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, 43 insertions, 0 deletions

diff --git a/gcc/configure.ac b/gcc/configure.ac
index 48c8000b249..a33f3a57bb4 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -4693,6 +4693,49 @@ if test x"$gcc_cv_ld_pie" = xyes; then
 fi
 AC_MSG_RESULT($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