Index: llvm/trunk/include/llvm/Support/GenericDomTree.h
===================================================================
--- llvm/trunk/include/llvm/Support/GenericDomTree.h
+++ llvm/trunk/include/llvm/Support/GenericDomTree.h
@@ -853,10 +853,15 @@
     assert(isReachableFromEntry(B));
     assert(isReachableFromEntry(A));
 
+    const unsigned ALevel = A->getLevel();
     const DomTreeNodeBase<NodeT> *IDom;
-    while ((IDom = B->getIDom()) != nullptr && IDom != A && IDom != B)
+
+    // Don't walk nodes above A's subtree. When we reach A's level, we must
+    // either find A or be in some other subtree not dominated by A.
+    while ((IDom = B->getIDom()) != nullptr && IDom->getLevel() >= ALevel)
       B = IDom;  // Walk up the tree
-    return IDom != nullptr;
+
+    return B == A;
   }
 
   /// Wipe this tree's state without releasing any resources.