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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 D0C89C43441 for ; Mon, 26 Nov 2018 01:23:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6F2F620865 for ; Mon, 26 Nov 2018 01:23:37 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6F2F620865 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=andestech.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726144AbeKZMQE (ORCPT ); Mon, 26 Nov 2018 07:16:04 -0500 Received: from 59-120-53-16.HINET-IP.hinet.net ([59.120.53.16]:11840 "EHLO ATCSQR.andestech.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726079AbeKZMQE (ORCPT ); Mon, 26 Nov 2018 07:16:04 -0500 Received: from mail.andestech.com (atcpcs16.andestech.com [10.0.1.222]) by ATCSQR.andestech.com with ESMTP id wAQ1M9h6037983; Mon, 26 Nov 2018 09:22:09 +0800 (GMT-8) (envelope-from vincentc@andestech.com) Received: from andestech.com (10.0.15.65) by ATCPCS16.andestech.com (10.0.1.222) with Microsoft SMTP Server id 14.3.123.3; Mon, 26 Nov 2018 09:23:13 +0800 Date: Mon, 26 Nov 2018 09:23:10 +0800 From: Vincent Chen To: David Laight CC: "green.hu@gmail.com" , "arnd@arndb.de" , "linux-kernel@vger.kernel.org" , "Nick Chun-Ming Hu(?????????)" , Subject: Re: [PATCH v4 2/5] nds32: Support FP emulation Message-ID: <20181126012310.GA26621@andestech.com> References: <1542856478-795-1-git-send-email-vincentc@andestech.com> <1542856478-795-3-git-send-email-vincentc@andestech.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) X-Originating-IP: [10.0.15.65] X-DNSRBL: X-MAIL: ATCSQR.andestech.com wAQ1M9h6037983 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 23, 2018 at 06:53:37PM +0800, David Laight wrote: > From: Vincent Chen > > Sent: 22 November 2018 03:15 > > > > The Andes FPU coprocessor does not support denormalized number handling. > > According to the specification, FPU generates a denorm input exception > > that requires the kernel to deal with this instrution operation when it > > encounters denormalized operands. Hence an nds32 FPU ISA emulator in the > > kernel is required to meet requirement. > > What does the FPU generate for results near zero? 1. The calculation result is a denormalized number Depending on the state of underflow trap, the FPU will raise an underflow exception or flash the result to zero. 2. One of the operands is a denormalized number Depending on the state of the flash-to-zero mode, the FPU will raise a denormalized input exception, which is a specific exception of the nds FPU, or directly treats the operand as 0. > If it doesn't generate denormalised results (but does detect them) > then I assume it never generates FP values with exponent zero. > (So the gap around zero is even larger than it would be if exponent > zero values had a hidden bit.) > So is it really worth doing anything other than treating > denormalised values as zero? > > David > Sorry, I can't fully understand what you mean, but we really have encountered the above two cases in some testsuites such as glibc testsuite. Hence we implement a FPU emulator in this commit which will recalculate the instructions again by software to enhance the precision. > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >