Index: include/llvm/ADT/StringMap.h
===================================================================
--- include/llvm/ADT/StringMap.h
+++ include/llvm/ADT/StringMap.h
@@ -142,11 +142,11 @@
 
   StringRef first() const { return StringRef(getKeyData(), getKeyLength()); }
 
-  /// Create - Create a StringMapEntry for the specified key and default
-  /// construct the value.
-  template <typename AllocatorTy, typename InitType>
+  /// Create a StringMapEntry for the specified key construct the value using
+  /// \p InitiVals.
+  template <typename AllocatorTy, typename... InitTypes>
   static StringMapEntry *Create(StringRef Key, AllocatorTy &Allocator,
-                                InitType &&InitVal) {
+                                InitTypes &&... InitVals) {
     unsigned KeyLength = Key.size();
 
     // Allocate a new item with space for the string at the end and a null
@@ -158,8 +158,9 @@
     StringMapEntry *NewItem =
       static_cast<StringMapEntry*>(Allocator.Allocate(AllocSize,Alignment));
 
-    // Default construct the value.
-    new (NewItem) StringMapEntry(KeyLength, std::forward<InitType>(InitVal));
+    // Construct the value.
+    new (NewItem)
+        StringMapEntry(KeyLength, std::forward<InitTypes>(InitVals)...);
 
     // Copy the string information.
     char *StrBuffer = const_cast<char*>(NewItem->getKeyData());
@@ -328,7 +329,16 @@
   /// if and only if the insertion takes place, and the iterator component of
   /// the pair points to the element with key equivalent to the key of the pair.
   std::pair<iterator, bool> insert(std::pair<StringRef, ValueTy> KV) {
-    unsigned BucketNo = LookupBucketFor(KV.first);
+    return emplace(KV.first, std::move(KV.second));
+  }
+
+  /// Emplace a new element for the specified key into the map if the key isn't
+  /// already in the map. The bool component of the returned pair is true
+  /// if and only if the insertion takes place, and the iterator component of
+  /// the pair points to the element with key equivalent to the key of the pair.
+  template <typename... ArgsTy>
+  std::pair<iterator, bool> emplace(StringRef Key, ArgsTy &&... Args) {
+    unsigned BucketNo = LookupBucketFor(Key);
     StringMapEntryBase *&Bucket = TheTable[BucketNo];
     if (Bucket && Bucket != getTombstoneVal())
       return std::make_pair(iterator(TheTable + BucketNo, false),
@@ -336,8 +346,7 @@
 
     if (Bucket == getTombstoneVal())
       --NumTombstones;
-    Bucket =
-        MapEntryTy::Create(KV.first, Allocator, std::move(KV.second));
+    Bucket = MapEntryTy::Create(Key, Allocator, std::forward<ArgsTy>(Args)...);
     ++NumItems;
     assert(NumItems + NumTombstones <= NumBuckets);
 
Index: include/llvm/IR/Metadata.h
===================================================================
--- include/llvm/IR/Metadata.h
+++ include/llvm/IR/Metadata.h
@@ -592,7 +592,7 @@
 
   StringMapEntry<MDString> *Entry;
   MDString() : Metadata(MDStringKind, Uniqued), Entry(nullptr) {}
-  MDString(MDString &&) : Metadata(MDStringKind, Uniqued) {}
+  MDString(MDString &&R) = default;
 
 public:
   static MDString *get(LLVMContext &Context, StringRef Str);
Index: lib/IR/Metadata.cpp
===================================================================
--- lib/IR/Metadata.cpp
+++ lib/IR/Metadata.cpp
@@ -397,17 +397,12 @@
 
 MDString *MDString::get(LLVMContext &Context, StringRef Str) {
   auto &Store = Context.pImpl->MDStringCache;
-  auto I = Store.find(Str);
-  if (I != Store.end())
-    return &I->second;
-
-  auto *Entry =
-      StringMapEntry<MDString>::Create(Str, Store.getAllocator(), MDString());
-  bool WasInserted = Store.insert(Entry);
-  (void)WasInserted;
-  assert(WasInserted && "Expected entry to be inserted");
-  Entry->second.Entry = Entry;
-  return &Entry->second;
+  auto I = Store.emplace(Str);
+  auto &MapEntry = I.first->getValue();
+  if (!I.second)
+    return &MapEntry;
+  MapEntry.Entry = &*I.first;
+  return &MapEntry;
 }
 
 StringRef MDString::getString() const {