Bug in '##fx*?' primitive ?

[bsaleil]

gsi trace:

> (##fx* 0 1)
0
> (##fx* 1 0)
0
> (##fx*? 0 1)
0
> (##fx*? 1 0)
Floating point exception

The same behavior appears each time the right operand is 0.

[gambiteer]

Not a bug. "Don't do that." ##fx* is an internal primitive that assumes that its right-hand argument is not zero.

For example, this code:

(define (times a b)
  (* a b))

is expanded by the compiler to

heine:~/lang/scheme/Arrays/srfi3/Binf-8> gsc -expansion crap
Expansion:

(define times
  (lambda (a b)
    (if ('#<procedure #2 ##eq?> * '#<procedure #3 *>)
        (if (and ('#<procedure #4 ##fixnum?> b)
                 ('#<procedure #4 ##fixnum?> a))
            (if ('#<procedure #5 ##eqv?> b 0)
                0
                (let ((temp.2 (if ('#<procedure #5 ##eqv?> b -1)
                                  ('#<procedure #6 ##fx-?> a)
                                  ('#<procedure #7 ##fx*?> a b))))
                  (if temp.2 temp.2 (* a b))))
            (if (and ('#<procedure #8 ##flonum?> b)
                     ('#<procedure #8 ##flonum?> a))
                ('#<procedure #9 ##fl*> a b)
                (* a b)))
        (* a b))))

[feeley]

As Brad has pointed out, it is an error to pass 0 as a second argument to ##fx*?. The reason is that it uses division to check if the multiplication overflowed. That is (##fx*? a b) computes r = a*b (modulo the word size), and then checks if r/b=a (if it does there was no overflow).