栈顶的地址就代表最后一次被调用的函数（也就是当前的活动函数）。
The address at the top of the stack represents the function that was last called (that is, the active function).
在碰到退出函数时，当前的活动函数就会返回，并释放栈顶元素。
When an exit address is encountered, the current active function has returned and the top element on the stack is discarded.
在碰到退出函数时，当前的活动函数就会返回，并释放栈顶元素。
When an exit address is encountered, the current active function has returned and the top element on the stack is discarded.
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