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 3557AC3A5A0 for ; Mon, 19 Aug 2019 14:37:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0E9742070D for ; Mon, 19 Aug 2019 14:37:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727398AbfHSOh0 (ORCPT ); Mon, 19 Aug 2019 10:37:26 -0400 Received: from gate.crashing.org ([63.228.1.57]:59023 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725958AbfHSOh0 (ORCPT ); Mon, 19 Aug 2019 10:37:26 -0400 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 Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org 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 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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