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.
Thanks
Sri
On Fri, May 22, 2015 at 2:00 AM, Pedro Alves <[email protected]> wrote:
> On 05/21/2015 11:02 PM, Sriraman Tallam wrote:
>> On Thu, May 21, 2015 at 2:51 PM, Pedro Alves <[email protected]> wrote:
>>> On 05/21/2015 10:12 PM, Sriraman Tallam wrote:
>>>>
>>>> My original proposal, for x86_64 only, was to add
>>>> -fno-plt=<function-name>. This lets the user decide for which
>>>> functions PLT must be avoided. Let the compiler always generate an
>>>> indirect call using call *func@GOTPCREL(%rip). We could do this for
>>>> non-PIC code too. No need for linker fixups since this relies on the
>>>> user to know that func is from a shared object.
>>>
>>> Having to pass function names on the command line seems like an odd
>>> interface. E.g, you'll need to pass the mangled name for
>>> C++ functions. Any reason this isn't a function attribute?
>>
>> It is not clear to me where I would stick the attribute. Example
>> usage in foo.cc:
>>
>> #include<string.h>
>>
>> int main() {
>> int n = memcmp(....);
>> }
>>
>> I want memcmp to not go through PLT, do you propose explicitly
>> re-declaring it in foo.cc with the attribute?
>
> I guess you'd do:
>
> #include<string.h>
>
> __attribute__((no_plt)) typeof (memcpy) memcpy;
>
> int main() {
> int n = memcmp(....);
> }
>
> or even:
>
> #include<string.h>
>
> int main() {
> if (hotpath) {
> __attribute__((no_plt)) typeof (memcpy) memcpy;
> for (..) {
> int n = memcmp(....);
> }
> } else {
> int n = memcmp(....);
> }
> }
>
> or globally:
>
> $ cat no-plt/string.h:
> #include_next <string.h>
> __attribute__((no_plt)) typeof (memcpy) memcpy;
>
> $ gcc -I no-plt/ ...
>
> Thanks,
> Pedro Alves
>
* 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 +25599,25 @@ 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 (GET_CODE (call_op) != SYMBOL_REF
+ || 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 +25630,12 @@ ix86_output_call_insn (rtx_insn *insn, rtx call_op
if (SIBLING_CALL_P (insn))
{
if (direct_p)
- xasm = "%!jmp\t%P0";
+ {
+ if (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 +25678,12 @@ ix86_output_call_insn (rtx_insn *insn, rtx call_op
}
if (direct_p)
- xasm = "%!call\t%P0";
+ {
+ if (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 +46657,9 @@ 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\\)" } }
*/
