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LibJS: Make the GC marking phase cycle-proof

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Don't visit cells that are already marked. This prevents the marking
phase from looping forever when two cells refer to each other.

Also do the marking directly from the CellVisitor, removing another
unnecessary phase of the collector. :^)
This commit is contained in:
Andreas Kling 2020-03-09 22:11:22 +01:00
parent 70a3e738f5
commit 05c80cac20
5 changed files with 24 additions and 51 deletions
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59
Libraries/LibJS/Heap.cpp
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@ -63,11 +63,7 @@ void Heap::collect_garbage()
{
HashTable<Cell*> roots;
collect_roots(roots);
HashTable<Cell*> live_cells;
visit_live_cells(roots, live_cells);
mark_live_cells(live_cells);
mark_live_cells(roots);
sweep_dead_cells();
}
@ -83,48 +79,30 @@ void Heap::collect_roots(HashTable<Cell*>& roots)
#endif
}
class LivenessVisitor final : public Cell::Visitor {
class MarkingVisitor final : public Cell::Visitor {
public:
LivenessVisitor(HashTable<Cell*>& live_cells)
: m_live_cells(live_cells)
MarkingVisitor() {}
virtual void visit(Cell* cell)
{
}
virtual void did_visit(Cell* cell) override
{
m_live_cells.set(cell);
}
private:
HashTable<Cell*>& m_live_cells;
};
void Heap::visit_live_cells(const HashTable<Cell*>& roots, HashTable<Cell*>& live_cells)
{
LivenessVisitor visitor(live_cells);
for (auto* root : roots) {
root->visit_graph(visitor);
}
#ifdef HEAP_DEBUG
dbg() << "visit_live_cells:";
for (auto* cell : live_cells) {
dbg() << " @ " << cell;
}
#endif
}
void Heap::mark_live_cells(const HashTable<Cell*>& live_cells)
{
#ifdef HEAP_DEBUG
dbg() << "mark_live_cells:";
#endif
for (auto& cell : live_cells) {
if (cell->is_marked())
return;
#ifdef HEAP_DEBUG
dbg() << " ! " << cell;
#endif
cell->set_marked(true);
cell->visit_children(*this);
}
};
void Heap::mark_live_cells(const HashTable<Cell*>& roots)
{
#ifdef HEAP_DEBUG
dbg() << "mark_live_cells:";
#endif
MarkingVisitor visitor;
for (auto* root : roots)
visitor.visit(root);
}
void Heap::sweep_dead_cells()
@ -147,5 +125,4 @@ void Heap::sweep_dead_cells()
});
}
}
}