From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-x335.google.com (mail-wm1-x335.google.com [IPv6:2a00:1450:4864:20::335]) by sourceware.org (Postfix) with ESMTPS id 1AF153858403 for ; Fri, 14 Jan 2022 19:54:30 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1AF153858403 Received: by mail-wm1-x335.google.com with SMTP id l4so8038540wmq.3 for ; Fri, 14 Jan 2022 11:54:30 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=obA7xLpdjt/JUtce/LuvGn9lSf87GQmPWMLlMUVj7Jk=; b=lYXrf++2mFxvBAu38o+Ts8ybZ0ggX6ylbTo7X+ocWDmxeYiLAUIVMVkIFkIAzGkkBN Iocie/Twtb3a1tmRbKvRrRwSEmu0DDumaj9IaMUiuTEFfQQceviq5/KhDTwZ3MARqgHy Q9btR8ocXYsxrvPsqnk6oui5mzZGO2PwoDfaMhwrj1cx2YpRLO0fP6P50dlBEkoNBhs0 RUBZPodRRePpr2nZfGf+JluN73M4UBytmvZLk9syCSGtpnk8CgJWrnFuepQy6EmyObSi aeZ6dgEiTmJxxFUxpBYWfS4M+biFD/resgs31b9Gp7G2E7SUMja3MUpTnpp+VHuEK7Qv Ev3g== X-Gm-Message-State: AOAM531b5cY1JGp9R1WfppVR2WhfXHLqeflrYTLrbCeePyEk91wT04u5 nds+kbTPyXJrBhVcwBgbwBJSXMf1M+Mwr9YX+Dk= X-Google-Smtp-Source: ABdhPJyuK/3lrzePc7IDt0IneMMRF6rqmS5uQTFQRiw4Fb+LgCTIkqZLuWluD30IEfgZv52/CciFYQt1h9Oo98eEV30= X-Received: by 2002:a7b:c310:: with SMTP id k16mr16906422wmj.169.1642190068885; Fri, 14 Jan 2022 11:54:28 -0800 (PST) MIME-Version: 1.0 References: <832b1b3957a0243ca37378a774effe537642eed3.camel@gmail.com> <40fd9a2f078cd6e87fedbc5f1e77baf8445a7356.camel@gmail.com> <02f4b13397f1d77db433ffc0c9401a6e66fb706d.camel@gmail.com> In-Reply-To: From: Jonathan Wakely Date: Fri, 14 Jan 2022 19:54:14 +0000 Message-ID: Subject: Re: reordering of trapping operations and volatile To: Michael Matz Cc: Martin Uecker , "gcc@gcc.gnu.org" X-Spam-Status: No, score=-1.5 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, HTML_MESSAGE, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org Content-Type: text/plain; charset="UTF-8" X-Content-Filtered-By: Mailman/MimeDel 2.1.29 X-BeenThere: gcc@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 14 Jan 2022 19:54:32 -0000 On Fri, 14 Jan 2022, 14:17 Michael Matz via Gcc, wrote: > Hello, > > On Thu, 13 Jan 2022, Martin Uecker wrote: > > > > > > Handling all volatile accesses in the very same way would be > > > > > possible but quite some work I don't see much value in. > > > > > > > > I see some value. > > > > > > > > But an alternative could be to remove volatile > > > > from the observable behavior in the standard > > > > or make it implementation-defined whether it > > > > is observable or not. > > > > > > But you are actually arguing for making UB be observable > > > > No, I am arguing for UB not to have the power > > to go back in time and change previous defined > > observable behavior. > > But right now that's equivalent to making it observable, > because we don't have any other terms than observable or > undefined. As aluded to later you would have to > introduce a new concept, something pseudo-observable, > which you then started doing. So, see below. > > > > That's > > > much different from making volatile not be > > > observable anymore (which obviously would > > > be a bad idea), and is also much harder to > > > > I tend to agree that volatile should be > > considered observable. But volatile is > > a bit implementation-defined anyway, so this > > would be a compromise so that implementations > > do not have to make all the implied changes > > if we revise the meaning of UB. > > Using volatile accesses for memory mapped IO is a much stronger use-case > than your wish of using volatile accesses to block moving of UB as a > debugging aid, and the former absolutely needs some guarantees, so I don't > think it would be a compromise at all. Mkaing volatile not be observable > would break the C language. > > > > Well, what you _actually_ want is an implied > > > dependency between some UB and volatile accesses > > > (and _only_ those, not e.g. with other UB), and the > > > difficulty now is to define "some" and to create > > > the dependency without making that specific UB > > > to be properly observable. > > > > Yes, this is what I actually want. > > > > > I think to define this > > > all rigorously seems futile (you need a new > > > category between observable and UB), so it comes > > > down to compiler QoI on a case by case basis. > > > > We would simply change UB to mean "arbitrary > > behavior at the point of time the erraneous > > construct is encountered at run-time" and > > not "the complete program is invalid all > > together". I see no problem in specifying this > > (even in a formally precise way) > > First you need to define "point in time", a concept which doesn't exist > yet in C. The obvious choice is of course observable behaviour in the > execution environment and its specified ordering from the abstract > machine, as clarified via sequence points. With that your "at the point > in time" becomes something like "after all side effects of previous > sequence point, but strictly before all side effects of next sequence > point". > > But doing that would have very far reaching consequences, as already > stated in this thread. The above would basically make undefined behaviour > be reliably countable, and all implementations would need to produce the > same counts of UB. That in turn disables many code movement and > commonization transformations, e.g. this: > > int a = ..., b = ...; > int x = a + b; > int y = a + b; > > can't be transformed into "y = x = a + b" anymore, because the addition > _might_ overflow, and if it does you have two UBs originally but would > have one UB after. I know that you don't want to inhibit this or similar > transformations, but that would be the result of making UB countable, > which is the result of forcing UB to happen at specific points in time. > So, I continue to see problems in precisely specifying what you want, _but > not more_. > > I think all models in which you order the happening of UB with respect to > existing side effects (per abstract machine, so it includes modification > of objects!) have this same problem, it always becomes a side effect > itself (one where you don't specify what actually happens, but a side > effect nontheless) and hence becomes observable. > The C++ committee is currently considering this paper: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1494r2.html I think this explicit barrier-like solution is better than trying to use volatile accesses to achieve something similar. > > >