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

[Philip Levis]

On Jun 25, 2009, at 12:06 PM, Eric Decker wrote:

>
>
> On Thu, Jun 25, 2009 at 11:54 AM, Philip Levis <pal at cs.stanford.edu>  
> wrote:
>
> On Jun 25, 2009, at 11:42 AM, Vlado Handziski wrote:
>
> Speaking about optimization, and looking at your example snippet:  
> For the msp430 platforms, it is always much more efficient to use  
> word size array indices. The code is a lot simpler and some nasty  
> gcc bugs can be avoided more easy.
>
> But this is platform independent code. The cost ratio of an  
> inequality and jump compared to a hardware modulo is much less than  
> a software modulo and an inequality and jump.
>
> Phil
>
> I'm not following the analysis because I'm not sure what is meant by  
> hardware vs. software modulo.   % on the MSP430 is implemented by a  
> library routine that takes a significant number of instructions.    
> Whereas the comparsion, jump, assignment take much fewer.

By "software modulo" I mean "a library routine." By "hardware modulo"  
I mean an ALU that computes modulo, e.g., as the iMote2 has. The point  
is that making an iMote2 do a compare and jump is not a big overhead  
over a hardware modulo, while making an msp430 (or other MCUs) do a  
software modulo is a big overhead over a compare and jump. But  
starting to set the width of variables in platform-independent code  
due to some particulars of a popular MCU's compiler (as Vlado  
suggested) seems like a bad idea.

Phil