On Thu, Dec 4, 2014 at 2:19 PM, Dominique Dhumieres <domi...@lps.ens.fr> wrote:
>> 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.
>
> It caused pr64189.
>
I checked in this as an obvious fix. Sorry for the inconvenience.
--
H.J.
---
Index: ChangeLog
===================================================================
--- ChangeLog (revision 218407)
+++ ChangeLog (working copy)
@@ -1,3 +1,9 @@
+2014-12-04 H.J. Lu <hongjiu...@intel.com>
+
+ PR bootstrap/64189
+ * configure.ac (HAVE_LD_PIE_COPYRELOC): Always define.
+ * configure: Regenerated.
+
2014-12-04 Manuel López-Ibáñez <m...@gcc.gnu.org>
* diagnostic.c (diagnostic_color_init): New.
Index: configure
===================================================================
--- configure (revision 218407)
+++ configure (working copy)
@@ -27063,12 +27063,12 @@ EOF
;;
esac
fi
+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; }
Index: configure.ac
===================================================================
--- configure.ac (revision 218407)
+++ configure.ac (working copy)
@@ -4730,10 +4730,10 @@ EOF
;;
esac
fi
- AC_DEFINE_UNQUOTED(HAVE_LD_PIE_COPYRELOC,
- [`if test x"$gcc_cv_ld_pie_copyreloc" = xyes; then echo 1; else
echo 0; fi`],
- [Define 0/1 if your linker supports -pie option with copy reloc.])
fi
+AC_DEFINE_UNQUOTED(HAVE_LD_PIE_COPYRELOC,
+ [`if test x"$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.])
AC_MSG_RESULT($gcc_cv_ld_pie_copyreloc)
AC_MSG_CHECKING(linker EH-compatible garbage collection of sections)
