MarkedBlock.cpp

/*
 * Copyright (C) 2011 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
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 *
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 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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#include "config.h"
#include "MarkedBlock.h"

#include "JSCell.h"
#include "JSObject.h"
#include "ScopeChain.h"

namespace JSC {

MarkedBlock* MarkedBlock::create(const PageAllocationAligned& allocation, Heap* heap, size_t cellSize, bool cellsNeedDestruction)
{
    return new (NotNull, allocation.base()) MarkedBlock(allocation, heap, cellSize, cellsNeedDestruction);
}

PageAllocationAligned MarkedBlock::destroy(MarkedBlock* block)
{
    PageAllocationAligned allocation;
    swap(allocation, block->m_allocation);

    block->~MarkedBlock();
    return allocation;
}

MarkedBlock::MarkedBlock(const PageAllocationAligned& allocation, Heap* heap, size_t cellSize, bool cellsNeedDestruction)
    : HeapBlock(allocation)
    , m_atomsPerCell((cellSize + atomSize - 1) / atomSize)
    , m_endAtom(atomsPerBlock - m_atomsPerCell + 1)
    , m_cellsNeedDestruction(cellsNeedDestruction)
    , m_state(New) // All cells start out unmarked.
    , m_weakSet(heap)
{
    ASSERT(heap);
    HEAP_LOG_BLOCK_STATE_TRANSITION(this);
}

inline void MarkedBlock::callDestructor(JSCell* cell)
{
    // A previous eager sweep may already have run cell's destructor.
    if (cell->isZapped())
        return;

#if ENABLE(SIMPLE_HEAP_PROFILING)
    m_heap->m_destroyedTypeCounts.countVPtr(vptr);
#endif

#if !ASSERT_DISABLED || ENABLE(GC_VALIDATION)
    cell->clearStructure();
#endif

    cell->methodTable()->destroy(cell);
    cell->zap();
}

template<MarkedBlock::BlockState blockState, MarkedBlock::SweepMode sweepMode, bool destructorCallNeeded>
MarkedBlock::FreeList MarkedBlock::specializedSweep()
{
    ASSERT(blockState != Allocated && blockState != FreeListed);
    ASSERT(destructorCallNeeded || sweepMode != SweepOnly);

    // This produces a free list that is ordered in reverse through the block.
    // This is fine, since the allocation code makes no assumptions about the
    // order of the free list.
    FreeCell* head = 0;
    size_t count = 0;
    for (size_t i = firstAtom(); i < m_endAtom; i += m_atomsPerCell) {
        if (blockState == Marked && m_marks.get(i))
            continue;

        JSCell* cell = reinterpret_cast_ptr<JSCell*>(&atoms()[i]);
        if (blockState == Zapped && !cell->isZapped())
            continue;

        if (destructorCallNeeded && blockState != New)
            callDestructor(cell);

        if (sweepMode == SweepToFreeList) {
            FreeCell* freeCell = reinterpret_cast<FreeCell*>(cell);
            freeCell->next = head;
            head = freeCell;
            ++count;
        }
    }

    m_state = ((sweepMode == SweepToFreeList) ? FreeListed : Zapped);
    return FreeList(head, count * cellSize());
}

MarkedBlock::FreeList MarkedBlock::sweep(SweepMode sweepMode)
{
    HEAP_LOG_BLOCK_STATE_TRANSITION(this);

    if (sweepMode == SweepOnly && !m_cellsNeedDestruction)
        return FreeList();

    if (m_cellsNeedDestruction)
        return sweepHelper<true>(sweepMode);
    return sweepHelper<false>(sweepMode);
}

template<bool destructorCallNeeded>
MarkedBlock::FreeList MarkedBlock::sweepHelper(SweepMode sweepMode)
{
    switch (m_state) {
    case New:
        ASSERT(sweepMode == SweepToFreeList);
        return specializedSweep<New, SweepToFreeList, destructorCallNeeded>();
    case FreeListed:
        // Happens when a block transitions to fully allocated.
        ASSERT(sweepMode == SweepToFreeList);
        return FreeList();
    case Allocated:
        ASSERT_NOT_REACHED();
        return FreeList();
    case Marked:
        return sweepMode == SweepToFreeList
            ? specializedSweep<Marked, SweepToFreeList, destructorCallNeeded>()
            : specializedSweep<Marked, SweepOnly, destructorCallNeeded>();
    case Zapped:
        return sweepMode == SweepToFreeList
            ? specializedSweep<Zapped, SweepToFreeList, destructorCallNeeded>()
            : specializedSweep<Zapped, SweepOnly, destructorCallNeeded>();
    }

    ASSERT_NOT_REACHED();
    return FreeList();
}

void MarkedBlock::zapFreeList(const FreeList& freeList)
{
    HEAP_LOG_BLOCK_STATE_TRANSITION(this);
    FreeCell* head = freeList.head;

    if (m_state == Marked) {
        // If the block is in the Marked state then we know that:
        // 1) It was not used for allocation during the previous allocation cycle.
        // 2) It may have dead objects, and we only know them to be dead by the
        //    fact that their mark bits are unset.
        // Hence if the block is Marked we need to leave it Marked.
        
        ASSERT(!head);
        
        return;
    }
    
    if (m_state == Zapped) {
        // If the block is in the Zapped state then we know that someone already
        // zapped it for us. This could not have happened during a GC, but might
        // be the result of someone having done a GC scan to perform some operation
        // over all live objects (or all live blocks). It also means that somebody
        // had allocated in this block since the last GC, swept all dead objects
        // onto the free list, left the block in the FreeListed state, then the heap
        // scan happened, and canonicalized the block, leading to all dead objects
        // being zapped. Therefore, it is safe for us to simply do nothing, since
        // dead objects will have 0 in their vtables and live objects will have
        // non-zero vtables, which is consistent with the block being zapped.
        
        ASSERT(!head);
        
        return;
    }
    
    ASSERT(m_state == FreeListed);
    
    // Roll back to a coherent state for Heap introspection. Cells newly
    // allocated from our free list are not currently marked, so we need another
    // way to tell what's live vs dead. We use zapping for that.
    
    FreeCell* next;
    for (FreeCell* current = head; current; current = next) {
        next = current->next;
        reinterpret_cast<JSCell*>(current)->zap();
    }
    
    m_state = Zapped;
}

} // namespace JSC