From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: [musl] casinh function accuracy problem
Date: Tue, 11 Feb 2020 23:19:12 -0500 [thread overview]
Message-ID: <20200212041912.GW1663@brightrain.aerifal.cx> (raw)
In-Reply-To: <alpine.LRH.2.02.2002121450170.16988@key0.esi.com.au>
On Wed, Feb 12, 2020 at 03:07:17PM +1100, Damian McGuckin wrote:
> On Tue, 11 Feb 2020, Rich Felker wrote:
>
> >My minimal criterion for large-scale improvements of src/complex would
> >be fixing any remaining cases where inf/nan behavior is badly wrong or
> >there's catastrophic error (>2^52 ulp, or even just >2^20 ulp or so).
> >Beyond that, I think "reducing ulp error" would be nice but hard to
> >quantify and make a goal without having an idea how bad it is now, not
> >to mention without having rigorous error bounds on the real math
> >library functions.
>
> I think INF/NaN behaviour at the fundamental level is flawed.
>
> This initialization:
>
> double complex x = 1.0e+200 + INFINITY * I;
>
> on every compiler I try, yields an 'x' of
>
> NaN + INFINITY i
This is expected unless the implementation supports pure-imaginary
types and a _Imaginary_I (as opposed to _Complex_I). Note that Annex G
requires this, but GCC treats it as optional and does not support it
despite claiming Annex G conformance.
In this particular kind of case, the right solution is to use the
CMLPX macro instead of the * operator with I:
double complex x = CMPLX(1.0e+200, INFINITY);
> Whereas if I compute
>
> double complex a = 2.0 + 1.0e200 * I;
> double complex b = 1.0e200 + 1.0 * I;
> double x = a * b;
>
> then 'x' prints correctly as
>
> 1.0e+200 + INF * I;
Indeed the compiler is required to handle these overflow cases unless
#pragma STDC CX_LIMITED_RANGE ON is in effect. I don't know if GCC
honors the pragma but it has -fcx-limited-range to set it globally.
Note that without CX_LIMITED_RANGE, complex math performance is
atrociously bad due to extensive overflow checking.
Rich
prev parent reply other threads:[~2020-02-12 4:19 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-11 15:51 mg1633068
2020-02-11 19:30 ` Szabolcs Nagy
2020-02-11 23:24 ` Rich Felker
2020-02-12 0:46 ` Damian McGuckin
2020-02-12 2:00 ` Rich Felker
2020-02-12 4:07 ` Damian McGuckin
2020-02-12 4:19 ` Rich Felker [this message]
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