// This version is lock-free.

HeapWord* MutableSpace::cas_allocate(size_t size) {

  {- -------------------------------------------
  (1) CAS で top を size 分増加させて領域を確保し, それをリターンするだけ.
      (CAS が失敗すれば, 成功するまでループで繰り返す.
       なお, 残りの領域長が size 未満であれば NULL を返す.)
      ---------------------------------------- -}

      do {
        HeapWord* obj = top();
        if (pointer_delta(end(), obj) >= size) {
          HeapWord* new_top = obj + size;
          HeapWord* result = (HeapWord*)Atomic::cmpxchg_ptr(new_top, top_addr(), obj);
          // result can be one of two:
          //  the old top value: the exchange succeeded
          //  otherwise: the new value of the top is returned.
          if (result != obj) {
            continue; // another thread beat us to the allocation, try again
          }
          assert(is_object_aligned((intptr_t)obj) && is_object_aligned((intptr_t)new_top),
                 "checking alignment");
          return obj;
        } else {
          return NULL;
        }
      } while (true);
    }