From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752550Ab3AAVKs (ORCPT ); Tue, 1 Jan 2013 16:10:48 -0500 Received: from nm17.access.bullet.mail.mud.yahoo.com ([66.94.237.218]:35922 "EHLO nm17.access.bullet.mail.mud.yahoo.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752458Ab3AAVKU (ORCPT ); Tue, 1 Jan 2013 16:10:20 -0500 X-Yahoo-Newman-Id: 3645.17119.bm@smtp109.sbc.mail.ne1.yahoo.com X-Yahoo-Newman-Property: ymail-3 X-YMail-OSG: dj9N4gkVM1kmbWNC1ygcfqI6acxA9rzAHDa0HzJBWYLceke dVA88YnaUVXdMcTbP4a1iOtmPykG3.75t_ZX2HMwBmfzCtmNh2GsexO9TBKY XAJ9lMAqrbeoMp7FUwpRjz2pqyWS.QOHAmSX44.QR05XrLsPjzf044PSbDRG HUGqw2JK7vSuuRz0cdKcBJFWt6LKwEOJRZW_Jhd2tSjmEKsLb3Qk_YgC4Cuz UzOQ5zzJvI3hBTUFRdjXjS_4kvXawgUwnZH7XEQpEm3wby8xWj1R4vUTovVv sQmVzeu3Qq2Rw3lghSIKbqYx4sPIg0iUYVUdGA6UeG3OLowSCowsOgDP_rkG ZW28necRitx050b_0hkbVHfCo4jpaj4XmQ5S1bmMWdjFi4KBBSadEoxrkTnB Z9uOdTwzyspgKWhs02qwAswMZk2r8bKz423IuX0IBpvorZEndm93CPa6ejLe Mg2TtAF.ql9LaUnuFHe3S19iUUGgs X-Yahoo-SMTP: xXkkXk6swBBAi.5wfkIWFW3ugxbrqyhyk_b4Z25Sfu.XGQ-- From: danielfsantos@att.net To: LKML , Andi Kleen , Andrea Arcangeli , Andrew Morton , Borislav Petkov , Christopher Li , David Daney , David Rientjes , Joe Perches , Josh Triplett , linux-sparse@vger.kernel.org, Michel Lespinasse , Paul Gortmaker , Peter Zijlstra , Steven Rostedt Cc: Daniel Santos Subject: [PATCH v7 8/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON} Date: Tue, 1 Jan 2013 15:09:56 -0600 Message-Id: <1357074597-21722-8-git-send-email-daniel.santos@pobox.com> X-Mailer: git-send-email 1.7.8.6 In-Reply-To: <1357074597-21722-1-git-send-email-daniel.santos@pobox.com> References: <1357074491-21669-1-git-send-email-daniel.santos@pobox.com> <1357074597-21722-1-git-send-email-daniel.santos@pobox.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3, creating compile-time errors required a little trickery. BUILD_BUG{,_ON} uses this attribute when available to generate compile-time errors, but also uses the negative-sized array trick for older compilers, resulting in two error messages in some cases. The reason it's "some" cases is that as of gcc 4.4, the negative-sized array will not create an error in some situations, like inline functions. This patch replaces the negative-sized array code with the new __compiletime_error_fallback() macro which expands to the same thing unless the the error attribute is available, in which case it expands to do{}while(0), resulting in exactly one compile-time error on all versions of gcc. Note that we are not changing the negative-sized array code for the unoptimized version of BUILD_BUG_ON, since it has the potential to catch problems that would be disabled in later versions of gcc were __compiletime_error_fallback used. The reason is that that an unoptimized build can't always remove calls to an error-attributed function call (like we are using) that should effectively become dead code if it were optimized. However, using a negative-sized array with a similar value will not result in an false-positive (error). The only caveat being that it will also fail to catch valid conditions, which we should be expecting in an unoptimized build anyway. Signed-off-by: Daniel Santos --- include/linux/bug.h | 2 +- include/linux/compiler.h | 5 +++++ 2 files changed, 6 insertions(+), 1 deletions(-) diff --git a/include/linux/bug.h b/include/linux/bug.h index 08d97e9..57c7688 100644 --- a/include/linux/bug.h +++ b/include/linux/bug.h @@ -67,7 +67,7 @@ struct pt_regs; __compiletime_error("BUILD_BUG_ON failed"); \ if (__cond) \ __build_bug_on_failed(); \ - ((void)sizeof(char[1 - 2 * __cond])); \ + __compiletime_error_fallback(__cond); \ } while(0) #endif diff --git a/include/linux/compiler.h b/include/linux/compiler.h index 4c638be..423bb6b 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -307,7 +307,12 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); #endif #ifndef __compiletime_error # define __compiletime_error(message) +# define __compiletime_error_fallback(condition) \ + do { ((void)sizeof(char[1 - 2*!!(condition)])); } while (0) +#else +# define __compiletime_error_fallback(condition) do { } while (0) #endif + /* * Prevent the compiler from merging or refetching accesses. The compiler * is also forbidden from reordering successive instances of ACCESS_ONCE(), -- 1.7.8.6