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On Sat, May 28, 2016 at 10:44 AM, H.J. Lu <hjl.tools@gmail.com> wrote: > 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? Ok, patch changed to not check for this and refactored a bit. Thanks Sri > > -- > H.J.
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convert_indirect_call_patch.txt
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