char *
ReservedSpace::align_reserved_region(char* addr, const size_t len,
                                     const size_t prefix_size,
                                     const size_t prefix_align,
                                     const size_t suffix_size,
                                     const size_t suffix_align)
{

  {- -------------------------------------------
  (1) 指定された領域内から条件に合う開始アドレスを計算する.
        (正確には先頭から数えた開始位置のオフセットを計算 (以下の beg_delta))
      ---------------------------------------- -}

      assert(addr != NULL, "sanity");
      const size_t required_size = prefix_size + suffix_size;
      assert(len >= required_size, "len too small");

      const size_t s = size_t(addr);
      const size_t beg_ofs = s + prefix_size & suffix_align - 1;
      const size_t beg_delta = beg_ofs == 0 ? 0 : suffix_align - beg_ofs;

      if (len < beg_delta + required_size) {
         return NULL; // Cannot do proper alignment.
      }

  {- -------------------------------------------
  (1) その開始アドレスから required_size 分だけを残して
      残りの領域は os::release_memory() で OS に返却し, 
      開始アドレスをリターンする.
      ---------------------------------------- -}

      const size_t end_delta = len - (beg_delta + required_size);

      if (beg_delta != 0) {
        os::release_memory(addr, beg_delta);
      }

      if (end_delta != 0) {
        char* release_addr = (char*) (s + beg_delta + required_size);
        os::release_memory(release_addr, end_delta);
      }

      return (char*) (s + beg_delta);
    }