diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index 2012ab473c9..1cd16bb06de 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -19,6 +19,7 @@
 #include "llvm/Instructions.h"
 #include "llvm/Function.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/Dominators.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetData.h"
@@ -82,6 +83,7 @@ void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   AU.addRequiredTransitive<AliasAnalysis>();
   AU.addRequiredTransitive<TargetData>();
+  AU.addRequiredTransitive<DominatorTree>();
 }
 
 /// getCallSiteDependency - Private helper for finding the local dependencies
@@ -222,6 +224,17 @@ void MemoryDependenceAnalysis::nonLocalHelper(Instruction* query,
       continue;
     }
     
+    // Don't recur upwards if the current block is unreachable.
+    // Instead, mark it as having no dependency on this path,
+    // which will block optzns from occuring.  For this reason,
+    // eliminating unreachable blocks before running a memdep
+    // based optimization is recommended.
+    DominatorTree& DT = getAnalysis<DominatorTree>();
+    if (!DT.isReachableFromEntry(BB)) {
+      resp.insert(std::make_pair(BB, None));
+      continue;
+    }
+    
     // If we didn't find anything, recurse on the precessors of this block
     // Only do this for blocks with a small number of predecessors.
     bool predOnStack = false;