[all-commits] [llvm/llvm-project] 0aeb37: [SimpleLoopUnswitch] Re-fix introduction of UB whe...

[Hyeongyu Kim via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 0aeb37324dbb83d442b9222f465cece691fe29e0
      https://github.com/llvm/llvm-project/commit/0aeb37324dbb83d442b9222f465cece691fe29e0
  Author: hyeongyu kim <gusrb406 at snu.ac.kr>
  Date:   2021-10-12 (Tue, 12 Oct 2021)

  Changed paths:
    M llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
    A llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-freeze.ll

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

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.
However, 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)
Performance regression occurred in D104569, and patches such as D105344 and D105392 were written to minimize it.

This patch will correct the SimpleLoopUnswitch as D104569 handles the SimplyCFG while minimizing performance loss by introducing patches like D105344 and D105392(This patch was rebased with the author's permission)

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D106041