// If could fit into free regions w/o expansion, try.
// Otherwise, if can expand, do so.
// Otherwise, if using ex regions might help, try with ex given back.

HeapWord* G1CollectedHeap::humongous_obj_allocate(size_t word_size) {

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

      assert_heap_locked_or_at_safepoint(true /* should_be_vm_thread */);

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

      verify_region_sets_optional();

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

      size_t num_regions =
             round_to(word_size, HeapRegion::GrainWords) / HeapRegion::GrainWords;
      size_t x_size = expansion_regions();
      size_t fs = _hrs->free_suffix();

  {- -------------------------------------------
  (1) G1CollectedHeap::humongous_obj_allocate_find_first() を呼んで, 
      メモリ確保用にフリーリストからの空きリージョンの取得を試みる.
      ---------------------------------------- -}

      int first = humongous_obj_allocate_find_first(num_regions, word_size);

  {- -------------------------------------------
  (1) 失敗した場合は, ...#TODO であれば
      G1CollectedHeap::expand() でヒープ領域を拡張した後で, 
      もう一度 G1CollectedHeap::humongous_obj_allocate_find_first() によるリージョンの取得を試みる.
      ---------------------------------------- -}

      if (first == -1) {
        // The only thing we can do now is attempt expansion.
        if (fs + x_size >= num_regions) {
          // If the number of regions we're trying to allocate for this
          // object is at most the number of regions in the free suffix,
          // then the call to humongous_obj_allocate_find_first() above
          // should have succeeded and we wouldn't be here.
          //
          // We should only be trying to expand when the free suffix is
          // not sufficient for the object _and_ we have some expansion
          // room available.
          assert(num_regions > fs, "earlier allocation should have succeeded");

          if (expand((num_regions - fs) * HeapRegion::GrainBytes)) {
            first = humongous_obj_allocate_find_first(num_regions, word_size);
            // If the expansion was successful then the allocation
            // should have been successful.
            assert(first != -1, "this should have worked");
          }
        }
      }

  {- -------------------------------------------
  (1) リージョンの取得が成功していれば, 
      G1CollectedHeap::humongous_obj_allocate_initialize_regions() を呼んで, 
      メモリ確保を試みる.
      ---------------------------------------- -}

      HeapWord* result = NULL;
      if (first != -1) {
        result =
          humongous_obj_allocate_initialize_regions(first, num_regions, word_size);
        assert(result != NULL, "it should always return a valid result");
      }

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

      verify_region_sets_optional();

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

      return result;
    }