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支持减、乘，以及除

 4 years ago
source link: https://www.tuicool.com/articles/7neyYnj
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在上一篇文章中，初步搭建了一个输入Common Lisp代码，输出汇编代码的编译器的骨架，实现了二元整数的加法运算。在这个基础上，要想实现减法、乘法，以及除法就是手到擒来的事情了。只需依葫芦画瓢，补充更多的分支情况即可。

我自己模仿着x64的调用约定，规定四则运算的结果始终放在 EAX 这个寄存器中。在稍后给出的代码中，对于减法和除法运算，都是把运算符的左操作数放到 EAX 寄存器中，再从 EAX 中减去或者除掉右操作数。

在摸索除法的汇编代码怎么生成时，遇到了个费解的问题，最后才知道，原来需要把 EAX 寄存器的符号扩展到高位的 EDX 寄存器中去。对于 as 这个汇编器来说，需要用到 CLTD 指令。

最后， jjcc2 和 stringify 两个函数被修改为如下的样子 

(defun jjcc2 (expr)
  "支持两个数的四则运算的编译器"
  (cond ((eq (first expr) '+)
         `((movl ,(second expr) %eax)
           (movl ,(third expr) %ebx)
           (addl %ebx %eax)))
        ((eq (first expr) '-)
         `((movl ,(second expr) %eax)
           (movl ,(third expr) %ebx)
           (subl %ebx %eax)))
        ((eq (first expr) '*)
         ;; 将两个数字相乘的结果放到第二个操作数所在的寄存器中
         ;; 因为约定了用EAX寄存器作为存放最终结果给continuation用的寄存器，所以第二个操作数应当为EAX
         `((movl ,(second expr) %eax)
           (movl ,(third expr) %ebx)
           (imull %ebx %eax)))
        ((eq (first expr) '/)
         `((movl ,(second expr) %eax)
           (cltd)
           (movl ,(third expr) %ebx)
           (idivl %ebx)))))

(defun stringify (asm)
  "根据jjcc2产生的S表达式生成汇编代码字符串"
  (format t "        .section __TEXT,__text,regular,pure_instructions~%")
  (format t "        .globl _main~%")
  (format t "_main:~%")
  (dolist (ins asm)
    (cond ((= (length ins) 3)
           (format t "        ~A ~A, ~A~%"
                   (first ins)
                   (if (numberp (second ins))
                       (format nil "$~A" (second ins))
                       (second ins))
                   (if (numberp (third ins))
                       (format nil "$~A" (third ins))
                       (third ins))))
          ((= (length ins) 2)
           (format t "        ~A ~A~%"
                   (first ins)
                   (if (numberp (second ins))
                       (format nil "$~A" (second ins))
                       (second ins))))
          ((= (length ins) 1)
           (format t "        ~A~%" (first ins)))))
  (format t "        movl %eax, %edi~%")
  (format t "        movl $0x2000001, %eax~%")
  (format t "        syscall~%"))

全文完。


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