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diff --git a/memory/build/Mutex.h b/memory/build/Mutex.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef Mutex_h
+#define Mutex_h
+
+#if defined(XP_WIN)
+#  include <windows.h>
+#elif defined(XP_DARWIN)
+#  include <libkern/OSAtomic.h>
+#else
+#  include <pthread.h>
+#endif
+#include "mozilla/Attributes.h"
+
+// Mutexes based on spinlocks.  We can't use normal pthread spinlocks in all
+// places, because they require malloc()ed memory, which causes bootstrapping
+// issues in some cases.  We also can't use constructors, because for statics,
+// they would fire after the first use of malloc, resetting the locks.
+struct Mutex {
+#if defined(XP_WIN)
+  CRITICAL_SECTION mMutex;
+#elif defined(XP_DARWIN)
+  OSSpinLock mMutex;
+#else
+  pthread_mutex_t mMutex;
+#endif
+
+  // Initializes a mutex. Returns whether initialization succeeded.
+  inline bool Init() {
+#if defined(XP_WIN)
+    if (!InitializeCriticalSectionAndSpinCount(&mMutex, 5000)) {
+      return false;
+    }
+#elif defined(XP_DARWIN)
+    mMutex = OS_SPINLOCK_INIT;
+#elif defined(XP_LINUX) && !defined(ANDROID)
+    pthread_mutexattr_t attr;
+    if (pthread_mutexattr_init(&attr) != 0) {
+      return false;
+    }
+    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ADAPTIVE_NP);
+    if (pthread_mutex_init(&mMutex, &attr) != 0) {
+      pthread_mutexattr_destroy(&attr);
+      return false;
+    }
+    pthread_mutexattr_destroy(&attr);
+#else
+    if (pthread_mutex_init(&mMutex, nullptr) != 0) {
+      return false;
+    }
+#endif
+    return true;
+  }
+
+  inline void Lock() {
+#if defined(XP_WIN)
+    EnterCriticalSection(&mMutex);
+#elif defined(XP_DARWIN)
+    OSSpinLockLock(&mMutex);
+#else
+    pthread_mutex_lock(&mMutex);
+#endif
+  }
+
+  inline void Unlock() {
+#if defined(XP_WIN)
+    LeaveCriticalSection(&mMutex);
+#elif defined(XP_DARWIN)
+    OSSpinLockUnlock(&mMutex);
+#else
+    pthread_mutex_unlock(&mMutex);
+#endif
+  }
+};
+
+// Mutex that can be used for static initialization.
+// On Windows, CRITICAL_SECTION requires a function call to be initialized,
+// but for the initialization lock, a static initializer calling the
+// function would be called too late. We need no-function-call
+// initialization, which SRWLock provides.
+// Ideally, we'd use the same type of locks everywhere, but SRWLocks
+// everywhere incur a performance penalty. See bug 1418389.
+#if defined(XP_WIN)
+struct StaticMutex {
+  SRWLOCK mMutex;
+
+  inline void Lock() { AcquireSRWLockExclusive(&mMutex); }
+
+  inline void Unlock() { ReleaseSRWLockExclusive(&mMutex); }
+};
+
+// Normally, we'd use a constexpr constructor, but MSVC likes to create
+// static initializers anyways.
+#  define STATIC_MUTEX_INIT SRWLOCK_INIT
+
+#else
+typedef Mutex StaticMutex;
+
+#  if defined(XP_DARWIN)
+#    define STATIC_MUTEX_INIT OS_SPINLOCK_INIT
+#  elif defined(XP_LINUX) && !defined(ANDROID)
+#    define STATIC_MUTEX_INIT PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP
+#  else
+#    define STATIC_MUTEX_INIT PTHREAD_MUTEX_INITIALIZER
+#  endif
+
+#endif
+
+template <typename T>
+struct MOZ_RAII AutoLock {
+  explicit AutoLock(T& aMutex) : mMutex(aMutex) { mMutex.Lock(); }
+
+  ~AutoLock() { mMutex.Unlock(); }
+
+ private:
+  T& mMutex;
+};
+
+using MutexAutoLock = AutoLock<Mutex>;
+
+#endif