diff --git a/llvm/include/llvm/Analysis/Dominators.h b/llvm/include/llvm/Analysis/Dominators.h
index b539a788a220..587307314736 100644
--- a/llvm/include/llvm/Analysis/Dominators.h
+++ b/llvm/include/llvm/Analysis/Dominators.h
@@ -820,7 +820,7 @@ public:
     DT->splitBlock(NewBB);
   }
 
-  bool isReachableFromEntry(const BasicBlock* A) {
+  bool isReachableFromEntry(const BasicBlock* A) const {
     return DT->isReachableFromEntry(A);
   }
 
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 37253f72cf6b..b518d0ca71cd 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -92,7 +92,8 @@ static bool ValueDominatesPHI(Value *V, PHINode *P, const DominatorTree *DT) {
 
   // If we have a DominatorTree then do a precise test.
   if (DT)
-    return DT->dominates(I, P);
+    return !DT->isReachableFromEntry(P->getParent()) ||
+      !DT->isReachableFromEntry(I->getParent()) || DT->dominates(I, P);
 
   // Otherwise, if the instruction is in the entry block, and is not an invoke,
   // then it obviously dominates all phi nodes.