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

[Sean Silva]

On Tue, Nov 19, 2013 at 3:48 PM, Reid Kleckner <rnk at google.com> wrote:

> 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.
>

Wow, even for aligning loops? Seems like a pointless waste of decoding
bandwidth vs. an appropriately sized nop.

-- Sean Silva


>
>
> 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.
>>>
>>>
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>
>>
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20131119/6b477fbc/attachment.html>