From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from fg-out-1718.google.com (fg-out-1718.google.com [72.14.220.154]) by ozlabs.org (Postfix) with ESMTP id 5F731B7B7C for ; Wed, 21 Oct 2009 00:58:41 +1100 (EST) Received: by fg-out-1718.google.com with SMTP id l26so4211901fgb.14 for ; Tue, 20 Oct 2009 06:58:38 -0700 (PDT) Date: Tue, 20 Oct 2009 21:58:35 +0800 From: =?utf-8?Q?Am=C3=A9rico?= Wang To: Rusty Russell Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases Message-ID: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <200910201415.34361.rusty@rustcorp.com.au> Cc: sfr@canb.auug.org.au, Hollis Blanchard , 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 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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? >+#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__) > >-- >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/ -- Live like a child, think like the god.