Subj : Re: faster approach to division
To   : comp.programming
From : Mark P
Date : Tue Jul 12 2005 08:29 pm

goose wrote:
> Mark P wrote:
> 
>> 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?)
>>
>> Thanks for your advice,
>> Mark
> 
> 
> I've had need for something similar in the past, I've found
> that the fastest way for me to do it was by #define a macro
> to do the division.

I assume then that the divisor was known to you at compile time.  My 
situation is more like: x / f(...) where f(...) will always return +/-1 
or +/-2, determined at run time.

.