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Message-ID: <ada3a2jugqu.fsf@roland-alpha.cisco.com>
Date: Wed, 06 Jan 2010 12:21:13 -0800
From: Roland Dreier <rdreier@...co.com>
To: akpm@...ux-foundation.org
Cc: linux-kernel@...r.kernel.org,
Bart Van Assche <bart.vanassche@...il.com>,
David Dillow <dave@...dillows.org>
Subject: [PATCH] log2.h: Macro-ize is_power_of_2() for use in BUILD_BUG_ON
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@...co.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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