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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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