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

[Sriraman Tallam <tmsriram at google dot com>]

On Thu, May 28, 2015 at 2:52 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 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.

I should have realized that :), sorry.  Patch fixed.

Thanks
Sri

>
>
>
> --
> H.J.

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