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 [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id BE7D53858D39 for ; Fri, 26 Nov 2021 12:50:12 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org BE7D53858D39 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-441-JVHZtPfZMzuobdNIipJulg-1; Fri, 26 Nov 2021 07:50:11 -0500 X-MC-Unique: JVHZtPfZMzuobdNIipJulg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0C1D78024F5; Fri, 26 Nov 2021 12:50:10 +0000 (UTC) Received: from tucnak.zalov.cz (unknown [10.39.192.23]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 983F47DE4D; Fri, 26 Nov 2021 12:49:56 +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 1AQCns313959451 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Fri, 26 Nov 2021 13:49:54 +0100 Received: (from jakub@localhost) by tucnak.zalov.cz (8.16.1/8.16.1/Submit) id 1AQCnrcJ3959450; Fri, 26 Nov 2021 13:49:53 +0100 Date: Fri, 26 Nov 2021 13:49:53 +0100 From: Jakub Jelinek To: Jonathan Wakely Cc: libstdc++ , gcc Patches Subject: Re: [PATCH] libstdc++: Define std::__is_constant_evaluated() for internal use Message-ID: <20211126124953.GH2646553@tucnak> Reply-To: Jakub Jelinek References: <20211126122925.1626170-1-jwakely@redhat.com> <20211126123950.GG2646553@tucnak> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 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=-5.7 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_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, 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: 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: Fri, 26 Nov 2021 12:50:13 -0000 On Fri, Nov 26, 2021 at 12:43:44PM +0000, Jonathan Wakely wrote: > > On Fri, Nov 26, 2021 at 12:29:25PM +0000, Jonathan Wakely via Gcc-patches wrote: > > > + // Internal version of std::is_constant_evaluated() for C++11. > > > + // This can be used without checking if the compiler supports the built-in. > > > + constexpr inline bool > > > + __is_constant_evaluated() noexcept > > > + { > > > > When you have such a nice one spot, shouldn't it: > > #if __cpp_if_consteval >= 202106L > > if consteval > > { > > return true; > > } > > else > > { > > return false; > > } > > #elif __has_builtin(__builtin_is_constant_evaluated) > > ... > > > > Theoretically not all compilers need to support the builtin and in C++23 > > mode if consteval should be slightly more efficient. > > Yes, good idea. We actually still have two spots, because we still > have std::is_constant_evaluated as well, which is only defined if it > actually works. But we can use the same implementation in there (or > make it call std::__is_constant_evaluated()). I'll prepare a new patch > soon. While calling the latter might be more maintainable, I think having it implemented twice would be better, so that std::is_constant_evaluated() doesn't need to hop through another inline call. I'd expect people to use std::is_constant_evaluated() in their code quite a lot, it will take time until they start to use if consteval instead. Jakub