[Local] Fix a bug in the domtree update logic for MergeBasicBlockIntoOnlyPred.

Summary: For some irreducible CFG the domtree nodes might be dead, do not update domtree for dead nodes.

Reviewers: kuhar, dberlin, hfinkel

Reviewed By: kuhar

Subscribers: llvm-commits, mcrosier

Differential Revision: https://reviews.llvm.org/D38960

llvm-svn: 316582

1 files changed, 7 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index fd3367710f3..8c643c93ec4 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -649,9 +649,13 @@ void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, DominatorTree *DT) {
     DestBB->moveAfter(PredBB);
 
   if (DT) {
-    BasicBlock *PredBBIDom = DT->getNode(PredBB)->getIDom()->getBlock();
-    DT->changeImmediateDominator(DestBB, PredBBIDom);
-    DT->eraseNode(PredBB);
+    // For some irreducible CFG we end up having forward-unreachable blocks
+    // so check if getNode returns a valid node before updating the domtree.
+    if (DomTreeNode *DTN = DT->getNode(PredBB)) {
+      BasicBlock *PredBBIDom = DTN->getIDom()->getBlock();
+      DT->changeImmediateDominator(DestBB, PredBBIDom);
+      DT->eraseNode(PredBB);
+    }
   }
   // Nuke BB.
   PredBB->eraseFromParent();