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LibJS: Use a HashTable to identify potential cell pointers in GC scan

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Previously we would iterate over all the live HeapBlocks in order to
learn if an arbitrary pointer-sized value was a pointer into a live
HeapBlock. This was quite time-consuming.

Instead of that, just put all the live HeapBlock*'s in a HashTable
and identify pointers by doing a bit-masked lookup into the table.
This commit is contained in:
Andreas Kling 2020-11-10 20:24:08 +01:00
parent 50aa726db7
commit 1745e503aa
2 changed files with 15 additions and 24 deletions
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37
Libraries/LibJS/Heap/Heap.cpp
View file

@ -151,43 +151,36 @@ void Heap::gather_conservative_roots(HashTable<Cell*>& roots)
possible_pointers.set(data); possible_pointers.set(data);
} }
HashTable<HeapBlock*> all_live_heap_blocks;
for_each_block([&](auto& block) {
all_live_heap_blocks.set(&block);
return IterationDecision::Continue;
});
for (auto possible_pointer : possible_pointers) { for (auto possible_pointer : possible_pointers) {
if (!possible_pointer) if (!possible_pointer)
continue; continue;
#ifdef HEAP_DEBUG #ifdef HEAP_DEBUG
dbg() << " ? " << (const void*)possible_pointer; dbg() << " ? " << (const void*)possible_pointer;
#endif #endif
if (auto* cell = cell_from_possible_pointer(possible_pointer)) { auto* possible_heap_block = HeapBlock::from_cell(reinterpret_cast<const Cell*>(possible_pointer));
if (cell->is_live()) { if (all_live_heap_blocks.contains(possible_heap_block)) {
if (auto* cell = possible_heap_block->cell_from_possible_pointer(possible_pointer)) {
if (cell->is_live()) {
#ifdef HEAP_DEBUG #ifdef HEAP_DEBUG
dbg() << " ?-> " << (const void*)cell; dbg() << " ?-> " << (const void*)cell;
#endif #endif
roots.set(cell); roots.set(cell);
} else { } else {
#ifdef HEAP_DEBUG #ifdef HEAP_DEBUG
dbg() << " #-> " << (const void*)cell; dbg() << " #-> " << (const void*)cell;
#endif #endif
}
} }
} }
} }
} }
Cell* Heap::cell_from_possible_pointer(FlatPtr pointer)
{
auto* possible_heap_block = HeapBlock::from_cell(reinterpret_cast<const Cell*>(pointer));
bool found = false;
for_each_block([&](auto& block) {
if (&block == possible_heap_block) {
found = true;
return IterationDecision::Break;
}
return IterationDecision::Continue;
});
if (!found)
return nullptr;
return possible_heap_block->cell_from_possible_pointer(pointer);
}
class MarkingVisitor final : public Cell::Visitor { class MarkingVisitor final : public Cell::Visitor {
public: public:
MarkingVisitor() { } MarkingVisitor() { }

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

@ -100,8 +100,6 @@ private:
void mark_live_cells(const HashTable<Cell*>& live_cells); void mark_live_cells(const HashTable<Cell*>& live_cells);
void sweep_dead_cells(bool print_report, const Core::ElapsedTimer&); void sweep_dead_cells(bool print_report, const Core::ElapsedTimer&);
Cell* cell_from_possible_pointer(FlatPtr);
Allocator& allocator_for_size(size_t); Allocator& allocator_for_size(size_t);
template<typename Callback> template<typename Callback>