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


On Thu, Dec 4, 2014 at 5:46 PM, H.J. Lu <hjl.tools@gmail.com> wrote:

>>>>>>>> It would probably help reviewers if you pointed to actual path
>>>>>>>> submission [1], which unfortunately contains the explanation in the
>>>>>>>> patch itself [2], which further explains that this functionality is
>>>>>>>> currently only supported with gold, patched with [3].
>>>>>>>>
>>>>>>>> [1] https://gcc.gnu.org/ml/gcc-patches/2014-09/msg00645.html
>>>>>>>> [2] https://gcc.gnu.org/ml/gcc-patches/2014-09/txt2CHtu81P1O.txt
>>>>>>>> [3] https://sourceware.org/ml/binutils/2014-05/msg00092.html
>>>>>>>>
>>>>>>>> After a bit of the above detective work, I think that new gcc option
>>>>>>>> is not necessary. The configure should detect if new functionality is
>>>>>>>> supported in the linker, and auto-configure gcc to use it when
>>>>>>>> appropriate.
>>>>>>>
>>>>>>> I think GCC option is needed since one can use -fuse-ld= to
>>>>>>> change linker.
>>>>>>
>>>>>> IMO, nobody will use this highly special x86_64-only option. It would
>>>>>> be best for gnu-ld to reach feature parity with gold as far as this
>>>>>> functionality is concerned. In this case, the optimization would be
>>>>>> auto-configured, and would fire automatically, without any user
>>>>>> intervention.
>>>>>>
>>>>>
>>>>> Let's do it.  I implemented the same feature in bfd linker on both
>>>>> master and 2.25 branch.
>>>>>
>>>>
>>>> +bool
>>>> +i386_binds_local_p (const_tree exp)
>>>> +{
>>>> +  /* Globals marked extern are treated as local when linker copy relocations
>>>> +     support is available with -f{pie|PIE}.  */
>>>> +  if (TARGET_64BIT && ix86_copyrelocs && flag_pie
>>>> +      && TREE_CODE (exp) == VAR_DECL
>>>> +      && DECL_EXTERNAL (exp) && !DECL_WEAK (exp))
>>>> +    return true;
>>>> +  return default_binds_local_p (exp);
>>>> +}
>>>> +
>>>>
>>>> It returns true with -fPIE and false without -fPIE.  It is lying to compiler.
>>>> Maybe legitimate_pic_address_disp_p is a better place.
>>
>> Agreed.
>>
>>> Something like this?
>>
>> Yes.
>>
>> OK, if Jakub doesn't have any objections here. Please also add
>> Sriraman as author to ChangeLog entry.
>>
>> Thanks,
>> Uros.
>
> Here is the patch.   OK to install?
>
> Thanks.
>
> --
> H.J.
> ---
> 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.

OK.

Thanks,
Uros.


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