linux-kernel - Re: [RFC] div64

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Message-ID: <20070226154413.4ead571a@freekitty>
Date:	Mon, 26 Feb 2007 15:44:13 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RFC] div64_64 support

On Tue, 27 Feb 2007 00:02:50 +0100 (MET)
Jan Engelhardt <jengelh@...ux01.gwdg.de> wrote:

> 
> On Feb 26 2007 13:28, Stephen Hemminger wrote:
> >> 
> >> ./arch/arm26/lib/udivdi3.c
> >> ./arch/sh/lib/udivdi3.c
> >> ./arch/sparc/lib/udivdi3.S
> >> 
> >> should not this be consolidated too?
> >
> >Hmm. Those are the GCC internal versions, that are picked up but
> >doing divide in place. Do we want to allow general 64 bit in kernel to
> >be easily used? It could cause sloppy slow code, but it would look
> >cleaner.
> 
> Then our reviewers should catch it, and if not, the janitors will
> (/me winks at R.P.J.Day and trivial@).
> 
> >@@ -134,7 +112,7 @@
> > 	 */
> > 	do {
> > 		x1 = x;
> >-		x = (2 * x + (uint32_t) div64_64(a, x*x)) / 3;
> >+		x = (2 * x + (u32) (a / x*x)) / 3;
> 
> Eye see a bug.
> 
> Previously there was div64_64(a, x*x) which is equivalent to
> (a)/(x*x), or just: a/(x^2). But now you do a/x*x, which is
> equivalent to a*x/x (in the domain of real numbers). Furthermore,
> a/x*x is a-(a%x), which does not even remotely match a/(x^2).
> 
> Please keep the math intact, thank you ;-)

Been there, done that, don't want to repeat it...

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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