From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-ID: <47B72BFE.9060302@am.sony.com> Date: Sat, 16 Feb 2008 10:31:26 -0800 From: Geoff Levand MIME-Version: 1.0 To: Arjan van de Ven Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y References: <47B70A61.9030306@tiscali.nl> <20080216092552.325e5726@laptopd505.fenrus.org> <20080216173315.GU8953@1wt.eu> <20080216094226.1e8eede1@laptopd505.fenrus.org> In-Reply-To: <20080216094226.1e8eede1@laptopd505.fenrus.org> Content-Type: text/plain; charset=ISO-8859-1 Cc: linuxppc-dev@ozlabs.org, Roel Kluin <12o3l@tiscali.nl>, Willy Tarreau , lkml , cbe-oss-dev@ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 02/16/2008 09:42 AM, Arjan van de Ven wrote: > On Sat, 16 Feb 2008 18:33:16 +0100 > Willy Tarreau wrote: > >> On Sat, Feb 16, 2008 at 09:25:52AM -0800, Arjan van de Ven wrote: >> > On Sat, 16 Feb 2008 17:08:01 +0100 >> > Roel Kluin <12o3l@tiscali.nl> wrote: >> > >> > > The patch below was not yet tested. If it's correct as it is, >> > > please comment. --- >> > > Fix Unlikely(x) == y >> > > >> > >> > you found a great set of bugs.. >> > but to be honest... I suspect it's just best to remove unlikely >> > altogether for these cases; unlikely() is almost a >> > go-faster-stripes thing, and if you don't know how to use it you >> > shouldn't be using it... so just removing it for all wrong cases is >> > actually the best thing to do imo. >> >> Well, eventhough the author may not know how to use it, "unlikely" at >> least indicates the intention of the author, or his knowledge of what >> should happen here. I'd suggest leaving it where it is because the >> authot of this code is in best position to know that this branch is >> unlikely to happen, eventhough he does not correctly use the macro. >> > > you have more faith in the authors knowledge of how his code actually behaves than I think is warranted :) > Or faith in that he knows what "unlikely" means. > I should write docs about this; but unlikely() means: > 1) It happens less than 0.01% of the cases. > 2) The compiler couldn't have figured this out by itself > (NULL pointer checks are compiler done already, same for some other conditions) > 3) It's a hot codepath where shaving 0.5 cycles (less even on x86) matters > (and the author is ok with taking a 500 cycles hit if he's wrong) > > If you think unlikely() means something else, we should fix what it maps to towards gcc ;) > (to.. be empty ;) Well, I didn't consider what today's compiler does, but used it as a general indicator, because I think that code will be around a long time. If you show me some test results that prove it causes harm I might consider removing it. -Geoff