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 ESMTPS id 3865B3858D32 for ; Mon, 18 Sep 2023 16:42:00 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 3865B3858D32 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=1695055319; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0QzBMFwTkYLLNP/nNbJmA4RP3YeO6WZAdVdgFtq4EhI=; b=PMpJ6o7dLYyt5bssBJIgNfgPp1Tq9VIGi5oq5y6z9SQbItibUCI2RIG8gzL0c+iyyrnVt+ y3RyLGf1IhdcPlHygcMk3LsCArjEIx6IaXuJ8YdHd+G0kJBKlMzf60tEdhLdNK/1cT3+lz bvtdtHcuDQDU8bu3lMg2z5Yv0zKuuk8= 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-680-ey3mZrQuPtuX8HYAaGgyQA-1; Mon, 18 Sep 2023 12:41:58 -0400 X-MC-Unique: ey3mZrQuPtuX8HYAaGgyQA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (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 70BCE80D651; Mon, 18 Sep 2023 16:41:57 +0000 (UTC) Received: from oldenburg.str.redhat.com (unknown [10.2.16.9]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 00A0E20268CC; Mon, 18 Sep 2023 16:41:56 +0000 (UTC) From: Florian Weimer To: Alexander Monakov Cc: gcc@gcc.gnu.org Subject: Re: Concerns regarding the -ffp-contract=fast default References: <87r0n01z18.fsf@oldenburg3.str.redhat.com> <21e46cef-1fbf-df87-608d-52b7f894dea7@ispras.ru> <877condc69.fsf@oldenburg.str.redhat.com> <87y1h3btqm.fsf@oldenburg.str.redhat.com> Date: Mon, 18 Sep 2023 18:41:55 +0200 In-Reply-To: (Alexander Monakov's message of "Mon, 18 Sep 2023 14:55:09 +0300 (MSK)") Message-ID: <87o7hzbfho.fsf@oldenburg.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 10.11.54.4 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=-4.6 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_H3,RCVD_IN_MSPIKE_WL,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: * Alexander Monakov: >> > Contracting 'x + x - x' to fma(x, 2, -x) would be fine. >>=20 >> It still changes the result, doesn't it? > > I don't follow. I doesn't change the result for infinities (produces > a NaN). It changes the result when x is so large that 'x + x' is > not representable (exponent would overflow), but that's exactly what > contraction is about? Okay, you meant =E2=80=9Cchanging the result=E2=80=9D as in =E2=80=9Cchangi= ng the result in a permitted way=E2=80=9D. Sorry, was confused. So this is a bad example all around. Are there better ones (that don't involve FMA)? Thanks, Florian