HeapWord* CompactibleSpace::forward(oop q, size_t size,
                                    CompactPoint* cp, HeapWord* compact_top) {

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

      // q is alive
      // First check if we should switch compaction space
      assert(this == cp->space, "'this' should be current compaction space.");

  {- -------------------------------------------
  (1) もし現在のコンパクション先の領域に空きがなければ, 
      CompactPoint で管理しているコンパクト先領域を(同じ世代内の)別の領域に切り替える.
      同じ世代内で空き領域がなくなれば, 一つ若い世代の領域に切り替える.
      ---------------------------------------- -}

      size_t compaction_max_size = pointer_delta(end(), compact_top);
      while (size > compaction_max_size) {

    {- -------------------------------------------
  (1.1) (同じ世代内の別の領域へ切り替え)
        ---------------------------------------- -}

        // switch to next compaction space
        cp->space->set_compaction_top(compact_top);
        cp->space = cp->space->next_compaction_space();

    {- -------------------------------------------
  (1.1) (同じ世代内にもう領域がなければ, 一つ若い世代に切り替え)
        ---------------------------------------- -}

        if (cp->space == NULL) {
          cp->gen = GenCollectedHeap::heap()->prev_gen(cp->gen);
          assert(cp->gen != NULL, "compaction must succeed");
          cp->space = cp->gen->first_compaction_space();
          assert(cp->space != NULL, "generation must have a first compaction space");
        }

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

        compact_top = cp->space->bottom();
        cp->space->set_compaction_top(compact_top);
        cp->threshold = cp->space->initialize_threshold();
        compaction_max_size = pointer_delta(cp->space->end(), compact_top);
      }

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

      // store the forwarding pointer into the mark word
      if ((HeapWord*)q != compact_top) {
        q->forward_to(oop(compact_top));
        assert(q->is_gc_marked(), "encoding the pointer should preserve the mark");
      } else {
        // if the object isn't moving we can just set the mark to the default
        // mark and handle it specially later on.
        q->init_mark();
        assert(q->forwardee() == NULL, "should be forwarded to NULL");
      }

      VALIDATE_MARK_SWEEP_ONLY(MarkSweep::register_live_oop(q, size));
      compact_top += size;

      // we need to update the offset table so that the beginnings of objects can be
      // found during scavenge.  Note that we are updating the offset table based on
      // where the object will be once the compaction phase finishes.
      if (compact_top > cp->threshold)
        cp->threshold =
          cp->space->cross_threshold(compact_top - size, compact_top);
      return compact_top;
    }