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diff --git a/Source/JavaScriptCore/heap/HeapCell.h b/Source/JavaScriptCore/heap/HeapCell.h
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+++ b/Source/JavaScriptCore/heap/HeapCell.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 "DestructionMode.h"
+
+namespace JSC {
+
+class CellContainer;
+class Heap;
+class LargeAllocation;
+class MarkedBlock;
+class VM;
+struct AllocatorAttributes;
+
+class HeapCell {
+public:
+    enum Kind : int8_t {
+        JSCell,
+        Auxiliary
+    };
+    
+    HeapCell() { }
+    
+    void zap() { *reinterpret_cast_ptr<uintptr_t**>(this) = 0; }
+    bool isZapped() const { return !*reinterpret_cast_ptr<uintptr_t* const*>(this); }
+    
+    bool isLargeAllocation() const;
+    CellContainer cellContainer() const;
+    MarkedBlock& markedBlock() const;
+    LargeAllocation& largeAllocation() const;
+
+    // If you want performance and you know that your cell is small, you can do this instead:
+    // ASSERT(!cell->isLargeAllocation());
+    // cell->markedBlock().vm()
+    // We currently only use this hack for callees to make ExecState::vm() fast. It's not
+    // recommended to use it for too many other things, since the large allocation cutoff is
+    // a runtime option and its default value is small (400 bytes).
+    Heap* heap() const;
+    VM* vm() const;
+    
+    size_t cellSize() const;
+    AllocatorAttributes allocatorAttributes() const;
+    DestructionMode destructionMode() const;
+    Kind cellKind() const;
+    
+    // Call use() after the last point where you need `this` pointer to be kept alive. You usually don't
+    // need to use this, but it might be necessary if you're otherwise referring to an object's innards
+    // but not the object itself.
+#if COMPILER(GCC_OR_CLANG)
+    void use() const
+    {
+        asm volatile ("" : : "r"(this) : "memory");
+    }
+#else
+    void use() const;
+#endif
+};
+
+} // namespace JSC
+
+namespace WTF {
+
+class PrintStream;
+
+void printInternal(PrintStream&, JSC::HeapCell::Kind);
+
+} // namespace WTF
+