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 [170.10.133.124]) by sourceware.org (Postfix) with ESMTP id 8B6C13857C47 for ; Wed, 29 Sep 2021 18:29:54 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 8B6C13857C47 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-152-NYOXjrO2OK-q-5G44L42LA-1; Wed, 29 Sep 2021 14:29:52 -0400 X-MC-Unique: NYOXjrO2OK-q-5G44L42LA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C89AA18D6A25; Wed, 29 Sep 2021 18:29:51 +0000 (UTC) Received: from tucnak.zalov.cz (unknown [10.39.193.109]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5B1D960C81; Wed, 29 Sep 2021 18:29:51 +0000 (UTC) Received: from tucnak.zalov.cz (localhost [127.0.0.1]) by tucnak.zalov.cz (8.16.1/8.16.1) with ESMTPS id 18TITmw2387166 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Wed, 29 Sep 2021 20:29:49 +0200 Received: (from jakub@localhost) by tucnak.zalov.cz (8.16.1/8.16.1/Submit) id 18TITlmV387165; Wed, 29 Sep 2021 20:29:47 +0200 Date: Wed, 29 Sep 2021 20:29:47 +0200 From: Jakub Jelinek To: Thomas Rodgers Cc: Jonathan Wakely , gcc Patches , libstdc++ , Thomas Rodgers Subject: Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange Message-ID: <20210929182947.GQ304296@tucnak> Reply-To: Jakub Jelinek References: <20210923180837.633173-1-rodgert@appliantology.com> <20210927141031.651313-1-rodgert@appliantology.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-3.7 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=unavailable autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: libstdc++@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++ mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 29 Sep 2021 18:29:55 -0000 On Wed, Sep 29, 2021 at 11:22:29AM -0700, Thomas Rodgers via Gcc-patches wrote: > > The MSVC implementation uses !__has_unique_object_representations(_Tp) > > && !is_floating_point<_Tp>::value here, which is better than mine > > above (FP types don't have unique object reps, but also don't have > > padding bits). Except the 80-bit long double, that one has padding bits (similarly _Complex long double). As I said earlier, if you want a builtin using the __builtin_clear_padding infrastructure that will return bool if __builtin_clear_padding expands to something or not, it can be easily added. GCC already uses something like that internally. Jakub