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Date: Wed, 11 Apr 2018 02:52:11 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: catan errors
* Rich Felker <dalias@...c.org> [2018-04-10 19:23:28 -0400]:
> On Wed, Apr 11, 2018 at 01:08:50AM +0200, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@...c.org> [2018-04-10 15:50:07 -0400]:
> > > t = y - 1.0;
> > > a = x2 + (t * t);
> > > - if (a == 0.0)
> > > - goto ovrf;
> >
> > why does a==0 imply x==0?
>
> OK, at least by my reasoning it's only algebraically true not
> necessarily as doubles.
>
> > if |x| < sqrt(2)*0x1p-538, x2 underflows to 0 in nearest rounding mode.
> >
> > to handle this correctly extra work would need to be done, so i think
> > either way is fine (leaving the goto there or not are both wrong, but
> > we dont guarantee correct complex functions yet)
>
> I'm fine with just moving the check back here. But the special case
> (maybe not near-special cases with internal over/underflow though)
> works fine just replacing the *I with CMPLX. See attached.
>
i think this makes sense for now.
> Rich
> diff --git a/src/complex/catan.c b/src/complex/catan.c
> index 39ce6cf..7dc2afe 100644
> --- a/src/complex/catan.c
> +++ b/src/complex/catan.c
> @@ -91,29 +91,17 @@ double complex catan(double complex z)
> x = creal(z);
> y = cimag(z);
>
> - if (x == 0.0 && y > 1.0)
> - goto ovrf;
> -
> x2 = x * x;
> a = 1.0 - x2 - (y * y);
> - if (a == 0.0)
> - goto ovrf;
>
> t = 0.5 * atan2(2.0 * x, a);
> w = _redupi(t);
>
> t = y - 1.0;
> a = x2 + (t * t);
> - if (a == 0.0)
> - goto ovrf;
>
> t = y + 1.0;
> a = (x2 + t * t)/a;
> - w = w + (0.25 * log(a)) * I;
> - return w;
> -
> -ovrf:
> - // FIXME
> - w = MAXNUM + MAXNUM * I;
> + w = CMPLX(w, 0.25 * log(a));
> return w;
> }
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