[PATCH] D79783: [LoopVectorize] Fallback to a scalar epilogue when TP fails

[Sjoerd Meijer via Phabricator via llvm-commits]

SjoerdMeijer added a comment.

> There are indeed loops which can be vectorized only with a scalar remainder loop, i.e., w/o foldTail. But the tests attached could easily be made an exception: LV handles induction variables that are live-out by pre-computing their values in the pre-header. So foldTail should work fine in their presence, provided the live-out value is computed using the original trip-count rather than the one foldTail rounds up.
> 
> Other live-outs could also be made to work under foldTail, such as selecting the last live element from the last live vector instead of assuming it's the last element of the last part.

Thanks for elaborating and explaining, and yes, got it about the live-out values and precomputing them.  Other than the live-out values, I think we have an additional challenge because target hook TTI->preferPredicateOverEpilogue that can reject tail-folding for loops that can be vectorised with a scalar remainder because of target dependent restrictions. I think that's why we also need a fallback. And judging from your other comments, please correct me if I'm wrong, it looks like you're happy with the general direction of this, and so we will address your other comments.


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  https://reviews.llvm.org/D79783/new/

https://reviews.llvm.org/D79783