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Re: [PATCH x86_64] Optimize access to globals in "-fpie -pie" builds with copy relocations
- From: Uros Bizjak <ubizjak at gmail dot com>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, Sriraman Tallam <tmsriram at google dot com>, Jakub Jelinek <jakub at redhat dot com>
- Date: Thu, 4 Dec 2014 13:44:05 +0100
- Subject: Re: [PATCH x86_64] Optimize access to globals in "-fpie -pie" builds with copy relocations
- Authentication-results: sourceware.org; auth=none
- References: <CAFULd4aMopCHZHTWa3V=6-CwVLq1grorwRqT7yCaOfpLWYZibw at mail dot gmail dot com> <CAMe9rOok=sUB=OVdFNt369gkpNWxVgzQKjeDVQT96SrSNRS=Vw at mail dot gmail dot com> <CAFULd4Z=WaenYMr=Ke1QLQAq4bVQpkOD_Wt3nW4w9F71fuaPAw at mail dot gmail dot com> <CAMe9rOpXge0X3Dz0OvnkWw1Q9HRPmNAFhYphVuquZj856J_RXQ at mail dot gmail dot com> <CAMe9rOr6BsvSgOtM2K+JfTgLktnTa19YFrfRUdRQpRPjDjOdmg at mail dot gmail dot com> <CAMe9rOpXupztWPgTWAxGx7yP7aynAjYoJbh-5L+wJ3S=qk8CvQ at mail dot gmail dot com>
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.