From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753214AbbETMCh (ORCPT ); Wed, 20 May 2015 08:02:37 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:46079 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752324AbbETMCe (ORCPT ); Wed, 20 May 2015 08:02:34 -0400 Date: Wed, 20 May 2015 05:02:29 -0700 From: "Paul E. McKenney" To: Richard Biener Cc: Jens Maurer , c++std-parallel@accu.org, 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-1614] Re: Compilers and RCU readers: Once more unto the breach! Message-ID: <20150520120229.GG6776@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: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15052012-0033-0000-0000-000004915F05 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 20, 2015 at 11:03:00AM +0200, Richard Biener wrote: > On Wed, May 20, 2015 at 9:34 AM, 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 > > Of course you can try to circumvent that by doing > (char*)((intptr_t)ptr - (intptr_t)ptr + (intptr_t)ptr) > (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752 for extra fun). > > Which (IMHO) gets you into the standard language that only makes conversion of > the exact same integer back to a pointer well-defined(?) I am feeling good about leaving the restriction and calling out the two paragraphs in a footnote, then. ;-) Thanx, Paul