Index: lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- lib/Bitcode/Reader/BitcodeReader.cpp
+++ lib/Bitcode/Reader/BitcodeReader.cpp
@@ -170,7 +170,7 @@
 
   // When intrinsic functions are encountered which require upgrading they are
   // stored here with their replacement function.
-  typedef std::vector<std::pair<Function*, Function*> > UpgradedIntrinsicMap;
+  typedef DenseMap<Function*, Function*> UpgradedIntrinsicMap;
   UpgradedIntrinsicMap UpgradedIntrinsics;
 
   // Map the bitcode's custom MDKind ID to the Module's MDKind ID.
@@ -2710,7 +2710,7 @@
   for (Function &F : *TheModule) {
     Function *NewFn;
     if (UpgradeIntrinsicFunction(&F, NewFn))
-      UpgradedIntrinsics.push_back(std::make_pair(&F, NewFn));
+      UpgradedIntrinsics[&F] = NewFn;
   }
 
   // Look for global variables which need to be renamed.
@@ -4457,13 +4457,11 @@
     stripDebugInfo(*F);
 
   // Upgrade any old intrinsic calls in the function.
-  for (UpgradedIntrinsicMap::iterator I = UpgradedIntrinsics.begin(),
-       E = UpgradedIntrinsics.end(); I != E; ++I) {
-    if (I->first != I->second) {
-      for (auto UI = I->first->user_begin(), UE = I->first->user_end();
-           UI != UE;) {
-        if (CallInst* CI = dyn_cast<CallInst>(*UI++))
-          UpgradeIntrinsicCall(CI, I->second);
+  for (auto &I : UpgradedIntrinsics) {
+    if (I.first != I.second) {
+      for (auto *UI : I.first->users()) {
+        if (CallInst* CI = dyn_cast<CallInst>(UI))
+          UpgradeIntrinsicCall(CI, I.second);
       }
     }
   }
@@ -4531,20 +4529,18 @@
   // delete the old functions to clean up. We can't do this unless the entire
   // module is materialized because there could always be another function body
   // with calls to the old function.
-  for (std::vector<std::pair<Function*, Function*> >::iterator I =
-       UpgradedIntrinsics.begin(), E = UpgradedIntrinsics.end(); I != E; ++I) {
-    if (I->first != I->second) {
-      for (auto UI = I->first->user_begin(), UE = I->first->user_end();
-           UI != UE;) {
-        if (CallInst* CI = dyn_cast<CallInst>(*UI++))
-          UpgradeIntrinsicCall(CI, I->second);
+  for (auto &I : UpgradedIntrinsics) {
+    if (I.first != I.second) {
+      for (auto *U : I.first->users()) {
+        if (CallInst* CI = dyn_cast<CallInst>(U))
+          UpgradeIntrinsicCall(CI, I.second);
       }
-      if (!I->first->use_empty())
-        I->first->replaceAllUsesWith(I->second);
-      I->first->eraseFromParent();
+      if (!I.first->use_empty())
+        I.first->replaceAllUsesWith(I.second);
+      I.first->eraseFromParent();
     }
   }
-  std::vector<std::pair<Function*, Function*> >().swap(UpgradedIntrinsics);
+  UpgradedIntrinsics.clear();
 
   for (unsigned I = 0, E = InstsWithTBAATag.size(); I < E; I++)
     UpgradeInstWithTBAATag(InstsWithTBAATag[I]);
Index: test/Bitcode/PR23310.test
===================================================================
--- /dev/null
+++ test/Bitcode/PR23310.test
@@ -0,0 +1 @@
+RUN: llvm-dis -disable-output %p/Inputs/PR23310.bc