From 6e1f7bf316424f3b3af108db4911c09cb24c421c Mon Sep 17 00:00:00 2001 From: Davide Italiano Date: Fri, 11 May 2018 15:45:36 +0000 Subject: [Reassociate] Prevent infinite loops when processing PHIs. 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. llvm-svn: 332100 --- llvm/lib/Transforms/Scalar/Reassociate.cpp | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'llvm/lib/Transforms/Scalar/Reassociate.cpp') diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp index ad00552f04a..0d1d57d6486 100644 --- a/llvm/lib/Transforms/Scalar/Reassociate.cpp +++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp @@ -1905,7 +1905,14 @@ void ReassociatePass::EraseInst(Instruction *I) { while (Op->hasOneUse() && Op->user_back()->getOpcode() == Opcode && Visited.insert(Op).second) Op = Op->user_back(); - RedoInsts.insert(Op); + + // The instruction we're going to push may be coming from a + // dead block, and Reassociate skips the processing of unreachable + // blocks because it's a waste of time and also because it can + // lead to infinite loop due to LLVM's non-standard definition + // of dominance. + if (ValueRankMap.find(Op) != ValueRankMap.end()) + RedoInsts.insert(Op); } MadeChange = true; -- cgit v1.2.3