Subj : Re: faster approach to division
To   : comp.programming
From : Walter
Date : Mon Jul 11 2005 01:06 pm


"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?)

The behavior of CPUs on various operations is often not intuitive, so you
should be prepared to test. If you're not seeing a worthwhile difference in
your tests, then it's almost surely not an optimization worth spending time
on.

For some techniques for timing code, check out
www.digitalmars.com/techtips/timing_code.html

-Walter
www.digitalmars.com C, C++, D compilers
"code of the nerds"

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