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

Wed May 11 07:27:56 PDT 2022

ABataev added a comment.

In D124867#3506211 <https://reviews.llvm.org/D124867#3506211>, @wjschmidt wrote:

> 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.

Why, could you investigate it?

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