[Tinyos-devel] One should make limited use of the "%" operator

[Philip Levis]

On Jun 25, 2009, at 11:30 AM, Eric Decker wrote:

> While perusing the code and paying attention to the kind of code  
> generated I noticed constructs of the form:
>
>   command queue_t Queue.element(uint8_t idx) {
>     idx += head;
>     idx %= QUEUE_SIZE;
>     return queue[idx];
>   }
>
> I'm referring to "idx %= QUEUE_SIZE"
>
>
> The use of the % operator can be expensive and more often than not  
> is.  How is it expensive?  Well what
> it does it generates a call to the math library (which pull that  
> piece in which chews precious rom space) and
> the math code take a bit of time to execute.
>
> The above could might be rewritten as:
>
> command queue_t Queue.element(uint8_t idx) {
>   if (idx > QUEUE_SIZE)
>     return NULL;
>    return queue[idx];
> }
>
> or
>
> while (idx > QUEUE_SIZE)
>    idx -= QUEUE_SIZE;
>
>
> I prefer the former because it is well bounded in time execution and  
> detects odd requests.

Agreed. Except that the former is buggy. The latter is the correct  
approach. I'll check in the fix.

Phil