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Message-ID: <20180410232328.GN3094@brightrain.aerifal.cx>
Date: Tue, 10 Apr 2018 19:23:28 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: catan errors
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]:
> >
> > 3. Is only possible if y==1.0 and x==0.0, which is the only real
> > exceptional case for atan: z==I.
> >
> > I opted to replace the non-obvious (3) with an explicit check for z==I
> > but this isn't necessary.
> >
> ....
> > 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.
Rich
View attachment "catan2.diff" of type "text/plain" (637 bytes)
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