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 <pal...@redhat.com> wrote: > On 05/21/2015 11:02 PM, Sriraman Tallam wrote: >> On Thu, May 21, 2015 at 2:51 PM, Pedro Alves <pal...@redhat.com> 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\\)" } } */