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Date: Fri, 27 Jan 2012 20:01:18 +0100
From: Pascal Cuoq <pascal.cuoq@...il.com>
To: musl@...ts.openwall.com
Subject: Re: libm
On Fri, Jan 27, 2012 at 5:02 PM, Szabolcs Nagy <nsz@...t70.net> wrote:
> meanwhile i compared some implementations
> openbsd, freebsd and glibc are worth to look at
>
Sorry to intrude in your conversation, but for reasons of
my own I was recently looking for an aesthetically pleasing
single-precision implementation of trigonometric functions,
and some of the single-precisions functions out there are
so ugly that I would seriously consider implementing
these in musl by a call to their respective double-precision
counterparts, despite the disadvantages. At least it's compact.
Consider:
http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libm/src/k_cosf.c?rev=1.3;content-type=text%2Fplain
The code written at Sun and that many libms reuse must
have been double-precision only, and someone had to
make a single-precision version of them at some point.
Without the analysis that led to the original code, one
cannot blame Ian Lance Taylor for staying close
to the double-precision function:
http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libm/src/k_cos.c?rev=1.4;content-type=text%2Fplain
But the degree of the polynomial in the single-precision version
is about twice what is necessary, and the coefficients
look like coefficients of the Taylor expansion rounded to
nearest float, when they should be Chebyshev coefficients.
Criticism is easy and art is difficult. I don't know how
to write a better single-precision function, I just know
that this one, for instance, cannot be good.
Pascal
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