A very common use of __builtin_constant_p is to make sure that a certain value is a compile time constant and generate a build-time error if it is not. However, __builtin_constant_p is broken in a variety of ways in various situations (on various versions of gcc) and never returns one in an unoptimized build. This macro provide a mechanism to perform these build-time checks, but not break unoptimized builds (or modules being build with -O0), of which there probably aren't many people that care anyway. This patch documents all of the relevant quirks I could find in the "Gory Details" section of the doc-comments. For almost all cases, BUILD_BUG_ON_NON_CONST() should never fail on a primitive, non-pointer type variable declared const. A subsequent patch provides a separate macro for performing tests which are known to be broken in older compilers (pretty much, using __builtin_constant_p on arrays, pointers & structs as well as testing those values). Signed-off-by: Daniel Santos <daniel.santos@xxxxxxxxx> --- include/linux/bug.h | 48 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 48 insertions(+), 0 deletions(-) diff --git a/include/linux/bug.h b/include/linux/bug.h index c70b833..e30f600 100644 --- a/include/linux/bug.h +++ b/include/linux/bug.h @@ -81,6 +81,54 @@ struct pt_regs; __build_bug_failed(); \ } while (0) +/** + * BUILD_BUG_ON_NON_CONST - break compile if expression cannot be determined + * to be a compile-time constant. + * @exp: value to test for compile-time constness + * + * __builtin_constant_p() is a work in progress and is broken in various ways + * on various versions of gcc and optimization levels. It can fail, even when + * gcc otherwise determines that the expression is compile-time constant when + * performing actual optimizations and thus, compile out the value anyway. Do + * not use this macro for struct members or dereferenced pointers and arrays, + * as these are broken in many versions of gcc -- use BUILD_BUG_ON_NON_CONST42 + * or another gcc-version-checked macro instead. + * + * As long as you are passing a variable declared const (and not modified), + * this macro should never fail (except for floats). For information on gcc's + * behavior in other cases, see below. + * + * Gory Details: + * + * Normal primitive variables + * - global non-static non-const values are never compile-time constants (but + * you should already know that) + * - all const values (global/local, non/static) should never fail this test + * (3.4+) with one exception (below) + * - floats (which we wont use anyway) are broken in various ways until 4.2 + * (-O1 broken until 4.4) + * - local static non-const broken until 4.2 (-O1 broken until 4.3) + * - local non-static non-const broken until 4.0 + * + * Dereferencing pointers & arrays + * - all static const derefs broken until 4.4 (except arrays at -O2 or better, + * which are fixed in 4.2) + * - global non-static const pointer derefs always fail (<=4.7) + * - local non-static const derefs broken until 4.3, except for array derefs + * to a zero value, which works from 4.0+ + * - local static non-const pointers always fail (<=4.7) + * - local static non-const arrays broken until 4.4 + * - local non-static non-const arrays broken until 4.0 (unless zero deref, + * works in 3.4+) + + */ +#ifdef __OPTIMIZE__ +#define BUILD_BUG_ON_NON_CONST(exp) \ + BUILD_BUG_ON(!__builtin_constant_p(exp)) +#else +#define BUILD_BUG_ON_NON_CONST(exp) +#endif + #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
