[PATCH] D147735: [LV] Adds simple analysis that improves VF-aware uniformity checks.

[Vasileios Porpodas via Phabricator via llvm-commits]

vporpo created this revision.
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This helps generate broadcasts instead of gathers in cases like:

  for (i = 0; i != N; ++i)
    ... = ... B[i/4]

when VF is a multiple of 4.

And will generate a single store in cases like this:

  for (i = 0; i != N; ++i)
    A[i/4] = ...

when VF is a multiple of 4.

Without this patch the uniformity checks only test for loop-invariance, so they
return false for `i/4`, even though `i/4` will always return `i/4` for all
`i/4`, `(i+1)/4`, `(i+2)/4` and `(i+3)/4`.

This patch adds a simple analysis that checks if a value is a linear expression
of the induction variable divided by a constant and that the VF is a multiple
of this constant. If so, the value is treaded as uniform.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D147735

Files:
  llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
  llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
  llvm/test/Transforms/LoopVectorize/X86/uniform_across_vf.ll
  llvm/test/Transforms/LoopVectorize/X86/uniform_mem_op.ll
  llvm/test/Transforms/LoopVectorize/pr47343-expander-lcssa-after-cfg-update.ll

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