C语言实现线索二叉树

Question

线索二叉树节点结构：

typedef struct bstree_node bstree_node_t;
struct bstree_node {

    uintptr_t left, right;

} __attribute__((aligned(2)));

使用时将该结构体嵌入用户结构体即可：

struct leaf {

    int key;

    struct bstree_node node;

};

取得线索与指向普通叶子节点指针的方法：

static inline void _bstree_set_thread(bstree_node_t *t, uintptr_t *p)

{

    *p = (uintptr_t) t | 1;

}

static inline bstree_node_t *_bstree_get_thread(uintptr_t u)

{

    return (bstree_node_t *) ((u & -(int) (u & 1)) & ~1UL);

}

static inline void _bstree_set_link(bstree_node_t *n, uintptr_t *p)

{

    *p = (uintptr_t) n;

}

static inline bstree_node_t *_bstree_get_link(uintptr_t u)

{

    return (bstree_node_t *) (u & ((int) (u & 1) - 1));

}

我认为与__attribute__((aligned(2)));有关，
该怎么理解这段代码？