Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -1328,12 +1328,14 @@
   }
 
   // If this load follows a GEP, see if we can PRE the indices before analyzing.
-  if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(LI->getOperand(0))) {
-    for (GetElementPtrInst::op_iterator OI = GEP->idx_begin(),
-                                        OE = GEP->idx_end();
-         OI != OE; ++OI)
-      if (Instruction *I = dyn_cast<Instruction>(OI->get()))
-        performScalarPRE(I);
+  if (EnablePRE) {
+    if (auto GEP = dyn_cast<GetElementPtrInst>(LI->getOperand(0))) {
+      for (GetElementPtrInst::op_iterator OI = GEP->idx_begin(),
+                                          OE = GEP->idx_end();
+           OI != OE; ++OI)
+        if (Instruction *I = dyn_cast<Instruction>(OI->get()))
+          performScalarPRE(I);
+    }
   }
 
   // Step 2: Analyze the availability of the load
@@ -1645,6 +1647,11 @@
 }
 
 void GVN::assignBlockRPONumber(Function &F) {
+  // Initialize BlockRPONumber if it has never been populated, or
+  // if some blocks have been split and added to F.
+  if (F.size() == BlockRPONumber.size())
+    return;
+  BlockRPONumber.clear();
   uint32_t NextBlockNumber = 1;
   ReversePostOrderTraversal<Function *> RPOT(&F);
   for (BasicBlock *BB : RPOT)
@@ -2021,7 +2028,6 @@
     // Fabricate val-num for dead-code in order to suppress assertion in
     // performPRE().
     assignValNumForDeadCode();
-    assignBlockRPONumber(F);
     bool PREChanged = true;
     while (PREChanged) {
       PREChanged = performPRE(F);
@@ -2143,6 +2149,7 @@
 }
 
 bool GVN::performScalarPRE(Instruction *CurInst) {
+  assert(EnablePRE && "GVN: Calling performScalarPRE with -enable-pre=false.");
   if (isa<AllocaInst>(CurInst) || CurInst->isTerminator() ||
       isa<PHINode>(CurInst) || CurInst->getType()->isVoidTy() ||
       CurInst->mayReadFromMemory() || CurInst->mayHaveSideEffects() ||
@@ -2184,6 +2191,9 @@
   BasicBlock *PREPred = nullptr;
   BasicBlock *CurrentBlock = CurInst->getParent();
 
+  // Update the RPO numbers for this function.
+  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
Index: test/Transforms/GVN/nonescaping-malloc.ll
===================================================================
--- test/Transforms/GVN/nonescaping-malloc.ll
+++ test/Transforms/GVN/nonescaping-malloc.ll
@@ -2,6 +2,10 @@
 ; RUN: opt < %s -basicaa -gvn -stats -disable-output 2>&1 | FileCheck %s
 ; rdar://7363102
 
+; Sanity check that GVN::performScalarPRE is not called with
+; enable-pre=false (asserts)
+; RUN: opt < %s -basicaa -gvn -enable-pre=false -disable-output 2>&1
+
 ; CHECK: Number of loads deleted
 
 ; GVN should be able to eliminate load %tmp22.i, because it is redundant with