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From: Paul Zimmermann <>
To: Szabolcs Nagy <>
Subject: Re: [musl] correctly rounded mathematical functions
Date: Thu, 06 Jan 2022 09:55:59 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (message from Szabolcs Nagy on Tue, 4 Jan 2022 22:12:02 +0100)

       Dear Szabolcs,

> Date: Tue, 4 Jan 2022 22:12:02 +0100
> From: Szabolcs Nagy <>
> 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,

      reply	other threads:[~2022-01-06  8:56 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-03 13:07 Paul Zimmermann
2022-01-04  0:04 ` Damian McGuckin
2022-01-04  2:29 ` Rich Felker
2022-01-04 14:19   ` Paul Zimmermann
2022-01-04 21:12     ` Szabolcs Nagy
2022-01-06  8:55       ` Paul Zimmermann [this message]

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