Re: [musl] issue with acoshf

[Rich Felker <dalias@libc.org>]

On Fri, Feb 05, 2021 at 08:18:02AM +0100, Paul Zimmermann wrote:
>        Hi,
> 
> while updating to my comparison of the accuracy of mathematical functions [1],
> I have noticed an issue with acoshf in musl-1.2.2:
> 
> $ cat test_acosh_musl.c
> #include <stdio.h>
> #include <stdlib.h>
> #include <math.h>
> 
> int
> main (int argc, char *argv[])
> {
>   float x = -0x1.1e6ae8p+5;
>   float y;
>   y = acoshf (x);
>   printf ("x=%a y=%a\n", x, y);
> }
> 
> With gcc I get NaN as expected:
> 
> $ gcc -fno-builtin test_acosh_musl.c -lm
> $ ./a.out
> x=-0x1.1e6ae8p+5 y=-nan
> 
> With musl-1.2.2 I get -0x1.2f63acp+3:
> 
> $ gcc -fno-builtin test_acosh_musl.c $FILES
> $ ./a.out
> x=-0x1.1e6ae8p+5 y=-0x1.2f63acp+3
> 
> Please can someone confirm?

I can't reproduce it on i386 but can on sh w/softfloat. I'm guessing
you're using an arch without its own special definition of sqrtf or
logf, so it looks like it would have to be a bug in the
non-arch-specific version of one of those, but I haven't been able to
reproduce it in isolation just passing the values passed to them
(-0x1.c9b6fcp-7 to logf or 0x1.40330cp+10 to sqrtf) manually building
the generic C versions.

Thanks for the report. I'll keep looking.

Rich