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

[Sriraman Tallam <tmsriram@google.com>]

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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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	* 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.

Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c	(revision 223720)
+++ config/i386/i386.c	(working copy)
@@ -25599,6 +25599,24 @@ ix86_expand_call (rtx retval, rtx fnaddr, rtx call
   return call;
 }
 
+/* Return true if the function being called was marked with attribute
+   "noplt".  If this function is defined, this should return false.  */
+static bool
+avoid_plt_to_call (rtx call_op)
+{
+  if (SYMBOL_REF_LOCAL_P (call_op))
+    return false;
+
+  tree symbol_decl = SYMBOL_REF_DECL (call_op);
+
+  if (symbol_decl != NULL_TREE
+      && TREE_CODE (symbol_decl) == FUNCTION_DECL
+      && lookup_attribute ("noplt", DECL_ATTRIBUTES (symbol_decl)))
+    return true;
+
+  return false;
+}
+
 /* Output the assembly for a call instruction.  */
 
 const char *
@@ -25611,7 +25629,13 @@ ix86_output_call_insn (rtx_insn *insn, rtx call_op
   if (SIBLING_CALL_P (insn))
     {
       if (direct_p)
-	xasm = "%!jmp\t%P0";
+	{
+	  if (!TARGET_MACHO && !TARGET_SEH && !TARGET_PECOFF
+	      && TARGET_64BIT && avoid_plt_to_call (call_op))
+	    xasm = "%!jmp\t*%p0@GOTPCREL(%%rip)";
+	  else
+	    xasm = "%!jmp\t%P0";
+	}
       /* SEH epilogue detection requires the indirect branch case
 	 to include REX.W.  */
       else if (TARGET_SEH)
@@ -25654,7 +25678,13 @@ ix86_output_call_insn (rtx_insn *insn, rtx call_op
     }
 
   if (direct_p)
-    xasm = "%!call\t%P0";
+    {
+      if (!TARGET_MACHO && !TARGET_SEH && !TARGET_PECOFF
+	  && TARGET_64BIT && avoid_plt_to_call (call_op))
+        xasm = "%!call\t*%p0@GOTPCREL(%%rip)";
+      else
+        xasm = "%!call\t%P0";
+    }
   else
     xasm = "%!call\t%A0";
 
@@ -46628,6 +46658,9 @@ static const struct attribute_spec ix86_attribute_
     false },
   { "callee_pop_aggregate_return", 1, 1, false, true, true,
     ix86_handle_callee_pop_aggregate_return, true },
+  /* Attribute to avoid calling function via PLT.  */
+  { "noplt", 0, 0, true, false, false, ix86_handle_fndecl_attribute,
+    false },
   /* End element.  */
   { NULL,        0, 0, false, false, false, NULL, false }
 };
Index: doc/extend.texi
===================================================================
--- doc/extend.texi	(revision 223720)
+++ doc/extend.texi	(working copy)
@@ -4858,6 +4858,13 @@ On x86-32 targets, the @code{stdcall} attribute ca
 assume that the called function pops off the stack space used to
 pass arguments, unless it takes a variable number of arguments.
 
+@item noplt
+@cindex @code{noplt} function attribute, x86-64
+@cindex functions whose calls do not go via PLT
+On x86-64 targets. the @code{noplt} attribute causes the compiler to
+call this external function indirectly using a GOT entry and avoid the
+PLT.
+
 @item target (@var{options})
 @cindex @code{target} function attribute
 As discussed in @ref{Common Function Attributes}, this attribute 
Index: testsuite/gcc.target/i386/noplt-1.c
===================================================================
--- 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-*-* } } */
+
+
+__attribute__ ((noplt))
+void foo();
+
+int main()
+{
+  foo();
+  return 0;
+}
+
+/* { dg-final { scan-assembler "call\[ \t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } } */ 
Index: testsuite/gcc.target/i386/noplt-2.c
===================================================================
--- testsuite/gcc.target/i386/noplt-2.c	(revision 0)
+++ testsuite/gcc.target/i386/noplt-2.c	(working copy)
@@ -0,0 +1,13 @@
+/* { dg-do compile { target x86_64-*-* } } */
+/* { dg-options "-O2" } */
+
+
+__attribute__ ((noplt))
+int foo();
+
+int main()
+{
+  return foo();
+}
+
+/* { dg-final { scan-assembler "jmp\[ \t\]\\*.*foo.*@GOTPCREL\\(%rip\\)" } } */