From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: [RFC] div64_64 support Date: Mon, 26 Feb 2007 15:44:13 -0800 Message-ID: <20070226154413.4ead571a@freekitty> References: <20070223170527.4ca695b2@freekitty> <20070226143127.5c74bec9@freekitty> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org To: Jan Engelhardt Return-path: Received: from smtp.osdl.org ([65.172.181.24]:38533 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932899AbXBZXoT (ORCPT ); Mon, 26 Feb 2007 18:44:19 -0500 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, 27 Feb 2007 00:02:50 +0100 (MET) Jan Engelhardt 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