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diff --git a/js/src/gc/ParallelWork.h b/js/src/gc/ParallelWork.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_ParallelWork_h
+#define gc_ParallelWork_h
+
+#include "mozilla/Maybe.h"
+
+#include <algorithm>
+
+#include "gc/GCParallelTask.h"
+#include "gc/GCRuntime.h"
+#include "js/SliceBudget.h"
+#include "vm/HelperThreads.h"
+
+namespace js {
+
+namespace gcstats {
+enum class PhaseKind : uint8_t;
+}
+
+namespace gc {
+
+template <typename WorkItem>
+using ParallelWorkFunc = size_t (*)(GCRuntime*, const WorkItem&);
+
+// A GCParallelTask task that executes WorkItems from a WorkItemIterator.
+//
+// The WorkItemIterator class must supply done(), next() and get() methods. The
+// get() method must return WorkItems objects.
+template <typename WorkItem, typename WorkItemIterator>
+class ParallelWorker : public GCParallelTask {
+ public:
+  using WorkFunc = ParallelWorkFunc<WorkItem>;
+
+  ParallelWorker(GCRuntime* gc, gcstats::PhaseKind phaseKind, GCUse use,
+                 WorkFunc func, WorkItemIterator& work,
+                 const SliceBudget& budget, AutoLockHelperThreadState& lock)
+      : GCParallelTask(gc, phaseKind, use),
+        func_(func),
+        work_(work),
+        budget_(budget),
+        item_(work.get()) {
+    // Consume a work item on creation so that we can stop creating workers if
+    // the number of workers exceeds the number of work items.
+    work.next();
+  }
+
+  void run(AutoLockHelperThreadState& lock) {
+    AutoUnlockHelperThreadState unlock(lock);
+
+    for (;;) {
+      size_t steps = func_(gc, item_);
+      budget_.step(std::max(steps, size_t(1)));
+      if (budget_.isOverBudget()) {
+        break;
+      }
+
+      AutoLockHelperThreadState lock;
+      if (work().done()) {
+        break;
+      }
+
+      item_ = work().get();
+      work().next();
+    }
+  }
+
+ private:
+  WorkItemIterator& work() { return work_.ref(); }
+
+  // A function to execute work items on the helper thread.
+  WorkFunc func_;
+
+  // An iterator which produces work items to execute.
+  HelperThreadLockData<WorkItemIterator&> work_;
+
+  // The budget that determines how long to run for.
+  SliceBudget budget_;
+
+  // The next work item to process.
+  WorkItem item_;
+};
+
+static constexpr size_t MaxParallelWorkers = 8;
+
+// An RAII class that starts a number of ParallelWorkers and waits for them to
+// finish.
+template <typename WorkItem, typename WorkItemIterator>
+class MOZ_RAII AutoRunParallelWork {
+ public:
+  using Worker = ParallelWorker<WorkItem, WorkItemIterator>;
+  using WorkFunc = ParallelWorkFunc<WorkItem>;
+
+  AutoRunParallelWork(GCRuntime* gc, WorkFunc func,
+                      gcstats::PhaseKind phaseKind, GCUse use,
+                      WorkItemIterator& work, const SliceBudget& budget,
+                      AutoLockHelperThreadState& lock)
+      : gc(gc), phaseKind(phaseKind), lock(lock), tasksStarted(0) {
+    size_t workerCount = gc->parallelWorkerCount();
+    MOZ_ASSERT(workerCount <= MaxParallelWorkers);
+    MOZ_ASSERT_IF(workerCount == 0, work.done());
+
+    for (size_t i = 0; i < workerCount && !work.done(); i++) {
+      tasks[i].emplace(gc, phaseKind, use, func, work, budget, lock);
+      gc->startTask(*tasks[i], lock);
+      tasksStarted++;
+    }
+  }
+
+  ~AutoRunParallelWork() {
+    gHelperThreadLock.assertOwnedByCurrentThread();
+
+    for (size_t i = 0; i < tasksStarted; i++) {
+      gc->joinTask(*tasks[i], lock);
+    }
+    for (size_t i = tasksStarted; i < MaxParallelWorkers; i++) {
+      MOZ_ASSERT(tasks[i].isNothing());
+    }
+  }
+
+ private:
+  GCRuntime* gc;
+  gcstats::PhaseKind phaseKind;
+  AutoLockHelperThreadState& lock;
+  size_t tasksStarted;
+  mozilla::Maybe<Worker> tasks[MaxParallelWorkers];
+};
+
+} /* namespace gc */
+} /* namespace js */
+
+#endif /* gc_ParallelWork_h */