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Message-Id: <mwwnjdz1i8.fsf@tomate.loria.fr>
Date: Thu, 06 Jan 2022 09:55:59 +0100
From: Paul Zimmermann <Paul.Zimmermann@...ia.fr>
To: Szabolcs Nagy <nsz@...t70.net>
Cc: dalias@...c.org, musl@...ts.openwall.com, sibid@...c.ca,
christoph.lauter@...istoph-lauter.org,
Jean-Michel.Muller@...-LYON.FR
Subject: Re: correctly rounded mathematical functions
Dear Szabolcs,
> Date: Tue, 4 Jan 2022 22:12:02 +0100
> From: Szabolcs Nagy <nsz@...t70.net>
>
> i only looked at cr_asinf:
thank you for looking! This is much appreciated.
> float
> cr_asinf (float x)
> {
> /* deal here with NaN, +Inf and -Inf */
>
> yeah that can be tricky and different across math libs.
> (errno vs no errno, wrapper to handle special cases vs no wrapper etc)
indeed, and that will be specific to each libm
> to minimize branches you may want to merge it with the |x|>1 check below.
>
> double absx = fabsf (x), y;
> if (absx > 1)
> return sqrt (-1.0); /* NaN */
> ...
I tried to merge the detection of NaN, Inf with the case |x| > 1 using
the glibc macro asuint, but it was not faster (at least for glibc make bench).
> this reminds me that musl does not use compiler builtins at build time,
> which means fabsf, sqrt are extern calls, which means this will not be
> a leaf function (note: sqrt is not a tail call here because of the
> implicit conversion, i think it should be sqrtf).
then probably for musl it would be faster to have the following:
double absx = (x > 0) ? x : -x;
we can probably define macros for this, say CORE_MATH_FABS, that would
be instantiated differently for each libm.
> i prefer to tail call a function with descriptive name when handling
> errors, but it does not always work out well: e.g. underflow can be
> tricky if you want to ensure that an exact subnormal result does not
> raise it (e.g. powf(2, -140)), but inexact does.
yes, sqrt (-1.0) was just a portable way to generate NaN. This code
will evolve over time with the feedback we get.
Best regards,
Paul
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