Ping.
On Mon, May 19, 2014 at 11:11 AM, Sriraman Tallam <tmsri...@google.com> wrote:
> Ping.
>
> On Thu, May 15, 2014 at 11:34 AM, Sriraman Tallam <tmsri...@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
