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diff --git a/js/src/gc/GCParallelTask.h b/js/src/gc/GCParallelTask.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 gc_GCParallelTask_h
+#define gc_GCParallelTask_h
+
+#include "mozilla/LinkedList.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/TimeStamp.h"
+
+#include <utility>
+
+#include "gc/GCContext.h"
+#include "js/Utility.h"
+#include "threading/ProtectedData.h"
+#include "vm/HelperThreads.h"
+#include "vm/HelperThreadTask.h"
+
+#define JS_MEMBER_FN_PTR_TYPE(ClassT, ReturnT, /* ArgTs */...) \
+  ReturnT (ClassT::*)(__VA_ARGS__)
+
+#define JS_CALL_MEMBER_FN_PTR(Receiver, Ptr, /* Args */...) \
+  ((Receiver)->*(Ptr))(__VA_ARGS__)
+
+namespace js {
+
+namespace gcstats {
+enum class PhaseKind : uint8_t;
+}
+
+namespace gc {
+
+class GCRuntime;
+
+static inline mozilla::TimeDuration TimeSince(mozilla::TimeStamp prev) {
+  mozilla::TimeStamp now = mozilla::TimeStamp::Now();
+  // Sadly this happens sometimes.
+  MOZ_ASSERT(now >= prev);
+  if (now < prev) {
+    now = prev;
+  }
+  return now - prev;
+}
+
+}  // namespace gc
+
+class AutoLockHelperThreadState;
+class GCParallelTask;
+class HelperThread;
+
+// A wrapper around a linked list to enforce synchronization.
+class GCParallelTaskList {
+  mozilla::LinkedList<GCParallelTask> tasks;
+
+ public:
+  bool isEmpty(const AutoLockHelperThreadState& lock) {
+    gHelperThreadLock.assertOwnedByCurrentThread();
+    return tasks.isEmpty();
+  }
+
+  void insertBack(GCParallelTask* task, const AutoLockHelperThreadState& lock) {
+    gHelperThreadLock.assertOwnedByCurrentThread();
+    tasks.insertBack(task);
+  }
+
+  GCParallelTask* popFirst(const AutoLockHelperThreadState& lock) {
+    gHelperThreadLock.assertOwnedByCurrentThread();
+    return tasks.popFirst();
+  }
+
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf,
+                             const AutoLockHelperThreadState& lock) const {
+    gHelperThreadLock.assertOwnedByCurrentThread();
+    return tasks.sizeOfExcludingThis(aMallocSizeOf);
+  }
+};
+
+// A generic task used to dispatch work to the helper thread system.
+// Users override the pure-virtual run() method.
+class GCParallelTask : private mozilla::LinkedListElement<GCParallelTask>,
+                       public HelperThreadTask {
+  friend class mozilla::LinkedList<GCParallelTask>;
+  friend class mozilla::LinkedListElement<GCParallelTask>;
+
+ public:
+  gc::GCRuntime* const gc;
+
+  // This can be PhaseKind::NONE for tasks that take place outside a GC.
+  const gcstats::PhaseKind phaseKind;
+
+  gc::GCUse use;
+
+ private:
+  // The state of the parallel computation.
+  enum class State {
+    // The task is idle. Either start() has not been called or join() has
+    // returned.
+    Idle,
+
+    // The task is waiting in the per-runtime queue.
+    Queued,
+
+    // The task has been started but has not yet begun running on a helper
+    // thread.
+    Dispatched,
+
+    // The task is currently running on a helper thread.
+    Running,
+
+    // The task has finished running but has not yet been joined by the main
+    // thread.
+    Finished
+  };
+
+  UnprotectedData<State> state_;
+
+  // May be set to the time this task was queued to collect telemetry.
+  mozilla::TimeStamp maybeQueueTime_;
+
+  // Amount of time this task took to execute.
+  MainThreadOrGCTaskData<mozilla::TimeDuration> duration_;
+
+ protected:
+  // A flag to signal a request for early completion of the off-thread task.
+  mozilla::Atomic<bool, mozilla::MemoryOrdering::ReleaseAcquire> cancel_;
+
+ public:
+  explicit GCParallelTask(gc::GCRuntime* gc, gcstats::PhaseKind phaseKind,
+                          gc::GCUse use = gc::GCUse::Unspecified)
+      : gc(gc),
+        phaseKind(phaseKind),
+        use(use),
+        state_(State::Idle),
+        cancel_(false) {}
+  GCParallelTask(GCParallelTask&& other) noexcept
+      : gc(other.gc),
+        phaseKind(other.phaseKind),
+        use(other.use),
+        state_(other.state_),
+        cancel_(false) {}
+
+  explicit GCParallelTask(const GCParallelTask&) = delete;
+
+  // Derived classes must override this to ensure that join() gets called
+  // before members get destructed.
+  virtual ~GCParallelTask();
+
+  // Time spent in the most recent invocation of this task.
+  mozilla::TimeDuration duration() const { return duration_; }
+
+  // The simple interface to a parallel task works exactly like pthreads.
+  void start();
+  void join(mozilla::Maybe<mozilla::TimeStamp> deadline = mozilla::Nothing());
+
+  // If multiple tasks are to be started or joined at once, it is more
+  // efficient to take the helper thread lock once and use these methods.
+  void startWithLockHeld(AutoLockHelperThreadState& lock);
+  void joinWithLockHeld(
+      AutoLockHelperThreadState& lock,
+      mozilla::Maybe<mozilla::TimeStamp> deadline = mozilla::Nothing());
+  void joinNonIdleTask(mozilla::Maybe<mozilla::TimeStamp> deadline,
+                       AutoLockHelperThreadState& lock);
+
+  // Instead of dispatching to a helper, run the task on the current thread.
+  void runFromMainThread();
+  void runFromMainThread(AutoLockHelperThreadState& lock);
+
+  // If the task is not already running, either start it or run it on the main
+  // thread if that fails.
+  void startOrRunIfIdle(AutoLockHelperThreadState& lock);
+
+  // Set the cancel flag and wait for the task to finish.
+  void cancelAndWait();
+
+  // Report whether the task is idle. This means either before start() has been
+  // called or after join() has been called.
+  bool isIdle() const;
+  bool isIdle(const AutoLockHelperThreadState& lock) const {
+    return state_ == State::Idle;
+  }
+
+  // Report whether the task has been started. This means after start() has been
+  // called but before the task has run to completion. The task may not yet have
+  // started running.
+  bool wasStarted() const;
+  bool wasStarted(const AutoLockHelperThreadState& lock) const {
+    return isDispatched(lock) || isRunning(lock);
+  }
+
+  bool isQueued(const AutoLockHelperThreadState& lock) const {
+    return state_ == State::Queued;
+  }
+
+  bool isDispatched(const AutoLockHelperThreadState& lock) const {
+    return state_ == State::Dispatched;
+  }
+
+  bool isNotYetRunning(const AutoLockHelperThreadState& lock) const {
+    return state_ == State::Idle || state_ == State::Queued ||
+           state_ == State::Dispatched;
+  }
+
+ protected:
+  // Override this method to provide the task's functionality.
+  virtual void run(AutoLockHelperThreadState& lock) = 0;
+
+  virtual void recordDuration();
+
+  bool isCancelled() const { return cancel_; }
+
+ private:
+  void assertIdle() const {
+    // Don't lock here because that adds extra synchronization in debug
+    // builds that may hide bugs. There's no race if the assertion passes.
+    MOZ_ASSERT(state_ == State::Idle);
+  }
+
+  bool isRunning(const AutoLockHelperThreadState& lock) const {
+    return state_ == State::Running;
+  }
+  bool isFinished(const AutoLockHelperThreadState& lock) const {
+    return state_ == State::Finished;
+  }
+
+  void setQueued(const AutoLockHelperThreadState& lock) {
+    MOZ_ASSERT(isIdle(lock));
+    state_ = State::Queued;
+  }
+  void setDispatched(const AutoLockHelperThreadState& lock) {
+    MOZ_ASSERT(isIdle(lock) || isQueued(lock));
+    state_ = State::Dispatched;
+  }
+  void setRunning(const AutoLockHelperThreadState& lock) {
+    MOZ_ASSERT(isNotYetRunning(lock));
+    state_ = State::Running;
+  }
+  void setFinished(const AutoLockHelperThreadState& lock) {
+    MOZ_ASSERT(isRunning(lock));
+    state_ = State::Finished;
+  }
+  void setIdle(const AutoLockHelperThreadState& lock) {
+    MOZ_ASSERT(!isRunning(lock));
+    state_ = State::Idle;
+  }
+  friend class gc::GCRuntime;
+
+  void runTask(JS::GCContext* gcx, AutoLockHelperThreadState& lock);
+
+  // Implement the HelperThreadTask interface.
+  ThreadType threadType() override {
+    return ThreadType::THREAD_TYPE_GCPARALLEL;
+  }
+  void runHelperThreadTask(AutoLockHelperThreadState& locked) override;
+};
+
+} /* namespace js */
+#endif /* gc_GCParallelTask_h */