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diff --git a/Source/JavaScriptCore/heap/CollectorPhase.h b/Source/JavaScriptCore/heap/CollectorPhase.h
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+/*
+ * Copyright (C) 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. AND ITS CONTRIBUTORS ``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 ITS 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
+
+namespace JSC {
+
+// We track collector phase in order to allow either the collector thread or the mutator thread to
+// jump in and do work. The collector and mutator trade the conn
+// (https://en.wikipedia.org/wiki/Conn_(nautical)) with each other based on who is available to do work,
+// and they use the phase to help each other know what to do once they take the conn.
+//
+// The collector relinquishes the conn whenever it stopTheMutator's and the mutator is running. Then the
+// collector thread goes to sleep.
+//
+// The mutator relinquishes the conn whenever it releaseAccess's. That wakes up the collector thread.
+enum class CollectorPhase : uint8_t {
+    // We use this phase when the collector is not running at all. After this state is Begin.
+    NotRunning,
+    
+    // This is everything from when the collector begins to when it first yields to the mutator for
+    // marking. After this is Fixpoint.
+    Begin,
+    
+    // This means that we should try to do some progress with the world stopped. This usually means
+    // doing an iteration of MarkingConstraintSet::executeConvergence, but it could also mean marking
+    // with the world stopped. After this is either Concurrent or End.
+    Fixpoint,
+    
+    // In this state the collector is relying on the parallel helpers and incremental mutator work to
+    // make progress. After this is Reloop, once marking stalls.
+    Concurrent,
+        
+    // We did some concurrent marking and now we ran out of work. This phase prepares the GC for another
+    // Fixpoint. After this is Fixpoint.
+    Reloop,
+    
+    // The collector is trying to finish up. After this state is NotRunning.
+    End
+};
+
+bool worldShouldBeSuspended(CollectorPhase phase);
+
+} // namespace JSC
+
+namespace WTF {
+
+class PrintStream;
+
+void printInternal(PrintStream&, JSC::CollectorPhase);
+
+} // namespace WTF
+