From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id D39C6438E5; Tue, 16 Jan 2024 22:50:37 +0100 (CET) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 9E5C240278; Tue, 16 Jan 2024 22:50:37 +0100 (CET) Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by mails.dpdk.org (Postfix) with ESMTP id CD07F400D6 for ; Tue, 16 Jan 2024 22:50:35 +0100 (CET) Received: by linux.microsoft.com (Postfix, from userid 1086) id ED9DE20DF574; Tue, 16 Jan 2024 13:50:34 -0800 (PST) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com ED9DE20DF574 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1705441834; bh=yhTbiqNjAtqQiWfcEgGJ+8pgbAbZKvnJM5t+i7FZ7Os=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=D8zJK5EL/YfuBMXjd1J2OvNePv5QuG/43WNw+AGwEiNgaJO3ePmf+nm5A3syupSlW RbVBkab00g4hcEcoT2HUAsZT8SDRRZ4qK+T7GiJglHP6WVw6FaF31Kgz+6vydTtss2 ePld7YWO98firWYCC+mNHTQv1tNAQGWFaZWt4iDQ= Date: Tue, 16 Jan 2024 13:50:34 -0800 From: Tyler Retzlaff To: Stephen Hemminger Cc: Andrew Rybchenko , dev@dpdk.org Subject: Re: static_assert, sfc, and clang issues Message-ID: <20240116215034.GA351@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> References: <20240116090301.4c29d5b7@hermes.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240116090301.4c29d5b7@hermes.local> User-Agent: Mutt/1.5.21 (2010-09-15) X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org On Tue, Jan 16, 2024 at 09:03:01AM -0800, Stephen Hemminger wrote: > Ran into a corner case issue, so sending to mailing list for wider discussion. > > One improvement to DPDK code base planned is getting rid of variable length arrays. > VLA's can cause bugs and are not supported by the Windows compiler. > Gcc and Clang have a flag to warn on use of VLA's (-Wvla). > > In DPDK one use of VLA's is in the RTE_BUILD_BUG_ON macro. > The best path is to replace the undefined array access with a better > static_assert() which is builtin to compilers. > > Using static_assert() is relatively easy, there are a few places where > it does detect use of non-constant expressions in existing code but these > are fixable. > > But there is one case where use static_assert() runs into a Clang bug > which is not fixed in distros: > > https://github.com/llvm/llvm-project/issues/55821 > > The code in question is in the sfc driver which for better or worse > has lots of assertions. The problem is that clang (except in unreleased trunk) > can not handle static_assert in a switch leg. > > switch (actions->type) { > case RTE_FLOW_ACTION_TYPE_VOID: > SFC_BUILD_SET_OVERFLOW(RTE_FLOW_ACTION_TYPE_VOID, > actions_set); > break; > > ../drivers/net/sfc/sfc_flow.c:1635:4: error: expected expression > SFC_BUILD_SET_OVERFLOW(RTE_FLOW_ACTION_TYPE_VOID, > ^ > ../drivers/net/sfc/sfc_flow.h:36:2: note: expanded from macro 'SFC_BUILD_SET_OVERFLOW' > RTE_BUILD_BUG_ON((_action) >= sizeof(_set) * CHAR_BIT) > ^ > > > There are some workarounds: > 0. Ignore it, works on Gcc, and Clang fix is pending. > 1. Remove many of these RTE_BUILD_BUG_ON's. They are really not that helpful. > 2. Add additional { } to these switch cases so they become basic blocks > which works around the bug. > 3. Move the assertions out of the switch > > My preference is #2 but open to other options. +1 for #2 just make it a block.