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

[Sriraman Tallam <tmsriram at google dot com>]

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.

Is this committed to ld?, trunk ld does not seem to do this.

Also a quick thing about -fPIE and -fno-plt.  The assembly looks like
this for the call:

movq foo@GOTPCREL(%rip), %rax
jmp *%rax

Why can't we make this a single jmp *foo@GOTPCREL(%rip)

This goes via the GOT if foo is external and that is always reachable
with a 32-bit offset. Did I miss anything obvious?


> Also for
>
> jmp *foo#GOTPCREL(%rip)
>
>  ld converts it to
>
> jmp foo
> nop
>
> --
> H.J.