Source/JavaScriptCore/heap/IsoCellSet.h

/*
 * Copyright (C) 2017-2021 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 * 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
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#pragma once

#include "MarkedBlock.h"
#include <wtf/Bitmap.h>
#include <wtf/ConcurrentVector.h>
#include <wtf/FastBitVector.h>
#include <wtf/SentinelLinkedList.h>
#include <wtf/SharedTask.h>

namespace JSC {

class HeapCell;
class IsoSubspace;

// Create a set of cells that are in an IsoSubspace. This allows concurrent O(1) set insertion and
// removal. Each such set should be thought of as a 0.8% increase in object size for objects in that
// IsoSubspace (it's like adding 1 bit every 16 bytes, or 1 bit every 128 bits).
class IsoCellSet : public PackedRawSentinelNode<IsoCellSet> {
public:
    IsoCellSet(IsoSubspace& subspace);
    ~IsoCellSet();
    
    bool add(HeapCell* cell); // Returns true if the cell was newly added.
    
    bool remove(HeapCell* cell); // Returns true if the cell was previously present and got removed.
    
    bool contains(HeapCell* cell) const;
    
    JS_EXPORT_PRIVATE Ref<SharedTask<MarkedBlock::Handle*()>> parallelNotEmptyMarkedBlockSource();
    
    // This will have to do a combined search over whatever Subspace::forEachMarkedCell uses and
    // our m_blocksWithBits.
    template<typename Func>
    void forEachMarkedCell(const Func&);

    template<typename Visitor, typename Func>
    Ref<SharedTask<void(Visitor&)>> forEachMarkedCellInParallel(const Func&);
    
    template<typename Func>
    void forEachLiveCell(const Func&);
    
private:
    friend class IsoSubspace;
    
    Bitmap<MarkedBlock::atomsPerBlock>* addSlow(unsigned blockIndex);
    
    void didResizeBits(unsigned newSize);
    void didRemoveBlock(unsigned blockIndex);
    void sweepToFreeList(MarkedBlock::Handle*);
    void clearLowerTierCell(unsigned);
    
    Bitmap<MarkedBlock::maxNumberOfLowerTierCells> m_lowerTierBits;

    IsoSubspace& m_subspace;
    
    // Idea: sweeping to free-list clears bits for those cells that were free-listed. The first time
    // we add a cell in a block, that block gets a free-list. Unless we do something that obviously
    // clears all bits for a block, we keep it set in blocksWithBits.
    
    FastBitVector m_blocksWithBits;
    ConcurrentVector<std::unique_ptr<Bitmap<MarkedBlock::atomsPerBlock>>> m_bits;
};

} // namespace JSC