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 9A2C0C2D0CE for ; Tue, 21 Jan 2020 19:55:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 71D2E217F4 for ; Tue, 21 Jan 2020 19:55:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728899AbgAUTzd (ORCPT ); Tue, 21 Jan 2020 14:55:33 -0500 Received: from gate.crashing.org ([63.228.1.57]:58169 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726926AbgAUTzd (ORCPT ); Tue, 21 Jan 2020 14:55:33 -0500 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 00LJt2oG006385; Tue, 21 Jan 2020 13:55:02 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 00LJt16T006382; Tue, 21 Jan 2020 13:55:01 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Tue, 21 Jan 2020 13:55:01 -0600 From: Segher Boessenkool To: Christophe Leroy Cc: Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , ruscur@russell.cc, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: GCC bug ? Re: [PATCH v2 10/10] powerpc/32s: Implement Kernel Userspace Access Protection Message-ID: <20200121195501.GJ3191@gate.crashing.org> References: <87ftqfu7j1.fsf@concordia.ellerman.id.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 Tue, Jan 21, 2020 at 05:22:32PM +0000, Christophe Leroy wrote: > g1() should return 3, not 5. What makes you say that? "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 the rules it uses for this are *not* the same as C "constant expressions" or C "integer constant expression" or C "arithmetic constant expression" or anything like that -- which should be already obvious from that it changes with different -Ox). You can use builtin_constant_p to have the compiler do something better if the compiler feels like it, but not anything more. Often people want stronger guarantees, but when they see how much less often it then returns "true", they do not want that either. Segher