void G1CollectedHeap::retire_mutator_alloc_region(HeapRegion* alloc_region,
                                                  size_t allocated_bytes) {

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);
      assert(alloc_region->is_young(), "all mutator alloc regions should be young");

  {- -------------------------------------------
  (1) G1CollectorPolicy::add_region_to_incremental_cset_lhs() を呼んで, 
      引数で渡された HeapRegion (以下の alloc_region) を incremental collection set の左端に追加する.
      ---------------------------------------- -}

      g1_policy()->add_region_to_incremental_cset_lhs(alloc_region);

  {- -------------------------------------------
  (1) _summary_bytes_used フィールドをインクリメントしておく.
      ---------------------------------------- -}

      _summary_bytes_used += allocated_bytes;
    }