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Re: stack size info
From: |
A Soare |
Subject: |
Re: stack size info |
Date: |
Mon, 26 Mar 2007 12:42:40 +0200 (CEST) |
> 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.