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
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;
--- /dev/null
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -reassociate %s -S | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[DONE:%.*]]
+; CHECK: dead:
+; CHECK-NEXT: [[XOR0:%.*]] = xor i16 [[XOR1:%.*]], undef
+; CHECK-NEXT: [[XOR1]] = xor i16 [[XOR0]], undef
+; CHECK-NEXT: br i1 undef, label [[DEAD:%.*]], label [[DONE]]
+; CHECK: done:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %done
+
+dead:
+ %xor0 = xor i16 %xor1, undef
+ %xor1 = xor i16 %xor0, undef
+ br i1 undef, label %dead, label %done
+
+done:
+ %e = phi i16 [ %xor1, %dead ], [ 0, %entry ]
+ ret void
+}