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

H.J. Lu hjl.tools@gmail.com
Wed Dec 3 21:35:00 GMT 2014


On Wed, Dec 3, 2014 at 7:01 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Wed, Dec 3, 2014 at 5:47 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Tue, Dec 2, 2014 at 12:01 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
>>> On Tue, Dec 2, 2014 at 8:40 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Tue, Dec 2, 2014 at 11:19 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
>>>>> Hello!
>>>>>
>>>>>> Ping.
>>>>>>> Ping.
>>>>>>>> Ping.
>>>>>>>>> Ping.
>>>>>
>>>>> 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.
>

Something like this?

-- 
H.J.
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