[PATCH] D54239: [JumpThreading] Fix exponential time algorithm computing known values.

[Eli Friedman via Phabricator via llvm-commits]

efriedma created this revision.
efriedma added reviewers: mkazantsev, mzolotukhin, wmi.
Herald added a subscriber: jfb.

ComputeValueKnownInPredecessors has a "visited" set to prevent infinite loops, since a value can be visited more than once.  However, the implementation didn't prevent the algorithm from taking exponential time. Instead of removing elements from the RecursionSet one at a time, we should keep around the whole set until ComputeValueKnownInPredecessors finishes, then discard it.

The testcase is synthetic because I was having trouble effectively reducing the original.  But it's basically the same idea.

Instead of failing, we could theoretically cache the result instead. But I don't think it would help substantially in practice.


Repository:
  rL LLVM

https://reviews.llvm.org/D54239

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

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