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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E4A5C433F5 for ; Tue, 26 Oct 2021 19:22:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1E13760EFF for ; Tue, 26 Oct 2021 19:22:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237066AbhJZTYq (ORCPT ); Tue, 26 Oct 2021 15:24:46 -0400 Received: from mga03.intel.com ([134.134.136.65]:54678 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235162AbhJZTYn (ORCPT ); Tue, 26 Oct 2021 15:24:43 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10149"; a="229942638" X-IronPort-AV: E=Sophos;i="5.87,184,1631602800"; d="scan'208";a="229942638" Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Oct 2021 12:22:19 -0700 X-IronPort-AV: E=Sophos;i="5.87,184,1631602800"; d="scan'208";a="486323056" Received: from smile.fi.intel.com ([10.237.72.184]) by orsmga007-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Oct 2021 12:22:18 -0700 Received: from andy by smile.fi.intel.com with local (Exim 4.95) (envelope-from ) id 1mfS1G-001IBg-Au; Tue, 26 Oct 2021 22:21:58 +0300 Date: Tue, 26 Oct 2021 22:21:58 +0300 From: Andy Shevchenko To: Yury Norov Cc: Qian Cai , Rasmus Villemoes , linux-kernel@vger.kernel.org Subject: Re: [PATCH] bitmap: simplify GENMASK(size - 1, 0) lines Message-ID: References: <20211026144108.35373-1-quic_qiancai@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 26, 2021 at 11:54:16AM -0700, Yury Norov wrote: > On Tue, Oct 26, 2021 at 10:41:08AM -0400, Qian Cai wrote: > > Since "size" is an "unsigned int", the rvalue "size - 1" will still be > > "unsigned int" according to the C standard (3.2.1.5 Usual arithmetic > > conversions). Therefore, GENMASK(size - 1, 0) will always return 0UL. Those > > are also caught by GCC (W=2): > > > > ./include/linux/find.h: In function 'find_first_bit': > > ./include/linux/bits.h:25:22: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] > > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > > | ^ > > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > > | ^ > > ./include/linux/bits.h:25:3: note: in expansion of macro '__is_constexpr' > > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > > | ^~~~~~~~~~~~~~ > > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > > 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > | ^~~~~~~~~~~~~~~~~~~ > > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK' > > 119 | unsigned long val = *addr & GENMASK(size - 1, 0); > > | ^~~~~~~ > > ./include/linux/bits.h:25:34: warning: comparison of unsigned expression in '< 0' is always false [-Wtype-limits] > > 25 | __is_constexpr((l) > (h)), (l) > (h), 0))) > > | ^ > > ./include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > > | ^ > > ./include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > > 38 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > | ^~~~~~~~~~~~~~~~~~~ > > ./include/linux/find.h:119:31: note: in expansion of macro 'GENMASK' > > 119 | unsigned long val = *addr & GENMASK(size - 1, 0); > > | ^~~~~~~ > > > > Signed-off-by: Qian Cai > > --- > > include/linux/find.h | 28 ++++++++-------------------- > > 1 file changed, 8 insertions(+), 20 deletions(-) > > > > diff --git a/include/linux/find.h b/include/linux/find.h > > index 5bb6db213bcb..5ce2b17aea42 100644 > > --- a/include/linux/find.h > > +++ b/include/linux/find.h > > @@ -115,11 +115,8 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size, > > static inline > > unsigned long find_first_bit(const unsigned long *addr, unsigned long size) > > { > > - if (small_const_nbits(size)) { > > - unsigned long val = *addr & GENMASK(size - 1, 0); > > - > > - return val ? __ffs(val) : size; > > - } > > + if (small_const_nbits(size)) > > + return size; > > > > return _find_first_bit(addr, size); > > } > > [...] > > Nice catch! I'm a bit concerned that small_const_nbits() will never > allow GENMASK() to be passed with size == 0, but the patch looks > good to me overall. Can you explain to me how it is supposed to work? For example, x = 0xaa55; size = 5; printf("%lu\n", find_first_bit(&x, size)); In the resulting code we will always have 5 as the result, but is it correct one? -- With Best Regards, Andy Shevchenko