[all-commits] [llvm/llvm-project] 3e5ce4: [LV] Unconditionally branch from middle to scalar ...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3e5ce49e5371ce4feadbf97dd5c2b652d9db3d1d
      https://github.com/llvm/llvm-project/commit/3e5ce49e5371ce4feadbf97dd5c2b652d9db3d1d
  Author: Philip Reames <listmail at philipreames.com>
  Date:   2021-02-04 (Thu, 04 Feb 2021)

  Changed paths:
    M llvm/lib/Transforms/Utils/LoopVersioning.cpp
    M llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
    M llvm/test/Transforms/LoopVectorize/first-order-recurrence-complex.ll
    M llvm/test/Transforms/LoopVectorize/interleaved-accesses.ll
    M llvm/test/Transforms/LoopVectorize/loop-form.ll

  Log Message:
  -----------
  [LV] Unconditionally branch from middle to scalar preheader if the scalar loop must execute

If we know that the scalar epilogue is required to run, modify the CFG to end the middle block with an unconditional branch to scalar preheader. This is instead of a conditional branch to either the preheader or the exit block.

The motivation to do this is to support multiple exit blocks. Specifically, the current structure forces us to identify immediate dominators and *which* exit block to branch from in the middle terminator. For the multiple exit case - where we know require scalar will hold - these questions are ill formed.

This is the last change needed to support multiple exit loops, but since the diffs are already large enough, I'm going to land this, and then enable separately. You can think of this as being NFCI-ish prep work, but the changes are a bit too involved for me to feel comfortable tagging the change that way.

Differential Revision: https://reviews.llvm.org/D94892