Index: llvm/trunk/include/llvm/Transforms/Scalar/GVN.h
===================================================================
--- llvm/trunk/include/llvm/Transforms/Scalar/GVN.h
+++ llvm/trunk/include/llvm/Transforms/Scalar/GVN.h
@@ -27,6 +27,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/PassManager.h"
+#include "llvm/IR/ValueHandle.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Compiler.h"
 #include <cstdint>
@@ -180,7 +181,12 @@
 
   // Map the block to reversed postorder traversal number. It is used to
   // find back edge easily.
-  DenseMap<const BasicBlock *, uint32_t> BlockRPONumber;
+  DenseMap<AssertingVH<BasicBlock>, uint32_t> BlockRPONumber;
+
+  // This is set 'true' initially and also when new blocks have been added to
+  // the function being analyzed. This boolean is used to control the updating
+  // of BlockRPONumber prior to accessing the contents of BlockRPONumber.
+  bool InvalidBlockRPONumbers = true;
 
   using LoadDepVect = SmallVector<NonLocalDepResult, 64>;
   using AvailValInBlkVect = SmallVector<gvn::AvailableValueInBlock, 64>;
Index: llvm/trunk/lib/Transforms/Scalar/GVN.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/GVN.cpp
+++ llvm/trunk/lib/Transforms/Scalar/GVN.cpp
@@ -1645,10 +1645,12 @@
 }
 
 void GVN::assignBlockRPONumber(Function &F) {
+  BlockRPONumber.clear();
   uint32_t NextBlockNumber = 1;
   ReversePostOrderTraversal<Function *> RPOT(&F);
   for (BasicBlock *BB : RPOT)
     BlockRPONumber[BB] = NextBlockNumber++;
+  InvalidBlockRPONumbers = false;
 }
 
 // Tries to replace instruction with const, using information from
@@ -1992,6 +1994,7 @@
   ICF = &ImplicitCFT;
   VN.setMemDep(MD);
   ORE = RunORE;
+  InvalidBlockRPONumbers = true;
 
   bool Changed = false;
   bool ShouldContinue = true;
@@ -2021,7 +2024,6 @@
     // Fabricate val-num for dead-code in order to suppress assertion in
     // performPRE().
     assignValNumForDeadCode();
-    assignBlockRPONumber(F);
     bool PREChanged = true;
     while (PREChanged) {
       PREChanged = performPRE(F);
@@ -2183,6 +2185,10 @@
   BasicBlock *PREPred = nullptr;
   BasicBlock *CurrentBlock = CurInst->getParent();
 
+  // Update the RPO numbers for this function.
+  if (InvalidBlockRPONumbers)
+    assignBlockRPONumber(*CurrentBlock->getParent());
+
   SmallVector<std::pair<Value *, BasicBlock *>, 8> predMap;
   for (BasicBlock *P : predecessors(CurrentBlock)) {
     // We're not interested in PRE where blocks with predecessors that are
@@ -2194,6 +2200,8 @@
     // It is not safe to do PRE when P->CurrentBlock is a loop backedge, and
     // when CurInst has operand defined in CurrentBlock (so it may be defined
     // by phi in the loop header).
+    assert(BlockRPONumber.count(P) && BlockRPONumber.count(CurrentBlock) &&
+           "Invalid BlockRPONumber map.");
     if (BlockRPONumber[P] >= BlockRPONumber[CurrentBlock] &&
         llvm::any_of(CurInst->operands(), [&](const Use &U) {
           if (auto *Inst = dyn_cast<Instruction>(U.get()))
@@ -2341,6 +2349,7 @@
       SplitCriticalEdge(Pred, Succ, CriticalEdgeSplittingOptions(DT));
   if (MD)
     MD->invalidateCachedPredecessors();
+  InvalidBlockRPONumbers = true;
   return BB;
 }
 
@@ -2355,6 +2364,7 @@
                       CriticalEdgeSplittingOptions(DT));
   } while (!toSplit.empty());
   if (MD) MD->invalidateCachedPredecessors();
+  InvalidBlockRPONumbers = true;
   return true;
 }
 
@@ -2381,6 +2391,7 @@
   BlockRPONumber.clear();
   TableAllocator.Reset();
   ICF->clear();
+  InvalidBlockRPONumbers = true;
 }
 
 /// Verify that the specified instruction does not occur in our