LibJS: Replace Cell live bit with a cell state

So far we only have two states: Live and Dead. In the future, we can
add additional states to support incremental sweeping and/or multi-
stage cell destruction.

Userland/Libraries/LibJS/Heap/Cell.h

@@ -26,8 +26,13 @@ public:
     bool is_marked() const { return m_mark; }
     void set_marked(bool b) { m_mark = b; }
 
-    bool is_live() const { return m_live; }
-    void set_live(bool b) { m_live = b; }
+    enum class State {
+        Live,
+        Dead,
+    };
+
+    State state() const { return m_state; }
+    void set_state(State state) { m_state = state; }
 
     virtual const char* class_name() const = 0;
 
@@ -54,8 +59,8 @@ protected:
     Cell() { }
 
 private:
-    bool m_mark { false };
-    bool m_live { true };
+    bool m_mark : 1 { false };
+    State m_state : 7 { State::Live };
 };
 
 }

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -140,7 +140,7 @@ __attribute__((no_sanitize("address"))) void Heap::gather_conservative_roots(Has
         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()) {
+                if (cell->state() == Cell::State::Live) {
                     dbgln_if(HEAP_DEBUG, "  ?-> {}", (const void*)cell);
                     roots.set(cell);
                 } else {
@@ -187,19 +187,17 @@ void Heap::sweep_dead_cells(bool print_report, const Core::ElapsedTimer& measure
     for_each_block([&](auto& block) {
         bool block_has_live_cells = false;
         bool block_was_full = block.is_full();
-        block.for_each_cell([&](Cell* cell) {
-            if (cell->is_live()) {
-                if (!cell->is_marked()) {
-                    dbgln_if(HEAP_DEBUG, "  ~ {}", cell);
-                    block.deallocate(cell);
-                    ++collected_cells;
-                    collected_cell_bytes += block.cell_size();
-                } else {
-                    cell->set_marked(false);
-                    block_has_live_cells = true;
-                    ++live_cells;
-                    live_cell_bytes += block.cell_size();
-                }
+        block.template for_each_cell_in_state<Cell::State::Live>([&](Cell* cell) {
+            if (!cell->is_marked()) {
+                dbgln_if(HEAP_DEBUG, "  ~ {}", cell);
+                block.deallocate(cell);
+                ++collected_cells;
+                collected_cell_bytes += block.cell_size();
+            } else {
+                cell->set_marked(false);
+                block_has_live_cells = true;
+                ++live_cells;
+                live_cell_bytes += block.cell_size();
             }
         });
         if (!block_has_live_cells)

Userland/Libraries/LibJS/Heap/HeapBlock.cpp

@@ -47,11 +47,11 @@ void HeapBlock::deallocate(Cell* cell)
 {
     VERIFY(is_valid_cell_pointer(cell));
     VERIFY(!m_freelist || is_valid_cell_pointer(m_freelist));
-    VERIFY(cell->is_live());
+    VERIFY(cell->state() == Cell::State::Live);
     VERIFY(!cell->is_marked());
     cell->~Cell();
     auto* freelist_entry = new (cell) FreelistEntry();
-    freelist_entry->set_live(false);
+    freelist_entry->set_state(Cell::State::Dead);
     freelist_entry->next = m_freelist;
     m_freelist = freelist_entry;
 }

Userland/Libraries/LibJS/Heap/HeapBlock.h

@@ -48,6 +48,15 @@ public:
             callback(cell(i));
     }
 
+    template<Cell::State state, typename Callback>
+    void for_each_cell_in_state(Callback callback)
+    {
+        for_each_cell([&](auto* cell) {
+            if (cell->state() == state)
+                callback(cell);
+        });
+    }
+
     Heap& heap() { return m_heap; }
 
     static HeapBlock* from_cell(const Cell* cell)