From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pw0-f43.google.com (mail-pw0-f43.google.com [209.85.160.43]) by ozlabs.org (Postfix) with ESMTP id 393CEB7BC3 for ; Fri, 23 Oct 2009 12:50:07 +1100 (EST) Received: by pwj6 with SMTP id 6so285266pwj.2 for ; Thu, 22 Oct 2009 18:50:05 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <9b2b86520910200743h4e134cf8jd2860d42b3936597@mail.gmail.com> 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> <9b2b86520910200743h4e134cf8jd2860d42b3936597@mail.gmail.com> Date: Fri, 23 Oct 2009 09:50:05 +0800 Message-ID: <2375c9f90910221850r584e16a9k924aa2e0861bd358@mail.gmail.com> Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases From: =?UTF-8?Q?Am=C3=A9rico_Wang?= To: Alan Jenkins Content-Type: text/plain; charset=UTF-8 Cc: sfr@canb.auug.org.au, Hollis Blanchard , Rusty Russell , linux-kernel@vger.kernel.org, Jan Beulich , linux-next@vger.kernel.org, kvm-ppc@vger.kernel.org, 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 10:43 PM, Alan Jenkins wrote: > On 10/20/09, Am=C3=A9rico 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 unde= r >>> =C2=A0 =C2=A0 =C2=A0"if (__builtin_constant_p(x))" for example. >>>negative array: can handle anything, but if the compiler can't tell it's >>> =C2=A0 =C2=A0 =C2=A0a constant, silently has no effect. >>>link time: breaks link if the compiler can't determine the value, but th= e >>> =C2=A0 =C2=A0 =C2=A0linker output is not usually as informative as a co= mpiler 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 { >>> =C2=A0 =C2=A0 =C2=A0char _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 >>> =C2=A0 =C2=A0result (of value 0 and type size_t), so the expression can= be used >>> =C2=A0 =C2=A0e.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). =C2=A0So as a fallback we use the optimizer; i= f it >>> can't >>>+ * prove the condition is false, it will cause a link error on the >>> undefined >>>+ * "__build_bug_on_failed". =C2=A0This error message can be harder to t= rack >>> 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. =C2=A0I'm sure > grep will tell you it wasn't defined before hand either. =C2=A0So any > reference to it is an error - which will be reported at link time. > >>>+#define BUILD_BUG_ON(condition) =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0\ >>>+ =C2=A0 =C2=A0 do { =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0\ >>>+ =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 ((void)sizeof(char[1 - 2*!!(= condition)])); =C2=A0 =C2=A0 =C2=A0\ >>>+ =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (condition) __build_bug_o= n_failed =3D 1; =C2=A0 =C2=A0 =C2=A0 \ > > If "condition" is known false at compile time, gcc -O will eliminate > the code which refers to __build_bug_on_failed. =C2=A0If it's not proved = to > be false - it will break the build, which is exactly what we want > BUILD_BUG_ON to do. Ah, clever trick! Got it. Thanks! Reviewed-by: WANG Cong