void G1CollectedHeap::release_mutator_alloc_region() {

  {- -------------------------------------------
  (1) G1AllocRegion::release() を呼んで, 
      確保に使用している HeapRegion (G1AllocRegion) を 
      GC に備えて retire させておく
      ---------------------------------------- -}

      _mutator_alloc_region.release();

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

      assert(_mutator_alloc_region.get() == NULL, "post-condition");
    }