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Date: Tue, 10 Apr 2018 17:41:46 -0300
From: "dgutson ." <danielgutson@...il.com>
To: musl@...ts.openwall.com
Subject: Re: catan errors

On Tue, Apr 10, 2018 at 5:32 PM, Rich Felker <dalias@...c.org> wrote:

> On Tue, Apr 10, 2018 at 05:23:12PM -0300, dgutson . wrote:
> > On Tue, Apr 10, 2018 at 4:50 PM, Rich Felker <dalias@...c.org> wrote:
> >
> > > The OpenBSD catan implementation we're using has a number of
> > > nonsensical "overflow" (goto ovrf) conditions that aren't errors,
> > > reported by mepholic on irc. I think the attached patch fixes them
> > > without introducing new problems, but I'm not sure if any other
> > > problems remain.
> > >
> > > Note that, of the three cases removed:
> > >
> > > 1. Is not an exceptional case at all, and made no sense to begin with.
> > >
> > > 2. Is only exceptional if x and a are both zero; atan(2x,0) is
> > >    perfectly well-defined.
> > >
> > > 3. Is only possible if y==1.0 and x==0.0, which is the only real
> > >    exceptional case for atan: z==I.
> > >
> >
> >
> > Besides the trigonometric case, are you considering de-normalized
> numbers,
> > such as 4.94066e-324 as divisor?
> > For example:
> >     double x = 1.0;
> >     double y = 5E-324;
> > x / y is inf, and y != 0.0.
> > Shouldn't 'a' be checked against that number or its absolute value >=
> > minimum?
>
> Can you clarify where you think something goes wrong?
>

-	if (a == 0.0)
-		goto ovrf;

 	t = y + 1.0;
 	a = (x2 + t * t)/a;


The check you removed does not look correct for me because what I mentioned.
However, shouldn't you check, before the division, that a is not the
nearest to zero (+ or -) denormalized representable double,
in order to avoid ending in inf?



>
> Rich
>



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