Activation Records

:material-circle-edit-outline: 约 1146 个字 :material-clock-time-two-outline: 预计阅读时间 4 分钟

An invocation of function f is an activation of f，每次 f 被调用时，其内部变量会被创建新的实例，并在函数返回时被摧毁，我们该如何维护这些本地变量呢——用 stack

Procedure calls and returns are usually managed by a run time stack called control stack
- Each live activation has an activation record (sometimes called a frame) on the control stack

Each time a procedure（过程） is called, space for its local variables is pushed onto the stack
- Procedure calls are also called activations of procedures.
过程终止后对应的 space 就会 pop off

Stack Frames

栈如果只支持 push 和 pop 是还不能用于存储局部变量的

在实际使用中会将 stack 看成一个大数组，用 sp 指向栈顶

Stack Pointer– a special register that points at some location

Run-time stacks 由高地址向低地址增长

Registers hold
1. some parameters
2. return address
3. return value
4. some local variables and temporaries
Stack frame holds
- variables passed by reference or have their address taken (&)
- variables that are accessed by procedures nested within the current one
- variables that are too large to fit into register file
- array variables (address arithmetic needed to access array elements)
- spilled registers (too many local variables to fit into register file, so some must be stored in stack frame)

Frame Pointer

Frame Pointer 指向 caller 帧头
当 caller call callee 时，sp 指向 caller 传递给 callee 的第一个参数
- 实际上是指向 static link

When enter f：
1. save the old frame pointer FP in memory in the frame
2. let FP = SP
3. SP = SP – framesize
When f exits:
1. let SP = FP
2. let FP = saved old FP

Saved Registers

caller-save register: if the caller (in this case, f) must save and restore it

callee-save register: if it is the responsibility of the callee (in this case, g)

Parameter Passing

the first k arguments (for k = 4 or k = 6, typically) of a function are passed in registers rp, …, r(p+k−1), and the rest of the arguments are passed in memory.

但是，如果在函数中调用函数，那么这两个函数的第一个参数的值都在同一个寄存器中，前者会被后者覆盖，如果第二个函数返回后，第一个函数还用第一个参数，就会用到错误的值

解决方法如下：

Parameter a is a dead variable at the point where h(z) is called. Then f can overwrite r1 without saving it.
Leaf procedures need not write their incoming arguments to memory
- leaf procedures: the procedures that don’t call other procedures
分析全局，在每个地方进行抉择
每次分配新的寄存器

Return Address

略

Frame-Resident Variables

需要存在栈帧里的变量，如本地变量和临时变量，

Values are written to memory (in the stack frame) only when necessary for one of these reasons:

the variable will be passed by reference, so it must have a memory address (e.g., & in the C language);
the variable is accessed by a procedure nested inside the current one;
the value is too big to fit into a single register;
the variable is an array, for which address arithmetic is necessary to extract components;
the register holding the variable is needed for a specific purpose, such as parameter passing (as described above); - 原先存储值的寄存器要用于新的用途，就得将其值存入栈帧
or there are so many local variables and temporary values that they won’t all fit in registers, in which case some of them are “spilled” into the frame.