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diff --git a/js/src/threading/ConditionVariable.h b/js/src/threading/ConditionVariable.h
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+++ b/js/src/threading/ConditionVariable.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 threading_ConditionVariable_h
+#define threading_ConditionVariable_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Move.h"
+#include "mozilla/TimeStamp.h"
+
+#include <stdint.h>
+#ifndef XP_WIN
+# include <pthread.h>
+#endif
+
+#include "threading/LockGuard.h"
+#include "threading/Mutex.h"
+
+namespace js {
+
+template <typename T> using UniqueLock = LockGuard<T>;
+
+enum class CVStatus {
+  NoTimeout,
+  Timeout
+};
+
+// A poly-fill for std::condition_variable.
+class ConditionVariable
+{
+public:
+  struct PlatformData;
+
+  ConditionVariable();
+  ~ConditionVariable();
+
+  // Wake one thread that is waiting on this condition.
+  void notify_one();
+
+  // Wake all threads that are waiting on this condition.
+  void notify_all();
+
+  // Block the current thread of execution until this condition variable is
+  // woken from another thread via notify_one or notify_all.
+  void wait(UniqueLock<Mutex>& lock);
+
+  // As with |wait|, block the current thread of execution until woken from
+  // another thread. This method will resume waiting once woken until the given
+  // Predicate |pred| evaluates to true.
+  template <typename Predicate>
+  void wait(UniqueLock<Mutex>& lock, Predicate pred) {
+    while (!pred()) {
+      wait(lock);
+    }
+  }
+
+  // Block the current thread of execution until woken from another thread, or
+  // the given absolute time is reached. The given absolute time is evaluated
+  // when this method is called, so will wake up after (abs_time - now),
+  // independent of system clock changes. While insulated from clock changes,
+  // this API is succeptible to the issues discussed above wait_for.
+  CVStatus wait_until(UniqueLock<Mutex>& lock,
+                      const mozilla::TimeStamp& abs_time);
+
+  // As with |wait_until|, block the current thread of execution until woken
+  // from another thread, or the given absolute time is reached. This method
+  // will resume waiting once woken until the given Predicate |pred| evaluates
+  // to true.
+  template <typename Predicate>
+  bool wait_until(UniqueLock<Mutex>& lock, const mozilla::TimeStamp& abs_time,
+                  Predicate pred) {
+    while (!pred()) {
+      if (wait_until(lock, abs_time) == CVStatus::Timeout) {
+        return pred();
+      }
+    }
+    return true;
+  }
+
+  // Block the current thread of execution until woken from another thread, or
+  // the given time duration has elapsed. Given that the system may be
+  // interrupted between the callee and the actual wait beginning, this call
+  // has a minimum granularity of the system's scheduling interval, and may
+  // encounter substantially longer delays, depending on system load.
+  CVStatus wait_for(UniqueLock<Mutex>& lock,
+                    const mozilla::TimeDuration& rel_time);
+
+  // As with |wait_for|, block the current thread of execution until woken from
+  // another thread or the given time duration has elapsed. This method will
+  // resume waiting once woken until the given Predicate |pred| evaluates to
+  // true.
+  template <typename Predicate>
+  bool wait_for(UniqueLock<Mutex>& lock, const mozilla::TimeDuration& rel_time,
+                Predicate pred) {
+    return wait_until(lock, mozilla::TimeStamp::Now() + rel_time,
+                      mozilla::Move(pred));
+  }
+
+
+private:
+  ConditionVariable(const ConditionVariable&) = delete;
+  ConditionVariable& operator=(const ConditionVariable&) = delete;
+
+  PlatformData* platformData();
+
+#ifndef XP_WIN
+  void* platformData_[sizeof(pthread_cond_t) / sizeof(void*)];
+  static_assert(sizeof(pthread_cond_t) / sizeof(void*) != 0 &&
+                sizeof(pthread_cond_t) % sizeof(void*) == 0,
+                "pthread_cond_t must have pointer alignment");
+#else
+  void* platformData_[4];
+#endif
+};
+
+} // namespace js
+
+#endif // threading_ConditionVariable_h