[PATCH] log2.h: Macro-ize is_power_of_2() for use in BUILD_BUG_ON

[Roland Dreier <rdreier@cisco.com>]

When code relies on a constant being a power of 2:

	#define FOO	512	/* must be a power of 2 */

it would be nice to be able to do:

	BUILD_BUG_ON(!is_power_of_2(FOO));

However applying an inline function does not result in a compile-time
constant that can be used with BUILD_BUG_ON(), so trying that gives
results in:

	error: bit-field '<anonymous>' width not an integer constant

We can fix this by changing is_power_of_2() to a macro; we leave the
inline function for the non-constant case, to avoid evaluating the
parameter more than once.  (gcc does not accept a multi-statement
expression like "({ unsigned long __n = n; ... })" as a compile-time
constant so that solution doesn't work)

Signed-off-by: Roland Dreier <rolandd@cisco.com>
---
This is somewhat of an RFC -- I'm a bit undecided whether it's really
worth making this change.  It's prompted by
http://www.mail-archive.com/linux-rdma@vger.kernel.org/msg01941.html and
http://www.mail-archive.com/linux-rdma@vger.kernel.org/msg01950.html
which do ugly things to work around is_power_of_2 not being usable in
BUILD_BUG_ON().

On the other hand maybe just

/* FOO must be a power of 2 */
#define FOO_SHIFT 9
#define FOO (1 << FOO_SHIFT)

is good enough.

 include/linux/log2.h |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index 25b8086..248f69c 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -49,11 +49,18 @@ int __ilog2_u64(u64 n)
  */
 
 static inline __attribute__((const))
-bool is_power_of_2(unsigned long n)
+bool __is_power_of_2(unsigned long n)
 {
 	return (n != 0 && ((n & (n - 1)) == 0));
 }
 
+#define is_power_of_2(n)					\
+(								\
+	__builtin_constant_p(n) ?				\
+		(((n) != 0) && (((n) & ((n) - 1)) == 0)) :	\
+		__is_power_of_2(n)				\
+)
+
 /*
  * round up to nearest power of two
  */