On 10/20/09, Américo Wang <xiyou.wangcong@xxxxxxxxx> 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 <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? 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-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html