Re: stack size info

[A Soare <alinsoar@voila.fr>]

>    I can not imagine an example in which this STACKSIZE is greater than
>    the actual real size. Can somebody help me with an example, please?
> 
> following is an example (created in the *scratch* buffer, btw, so
> you can try to recreate it for yourself, as well).  the annotations
> (comments) on the right were added by hand.
> 
> there are two functions and three function calls.  the second and
> third calls illustrate how one function might use a different amount
> of stack depending on its operating environment (in this case, the
> value of its arg OBJECT).  that amount is at most 4, which coincides
> (purposefully) with the STACKSIZE element in the byte-code function
> object for `maybe'.
> 
> to understand in more detail, you will need to understand how each
> operation interacts with the stack (if at all).  for more info,
> place the cursor after the following form and type `C-x C-e':
> 
>   (info "(elisp)Disassembly")
> 
> btw, the construct `(or (byte-compile FOO) (symbol-function FOO))'
> is because `byte-compile' returns nil if FOO is already compiled.
> 
> thi
> 
> ___________________________________________________
> 
> (defun no-op () t)
> no-op
> 
> (disassemble (or (byte-compile 'no-op)
>                  (symbol-function 'no-op))
>              (current-buffer))
> byte code:
>   args: nil                             ; stack usage
> 0       constant  t                     ; 1
> 1       return                          ; 0
> 
> (defun maybe (object)
>   (when object
>     (let* ((a 42)
>            (b 6))
>       (when (= a (+ b (* b b)))
>         object))))
> maybe
> 
> (disassemble (or (byte-compile 'maybe)
>                  (symbol-function 'maybe))
>              (current-buffer))
> byte code:                              ; OBJECT: nil
>   args: (object)                        ; stack usage
> 0       varref    object                ; 1
> 1       goto-if-nil-else-pop 2          ; (branch taken)
> 4       constant  42
> 5       varbind   a
> 6       constant  6
> 7       varbind   b
> 8       varref    a
> 9       varref    b
> 10      dup
> 11      dup
> 12      mult
> 13      plus
> 14      eqlsign
> 15      goto-if-nil-else-pop 1
> 18      varref    object
> 19:1    unbind    2
> 20:2    return                          ; 0
> 
> (disassemble (or (byte-compile 'maybe)
>                  (symbol-function 'maybe))
>              (current-buffer))
> byte code:                              ; OBJECT: 42
>   args: (object)                        ; stack usage
> 0       varref    object                ; 1
> 1       goto-if-nil-else-pop 2          ; 0 (branch not taken)
> 4       constant  42                    ; 1
> 5       varbind   a                     ; 0
> 6       constant  6                     ; 1
> 7       varbind   b                     ; 0
> 8       varref    a                     ; 1
> 9       varref    b                     ; 2
> 10      dup                             ; 3
> 11      dup                             ; 4
> 12      mult                            ; 3
> 13      plus                            ; 2
> 14      eqlsign                         ; 1
> 15      goto-if-nil-else-pop 1          ; 0 (branch not taken)
> 18      varref    object                ; 1
> 19:1    unbind    2
> 20:2    return                          ; 0
> 
> 
> 




Thank you very much.


Alin Soare.