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diff --git a/mfbt/ThreadLocal.h b/mfbt/ThreadLocal.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/. */
+
+/* Cross-platform lightweight thread local data wrappers. */
+
+#ifndef mozilla_ThreadLocal_h
+#define mozilla_ThreadLocal_h
+
+#if defined(XP_WIN)
+// This file will get included in any file that wants to add a profiler mark.
+// In order to not bring <windows.h> together we could include windef.h and
+// winbase.h which are sufficient to get the prototypes for the Tls* functions.
+// # include <windef.h>
+// # include <winbase.h>
+// Unfortunately, even including these headers causes us to add a bunch of ugly
+// stuff to our namespace e.g #define CreateEvent CreateEventW
+extern "C" {
+__declspec(dllimport) void* __stdcall TlsGetValue(unsigned long);
+__declspec(dllimport) int __stdcall TlsSetValue(unsigned long, void*);
+__declspec(dllimport) unsigned long __stdcall TlsAlloc();
+}
+#else
+#  include <pthread.h>
+#  include <signal.h>
+#endif
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/TypeTraits.h"
+
+namespace mozilla {
+
+// sig_safe_t denotes an atomic type which can be read or stored in a single
+// instruction.  This means that data of this type is safe to be manipulated
+// from a signal handler, or other similar asynchronous execution contexts.
+#if defined(XP_WIN)
+typedef unsigned long sig_safe_t;
+#else
+typedef sig_atomic_t sig_safe_t;
+#endif
+
+namespace detail {
+
+#if defined(HAVE_THREAD_TLS_KEYWORD)
+#define MOZ_HAS_THREAD_LOCAL
+#endif
+
+/*
+ * Thread Local Storage helpers.
+ *
+ * Usage:
+ *
+ * Do not directly instantiate this class.  Instead, use the
+ * MOZ_THREAD_LOCAL macro to declare or define instances.  The macro
+ * takes a type name as its argument.
+ *
+ * Declare like this:
+ * extern MOZ_THREAD_LOCAL(int) tlsInt;
+ * Define like this:
+ * MOZ_THREAD_LOCAL(int) tlsInt;
+ * or:
+ * static MOZ_THREAD_LOCAL(int) tlsInt;
+ *
+ * Only static-storage-duration (e.g. global variables, or static class members)
+ * objects of this class should be instantiated. This class relies on
+ * zero-initialization, which is implicit for static-storage-duration objects.
+ * It doesn't have a custom default constructor, to avoid static initializers.
+ *
+ * API usage:
+ *
+ * // Create a TLS item.
+ * //
+ * // Note that init() should be invoked before the first use of set()
+ * // or get().  It is ok to call it multiple times.  This must be
+ * // called in a way that avoids possible races with other threads.
+ * MOZ_THREAD_LOCAL(int) tlsKey;
+ * if (!tlsKey.init()) {
+ *   // deal with the error
+ * }
+ *
+ * // Set the TLS value
+ * tlsKey.set(123);
+ *
+ * // Get the TLS value
+ * int value = tlsKey.get();
+ */
+template<typename T>
+class ThreadLocal
+{
+#ifndef MOZ_HAS_THREAD_LOCAL
+#if defined(XP_WIN)
+  typedef unsigned long key_t;
+#else
+  typedef pthread_key_t key_t;
+#endif
+
+  // Integral types narrower than void* must be extended to avoid
+  // warnings from valgrind on some platforms.  This helper type
+  // achieves that without penalizing the common case of ThreadLocals
+  // instantiated using a pointer type.
+  template<typename S>
+  struct Helper
+  {
+    typedef uintptr_t Type;
+  };
+
+  template<typename S>
+  struct Helper<S *>
+  {
+    typedef S *Type;
+  };
+#endif
+
+  bool initialized() const {
+#ifdef MOZ_HAS_THREAD_LOCAL
+    return true;
+#else
+    return mInited;
+#endif
+  }
+
+public:
+  // __thread does not allow non-trivial constructors, but we can
+  // instead rely on zero-initialization.
+#ifndef MOZ_HAS_THREAD_LOCAL
+  ThreadLocal()
+    : mKey(0), mInited(false)
+  {}
+#endif
+
+  MOZ_MUST_USE inline bool init();
+
+  inline T get() const;
+
+  inline void set(const T aValue);
+
+private:
+#ifdef MOZ_HAS_THREAD_LOCAL
+  T mValue;
+#else
+  key_t mKey;
+  bool mInited;
+#endif
+};
+
+template<typename T>
+inline bool
+ThreadLocal<T>::init()
+{
+  static_assert(mozilla::IsPointer<T>::value || mozilla::IsIntegral<T>::value,
+                "mozilla::ThreadLocal must be used with a pointer or "
+                "integral type");
+  static_assert(sizeof(T) <= sizeof(void*),
+                "mozilla::ThreadLocal can't be used for types larger than "
+                "a pointer");
+
+#ifdef MOZ_HAS_THREAD_LOCAL
+  return true;
+#else
+  if (!initialized()) {
+#ifdef XP_WIN
+    mKey = TlsAlloc();
+    mInited = mKey != 0xFFFFFFFFUL; // TLS_OUT_OF_INDEXES
+#else
+    mInited = !pthread_key_create(&mKey, nullptr);
+#endif
+  }
+  return mInited;
+#endif
+}
+
+template<typename T>
+inline T
+ThreadLocal<T>::get() const
+{
+#ifdef MOZ_HAS_THREAD_LOCAL
+  return mValue;
+#else
+  MOZ_ASSERT(initialized());
+  void* h;
+#ifdef XP_WIN
+  h = TlsGetValue(mKey);
+#else
+  h = pthread_getspecific(mKey);
+#endif
+  return static_cast<T>(reinterpret_cast<typename Helper<T>::Type>(h));
+#endif
+}
+
+template<typename T>
+inline void
+ThreadLocal<T>::set(const T aValue)
+{
+#ifdef MOZ_HAS_THREAD_LOCAL
+  mValue = aValue;
+#else
+  MOZ_ASSERT(initialized());
+  void* h = reinterpret_cast<void*>(static_cast<typename Helper<T>::Type>(aValue));
+#ifdef XP_WIN
+  bool succeeded = TlsSetValue(mKey, h);
+#else
+  bool succeeded = !pthread_setspecific(mKey, h);
+#endif
+  if (!succeeded) {
+    MOZ_CRASH();
+  }
+#endif
+}
+
+#ifdef MOZ_HAS_THREAD_LOCAL
+#define MOZ_THREAD_LOCAL(TYPE) __thread mozilla::detail::ThreadLocal<TYPE>
+#else
+#define MOZ_THREAD_LOCAL(TYPE) mozilla::detail::ThreadLocal<TYPE>
+#endif
+
+} // namespace detail
+} // namespace mozilla
+
+#endif /* mozilla_ThreadLocal_h */