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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 5FEC7C3A5A1 for ; Mon, 19 Aug 2019 14:44:09 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D8AFD2082C for ; Mon, 19 Aug 2019 14:44:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D8AFD2082C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 46BxWk0yPbzDqFL for ; Tue, 20 Aug 2019 00:44:02 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=permerror (mailfrom) smtp.mailfrom=kernel.crashing.org (client-ip=63.228.1.57; helo=gate.crashing.org; envelope-from=segher@kernel.crashing.org; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 46BxMs4mW7zDqsj for ; Tue, 20 Aug 2019 00:37:13 +1000 (AEST) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id x7JEb1pp023153; Mon, 19 Aug 2019 09:37:01 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x7JEb09r023152; Mon, 19 Aug 2019 09:37:00 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 19 Aug 2019 09:37:00 -0500 From: Segher Boessenkool To: Christophe Leroy Subject: Re: [PATCH 3/3] powerpc: use __builtin_trap() in BUG/WARN macros. Message-ID: <20190819143700.GK31406@gate.crashing.org> References: <20510ce03cc9463f1c9e743c1d93b939de501b53.1566219503.git.christophe.leroy@c-s.fr> <20190819132313.GH31406@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.4.2.3i X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@lists.ozlabs.org, Paul Mackerras , linux-kernel@vger.kernel.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, Aug 19, 2019 at 04:08:43PM +0200, Christophe Leroy wrote: > Le 19/08/2019 à 15:23, Segher Boessenkool a écrit : > >On Mon, Aug 19, 2019 at 01:06:31PM +0000, Christophe Leroy wrote: > >>Note that we keep using an assembly text using "twi 31, 0, 0" for > >>inconditional traps because GCC drops all code after > >>__builtin_trap() when the condition is always true at build time. > > > >As I said, it can also do this for conditional traps, if it can prove > >the condition is always true. > > But we have another branch for 'always true' and 'always false' using > __builtin_constant_p(), which don't use __builtin_trap(). Is there > anything wrong with that ?: The compiler might not realise it is constant when it evaluates the __builtin_constant_p, but only realises it later. As the documentation for the builtin says: A return of 0 does not indicate that the value is _not_ a constant, but merely that GCC cannot prove it is a constant with the specified value of the '-O' option. (and there should be many more and more serious warnings here). > #define BUG_ON(x) do { \ > if (__builtin_constant_p(x)) { \ > if (x) \ > BUG(); \ > } else { \ > if (x) \ > __builtin_trap(); \ > BUG_ENTRY("", 0); \ > } \ > } while (0) I think it may work if you do #define BUG_ON(x) do { \ if (__builtin_constant_p(x)) { \ if (x) \ BUG(); \ } else { \ BUG_ENTRY("", 0); \ if (x) \ __builtin_trap(); \ } \ } while (0) or even just #define BUG_ON(x) do { \ BUG_ENTRY("", 0); \ if (x) \ __builtin_trap(); \ } \ } while (0) if BUG_ENTRY can work for the trap insn *after* it. > >Can you put the bug table asm *before* the __builtin_trap maybe? That > >should make it all work fine... If you somehow can tell what machine > >instruction is that trap, anyway. > > And how can I tell that ? I don't know how BUG_ENTRY works exactly. Segher