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 [216.145.221.124]) by sourceware.org (Postfix) with ESMTPS id C24F23858CDA for ; Thu, 14 Sep 2023 16:48:38 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org C24F23858CDA 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=1694710118; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=s2dQae2YTrIanTGKY6/FaOqm6RtHlN6eBe81wHPiHJU=; b=LjY5A2SchITdRGBsXXbkmoCzzrxBuvbqunPuHVc51xb0K6XFslzTiKu8XPTnn9NtquOIad lsZ1Pwc1BGT/bWLHU8hTHlGyo+tT7PAdoejgzLmYYoHoZL56/x1mvpGWFzCWOvdZ20cBDo dCihp0RaKAmxlhDDkQWM3W8SDLnKw5I= 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-2-dtCRxyfEM0OcsZBGsDTakQ-1; Thu, 14 Sep 2023 12:48:36 -0400 X-MC-Unique: dtCRxyfEM0OcsZBGsDTakQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id AD542802E5A for ; Thu, 14 Sep 2023 16:48:36 +0000 (UTC) Received: from oldenburg3.str.redhat.com (unknown [10.39.194.238]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 560BB40C6EA8 for ; Thu, 14 Sep 2023 16:48:36 +0000 (UTC) From: Florian Weimer To: gcc@gcc.gnu.org Subject: Concerns regarding the -ffp-contract=fast default Date: Thu, 14 Sep 2023 18:48:35 +0200 Message-ID: <87r0n01z18.fsf@oldenburg3.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 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=-4.5 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: While rebuilding CentOS Stream with -march=3Dx86-64-v3, I rediscovered several packages had test suite failures because x86-64 suddenly gained FMA support. I say =E2=80=9Crediscovered=E2=80=9D because these issues wer= e already visible on other architectures with FMA. So far, our package/architecture maintainers had just disabled test suites or had built the package with -fp-contract=3Doff because the failures did not reproduce on x86-64. I'm not sure if this is the right course of action. GCC contraction behavior is rather inconsistent. It does not contract x + x - x without -ffast-math, for example, although I believe it would be permissible under the rules that enable FMA contraction. This whole thing looks suspiciously like a quick hack to get a performance improvement from FMA instructions (sorry). I know that GCC 14 has -fp-contract=3Dstandard. Would it make sense to switch the default to that? If it fixes those package test suites, it probably has an observable performance impact. 8-/ Thanks, Florian