From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-x42e.google.com (mail-wr1-x42e.google.com [IPv6:2a00:1450:4864:20::42e]) by sourceware.org (Postfix) with ESMTPS id A41ED3858D37 for ; Mon, 10 Jan 2022 17:36:31 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org A41ED3858D37 Received: by mail-wr1-x42e.google.com with SMTP id r9so26431946wrg.0 for ; Mon, 10 Jan 2022 09:36:31 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:subject:from:to:cc:date:in-reply-to :references:user-agent:mime-version:content-transfer-encoding; bh=XFXZtMA9/bVuEJH5hjJAO67yGEtqxxJCwPl08BlPIQY=; b=2ld+vj0mUH6HmLwU9sHcwDhh7tFumSEjyQoCfnzJ8wrNXbiEpV7eQr8zWl4W2wrbsM NsHVw8KY7cmYBT+E3wKWUTfidlAd0zGzB2yIzwnsCh6/nniaNETpOm3OroIrrWS7El2j wSrB2P5SMu8MdBxbQkVAxBkhujDu6/55dLiQnzhR2DJiFcNqOrQixDPckcwCnD5VX9hK A/Ar9l4aBDoLcOH/HAIdErgF/B1wNNw68Pgo1+uABrZgWTa7IQZxNT2t2NjsseW3v+uv SlAlDHljChpxW7GnveZ5ayxnVoAF9cVLEuxVFXpczDT7xwZdx55B18N186CAqNakRJjF j5xw== X-Gm-Message-State: AOAM530tBbVmKrMiLmUP9xDY3DZAt6ekxrOPuELfqxj/Ng5m9/aZmP+u 5CKNUGa4p7f1lxLv9iggFtE= X-Google-Smtp-Source: ABdhPJw+CKd+0NuCZbFnpXuD04VrfeWYeyYuugtGC2YURyv3I6rtMYgwPXjRmi/SYEeeDDZ36VBlnA== X-Received: by 2002:adf:e3c9:: with SMTP id k9mr550089wrm.193.1641836190745; Mon, 10 Jan 2022 09:36:30 -0800 (PST) Received: from 2a02-8388-e205-e880-569c-680a-c69b-a1ad.cable.dynamic.v6.surfer.at (2a02-8388-e205-e880-569c-680a-c69b-a1ad.cable.dynamic.v6.surfer.at. [2a02:8388:e205:e880:569c:680a:c69b:a1ad]) by smtp.gmail.com with ESMTPSA id m7sm1606402wmi.13.2022.01.10.09.36.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Jan 2022 09:36:30 -0800 (PST) Message-ID: <40fd9a2f078cd6e87fedbc5f1e77baf8445a7356.camel@gmail.com> Subject: Re: reordering of trapping operations and volatile From: Martin Uecker To: Richard Biener Cc: Andrew Pinski , "gcc@gcc.gnu.org" Date: Mon, 10 Jan 2022 18:36:29 +0100 In-Reply-To: References: <832b1b3957a0243ca37378a774effe537642eed3.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.30.5-1.1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-1.8 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 17:36:33 -0000 Am Montag, den 10.01.2022, 10:04 +0100 schrieb Richard Biener: > 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. Yes. > > For volatile, it seems this would need some tweaks. > > Yes, likewise when re-ordering (observable) traps like > > r = a / b; > q = c / d; I think this could also be useful. But at the moment I am concerned about the effect previous defined behavior being affected. For this, reordering traps is OK. Also sinking traps across observable behavior is OK. Just hoisting it up across observable behavior would be a problem. > > 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. What other passes would need to be checked? And do you think there is any negative impact on an important optimization (considering this affects only volatile accesses)? > > > 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. Thanks, Martin > > > Note I thought -fno-delete-dead-exceptions would fix the sink > > > but it didn't. > > > > Martin > > > >