Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753033AbbETOPw (ORCPT ); Wed, 20 May 2015 10:15:52 -0400 Received: from foss.arm.com ([217.140.101.70]:38435 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752494AbbETOPu (ORCPT ); Wed, 20 May 2015 10:15:50 -0400 Message-ID: <555C9714.6020001@arm.com> Date: Wed, 20 May 2015 15:15:48 +0100 From: Ramana Radhakrishnan User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: "paulmck@linux.vnet.ibm.com" 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! 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> <20150520140343.GO6776@linux.vnet.ibm.com> In-Reply-To: <20150520140343.GO6776@linux.vnet.ibm.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1437 Lines: 58 On 20/05/15 15:03, Paul E. McKenney wrote: > 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. Well, reads if any of x (assuming x was initialized elsewhere) would need to happen before x got assigned to z. I understood the original "maintain the ordering" as between the statements `x = z' and `y'. > > Or am I missing something subtle here? No, it sounds like we are on the same page here. regards Ramana > > 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/