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

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, May 28, 2015 at 4:54 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> 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.
>

--- testsuite/gcc.target/i386/noplt-1.c (revision 0)
+++ testsuite/gcc.target/i386/noplt-1.c (working copy)
@@ -0,0 +1,13 @@
+/* { dg-do compile { target x86_64-*-* } } */
...
+/* { dg-final { scan-assembler "call\[
\t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } } */

The test will fail on Windows and Darwin.


-- 
H.J.