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=-7.9 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,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 E9130C433DB for ; Fri, 5 Feb 2021 14:08:26 +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 1D70564FCA for ; Fri, 5 Feb 2021 14:08:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1D70564FCA 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]:40930 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l81mb-0007bx-0i for qemu-devel@archiver.kernel.org; Fri, 05 Feb 2021 09:08:25 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:34504) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l81lm-000762-34 for qemu-devel@nongnu.org; Fri, 05 Feb 2021 09:07:34 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:47179) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l81lk-0002rj-6p for qemu-devel@nongnu.org; Fri, 05 Feb 2021 09:07:33 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1612534050; 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=8osqUjNLad5Li8K/E9Elc/gsq4TtN4e1Pvyf+o0K8b4=; b=hUAN0FH2j4T2q9ssmy/+xo4SnoaQZCD01PUj6ntaGRPLynqvNEyJ1cghB1RfBa8lJfvEN/ lCO6SaZc9La4FFj1an9jbsBL+gg4511FQbRP9c9EEBe2z+f4OdC6vnxrPV1xc+lKrxPUfm Pj4fYE0lF4KzyaLZ4cv559mmYeFB1pI= 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-516-gKuy0bqpOT-5KQmBl1TXGA-1; Fri, 05 Feb 2021 09:07:28 -0500 X-MC-Unique: gKuy0bqpOT-5KQmBl1TXGA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8699C10866AE; Fri, 5 Feb 2021 14:06:55 +0000 (UTC) Received: from [10.3.112.253] (ovpn-112-253.phx2.redhat.com [10.3.112.253]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D7D0C63623; Fri, 5 Feb 2021 14:06:53 +0000 (UTC) To: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org References: <20210204190708.1306296-1-eblake@redhat.com> <20210204190708.1306296-2-eblake@redhat.com> <21fcd50e-b5c2-d35c-0555-7d26014370ee@virtuozzo.com> From: Eric Blake Organization: Red Hat, Inc. Subject: Re: [PATCH 1/3] utils: Improve qemu_strtosz() to have 64 bits of precision Message-ID: Date: Fri, 5 Feb 2021 08:06:53 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <21fcd50e-b5c2-d35c-0555-7d26014370ee@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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=63.128.21.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 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, NICE_REPLY_A=-0.33, 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: , Cc: Kevin Wolf , berrange@redhat.com, qemu-block@nongnu.org, tao3.xu@intel.com, armbru@redhat.com, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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? > >> >>>           retval = -ERANGE; >>>           goto out; >>>       } >>> -    *result = val * mul; >>> +    *result = val * mul + (uint64_t) (fraction * mul); >>>       retval = 0; >>> >>>   out: > > -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org