Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755898AbZKEGBv (ORCPT ); Thu, 5 Nov 2009 01:01:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750847AbZKEGBv (ORCPT ); Thu, 5 Nov 2009 01:01:51 -0500 Received: from gate.crashing.org ([63.228.1.57]:36979 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750825AbZKEGBu (ORCPT ); Thu, 5 Nov 2009 01:01:50 -0500 Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases From: Benjamin Herrenschmidt To: Rusty Russell Cc: Hollis Blanchard , sfr@canb.auug.org.au, linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-next@vger.kernel.org, Jan Beulich , akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org In-Reply-To: <200910201415.34361.rusty@rustcorp.com.au> References: <4AC1E15502000078000516B5@vpn.id2.novell.com> <200910201142.34006.rusty@rustcorp.com.au> <1256002193.6546.2.camel@slab> <200910201415.34361.rusty@rustcorp.com.au> Content-Type: text/plain; charset="UTF-8" Date: Thu, 05 Nov 2009 17:01:26 +1100 Message-ID: <1257400886.13611.98.camel@pasglop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3691 Lines: 87 On Tue, 2009-10-20 at 14:15 +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. What's the status with this patch ? The lack of it breaks my KVM stuff in powerpc... Cheers, Ben. > 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; > +#define BUILD_BUG_ON(condition) \ > + do { \ > + ((void)sizeof(char[1 - 2*!!(condition)])); \ > + if (condition) __build_bug_on_failed = 1; \ > + } while(0) > +#endif > +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition) > + > /* Trap pasters of __FUNCTION__ at compile-time */ > #define __FUNCTION__ (__func__) > > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev -- 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/