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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) && !defined(__wasi__)
+#  include <pthread.h>
+#endif
+
+#include <type_traits>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+
+namespace mozilla {
+
+namespace detail {
+
+#ifdef XP_MACOSX
+#  if defined(__has_feature)
+#    if __has_feature(cxx_thread_local)
+#      define MACOSX_HAS_THREAD_LOCAL
+#    endif
+#  endif
+#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();
+ */
+
+// 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;
+};
+
+#if defined(XP_WIN)
+/*
+ * ThreadLocalKeyStorage uses Thread Local APIs that are declared in
+ * processthreadsapi.h. To use this class on Windows, include that file
+ * (or windows.h) before including ThreadLocal.h.
+ *
+ * TLS_OUT_OF_INDEXES is a #define that is used to detect whether
+ * an appropriate header has been included prior to this file
+ */
+#  if defined(TLS_OUT_OF_INDEXES)
+/* Despite not being used for MOZ_THREAD_LOCAL, we expose an implementation for
+ * Windows for cases where it's not desirable to use thread_local */
+template <typename T>
+class ThreadLocalKeyStorage {
+ public:
+  ThreadLocalKeyStorage() : mKey(TLS_OUT_OF_INDEXES) {}
+
+  inline bool initialized() const { return mKey != TLS_OUT_OF_INDEXES; }
+
+  inline void init() { mKey = TlsAlloc(); }
+
+  inline T get() const {
+    void* h = TlsGetValue(mKey);
+    return static_cast<T>(reinterpret_cast<typename Helper<T>::Type>(h));
+  }
+
+  inline bool set(const T aValue) {
+    void* h = const_cast<void*>(reinterpret_cast<const void*>(
+        static_cast<typename Helper<T>::Type>(aValue)));
+    return TlsSetValue(mKey, h);
+  }
+
+ private:
+  unsigned long mKey;
+};
+#  endif
+#elif defined(__wasi__)
+// There are no threads on WASI, so we just use a global variable.
+template <typename T>
+class ThreadLocalKeyStorage {
+ public:
+  constexpr ThreadLocalKeyStorage() : mInited(false) {}
+
+  inline bool initialized() const { return mInited; }
+
+  inline void init() { mInited = true; }
+
+  inline T get() const { return mVal; }
+
+  inline bool set(const T aValue) {
+    mVal = aValue;
+    return true;
+  }
+
+ private:
+  bool mInited;
+  T mVal;
+};
+#else
+template <typename T>
+class ThreadLocalKeyStorage {
+ public:
+  constexpr ThreadLocalKeyStorage() : mKey(0), mInited(false) {}
+
+  inline bool initialized() const { return mInited; }
+
+  inline void init() { mInited = !pthread_key_create(&mKey, nullptr); }
+
+  inline T get() const {
+    void* h = pthread_getspecific(mKey);
+    return static_cast<T>(reinterpret_cast<typename Helper<T>::Type>(h));
+  }
+
+  inline bool set(const T aValue) {
+    const void* h = reinterpret_cast<const void*>(
+        static_cast<typename Helper<T>::Type>(aValue));
+    return !pthread_setspecific(mKey, h);
+  }
+
+ private:
+  pthread_key_t mKey;
+  bool mInited;
+};
+#endif
+
+template <typename T>
+class ThreadLocalNativeStorage {
+ public:
+  // __thread does not allow non-trivial constructors, but we can
+  // instead rely on zero-initialization.
+  inline bool initialized() const { return true; }
+
+  inline void init() {}
+
+  inline T get() const { return mValue; }
+
+  inline bool set(const T aValue) {
+    mValue = aValue;
+    return true;
+  }
+
+ private:
+  T mValue;
+};
+
+template <typename T, template <typename U> class Storage>
+class ThreadLocal : public Storage<T> {
+ public:
+  [[nodiscard]] inline bool init();
+
+  void infallibleInit() {
+    MOZ_RELEASE_ASSERT(init(), "Infallible TLS initialization failed");
+  }
+
+  inline T get() const;
+
+  inline void set(const T aValue);
+
+  using Type = T;
+};
+
+template <typename T, template <typename U> class Storage>
+inline bool ThreadLocal<T, Storage>::init() {
+  static_assert(std::is_pointer_v<T> || std::is_integral_v<T>,
+                "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");
+
+  if (!Storage<T>::initialized()) {
+    Storage<T>::init();
+  }
+  return Storage<T>::initialized();
+}
+
+template <typename T, template <typename U> class Storage>
+inline T ThreadLocal<T, Storage>::get() const {
+  MOZ_ASSERT(Storage<T>::initialized());
+  return Storage<T>::get();
+}
+
+template <typename T, template <typename U> class Storage>
+inline void ThreadLocal<T, Storage>::set(const T aValue) {
+  MOZ_ASSERT(Storage<T>::initialized());
+  bool succeeded = Storage<T>::set(aValue);
+  if (!succeeded) {
+    MOZ_CRASH();
+  }
+}
+
+#if (defined(XP_WIN) || defined(MACOSX_HAS_THREAD_LOCAL)) && \
+    !defined(__MINGW32__)
+#  define MOZ_THREAD_LOCAL(TYPE)                 \
+    thread_local ::mozilla::detail::ThreadLocal< \
+        TYPE, ::mozilla::detail::ThreadLocalNativeStorage>
+#elif defined(HAVE_THREAD_TLS_KEYWORD)
+#  define MOZ_THREAD_LOCAL(TYPE)             \
+    __thread ::mozilla::detail::ThreadLocal< \
+        TYPE, ::mozilla::detail::ThreadLocalNativeStorage>
+#else
+#  define MOZ_THREAD_LOCAL(TYPE)         \
+    ::mozilla::detail::ThreadLocal<TYPE, \
+                                   ::mozilla::detail::ThreadLocalKeyStorage>
+#endif
+
+}  // namespace detail
+}  // namespace mozilla
+
+#endif /* mozilla_ThreadLocal_h */