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.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@218397 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/config.in b/gcc/config.in
index 65d5e421a90..f34adb57e6d 100644
--- a/gcc/config.in
+++ b/gcc/config.in
@@ -1411,6 +1411,12 @@
 #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