Normally, with -fPIE/-fpie, GCC accesses globals that are extern to the
module using the GOT. This is two instructions, one to get the address
of the global from the GOT and the other to get the value. If it turns
out that the global gets defined in the executable at link-time, it still
needs to go through the GOT as it is too late then to generate a direct
access.
Examples:
foo.cc
------
int a_glob;
int main () {
return a_glob; // defined in this file
}
With -O2 -fpie -pie, the generated code directly accesses the global via
PC-relative insn:
5e0 <main>:
mov 0x165a(%rip),%eax # 1c40 <a_glob>
foo.cc
------
extern int a_glob;
int main () {
return a_glob; // defined in this file
}
With -O2 -fpie -pie, the generated code accesses global via GOT using
two memory loads:
6f0 <main>:
mov 0x1609(%rip),%rax # 1d00 <_DYNAMIC+0x230>
mov (%rax),%eax
This is true even if in the latter case the global was defined in the
executable through a different file.
Some experiments on google benchmarks shows that the extra memory loads
affects performance by 1% to 5%.
Solution - Copy Relocations:
When the linker supports copy relocations, GCC can always assume that
the global will be defined in the executable. For globals that are truly
extern (come from shared objects), the linker will create copy relocations
and have them defined in the executable. Result is that no global access
needs to go through the GOT and hence improves performance.
This optimization only applies to undefined, non-weak global data.
Undefined, weak global data access still must go through the GOT.
This patch checks if linker supports PIE with copy reloc, which is
enabled in gold and bfd linker in bininutils 2.25, at configure time
and enables this optimization if the linker support is available.
gcc/
* configure.ac (HAVE_LD_PIE_COPYRELOC): Defined to 1 if
Linux/x86-64 linker supports PIE with copy reloc.
* config.in: Regenerated.
* configure: Likewise.
* config/i386/i386.c (legitimate_pic_address_disp_p): Allow
pc-relative address for undefined, non-weak, non-function
symbol reference in 64-bit PIE if linker supports PIE with
copy reloc.
* doc/sourcebuild.texi: Document pie_copyreloc target.
gcc/testsuite/
* gcc.target/i386/pie-copyrelocs-1.c: New test.
* gcc.target/i386/pie-copyrelocs-2.c: Likewise.
* gcc.target/i386/pie-copyrelocs-3.c: Likewise.
* gcc.target/i386/pie-copyrelocs-4.c: Likewise.
* lib/target-supports.exp (check_effective_target_pie_copyreloc):
New procedure.
Co-Authored-By: H.J. Lu <hongjiu.lu@intel.com>
From-SVN: r218397
+2014-12-04 Sriraman Tallam <tmsriram@google.com>
+ H.J. Lu <hongjiu.lu@intel.com>
+
+ * configure.ac (HAVE_LD_PIE_COPYRELOC): Defined to 1 if
+ Linux/x86-64 linker supports PIE with copy reloc.
+ * config.in: Regenerated.
+ * configure: Likewise.
+
+ * config/i386/i386.c (legitimate_pic_address_disp_p): Allow
+ pc-relative address for undefined, non-weak, non-function
+ symbol reference in 64-bit PIE if linker supports PIE with
+ copy reloc.
+
+ * doc/sourcebuild.texi: Document pie_copyreloc target.
+
2014-12-04 Marek Polacek <polacek@redhat.com>
PR middle-end/56917
#endif
+/* Define 0/1 if your linker supports -pie option with copy reloc. */
+#ifndef USED_FOR_TARGET
+#undef HAVE_LD_PIE_COPYRELOC
+#endif
+
+
/* Define if your linker links a mix of read-only and read-write sections into
a read-write section. */
#ifndef USED_FOR_TARGET
return true;
}
else if (!SYMBOL_REF_FAR_ADDR_P (op0)
- && SYMBOL_REF_LOCAL_P (op0)
+ && (SYMBOL_REF_LOCAL_P (op0)
+ || (HAVE_LD_PIE_COPYRELOC
+ && flag_pie
+ && !SYMBOL_REF_WEAK (op0)
+ && !SYMBOL_REF_FUNCTION_P (op0)))
&& ix86_cmodel != CM_LARGE_PIC)
return true;
break;
{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gcc_cv_ld_pie" >&5
$as_echo "$gcc_cv_ld_pie" >&6; }
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking linker PIE support with copy reloc" >&5
+$as_echo_n "checking linker PIE support with copy reloc... " >&6; }
+gcc_cv_ld_pie_copyreloc=no
+if test $gcc_cv_ld_pie = yes ; then
+ if test $in_tree_ld = yes ; then
+ if test "$gcc_cv_gld_major_version" -eq 2 -a "$gcc_cv_gld_minor_version" -ge 25 -o "$gcc_cv_gld_major_version" -gt 2; then
+ gcc_cv_ld_pie_copyreloc=yes
+ fi
+ elif test x$gcc_cv_as != x -a x$gcc_cv_ld != x ; then
+ # Check if linker supports -pie option with copy reloc
+ case "$target" in
+ i?86-*-linux* | x86_64-*-linux*)
+ cat > conftest1.s <<EOF
+ .globl a_glob
+ .data
+ .type a_glob, @object
+ .size a_glob, 4
+a_glob:
+ .long 2
+EOF
+ cat > conftest2.s <<EOF
+ .text
+ .globl main
+ .type main, @function
+main:
+ movl %eax, a_glob(%rip)
+ .size main, .-main
+EOF
+ if $gcc_cv_as --64 -o conftest1.o conftest1.s > /dev/null 2>&1 \
+ && $gcc_cv_ld -shared -melf_x86_64 -o conftest1.so conftest1.o > /dev/null 2>&1 \
+ && $gcc_cv_as --64 -o conftest2.o conftest2.s > /dev/null 2>&1 \
+ && $gcc_cv_ld -pie -melf_x86_64 -o conftest conftest2.o conftest1.so > /dev/null 2>&1; then
+ gcc_cv_ld_pie_copyreloc=yes
+ fi
+ rm -f conftest conftest1.so conftest1.o conftest2.o conftest1.s conftest2.s
+ ;;
+ esac
+ fi
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_LD_PIE_COPYRELOC `if test x"$gcc_cv_ld_pie_copyreloc" = xyes; then echo 1; else echo 0; fi`
+_ACEOF
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gcc_cv_ld_pie_copyreloc" >&5
+$as_echo "$gcc_cv_ld_pie_copyreloc" >&6; }
+
{ $as_echo "$as_me:${as_lineno-$LINENO}: checking linker EH-compatible garbage collection of sections" >&5
$as_echo_n "checking linker EH-compatible garbage collection of sections... " >&6; }
gcc_cv_ld_eh_gc_sections=no
fi
AC_MSG_RESULT($gcc_cv_ld_pie)
+AC_MSG_CHECKING(linker PIE support with copy reloc)
+gcc_cv_ld_pie_copyreloc=no
+if test $gcc_cv_ld_pie = yes ; then
+ if test $in_tree_ld = yes ; then
+ if test "$gcc_cv_gld_major_version" -eq 2 -a "$gcc_cv_gld_minor_version" -ge 25 -o "$gcc_cv_gld_major_version" -gt 2; then
+ gcc_cv_ld_pie_copyreloc=yes
+ fi
+ elif test x$gcc_cv_as != x -a x$gcc_cv_ld != x ; then
+ # Check if linker supports -pie option with copy reloc
+ case "$target" in
+ i?86-*-linux* | x86_64-*-linux*)
+ cat > conftest1.s <<EOF
+ .globl a_glob
+ .data
+ .type a_glob, @object
+ .size a_glob, 4
+a_glob:
+ .long 2
+EOF
+ cat > conftest2.s <<EOF
+ .text
+ .globl main
+ .type main, @function
+main:
+ movl %eax, a_glob(%rip)
+ .size main, .-main
+EOF
+ if $gcc_cv_as --64 -o conftest1.o conftest1.s > /dev/null 2>&1 \
+ && $gcc_cv_ld -shared -melf_x86_64 -o conftest1.so conftest1.o > /dev/null 2>&1 \
+ && $gcc_cv_as --64 -o conftest2.o conftest2.s > /dev/null 2>&1 \
+ && $gcc_cv_ld -pie -melf_x86_64 -o conftest conftest2.o conftest1.so > /dev/null 2>&1; then
+ gcc_cv_ld_pie_copyreloc=yes
+ fi
+ rm -f conftest conftest1.so conftest1.o conftest2.o conftest1.s conftest2.s
+ ;;
+ esac
+ fi
+ AC_DEFINE_UNQUOTED(HAVE_LD_PIE_COPYRELOC,
+ [`if test x"$gcc_cv_ld_pie_copyreloc" = xyes; then echo 1; else echo 0; fi`],
+ [Define 0/1 if your linker supports -pie option with copy reloc.])
+fi
+AC_MSG_RESULT($gcc_cv_ld_pie_copyreloc)
+
AC_MSG_CHECKING(linker EH-compatible garbage collection of sections)
gcc_cv_ld_eh_gc_sections=no
if test $in_tree_ld = yes ; then
@item vect_cmdline_needed
Target requires a command line argument to enable a SIMD instruction set.
+
+@item pie_copyreloc
+The x86-64 target linker supports PIE with copy reloc.
@end table
@subsubsection Environment attributes
+2014-12-04 Sriraman Tallam <tmsriram@google.com>
+ H.J. Lu <hongjiu.lu@intel.com>
+
+ * gcc.target/i386/pie-copyrelocs-1.c: New test.
+ * gcc.target/i386/pie-copyrelocs-2.c: Likewise.
+ * gcc.target/i386/pie-copyrelocs-3.c: Likewise.
+ * gcc.target/i386/pie-copyrelocs-4.c: Likewise.
+
+ * lib/target-supports.exp (check_effective_target_pie_copyreloc):
+ New procedure.
+
2014-12-04 Marek Polacek <polacek@redhat.com>
PR middle-end/56917
--- /dev/null
+/* Check that GOTPCREL isn't used to access glob_a. */
+/* { dg-do compile { target *-*-linux* } } */
+/* { dg-require-effective-target pie_copyreloc } */
+/* { dg-options "-O2 -fpie" } */
+
+extern int glob_a;
+
+int foo ()
+{
+ return glob_a;
+}
+
+/* glob_a should never be accessed with a GOTPCREL. */
+/* { dg-final { scan-assembler-not "glob_a@GOTPCREL" { target { ! ia32 } } } } */
--- /dev/null
+/* Check that GOTPCREL isn't used to access glob_a. */
+/* { dg-do compile { target *-*-linux* } } */
+/* { dg-require-effective-target pie_copyreloc } */
+/* { dg-options "-O2 -fpie" } */
+
+int glob_a;
+
+int foo ()
+{
+ return glob_a;
+}
+
+/* glob_a should never be accessed with a GOTPCREL. */
+/* { dg-final { scan-assembler-not "glob_a@GOTPCREL" { target { ! ia32 } } } } */
--- /dev/null
+/* Check that PLT is used to access glob_a. */
+/* { dg-do compile { target *-*-linux* } } */
+/* { dg-require-effective-target pie_copyreloc } */
+/* { dg-options "-O2 -fpie" } */
+
+extern int glob_a (void);
+
+int foo ()
+{
+ return glob_a ();
+}
+
+/* glob_a should be accessed with a PLT. */
+/* { dg-final { scan-assembler "glob_a@PLT" { target { ! ia32 } } } } */
--- /dev/null
+/* Check that GOTPCREL is used to access glob_a. */
+/* { dg-do compile { target *-*-linux* } } */
+/* { dg-require-effective-target pie_copyreloc } */
+/* { dg-options "-O2 -fpie" } */
+
+extern int glob_a __attribute__((weak));
+
+int foo ()
+{
+ if (&glob_a != 0)
+ return glob_a;
+ else
+ return 0;
+}
+
+/* weak glob_a should be accessed with a GOTPCREL. */
+/* { dg-final { scan-assembler "glob_a@GOTPCREL" { target { ! ia32 } } } } */
}
}
+# Return 1 if the x86-64 target supports PIE with copy reloc, 0
+# otherwise. Cache the result.
+
+proc check_effective_target_pie_copyreloc { } {
+ global pie_copyreloc_available_saved
+ global tool
+ global GCC_UNDER_TEST
+
+ if { !([istarget x86_64-*-*] || [istarget i?86-*-*]) } {
+ return 0
+ }
+
+ # Need auto-host.h to check linker support.
+ if { ![file exists ../../auto-host.h ] } {
+ return 0
+ }
+
+ if [info exists pie_copyreloc_available_saved] {
+ verbose "check_effective_target_pie_copyreloc returning saved $pie_copyreloc_available_saved" 2
+ } else {
+ # Set up and compile to see if linker supports PIE with copy
+ # reloc. Include the current process ID in the file names to
+ # prevent conflicts with invocations for multiple testsuites.
+
+ set src pie[pid].c
+ set obj pie[pid].o
+
+ set f [open $src "w"]
+ puts $f "#include \"../../auto-host.h\""
+ puts $f "#if HAVE_LD_PIE_COPYRELOC == 0"
+ puts $f "# error Linker does not support PIE with copy reloc."
+ puts $f "#endif"
+ close $f
+
+ verbose "check_effective_target_pie_copyreloc compiling testfile $src" 2
+ set lines [${tool}_target_compile $src $obj object ""]
+
+ file delete $src
+ file delete $obj
+
+ if [string match "" $lines] then {
+ verbose "check_effective_target_pie_copyreloc testfile compilation passed" 2
+ set pie_copyreloc_available_saved 1
+ } else {
+ verbose "check_effective_target_pie_copyreloc testfile compilation failed" 2
+ set pie_copyreloc_available_saved 0
+ }
+ }
+
+ return $pie_copyreloc_available_saved
+}
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