Re: GCOV_PROFILE_ALL breaks BUILD_BUG_ON(!is_power_of_2(8))

On Fri, Aug 14, 2015 at 10:29:04AM +0200, Johannes Berg wrote:

+linux-kernel

quoted

+#define DECLARE_EWMA(name, _factor, _weight)> 	> 	> 	> 	> \
+> 	> struct ewma_##name {> 	> 	> 	> 	> 	> 	> \
+> 	> 	> unsigned long internal;> 	> 	> 	> 	> 	> \
+> 	> };> 	> 	> 	> 	> 	> 	> 	> 	> \
+> 	> static inline void ewma_##name##_init(struct ewma_##name *e)> 	> \
+> 	> {> 	> 	> 	> 	> 	> 	> 	> 	> \
+> 	> 	> BUILD_BUG_ON(!__builtin_constant_p(_factor));> 	> 	> \
+> 	> 	> BUILD_BUG_ON(!__builtin_constant_p(_weight));> 	> 	> \
+> 	> 	> BUILD_BUG_ON(!is_power_of_2(_factor));> 	> 	> 	> \
+> 	> 	> BUILD_BUG_ON(!is_power_of_2(_weight));> 	> 	> 	> \

So this seemed fine to me, but for some reason the compiler is saying
the BUILD_BUG_ON(!is_power_of_2(x)) fails, if and only if (!)
CONFIG_GCOV_PROFILE_ALL is enabled, which seems to boil down to the
compiler option -fprofile-arcs.

I'm going to replace this with just the code itself, i.e.

                /* both must be a power of 2 */
                BUILD_BUG_ON(_factor & (_factor - 1));
                BUILD_BUG_ON(_weight & (_weight - 1));

but should I have expected this?

It might have something to do with the fact that is_power_of_2() being
an inline function, perhaps with this compiler option it translates to
something that can't be used in the context BUILD_BUG_ON() uses it in.

There is a BUILD_BUG_ON_NOT_POWER_OF_2() macro you could use.

                                                        Michal Kubecek