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

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.

Libraries/LibJS/Heap/Heap.cpp

@@ -151,43 +151,36 @@ void Heap::gather_conservative_roots(HashTable<Cell*>& roots)
  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) {
  if (!possible_pointer)
  continue;
 #ifdef HEAP_DEBUG
  dbg() << " ? " << (const void*)possible_pointer;
 #endif
- if (auto* cell = cell_from_possible_pointer(possible_pointer)) {
- if (cell->is_live()) {
+ auto* possible_heap_block = HeapBlock::from_cell(reinterpret_cast<const Cell*>(possible_pointer));
+ 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
- dbg() << " ?-> " << (const void*)cell;
+  dbg() << " ?-> " << (const void*)cell;
 #endif
- roots.set(cell);
- } else {
+  roots.set(cell);
+  } else {
 #ifdef HEAP_DEBUG
- dbg() << " #-> " << (const void*)cell;
+  dbg() << " #-> " << (const void*)cell;
 #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 {
 public:
  MarkingVisitor() { }

Libraries/LibJS/Heap/Heap.h

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