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Date: Tue, 10 Apr 2018 17:23:12 -0300
From: "dgutson ." <danielgutson@...il.com>
To: musl@...ts.openwall.com
Subject: Re: catan errors

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?


>
> I opted to replace the non-obvious (3) with an explicit check for z==I
> but this isn't necessary.
>
> As written the patch does not address raising exception flags; that
> should be fixed before it's committed but right now I'm just
> submitting this for comments.
>
> Prior to the patch, at least the following (utterly dumb -- the first
> is a very real input!) cases are broken:
>
> - catan(1.0)
> - catan(2*I)
> - catan(I)
>
> Rich
>



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