[PATCH] Improvde code quality of vselect

[Nadav Rotem]

Michael, 

I think that the best way to handle the redundant sign-extending is to improve the simplify-demanded-bits optimization.  Last year I added some code to optimize the X86::VSELECT using simplify-demanded-bits.  Do you know why it does not work ?

Thanks,
Nadav

On Apr 10, 2013, at 4:48 PM, Michael Liao <michael.liao at intel.com> wrote:

> Hi,
> 
> As we canonicalize sext from i1 to select in instcomb, it generates redundant code to prepare all 0s and all 1s if we don't take it into account that, in SSE/AVX, packed comparisons output all 0s or 1s in SIMD lanes. The attached patch adds optimization for those cases and transforms select into AND/OR to reduce BLEND pressure when profitable.
> 
> Thanks for review
> - Michael
> 
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