From: Jakub Jelinek <jakub@xxxxxxxxxx> Subject: ilog2: improve ilog2 for constant arguments As discussed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445 the const_ilog2 macro generates a lot of code which interferes badly with GCC inlining heuristics, until it can be proven that the ilog2 argument can or can't be simplified into a constant. It can be expressed using __builtin_clzll builtin which is supported by GCC 3.4 and later and when used only in the __builtin_constant_p guarded code it ought to always fold back to a constant. Other compilers support the same builtin for many years too. Other option would be to change the const_ilog2 macro, though as the description says it is meant to be used also in C constant expressions, and while GCC will fold it to constant with constant argument even in those, perhaps it is better to avoid using extensions in that case. [akpm@xxxxxxxxxxxxxxxxxxxx: coding style fixes] Link: https://lkml.kernel.org/r/20201120125154.GB3040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Link: https://lkml.kernel.org/r/20201021132718.GB2176@tucnak Signed-off-by: Jakub Jelinek <jakub@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/log2.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/include/linux/log2.h~ilog2-improve-ilog2-for-constant-arguments +++ a/include/linux/log2.h @@ -156,7 +156,8 @@ unsigned long __rounddown_pow_of_two(uns #define ilog2(n) \ ( \ __builtin_constant_p(n) ? \ - const_ilog2(n) : \ + ((n) < 2 ? 0 : \ + 63 - __builtin_clzll(n)) : \ (sizeof(n) <= 4) ? \ __ilog2_u32(n) : \ __ilog2_u64(n) \ _
