Index: llvm/trunk/include/llvm/DebugInfo/CodeView/TypeSerializer.h
===================================================================
--- llvm/trunk/include/llvm/DebugInfo/CodeView/TypeSerializer.h
+++ llvm/trunk/include/llvm/DebugInfo/CodeView/TypeSerializer.h
@@ -17,7 +17,6 @@
 
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Error.h"
@@ -26,6 +25,8 @@
 
 namespace codeview {
 
+class TypeHasher;
+
 class TypeSerializer : public TypeVisitorCallbacks {
   struct SubRecord {
     SubRecord(TypeLeafKind K, uint32_t S) : Kind(K), Size(S) {}
@@ -45,14 +46,13 @@
     }
   };
 
-  typedef SmallVector<MutableArrayRef<uint8_t>, 2> RecordList;
+  typedef SmallVector<MutableArrayRef<uint8_t>, 2> MutableRecordList;
 
   static constexpr uint8_t ContinuationLength = 8;
   BumpPtrAllocator &RecordStorage;
   RecordSegment CurrentSegment;
-  RecordList FieldListSegments;
+  MutableRecordList FieldListSegments;
 
-  TypeIndex LastTypeIndex;
   Optional<TypeLeafKind> TypeKind;
   Optional<TypeLeafKind> MemberKind;
   std::vector<uint8_t> RecordBuffer;
@@ -60,28 +60,23 @@
   BinaryStreamWriter Writer;
   TypeRecordMapping Mapping;
 
-  RecordList SeenRecords;
-  StringMap<TypeIndex> HashedRecords;
+  /// Private type record hashing implementation details are handled here.
+  std::unique_ptr<TypeHasher> Hasher;
 
   bool isInFieldList() const;
-  TypeIndex calcNextTypeIndex() const;
-  TypeIndex incrementTypeIndex();
   MutableArrayRef<uint8_t> getCurrentSubRecordData();
   MutableArrayRef<uint8_t> getCurrentRecordData();
   Error writeRecordPrefix(TypeLeafKind Kind);
-  TypeIndex insertRecordBytesPrivate(MutableArrayRef<uint8_t> Record);
-  TypeIndex insertRecordBytesWithCopy(CVType &Record,
-                                      MutableArrayRef<uint8_t> Data);
 
   Expected<MutableArrayRef<uint8_t>>
   addPadding(MutableArrayRef<uint8_t> Record);
 
 public:
   explicit TypeSerializer(BumpPtrAllocator &Storage);
+  ~TypeSerializer();
 
-  ArrayRef<MutableArrayRef<uint8_t>> records() const;
-  TypeIndex getLastTypeIndex() const;
-  TypeIndex insertRecordBytes(MutableArrayRef<uint8_t> Record);
+  ArrayRef<ArrayRef<uint8_t>> records() const;
+  TypeIndex insertRecordBytes(ArrayRef<uint8_t> Record);
   Expected<TypeIndex> visitTypeEndGetIndex(CVType &Record);
 
   Error visitTypeBegin(CVType &Record) override;
Index: llvm/trunk/include/llvm/DebugInfo/CodeView/TypeTableBuilder.h
===================================================================
--- llvm/trunk/include/llvm/DebugInfo/CodeView/TypeTableBuilder.h
+++ llvm/trunk/include/llvm/DebugInfo/CodeView/TypeTableBuilder.h
@@ -64,7 +64,7 @@
     return *ExpectedIndex;
   }
 
-  TypeIndex writeSerializedRecord(MutableArrayRef<uint8_t> Record) {
+  TypeIndex writeSerializedRecord(ArrayRef<uint8_t> Record) {
     return Serializer.insertRecordBytes(Record);
   }
 
@@ -77,9 +77,7 @@
     }
   }
 
-  ArrayRef<MutableArrayRef<uint8_t>> records() const {
-    return Serializer.records();
-  }
+  ArrayRef<ArrayRef<uint8_t>> records() const { return Serializer.records(); }
 };
 
 class FieldListRecordBuilder {
Index: llvm/trunk/include/llvm/DebugInfo/CodeView/TypeTableCollection.h
===================================================================
--- llvm/trunk/include/llvm/DebugInfo/CodeView/TypeTableCollection.h
+++ llvm/trunk/include/llvm/DebugInfo/CodeView/TypeTableCollection.h
@@ -18,7 +18,7 @@
 
 class TypeTableCollection : public TypeCollection {
 public:
-  explicit TypeTableCollection(ArrayRef<MutableArrayRef<uint8_t>> Records);
+  explicit TypeTableCollection(ArrayRef<ArrayRef<uint8_t>> Records);
 
   Optional<TypeIndex> getFirst() override;
   Optional<TypeIndex> getNext(TypeIndex Prev) override;
@@ -33,7 +33,7 @@
   bool hasCapacityFor(TypeIndex Index) const;
   void ensureTypeExists(TypeIndex Index);
 
-  ArrayRef<MutableArrayRef<uint8_t>> Records;
+  ArrayRef<ArrayRef<uint8_t>> Records;
   TypeDatabase Database;
 };
 }
Index: llvm/trunk/lib/DebugInfo/CodeView/TypeSerializer.cpp
===================================================================
--- llvm/trunk/lib/DebugInfo/CodeView/TypeSerializer.cpp
+++ llvm/trunk/lib/DebugInfo/CodeView/TypeSerializer.cpp
@@ -9,6 +9,7 @@
 
 #include "llvm/DebugInfo/CodeView/TypeSerializer.h"
 
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/Support/BinaryStreamWriter.h"
 
 #include <string.h>
@@ -16,21 +17,116 @@
 using namespace llvm;
 using namespace llvm::codeview;
 
-bool TypeSerializer::isInFieldList() const {
-  return TypeKind.hasValue() && *TypeKind == TypeLeafKind::LF_FIELDLIST;
-}
+namespace {
+struct HashedType {
+  uint64_t Hash;
+  const uint8_t *Data;
+  unsigned Size; // FIXME: Go to uint16_t?
+  TypeIndex Index;
+};
+
+/// Wrapper around a poitner to a HashedType. Hash and equality operations are
+/// based on data in the pointee.
+struct HashedTypePtr {
+  HashedTypePtr() = default;
+  HashedTypePtr(HashedType *Ptr) : Ptr(Ptr) {}
+  HashedType *Ptr = nullptr;
+};
+} // namespace
+
+template <> struct DenseMapInfo<HashedTypePtr> {
+  static inline HashedTypePtr getEmptyKey() { return HashedTypePtr(nullptr); }
+  static inline HashedTypePtr getTombstoneKey() {
+    return HashedTypePtr(reinterpret_cast<HashedType *>(1));
+  }
+  static unsigned getHashValue(HashedTypePtr Val) {
+    assert(Val.Ptr != getEmptyKey().Ptr && Val.Ptr != getTombstoneKey().Ptr);
+    return Val.Ptr->Hash;
+  }
+  static bool isEqual(HashedTypePtr LHSP, HashedTypePtr RHSP) {
+    HashedType *LHS = LHSP.Ptr;
+    HashedType *RHS = RHSP.Ptr;
+    if (RHS == getEmptyKey().Ptr || RHS == getTombstoneKey().Ptr)
+      return LHS == RHS;
+    if (LHS->Hash != RHS->Hash || LHS->Size != RHS->Size)
+      return false;
+    return ::memcmp(LHS->Data, RHS->Data, LHS->Size) == 0;
+  }
+};
+
+/// Private implementation so that we don't leak our DenseMap instantiations to
+/// users.
+class llvm::codeview::TypeHasher {
+private:
+  /// Storage for type record provided by the caller. Records will outlive the
+  /// hasher object, so they should be allocated here.
+  BumpPtrAllocator &RecordStorage;
+
+  /// Storage for hash keys. These only need to live as long as the hashing
+  /// operation.
+  BumpPtrAllocator KeyStorage;
+
+  /// Hash table. We really want a DenseMap<ArrayRef<uint8_t>, TypeIndex> here,
+  /// but DenseMap is inefficient when the keys are long (like type records)
+  /// because it recomputes the hash value of every key when it grows. This
+  /// value type stores the hash out of line in KeyStorage, so that table
+  /// entries are small and easy to rehash.
+  DenseSet<HashedTypePtr> HashedRecords;
+
+  SmallVector<ArrayRef<uint8_t>, 2> SeenRecords;
+
+  TypeIndex NextTypeIndex = TypeIndex(TypeIndex::FirstNonSimpleIndex);
+
+public:
+  TypeHasher(BumpPtrAllocator &RecordStorage) : RecordStorage(RecordStorage) {}
+
+  ArrayRef<ArrayRef<uint8_t>> records() const { return SeenRecords; }
+
+  /// Takes the bytes of type record, inserts them into the hash table, saves
+  /// them, and returns a pointer to an identical stable type record along with
+  /// its type index in the destination stream.
+  TypeIndex getOrCreateRecord(ArrayRef<uint8_t> &Record);
+};
+
+TypeIndex TypeHasher::getOrCreateRecord(ArrayRef<uint8_t> &Record) {
+  assert(Record.size() < UINT32_MAX && "Record too big");
+  assert(Record.size() % 4 == 0 && "Record is not aligned to 4 bytes!");
+
+  // Compute the hash up front so we can store it in the key.
+  HashedType TempHashedType = {hash_value(Record), Record.data(),
+                               unsigned(Record.size()), NextTypeIndex};
+
+  auto Result = HashedRecords.insert(HashedTypePtr(&TempHashedType));
+  HashedType *&Hashed = Result.first->Ptr;
+
+  if (Result.second) {
+    // This was a new type record. We need stable storage for both the key and
+    // the record. The record should outlive the hashing operation.
+    Hashed = KeyStorage.Allocate<HashedType>();
+    *Hashed = TempHashedType;
+
+    uint8_t *Stable = RecordStorage.Allocate<uint8_t>(Record.size());
+    memcpy(Stable, Record.data(), Record.size());
+    Hashed->Data = Stable;
+    assert(Hashed->Size == Record.size());
+
+    // This was a new record, so increment our next type index.
+    ++NextTypeIndex;
+  }
+
+  // Update the caller's copy of Record to point a stable copy.
+  Record = ArrayRef<uint8_t>(Hashed->Data, Hashed->Size);
+
+  if (Result.second) {
+    // FIXME: Can we record these in a more efficient way?
+    SeenRecords.push_back(Record);
+  }
 
-TypeIndex TypeSerializer::calcNextTypeIndex() const {
-  if (LastTypeIndex.isNoneType())
-    return TypeIndex(TypeIndex::FirstNonSimpleIndex);
-  else
-    return TypeIndex(LastTypeIndex.getIndex() + 1);
+  return TypeIndex(Hashed->Index);
 }
 
-TypeIndex TypeSerializer::incrementTypeIndex() {
-  TypeIndex Previous = LastTypeIndex;
-  LastTypeIndex = calcNextTypeIndex();
-  return Previous;
+bool TypeSerializer::isInFieldList() const {
+  return TypeKind.hasValue() && *TypeKind == TypeLeafKind::LF_FIELDLIST;
 }
 
 MutableArrayRef<uint8_t> TypeSerializer::getCurrentSubRecordData() {
@@ -51,46 +147,6 @@
   return Error::success();
 }
 
-TypeIndex
-TypeSerializer::insertRecordBytesPrivate(MutableArrayRef<uint8_t> Record) {
-  assert(Record.size() % 4 == 0 && "Record is not aligned to 4 bytes!");
-
-  StringRef S(reinterpret_cast<const char *>(Record.data()), Record.size());
-
-  TypeIndex NextTypeIndex = calcNextTypeIndex();
-  auto Result = HashedRecords.try_emplace(S, NextTypeIndex);
-  if (Result.second) {
-    LastTypeIndex = NextTypeIndex;
-    SeenRecords.push_back(Record);
-  }
-  return Result.first->getValue();
-}
-
-TypeIndex
-TypeSerializer::insertRecordBytesWithCopy(CVType &Record,
-                                          MutableArrayRef<uint8_t> Data) {
-  assert(Data.size() % 4 == 0 && "Record is not aligned to 4 bytes!");
-
-  StringRef S(reinterpret_cast<const char *>(Data.data()), Data.size());
-
-  // Do a two state lookup / insert so that we don't have to allocate unless
-  // we're going
-  // to do an insert.  This is a big memory savings.
-  auto Iter = HashedRecords.find(S);
-  if (Iter != HashedRecords.end())
-    return Iter->second;
-
-  LastTypeIndex = calcNextTypeIndex();
-  uint8_t *Copy = RecordStorage.Allocate<uint8_t>(Data.size());
-  ::memcpy(Copy, Data.data(), Data.size());
-  Data = MutableArrayRef<uint8_t>(Copy, Data.size());
-  S = StringRef(reinterpret_cast<const char *>(Data.data()), Data.size());
-  HashedRecords.insert(std::make_pair(S, LastTypeIndex));
-  SeenRecords.push_back(Data);
-  Record.RecordData = Data;
-  return LastTypeIndex;
-}
-
 Expected<MutableArrayRef<uint8_t>>
 TypeSerializer::addPadding(MutableArrayRef<uint8_t> Record) {
   uint32_t Align = Record.size() % 4;
@@ -109,26 +165,25 @@
 }
 
 TypeSerializer::TypeSerializer(BumpPtrAllocator &Storage)
-    : RecordStorage(Storage), LastTypeIndex(),
-      RecordBuffer(MaxRecordLength * 2),
+    : RecordStorage(Storage), RecordBuffer(MaxRecordLength * 2),
       Stream(RecordBuffer, llvm::support::little), Writer(Stream),
-      Mapping(Writer) {
+      Mapping(Writer), Hasher(make_unique<TypeHasher>(Storage)) {
   // RecordBuffer needs to be able to hold enough data so that if we are 1
   // byte short of MaxRecordLen, and then we try to write MaxRecordLen bytes,
   // we won't overflow.
 }
 
-ArrayRef<MutableArrayRef<uint8_t>> TypeSerializer::records() const {
-  return SeenRecords;
-}
+TypeSerializer::~TypeSerializer() = default;
 
-TypeIndex TypeSerializer::getLastTypeIndex() const { return LastTypeIndex; }
+ArrayRef<ArrayRef<uint8_t>> TypeSerializer::records() const {
+  return Hasher->records();
+}
 
-TypeIndex TypeSerializer::insertRecordBytes(MutableArrayRef<uint8_t> Record) {
+TypeIndex TypeSerializer::insertRecordBytes(ArrayRef<uint8_t> Record) {
   assert(!TypeKind.hasValue() && "Already in a type mapping!");
   assert(Writer.getOffset() == 0 && "Stream has data already!");
 
-  return insertRecordBytesPrivate(Record);
+  return Hasher->getOrCreateRecord(Record);
 }
 
 Error TypeSerializer::visitTypeBegin(CVType &Record) {
@@ -163,8 +218,8 @@
   Prefix->RecordLen = ThisRecordData.size() - sizeof(uint16_t);
 
   Record.Type = *TypeKind;
-  TypeIndex InsertedTypeIndex =
-      insertRecordBytesWithCopy(Record, ThisRecordData);
+  Record.RecordData = ThisRecordData;
+  TypeIndex InsertedTypeIndex = Hasher->getOrCreateRecord(Record.RecordData);
 
   // Write out each additional segment in reverse order, and update each
   // record's continuation index to point to the previous one.
@@ -174,7 +229,7 @@
         reinterpret_cast<support::ulittle32_t *>(CIBytes.data());
     assert(*CI == 0xB0C0B0C0 && "Invalid TypeIndex placeholder");
     *CI = InsertedTypeIndex.getIndex();
-    InsertedTypeIndex = insertRecordBytesPrivate(X);
+    InsertedTypeIndex = Hasher->getOrCreateRecord(X);
   }
 
   TypeKind.reset();
Index: llvm/trunk/lib/DebugInfo/CodeView/TypeTableCollection.cpp
===================================================================
--- llvm/trunk/lib/DebugInfo/CodeView/TypeTableCollection.cpp
+++ llvm/trunk/lib/DebugInfo/CodeView/TypeTableCollection.cpp
@@ -24,8 +24,7 @@
     consumeError(std::move(EC));
 }
 
-TypeTableCollection::TypeTableCollection(
-    ArrayRef<MutableArrayRef<uint8_t>> Records)
+TypeTableCollection::TypeTableCollection(ArrayRef<ArrayRef<uint8_t>> Records)
     : Records(Records), Database(Records.size()) {}
 
 Optional<TypeIndex> TypeTableCollection::getFirst() {
Index: llvm/trunk/tools/llvm-pdbdump/llvm-pdbdump.cpp
===================================================================
--- llvm/trunk/tools/llvm-pdbdump/llvm-pdbdump.cpp
+++ llvm/trunk/tools/llvm-pdbdump/llvm-pdbdump.cpp
@@ -877,14 +877,12 @@
 
   auto &DestTpi = Builder.getTpiBuilder();
   auto &DestIpi = Builder.getIpiBuilder();
-  MergedTpi.ForEachRecord(
-      [&DestTpi](TypeIndex TI, MutableArrayRef<uint8_t> Data) {
-        DestTpi.addTypeRecord(Data, None);
-      });
-  MergedIpi.ForEachRecord(
-      [&DestIpi](TypeIndex TI, MutableArrayRef<uint8_t> Data) {
-        DestIpi.addTypeRecord(Data, None);
-      });
+  MergedTpi.ForEachRecord([&DestTpi](TypeIndex TI, ArrayRef<uint8_t> Data) {
+    DestTpi.addTypeRecord(Data, None);
+  });
+  MergedIpi.ForEachRecord([&DestIpi](TypeIndex TI, ArrayRef<uint8_t> Data) {
+    DestIpi.addTypeRecord(Data, None);
+  });
 
   SmallString<64> OutFile(opts::merge::PdbOutputFile);
   if (OutFile.empty()) {