[PATCH v2 3/5] math: Improve fmod

[H.J. Lu]

On Fri, Mar 17, 2023 at 7:55 AM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>
> Hi Adhemerval,
>
> >> It's these cases where x87 is still faster than the generic version:
> >>
> >>> E5-2640          | close-exponents | 39.298   | 22.2742
> >>>
> >>> i7-4510U         | close-exponents | 29.463   | 22.8572
> >>
> >> Are these mostly x < y or cases where the exponent difference is just over 11 and
> >> thus we do not use the fast path?
> >
> > In fact the fast path will be used on ~83% of the cases (849 from 1024 entries).
> > Profiling shows that the initial checks might be the culprit, since generic
> > compat wrapper uses compiler builtins that might map to fp instructions. But even
> > trying to mimic did not improve much.  It seems that for some CPU the integer
> > operations to create the final floating number is what is costly.
>
> If it is mostly the fast path we could further tune it and reduce instruction counts.
> It takes 6 if statements to enter this fast path, we could reduce that to 3. There are
> several large constants which could be simplified (older x86 cores might have
> issues with multiple 10-byte MOVABS in the instruction stream).
>
> Also I think your results for generic above use the wrapper, so we'd still get the
> > 20% speedup which should make things closer.
>

The current __ieee754_fmod doesn't set errno nor does x87 __ieee754_fmod.
A wrapper will avoid setting errno.

-- 
H.J.