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 [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 707DE3896C24 for ; Thu, 10 Jun 2021 10:34:40 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 707DE3896C24 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-9-Lpr9LzxZPUWgiiK1Fr8qEA-1; Thu, 10 Jun 2021 06:34:39 -0400 X-MC-Unique: Lpr9LzxZPUWgiiK1Fr8qEA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1FD2B801B1A for ; Thu, 10 Jun 2021 10:34:38 +0000 (UTC) Received: from tucnak.zalov.cz (ovpn-114-102.ams2.redhat.com [10.36.114.102]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BBCD85C1C5; Thu, 10 Jun 2021 10:34:37 +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 15AAYZdY2291955 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Thu, 10 Jun 2021 12:34:35 +0200 Received: (from jakub@localhost) by tucnak.zalov.cz (8.16.1/8.16.1/Submit) id 15AAYYDS2291954; Thu, 10 Jun 2021 12:34:34 +0200 Date: Thu, 10 Jun 2021 12:34:34 +0200 From: Jakub Jelinek To: Jonathan Wakely Cc: Jason Merrill , gcc Patches Subject: Re: [PATCH] c++: Add C++23 consteval if support - P1938R3 [PR100974] Message-ID: <20210610103434.GE7746@tucnak> Reply-To: Jakub Jelinek References: <20210610083416.GC7746@tucnak> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 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=-5.8 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_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-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 10 Jun 2021 10:34:41 -0000 On Thu, Jun 10, 2021 at 11:24:43AM +0100, Jonathan Wakely wrote: > > And I'm not changing the libstdc++ side, where perhaps we could change > > std::is_constant_evaluated definition for > > #ifdef __cpp_if_consteval > > case to if consteval { return true; } else { return false; } > > but we need to keep it defined to __builtin_is_constant_evaluated () > > for C++20 or older. > > Is there any advantage to changing that (cheaper for GCC to evaluate?) I guess compile-time lost in the noise. > or should we just continue to use the __builtin unconditionally? > > I suppose it's theoretically possible that there could be a non-GCC > compiler where defined(__cpp_if_consteval) is true but > __has_builtin(__builtin_is_constant_evaluated) is false. Up to you. The wording says Equivalent to if consteval { return true; } else { return false; } and return __builtin_is_constant_evaluated (); is equivalent to that. Perhaps some people could appreciate to see it literally there, but we can't use it for C++20 (due to the -Wpedantic warnings or -pedantic-errors errors) and for C++17 and earlier (consteval is not a keyword). Jakub