Re: Man page issues: logb, significand, cbrt, log2, log10, exp10

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 02/03/24 23:02, Morten Welinder wrote:
> Thanks.
> 
> There is (was?) already crlibm out there.
> https://core-math.gitlabpages.inria.fr/  No particular need for wheel
> reinvention here.
> 
> FWIW, it appears that the author of the glibc exp10 implementation
> agrees with me that the implementation is sub-standard:
> 
> https://codebrowser.dev/glibc/glibc/math/e_exp10.c.html

This code was not used by any port and we recently removed to avoid this
very issue [1]. The exp10 implementation used by all ports
is at sysdeps/ieee754/dbl-64/e_exp10.c (i386/m68k are exceptions and my
plan to eventually remove this implementation in favor the generic
one [2]).

And the exp10 implementation was recently improved [3], with the
author suggesting the worst-case error in round-to- should be
nearest to 0.513 ULP (I am not sure if he did some empirical testing
to verify this value, at least with libm-test-ulps the resuts are
bounded to 2 ulp max).

> 
> /* This is a very stupid and inprecise implementation. It'll get
> replaced sometime (soon?). */
> return __ieee754_exp (M_LN10 * arg);
> 
> 
> Compare with musl:
> 
> https://github.com/rofl0r/musl/blob/master/src/math/exp10.c
> 

[1] https://sourceware.org/git/?p=glibc.git;a=commit;h=9c61303ebbdc6e727c89591bff3229c9fbfa438b
[2] https://sourceware.org/pipermail/libc-alpha/2024-January/154107.html
[3] https://sourceware.org/git/?p=glibc.git;a=commit;h=63d0a35d5f223a3f4b68190567b7d4d44545bce5