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Date: Wed, 12 Feb 2020 15:07:17 +1100 (AEDT)
From: Damian McGuckin <damianm@....com.au>
To: musl@...ts.openwall.com
Subject: Re: casinh function accuracy problem
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
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;
Regards - Damian
Pacific Engineering Systems International, 277-279 Broadway, Glebe NSW 2037
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