[PATCH] D151658: [LV] Check if value was already not uniform for previous VF.

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:4684
+        return false;
+    }
     if (!Legal->isUniformMemOp(*I))
----------------
fhahn wrote:
> Ayal wrote:
> > This looks right.
> > Also holds for non-MemOpUse?
> > Furthermore, if value is uniform for PrevVF, suffice to only check if SCEVs of lanes 0 and VF-1 are equal.
> > Also holds for non-MemOpUse?
> 
> Yes, but the loop that uses `isUniformMemOpUse` also collects pointers where only lane 0 is demanded (for widened memory ops), so checking in the loop directly isn't possible without first dis-entangling this logic.
> 
> 
> > Furthermore, if value is uniform for PrevVF, suffice to only check if SCEVs of lanes 0 and VF-1 are equal.
> 
> Hmm, I think we would have to check if lane 0 matches the lanes PrevVF..VF-1? Might be good as follow-up, as it will require adjusting the API a bit I think to provide the additional info.
>> Also holds for non-MemOpUse?

> Yes, but the loop that uses isUniformMemOpUse also collects pointers where only lane 0 is demanded (for widened memory ops), so checking in the loop directly isn't possible without first dis-entangling this logic.

Agreed, uniformness should be disentangled from first-lane-used-ness.

>> Furthermore, if value is uniform for PrevVF, suffice to only check if SCEVs of lanes 0 and VF-1 are equal.

> Hmm, I think we would have to check if lane 0 matches the lanes PrevVF..VF-1? Might be good as follow-up, as it will require adjusting the API a bit I think to provide the additional info.

Stitching piecewise uniformity - if a value is uniform across PrevVF then its SCEVs for lanes {0,1,..,PrevVF-1} are equal to each other, and so are those of lanes/offsets {PrevVF,PrevVF+1...,VF-1}, right? If so then suffice the select and compare one sample from each for the value to be uniform across VF.

In any case these can be dealt with in separate follow-up patches.


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  https://reviews.llvm.org/D151658/new/

https://reviews.llvm.org/D151658