Re: [Maxima-discuss] SIGN1 strangeness, was: Problem with bfloat

[Robert D. <rob...@gm...>]

On 2014-09-01, David Billinghurst <dbm...@gm...> wrote:

>     I:87^611$
>     M:91^211$
>     sign(sqrt(I)-M);

Thanks for the example. Here's a proposed fix. This causes $FLOAT to
always fail (even for Lisps which otherwise allow float infinity)
and therefore SIGN1 takes a different route and sign returns 'pos.

The patch assumes that (float inf) minus (float inf) != 0.
I'm pretty sure that's required by IEEE 754 (in fact I think the
result is supposed to be NaN) but I can't quote chapter & verse
at the moment. 

best

Robert Dodier

PS.
$ git diff src/comm.lisp
diff --git a/src/comm.lisp b/src/comm.lisp
index 42bdc12..1d6d178 100644
--- a/src/comm.lisp
+++ b/src/comm.lisp
@@ -1178,7 +1178,7 @@
            (car result))))))
            
 (defmfun $float (e)
-  (cond ((numberp e) (float e))
+  (cond ((numberp e) (let ((e1 (float e))) (if (= (- e1 e1) 0) e1 (signal 'floating-point-overflow))))
        ((and (symbolp e) (mget e '$numer)))
        ((or (atom e) (member 'array (cdar e) :test #'eq)) e)
        ((eq (caar e) 'rat) (fpcofrat e))