Index: include/llvm/Transforms/Scalar/GVN.h
===================================================================
--- include/llvm/Transforms/Scalar/GVN.h
+++ include/llvm/Transforms/Scalar/GVN.h
@@ -111,6 +111,7 @@
   const TargetLibraryInfo *TLI;
   AssumptionCache *AC;
   SetVector<BasicBlock *> DeadBlocks;
+  SmallSet<BasicBlock *, 32> NumberedBlocks;
   OptimizationRemarkEmitter *ORE;
 
   ValueTable VN;
Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -2053,6 +2053,11 @@
       toSplit.push_back(std::make_pair(PREPred->getTerminator(), SuccNum));
       return false;
     }
+
+    // Don't PRE on blocks that have not been numbered.
+    if (NumberedBlocks.count(PREPred) == 0)
+      return false;
+
     // We need to insert somewhere, so let's give it a shot
     PREInstr = CurInst->clone();
     if (!performScalarPREInsertion(PREInstr, PREPred, ValNo)) {
@@ -2103,7 +2108,9 @@
 /// control flow patterns and attempts to perform simple PRE at the join point.
 bool GVN::performPRE(Function &F) {
   bool Changed = false;
+  NumberedBlocks.clear();
   for (BasicBlock *CurrentBlock : depth_first(&F.getEntryBlock())) {
+    NumberedBlocks.insert(CurrentBlock);
     // Nothing to PRE in the entry block.
     if (CurrentBlock == &F.getEntryBlock())
       continue;
@@ -2153,6 +2160,7 @@
 /// Executes one iteration of GVN
 bool GVN::iterateOnFunction(Function &F) {
   cleanupGlobalSets();
+  NumberedBlocks.clear();
 
   // Top-down walk of the dominator tree
   bool Changed = false;
@@ -2160,8 +2168,10 @@
   // RPOT walks the graph in its constructor and will not be invalidated during
   // processBlock.
   ReversePostOrderTraversal<Function *> RPOT(&F);
-  for (BasicBlock *BB : RPOT)
+  for (BasicBlock *BB : RPOT) {
+    NumberedBlocks.insert(BB);
     Changed |= processBlock(BB);
+  }
 
   return Changed;
 }