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=-17.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 437A4C433E0 for ; Mon, 15 Mar 2021 11:41:04 +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 DCC3364E81 for ; Mon, 15 Mar 2021 11:41:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org DCC3364E81 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]:36144 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lLlao-0000ru-Sh for qemu-devel@archiver.kernel.org; Mon, 15 Mar 2021 07:41:02 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40212) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lLlYp-0007ej-4y for qemu-devel@nongnu.org; Mon, 15 Mar 2021 07:38:59 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:27180) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lLlYn-0001hI-Hi for qemu-devel@nongnu.org; Mon, 15 Mar 2021 07:38:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615808336; 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=xi87ZN5Yk4mvo2Ho+rUgNUOfwgzAGqT8Cx9cahqqLx4=; b=EqxW0EEGx4VLH+y2+4SDEJvsTnr4N0ZAJ/ySAYraBX1SKxPPEwOldgsFcaHVCk2gjCSWMA wApPORgR3pd8D5+jWH3Ooca8D64/gclxUzKHZGyXR1jcu8RgdfplOm4yZ3sMshLNrsTyLs B+elsXLWPYt4Vyb8Knvz3DcBQz/O1OA= 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-540-iQmsjI9JPGmnDM6GE1fU_A-1; Mon, 15 Mar 2021 07:38:54 -0400 X-MC-Unique: iQmsjI9JPGmnDM6GE1fU_A-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id D4EDD801817; Mon, 15 Mar 2021 11:38:53 +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 806C719719; Mon, 15 Mar 2021 11:38:53 +0000 (UTC) Subject: Re: [PATCH] utils: Use fma in qemu_strtosz To: Richard Henderson , qemu-devel@nongnu.org References: <20210314234821.1954428-1-richard.henderson@linaro.org> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <5510483b-b38c-ad82-c138-6c757e4dac3a@redhat.com> Date: Mon, 15 Mar 2021 06:38:52 -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: <20210314234821.1954428-1-richard.henderson@linaro.org> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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: 7bit 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: -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_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: peter.maydell@linaro.org, berrange@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/14/21 6:48 PM, Richard Henderson wrote: > Use fma to simulatneously scale and round up fraction. > > The libm function will always return a properly rounded double precision > value, which will eliminate any extra precision the x87 co-processor may > give us, which will keep the output predictable vs other hosts. > > Adding DBL_EPSILON while scaling should help with fractions like > 12.345, where the closest representable number is actually 12.3449*. > > Signed-off-by: Richard Henderson > --- > util/cutils.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/util/cutils.c b/util/cutils.c > index d89a40a8c3..f7f8e48a68 100644 > --- a/util/cutils.c > +++ b/util/cutils.c > @@ -342,7 +342,7 @@ static int do_strtosz(const char *nptr, const char **end, > retval = -ERANGE; > goto out; > } > - *result = val * mul + (uint64_t) (fraction * mul); > + *result = val * mul + (uint64_t)fma(fraction, mul, DBL_EPSILON); Don't you need to include to get DBL_EPSILON? More importantly, this patch seems wrong. fma(a, b, c) performs (a * b) + c without intermediate rounding errors, but given our values for a and b, where mul > 1 in any situation we care about, adding 2^-53 is so much smaller than a*b that it not going to round the result up to the next integer. Don't you want to do fma(fraction, mul, 0.5) instead? -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org