HeapWord* DefNewGeneration::allocate(size_t word_size,
                                     bool is_tlab) {

  {- -------------------------------------------
  (1) まずは, EdenSpace::par_allocate() で確保を試みる.
      成功すればここでリターン.
      ---------------------------------------- -}

      // This is the slow-path allocation for the DefNewGeneration.
      // Most allocations are fast-path in compiled code.
      // We try to allocate from the eden.  If that works, we are happy.
      // Note that since DefNewGeneration supports lock-free allocation, we
      // have to use it here, as well.
      HeapWord* result = eden()->par_allocate(word_size);
      if (result != NULL) {
        return result;
      }

  {- -------------------------------------------
  (1) Eden の soft limit (soft_end()) が hard limit (end()) より小さければ, soft limit を増加させて確保を試みる.
      (以下, 確保が成功するか soft limit が hard limit に等しくなるまでループ) (詳細は以下参照)
      確保が成功すればここでリターン.
      ---------------------------------------- -}

      do {

    {- -------------------------------------------
  (1.1) soft limit が hard limit より小さければ, soft limit を増加させる.
        ---------------------------------------- -}

        HeapWord* old_limit = eden()->soft_end();
        if (old_limit < eden()->end()) {
          // Tell the next generation we reached a limit.
          HeapWord* new_limit =
            next_gen()->allocation_limit_reached(eden(), eden()->top(), word_size);
          if (new_limit != NULL) {
            Atomic::cmpxchg_ptr(new_limit, eden()->soft_end_addr(), old_limit);
          } else {
            assert(eden()->soft_end() == eden()->end(),
                   "invalid state after allocation_limit_reached returned null");
          }

    {- -------------------------------------------
  (1.1) soft limit が hard limit と等しければ, ループを抜ける.
        ---------------------------------------- -}

        } else {
          // The allocation failed and the soft limit is equal to the hard limit,
          // there are no reasons to do an attempt to allocate
          assert(old_limit == eden()->end(), "sanity check");
          break;
        }

    {- -------------------------------------------
  (1.1) EdenSpace::par_allocate() で確保を試みる.
        ---------------------------------------- -}

        // Try to allocate until succeeded or the soft limit can't be adjusted
        result = eden()->par_allocate(word_size);
      } while (result == NULL);

  {- -------------------------------------------
  (1) もしまだ確保が成功していなければ, 
      DefNewGeneration::allocate_from_space() で from 領域からの確保を試みる.
      ---------------------------------------- -}

      // If the eden is full and the last collection bailed out, we are running
      // out of heap space, and we try to allocate the from-space, too.
      // allocate_from_space can't be inlined because that would introduce a
      // circular dependency at compile time.
      if (result == NULL) {
        result = allocate_from_space(word_size);
      }

  {- -------------------------------------------
  (1) 結果をリターンする
      ---------------------------------------- -}

      return result;
    }