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_HELO_NONE,SPF_PASS 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 97E67C35DE1 for ; Tue, 25 Feb 2020 11:52:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7437E21556 for ; Tue, 25 Feb 2020 11:52:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730132AbgBYLwV (ORCPT ); Tue, 25 Feb 2020 06:52:21 -0500 Received: from mga05.intel.com ([192.55.52.43]:11152 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729976AbgBYLwU (ORCPT ); Tue, 25 Feb 2020 06:52:20 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga105.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 25 Feb 2020 03:52:20 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,484,1574150400"; d="scan'208";a="271280971" Received: from smile.fi.intel.com (HELO smile) ([10.237.68.40]) by fmsmga002.fm.intel.com with ESMTP; 25 Feb 2020 03:52:18 -0800 Received: from andy by smile with local (Exim 4.93) (envelope-from ) id 1j6Yl9-004eDR-MA; Tue, 25 Feb 2020 13:52:19 +0200 Date: Tue, 25 Feb 2020 13:52:19 +0200 From: Andy Shevchenko To: Jesse Brandeburg Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, linux-kernel@vger.kernel.org, linux@rasmusvillemoes.dk, dan.j.williams@intel.com, peterz@infradead.org Subject: Re: [PATCH v5 1/2] x86: fix bitops.h warning with a moved cast Message-ID: <20200225115219.GI10400@smile.fi.intel.com> References: <20200224225020.2212544-1-jesse.brandeburg@intel.com> <20200225103050.GD10400@smile.fi.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200225103050.GD10400@smile.fi.intel.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 25, 2020 at 12:30:50PM +0200, Andy Shevchenko wrote: > On Mon, Feb 24, 2020 at 02:50:19PM -0800, Jesse Brandeburg wrote: > > Fix many sparse warnings when building with C=1. > > > > When the kernel is compiled with C=1, there are lots of messages like: > > arch/x86/include/asm/bitops.h:77:37: warning: cast truncates bits from constant value (ffffff7f becomes 7f) > > > > CONST_MASK() is using a signed integer "1" to create the mask which > > is later cast to (u8) when used, in order to yield an 8-bit value > > for the assembly instructions to use. Simplify the expressions used to > > clearly indicate they are working on 8-bit values only, which still > > keeps sparse happy without an accidental promotion to a 32 bit integer. > > > > > The reason the warning was occurring is because certain bitmasks that > > end with a mask next to a natural boundary like 7, 15, 23, 31, end up > > with a mask like 0x7f, which then results in sign extension when doing > > an invert (but I'm not a compiler expert). It was really only > > "clear_bit" that was having problems, and it was only on bit checks next > > to a byte boundary (top bit). > > I guess this describes it incorrectly. Forget about it, I looked at the warning again and there is the 0x7f byte at the end. Sorry for noise. > I think it is a C standard which dictates this, compiler just follows. -- With Best Regards, Andy Shevchenko