[all-commits] [llvm/llvm-project] 8fa02d: [llvm][SLP] Exit early if inputs to comparator are...

[David Berard via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8fa02db8cf82edb136d905ce44ffbc00f1cb6dbd
      https://github.com/llvm/llvm-project/commit/8fa02db8cf82edb136d905ce44ffbc00f1cb6dbd
  Author: David Berard <dberard at meta.com>
  Date:   2023-07-21 (Fri, 21 Jul 2023)

  Changed paths:
    M llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

  Log Message:
  -----------
  [llvm][SLP] Exit early if inputs to comparator are equal

**TL;DR:** This PR modifies a comparator. The comparator is used in a subsequent call to llvm::stable_sort. Sorting comparators should follow strict weak ordering - in particular, (x < x) should return false. This PR adds a fix to avoid an infinite loop when the inputs to the comparator are equal.

**Details**:

Sometimes when two equivalent tensors passed into the comparator, we encounter infinite looping (at https://github.com/llvm/llvm-project/blob/aae2eaae2cefd3132059925c4592276defdb1faa/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp#L4049)

Although it seems like this comparator will never be called with two equivalent pointers, some sanitizers, e.g. https://chromium.googlesource.com/chromiumos/third_party/gcc/+/refs/heads/stabilize-zako-5712.88.B/libstdc++-v3/include/bits/stl_algo.h#360, will add checks for (x < x). When this sanitizer is used with the current implementation, it triggers a comparator check for (x < x) which runs into the infinite loop

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D155874