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Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=


On Thu, May 28, 2015 at 2:27 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Thu, May 28, 2015 at 2:01 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Thu, May 28, 2015 at 1:54 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>> On Thu, May 28, 2015 at 12:05 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Thu, May 28, 2015 at 11:50 AM, Sriraman Tallam <tmsriram@google.com> wrote:
>>>>> On Thu, May 28, 2015 at 11:42 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> On Thu, May 28, 2015 at 11:34 AM, Sriraman Tallam <tmsriram@google.com> wrote:
>>>>>>> I have attached a patch that adds the new attribute "noplt".  Please review.
>>>>>>>
>>>>>>> * config/i386/i386.c (avoid_plt_to_call): New function.
>>>>>>> (ix86_output_call_insn): Generate indirect call for functions
>>>>>>> marked with "noplt" attribute.
>>>>>>> (attribute_spec ix86_attribute_): Define new attribute "noplt".
>>>>>>> * doc/extend.texi: Document new attribute "noplt".
>>>>>>> * gcc.target/i386/noplt-1.c: New testcase.
>>>>>>> * gcc.target/i386/noplt-2.c: New testcase.
>>>>>>>
>>>>>>
>>>>>> 2 comments:
>>>>>>
>>>>>> 1. Don't remove "%!" prefix before call/jmp.  It is needed for MPX.
>>>>>> 2. Don't you need to check
>>>>>>
>>>>>>       && !TARGET_MACHO
>>>>>>       && !TARGET_SEH
>>>>>>       && !TARGET_PECOFF
>>>>>>
>>>>>> since it only works for ELF.
>>>>>
>>>>> Ok, I will make this change. OTOH, is it just better to piggy-back on
>>>>> existing -fno-plt change by Alex in calls.c
>>>>> and do this:
>>>>>
>>>>> Index: calls.c
>>>>> ===================================================================
>>>>> --- calls.c (revision 223720)
>>>>> +++ calls.c (working copy)
>>>>> @@ -226,9 +226,11 @@ prepare_call_address (tree fndecl_or_type, rtx fun
>>>>>         && targetm.small_register_classes_for_mode_p (FUNCTION_MODE))
>>>>>        ? force_not_mem (memory_address (FUNCTION_MODE, funexp))
>>>>>        : memory_address (FUNCTION_MODE, funexp));
>>>>> -  else if (flag_pic && !flag_plt && fndecl_or_type
>>>>> +  else if (fndecl_or_type
>>>>>     && TREE_CODE (fndecl_or_type) == FUNCTION_DECL
>>>>> -   && !targetm.binds_local_p (fndecl_or_type))
>>>>> +   && !targetm.binds_local_p (fndecl_or_type)
>>>>> +   && ((flag_pic && !flag_plt)
>>>>> +       || (lookup_attribute ("noplt", DECL_ATTRIBUTES(fndecl_or_type)))))
>>>>>      {
>>>>>        funexp = force_reg (Pmode, funexp);
>>>>>      }
>>>>>
>>>>
>>>> Does it work on non-PIC calls?
>>>
>>> You are right, it doesnt work.  I have attached the patch with the
>>> changes you mentioned.
>>>
>>
>> Since direct_p is true, do wee need
>>
>> +  if (GET_CODE (call_op) != SYMBOL_REF
>> +      || SYMBOL_REF_LOCAL_P (call_op))
>> +    return false;
>
> We do need it right because  for this case below, I do not want an
> indirect call:
>
> __attribute__((noplt))
> int foo() {
>   return 0;
> }
>
> int main()
> {
>   return foo();
> }
>
> Assuming foo is not inlined, if I remove the lines you mentioned, I
> will get an indirect call which is unnecessary.
>

I meant the "GET_CODE (call_op) != SYMBOL_REF" part isn't
needed.



-- 
H.J.


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