Re: Optimize numeric multiplication for one and two base-NBASE digit multiplicands.

Em qua., 3 de jul. de 2024 às 08:18, Dean Rasheed <dean.a.rasheed@gmail.com>
escreveu:

> On Tue, 2 Jul 2024 at 21:10, Joel Jacobson <joel@compiler.org> wrote:
> >
> > I found the bug in the case 3 code,
> > and it turns out the same type of bug also exists in the case 2 code:
> >
> >                         case 2:
> >                                 newdig = (int) var1digits[1] *
> var2digits[res_ndigits - 4];
> >
> > The problem here is that res_ndigits could become less than 4,
>
> Yes. It can't be less than 3 though (per an earlier test), so the case
> 2 code was correct.
>
> I've been hacking on this a bit and trying to tidy it up. Firstly, I
> moved it to a separate function, because it was starting to look messy
> having so much extra code in mul_var(). Then I added a bunch more
> comments to explain what's going on, and the limits of the various
> variables. Note that most of the boundary checks are actually
> unnecessary -- in particular all the ones in or after the main loop,
> provided you pull out the first 2 result digits from the main loop in
> the 3-digit case. That does seem to work very well, but...
>
> I wasn't entirely happy with how messy that code is getting, so I
> tried a different approach. Similar to div_var_int(), I tried writing
> a mul_var_int() function instead. This can be used for 1 and 2 digit
> factors, and we could add a similar mul_var_int64() function on
> platforms with 128-bit integers. The code looks quite a lot neater, so
> it's probably less likely to contain bugs (though I have just written
> it in a hurry,so it might still have bugs). In testing, it seemed to
> give a decent speedup, but perhaps a little less than before. But
> that's to be balanced against having more maintainable code, and also
> a function that might be useful elsewhere in numeric.c.
>
> Anyway, here are both patches for comparison. I'll stop hacking for a
> while and let you see what you make of these.
>
I liked v5-add-mul_var_int.patch better.

I think that *var_digits can be const too.
+ const NumericDigit *var_digits = var->digits;

Typo In the comments:
- by procssing
+ by processing

best regards,
Ranier Vilela