[lld] r194860 - [PECOFF] Use INT3 instead of NOP.

[Sean Silva]

The CPU is probably still decoding these and it might hiccup. Could you try
running some microbenchmarks to verify that this isn't a performance
issue? (2 otherwise identical tight asm loops under `perf stat` is probably
enough)
It may not be a problem, but in case it is, you're going to have a *really*
hard time tracking down this performance bug later; all you would basically
have to go on from a bug report is "when I link my program with lld, it
runs slower".

-- Sean Silva


On Fri, Nov 15, 2013 at 5:11 PM, Rui Ueyama <ruiu at google.com> wrote:

> Author: ruiu
> Date: Fri Nov 15 16:11:43 2013
> New Revision: 194860
>
> URL: http://llvm.org/viewvc/llvm-project?rev=194860&view=rev
> Log:
> [PECOFF] Use INT3 instead of NOP.
>
> This patch does not change the meaning of the program, but if something's
> wrong
> in the linker or the compiler and the control reaches to the gap of
> imported
> function table, it will stop immediately because of the presence of INT3.
> If
> NOP, it'd fall through to the next call instruction, which is usually a
> completely foreign function call.
>
> Modified:
>     lld/trunk/lib/ReaderWriter/PECOFF/ReaderImportHeader.cpp
>
> Modified: lld/trunk/lib/ReaderWriter/PECOFF/ReaderImportHeader.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/PECOFF/ReaderImportHeader.cpp?rev=194860&r1=194859&r2=194860&view=diff
>
> ==============================================================================
> --- lld/trunk/lib/ReaderWriter/PECOFF/ReaderImportHeader.cpp (original)
> +++ lld/trunk/lib/ReaderWriter/PECOFF/ReaderImportHeader.cpp Fri Nov 15
> 16:11:43 2013
> @@ -152,7 +152,7 @@ namespace {
>
>  uint8_t FuncAtomContent[] = {
>    0xff, 0x25, 0x00, 0x00, 0x00, 0x00,  // jmp *0x0
> -  0x90, 0x90                           // nop; nop
> +  0xcc, 0xcc                           // int 3; int 3
>  };
>
>  /// The defined atom for jump table.
>
>
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