[NFC][libcxxabi] Refactor libcxxabi/src/cxa_guard_impl.h

[DanielMcIntosh-IBM]

Summary

The new version doesn't need the curiously recurring template pattern, is
consistent about naming of GlobalMutex vs GlobalLock, has a lot more comments,
and has a much better separation of concerns than before.

In addition to being much cleaner and easier to understand, these changes are
also going to be needed later, when another set of changes gets put upstream
(pending approval by IBM management).

If this set of changes it too large for a single PR/commit, I can easily break
it down into the following 5 change sets:

• Rename GlobalLock to GlobalMutex
• Make InitByteGlobalMutex check GetThreadID instead of PlatformThreadID
• Split InitByteXXX classes
• Added GuardBase class
• Moved cxa_guard_acquire/release/abort into GuardBase

[DanielMcIntosh-IBM]

Ping

[ldionne]

Sorry for the delay.

As you mention, this change is kind of large to review as-is. If you can split it into a few changes that are easier to review, it would be helpful. As it stands, it's difficult for me to validate that this is good since I don't know the code by heart and there's several unrelated changes bunched together.

It would also be awesome if you could put up for review the subsequent change that this will all be useful for (although I understand you might not be allowed to yet).

[DanielMcIntosh-IBM]

This comment was removed by DanielMcIntosh-IBM.

[DanielMcIntosh-IBM]

Added a 6th set of changes, getting rid of the multiple inheritance

[DanielMcIntosh-IBM]

libcxxabi/src/cxa_guard_impl.h

 uint32_t PlatformThreadID() {
   static_assert(sizeof(pid_t) == sizeof(uint32_t), "");
   return static_cast<uint32_t>(syscall(SYS_gettid));
 }
 #else
 constexpr uint32_t (*PlatformThreadID)() = nullptr;
 #endif
 
-constexpr bool PlatformSupportsThreadID() { return +PlatformThreadID != nullptr; }
-
 //===----------------------------------------------------------------------===//
-//                          GuardBase
+//                          GuardByte
 //===----------------------------------------------------------------------===//
 
-enum class AcquireResult {
-  INIT_IS_DONE,
-  INIT_IS_PENDING,
-};
-constexpr AcquireResult INIT_IS_DONE = AcquireResult::INIT_IS_DONE;
-constexpr AcquireResult INIT_IS_PENDING = AcquireResult::INIT_IS_PENDING;
-
 static constexpr uint8_t UNSET = 0;
 static constexpr uint8_t COMPLETE_BIT = (1 << 0);
 static constexpr uint8_t PENDING_BIT = (1 << 1);
 static constexpr uint8_t WAITING_BIT = (1 << 2);
 
-template <class Derived>
-struct GuardObject {
-  GuardObject() = delete;
-  GuardObject(GuardObject const&) = delete;
-  GuardObject& operator=(GuardObject const&) = delete;
-
-  explicit GuardObject(uint32_t* g)
-      : base_address(g), guard_byte_address(reinterpret_cast<uint8_t*>(g)),
-        init_byte_address(reinterpret_cast<uint8_t*>(g) + 1), thread_id_address(nullptr) {}
-
-  explicit GuardObject(uint64_t* g)
-      : base_address(g), guard_byte_address(reinterpret_cast<uint8_t*>(g)),
-        init_byte_address(reinterpret_cast<uint8_t*>(g) + 1), thread_id_address(reinterpret_cast<uint32_t*>(g) + 1) {}
+/// Manages reads and writes to the guard byte.
+struct GuardByte {
+  GuardByte() = delete;
+  GuardByte(GuardByte const&) = delete;
+  GuardByte& operator=(GuardByte const&) = delete;
 
-public:
-  /// Implements __cxa_guard_acquire
-  AcquireResult cxa_guard_acquire() {
-    AtomicInt<uint8_t> guard_byte(guard_byte_address);
-    if (guard_byte.load(std::_AO_Acquire) != UNSET)
-      return INIT_IS_DONE;
-    return derived()->acquire_init_byte();
-  }
-
-  /// Implements __cxa_guard_release
-  void cxa_guard_release() {
-    AtomicInt<uint8_t> guard_byte(guard_byte_address);
-    // Store complete first, so that when release wakes other folks, they see
-    // it as having been completed.
-    guard_byte.store(COMPLETE_BIT, std::_AO_Release);
-    derived()->release_init_byte();
-  }
-
-  /// Implements __cxa_guard_abort
-  void cxa_guard_abort() { derived()->abort_init_byte(); }
+  explicit GuardByte(uint8_t* const guard_byte_address) : guard_byte(guard_byte_address) {}
 
 public:
-  /// base_address - the address of the original guard object.
-  void* const base_address;
-  /// The address of the guard byte at offset 0.
-  uint8_t* const guard_byte_address;
-  /// The address of the byte used by the implementation during initialization.
-  uint8_t* const init_byte_address;
-  /// An optional address storing an identifier for the thread performing initialization.
-  /// It's used to detect recursive initialization.
-  uint32_t* const thread_id_address;
+  /// The guard_byte portion of cxa_guard_acquire. Returns true if
+  /// initialization has already been completed.
+  /// Note: On completion, we haven't 'acquired' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_acquire.
+  bool acquire_guard_byte() {
+    // if guard_byte is non-zero, we have already completed initialization
+    // (i.e. release_guard_byte has been called)
+    return guard_byte.load(std::_AO_Acquire) != UNSET;
+  }
+
+  /// The guard_byte portion of cxa_guard_release.
+  /// Note: On completion, we haven't 'released' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_release.
+  void release_guard_byte() { guard_byte.store(COMPLETE_BIT, std::_AO_Release); }
+
+  /// The guard_byte portion of cxa_guard_abort.
+  void abort_guard_byte() {} // Nothing to do
 
 private:
-  Derived* derived() { return static_cast<Derived*>(this); }
+  AtomicInt<uint8_t> guard_byte;
 };
 
 //===----------------------------------------------------------------------===//
 //                    Single Threaded Implementation
 //===----------------------------------------------------------------------===//
 
-struct InitByteNoThreads : GuardObject<InitByteNoThreads> {
-  using GuardObject::GuardObject;
-
-  AcquireResult acquire_init_byte() {
+/// InitByteNoThreads - Doesn't use any inter-thread synchronization when
+/// managing reads and writes to the init byte.
+struct InitByteNoThreads {
+  InitByteNoThreads() = delete;
+  InitByteNoThreads(InitByteNoThreads const&) = delete;
+  InitByteNoThreads& operator=(InitByteNoThreads const&) = delete;
+
+  explicit InitByteNoThreads(uint8_t* _init_byte_address, uint32_t*) : init_byte_address(_init_byte_address) {}
+
+  /// The init_byte portion of cxa_guard_acquire. Returns true if
+  /// initialization has already been completed.
+  /// Note: On completion, we haven't 'acquired' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_acquire.
+  bool acquire_init_byte() {
     if (*init_byte_address == COMPLETE_BIT)
-      return INIT_IS_DONE;
+      return true;
     if (*init_byte_address & PENDING_BIT)
       ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
     *init_byte_address = PENDING_BIT;
-    return INIT_IS_PENDING;
+    return false;
   }
 
+  /// The init_byte portion of cxa_guard_release.
+  /// Note: On completion, we haven't 'released' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_release.
   void release_init_byte() { *init_byte_address = COMPLETE_BIT; }
+  /// The init_byte portion of cxa_guard_abort.
   void abort_init_byte() { *init_byte_address = UNSET; }
+
+private:
+  /// The address of the byte used during initialization.
+  uint8_t* const init_byte_address;
 };
 
 //===----------------------------------------------------------------------===//
 //                     Global Mutex Implementation
 //===----------------------------------------------------------------------===//
 
 struct LibcppMutex;
 struct LibcppCondVar;

 private:
   std::__libcpp_condvar_t cond = _LIBCPP_CONDVAR_INITIALIZER;
 };
 #else
 struct LibcppMutex {};
 struct LibcppCondVar {};
 #endif // !defined(_LIBCXXABI_HAS_NO_THREADS)
 
+/// InitByteGlobalMutex - Uses a global mutex and condition variable (common to
+/// all static local variables) to manage reads and writes to the init byte.
 template <class Mutex, class CondVar, Mutex& global_mutex, CondVar& global_cond,
           uint32_t (*GetThreadID)() = PlatformThreadID>
-struct InitByteGlobalMutex : GuardObject<InitByteGlobalMutex<Mutex, CondVar, global_mutex, global_cond, GetThreadID>> {
+struct InitByteGlobalMutex {
 
-  using BaseT = typename InitByteGlobalMutex::GuardObject;
-  using BaseT::BaseT;
-
-  explicit InitByteGlobalMutex(uint32_t* g) : BaseT(g), has_thread_id_support(false) {}
-  explicit InitByteGlobalMutex(uint64_t* g) : BaseT(g), has_thread_id_support(PlatformSupportsThreadID()) {}
+  explicit InitByteGlobalMutex(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
+      : init_byte_address(_init_byte_address), thread_id_address(_thread_id_address),
+        has_thread_id_support(_thread_id_address != nullptr && GetThreadID != nullptr) {}
 
 public:
-  AcquireResult acquire_init_byte() {
+  /// The init_byte portion of cxa_guard_acquire. Returns true if
+  /// initialization has already been completed.
+  /// Note: On completion, we haven't 'acquired' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_acquire.
+  bool acquire_init_byte() {
     LockGuard g("__cxa_guard_acquire");
     // Check for possible recursive initialization.
     if (has_thread_id_support && (*init_byte_address & PENDING_BIT)) {
       if (*thread_id_address == current_thread_id.get())
         ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
     }
 
     // Wait until the pending bit is not set.
     while (*init_byte_address & PENDING_BIT) {
       *init_byte_address |= WAITING_BIT;
       global_cond.wait(global_mutex);
     }
 
     if (*init_byte_address == COMPLETE_BIT)
-      return INIT_IS_DONE;
+      return true;
 
     if (has_thread_id_support)
       *thread_id_address = current_thread_id.get();
 
     *init_byte_address = PENDING_BIT;
-    return INIT_IS_PENDING;
+    return false;
   }
 
+  /// The init_byte portion of cxa_guard_release.
+  /// Note: On completion, we haven't 'released' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_release.
   void release_init_byte() {
     bool has_waiting;
     {
       LockGuard g("__cxa_guard_release");
       has_waiting = *init_byte_address & WAITING_BIT;
       *init_byte_address = COMPLETE_BIT;
     }
     if (has_waiting) {
       if (global_cond.broadcast()) {
         ABORT_WITH_MESSAGE("%s failed to broadcast", "__cxa_guard_release");
       }
     }
   }
 
+  /// The init_byte portion of cxa_guard_abort.
   void abort_init_byte() {
     bool has_waiting;
     {
       LockGuard g("__cxa_guard_abort");
       if (has_thread_id_support)
         *thread_id_address = 0;
       has_waiting = *init_byte_address & WAITING_BIT;
       *init_byte_address = UNSET;
     }
     if (has_waiting) {
       if (global_cond.broadcast()) {
         ABORT_WITH_MESSAGE("%s failed to broadcast", "__cxa_guard_abort");
       }
     }
   }
 
 private:
-  using BaseT::init_byte_address;
-  using BaseT::thread_id_address;
+  /// The address of the byte used during initialization.
+  uint8_t* const init_byte_address;
+  /// An optional address storing an identifier for the thread performing initialization.
+  /// It's used to detect recursive initialization.
+  uint32_t* const thread_id_address;
+
   const bool has_thread_id_support;
   LazyValue<uint32_t, GetThreadID> current_thread_id;
 
 private:
   struct LockGuard {
     LockGuard() = delete;
     LockGuard(LockGuard const&) = delete;
     LockGuard& operator=(LockGuard const&) = delete;

 }
 #else
 constexpr void (*PlatformFutexWait)(int*, int) = nullptr;
 constexpr void (*PlatformFutexWake)(int*) = nullptr;
 #endif
 
 constexpr bool PlatformSupportsFutex() { return +PlatformFutexWait != nullptr; }
 
-/// InitByteFutex - Manages initialization using atomics and the futex syscall
-/// for waiting and waking.
+/// InitByteFutex - Uses a futex to manage reads and writes to the init byte.
 template <void (*Wait)(int*, int) = PlatformFutexWait, void (*Wake)(int*) = PlatformFutexWake,
           uint32_t (*GetThreadIDArg)() = PlatformThreadID>
-struct InitByteFutex : GuardObject<InitByteFutex<Wait, Wake, GetThreadIDArg>> {
-  using BaseT = typename InitByteFutex::GuardObject;
-
-  /// ARM Constructor
-  explicit InitByteFutex(uint32_t* g)
-      : BaseT(g), init_byte(this->init_byte_address), has_thread_id_support(this->thread_id_address && GetThreadIDArg),
-        thread_id(this->thread_id_address) {}
+struct InitByteFutex {
 
-  /// Itanium Constructor
-  explicit InitByteFutex(uint64_t* g)
-      : BaseT(g), init_byte(this->init_byte_address), has_thread_id_support(this->thread_id_address && GetThreadIDArg),
-        thread_id(this->thread_id_address) {}
+  explicit InitByteFutex(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
+      : init_byte(_init_byte_address),
+        has_thread_id_support(_thread_id_address != nullptr && GetThreadIDArg != nullptr),
+        thread_id(_thread_id_address), base_address(reinterpret_cast<int*>(
+                                           /*_init_byte_address & ~0x3*/ _init_byte_address - 1)) {}
 
 public:
-  AcquireResult acquire_init_byte() {
+  /// The init_byte portion of cxa_guard_acquire. Returns true if
+  /// initialization has already been completed.
+  /// Note: On completion, we haven't 'acquired' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_acquire.
+  bool acquire_init_byte() {
     while (true) {
       uint8_t last_val = UNSET;
       if (init_byte.compare_exchange(&last_val, PENDING_BIT, std::_AO_Acq_Rel, std::_AO_Acquire)) {
         if (has_thread_id_support) {
           thread_id.store(current_thread_id.get(), std::_AO_Relaxed);
         }
-        return INIT_IS_PENDING;
+        return false;
       }
 
       if (last_val == COMPLETE_BIT)
-        return INIT_IS_DONE;
+        return true;
 
       if (last_val & PENDING_BIT) {
 
         // Check for recursive initialization
         if (has_thread_id_support && thread_id.load(std::_AO_Relaxed) == current_thread_id.get()) {
           ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
         }
 
         if ((last_val & WAITING_BIT) == 0) {
           // This compare exchange can fail for several reasons
           // (1) another thread finished the whole thing before we got here
           // (2) another thread set the waiting bit we were trying to thread
           // (3) another thread had an exception and failed to finish
           if (!init_byte.compare_exchange(&last_val, PENDING_BIT | WAITING_BIT, std::_AO_Acq_Rel, std::_AO_Release)) {
             // (1) success, via someone else's work!
             if (last_val == COMPLETE_BIT)
-              return INIT_IS_DONE;
+              return true;
 
             // (3) someone else, bailed on doing the work, retry from the start!
             if (last_val == UNSET)
               continue;
 
             // (2) the waiting bit got set, so we are happy to keep waiting
           }
         }
         wait_on_initialization();
       }
     }
   }
 
+  /// The init_byte portion of cxa_guard_release.
+  /// Note: On completion, we haven't 'released' ownership of anything mutex
+  /// related. The name is simply referring to __cxa_guard_release.
   void release_init_byte() {
     uint8_t old = init_byte.exchange(COMPLETE_BIT, std::_AO_Acq_Rel);
     if (old & WAITING_BIT)
       wake_all();
   }
 
+  /// The init_byte portion of cxa_guard_abort.
   void abort_init_byte() {
     if (has_thread_id_support)
       thread_id.store(0, std::_AO_Relaxed);
 
     uint8_t old = init_byte.exchange(UNSET, std::_AO_Acq_Rel);
     if (old & WAITING_BIT)
       wake_all();
   }
 
 private:
   /// Use the futex to wait on the current guard variable. Futex expects a
-  /// 32-bit 4-byte aligned address as the first argument, so we have to use use
-  /// the base address of the guard variable (not the init byte).
-  void wait_on_initialization() {
-    Wait(static_cast<int*>(this->base_address), expected_value_for_futex(PENDING_BIT | WAITING_BIT));
-  }
-  void wake_all() { Wake(static_cast<int*>(this->base_address)); }
+  /// 32-bit 4-byte aligned address as the first argument, so we use the 4-byte
+  /// aligned address that encompasses the init byte (i.e. the address of the
+  /// raw guard object that was passed to __cxa_guard_acquire/release/abort).
+  void wait_on_initialization() { Wait(base_address, expected_value_for_futex(PENDING_BIT | WAITING_BIT)); }
+  void wake_all() { Wake(base_address); }
 
 private:
   AtomicInt<uint8_t> init_byte;
 
   const bool has_thread_id_support;
   // Unsafe to use unless has_thread_id_support
   AtomicInt<uint32_t> thread_id;
   LazyValue<uint32_t, GetThreadIDArg> current_thread_id;
 
+  /// the 4-byte-aligned address that encompasses the init byte (i.e. the
+  /// address of the raw guard object).
+  int* const base_address;
+
   /// Create the expected integer value for futex `wait(int* addr, int expected)`.
   /// We pass the base address as the first argument, So this function creates
   /// an zero-initialized integer  with `b` copied at the correct offset.
   static int expected_value_for_futex(uint8_t b) {
     int dest_val = 0;
     std::memcpy(reinterpret_cast<char*>(&dest_val) + 1, &b, 1);
     return dest_val;
   }
 
   static_assert(Wait != nullptr && Wake != nullptr, "");
 };
 
 //===----------------------------------------------------------------------===//
+//                                   Guards
+//===----------------------------------------------------------------------===//
+
+enum class AcquireResult {
+  INIT_IS_DONE,
+  INIT_IS_PENDING,
+};
+constexpr AcquireResult INIT_IS_DONE = AcquireResult::INIT_IS_DONE;
+constexpr AcquireResult INIT_IS_PENDING = AcquireResult::INIT_IS_PENDING;
+
+/// Co-ordinates between GuardByte and InitByte.
+template <class InitByteT>
+struct GuardBase : GuardByte, InitByteT {
+  GuardBase() = delete;
+  GuardBase(GuardBase const&) = delete;
+  GuardBase& operator=(GuardBase const&) = delete;
+
+  /// ARM Constructor
+  explicit GuardBase(uint32_t* raw_guard_object)
+      : GuardByte(reinterpret_cast<uint8_t*>(raw_guard_object)),
+        InitByteT(reinterpret_cast<uint8_t*>(raw_guard_object) + 1, nullptr) {}
+
+  /// Itanium Constructor
+  explicit GuardBase(uint64_t* raw_guard_object)
+      : GuardByte(reinterpret_cast<uint8_t*>(raw_guard_object)),
+        InitByteT(reinterpret_cast<uint8_t*>(raw_guard_object) + 1, reinterpret_cast<uint32_t*>(raw_guard_object) + 1) {
+  }
+
+  /// Implements __cxa_guard_acquire.
+  AcquireResult cxa_guard_acquire() {
+    // use short-circuit evalutation to avoid calling acquire_init_byte when
+    // acquire_guard_byte returns true. (i.e. don't call it when we know from
+    // the guard byte that initialization has already been completed)
+    if (acquire_guard_byte() || InitByteT::acquire_init_byte())
+      return INIT_IS_DONE;
+    return INIT_IS_PENDING;
+  }
+
+  /// Implements __cxa_guard_release.
+  void cxa_guard_release() {
+    // Update guard byte first, so if somebody is woken up by release_init_byte
+    // and comes all the way back around to __cxa_guard_acquire again, they see
+    // it as having completed initialization.
+    release_guard_byte();
+    InitByteT::release_init_byte();
+  }
+
+  /// Implements __cxa_guard_abort.
+  void cxa_guard_abort() {
+    abort_guard_byte();
+    InitByteT::abort_init_byte();
+  }
+};
+
+/// NoThreadsGuard - Manages initialization without performing any inter-thread
+/// synchronization.
+struct NoThreadsGuard : GuardBase<InitByteNoThreads> {
+  using BaseT = typename NoThreadsGuard::GuardBase;
+  using BaseT::BaseT;
+};
+
+/// GlobalMutexGuard - Manages initialization using a global mutex and
+/// condition variable.
+template <class Mutex, class CondVar, Mutex& global_mutex, CondVar& global_cond,
+          uint32_t (*GetThreadID)() = PlatformThreadID>
+struct GlobalMutexGuard : GuardBase<InitByteGlobalMutex<Mutex, CondVar, global_mutex, global_cond, GetThreadID>> {
+  using BaseT = typename GlobalMutexGuard::GuardBase;
+  using BaseT::BaseT;
+};
+
+/// FutexGuard - Manages initialization using atomics and the futex syscall for
+/// waiting and waking.
+template <void (*Wait)(int*, int) = PlatformFutexWait, void (*Wake)(int*) = PlatformFutexWake,
+          uint32_t (*GetThreadIDArg)() = PlatformThreadID>
+struct FutexGuard : GuardBase<InitByteFutex<Wait, Wake, GetThreadIDArg>> {
+  using BaseT = typename FutexGuard::GuardBase;
+  using BaseT::BaseT;
+};
+
+//===----------------------------------------------------------------------===//
 //
 //===----------------------------------------------------------------------===//
 
 template <class T>
 struct GlobalStatic {
   static T instance;
 };
 template <class T>
 _LIBCPP_SAFE_STATIC T GlobalStatic<T>::instance = {};
 
-enum class Implementation { NoThreads, GlobalLock, Futex };
+enum class Implementation { NoThreads, GlobalMutex, Futex };
 
 template <Implementation Impl>
 struct SelectImplementation;
 
 template <>
 struct SelectImplementation<Implementation::NoThreads> {
-  using type = InitByteNoThreads;
+  using type = NoThreadsGuard;
 };
 
 template <>
-struct SelectImplementation<Implementation::GlobalLock> {
-  using type = InitByteGlobalMutex<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
-                                   GlobalStatic<LibcppCondVar>::instance, PlatformThreadID>;
+struct SelectImplementation<Implementation::GlobalMutex> {
+  using type = GlobalMutexGuard<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
 };
 
 template <>
 struct SelectImplementation<Implementation::Futex> {
-  using type = InitByteFutex<PlatformFutexWait, PlatformFutexWake, PlatformThreadID>;
+  using type = FutexGuard<PlatformFutexWait, PlatformFutexWake, PlatformThreadID>;
 };
 
 // TODO(EricWF): We should prefer the futex implementation when available. But
 // it should be done in a separate step from adding the implementation.
 constexpr Implementation CurrentImplementation =
 #if defined(_LIBCXXABI_HAS_NO_THREADS)
     Implementation::NoThreads;
 #elif defined(_LIBCXXABI_USE_FUTEX)
     Implementation::Futex;
 #else
-    Implementation::GlobalLock;
+    Implementation::GlobalMutex;
 #endif
 
 static_assert(CurrentImplementation != Implementation::Futex || PlatformSupportsFutex(),
               "Futex selected but not supported");
 
 using SelectedImplementation = SelectImplementation<CurrentImplementation>::type;
 
 } // end namespace

libcxxabi/test/guard_test_basic.pass.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
 //
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: c++03
 
 #define TESTING_CXA_GUARD
 #include "../src/cxa_guard_impl.h"
 #include <cassert>
 
-// Disable GCC warning about tautological comparison of a function's address
-#if defined(__GNUC__) && !defined(__clang__)
+#if defined(__clang__)
+#pragma clang diagnostic ignored "-Wtautological-pointer-compare"
+#elif defined(__GNUC__)
 # pragma GCC diagnostic ignored "-Waddress"
 #endif
 
 using namespace __cxxabiv1;
 
 template <class GuardType, class Impl>
 struct Tests {
 private:

 void NopFutexWait(int*, int) { assert(false); }
 void NopFutexWake(int*) { assert(false); }
 uint32_t MockGetThreadID() { return 0; }
 
 int main(int, char**) {
   {
 #if defined(_LIBCXXABI_HAS_NO_THREADS)
     static_assert(CurrentImplementation == Implementation::NoThreads, "");
-    static_assert(
-        std::is_same<SelectedImplementation, InitByteNoThreads>::value, "");
+    static_assert(std::is_same<SelectedImplementation, NoThreadsGuard>::value, "");
 #else
-    static_assert(CurrentImplementation == Implementation::GlobalLock, "");
-    static_assert(
-        std::is_same<
-            SelectedImplementation,
-            InitByteGlobalMutex<LibcppMutex, LibcppCondVar,
-                                GlobalStatic<LibcppMutex>::instance,
+    static_assert(CurrentImplementation == Implementation::GlobalMutex, "");
+    static_assert(std::is_same<SelectedImplementation,
+                               GlobalMutexGuard<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
                                                 GlobalStatic<LibcppCondVar>::instance>>::value,
                   "");
 #endif
   }
   {
 #if (defined(__APPLE__) || defined(__linux__))  && !defined(_LIBCXXABI_HAS_NO_THREADS)
     assert(PlatformThreadID);
 #endif
-    if (PlatformSupportsThreadID()) {
+    if (PlatformThreadID != nullptr) {
       assert(PlatformThreadID() != 0);
       assert(PlatformThreadID() == PlatformThreadID());
     }
   }
   {
-    Tests<uint32_t, InitByteNoThreads>::test();
-    Tests<uint64_t, InitByteNoThreads>::test();
+    Tests<uint32_t, NoThreadsGuard>::test();
+    Tests<uint64_t, NoThreadsGuard>::test();
   }
   {
-    using MutexImpl =
-        InitByteGlobalMutex<NopMutex, NopCondVar, global_nop_mutex,
-                            global_nop_cond, MockGetThreadID>;
+    using MutexImpl = GlobalMutexGuard<NopMutex, NopCondVar, global_nop_mutex, global_nop_cond, MockGetThreadID>;
     Tests<uint32_t, MutexImpl>::test();
     Tests<uint64_t, MutexImpl>::test();
   }
   {
-    using FutexImpl =
-        InitByteFutex<&NopFutexWait, &NopFutexWake, &MockGetThreadID>;
+    using FutexImpl = FutexGuard<&NopFutexWait, &NopFutexWake, &MockGetThreadID>;
     Tests<uint32_t, FutexImpl>::test();
     Tests<uint64_t, FutexImpl>::test();
   }
 
   return 0;
 }

libcxxabi/test/guard_threaded_test.pass.cpp

for (auto test_func : TestList) {
           test_func(ThreadsPerTest);
         }
       });
       test_threads.JoinAll();
     }
   }
 
 void test_all_impls() {
-  using MutexImpl = SelectImplementation<Implementation::GlobalLock>::type;
+  using MutexImpl = SelectImplementation<Implementation::GlobalMutex>::type;
 
   // Attempt to test the Futex based implementation if it's supported on the
   // target platform.
   using RealFutexImpl = SelectImplementation<Implementation::Futex>::type;
   using FutexImpl = typename std::conditional<
       PlatformSupportsFutex(),
       RealFutexImpl,
       MutexImpl