Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753351AbbETJDJ (ORCPT ); Wed, 20 May 2015 05:03:09 -0400 Received: from mail-oi0-f54.google.com ([209.85.218.54]:32979 "EHLO mail-oi0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753120AbbETJDB (ORCPT ); Wed, 20 May 2015 05:03:01 -0400 MIME-Version: 1.0 In-Reply-To: <555C38F2.7060402@gmx.net> References: <20150520005510.GA23559@linux.vnet.ibm.com> <20150520023402.GC6776@linux.vnet.ibm.com> <555C38F2.7060402@gmx.net> Date: Wed, 20 May 2015 11:03:00 +0200 Message-ID: Subject: Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more unto the breach! From: Richard Biener To: Jens Maurer Cc: c++std-parallel@accu.org, "Paul E. McKenney" , 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 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1893 Lines: 45 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(?) Richard. > Jens -- 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/