Subj : Re: faster approach to division
To   : comp.programming
From : Rob Thorpe
Date : Fri Jul 15 2005 02:23 am

William wrote:
> "Mark P" <not@my.real.email> wrote in message
> news:9JyAe.351$Rv7.249@newssvr21.news.prodigy.com...
> > I'm working on a problem which involves a fair amount of integer
> > division but in essentially all cases the divisor is +/- 1 or +/- 2.
> > I'm using C++ and trying to figure out whether I should code this as
> > just ordinary division or whether it makes sense to create a special
> > class (basically a wrapped enumeration) to hold my possible divisor
> > values.  Then I would overload operator/() to recast division as a null
> > operation or a sign flip and/or right shift.
> >
> > Anyone have any intuition on this?  I've tried a few very simple tests
> > comparing division and shifting and it's hard to discern a difference,
> > but I'm not sure my tests are very effective.  (Are modern machines
> > optimized to divide by 1 and 2 quickly?)
>
> I assume you've got your compiler set to optimize for speed,
> so what kind of assembly is it actually outputting? I'd
> start there - you might find it's already made some good
> choices. (For example, it probably has turned the integer
> division into a right shift or something faster already. If
> it didn't, it's either broken or your code didn't make that
> replacement seem reasonable to it.) -Wm

Or the language didn't make it reasonable to the compiler, as happens
in some cases.  Many languages round towards zero, right shifts round
towards negative infinity for signed numbers.

.