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

From: Paulo Marques
Subject: Re: [avr-gcc-list] dev and mod may not be optimized
Date: Wed, 12 Dec 2007 16:48:52 +0000
User-agent: Thunderbird (X11/20070509)

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:


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:

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."

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