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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 ACE0AC677FF for ; Thu, 11 Oct 2018 16:17:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6F8262085B for ; Thu, 11 Oct 2018 16:17:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6F8262085B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=huawei.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 S1729844AbeJKXox (ORCPT ); Thu, 11 Oct 2018 19:44:53 -0400 Received: from szxga06-in.huawei.com ([45.249.212.32]:59768 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726700AbeJKXox (ORCPT ); Thu, 11 Oct 2018 19:44:53 -0400 Received: from DGGEMS408-HUB.china.huawei.com (unknown [172.30.72.58]) by Forcepoint Email with ESMTP id B381CEB5075A2; Fri, 12 Oct 2018 00:16:55 +0800 (CST) Received: from [127.0.0.1] (10.202.226.41) by DGGEMS408-HUB.china.huawei.com (10.3.19.208) with Microsoft SMTP Server id 14.3.399.0; Fri, 12 Oct 2018 00:17:17 +0800 Subject: Re: Question on FIELD_PREP() for static array To: Johannes Berg , Joe Perches , Andrew Morton , Andy Shevchenko , Kalle Valo , , , "Arnd Bergmann" , , "linux-kernel@vger.kernel.org" References: <40fd3d963820bf96547fa9b5e8c171c6a339674e.camel@perches.com> <1539195207.3687.178.camel@sipsolutions.net> <830ff9fb-07b1-86b6-4f57-af549a37a8c0@huawei.com> <1539271422.3687.217.camel@sipsolutions.net> CC: , From: John Garry Message-ID: Date: Thu, 11 Oct 2018 17:16:41 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 MIME-Version: 1.0 In-Reply-To: <1539271422.3687.217.camel@sipsolutions.net> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.202.226.41] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/10/2018 16:23, Johannes Berg wrote: Hi > On Thu, 2018-10-11 at 15:24 +0100, John Garry wrote: >> >>> +#define BUILD_BUG_ON_RET_ZERO(cond) (sizeof(char[1 - 2*!!(cond)]) - 1) >>> +#define BUILD_BUG_ON_NOT_POW2_RET_ZERO(n) BUILD_BUG_ON_RET_ZERO(((n) & ((n) - 1)) != 0) > >> Seems reasonable. However I did try this and was getting compiler >> warnings about VLA, from a non-constant being fed into >> BUILD_BUG_ON_RET_ZERO(), related to sizeof char[]: >> drivers/iio/adc/meson_saradc.c:375:2: warning: ISO C90 forbids variable >> length array [-Wvla] >> regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0), > > Hmm, what's the code there? Nothing special, it was just a sample. Here'e the code: regval = FIELD_PREP(MESON_SAR_ADC_CHAN_LIST_ENTRY_MASK(0), chan->address); So val is a variable, and I find if remove both of the BUILD_BUG_ON_RET_ZERO()' which use __bf_shf() then it goes away. > > I don't see why the compiler should think it's a variable length? > >> Surely __NLA_ENSURE is getting a similar issue as it uses a similar >> principle, no? I see that this is in -next now, but could not this macro >> or derivatives being referenced. > > Yeah, I have a patch now to reference it, but I don't see anything from > -Wvla with gcc 8.1? I'm using a 7.3.1-based toolchain > > See > https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942 > and its parent. > > Do you see -Wvla warnings there? Any idea how I could reproduce them? I'll try it, thanks John > > johannes > > . >