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=-7.3 required=3.0 tests=BAYES_00,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 83528C433DB for ; Sat, 13 Mar 2021 21:49:38 +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 D309864E62 for ; Sat, 13 Mar 2021 21:49:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D309864E62 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linaro.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:48638 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lLC8e-0006LC-IU for qemu-devel@archiver.kernel.org; Sat, 13 Mar 2021 16:49:36 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:44544) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lLC7t-0005ss-Rq for qemu-devel@nongnu.org; Sat, 13 Mar 2021 16:48:49 -0500 Received: from mail-qt1-x82b.google.com ([2607:f8b0:4864:20::82b]:43702) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1lLC7s-0003uw-4n for qemu-devel@nongnu.org; Sat, 13 Mar 2021 16:48:49 -0500 Received: by mail-qt1-x82b.google.com with SMTP id s2so6613831qtx.10 for ; Sat, 13 Mar 2021 13:48:47 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=uuJlbcDm0GICpe7tIhk3wVkNNj1wLEZjhcQ1R68OqB4=; b=llT0PL8HhjOvF1jitDT+FTtd1HmpPdFTa3F6u8aqZKMiOLiqUeIzKw1087zFJlAPLc DXE80aYlOsDEoPhf5FGq9CUYLUwwyufIc0ccpx5uw3obxbLcPFlzdSguE3RROiMwxU49 DKklTIqZA47en/DHy+IcIkJdNb/yxn24/p5XuE3XvVUNqEQDdk1bcxpCtxwn+2hu2/1Z +Zp5/Jl8dNmHhnanrwtAgm1nvgYZARmA6P3pFCJFw+B04xW3uhFxV2xCEe2/GvELHU5d yqjvrE/9U4HoF0HDFpHYi1mzsTKI+89LN0EClZamh93vSPrYmqc/duAwcnbC5Uwj7rqB gtYw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=uuJlbcDm0GICpe7tIhk3wVkNNj1wLEZjhcQ1R68OqB4=; b=IPNy++vKU9vrZ5cScVJaejzj4KnpYUFjqUdIOk3oCHSaInP+HioO25TBVl9mzGd/AH DGWCseSDqFsY3re6kLtWJ5nNWxnrbOPvy3gnP7tF0iuF6oEsoz5zXJvMJx+yJSdKfneN QrTotVk3Z6X0ZWgNoJ8rx0/OZ/s84cc02EtscJGw2p/PST49skoLlY+ugLnR5Lut5tER VyeAmLQd58/uGEwFZxHvo42kLGqFtwkMKwTaHoJnuZEkYvyblM6mqvZs4ZAaUMt8ldAZ J/TGtL8nVn7g/3NGLupMUjb3l+G0hvaXN2JcFfCidNX/JEQK4DCD3sAw309/4F0JQLT2 DgDQ== X-Gm-Message-State: AOAM532892kedx0ndnxrM0Xwi79Srh0zjYHPnEdsQtlqXWQd1QOaOvrt Mf2CqB2ltENm4OZwaoqNBZ+srQ== X-Google-Smtp-Source: ABdhPJx2t36spSfxbPrRZ6QCj+I0t1wOmw+UqdyluJc0xHordp/LEpjg1CvssV9I2nr4DBMbuKF9vg== X-Received: by 2002:ac8:7b8b:: with SMTP id p11mr8120768qtu.57.1615672127031; Sat, 13 Mar 2021 13:48:47 -0800 (PST) Received: from [10.10.121.52] (fixed-187-189-51-144.totalplay.net. [187.189.51.144]) by smtp.gmail.com with ESMTPSA id m13sm7330409qkm.103.2021.03.13.13.48.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 13 Mar 2021 13:48:46 -0800 (PST) Subject: Re: [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz. To: Eric Blake , qemu-devel@nongnu.org References: <20210311200702.1302855-1-eblake@redhat.com> From: Richard Henderson Message-ID: Date: Sat, 13 Mar 2021 15:48:43 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210311200702.1302855-1-eblake@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::82b; envelope-from=richard.henderson@linaro.org; helo=mail-qt1-x82b.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, 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_NONE=-0.0001, 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 3/11/21 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. > 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). r~