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6.Activation Records

Storage Organization

High-Order function

Stack Frame

栈帧(frame)

frame-resident variables

A variable escapes if:(需要被放在内存上)

  • it is passed by reference,
  • its address is taken (using C’s & operator)
  • or it is accessed from a nested function Escaping variables must be written to memory.

如何处理嵌套函数，一般有三种方法

  1. Whenever a function f is called, it can be passed a pointer to the frame of the function statically enclosing f; thispointer is the static link.
  2. A global array can be maintained, containing-in position i-a pointer to the frame of the most recently entered procedure whose static nesting depth is i. This array is called a display.
  3. When g calls f, each variable of g that is actually accessedby f (or by any function nested inside f) is passed to f as an extra argument. This is called lambda lifting.

重点讲static link. 习题6.7 讲的是display

Warning

Whenever a function f is called, it is passed a pointer to the stackframe of the “current"(most recently entered) activation of the function g that immediately encloses f in the text of the program

  • g immediately enclose f 的定义: 函数f是在函数g里 声明 的, 只差一层, 所以叫g直接包含f
  • current activation: 因为g可能被调用多次(如递归, g在栈里有多个栈帧. 所以指的是最近一次g的调用
int g(int x){
    int f(int y){

    }
    return f(x)+1;
}

Note

static link是一个指针。每个函数的栈帧里面都有一个static link

注意indent调用write, 但是write不是在indent里面声明的，先找到indent的static link show, 再找到pp. 最终因为 write是在pp里声明的 所以write的static link=pp的fp

  • 在函数调用时，沿着static link往上找。
    • 如果不是递归调用,将被调用函数的static link赋值为 声明 它的函数 最近使用的栈帧的 fp
    • 如果是递归调用，直接把传递自己的static link
  • 在访问变量时，同样沿着static link往上

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