Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752475AbZJTOnO (ORCPT ); Tue, 20 Oct 2009 10:43:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752455AbZJTOnN (ORCPT ); Tue, 20 Oct 2009 10:43:13 -0400 Received: from mail-fx0-f218.google.com ([209.85.220.218]:46536 "EHLO mail-fx0-f218.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752452AbZJTOnL convert rfc822-to-8bit (ORCPT ); Tue, 20 Oct 2009 10:43:11 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=YtherSlma3bhCwZsO5vp/vdQ/H3T8cZCd7ppeJcGNf6ASi+5bnoyrXCElqGcXYyGdF EbmowA9/R+M7phfyzeEoig+WhJnrWHl2PxYhffZUwcRwAvokVX80kvqSBZiQ6UjbnHW6 AWsTmyx4AsxgcdJGOE7ShwC4234JDlIIptFHw= MIME-Version: 1.0 In-Reply-To: <20091020135835.GB2462@hack> References: <4AC1E15502000078000516B5@vpn.id2.novell.com> <200910201142.34006.rusty@rustcorp.com.au> <1256002193.6546.2.camel@slab> <200910201415.34361.rusty@rustcorp.com.au> <20091020135835.GB2462@hack> Date: Tue, 20 Oct 2009 15:43:13 +0100 Message-ID: <9b2b86520910200743h4e134cf8jd2860d42b3936597@mail.gmail.com> Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases From: Alan Jenkins To: =?ISO-8859-1?Q?Am=E9rico_Wang?= Cc: Rusty Russell , Hollis Blanchard , Jan Beulich , sfr@canb.auug.org.au, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-next@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3991 Lines: 94 On 10/20/09, Am?rico Wang wrote: > On Tue, Oct 20, 2009 at 02:15:33PM +1030, Rusty Russell wrote: >>BUILD_BUG_ON used to use the optimizer to do code elimination or fail >>at link time; it was changed to first the size of a negative array (a >>nicer compile time error), then (in >>8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield. >> >>bitfields: needs a literal constant at parse time, and can't be put under >> "if (__builtin_constant_p(x))" for example. >>negative array: can handle anything, but if the compiler can't tell it's >> a constant, silently has no effect. >>link time: breaks link if the compiler can't determine the value, but the >> linker output is not usually as informative as a compiler error. >> >>If we use the negative-array-size method *and* the link time trick, >>we get the ability to use BUILD_BUG_ON() under __builtin_constant_p() >>branches, and maximal ability for the compiler to detect errors at >>build time. >> >>Signed-off-by: Rusty Russell >> >>diff --git a/include/linux/kernel.h b/include/linux/kernel.h >>--- a/include/linux/kernel.h >>+++ b/include/linux/kernel.h >>@@ -683,12 +683,6 @@ struct sysinfo { >> char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */ >> }; >> >>-/* Force a compilation error if condition is true */ >>-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition)) >>- >>-/* Force a compilation error if condition is constant and true */ >>-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)])) >>- >> /* Force a compilation error if condition is true, but also produce a >> result (of value 0 and type size_t), so the expression can be used >> e.g. in a structure initializer (or where-ever else comma expressions >>@@ -696,6 +690,33 @@ struct sysinfo { >> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) >> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); })) >> >>+/** >>+ * BUILD_BUG_ON - break compile if a condition is true. >>+ * @cond: the condition which the compiler should know is false. >>+ * >>+ * If you have some code which relies on certain constants being equal, or >>+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to >>+ * detect if someone changes it. >>+ * >>+ * The implementation uses gcc's reluctance to create a negative array, >> but >>+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not >> arguments >>+ * to inline functions). So as a fallback we use the optimizer; if it >> can't >>+ * prove the condition is false, it will cause a link error on the >> undefined >>+ * "__build_bug_on_failed". This error message can be harder to track >> down >>+ * though, hence the two different methods. >>+ */ >>+#ifndef __OPTIMIZE__ >>+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) >>+#else >>+extern int __build_bug_on_failed; > > Hmm, what exactly is __build_bug_on_failed? Well, we haven't added a definition for it in this patch. I'm sure grep will tell you it wasn't defined before hand either. So any reference to it is an error - which will be reported at link time. >>+#define BUILD_BUG_ON(condition) \ >>+ do { \ >>+ ((void)sizeof(char[1 - 2*!!(condition)])); \ >>+ if (condition) __build_bug_on_failed = 1; \ If "condition" is known false at compile time, gcc -O will eliminate the code which refers to __build_bug_on_failed. If it's not proved to be false - it will break the build, which is exactly what we want BUILD_BUG_ON to do. >>+ } while(0) >>+#endif >>+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition) >>+ >> /* Trap pasters of __FUNCTION__ at compile-time */ >> #define __FUNCTION__ (__func__) >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/