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25a5ed94d6
This is a weak pointer that integrates with the garbage collector. It has a number of differences compared to AK::WeakPtr, including: - The "control block" is allocated from a well-packed WeakBlock owned by the GC heap, not just a generic malloc allocation. - Pointers to dead cells are nulled out by the garbage collector immediately before running destructors. - It works on any GC::Cell derived type, meaning you don't have to inherit from AK::Weakable for the ability to be weakly referenced. - The Weak always points to a control block, even when "null" (it then points to a null WeakImpl), which means one less null check when chasing pointers.
77 lines
1.9 KiB
C++
77 lines
1.9 KiB
C++
/*
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* Copyright (c) 2025, Andreas Kling <andreas@ladybird.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <LibGC/Cell.h>
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#include <LibGC/WeakBlock.h>
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#include <sys/mman.h>
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#if defined(AK_OS_WINDOWS)
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# include <AK/Windows.h>
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# include <memoryapi.h>
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#endif
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namespace GC {
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WeakImpl WeakImpl::the_null_weak_impl;
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WeakBlock* WeakBlock::create()
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{
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#if !defined(AK_OS_WINDOWS)
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auto* block = (HeapBlock*)mmap(nullptr, WeakBlock::BLOCK_SIZE, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
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VERIFY(block != MAP_FAILED);
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#else
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auto* block = (HeapBlock*)VirtualAlloc(NULL, WeakBlock::BLOCK_SIZE, MEM_COMMIT, PAGE_READWRITE);
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VERIFY(block);
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#endif
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return new (block) WeakBlock;
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}
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WeakBlock::WeakBlock()
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{
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for (size_t i = 0; i < IMPL_COUNT; ++i) {
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m_impls[i].set_ptr({}, i + 1 < IMPL_COUNT ? &m_impls[i + 1] : nullptr);
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m_impls[i].set_state(WeakImpl::State::Freelist);
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}
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m_freelist = &m_impls[0];
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}
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WeakBlock::~WeakBlock() = default;
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WeakImpl* WeakBlock::allocate(Cell* cell)
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{
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auto* impl = m_freelist;
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if (!impl)
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return nullptr;
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VERIFY(impl->ref_count() == 0);
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m_freelist = impl->ptr() ? static_cast<WeakImpl*>(impl->ptr()) : nullptr;
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impl->set_ptr({}, cell);
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impl->set_state(WeakImpl::State::Allocated);
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return impl;
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}
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void WeakBlock::deallocate(WeakImpl* impl)
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{
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VERIFY(impl->ref_count() == 0);
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impl->set_ptr({}, m_freelist);
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impl->set_state(WeakImpl::State::Freelist);
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m_freelist = impl;
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}
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void WeakBlock::sweep()
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{
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for (size_t i = 0; i < IMPL_COUNT; ++i) {
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auto& impl = m_impls[i];
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if (impl.state() == WeakImpl::State::Freelist)
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continue;
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auto* cell = static_cast<Cell*>(impl.ptr());
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if (!cell || !cell->is_marked())
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impl.set_ptr({}, nullptr);
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if (impl.ref_count() == 0)
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deallocate(&impl);
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}
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}
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}
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