[llvm-commits] [PATCH] Re: Solicit code review (change to CodeGen)

[Evan Cheng]

On Oct 12, 2012, at 12:11 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:

> Hi,
> 
>    My previous change has a bug: I negated the condition code of a CMOV, and go ahead creating a new CMOV using the *ORIGINAL* condition code.
> 
>   This diff.patch passes the tests of "make check-all -C <buildroot>test", the SingleSource and MultiSource testing under projects/test_suite/.

Please also test External (which include SPEC and others). Another important test is clang self-hosting.

Thanks,

Evan

> 
> Thanks
> 
> Shuxin
> 
> 
> On 10/10/12 1:20 PM, Shuxin Yang wrote:
>> Hi,
>> 
>>  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".
>> 
>> Thanks
>> Shuxin
> 
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