Re: [PATCH x86_64] Optimize access to globals in "-fpie -pie" builds with copy relocations

[Sriraman Tallam <tmsriram at google dot com>]

Ping.

On Thu, May 15, 2014 at 11:34 AM, Sriraman Tallam <tmsriram@google.com> wrote:
> Optimize access to globals with -fpie, x86_64 only:
>
> Currently, 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 patch to the gold linker :
> https://sourceware.org/ml/binutils/2014-05/msg00092.html
> submitted recently allows gold to generate copy relocations for -pie mode when
> necessary.
>
> I have added option -mld-pie-copyrelocs which when combined with -fpie would do
> this.  Note that the BFD linker does not support pie copyrelocs yet and this
> option cannot be used there.
>
> Please review.
>
>
> ChangeLog:
>
> * config/i386/i36.opt (mld-pie-copyrelocs): New option.
> * config/i386/i386.c (legitimate_pic_address_disp_p): Check if this
>  address is still legitimate in the presence of copy relocations
>  and -fpie.
> * testsuite/gcc.target/i386/ld-pie-copyrelocs-1.c: New test.
> * testsuite/gcc.target/i386/ld-pie-copyrelocs-2.c: New test.
>
>
>
> Patch attached.
> Thanks
> Sri