[PATCH] D124867: [SLP][NFC] Pre-commit test showing horizontal reduction preventing FMA

[Bill Schmidt via Phabricator via llvm-commits]

wjschmidt added a comment.

In D124867#3506146 <https://reviews.llvm.org/D124867#3506146>, @ABataev wrote:

> The undefs still here.

Yes, see above -- I was unable to find a sequence without the undefs that causes the horizontal reduction to kick in.

> Also, why these sequences are not optimized by InsrtuctionCombiner to FMA?

Phase ordering -- it seems the FMA combining happens quite late in the pipeline.  When we replace the adds with a horizontal reduction, the opportunity is removed.


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