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

[Reid Kleckner]

MSVC uses int3 fill for alignment everywhere in .text.  I agree we should
benchmark eventually, but there's nothing wrong with being consistent here
for now.


On Sat, Nov 16, 2013 at 11:31 PM, Sean Silva <silvas at purdue.edu> wrote:

> 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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