Source/JavaScriptCore/heap/Subspace.h

/*
 * Copyright (C) 2017 Apple Inc. All rights reserved.
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 * 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
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 *    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
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
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#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