Maintain a HashTable<HeapBlock*> of live heap blocks in the Heap,
updated on block creation and destruction.
Weak containers (WeakMap, WeakSet, WeakRef, FinalizationRegistry)
now check block liveness before accessing cell memory in their
remove_dead_cells() methods. This prevents use-after-free when
blocks have been freed during incremental sweeping.
Instead of sweeping all heap blocks in one go after marking, sweep
incrementally, one block at a time, interleaved with program execution.
This significantly reduces worst-case GC pause times by spreading
sweep work across multiple smaller time slices.
Sweep is driven by two complementary mechanisms:
1. Timer-based sweeping: A 16ms repeating timer drives background
sweep work, processing blocks for up to 5ms per timer fire.
2. Allocation-directed sweeping: Each allocator sweeps its own
pending blocks before creating new ones, ensuring forward
progress even without timer events.
Each allocator maintains its own list of blocks pending sweep,
and allocators with pending work are tracked in a separate list
for efficient timer-driven sweeping.
Key implementation details:
- Newly allocated cells during sweep are marked immediately to
prevent premature collection.
- Mark bits are cleared incrementally as each block is swept,
rather than in a separate pass over the entire heap.
- Finalization and weak reference processing remain stop-the-world
since they must complete atomically before any sweeping occurs.
Instead of checking if every single cell overrides the "must survive GC"
virtual, we can make this a HeapBlock level thing.
This avoids almost an entire GC heap traversal during the mark phase.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
