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LibJS: Add Cell::must_survive_garbage_collection() mechanism

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This allows cells to prevent themselves from being garbage collected,
even when there are no references to them.
This commit is contained in:
Andreas Kling 2022-10-24 12:37:54 +02:00
parent 8ace6b4f1d
commit 51579810bd
Notes: sideshowbarker 2024-07-17 05:09:31 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/51579810bd
3 changed files with 24 additions and 4 deletions
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15
Userland/Libraries/LibJS/Heap/Cell.h
View file

@ -1,11 +1,12 @@
/*
* Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2020-2022, Andreas Kling <kling@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/Badge.h>
#include <AK/Format.h>
#include <AK/Forward.h>
#include <AK/Noncopyable.h>
@ -80,15 +81,25 @@ public:
// This will be called on unmarked objects by the garbage collector in a separate pass before destruction.
virtual void finalize() { }
// This allows cells to survive GC by choice, even if nothing points to them.
// It's used to implement special rules in the web platform.
// NOTE: Cells must call set_overrides_must_survive_garbage_collection() for this to be honored.
virtual bool must_survive_garbage_collection() const { return false; }
bool overrides_must_survive_garbage_collection(Badge<Heap>) const { return m_overrides_must_survive_garbage_collection; }
Heap& heap() const;
VM& vm() const;
protected:
Cell() = default;
void set_overrides_must_survive_garbage_collection(bool b) { m_overrides_must_survive_garbage_collection = b; }
private:
bool m_mark : 1 { false };
State m_state : 7 { State::Live };
bool m_overrides_must_survive_garbage_collection { false };
State m_state : 1 { State::Live };
};
}

11
Userland/Libraries/LibJS/Heap/Heap.cpp
View file

@ -232,11 +232,18 @@ void Heap::mark_live_cells(HashTable<Cell*> const& roots)
m_uprooted_cells.clear();
}
bool Heap::cell_must_survive_garbage_collection(Cell const& cell)
{
if (!cell.overrides_must_survive_garbage_collection({}))
return false;
return cell.must_survive_garbage_collection();
}
void Heap::finalize_unmarked_cells()
{
for_each_block([&](auto& block) {
block.template for_each_cell_in_state<Cell::State::Live>([](Cell* cell) {
if (!cell->is_marked())
if (!cell->is_marked() && !cell_must_survive_garbage_collection(*cell))
cell->finalize();
});
return IterationDecision::Continue;
@ -258,7 +265,7 @@ void Heap::sweep_dead_cells(bool print_report, Core::ElapsedTimer const& measure
bool block_has_live_cells = false;
bool block_was_full = block.is_full();
block.template for_each_cell_in_state<Cell::State::Live>([&](Cell* cell) {
if (!cell->is_marked()) {
if (!cell->is_marked() && !cell_must_survive_garbage_collection(*cell)) {
dbgln_if(HEAP_DEBUG, " ~ {}", cell);
block.deallocate(cell);
++collected_cells;

2
Userland/Libraries/LibJS/Heap/Heap.h
View file

@ -79,6 +79,8 @@ public:
void uproot_cell(Cell* cell);
private:
static bool cell_must_survive_garbage_collection(Cell const&);
Cell* allocate_cell(size_t);
void gather_roots(HashTable<Cell*>&);