[PATCH][x86_64] Convert indirect call via GOT to direct when possible

[Sriraman Tallam <tmsriram at google dot com>]

Hi,

   GCC has option -fno-plt which converts all extern calls to indirect
calls via GOT to prevent the linker for generating any PLT stubs.
However, if the function ends up defined in the executable this patch
will convert those indirect calls/jumps to direct.  Since the indirect
calls are one byte longer, an extra nop is needed at the beginning.

Here is a simple example:

main.c
---------
extern int foo();
int main() {
  return foo();
}

deffoo.c
-----------
int foo() {
  return 0;
}

$ gcc -fno-plt main.c deffoo.c
$objdump -d a.out

0000000000400626 <main>:
  ...
  40062a:       ff 15 28 14 00 00       callq  *0x1428(%rip)        #
401a58 <_DYNAMIC+0x1d8>

The call is indirect even though foo is defined in the executable.

With this patch,
0000000000400606 <main>:
   ....
   40060a:       90                      nop
  40060b:       e8 03 00 00 00          callq  400613 <foo>

The call is now direct with an extra nop.

   Please review.

Thanks
Sri

* x86_64.cc (can_convert_callq_to_direct): New function.
Target_x86_64<size>::Scan::global: Check if an indirect call via
GOT can be converted to direct.
Target_x86_64<size>::Relocate::relocate: Change any indirect call
via GOT that can be converted.
* testsuite/Makefile.am (x86_64_indirect_call_to_direct.sh): New test.
* testsuite/Makefile.in: Regenerate.
* testsuite/x86_64_indirect_call_to_direct1.s: New file.
* testsuite/x86_64_indirect_jump_to_direct1.s: New file.

Attachment: convert_indirect_call_patch.txt
Description: Text document