From 7ef88ad561457c0346355dfd1f53e503ddfde719 Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Mon, 24 Jan 2011 14:45:10 -0600 Subject: [PATCH] BUILD_BUG_ON: make it handle more cases 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. This forced us to change some non-constant cases to MAYBE_BUILD_BUG_ON(); as Jan points out in that commit, it didn't work as intended anyway. 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. We also document it thoroughly. Signed-off-by: Rusty Russell Cc: Jan Beulich Acked-by: Hollis Blanchard --- include/linux/kernel.h | 33 +++++++++++++++++++++++++++------ 1 file changed, 27 insertions(+), 6 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index d07d8057e440..864712f3653d 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -575,12 +575,6 @@ 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 a constant expression is not a power of 2 */ #define BUILD_BUG_ON_NOT_POWER_OF_2(n) \ BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0)) @@ -592,6 +586,33 @@ 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; +#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__) -- 2.20.1