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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Feb 27, 2015 at 3:23 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Dec 4, 2014 at 8:46 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Thu, Dec 4, 2014 at 4:44 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
>>> On Wed, Dec 3, 2014 at 10:35 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.
>
> This caused:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65248
>
> Should we turn it off by default?
>

Or we can provide a command line option to turn it off.


-- 
H.J.