# Efficient 64 bit arithmetic

Michael Haufe tno at thenewobjective.com
Tue Jul 8 09:19:34 PDT 2014

```Latest news on "div" AFAIK:

On Tue, Jul 8, 2014 at 10:45 AM, Filip Pizlo <fpizlo at apple.com> wrote:

> I like this a lot.
>
> It looks like it will have a fairly mellow performance cliff in cases
> where VM optimizations fail for whatever reason, since even in a lower-tier
> compiler incapable of sophisticated optimizations all you really have to do
> is make sure that these new math function calls bottom out in something
> efficient.
>
> This would be even better if there was a story for div and mod. Is there
> one?
>
> -Filip
>
> > On Jul 8, 2014, at 7:59 AM, Fabrice Bellard <fabrice at bellard.org> wrote:
> >
> > Hi all,
> >
> > Regarding 64 bit arithmetic, there is a simpler solution compared to
> what was proposed in [1]. It is already possible to get the low 32 bit part
> of the 64 bit operations for addition, subtraction and multiplication. So
> it is enough to add a few more functions to get the high 32 bit part of 64
> bit additions, subtractions and multiplications. Hence I propose to add the
> following functions:
> >
> > Math.iaddh(lo0, hi0, lo1, hi1) : return the high 32 bit part of the 64
> bit addition of (hi0, lo0) and (hi1, lo1).
> >
> > Math.isubh(lo0, hi0, lo1, hi1) : return the high 32 bit part of the 64
> bit subtraction of (hi0, lo0) and (hi1, lo1).
> >
> > Math.imulh(a, b) : return the high 32 bit part of the signed 64 bit
> product of the 32 bit numbers a and b.
> >
> > Math.imuluh(a, b) : return the high 32 bit part of the unsigned 64 bit
> product of the 32 bit numbers a and b.
> >
> > All these functions convert their argument to 32 bit integers. They
> return a signed 32 bit integer.
> >
> > With these functions, the 64 bit operations are easy to implement :
> >
> > - (hi_res, lo_res) = (hi0, lo0) + (hi1, lo1) (64 bit addition):
> >
> >   lo_res = (lo0 + lo1) | 0;
> >   hi_res = Math.iaddh(lo0, hi0, lo1, hi1);
> >
> > - (hi_res, lo_res) = (hi0, lo0) - (hi1, lo1) (64 bit subtraction):
> >
> >   lo_res = (lo0 - lo1) | 0;
> >   hi_res = Math.isubh(lo0, hi0, lo1, hi1);
> >
> > -  (hi_res, lo_res) = a * b (signed 64 bit product of 32 bit integers):
> >
> >   lo_res = Math.imul(a, b);
> >   hi_res = Math.imulh(a, b);
> >
> > -  (hi_res, lo_res) = a * b (unsigned 64 bit product of 32 bit integers):
> >
> >   lo_res = Math.imul(a, b);
> >   hi_res = Math.imuluh(a, b);
> >
> > -  (hi_res, lo_res) = (hi0, lo0) * (hi1, lo1) (signed 64 bit product of
> 64 bit integers):
> >
> >   lo_res = Math.imul(lo0, lo1);
> >   hi_res = (Math.imulh(lo0, lo1) + Math.imul(lo0, hi1)) | 0;
> >   hi_res = (hi_res + Math.imul(lo1, hi0)) | 0;
> >
> > -  (hi_res, lo_res) = (hi0, lo0) * (hi1, lo1) (unsigned 64 bit product
> of 64 bit integers):
> >
> >   lo_res = Math.imul(lo0, lo1);
> >   hi_res = (Math.imuluh(lo0, lo1) + Math.imul(lo0, hi1)) | 0;
> >   hi_res = (hi_res + Math.imul(lo1, hi0)) | 0;
> >
> > It is easy for the compiler to optimize the code because only 32 bit
> integers are used and because the functions have no side effect. Even if
> the compiler does not remove the duplicate operation for the low 32 bit
> parts, the overhead is very small on a 32 bit CPU (1 more instruction than
> the optimal code).
> >
> > Fabrice.
> >
> > [1] https://www.mail-archive.com/es-discuss%40mozilla.org/msg27237.html
> > _______________________________________________
> > es-discuss mailing list
> > es-discuss at mozilla.org
> > https://mail.mozilla.org/listinfo/es-discuss
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