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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 BE1A6C35DF9 for ; Tue, 25 Feb 2020 10:30:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9855521927 for ; Tue, 25 Feb 2020 10:30:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730297AbgBYKav (ORCPT ); Tue, 25 Feb 2020 05:30:51 -0500 Received: from mga01.intel.com ([192.55.52.88]:21797 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729698AbgBYKav (ORCPT ); Tue, 25 Feb 2020 05:30:51 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 25 Feb 2020 02:30:50 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,483,1574150400"; d="scan'208";a="230108009" Received: from smile.fi.intel.com (HELO smile) ([10.237.68.40]) by fmsmga007.fm.intel.com with ESMTP; 25 Feb 2020 02:30:48 -0800 Received: from andy by smile with local (Exim 4.93) (envelope-from ) id 1j6XUI-004dNp-0i; Tue, 25 Feb 2020 12:30:50 +0200 Date: Tue, 25 Feb 2020 12:30:50 +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: <20200225103050.GD10400@smile.fi.intel.com> References: <20200224225020.2212544-1-jesse.brandeburg@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200224225020.2212544-1-jesse.brandeburg@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 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. The problem is integer promotion of negation operation for any bit basically. For example, if we have bit 3 (nr = 3) and got it in the clear bit we will get (according to what I see in the warnings) 0xfffffff7. Which is simple ~(1 << 3) promoted to integer. I think it is a C standard which dictates this, compiler just follows. > Verified with a test module (see next patch) and assembly inspection > that the patch doesn't introduce any change in generated code. > > Signed-off-by: Jesse Brandeburg > Reviewed-by: Andy Shevchenko > --- > v5: changed code to use simple AND and XOR, updated commit message > v4: reverse argument order as suggested by David Laight, added reviewed-by > v3: Clean up the header file changes as per peterz. > v2: use correct CC: list > --- > arch/x86/include/asm/bitops.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h > index 062cdecb2f24..53f246e9df5a 100644 > --- a/arch/x86/include/asm/bitops.h > +++ b/arch/x86/include/asm/bitops.h > @@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr) > if (__builtin_constant_p(nr)) { > asm volatile(LOCK_PREFIX "orb %1,%0" > : CONST_MASK_ADDR(nr, addr) > - : "iq" ((u8)CONST_MASK(nr)) > + : "iq" (CONST_MASK(nr) & 0xff) > : "memory"); > } else { > asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0" > @@ -74,7 +74,7 @@ arch_clear_bit(long nr, volatile unsigned long *addr) > if (__builtin_constant_p(nr)) { > asm volatile(LOCK_PREFIX "andb %1,%0" > : CONST_MASK_ADDR(nr, addr) > - : "iq" ((u8)~CONST_MASK(nr))); > + : "iq" (CONST_MASK(nr) ^ 0xff)); > } else { > asm volatile(LOCK_PREFIX __ASM_SIZE(btr) " %1,%0" > : : RLONG_ADDR(addr), "Ir" (nr) : "memory"); > > base-commit: ca7e1fd1026c5af6a533b4b5447e1d2f153e28f2 > -- > 2.24.1 > -- With Best Regards, Andy Shevchenko