GCC Command Options
*******************

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking.  The ``overall options'' allow you to stop this process at an intermediate stage.  For example, the -c option says not to run the linker.  Then the output consists of object files output by the assembler.

Other options are passed on to one stage of processing.  Some options control the preprocessor and others the compiler itself.  Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly.  If the description for a particular option does not mention a source language, you can use that option with all supported languages.

See Compiling C++ Programs, for a summary of special options for compiling C++ programs.

The gcc program accepts options and file names as operands.  Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dr is very different from -d -r.

You can mix options and other arguments.  For the most part, the order you use doesn't matter.  Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified.

Many options have long names starting with -f or with -W---for example, -fforce-mem, -fstrength-reduce, -Wformat and so on.  Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo.  This manual documents only one of these two forms, whichever one is not the default.

See Option Index, for an index to GCC's options.

Option Summary
==============

Here is a summary of all the options, grouped by type.  Explanations are in the following sections.

Overall Options
     See Options Controlling the Kind of Output. -c  -S  -E  -o FILE  -combine -pipe  -pass-exit-codes -x LANGUAGE  -v  -###  --help  --target-help  --version

C Language Options
     See Options Controlling C Dialect. -ansi  -std=STANDARD  -aux-info FILENAME -fno-asm  -fno-builtin  -fno-builtin-FUNCTION -fhosted  -ffreestanding  -fms-extensions -trigraphs  -no-integrated-cpp  -traditional  -traditional-cpp -fallow-single-precision  -fcond-mismatch -fsigned-bitfields  -fsigned-char -funsigned-bitfields  -funsigned-char

C++ Language Options
     See Options Controlling C++ Dialect. -fabi-version=N  -fno-access-control  -fcheck-new -fconserve-space  -fno-const-strings -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope  -fno-for-scope  -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines  -fms-extensions -fno-nonansi-builtins  -fno-operator-names -fno-optional-diags  -fpermissive -frepo  -fno-rtti  -fstats  -ftemplate-depth-N -fno-threadsafe-statics -fuse-cxa-atexit  -fno-weak  -nostdinc++ -fno-default-inline  -fvisibility-inlines-hidden -Wabi  -Wctor-dtor-privacy -Wnon-virtual-dtor  -Wreorder -Weffc++  -Wno-deprecated -Wno-non-template-friend  -Wold-style-cast -Woverloaded-virtual  -Wno-pmf-conversions -Wsign-promo  -Wsynth

Objective-C and Objective-C++ Language Options
     See Options Controlling Objective-C and Objective-C++ Dialects.

     -fconstant-string-class=CLASS-NAME -fgnu-runtime  -fnext-runtime -fno-nil-receivers -fobjc-exceptions -freplace-objc-classes -fzero-link -gen-decls -Wno-protocol  -Wselector -Wundeclared-selector

Language Independent Options
     See Options to Control Diagnostic Messages Formatting. -fmessage-length=N -fdiagnostics-show-location=[once|every-line]

Warning Options
     See Options to Request or Suppress Warnings. -fsyntax-only  -pedantic  -pedantic-errors -w  -Wextra  -Wall  -Waggregate-return -Wcast-align  -Wcast-qual  -Wchar-subscripts  -Wcomment -Wconversion  -Wno-deprecated-declarations -Wdisabled-optimization  -Wno-div-by-zero  -Wno-endif-labels -Werror  -Werror-implicit-function-declaration -Wfatal-errors  -Wfloat-equal  -Wformat  -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-security  -Wformat-y2k -Wimplicit  -Wimplicit-function-declaration  -Wimplicit-int -Wimport  -Wno-import  -Winit-self  -Winline -Wno-invalid-offsetof  -Winvalid-pch -Wlarger-than-LEN  -Wlong-long -Wmain  -Wmissing-braces  -Wmissing-field-initializers -Wmissing-format-attribute  -Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar  -Wnonnull  -Wpacked  -Wpadded -Wparentheses  -Wpointer-arith  -Wredundant-decls -Wreturn-type  -Wsequence-point  -Wshadow -Wsign-compare  -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch  -Wswitch-default  -Wswitch-enum -Wsystem-headers  -Wtrigraphs  -Wundef  -Wuninitialized -Wunknown-pragmas  -Wunreachable-code -Wunused  -Wunused-function  -Wunused-label  -Wunused-parameter -Wunused-value  -Wunused-variable  -Wwrite-strings -Wvariadic-macros

C-only Warning Options
     -Wbad-function-cast  -Wmissing-declarations -Wmissing-prototypes  -Wnested-externs  -Wold-style-definition -Wstrict-prototypes  -Wtraditional -Wdeclaration-after-statement -Wno-pointer-sign

Debugging Options
     See Options for Debugging Your Program or GCC. -dLETTERS  -dumpspecs  -dumpmachine  -dumpversion -fdump-unnumbered  -fdump-translation-unit[-N] -fdump-class-hierarchy[-N] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-N] -fdump-tree-optimized[-N] -fdump-tree-inlined[-N] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-N] -fdump-tree-pre[-N] -fdump-tree-ccp[-N] -fdump-tree-dce[-N] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-N] -fdump-tree-dom[-N] -fdump-tree-dse[-N] -fdump-tree-phiopt[-N] -fdump-tree-forwprop[-N] -fdump-tree-copyrename[-N] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink -fdump-tree-sra[-N] -fdump-tree-salias -fdump-tree-fre[-N] -ftree-vectorizer-verbose=N -feliminate-dwarf2-dups -feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -ftree-based-profiling -frandom-seed=STRING -fsched-verbose=N -ftest-coverage  -ftime-report -fvar-tracking -g  -gLEVEL  -gcoff -gdwarf-2 -ggdb  -gstabs  -gstabs+  -gvms  -gxcoff  -gxcoff+ -p  -pg  -print-file-name=LIBRARY  -print-libgcc-file-name -print-multi-directory  -print-multi-lib -print-prog-name=PROGRAM  -print-search-dirs  -Q -save-temps  -time

Optimization Options
     See Options that Control Optimization. -falign-functions=N  -falign-jumps=N -falign-labels=N  -falign-loops=N -fbounds-check -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves  -fcprop-registers  -fcse-follow-jumps -fcse-follow-jumps  -fcse-skip-blocks  -fcx-limited-range  -fdata-sections -fdelayed-branch  -fdelete-null-pointer-checks -fexpensive-optimizations  -ffast-math  -ffloat-store -fforce-addr  -fforce-mem  -ffunction-sections -fgcse  -fgcse-lm  -fgcse-sm  -fgcse-las  -fgcse-after-reload -floop-optimize -fcrossjumping  -fif-conversion  -fif-conversion2 -finline-functions  -finline-limit=N  -fkeep-inline-functions -fkeep-static-consts  -fmerge-constants  -fmerge-all-constants -fmodulo-sched -fno-branch-count-reg -fno-default-inline  -fno-defer-pop -floop-optimize2 -fmove-loop-invariants -fno-function-cse  -fno-guess-branch-probability -fno-inline  -fno-math-errno  -fno-peephole  -fno-peephole2 -funsafe-math-optimizations  -ffinite-math-only -fno-trapping-math  -fno-zero-initialized-in-bss -fomit-frame-pointer  -foptimize-register-move -foptimize-sibling-calls  -fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove  -frename-registers -freorder-blocks  -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop  -frerun-loop-opt -frounding-math -fschedule-insns  -fschedule-insns2 -fno-sched-interblock  -fno-sched-spec  -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns=N -sched-stalled-insns-dep=N -fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans -fsingle-precision-constant  -fspeculative-prefetching -fstrength-reduce  -fstrict-aliasing  -ftracer  -fthread-jumps -funroll-all-loops  -funroll-loops  -fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -fvariable-expansion-in-unroller -ftree-pre  -ftree-ccp  -ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize -ftree-salias --param NAME=VALUE -O  -O0  -O1  -O2  -O3  -Os

Preprocessor Options
     See Options Controlling the Preprocessor. -AQUESTION=ANSWER -A-QUESTION[=ANSWER] -C  -dD  -dI  -dM  -dN -DMACRO[=DEFN]  -E  -H -idirafter DIR -include FILE  -imacros FILE -iprefix FILE  -iwithprefix DIR -iwithprefixbefore DIR  -isystem DIR -M  -MM  -MF  -MG  -MP  -MQ  -MT  -nostdinc -P  -fworking-directory  -remap -trigraphs  -undef  -UMACRO  -Wp,OPTION -Xpreprocessor OPTION

Assembler Option
     See Passing Options to the Assembler. -Wa,OPTION  -Xassembler OPTION

Linker Options
     See Options for Linking. OBJECT-FILE-NAME  -lLIBRARY -nostartfiles  -nodefaultlibs  -nostdlib -pie -s  -static  -static-libgcc  -shared  -shared-libgcc  -symbolic -Wl,OPTION  -Xlinker OPTION -u SYMBOL

Directory Options
     See Options for Directory Search. -BPREFIX  -IDIR  -iquoteDIR  -LDIR  -specs=FILE  -I-

Target Options
     See Target Options. -V VERSION  -b MACHINE

Machine Dependent Options
     See Hardware Models and Configurations.

     ARC Options -EB  -EL -mmangle-cpu  -mcpu=CPU  -mtext=TEXT-SECTION -mdata=DATA-SECTION  -mrodata=READONLY-DATA-SECTION

     ARM Options -mapcs-frame  -mno-apcs-frame -mabi=NAME -mapcs-stack-check  -mno-apcs-stack-check -mapcs-float  -mno-apcs-float -mapcs-reentrant  -mno-apcs-reentrant -msched-prolog  -mno-sched-prolog -mlittle-endian  -mbig-endian  -mwords-little-endian -mfloat-abi=NAME  -msoft-float  -mhard-float  -mfpe -mthumb-interwork  -mno-thumb-interwork -mcpu=NAME  -march=NAME  -mfpu=NAME -mstructure-size-boundary=N -mabort-on-noreturn -mlong-calls  -mno-long-calls -msingle-pic-base  -mno-single-pic-base -mpic-register=REG -mnop-fun-dllimport -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb  -marm -mtpcs-frame  -mtpcs-leaf-frame -mcaller-super-interworking  -mcallee-super-interworking

     AVR Options -mmcu=MCU  -msize  -minit-stack=N  -mno-interrupts -mcall-prologues  -mno-tablejump  -mtiny-stack  -mint8

     CRIS Options -mcpu=CPU  -march=CPU  -mtune=CPU -mmax-stack-frame=N  -melinux-stacksize=N -metrax4  -metrax100  -mpdebug  -mcc-init  -mno-side-effects -mstack-align  -mdata-align  -mconst-align -m32-bit  -m16-bit  -m8-bit  -mno-prologue-epilogue  -mno-gotplt -melf  -maout  -melinux  -mlinux  -sim  -sim2 -mmul-bug-workaround  -mno-mul-bug-workaround

     Darwin Options -all_load  -allowable_client  -arch  -arch_errors_fatal -arch_only  -bind_at_load  -bundle  -bundle_loader -client_name  -compatibility_version  -current_version -dead_strip -dependency-file  -dylib_file  -dylinker_install_name -dynamic  -dynamiclib  -exported_symbols_list -filelist  -flat_namespace  -force_cpusubtype_ALL -force_flat_namespace  -headerpad_max_install_names -image_base  -init  -install_name  -keep_private_externs -multi_module  -multiply_defined  -multiply_defined_unused -noall_load   -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs  -noprebind  -noseglinkedit -pagezero_size  -prebind  -prebind_all_twolevel_modules -private_bundle  -read_only_relocs  -sectalign -sectobjectsymbols  -whyload  -seg1addr -sectcreate  -sectobjectsymbols  -sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table  -seg_addr_table_filename  -seglinkedit -segprot  -segs_read_only_addr  -segs_read_write_addr -single_module  -static  -sub_library  -sub_umbrella -twolevel_namespace  -umbrella  -undefined -unexported_symbols_list  -weak_reference_mismatches -whatsloaded -F -gused -gfull -mone-byte-bool

     DEC Alpha Options -mno-fp-regs  -msoft-float  -malpha-as  -mgas -mieee  -mieee-with-inexact  -mieee-conformant -mfp-trap-mode=MODE  -mfp-rounding-mode=MODE -mtrap-precision=MODE  -mbuild-constants -mcpu=CPU-TYPE  -mtune=CPU-TYPE -mbwx  -mmax  -mfix  -mcix -mfloat-vax  -mfloat-ieee -mexplicit-relocs  -msmall-data  -mlarge-data -msmall-text  -mlarge-text -mmemory-latency=TIME

     DEC Alpha/VMS Options -mvms-return-codes

     FRV Options -mgpr-32  -mgpr-64  -mfpr-32  -mfpr-64 -mhard-float  -msoft-float -malloc-cc  -mfixed-cc  -mdword  -mno-dword -mdouble  -mno-double -mmedia  -mno-media  -mmuladd  -mno-muladd -mfdpic  -minline-plt -mgprel-ro  -multilib-library-pic -mlinked-fp  -mlong-calls  -malign-labels -mlibrary-pic  -macc-4  -macc-8 -mpack  -mno-pack  -mno-eflags  -mcond-move  -mno-cond-move -mscc  -mno-scc  -mcond-exec  -mno-cond-exec -mvliw-branch  -mno-vliw-branch -mmulti-cond-exec  -mno-multi-cond-exec  -mnested-cond-exec -mno-nested-cond-exec  -mtomcat-stats -mTLS -mtls -mcpu=CPU

     H8/300 Options -mrelax  -mh  -ms  -mn  -mint32  -malign-300

     HPPA Options -march=ARCHITECTURE-TYPE -mbig-switch  -mdisable-fpregs  -mdisable-indexing -mfast-indirect-calls  -mgas  -mgnu-ld   -mhp-ld -mfixed-range=REGISTER-RANGE -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store  -mno-big-switch  -mno-disable-fpregs -mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas -mno-jump-in-delay  -mno-long-load-store -mno-portable-runtime  -mno-soft-float -mno-space-regs  -msoft-float  -mpa-risc-1-0 -mpa-risc-1-1  -mpa-risc-2-0  -mportable-runtime -mschedule=CPU-TYPE  -mspace-regs  -msio  -mwsio -munix=UNIX-STD  -nolibdld  -static  -threads

     i386 and x86-64 Options -mtune=CPU-TYPE  -march=CPU-TYPE -mfpmath=UNIT -masm=DIALECT  -mno-fancy-math-387 -mno-fp-ret-in-387  -msoft-float  -msvr3-shlib -mno-wide-multiply  -mrtd  -malign-double -mpreferred-stack-boundary=NUM -mmmx  -msse  -msse2 -msse3 -m3dnow -mthreads  -mno-align-stringops  -minline-all-stringops -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double -m96bit-long-double  -mregparm=NUM  -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=CODE-MODEL -m32  -m64

     IA-64 Options -mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic -mvolatile-asm-stop  -mregister-names  -mno-sdata -mconstant-gp  -mauto-pic  -minline-float-divide-min-latency -minline-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=REGISTER-RANGE -mtls-size=TLS-SIZE -mtune=CPU-TYPE -mt -pthread -milp32 -mlp64

     M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=NUMBER -mbranch-cost=NUMBER -mmodel=CODE-SIZE-MODEL-TYPE -msdata=SDATA-TYPE -mno-flush-func -mflush-func=NAME -mno-flush-trap -mflush-trap=NUMBER -G NUM

     M680x0 Options -m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040 -m68060  -mcpu32  -m5200  -m68881  -mbitfield  -mc68000  -mc68020 -mnobitfield  -mrtd  -mshort  -msoft-float  -mpcrel -malign-int  -mstrict-align  -msep-data  -mno-sep-data -mshared-library-id=n  -mid-shared-library  -mno-id-shared-library

     M68hc1x Options -m6811  -m6812  -m68hc11  -m68hc12   -m68hcs12 -mauto-incdec  -minmax  -mlong-calls  -mshort -msoft-reg-count=COUNT

     MCore Options -mhardlit  -mno-hardlit  -mdiv  -mno-div  -mrelax-immediates -mno-relax-immediates  -mwide-bitfields  -mno-wide-bitfields -m4byte-functions  -mno-4byte-functions  -mcallgraph-data -mno-callgraph-data  -mslow-bytes  -mno-slow-bytes  -mno-lsim -mlittle-endian  -mbig-endian  -m210  -m340  -mstack-increment

     MIPS Options -EL  -EB  -march=ARCH  -mtune=ARCH -mips1  -mips2  -mips3  -mips4  -mips32  -mips32r2  -mips64 -mips16  -mno-mips16  -mabi=ABI  -mabicalls  -mno-abicalls -mxgot  -mno-xgot  -mgp32  -mgp64  -mfp32  -mfp64 -mhard-float  -msoft-float  -msingle-float  -mdouble-float -mpaired-single  -mips3d -mint64  -mlong64  -mlong32  -msym32  -mno-sym32 -GNUM  -membedded-data  -mno-embedded-data -muninit-const-in-rodata  -mno-uninit-const-in-rodata -msplit-addresses  -mno-split-addresses -mexplicit-relocs  -mno-explicit-relocs -mcheck-zero-division  -mno-check-zero-division -mdivide-traps  -mdivide-breaks -mmemcpy  -mno-memcpy  -mlong-calls  -mno-long-calls -mmad  -mno-mad  -mfused-madd  -mno-fused-madd  -nocpp -mfix-r4000  -mno-fix-r4000  -mfix-r4400  -mno-fix-r4400 -mfix-vr4120  -mno-fix-vr4120  -mfix-vr4130 -mfix-sb1  -mno-fix-sb1 -mflush-func=FUNC  -mno-flush-func -mbranch-likely  -mno-branch-likely -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align

     MMIX Options -mlibfuncs  -mno-libfuncs  -mepsilon  -mno-epsilon  -mabi=gnu -mabi=mmixware  -mzero-extend  -mknuthdiv  -mtoplevel-symbols -melf  -mbranch-predict  -mno-branch-predict  -mbase-addresses -mno-base-addresses  -msingle-exit  -mno-single-exit

     MN10300 Options -mmult-bug  -mno-mult-bug -mam33  -mno-am33 -mam33-2  -mno-am33-2 -mno-crt0  -mrelax

     NS32K Options -m32032  -m32332  -m32532  -m32081  -m32381 -mmult-add  -mnomult-add  -msoft-float  -mrtd  -mnortd -mregparam  -mnoregparam  -msb  -mnosb -mbitfield  -mnobitfield  -mhimem  -mnohimem

     PDP-11 Options -mfpu  -msoft-float  -mac0  -mno-ac0  -m40  -m45  -m10 -mbcopy  -mbcopy-builtin  -mint32  -mno-int16 -mint16  -mno-int32  -mfloat32  -mno-float64 -mfloat64  -mno-float32  -mabshi  -mno-abshi -mbranch-expensive  -mbranch-cheap -msplit  -mno-split  -munix-asm  -mdec-asm

     PowerPC Options See RS/6000 and PowerPC Options.

     RS/6000 and PowerPC Options -mcpu=CPU-TYPE -mtune=CPU-TYPE -mpower  -mno-power  -mpower2  -mno-power2 -mpowerpc  -mpowerpc64  -mno-powerpc -maltivec  -mno-altivec -mpowerpc-gpopt  -mno-powerpc-gpopt -mpowerpc-gfxopt  -mno-powerpc-gfxopt -mnew-mnemonics  -mold-mnemonics -mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc -m64  -m32  -mxl-compat  -mno-xl-compat  -mpe -malign-power  -malign-natural -msoft-float  -mhard-float  -mmultiple  -mno-multiple -mstring  -mno-string  -mupdate  -mno-update -mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align -mstrict-align  -mno-strict-align  -mrelocatable -mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib -mtoc  -mno-toc  -mlittle  -mlittle-endian  -mbig  -mbig-endian -mdynamic-no-pic -mprioritize-restricted-insns=PRIORITY -msched-costly-dep=DEPENDENCE_TYPE -minsert-sched-nops=SCHEME -mcall-sysv  -mcall-netbsd -maix-struct-return  -msvr4-struct-return -mabi=altivec  -mabi=no-altivec -mabi=spe  -mabi=no-spe -misel=yes  -misel=no -mspe=yes  -mspe=no -mfloat-gprs=yes  -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype  -mno-prototype -msim  -mmvme  -mads  -myellowknife  -memb  -msdata -msdata=OPT  -mvxworks  -mwindiss  -G NUM  -pthread

     S/390 and zSeries Options -mtune=CPU-TYPE  -march=CPU-TYPE -mhard-float  -msoft-float  -mbackchain  -mno-backchain -mpacked-stack  -mno-packed-stack -msmall-exec  -mno-small-exec  -mmvcle -mno-mvcle -m64  -m31  -mdebug  -mno-debug  -mesa  -mzarch -mtpf-trace -mno-tpf-trace  -mfused-madd  -mno-fused-madd -mwarn-framesize  -mwarn-dynamicstack  -mstack-size -mstack-guard

     SH Options -m1  -m2  -m2e  -m3  -m3e -m4-nofpu  -m4-single-only  -m4-single  -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media  -m5-64media-nofpu -m5-32media  -m5-32media-nofpu -m5-compact  -m5-compact-nofpu -mb  -ml  -mdalign  -mrelax -mbigtable  -mfmovd  -mhitachi -mrenesas -mno-renesas -mnomacsave -mieee  -misize  -mpadstruct  -mspace -mprefergot  -musermode

     SPARC Options -mcpu=CPU-TYPE -mtune=CPU-TYPE -mcmodel=CODE-MODEL -m32  -m64  -mapp-regs  -mno-app-regs -mfaster-structs  -mno-faster-structs -mfpu  -mno-fpu  -mhard-float  -msoft-float -mhard-quad-float  -msoft-quad-float -mimpure-text  -mno-impure-text  -mlittle-endian -mstack-bias  -mno-stack-bias -munaligned-doubles  -mno-unaligned-doubles -mv8plus  -mno-v8plus  -mvis  -mno-vis -threads -pthreads

     System V Options -Qy  -Qn  -YP,PATHS  -Ym,DIR

     TMS320C3x/C4x Options -mcpu=CPU  -mbig  -msmall  -mregparm  -mmemparm -mfast-fix  -mmpyi  -mbk  -mti  -mdp-isr-reload -mrpts=COUNT  -mrptb  -mdb  -mloop-unsigned -mparallel-insns  -mparallel-mpy  -mpreserve-float

     V850 Options -mlong-calls  -mno-long-calls  -mep  -mno-ep -mprolog-function  -mno-prolog-function  -mspace -mtda=N  -msda=N  -mzda=N -mapp-regs  -mno-app-regs -mdisable-callt  -mno-disable-callt -mv850e1 -mv850e -mv850  -mbig-switch

     VAX Options -mg  -mgnu  -munix

     x86-64 Options See i386 and x86-64 Options.

     Xstormy16 Options -msim

     Xtensa Options -mconst16 -mno-const16 -mfused-madd  -mno-fused-madd -mtext-section-literals  -mno-text-section-literals -mtarget-align  -mno-target-align -mlongcalls  -mno-longcalls

     zSeries Options See S/390 and zSeries Options.

Code Generation Options
     See Options for Code Generation Conventions. -fcall-saved-REG  -fcall-used-REG -ffixed-REG  -fexceptions -fnon-call-exceptions  -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive  -finstrument-functions -fno-common  -fno-ident -fpcc-struct-return  -fpic  -fPIC -fpie -fPIE -freg-struct-return  -fshared-data  -fshort-enums -fshort-double  -fshort-wchar -fverbose-asm  -fpack-struct[=N]  -fstack-check -fstack-limit-register=REG  -fstack-limit-symbol=SYM -fargument-alias  -fargument-noalias -fargument-noalias-global  -fleading-underscore -ftls-model=MODEL -ftrapv  -fwrapv  -fbounds-check -fvisibility

Options Controlling the Kind of Output
======================================

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order.  GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:

FILE.c
     C source code which must be preprocessed.

FILE.i
     C source code which should not be preprocessed.

FILE.ii
     C++ source code which should not be preprocessed.

FILE.m
     Objective-C source code.  Note that you must link with the libobjc library to make an Objective-C program work.

FILE.mi
     Objective-C source code which should not be preprocessed.

FILE.mm
FILE.M
     Objective-C++ source code.  Note that you must link with the libobjc library to make an Objective-C++ program work.  Note that .M refers to a literal capital M.

FILE.mii
     Objective-C++ source code which should not be preprocessed.

FILE.h
     C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.

FILE.cc
FILE.cp
FILE.cxx
FILE.cpp
FILE.CPP
FILE.c++
FILE.C
     C++ source code which must be preprocessed.  Note that in .cxx, the last two letters must both be literally x.  Likewise, .C refers to a literal capital C.

FILE.hh
FILE.H
     C++ header file to be turned into a precompiled header.

FILE.f
FILE.for
FILE.FOR
     Fortran source code which should not be preprocessed.

FILE.F
FILE.fpp
FILE.FPP
     Fortran source code which must be preprocessed (with the traditional preprocessor).

FILE.r
     Fortran source code which must be preprocessed with a RATFOR preprocessor (not included with GCC).

FILE.f90
FILE.f95
     Fortran 90/95 source code which should not be preprocessed.

FILE.ads
     Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration).  Such files are also called specs.

FILE.adb
     Ada source code file containing a library unit body (a subprogram or package body).  Such files are also called bodies.

FILE.s
     Assembler code.

FILE.S
     Assembler code which must be preprocessed.

OTHER
     An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the -x option:

-x LANGUAGE
     Specify explicitly the LANGUAGE for the following input files (rather than letting the compiler choose a default based on the file name suffix).  This option applies to all following input files until the next -x option.  Possible values for LANGUAGE are:

          c  c-header  c-cpp-output
          c++  c++-header  c++-cpp-output
          objective-c  objective-c-header  objective-c-cpp-output
          objective-c++ objective-c++-header objective-c++-cpp-output
          assembler  assembler-with-cpp
          ada
          f77  f77-cpp-input  ratfor
          f95
          java
          treelang

-x none
     Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

-pass-exit-codes
     Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success return code.  If you specify -pass-exit-codes, the gcc program will instead return with numerically highest error produced by any phase that returned an error indication.

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop.  Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.

-c
     Compile or assemble the source files, but do not link.  The linking stage simply is not done.  The ultimate output is in the form of an object file for each source file.

     By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

     Unrecognized input files, not requiring compilation or assembly, are ignored.

-S
     Stop after the stage of compilation proper; do not assemble.  The output is in the form of an assembler code file for each non-assembler input file specified.

     By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

     Input files that don't require compilation are ignored.

-E
     Stop after the preprocessing stage; do not run the compiler proper.  The output is in the form of preprocessed source code, which is sent to the standard output.

     Input files which don't require preprocessing are ignored.

-o FILE
     Place output in file FILE.  This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

     If -o is not specified, the default is to put an executable file in a.out, the object file for SOURCE.SUFFIX in SOURCE.o, its assembler file in SOURCE.s, a precompiled header file in SOURCE.SUFFIX.gch, and all preprocessed C source on standard output.

-v
     Print (on standard error output) the commands executed to run the stages of compilation.  Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

-###
     Like -v except the commands are not executed and all command arguments are quoted.  This is useful for shell scripts to capture the driver-generated command lines.

-pipe
     Use pipes rather than temporary files for communication between the various stages of compilation.  This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.

-combine
     If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this).  This will allow intermodule analysis (IMA) to be performed by the compiler.  Currently the only language for which this is supported is C.  If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it.  For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language.  If you use this option in conjunction with -save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.

--help
     Print (on the standard output) a description of the command line options understood by gcc.  If the -v option is also specified then --help will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept.  If the -Wextra option is also specified then command line options which have no documentation associated with them will also be displayed.

--target-help
     Print (on the standard output) a description of target specific command line options for each tool.

--version
     Display the version number and copyrights of the invoked GCC.

Compiling C++ Programs
======================

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh or .H; and preprocessed C++ files use the suffix .ii.  GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, C++ programs often require class libraries as well as a compiler that understands the C++ language---and under some circumstances, you might want to compile programs or header files from standard input, or otherwise without a suffix that flags them as C++ programs.  You might also like to precompile a C header file with a .h extension to be used in C++ compilations.  g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library.  On many systems, g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.

Options Controlling C Dialect
=============================

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:

-ansi
     In C mode, support all ISO C90 programs.  In C++ mode, remove GNU extensions that conflict with ISO C++.

     This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using.  It also enables the undesirable and rarely used ISO trigraph feature.  For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

     The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi.  You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi.  Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

     The -ansi option does not cause non-ISO programs to be rejected gratuitously.  For that, -pedantic is required in addition to -ansi.  See Warning Options.

     The macro __STRICT_ANSI__ is predefined when the -ansi option is used.  Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

     Functions which would normally be built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions with -ansi is used.  See Other built-in functions provided by GCC, for details of the functions affected.

-std=
     Determine the language standard.  This option is currently only supported when compiling C or C++.  A value for this option must be provided; possible values are

     c89
     iso9899:1990
          ISO C90 (same as -ansi).

     iso9899:199409
          ISO C90 as modified in amendment 1.

     c99
     c9x
     iso9899:1999
     iso9899:199x
          ISO C99.  Note that this standard is not yet fully supported; see http://gcc.gnu.org/c99status.html for more information.  The names c9x and iso9899:199x are deprecated.

     gnu89
          Default, ISO C90 plus GNU extensions (including some C99 features).

     gnu99
     gnu9x
          ISO C99 plus GNU extensions.  When ISO C99 is fully implemented in GCC, this will become the default.  The name gnu9x is deprecated.

     c++98
          The 1998 ISO C++ standard plus amendments.

     gnu++98
          The same as -std=c++98 plus GNU extensions.  This is the default for C++ code.

     Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards.  For example, you may use __restrict__ even when -std=c99 is not specified.

     The -std options specifying some version of ISO C have the same effects as -ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the inline keyword in ISO C99) are not disabled.

     See Language Standards Supported by GCC, for details of these standard versions.

-aux-info FILENAME
     Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files.  This option is silently ignored in any language other than C.

     Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character).  In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

-fno-asm
     Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers.  You can use the keywords __asm__, __inline__ and __typeof__ instead.  -ansi implies -fno-asm.

     In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords.  You may want to use the -fno-gnu-keywords flag instead, which has the same effect.  In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

-fno-builtin
-fno-builtin-FUNCTION
     Don't recognize built-in functions that do not begin with __builtin_ as prefix.  See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning.

     GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions that adjust the stack directly, and calls to memcpy may become inline copy loops.  The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.  In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function.  For example, warnings are given with -Wformat for bad calls to printf, when printf is built in, and strlen is known not to modify global memory.

     With the -fno-builtin-FUNCTION option only the built-in function FUNCTION is disabled.  FUNCTION must not begin with __builtin_.  If a function is named this is not built-in in this version of GCC, this option is ignored.  There is no corresponding -fbuiltin-FUNCTION option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

          #define abs(n)          __builtin_abs ((n))
          #define strcpy(d, s)    __builtin_strcpy ((d), (s))

-fhosted

     Assert that compilation takes place in a hosted environment.  This implies -fbuiltin.  A hosted environment is one in which the entire standard library is available, and in which main has a return type of int.  Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.

-ffreestanding

     Assert that compilation takes place in a freestanding environment.  This implies -fno-builtin.  A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main.  The most obvious example is an OS kernel. This is equivalent to -fno-hosted.

     See Language Standards Supported by GCC, for details of freestanding and hosted environments.

-fms-extensions
     Accept some non-standard constructs used in Microsoft header files.

     Some cases of unnamed fields in structures and unions are only accepted with this option.  See Unnamed struct/union fields within structs/unions, for details.

-trigraphs
     Support ISO C trigraphs.  The -ansi option (and -std options for strict ISO C conformance) implies -trigraphs.

-no-integrated-cpp
     Performs a compilation in two passes: preprocessing and compiling.  This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option.  The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling.  The default is to use the integrated cpp (internal cpp)

     The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.

-traditional
-traditional-cpp
     Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler.  They are now only supported with the -E switch. The preprocessor continues to support a pre-standard mode.  See the GNU CPP manual for details.

-fcond-mismatch
     Allow conditional expressions with mismatched types in the second and third arguments.  The value of such an expression is void.  This option is not supported for C++.

-funsigned-char
     Let the type char be unsigned, like unsigned char.

     Each kind of machine has a default for what char should be.  It is either like unsigned char by default or like signed char by default.

     Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for.  This option, and its inverse, let you make such a program work with the opposite default.

     The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

-fsigned-char
     Let the type char be signed, like signed char.

     Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char.  Likewise, the option -fno-signed-char is equivalent to -funsigned-char.

-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
     These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned.  By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types.

Options Controlling C++ Dialect
===============================

This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in.  For example, you might compile a file firstClass.C like this:

     g++ -g -frepo -O -c firstClass.C

In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=N
     Use version N of the C++ ABI.  Version 2 is the version of the C++ ABI that first appeared in G++ 3.4.  Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.  Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

     The default is version 2.

-fno-access-control
     Turn off all access checking.  This switch is mainly useful for working around bugs in the access control code.

-fcheck-new
     Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated.  This check is normally unnecessary because the C++ standard specifies that operator new will only return 0 if it is declared throw(), in which case the compiler will always check the return value even without this option.  In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc.  See also new (nothrow).

-fconserve-space
     Put uninitialized or runtime-initialized global variables into the common segment, as C does.  This saves space in the executable at the cost of not diagnosing duplicate definitions.  If you compile with this flag and your program mysteriously crashes after main() has completed, you may have an object that is being destroyed twice because two definitions were merged.

     This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

-fno-const-strings
     Give string constants type char * instead of type const char *.  By default, G++ uses type const char * as required by the standard.  Even if you use -fno-const-strings, you cannot actually modify the value of a string constant.

     This option might be removed in a future release of G++.  For maximum portability, you should structure your code so that it works with string constants that have type const char *.

-fno-elide-constructors
     The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

-fno-enforce-eh-specs
     Don't check for violation of exception specifications at runtime.  This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG.  The compiler will still optimize based on the exception specifications.

-ffor-scope
-fno-for-scope
     If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C++.

     The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-fno-gnu-keywords
     Do not recognize typeof as a keyword, so that code can use this word as an identifier.  You can use the keyword __typeof__ instead. -ansi implies -fno-gnu-keywords.

-fno-implicit-templates
     Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.

-fno-implicit-inline-templates
     Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization will need the same set of explicit instantiations.

-fno-implement-inlines
     To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation.  This will cause linker errors if these functions are not inlined everywhere they are called.

-fms-extensions
     Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

-fno-nonansi-builtins
     Disable built-in declarations of functions that are not mandated by ANSI/ISO C.  These include ffs, alloca, _exit, index, bzero, conjf, and other related functions.

-fno-operator-names
     Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.

-fno-optional-diags
     Disable diagnostics that the standard says a compiler does not need to issue.  Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.

-fpermissive
     Downgrade some diagnostics about nonconformant code from errors to warnings.  Thus, using -fpermissive will allow some nonconforming code to compile.

-frepo
     Enable automatic template instantiation at link time.  This option also implies -fno-implicit-templates.  See Template Instantiation, for more information.

-fno-rtti
     Disable generation of information about every class with virtual functions for use by the C++ runtime type identification features (dynamic_cast and typeid).  If you don't use those parts of the language, you can save some space by using this flag.  Note that exception handling uses the same information, but it will generate it as needed.

-fstats
     Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

-ftemplate-depth-N
     Set the maximum instantiation depth for template classes to N. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation.  ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.

-fno-threadsafe-statics
     Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics.  You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.

-fuse-cxa-atexit
     Register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but will only work if your C library supports __cxa_atexit.

-fvisibility-inlines-hidden
     Causes all inlined methods to be marked with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO.  Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.  While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces.  For even more savings, combine with the -fvisibility=hidden switch.

-fno-weak
     Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available.  This option exists only for testing, and should not be used by end-users; it will result in inferior code and has no benefits.  This option may be removed in a future release of G++.

-nostdinc++
     Do not search for header files in the standard directories specific to C++, but do still search the other standard directories.  (This option is used when building the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

-fno-default-inline
     Do not assume inline for functions defined inside a class scope. See Options That Control Optimization.  Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wabi (C++ only)
     Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI.  Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code.  There may also be cases where warnings are emitted even though the code that is generated will be compatible.

     You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

     The known incompatibilities at this point include:

       * Incorrect handling of tail-padding for bit-fields.  G++ may attempt to pack data into the same byte as a base class.  For example:

               struct A { virtual void f(); int f1 : 1; };
               struct B : public A { int f2 : 1; };

          In this case, G++ will place B::f2 into the same byte asA::f1; other compilers will not.  You can avoid this problem by explicitly padding A so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout B identically.

       * Incorrect handling of tail-padding for virtual bases.  G++ does not use tail padding when laying out virtual bases.  For example:

               struct A { virtual void f(); char c1; };
               struct B { B(); char c2; };
               struct C : public A, public virtual B {};

          In this case, G++ will not place B into the tail-padding for A; other compilers will.  You can avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout C identically.

       * Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union.  For example:

               union U { int i : 4096; };

          Assuming that an int does not have 4096 bits, G++ will make the union too small by the number of bits in an int.

       * Empty classes can be placed at incorrect offsets.  For example:

               struct A {};

               struct B {
                 A a;
                 virtual void f ();
               };

               struct C : public B, public A {};

          G++ will place the A base class of C at a nonzero offset; it should be placed at offset zero.  G++ mistakenly believes that the A data member of B is already at offset zero.

       * Names of template functions whose types involve typename or template template parameters can be mangled incorrectly.

               template <typename Q>
               void f(typename Q::X) {}

               template <template <typename> class Q>
               void f(typename Q<int>::X) {}

          Instantiations of these templates may be mangled incorrectly.

-Wctor-dtor-privacy (C++ only)
     Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.

-Wnon-virtual-dtor (C++ only)
     Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by -Wall.

-Wreorder (C++ only)
     Warn when the order of member initializers given in the code does not match the order in which they must be executed.  For instance:

          struct A {
            int i;
            int j;
            A(): j (0), i (1) { }
          };

     The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect.  This warning is enabled by -Wall.

The following -W... options are not affected by -Wall.

-Weffc++ (C++ only)
     Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:

       * Item 11:  Define a copy constructor and an assignment operator for classes with dynamically allocated memory.

       * Item 12:  Prefer initialization to assignment in constructors.

       * Item 14:  Make destructors virtual in base classes.

       * Item 15:  Have operator= return a reference to *this.

       * Item 23:  Don't try to return a reference when you must return an object.

     Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

       * Item 6:  Distinguish between prefix and postfix forms of increment and decrement operators.

       * Item 7:  Never overload &&, ||, or ,.

     When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings.

-Wno-deprecated (C++ only)
     Do not warn about usage of deprecated features.  See Deprecated Features.

-Wno-non-template-friend (C++ only)
     Disable warnings when non-templatized friend functions are declared within a template.  Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function.  (Section 14.5.3).  Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function.  Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend which keeps the conformant compiler code but disables the helpful warning.

-Wold-style-cast (C++ only)
     Warn if an old-style (C-style) cast to a non-void type is used within a C++ program.  The new-style casts (static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.

-Woverloaded-virtual (C++ only)
     Warn when a function declaration hides virtual functions from a base class.  For example, in:

          struct A {
            virtual void f();
          };

          struct B: public A {
            void f(int);
          };

     the A class version of f is hidden in B, and code like:

          B* b;
          b->f();

     will fail to compile.

-Wno-pmf-conversions (C++ only)
     Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

-Wsign-promo (C++ only)
     Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size.  Previous versions of G++ would try to preserve unsignedness, but the standard mandates the current behavior.

-Wsynth (C++ only)
     Warn when G++'s synthesis behavior does not match that of cfront.  For instance:

          struct A {
            operator int ();
            A& operator = (int);
          };

          main ()
          {
            A a,b;
            a = b;
          }

     In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =.

Options Controlling Objective-C and Objective-C++ Dialects
==========================================================

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.  See Language Standards Supported by GCC, for references.)

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs, but you can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:

     gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional).  Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

-fconstant-string-class=CLASS-NAME
     Use CLASS-NAME as the name of the class to instantiate for each literal string specified with the syntax @"...".  The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below).  The -fconstant-cfstrings option, if also present, will override the -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.

-fgnu-runtime
     Generate object code compatible with the standard GNU Objective-C runtime.  This is the default for most types of systems.

-fnext-runtime
     Generate output compatible with the NeXT runtime.  This is the default for NeXT-based systems, including Darwin and Mac OS X.  The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.

-fno-nil-receivers
     Assume that all Objective-C message dispatches (e.g., [receiver message:arg]) in this translation unit ensure that the receiver is not nil.  This allows for more efficient entry points in the runtime to be used.  Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fobjc-exceptions
     Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java.  Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

            @try {
              ...
                 @throw expr;
              ...
            }
            @catch (AnObjCClass *exc) {
              ...
                @throw expr;
              ...
                @throw;
              ...
            }
            @catch (AnotherClass *exc) {
              ...
            }
            @catch (id allOthers) {
              ...
            }
            @finally {
              ...
                @throw expr;
              ...
            }

     The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.

     Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme.  When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how catch blocks work in C++ and Java.  A @catch(id ...) clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).

     The @finally clause, if present, will be executed upon exit from the immediately preceding @try ... @catch section.  This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the @try ... @catch section, analogously to the behavior of the finally clause in Java.

     There are several caveats to using the new exception mechanism:

       * Although currently designed to be binary compatible with NS_HANDLER-style idioms provided by the NSException class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.

       * As mentioned above, the new exceptions do not support handling types other than Objective-C objects.   Furthermore, when used from Objective-C++, the Objective-C exception model does not interoperate with C++ exceptions at this time.  This means you cannot @throw an exception from Objective-C and catch it in C++, or vice versa (i.e., throw ... @catch).

     The -fobjc-exceptions switch also enables the use of synchronization blocks for thread-safe execution:

            @synchronized (ObjCClass *guard) {
              ...
            }

     Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding guard object by another thread.  If it has, the current thread shall wait until the other thread relinquishes its lock.  Once guard becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making guard available to other threads).

     Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized.  Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.

-freplace-objc-classes
     Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead.  This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself.  Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

-fzero-link
     When compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance.  Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained.  This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution.

-gen-decls
     Dump interface declarations for all classes seen in the source file to a file named SOURCENAME.decl.

-Wno-protocol
     If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class.  The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass.  If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.

-Wselector
     Warn if multiple methods of different types for the same selector are found during compilation.  The check is performed on the list of methods in the final stage of compilation.  Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation.  Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the -fsyntax-only option is being used.

-Wundeclared-selector
     Warn if a @selector(...) expression referring to an undeclared selector is found.  A selector is considered undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section.  This option always performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in the final stage of compilation.  This also enforces the coding style convention that methods and selectors must be declared before being used.

-print-objc-runtime-info
     Generate C header describing the largest structure that is passed by value, if any.

Options to Control Diagnostic Messages Formatting
=================================================

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...).  The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported.  Right now, only the C++ front end can honor these options.  However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.

-fmessage-length=N
     Try to format error messages so that they fit on lines of about N characters.  The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC.  If N is zero, then no line-wrapping will be done; each error message will appear on a single line.

-fdiagnostics-show-location=once
     Only meaningful in line-wrapping mode.  Instructs the diagnostic messages reporter to emit once source location information; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines.  This is the default behavior.

-fdiagnostics-show-location=every-line
     Only meaningful in line-wrapping mode.  Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.

Options to Request or Suppress Warnings
=======================================

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.

You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations.  Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit.  This manual lists only one of the two forms, whichever is not the default.

The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-fsyntax-only
     Check the code for syntax errors, but don't do anything beyond that.

-pedantic
     Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++.  For ISO C, follows the version of the ISO C standard specified by any -std option used.

     Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require -ansi or a -std option specifying the required version of ISO C).  However, without this option, certain GNU extensions and traditional C and C++ features are supported as well.  With this option, they are rejected.

     -pedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __.  Pedantic warnings are also disabled in the expression that follows __extension__.  However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

     Some users try to use -pedantic to check programs for strict ISO C conformance.  They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all---only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

     A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -pedantic.  We don't have plans to support such a feature in the near future.

     Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based.  Warnings from -pedantic are given where they are required by the base standard.  (It would not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors
     Like -pedantic, except that errors are produced rather than warnings.

-w
     Inhibit all warning messages.

-Wno-import
     Inhibit warning messages about the use of #import.

-Wchar-subscripts
     Warn if an array subscript has type char.  This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by -Wall.

-Wcomment
     Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment. This warning is enabled by -Wall.

-Wfatal-errors
     This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.

-Wformat
     Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense.  This includes standard functions, and others specified by format attributes (see Function Attributes), in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

     The formats are checked against the format features supported by GNU libc version 2.2.  These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions.  Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations.  However, if -pedantic is used with -Wformat, warnings will be given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard).  See Options Controlling C Dialect.

     Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.

     -Wformat is included in -Wall.  For more control over some aspects of format checking, the options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not included in -Wall.

-Wformat-y2k
     If -Wformat is specified, also warn about strftime formats which may yield only a two-digit year.

-Wno-format-extra-args
     If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function.  The C standard specifies that such arguments are ignored.

     Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments.  However, in the case of scanf formats, this option will suppress the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

-Wno-format-zero-length
     If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

-Wformat-nonliteral
     If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list.

-Wformat-security
     If -Wformat is specified, also warn about uses of format functions that represent possible security problems.  At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);.  This may be a security hole if the format string came from untrusted input and contains %n.  (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

-Wformat=2
     Enable -Wformat plus format checks not included in -Wformat.  Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

-Wnonnull
     Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

     -Wnonnull is included in -Wall and -Wformat.  It can be disabled with the -Wno-nonnull option.

-Winit-self (C, C++, Objective-C and Objective-C++ only)
     Warn about uninitialized variables which are initialized with themselves. Note this option can only be used with the -Wuninitialized option, which in turn only works with -O1 and above.

     For example, GCC will warn about i being uninitialized in the following snippet only when -Winit-self has been specified:

          int f()
          {
            int i = i;
            return i;
          }

-Wimplicit-int
     Warn when a declaration does not specify a type. This warning is enabled by -Wall.

-Wimplicit-function-declaration
-Werror-implicit-function-declaration
     Give a warning (or error) whenever a function is used before being declared.  The form -Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning, not an error).

-Wimplicit
     Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.

-Wmain
     Warn if the type of main is suspicious.  main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by -Wall.

-Wmissing-braces
     Warn if an aggregate or union initializer is not fully bracketed.  In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.

          int a[2][2] = { 0, 1, 2, 3 };
          int b[2][2] = { { 0, 1 }, { 2, 3 } };

     This warning is enabled by -Wall.

-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
     Warn if a user-supplied include directory does not exist.

-Wparentheses
     Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.  Only the warning for an assignment used as a truth value is supported when compiling C++; the other warnings are only supported when compiling C.

     Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.

     Also warn about constructions where there may be confusion to which if statement an else branch belongs.  Here is an example of such a case:

          {
            if (a)
              if (b)
                foo ();
            else
              bar ();
          }

     In C, every else branch belongs to the innermost possible if statement, which in this example is if (b).  This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose.  When there is the potential for this confusion, GCC will issue a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else could belong to the enclosing if.  The resulting code would look like this:

          {
            if (a)
              {
                if (b)
                  foo ();
                else
                  bar ();
              }
          }

     This warning is enabled by -Wall.

-Wsequence-point
     Warn about code that may have undefined semantics because of violations of sequence point rules in the C standard.

     The C standard defines the order in which expressions in a C program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it.  These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified.  All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified.  However, the standards committee have ruled that function calls do not overlap.

     It is not specified when between sequence points modifications to the values of objects take effect.  Programs whose behavior depends on this have undefined behavior; the C standard specifies that ``Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression.  Furthermore, the prior value shall be read only to determine the value to be stored.''.  If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

     Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;.  Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

     The present implementation of this option only works for C programs.  A future implementation may also work for C++ programs.

     The C standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at http://gcc.gnu.org/readings.html.

     This warning is enabled by -Wall.

-Wreturn-type
     Warn whenever a function is defined with a return-type that defaults to int.  Also warn about any return statement with no return-value in a function whose return-type is not void.

     For C, also warn if the return type of a function has a type qualifier such as const.  Such a type qualifier has no effect, since the value returned by a function is not an lvalue.  ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option.

     For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is specified.  The only exceptions are main and functions defined in system headers.

     This warning is enabled by -Wall.

-Wswitch
     Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration.  (The presence of a default label prevents this warning.)  case labels outside the enumeration range also provoke warnings when this option is used. This warning is enabled by -Wall.

-Wswitch-default
     Warn whenever a switch statement does not have a default case.

-Wswitch-enum
     Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration.  case labels outside the enumeration range also provoke warnings when this option is used.

-Wtrigraphs
     Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning is enabled by -Wall.

-Wunused-function
     Warn whenever a static function is declared but not defined or a non\-inline static function is unused. This warning is enabled by -Wall.

-Wunused-label
     Warn whenever a label is declared but not used. This warning is enabled by -Wall.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-parameter
     Warn whenever a function parameter is unused aside from its declaration.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-variable
     Warn whenever a local variable or non-constant static variable is unused aside from its declaration This warning is enabled by -Wall.

     To suppress this warning use the unused attribute (see Variable Attributes).

-Wunused-value
     Warn whenever a statement computes a result that is explicitly not used. This warning is enabled by -Wall.

     To suppress this warning cast the expression to void.

-Wunused
     All the above -Wunused options combined.

     In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.

-Wuninitialized
     Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call.

     These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing.  If you don't specify -O, you simply won't get these warnings.

     If you want to warn about code which uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

     These warnings occur only for variables that are candidates for register allocation.  Therefore, they do not occur for a variable that is declared volatile, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes.  Also, they do not occur for structures, unions or arrays, even when they are in registers.

     Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

     These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error.  Here is one example of how this can happen:

          {
            int x;
            switch (y)
              {
              case 1: x = 1;
                break;
              case 2: x = 4;
                break;
              case 3: x = 5;
              }
            foo (x);
          }

     If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn't know this.  Here is another common case:

          {
            int save_y;
            if (change_y) save_y = y, y = new_y;
            ...
            if (change_y) y = save_y;
          }

     This has no bug because save_y is used only if it is set.

     This option also warns when a non-volatile automatic variable might be changed by a call to longjmp.  These warnings as well are possible only in optimizing compilation.

     The compiler sees only the calls to setjmp.  It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code.  As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place which would cause a problem.

     Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn.  See Function Attributes.

     This warning is enabled by -Wall.

-Wunknown-pragmas
     Warn when a #pragma directive is encountered which is not understood by GCC.  If this command line option is used, warnings will even be issued for unknown pragmas in system header files.  This is not the case if the warnings were only enabled by the -Wall command line option.

-Wstrict-aliasing
     This option is only active when -fstrict-aliasing is active. It warns about code which might break the strict aliasing rules that the compiler is using for optimization.  The warning does not catch all cases, but does attempt to catch the more common pitfalls.  It is included in -Wall.

-Wstrict-aliasing=2
     This option is only active when -fstrict-aliasing is active. It warns about all code which might break the strict aliasing rules that the compiler is using for optimization.  This warning catches all cases, but it will also give a warning for some ambiguous cases that are safe.

-Wall
     All of the above -W options combined.  This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros.  This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.

-Wextra
     (This option used to be called -W.  The older name is still supported, but the newer name is more descriptive.)  Print extra warning messages for these events:

       * A function can return either with or without a value.  (Falling off the end of the function body is considered returning without a value.)  For example, this function would evoke such a warning:

               foo (a)
               {
                 if (a > 0)
                   return a;
               }

       * An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] will cause a warning, but x[(void)i,j] will not.

       * An unsigned value is compared against zero with < or >=.

       * Storage-class specifiers like static are not the first things in a declaration.  According to the C Standard, this usage is obsolescent.

       * If -Wall or -Wunused is also specified, warn about unused arguments.

       * A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But don't warn if -Wno-sign-compare is also specified.)

       * An aggregate has an initializer which does not initialize all members. This warning can be independently controlled by -Wmissing-field-initializers.

       * A function parameter is declared without a type specifier in K&R-style functions:

               void foo(bar) { }

       * An empty body occurs in an if or else statement.

       * A pointer is compared against integer zero with <, <=, >, or >=.

       * A variable might be changed by longjmp or vfork.

       * Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of precision, etc.

       * (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.

       * (C++ only) A non-static reference or non-static const member appears in a class without constructors.

       * (C++ only) Ambiguous virtual bases.

       * (C++ only) Subscripting an array which has been declared register.

       * (C++ only) Taking the address of a variable which has been declared register.

       * (C++ only) A base class is not initialized in a derived class' copy constructor.

-Wno-div-by-zero
     Do not warn about compile-time integer division by zero.  Floating point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

-Wsystem-headers
     Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read.  Using this command line option tells GCC to emit warnings from system headers as if they occurred in user code.  However, note that using -Wall in conjunction with this option will not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.

-Wfloat-equal
     Warn if floating point values are used in equality comparisons.

     The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers.  If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem).  In particular, instead of testing for equality, you would check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

-Wtraditional (C only)
     Warn about certain constructs that behave differently in traditional and ISO C.  Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs which should be avoided.

       * Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but does not in ISO C.

       * In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line to be a directive if the # appeared in column 1 on the line.  Therefore -Wtraditional warns about directives that traditional C understands but would ignore because the # does not appear as the first character on the line.  It also suggests you hide directives like #pragma not understood by traditional C by indenting them.  Some traditional implementations would not recognize #elif, so it suggests avoiding it altogether.

       * A function-like macro that appears without arguments.

       * The unary plus operator.

       * The U integer constant suffix, or the F or L floating point constant suffixes.  (Traditional C does support the L suffix on integer constants.)  Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.

       * A function declared external in one block and then used after the end of the block.

       * A switch statement has an operand of type long.

       * A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.

       * The ISO type of an integer constant has a different width or signedness from its traditional type.  This warning is only issued if the base of the constant is ten.  I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.

       * Usage of ISO string concatenation is detected.

       * Initialization of automatic aggregates.

       * Identifier conflicts with labels.  Traditional C lacks a separate namespace for labels.

       * Initialization of unions.  If the initializer is zero, the warning is omitted.  This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.

       * Conversions by prototypes between fixed/floating point values and vice versa.  The absence of these prototypes when compiling with traditional C would cause serious problems.  This is a subset of the possible conversion warnings, for the full set use -Wconversion.

       * Use of ISO C style function definitions.  This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features will appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS.  This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.

-Wdeclaration-after-statement (C only)
     Warn when a declaration is found after a statement in a block.  This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC.  It is not supported by ISO C90 and was not supported by GCC versions before GCC 3.0.  See Mixed Declarations.

-Wundef
     Warn if an undefined identifier is evaluated in an #if directive.

-Wno-endif-labels
     Do not warn whenever an #else or an #endif are followed by text.

-Wshadow
     Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a built-in function is shadowed.

-Wlarger-than-LEN
     Warn whenever an object of larger than LEN bytes is defined.

-Wpointer-arith
     Warn about anything that depends on the ``size of'' a function type or of void.  GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions.

-Wbad-function-cast (C only)
     Warn whenever a function call is cast to a non-matching type. For example, warn if int malloc() is cast to anything *.

-Wcast-qual
     Warn whenever a pointer is cast so as to remove a type qualifier from the target type.  For example, warn if a const char * is cast to an ordinary char *.

-Wcast-align
     Warn whenever a pointer is cast such that the required alignment of the target is increased.  For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

-Wwrite-strings
     When compiling C, give string constants the type const char[LENGTH] so that copying the address of one into a non-const char * pointer will get a warning; when compiling C++, warn about the deprecated conversion from string constants to char *. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes.  Otherwise, it will just be a nuisance; this is why we did not make -Wall request these warnings.

-Wconversion
     Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype.  This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion.

     Also, warn if a negative integer constant expression is implicitly converted to an unsigned type.  For example, warn about the assignment x = -1 if x is unsigned.  But do not warn about explicit casts like (unsigned) -1.

-Wsign-compare
     Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get the other warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare.

-Waggregate-return
     Warn if any functions that return structures or unions are defined or called.  (In languages where you can return an array, this also elicits a warning.)

-Wstrict-prototypes (C only)
     Warn if a function is declared or defined without specifying the argument types.  (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.)

-Wold-style-definition (C only)
     Warn if an old-style function definition is used.  A warning is given even if there is a previous prototype.

-Wmissing-prototypes (C only)
     Warn if a global function is defined without a previous prototype declaration.  This warning is issued even if the definition itself provides a prototype.  The aim is to detect global functions that fail to be declared in header files.

-Wmissing-declarations (C only)
     Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files.

-Wmissing-field-initializers
     Warn if a structure's initializer has some fields missing.  For example, the following code would cause such a warning, because x.h is implicitly zero:

          struct s { int f, g, h; };
          struct s x = { 3, 4 };

     This option does not warn about designated initializers, so the following modification would not trigger a warning:

          struct s { int f, g, h; };
          struct s x = { .f = 3, .g = 4 };

     This warning is included in -Wextra.  To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers.

-Wmissing-noreturn
     Warn about functions which might be candidates for attribute noreturn. Note these are only possible candidates, not absolute ones.  Care should be taken to manually verify functions actually do not ever return before adding the noreturn attribute, otherwise subtle code generation bugs could be introduced.  You will not get a warning for main in hosted C environments.

-Wmissing-format-attribute
     If -Wformat is enabled, also warn about functions which might be candidates for format attributes.  Note these are only possible candidates, not absolute ones.  GCC will guess that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected.  This option has no effect unless -Wformat is enabled (possibly by -Wall).

-Wno-multichar
     Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code.

-Wnormalized=<none|id|nfc|nfkc>
     In ISO C and ISO C++, two identifiers are different if they are different sequences of characters.  However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same.  To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence.  GCC can warn you if you are using identifiers which have not been normalized; this option controls that warning.

     There are four levels of warning that GCC supports.  The default is -Wnormalized=nfc, which warns about any identifier which is not in the ISO 10646 ``C'' normalized form, NFC.  NFC is the recommended form for most uses.

     Unfortunately, there are some characters which ISO C and ISO C++ allow in identifiers that when turned into NFC aren't allowable as identifiers.  That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.

     You can switch the warning off for all characters by writing -Wnormalized=none.  You would only want to do this if you were using some other normalization scheme (like ``D''), because otherwise you can easily create bugs that are literally impossible to see.

     Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied.  For instance \u207F, ``SUPERSCRIPT LATIN SMALL LETTER N'', will display just like a regular n which has been placed in a superscript.  ISO 10646 defines the NFKC normalisation scheme to convert all these into a standard form as well, and GCC will warn if your code is not in NFKC if you use -Wnormalized=nfkc.  This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment is unable to be fixed to display these characters distinctly.

-Wno-deprecated-declarations
     Do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. (see Function Attributes, see Variable Attributes, see Type Attributes.)

-Wpacked
     Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit.  For instance, in this code, the variable f.x in struct bar will be misaligned even though struct bar does not itself have the packed attribute:

          struct foo {
            int x;
            char a, b, c, d;
          } __attribute__((packed));
          struct bar {
            char z;
            struct foo f;
          };

-Wpadded
     Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure.  Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.

-Wredundant-decls
     Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.

-Wnested-externs (C only)
     Warn if an extern declaration is encountered within a function.

-Wunreachable-code
     Warn if the compiler detects that code will never be executed.

     This option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.

     It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.

     For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.

     This option is not made part of -Wall because in a debugging version of a program there is often substantial code which checks correct functioning of the program and is, hopefully, unreachable because the program does work.  Another common use of unreachable code is to provide behavior which is selectable at compile-time.

-Winline
     Warn if a function can not be inlined and it was declared as inline. Even with this option, the compiler will not warn about failures to inline functions declared in system headers.

     The compiler uses a variety of heuristics to determine whether or not to inline a function.  For example, the compiler takes into account the size of the function being inlined and the the amount of inlining that has already been done in the current function.  Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear.

-Wno-invalid-offsetof (C++ only)
     Suppress warnings from applying the offsetof macro to a non-POD type.  According to the 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined.  In existing C++ implementations, however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only by virtue of having a constructor.)  This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

     The restrictions on offsetof may be relaxed in a future version of the C++ standard.

-Winvalid-pch
     Warn if a precompiled header (see Precompiled Headers) is found in the search path but can't be used.

-Wlong-long
     Warn if long long type is used.  This is default.  To inhibit the warning messages, use -Wno-long-long.  Flags -Wlong-long and -Wno-long-long are taken into account only when -pedantic flag is used.

-Wvariadic-macros
     Warn if variadic macros are used in pedantic ISO C90 mode, or the GNU alternate syntax when in pedantic ISO C99 mode.  This is default. To inhibit the warning messages, use -Wno-variadic-macros.

-Wdisabled-optimization
     Warn if a requested optimization pass is disabled.  This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers were unable to handle the code effectively.  Often, the problem is that your code is too big or too complex; GCC will refuse to optimize programs when the optimization itself is likely to take inordinate amounts of time.

-Wno-pointer-sign
     Don't warn for pointer argument passing or assignment with different signedness. Only useful in the negative form since this warning is enabled by default. This option is only supported for C and Objective-C.

-Werror
     Make all warnings into errors.

Options for Debugging Your Program or GCC
=========================================

GCC has various special options that are used for debugging either your program or GCC:

-g
     Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2).  GDB can work with this debugging information.

     On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program.  If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).

     GCC allows you to use -g with -O.  The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.

     Nevertheless it proves possible to debug optimized output.  This makes it reasonable to use the optimizer for programs that might have bugs.

     The following options are useful when GCC is generated with the capability for more than one debugging format.

-ggdb
     Produce debugging information for use by GDB.  This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.

-gstabs
     Produce debugging information in stabs format (if that is supported), without GDB extensions.  This is the format used by DBX on most BSD systems.  On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler.

-feliminate-unused-debug-symbols
     Produce debugging information in stabs format (if that is supported), for only symbols that are actually used.

-gstabs+
     Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB).  The use of these extensions is likely to make other debuggers crash or refuse to read the program.

-gcoff
     Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

-gxcoff
     Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

-gxcoff+
     Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB).  The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.

-gdwarf-2
     Produce debugging information in DWARF version 2 format (if that is supported).  This is the format used by DBX on IRIX 6.  With this option, GCC uses features of DWARF version 3 when they are useful; version 3 is upward compatible with version 2, but may still cause problems for older debuggers.

-gvms
     Produce debugging information in VMS debug format (if that is supported).  This is the format used by DEBUG on VMS systems.

-gLEVEL
-ggdbLEVEL
-gstabsLEVEL
-gcoffLEVEL
-gxcoffLEVEL
-gvmsLEVEL
     Request debugging information and also use LEVEL to specify how much information.  The default level is 2.

     Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug.  This includes descriptions of functions and external variables, but no information about local variables and no line numbers.

     Level 3 includes extra information, such as all the macro definitions present in the program.  Some debuggers support macro expansion when you use -g3.

     -gdwarf-2 does not accept a concatenated debug level, because GCC used to support an option -gdwarf that meant to generate debug information in version 1 of the DWARF format (which is very different from version 2), and it would have been too confusing.  That debug format is long obsolete, but the option cannot be changed now. Instead use an additional -gLEVEL option to change the debug level for DWARF2.

-feliminate-dwarf2-dups
     Compress DWARF2 debugging information by eliminating duplicated information about each symbol.  This option only makes sense when generating DWARF2 debugging information with -gdwarf-2.

-p
     Generate extra code to write profile information suitable for the analysis program prof.  You must use this option when compiling the source files you want data about, and you must also use it when linking.

-pg
     Generate extra code to write profile information suitable for the analysis program gprof.  You must use this option when compiling the source files you want data about, and you must also use it when linking.

-Q
     Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ftime-report
     Makes the compiler print some statistics about the time consumed by each pass when it finishes.

-fmem-report
     Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fprofile-arcs
     Add code so that program flow arcs are instrumented.  During execution the program records how many times each branch and call is executed and how many times it is taken or returns.  When the compiled program exits it saves this data to a file called AUXNAME.gcda for each source file.  The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage).  Each object file's AUXNAME is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file.  In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

--coverage

     This option is used to compile and link code instrumented for coverage analysis.  The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking).  See the documentation for those options for more details.

       * Compile the source files with -fprofile-arcs plus optimization and code generation options.  For test coverage analysis, use the additional -ftest-coverage option.  You do not need to profile every source file in a program.

       * Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).

       * Run the program on a representative workload to generate the arc profile information.  This may be repeated any number of times.  You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated.  Also fork calls are detected and correctly handled (double counting will not happen).

       * For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities (see Options that Control Optimization).

       * For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files.  Refer to the gcov documentation for further information.

     With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed.  When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

-ftree-based-profiling
     This option is used in addition to -fprofile-arcs or -fbranch-probabilities to control whether those optimizations are performed on a tree-based or rtl-based internal representation. If you use this option when compiling with -fprofile-arcs, you must also use it when compiling later with -fbranch-probabilities. Currently the tree-based optimization is in an early stage of development, and this option is recommended only for those people working on improving it.

-ftest-coverage
     Produce a notes file that the gcov code-coverage utility (see gcov---a Test Coverage Program) can use to show program coverage.  Each source file's note file is called AUXNAME.gcno.  Refer to the -fprofile-arcs option above for a description of AUXNAME and instructions on how to generate test coverage data.  Coverage data will match the source files more closely, if you do not optimize.

-dLETTERS

-fdump-rtl-PASS
     Says to make debugging dumps during compilation at times specified by LETTERS.    This is used for debugging the RTL-based passes of the compiler.  The file names for most of the dumps are made by appending a pass number and a word to the DUMPNAME.  DUMPNAME is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file.

     Most debug dumps can be enabled either passing a letter to the -d option, or with a long -fdump-rtl switch; here are the possible letters for use in LETTERS and PASS, and their meanings:

     -dA
          Annotate the assembler output with miscellaneous debugging information.

     -db
     -fdump-rtl-bp
          Dump after computing branch probabilities, to FILE.09.bp.

     -dB
     -fdump-rtl-bbro
          Dump after block reordering, to FILE.30.bbro.

     -dc
     -fdump-rtl-combine
          Dump after instruction combination, to the file FILE.17.combine.

     -dC
     -fdump-rtl-ce1
     -fdump-rtl-ce2
          -dC and -fdump-rtl-ce1 enable dumping after the first if conversion, to the file FILE.11.ce1.  -dC and -fdump-rtl-ce2 enable dumping after the second if conversion, to the file FILE.18.ce2.

     -dd
     -fdump-rtl-btl
     -fdump-rtl-dbr
          -dd and -fdump-rtl-btl enable dumping after branch target load optimization, to to FILE.31.btl.  -dd and -fdump-rtl-dbr enable dumping after delayed branch scheduling, to FILE.36.dbr.

     -dD
          Dump all macro definitions, at the end of preprocessing, in addition to normal output.

     -dE
     -fdump-rtl-ce3
          Dump after the third if conversion, to FILE.28.ce3.

     -df
     -fdump-rtl-cfg
     -fdump-rtl-life
          -df and -fdump-rtl-cfg enable dumping after control and data flow analysis, to FILE.08.cfg.  -df and -fdump-rtl-cfg enable dumping dump after life analysis, to FILE.16.life.

     -dg
     -fdump-rtl-greg
          Dump after global register allocation, to FILE.23.greg.

     -dG
     -fdump-rtl-gcse
     -fdump-rtl-bypass
          -dG and -fdump-rtl-gcse enable dumping after GCSE, to FILE.05.gcse.  -dG and -fdump-rtl-bypass enable dumping after jump bypassing and control flow optimizations, to FILE.07.bypass.

     -dh
     -fdump-rtl-eh
          Dump after finalization of EH handling code, to FILE.02.eh.

     -di
     -fdump-rtl-sibling
          Dump after sibling call optimizations, to FILE.01.sibling.

     -dj
     -fdump-rtl-jump
          Dump after the first jump optimization, to FILE.03.jump.

     -dk
     -fdump-rtl-stack
          Dump after conversion from registers to stack, to FILE.33.stack.

     -dl
     -fdump-rtl-lreg
          Dump after local register allocation, to FILE.22.lreg.

     -dL
     -fdump-rtl-loop
     -fdump-rtl-loop2
          -dL and -fdump-rtl-loop enable dumping after the first loop optimization pass, to FILE.06.loop.  -dL and -fdump-rtl-loop2 enable dumping after the second pass, to FILE.13.loop2.

     -dm
     -fdump-rtl-sms
          Dump after modulo scheduling, to FILE.20.sms.

     -dM
     -fdump-rtl-mach
          Dump after performing the machine dependent reorganization pass, to FILE.35.mach.

     -dn
     -fdump-rtl-rnreg
          Dump after register renumbering, to FILE.29.rnreg.

     -dN
     -fdump-rtl-regmove
          Dump after the register move pass, to FILE.19.regmove.

     -do
     -fdump-rtl-postreload
          Dump after post-reload optimizations, to FILE.24.postreload.

     -dr
     -fdump-rtl-expand
          Dump after RTL generation, to FILE.00.expand.

     -dR
     -fdump-rtl-sched2
          Dump after the second scheduling pass, to FILE.32.sched2.

     -ds
     -fdump-rtl-cse
          Dump after CSE (including the jump optimization that sometimes follows CSE), to FILE.04.cse.

     -dS
     -fdump-rtl-sched
          Dump after the first scheduling pass, to FILE.21.sched.

     -dt
     -fdump-rtl-cse2
          Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to FILE.15.cse2.

     -dT
     -fdump-rtl-tracer
          Dump after running tracer, to FILE.12.tracer.

     -dV
     -fdump-rtl-vpt
     -fdump-rtl-vartrack
          -dV and -fdump-rtl-vpt enable dumping after the value profile transformations, to FILE.10.vpt.  -dV and -fdump-rtl-vartrack enable dumping after variable tracking, to FILE.34.vartrack.

     -dw
     -fdump-rtl-flow2
          Dump after the second flow pass, to FILE.26.flow2.

     -dz
     -fdump-rtl-peephole2
          Dump after the peephole pass, to FILE.27.peephole2.

     -dZ
     -fdump-rtl-web
          Dump after live range splitting, to FILE.14.web.

     -da
     -fdump-rtl-all
          Produce all the dumps listed above.

     -dH
          Produce a core dump whenever an error occurs.

     -dm
          Print statistics on memory usage, at the end of the run, to standard error.

     -dp
          Annotate the assembler output with a comment indicating which pattern and alternative was used.  The length of each instruction is also printed.

     -dP
          Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.

     -dv
          For each of the other indicated dump files (either with -d or -fdump-rtl-PASS), dump a representation of the control flow graph suitable for viewing with VCG to FILE.PASS.vcg.

     -dx
          Just generate RTL for a function instead of compiling it.  Usually used with r (-fdump-rtl-expand).

     -dy
          Dump debugging information during parsing, to standard error.

-fdump-unnumbered
     When doing debugging dumps (see -d option above), suppress instruction numbers and line number note output.  This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g.

-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-OPTIONS (C and C++ only)
     Dump a representation of the tree structure for the entire translation unit to a file.  The file name is made by appending .tu to the source file name.  If the -OPTIONS form is used, OPTIONS controls the details of the dump as described for the -fdump-tree options.

-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-OPTIONS (C++ only)
     Dump a representation of each class's hierarchy and virtual function table layout to a file.  The file name is made by appending .class to the source file name.  If the -OPTIONS form is used, OPTIONS controls the details of the dump as described for the -fdump-tree options.

-fdump-ipa-SWITCH
     Control the dumping at various stages of inter-procedural analysis language tree to a file.  The file name is generated by appending a switch specific suffix to the source file name.  The following dumps are possible:

     all
          Enables all inter-procedural analysis dumps; currently the only produced dump is the cgraph dump.

     cgraph
          Dumps information about call-graph optimization, unused function removal, and inlining decisions.

-fdump-tree-SWITCH (C and C++ only)
-fdump-tree-SWITCH-OPTIONS (C and C++ only)
     Control the dumping at various stages of processing the intermediate language tree to a file.  The file name is generated by appending a switch specific suffix to the source file name.  If the -OPTIONS form is used, OPTIONS is a list of - separated options that control the details of the dump.  Not all options are applicable to all dumps, those which are not meaningful will be ignored.  The following options are available

     address
          Print the address of each node.  Usually this is not meaningful as it changes according to the environment and source file.  Its primary use is for tying up a dump file with a debug environment.

     slim
          Inhibit dumping of members of a scope or body of a function merely because that scope has been reached.  Only dump such items when they are directly reachable by some other path.  When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

     raw
          Print a raw representation of the tree.  By default, trees are pretty-printed into a C-like representation.

     details
          Enable more detailed dumps (not honored by every dump option).

     stats
          Enable dumping various statistics about the pass (not honored by every dump option).

     blocks
          Enable showing basic block boundaries (disabled in raw dumps).

     vops
          Enable showing virtual operands for every statement.

     lineno
          Enable showing line numbers for statements.

     uid
          Enable showing the unique ID (DECL_UID) for each variable.

     all
          Turn on all options, except raw, slim and lineno.

     The following tree dumps are possible:

     original
          Dump before any tree based optimization, to FILE.original.

     optimized
          Dump after all tree based optimization, to FILE.optimized.

     inlined
          Dump after function inlining, to FILE.inlined.

     gimple
          Dump each function before and after the gimplification pass to a file.  The file name is made by appending .gimple to the source file name.

     cfg
          Dump the control flow graph of each function to a file.  The file name is made by appending .cfg to the source file name.

     vcg
          Dump the control flow graph of each function to a file in VCG format.  The file name is made by appending .vcg to the source file name.  Note that if the file contains more than one function, the generated file cannot be used directly by VCG.  You will need to cut and paste each function's graph into its own separate file first.

     ch
          Dump each function after copying loop headers.  The file name is made by appending .ch to the source file name.

     ssa
          Dump SSA related information to a file.  The file name is made by appending .ssa to the source file name.

     salias
          Dump structure aliasing variable information to a file.  This file name is made by appending .salias to the source file name.

     alias
          Dump aliasing information for each function.  The file name is made by appending .alias to the source file name.

     ccp
          Dump each function after CCP.  The file name is made by appending .ccp to the source file name.

     pre
          Dump trees after partial redundancy elimination.  The file name is made by appending .pre to the source file name.

     fre
          Dump trees after full redundancy elimination.  The file name is made by appending .fre to the source file name.

     dce
          Dump each function after dead code elimination.  The file name is made by appending .dce to the source file name.

     mudflap
          Dump each function after adding mudflap instrumentation.  The file name is made by appending .mudflap to the source file name.

     sra
          Dump each function after performing scalar replacement of aggregates.  The file name is made by appending .sra to the source file name.

     sink
          Dump each function after performing code sinking.  The file name is made by appending .sink to the source file name.

     dom
          Dump each function after applying dominator tree optimizations.  The file name is made by appending .dom to the source file name.

     dse
          Dump each function after applying dead store elimination.  The file name is made by appending .dse to the source file name.

     phiopt
          Dump each function after optimizing PHI nodes into straightline code.  The file name is made by appending .phiopt to the source file name.

     forwprop
          Dump each function after forward propagating single use variables.  The file name is made by appending .forwprop to the source file name.

     copyrename
          Dump each function after applying the copy rename optimization.  The file name is made by appending .copyrename to the source file name.

     nrv
          Dump each function after applying the named return value optimization on generic trees.  The file name is made by appending .nrv to the source file name.

     vect
          Dump each function after applying vectorization of loops.  The file name is made by appending .vect to the source file name.

     all
          Enable all the available tree dumps with the flags provided in this option.

-ftree-vectorizer-verbose=N
     This option controls the amount of debugging output the vectorizer prints. This information is written to standard error, unless -fdump-tree-all or -fdump-tree-vect is specified, in which case it is output to the usual dump listing file, .vect.

-frandom-seed=STRING
     This option provides a seed that GCC uses when it would otherwise use random numbers.  It is used to generate certain symbol names that have to be different in every compiled file.  It is also used to place unique stamps in coverage data files and the object files that produce them.  You can use the -frandom-seed option to produce reproducibly identical object files.

     The STRING should be different for every file you compile.

-fsched-verbose=N
     On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints.  This information is written to standard error, unless -dS or -dR is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively.  However for N greater than nine, the output is always printed to standard error.

     For N greater than zero, -fsched-verbose outputs the same information as -dRS.  For N greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info.  For N greater than two, it includes RTL at abort point, control-flow and regions info.  And for N over four, -fsched-verbose also includes dependence info.

-save-temps
     Store the usual ``temporary'' intermediate files permanently; place them in the current directory and name them based on the source file.  Thus, compiling foo.c with -c -save-temps would produce files foo.i and foo.s, as well as foo.o.  This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor.

     When used in combination with the -x command line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.

-time
     Report the CPU time taken by each subprocess in the compilation sequence.  For C source files, this is the compiler proper and assembler (plus the linker if linking is done).  The output looks like this:

          # cc1 0.12 0.01
          # as 0.00 0.01

     The first number on each line is the ``user time'', that is time spent executing the program itself.  The second number is ``system time'', time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

-fvar-tracking
     Run variable tracking pass.  It computes where variables are stored at each position in code.  Better debugging information is then generated (if the debugging information format supports this information).

     It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.

-print-file-name=LIBRARY
     Print the full absolute name of the library file LIBRARY that would be used when linking---and don't do anything else.  With this option, GCC does not compile or link anything; it just prints the file name.

-print-multi-directory
     Print the directory name corresponding to the multilib selected by any other switches present in the command line.  This directory is supposed to exist in GCC_EXEC_PREFIX.

-print-multi-lib
     Print the mapping from multilib directory names to compiler switches that enable them.  The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches.  This is supposed to ease shell-processing.

-print-prog-name=PROGRAM
     Like -print-file-name, but searches for a program such as cpp.

-print-libgcc-file-name
     Same as -print-file-name=libgcc.a.

     This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a.  You can do

          gcc -nostdlib FILES... `gcc -print-libgcc-file-name`

-print-search-dirs
     Print the name of the configured installation directory and a list of program and library directories gcc will search---and don't do anything else.

     This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. See Environment Variables.

-dumpmachine
     Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.

-dumpversion
     Print the compiler version (for example, 3.0)---and don't do anything else.

-dumpspecs
     Print the compiler's built-in specs---and don't do anything else.  (This is used when GCC itself is being built.)  See Spec Files.

-feliminate-unused-debug-types
     Normally, when producing DWARF2 output, GCC will emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit.  Sometimes this is useful, such as if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared).  More often, however, this results in a significant amount of wasted space. With this option, GCC will avoid producing debug symbol output for types that are nowhere used in the source file being compiled.

Options That Control Optimization
=================================

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results.  Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program.  Optimization levels -O2 and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function.  Compiling multiple files at once to a single output file in unit-at-a-time mode allows the compiler to use information gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag.  Only optimizations that have a flag are listed.

-O
-O1
     Optimize.  Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

     With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

     -O turns on the following optimization flags: -fdefer-pop -fdelayed-branch -fguess-branch-probability -fcprop-registers -floop-optimize -fif-conversion -fif-conversion2 -ftree-ccp -ftree-dce -ftree-dom -ftree-dse -ftree-ter -ftree-live_range_split -ftree-sra -ftree-copyrename -ftree-fre -ftree-ch -fmerge-constants

     -O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

-O2
     Optimize even more.  GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff.  The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.

     -O2 turns on all optimization flags specified by -O.  It also turns on the following optimization flags: -fthread-jumps -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps  -fcse-skip-blocks -fgcse  -fgcse-lm -fexpensive-optimizations -fstrength-reduce -frerun-cse-after-loop  -frerun-loop-opt -fcaller-saves -fforce-mem -fpeephole2 -fschedule-insns  -fschedule-insns2 -fsched-interblock  -fsched-spec -fregmove -fstrict-aliasing -fdelete-null-pointer-checks -freorder-blocks  -freorder-functions -funit-at-a-time -falign-functions  -falign-jumps -falign-loops  -falign-labels -ftree-pre

     Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.

-O3
     Optimize yet more.  -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops and -fgcse-after-reload options.

-O0
     Do not optimize.  This is the default.

-Os
     Optimize for size.  -Os enables all -O2 optimizations that do not typically increase code size.  It also performs further optimizations designed to reduce code size.

     -Os disables the following optimization flags: -falign-functions  -falign-jumps  -falign-loops -falign-labels  -freorder-blocks  -freorder-blocks-and-partition -fprefetch-loop-arrays

     If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

Options of the form -fFLAG specify machine-independent flags.  Most flags have both positive and negative forms; the negative form of -ffoo would be -fno-foo.  In the table below, only one of the forms is listed---the one you typically will use.  You can figure out the other form by either removing no- or adding it.

The following options control specific optimizations.  They are either activated by -O options or are related to ones that are.  You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.

-fno-default-inline
     Do not make member functions inline by default merely because they are defined inside the class scope (C++ only).  Otherwise, when you specify -O, member functions defined inside class scope are compiled inline by default; i.e., you don't need to add inline in front of the member function name.

-fno-defer-pop
     Always pop the arguments to each function call as soon as that function returns.  For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.

     Disabled at levels -O, -O2, -O3, -Os.

-fforce-mem
     Force memory operands to be copied into registers before doing arithmetic on them.  This produces better code by making all memory references potential common subexpressions.  When they are not common subexpressions, instruction combination should eliminate the separate register-load.

     Enabled at levels -O2, -O3, -Os.

-fforce-addr
     Force memory address constants to be copied into registers before doing arithmetic on them.  This may produce better code just as -fforce-mem may.

-fomit-frame-pointer
     Don't keep the frame pointer in a register for functions that don't need one.  This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions.  It also makes debugging impossible on some machines.

     On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist.  The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag.  See Register Usage (GNU Compiler Collection (GCC) Internals).

     Enabled at levels -O, -O2, -O3, -Os.

-foptimize-sibling-calls
     Optimize sibling and tail recursive calls.

     Enabled at levels -O2, -O3, -Os.

-fno-inline
     Don't pay attention to the inline keyword.  Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.

-finline-functions
     Integrate all simple functions into their callers.  The compiler heuristically decides which functions are simple enough to be worth integrating in this way.

     If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

     Enabled at level -O3.

-finline-limit=N
     By default, GCC limits the size of functions that can be inlined.  This flag allows the control of this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword or defined within the class definition in c++).  N is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling).  The default value of N is 600. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption.  Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs).  This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with C++.

     Inlining is actually controlled by a number of parameters, which may be specified individually by using --param NAME=VALUE. The -finline-limit=N option sets some of these parameters as follows:

     max-inline-insns-single
          is set to N/2.

     max-inline-insns-auto
          is set to N/2.

     min-inline-insns
          is set to 130 or N/4, whichever is smaller.

     max-inline-insns-rtl
          is set to N.

     See below for a documentation of the individual parameters controlling inlining.

     Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size.  In no way, it represents a count of assembly instructions and as such its exact meaning might change from one release to an another.

-fkeep-inline-functions
     In C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers.  This switch does not affect functions using the extern inline extension in GNU C.  In C++, emit any and all inline functions into the object file.

-fkeep-static-consts
     Emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

     GCC enables this option by default.  If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

-fmerge-constants
     Attempt to merge identical constants (string constants and floating point constants) across compilation units.

     This option is the default for optimized compilation if the assembler and linker support it.  Use -fno-merge-constants to inhibit this behavior.

     Enabled at levels -O, -O2, -O3, -Os.

-fmerge-all-constants
     Attempt to merge identical constants and identical variables.

     This option implies -fmerge-constants.  In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types.  Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conforming behavior.

-fmodulo-sched
     Perform swing modulo scheduling immediately before the first scheduling pass.  This pass looks at innermost loops and reorders their instructions by overlapping different iterations.

-fno-branch-count-reg
     Do not use ``decrement and branch'' instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.

     The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.

-fno-function-cse
     Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.

     This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.

     The default is -ffunction-cse

-fno-zero-initialized-in-bss
     If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS.  This can save space in the resulting code.

     This option turns off this behavior because some programs explicitly rely on variables going to the data section.  E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.

     The default is -fzero-initialized-in-bss.

-fbounds-check
     For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range.  This is currently only supported by the Java and Fortran front-ends, where this option defaults to true and false respectively.

-fmudflap -fmudflapth -fmudflapir
     For front-ends that support it (C and C++), instrument all risky pointer/array dereferencing operations, some standard library string/heap functions, and some other associated constructs with range/validity tests.  Modules so instrumented should be immune to buffer overflows, invalid heap use, and some other classes of C/C++ programming errors.  The instrumentation relies on a separate runtime library (libmudflap), which will be linked into a program if -fmudflap is given at link time.  Run-time behavior of the instrumented program is controlled by the MUDFLAP_OPTIONS environment variable.  See env MUDFLAP_OPTIONS=-help a.out for its options.

     Use -fmudflapth instead of -fmudflap to compile and to link if your program is multi-threaded.  Use -fmudflapir, in addition to -fmudflap or -fmudflapth, if instrumentation should ignore pointer reads.  This produces less instrumentation (and therefore faster execution) and still provides some protection against outright memory corrupting writes, but allows erroneously read data to propagate within a program.

-fstrength-reduce
     Perform the optimizations of loop strength reduction and elimination of iteration variables.

     Enabled at levels -O2, -O3, -Os.

-fthread-jumps
     Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found.  If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

     Enabled at levels -O2, -O3, -Os.

-fcse-follow-jumps
     In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path.  For example, when CSE encounters an if statement with an else clause, CSE will follow the jump when the condition tested is false.

     Enabled at levels -O2, -O3, -Os.

-fcse-skip-blocks
     This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which conditionally skip over blocks.  When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

     Enabled at levels -O2, -O3, -Os.

-frerun-cse-after-loop
     Re-run common subexpression elimination after loop optimizations has been performed.

     Enabled at levels -O2, -O3, -Os.

-frerun-loop-opt
     Run the loop optimizer twice.

     Enabled at levels -O2, -O3, -Os.

-fgcse
     Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

     Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

     Enabled at levels -O2, -O3, -Os.

-fgcse-lm
     When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads which are only killed by stores into themselves.  This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.

     Enabled by default when gcse is enabled.

-fgcse-sm
     When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination.  This pass will attempt to move stores out of loops.  When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.

     Not enabled at any optimization level.

-fgcse-las
     When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).

     Not enabled at any optimization level.

-fgcse-after-reload
     When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload.  The purpose of this pass is to cleanup redundant spilling.

-floop-optimize
     Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction as well.

     Enabled at levels -O, -O2, -O3, -Os.

-floop-optimize2
     Perform loop optimizations using the new loop optimizer.  The optimizations (loop unrolling, peeling and unswitching, loop invariant motion) are enabled by separate flags.

-fcrossjumping
     Perform cross-jumping transformation.  This transformation unifies equivalent code and save code size.  The resulting code may or may not perform better than without cross-jumping.

     Enabled at levels -O2, -O3, -Os.

-fif-conversion
     Attempt to transform conditional jumps into branch-less equivalents.  This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard arithmetics.  The use of conditional execution on chips where it is available is controlled by if-conversion2.

     Enabled at levels -O, -O2, -O3, -Os.

-fif-conversion2
     Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.

     Enabled at levels -O, -O2, -O3, -Os.

-fdelete-null-pointer-checks
     Use global dataflow analysis to identify and eliminate useless checks for null pointers.  The compiler assumes that dereferencing a null pointer would have halted the program.  If a pointer is checked after it has already been dereferenced, it cannot be null.

     In some environments, this assumption is not true, and programs can safely dereference null pointers.  Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.

     Enabled at levels -O2, -O3, -Os.

-fexpensive-optimizations
     Perform a number of minor optimizations that are relatively expensive.

     Enabled at levels -O2, -O3, -Os.

-foptimize-register-move
-fregmove
     Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying.  This is especially helpful on machines with two-operand instructions.

     Note -fregmove and -foptimize-register-move are the same optimization.

     Enabled at levels -O2, -O3, -Os.

-fdelayed-branch
     If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

     Enabled at levels -O, -O2, -O3, -Os.

-fschedule-insns
     If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable.  This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.

     Enabled at levels -O2, -O3, -Os.

-fschedule-insns2
     Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done.  This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

     Enabled at levels -O2, -O3, -Os.

-fno-sched-interblock
     Don't schedule instructions across basic blocks.  This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fno-sched-spec
     Don't allow speculative motion of non-load instructions.  This is normally enabled by default when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load
     Allow speculative motion of some load instructions.  This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-spec-load-dangerous
     Allow speculative motion of more load instructions.  This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.

-fsched-stalled-insns=N
     Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list, during the second scheduling pass.

-fsched-stalled-insns-dep=N
     Define how many insn groups (cycles) will be examined for a dependency on a stalled insn that is candidate for premature removal from the queue of stalled insns.  Has an effect only during the second scheduling pass, and only if -fsched-stalled-insns is used and its value is not zero.

-fsched2-use-superblocks
     When scheduling after register allocation, do use superblock scheduling algorithm.  Superblock scheduling allows motion across basic block boundaries resulting on faster schedules.  This option is experimental, as not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm.

     This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-fsched2-use-traces
     Use -fsched2-use-superblocks algorithm when scheduling after register allocation and additionally perform code duplication in order to increase the size of superblocks using tracer pass.  See -ftracer for details on trace formation.

     This mode should produce faster but significantly longer programs.  Also without -fbranch-probabilities the traces constructed may not match the reality and hurt the performance.  This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

-freschedule-modulo-scheduled-loops
     The modulo scheduling comes before the traditional scheduling, if a loop was modulo scheduled we may want to prevent the later scheduling passes from changing its schedule, we use this option to control that.

-fcaller-saves
     Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls.  Such allocation is done only when it seems to result in better code than would otherwise be produced.

     This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

     Enabled at levels -O2, -O3, -Os.

-ftree-pre
     Perform Partial Redundancy Elimination (PRE) on trees.  This flag is enabled by default at -O2 and -O3.

-ftree-fre
     Perform Full Redundancy Elimination (FRE) on trees.  The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O and higher.

-ftree-salias
     Perform structural alias analysis on trees.  This flag is enabled by default at -O and higher.

-ftree-sink
     Perform forward store motion  on trees.  This flag is enabled by default at -O and higher.

-ftree-ccp
     Perform sparse conditional constant propagation (CCP) on trees.  This flag is enabled by default at -O and higher.

-ftree-dce
     Perform dead code elimination (DCE) on trees.  This flag is enabled by default at -O and higher.

-ftree-dominator-opts
     Perform dead code elimination (DCE) on trees.  This flag is enabled by default at -O and higher.

-ftree-ch
     Perform loop header copying on trees.  This is beneficial since it increases effectiveness of code motion optimizations.  It also saves one jump.  This flag is enabled by default at -O and higher.  It is not enabled for -Os, since it usually increases code size.

-ftree-loop-optimize
     Perform loop optimizations on trees.  This flag is enabled by default at -O and higher.

-ftree-loop-linear
     Perform linear loop transformations on tree.  This flag can improve cache performance and allow further loop optimizations to take place.

-ftree-lim
     Perform loop invariant motion on trees.  This pass moves only invartiants that would be hard to handle on rtl level (function calls, operations that expand to nontrivial sequences of insns).  With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching.  The pass also includes store motion.

-fivcanon
     Create a canonical counter for number of iterations in the loop for that determining number of iterations requires complicated analysis.  Later optimizations then may determine the number easily.  Useful especially in connection with unrolling.

-fivopts
     Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.

-ftree-sra
     Perform scalar replacement of aggregates.  This pass replaces structure references with scalars to prevent committing structures to memory too early.  This flag is enabled by default at -O and higher.

-ftree-copyrename
     Perform copy renaming on trees.  This pass attempts to rename compiler temporaries to other variables at copy locations, usually resulting in variable names which more closely resemble the original variables.  This flag is enabled by default at -O and higher.

-ftree-ter
     Perform temporary expression replacement during the SSA->normal phase.  Single use/single def temporaries are replaced at their use location with their defining expression.  This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation.  This is enabled by default at -O and higher.

-ftree-lrs
     Perform live range splitting during the SSA->normal phase.  Distinct live ranges of a variable are split into unique variables, allowing for better optimization later.  This is enabled by default at -O and higher.

-ftree-vectorize
     Perform loop vectorization on trees.

-ftracer
     Perform tail duplication to enlarge superblock size.  This transformation simplifies the control flow of the function allowing other optimizations to do better job.

-funroll-loops
     Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.  -funroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop.  This option makes code larger, and may or may not make it run faster.

-funroll-all-loops
     Unroll all loops, even if their number of iterations is uncertain when the loop is entered.  This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops,

-fsplit-ivs-in-unroller
     Enables expressing of values of induction variables in later iterations of the unrolled loop using the value in the first iteration.  This breaks long dependency chains, thus improving efficiency of the scheduling passes (for best results, -fweb should be used as well).

     Combination of -fweb and CSE is often sufficient to obtain the same effect.  However in cases the loop body is more complicated than a single basic block, this is not reliable.  It also does not work at all on some of the architectures due to restrictions in the CSE pass.

     This optimization is enabled by default.

-fvariable-expansion-in-unroller
     With this option, the compiler will create multiple copies of some local variables when unrolling a loop which can result in superior code.

-fprefetch-loop-arrays
     If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

     These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.

-fno-peephole
-fno-peephole2
     Disable any machine-specific peephole optimizations.  The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.

     -fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.

-fno-guess-branch-probability
     Do not guess branch probabilities using heuristics.

     GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs).  These heuristics are based on the control flow graph.  If some branch probabilities are specified by __builtin_expect, then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account.  The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.

     The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

-freorder-blocks
     Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.

     Enabled at levels -O2, -O3.

-freorder-blocks-and-partition
     In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance.

     This optimization is automatically turned off in the presence of exception handling, for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

-freorder-functions
     Reorder functions in the object file in order to improve code locality.  This is implemented by using special subsections .text.hot for most frequently executed functions and .text.unlikely for unlikely executed functions.  Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.

     Also profile feedback must be available in to make this option effective.  See -fprofile-arcs for details.

     Enabled at levels -O2, -O3, -Os.

-fstrict-aliasing
     Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled.  For C (and C++), this activates optimizations based on the type of expressions.  In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same.  For example, an unsigned int can alias an int, but not a void* or a double.  A character type may alias any other type.

     Pay special attention to code like this:

          union a_union {
            int i;
            double d;
          };

          int f() {
            a_union t;
            t.d = 3.0;
            return t.i;
          }

     The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common.  Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type.  So, the code above will work as expected.  However, this code might not:

          int f() {
            a_union t;
            int* ip;
            t.d = 3.0;
            ip = &t.i;
            return *ip;
          }

     Every language that wishes to perform language-specific alias analysis should define a function that computes, given an tree node, an alias set for the node.  Nodes in different alias sets are not allowed to alias.  For an example, see the C front-end function c_get_alias_set.

     Enabled at levels -O2, -O3, -Os.

-falign-functions
-falign-functions=N
     Align the start of functions to the next power-of-two greater than N, skipping up to N bytes.  For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but -falign-functions=24 would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less.

     -fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.

     Some assemblers only support this flag when N is a power of two; in that case, it is rounded up.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-falign-labels
-falign-labels=N
     Align all branch targets to a power-of-two boundary, skipping up to N bytes like -falign-functions.  This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code.

     -fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.

     If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.

     If N is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.

     Enabled at levels -O2, -O3.

-falign-loops
-falign-loops=N
     Align loops to a power-of-two boundary, skipping up to N bytes like -falign-functions.  The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations.

     -fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-falign-jumps
-falign-jumps=N
     Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to N bytes like -falign-functions.  In this case, no dummy operations need be executed.

     -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.

     If N is not specified or is zero, use a machine-dependent default.

     Enabled at levels -O2, -O3.

-funit-at-a-time
     Parse the whole compilation unit before starting to produce code. This allows some extra optimizations to take place but consumes more memory (in general).  There are some compatibility issues with unit-at-at-time mode:

       * enabling unit-at-a-time mode may change the order in which functions, variables, and top-level asm statements are emitted, and will likely break code relying on some particular ordering.  The majority of such top-level asm statements, though, can be replaced by section attributes.

       * unit-at-a-time mode removes unreferenced static variables and functions are removed.  This may result in undefined references when an asm statement refers directly to variables or functions that are otherwise unused.  In that case either the variable/function shall be listed as an operand of the asm statement operand or, in the case of top-level asm statements the attribute used shall be used on the declaration.

       * Static functions now can use non-standard passing conventions that may break asm statements calling functions directly.  Again, attribute used will prevent this behavior.

     As a temporary workaround, -fno-unit-at-a-time can be used, but this scheme may not be supported by future releases of GCC.

     Enabled at levels -O2, -O3.

-fweb
     Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register.  This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover.  It can, however, make debugging impossible, since variables will no longer stay in a ``home register''.

     Enabled at levels -O2, -O3, -Os, on targets where the default format for debugging information supports variable tracking.

-fno-cprop-registers
     After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.

     Disabled at levels -O, -O2, -O3, -Os.

-fprofile-generate

     Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization.  You must use -fprofile-generate both when compiling and when linking your program.

     The following options are enabled: -fprofile-arcs, -fprofile-values, -fvpt.

-fprofile-use
     Enable profile feedback directed optimizations, and optimizations generally profitable only with profile feedback available.

     The following options are enabled: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer.

The following options control compiler behavior regarding floating point arithmetic.  These options trade off between speed and correctness.  All must be specifically enabled.

-ffloat-store
     Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory.

     This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have.  Similarly for the x86 architecture.  For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point.  Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.

-ffast-math
     Sets -fno-math-errno, -funsafe-math-optimizations,

     -fno-trapping-math, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and fcx-limited-range.

     This option causes the preprocessor macro __FAST_MATH__ to be defined.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

-fno-math-errno
     Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt.  A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -fmath-errno.

-funsafe-math-optimizations
     Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards.  When used at link-time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -fno-unsafe-math-optimizations.

-ffinite-math-only
     Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.

     The default is -fno-finite-math-only.

-fno-trapping-math
     Compile code assuming that floating-point operations cannot generate user-visible traps.  These traps include division by zero, overflow, underflow, inexact result and invalid operation.  This option implies -fno-signaling-nans.  Setting this option may allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.

     This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

     The default is -ftrapping-math.

-frounding-math
     Disable transformations and optimizations that assume default floating point rounding behavior.  This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations.  This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode.  This option disables constant folding of floating point expressions at compile-time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

     The default is -fno-rounding-math.

     This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma.  This command line option will be used to specify the default state for FENV_ACCESS.

-fsignaling-nans
     Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations.  Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs.  This option implies -ftrapping-math.

     This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

     The default is -fno-signaling-nans.

     This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

-fsingle-precision-constant
     Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant.

-fcx-limited-range
-fno-cx-limited-range
     When enabled, this option states that a range reduction step is not needed when performing complex division.  The default is -fno-cx-limited-range, but is enabled by -ffast-math.

     This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma.  Nevertheless, the option applies to all languages.

The following options control optimizations that may improve performance, but are not enabled by any -O options.  This section includes experimental options that may produce broken code.

-fbranch-probabilities
     After running a program compiled with -fprofile-arcs (see Options for Debugging Your Program or gcc), you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken.  When the program compiled with -fprofile-arcs exits it saves arc execution counts to a file called SOURCENAME.gcda for each source file  The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations.

     With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization.  Currently, they are only used in one place: in reorg.c, instead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.

-fprofile-values
     If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

     With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions and adds REG_VALUE_PROFILE notes to instructions for their later usage in optimizations.

     Enabled with -fprofile-generate and -fprofile-use.

-fvpt
     If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about values of expressions.

     With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operation using the knowledge about the value of the denominator.

-fspeculative-prefetching
     If combined with -fprofile-arcs, it instructs the compiler to add a code to gather information about addresses of memory references in the program.

     With -fbranch-probabilities, it reads back the data gathered and issues prefetch instructions according to them.  In addition to the opportunities noticed by -fprefetch-loop-arrays, it also notices more complicated memory access patterns---for example accesses to the data stored in linked list whose elements are usually allocated sequentially.

     In order to prevent issuing double prefetches, usage of -fspeculative-prefetching implies -fno-prefetch-loop-arrays.

     Enabled with -fprofile-generate and -fprofile-use.

-frename-registers
     Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation.  This optimization will most benefit processors with lots of registers.  Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables will no longer stay in a ``home register''.

     Not enabled by default at any level because it has known bugs.

-ftracer
     Perform tail duplication to enlarge superblock size.  This transformation simplifies the control flow of the function allowing other optimizations to do better job.

     Enabled with -fprofile-use.

-funroll-loops
     Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.  -funroll-loops implies -frerun-cse-after-loop.  It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). This option makes code larger, and may or may not make it run faster.

     Enabled with -fprofile-use.

-funroll-all-loops
     Unroll all loops, even if their number of iterations is uncertain when the loop is entered.  This usually makes programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.

-fpeel-loops
     Peels the loops for that there is enough information that they do not roll much (from profile feedback).  It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).

     Enabled with -fprofile-use.

-fmove-loop-invariants
     Enables the loop invariant motion pass in the new loop optimizer.  Enabled at level -O1

-funswitch-loops
     Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).

-fprefetch-loop-arrays
     If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

     Disabled at level -Os.

-ffunction-sections
-fdata-sections
     Place each function or data item into its own section in the output file if the target supports arbitrary sections.  The name of the function or the name of the data item determines the section's name in the output file.

     Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space.  Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations.  AIX may have these optimizations in the future.

     Only use these options when there are significant benefits from doing so.  When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use gprof on all systems if you specify this option and you may have problems with debugging if you specify both this option and -g.

-fbranch-target-load-optimize
     Perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.

-fbranch-target-load-optimize2
     Perform branch target register load optimization after prologue / epilogue threading.

-fbtr-bb-exclusive
     When performing branch target register load optimization, don't reuse branch target registers in within any basic block.

--param NAME=VALUE
     In some places, GCC uses various constants to control the amount of optimization that is done.  For example, GCC will not inline functions that contain more that a certain number of instructions.  You can control some of these constants on the command-line using the --param option.

     The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.

     In each case, the VALUE is an integer.  The allowable choices for NAME are given in the following table:

     sra-max-structure-size
          The maximum structure size, in bytes, at which the scalar replacement of aggregates (SRA) optimization will perform block copies.  The default value, 0, implies that GCC will select the most appropriate size itself.

     sra-field-structure-ratio
          The threshold ratio (as a percentage) between instantiated fields and the complete structure size.  We say that if the ratio of the number of bytes in instantiated fields to the number of bytes in the complete structure exceeds this parameter, then block copies are not used.  The default is 75.

     max-crossjump-edges
          The maximum number of incoming edges to consider for crossjumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block.  Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable size.

     min-crossjump-insns
          The minimum number of instructions which must be matched at the end of two blocks before crossjumping will be performed on them.  This value is ignored in the case where all instructions in the block being crossjumped from are matched.  The default value is 5.

     max-goto-duplication-insns
          The maximum number of instructions to duplicate to a block that jumps to a computed goto.  To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible.  Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored.  The default value is 8.

     max-delay-slot-insn-search
          The maximum number of instructions to consider when looking for an instruction to fill a delay slot.  If more than this arbitrary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching.  Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run time.

     max-delay-slot-live-search
          When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information.  Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time.  This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.

     max-gcse-memory
          The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression elimination optimization.  If more memory than specified is required, the optimization will not be done.

     max-gcse-passes
          The maximum number of passes of GCSE to run.  The default is 1.

     max-pending-list-length
          The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over.  Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.

     max-inline-insns-single
          Several parameters control the tree inliner used in gcc. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining.  This only affects functions declared inline and methods implemented in a class declaration (C++). The default value is 450.

     max-inline-insns-auto
          When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler will be investigated.  To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 90.

     large-function-insns
          The limit specifying really large functions.  For functions larger than this limit after inlining inlining is constrained by --param large-function-growth.  This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the backend. This parameter is ignored when -funit-at-a-time is not used. The default value is 2700.

     large-function-growth
          Specifies maximal growth of large function caused by inlining in percents. This parameter is ignored when -funit-at-a-time is not used. The default value is 100 which limits large function growth to 2.0 times the original size.

     inline-unit-growth
          Specifies maximal overall growth of the compilation unit caused by inlining. This parameter is ignored when -funit-at-a-time is not used. The default value is 50 which limits unit growth to 1.5 times the original size.

     max-inline-insns-recursive
     max-inline-insns-recursive-auto
          Specifies maximum number of instructions out-of-line copy of self recursive inline function can grow into by performing recursive inlining.

          For functions declared inline --param max-inline-insns-recursive is taken into acount.  For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-insns-recursive-auto is used.  The default value is 450.

     max-inline-recursive-depth
     max-inline-recursive-depth-auto
          Specifies maximum recursion depth used by the recursive inlining.

          For functions declared inline --param max-inline-recursive-depth is taken into acount.  For function not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled and --param max-inline-recursive-depth-auto is used.  The default value is 450.

     inline-call-cost
          Specify cost of call instruction relative to simple arithmetics operations (having cost of 1).  Increasing this cost disqualifies inlining of non-leaf functions and at the same time increases size of leaf function that is believed to reduce function size by being inlined.  In effect it increases amount of inlining for code having large abstraction penalty (many functions that just pass the arguments to other functions) and decrease inlining for code with low abstraction penalty.  The default value is 16.

     max-unrolled-insns
          The maximum number of instructions that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.

     max-average-unrolled-insns
          The maximum number of instructions biased by probabilities of their execution that a loop should have if that loop is unrolled, and if the loop is unrolled, it determines how many times the loop code is unrolled.

     max-unroll-times
          The maximum number of unrollings of a single loop.

     max-peeled-insns
          The maximum number of instructions that a loop should have if that loop is peeled, and if the loop is peeled, it determines how many times the loop code is peeled.

     max-peel-times
          The maximum number of peelings of a single loop.

     max-completely-peeled-insns
          The maximum number of insns of a completely peeled loop.

     max-completely-peel-times
          The maximum number of iterations of a loop to be suitable for complete peeling.

     max-unswitch-insns
          The maximum number of insns of an unswitched loop.

     max-unswitch-level
          The maximum number of branches unswitched in a single loop.

     lim-expensive
          The minimum cost of an expensive expression in the loop invariant motion.

     iv-consider-all-candidates-bound
          Bound on number of candidates for induction variables below that all candidates are considered for each use in induction variable optimizations.  Only the most relevant candidates are considered if there are more candidates, to avoid quadratic time complexity.

     iv-max-considered-uses
          The induction variable optimizations give up on loops that contain more induction variable uses.

     iv-always-prune-cand-set-bound
          If number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set during its optimization when a new iv is added to the set.

     max-iterations-to-track

          The maximum number of iterations of a loop the brute force algorithm for analysis of # of iterations of the loop tries to evaluate.

     hot-bb-count-fraction
          Select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot.

     hot-bb-frequency-fraction
          Select fraction of the maximal frequency of executions of basic block in function given basic block needs to have to be considered hot

     tracer-dynamic-coverage
     tracer-dynamic-coverage-feedback

          This value is used to limit superblock formation once the given percentage of executed instructions is covered.  This limits unnecessary code size expansion.

          The tracer-dynamic-coverage-feedback is used only when profile feedback is available.  The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.

     tracer-max-code-growth
          Stop tail duplication once code growth has reached given percentage.  This is rather hokey argument, as most of the duplicates will be eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.

     tracer-min-branch-ratio

          Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).

     tracer-min-branch-ratio
     tracer-min-branch-ratio-feedback

          Stop forward growth if the best edge do have probability lower than this threshold.

          Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile feedback and one for compilation without.  The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

     max-cse-path-length

          Maximum number of basic blocks on path that cse considers.  The default is 10.

     global-var-threshold

          Counts the number of function calls (N) and the number of call-clobbered variables (V).  If NxV is larger than this limit, a single artificial variable will be created to represent all the call-clobbered variables at function call sites.  This artificial variable will then be made to alias every call-clobbered variable. (done as int * size_t on the host machine; beware overflow).

     max-aliased-vops

          Maximum number of virtual operands allowed to represent aliases before triggering the alias grouping heuristic.  Alias grouping reduces compile times and memory consumption needed for aliasing at the expense of precision loss in alias information.

     ggc-min-expand

          GCC uses a garbage collector to manage its own memory allocation.  This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.

          The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB.  If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS.  If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used.  Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity.  This is extremely slow, but can be useful for debugging.

     ggc-min-heapsize

          Minimum size of the garbage collector's heap before it begins bothering to collect garbage.  The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize.  Again, tuning this may improve compilation speed, and has no effect on code generation.

          The default is the smaller of RAM/8, RLIMIT_RSS, or a limit which tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes).  If GCC is not able to calculate RAM on a particular platform, the lower bound is used.  Setting this parameter very large effectively disables garbage collection.  Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.

     max-reload-search-insns
          The maximum number of instruction reload should look backward for equivalent register.  Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance.  The default value is 100.

     max-cselib-memory-location
          The maximum number of memory locations cselib should take into acount. Increasing values mean more aggressive optimization, making the compile time increase with probably slightly better performance.  The default value is 500.

     reorder-blocks-duplicate
     reorder-blocks-duplicate-feedback

          Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on its destination.  Code is duplicated when its estimated size is smaller than this value multiplied by the estimated size of unconditional jump in the hot spots of the program.

          The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than reorder-block-duplicate since information about the hot spots is more accurate.

     max-sched-region-blocks
          The maximum number of blocks in a region to be considered for interblock scheduling.  The default value is 10.

     max-sched-region-insns
          The maximum number of insns in a region to be considered for interblock scheduling.  The default value is 100.

     max-last-value-rtl

          The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register.  The default is 10000.

     integer-share-limit
          Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed.  This sets the maximum value of a shared integer constant's.  The default value is 256.

Options Controlling the Preprocessor
====================================

These options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.

     You can use -Wp,OPTION to bypass the compiler driver and pass OPTION directly through to the preprocessor.  If OPTION contains commas, it is split into multiple options at the commas.  However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase.  The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.

-Xpreprocessor OPTION
     Pass OPTION as an option to the preprocessor.  You can use this to supply system-specific preprocessor options which GCC does not know how to recognize.

     If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.

     [file cppopts.texi not included]

Passing Options to the Assembler
================================

You can pass options to the assembler.

-Wa,OPTION
     Pass OPTION as an option to the assembler.  If OPTION contains commas, it is split into multiple options at the commas.

-Xassembler OPTION
     Pass OPTION as an option to the assembler.  You can use this to supply system-specific assembler options which GCC does not know how to recognize.

     If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument.

Options for Linking
===================

These options come into play when the compiler links object files into an executable output file.  They are meaningless if the compiler is not doing a link step.

OBJECT-FILE-NAME
     A file name that does not end in a special recognized suffix is considered to name an object file or library.  (Object files are distinguished from libraries by the linker according to the file contents.)  If linking is done, these object files are used as input to the linker.

-c
-S
-E
     If any of these options is used, then the linker is not run, and object file names should not be used as arguments.  See Overall Options.

-lLIBRARY
-l LIBRARY
     Search the library named LIBRARY when linking.  (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

     It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified.  Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o.  If bar.o refers to functions in z, those functions may not be loaded.

     The linker searches a standard list of directories for the library, which is actually a file named libLIBRARY.a.  The linker then uses this file as if it had been specified precisely by name.

     The directories searched include several standard system directories plus any that you specify with -L.

     Normally the files found this way are library files---archive files whose members are object files.  The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined.  But if the file that is found is an ordinary object file, it is linked in the usual fashion.  The only difference between using an -l option and specifying a file name is that -l surrounds LIBRARY with lib and .a and searches several directories.

-lobjc
     You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.

-nostartfiles
     Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used.

-nodefaultlibs
     Do not use the standard system libraries when linking. Only the libraries you specify will be passed to the linker. The standard startup files are used normally, unless -nostartfiles is used.  The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.  These entry points should be supplied through some other mechanism when this option is specified.

-nostdlib
     Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify will be passed to the linker.  The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc.  These entry points should be supplied through some other mechanism when this option is specified.

     One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (see Interfacing to GCC Output (GNU Compiler Collection (GCC) Internals), for more discussion of libgcc.a.) In most cases, you need libgcc.a even when you want to avoid other standard libraries.  In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines.  (For example, __main, used to ensure C++ constructors will be called; see collect2 (GNU Compiler Collection (GCC) Internals).)

-pie
     Produce a position independent executable on targets which support it. For predictable results, you must also specify the same set of options that were used to generate code (-fpie, -fPIE, or model suboptions) when you specify this option.

-s
     Remove all symbol table and relocation information from the executable.

-static
     On systems that support dynamic linking, this prevents linking with the shared libraries.  On other systems, this option has no effect.

-shared
     Produce a shared object which can then be linked with other objects to form an executable.  Not all systems support this option.  For predictable results, you must also specify the same set of options that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this option. (On some systems, gcc -shared needs to build supplementary stub code for constructors to work.  On multi-libbed systems, gcc -shared must select the correct support libraries to link against.  Failing to supply the correct flags may lead to subtle defects.  Supplying them in cases where they are not necessary is innocuous.)

-shared-libgcc
-static-libgcc
     On systems that provide libgcc as a shared library, these options force the use of either the shared or static version respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.

     There are several situations in which an application should use the shared libgcc instead of the static version.  The most common of these is when the application wishes to throw and catch exceptions across different shared libraries.  In that case, each of the libraries as well as the application itself should use the shared libgcc.

     Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.

     If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into shared libraries by default.  Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of libgcc, linking with the static version of libgcc by default.  This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

     However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

-symbolic
     Bind references to global symbols when building a shared object.  Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs).  Only a few systems support this option.

-Xlinker OPTION
     Pass OPTION as an option to the linker.  You can use this to supply system-specific linker options which GCC does not know how to recognize.

     If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions.  It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects.

-Wl,OPTION
     Pass OPTION as an option to the linker.  If OPTION contains commas, it is split into multiple options at the commas.

-u SYMBOL
     Pretend the symbol SYMBOL is undefined, to force linking of library modules to define it.  You can use -u multiple times with different symbols to force loading of additional library modules.

Options for Directory Search
============================

These options specify directories to search for header files, for libraries and for parts of the compiler:

-IDIR
     Add the directory DIR to the head of the list of directories to be searched for header files.  This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories.  However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that).  If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.

     If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option will be ignored.  The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-iquoteDIR
     Add the directory DIR to the head of the list of directories to be searched for header files only for the case of #include "FILE"; they are not searched for #include <FILE>, otherwise just like -I.

-LDIR
     Add directory DIR to the list of directories to be searched for -l.

-BPREFIX
     This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

     The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld.  It tries PREFIX as a prefix for each program it tries to run, both with and without MACHINE/VERSION/ (see Target Options).

     For each subprogram to be run, the compiler driver first tries the -B prefix, if any.  If that name is not found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and /usr/local/lib/gcc/.  If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

     The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add a directory separator character at the end of the path.

     -B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker.  They also apply to includes files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor.  In this case, the compiler appends include to the prefix.

     The run-time support file libgcc.a can also be searched for using the -B prefix, if needed.  If it is not found there, the two standard prefixes above are tried, and that is all.  The file is left out of the link if it is not found by those means.

     Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.  See Environment Variables.

     As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it will be replaced by [dir/]include.  This is to help with boot-strapping the compiler.

-specs=FILE
     Process FILE after the compiler reads in the standard specs file, in order to override the defaults that the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc.  More than one -specs=FILE can be specified on the command line, and they are processed in order, from left to right.

-I-
     This option has been deprecated.  Please use -iquote instead for -I directories before the -I- and remove the -I-. Any directories you specify with -I options before the -I- option are searched only for the case of #include "FILE"; they are not searched for #include <FILE>.

     If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives.  (Ordinarily all -I directories are used this way.)

     In addition, the -I- option inhibits the use of the current directory (where the current input file came from) as the first search directory for #include "FILE".  There is no way to override this effect of -I-.  With -I. you can specify searching the directory which was current when the compiler was invoked.  That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.

     -I- does not inhibit the use of the standard system directories for header files.  Thus, -I- and -nostdinc are independent.

Specifying subprocesses and the switches to pass to them
========================================================

gcc is a driver program.  It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking.  GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines.  This behavior is controlled by spec strings.  In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior.  The spec strings built into GCC can be overridden by using the -specs= command-line switch to specify a spec file.

Spec files are plaintext files that are used to construct spec strings.  They consist of a sequence of directives separated by blank lines.  The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:

%COMMAND
     Issues a COMMAND to the spec file processor.  The commands that can appear here are:

     %include <FILE>
          Search for FILE and insert its text at the current point in the specs file.

     %include_noerr <FILE>
          Just like %include, but do not generate an error message if the include file cannot be found.

     %rename OLD_NAME NEW_NAME
          Rename the spec string OLD_NAME to NEW_NAME.

*[SPEC_NAME]:
     This tells the compiler to create, override or delete the named spec string.  All lines after this directive up to the next directive or blank line are considered to be the text for the spec string.  If this results in an empty string then the spec will be deleted.  (Or, if the spec did not exist, then nothing will happened.)  Otherwise, if the spec does not currently exist a new spec will be created.  If the spec does exist then its contents will be overridden by the text of this directive, unless the first character of that text is the + character, in which case the text will be appended to the spec.

[SUFFIX]:
     Creates a new [SUFFIX] spec pair.  All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix.  When the compiler encounters an input file with the named suffix, it will processes the spec string in order to work out how to compile that file.  For example:

          .ZZ:
          z-compile -input %i

     This says that any input file whose name ends in .ZZ should be passed to the program z-compile, which should be invoked with the command-line switch -input and with the result of performing the %i substitution.  (See below.)

     As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:

     @LANGUAGE
          This says that the suffix is an alias for a known LANGUAGE.  This is similar to using the -x command-line switch to GCC to specify a language explicitly.  For example:

               .ZZ:
               @c++

          Says that .ZZ files are, in fact, C++ source files.

     #NAME
          This causes an error messages saying:

               NAME compiler not installed on this system.

     GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.

GCC has the following spec strings built into it.  Spec files can override these strings or create their own.  Note that individual targets can also add their own spec strings to this list.

     asm          Options to pass to the assembler
     asm_final    Options to pass to the assembler post-processor
     cpp          Options to pass to the C preprocessor
     cc1          Options to pass to the C compiler
     cc1plus      Options to pass to the C++ compiler
     endfile      Object files to include at the end of the link
     link         Options to pass to the linker
     lib          Libraries to include on the command line to the linker
     libgcc       Decides which GCC support library to pass to the linker
     linker       Sets the name of the linker
     predefines   Defines to be passed to the C preprocessor
     signed_char  Defines to pass to CPP to say whether char is signed
                  by default
     startfile    Object files to include at the start of the link

Here is a small example of a spec file:

     %rename lib                 old_lib

     *lib:
     --start-group -lgcc -lc -leval1 --end-group %(old_lib)

This example renames the spec called lib to old_lib and then overrides the previous definition of lib with a new one. The new definition adds in some extra command-line options before including the text of the old definition.

Spec strings are a list of command-line options to be passed to their corresponding program.  In addition, the spec strings can contain %-prefixed sequences to substitute variable text or to conditionally insert text into the command line.  Using these constructs it is possible to generate quite complex command lines.

Here is a table of all defined %-sequences for spec strings.  Note that spaces are not generated automatically around the results of expanding these sequences.  Therefore you can concatenate them together or combine them with constant text in a single argument.

%%
     Substitute one % into the program name or argument.

%i
     Substitute the name of the input file being processed.

%b
     Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory.

%B
     This is the same as %b, but include the file suffix (text after the last period).

%d
     Marks the argument containing or following the %d as a temporary file name, so that that file will be deleted if GCC exits successfully.  Unlike %g, this contributes no text to the argument.

%gSUFFIX
     Substitute a file name that has suffix SUFFIX and is chosen once per compilation, and mark the argument in the same way as %d.  To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known.  For example, %g.s ... %g.o ... %g.s might turn into ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s.  SUFFIX matches the regexp [.A-Za-z]* or the special string %O, which is treated exactly as if %O had been preprocessed.  Previously, %g was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.

%uSUFFIX
     Like %g, but generates a new temporary file name even if %uSUFFIX was already seen.

%USUFFIX
     Substitutes the last file name generated with %uSUFFIX, generating a new one if there is no such last file name.  In the absence of any %uSUFFIX, this is just like %gSUFFIX, except they don't share the same suffix space, so %g.s ... %U.s ... %g.s ... %U.s would involve the generation of two distinct file names, one for each %g.s and another for each %U.s.  Previously, %U was simply substituted with a file name chosen for the previous %u, without regard to any appended suffix.

%jSUFFIX
     Substitutes the name of the HOST_BIT_BUCKET, if any, and if it is writable, and if save-temps is off; otherwise, substitute the name of a temporary file, just like %u.  This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.

%|SUFFIX
%mSUFFIX
     Like %g, except if -pipe is in effect.  In that case %| substitutes a single dash and %m substitutes nothing at all.  These are the two most common ways to instruct a program that it should read from standard input or write to standard output.  If you need something more elaborate you can use an %{pipe:X} construct: see for example f/lang-specs.h.

%.SUFFIX
     Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with %*.  SUFFIX is terminated by the next space or %.

%w
     Marks the argument containing or following the %w as the designated output file of this compilation.  This puts the argument into the sequence of arguments that %o will substitute later.

%o
     Substitutes the names of all the output files, with spaces automatically placed around them.  You should write spaces around the %o as well or the results are undefined. %o is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they will be linked.

%O
     Substitutes the suffix for object files.  Note that this is handled specially when it immediately follows %g, %u, or %U, because of the need for those to form complete file names.  The handling is such that %O is treated exactly as if it had already been substituted, except that %g, %u, and %U do not currently support additional SUFFIX characters following %O as they would following, for example, .o.

%p
     Substitutes the standard macro predefinitions for the current target machine.  Use this when running cpp.

%P
     Like %p, but puts __ before and after the name of each predefined macro, except for macros that start with __ or with _L, where L is an uppercase letter.  This is for ISO C.

%I
     Substitute any of -iprefix (made from GCC_EXEC_PREFIX), -isysroot (made from TARGET_SYSTEM_ROOT), and -isystem (made from COMPILER_PATH and -B options) as necessary.

%s
     Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.

%eSTR
     Print STR as an error message.  STR is terminated by a newline. Use this when inconsistent options are detected.

%(NAME)
     Substitute the contents of spec string NAME at this point.

%[NAME]
     Like %(...) but put __ around -D arguments.

%x{OPTION}
     Accumulate an option for %X.

%X
     Output the accumulated linker options specified by -Wl or a %x spec string.

%Y
     Output the accumulated assembler options specified by -Wa.

%Z
     Output the accumulated preprocessor options specified by -Wp.

%a
     Process the asm spec.  This is used to compute the switches to be passed to the assembler.

%A
     Process the asm_final spec.  This is a spec string for passing switches to an assembler post-processor, if such a program is needed.

%l
     Process the link spec.  This is the spec for computing the command line passed to the linker.  Typically it will make use of the %L %G %S %D and %E sequences.

%D
     Dump out a -L option for each directory that GCC believes might contain startup files.  If the target supports multilibs then the current multilib directory will be prepended to each of these paths.

%L
     Process the lib spec.  This is a spec string for deciding which libraries should be included on the command line to the linker.

%G
     Process the libgcc spec.  This is a spec string for deciding which GCC support library should be included on the command line to the linker.

%S
     Process the startfile spec.  This is a spec for deciding which object files should be the first ones passed to the linker.  Typically this might be a file named crt0.o.

%E
     Process the endfile spec.  This is a spec string that specifies the last object files that will be passed to the linker.

%C
     Process the cpp spec.  This is used to construct the arguments to be passed to the C preprocessor.

%1
     Process the cc1 spec.  This is used to construct the options to be passed to the actual C compiler (cc1).

%2
     Process the cc1plus spec.  This is used to construct the options to be passed to the actual C++ compiler (cc1plus).

%*
     Substitute the variable part of a matched option.  See below. Note that each comma in the substituted string is replaced by a single space.

%<S
     Remove all occurrences of -S from the command line.  Note---this command is position dependent.  % commands in the spec string before this one will see -S, % commands in the spec string after this one will not.

%:FUNCTION(ARGS)
     Call the named function FUNCTION, passing it ARGS. ARGS is first processed as a nested spec string, then split into an argument vector in the usual fashion.  The function returns a string which is processed as if it had appeared literally as part of the current spec.

     The following built-in spec functions are provided:

     if-exists
          The if-exists spec function takes one argument, an absolute pathname to a file.  If the file exists, if-exists returns the pathname.  Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s

     if-exists-else
          The if-exists-else spec function is similar to the if-exists spec function, except that it takes two arguments.  The first argument is an absolute pathname to a file.  If the file exists, if-exists-else returns the pathname.  If it does not exist, it returns the second argument. This way, if-exists-else can be used to select one file or another, based on the existence of the first.  Here is a small example of its usage:

               *startfile:
               crt0%O%s %:if-exists(crti%O%s) \
               %:if-exists-else(crtbeginT%O%s crtbegin%O%s)

     replace-outfile
          The replace-outfile spec function takes two arguments.  It looks for the first argument in the outfiles array and replaces it with the second argument.  Here is a small example of its usage:

               %{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)}

%{S}
     Substitutes the -S switch, if that switch was given to GCC. If that switch was not specified, this substitutes nothing.  Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed.  Thus the spec string %{foo} would match the command-line option -foo and would output the command line option -foo.

%W{S}
     Like %{S} but mark last argument supplied within as a file to be deleted on failure.

%{S*}
     Substitutes all the switches specified to GCC whose names start with -S, but which also take an argument.  This is used for switches like -o, -D, -I, etc. GCC considers -o foo as being one switch whose names starts with o.  %{o*} would substitute this text, including the space.  Thus two arguments would be generated.

%{S*&T*}
     Like %{S*}, but preserve order of S and T options (the order of S and T in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional.  Useful for CPP as %{D*&U*&A*}.

%{S:X}
     Substitutes X, if the -S switch was given to GCC.

%{!S:X}
     Substitutes X, if the -S switch was not given to GCC.

%{S*:X}
     Substitutes X if one or more switches whose names start with -S are specified to GCC.  Normally X is substituted only once, no matter how many such switches appeared.  However, if %* appears somewhere in X, then X will be substituted once for each matching switch, with the %* replaced by the part of that switch that matched the *.

%{.S:X}
     Substitutes X, if processing a file with suffix S.

%{!.S:X}
     Substitutes X, if not processing a file with suffix S.

%{S|P:X}
     Substitutes X if either -S or -P was given to GCC. This may be combined with !, ., and * sequences as well, although they have a stronger binding than the |.  If %* appears in X, all of the alternatives must be starred, and only the first matching alternative is substituted.

     For example, a spec string like this:

          %{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}

     will output the following command-line options from the following input command-line options:

          fred.c        -foo -baz
          jim.d         -bar -boggle
          -d fred.c     -foo -baz -boggle
          -d jim.d      -bar -baz -boggle

%{S:X; T:Y; :D}

     If S was given to GCC, substitutes X; else if T was given to GCC, substitutes Y; else substitutes D.  There can be as many clauses as you need.  This may be combined with ., !, |, and * as needed.

The conditional text X in a %{S:X} or similar construct may contain other nested % constructs or spaces, or even newlines.  They are processed as usual, as described above. Trailing white space in X is ignored.  White space may also appear anywhere on the left side of the colon in these constructs, except between . or * and the corresponding word.

The -O, -f, -m, and -W switches are handled specifically in these constructs.  If another value of -O or the negated form of a -f, -m, or -W switch is found later in the command line, the earlier switch value is ignored, except with {S*} where S is just one letter, which passes all matching options.

The character | at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if -pipe is specified.

It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments.  But this cannot be done in a consistent fashion.  GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).

GCC also knows implicitly that arguments starting in -l are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.

Specifying Target Machine and Compiler Version
==============================================

The usual way to run GCC is to run the executable called gcc, or <machine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a version other than the one that was installed last.  Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.

-b MACHINE
     The argument MACHINE specifies the target machine for compilation.

     The value to use for MACHINE is the same as was specified as the machine type when configuring GCC as a cross-compiler.  For example, if a cross-compiler was configured with configure i386v, meaning to compile for an 80386 running System V, then you would specify -b i386v to run that cross compiler.

-V VERSION
     The argument VERSION specifies which version of GCC to run. This is useful when multiple versions are installed.  For example, VERSION might be 2.0, meaning to run GCC version 2.0.

The -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real reason to use them if you can just run that directly.

Hardware Models and Configurations
==================================

Earlier we discussed the standard option -b which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.

In addition, each of these target machine types can have its own special options, starting with -m, to choose among various hardware models or configurations---for example, 68010 vs 68020, floating coprocessor or none.  A single installed version of the compiler can compile for any model or configuration, according to the options specified.

Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.

These options are defined by the macro TARGET_SWITCHES in the machine description.  The default for the options is also defined by that macro, which enables you to change the defaults.

ARC Options
-----------

These options are defined for ARC implementations:

-EL
     Compile code for little endian mode.  This is the default.

-EB
     Compile code for big endian mode.

-mmangle-cpu
     Prepend the name of the cpu to all public symbol names. In multiple-processor systems, there are many ARC variants with different instruction and register set characteristics.  This flag prevents code compiled for one cpu to be linked with code compiled for another. No facility exists for handling variants that are ``almost identical''. This is an all or nothing option.

-mcpu=CPU
     Compile code for ARC variant CPU. Which variants are supported depend on the configuration. All variants support -mcpu=base, this is the default.

-mtext=TEXT-SECTION
-mdata=DATA-SECTION
-mrodata=READONLY-DATA-SECTION
     Put functions, data, and readonly data in TEXT-SECTION, DATA-SECTION, and READONLY-DATA-SECTION respectively by default.  This can be overridden with the section attribute. See Variable Attributes.

ARM Options
-----------

These -m options are defined for Advanced RISC Machines (ARM) architectures:

-mabi=NAME
     Generate code for the specified ABI.  Permissible values are: apcs-gnu, atpcs, aapcs and iwmmxt.

-mapcs-frame
     Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code.  Specifying -fomit-frame-pointer with this option will cause the stack frames not to be generated for leaf functions.  The default is -mno-apcs-frame.

-mapcs
     This is a synonym for -mapcs-frame.

-mthumb-interwork
     Generate code which supports calling between the ARM and Thumb instruction sets.  Without this option the two instruction sets cannot be reliably used inside one program.  The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified.

-mno-sched-prolog
     Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body.  This means that all functions will start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start if functions inside an executable piece of code.  The default is -msched-prolog.

-mhard-float
     Generate output containing floating point instructions.  This is the default.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all ARM targets.  Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.

     -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.  In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mfloat-abi=NAME
     Specifies which ABI to use for floating point values.  Permissible values are: soft, softfp and hard.

     soft and hard are equivalent to -msoft-float and -mhard-float respectively.  softfp allows the generation of floating point instructions, but still uses the soft-float calling conventions.

-mlittle-endian
     Generate code for a processor running in little-endian mode.  This is the default for all standard configurations.

-mbig-endian
     Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

-mwords-little-endian
     This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order.  That is, a byte order of the form 32107654.  Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.

-mcpu=NAME
     This specifies the name of the target ARM processor.  GCC uses this name to determine what kind of instructions it can emit when generating assembly code.  Permissible names are: arm2, arm250, arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm, strongarm110, strongarm1100, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312.

-mtune=NAME
     This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instructions that it will generate based on the cpu specified by a -mcpu= option. For some ARM implementations better performance can be obtained by using this option.

-march=NAME
     This specifies the name of the target ARM architecture.  GCC uses this name to determine what kind of instructions it can emit when generating assembly code.  This option can be used in conjunction with or instead of the -mcpu= option.  Permissible names are: armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t, armv5te, armv6, armv6j, iwmmxt, ep9312.

-mfpu=NAME
-mfpe=NUMBER
-mfp=NUMBER
     This specifies what floating point hardware (or hardware emulation) is available on the target.  Permissible names are: fpa, fpe2, fpe3, maverick, vfp.  -mfp and -mfpe are synonyms for -mfpu=fpeNUMBER, for compatibility with older versions of GCC.

     If -msoft-float is specified this specifies the format of floating point values.

-mstructure-size-boundary=N
     The size of all structures and unions will be rounded up to a multiple of the number of bits set by this option.  Permissible values are 8, 32 and 64.  The default value varies for different toolchains.  For the COFF targeted toolchain the default value is 8.  A value of 64 is only allowed if the underlying ABI supports it.

     Specifying the larger number can produce faster, more efficient code, but can also increase the size of the program.  Different values are potentially incompatible.  Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

-mabort-on-noreturn
     Generate a call to the function abort at the end of a noreturn function.  It will be executed if the function tries to return.

-mlong-calls
-mno-long-calls
     Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register.  This switch is needed if the target function will lie outside of the 64 megabyte addressing range of the offset based version of subroutine call instruction.

     Even if this switch is enabled, not all function calls will be turned into long calls.  The heuristic is that static functions, functions which have the short-call attribute, functions that are inside the scope of a #pragma no_long_calls directive and functions whose definitions have already been compiled within the current compilation unit, will not be turned into long calls.  The exception to this rule is that weak function definitions, functions with the long-call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive, will always be turned into long calls.

     This feature is not enabled by default.  Specifying -mno-long-calls will restore the default behavior, as will placing the function calls within the scope of a #pragma long_calls_off directive.  Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

-mnop-fun-dllimport
     Disable support for the dllimport attribute.

-msingle-pic-base
     Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function.  The run-time system is responsible for initializing this register with an appropriate value before execution begins.

-mpic-register=REG
     Specify the register to be used for PIC addressing.  The default is R10 unless stack-checking is enabled, when R9 is used.

-mcirrus-fix-invalid-insns
     Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction combinations.  This option is only valid if the -mcpu=ep9312 option has been used to enable generation of instructions for the Cirrus Maverick floating point co-processor.  This option is not enabled by default, since the problem is only present in older Maverick implementations.  The default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns switch.

-mpoke-function-name
     Write the name of each function into the text section, directly preceding the function prologue.  The generated code is similar to this:

               t0
                   .ascii "arm_poke_function_name", 0
                   .align
               t1
                   .word 0xff000000 + (t1 - t0)
               arm_poke_function_name
                   mov     ip, sp
                   stmfd   sp!, {fp, ip, lr, pc}
                   sub     fp, ip, #4

     When performing a stack backtrace, code can inspect the value of pc stored at fp + 0.  If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000).

-mthumb
     Generate code for the 16-bit Thumb instruction set.  The default is to use the 32-bit ARM instruction set.

-mtpcs-frame
     Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions.  (A leaf function is one that does not call any other functions.)  The default is -mno-tpcs-frame.

-mtpcs-leaf-frame
     Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions.  (A leaf function is one that does not call any other functions.)  The default is -mno-apcs-leaf-frame.

-mcallee-super-interworking
     Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function.  This allows these functions to be called from non-interworking code.

-mcaller-super-interworking
     Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not.  There is a small overhead in the cost of executing a function pointer if this option is enabled.

AVR Options
-----------

These options are defined for AVR implementations:

-mmcu=MCU
     Specify ATMEL AVR instruction set or MCU type.

     Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler programs (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).

     Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).

     Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).

     Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85).

     Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).

-msize
     Output instruction sizes to the asm file.

-minit-stack=N
     Specify the initial stack address, which may be a symbol or numeric value, __stack is the default.

-mno-interrupts
     Generated code is not compatible with hardware interrupts. Code size will be smaller.

-mcall-prologues
     Functions prologues/epilogues expanded as call to appropriate subroutines.  Code size will be smaller.

-mno-tablejump
     Do not generate tablejump insns which sometimes increase code size.

-mtiny-stack
     Change only the low 8 bits of the stack pointer.

-mint8
     Assume int to be 8 bit integer.  This affects the sizes of all types: A char will be 1 byte, an int will be 1 byte, an long will be 2 bytes and long long will be 4 bytes.  Please note that this option does not comply to the C standards, but it will provide you with smaller code size.

CRIS Options
------------

These options are defined specifically for the CRIS ports.

-march=ARCHITECTURE-TYPE
-mcpu=ARCHITECTURE-TYPE
     Generate code for the specified architecture.  The choices for ARCHITECTURE-TYPE are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0 except for cris-axis-linux-gnu, where the default is v10.

-mtune=ARCHITECTURE-TYPE
     Tune to ARCHITECTURE-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions.  The choices for ARCHITECTURE-TYPE are the same as for -march=ARCHITECTURE-TYPE.

-mmax-stack-frame=N
     Warn when the stack frame of a function exceeds N bytes.

-melinux-stacksize=N
     Only available with the cris-axis-aout target.  Arranges for indications in the program to the kernel loader that the stack of the program should be set to N bytes.

-metrax4
-metrax100
     The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.

-mmul-bug-workaround
-mno-mul-bug-workaround
     Work around a bug in the muls and mulu instructions for CPU models where it applies.  This option is active by default.

-mpdebug
     Enable CRIS-specific verbose debug-related information in the assembly code.  This option also has the effect to turn off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.

-mcc-init
     Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.

-mno-side-effects
     Do not emit instructions with side-effects in addressing modes other than post-increment.

-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
     These options (no-options) arranges (eliminate arrangements) for the stack-frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model.  The default is to arrange for 32-bit alignment.  ABI details such as structure layout are not affected by these options.

-m32-bit
-m16-bit
-m8-bit
     Similar to the stack- data- and const-align options above, these options arrange for stack-frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned.  The default is 32-bit alignment.

-mno-prologue-epilogue
-mprologue-epilogue
     With -mno-prologue-epilogue, the normal function prologue and epilogue that sets up the stack-frame are omitted and no return instructions or return sequences are generated in the code.  Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variable needs to be allocated.

-mno-gotplt
-mgotplt
     With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than (traditional on other architectures) calls to the PLT.  The default is -mgotplt.

-maout
     Legacy no-op option only recognized with the cris-axis-aout target.

-melf
     Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linux-gnu targets.

-melinux
     Only recognized with the cris-axis-aout target, where it selects a GNU/linux-like multilib, include files and instruction set for -march=v8.

-mlinux
     Legacy no-op option only recognized with the cris-axis-linux-gnu target.

-sim
     This option, recognized for the cris-axis-aout and cris-axis-elf arranges to link with input-output functions from a simulator library.  Code, initialized data and zero-initialized data are allocated consecutively.

-sim2
     Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.

Darwin Options
--------------

These options are defined for all architectures running the Darwin operating system.

FSF GCC on Darwin does not create ``fat'' object files; it will create an object file for the single architecture that it was built to target.  Apple's GCC on Darwin does create ``fat'' files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targetting, like -mcpu or -march.  The -force_cpusubtype_ALL option can be used to override this.

The Darwin tools vary in their behaviour when presented with an ISA mismatch.  The assembler, as, will only permit instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in an ppc750 object file. The linker for shared libraries, /usr/bin/libtool, will fail and print an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library).  The linker for executables, ld, will quietly give the executable the most restrictive subtype of any of its input files.

-FDIR
     Add the framework directory DIR to the head of the list of directories to be searched for header files.  These directories are interleaved with those specified by -I options and are scanned in a left-to-right order.

     A framework directory is a directory with frameworks in it.  A framework is a directory with a "Headers" and/or "PrivateHeaders" directory contained directly in it that ends in ".framework".  The name of a framework is the name of this directory excluding the ".framework".  Headers associated with the framework are found in one of those two directories, with "Headers" being searched first.  A subframework is a framework directory that is in a framework's "Frameworks" directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header.  Two subframeworks are siblings if they occur in the same framework.  A subframework should not have the same name as a framework, a warning will be issued if this is violated.  Currently a subframework cannot have subframeworks, in the future, the mechanism may be extended to support this.  The standard frameworks can be found in "/System/Library/Frameworks" and "/Library/Frameworks".  An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the "PrivateHeaders" or "Headers" directory.

-gused
     Emit debugging information for symbols that are used.  For STABS debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON.

-gfull
     Emit debugging information for all symbols and types.

-mone-byte-bool
     Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.

     Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch.  Using this switch may require recompiling all other modules in a program, including system libraries.  Use this switch to conform to a non-default data model.

-mfix-and-continue
-ffix-and-continue
-findirect-data
     Generate code suitable for fast turn around development.  Needed to enable gdb to dynamically load .o files into already running programs.  -findirect-data and -ffix-and-continue are provided for backwards compatibility.

-all_load
     Loads all members of static archive libraries. See man ld(1) for more information.

-arch_errors_fatal
     Cause the errors having to do with files that have the wrong architecture to be fatal.

-bind_at_load
     Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.

-bundle
     Produce a Mach-o bundle format file. See man ld(1) for more information.

-bundle_loader EXECUTABLE
     This option specifies the EXECUTABLE that will be loading the build output file being linked.  See man ld(1) for more information.

-dynamiclib
     When passed this option, GCC will produce a dynamic library instead of an executable when linking, using the Darwin libtool command.

-force_cpusubtype_ALL
     This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.

-allowable_client  CLIENT_NAME
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded

     These options are passed to the Darwin linker.  The Darwin linker man page describes them in detail.

DEC Alpha Options
-----------------

These -m options are defined for the DEC Alpha implementations:

-mno-soft-float
-msoft-float
     Use (do not use) the hardware floating-point instructions for floating-point operations.  When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations.  Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations.   If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

     Note that Alpha implementations without floating-point operations are required to have floating-point registers.

-mfp-reg
-mno-fp-regs
     Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float.  If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0.  This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.

     A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

-mieee
     The Alpha architecture implements floating-point hardware optimized for maximum performance.  It is mostly compliant with the IEEE floating point standard.  However, for full compliance, software assistance is required.  This option generates code fully IEEE compliant code except that the INEXACT-FLAG is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation.  The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity.  Other Alpha compilers call this option -ieee_with_no_inexact.

-mieee-with-inexact
     This is like -mieee except the generated code also maintains the IEEE INEXACT-FLAG.  Turning on this option causes the generated code to implement fully-compliant IEEE math.  In addition to _IEEE_FP, _IEEE_FP_EXACT is defined as a preprocessor macro.  On some Alpha implementations the resulting code may execute significantly slower than the code generated by default.  Since there is very little code that depends on the INEXACT-FLAG, you should normally not specify this option.  Other Alpha compilers call this option -ieee_with_inexact.

-mfp-trap-mode=TRAP-MODE
     This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm TRAP-MODE. The trap mode can be set to one of four values:

     n
          This is the default (normal) setting.  The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).

     u
          In addition to the traps enabled by n, underflow traps are enabled as well.

     su
          Like su, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).

     sui
          Like su, but inexact traps are enabled as well.

-mfp-rounding-mode=ROUNDING-MODE
     Selects the IEEE rounding mode.  Other Alpha compilers call this option -fprm ROUNDING-MODE.  The ROUNDING-MODE can be one of:

     n
          Normal IEEE rounding mode.  Floating point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.

     m
          Round towards minus infinity.

     c
          Chopped rounding mode.  Floating point numbers are rounded towards zero.

     d
          Dynamic rounding mode.  A field in the floating point control register (FPCR, see Alpha architecture reference manual) controls the rounding mode in effect.  The C library initializes this register for rounding towards plus infinity.  Thus, unless your program modifies the FPCR, d corresponds to round towards plus infinity.

-mtrap-precision=TRAP-PRECISION
     In the Alpha architecture, floating point traps are imprecise.  This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating point trap. Depending on the requirements of an application, different levels of precisions can be selected:

     p
          Program precision.  This option is the default and means a trap handler can only identify which program caused a floating point exception.

     f
          Function precision.  The trap handler can determine the function that caused a floating point exception.

     i
          Instruction precision.  The trap handler can determine the exact instruction that caused a floating point exception.

     Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

-mieee-conformant
     This option marks the generated code as IEEE conformant.  You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui.  Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.  Under DEC Unix, this has the effect that IEEE-conformant math library routines will be linked in.

-mbuild-constants
     Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions.  If it cannot, it will output the constant as a literal and generate code to load it from the data segment at runtime.

     Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).

     You would typically use this option to build a shared library dynamic loader.  Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

-malpha-as
-mgas
     Select whether to generate code to be assembled by the vendor-supplied assembler (-malpha-as) or by the GNU assembler -mgas.

-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
     Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets.  The default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none was specified.

-mfloat-vax
-mfloat-ieee
     Generate code that uses (does not use) VAX F and G floating point arithmetic instead of IEEE single and double precision.

-mexplicit-relocs
-mno-explicit-relocs
     Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros.  Use of these macros does not allow optimal instruction scheduling.  GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions.  This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.

-msmall-data
-mlarge-data
     When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations.  When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register.  This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.

     The default is -mlarge-data.  With this option the data area is limited to just below 2GB.  Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.

     When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

-msmall-text
-mlarge-text
     When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction.  When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.

     The default is -mlarge-text.

-mcpu=CPU_TYPE
     Set the instruction set and instruction scheduling parameters for machine type CPU_TYPE.  You can specify either the EV style name or the corresponding chip number.  GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and will choose the default values for the instruction set from the processor you specify.  If you do not specify a processor type, GCC will default to the processor on which the compiler was built.

     Supported values for CPU_TYPE are

     ev4
     ev45
     21064
          Schedules as an EV4 and has no instruction set extensions.

     ev5
     21164
          Schedules as an EV5 and has no instruction set extensions.

     ev56
     21164a
          Schedules as an EV5 and supports the BWX extension.

     pca56
     21164pc
     21164PC
          Schedules as an EV5 and supports the BWX and MAX extensions.

     ev6
     21264
          Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.

     ev67
     21264a
          Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

-mtune=CPU_TYPE
     Set only the instruction scheduling parameters for machine type CPU_TYPE.  The instruction set is not changed.

-mmemory-latency=TIME
     Sets the latency the scheduler should assume for typical memory references as seen by the application.  This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

     Valid options for TIME are

     NUMBER
          A decimal number representing clock cycles.

     L1
     L2
     L3
     main
          The compiler contains estimates of the number of clock cycles for ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

DEC Alpha/VMS Options
---------------------

These -m options are defined for the DEC Alpha/VMS implementations:

-mvms-return-codes
     Return VMS condition codes from main.  The default is to return POSIX style condition (e.g. error) codes.

FRV Options
-----------

-mgpr-32

     Only use the first 32 general purpose registers.

-mgpr-64

     Use all 64 general purpose registers.

-mfpr-32

     Use only the first 32 floating point registers.

-mfpr-64

     Use all 64 floating point registers

-mhard-float

     Use hardware instructions for floating point operations.

-msoft-float

     Use library routines for floating point operations.

-malloc-cc

     Dynamically allocate condition code registers.

-mfixed-cc

     Do not try to dynamically allocate condition code registers, only use icc0 and fcc0.

-mdword

     Change ABI to use double word insns.

-mno-dword

     Do not use double word instructions.

-mdouble

     Use floating point double instructions.

-mno-double

     Do not use floating point double instructions.

-mmedia

     Use media instructions.

-mno-media

     Do not use media instructions.

-mmuladd

     Use multiply and add/subtract instructions.

-mno-muladd

     Do not use multiply and add/subtract instructions.

-mfdpic

     Select the FDPIC ABI, that uses function descriptors to represent pointers to functions.  Without any PIC/PIE-related options, it implies -fPIE.  With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits.

-minline-plt

     Enable inlining of PLT entries in function calls to functions that are not known to bind locally.  It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.

-mTLS

     Assume a large TLS segment when generating thread-local code.

-mtls

     Do not assume a large TLS segment when generating thread-local code.

-mgprel-ro

     Enable the use of GPREL relocations in the FDPIC ABI for data that is known to be in read-only sections.  It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win.  If it is not, -mno-gprel-ro can be used to disable it.

-multilib-library-pic

     Link with the (library, not FD) pic libraries.  It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic.  You should never have to use it explicitly.

-mlinked-fp

     Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated.  This option is enabled by default and can be disabled with -mno-linked-fp.

-mlong-calls

     Use indirect addressing to call functions outside the current compilation unit.  This allows the functions to be placed anywhere within the 32-bit address space.

-malign-labels

     Try to align labels to an 8-byte boundary by inserting nops into the previous packet.  This option only has an effect when VLIW packing is enabled.  It doesn't create new packets; it merely adds nops to existing ones.

-mlibrary-pic

     Generate position-independent EABI code.

-macc-4

     Use only the first four media accumulator registers.

-macc-8

     Use all eight media accumulator registers.

-mpack

     Pack VLIW instructions.

-mno-pack

     Do not pack VLIW instructions.

-mno-eflags

     Do not mark ABI switches in e_flags.

-mcond-move

     Enable the use of conditional-move instructions (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-cond-move

     Disable the use of conditional-move instructions.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mscc

     Enable the use of conditional set instructions (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-scc

     Disable the use of conditional set instructions.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mcond-exec

     Enable the use of conditional execution (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-cond-exec

     Disable the use of conditional execution.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mvliw-branch

     Run a pass to pack branches into VLIW instructions (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-vliw-branch

     Do not run a pass to pack branches into VLIW instructions.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mmulti-cond-exec

     Enable optimization of && and || in conditional execution (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-multi-cond-exec

     Disable optimization of && and || in conditional execution.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mnested-cond-exec

     Enable nested conditional execution optimizations (default).

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mno-nested-cond-exec

     Disable nested conditional execution optimizations.

     This switch is mainly for debugging the compiler and will likely be removed in a future version.

-mtomcat-stats

     Cause gas to print out tomcat statistics.

-mcpu=CPU

     Select the processor type for which to generate code.  Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

H8/300 Options
--------------

These -m options are defined for the H8/300 implementations:

-mrelax
     Shorten some address references at link time, when possible; uses the linker option -relax.  See ld and the H8/300 (Using ld), for a fuller description.

-mh
     Generate code for the H8/300H.

-ms
     Generate code for the H8S.

-mn
     Generate code for the H8S and H8/300H in the normal mode.  This switch must be used either with -mh or -ms.

-ms2600
     Generate code for the H8S/2600.  This switch must be used with -ms.

-mint32
     Make int data 32 bits by default.

-malign-300
     On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4 byte boundaries. -malign-300 causes them to be aligned on 2 byte boundaries. This option has no effect on the H8/300.

HPPA Options
------------

These -m options are defined for the HPPA family of computers:

-march=ARCHITECTURE-TYPE
     Generate code for the specified architecture.  The choices for ARCHITECTURE-TYPE are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors.  Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine.  Code compiled for lower numbered architectures will run on higher numbered architectures, but not the other way around.

-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
     Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

-mbig-switch
     Generate code suitable for big switch tables.  Use this option only if the assembler/linker complain about out of range branches within a switch table.

-mjump-in-delay
     Fill delay slots of function calls with unconditional jump instructions by modifying the return pointer for the function call to be the target of the conditional jump.

-mdisable-fpregs
     Prevent floating point registers from being used in any manner.  This is necessary for compiling kernels which perform lazy context switching of floating point registers.  If you use this option and attempt to perform floating point operations, the compiler will abort.

-mdisable-indexing
     Prevent the compiler from using indexing address modes.  This avoids some rather obscure problems when compiling MIG generated code under MACH.

-mno-space-regs
     Generate code that assumes the target has no space registers.  This allows GCC to generate faster indirect calls and use unscaled index address modes.

     Such code is suitable for level 0 PA systems and kernels.

-mfast-indirect-calls
     Generate code that assumes calls never cross space boundaries.  This allows GCC to emit code which performs faster indirect calls.

     This option will not work in the presence of shared libraries or nested functions.

-mfixed-range=REGISTER-RANGE
     Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use.  This is useful when compiling kernel code.  A register range is specified as two registers separated by a dash.  Multiple register ranges can be specified separated by a comma.

-mlong-load-store
     Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker.  This is equivalent to the +k option to the HP compilers.

-mportable-runtime
     Use the portable calling conventions proposed by HP for ELF systems.

-mgas
     Enable the use of assembler directives only GAS understands.

-mschedule=CPU-TYPE
     Schedule code according to the constraints for the machine type CPU-TYPE.  The choices for CPU-TYPE are 700 7100, 7100LC, 7200, 7300 and 8000.  Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine.  The default scheduling is 8000.

-mlinker-opt
     Enable the optimization pass in the HP-UX linker.  Note this makes symbolic debugging impossible.  It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets.  Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.  The embedded target hppa1.1-*-pro does provide software floating point support.

     -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.  In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-msio
     Generate the predefine, _SIO, for server IO.  The default is -mwsio.  This generates the predefines, __hp9000s700, __hp9000s700__ and _WSIO, for workstation IO.  These options are available under HP-UX and HI-UX.

-mgnu-ld
     Use GNU ld specific options.  This passes -shared to ld when building a shared library.  It is the default when GCC is configured, explicitly or implicitly, with the GNU linker.  This option does not have any affect on which ld is called, it only changes what parameters are passed to that ld.  The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH.  The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.

-mhp-ld
     Use HP ld specific options.  This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links.  It is the default when GCC is configured, explicitly or implicitly, with the HP linker.  This option does not have any affect on which ld is called, it only changes what parameters are passed to that ld.  The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH.  The linker used by GCC can be printed using which `gcc -print-prog-name=ld`.

-mlong-calls
     Generate code that uses long call sequences.  This ensures that a call is always able to reach linker generated stubs.  The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used.  The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures.  Sibcalls are always limited at 240,000 bytes.

     Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

     It is normally not desirable to use this option as it will degrade performance.  However, it may be useful in large applications, particularly when partial linking is used to build the application.

     The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated.  The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.

-munix=UNIX-STD
     Generate compiler predefines and select a startfile for the specified UNIX standard.  The choices for UNIX-STD are 93, 95 and 98.  93 is supported on all HP-UX versions.  95 is available on HP-UX 10.10 and later.  98 is available on HP-UX 11.11 and later.  The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98 for HP-UX 11.11 and later.

     -munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for XOPEN_UNIX and _XOPEN_SOURCE_EXTENDED, and the startfile unix95.o. -munix=98 provides additional predefines for _XOPEN_UNIX, _XOPEN_SOURCE_EXTENDED, _INCLUDE__STDC_A1_SOURCE and _INCLUDE_XOPEN_SOURCE_500, and the startfile unix98.o.

     It is important to note that this option changes the interfaces for various library routines.  It also affects the operational behavior of the C library.  Thus, extreme care is needed in using this option.

     Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __XPG4_EXTENDED_MASK as appropriate.  Most GNU software doesn't provide this capability.

-nolibdld
     Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.

-static
     The HP-UX implementation of setlocale in libc has a dependency on libdld.sl.  There isn't an archive version of libdld.sl.  Thus, when the -static option is specified, special link options are needed to resolve this dependency.

     On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic.  On the 64-bit port, the linkers generate dynamic binaries by default in any case.  The -nolibdld option can be used to prevent the GCC driver from adding these link options.

-threads
     Add support for multithreading with the dce thread library under HP-UX.  This option sets flags for both the preprocessor and linker.

Intel 386 and AMD x86-64 Options
--------------------------------

These -m options are defined for the i386 and x86-64 family of computers:

-mtune=CPU-TYPE
     Tune to CPU-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions.  The choices for CPU-TYPE are:

     i386
          Original Intel's i386 CPU.

     i486
          Intel's i486 CPU.  (No scheduling is implemented for this chip.)

     i586, pentium
          Intel Pentium CPU with no MMX support.

     pentium-mmx
          Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.

     i686, pentiumpro
          Intel PentiumPro CPU.

     pentium2
          Intel Pentium2 CPU based on PentiumPro core with MMX instruction set support.

     pentium3, pentium3m
          Intel Pentium3 CPU based on PentiumPro core with MMX and SSE instruction set support.

     pentium-m
          Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2 instruction set support.  Used by Centrino notebooks.

     pentium4, pentium4m
          Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set support.

     prescott
          Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.

     nocona
          Improved version of Intel Pentium4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.

     k6
          AMD K6 CPU with MMX instruction set support.

     k6-2, k6-3
          Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction set support.

     athlon, athlon-tbird
          AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE prefetch instructions support.

     athlon-4, athlon-xp, athlon-mp
          Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and full SSE instruction set support.

     k8, opteron, athlon64, athlon-fx
          AMD K8 core based CPUs with x86-64 instruction set support.  (This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and 64-bit instruction set extensions.)

     winchip-c6
          IDT Winchip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.

     winchip2
          IDT Winchip2 CPU, dealt in same way as i486 with additional MMX and 3dNOW! instruction set support.

     c3
          Via C3 CPU with MMX and 3dNOW! instruction set support.  (No scheduling is implemented for this chip.)

     c3-2
          Via C3-2 CPU with MMX and SSE instruction set support.  (No scheduling is implemented for this chip.)

     While picking a specific CPU-TYPE will schedule things appropriately for that particular chip, the compiler will not generate any code that does not run on the i386 without the -march=CPU-TYPE option being used.

-march=CPU-TYPE
     Generate instructions for the machine type CPU-TYPE.  The choices for CPU-TYPE are the same as for -mtune.  Moreover, specifying -march=CPU-TYPE implies -mtune=CPU-TYPE.

-mcpu=CPU-TYPE
     A deprecated synonym for -mtune.

-m386
-m486
-mpentium
-mpentiumpro
     These options are synonyms for -mtune=i386, -mtune=i486, -mtune=pentium, and -mtune=pentiumpro respectively. These synonyms are deprecated.

-mfpmath=UNIT
     Generate floating point arithmetics for selected unit UNIT.  The choices for UNIT are:

     387
          Use the standard 387 floating point coprocessor present majority of chips and emulated otherwise.  Code compiled with this option will run almost everywhere. The temporary results are computed in 80bit precision instead of precision specified by the type resulting in slightly different results compared to most of other chips.  See -ffloat-store for more detailed description.

          This is the default choice for i386 compiler.

     sse
          Use scalar floating point instructions present in the SSE instruction set. This instruction set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and Athlon-mp chips.  The earlier version of SSE instruction set supports only single precision arithmetics, thus the double and extended precision arithmetics is still done using 387.  Later version, present only in Pentium4 and the future AMD x86-64 chips supports double precision arithmetics too.

          For the i386 compiler, you need to use -march=CPU-TYPE, -msse or -msse2 switches to enable SSE extensions and make this option effective.  For the x86-64 compiler, these extensions are enabled by default.

          The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80bit.

          This is the default choice for the x86-64 compiler.

     sse,387
          Attempt to utilize both instruction sets at once.  This effectively double the amount of available registers and on chips with separate execution units for 387 and SSE the execution resources too.  Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well resulting in instable performance.

-masm=DIALECT
     Output asm instructions using selected DIALECT.  Supported choices are intel or att (the default one).

-mieee-fp
-mno-ieee-fp
     Control whether or not the compiler uses IEEE floating point comparisons.  These handle correctly the case where the result of a comparison is unordered.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.

     On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if -msoft-float is used.

-mno-fp-ret-in-387
     Do not use the FPU registers for return values of functions.

     The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU.  The idea is that the operating system should emulate an FPU.

     The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

-mno-fancy-math-387
     Some 387 emulators do not support the sin, cos and sqrt instructions for the 387.  Specify this option to avoid generating those instructions.  This option is the default on FreeBSD, OpenBSD and NetBSD.  This option is overridden when -march indicates that the target cpu will always have an FPU and so the instruction will not need emulation.  As of revision 2.6.1, these instructions are not generated unless you also use the -funsafe-math-optimizations switch.

-malign-double
-mno-align-double
     Control whether GCC aligns double, long double, and long long variables on a two word boundary or a one word boundary.  Aligning double variables on a two word boundary will produce code that runs somewhat faster on a Pentium at the expense of more memory.

     Warning: if you use the -malign-double switch, structures containing the above types will be aligned differently than the published application binary interface specifications for the 386 and will not be binary compatible with structures in code compiled without that switch.

-m96bit-long-double
-m128bit-long-double
     These switches control the size of long double type.  The i386 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32 bit mode.

     Modern architectures (Pentium and newer) would prefer long double to be aligned to an 8 or 16 byte boundary.  In arrays or structures conforming to the ABI, this would not be possible.  So specifying a -m128bit-long-double will align long double to a 16 byte boundary by padding the long double with an additional 32 bit zero.

     In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is to be aligned on 16 byte boundary.

     Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.

     Warning: if you override the default value for your target ABI, the structures and arrays containing long double variables will change their size as well as function calling convention for function taking long double will be modified.  Hence they will not be binary compatible with arrays or structures in code compiled without that switch.

-msvr3-shlib
-mno-svr3-shlib
     Control whether GCC places uninitialized local variables into the bss or data segments.  -msvr3-shlib places them into bss.  These options are meaningful only on System V Release 3.

-mrtd
     Use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret NUM instruction, which pops their arguments while returning.  This saves one instruction in the caller since there is no need to pop the arguments there.

     You can specify that an individual function is called with this calling sequence with the function attribute stdcall.  You can also override the -mrtd option by using the function attribute cdecl.  See Function Attributes.

     Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

     Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions.

     In addition, seriously incorrect code will result if you call a function with too many arguments.  (Normally, extra arguments are harmlessly ignored.)

-mregparm=NUM
     Control how many registers are used to pass integer arguments.  By default, no registers are used to pass arguments, and at most 3 registers can be used.  You can control this behavior for a specific function by using the function attribute regparm. See Function Attributes.

     Warning: if you use this switch, and NUM is nonzero, then you must build all modules with the same value, including any libraries.  This includes the system libraries and startup modules.

-mpreferred-stack-boundary=NUM
     Attempt to keep the stack boundary aligned to a 2 raised to NUM byte boundary.  If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits), except when optimizing for code size (-Os), in which case the default is the minimum correct alignment (4 bytes for x86, and 8 bytes for x86-64).

     On Pentium and PentiumPro, double and long double values should be aligned to an 8 byte boundary (see -malign-double) or suffer significant run time performance penalties.  On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 suffers similar penalties if it is not 16 byte aligned.

     To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned.  Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack.  It is recommended that libraries that use callbacks always use the default setting.

     This extra alignment does consume extra stack space, and generally increases code size.  Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

-mmmx
-mno-mmx

-msse
-mno-sse

-msse2
-mno-sse2

-msse3
-mno-sse3

-m3dnow
-mno-3dnow
     These switches enable or disable the use of built-in functions that allow direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow extensions of the instruction set.

     See X86 Built-in Functions, for details of the functions enabled and disabled by these switches.

     To have SSE/SSE2 instructions generated automatically from floating-point code, see -mfpmath=sse.

-mpush-args
-mno-push-args
     Use PUSH operations to store outgoing parameters.  This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default.  In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.

-maccumulate-outgoing-args
     If enabled, the maximum amount of space required for outgoing arguments will be computed in the function prologue.  This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when preferred stack boundary is not equal to 2.  The drawback is a notable increase in code size.  This switch implies -mno-push-args.

-mthreads
     Support thread-safe exception handling on Mingw32.  Code that relies on thread-safe exception handling must compile and link all code with the -mthreads option.  When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per thread exception handling data.

-mno-align-stringops
     Do not align destination of inlined string operations.  This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it.

-minline-all-stringops
     By default GCC inlines string operations only when destination is known to be aligned at least to 4 byte boundary.  This enables more inlining, increase code size, but may improve performance of code that depends on fast memcpy, strlen and memset for short lengths.

-momit-leaf-frame-pointer
     Don't keep the frame pointer in a register for leaf functions.  This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions.  The option -fomit-frame-pointer removes the frame pointer for all functions which might make debugging harder.

-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
     Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added.  Whether or not this is legal depends on the operating system, and whether it maps the segment to cover the entire TLS area.

     For systems that use GNU libc, the default is on.

These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.

-m32
-m64
     Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.

-mno-red-zone
     Do not use a so called red zone for x86-64 code.  The red zone is mandated by the x86-64 ABI, it is a 128-byte area beyond the location of the stack pointer that will not be modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer.  The flag -mno-red-zone disables this red zone.

-mcmodel=small
     Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space.  Pointers are 64 bits. Programs can be statically or dynamically linked.  This is the default code model.

-mcmodel=kernel
     Generate code for the kernel code model.  The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.

-mcmodel=medium
     Generate code for the medium model: The program is linked in the lower 2 GB of the address space but symbols can be located anywhere in the address space.  Programs can be statically or dynamically linked, but building of shared libraries are not supported with the medium model.

-mcmodel=large
     Generate code for the large model: This model makes no assumptions about addresses and sizes of sections.  Currently GCC does not implement this model.

IA-64 Options
-------------

These are the -m options defined for the Intel IA-64 architecture.

-mbig-endian
     Generate code for a big endian target.  This is the default for HP-UX.

-mlittle-endian
     Generate code for a little endian target.  This is the default for AIX5 and GNU/Linux.

-mgnu-as
-mno-gnu-as
     Generate (or don't) code for the GNU assembler.  This is the default.

-mgnu-ld
-mno-gnu-ld
     Generate (or don't) code for the GNU linker.  This is the default.

-mno-pic
     Generate code that does not use a global pointer register.  The result is not position independent code, and violates the IA-64 ABI.

-mvolatile-asm-stop
-mno-volatile-asm-stop
     Generate (or don't) a stop bit immediately before and after volatile asm statements.

-mregister-names
-mno-register-names
     Generate (or don't) in, loc, and out register names for the stacked registers.  This may make assembler output more readable.

-mno-sdata
-msdata
     Disable (or enable) optimizations that use the small data section.  This may be useful for working around optimizer bugs.

-mconstant-gp
     Generate code that uses a single constant global pointer value.  This is useful when compiling kernel code.

-mauto-pic
     Generate code that is self-relocatable.  This implies -mconstant-gp. This is useful when compiling firmware code.

-minline-float-divide-min-latency
     Generate code for inline divides of floating point values using the minimum latency algorithm.

-minline-float-divide-max-throughput
     Generate code for inline divides of floating point values using the maximum throughput algorithm.

-minline-int-divide-min-latency
     Generate code for inline divides of integer values using the minimum latency algorithm.

-minline-int-divide-max-throughput
     Generate code for inline divides of integer values using the maximum throughput algorithm.

-minline-sqrt-min-latency
     Generate code for inline square roots using the minimum latency algorithm.

-minline-sqrt-max-throughput
     Generate code for inline square roots using the maximum throughput algorithm.

-mno-dwarf2-asm
-mdwarf2-asm
     Don't (or do) generate assembler code for the DWARF2 line number debugging info.  This may be useful when not using the GNU assembler.

-mearly-stop-bits
-mno-early-stop-bits
     Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit.  This can improve instruction scheduling, but does not always do so.

-mfixed-range=REGISTER-RANGE
     Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator can not use.  This is useful when compiling kernel code.  A register range is specified as two registers separated by a dash.  Multiple register ranges can be specified separated by a comma.

-mtls-size=TLS-SIZE
     Specify bit size of immediate TLS offsets.  Valid values are 14, 22, and 64.

-mtune-arch=CPU-TYPE
     Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley.

-mt
-pthread
     Add support for multithreading using the POSIX threads library.  This option sets flags for both the preprocessor and linker.  It does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.  These are HP-UX specific flags.

-milp32
-mlp64
     Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.  These are HP-UX specific flags.

M32R/D Options
--------------

These -m options are defined for Renesas M32R/D architectures:

-m32r2
     Generate code for the M32R/2.

-m32rx
     Generate code for the M32R/X.

-m32r
     Generate code for the M32R.  This is the default.

-mmodel=small
     Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

     The addressability of a particular object can be set with the model attribute.

-mmodel=medium
     Assume objects may be anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.

-mmodel=large
     Assume objects may be anywhere in the 32-bit address space (the compiler will generate seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler will generate the much slower seth/add3/jl instruction sequence).

-msdata=none
     Disable use of the small data area.  Variables will be put into one of .data, bss, or .rodata (unless the section attribute has been specified). This is the default.

     The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the section attribute using one of these sections.

-msdata=sdata
     Put small global and static data in the small data area, but do not generate special code to reference them.

-msdata=use
     Put small global and static data in the small data area, and generate special instructions to reference them.

-G NUM
     Put global and static objects less than or equal to NUM bytes into the small data or bss sections instead of the normal data or bss sections.  The default value of NUM is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.

     All modules should be compiled with the same -G NUM value. Compiling with different values of NUM may or may not work; if it doesn't the linker will give an error message---incorrect code will not be generated.

-mdebug
     Makes the M32R specific code in the compiler display some statistics that might help in debugging programs.

-malign-loops
     Align all loops to a 32-byte boundary.

-mno-align-loops
     Do not enforce a 32-byte alignment for loops.  This is the default.

-missue-rate=NUMBER
     Issue NUMBER instructions per cycle.  NUMBER can only be 1 or 2.

-mbranch-cost=NUMBER
     NUMBER can only be 1 or 2.  If it is 1 then branches will be preferred over conditional code, if it is 2, then the opposite will apply.

-mflush-trap=NUMBER
     Specifies the trap number to use to flush the cache.  The default is 12.  Valid numbers are between 0 and 15 inclusive.

-mno-flush-trap
     Specifies that the cache cannot be flushed by using a trap.

-mflush-func=NAME
     Specifies the name of the operating system function to call to flush the cache.  The default is _flush_cache, but a function call will only be used if a trap is not available.

-mno-flush-func
     Indicates that there is no OS function for flushing the cache.

M680x0 Options
--------------

These are the -m options defined for the 68000 series.  The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m68000
-mc68000
     Generate output for a 68000.  This is the default when the compiler is configured for 68000-based systems.

     Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.

-m68020
-mc68020
     Generate output for a 68020.  This is the default when the compiler is configured for 68020-based systems.

-m68881
     Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless --nfp was specified when the compiler was configured.

-m68030
     Generate output for a 68030.  This is the default when the compiler is configured for 68030-based systems.

-m68040
     Generate output for a 68040.  This is the default when the compiler is configured for 68040-based systems.

     This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040.  Use this option if your 68040 does not have code to emulate those instructions.

-m68060
     Generate output for a 68060.  This is the default when the compiler is configured for 68060-based systems.

     This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060.  Use this option if your 68060 does not have code to emulate those instructions.

-mcpu32
     Generate output for a CPU32.  This is the default when the compiler is configured for CPU32-based systems.

     Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

-m5200
     Generate output for a 520X ``coldfire'' family cpu.  This is the default when the compiler is configured for 520X-based systems.

     Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.

-m68020-40
     Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.  The generated code does use the 68881 instructions that are emulated on the 68040.

-m68020-60
     Generate output for a 68060, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.  The generated code does use the 68881 instructions that are emulated on the 68060.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all m68k targets.  Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.  The embedded targets m68k-*-aout and m68k-*-coff do provide software floating point support.

-mshort
     Consider type int to be 16 bits wide, like short int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.

-mnobitfield
     Do not use the bit-field instructions.  The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.

-mbitfield
     Do use the bit-field instructions.  The -m68020 option implies -mbitfield.  This is the default if you use a configuration designed for a 68020.

-mrtd
     Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning.  This saves one instruction in the caller since there is no need to pop the arguments there.

     This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

     Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions.

     In addition, seriously incorrect code will result if you call a function with too many arguments.  (Normally, extra arguments are harmlessly ignored.)

     The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.

-malign-int
-mno-align-int
     Control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

     Warning: if you use the -malign-int switch, GCC will align structures containing the above types  differently than most published application binary interface specifications for the m68k.

-mpcrel
     Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table.  At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing.  -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.

-mno-strict-align
-mstrict-align
     Do not (do) assume that unaligned memory references will be handled by the system.

-msep-data
     Generate code that allows the data segment to be located in a different area of memory from the text segment.  This allows for execute in place in an environment without virtual memory management.  This option implies -fPIC.

-mno-sep-data
     Generate code that assumes that the data segment follows the text segment. This is the default.

-mid-shared-library
     Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management.  This option implies -fPIC.

-mno-id-shared-library
     Generate code that doesn't assume ID based shared libraries are being used. This is the default.

-mshared-library-id=n
     Specified the identification number of the ID based shared library being compiled.  Specifying a value of 0 will generate more compact code, specifying other values will force the allocation of that number to the current library but is no more space or time efficient than omitting this option.

M68hc1x Options
---------------

These are the -m options defined for the 68hc11 and 68hc12 microcontrollers.  The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.

-m6811
-m68hc11
     Generate output for a 68HC11.  This is the default when the compiler is configured for 68HC11-based systems.

-m6812
-m68hc12
     Generate output for a 68HC12.  This is the default when the compiler is configured for 68HC12-based systems.

-m68S12
-m68hcs12
     Generate output for a 68HCS12.

-mauto-incdec
     Enable the use of 68HC12 pre and post auto-increment and auto-decrement addressing modes.

-minmax
-nominmax
     Enable the use of 68HC12 min and max instructions.

-mlong-calls
-mno-long-calls
     Treat all calls as being far away (near).  If calls are assumed to be far away, the compiler will use the call instruction to call a function and the rtc instruction for returning.

-mshort
     Consider type int to be 16 bits wide, like short int.

-msoft-reg-count=COUNT
     Specify the number of pseudo-soft registers which are used for the code generation.  The maximum number is 32.  Using more pseudo-soft register may or may not result in better code depending on the program. The default is 4 for 68HC11 and 2 for 68HC12.

MCore Options
-------------

These are the -m options defined for the Motorola M*Core processors.

-mhardlit
-mno-hardlit
     Inline constants into the code stream if it can be done in two instructions or less.

-mdiv
-mno-div
     Use the divide instruction.  (Enabled by default).

-mrelax-immediate
-mno-relax-immediate
     Allow arbitrary sized immediates in bit operations.

-mwide-bitfields
-mno-wide-bitfields
     Always treat bit-fields as int-sized.

-m4byte-functions
-mno-4byte-functions
     Force all functions to be aligned to a four byte boundary.

-mcallgraph-data
-mno-callgraph-data
     Emit callgraph information.

-mslow-bytes
-mno-slow-bytes
     Prefer word access when reading byte quantities.

-mlittle-endian
-mbig-endian
     Generate code for a little endian target.

-m210
-m340
     Generate code for the 210 processor.

MIPS Options
------------

-EB
     Generate big-endian code.

-EL
     Generate little-endian code.  This is the default for mips*el-*-* configurations.

-march=ARCH
     Generate code that will run on ARCH, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, and mips64. The processor names are: 4kc, 4kp, 5kc, 20kc, m4k, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r6000, r8000, rm7000, rm9000, orion, sb1, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400 and vr5500. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).

     In processor names, a final 000 can be abbreviated as k (for example, -march=r2k).  Prefixes are optional, and vr may be written r.

     GCC defines two macros based on the value of this option.  The first is _MIPS_ARCH, which gives the name of target architecture, as a string.  The second has the form _MIPS_ARCH_FOO, where FOO is the capitalized value of _MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to "r2000" and define the macro _MIPS_ARCH_R2000.

     Note that the _MIPS_ARCH macro uses the processor names given above.  In other words, it will have the full prefix and will not abbreviate 000 as k.  In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3").  It names the default architecture when no -march option is given.

-mtune=ARCH
     Optimize for ARCH.  Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations.  The list of ARCH values is the same as for -march.

     When this option is not used, GCC will optimize for the processor specified by -march.  By using -march and -mtune together, it is possible to generate code that will run on a family of processors, but optimize the code for one particular member of that family.

     -mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_FOO, which work in the same way as the -march ones described above.

-mips1
     Equivalent to -march=mips1.

-mips2
     Equivalent to -march=mips2.

-mips3
     Equivalent to -march=mips3.

-mips4
     Equivalent to -march=mips4.

-mips32
     Equivalent to -march=mips32.

-mips32r2
     Equivalent to -march=mips32r2.

-mips64
     Equivalent to -march=mips64.

-mips16
-mno-mips16
     Use (do not use) the MIPS16 ISA.

-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
     Generate code for the given ABI.

     Note that the EABI has a 32-bit and a 64-bit variant.  GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

     For information about the O64 ABI, see http://gcc.gnu.org/projects/mipso64-abi.html.

-mabicalls
-mno-abicalls
     Generate (do not generate) SVR4-style position-independent code. -mabicalls is the default for SVR4-based systems.

-mxgot
-mno-xgot
     Lift (do not lift) the usual restrictions on the size of the global offset table.

     GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it will only work if the GOT is smaller than about 64k.  Anything larger will cause the linker to report an error such as:

          relocation truncated to fit: R_MIPS_GOT16 foobar

     If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs, although it will also be less efficient, since it will take three instructions to fetch the value of a global symbol.

     Note that some linkers can create multiple GOTs.  If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries.  Very few do.

     These options have no effect unless GCC is generating position independent code.

-mgp32
     Assume that general-purpose registers are 32 bits wide.

-mgp64
     Assume that general-purpose registers are 64 bits wide.

-mfp32
     Assume that floating-point registers are 32 bits wide.

-mfp64
     Assume that floating-point registers are 64 bits wide.

-mhard-float
     Use floating-point coprocessor instructions.

-msoft-float
     Do not use floating-point coprocessor instructions.  Implement floating-point calculations using library calls instead.

-msingle-float
     Assume that the floating-point coprocessor only supports single-precision operations.

-mdouble-float
     Assume that the floating-point coprocessor supports double-precision operations.  This is the default.

-mpaired-single
-mno-paired-single
     Use (do not use) paired-single floating-point instructions. See MIPS Paired-Single Support.  This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.

-mips3d
-mno-mips3d
     Use (do not use) the MIPS-3D ASE.  See MIPS-3D Built-in Functions. The option -mips3d implies -mpaired-single.

-mint64
     Force int and long types to be 64 bits wide.  See -mlong32 for an explanation of the default and the way that the pointer size is determined.

     This option has been deprecated and will be removed in a future release.

-mlong64
     Force long types to be 64 bits wide.  See -mlong32 for an explanation of the default and the way that the pointer size is determined.

-mlong32
     Force long, int, and pointer types to be 32 bits wide.

     The default size of ints, longs and pointers depends on the ABI.  All the supported ABIs use 32-bit ints.  The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs.  Pointers are the same size as longs, or the same size as integer registers, whichever is smaller.

-msym32
-mno-sym32
     Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI.  This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses.

-G NUM
     Put global and static items less than or equal to NUM bytes into the small data or bss section instead of the normal data or bss section. This allows the data to be accessed using a single instruction.

     All modules should be compiled with the same -G NUM value.

-membedded-data
-mno-embedded-data
     Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data.  This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.

-muninit-const-in-rodata
-mno-uninit-const-in-rodata
     Put uninitialized const variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data.

-msplit-addresses
-mno-split-addresses
     Enable (disable) use of the %hi() and %lo() assembler relocation operators.  This option has been superceded by -mexplicit-relocs but is retained for backwards compatibility.

-mexplicit-relocs
-mno-explicit-relocs
     Use (do not use) assembler relocation operators when dealing with symbolic addresses.  The alternative, selected by -mno-explicit-relocs, is to use assembler macros instead.

     -mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators.

-mcheck-zero-division
-mno-check-zero-division
     Trap (do not trap) on integer division by zero.  The default is -mcheck-zero-division.

-mdivide-traps
-mdivide-breaks
     MIPS systems check for division by zero by generating either a conditional trap or a break instruction.  Using traps results in smaller code, but is only supported on MIPS II and later.  Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (SIGFPE).  Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.

     The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.

-mmemcpy
-mno-memcpy
     Force (do not force) the use of memcpy() for non-trivial block moves.  The default is -mno-memcpy, which allows GCC to inline most constant-sized copies.

-mlong-calls
-mno-long-calls
     Disable (do not disable) use of the jal instruction.  Calling functions using jal is more efficient but requires the caller and callee to be in the same 256 megabyte segment.

     This option has no effect on abicalls code.  The default is -mno-long-calls.

-mmad
-mno-mad
     Enable (disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA.

-mfused-madd
-mno-fused-madd
     Enable (disable) use of the floating point multiply-accumulate instructions, when they are available.  The default is -mfused-madd.

     When multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit.  This may be undesirable in some circumstances.

-nocpp
     Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them.

-mfix-r4000
-mno-fix-r4000
     Work around certain R4000 CPU errata:

       - A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.

       - A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.

       - An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.

-mfix-r4400
-mno-fix-r4400
     Work around certain R4400 CPU errata:

       - A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.

-mfix-vr4120
-mno-fix-vr4120
     Work around certain VR4120 errata:

       - dmultu does not always produce the correct result.

       - div and ddiv do not always produce the correct result if one of the operands is negative.

     The workarounds for the division errata rely on special functions in libgcc.a.  At present, these functions are only provided by the mips64vr*-elf configurations.

     Other VR4120 errata require a nop to be inserted between certain pairs of instructions.  These errata are handled by the assembler, not by GCC itself.

-mfix-vr4130
     Work around the VR4130 mflo/mfhi errata.  The workarounds are implemented by the assembler rather than by GCC, although GCC will avoid using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead.

-mfix-sb1
-mno-fix-sb1
     Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 ``F1'' and ``F2'' floating point errata.)

-mflush-func=FUNC
-mno-flush-func
     Specifies the function to call to flush the I and D caches, or to not call any such function.  If called, the function must take the same arguments as the common _flush_func(), that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches).  The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush.

-mbranch-likely
-mno-branch-likely
     Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture.  By default, Branch Likely instructions may be generated if they are supported by the selected architecture.  An exception is for the MIPS32 and MIPS64 architectures and processors which implement those architectures; for those, Branch Likely instructions will not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.

-mfp-exceptions
-mno-fp-exceptions
     Specifies whether FP exceptions are enabled.  This affects how we schedule FP instructions for some processors.  The default is that FP exceptions are enabled.

     For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes.  Otherwise, we can only use one FP pipe.

-mvr4130-align
-mno-vr4130-align
     The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned.  When this option is enabled, GCC will align pairs of instructions that it thinks should execute in parallel.

     This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3.

MMIX Options
------------

These options are defined for the MMIX:

-mlibfuncs
-mno-libfuncs
     Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.

-mepsilon
-mno-epsilon
     Generate floating-point comparison instructions that compare with respect to the rE epsilon register.

-mabi=mmixware
-mabi=gnu
     Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.

-mzero-extend
-mno-zero-extend
     When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.

-mknuthdiv
-mno-knuthdiv
     Make the result of a division yielding a remainder have the same sign as the divisor.  With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend.  Both methods are arithmetically valid, the latter being almost exclusively used.

-mtoplevel-symbols
-mno-toplevel-symbols
     Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.

-melf
     Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.

-mbranch-predict
-mno-branch-predict
     Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.

-mbase-addresses
-mno-base-addresses
     Generate (do not generate) code that uses base addresses.  Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register.  The register is used for one or more base address requests within the range 0 to 255 from the value held in the register.  The generally leads to short and fast code, but the number of different data items that can be addressed is limited.  This means that a program that uses lots of static data may require -mno-base-addresses.

-msingle-exit
-mno-single-exit
     Force (do not force) generated code to have a single exit point in each function.

MN10300 Options
---------------

These -m options are defined for Matsushita MN10300 architectures:

-mmult-bug
     Generate code to avoid bugs in the multiply instructions for the MN10300 processors.  This is the default.

-mno-mult-bug
     Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.

-mam33
     Generate code which uses features specific to the AM33 processor.

-mno-am33
     Do not generate code which uses features specific to the AM33 processor.  This is the default.

-mno-crt0
     Do not link in the C run-time initialization object file.

-mrelax
     Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses.  This option only has an effect when used on the command line for the final link step.

     This option makes symbolic debugging impossible.

NS32K Options
-------------

These are the -m options defined for the 32000 series.  The default values for these options depends on which style of 32000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m32032
-m32032
     Generate output for a 32032.  This is the default when the compiler is configured for 32032 and 32016 based systems.

-m32332
-m32332
     Generate output for a 32332.  This is the default when the compiler is configured for 32332-based systems.

-m32532
-m32532
     Generate output for a 32532.  This is the default when the compiler is configured for 32532-based systems.

-m32081
     Generate output containing 32081 instructions for floating point. This is the default for all systems.

-m32381
     Generate output containing 32381 instructions for floating point.  This also implies -m32081.  The 32381 is only compatible with the 32332 and 32532 cpus.  This is the default for the pc532-netbsd configuration.

-mmulti-add
     Try and generate multiply-add floating point instructions polyF and dotF.  This option is only available if the -m32381 option is in effect.  Using these instructions requires changes to register allocation which generally has a negative impact on performance.  This option should only be enabled when compiling code particularly likely to make heavy use of multiply-add instructions.

-mnomulti-add
     Do not try and generate multiply-add floating point instructions polyF and dotF.  This is the default on all platforms.

-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries may not be available.

-mieee-compare
-mno-ieee-compare
     Control whether or not the compiler uses IEEE floating point comparisons.  These handle correctly the case where the result of a comparison is unordered. Warning: the requisite kernel support may not be available.

-mnobitfield
     Do not use the bit-field instructions.  On some machines it is faster to use shifting and masking operations.  This is the default for the pc532.

-mbitfield
     Do use the bit-field instructions.  This is the default for all platforms except the pc532.

-mrtd
     Use a different function-calling convention, in which functions that take a fixed number of arguments return pop their arguments on return with the ret instruction.

     This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

     Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions.

     In addition, seriously incorrect code will result if you call a function with too many arguments.  (Normally, extra arguments are harmlessly ignored.)

     This option takes its name from the 680x0 rtd instruction.

-mregparam
     Use a different function-calling convention where the first two arguments are passed in registers.

     This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

-mnoregparam
     Do not pass any arguments in registers.  This is the default for all targets.

-msb
     It is OK to use the sb as an index register which is always loaded with zero.  This is the default for the pc532-netbsd target.

-mnosb
     The sb register is not available for use or has not been initialized to zero by the run time system.  This is the default for all targets except the pc532-netbsd.  It is also implied whenever -mhimem or -fpic is set.

-mhimem
     Many ns32000 series addressing modes use displacements of up to 512MB. If an address is above 512MB then displacements from zero can not be used. This option causes code to be generated which can be loaded above 512MB. This may be useful for operating systems or ROM code.

-mnohimem
     Assume code will be loaded in the first 512MB of virtual address space. This is the default for all platforms.

PDP-11 Options
--------------

These options are defined for the PDP-11:

-mfpu
     Use hardware FPP floating point.  This is the default.  (FIS floating point on the PDP-11/40 is not supported.)

-msoft-float
     Do not use hardware floating point.

-mac0
     Return floating-point results in ac0 (fr0 in Unix assembler syntax).

-mno-ac0
     Return floating-point results in memory.  This is the default.

-m40
     Generate code for a PDP-11/40.

-m45
     Generate code for a PDP-11/45.  This is the default.

-m10
     Generate code for a PDP-11/10.

-mbcopy-builtin
     Use inline movmemhi patterns for copying memory.  This is the default.

-mbcopy
     Do not use inline movmemhi patterns for copying memory.

-mint16
-mno-int32
     Use 16-bit int.  This is the default.

-mint32
-mno-int16
     Use 32-bit int.

-mfloat64
-mno-float32
     Use 64-bit float.  This is the default.

-mfloat32
-mno-float64
     Use 32-bit float.

-mabshi
     Use abshi2 pattern.  This is the default.

-mno-abshi
     Do not use abshi2 pattern.

-mbranch-expensive
     Pretend that branches are expensive.  This is for experimenting with code generation only.

-mbranch-cheap
     Do not pretend that branches are expensive.  This is the default.

-msplit
     Generate code for a system with split I&D.

-mno-split
     Generate code for a system without split I&D.  This is the default.

-munix-asm
     Use Unix assembler syntax.  This is the default when configured for pdp11-*-bsd.

-mdec-asm
     Use DEC assembler syntax.  This is the default when configured for any PDP-11 target other than pdp11-*-bsd.

PowerPC Options
---------------

These are listed under see RS/6000 and PowerPC Options.

IBM RS/6000 and PowerPC Options
-------------------------------

These -m options are defined for the IBM RS/6000 and PowerPC:

-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
     GCC supports two related instruction set architectures for the RS/6000 and PowerPC.  The POWER instruction set are those instructions supported by the rios chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors.

     Neither architecture is a subset of the other.  However there is a large common subset of instructions supported by both.  An MQ register is included in processors supporting the POWER architecture.

     You use these options to specify which instructions are available on the processor you are using.  The default value of these options is determined when configuring GCC.  Specifying the -mcpu=CPU_TYPE overrides the specification of these options.  We recommend you use the -mcpu=CPU_TYPE option rather than the options listed above.

     The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.

     The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root.  Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

     The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities.  GCC defaults to -mno-powerpc64.

     If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register.  Specifying both -mpower and -mpowerpc permits GCC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

-mnew-mnemonics
-mold-mnemonics
     Select which mnemonics to use in the generated assembler code.  With -mnew-mnemonics, GCC uses the assembler mnemonics defined for the PowerPC architecture.  With -mold-mnemonics it uses the assembler mnemonics defined for the POWER architecture.  Instructions defined in only one architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these options is specified.

     GCC defaults to the mnemonics appropriate for the architecture in use.  Specifying -mcpu=CPU_TYPE sometimes overrides the value of these option.  Unless you are building a cross-compiler, you should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept the default.

-mcpu=CPU_TYPE
     Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type CPU_TYPE. Supported values for CPU_TYPE are 401, 403, 405, 405fp, 440, 440fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, common, ec603e, G3, G4, G5, power, power2, power3, power4, power5, powerpc, powerpc64, rios, rios1, rios2, rsc, and rs64a.

     -mcpu=common selects a completely generic processor.  Code generated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures, and will not use the MQ register.  GCC assumes a generic processor model for scheduling purposes.

     -mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

     The other options specify a specific processor.  Code generated under those options will run best on that processor, and may not run at all on others.

     The -mcpu options automatically enable or disable the following options: -maltivec, -mhard-float, -mmfcrf, -mmultiple, -mnew-mnemonics, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt, -mpowerpc-gfxopt, -mstring.  The particular options set for any particular CPU will vary between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities.  If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec.

     On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, since AIX does not have full support for these options.  You may still enable or disable them individually if you're sure it'll work in your environment.

-mtune=CPU_TYPE
     Set the instruction scheduling parameters for machine type CPU_TYPE, but do not set the architecture type, register usage, or choice of mnemonics, as -mcpu=CPU_TYPE would.  The same values for CPU_TYPE are used for -mtune as for -mcpu.  If both are specified, the code generated will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters set by -mtune.

-maltivec
-mno-altivec
     Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set.  You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

-mabi=spe
     Extend the current ABI with SPE ABI extensions.  This does not change the default ABI, instead it adds the SPE ABI extensions to the current ABI.

-mabi=no-spe
     Disable Booke SPE ABI extensions for the current ABI.

-misel=YES/NO
-misel
     This switch enables or disables the generation of ISEL instructions.

-mspe=YES/NO
-mspe
     This switch enables or disables the generation of SPE simd instructions.

-mfloat-gprs=YES/SINGLE/DOUBLE/NO
-mfloat-gprs
     This switch enables or disables the generation of floating point operations on the general purpose registers for architectures that support it.

     The argument YES or SINGLE enables the use of single-precision floating point operations.

     The argument DOUBLE enables the use of single and double-precision floating point operations.

     The argument NO disables floating point operations on the general purpose registers.

     This option is currently only available on the MPC854x.

-m32
-m64
     Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux).  The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant.  The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64.

-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
     Modify generation of the TOC (Table Of Contents), which is created for every executable file.  The -mfull-toc option is selected by default.  In that case, GCC will allocate at least one TOC entry for each unique non-automatic variable reference in your program.  GCC will also place floating-point constants in the TOC.  However, only 16,384 entries are available in the TOC.

     If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC.  You may specify one or both of these options.  Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

     If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead.  This option causes GCC to make only one TOC entry for every file.  When you specify this option, GCC will produce code that is slower and larger but which uses extremely little TOC space.  You may wish to use this option only on files that contain less frequently executed code.

-maix64
-maix32
     Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64.  GCC defaults to -maix32.

-mxl-compat
-mno-xl-compat
     Produce code that conforms more closely to IBM XLC semantics when using AIX-compatible ABI.  Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs.  Do not assume that most significant double in 128 bit long double value is properly rounded when comparing values.

     The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared.  AIX XL compilers access floating point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization.  Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization.

-mpe
     Support IBM RS/6000 SP Parallel Environment (PE).  Link an application written to use message passing with special startup code to enable the application to run.  The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location.  The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible.

-malign-natural
-malign-power
     On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules.  GCC defaults to the standard alignment defined in the ABI.

     On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.

-msoft-float
-mhard-float
     Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.

-mmultiple
-mno-multiple
     Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions.  These instructions are generated by default on POWER systems, and not generated on PowerPC systems.  Do not use -mmultiple on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.  The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

-mstring
-mno-string
     Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves.  These instructions are generated by default on POWER systems, and not generated on PowerPC systems.  Do not use -mstring on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in little endian mode.

-mupdate
-mno-update
     Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location.  These instructions are generated by default.  If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.

-mfused-madd
-mno-fused-madd
     Generate code that uses (does not use) the floating point multiply and accumulate instructions.  These instructions are generated by default if hardware floating is used.

-mno-bit-align
-mbit-align
     On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

     For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes.  By using -mno-bit-align, the structure would be aligned to a 1 byte boundary and be one byte in size.

-mno-strict-align
-mstrict-align
     On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.

-mrelocatable
-mno-relocatable
     On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime.  If you use -mrelocatable on any module, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib.

-mrelocatable-lib
-mno-relocatable-lib
     On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime.  Modules compiled with -mrelocatable-lib can be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules compiled with the -mrelocatable options.

-mno-toc
-mtoc
     On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

-mlittle
-mlittle-endian
     On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode.  The -mlittle-endian option is the same as -mlittle.

-mbig
-mbig-endian
     On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode.  The -mbig-endian option is the same as -mbig.

-mdynamic-no-pic
     On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable.  The resulting code is suitable for applications, but not shared libraries.

-mprioritize-restricted-insns=PRIORITY
     This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass.  The argument PRIORITY takes the value 0/1/2 to assign NO/HIGHEST/SECOND-HIGHEST priority to dispatch slot restricted instructions.

-msched-costly-dep=DEPENDENCE_TYPE
     This option controls which dependences are considered costly by the target during instruction scheduling.  The argument DEPENDENCE_TYPE takes one of the following values: NO: no dependence is costly, ALL: all dependences are costly, TRUE_STORE_TO_LOAD: a true dependence from store to load is costly, STORE_TO_LOAD: any dependence from store to load is costly, NUMBER: any dependence which latency >= NUMBER is costly.

-minsert-sched-nops=SCHEME
     This option controls which nop insertion scheme will be used during the second scheduling pass.  The argument SCHEME takes one of the following values: NO: Don't insert nops. PAD: Pad with nops any dispatch group which has vacant issue slots, according to the scheduler's grouping. REGROUP_EXACT: Insert nops to force costly dependent insns into separate groups.  Insert exactly as many nops as needed to force an insn to a new group, according to the estimated processor grouping. NUMBER: Insert nops to force costly dependent insns into separate groups.  Insert NUMBER nops to force an insn to a new group.

-mcall-sysv
     On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement.  This is the default unless you configured GCC using powerpc-*-eabiaix.

-mcall-sysv-eabi
     Specify both -mcall-sysv and -meabi options.

-mcall-sysv-noeabi
     Specify both -mcall-sysv and -mno-eabi options.

-mcall-solaris
     On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.

-mcall-linux
     On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.

-mcall-gnu
     On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.

-mcall-netbsd
     On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.

-maix-struct-return
     Return all structures in memory (as specified by the AIX ABI).

-msvr4-struct-return
     Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).

-mabi=altivec
     Extend the current ABI with AltiVec ABI extensions.  This does not change the default ABI, instead it adds the AltiVec ABI extensions to the current ABI.

-mabi=no-altivec
     Disable AltiVec ABI extensions for the current ABI.

-mprototype
-mno-prototype
     On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped.  Otherwise, the compiler must insert an instruction before every non prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values were passed in the floating point registers in case the function takes a variable arguments.  With -mprototype, only calls to prototyped variable argument functions will set or clear the bit.

-msim
     On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a.  This is the default for powerpc-*-eabisim. configurations.

-mmvme
     On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.

-mads
     On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.

-myellowknife
     On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a.

-mvxworks
     On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.

-mwindiss
     Specify that you are compiling for the WindISS simulation environment.

-memb
     On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi extended relocations are used.

-meabi
-mno-eabi
     On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications.  Selecting -meabi means that the stack is aligned to an 8 byte boundary, a function __eabi is called to from main to set up the eabi environment, and the -msdata option can use both r2 and r13 to point to two separate small data areas.  Selecting -mno-eabi means that the stack is aligned to a 16 byte boundary, do not call an initialization function from main, and the -msdata option will only use r13 to point to a single small data area.  The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.

-msdata=eabi
     On System V.4 and embedded PowerPC systems, put small initialized const global and static data in the .sdata2 section, which is pointed to by register r2.  Put small initialized non-const global and static data in the .sdata section, which is pointed to by register r13.  Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section.  The -msdata=eabi option is incompatible with the -mrelocatable option.  The -msdata=eabi option also sets the -memb option.

-msdata=sysv
     On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register r13.  Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the -mrelocatable option.

-msdata=default
-msdata
     On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.

-msdata-data
     On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section.  Put small uninitialized global and static data in the .sbss section.  Do not use register r13 to address small data however.  This is the default behavior unless other -msdata options are used.

-msdata=none
-mno-sdata
     On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.

-G NUM
     On embedded PowerPC systems, put global and static items less than or equal to NUM bytes into the small data or bss sections instead of the normal data or bss section.  By default, NUM is 8.  The -G NUM switch is also passed to the linker. All modules should be compiled with the same -G NUM value.

-mregnames
-mno-regnames
     On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.

-mlongcall
-mno-longcall
     Default to making all function calls indirectly, using a register, so that functions which reside further than 32 megabytes (33,554,432 bytes) from the current location can be called.  This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0).

     Some linkers are capable of detecting out-of-range calls and generating glue code on the fly.  On these systems, long calls are unnecessary and generate slower code.  As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64.  It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

     On Darwin/PPC systems, #pragma longcall will generate ``jbsr callee, L42'', plus a ``branch island'' (glue code).  The two target addresses represent the callee and the ``branch island''.  The Darwin/PPC linker will prefer the first address and generate a ``bl callee'' if the PPC ``bl'' instruction will reach the callee directly; otherwise, the linker will generate ``bl L42'' to call the ``branch island''.  The ``branch island'' is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

     On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

     In the future, we may cause GCC to ignore all longcall specifications when the linker is known to generate glue.

-pthread
     Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

S/390 and zSeries Options
-------------------------

These are the -m options defined for the S/390 and zSeries architecture.

-mhard-float
-msoft-float
     Use (do not use) the hardware floating-point instructions and registers for floating-point operations.  When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations.  When -mhard-float is specified, the compiler generates IEEE floating-point instructions.  This is the default.

-mbackchain
-mno-backchain
     Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF-2 call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

     In general, code compiled with -mbackchain is call-compatible with code compiled with -mmo-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain.  Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.  In order to build a linux kernel use -msoft-float.

     The default is to not maintain the backchain.

-mpacked-stack

-mno-packed-stack
     Use (do not use) the packed stack layout.  When -mno-packed-stack is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.

     As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack.  Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes.  Such code is not call-compatible with code compiled with -mpacked-stack.  Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported.  In order to build a linux kernel use -msoft-float.

     The default is to not use the packed stack layout.

-msmall-exec
-mno-small-exec
     Generate (or do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k.  The default is to use the basr instruction instead, which does not have this limitation.

-m64
-m31
     When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI.  When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI.  This allows GCC in particular to generate 64-bit instructions.  For the s390 targets, the default is -m31, while the s390x targets default to -m64.

-mzarch
-mesa
     When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390.  Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa.  When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.

-mmvcle
-mno-mvcle
     Generate (or do not generate) code using the mvcle instruction to perform block moves.  When -mno-mvcle is specified, use a mvc loop instead.  This is the default.

-mdebug
-mno-debug
     Print (or do not print) additional debug information when compiling. The default is to not print debug information.

-march=CPU-TYPE
     Generate code that will run on CPU-TYPE, which is the name of a system representing a certain processor type.  Possible values for CPU-TYPE are g5, g6, z900, and z990. When generating code using the instructions available on z/Architecture, the default is -march=z900.  Otherwise, the default is -march=g5.

-mtune=CPU-TYPE
     Tune to CPU-TYPE everything applicable about the generated code, except for the ABI and the set of available instructions. The list of CPU-TYPE values is the same as for -march. The default is the value used for -march.

-mtpf-trace
-mno-tpf-trace
     Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system.  This option is off by default, even when compiling for the TPF OS.

-mfused-madd
-mno-fused-madd
     Generate code that uses (does not use) the floating point multiply and accumulate instructions.  These instructions are generated by default if hardware floating point is used.

-mwarn-framesize=FRAMESIZE
     Emit a warning if the current function exceeds the given frame size.  Because this is a compile time check it doesn't need to be a real problem when the program runs.  It is intended to identify functions which most probably cause a stack overflow.  It is useful to be used in an environment with limited stack size e.g. the linux kernel.

-mwarn-dynamicstack
     Emit a warning if the function calls alloca or uses dynamically sized arrays.  This is generally a bad idea with a limited stack size.

-mstack-guard=STACK-GUARD

-mstack-size=STACK-SIZE
     These arguments always have to be used in conjunction.  If they are present the s390 back end emits additional instructions in the function prologue which trigger a trap if the stack size is STACK-GUARD bytes above the STACK-SIZE (remember that the stack on s390 grows downward).  These options are intended to be used to help debugging stack overflow problems.  The additionally emitted code cause only little overhead and hence can also be used in production like systems without greater performance degradation.  The given values have to be exact powers of 2 and STACK-SIZE has to be greater than STACK-GUARD. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by STACK-SIZE.

SH Options
----------

These -m options are defined for the SH implementations:

-m1
     Generate code for the SH1.

-m2
     Generate code for the SH2.

-m2e
     Generate code for the SH2e.

-m3
     Generate code for the SH3.

-m3e
     Generate code for the SH3e.

-m4-nofpu
     Generate code for the SH4 without a floating-point unit.

-m4-single-only
     Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.

-m4-single
     Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.

-m4
     Generate code for the SH4.

-m4a-nofpu
     Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.

-m4a-single-only
     Generate code for the SH4a, in such a way that no double-precision floating point operations are used.

-m4a-single
     Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.

-m4a
     Generate code for the SH4a.

-m4al
     Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler.  GCC doesn't generate any DSP instructions at the moment.

-mb
     Compile code for the processor in big endian mode.

-ml
     Compile code for the processor in little endian mode.

-mdalign
     Align doubles at 64-bit boundaries.  Note that this changes the calling conventions, and thus some functions from the standard C library will not work unless you recompile it first with -mdalign.

-mrelax
     Shorten some address references at link time, when possible; uses the linker option -relax.

-mbigtable
     Use 32-bit offsets in switch tables.  The default is to use 16-bit offsets.

-mfmovd
     Enable the use of the instruction fmovd.

-mhitachi
     Comply with the calling conventions defined by Renesas.

-mrenesas
     Comply with the calling conventions defined by Renesas.

-mno-renesas
     Comply with the calling conventions defined for GCC before the Renesas conventions were available.  This option is the default for all targets of the SH toolchain except for sh-symbianelf.

-mnomacsave
     Mark the MAC register as call-clobbered, even if -mhitachi is given.

-mieee
     Increase IEEE-compliance of floating-point code.

-misize
     Dump instruction size and location in the assembly code.

-mpadstruct
     This option is deprecated.  It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.

-mspace
     Optimize for space instead of speed.  Implied by -Os.

-mprefergot
     When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table.

-musermode
     Generate a library function call to invalidate instruction cache entries, after fixing up a trampoline.  This library function call doesn't assume it can write to the whole memory address space.  This is the default when the target is sh-*-linux*.

SPARC Options
-------------

These -m options are supported on the SPARC:

-mno-app-regs
-mapp-regs
     Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications.  This is the default.

     To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs.  You should compile libraries and system software with this option.

-mfpu
-mhard-float
     Generate output containing floating point instructions.  This is the default.

-mno-fpu
-msoft-float
     Generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets.  Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation.  You must make your own arrangements to provide suitable library functions for cross-compilation.  The embedded targets sparc-*-aout and sparclite-*-* do provide software floating point support.

     -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option.  In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

-mhard-quad-float
     Generate output containing quad-word (long double) floating point instructions.

-msoft-quad-float
     Generate output containing library calls for quad-word (long double) floating point instructions.  The functions called are those specified in the SPARC ABI.  This is the default.

     As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating point instructions.  They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction.  Because of the trap handler overhead, this is much slower than calling the ABI library routines.  Thus the -msoft-quad-float option is the default.

-mno-unaligned-doubles
-munaligned-doubles
     Assume that doubles have 8 byte alignment.  This is the default.

     With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained in another type, or if they have an absolute address.  Otherwise, it assumes they have 4 byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers.  It is not the default because it results in a performance loss, especially for floating point code.

-mno-faster-structs
-mfaster-structs
     With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment.  This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI.  Thus, it's intended only for use on targets where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI.

-mimpure-text
     -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object.  Using this option, you can link position-dependent code into a shared object.

     -mimpure-text suppresses the ``relocations remain against allocatable but non-writable sections'' linker error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not actually shared across processes.  Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

     This option is only available on SunOS and Solaris.

-mcpu=CPU_TYPE
     Set the instruction set, register set, and instruction scheduling parameters for machine type CPU_TYPE.  Supported values for CPU_TYPE are v7, cypress, v8, supersparc, sparclite, f930, f934, hypersparc, sparclite86x, sparclet, tsc701, v9, ultrasparc, and ultrasparc3.

     Default instruction scheduling parameters are used for values that select an architecture and not an implementation.  These are v7, v8, sparclite, sparclet, v9.

     Here is a list of each supported architecture and their supported implementations.

              v7:             cypress
              v8:             supersparc, hypersparc
              sparclite:      f930, f934, sparclite86x
              sparclet:       tsc701
              v9:             ultrasparc, ultrasparc3

     By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture.  With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series.  This is also appropriate for the older SPARCStation 1, 2, IPX etc.

     With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture.  The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7.  With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

     With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture.  This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU.  With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

     With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture.  This adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions which exist in SPARClet but not in SPARC-V7.  With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

     With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture.  This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions.  With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II chips.  With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III chip.

-mtune=CPU_TYPE
     Set the instruction scheduling parameters for machine type CPU_TYPE, but do not set the instruction set or register set that the option -mcpu=CPU_TYPE would.

     The same values for -mcpu=CPU_TYPE can be used for -mtune=CPU_TYPE, but the only useful values are those that select a particular cpu implementation.  Those are cypress, supersparc, hypersparc, f930, f934, sparclite86x, tsc701, ultrasparc, and ultrasparc3.

-mv8plus
-mno-v8plus
     With -mv8plus, GCC generates code for the SPARC-V8+ ABI.  The difference from the V8 ABI is that the global and out registers are considered 64-bit wide.  This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.

-mvis
-mno-vis
     With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions.  The default is -mno-vis.

These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:

-mlittle-endian
     Generate code for a processor running in little-endian mode.  It is only available for a few configurations and most notably not on Solaris.

-m32
-m64
     Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.

-mcmodel=medlow
     Generate code for the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory.  Programs can be statically or dynamically linked.

-mcmodel=medmid
     Generate code for the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.

-mcmodel=medany
     Generate code for the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.

-mcmodel=embmedany
     Generate code for the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time).  The global register %g4 points to the base of the data segment.  Programs are statically linked and PIC is not supported.

-mstack-bias
-mno-stack-bias
     With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references.  This is the default in 64-bit mode. Otherwise, assume no such offset is present.

These switches are supported in addition to the above on Solaris:

-threads
     Add support for multithreading using the Solaris threads library.  This option sets flags for both the preprocessor and linker.  This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.

-pthreads
     Add support for multithreading using the POSIX threads library.  This option sets flags for both the preprocessor and linker.  This option does not affect the thread safety of object code produced  by the compiler or that of libraries supplied with it.

Options for System V
--------------------

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

-G
     Create a shared object. It is recommended that -symbolic or -shared be used instead.

-Qy
     Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.

-Qn
     Refrain from adding .ident directives to the output file (this is the default).

-YP,DIRS
     Search the directories DIRS, and no others, for libraries specified with -l.

-Ym,DIR
     Look in the directory DIR to find the M4 preprocessor. The assembler uses this option.

TMS320C3x/C4x Options
---------------------

These -m options are defined for TMS320C3x/C4x implementations:

-mcpu=CPU_TYPE
     Set the instruction set, register set, and instruction scheduling parameters for machine type CPU_TYPE.  Supported values for CPU_TYPE are c30, c31, c32, c40, and c44.  The default is c40 to generate code for the TMS320C40.

-mbig-memory
-mbig
-msmall-memory
-msmall
     Generates code for the big or small memory model.  The small memory model assumed that all data fits into one 64K word page.  At run-time the data page (DP) register must be set to point to the 64K page containing the .bss and .data program sections.  The big memory model is the default and requires reloading of the DP register for every direct memory access.

-mbk
-mno-bk
     Allow (disallow) allocation of general integer operands into the block count register BK.

-mdb
-mno-db
     Enable (disable) generation of code using decrement and branch, DBcond(D), instructions.  This is enabled by default for the C4x.  To be on the safe side, this is disabled for the C3x, since the maximum iteration count on the C3x is 2^23 + 1 (but who iterates loops more than 2^23 times on the C3x?).  Note that GCC will try to reverse a loop so that it can utilize the decrement and branch instruction, but will give up if there is more than one memory reference in the loop.  Thus a loop where the loop counter is decremented can generate slightly more efficient code, in cases where the RPTB instruction cannot be utilized.

-mdp-isr-reload
-mparanoid
     Force the DP register to be saved on entry to an interrupt service routine (ISR), reloaded to point to the data section, and restored on exit from the ISR.  This should not be required unless someone has violated the small memory model by modifying the DP register, say within an object library.

-mmpyi
-mno-mpyi
     For the C3x use the 24-bit MPYI instruction for integer multiplies instead of a library call to guarantee 32-bit results.  Note that if one of the operands is a constant, then the multiplication will be performed using shifts and adds.  If the -mmpyi option is not specified for the C3x, then squaring operations are performed inline instead of a library call.

-mfast-fix
-mno-fast-fix
     The C3x/C4x FIX instruction to convert a floating point value to an integer value chooses the nearest integer less than or equal to the floating point value rather than to the nearest integer.  Thus if the floating point number is negative, the result will be incorrectly truncated an additional code is necessary to detect and correct this case.  This option can be used to disable generation of the additional code required to correct the result.

-mrptb
-mno-rptb
     Enable (disable) generation of repeat block sequences using the RPTB instruction for zero overhead looping.  The RPTB construct is only used for innermost loops that do not call functions or jump across the loop boundaries.  There is no advantage having nested RPTB loops due to the overhead required to save and restore the RC, RS, and RE registers. This is enabled by default with -O2.

-mrpts=COUNT
-mno-rpts
     Enable (disable) the use of the single instruction repeat instruction RPTS.  If a repeat block contains a single instruction, and the loop count can be guaranteed to be less than the value COUNT, GCC will emit a RPTS instruction instead of a RPTB.  If no value is specified, then a RPTS will be emitted even if the loop count cannot be determined at compile time.  Note that the repeated instruction following RPTS does not have to be reloaded from memory each iteration, thus freeing up the CPU buses for operands.  However, since interrupts are blocked by this instruction, it is disabled by default.

-mloop-unsigned
-mno-loop-unsigned
     The maximum iteration count when using RPTS and RPTB (and DB on the C40) is 2^31 + 1 since these instructions test if the iteration count is negative to terminate the loop.  If the iteration count is unsigned there is a possibility than the 2^31 + 1 maximum iteration count may be exceeded.  This switch allows an unsigned iteration count.

-mti
     Try to emit an assembler syntax that the TI assembler (asm30) is happy with.  This also enforces compatibility with the API employed by the TI C3x C compiler.  For example, long doubles are passed as structures rather than in floating point registers.

-mregparm
-mmemparm
     Generate code that uses registers (stack) for passing arguments to functions. By default, arguments are passed in registers where possible rather than by pushing arguments on to the stack.

-mparallel-insns
-mno-parallel-insns
     Allow the generation of parallel instructions.  This is enabled by default with -O2.

-mparallel-mpy
-mno-parallel-mpy
     Allow the generation of MPY||ADD and MPY||SUB parallel instructions, provided -mparallel-insns is also specified.  These instructions have tight register constraints which can pessimize the code generation of large functions.

V850 Options
------------

These -m options are defined for V850 implementations:

-mlong-calls
-mno-long-calls
     Treat all calls as being far away (near).  If calls are assumed to be far away, the compiler will always load the functions address up into a register, and call indirect through the pointer.

-mno-ep
-mep
     Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the ep register, and use the shorter sld and sst instructions.  The -mep option is on by default if you optimize.

-mno-prolog-function
-mprolog-function
     Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function.  The external functions are slower, but use less code space if more than one function saves the same number of registers.  The -mprolog-function option is on by default if you optimize.

-mspace
     Try to make the code as small as possible.  At present, this just turns on the -mep and -mprolog-function options.

-mtda=N
     Put static or global variables whose size is N bytes or less into the tiny data area that register ep points to.  The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).

-msda=N
     Put static or global variables whose size is N bytes or less into the small data area that register gp points to.  The small data area can hold up to 64 kilobytes.

-mzda=N
     Put static or global variables whose size is N bytes or less into the first 32 kilobytes of memory.

-mv850
     Specify that the target processor is the V850.

-mbig-switch
     Generate code suitable for big switch tables.  Use this option only if the assembler/linker complain about out of range branches within a switch table.

-mapp-regs
     This option will cause r2 and r5 to be used in the code generated by the compiler.  This setting is the default.

-mno-app-regs
     This option will cause r2 and r5 to be treated as fixed registers.

-mv850e1
     Specify that the target processor is the V850E1.  The preprocessor constants __v850e1__ and __v850e__ will be defined if this option is used.

-mv850e
     Specify that the target processor is the V850E.  The preprocessor constant __v850e__ will be defined if this option is used.

     If neither -mv850 nor -mv850e nor -mv850e1 are defined then a default target processor will be chosen and the relevant __v850*__ preprocessor constant will be defined.

     The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target.

-mdisable-callt
     This option will suppress generation of the CALLT instruction for the v850e and v850e1 flavors of the v850 architecture.  The default is -mno-disable-callt which allows the CALLT instruction to be used.

VAX Options
-----------

These -m options are defined for the VAX:

-munix
     Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.

-mgnu
     Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.

-mg
     Output code for g-format floating point numbers instead of d-format.

x86-64 Options
--------------

These are listed under see i386 and x86-64 Options.

Xstormy16 Options
-----------------

These options are defined for Xstormy16:

-msim
     Choose startup files and linker script suitable for the simulator.

Xtensa Options
--------------

These options are supported for Xtensa targets:

-mconst16
-mno-const16
     Enable or disable use of CONST16 instructions for loading constant values.  The CONST16 instruction is currently not a standard option from Tensilica.  When enabled, CONST16 instructions are always used in place of the standard L32R instructions.  The use of CONST16 is enabled by default only if the L32R instruction is not available.

-mfused-madd
-mno-fused-madd
     Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option.  This has no effect if the floating-point option is not also enabled.  Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations.  This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard.  Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations.

-mtext-section-literals
-mno-text-section-literals
     Control the treatment of literal pools.  The default is -mno-text-section-literals, which places literals in a separate section in the output file.  This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size.  With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references.  This may be necessary for large assembly files.

-mtarget-align
-mno-target-align
     When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density.  The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions.  If there are not enough preceding safe density instructions to align a target, no widening will be performed.  The default is -mtarget-align.  These options do not affect the treatment of auto-aligned instructions like LOOP, which the assembler will always align, either by widening density instructions or by inserting no-op instructions.

-mlongcalls
-mno-longcalls
     When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction.  This translation typically occurs for calls to functions in other source files.  Specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. The default is -mno-longcalls.  This option should be used in programs where the call target can potentially be out of range.  This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC will still show direct call instructions---look at the disassembled object code to see the actual instructions.  Note that the assembler will use an indirect call for every cross-file call, not just those that really will be out of range.

zSeries Options
---------------

These are listed under see S/390 and zSeries Options.

Options for Code Generation Conventions
=======================================

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo.  In the table below, only one of the forms is listed---the one which is not the default.  You can figure out the other form by either removing no- or adding it.

-fbounds-check
     For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range.  This is currently only supported by the Java and Fortran 77 front-ends, where this option defaults to true and false respectively.

-ftrapv
     This option generates traps for signed overflow on addition, subtraction, multiplication operations.

-fwrapv
     This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation.  This flag enables some optimizations and disables other.  This option is enabled by default for the Java front-end, as required by the Java language specification.

-fexceptions
     Enable exception handling.  Generates extra code needed to propagate exceptions.  For some targets, this implies GCC will generate frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution.  If you do not specify this option, GCC will enable it by default for languages like C++ which normally require exception handling, and disable it for languages like C that do not normally require it.  However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++.  You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling.

-fnon-call-exceptions
     Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere.  Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating point instructions.  It does not allow exceptions to be thrown from arbitrary signal handlers such as SIGALRM.

-funwind-tables
     Similar to -fexceptions, except that it will just generate any needed static data, but will not affect the generated code in any other way. You will normally not enable this option; instead, a language processor that needs this handling would enable it on your behalf.

-fasynchronous-unwind-tables
     Generate unwind table in dwarf2 format, if supported by target machine.  The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).

-fpcc-struct-return
     Return ``short'' struct and union values in memory like longer ones, rather than in registers.  This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).

     The precise convention for returning structures in memory depends on the target configuration macros.

     Short structures and unions are those whose size and alignment match that of some integer type.

     Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface.

-freg-struct-return
     Return struct and union values in registers when possible. This is more efficient for small structures than -fpcc-struct-return.

     If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target.  If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler.  In those cases, we can choose the standard, and we chose the more efficient register return alternative.

     Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

-fshort-enums
     Allocate to an enum type only as many bytes as it needs for the declared range of possible values.  Specifically, the enum type will be equivalent to the smallest integer type which has enough room.

     Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-double
     Use the same size for double as for float.

     Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshort-wchar
     Override the underlying type for wchar_t to be short unsigned int instead of the default for the target.  This option is useful for building programs to run under WINE.

     Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

-fshared-data
     Requests that the data and non-const variables of this compilation be shared data rather than private data.  The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.

-fno-common
     In C, allocate even uninitialized global variables in the data section of the object file, rather than generating them as common blocks.  This has the effect that if the same variable is declared (without extern) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.

-fno-ident
     Ignore the #ident directive.

-finhibit-size-directive
     Don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory.  This option is used when compiling crtstuff.c; you should not need to use it for anything else.

-fverbose-asm
     Put extra commentary information in the generated assembly code to make it more readable.  This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).

     -fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

-fpic
     Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine.  Such code accesses all constant addresses through a global offset table (GOT).  The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system).  If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead.  (These maximums are 8k on the SPARC and 32k on the m68k and RS/6000.  The 386 has no such limit.)

     Position-independent code requires special support, and therefore works only on certain machines.  For the 386, GCC supports PIC for System V but not for the Sun 386i.  Code generated for the IBM RS/6000 is always position-independent.

-fPIC
     If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table.  This option makes a difference on the m68k, PowerPC and SPARC.

     Position-independent code requires special support, and therefore works only on certain machines.

-fpie
-fPIE
     These options are similar to -fpic and -fPIC, but generated position independent code can be only linked into executables. Usually these options are used when -pie GCC option will be used during linking.

-ffixed-REG
     Treat the register named REG as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).

     REG must be the name of a register.  The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.

     This flag does not have a negative form, because it specifies a three-way choice.

-fcall-used-REG
     Treat the register named REG as an allocable register that is clobbered by function calls.  It may be allocated for temporaries or variables that do not live across a call.  Functions compiled this way will not save and restore the register REG.

     It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

     This flag does not have a negative form, because it specifies a three-way choice.

-fcall-saved-REG
     Treat the register named REG as an allocable register saved by functions.  It may be allocated even for temporaries or variables that live across a call.  Functions compiled this way will save and restore the register REG if they use it.

     It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.

     A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

     This flag does not have a negative form, because it specifies a three-way choice.

-fpack-struct[=N]
     Without a value specified, pack all structure members together without holes.  When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this will be output potentially unaligned at the next fitting location.

     Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

-finstrument-functions
     Generate instrumentation calls for entry and exit to functions.  Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site.  (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

          void __cyg_profile_func_enter (void *this_fn,
                                         void *call_site);
          void __cyg_profile_func_exit  (void *this_fn,
                                         void *call_site);

     The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

     This instrumentation is also done for functions expanded inline in other functions.  The profiling calls will indicate where, conceptually, the inline function is entered and exited.  This means that addressable versions of such functions must be available.  If all your uses of a function are expanded inline, this may mean an additional expansion of code size.  If you use extern inline in your C code, an addressable version of such functions must be provided.  (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

     A function may be given the attribute no_instrument_function, in which case this instrumentation will not be done.  This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory).

-fstack-check
     Generate code to verify that you do not go beyond the boundary of the stack.  You should specify this flag if you are running in an environment with multiple threads, but only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

     Note that this switch does not actually cause checking to be done; the operating system must do that.  The switch causes generation of code to ensure that the operating system sees the stack being extended.

-fstack-limit-register=REG
-fstack-limit-symbol=SYM
-fno-stack-limit
     Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol.  If the stack would grow beyond the value, a signal is raised.  For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

     For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB.  Note that this may only work with the GNU linker.

-fargument-alias
-fargument-noalias
-fargument-noalias-global
     Specify the possible relationships among parameters and between parameters and global data.

     -fargument-alias specifies that arguments (parameters) may alias each other and may alias global storage.

     -fargument-noalias specifies that arguments do not alias each other, but may alias global storage.

     -fargument-noalias-global specifies that arguments do not alias each other and do not alias global storage.

     Each language will automatically use whatever option is required by the language standard.  You should not need to use these options yourself.

-fleading-underscore
     This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file.  One use is to help link with legacy assembly code.

     Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch.  Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

-ftls-model=MODEL
     Alter the thread-local storage model to be used (see Thread-Local). The MODEL argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec.

     The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

-fvisibility=DEFAULT|INTERNAL|HIDDEN|PROTECTED
     Set the default ELF image symbol visibility to the specified option---all symbols will be marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.

     Despite the nomenclature, default always means public ie; available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option will be hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public---this causes the same behavior as previous versions of GCC.

     A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by ``How To Write Shared Libraries'' by Ulrich Drepper (which can be found at http://people.redhat.com/~drepper/)---however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public.  This is the norm with DLL's on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax.  This is a great boon to those working with cross-platform projects.

     For those adding visibility support to existing code, you may find #pragma GCC visibility of use.  This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop.  These can be nested up to sixteen times.  Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default ie; declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility.

     An overview of these techniques, their benefits and how to use them is at http://gcc.gnu.org/wiki/Visibility.

Environment Variables Affecting GCC
===================================

This section describes several environment variables that affect how GCC operates.  Some of them work by specifying directories or prefixes to use when searching for various kinds of files.  Some are used to specify other aspects of the compilation environment.

Note that you can also specify places to search using options such as -B, -I and -L (see Directory Options).  These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See Controlling the Compilation Driver gcc (GNU Compiler Collection (GCC) Internals).

LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
     These environment variables control the way that GCC uses localization information that allow GCC to work with different national conventions.  GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so.  These locale categories can be set to any value supported by your installation.  A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.

     The LC_CTYPE environment variable specifies character classification.  GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.

     The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.

     If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable.  If none of these variables are set, GCC defaults to traditional C English behavior.

TMPDIR
     If TMPDIR is set, it specifies the directory to use for temporary files.  GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.

GCC_EXEC_PREFIX
     If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler.  No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

     If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with.

     If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

     The default value of GCC_EXEC_PREFIX is PREFIX/lib/gcc/ where PREFIX is the value of prefix when you ran the configure script.

     Other prefixes specified with -B take precedence over this prefix.

     This prefix is also used for finding files such as crt0.o that are used for linking.

     In addition, the prefix is used in an unusual way in finding the directories to search for header files.  For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name.  Thus, with -Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These alternate directories are searched first; the standard directories come next.

COMPILER_PATH
     The value of COMPILER_PATH is a colon-separated list of directories, much like PATH.  GCC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH
     The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH.  When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX.  Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first).

LANG
     This variable is used to pass locale information to the compiler.  One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

     C-JIS
          Recognize JIS characters.

     C-SJIS
          Recognize SJIS characters.

     C-EUCJP
          Recognize EUCJP characters.

     If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.

Some additional environments variables affect the behavior of the preprocessor.

     [file cppenv.texi not included]

Using Precompiled Headers
=========================

Often large projects have many header files that are included in every source file.  The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project.  To make builds faster, GCC allows users to `precompile' a header file; then, if builds can use the precompiled header file they will be much faster.

Caution: There are a few known situations where GCC will crash when trying to use a precompiled header.  If you have trouble with a precompiled header, you should remove the precompiled header and compile without it.  In addition, please use GCC's on-line defect-tracking system to report any problems you encounter with precompiled headers.  See Bugs.

To create a precompiled header file, simply compile it as you would any other file, if necessary using the -x option to make the driver treat it as a C or C++ header file.  You will probably want to use a tool like make to keep the precompiled header up-to-date when the headers it contains change.

A precompiled header file will be searched for when #include is seen in the compilation.  As it searches for the included file (see Search Path (The C Preprocessor)) the compiler looks for a precompiled header in each directory just before it looks for the include file in that directory.  The name searched for is the name specified in the #include with .gch appended.  If the precompiled header file can't be used, it is ignored.

For instance, if you have #include "all.h", and you have all.h.gch in the same directory as all.h, then the precompiled header file will be used if possible, and the original header will be used otherwise.

Alternatively, you might decide to put the precompiled header file in a directory and use -I to ensure that directory is searched before (or instead of) the directory containing the original header. Then, if you want to check that the precompiled header file is always used, you can put a file of the same name as the original header in this directory containing an #error command.

This also works with -include.  So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and -include the precompiled header.  If the header files have guards against multiple inclusion, they will be skipped because they've already been included (in the precompiled header).

If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like all.h.gch, and put each precompiled header in the directory, perhaps using -o.  It doesn't matter what you call the files in the directory, every precompiled header in the directory will be considered.  The first precompiled header encountered in the directory that is valid for this compilation will be used; they're searched in no particular order.

There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.

A precompiled header file can be used only when these conditions apply:

  * Only one precompiled header can be used in a particular compilation.

  * A precompiled header can't be used once the first C token is seen.  You can have preprocessor directives before a precompiled header; you can even include a precompiled header from inside another header, so long as there are no C tokens before the #include.

  * The precompiled header file must be produced for the same language as the current compilation.  You can't use a C precompiled header for a C++ compilation.

  * The precompiled header file must be produced by the same compiler version and configuration as the current compilation is using. The easiest way to guarantee this is to use the same compiler binary for creating and using precompiled headers.

  * Any macros defined before the precompiled header is included must either be defined in the same way as when the precompiled header was generated, or must not affect the precompiled header, which usually means that the they don't appear in the precompiled header at all.

     The -D option is one way to define a macro before a precompiled header is included; using a #define can also do it. There are also some options that define macros implicitly, like -O and -Wdeprecated; the same rule applies to macros defined this way.

  * If debugging information is output when using the precompiled header, using -g or similar, the same kind of debugging information must have been output when building the precompiled header.  However, a precompiled header built using -g can be used in a compilation when no debugging information is being output.

  * The same -m options must generally be used when building and using the precompiled header.  See Submodel Options, for any cases where this rule is relaxed.

  * Each of the following options must be the same when building and using the precompiled header:

     -fexceptions -funit-at-a-time

  * Some other command-line options starting with -f, -p, or -O must be defined in the same way as when the precompiled header was generated.  At present, it's not clear which options are safe to change and which are not; the safest choice is to use exactly the same options when generating and using the precompiled header.  The following are known to be safe:

     -fpreprocessed -pedantic-errors

For all of these except the last, the compiler will automatically ignore the precompiled header if the conditions aren't met.  If you find an option combination that doesn't work and doesn't cause the precompiled header to be ignored, please consider filing a bug report, see Bugs.

If you do use differing options when generating and using the precompiled header, the actual behavior will be a mixture of the behavior for the options.  For instance, if you use -g to generate the precompiled header but not when using it, you may or may not get debugging information for routines in the precompiled header.

Running Protoize
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The program protoize is an optional part of GCC.  You can use it to add prototypes to a program, thus converting the program to ISO C in one respect.  The companion program unprotoize does the reverse: it removes argument types from any prototypes that are found.

When you run these programs, you must specify a set of source files as command line arguments.  The conversion programs start out by compiling these files to see what functions they define.  The information gathered about a file FOO is saved in a file named FOO.X.

After scanning comes actual conversion.  The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.

But not all the eligible files are converted.  By default, protoize and unprotoize convert only source and header files in the current directory.  You can specify additional directories whose files should be converted with the -d DIRECTORY option.  You can also specify particular files to exclude with the -x FILE option.  A file is converted if it is eligible, its directory name matches one of the specified directory names, and its name within the directory has not been excluded.

Basic conversion with protoize consists of rewriting most function definitions and function declarations to specify the types of the arguments.  The only ones not rewritten are those for varargs functions.

protoize optionally inserts prototype declarations at the beginning of the source file, to make them available for any calls that precede the function's definition.  Or it can insert prototype declarations with block scope in the blocks where undeclared functions are called.

Basic conversion with unprotoize consists of rewriting most function declarations to remove any argument types, and rewriting function definitions to the old-style pre-ISO form.

Both conversion programs print a warning for any function declaration or definition that they can't convert.  You can suppress these warnings with -q.

The output from protoize or unprotoize replaces the original source file.  The original file is renamed to a name ending with .save (for DOS, the saved filename ends in .sav without the original .c suffix).  If the .save (.sav for DOS) file already exists, then the source file is simply discarded.

protoize and unprotoize both depend on GCC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GCC is installed.

Here is a table of the options you can use with protoize and unprotoize.  Each option works with both programs unless otherwise stated.

-B DIRECTORY
     Look for the file SYSCALLS.c.X in DIRECTORY, instead of the usual directory (normally /usr/local/lib).  This file contains prototype information about standard system functions.  This option applies only to protoize.

-c COMPILATION-OPTIONS
     Use COMPILATION-OPTIONS as the options when running gcc to produce the .X files.  The special option -aux-info is always passed in addition, to tell gcc to write a .X file.

     Note that the compilation options must be given as a single argument to protoize or unprotoize.  If you want to specify several gcc options, you must quote the entire set of compilation options to make them a single word in the shell.

     There are certain gcc arguments that you cannot use, because they would produce the wrong kind of output.  These include -g, -O, -c, -S, and -o If you include these in the COMPILATION-OPTIONS, they are ignored.

-C
     Rename files to end in .C (.cc for DOS-based file systems) instead of .c.  This is convenient if you are converting a C program to C++.  This option applies only to protoize.

-g
     Add explicit global declarations.  This means inserting explicit declarations at the beginning of each source file for each function that is called in the file and was not declared.  These declarations precede the first function definition that contains a call to an undeclared function.  This option applies only to protoize.

-i STRING
     Indent old-style parameter declarations with the string STRING. This option applies only to protoize.

     unprotoize converts prototyped function definitions to old-style function definitions, where the arguments are declared between the argument list and the initial {.  By default, unprotoize uses five spaces as the indentation.  If you want to indent with just one space instead, use -i " ".

-k
     Keep the .X files.  Normally, they are deleted after conversion is finished.

-l
     Add explicit local declarations.  protoize with -l inserts a prototype declaration for each function in each block which calls the function without any declaration.  This option applies only to protoize.

-n
     Make no real changes.  This mode just prints information about the conversions that would have been done without -n.

-N
     Make no .save files.  The original files are simply deleted. Use this option with caution.

-p PROGRAM
     Use the program PROGRAM as the compiler.  Normally, the name gcc is used.

-q
     Work quietly.  Most warnings are suppressed.

-v
     Print the version number, just like -v for gcc.

If you need special compiler options to compile one of your program's source files, then you should generate that file's .X file specially, by running gcc on that source file with the appropriate options and the option -aux-info.  Then run protoize on the entire set of files.  protoize will use the existing .X file because it is newer than the source file. For example:

     gcc -Dfoo=bar file1.c -aux-info file1.X
     protoize *.c

You need to include the special files along with the rest in the protoize command, even though their .X files already exist, because otherwise they won't get converted.

See Protoize Caveats, for more information on how to use protoize successfully.