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@linux-foundation.org: coding style fixes]

Link: https://lkml.kernel.org/r/20201120125154.GB3040@hirez.programming.kicks-ass.net
Link: https://lkml.kernel.org/r/20201021132718.GB2176@tucnak
Signed-off-by: Jakub Jelinek <jakub@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/log2.h

@@ -156,7 +156,8 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 #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)			\