[llvm-commits] [PATCH] Re: Solicit code review (change to CodeGen)
evan.cheng at apple.com
Fri Oct 12 17:56:56 PDT 2012
On Oct 12, 2012, at 12:11 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
> 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.
> On 10/10/12 1:20 PM, Shuxin Yang wrote:
>> 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".
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