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Date: Mon, 29 Apr 2013 14:04:00 -0000
From: =?utf-8?B?T25kxZllaiBCw61sa2E=?= <neleai@seznam.cz>
To: bugdal at aerifal dot cx <sourceware-bugzilla@sourceware.org>
Cc: glibc-bugs@sourceware.org
Subject: Re: [Bug math/14412] Removal of sysdeps/x86_64/fpu/s_sincos.S causes regressions
Message-ID: <20130429140349.GB24033@domone.kolej.mff.cuni.cz>
References: <bug-14412-131@http.sourceware.org/bugzilla/> <bug-14412-131-pCCovGlIGm@http.sourceware.org/bugzilla/>
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On Mon, Apr 29, 2013 at 01:28:10PM +0000, bugdal at aerifal dot cx wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=14412
> 
> --- Comment #33 from Rich Felker <bugdal at aerifal dot cx> 2013-04-29 13:28:10 UTC ---
> Ondrej, the results are not pure garbage. There are only two correct results
> sin(0x1p1000) can give, either of the nearest representable neighbors (less
> than 1ulp error) of the number 2**1000. Your fallacy is writing PI in your
> example. There is no such floating point number as PI, and this is part of why
> implementing correct trig functions is nontrivial; implementing ones that work
> in degree units, or a base-2 division of the unit circle, would be much easier.
>
Ok, assume you wrote function cos_deg that takes input degree in
degrees. You still have
cosdeg(360*(2**60)) == cosdeg(360*(2**60)+180)
In short when you have inputs with zero significant digits you can
expect only garbage as output.