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Re: [PATCH][x86_64] Convert indirect call via GOT to direct when possible
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Sriraman Tallam <tmsriram at google dot com>
- Cc: binutils <binutils at sourceware dot org>, Cary Coutant <ccoutant at gmail dot com>, David Li <davidxl at google dot com>
- Date: Sat, 28 May 2016 10:44:08 -0700
- Subject: Re: [PATCH][x86_64] Convert indirect call via GOT to direct when possible
- Authentication-results: sourceware.org; auth=none
- References: <CAAs8HmxxdBpS7w8udZgK0QFi5TnenU3wGhpPfhWeKE8Tr=thvA at mail dot gmail dot com> <CAMe9rOpk3aOK5mMkKvYQyzeQxJ-h8o+3KjLRikKSkLmMfqoUtg at mail dot gmail dot com> <CAAs8Hmw2KQ2neDNP5cnQPBVBZMJthvQGTARPiwa-NfAx5R6ugw at mail dot 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.