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16.8 Disassembled Byte-Code

People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into humanly readable form.

The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.

In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.

Command: disassemble object &optional buffer-or-name

This command displays the disassembled code for object. In interactive use, or if buffer-or-name is nil or omitted, the output goes in a buffer named ‘*Disassemble*’. If buffer-or-name is non-nil, it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output.

The argument object can be a function name, a lambda expression or a byte-code object. If it is a lambda expression, disassemble compiles it and disassembles the resulting compiled code.

Here are two examples of using the disassemble function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of disassemble. These examples show unoptimized byte-code. Nowadays byte-code is usually optimized, but we did not want to rewrite these examples, since they still serve their purpose.

 
(defun factorial (integer)
  "Compute factorial of an integer."
  (if (= 1 integer) 1
    (* integer (factorial (1- integer)))))
     ⇒ factorial

(factorial 4)
     ⇒ 24

(disassemble 'factorial)
     -| byte-code for factorial:
 doc: Compute factorial of an integer.
 args: (integer)

0   constant 1              ; Push 1 onto stack.

1   varref   integer        ; Get value of integer
                            ;   from the environment
                            ;   and push the value
                            ;   onto the stack.

2   eqlsign                 ; Pop top two values off stack,
                            ;   compare them,
                            ;   and push result onto stack.

3   goto-if-nil 10          ; Pop and test top of stack;
                            ;   if nil, go to 10,
                            ;   else continue.

6   constant 1              ; Push 1 onto top of stack.

7   goto     17             ; Go to 17 (in this case, 1 will be
                            ;   returned by the function).

10  constant *              ; Push symbol * onto stack.

11  varref   integer        ; Push value of integer onto stack.

12  constant factorial      ; Push factorial onto stack.

13  varref   integer        ; Push value of integer onto stack.

14  sub1                    ; Pop integer, decrement value,
                            ;   push new value onto stack.

                            ; Stack now contains:
                            ;   - decremented value of integer
                            ;   - factorial
                            ;   - value of integer
                            ;   - *

15  call     1              ; Call function factorial using
                            ;   the first (i.e., the top) element
                            ;   of the stack as the argument;
                            ;   push returned value onto stack.

                            ; Stack now contains:
                            ;   - result of recursive
                            ;        call to factorial
                            ;   - value of integer
                            ;   - *

16  call     2              ; Using the first two
                            ;   (i.e., the top two)
                            ;   elements of the stack
                            ;   as arguments,
                            ;   call the function *,
                            ;   pushing the result onto the stack.

17  return                  ; Return the top element
                            ;   of the stack.
     ⇒ nil

The silly-loop function is somewhat more complex:

 
(defun silly-loop (n)
  "Return time before and after N iterations of a loop."
  (let ((t1 (current-time-string)))
    (while (> (setq n (1- n))
              0))
    (list t1 (current-time-string))))
     ⇒ silly-loop

(disassemble 'silly-loop)
     -| byte-code for silly-loop:
 doc: Return time before and after N iterations of a loop.
 args: (n)

0   constant current-time-string  ; Push
                                  ;   current-time-string
                                  ;   onto top of stack.

1   call     0              ; Call current-time-string
                            ;    with no argument,
                            ;    pushing result onto stack.

2   varbind  t1             ; Pop stack and bind t1
                            ;   to popped value.

3   varref   n              ; Get value of n from
                            ;   the environment and push
                            ;   the value onto the stack.

4   sub1                    ; Subtract 1 from top of stack.

5   dup                     ; Duplicate the top of the stack;
                            ;   i.e., copy the top of
                            ;   the stack and push the
                            ;   copy onto the stack.

6   varset   n              ; Pop the top of the stack,
                            ;   and bind n to the value.

                            ; In effect, the sequence dup varset
                            ;   copies the top of the stack
                            ;   into the value of n
                            ;   without popping it.

7   constant 0              ; Push 0 onto stack.

8   gtr                     ; Pop top two values off stack,
                            ;   test if n is greater than 0
                            ;   and push result onto stack.

9   goto-if-nil-else-pop 17 ; Goto 17 if n <= 0
                            ;   (this exits the while loop).
                            ;   else pop top of stack
                            ;   and continue

12  constant nil            ; Push nil onto stack
                            ;   (this is the body of the loop).

13  discard                 ; Discard result of the body
                            ;   of the loop (a while loop
                            ;   is always evaluated for
                            ;   its side effects).

14  goto     3              ; Jump back to beginning
                            ;   of while loop.

17  discard                 ; Discard result of while loop
                            ;   by popping top of stack.
                            ;   This result is the value nil that
                            ;   was not popped by the goto at 9.

18  varref   t1             ; Push value of t1 onto stack.

19  constant current-time-string  ; Push
                                  ;   current-time-string
                                  ;   onto top of stack.

20  call     0              ; Call current-time-string again.

21  list2                   ; Pop top two elements off stack,
                            ;   create a list of them,
                            ;   and push list onto stack.

22  unbind   1              ; Unbind t1 in local environment.

23  return                  ; Return value of the top of stack.

     ⇒ nil
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Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.