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diff --git a/Source/JavaScriptCore/heap/SpaceTimeMutatorScheduler.h b/Source/JavaScriptCore/heap/SpaceTimeMutatorScheduler.h
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+++ b/Source/JavaScriptCore/heap/SpaceTimeMutatorScheduler.h
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+/*
+ * Copyright (C) 2016-2017 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
+ */
+
+#pragma once
+
+#include "MutatorScheduler.h"
+#include <wtf/Seconds.h>
+
+namespace JSC {
+
+class Heap;
+
+// The JSC concurrent GC sometimes stops the world in order to stay ahead of it. These deliberate,
+// synthetic pauses ensure that the GC won't have to do one huge pause in order to catch up to the
+// retreating wavefront. The scheduler is called "space-time" because it links the amount of time
+// that the world is paused for to the amount of space that the world allocated since the GC cycle
+// began.
+
+class SpaceTimeMutatorScheduler : public MutatorScheduler {
+public:
+    SpaceTimeMutatorScheduler(Heap&);
+    ~SpaceTimeMutatorScheduler();
+    
+    State state() const override;
+    
+    void beginCollection() override;
+    
+    void didStop() override;
+    void willResume() override;
+    void didExecuteConstraints() override;
+    
+    MonotonicTime timeToStop() override;
+    MonotonicTime timeToResume() override;
+    
+    void log() override;
+    
+    void endCollection() override;
+    
+private:
+    class Snapshot;
+    friend class Snapshot;
+    
+    double bytesAllocatedThisCycleImpl();
+    
+    double bytesSinceBeginningOfCycle(const Snapshot&);
+    double maxHeadroom();
+    double headroomFullness(const Snapshot&);
+    double mutatorUtilization(const Snapshot&);
+    double collectorUtilization(const Snapshot&);
+    Seconds elapsedInPeriod(const Snapshot&);
+    double phase(const Snapshot&);
+    bool shouldBeResumed(const Snapshot&);
+    
+    Heap& m_heap;
+    Seconds m_period;
+    State m_state { Normal };
+    
+    double m_bytesAllocatedThisCycleAtTheBeginning { 0 };
+    double m_bytesAllocatedThisCycleAtTheEnd { 0 };
+    MonotonicTime m_startTime;
+};
+
+} // namespace JSC
+