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

[Quentin Colombet]

Committed revision 201906.

Thanks,
-Quentin

On Feb 21, 2014, at 3:21 PM, Quentin Colombet <qcolombet at apple.com> wrote:

> Hi Adam,
> 
> This LGTM with two minors comments:
> 
> +define <8 x i16> @foo(<8 x i16> %a, <8 x i16> %b) #0 {
> +; CHECK: .short	      32
> 
> - Add a check LABEL here.
> - Remove the #0, it seems you do not have any related attribute (same for bar).
> 
> If you still do not have commit access, sent the updated patch, I’ll commit for you!
> 
> Thanks,
> -Quentin
> 
> On Feb 21, 2014, at 3:12 PM, Adam Nemet <anemet at apple.com> wrote:
> 
>> 
>> On Feb 20, 2014, at 8:13 PM, Adam Nemet <anemet at apple.com> wrote:
>> 
>>> 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.
>> 
>> Even BuildVectors that return true in isConstant can have undef elements. In this case constant-folding returns NULL so my new assert triggers.  Function bar in the test case is an example.
>> 
>> In this new version, I bail if constant-folding failed, i.e. when the const vector has an undef element.
>> 
>> Retested as before.
>> 
>> Adam
>> 
>> <redundant-psll-4.patch>
> 
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