[glibc] powerpc: Update acosf ulps

[Alexander Monakov]

Hello Paul,

On Thu, 2 Jan 2025, Paul Zimmermann wrote:

>        Hi Florian,
> 
> > Isn't the new acosf implementation supposed to be correctly rounded?
> 
> yes, but due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57245,
> the new implementation might be miscompiled by gcc < 12.

can you kindly point me where the new acosf employs problematic conversions?
I see a couple of instances where the 'f' suffix is missing, producing a
double literal where a float could work as well:

 * in as_special there is

  const float pih = 0x1.921fb6p+1;

 * and in __ieee754_acosf, in 0x1p-25:

      if (t == 0x328885a3u)
       return 0x1.921fb6p+0f + 0x1p-25;
      if (t == 0x39826222u)
       return 0x1.920f6ap+0f + 0x1p-25;
 
but I don't see any instance where a double-precision constant is converted
to float such that result depends on rounding mode.

I'm guessing the '+ 0x1p-25' trick above is a workaround for this exact bug?

Also, should the above instances be fixed to use float literals?

Thank you.
Alexander