Index: clang-tools-extra/clangd/support/Threading.cpp
===================================================================
--- clang-tools-extra/clangd/support/Threading.cpp
+++ clang-tools-extra/clangd/support/Threading.cpp
@@ -2,6 +2,7 @@
 #include "support/Trace.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/thread.h"
 #include "llvm/Support/Threading.h"
 #include <atomic>
 #include <thread>
@@ -95,8 +96,10 @@
   };
 
   // Ensure our worker threads have big enough stacks to run clang.
-  llvm::llvm_execute_on_thread_async(std::move(Task),
-                                     /*clang::DesiredStackSize*/ 8 << 20);
+  llvm::thread Thread(
+      /*clang::DesiredStackSize*/ llvm::Optional<unsigned>(8 << 20),
+      std::move(Task));
+  Thread.detach();
 }
 
 Deadline timeoutSeconds(llvm::Optional<double> Seconds) {
Index: clang/tools/libclang/CIndex.cpp
===================================================================
--- clang/tools/libclang/CIndex.cpp
+++ clang/tools/libclang/CIndex.cpp
@@ -52,6 +52,7 @@
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/TargetSelect.h"
+#include "llvm/Support/thread.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/Timer.h"
 #include "llvm/Support/raw_ostream.h"
@@ -6770,10 +6771,10 @@
 
 void clang_executeOnThread(void (*fn)(void *), void *user_data,
                            unsigned stack_size) {
-  llvm::llvm_execute_on_thread(fn, user_data,
-                               stack_size == 0
-                                   ? clang::DesiredStackSize
-                                   : llvm::Optional<unsigned>(stack_size));
+  llvm::thread Thread(stack_size == 0 ? clang::DesiredStackSize
+                                      : llvm::Optional<unsigned>(stack_size),
+                      fn, user_data);
+  Thread.join();
 }
 
 //===----------------------------------------------------------------------===//
Index: llvm/include/llvm/Support/CrashRecoveryContext.h
===================================================================
--- llvm/include/llvm/Support/CrashRecoveryContext.h
+++ llvm/include/llvm/Support/CrashRecoveryContext.h
@@ -87,7 +87,7 @@
   /// a protected context which is run in another thread (optionally with a
   /// requested stack size).
   ///
-  /// See RunSafely() and llvm_execute_on_thread().
+  /// See RunSafely().
   ///
   /// On Darwin, if PRIO_DARWIN_BG is set on the calling thread, it will be
   /// propagated to the new thread as well.
Index: llvm/include/llvm/Support/Threading.h
===================================================================
--- llvm/include/llvm/Support/Threading.h
+++ llvm/include/llvm/Support/Threading.h
@@ -54,36 +54,6 @@
 /// false otherwise.
 bool llvm_is_multithreaded();
 
-/// Execute the given \p UserFn on a separate thread, passing it the provided \p
-/// UserData and waits for thread completion.
-///
-/// This function does not guarantee that the code will actually be executed
-/// on a separate thread or honoring the requested stack size, but tries to do
-/// so where system support is available.
-///
-/// \param UserFn - The callback to execute.
-/// \param UserData - An argument to pass to the callback function.
-/// \param StackSizeInBytes - A requested size (in bytes) for the thread stack
-/// (or None for default)
-void llvm_execute_on_thread(
-    void (*UserFn)(void *), void *UserData,
-    llvm::Optional<unsigned> StackSizeInBytes = llvm::None);
-
-/// Schedule the given \p Func for execution on a separate thread, then return
-/// to the caller immediately. Roughly equivalent to
-/// `std::thread(Func).detach()`, except it allows requesting a specific stack
-/// size, if supported for the platform.
-///
-/// This function would report a fatal error if it can't execute the code
-/// on a separate thread.
-///
-/// \param Func - The callback to execute.
-/// \param StackSizeInBytes - A requested size (in bytes) for the thread stack
-/// (or None for default)
-void llvm_execute_on_thread_async(
-    llvm::unique_function<void()> Func,
-    llvm::Optional<unsigned> StackSizeInBytes = llvm::None);
-
 #if LLVM_THREADING_USE_STD_CALL_ONCE
 
   typedef std::once_flag once_flag;
Index: llvm/include/llvm/Support/thread.h
===================================================================
--- llvm/include/llvm/Support/thread.h
+++ llvm/include/llvm/Support/thread.h
@@ -16,16 +16,215 @@
 #ifndef LLVM_SUPPORT_THREAD_H
 #define LLVM_SUPPORT_THREAD_H
 
+#include "llvm/ADT/Optional.h"
 #include "llvm/Config/llvm-config.h"
 
+#ifdef _WIN32
+#include "llvm/Support/Windows/WindowsSupport.h"
+#endif
+
 #if LLVM_ENABLE_THREADS
 
 #include <thread>
 
 namespace llvm {
-typedef std::thread thread;
+
+# if LLVM_ON_UNIX || _WIN32
+
+/// LLVM thread following std::thread interface with added constructor to
+/// specify stack size.
+class thread {
+  template<typename FPtr, typename ...Args, size_t ...Indices>
+  static void Apply(std::tuple<FPtr, Args...> &Callee, std::index_sequence<Indices...>) {
+    std::move(std::get<0>(Callee))(std::move(std::get<Indices + 1>(Callee))...);
+  }
+
+  template<typename CalleeTuple>
+  static void GenericThreadProxy(void *Ptr) {
+    std::unique_ptr<CalleeTuple> Callee(static_cast<CalleeTuple *>(Ptr));
+
+    // FIXME: use std::apply when C++17 is allowed.
+    std::make_index_sequence<std::tuple_size<CalleeTuple>() - 1> Indices{};
+    Apply(*Callee.get(), Indices);
+  }
+public:
+#if LLVM_ON_UNIX
+  using native_handle_type = pthread_t;
+  using id = pthread_t;
+  using start_routine_type = void *(*)(void *);
+
+  template <typename CalleeTuple> static void *ThreadProxy(void *Ptr) {
+    GenericThreadProxy<CalleeTuple>(Ptr);
+    return nullptr;
+  }
+#elif _WIN32
+  using native_handle_type = HANDLE;
+  using id = HANDLE;
+  using start_routine_type = unsigned (__stdcall *)(void *);
+
+  template <typename CalleeTuple> static unsigned __stdcall ThreadProxy(void *Ptr) {
+    GenericThreadProxy<CalleeTuple>(Ptr);
+    return 0;
+  }
+#endif
+
+#if defined(__APPLE__)
+  // Darwin's default stack size for threads except the main one is only 512KB,
+  // which is not enough for some/many normal LLVM compilations. This implements
+  // the same interface as std::thread but requests the same stack size as the
+  // main thread (8MB) before creation.
+  static const constexpr llvm::Optional<unsigned> DefaultStackSize =
+      8 * 1024 * 1024;
+#else
+  static const constexpr llvm::Optional<unsigned> DefaultStackSize = None;
+#endif
+
+  thread() : Thread(native_handle_type()) {}
+  thread(thread &&Other) noexcept
+      : Thread(std::exchange(Other.Thread, native_handle_type())) {}
+
+  template <class Function, class... Args>
+  explicit thread(Function &&f, Args &&...args)
+      : thread(DefaultStackSize, f, args...) {}
+
+  template <class Function, class... Args>
+  explicit thread(llvm::Optional<unsigned> StackSizeInBytes, Function &&f,
+                  Args &&...args);
+  thread(const thread &) = delete;
+
+  ~thread() {
+    if (joinable())
+      std::terminate();
+  }
+
+  thread &operator=(thread &&Other) noexcept {
+    if (joinable())
+      std::terminate();
+    Thread = std::exchange(Other.Thread, native_handle_type());
+    return *this;
+  }
+
+  bool joinable() const noexcept {
+    return Thread != native_handle_type();
+  }
+
+  id get_id() const noexcept {
+    return Thread;
+  }
+
+  native_handle_type native_handle() const noexcept {
+    return Thread;
+  }
+
+  static unsigned hardware_concurrency() {
+    return std::thread::hardware_concurrency();
+  };
+
+  inline void join();
+  inline void detach();
+
+  void swap(llvm::thread &Other) noexcept {
+    std::swap(Thread, Other.Thread);
+  }
+private:
+  native_handle_type Thread;
+};
+
+thread::native_handle_type
+llvm_execute_on_thread_impl(thread::start_routine_type ThreadFunc, void *Arg,
+                            llvm::Optional<unsigned> StackSizeInBytes);
+void llvm_thread_join_impl(thread::native_handle_type Thread);
+void llvm_thread_detach_impl(thread::native_handle_type Thread);
+
+template <class Function, class... Args>
+thread::thread(llvm::Optional<unsigned> StackSizeInBytes, Function &&f,
+               Args &&...args) {
+  typedef std::tuple<typename std::decay<Function>::type,
+                     typename std::decay<Args>::type...>
+      CalleeTuple;
+  std::unique_ptr<CalleeTuple> Callee(
+      new CalleeTuple(std::forward<Function>(f), std::forward<Args>(args)...));
+
+  Thread = llvm_execute_on_thread_impl(ThreadProxy<CalleeTuple>, Callee.get(),
+                                       StackSizeInBytes);
+  if (Thread != native_handle_type())
+    Callee.release();
+}
+
+void thread::join() {
+  llvm_thread_join_impl(Thread);
+  Thread = native_handle_type();
+}
+
+void thread::detach() {
+  llvm_thread_detach_impl(Thread);
+  Thread = native_handle_type();
 }
 
+#else // !LLVM_ON_UNIX && !_WIN32
+
+/// std::thread backed implementation of llvm::thread interface that ignores the
+/// stack size request.
+class thread {
+public:
+  using native_handle_type = std::thread::native_handle_type;
+  using id = std::thread::id;
+
+  thread() : Thread(std::thread()) {}
+  thread(thread &&Other) noexcept
+      : Thread(std::exchange(Other.Thread, std::thread())) {}
+
+  template <class Function, class... Args>
+  explicit thread(llvm::Optional<unsigned> StackSizeInBytes, Function &&f,
+                  Args &&...args)
+      : Thread(std::forward<Function>(f), std::forward<Args>(args)...) {}
+
+  template <class Function, class... Args>
+  explicit thread(Function &&f, Args &&...args) : Thread(f, args...) {}
+
+  thread(const thread &) = delete;
+
+  ~thread() {}
+
+  thread &operator=(thread &&Other) noexcept {
+    Thread = std::exchange(Other.Thread, std::thread());
+    return *this;
+  }
+
+  bool joinable() const noexcept {
+    return Thread.joinable();
+  }
+
+  id get_id() const noexcept {
+    return Thread.get_id();
+  }
+
+  native_handle_type native_handle() noexcept {
+    return Thread.native_handle();
+  }
+
+  static unsigned hardware_concurrency() {
+    return std::thread::hardware_concurrency();
+  };
+
+  inline void join() {
+    Thread.join();
+  }
+  inline void detach() {
+    Thread.detach();
+  }
+
+  void swap(llvm::thread &Other) noexcept {
+    std::swap(Thread, Other.Thread);
+  }
+private:
+  std::thread Thread;
+};
+
+#endif // LLVM_ON_UNIX || _WIN32
+
+} // namespace llvm
+
 #else // !LLVM_ENABLE_THREADS
 
 #include <utility>
@@ -36,11 +235,20 @@
   thread() {}
   thread(thread &&other) {}
   template <class Function, class... Args>
+  explicit thread(llvm::Optional<unsigned> StackSizeInBytes, Function &&f,
+                  Args &&...args) {
+    f(std::forward<Args>(args)...);
+  }
+  template <class Function, class... Args>
   explicit thread(Function &&f, Args &&... args) {
     f(std::forward<Args>(args)...);
   }
   thread(const thread &) = delete;
 
+  void detach() {
+    report_fatal_error("Detaching from a thread does not make sense with no "
+                       "threading support");
+  }
   void join() {}
   static unsigned hardware_concurrency() { return 1; };
 };
@@ -49,4 +257,4 @@
 
 #endif // LLVM_ENABLE_THREADS
 
-#endif
+#endif // LLVM_SUPPORT_THREAD_H
\ No newline at end of file
Index: llvm/lib/Support/CrashRecoveryContext.cpp
===================================================================
--- llvm/lib/Support/CrashRecoveryContext.cpp
+++ llvm/lib/Support/CrashRecoveryContext.cpp
@@ -12,6 +12,7 @@
 #include "llvm/Support/ExitCodes.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/Signals.h"
+#include "llvm/Support/thread.h"
 #include "llvm/Support/ThreadLocal.h"
 #include <mutex>
 #include <setjmp.h>
@@ -500,10 +501,12 @@
                                              unsigned RequestedStackSize) {
   bool UseBackgroundPriority = hasThreadBackgroundPriority();
   RunSafelyOnThreadInfo Info = { Fn, this, UseBackgroundPriority, false };
-  llvm_execute_on_thread(RunSafelyOnThread_Dispatch, &Info,
-                         RequestedStackSize == 0
-                             ? llvm::None
-                             : llvm::Optional<unsigned>(RequestedStackSize));
+  llvm::thread Thread(RequestedStackSize == 0
+                          ? llvm::None
+                          : llvm::Optional<unsigned>(RequestedStackSize),
+                      RunSafelyOnThread_Dispatch, &Info);
+  Thread.join();
+
   if (CrashRecoveryContextImpl *CRC = (CrashRecoveryContextImpl *)Impl)
     CRC->setSwitchedThread();
   return Info.Result;
Index: llvm/lib/Support/Threading.cpp
===================================================================
--- llvm/lib/Support/Threading.cpp
+++ llvm/lib/Support/Threading.cpp
@@ -11,6 +11,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/Support/thread.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/Config/config.h"
@@ -38,13 +39,6 @@
 
 #if LLVM_ENABLE_THREADS == 0 ||                                                \
     (!defined(_WIN32) && !defined(HAVE_PTHREAD_H))
-// Support for non-Win32, non-pthread implementation.
-void llvm::llvm_execute_on_thread(void (*Fn)(void *), void *UserData,
-                                  llvm::Optional<unsigned> StackSizeInBytes) {
-  (void)StackSizeInBytes;
-  Fn(UserData);
-}
-
 uint64_t llvm::get_threadid() { return 0; }
 
 uint32_t llvm::get_max_thread_name_length() { return 0; }
@@ -60,25 +54,6 @@
   return 1;
 }
 
-#if LLVM_ENABLE_THREADS == 0
-void llvm::llvm_execute_on_thread_async(
-    llvm::unique_function<void()> Func,
-    llvm::Optional<unsigned> StackSizeInBytes) {
-  (void)Func;
-  (void)StackSizeInBytes;
-  report_fatal_error("Spawning a detached thread doesn't make sense with no "
-                     "threading support");
-}
-#else
-// Support for non-Win32, non-pthread implementation.
-void llvm::llvm_execute_on_thread_async(
-    llvm::unique_function<void()> Func,
-    llvm::Optional<unsigned> StackSizeInBytes) {
-  (void)StackSizeInBytes;
-  std::thread(std::move(Func)).detach();
-}
-#endif
-
 #else
 
 int computeHostNumHardwareThreads();
@@ -95,17 +70,6 @@
   return std::min((unsigned)MaxThreadCount, ThreadsRequested);
 }
 
-namespace {
-struct SyncThreadInfo {
-  void (*UserFn)(void *);
-  void *UserData;
-};
-
-using AsyncThreadInfo = llvm::unique_function<void()>;
-
-enum class JoiningPolicy { Join, Detach };
-} // namespace
-
 // Include the platform-specific parts of this class.
 #ifdef LLVM_ON_UNIX
 #include "Unix/Threading.inc"
@@ -114,22 +78,6 @@
 #include "Windows/Threading.inc"
 #endif
 
-void llvm::llvm_execute_on_thread(void (*Fn)(void *), void *UserData,
-                                  llvm::Optional<unsigned> StackSizeInBytes) {
-
-  SyncThreadInfo Info = {Fn, UserData};
-  llvm_execute_on_thread_impl(threadFuncSync, &Info, StackSizeInBytes,
-                              JoiningPolicy::Join);
-}
-
-void llvm::llvm_execute_on_thread_async(
-    llvm::unique_function<void()> Func,
-    llvm::Optional<unsigned> StackSizeInBytes) {
-  llvm_execute_on_thread_impl(&threadFuncAsync,
-                              new AsyncThreadInfo(std::move(Func)),
-                              StackSizeInBytes, JoiningPolicy::Detach);
-}
-
 #endif
 
 Optional<ThreadPoolStrategy>
Index: llvm/lib/Support/Unix/Threading.inc
===================================================================
--- llvm/lib/Support/Unix/Threading.inc
+++ llvm/lib/Support/Unix/Threading.inc
@@ -48,22 +48,9 @@
 #include <unistd.h>      // For syscall()
 #endif
 
-static void *threadFuncSync(void *Arg) {
-  SyncThreadInfo *TI = static_cast<SyncThreadInfo *>(Arg);
-  TI->UserFn(TI->UserData);
-  return nullptr;
-}
-
-static void *threadFuncAsync(void *Arg) {
-  std::unique_ptr<AsyncThreadInfo> Info(static_cast<AsyncThreadInfo *>(Arg));
-  (*Info)();
-  return nullptr;
-}
-
-static void
-llvm_execute_on_thread_impl(void *(*ThreadFunc)(void *), void *Arg,
-                            llvm::Optional<unsigned> StackSizeInBytes,
-                            JoiningPolicy JP) {
+pthread_t
+llvm::llvm_execute_on_thread_impl(void *(*ThreadFunc)(void *), void *Arg,
+                                  llvm::Optional<unsigned> StackSizeInBytes) {
   int errnum;
 
   // Construct the attributes object.
@@ -90,15 +77,22 @@
   if ((errnum = ::pthread_create(&Thread, &Attr, ThreadFunc, Arg)) != 0)
     ReportErrnumFatal("pthread_create failed", errnum);
 
-  if (JP == JoiningPolicy::Join) {
-    // Wait for the thread
-    if ((errnum = ::pthread_join(Thread, nullptr)) != 0) {
-      ReportErrnumFatal("pthread_join failed", errnum);
-    }
-  } else if (JP == JoiningPolicy::Detach) {
-    if ((errnum = ::pthread_detach(Thread)) != 0) {
-      ReportErrnumFatal("pthread_detach failed", errnum);
-    }
+  return Thread;
+}
+
+void llvm::llvm_thread_detach_impl(pthread_t Thread) {
+  int errnum;
+
+  if ((errnum = ::pthread_detach(Thread)) != 0) {
+    ReportErrnumFatal("pthread_detach failed", errnum);
+  }
+}
+
+void llvm::llvm_thread_join_impl(pthread_t Thread) {
+  int errnum;
+
+  if ((errnum = ::pthread_join(Thread, nullptr)) != 0) {
+    ReportErrnumFatal("pthread_join failed", errnum);
   }
 }
 
Index: llvm/lib/Support/Windows/Threading.inc
===================================================================
--- llvm/lib/Support/Windows/Threading.inc
+++ llvm/lib/Support/Windows/Threading.inc
@@ -23,22 +23,10 @@
 #undef MemoryFence
 #endif
 
-static unsigned __stdcall threadFuncSync(void *Arg) {
-  SyncThreadInfo *TI = static_cast<SyncThreadInfo *>(Arg);
-  TI->UserFn(TI->UserData);
-  return 0;
-}
-
-static unsigned __stdcall threadFuncAsync(void *Arg) {
-  std::unique_ptr<AsyncThreadInfo> Info(static_cast<AsyncThreadInfo *>(Arg));
-  (*Info)();
-  return 0;
-}
-
-static void
-llvm_execute_on_thread_impl(unsigned (__stdcall *ThreadFunc)(void *), void *Arg,
-                            llvm::Optional<unsigned> StackSizeInBytes,
-                            JoiningPolicy JP) {
+HANDLE
+llvm::llvm_execute_on_thread_impl(unsigned(__stdcall *ThreadFunc)(void *),
+                                  void *Arg,
+                                  llvm::Optional<unsigned> StackSizeInBytes) {
   HANDLE hThread = (HANDLE)::_beginthreadex(
       NULL, StackSizeInBytes.getValueOr(0), ThreadFunc, Arg, 0, NULL);
 
@@ -46,11 +34,16 @@
     ReportLastErrorFatal("_beginthreadex failed");
   }
 
-  if (JP == JoiningPolicy::Join) {
-    if (::WaitForSingleObject(hThread, INFINITE) == WAIT_FAILED) {
-      ReportLastErrorFatal("WaitForSingleObject failed");
-    }
+  return hThread;
+}
+
+void llvm::llvm_thread_join_impl(HANDLE hThread) {
+  if (::WaitForSingleObject(hThread, INFINITE) == WAIT_FAILED) {
+    ReportLastErrorFatal("WaitForSingleObject failed");
   }
+}
+
+void llvm::llvm_thread_detach_impl(HANDLE hThread) {
   if (::CloseHandle(hThread) == FALSE) {
     ReportLastErrorFatal("CloseHandle failed");
   }
Index: llvm/unittests/Support/Threading.cpp
===================================================================
--- llvm/unittests/Support/Threading.cpp
+++ llvm/unittests/Support/Threading.cpp
@@ -63,7 +63,8 @@
     ThreadFinished.notify();
   };
 
-  llvm::llvm_execute_on_thread_async(ThreadFunc);
+  llvm::thread Thread(ThreadFunc);
+  Thread.detach();
   ASSERT_TRUE(ThreadStarted.wait());
   ThreadAdvanced.notify();
   ASSERT_TRUE(ThreadFinished.wait());
@@ -71,11 +72,23 @@
 
 TEST(Threading, RunOnThreadSync) {
   std::atomic_bool Executed(false);
-  llvm::llvm_execute_on_thread(
+  llvm::thread Thread(
       [](void *Arg) { *static_cast<std::atomic_bool *>(Arg) = true; },
       &Executed);
+  Thread.join();
   ASSERT_EQ(Executed, true);
 }
+
+#if defined(__APPLE__)
+TEST(Threading, AppleStackSize) {
+  llvm::thread Thread([] {
+    volatile unsigned char Var[8 * 1024 * 1024 - 1024];
+    Var[0] = 0xff;
+    ASSERT_EQ(Var[0], 0xff);
+  });
+  Thread.join();
+}
+#endif
 #endif
 
 } // end anon namespace