[PATCH] D120330: [MachineSink] Fix CFG walk in clobber check (PR53990)

[Nikita Popov via Phabricator via llvm-commits]

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MachineSink::hasStoreBetween() determines that the code is "straight-line" by checking that From dominates To and To post-dominates From. Then it does a DFS walk starting at To, but only considers MBBs post-dominated by From.

The critical part this misses, is while the code is, in a certain sense of the word, "straight-line", it may still be part of a loop. To give a simple example:

  From ->  BB2
     \    /  A
      v  v   |
       To -> BB3

Here From dominates To, and To post-dominates From, and the current code would only check for a clobber in BB2, but not in BB3. (Unfortunately, I was not able to get MachineSink to trigger with a CFG that simple.)

What we actually want to determine is that no clobber is reverse-reachable from To (without going through From), so implement that, without the post-dominance check.

Fixes https://github.com/llvm/llvm-project/issues/53990.


https://reviews.llvm.org/D120330

Files:
  llvm/lib/CodeGen/MachineSink.cpp
  llvm/test/CodeGen/X86/pr53990-incorrect-machine-sink.ll

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