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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,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 56A99C43441 for ; Wed, 10 Oct 2018 18:14:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 298F52087A for ; Wed, 10 Oct 2018 18:14:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 298F52087A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=sipsolutions.net 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 S1726995AbeJKBhP (ORCPT ); Wed, 10 Oct 2018 21:37:15 -0400 Received: from s3.sipsolutions.net ([144.76.43.62]:44656 "EHLO sipsolutions.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726656AbeJKBhP (ORCPT ); Wed, 10 Oct 2018 21:37:15 -0400 Received: by sipsolutions.net with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.91) (envelope-from ) id 1gAIzN-0007tm-CE; Wed, 10 Oct 2018 20:13:42 +0200 Message-ID: <1539195207.3687.178.camel@sipsolutions.net> Subject: Re: Question on FIELD_PREP() for static array From: Johannes Berg To: Joe Perches , John Garry , Andrew Morton , Andy Shevchenko , Kalle Valo , jakub.kicinski@netronome.com, yamada.masahiro@socionext.com, Arnd Bergmann , viro@zeniv.linux.org.uk, "linux-kernel@vger.kernel.org" Cc: linux-wireless@vger.kernel.org, nbd@nbd.name Date: Wed, 10 Oct 2018 20:13:27 +0200 In-Reply-To: <40fd3d963820bf96547fa9b5e8c171c6a339674e.camel@perches.com> References: <40fd3d963820bf96547fa9b5e8c171c6a339674e.camel@perches.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.6 (3.26.6-1.fc27) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2018-10-10 at 10:33 -0700, Joe Perches wrote: > > > Specifically it doesn't like the __BF_FIELD_CHECK() in FIELD_PREP(). > > > > Any ideas on compiler trickery we could do with the FIELD_PREP() > > definition to avoid this issue (i.e. enforce the check but only use the > > constant value)? > > Perhaps __bf_shf should not use __builtin_ffsll. __bf_shf() is a constant expression, and is fine in this context. The problem is the use of the compound statement here: static int x[2] = { ({ (void)(0); 1; }), 0, } similarly fails to compile. I've recently run into a similar situation, namely in include/net/netlink.h, and the applicable way to solve it here would be something like this: diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h index 3f1ef4450a7c..0680d641923f 100644 --- a/include/linux/bitfield.h +++ b/include/linux/bitfield.h @@ -49,19 +49,16 @@ #define __bf_shf(x) (__builtin_ffsll(x) - 1) +#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) + #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \ - ({ \ - BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \ - _pfx "mask is not constant"); \ - BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \ - BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \ - ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \ - _pfx "value too large for the field"); \ - BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull, \ - _pfx "type of reg too small for mask"); \ - __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \ - (1ULL << __bf_shf(_mask))); \ - }) + BUILD_BUG_ON_RET_ZERO(!__builtin_constant_p(_mask)) + \ + BUILD_BUG_ON_RET_ZERO((_mask) == 0) + \ + BUILD_BUG_ON_RET_ZERO(__builtin_constant_p(_val) ? \ + ~((_mask) >> __bf_shf(_mask)) & (_val) : 0) + \ + BUILD_BUG_ON_RET_ZERO((_mask) > (typeof(_reg))~0ull) + \ + BUILD_BUG_ON_NOT_POW2_RET_ZERO((_mask) + (1ULL << __bf_shf(_mask))) /** * FIELD_FIT() - check if value fits in the field @@ -85,10 +82,8 @@ * be combined with other fields of the bitfield using logical OR. */ #define FIELD_PREP(_mask, _val) \ - ({ \ - __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \ - ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \ - }) + (__BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: ") + \ + (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask))) /** * FIELD_GET() - extract a bitfield element Note that this is an incomplete patch - everything but FIELD_PREP will not compile with this. Also, BUILD_BUG_ON_RET_ZERO and BUILD_BUG_ON_NOT_POW2_RET_ZERO should probably have better names, or perhaps do the positive way that I did in __NLA_ENSURE, e.g. CONST_ASSERT()/CONST_ASSERT_IS_POWER_OF_2()? I guess they should go to build_bug.h as well... johannes