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diff --git a/js/public/SliceBudget.h b/js/public/SliceBudget.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 js_SliceBudget_h
+#define js_SliceBudget_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Variant.h"
+
+#include <stdint.h>
+
+#include "jstypes.h"
+
+namespace js {
+
+struct JS_PUBLIC_API TimeBudget {
+  const mozilla::TimeDuration budget;
+  mozilla::TimeStamp deadline;  // Calculated when SliceBudget is constructed.
+
+  explicit TimeBudget(mozilla::TimeDuration duration) : budget(duration) {}
+  explicit TimeBudget(int64_t milliseconds)
+      : budget(mozilla::TimeDuration::FromMilliseconds(milliseconds)) {}
+
+  void setDeadlineFromNow();
+};
+
+struct JS_PUBLIC_API WorkBudget {
+  const int64_t budget;
+
+  explicit WorkBudget(int64_t work) : budget(work) {}
+};
+
+struct UnlimitedBudget {};
+
+/*
+ * This class describes a limit to the amount of work to be performed in a GC
+ * slice, so that we can return to the mutator without pausing for too long. The
+ * budget may be based on a deadline time or an amount of work to be performed,
+ * or may be unlimited.
+ *
+ * To reduce the number of gettimeofday calls, we only check the time every 1000
+ * operations.
+ */
+class JS_PUBLIC_API SliceBudget {
+ public:
+  using InterruptRequestFlag = mozilla::Atomic<bool, mozilla::Relaxed>;
+
+  // Whether this slice is running in (predicted to be) idle time.
+  // Only used for recording in the profile.
+  bool idle = false;
+
+  // Whether this slice was given an extended budget, larger than
+  // the predicted idle time.
+  bool extended = false;
+
+ private:
+  static const intptr_t UnlimitedCounter = INTPTR_MAX;
+
+  // Most calls to isOverBudget will only check the counter value. Every N
+  // steps, do a more "expensive" check -- look at the current time and/or
+  // check the atomic interrupt flag.
+  static constexpr intptr_t StepsPerExpensiveCheck = 1000;
+
+  // Configuration
+
+  mozilla::Variant<TimeBudget, WorkBudget, UnlimitedBudget> budget;
+
+  // External flag to request the current slice to be interrupted
+  // (and return isOverBudget() early.) Applies only to time-based budgets.
+  InterruptRequestFlag* interruptRequested = nullptr;
+
+  // How many steps to count before checking the time and possibly the interrupt
+  // flag.
+  int64_t counter = StepsPerExpensiveCheck;
+
+  // This SliceBudget is considered interrupted from the time isOverBudget()
+  // finds the interrupt flag set.
+  bool interrupted = false;
+
+  explicit SliceBudget(InterruptRequestFlag* irqPtr)
+      : budget(UnlimitedBudget()),
+        interruptRequested(irqPtr),
+        counter(irqPtr ? StepsPerExpensiveCheck : UnlimitedCounter) {}
+
+  bool checkOverBudget();
+
+ public:
+  // Use to create an unlimited budget.
+  static SliceBudget unlimited() { return SliceBudget(nullptr); }
+
+  // Instantiate as SliceBudget(TimeBudget(n)).
+  explicit SliceBudget(TimeBudget time,
+                       InterruptRequestFlag* interrupt = nullptr);
+
+  explicit SliceBudget(mozilla::TimeDuration duration,
+                       InterruptRequestFlag* interrupt = nullptr)
+      : SliceBudget(TimeBudget(duration.ToMilliseconds()), interrupt) {}
+
+  // Instantiate as SliceBudget(WorkBudget(n)).
+  explicit SliceBudget(WorkBudget work);
+
+  // Register having performed the given number of steps (counted against a
+  // work budget, or progress towards the next time or callback check).
+  void step(uint64_t steps = 1) {
+    MOZ_ASSERT(steps > 0);
+    counter -= steps;
+  }
+
+  // Do enough steps to force an "expensive" (time and/or callback) check on
+  // the next call to isOverBudget. Useful when switching between major phases
+  // of an operation like a cycle collection.
+  void stepAndForceCheck() {
+    if (!isUnlimited()) {
+      counter = 0;
+    }
+  }
+
+  bool isOverBudget() { return counter <= 0 && checkOverBudget(); }
+
+  bool isWorkBudget() const { return budget.is<WorkBudget>(); }
+  bool isTimeBudget() const { return budget.is<TimeBudget>(); }
+  bool isUnlimited() const { return budget.is<UnlimitedBudget>(); }
+
+  mozilla::TimeDuration timeBudgetDuration() const {
+    return budget.as<TimeBudget>().budget;
+  }
+  int64_t timeBudget() const { return timeBudgetDuration().ToMilliseconds(); }
+  int64_t workBudget() const { return budget.as<WorkBudget>().budget; }
+
+  mozilla::TimeStamp deadline() const {
+    return budget.as<TimeBudget>().deadline;
+  }
+
+  int describe(char* buffer, size_t maxlen) const;
+};
+
+}  // namespace js
+
+#endif /* js_SliceBudget_h */