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[WIP][LazyValueInfo] Fix non-determinism in cache.
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Authored by efriedma on Mon, Nov 11, 4:34 PM.

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nikic
Summary

OverDefinedCache contains a set of values that are overdefined in a given basic block. However, we don't reliably update the set when one of the values is erased.

This is really two fixes: one, add a PoisoningVH to catch the issue, and two, fix the resulting issue.

I'm posting this now for two reasons: one, to verify this actually fixes https://bugs.llvm.org/show_bug.cgi?id=43909, and two, to ask for ideas on how to improve the algorithm. The current implementation is really inefficient, but I'm not sure what the best approach looks like.

I haven't reduced the testcase yet.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43909 (hopefully).

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