[PATCH] D106041: [SimpleLoopUnswitch] Re-fix introduction of UB when hoisted condition may be undef or poison

[Hyeongyu Kim via Phabricator via llvm-commits]

hyeongyukim created this revision.
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https://bugs.llvm.org/show_bug.cgi?id=27506
https://bugs.llvm.org/show_bug.cgi?id=31652
https://bugs.llvm.org/show_bug.cgi?id=51043

Problems with SimpleLoopUnswitch cause the bug reports above.

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

This problem can be solved by adding a freeze on hoisted branches(above transform) and has been solved by D29015 <https://reviews.llvm.org/D29015>.
However, D29015 <https://reviews.llvm.org/D29015> is now reverted by performance regression(https://github.com/llvm/llvm-project/commit/2b5a8976514de326bb84f0913d9d451089c11d22)

It is not the first time that an added freeze has caused performance regression.
SimplifyCFG also had a problem with UB caused by branching-on-undef, which was solved by adding freeze to the branching condition. (D104569 <https://reviews.llvm.org/D104569>)
Performance regression occurred in D104569 <https://reviews.llvm.org/D104569>, and patches such as D105344 <https://reviews.llvm.org/D105344> and D105392 <https://reviews.llvm.org/D105392> were written to minimize it.

This patch will correct the SimpleLoopUnswitch as D104569 <https://reviews.llvm.org/D104569> handles the SimplyCFG while minimizing performance loss by introducing patches like D105344 <https://reviews.llvm.org/D105344> and D105392 <https://reviews.llvm.org/D105392>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D106041

Files:
  llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
  llvm/test/Transforms/SimpleLoopUnswitch/ARM/nontrivial-unswitch-cost.ll
  llvm/test/Transforms/SimpleLoopUnswitch/implicit-null-checks.ll

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