Re: [avr-gcc-list] dev and mod may not be optimized

[Paulo Marques]

Gre7g Luterman wrote:
> --- Paulo Marques <address@hidden> wrote:
> > Actually after a few more digging, we can do this
> > all in plain C, by 
> > using a smaller reciprocal multiplier:
>
> <snipped>
>
> Wow, that's freakin' inspired! 

Thanks :)

It's not really a new idea, though. Using multiply by the reciprocal to 
do divisions by constants has been around for a while. For an in-depth 
analysis of the technique I suggest reading "Hacker's Delight".

The funny thing is that gcc for x86 does this optimization itself, 
replacing division by constants with multiplications when optimizing for 
speed. Unfortunately I don't know enough (or have time to get involved) 
to port the same optimization to the avr backend... :(

> And you're right it
> does optimize nicely.  Here's a 20 byte version that
> executes in 12 cycles:
> http://pastie.textmate.org/127541

Very nice :)

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com

"All I ask is a chance to prove that money can't make me happy."