G1 currently tracks collection set membership in a byte field in the HeapRegion structure and in the _cset_fast_test array (an array of bytes, one per region). There are also three methods for checking collection set membership: HeapRegion::in_collection_set(), G1CollectedHeap::obj_in_cs() and G1CollectedHeap::in_cset_fast_test(), along with a number of asserts to ensure the two data structures are in sync. Remove the HeapRegion byte field and associated accessors and track membership only in the cset array.