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

[Florian Hahn via Phabricator via llvm-commits]

fhahn added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:4684
+        return false;
+    }
     if (!Legal->isUniformMemOp(*I))
----------------
Ayal wrote:
> 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.
Sounds good, so should this initial patch go in as is for now?


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

https://reviews.llvm.org/D151658