From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [63.128.21.124]) by sourceware.org (Postfix) with ESMTP id 72F93386EC7D for ; Tue, 12 Jan 2021 13:36:18 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 72F93386EC7D Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-320-hS6_PeIvOQWW14F1t8fn3g-1; Tue, 12 Jan 2021 08:36:15 -0500 X-MC-Unique: hS6_PeIvOQWW14F1t8fn3g-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B492E100C66B; Tue, 12 Jan 2021 13:36:14 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-114-67.ams2.redhat.com [10.36.114.67]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DB5B210016FF; Tue, 12 Jan 2021 13:36:13 +0000 (UTC) From: Florian Weimer To: =?utf-8?Q?=E2=98=82Josh_Chia_=28=E8=AC=9D=E4=BB=BB=E4=B8=AD=29_via_Gcc?= =?utf-8?Q?-help?= Subject: Re: Failure to optimize? References: Date: Tue, 12 Jan 2021 14:36:12 +0100 In-Reply-To: (=?utf-8?Q?=22=E2=98=82Josh?= Chia =?utf-8?B?KOisneS7u+S4rSk=?= via Gcc-help"'s message of "Tue, 12 Jan 2021 21:32:20 +0800") Message-ID: <8735z6i8fn.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-4.9 required=5.0 tests=BAYES_00, BODY_8BITS, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gcc-help@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-help mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 12 Jan 2021 13:36:19 -0000 * =E2=98=82Josh Chia (=E8=AC=9D=E4=BB=BB=E4=B8=AD) via Gcc-help: > I have a code snippet that I'm wondering why GCC didn't optimize the way = I > think it should: > https://godbolt.org/z/1qKvax > > bar2() is a variant of bar1() that has been manually tweaked to avoid > branches. I haven't done any benchmarks but, I would expect the branchles= s > bar2() to perform better than bar1() but GCC does not automatically > optimize bar1() to be like bar2(); the generated code for bar1() and bar2= () > are different and the generated code for bar1() contains a branch. The optimization is probably valid for C99, but not for C11, where the memory model prevents the compiler from introducing spurious writes: Another thread may modify the variable concurrently, and if this happens only if foo returns NULL, the original bar1 function does not contain a data race, but the branchless version would. Thanks, Florian --=20 Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn, Commercial register: Amtsgericht Muenchen, HRB 153243, Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'N= eill