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diff --git a/Source/JavaScriptCore/heap/Subspace.h b/Source/JavaScriptCore/heap/Subspace.h
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+++ b/Source/JavaScriptCore/heap/Subspace.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. ``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 "MarkedBlock.h"
+#include "MarkedSpace.h"
+#include <wtf/text/CString.h>
+
+namespace JSC {
+
+// The idea of subspaces is that you can provide some custom behavior for your objects if you
+// allocate them from a custom Subspace in which you override some of the virtual methods. This
+// class is the baseclass of Subspaces and it provides a reasonable default implementation, where
+// sweeping assumes immortal structure. The common ways of overriding this are:
+//
+// - Provide customized destructor behavior. You can change how the destructor is called. You can
+//   also specialize the destructor call in the loop.
+//
+// - Use the Subspace as a quick way to iterate all of the objects in that subspace.
+class Subspace {
+    WTF_MAKE_NONCOPYABLE(Subspace);
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    JS_EXPORT_PRIVATE Subspace(CString name, Heap&, AllocatorAttributes);
+    JS_EXPORT_PRIVATE virtual ~Subspace();
+    
+    const char *name() const { return m_name.data(); }
+    MarkedSpace& space() const { return m_space; }
+    
+    const AllocatorAttributes& attributes() const { return m_attributes; }
+    
+    // The purpose of overriding this is to specialize the sweep for your destructors. This won't
+    // be called for no-destructor blocks. This must call MarkedBlock::finishSweepKnowingSubspace.
+    virtual FreeList finishSweep(MarkedBlock::Handle&, MarkedBlock::Handle::SweepMode);
+    
+    // These get called for large objects.
+    virtual void destroy(VM&, JSCell*);
+    
+    MarkedAllocator* tryAllocatorFor(size_t);
+    MarkedAllocator* allocatorFor(size_t);
+    
+    JS_EXPORT_PRIVATE void* allocate(size_t);
+    JS_EXPORT_PRIVATE void* allocate(GCDeferralContext*, size_t);
+    
+    JS_EXPORT_PRIVATE void* tryAllocate(size_t);
+    JS_EXPORT_PRIVATE void* tryAllocate(GCDeferralContext*, size_t);
+    
+    template<typename Func>
+    void forEachMarkedBlock(const Func&);
+    
+    template<typename Func>
+    void forEachNotEmptyMarkedBlock(const Func&);
+    
+    template<typename Func>
+    void forEachLargeAllocation(const Func&);
+    
+    template<typename Func>
+    void forEachMarkedCell(const Func&);
+    
+    static ptrdiff_t offsetOfAllocatorForSizeStep() { return OBJECT_OFFSETOF(Subspace, m_allocatorForSizeStep); }
+    
+    MarkedAllocator** allocatorForSizeStep() { return &m_allocatorForSizeStep[0]; }
+
+private:
+    MarkedAllocator* allocatorForSlow(size_t);
+    
+    // These slow paths are concerned with large allocations and allocator creation.
+    void* allocateSlow(GCDeferralContext*, size_t);
+    void* tryAllocateSlow(GCDeferralContext*, size_t);
+    
+    MarkedSpace& m_space;
+    
+    CString m_name;
+    AllocatorAttributes m_attributes;
+    
+    std::array<MarkedAllocator*, MarkedSpace::numSizeClasses> m_allocatorForSizeStep;
+    MarkedAllocator* m_firstAllocator { nullptr };
+    SentinelLinkedList<LargeAllocation, BasicRawSentinelNode<LargeAllocation>> m_largeAllocations;
+};
+
+ALWAYS_INLINE MarkedAllocator* Subspace::tryAllocatorFor(size_t size)
+{
+    if (size <= MarkedSpace::largeCutoff)
+        return m_allocatorForSizeStep[MarkedSpace::sizeClassToIndex(size)];
+    return nullptr;
+}
+
+ALWAYS_INLINE MarkedAllocator* Subspace::allocatorFor(size_t size)
+{
+    if (size <= MarkedSpace::largeCutoff) {
+        if (MarkedAllocator* result = m_allocatorForSizeStep[MarkedSpace::sizeClassToIndex(size)])
+            return result;
+        return allocatorForSlow(size);
+    }
+    return nullptr;
+}
+
+} // namespace JSC
+