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

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

On Fri, May 27, 2016 at 3:14 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Fri, May 20, 2016 at 1:32 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Fri, May 20, 2016 at 1:27 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>> 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 try ld, which uses 0x67 prefix (addr32) instead of nop.
>> Also for
>>
>> jmp *foo#GOTPCREL(%rip)
>>
>>  ld converts it to
>>
>> jmp foo
>> nop
>
> I have modified the patch to keep it consistent with what ld produces.
>
> Please take another look.
>
> * 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.
>

Do you need to check R_X86_64_REX_GOTPCRELX for branch?

-- 
H.J.