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=-5.0 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 84137C433E0 for ; Fri, 5 Feb 2021 14:11:56 +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 0E38865041 for ; Fri, 5 Feb 2021 14:11:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0E38865041 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]:46062 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l81pw-0001U3-Qh for qemu-devel@archiver.kernel.org; Fri, 05 Feb 2021 09:11:52 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:35148) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l81oV-0000tr-Mh for qemu-devel@nongnu.org; Fri, 05 Feb 2021 09:10:23 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:38319) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l81oQ-00046L-Nl for qemu-devel@nongnu.org; Fri, 05 Feb 2021 09:10:22 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1612534217; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=XvwSqEASe5ggxqoTcjliHdlSUMRc5j7e57HgpvcbXik=; b=CTs/k1pWHhsbnjoM3WnfQi8E9ingBvROF2AzKuRayfB1qiEMhFrPoz3Ib1CtwsUn6MiYNj eA38CrYZ8LHTszEvz7Kbl4mEu5tU0Hq0WagTU7LHStUpMKpPAgrSEIpfxDMuzZtfSqa89d /6bwUUQSvi87KdnRHjg2tZlXZfHovds= 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-354-I7bMTcisMqm0D84hVjrEZA-1; Fri, 05 Feb 2021 09:10:14 -0500 X-MC-Unique: I7bMTcisMqm0D84hVjrEZA-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 37FFDC7445; Fri, 5 Feb 2021 14:10:13 +0000 (UTC) Received: from redhat.com (ovpn-114-212.ams2.redhat.com [10.36.114.212]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 32A5D2C6F2; Fri, 5 Feb 2021 14:10:11 +0000 (UTC) Date: Fri, 5 Feb 2021 14:10:08 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Subject: Re: [PATCH 1/3] utils: Improve qemu_strtosz() to have 64 bits of precision Message-ID: <20210205141008.GO908621@redhat.com> References: <20210204190708.1306296-1-eblake@redhat.com> <20210204190708.1306296-2-eblake@redhat.com> <21fcd50e-b5c2-d35c-0555-7d26014370ee@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.352, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, tao3.xu@intel.com, qemu-devel@nongnu.org, armbru@redhat.com, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Feb 05, 2021 at 08:06:53AM -0600, Eric Blake wrote: > On 2/5/21 4:06 AM, Vladimir Sementsov-Ogievskiy wrote: > > >>> -    /* > >>> -     * Values near UINT64_MAX overflow to 2**64 when converting to > >>> double > >>> -     * precision.  Compare against the maximum representable double > >>> precision > >>> -     * value below 2**64, computed as "the next value after 2**64 > >>> (0x1p64) in > >>> -     * the direction of 0". > >>> -     */ > >>> -    if ((val * mul > nextafter(0x1p64, 0)) || val < 0) { > >>> +    if (val > UINT64_MAX / mul) { > >> > >> Hmm, do we care about: > >> 15.9999999999999999999999999999E > >> where the fractional portion becomes large enough to actually bump our > >> sum below to 16E which indeed overflows?  Then again, we rejected a > >> fraction of 1.0 above, and 0.9999999999999999999999999999 parses to 1.0 > >> due to rounding. > >> Maybe it's just worth a good comment why the overflow check here works > >> without consulting fraction. > > > > worth a good comment, because I don't follow :) > > > > If mul is big enough and fraction=0.5, why val*mul + fraction*mul will > > not overflow? > > When mul is a power of 2, we know that fraction*mul does not change the > number of significant bits, but merely moves the exponent, so starting > with fraction < 1.0, we know fraction*mul < mul. But when @unit is > 1000, there is indeed a rare possibility that the multiplication will > cause an inexact answer that will trigger rounding, so we could end up > with fraction*mul == mul. So I'm not yet 100% confident that there is > no possible combination where we can't cause an overflow to result in > val*mul + (uint64_t)(fraction*mul) resulting in 0 instead of UINT64_MAX, > and I think I will have to tighten this code up for v2. > > > > > > Also, if we find '.' in the number, why not just reparse the whole > > number with qemu_strtod_finite? And don't care about 1.0? > > Reparsing the whole number loses precision. Since we already have a > 64-bit precise integer, why throw it away? Yep, it isn't acceptable to throw away precision of the non-fractional part of the input IMHO. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|