[llvm-commits] [llvm] r165661 - in /llvm/trunk: lib/Target/X86/X86ISelLowering.cpp test/CodeGen/X86/select_const.ll

[NAKAMURA Takumi]

2012/10/11 Eli Friedman <eli.friedman at gmail.com>:
> On Wed, Oct 10, 2012 at 2:31 PM, Nadav Rotem <nrotem at apple.com> wrote:
>> Author: nadav
>> Date: Wed Oct 10 16:31:55 2012
>> New Revision: 165661
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=165661&view=rev
>> Log:
>> Patch by Shuxin Yang <shuxin.llvm at gmail.com>.
>>
>> Original message:
>>
>> The attached is the fix to radar://11663049. The optimization can be outlined by following rules:
>>
>>    (select (x != c), e, c) -> select (x != c), e, x),
>>    (select (x == c), c, e) -> select (x == c), x, e)
>> where the <c> is an integer constant.
>>
>>  The reason for this change is that : on x86, conditional-move-from-constant needs two instructions;
>> however, conditional-move-from-register need only one instruction.
>>
>>   While the LowerSELECT() sounds to be the most convenient place for this optimization, it turns out to be a bad place. The reason is that by replacing the constant <c> with a symbolic value, it obscure some instruction-combining opportunities which would otherwise be very easy to spot. For that reason, I have to postpone the change to last instruction-combining phase.
>>
>>   The change passes the test of "make check-all -C <build-root/test" and "make -C project/test-suite/SingleSource".
>
> This change is breaking
> test-suite/MultiSource/Benchmarks/mediabench/g721/g721encode/ in the
> test-suite; please fix or revert.

It broke selfhosting, too.
http://bb.pgr.jp/builders/clang-3stage-x86_64-linux/builds/619
Reverted in r165692, thank you.