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 29C9F385782B for ; Wed, 1 Feb 2023 11:07:22 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 29C9F385782B Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1675249641; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=lDp9DIfrUucBhwineYkTT8oKm+KJ5yFOMf2X0cijtms=; b=PIvbOQazrTDTBYtXiiT5xGEhAnBd01SElT+fRMqFIInnSjHIFHJesJINRJLmCqekcIqZld SYw/Ex9PFE4nuztCqToZjaNIBIl8t90LAmg+/9/aCFyR4aDw3X1xbJvIOOZIvxUDNicVQN zyCPyECGxcVJF0xd0iFyny1JDOFDiXk= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-557-hr6bJIr_MKaeZJJii4pgDA-1; Wed, 01 Feb 2023 06:07:18 -0500 X-MC-Unique: hr6bJIr_MKaeZJJii4pgDA-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4EFB285A588; Wed, 1 Feb 2023 11:07:18 +0000 (UTC) Received: from tucnak.zalov.cz (unknown [10.39.192.223]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 06077C16020; Wed, 1 Feb 2023 11:07:17 +0000 (UTC) Received: from tucnak.zalov.cz (localhost [127.0.0.1]) by tucnak.zalov.cz (8.17.1/8.17.1) with ESMTPS id 311B7ERW3214806 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Wed, 1 Feb 2023 12:07:14 +0100 Received: (from jakub@localhost) by tucnak.zalov.cz (8.17.1/8.17.1/Submit) id 311B7Dc63214805; Wed, 1 Feb 2023 12:07:13 +0100 Date: Wed, 1 Feb 2023 12:07:13 +0100 From: Jakub Jelinek To: Florian Weimer Cc: libc-alpha@sourceware.org, gcc@gcc.gnu.org, fortran@gcc.gnu.org Subject: Re: libquadmath, glibc, and the Q format flag Message-ID: Reply-To: Jakub Jelinek References: <87lelhzmad.fsf@oldenburg.str.redhat.com> MIME-Version: 1.0 In-Reply-To: <87lelhzmad.fsf@oldenburg.str.redhat.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 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=-2.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Wed, Feb 01, 2023 at 11:56:42AM +0100, Florian Weimer via Gcc wrote: > I recently discovered that libquadmath registers custom printf callbacks > on load. As far as I can tell, this is done so that the Q format flag > can be used to print floating point numbers, using format strings such > as "%Qf". To enable Q flag processing, libquadmath has to register > replacements for all floating point format specifiers (aAeEfFgG). > > Unfortunately, this has the side effect that even with out the Q flag, > printing any floating point number enters a generic, slow path in glibc, > prior to the number formatting itself. The effect is quite pronounced. > For example this admittedly silly benchmarking script > > for i=1,5000000 do > print(i, i * math.pi) > end > > runs for 5.8 seconds without libquadmath.so.0 loaded on my x86-64 > system. With libquadmath.so.0 from GCC 12 loaded, it runs for 6.3 > seconds. > > This impacts most (all?) Fortran code on GNU/Linux because libgfortran > depends on libquadmath. Not anymore. If GCC is configured against new enough glibc (with _Float128 support) libgfortran.so.5 is no longer linked against -lquadmath, and -lquadmath is added only --as-needed to ld command line, so if all the Fortran object files that need real(kind=16) (or quad complex) are built by such configured GCC, -lquadmath is not linked in (just needs to be present). And, even for C, users can just use the standard glibc _Float128 support (if they don't mind strfromf128) if they target new enough glibc. So, libquadmath should be left over for non-glibc uses or uses with old glibc. > Would it make sense to transplant the implementation of the Q specifier > from libquadmath to glibc, and disable the registration code in > libquadmath if glibc is recent enough? At least for the targets that > today have float128 support in glibc? Thus I'm not sure it is worth it. Jakub