[llvm-commits] [llvm] r165661 - in /llvm/trunk: lib/Target/X86/X86ISelLowering.cpp test/CodeGen/X86/select_const.ll

[Eli Friedman]

On Wed, Oct 10, 2012 at 2:31 PM, Nadav Rotem <nrotem at apple.com> wrote:
> Author: nadav
> Date: Wed Oct 10 16:31:55 2012
> New Revision: 165661
>
> URL: http://llvm.org/viewvc/llvm-project?rev=165661&view=rev
> Log:
> Patch by Shuxin Yang <shuxin.llvm at gmail.com>.
>
> Original message:
>
> 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".

This change is breaking
test-suite/MultiSource/Benchmarks/mediabench/g721/g721encode/ in the
test-suite; please fix or revert.

-Eli