[all-commits] [llvm/llvm-project] 181628: [SimpleLoopUnswitch] Fix introduction of UB when h...

[Juneyoung Lee via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 181628b52d390f2136fb5a63fe644d230a9b822d
      https://github.com/llvm/llvm-project/commit/181628b52d390f2136fb5a63fe644d230a9b822d
  Author: Juneyoung Lee <aqjune at gmail.com>
  Date:   2020-02-26 (Wed, 26 Feb 2020)

  Changed paths:
    M llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
    M llvm/test/Transforms/SimpleLoopUnswitch/exponential-nontrivial-unswitch-nested.ll
    M llvm/test/Transforms/SimpleLoopUnswitch/exponential-nontrivial-unswitch-nested2.ll
    M llvm/test/Transforms/SimpleLoopUnswitch/exponential-switch-unswitch.ll
    M llvm/test/Transforms/SimpleLoopUnswitch/guards.ll
    M llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-cost.ll
    M llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll

  Log Message:
  -----------
  [SimpleLoopUnswitch] Fix introduction of UB when hoisted condition may be undef or poison

Summary:
Loop unswitch hoists branches on loop-invariant conditions. However, if this
condition is poison/undef and the branch wasn't originally reachable, loop
unswitch introduces UB (since the optimized code will branch on poison/undef and
the original one didn't)).
We fix this problem by freezing the condition to ensure we don't introduce UB.

We will now transform the following:
  while (...) {
    if (C) { A }
    else   { B }
  }

Into:
  C' = freeze(C)
  if (C') {
    while (...) { A }
  } else {
    while (...) { B }
  }

This patch fixes the root cause of the following bug reports (which use the old loop unswitch, but can be reproduced with minor changes in the code and -enable-nontrivial-unswitch):
- https://llvm.org/bugs/show_bug.cgi?id=27506
- https://llvm.org/bugs/show_bug.cgi?id=31652

Reviewers: reames, majnemer, chenli, sanjoy, hfinkel

Reviewed By: reames

Subscribers: hiraditya, jvesely, nhaehnle, filcab, regehr, trentxintong, nlopes, llvm-commits, mzolotukhin

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D29015