Re: Optimizing jit_divi_u()

[Paulo César Pereira de Andrade]

Em sex., 18 de ago. de 2023 às 12:07, Paulo César Pereira de Andrade
<paulo.cesar.pereira.de.andrade@gmail.com> escreveu:
>
> Em sex., 18 de ago. de 2023 às 09:41, Paul Cercueil
> <paul@crapouillou.net> escreveu:
> >
> > Hi Paulo,
>
>   Hi Paul,
>
> > I'm implementing an algorithm in my JIT to handle divisions by known
> > immediate values:
> > https://ridiculousfish.com/blog/posts/labor-of-division-episode-i.html
> >
> > This could totally be a default implementation for jit_divi_u(), by the
> > way.

  We should create a single pass for this kind of optimization.
  Currently there are a few architecture based ones to convert division
into shift and some other minor optimizations.

  In gcc the computation of the constant is done in
gcc/expmed.c*:choose_multiplier()

> > It does require a "64 <- 32 * 32" multiplication and needs the high 32
> > bits. Since we already have jit_mulr() to retrieve the low bits, I
> > wonder if it would be a good idea to have a jit_mulhr() to only get the
> > high bits.

  For the x86 port it is trivial, and most other cpus have at least an
unsigned variant. The ports that do not have any "mulh" variant are
not widely used and a fallback can be added for completeness of
the Lightning instruction set.

Thanks!
Paulo