From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A0446C433E0 for ; Mon, 15 Mar 2021 11:35:04 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 1BE1764DE7 for ; Mon, 15 Mar 2021 11:35:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1BE1764DE7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:53008 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lLlV1-00049g-3I for qemu-devel@archiver.kernel.org; Mon, 15 Mar 2021 07:35:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:38962) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lLlTV-00034S-Ia for qemu-devel@nongnu.org; Mon, 15 Mar 2021 07:33:30 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:25019) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lLlTT-0006ot-SA for qemu-devel@nongnu.org; Mon, 15 Mar 2021 07:33:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615808006; 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=ZVQ+GjfmhiOppx0yxTT/ChqIPXVAecSeN000J7hW7/w=; b=UmzjR9yYGVjos8p4rn8ZumobcRLGi4bCKmNMk2kMV8kCFH/0LA5ats9PMQtlgj9DBMCAJu y0TKKr1ATpD7xWrGgxCd6DfVlSYKeXshOda0+1dEteIt7FtJhvxhLuLzAxRPheQWKh7bmr Q63chZ6h1RPPcmaM7QcXaqzoHFmJuWo= 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-371-9Zfw0yf0O3uPmpnfqcEpCg-1; Mon, 15 Mar 2021 07:33:22 -0400 X-MC-Unique: 9Zfw0yf0O3uPmpnfqcEpCg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C2F6483DB64; Mon, 15 Mar 2021 11:33:21 +0000 (UTC) Received: from [10.3.113.66] (ovpn-113-66.phx2.redhat.com [10.3.113.66]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 716AB100164C; Mon, 15 Mar 2021 11:33:21 +0000 (UTC) Subject: Re: [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz. To: Richard Henderson , qemu-devel@nongnu.org References: <20210311200702.1302855-1-eblake@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Mon, 15 Mar 2021 06:33:20 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , berrange@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/13/21 3:48 PM, Richard Henderson wrote: > On 3/11/21 2:07 PM, Eric Blake wrote: >> +        /* >> +         * Add in a fudge-factor (2^53 when double is IEEE format) for >> +         * all scales less than P (2^50), so that things like >> +         * 12.345M with unit 1000 produce 12345000 instead of >> +         * 12344999. >> +         */ >> +        if (mul > 1e49) { > > The comment says less than, the code says greater than. Shoot. A demonstration that I did not have an environment that actually reproduced the bug (and my request for help in figuring out how to kick off a gitlab CI run that would catch it). > > >> An alternative patch might be writing (uint64_t)(fraction * mul + 0.5) >> (that is, introduce the fudge factor after the multiplication instead >> of before).  Preferences? > > I think I would prefer this, or for further rounding error reduction, > fma(fraction, mul, 0.5). Indeed, fma() sounds a bit nicer at minimizing the chance for double rounding errors. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org