[glibc] powerpc: Update acosf ulps

[Alexander Monakov]

On Thu, 2 Jan 2025, Alexander Monakov wrote:

> 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?

Ah, this is answered in a change that was not carried over from core-math
to Glibc. It's in 'return (float) pi2;' where pi2 is a 'const double' local.

> 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?

I should clarify: without the 'f' suffix, the summation happens in 'double'
and triggers the GCC bug upon conversion to float. But with the 'f' suffix,
addition of 0x1p-25f is done at runtime, producing the necessary rounding.

> Also, should the above instances be fixed to use float literals?
> 
> Thank you.
> Alexander
>