void GenMarkSweep::mark_sweep_phase1(int level,
                                  bool clear_all_softrefs) {

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

      // Recursively traverse all live objects and mark them
      EventMark m("1 mark object");
      TraceTime tm("phase 1", PrintGC && Verbose, true, gclog_or_tty);
      trace(" 1");

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

      VALIDATE_MARK_SWEEP_ONLY(reset_live_oop_tracking(false));

  {- -------------------------------------------
  (1) (変数宣言など)
      ---------------------------------------- -}

      GenCollectedHeap* gch = GenCollectedHeap::heap();

  {- -------------------------------------------
  (1) まず, GenCollectedHeap::gen_process_strong_roots() を呼び出して root から参照されているオブジェクトを全て処理する.
      ---------------------------------------- -}

      // Because follow_root_closure is created statically, cannot
      // use OopsInGenClosure constructor which takes a generation,
      // as the Universe has not been created when the static constructors
      // are run.
      follow_root_closure.set_orig_generation(gch->get_gen(level));

      gch->gen_process_strong_roots(level,
                                    false, // Younger gens are not roots.
                                    true,  // activate StrongRootsScope
                                    true,  // Collecting permanent generation.
                                    SharedHeap::SO_SystemClasses,
                                    &follow_root_closure,
                                    true,   // walk code active on stacks
                                    &follow_root_closure);

  {- -------------------------------------------
  (1) 次に, ReferenceProcessor::process_discovered_references() を呼び出して, 
      上の処理中に発見された参照オブジェクト(java.lang.ref オブジェクト) の処理を行う.
      ---------------------------------------- -}

      // Process reference objects found during marking
      {
        ref_processor()->setup_policy(clear_all_softrefs);
        ref_processor()->process_discovered_references(
          &is_alive, &keep_alive, &follow_stack_closure, NULL);
      }

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

      // Follow system dictionary roots and unload classes
      bool purged_class = SystemDictionary::do_unloading(&is_alive);

      // Follow code cache roots
      CodeCache::do_unloading(&is_alive, &keep_alive, purged_class);
      follow_stack(); // Flush marking stack

      // Update subklass/sibling/implementor links of live klasses
      follow_weak_klass_links();
      assert(_marking_stack.is_empty(), "just drained");

      // Visit memoized MDO's and clear any unmarked weak refs
      follow_mdo_weak_refs();
      assert(_marking_stack.is_empty(), "just drained");

      // Visit interned string tables and delete unmarked oops
      StringTable::unlink(&is_alive);
      // Clean up unreferenced symbols in symbol table.
      SymbolTable::unlink();

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

      assert(_marking_stack.is_empty(), "stack should be empty by now");
    }