On Mon, Apr 16, 2018 at 10:54:26AM +0200, Philipp Klocke wrote: > Make sched_feat(x) return 1 or 0 instead of 2**x or 0 when > sysctl_sched_features is constant, by changing the left shift of the > mask-bit to a right shift of the bitmap. The behaviour of the code > remains unchanged. > We prove this by showing that the compiler can evaluate this shift > during compile time, which results in generating the same > machine code as before. > > 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)) > ~^~~~~~~~~~~~~~~~~~~~~~~~~~ > > This resolves the warning, leaves the code base largely as is. > > Tested with gcc 7.3.1 and clang 6.0.0 on x86_64, comparing resulting > binaries with diff. > Applicable to all modern compilers and architectures > > Signed-off-by: Philipp Klocke <Phil_K97@xxxxxx> > Suggested-by: Lukas Bulwahn <lukas.bulwahn@xxxxxxxxx> Reviewed-by: Nicholas Mc Guire <der.herr@xxxxxxx> > --- > kernel/sched/sched.h | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index fb5fc458547f..d2aedee6ab0f 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -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 > The changed sched_feat(y) line is fine but I do not get/like the of the ifdef - keeping the change minimal is ok if there is a relevant impact but here there is no effective difference (you write the object code is the same for the !CONFIG_SCHED_DEBUG case) So I think the ifdef should be kicked here and the proposed change seems fine to me. > #endif /* SCHED_DEBUG && HAVE_JUMP_LABEL */ > > -- > 2.17.0 > _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
