Subject: Re: JVM vs CLR
From: (Rob Warnock)
Date: Sun, 01 Feb 2009 22:28:56 -0600
Newsgroups: comp.lang.lisp
Message-ID: <>
Andr� Thieme  <> wrote:
| Jon Harrop schrieb:
| > William D Clinger wrote:
| >> This all started when Jon Harrop incorrectly stated that
| >> "Trampolines...only handle a few special cases of tail
| >> calls."
| > 
| > I wrote that specifically in the context of Clojure, where it is true.
| It�s very possible that you are right with this.
| I would really like to see an example, so that I can try to avoide these
| issues. If it is true that only some few special cases of tail calls
| can be done in a stack-safe way with trampolines it means that the great
| majority of the cases will run into problems with the stack.
| And if that is the case it should be easy to post one (or two) small
| examples, as a proof-of-concept.
| Could you please post some code, be it in Clojure or some other lang,
| that demonstrates this?
| I would need a concrete example.

I don't have an example at hand that will definitely fail for you
[I use CMUCL, which agressively optimizes tail calls even between
functions across separately-compiled files], but if Clojure *does*
have a problem with proper TCO you might be able to construct an
example by, say, defining three identical functions which tail call
each other in a circle, or, if necessary to defeat some optimization,
call one of the other two pseudo-randomly, something like this:

    (defun f1 (x y)
      (let ((x1 (1- x))
	    (y1 (1+ y)))
	(if (zerop x)
	  (if (zerop (random 2))
	   (f2 x1 y1)
	   (f3 x1 y1)))))

and the same, mutatis mutandis, for F2 & F3.

I was unable to get this to fail with CMUCL even with
(F1 MOST-POSITIVE-FIXNUM 0) [M-P-F is 536870911 there,
*way* larger than any reasonable stack size], even when
compiling each function separately in its own file.
But Clojure's MMV...


p.s. You might also try explicitly allocating something in
each function, e.g.:

    (defun f1 (x y z)
      (declare (ignorable z))
      (let ((x1 (1- x))
	    (y1 (1+ y))
	    (z1 (make-array 10)))
	(if (zerop x)
	  (if (zerop (random 2))
	   (f2 x1 y1 z1)
	   (f3 x1 y1 z1)))))

Though, again, this *didn't* cause CMUCL to break, but it *did*
slow things down a lot and force me to (SETF *GC-VERBOSE* NIL)!!  ;-}

Rob Warnock			<>
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