// We get called with "mr" representing the dirty region
// that we want to process. Because of imprecise marking,
// we may need to extend the incoming "mr" to the right,
// and scan more. However, because we may already have
// scanned some of that extended region, we may need to
// trim its right-end back some so we do not scan what
// we (or another worker thread) may already have scanned
// or planning to scan.

void DirtyCardToOopClosure::do_MemRegion(MemRegion mr) {

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

      // Some collectors need to do special things whenever their dirty
      // cards are processed. For instance, CMS must remember mutator updates
      // (i.e. dirty cards) so as to re-scan mutated objects.
      // Such work can be piggy-backed here on dirty card scanning, so as to make
      // it slightly more efficient than doing a complete non-detructive pre-scan
      // of the card table.
      MemRegionClosure* pCl = _sp->preconsumptionDirtyCardClosure();
      if (pCl != NULL) {
        pCl->do_MemRegion(mr);
      }

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

      HeapWord* bottom = mr.start();
      HeapWord* last = mr.last();
      HeapWord* top = mr.end();
      HeapWord* bottom_obj;
      HeapWord* top_obj;

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

      assert(_precision == CardTableModRefBS::ObjHeadPreciseArray ||
             _precision == CardTableModRefBS::Precise,
             "Only ones we deal with for now.");

      assert(_precision != CardTableModRefBS::ObjHeadPreciseArray ||
             _cl->idempotent() || _last_bottom == NULL ||
             top <= _last_bottom,
             "Not decreasing");
      NOT_PRODUCT(_last_bottom = mr.start());

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

      bottom_obj = _sp->block_start(bottom);
      top_obj    = _sp->block_start(last);

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

      assert(bottom_obj <= bottom, "just checking");
      assert(top_obj    <= top,    "just checking");

  {- -------------------------------------------
  (1) DirtyCardToOopClosure::get_actual_top(あるいはそれをサブクラスがオーバーライドしたもの)を呼んで, 
      top 変数の値を微調整しておく.
      ---------------------------------------- -}

      // Given what we think is the top of the memory region and
      // the start of the object at the top, get the actual
      // value of the top.
      top = get_actual_top(top, top_obj);

      // If the previous call did some part of this region, don't redo.
      if (_precision == CardTableModRefBS::ObjHeadPreciseArray &&
          _min_done != NULL &&
          _min_done < top) {
        top = _min_done;
      }

  {- -------------------------------------------
  (1) (変数宣言など)
      (extended_mr が実際の処理範囲を示す)
      ---------------------------------------- -}

      // Top may have been reset, and in fact may be below bottom,
      // e.g. the dirty card region is entirely in a now free object
      // -- something that could happen with a concurrent sweeper.
      bottom = MIN2(bottom, top);
      MemRegion extended_mr = MemRegion(bottom, top);

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

      assert(bottom <= top &&
             (_precision != CardTableModRefBS::ObjHeadPreciseArray ||
              _min_done == NULL ||
              top <= _min_done),
             "overlap!");

  {- -------------------------------------------
  (1) DirtyCardToOopClosure::walk_mem_region() を呼んで, 
      処理範囲内のポインタフィールドに対して
      コンストラクタ引数で指定された OopClosure を適用する.

      (ただし, 処理範囲(extended_mr)が空の場合には, 
       特にすることはないので, 何もしない).
      ---------------------------------------- -}

      // Walk the region if it is not empty; otherwise there is nothing to do.
      if (!extended_mr.is_empty()) {
        walk_mem_region(extended_mr, bottom_obj, top);
      }

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

      // An idempotent closure might be applied in any order, so we don't
      // record a _min_done for it.
      if (!_cl->idempotent()) {
        _min_done = bottom;
      } else {
        assert(_min_done == _last_explicit_min_done,
               "Don't update _min_done for idempotent cl");
      }
    }