// Traverse the list and remove any Refs that are not active, or
// whose referents are either alive or NULL.

void
ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
                             BoolObjectClosure* is_alive,
                             OopClosure*        keep_alive) {

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

      assert(discovery_is_atomic(), "Error");

  {- -------------------------------------------
  (1) (以下の while 文の中で, (DiscoveredListIterator を使って) 
       引数で渡された DiscoveredList を最初から最後まで辿って処理を行う)
      ---------------------------------------- -}

      DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
      while (iter.has_next()) {

  {- -------------------------------------------
  (1) もし, 参照オブジェクトの差し先が生きていれば, 
      その参照オブジェクトはリストから除外する.
      (ついでに, DiscoveredListIterator::make_referent_alive() で
       (必要に応じて) 差し先へのポインタの修正処理を行っておく)

      (ついでに, 除外処理を行う際には(トレース出力)も出している)
      ---------------------------------------- -}

        iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
        DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
        assert(next == NULL, "Should not discover inactive Reference");
        if (iter.is_referent_alive()) {
          if (TraceReferenceGC) {
            gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
                                   iter.obj(), iter.obj()->blueprint()->internal_name());
          }
          // The referent is reachable after all.
          // Remove Reference object from list.
          iter.remove();
          // Update the referent pointer as necessary: Note that this
          // should not entail any recursive marking because the
          // referent must already have been traversed.
          iter.make_referent_alive();
          iter.move_to_next();
        } else {
          iter.next();
        }
      }

  {- -------------------------------------------
  (1) (トレース出力)
      ---------------------------------------- -}

      NOT_PRODUCT(
        if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
          gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
            "Refs in discovered list " INTPTR_FORMAT,
            iter.removed(), iter.processed(), (address)refs_list.head());
        }
      )
    }