[all-commits] [llvm/llvm-project] 5d2b3d: [SLP] Avoid std::stable_sort(properlyDominates()).

[Harald van Dijk via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 5d2b3de284f46e4c1e60fc5f266b085f54391566
      https://github.com/llvm/llvm-project/commit/5d2b3de284f46e4c1e60fc5f266b085f54391566
  Author: Harald van Dijk <harald at gigawatt.nl>
  Date:   2021-06-03 (Thu, 03 Jun 2021)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
    A llvm/test/Transforms/SLPVectorizer/X86/ordering-bug.ll

  Log Message:
  -----------
  [SLP] Avoid std::stable_sort(properlyDominates()).

As noticed by NAKAMURA Takumi back in 2017, we cannot use
properlyDominates for std::stable_sort as properlyDominates only
partially orders blocks. That is, for blocks A, B, C, D, where A
dominates B and C dominates D, we have A == C, B == C, but A < B. This
is not a valid comparison function for std::stable_sort and causes
different results between libstdc++ and libc++. This change uses DFS
numbering to give deterministic results for all reachable blocks.
Unreachable blocks are ignored already, so do not need special
consideration.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D103441