Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753162AbbETMBV (ORCPT ); Wed, 20 May 2015 08:01:21 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:36120 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753030AbbETMBS (ORCPT ); Wed, 20 May 2015 08:01:18 -0400 Date: Wed, 20 May 2015 05:01:10 -0700 From: "Paul E. McKenney" To: c++std-parallel@accu.org Cc: Linus Torvalds , Linux Kernel Mailing List , "linux-arch@vger.kernel.org" , "gcc@gcc.gnu.org" , p796231 , "mark.batty@cl.cam.ac.uk" , Peter Zijlstra , Will Deacon , Ramana Radhakrishnan , David Howells , Andrew Morton , Ingo Molnar , michaelw@ca.ibm.com Subject: Re: [c++std-parallel-1616] Re: Compilers and RCU readers: Once more unto the breach! Message-ID: <20150520120110.GF6776@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150520005510.GA23559@linux.vnet.ibm.com> <20150520023402.GC6776@linux.vnet.ibm.com> <555C38F2.7060402@gmx.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <555C38F2.7060402@gmx.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15052012-8236-0000-0000-00000BA6DABE Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1652 Lines: 39 On Wed, May 20, 2015 at 09:34:10AM +0200, Jens Maurer wrote: > On 05/20/2015 04:34 AM, Paul E. McKenney wrote: > > On Tue, May 19, 2015 at 06:57:02PM -0700, Linus Torvalds wrote: > > >> - the "you can add/subtract integral values" still opens you up to > >> language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the > >> dependency, which it clearly doesn't. But language-lawyering it does, > >> since all those operations (cast to pointer, cast to integer, > >> subtracting an integer) claim to be dependency-preserving operations. > > [...] > > > There are some stranger examples, such as "(char *)ptr - ((intptr_t)ptr)/7", > > but in that case, if the resulting pointer happens by chance to reference > > valid memory, I believe a dependency would still be carried. > [...] > > >From a language lawyer standpoint, pointer arithmetic is only valid > within an array. These examples seem to go beyond the bounds of the > array and therefore have undefined behavior. > > C++ standard section 5.7 paragraph 4 > "If both the pointer operand and the result point to elements of the > same array object, or one past the last element of the array object, > the evaluation shall not produce an overflow; otherwise, the behavior > is undefined." > > C99 and C11 > identical phrasing in 6.5.6 paragraph 8 Even better! I added a footnote calling out these two paragraphs. Thax, 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/