On Tue, Oct 20, 2009 at 10:43 PM, Alan Jenkins <sourcejedi.lkml@xxxxxxxxxxxxxx> wrote: > 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. Ah, clever trick! Got it. Thanks! Reviewed-by: WANG Cong <xiyou.wangcong@xxxxxxxxx> -- 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