[Openmcl-devel] tagbody and go

Pascal Costanza pc at p-cos.net
Wed Dec 8 18:08:00 UTC 2010


There is no computed goto in Common Lisp, so what you want cannot be expressed with tagbody and go. case/ecase/ccase is the closest you can get in a straightforward way.

An alternative would be to compute a jump table manually. Roughly like this:

(defun table-step (step)
  (let ((table (vector (lambda () 0) (lambda () 1) (lambda () 2) ...)))
    (funcall (svref table step))))

...of course with the right kind of macrology around. But I doubt this buys you a substantial advantage.

The timings below don't tell you a lot because gostep and runstep don't perform the same kinds of computations.


Pascal

On 8 Dec 2010, at 18:58, Taoufik Dachraoui wrote:

> I run the following test to show that ecase is not jumping directly
> to the given step (less efficient then (go step)):
> 
> (defmacro gostep (step)
>     `(block nil (tagbody (go ,step)
> 	,@(loop for i = 0 then (1+ i)
> 	    while (< i 100)
> 	     append `(,i (return-from nil ,i) (go end)))
> 	end)))
> 
> (defmacro define-runstep ()
>     `(defun runstep (step)
> 	(block nil 
> 	  (ecase step
> 		,@(loop for i = 0 then (1+ i)
> 	    while (< i 100)
> 	     collect `(,i (return-from nil ,i)))
> 	))))
> 
> (define-runstep)
> 
> ? (time (dotimes (i 10000000) (gostep 99)))
> (DOTIMES (I 10000000) (GOSTEP 99)) took 30 milliseconds (0.030 seconds) to run 
>                     with 2 available CPU cores.
> During that period, 30 milliseconds (0.030 seconds) were spent in user mode
>                     0 milliseconds (0.000 seconds) were spent in system mode
> NIL
> ? (time (dotimes (i 10000000) (runstep 99)))
> (DOTIMES (I 10000000) (RUNSTEP 99)) took 988 milliseconds (0.988 seconds) to run 
>                     with 2 available CPU cores.
> During that period, 986 milliseconds (0.986 seconds) were spent in user mode
>                     0 milliseconds (0.000 seconds) were spent in system mode
> NIL
> 
> 
> -Taoufik
> 
> On Wed, Dec 8, 2010 at 6:11 PM, Alexander Repenning <ralex at cs.colorado.edu> wrote:
> 
> On Dec 8, 2010, at 9:52 AM, Taoufik Dachraoui wrote:
> 
> > It is not exactly what I am looking for, I would like to jump directly to the step.
> > The ecase will check all the clauses in order (like cond) until it finds a match.
> 
> 
> duno what CCL does in this particular case but most compilers could generate a jump and would not have to test all the previous clauses. In other words this is an implementation question.
> 
> alex
> 
> 
> 
> 

-- 
Pascal Costanza, mailto:pc at p-cos.net, http://p-cos.net
Vrije Universiteit Brussel
Software Languages Lab
Pleinlaan 2, B-1050 Brussel, Belgium






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