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

[NAKAMURA Takumi]

Please prune trailing whitespaces.

It passed selfhosting, for me.
http://bb.pgr.jp/builders/clang-3stage-x86_64-linux/builds/657
http://bb.pgr.jp/builders/clang-3stage-cygwin/builds/352

...Takumi


2012/10/13 Shuxin Yang <shuxin.llvm at gmail.com>:
> 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/.
>
> 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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