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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 2:01 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.

Thanks
Sri

>
> H.J.


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