BUILD_BUG_ON42(arg) BUILD_BUG_ON_CONST42(arg) Prior to gcc 4.2, the optimizer was unable to determine that many constant values stored in structs were indeed compile-time constants and optimize them out. Sometimes, it will find an intergral value to be a compile-time constant, but fail to perform a bit-wise AND at compile-time. These two macros provide a mechanism to perform these build-time checks, but not break on older compilers where we already know they can't be checked at compile time. For specific details, consult the doc comments for BUILD_BUG_ON_CONST. These macros are used in the generic rbtree code. Signed-off-by: Daniel Santos <daniel.santos@xxxxxxxxx> --- include/linux/bug.h | 35 +++++++++++++++++++++++++++++++++++ 1 files changed, 35 insertions(+), 0 deletions(-) diff --git a/include/linux/bug.h b/include/linux/bug.h index e30f600..8e30337 100644 --- a/include/linux/bug.h +++ b/include/linux/bug.h @@ -129,6 +129,41 @@ struct pt_regs; #define BUILD_BUG_ON_NON_CONST(exp) #endif + +#if GCC_VERSION >= 40200 +/** + * BUILD_BUG_ON_NON_CONST42 - break compile if expression cannot be determined + * to be a compile-time constant (disabled prior to + * gcc 4.2) + * @exp: value to test for compile-time constness + * + * Use this macro instead of BUILD_BUG_ON_NON_CONST when testing struct + * members or dereferenced arrays and pointers. Note that the version checks + * for this macro are not perfect. BUILD_BUG_ON_NON_CONST42 expands to nothing + * prior to gcc-4.2, after which it is the same as BUILD_BUG_ON_NON_CONST. + * However, there are still many checks that will break with this macro (see + * the Gory Details section of BUILD_BUG_ON_NON_CONST for more info). + * + * See also BUILD_BUG_ON_NON_CONST() + */ +# define BUILD_BUG_ON_NON_CONST42(exp) BUILD_BUG_ON_NON_CONST(exp) + +/** + * BUILD_BUG_ON42 - break compile if expression cannot be determined + * (disabled prior to gcc 4.2) + * + * This gcc-version check is necessary due to breakages in testing struct + * members prior to gcc 4.2. + * + * See also BUILD_BUG_ON() + */ +# define BUILD_BUG_ON42(arg) BUILD_BUG_ON(arg) +#else +# define BUILD_BUG_ON_NON_CONST42(exp) +# define BUILD_BUG_ON42(arg) +#endif /* GCC_VERSION >= 40200 */ + + #endif /* __CHECKER__ */ #ifdef CONFIG_GENERIC_BUG -- 1.7.3.4 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
