is eligible, its
12832 directory name matches one of the specified directory names, and its
12833 name within the directory has not been excluded.
12835 Basic conversion with @code{protoize} consists of rewriting most
12836 function definitions and function declarations to specify the types of
12837 the arguments. The only ones not rewritten are those for varargs
12840 @code{protoize} optionally inserts prototype declarations at the
12841 beginning of the source file, to make them available for any calls that
12842 precede the function's definition. Or it can insert prototype
12843 declarations with block scope in the blocks where undeclared functions
12846 Basic conversion with @code{unprotoize} consists of rewriting most
12847 function declarations to remove any argument types, and rewriting
12848 function definitions to the old-style pre-ISO form.
12850 Both conversion programs print a warning for any function declaration or
12851 definition that they can't convert. You can suppress these warnings
12854 The output from @code{protoize} or @code{unprotoize} replaces the
12855 original source file. The original file is renamed to a name ending
12856 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
12857 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
12858 for DOS) file already exists, then the source file is simply discarded.
12860 @code{protoize} and @code{unprotoize} both depend on GCC itself to
12861 scan the program and collect information about the functions it uses.
12862 So neither of these programs will work until GCC is installed.
12864 Here is a table of the options you can use with @code{protoize} and
12865 @code{unprotoize}. Each option works with both programs unless
12869 @item -B @var{directory}
12870 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
12871 usual directory (normally @file{/usr/local/lib}). This file contains
12872 prototype information about standard system functions. This option
12873 applies only to @code{protoize}.
12875 @item -c @var{compilation-options}
12876 Use @var{compilation-options} as the options when running @command{gcc} to
12877 produce the @samp{.X} files. The special option @option{-aux-info} is
12878 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
12880 Note that the compilation options must be given as a single argument to
12881 @code{protoize} or @code{unprotoize}. If you want to specify several
12882 @command{gcc} options, you must quote the entire set of compilation options
12883 to make them a single word in the shell.
12885 There are certain @command{gcc} arguments that you cannot use, because they
12886 would produce the wrong kind of output. These include @option{-g},
12887 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
12888 the @var{compilation-options}, they are ignored.
12891 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
12892 systems) instead of @samp{.c}. This is convenient if you are converting
12893 a C program to C++. This option applies only to @code{protoize}.
12896 Add explicit global declarations. This means inserting explicit
12897 declarations at the beginning of each source file for each function
12898 that is called in the file and was not declared. These declarations
12899 precede the first function definition that contains a call to an
12900 undeclared function. This option applies only to @code{protoize}.
12902 @item -i @var{string}
12903 Indent old-style parameter declarations with the string @var{string}.
12904 This option applies only to @code{protoize}.
12906 @code{unprotoize} converts prototyped function definitions to old-style
12907 function definitions, where the arguments are declared between the
12908 argument list and the initial @samp{@{}. By default, @code{unprotoize}
12909 uses five spaces as the indentation. If you want to indent with just
12910 one space instead, use @option{-i " "}.
12913 Keep the @samp{.X} files. Normally, they are deleted after conversion
12917 Add explicit local declarations. @code{protoize} with @option{-l} inserts
12918 a prototype declaration for each function in each block which calls the
12919 function without any declaration. This option applies only to
12923 Make no real changes. This mode just prints information about the conversions
12924 that would have been done without @option{-n}.
12927 Make no @samp{.save} files. The original files are simply deleted.
12928 Use this option with caution.
12930 @item -p @var{program}
12931 Use the program @var{program} as the compiler. Normally, the name
12932 @file{gcc} is used.
12935 Work quietly. Most warnings are suppressed.
12938 Print the version number, just like @option{-v} for @command{gcc}.
12941 If you need special compiler options to compile one of your program's
12942 source files, then you should generate that file's @samp{.X} file
12943 specially, by running @command{gcc} on that source file with the
12944 appropriate options and the option @option{-aux-info}. Then run
12945 @code{protoize} on the entire set of files. @code{protoize} will use
12946 the existing @samp{.X} file because it is newer than the source file.
12950 gcc -Dfoo=bar file1.c -aux-info file1.X
12955 You need to include the special files along with the rest in the
12956 @code{protoize} command, even though their @samp{.X} files already
12957 exist, because otherwise they won't get converted.
12959 @xref{Protoize Caveats}, for more information on how to use
12960 @code{protoize} successfully.