[PATCH] D93860: [SLP] delete unused pairwise reduction option

Wed Dec 30 05:48:49 PST 2020

spatel added a comment.

In D93860#2473348 <https://reviews.llvm.org/D93860#2473348>, @pengfei wrote:

> Hi Sanjay, pairwise reduction seems not adopted by any targets. But do we need to consider the FMF here? I found LangRef says the intrinsic is sequential for fadd and fmul if `reassoc` flag is not set.

Hi @pengfei - 
Sorry for the delayed reply - I did not get an email notification for your comment here in Phab.

We do not need to care about FMF directly here because neither of these shuffle variations corresponds to "sequential" reduction of the elements. In other words, we should require `reassoc` (at the least) to form an fadd or fmul reduction that is expected to need any shuffles. Let me know if you see any holes in that theory.

One motivation for this cleanup is to correct bugs with using/propagating FMF (that was also true for D87416 <https://reviews.llvm.org/D87416>). For example - https://llvm.org/PR35538

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https://reviews.llvm.org/D93860