From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed Date: Fri, 2 Nov 2018 06:37:28 -0700 Message-ID: <20181102133728.GR4170@linux.ibm.com> References: <20181031213240.zhh7dfcm47ucuyfl@pburton-laptop> <20181031220253.GA15505@roeck-us.net> <20181031233235.qbedw3pinxcuk7me@pburton-laptop> <4e2438a23d2edf03368950a72ec058d1d299c32e.camel@hammerspace.com> <20181101131846.biyilr2msonljmij@lakrids.cambridge.arm.com> <20181101145926.GE3178@hirez.programming.kicks-ass.net> <20181101163212.GF3159@hirez.programming.kicks-ass.net> <20181101170146.GQ4170@linux.ibm.com> <7d1ecd21c4c249138dfdd42b9aaa1cea@AcuMS.aculab.com> Reply-To: paulmck@linux.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Peter Zijlstra , Trond Myklebust , "mark.rutland@arm.com" , "linux-kernel@vger.kernel.org" , "ralf@linux-mips.org" , "jlayton@kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "bfields@fieldses.org" , "linux-mips@linux-mips.org" , "linux@roeck-us.net" , "linux-nfs@vger.kernel.org" , "akpm@linux-foundation.org" , "will.deacon@arm.com" , "boqun.feng@gmail.com" , "paul.burton@mips.com" , "anna.schumaker@netapp.com" Return-path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:53752 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727514AbeKBWot (ORCPT ); Fri, 2 Nov 2018 18:44:49 -0400 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wA2DXraP028226 for ; Fri, 2 Nov 2018 09:37:37 -0400 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0a-001b2d01.pphosted.com with ESMTP id 2ngnc8dv6e-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Fri, 02 Nov 2018 09:37:37 -0400 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 2 Nov 2018 13:37:36 -0000 Content-Disposition: inline In-Reply-To: <7d1ecd21c4c249138dfdd42b9aaa1cea@AcuMS.aculab.com> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Nov 02, 2018 at 10:56:31AM +0000, David Laight wrote: > From: Paul E. McKenney > > Sent: 01 November 2018 17:02 > ... > > And there is a push to define C++ signed arithmetic as 2s complement, > > but there are still 1s complement systems with C compilers. Just not > > C++ compilers. Legacy... > > Hmmm... I've used C compilers for DSPs where signed integer arithmetic > used the 'data registers' and would saturate, unsigned used the 'address > registers' and wrapped. > That was deliberate because it is much better to clip analogue values. There are no C++ compilers for those DSPs, correct? (Some of the C++ standards commmittee members believe that they have fully checked, but they might well have missed something.) > Then there was the annoying cobol run time that didn't update the > result variable if the result wouldn't fit. > Took a while to notice that the sum of a list of values was even wrong! > That would be perfectly valid for C - if unexpected. Heh! COBOL and FORTRAN also helped fund my first pass through university. > > > But for us using -fno-strict-overflow which actually defines signed > > > overflow > > I wonder how much real code 'strict-overflow' gets rid of? > IIRC gcc silently turns loops like: > int i; for (i = 1; i != 0; i *= 2) ... > into infinite ones. > Which is never what is required. The usual response is something like this: for (i = 1; i < n; i++) where the compiler has no idea what range of values "n" might take on. Can't say that I am convinced by that example, but at least we do have -fno-strict-overflow. Thanx, Paul