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

Wed May 11 07:56:18 PDT 2022

ABataev added a comment.

In D124867#3506289 <https://reviews.llvm.org/D124867#3506289>, @wjschmidt wrote:

> In D124867#3506218 <https://reviews.llvm.org/D124867#3506218>, @ABataev wrote:
>
>>>> 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?
>
> I'll have to refresh my memory, but my recollection is that the FMA combining is done in the MI level instruction combiner.

Why? Are there any target-caused limitations?

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