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diff --git a/js/src/builtin/AtomicsObject.h b/js/src/builtin/AtomicsObject.h
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+++ b/js/src/builtin/AtomicsObject.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 builtin_AtomicsObject_h
+#define builtin_AtomicsObject_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/TimeStamp.h"
+
+#include "threading/ConditionVariable.h"
+#include "threading/ProtectedData.h"  // js::ThreadData
+#include "vm/NativeObject.h"
+
+namespace js {
+
+class SharedArrayRawBuffer;
+
+class AtomicsObject : public NativeObject {
+ public:
+  static const JSClass class_;
+};
+
+class FutexThread {
+  friend class AutoLockFutexAPI;
+
+ public:
+  [[nodiscard]] static bool initialize();
+  static void destroy();
+
+  static void lock();
+  static void unlock();
+
+  FutexThread();
+  [[nodiscard]] bool initInstance();
+  void destroyInstance();
+
+  // Parameters to notify().
+  enum NotifyReason {
+    NotifyExplicit,       // Being asked to wake up by another thread
+    NotifyForJSInterrupt  // Interrupt requested
+  };
+
+  // Result codes from wait() and atomics_wait_impl().
+  enum class WaitResult {
+    Error,     // Error has been reported, just propagate error signal
+    NotEqual,  // Did not wait because the values differed
+    OK,        // Waited and was woken
+    TimedOut   // Waited and timed out
+  };
+
+  // Block the calling thread and wait.
+  //
+  // The futex lock must be held around this call.
+  //
+  // The timeout is the number of milliseconds, with fractional
+  // times allowed; specify mozilla::Nothing() for an indefinite
+  // wait.
+  //
+  // wait() will not wake up spuriously.
+  [[nodiscard]] WaitResult wait(
+      JSContext* cx, js::UniqueLock<js::Mutex>& locked,
+      const mozilla::Maybe<mozilla::TimeDuration>& timeout);
+
+  // Notify the thread this is associated with.
+  //
+  // The futex lock must be held around this call.  (The sleeping
+  // thread will not wake up until the caller of Atomics.notify()
+  // releases the lock.)
+  //
+  // If the thread is not waiting then this method does nothing.
+  //
+  // If the thread is waiting in a call to wait() and the
+  // reason is NotifyExplicit then the wait() call will return
+  // with Woken.
+  //
+  // If the thread is waiting in a call to wait() and the
+  // reason is NotifyForJSInterrupt then the wait() will return
+  // with WaitingNotifiedForInterrupt; in the latter case the caller
+  // of wait() must handle the interrupt.
+  void notify(NotifyReason reason);
+
+  bool isWaiting();
+
+  // If canWait() returns false (the default) then wait() is disabled
+  // on the thread to which the FutexThread belongs.
+  bool canWait() { return canWait_; }
+
+  void setCanWait(bool flag) { canWait_ = flag; }
+
+ private:
+  enum FutexState {
+    Idle,                         // We are not waiting or woken
+    Waiting,                      // We are waiting, nothing has happened yet
+    WaitingNotifiedForInterrupt,  // We are waiting, but have been interrupted,
+                                  //   and have not yet started running the
+                                  //   interrupt handler
+    WaitingInterrupted,           // We are waiting, but have been interrupted
+                                  //   and are running the interrupt handler
+    Woken                         // Woken by a script call to Atomics.notify
+  };
+
+  // Condition variable that this runtime will wait on.
+  js::ConditionVariable* cond_;
+
+  // Current futex state for this runtime.  When not in a wait this
+  // is Idle; when in a wait it is Waiting or the reason the futex
+  // is about to wake up.
+  FutexState state_;
+
+  // Shared futex lock for all runtimes.  We can perhaps do better,
+  // but any lock will need to be per-domain (consider SharedWorker)
+  // or coarser.
+  static mozilla::Atomic<js::Mutex*, mozilla::SequentiallyConsistent> lock_;
+
+  // A flag that controls whether waiting is allowed.
+  ThreadData<bool> canWait_;
+};
+
+// Go to sleep if the int32_t value at the given address equals `value`.
+[[nodiscard]] FutexThread::WaitResult atomics_wait_impl(
+    JSContext* cx, SharedArrayRawBuffer* sarb, size_t byteOffset, int32_t value,
+    const mozilla::Maybe<mozilla::TimeDuration>& timeout);
+
+// Go to sleep if the int64_t value at the given address equals `value`.
+[[nodiscard]] FutexThread::WaitResult atomics_wait_impl(
+    JSContext* cx, SharedArrayRawBuffer* sarb, size_t byteOffset, int64_t value,
+    const mozilla::Maybe<mozilla::TimeDuration>& timeout);
+
+// Notify some waiters on the given address.  If `count` is negative then notify
+// all.  The return value is nonnegative and is the number of waiters woken.  If
+// the number of waiters woken exceeds INT64_MAX then this never returns.  If
+// `count` is nonnegative then the return value is never greater than `count`.
+[[nodiscard]] int64_t atomics_notify_impl(SharedArrayRawBuffer* sarb,
+                                          size_t byteOffset, int64_t count);
+
+} /* namespace js */
+
+#endif /* builtin_AtomicsObject_h */