diff --git a/llvm/include/llvm/ADT/HashMappedTrie.h b/llvm/include/llvm/ADT/HashMappedTrie.h
new file mode 100644
--- /dev/null
+++ b/llvm/include/llvm/ADT/HashMappedTrie.h
@@ -0,0 +1,339 @@
+//===- HashMappedTrie.h -----------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_HASHMAPPEDTRIE_H
+#define LLVM_ADT_HASHMAPPEDTRIE_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Casting.h"
+#include <atomic>
+
+namespace llvm {
+
+/// Base class for a lock-free thread-safe hash-mapped trie.
+class ThreadSafeHashMappedTrieBase {
+public:
+  enum : size_t { TrieContentBaseSize = 4 };
+
+private:
+  template <class T> struct AllocValueType {
+    char Base[TrieContentBaseSize];
+    std::aligned_union_t<sizeof(T), T> Content;
+  };
+
+protected:
+  template <class T> static constexpr size_t getContentAllocSize() {
+    return sizeof(AllocValueType<T>);
+  }
+  template <class T> static constexpr size_t getContentAllocAlign() {
+    return alignof(AllocValueType<T>);
+  }
+  template <class T> static constexpr size_t getContentOffset() {
+    return offsetof(AllocValueType<T>, Content);
+  }
+
+public:
+  void operator delete(void *Ptr) { ::free(Ptr); }
+
+  static constexpr size_t DefaultNumRootBits = 6;
+  static constexpr size_t DefaultNumSubtrieBits = 4;
+
+  LLVM_DUMP_METHOD void dump() const;
+  void print(raw_ostream &OS) const;
+
+protected:
+  /// Result of a lookup. Suitable for an insertion hint. Maybe could be
+  /// expanded into an iterator of sorts, but likely not useful (visiting
+  /// everything in the trie should probably be done some way other than
+  /// through an iterator pattern).
+  class PointerBase {
+  protected:
+    void *get() const { return I == -2u ? P : nullptr; }
+
+  public:
+    PointerBase() noexcept {}
+    PointerBase(PointerBase &&) = default;
+    PointerBase(const PointerBase &) = default;
+    PointerBase &operator=(PointerBase &&) = default;
+    PointerBase &operator=(const PointerBase &) = default;
+
+  private:
+    friend class ThreadSafeHashMappedTrieBase;
+    explicit PointerBase(void *Content) : P(Content), I(-2u) {}
+    PointerBase(void *P, unsigned I, unsigned B) : P(P), I(I), B(B) {}
+
+    bool isHint() const { return I != -1u && I != -2u; }
+
+    void *P = nullptr;
+    unsigned I = -1u;
+    unsigned B = 0;
+  };
+
+  PointerBase find(ArrayRef<uint8_t> Hash) const;
+
+  /// Insert and return the stored content.
+  PointerBase
+  insert(PointerBase Hint, ArrayRef<uint8_t> Hash,
+         function_ref<const uint8_t *(void *Mem, ArrayRef<uint8_t> Hash)>
+             Constructor);
+
+  ThreadSafeHashMappedTrieBase() = delete;
+
+  ThreadSafeHashMappedTrieBase(size_t ContentAllocSize,
+                               size_t ContentAllocAlign, size_t ContentOffset,
+                               Optional<size_t> NumRootBits = None,
+                               Optional<size_t> NumSubtrieBits = None);
+
+  /// Destructor, which asserts if there's anything to do. Subclasses should
+  /// call \a destroyImpl().
+  ///
+  /// \pre \a destroyImpl() was already called.
+  ~ThreadSafeHashMappedTrieBase();
+  void destroyImpl(function_ref<void(void *ValueMem)> Destructor);
+
+  ThreadSafeHashMappedTrieBase(ThreadSafeHashMappedTrieBase &&RHS);
+
+  // Move assignment can be implemented in a thread-safe way if NumRootBits and
+  // NumSubtrieBits are stored inside the Root.
+  ThreadSafeHashMappedTrieBase &
+  operator=(ThreadSafeHashMappedTrieBase &&RHS) = delete;
+
+  // No copy.
+  ThreadSafeHashMappedTrieBase(const ThreadSafeHashMappedTrieBase &) = delete;
+  ThreadSafeHashMappedTrieBase &
+  operator=(const ThreadSafeHashMappedTrieBase &) = delete;
+
+private:
+  const unsigned short ContentAllocSize;
+  const unsigned short ContentAllocAlign;
+  const unsigned short ContentOffset;
+  unsigned short NumRootBits;
+  unsigned short NumSubtrieBits;
+  struct ImplType;
+  // ImplPtr is owned by ThreadSafeHashMappedTrieBase and needs to be freed in
+  // destoryImpl.
+  std::atomic<ImplType *> ImplPtr;
+  ImplType &getOrCreateImpl();
+  ImplType *getImpl() const;
+};
+
+/// Lock-free thread-safe hash-mapped trie.
+template <class T, size_t NumHashBytes>
+class ThreadSafeHashMappedTrie : ThreadSafeHashMappedTrieBase {
+public:
+  using HashT = std::array<uint8_t, NumHashBytes>;
+
+  class LazyValueConstructor;
+  struct value_type {
+    const HashT Hash;
+    T Data;
+
+    value_type(value_type &&) = default;
+    value_type(const value_type &) = default;
+
+    value_type(ArrayRef<uint8_t> Hash, const T &Data)
+        : Hash(makeHash(Hash)), Data(Data) {}
+    value_type(ArrayRef<uint8_t> Hash, T &&Data)
+        : Hash(makeHash(Hash)), Data(std::move(Data)) {}
+
+  private:
+    friend class LazyValueConstructor;
+
+    struct EmplaceTag {};
+    template <class... ArgsT>
+    value_type(ArrayRef<uint8_t> Hash, EmplaceTag, ArgsT &&...Args)
+        : Hash(makeHash(Hash)), Data(std::forward<ArgsT>(Args)...) {}
+
+    static HashT makeHash(ArrayRef<uint8_t> HashRef) {
+      HashT Hash;
+      std::copy(HashRef.begin(), HashRef.end(), Hash.data());
+      return Hash;
+    }
+  };
+
+  using ThreadSafeHashMappedTrieBase::operator delete;
+  using HashType = HashT;
+
+  using ThreadSafeHashMappedTrieBase::dump;
+  using ThreadSafeHashMappedTrieBase::print;
+
+private:
+  template <class ValueT> class PointerImpl : PointerBase {
+    friend class ThreadSafeHashMappedTrie;
+
+    ValueT *get() const {
+      if (void *B = PointerBase::get())
+        return reinterpret_cast<ValueT *>(B);
+      return nullptr;
+    }
+
+  public:
+    ValueT &operator*() const {
+      assert(get());
+      return *get();
+    }
+    ValueT *operator->() const {
+      assert(get());
+      return get();
+    }
+    explicit operator bool() const { return get(); }
+
+    PointerImpl() = default;
+    PointerImpl(PointerImpl &&) = default;
+    PointerImpl(const PointerImpl &) = default;
+    PointerImpl &operator=(PointerImpl &&) = default;
+    PointerImpl &operator=(const PointerImpl &) = default;
+
+  protected:
+    PointerImpl(PointerBase Result) : PointerBase(Result) {}
+  };
+
+public:
+  class pointer;
+  class const_pointer;
+  class pointer : public PointerImpl<value_type> {
+    friend class ThreadSafeHashMappedTrie;
+    friend class const_pointer;
+
+  public:
+    pointer() = default;
+    pointer(pointer &&) = default;
+    pointer(const pointer &) = default;
+    pointer &operator=(pointer &&) = default;
+    pointer &operator=(const pointer &) = default;
+
+  private:
+    pointer(PointerBase Result) : pointer::PointerImpl(Result) {}
+  };
+
+  class const_pointer : public PointerImpl<const value_type> {
+    friend class ThreadSafeHashMappedTrie;
+
+  public:
+    const_pointer() = default;
+    const_pointer(const_pointer &&) = default;
+    const_pointer(const const_pointer &) = default;
+    const_pointer &operator=(const_pointer &&) = default;
+    const_pointer &operator=(const const_pointer &) = default;
+
+    const_pointer(const pointer &P) : const_pointer::PointerImpl(P) {}
+
+  private:
+    const_pointer(PointerBase Result) : const_pointer::PointerImpl(Result) {}
+  };
+
+  class LazyValueConstructor {
+  public:
+    value_type &operator()(T &&RHS) {
+      assert(Mem && "Constructor already called, or moved away");
+      return assign(::new (Mem) value_type(Hash, std::move(RHS)));
+    }
+    value_type &operator()(const T &RHS) {
+      assert(Mem && "Constructor already called, or moved away");
+      return assign(::new (Mem) value_type(Hash, RHS));
+    }
+    template <class... ArgsT> value_type &emplace(ArgsT &&...Args) {
+      assert(Mem && "Constructor already called, or moved away");
+      return assign(::new (Mem)
+                        value_type(Hash, typename value_type::EmplaceTag{},
+                                   std::forward<ArgsT>(Args)...));
+    }
+
+    LazyValueConstructor(LazyValueConstructor &&RHS)
+        : Mem(RHS.Mem), Result(RHS.Result), Hash(RHS.Hash) {
+      RHS.Mem = nullptr; // Moved away, cannot call.
+    }
+    ~LazyValueConstructor() { assert(!Mem && "Constructor never called!"); }
+
+  private:
+    value_type &assign(value_type *V) {
+      Mem = nullptr;
+      Result = V;
+      return *V;
+    }
+    friend class ThreadSafeHashMappedTrie;
+    LazyValueConstructor() = delete;
+    LazyValueConstructor(void *Mem, value_type *&Result, ArrayRef<uint8_t> Hash)
+        : Mem(Mem), Result(Result), Hash(Hash) {
+      assert(Hash.size() == sizeof(HashT) && "Invalid hash");
+      assert(Mem && "Invalid memory for construction");
+    }
+    void *Mem;
+    value_type *&Result;
+    ArrayRef<uint8_t> Hash;
+  };
+
+  /// Insert with a hint. Default-constructed hint will work, but it's
+  /// recommended to start with a lookup to avoid overhead in object creation
+  /// if it already exists.
+  pointer insertLazy(const_pointer Hint, ArrayRef<uint8_t> Hash,
+                     function_ref<void(LazyValueConstructor)> OnConstruct) {
+    return pointer(ThreadSafeHashMappedTrieBase::insert(
+        Hint, Hash, [&](void *Mem, ArrayRef<uint8_t> Hash) {
+          value_type *Result = nullptr;
+          OnConstruct(LazyValueConstructor(Mem, Result, Hash));
+          return Result->Hash.data();
+        }));
+  }
+
+  pointer insertLazy(ArrayRef<uint8_t> Hash,
+                     function_ref<void(LazyValueConstructor)> OnConstruct) {
+    return insertLazy(const_pointer(), Hash, OnConstruct);
+  }
+
+  pointer insert(const_pointer Hint, value_type &&HashedData) {
+    return insertLazy(Hint, HashedData.Hash, [&](LazyValueConstructor C) {
+      C(std::move(HashedData.Data));
+    });
+  }
+
+  pointer insert(const_pointer Hint, const value_type &HashedData) {
+    return insertLazy(Hint, HashedData.Hash,
+                      [&](LazyValueConstructor C) { C(HashedData.Data); });
+  }
+
+  pointer find(ArrayRef<uint8_t> Hash) {
+    assert(Hash.size() == std::tuple_size<HashT>::value);
+    return ThreadSafeHashMappedTrieBase::find(Hash);
+  }
+
+  const_pointer find(ArrayRef<uint8_t> Hash) const {
+    assert(Hash.size() == std::tuple_size<HashT>::value);
+    return ThreadSafeHashMappedTrieBase::find(Hash);
+  }
+
+  ThreadSafeHashMappedTrie(Optional<size_t> NumRootBits = None,
+                           Optional<size_t> NumSubtrieBits = None)
+      : ThreadSafeHashMappedTrieBase(getContentAllocSize<value_type>(),
+                                     getContentAllocAlign<value_type>(),
+                                     getContentOffset<value_type>(),
+                                     NumRootBits, NumSubtrieBits) {}
+
+  ~ThreadSafeHashMappedTrie() {
+    if (std::is_trivially_destructible<value_type>::value)
+      this->destroyImpl(nullptr);
+    else
+      this->destroyImpl(
+          [](void *P) { static_cast<value_type *>(P)->~value_type(); });
+  }
+
+  // Move constructor okay.
+  ThreadSafeHashMappedTrie(ThreadSafeHashMappedTrie &&) = default;
+
+  // No move assignment or any copy.
+  ThreadSafeHashMappedTrie &operator=(ThreadSafeHashMappedTrie &&) = delete;
+  ThreadSafeHashMappedTrie(const ThreadSafeHashMappedTrie &) = delete;
+  ThreadSafeHashMappedTrie &
+  operator=(const ThreadSafeHashMappedTrie &) = delete;
+};
+
+} // namespace llvm
+
+#endif // LLVM_ADT_HASHMAPPEDTRIE_H
diff --git a/llvm/lib/Support/CMakeLists.txt b/llvm/lib/Support/CMakeLists.txt
--- a/llvm/lib/Support/CMakeLists.txt
+++ b/llvm/lib/Support/CMakeLists.txt
@@ -176,6 +176,7 @@
   FormatVariadic.cpp
   GlobPattern.cpp
   GraphWriter.cpp
+  HashMappedTrie.cpp
   Hashing.cpp
   InitLLVM.cpp
   InstructionCost.cpp
diff --git a/llvm/lib/Support/HashMappedTrie.cpp b/llvm/lib/Support/HashMappedTrie.cpp
new file mode 100644
--- /dev/null
+++ b/llvm/lib/Support/HashMappedTrie.cpp
@@ -0,0 +1,476 @@
+//===- HashMappedTrie.cpp -------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/HashMappedTrie.h"
+#include "HashMappedTrieIndexGenerator.h"
+#include "llvm/ADT/LazyAtomicPointer.h"
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ThreadSafeAllocator.h"
+#include "llvm/Support/raw_ostream.h"
+#include <memory>
+
+using namespace llvm;
+
+namespace {
+struct TrieNode {
+  const bool IsSubtrie = false;
+
+  TrieNode(bool IsSubtrie) : IsSubtrie(IsSubtrie) {}
+
+  static void *operator new(size_t Size) { return ::malloc(Size); }
+  void operator delete(void *Ptr) { ::free(Ptr); }
+};
+
+struct TrieContent final : public TrieNode {
+  const uint8_t ContentOffset;
+  const uint8_t HashSize;
+  const uint8_t HashOffset;
+
+  void *getValuePointer() const {
+    auto Content = reinterpret_cast<const uint8_t *>(this) + ContentOffset;
+    return const_cast<uint8_t *>(Content);
+  }
+
+  ArrayRef<uint8_t> getHash() const {
+    auto *Begin = reinterpret_cast<const uint8_t *>(this) + HashOffset;
+    return makeArrayRef(Begin, Begin + HashSize);
+  }
+
+  TrieContent(size_t ContentOffset, size_t HashSize, size_t HashOffset)
+      : TrieNode(/*IsSubtrie=*/false), ContentOffset(ContentOffset),
+        HashSize(HashSize), HashOffset(HashOffset) {}
+};
+static_assert(sizeof(TrieContent) ==
+                  ThreadSafeHashMappedTrieBase::TrieContentBaseSize,
+              "Check header assumption!");
+
+class TrieSubtrie final : public TrieNode {
+public:
+  TrieNode *get(size_t I) const { return Slots[I].load(); }
+
+  TrieSubtrie *
+  sink(size_t I, TrieContent &Content, size_t NumSubtrieBits, size_t NewI,
+       function_ref<TrieSubtrie *(std::unique_ptr<TrieSubtrie>)> Saver);
+
+  void printHash(raw_ostream &OS, ArrayRef<uint8_t> Bytes) const;
+  void print(raw_ostream &OS) const { print(OS, None); }
+  void print(raw_ostream &OS, Optional<std::string> Prefix) const;
+  void dump() const { print(dbgs()); }
+
+  static std::unique_ptr<TrieSubtrie> create(size_t StartBit, size_t NumBits);
+
+  explicit TrieSubtrie(size_t StartBit, size_t NumBits);
+
+private:
+  // FIXME: Use a bitset to speed up access:
+  //
+  //     std::array<std::atomic<uint64_t>, NumSlots/64> IsSet;
+  //
+  // This will avoid needing to visit sparsely filled slots in
+  // \a ThreadSafeHashMappedTrieBase::destroyImpl() when there's a non-trivial
+  // destructor.
+  //
+  // It would also greatly speed up iteration, if we add that some day, and
+  // allow get() to return one level sooner.
+  //
+  // This would be the algorithm for updating IsSet (after updating Slots):
+  //
+  //     std::atomic<uint64_t> &Bits = IsSet[I.High];
+  //     const uint64_t NewBit = 1ULL << I.Low;
+  //     uint64_t Old = 0;
+  //     while (!Bits.compare_exchange_weak(Old, Old | NewBit))
+  //       ;
+
+  // For debugging.
+  unsigned StartBit = 0;
+  unsigned NumBits = 0;
+
+public:
+  /// Linked list for ownership of tries. The pointer is owned by TrieSubtrie.
+  std::atomic<TrieSubtrie *> Next;
+
+  /// The (co-allocated) slots of the subtrie.
+  MutableArrayRef<LazyAtomicPointer<TrieNode>> Slots;
+};
+} // end namespace
+
+namespace llvm {
+template <> struct isa_impl<TrieContent, TrieNode> {
+  static inline bool doit(const TrieNode &TN) { return !TN.IsSubtrie; }
+};
+template <> struct isa_impl<TrieSubtrie, TrieNode> {
+  static inline bool doit(const TrieNode &TN) { return TN.IsSubtrie; }
+};
+} // end namespace llvm
+
+static size_t getTrieTailSize(size_t StartBit, size_t NumBits) {
+  assert(NumBits < 20 && "Tries should have fewer than ~1M slots");
+  return sizeof(TrieNode *) * (1u << NumBits);
+}
+
+std::unique_ptr<TrieSubtrie> TrieSubtrie::create(size_t StartBit,
+                                                 size_t NumBits) {
+  size_t Size = sizeof(TrieSubtrie) + getTrieTailSize(StartBit, NumBits);
+  void *Memory = ::malloc(Size);
+  TrieSubtrie *S = ::new (Memory) TrieSubtrie(StartBit, NumBits);
+  return std::unique_ptr<TrieSubtrie>(S);
+}
+
+TrieSubtrie::TrieSubtrie(size_t StartBit, size_t NumBits)
+    : TrieNode(true), StartBit(StartBit), NumBits(NumBits), Next(nullptr),
+      Slots(reinterpret_cast<LazyAtomicPointer<TrieNode> *>(
+                reinterpret_cast<char *>(this) + sizeof(TrieSubtrie)),
+            (1u << NumBits)) {
+  for (auto *I = Slots.begin(), *E = Slots.end(); I != E; ++I)
+    new (I) LazyAtomicPointer<TrieNode>(nullptr);
+
+  static_assert(
+      std::is_trivially_destructible<LazyAtomicPointer<TrieNode>>::value,
+      "Expected no work in destructor for TrieNode");
+}
+
+TrieSubtrie *TrieSubtrie::sink(
+    size_t I, TrieContent &Content, size_t NumSubtrieBits, size_t NewI,
+    function_ref<TrieSubtrie *(std::unique_ptr<TrieSubtrie>)> Saver) {
+  assert(NumSubtrieBits > 0);
+  std::unique_ptr<TrieSubtrie> S = create(StartBit + NumBits, NumSubtrieBits);
+
+  assert(NewI < S->Slots.size());
+  S->Slots[NewI].store(&Content);
+
+  TrieNode *ExistingNode = &Content;
+  assert(I < Slots.size());
+  if (Slots[I].compare_exchange_strong(ExistingNode, S.get()))
+    return Saver(std::move(S));
+
+  // Another thread created a subtrie already. Return it and let "S" be
+  // destructed.
+  return cast<TrieSubtrie>(ExistingNode);
+}
+
+struct ThreadSafeHashMappedTrieBase::ImplType {
+  static ImplType *create(size_t StartBit, size_t NumBits) {
+    size_t Size = sizeof(ImplType) + getTrieTailSize(StartBit, NumBits);
+    void *Memory = ::malloc(Size);
+    return ::new (Memory) ImplType(StartBit, NumBits);
+  }
+
+  TrieSubtrie *save(std::unique_ptr<TrieSubtrie> S) {
+    assert(!S->Next && "Expected S to a freshly-constructed leaf");
+
+    TrieSubtrie *CurrentHead = nullptr;
+    // Add ownership of "S" to front of the list, so that Root -> S ->
+    // Root.Next. This works by repeatedly setting S->Next to a candidate value
+    // of Root.Next (initially nullptr), then setting Root.Next to S once the
+    // candidate matches reality.
+    while (!Root.Next.compare_exchange_weak(CurrentHead, S.get()))
+      S->Next.exchange(CurrentHead);
+
+    // Ownership transferred to subtrie.
+    return S.release();
+  }
+
+  static void *operator new(size_t Size) { return ::malloc(Size); }
+  void operator delete(void *Ptr) { ::free(Ptr); }
+
+  /// FIXME: This should take a function that allocates and constructs the
+  /// content lazily (taking the hash as a separate parameter), in case of
+  /// collision.
+  ThreadSafeAllocator<BumpPtrAllocator> ContentAlloc;
+  TrieSubtrie Root; // Must be last! Tail-allocated.
+
+private:
+  ImplType(size_t StartBit, size_t NumBits) : Root(StartBit, NumBits) {}
+};
+
+ThreadSafeHashMappedTrieBase::ImplType &
+ThreadSafeHashMappedTrieBase::getOrCreateImpl() {
+  if (ImplType *Impl = ImplPtr.load())
+    return *Impl;
+
+  // Create a new ImplType and store it if another thread doesn't do so first.
+  // If another thread wins this one is destroyed locally.
+  std::unique_ptr<ImplType> Impl(ImplType::create(0, NumRootBits));
+  ImplType *ExistingImpl = nullptr;
+  if (ImplPtr.compare_exchange_strong(ExistingImpl, Impl.get()))
+    return *Impl.release();
+
+  return *ExistingImpl;
+}
+
+ThreadSafeHashMappedTrieBase::PointerBase
+ThreadSafeHashMappedTrieBase::find(ArrayRef<uint8_t> Hash) const {
+  assert(!Hash.empty() && "Uninitialized hash");
+
+  ImplType *Impl = ImplPtr.load();
+  if (!Impl)
+    return PointerBase();
+
+  TrieSubtrie *S = &Impl->Root;
+  IndexGenerator IndexGen{NumRootBits, NumSubtrieBits, Hash};
+  size_t Index = IndexGen.next();
+  for (;;) {
+    // Try to set the content.
+    TrieNode *Existing = S->get(Index);
+    if (!Existing)
+      return PointerBase(S, Index, *IndexGen.StartBit);
+
+    // Check for an exact match.
+    if (auto *ExistingContent = dyn_cast<TrieContent>(Existing))
+      return ExistingContent->getHash() == Hash
+                 ? PointerBase(ExistingContent->getValuePointer())
+                 : PointerBase(S, Index, *IndexGen.StartBit);
+
+    Index = IndexGen.next();
+    S = cast<TrieSubtrie>(Existing);
+  }
+}
+
+ThreadSafeHashMappedTrieBase::PointerBase ThreadSafeHashMappedTrieBase::insert(
+    PointerBase Hint, ArrayRef<uint8_t> Hash,
+    function_ref<const uint8_t *(void *Mem, ArrayRef<uint8_t> Hash)>
+        Constructor) {
+  assert(!Hash.empty() && "Uninitialized hash");
+
+  ImplType &Impl = getOrCreateImpl();
+  TrieSubtrie *S = &Impl.Root;
+  IndexGenerator IndexGen{NumRootBits, NumSubtrieBits, Hash};
+  size_t Index;
+  if (Hint.isHint()) {
+    S = static_cast<TrieSubtrie *>(Hint.P);
+    Index = IndexGen.hint(Hint.I, Hint.B);
+  } else {
+    Index = IndexGen.next();
+  }
+
+  for (;;) {
+    // Load the node from the slot, allocating and calling the constructor if
+    // the slot is empty.
+    bool Generated = false;
+    TrieNode &Existing = S->Slots[Index].loadOrGenerate([&]() {
+      Generated = true;
+
+      // Construct the value itself at the tail.
+      uint8_t *Memory = reinterpret_cast<uint8_t *>(
+          Impl.ContentAlloc.Allocate(ContentAllocSize, ContentAllocAlign));
+      const uint8_t *HashStorage = Constructor(Memory + ContentOffset, Hash);
+
+      // Construct the TrieContent header, passing in the offset to the hash.
+      TrieContent *Content = ::new (Memory)
+          TrieContent(ContentOffset, Hash.size(), HashStorage - Memory);
+      assert(Hash == Content->getHash() && "Hash not properly initialized");
+      return Content;
+    });
+    // If we just generated it, return it!
+    if (Generated)
+      return PointerBase(cast<TrieContent>(Existing).getValuePointer());
+
+    if (isa<TrieSubtrie>(Existing)) {
+      S = &cast<TrieSubtrie>(Existing);
+      Index = IndexGen.next();
+      continue;
+    }
+
+    // Return the existing content if it's an exact match!
+    auto &ExistingContent = cast<TrieContent>(Existing);
+    if (ExistingContent.getHash() == Hash)
+      return PointerBase(ExistingContent.getValuePointer());
+
+    // Sink the existing content as long as the indexes match.
+    for (;;) {
+      size_t NextIndex = IndexGen.next();
+      size_t NewIndexForExistingContent =
+          IndexGen.getCollidingBits(ExistingContent.getHash());
+      S = S->sink(Index, ExistingContent, IndexGen.getNumBits(),
+                  NewIndexForExistingContent,
+                  [&Impl](std::unique_ptr<TrieSubtrie> S) {
+                    return Impl.save(std::move(S));
+                  });
+      Index = NextIndex;
+
+      // Found the difference.
+      if (NextIndex != NewIndexForExistingContent)
+        break;
+    }
+  }
+}
+
+static void printHexDigit(raw_ostream &OS, uint8_t Digit) {
+  if (Digit < 10)
+    OS << char(Digit + '0');
+  else
+    OS << char(Digit - 10 + 'a');
+}
+
+static void printHexDigits(raw_ostream &OS, ArrayRef<uint8_t> Bytes,
+                           size_t StartBit, size_t NumBits) {
+  assert(StartBit % 4 == 0);
+  assert(NumBits % 4 == 0);
+  for (size_t I = StartBit, E = StartBit + NumBits; I != E; I += 4) {
+    uint8_t HexPair = Bytes[I / 8];
+    uint8_t HexDigit = I % 8 == 0 ? HexPair >> 4 : HexPair & 0xf;
+    printHexDigit(OS, HexDigit);
+  }
+}
+
+static void printBits(raw_ostream &OS, ArrayRef<uint8_t> Bytes, size_t StartBit,
+                      size_t NumBits) {
+  assert(StartBit + NumBits <= Bytes.size() * 8u);
+  for (size_t I = StartBit, E = StartBit + NumBits; I != E; ++I) {
+    uint8_t Byte = Bytes[I / 8];
+    size_t ByteOffset = I % 8;
+    if (size_t ByteShift = 8 - ByteOffset - 1)
+      Byte >>= ByteShift;
+    OS << (Byte & 0x1 ? '1' : '0');
+  }
+}
+
+void TrieSubtrie::printHash(raw_ostream &OS, ArrayRef<uint8_t> Bytes) const {
+  // afb[1c:00*01110*0]def
+  size_t EndBit = StartBit + NumBits;
+  size_t HashEndBit = Bytes.size() * 8u;
+
+  size_t FirstBinaryBit = StartBit & ~0x3u;
+  printHexDigits(OS, Bytes, 0, FirstBinaryBit);
+
+  size_t LastBinaryBit = (EndBit + 3u) & ~0x3u;
+  OS << "[";
+  printBits(OS, Bytes, FirstBinaryBit, LastBinaryBit - FirstBinaryBit);
+  OS << "]";
+
+  printHexDigits(OS, Bytes, LastBinaryBit, HashEndBit - LastBinaryBit);
+}
+
+static void appendIndexBits(std::string &Prefix, size_t Index,
+                            size_t NumSlots) {
+  std::string Bits;
+  for (size_t NumBits = 1u; NumBits < NumSlots; NumBits <<= 1) {
+    Bits.push_back('0' + (Index & 0x1));
+    Index >>= 1;
+  }
+  for (char Ch : llvm::reverse(Bits))
+    Prefix += Ch;
+}
+
+static void printPrefix(raw_ostream &OS, StringRef Prefix) {
+  while (Prefix.size() >= 4) {
+    uint8_t Digit;
+    bool ErrorParsingBinary = Prefix.take_front(4).getAsInteger(2, Digit);
+    assert(!ErrorParsingBinary);
+    (void)ErrorParsingBinary;
+    printHexDigit(OS, Digit);
+    Prefix = Prefix.drop_front(4);
+  }
+  if (!Prefix.empty())
+    OS << "[" << Prefix << "]";
+}
+
+void TrieSubtrie::print(raw_ostream &OS, Optional<std::string> Prefix) const {
+  if (!Prefix) {
+    OS << "root";
+    Prefix.emplace();
+  } else {
+    OS << "subtrie=";
+    printPrefix(OS, *Prefix);
+  }
+
+  OS << " num-slots=" << Slots.size() << "\n";
+  SmallVector<TrieSubtrie *> Subs;
+  SmallVector<std::string> Prefixes;
+  for (size_t I = 0, E = Slots.size(); I != E; ++I) {
+    TrieNode *N = get(I);
+    if (!N)
+      continue;
+    OS << "- index=" << I << " ";
+    if (auto *S = dyn_cast<TrieSubtrie>(N)) {
+      std::string SubtriePrefix = *Prefix;
+      appendIndexBits(SubtriePrefix, I, Slots.size());
+      OS << "subtrie=";
+      printPrefix(OS, SubtriePrefix);
+      OS << "\n";
+      Subs.push_back(S);
+      Prefixes.push_back(SubtriePrefix);
+      continue;
+    }
+    auto *Content = cast<TrieContent>(N);
+    OS << "content=";
+    printHash(OS, Content->getHash());
+    OS << "\n";
+  }
+  for (size_t I = 0, E = Subs.size(); I != E; ++I)
+    Subs[I]->print(OS, Prefixes[I]);
+}
+
+void ThreadSafeHashMappedTrieBase::print(raw_ostream &OS) const {
+  OS << "root-bits=" << NumRootBits << " subtrie-bits=" << NumSubtrieBits
+     << "\n";
+  if (ImplType *Impl = ImplPtr.load())
+    Impl->Root.print(OS);
+  else
+    OS << "[no-root]\n";
+}
+
+LLVM_DUMP_METHOD void ThreadSafeHashMappedTrieBase::dump() const {
+  print(dbgs());
+}
+
+ThreadSafeHashMappedTrieBase::ThreadSafeHashMappedTrieBase(
+    size_t ContentAllocSize, size_t ContentAllocAlign, size_t ContentOffset,
+    Optional<size_t> NumRootBits, Optional<size_t> NumSubtrieBits)
+    : ContentAllocSize(ContentAllocSize), ContentAllocAlign(ContentAllocAlign),
+      ContentOffset(ContentOffset),
+      NumRootBits(NumRootBits ? *NumRootBits : DefaultNumRootBits),
+      NumSubtrieBits(NumSubtrieBits ? *NumSubtrieBits : DefaultNumSubtrieBits),
+      ImplPtr(nullptr) {
+  assert((!NumRootBits || *NumRootBits < 20) &&
+         "Root should have fewer than ~1M slots");
+  assert((!NumSubtrieBits || *NumSubtrieBits < 10) &&
+         "Subtries should have fewer than ~1K slots");
+}
+
+ThreadSafeHashMappedTrieBase::ThreadSafeHashMappedTrieBase(
+    ThreadSafeHashMappedTrieBase &&RHS)
+    : ContentAllocSize(RHS.ContentAllocSize),
+      ContentAllocAlign(RHS.ContentAllocAlign),
+      ContentOffset(RHS.ContentOffset), NumRootBits(RHS.NumRootBits),
+      NumSubtrieBits(RHS.NumSubtrieBits) {
+  // Steal the root from RHS.
+  ImplPtr = RHS.ImplPtr.exchange(nullptr);
+}
+
+ThreadSafeHashMappedTrieBase::~ThreadSafeHashMappedTrieBase() {
+  assert(!ImplPtr.load() && "Expected subclass to call destroyImpl()");
+}
+
+void ThreadSafeHashMappedTrieBase::destroyImpl(
+    function_ref<void(void *)> Destructor) {
+  std::unique_ptr<ImplType> Impl(ImplPtr.exchange(nullptr));
+  if (!Impl)
+    return;
+
+  // Destroy content nodes throughout trie. Avoid destroying any subtries since
+  // we need TrieNode::classof() to find the content nodes.
+  //
+  // FIXME: Once we have bitsets (see FIXME in TrieSubtrie class), use them
+  // facilitate sparse iteration here.
+  if (Destructor)
+    for (TrieSubtrie *Trie = &Impl->Root; Trie; Trie = Trie->Next.load())
+      for (auto &Slot : Trie->Slots)
+        if (auto *Content = dyn_cast_or_null<TrieContent>(Slot.load()))
+          Destructor(Content->getValuePointer());
+
+  // Destroy the subtries. Incidentally, this destroys them in the reverse order
+  // of saving.
+  TrieSubtrie *Trie = Impl->Root.Next;
+  while (Trie) {
+    TrieSubtrie *Next = Trie->Next.exchange(nullptr);
+    delete Trie;
+    Trie = Next;
+  }
+}
diff --git a/llvm/lib/Support/HashMappedTrieIndexGenerator.h b/llvm/lib/Support/HashMappedTrieIndexGenerator.h
new file mode 100644
--- /dev/null
+++ b/llvm/lib/Support/HashMappedTrieIndexGenerator.h
@@ -0,0 +1,89 @@
+//===- HashMappedTrieIndexGenerator.h ---------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_SUPPORT_HASHMAPPEDTRIEINDEXGENERATOR_H
+#define LLVM_LIB_SUPPORT_HASHMAPPEDTRIEINDEXGENERATOR_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/Optional.h"
+
+namespace llvm {
+
+struct IndexGenerator {
+  size_t NumRootBits;
+  size_t NumSubtrieBits;
+  ArrayRef<uint8_t> Bytes;
+  Optional<size_t> StartBit = None;
+
+  size_t getNumBits() const {
+    assert(StartBit);
+    size_t TotalNumBits = Bytes.size() * 8;
+    assert(*StartBit <= TotalNumBits);
+    return std::min(*StartBit ? NumSubtrieBits : NumRootBits,
+                    TotalNumBits - *StartBit);
+  }
+  size_t next() {
+    size_t Index;
+    if (!StartBit) {
+      StartBit = 0;
+      Index = getIndex(Bytes, *StartBit, NumRootBits);
+    } else {
+      *StartBit += *StartBit ? NumSubtrieBits : NumRootBits;
+      assert((*StartBit - NumRootBits) % NumSubtrieBits == 0);
+      Index = getIndex(Bytes, *StartBit, NumSubtrieBits);
+    }
+    return Index;
+  }
+
+  size_t hint(unsigned Index, unsigned Bit) {
+    assert(Index >= 0);
+    assert(Bit < Bytes.size() * 8);
+    assert(Bit == 0 || (Bit - NumRootBits) % NumSubtrieBits == 0);
+    StartBit = Bit;
+    return Index;
+  }
+
+  size_t getCollidingBits(ArrayRef<uint8_t> CollidingBits) const {
+    assert(StartBit);
+    return getIndex(CollidingBits, *StartBit, NumSubtrieBits);
+  }
+
+  static size_t getIndex(ArrayRef<uint8_t> Bytes, size_t StartBit,
+                         size_t NumBits) {
+    assert(StartBit < Bytes.size() * 8);
+
+    Bytes = Bytes.drop_front(StartBit / 8u);
+    StartBit %= 8u;
+    size_t Index = 0;
+    for (uint8_t Byte : Bytes) {
+      size_t ByteStart = 0, ByteEnd = 8;
+      if (StartBit) {
+        ByteStart = StartBit;
+        Byte &= (1u << (8 - StartBit)) - 1u;
+        StartBit = 0;
+      }
+      size_t CurrentNumBits = ByteEnd - ByteStart;
+      if (CurrentNumBits > NumBits) {
+        Byte >>= CurrentNumBits - NumBits;
+        CurrentNumBits = NumBits;
+      }
+      Index <<= CurrentNumBits;
+      Index |= Byte & ((1u << CurrentNumBits) - 1u);
+
+      assert(NumBits >= CurrentNumBits);
+      NumBits -= CurrentNumBits;
+      if (!NumBits)
+        break;
+    }
+    return Index;
+  }
+};
+
+} // namespace llvm
+
+#endif // LLVM_LIB_SUPPORT_HASHMAPPEDTRIEINDEXGENERATOR_H
diff --git a/llvm/unittests/ADT/CMakeLists.txt b/llvm/unittests/ADT/CMakeLists.txt
--- a/llvm/unittests/ADT/CMakeLists.txt
+++ b/llvm/unittests/ADT/CMakeLists.txt
@@ -31,6 +31,7 @@
   FoldingSet.cpp
   FunctionExtrasTest.cpp
   FunctionRefTest.cpp
+  HashMappedTrieTest.cpp
   HashingTest.cpp
   IListBaseTest.cpp
   IListIteratorTest.cpp
diff --git a/llvm/unittests/ADT/HashMappedTrieTest.cpp b/llvm/unittests/ADT/HashMappedTrieTest.cpp
new file mode 100644
--- /dev/null
+++ b/llvm/unittests/ADT/HashMappedTrieTest.cpp
@@ -0,0 +1,335 @@
+//===- HashMappedTrieTest.cpp ---------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/HashMappedTrie.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/Support/Endian.h"
+#include "llvm/Support/SHA1.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+static StringRef takeNextLine(StringRef &Lines) {
+  size_t Newline = Lines.find('\n');
+  StringRef Line = Lines.take_front(Newline);
+  Lines = Lines.drop_front(Newline + 1);
+  return Line;
+}
+
+namespace {
+
+TEST(HashMappedTrieTest, TrieStructure) {
+  using NumType = uint64_t;
+  using HashType = std::array<uint8_t, sizeof(NumType)>;
+  using TrieType = ThreadSafeHashMappedTrie<NumType, sizeof(HashType)>;
+  NumType Numbers[] = {
+      // Three numbers that will nest deeply to test (1) sinking subtries and
+      // (2) deep, non-trivial hints.
+      std::numeric_limits<NumType>::max(),
+      std::numeric_limits<NumType>::max() - 2u,
+      std::numeric_limits<NumType>::max() - 3u,
+      // One number to stay at the top-level.
+      0x37,
+  };
+
+  // Use the number itself as hash to test the pathological case.
+  auto hash = [](NumType Num) {
+    NumType HashN = llvm::support::endian::byte_swap(Num, llvm::support::big);
+    HashType Hash;
+    memcpy(&Hash[0], &HashN, sizeof(HashType));
+    return Hash;
+  };
+
+  // Use root and subtrie sizes of 1 so this gets sunk quite deep.
+  TrieType Trie(1, 1);
+  for (NumType N : Numbers) {
+    // Lookup first to exercise hint code for deep tries.
+    TrieType::pointer Lookup = Trie.find(hash(N));
+    EXPECT_FALSE(Lookup);
+
+    Trie.insert(Lookup, TrieType::value_type(hash(N), N));
+  }
+  for (NumType N : Numbers) {
+    TrieType::pointer Lookup = Trie.find(hash(N));
+    EXPECT_TRUE(Lookup);
+    if (!Lookup)
+      continue;
+    EXPECT_EQ(hash(N), Lookup->Hash);
+    EXPECT_EQ(N, Lookup->Data);
+
+    // Confirm a subsequent insertion fails to overwrite by trying to insert a
+    // bad value.
+    EXPECT_EQ(N,
+              Trie.insert(Lookup, TrieType::value_type(hash(N), N - 1))->Data);
+  }
+
+  // Dump out the trie so we can confirm the structure is correct. Each subtrie
+  // should have 2 slots. The root's index=0 should have the content for
+  // 0x37 directly, and index=1 should be a linked-list of subtries, finally
+  // ending with content for (max-2) and (max-3).
+  //
+  // Note: This structure is not exhaustive (too expensive to update tests),
+  // but it does test that the dump format is somewhat readable and that the
+  // basic structure is correct.
+  //
+  // Note: This test requires that the trie reads bytes starting from index 0
+  // of the array of uint8_t, and then reads each byte's bits from high to low.
+  SmallString<128> Dump;
+  {
+    raw_svector_ostream OS(Dump);
+    Trie.print(OS);
+  }
+
+  // Check the header.
+  StringRef DumpRef = Dump;
+  ASSERT_EQ("root-bits=1 subtrie-bits=1", takeNextLine(DumpRef));
+
+  // Check the root trie.
+  ASSERT_EQ("root num-slots=2", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=0 content=[0000]000000000000037", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=1 subtrie=[1]", takeNextLine(DumpRef));
+  ASSERT_EQ("subtrie=[1] num-slots=2", takeNextLine(DumpRef));
+
+  // Check the last subtrie.
+  size_t LastSubtrie = DumpRef.rfind("\nsubtrie=");
+  ASSERT_NE(StringRef::npos, LastSubtrie);
+  DumpRef = DumpRef.substr(LastSubtrie + 1);
+  ASSERT_EQ("subtrie=fffffffffffffff[110] num-slots=2", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=0 content=fffffffffffffff[1100]", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=1 content=fffffffffffffff[1101]", takeNextLine(DumpRef));
+  ASSERT_TRUE(DumpRef.empty());
+}
+
+TEST(HashMappedTrieTest, TrieStructureSmallFinalSubtrie) {
+  using NumType = uint64_t;
+  using HashType = std::array<uint8_t, sizeof(NumType)>;
+  using TrieType = ThreadSafeHashMappedTrie<NumType, sizeof(HashType)>;
+  NumType Numbers[] = {
+      // Three numbers that will nest deeply to test (1) sinking subtries and
+      // (2) deep, non-trivial hints.
+      std::numeric_limits<NumType>::max(),
+      std::numeric_limits<NumType>::max() - 2u,
+      std::numeric_limits<NumType>::max() - 3u,
+      // One number to stay at the top-level.
+      0x37,
+  };
+
+  // Use the number itself as hash to test the pathological case.
+  auto hash = [](NumType Num) {
+    NumType HashN = llvm::support::endian::byte_swap(Num, llvm::support::big);
+    HashType Hash;
+    memcpy(&Hash[0], &HashN, sizeof(HashType));
+    return Hash;
+  };
+
+  // Use subtrie size of 7 to avoid hitting 64 evenly, making the final subtrie
+  // small.
+  TrieType Trie(8, 5);
+  for (NumType N : Numbers) {
+    // Lookup first to exercise hint code for deep tries.
+    TrieType::pointer Lookup = Trie.find(hash(N));
+    EXPECT_FALSE(Lookup);
+
+    Trie.insert(Lookup, TrieType::value_type(hash(N), N));
+  }
+  for (NumType N : Numbers) {
+    TrieType::pointer Lookup = Trie.find(hash(N));
+    EXPECT_TRUE(Lookup);
+    if (!Lookup)
+      continue;
+    EXPECT_EQ(hash(N), Lookup->Hash);
+    EXPECT_EQ(N, Lookup->Data);
+
+    // Confirm a subsequent insertion fails to overwrite by trying to insert a
+    // bad value.
+    EXPECT_EQ(N,
+              Trie.insert(Lookup, TrieType::value_type(hash(N), N - 1))->Data);
+  }
+
+  // Dump out the trie so we can confirm the structure is correct. The root
+  // should have 2^8=256 slots, most subtries should have 2^5=32 slots, and the
+  // deepest subtrie should have 2^1=2 slots (since (64-8)mod(5)=1).
+  // should have 2 slots. The root's index=0 should have the content for
+  // 0x37 directly, and index=1 should be a linked-list of subtries, finally
+  // ending with content for (max-2) and (max-3).
+  //
+  // Note: This structure is not exhaustive (too expensive to update tests),
+  // but it does test that the dump format is somewhat readable and that the
+  // basic structure is correct.
+  //
+  // Note: This test requires that the trie reads bytes starting from index 0
+  // of the array of uint8_t, and then reads each byte's bits from high to low.
+  SmallString<128> Dump;
+  {
+    raw_svector_ostream OS(Dump);
+    Trie.print(OS);
+  }
+
+  // Check the header.
+  StringRef DumpRef = Dump;
+  ASSERT_EQ("root-bits=8 subtrie-bits=5", takeNextLine(DumpRef));
+
+  // Check the root trie.
+  ASSERT_EQ("root num-slots=256", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=0 content=[00000000]00000000000037",
+            takeNextLine(DumpRef));
+  ASSERT_EQ("- index=255 subtrie=ff", takeNextLine(DumpRef));
+  ASSERT_EQ("subtrie=ff num-slots=32", takeNextLine(DumpRef));
+
+  // Check the last subtrie.
+  size_t LastSubtrie = DumpRef.rfind("\nsubtrie=");
+  ASSERT_NE(StringRef::npos, LastSubtrie);
+  DumpRef = DumpRef.substr(LastSubtrie + 1);
+  ASSERT_EQ("subtrie=fffffffffffffff[110] num-slots=2", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=0 content=fffffffffffffff[1100]", takeNextLine(DumpRef));
+  ASSERT_EQ("- index=1 content=fffffffffffffff[1101]", takeNextLine(DumpRef));
+  ASSERT_TRUE(DumpRef.empty());
+}
+
+TEST(HashMappedTrieTest, TrieDestructionLoop) {
+  using NumT = uint64_t;
+  struct NumWithDestructorT {
+    NumT Num;
+    operator NumT() const { return Num; }
+    ~NumWithDestructorT() {}
+  };
+
+  using HashT = std::array<uint8_t, sizeof(NumT)>;
+  using TrieT = ThreadSafeHashMappedTrie<NumT, sizeof(HashT)>;
+  using TrieWithDestructorT =
+      ThreadSafeHashMappedTrie<NumWithDestructorT, sizeof(HashT)>;
+
+  // Use the number itself in big-endian order as the hash.
+  auto hash = [](NumT Num) {
+    NumT HashN = llvm::support::endian::byte_swap(Num, llvm::support::big);
+    HashT Hash;
+    memcpy(&Hash[0], &HashN, sizeof(HashT));
+    return Hash;
+  };
+
+  // Use optionals to control when destructors are called.
+  Optional<TrieT> Trie;
+  Optional<TrieWithDestructorT> TrieWithDestructor;
+
+  // Limit the tries to 2 slots (1 bit) to generate subtries at a higher rate.
+  Trie.emplace(/*NumRootBits=*/1, /*NumSubtrieBits=*/1);
+  TrieWithDestructor.emplace(/*NumRootBits=*/1, /*NumSubtrieBits=*/1);
+
+  // Fill them up. Pick a MaxN high enough to cause a stack overflow in debug
+  // builds.
+  static constexpr uint64_t MaxN = 100000;
+  for (uint64_t N = 0; N != MaxN; ++N) {
+    HashT Hash = hash(N);
+    Trie->insert(TrieT::pointer(), TrieT::value_type(Hash, N));
+    TrieWithDestructor->insert(
+        TrieWithDestructorT::pointer(),
+        TrieWithDestructorT::value_type(Hash, NumWithDestructorT{N}));
+  }
+
+  // Destroy tries. If destruction is recursive and MaxN is high enough, these
+  // will both fail.
+  Trie.reset();
+  TrieWithDestructor.reset();
+}
+
+namespace {
+using HasherT = SHA1;
+using HashType = decltype(HasherT::hash(std::declval<ArrayRef<uint8_t> &>()));
+template <class T>
+class ThreadSafeHashMappedTrieSet
+    : ThreadSafeHashMappedTrie<T, sizeof(HashType)> {
+public:
+  using TrieType =
+      typename ThreadSafeHashMappedTrieSet::ThreadSafeHashMappedTrie;
+  using LazyValueConstructor = typename ThreadSafeHashMappedTrieSet::
+      ThreadSafeHashMappedTrie::LazyValueConstructor;
+
+  class pointer : public TrieType::const_pointer {
+    using BaseType = typename TrieType::const_pointer;
+
+  public:
+    const T &operator*() const {
+      return TrieType::const_pointer::operator*().Data;
+    }
+    const T *operator->() const { return &operator*(); }
+
+    pointer() = default;
+    pointer(pointer &&) = default;
+    pointer(const pointer &) = default;
+    pointer &operator=(pointer &&) = default;
+    pointer &operator=(const pointer &) = default;
+
+  private:
+    pointer(BaseType Result) : BaseType(Result) {}
+    friend class ThreadSafeHashMappedTrieSet;
+  };
+
+  ThreadSafeHashMappedTrieSet(Optional<size_t> NumRootBits = None,
+                              Optional<size_t> NumSubtrieBits = None)
+      : TrieType(NumRootBits, NumSubtrieBits) {}
+
+  static HashType hash(const T &V) {
+    return HasherT::hash(ArrayRef<uint8_t>(
+        reinterpret_cast<const uint8_t *>(V.data()), V.size()));
+  }
+  pointer find(const T &Value) const {
+    return pointer(TrieType::find(hash(Value)));
+  }
+  pointer insert(pointer Hint, T &&Value) {
+    return pointer(TrieType::insertLazy(
+        typename pointer::BaseType(Hint),
+        [&](LazyValueConstructor C) { C(std::move(Value)); }));
+  }
+  pointer insert(pointer Hint, const T &Value) {
+    return pointer(
+        TrieType::insertLazy(typename pointer::BaseType(Hint), hash(Value),
+                             [&](LazyValueConstructor C) { C(Value); }));
+  }
+  pointer insert(T &&Value) { return insert(pointer(), Value); }
+  pointer insert(const T &Value) { return insert(pointer(), Value); }
+};
+} // end anonymous namespace
+
+TEST(HashMappedTrieTest, Strings) {
+  for (unsigned RootBits : {2, 3, 6, 10}) {
+    for (unsigned SubtrieBits : {2, 3, 4}) {
+      ThreadSafeHashMappedTrieSet<std::string> Strings(RootBits, SubtrieBits);
+      const std::string &A1 = *Strings.insert("A");
+      EXPECT_EQ(&A1, &*Strings.insert("A"));
+      std::string A2 = A1;
+      EXPECT_EQ(&A1, &*Strings.insert(A2));
+
+      const std::string &B1 = *Strings.insert("B");
+      EXPECT_EQ(&B1, &*Strings.insert(B1));
+      std::string B2 = B1;
+      EXPECT_EQ(&B1, &*Strings.insert(B2));
+
+      for (int I = 0, E = 1000; I != E; ++I) {
+        ThreadSafeHashMappedTrieSet<std::string>::pointer Lookup;
+        std::string S = Twine(I).str();
+        if (I & 1)
+          Lookup = Strings.find(S);
+        const std::string &S1 = *Strings.insert(Lookup, S);
+        EXPECT_EQ(&S1, &*Strings.insert(S1));
+        std::string S2 = S1;
+        EXPECT_EQ(&S1, &*Strings.insert(S2));
+      }
+      for (int I = 0, E = 1000; I != E; ++I) {
+        std::string S = Twine(I).str();
+        ThreadSafeHashMappedTrieSet<std::string>::pointer Lookup =
+            Strings.find(S);
+        EXPECT_TRUE(Lookup);
+        if (!Lookup)
+          continue;
+        EXPECT_EQ(S, *Lookup);
+      }
+    }
+  }
+}
+
+} // namespace