[PATCH] D90687: [LV] Ignore VF hint when unsafe

[Cullen Rhodes via Phabricator via llvm-commits]

c-rhodes created this revision.
c-rhodes added reviewers: sdesmalen, efriedma, fhahn, mkuper, RKSimon, spatel.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.
c-rhodes requested review of this revision.

In the following loop the dependence distance is 2 and can only be
vectorized if the vector length is no larger than this.

  void foo(int *a, int *b, int N) {
    #pragma clang loop vectorize(enable) vectorize_width(4)
    for (int i=0; i<N; ++i) {
      a[i + 2] = a[i] + b[i];
    }
  }

However, when specifying a VF of 4 via a loop hint this loop is
vectorized. According to [1][2], loop hints are ignored if the
optimization is not safe to apply.

This patch introduces a check to bail of vectorization if the user
specified VF is greater than the maximum feasible VF, unless explicitly
forced with '-force-vector-width=X'.

[1] https://llvm.org/docs/LangRef.html#llvm-loop-vectorize-and-llvm-loop-interleave
[2] https://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D90687

Files:
  llvm/include/llvm/Analysis/LoopAccessAnalysis.h
  llvm/lib/Analysis/LoopAccessAnalysis.cpp
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
  llvm/test/Transforms/LoopVectorize/X86/libm-vector-calls-VF2-VF8.ll
  llvm/test/Transforms/LoopVectorize/X86/libm-vector-calls-finite.ll
  llvm/test/Transforms/LoopVectorize/X86/libm-vector-calls.ll
  llvm/test/Transforms/LoopVectorize/X86/svml-calls-finite.ll
  llvm/test/Transforms/LoopVectorize/metadata-width.ll
  llvm/test/Transforms/LoopVectorize/preserve-dbg-loc-and-loop-metadata.ll
  llvm/test/Transforms/LoopVectorize/runtime-check.ll
  llvm/test/Transforms/LoopVectorize/unsafe-vf-remark.ll

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