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 <rusty@xxxxxxxxxxxxxxx> > >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@xxxxxxxxxxxxxxx >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. -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html