From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964851AbVLIS2q (ORCPT ); Fri, 9 Dec 2005 13:28:46 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964864AbVLIS2q (ORCPT ); Fri, 9 Dec 2005 13:28:46 -0500 Received: from waste.org ([64.81.244.121]:29351 "EHLO waste.org") by vger.kernel.org with ESMTP id S964851AbVLIS2p (ORCPT ); Fri, 9 Dec 2005 13:28:45 -0500 Date: Fri, 9 Dec 2005 10:22:55 -0800 From: Matt Mackall To: Oliver Neukum Cc: Ingo Molnar , Jesper Juhl , Linux Kernel Mailing List , Andrew Morton Subject: Re: [RFC][PATCH] Reduce number of pointer derefs in various files (kernel/exit.c used as example) Message-ID: <20051209182255.GQ8637@waste.org> References: <200512062302.06933.jesper.juhl@gmail.com> <20051206221528.GA12358@elte.hu> <20051209014658.GA11856@waste.org> <200512090914.21436.oliver@neukum.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200512090914.21436.oliver@neukum.org> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 09, 2005 at 09:14:21AM +0100, Oliver Neukum wrote: > Am Freitag, 9. Dezember 2005 02:46 schrieb Matt Mackall: > > On Tue, Dec 06, 2005 at 11:15:28PM +0100, Ingo Molnar wrote: > > > > > > * Jesper Juhl wrote: > > > > > > > Ohh, and before I forget, besides the fact that this should speed > > > > things up a little bit it also has the added benefit of reducing the > > > > size of the generated code. The original kernel/exit.o file was 19604 > > > > bytes in size, the patched one is 19508 bytes in size. > > > > > > nice. Just to underline your point, on x86, with gcc 4.0.2, i'm getting > > > this with your patch: > > > > > > text data bss dec hex filename > > > 11077 0 0 11077 2b45 exit.o.orig > > > 10997 0 0 10997 2af5 exit.o > > > > > > so 80 bytes shaved off. I think such patches also increase readability. > > > > Readability improved: good. > > 37 lines of patch for 80-100 bytes saved: not so good. > > > > So while this is a good style direction, I don't think it's worth the > > churn. And unlike kzalloc and the like, this particular optimization > > is perfectly doable by a compiler. So I'd rather wait for the compiler > > to get smarter than change code for such modest improvements. > > How can the compiler do it? If a function call is between two evaluations > of a pointer chain, the compiler would have to make sure no pointer in > the chain is touched. For the case of a computed function call, it is > impossible in principle. Excellent point. It'd require marking functions const, which might not be a bad idea. -- Mathematics is the supreme nostalgia of our time.