From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-x533.google.com (mail-ed1-x533.google.com [IPv6:2a00:1450:4864:20::533]) by sourceware.org (Postfix) with ESMTPS id E89E63858406 for ; Mon, 10 Jan 2022 09:04:59 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org E89E63858406 Received: by mail-ed1-x533.google.com with SMTP id u21so28143192edd.5 for ; Mon, 10 Jan 2022 01:04:59 -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=MlVoJi6n90BxF66lkPSZ6OwRbKzQjQ7R+GlmmEBNUls=; b=uDMoHgYDYD17SYtx27nlRa0/2Hm6HCttqGMoxtFgsdOaGzhjyj7Wt9rGusutylHzUz KG+D0jWEGGYrs2JTxfUi0VYhi1mS84SIdaMWIsMMrDpvCNhjCzLf0Z0w7GxwmK+aPopH uBU6Y+jEmlaQIe3jMP+kpx5my5wRLjPEegdzQwrZ4HG5BxAeoFZJtnseHXGspsBRhgl+ hiQsohI+VEgxb+JZmLRuDlVONiXDv3OpnrvoG+8b0GWwMDIWDTuXNsFur+x//QU1lZum TgBUM883k78W5cEFh9mpyquRpQUbuQIDheHZuIuroN4OC5lfQRG6sCvSENWEC9sgckCE QVGA== X-Gm-Message-State: AOAM533CtV0TAHw8CyG1Xn3LhlnbtsQNmt2l/ZqUfycC4j6h8512DymS AolKuje0eMQv7YPUEGyMimkc72TbVtJ1ZuZKXgY= X-Google-Smtp-Source: ABdhPJwVoWLP1A4Pg3eekiypWcx8/UEQtvZjlq+spQe8lr89I4fX7sa20FJZ7vWZpYRONWPVEqDtZBsxgtoKaQInBFk= X-Received: by 2002:a05:6402:27cf:: with SMTP id c15mr68519146ede.390.1641805498863; Mon, 10 Jan 2022 01:04:58 -0800 (PST) MIME-Version: 1.0 References: <832b1b3957a0243ca37378a774effe537642eed3.camel@gmail.com> In-Reply-To: From: Richard Biener Date: Mon, 10 Jan 2022 10:04:48 +0100 Message-ID: Subject: Re: reordering of trapping operations and volatile To: Martin Uecker Cc: Andrew Pinski , "gcc@gcc.gnu.org" Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.2 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, 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 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: Mon, 10 Jan 2022 09:05:01 -0000 On Sat, Jan 8, 2022 at 10:09 PM Martin Uecker via Gcc wrote: > > Am Samstag, den 08.01.2022, 10:35 -0800 schrieb Andrew Pinski: > > On Sat, Jan 8, 2022 at 12:33 AM Martin Uecker via Gcc wrote: > > > > > > Hi Richard, > > > > > > I have a question regarding reodering of volatile > > > accesses and trapping operations. My initial > > > assumption (and hope) was that compilers take > > > care to avoid creating traps that are incorrectly > > > ordered relative to observable behavior. > > > > > > I had trouble finding examples, and my cursory > > > glace at the code seemed to confirm that GCC > > > carefully avoids this. But then someone showed > > > me this example, where this can happen in GCC: > > > > > > > > > volatile int x; > > > > > > int foo(int a, int b, _Bool store_to_x) > > > { > > > if (!store_to_x) > > > return a / b; > > > x = b; > > > return a / b; > > > } > > > > > > > > > https://godbolt.org/z/vq3r8vjxr > > > > The question becomes what is a trapping instruction vs an undefined > > instruction? > > For floating point types, it is well defined what is a trapping > > instruction while for integer types it is not well defined. > > On some (many?) targets dividing by 0 is just undefined and does not > > trap (powerpc, aarch64, arm and many others; MIPS it depends on the > > options passed to GCC if the conditional trap should be inserted or > > not). > > > The other side is if there is undefined code on the path, should > > observable results happen first (stores to volatile/atomics, etc.)? > > I think for volatile stores and I/O, I think it would be > nice of we could guarantee that those happen before the UB > ruins the day. (I am not sure about atomics, those are > not directly obsevable) > > For I/O this is probably already the case (?). I/O usually happens through function calls where this is usually already guaranteed as GCC doesn't know whether the function will always return normally so the UB of a divide by zero might be properly guarded. > For volatile, it seems this would need some tweaks. Yes, likewise when re-ordering (observable) traps like r = a / b; q = c / d; > I am trying to figure out whether this is feasible. For PRE yes, you'd just need to include the observable stmts you care in the set of stmts that cause PRE to set BB_MAY_NOTRETURN. In general this is of course harder. > > GCC assumes by default that divide is trappable but stores not are not > > observable. This is where -fnon-call-exceptions come into play. > > Ok, thanks! I will look at this! > > > In the second case, GCC assumes reducing trappable instructions are > > fine. > > -fnon-call-exceptions would treat trapping instructions > as defined (and trapping) instead of UB? This is > then probably even stronger than the requirement above. No, I don't think it turns UB into defined behavior. Some frontends might expect that to some extent. So even with -fnon-call-exceptions we'd happily do the re-ordering unless the exception is catched in the same function. > > Note I thought -fno-delete-dead-exceptions would fix the sink > > but it didn't. > > > Martin > >