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diff --git a/mfbt/ThreadLocal.h b/mfbt/ThreadLocal.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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"
+
+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
+
+/*
+ * Thread Local Storage helpers.
+ *
+ * Usage:
+ *
+ * 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
+ * mozilla::ThreadLocal<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
+{
+#if defined(XP_WIN)
+    typedef unsigned long key_t;
+#else
+    typedef pthread_key_t key_t;
+#endif
+
+    union Helper {
+      void* ptr;
+      T value;
+    };
+
+  public:
+    MOZ_WARN_UNUSED_RESULT inline bool init();
+
+    inline T get() const;
+
+    inline void set(const T value);
+
+    bool initialized() const {
+      return inited;
+    }
+
+  private:
+    key_t key;
+    bool inited;
+};
+
+template<typename T>
+inline bool
+ThreadLocal<T>::init()
+{
+  MOZ_STATIC_ASSERT(sizeof(T) <= sizeof(void*),
+                    "mozilla::ThreadLocal can't be used for types larger than "
+                    "a pointer");
+  MOZ_ASSERT(!initialized());
+#ifdef XP_WIN
+  key = TlsAlloc();
+  inited = key != 0xFFFFFFFFUL; // TLS_OUT_OF_INDEXES
+#else
+  inited = !pthread_key_create(&key, NULL);
+#endif
+  return inited;
+}
+
+template<typename T>
+inline T
+ThreadLocal<T>::get() const
+{
+  MOZ_ASSERT(initialized());
+  Helper h;
+#ifdef XP_WIN
+  h.ptr = TlsGetValue(key);
+#else
+  h.ptr = pthread_getspecific(key);
+#endif
+  return h.value;
+}
+
+template<typename T>
+inline void
+ThreadLocal<T>::set(const T value)
+{
+  MOZ_ASSERT(initialized());
+  Helper h;
+  h.value = value;
+  bool succeeded;
+#ifdef XP_WIN
+  succeeded = TlsSetValue(key, h.ptr);
+#else
+  succeeded = !pthread_setspecific(key, h.ptr);
+#endif
+  if (!succeeded)
+    MOZ_CRASH();
+}
+
+} // namespace mozilla
+
+#endif // mozilla_ThreadLocal_h_