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


On Tue, Feb 3, 2015 at 1:29 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Feb 3, 2015 at 1:20 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>> On Tue, Feb 3, 2015 at 11:36 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>>> On Tue, Feb 03, 2015 at 11:25:38AM -0800, Sriraman Tallam wrote:
>>>> This was the original patch to i386.c to let global accesses take
>>>> advantage of copy relocations and avoid the GOT.
>>>>
>>>>
>>>> @@ -13113,7 +13113,11 @@ legitimate_pic_address_disp_p (rtx disp)
>>>>   return true;
>>>>      }
>>>>    else if (!SYMBOL_REF_FAR_ADDR_P (op0)
>>>> -   && SYMBOL_REF_LOCAL_P (op0)
>>>> +   && (SYMBOL_REF_LOCAL_P (op0)
>>>> +       || (HAVE_LD_PIE_COPYRELOC
>>>> +   && flag_pie
>>>> +   && !SYMBOL_REF_WEAK (op0)
>>>> +   && !SYMBOL_REF_FUNCTION_P (op0)))
>>>>     && ix86_cmodel != CM_LARGE_PIC)
>>>>
>>>> I do not understand here why weak global data access must go through
>>>> the GOT and not use copy relocations. Ultimately, there is only going
>>>> to be one copy of the global either defined in the executable or the
>>>> shared object right?
>>>>
>>>> Can we remove the check for SYMBOL_REF_WEAK?
>>>
>>> So, what will then happen if the weak undef symbol isn't defined anywhere?
>>> In non-PIE binaries that is fine, the linker will store 0.
>>> But in PIE binaries, the 0 would be biased by the PIE load bias and thus
>>> wouldn't be NULL.
>>
>> Thanks for clarifying.
>>
>>> You can only optimize weak vars if there is some weak definition in the
>>> current TU.
>>
>> Would this be fine then?  Replace !SYMBOL_REF_WEAK (op0) with
>>
>> !(SYMBOL_REF_WEAK (op0) && SYMBOL_REF_EXTERNAL_P (op0))
>>
>
> The full condition is:
>
>                   && (SYMBOL_REF_LOCAL_P (op0)
>                        || (HAVE_LD_PIE_COPYRELOC
>                            && flag_pie
>                            && !SYMBOL_REF_WEAK (op0)
>                            && !SYMBOL_REF_FUNCTION_P (op0)))
>
> If the weak op0 is defined in the current TU, shouldn't
> SYMBOL_REF_LOCAL_P (op0)  be true for PIE?

Thats not what I see for this:

zap.cc
---------
__attribute__((weak))
int glob;

int main()
{
   printf("%d\n", glob);
}

(gdb) p debug_rtx(op0)
(symbol_ref/i:DI ("glob") <var_decl 0x7ffff74f51c8 glob>)

(gdb) p SYMBOL_REF_LOCAL_P(op0)
$4 = false

(gdb) p SYMBOL_REF_WEAK (op0)
$5 = 1

(gdb) p SYMBOL_REF_EXTERNAL_P (op0)
$6 = false

Thanks
Sri




>
> --
> H.J.


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