[LLVMdev] Solicit code review (change to CodeGen)

Nadav Rotem nrotem at apple.com
Wed Oct 10 13:52:07 PDT 2012


I will commit. 

On Oct 10, 2012, at 1:20 PM, Shuxin Yang <shuxin.llvm at gmail.com> 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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