[LLVMdev] Solicit code review (change to CodeGen)
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:
> 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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