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From: Khilan Gudka <Khilan.Gudka@cl.cam.ac.uk>
Date: Tue, 21 Nov 2017 15:13:44 +0000
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Subject: Re: fabs(-0.0) returns -0.0
To: John Baldwin <jhb@freebsd.org>
Cc: freebsd-mips@freebsd.org, Khilan Gudka <Khilan.Gudka@cl.cam.ac.uk>
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Alex Richardson has submitted a patch to fix fabs() on FreeBSD:
https://reviews.freebsd.org/D13135

Thanks,
Khilan

On 14 November 2017 at 01:43, John Baldwin <jhb@freebsd.org> wrote:

> On Monday, November 13, 2017 10:52:10 PM Khilan Gudka wrote:
> > Hi,
> >
> > The implementation of fabs(3) for MIPS (at least) appears to not be
> giving
> > the right answer for when -0 is passed, as it returns -0 instead of +0.
> The
> > implementation of fabs is:
> >
> > double fabs(double x) {
> >   if (x < 0)
> >     return -x;
> >   return x;
> > }
> >
> > The if-test fails for -0 and thus it is returned. Is this a known issue?
> A
> > simple fix would be to return x+0.0 instead of just x.
> >
> > The implication of this is that other functions which rely on fabs, such
> as
> > hypot, also return -0. For example, hypot(-0.0, 0.0) returns -0 instead
> of
> > +0.
>
> The C version of fabs() for 32-bit arm has the same misimplementation btw.
> All other architectures implement fabs in assembly in libc.
>
> --
> John Baldwin
>