On 20.04.2018 09:57, Peter Zijlstra wrote: > On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote: > >> This patch is motivated by the clang warning Wconstant-logical-operand, >> issued when logically comparing a variable to a constant integer that is >> neither 1 nor 0. It happens for sched_feat(x) when sysctl_sched_features >> is constant, i.e., CONFIG_SCHED_DEBUG is not set. >> >> kernel/sched/fair.c:3927:14: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand] >> if (initial && sched_feat(START_DEBIT)) >> ^ ~~~~~~~~~~~~~~~~~~~~~~~ >> kernel/sched/fair.c:3927:14: note: use '&' for a bitwise operation >> if (initial && sched_feat(START_DEBIT)) >> ^~ >> & >> kernel/sched/fair.c:3927:14: note: remove constant to silence this warning >> if (initial && sched_feat(START_DEBIT)) >> ~^~~~~~~~~~~~~~~~~~~~~~~~~~ >> @@ -1305,7 +1305,11 @@ static const_debug __maybe_unused unsigned int sysctl_sched_features = >> 0; >> #undef SCHED_FEAT >> >> +#ifdef CONFIG_SCHED_DEBUG >> #define sched_feat(x) (sysctl_sched_features & (1UL << __SCHED_FEAT_##x)) >> +#else >> +#define sched_feat(x) ((sysctl_sched_features >> __SCHED_FEAT_##x) & 1UL) >> +#endif > So this is extra ugly, for no gain? The gain is stopping a warning that clutters the output log of clang. To improve readability, one can drop the ifdef-structure and just keep the right shift version, like Nicholas suggested. This will have a (very small) impact on performance in CONFIG_SCHED_DEBUG case, but when debugging, performance is no problem anyways. > WTH does clang complain about a constant? Can't you just disable that > stupid warning? There are 2 ways to disable the warning. Either disable it for this particular occurrence, which clutters the code with #pragma's. THIS is really ugly. Or disable it globally and maybe miss some important/helpful warnings. > Also, if sysctl_sched_features is a constant, the both expressions > _should_ really result in a constant and clang should still warn about > it. No, because clang only warns if the constant is neither 1 nor 0. (These being the 'best' integer representations of booleans) > I'm really not seeing why we'd want to do this. Just fix clang to not be > stupid. > _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
