Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936667AbXHOVH7 (ORCPT ); Wed, 15 Aug 2007 17:07:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761567AbXHOVHn (ORCPT ); Wed, 15 Aug 2007 17:07:43 -0400 Received: from gate.crashing.org ([63.228.1.57]:51294 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760483AbXHOVHl (ORCPT ); Wed, 15 Aug 2007 17:07:41 -0400 In-Reply-To: <20070815185724.GH9645@linux.vnet.ibm.com> References: <46C2350A.1010807@redhat.com> <20070815081841.GA16551@osiris.boeblingen.de.ibm.com> <46C30540.2070603@s5r6.in-berlin.de> <20070815145207.GA23106@gondor.apana.org.au> <46C3253F.5090707@s5r6.in-berlin.de> <20070815162722.GD9645@linux.vnet.ibm.com> <20070815185724.GH9645@linux.vnet.ibm.com> Mime-Version: 1.0 (Apple Message framework v623) Content-Type: text/plain; charset=US-ASCII; format=flowed Message-Id: <2c907d70a2267b887de346891758983d@kernel.crashing.org> Content-Transfer-Encoding: 7bit Cc: horms@verge.net.au, Stefan Richter , Satyam Sharma , Linux Kernel Mailing List , rpjday@mindspring.com, netdev@vger.kernel.org, ak@suse.de, cfriesen@nortel.com, Heiko Carstens , jesper.juhl@gmail.com, linux-arch@vger.kernel.org, Andrew Morton , zlynx@acm.org, clameter@sgi.com, schwidefsky@de.ibm.com, Chris Snook , Herbert Xu , davem@davemloft.net, Linus Torvalds , wensong@linux-vs.org, wjiang@resilience.com From: Segher Boessenkool Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Wed, 15 Aug 2007 23:05:35 +0200 To: paulmck@linux.vnet.ibm.com X-Mailer: Apple Mail (2.623) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1763 Lines: 44 >> No; compilation units have nothing to do with it, GCC can optimise >> across compilation unit boundaries just fine, if you tell it to >> compile more than one compilation unit at once. > > Last I checked, the Linux kernel build system did compile each .c file > as a separate compilation unit. I have some patches to use -combine -fwhole-program for Linux. Highly experimental, you need a patched bleeding edge toolchain. If there's interest I'll clean it up and put it online. David Woodhouse had some similar patches about a year ago. >>> In many cases, the compiler also has to assume that >>> msleep_interruptible() >>> might call back into a function in the current compilation unit, thus >>> possibly modifying global static variables. >> >> It most often is smart enough to see what compilation-unit-local >> variables might be modified that way, though :-) > > Yep. For example, if it knows the current value of a given such local > variable, and if all code paths that would change some other variable > cannot be reached given that current value of the first variable. Or the most common thing: if neither the address of the translation- unit local variable nor the address of any function writing to that variable can "escape" from that translation unit, nothing outside the translation unit can write to the variable. > At least given that gcc doesn't know about multiple threads of > execution! Heh, only about the threads it creates itself (not relevant to the kernel, for sure :-) ) Segher - 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/