On Tue, Feb 3, 2015 at 1:35 PM, Sriraman Tallam <tmsri...@google.com> wrote:
> On Tue, Feb 3, 2015 at 1:29 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>> On Tue, Feb 3, 2015 at 1:20 PM, Sriraman Tallam <tmsri...@google.com> wrote:
>>> On Tue, Feb 3, 2015 at 11:36 AM, Jakub Jelinek <ja...@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
So we aren't SYMBOL_REF_EXTERNAL_P nor
SYMBOL_REF_LOCAL_P. What do we reference?
--
H.J.
