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

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