From patchwork Mon Jun 26 15:24:35 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Nicola Vetrini X-Patchwork-Id: 13293059 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 lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id DD85CEB64DA for ; Mon, 26 Jun 2023 15:25:58 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.555578.867460 (Exim 4.92) (envelope-from ) id 1qDo5p-0002fd-Gu; Mon, 26 Jun 2023 15:25:29 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 555578.867460; Mon, 26 Jun 2023 15:25:29 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1qDo5p-0002fW-EP; Mon, 26 Jun 2023 15:25:29 +0000 Received: by outflank-mailman (input) for mailman id 555578; Mon, 26 Jun 2023 15:25:27 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1qDo5n-0002fM-KH for xen-devel@lists.xenproject.org; Mon, 26 Jun 2023 15:25:27 +0000 Received: from support.bugseng.com (mail.bugseng.com [162.55.131.47]) by se1-gles-flk1.inumbo.com (Halon) with ESMTPS id ab4645b6-1435-11ee-8611-37d641c3527e; Mon, 26 Jun 2023 17:25:23 +0200 (CEST) Received: from nico.bugseng.com (unknown [151.35.129.1]) by support.bugseng.com (Postfix) with ESMTPSA id 3C40E4EE0738; Mon, 26 Jun 2023 17:25:20 +0200 (CEST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: ab4645b6-1435-11ee-8611-37d641c3527e From: Nicola Vetrini To: xen-devel@lists.xenproject.org Cc: sstabellini@kernel.org, michal.orzel@amd.com, xenia.ragiadakou@amd.com, ayan.kumar.halder@amd.com, consulting@bugseng.com, jbeulich@suse.com, andrew.cooper3@citrix.com, roger.pau@citrix.com, bertrand.marquis@arm.com, julien@xen.org, Nicola Vetrini , George Dunlap , Wei Liu Subject: [XEN PATCH v5] xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO Date: Mon, 26 Jun 2023 17:24:35 +0200 Message-Id: <10f8685626fa4a9d131186a23b293479cb92da38.1687791626.git.nicola.vetrini@bugseng.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Redefine BUILD_BUG_ON_ZERO to avoid using a compiler extension that gives an acceptable semantics to C99 undefined behavior 58 ("A structure or union is defined as containing no named members (6.7.2.1)"). The first definition includes an additional named field of type char. The chosen ill-formed construct for the second definition is a struct with a named bitfield of width 0 when the condition is true, which prevents the UB without using the compiler extension while keeping the semantics of the construct. The choice of the bitwise AND operation to bring the result to 0 when cond is false boils down to possibly better portability. Signed-off-by: Nicola Vetrini --- Changes in V2: - Avoid using a VLA as the compile-time assertion - Do not drop _Static_assert Changes in V3: - Changed the operation to bring the result to 0 when the construct does not lead to a compilation error Changes in V4: - Switched to a shorter construct for the second definition. Changes in V5: - Dropped 'U's from the macro definitions --- xen/include/xen/lib.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h index 67fc7c1d7e..460cab6734 100644 --- a/xen/include/xen/lib.h +++ b/xen/include/xen/lib.h @@ -51,9 +51,10 @@ e.g. in a structure initializer (or where-ever else comma expressions aren't permitted). */ #define BUILD_BUG_ON_ZERO(cond) \ - sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); }) + (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) & 0U) #else -#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); }) +#define BUILD_BUG_ON_ZERO(cond) \ + (sizeof(struct { unsigned u : !(cond); }) & 0U) #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond)) #endif