[PATCH] D44177: [JumpThreading] use InvalidBlocks to avoid unreachable regions

[Brian Rzycki via Phabricator via llvm-commits]

brzycki created this revision.
brzycki added reviewers: dberlin, sebpop, kuhar, dinesh.d, rafael.
Herald added a subscriber: hiraditya.

It is possible to cause hangs in JumpThreading due to how the pass decides there is no more work to be done. An older attempt at fixing this problem was https://reviews.llvm.org/D3991 where `removeUnreachableBlocks(F)` is now called on the function before any JumpThreading is attempted. This has a few drawbacks though:

- It's an expensive transformation.
- It is a coarse tool: it does not comprehend or care about preserving Loops.
- It actually has nothing to do the JumpThreading pass.
- It does more than just remove unrechable blocks, it changes reachable blocks too, especially Terminator Instructions.

This patch foregoes calling `removeUnreachableBlocks` and uses a `SmallPtrSet` and our initial `DominatorTree` to track invalid blocks.


Repository:
  rL LLVM

https://reviews.llvm.org/D44177

Files:
  llvm/include/llvm/Transforms/Scalar/JumpThreading.h
  llvm/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/test/Transforms/JumpThreading/pr15851_hang.ll
  llvm/test/Transforms/JumpThreading/select.ll

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