[PATCH][X86] Optimize shifting of result of a vector-compare

[Adam Nemet]

On Feb 18, 2014, at 4:44 PM, Adam Nemet <anemet at apple.com> wrote:

> This change is motivated by the benchmark in SingleSource/Benchmarks/Misc/lowercase.c.
> 
>             pcmpgtw %xmm3, %xmm2
>             pand    %xmm7, %xmm2      // xmm7 is <1, 1, 1, 1, 1, 1, 1, 1>
>             psllw   $5, %xmm2
> 
> PCMP* sets its result to all ones or zeros so we can AND with the shifted mask rather than masking and shifting separately.
> 
> The patch adds this transformation to the DAGCombiner:
> 
>   (shl (and (setcc:i8v16 ...) N01C) N1C) -> (and (setcc:i8v16 ...) N01C<<N1C)
> 
> This is a similar transformation that is already done for sign-extending the result of setcc for targets where boolean values are either zero or all-ones.
> 
> If the patch looks good, please apply.

Quentin pointed out offline that a BuildVector is not necessarily constant.  This new version adds some extra checks to guarantee that.  Retested on x86_64-apple-darwin.

Adam

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