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[Bug libc/3783] tan(double x) takes excessive amount of time when x is a multiple of pi/2, x != 0
- From: "jakub at redhat dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: 22 Dec 2006 10:40:06 -0000
- Subject: [Bug libc/3783] tan(double x) takes excessive amount of time when x is a multiple of pi/2, x != 0
- References: <firstname.lastname@example.org>
- Reply-to: sourceware-bugzilla at sourceware dot org
------- Additional Comments From jakub at redhat dot com 2006-12-22 10:40 -------
Neither tanl nor tanf (unfortunately) are precise up to 0.5ulp, see
e.g. http://www.inf.ethz.ch/personal/gonnet/FPAccuracy/ testsuite for a proof.
And, when expecting exact 0 from a trigonometric function, the computation
needed for 0.5ulp precise result is obviously most expensive, not cheapest,
because .5ulp for 0.0 is of course orders of magnitude smaller than 0.5ulp
for 1.0 (you need error below or equal to 2.47033e-324 for IEEE double, while
if result is 1.0, the error needs to be below or equal to 1.110225e-16).
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