This is the mail archive of the
gcc-cvs@gcc.gnu.org
mailing list for the GCC project.
r218397 - in /trunk/gcc: ChangeLog config.in co...
- From: hjl at gcc dot gnu dot org
- To: gcc-cvs at gcc dot gnu dot org
- Date: Thu, 04 Dec 2014 19:40:51 -0000
- Subject: r218397 - in /trunk/gcc: ChangeLog config.in co...
Author: hjl
Date: Thu Dec 4 19:40:50 2014
New Revision: 218397
URL: https://gcc.gnu.org/viewcvs?rev=218397&root=gcc&view=rev
Log:
x86-64: Optimize access to globals in PIE with copy reloc
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.
Added:
trunk/gcc/testsuite/gcc.target/i386/pie-copyrelocs-1.c
trunk/gcc/testsuite/gcc.target/i386/pie-copyrelocs-2.c
trunk/gcc/testsuite/gcc.target/i386/pie-copyrelocs-3.c
trunk/gcc/testsuite/gcc.target/i386/pie-copyrelocs-4.c
Modified:
trunk/gcc/ChangeLog
trunk/gcc/config.in
trunk/gcc/config/i386/i386.c
trunk/gcc/configure
trunk/gcc/configure.ac
trunk/gcc/doc/sourcebuild.texi
trunk/gcc/testsuite/ChangeLog
trunk/gcc/testsuite/lib/target-supports.exp