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Phi nodes can reside in live blocks but one of their incoming arguments can come from a dead block. Dead blocks and reassociate don't play nice together. In fact, reassociate performs an RPO as a first step to avoid processing dead blocks. The reason why Reassociate might not fixpoint when examining dead blocks is that the following: %xor0 = xor i16 %xor1, undef %xor1 = xor i16 %xor0, undef is perfectly valid LLVM IR (if it appears in a dead block), so the worklist algorithm keeps pushing the two instructions for reexamination. Note that this is not Reassociate fault, at least not entirely. It's llvm that has a weird definition of dominance. Fixes PR37390. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332100 91177308-0d34-0410-b5e6-96231b3b80d8