Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758482AbZASVDu (ORCPT ); Mon, 19 Jan 2009 16:03:50 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754118AbZASVDj (ORCPT ); Mon, 19 Jan 2009 16:03:39 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:50479 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753403AbZASVDi (ORCPT ); Mon, 19 Jan 2009 16:03:38 -0500 Date: Tue, 20 Jan 2009 08:01:52 +1100 (EST) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: Nick Piggin cc: Ingo Molnar , Bernd Schmidt , Andi Kleen , David Woodhouse , Andrew Morton , Harvey Harrison , "H. Peter Anvin" , Chris Mason , Peter Zijlstra , Steven Rostedt , paulmck@linux.vnet.ibm.com, Gregory Haskins , Matthew Wilcox , Linux Kernel Mailing List , linux-fsdevel , linux-btrfs , Thomas Gleixner , Peter Morreale , Sven Dietrich , jh@suse.cz Subject: Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning In-Reply-To: <20090119062212.GC22584@wotan.suse.de> Message-ID: References: <20090112001255.GR26290@one.firstfloor.org> <20090112005228.GS26290@one.firstfloor.org> <496B86B5.3090707@t-online.de> <20090112193201.GA23848@one.firstfloor.org> <496BBE27.2020206@t-online.de> <20090119001345.GA9880@elte.hu> <20090119062212.GC22584@wotan.suse.de> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1429 Lines: 33 On Mon, 19 Jan 2009, Nick Piggin wrote: > > I want to know what is the problem with the restrict keyword? > I'm sure I've read Linus ranting about how bad it is in the > past... No, I don't think I've ranted about 'restrict'. I think it's a reasonable solution for performance-critical code, and unlike the whole type-aliasing model, it actually works for the _sane_ cases (ie doing some operation over two arrays of the same type, and letting the compiler know that it can access the arrays without fearing that writing to one would affect reading from the other). The problem with 'restrict' is that almost nobody uses it, and it does obviously require programmer input rather than the compiler doing it automatically. But it should work well as a way to get Fortran-like performance from HPC workloads written in C - which is where most of the people are who really want the alias analysis. > it seems like a nice opt-in thing that can be used where the aliases are > verified and the code is particularly performance critical... Yes. I think we could use it in the kernel, although I'm not sure how many cases we would ever find where we really care. Linus -- 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/