Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755538AbYBROOc (ORCPT ); Mon, 18 Feb 2008 09:14:32 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754556AbYBROON (ORCPT ); Mon, 18 Feb 2008 09:14:13 -0500 Received: from smtp5.pp.htv.fi ([213.243.153.39]:38151 "EHLO smtp5.pp.htv.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754537AbYBROOM (ORCPT ); Mon, 18 Feb 2008 09:14:12 -0500 Date: Mon, 18 Feb 2008 16:13:40 +0200 From: Adrian Bunk To: Geert Uytterhoeven Cc: Michael Ellerman , Roel Kluin <12o3l@tiscali.nl>, lkml , cbe-oss-dev@ozlabs.org, linuxppc-dev@ozlabs.org, Willy Tarreau , Arjan van de Ven Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y Message-ID: <20080218141340.GB667@cs181133002.pp.htv.fi> References: <47B70A61.9030306@tiscali.nl> <20080216092552.325e5726@laptopd505.fenrus.org> <20080216173315.GU8953@1wt.eu> <20080216094226.1e8eede1@laptopd505.fenrus.org> <47B72BFE.9060302@am.sony.com> <20080216103927.2a02352b@laptopd505.fenrus.org> <1203249003.6718.24.camel@concordia> <20080218135609.GD21080@cs181133002.pp.htv.fi> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1507 Lines: 43 On Mon, Feb 18, 2008 at 03:01:35PM +0100, Geert Uytterhoeven wrote: > On Mon, 18 Feb 2008, Adrian Bunk wrote: > > > > This means it generates faster code with a current gcc for your platform. > > > > But a future gcc might e.g. replace the whole loop with a division > > (gcc SVN head (that will soon become gcc 4.3) already does > > transformations like replacing loops with divisions [1]). > > Hence shouldn't we ask the gcc people what's the purpose of __builtin_expect(), > if it doesn't live up to its promise? That's a different issue. My point here is that we do not know how the latest gcc available in the year 2010 might transform this code, and how a likely/unlikely placed there might influence gcc's optimizations then. If this is in hotpath code with a measurable speedup when using likely/unlikely with a current gcc then it's worth it. But otherwise it brings no real advantage today and the longterm effects are not predictable. > With kind regards, > > Geert Uytterhoeven cu Adrian -- "Is there not promise of rain?" Ling Tan asked suddenly out of the darkness. There had been need of rain for many days. "Only a promise," Lao Er said. Pearl S. Buck - Dragon Seed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/