[PATCH] D71164: [InstCombine] Fix infinite loop due to bitcast <-> phi transforms

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

Okay, I think I got it. I don't have a standalone reproducer for this, but what seems to be happening is that we're iterating over `OldPN->users()` in https://github.com/llvm/llvm-project/blob/a506f7f9105eec4baac296d21c922457d6f4b52a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L2347, but https://github.com/llvm/llvm-project/blob/a506f7f9105eec4baac296d21c922457d6f4b52a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L2353 is going to modify the users. That makes us skip over a user (specifically, the original bitcast), which then refers to the old phi node.

That's already rather bad in itself (because it defeats the purpose of the transform and smells of memory unsafety), but with this change the undef replacement makes uses of the old phi actively incorrect.


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  https://reviews.llvm.org/D71164/new/

https://reviews.llvm.org/D71164