[PATCH] D61308: [X86][SSE] Fold add(shuffle(),shuffle()) on 'slow' targets (PR39920)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: andreadb, craig.topper, spatel.
Herald added a project: LLVM.

As reported on PR39920, "slow horizontal ops" targets tend to internally expand to 2*shuffle+add/sub - so if we can reduce 2*shuffle+add/sub to a hadd/sub then we should do it - similar port usage but reduced instruction count.

This works out in most cases, although the "PR22377" regression in vector-shuffle-combining.ll is annoying - going from 2*shuffle+add+shuffle to hadd+2*shuffle - I'm open to suggestions - I've been trying to think of ways to get foldShuffleOfHorizOp to work with general shuffles but haven't found anything yet.


Repository:
  rL LLVM

https://reviews.llvm.org/D61308

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/avx2-phaddsub.ll
  test/CodeGen/X86/haddsub-shuf.ll
  test/CodeGen/X86/haddsub-undef.ll
  test/CodeGen/X86/haddsub.ll
  test/CodeGen/X86/phaddsub.ll
  test/CodeGen/X86/vector-shuffle-combining.ll

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