[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
diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index ad00552..0d1d57d 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -1905,7 +1905,14 @@
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;