Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753471AbbETOEL (ORCPT ); Wed, 20 May 2015 10:04:11 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:42803 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753402AbbETOEH (ORCPT ); Wed, 20 May 2015 10:04:07 -0400 Date: Wed, 20 May 2015 07:03:43 -0700 From: "Paul E. McKenney" To: Ramana Radhakrishnan Cc: David Howells , Will Deacon , Linus Torvalds , Linux Kernel Mailing List , "c++std-parallel@accu.org" , "linux-arch@vger.kernel.org" , "gcc@gcc.gnu.org" , p796231 , "mark.batty@cl.cam.ac.uk" , Peter Zijlstra , Andrew Morton , Ingo Molnar , "michaelw@ca.ibm.com" Subject: Re: Compilers and RCU readers: Once more unto the breach! Message-ID: <20150520140343.GO6776@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150520133037.GK6776@linux.vnet.ibm.com> <20150520121522.GH6776@linux.vnet.ibm.com> <20150520005510.GA23559@linux.vnet.ibm.com> <20150520024148.GD6776@linux.vnet.ibm.com> <20150520114745.GC11498@arm.com> <31547.1432127917@warthog.procyon.org.uk> <31805.1432129025@warthog.procyon.org.uk> <555C8FBE.4020505@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <555C8FBE.4020505@arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15052014-0013-0000-0000-00000AEC30AA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1075 Lines: 40 On Wed, May 20, 2015 at 02:44:30PM +0100, Ramana Radhakrishnan wrote: > > > On 20/05/15 14:37, David Howells wrote: > >Paul E. McKenney wrote: > > > >>I was thinking of "y" as a simple variable, but if it is something more > >>complex, then the compiler could do this, right? > >> > >> char *x; > >> > >> y; > >> x = z; > > > >Yeah. I presume it has to maintain the ordering, though. > > The scheduler for e.g. is free to reorder if it can prove there is > no dependence (or indeed side-effects for y) between insns produced > for y and `x = z'. So for example, if y is independent of z, the compiler can do the following: char *x; x = z; y; But the dependency ordering is still maintained from z to x, so this is not a problem. Or am I missing something subtle here? Thanx, Paul -- 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/