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Message-ID: <adaeim2supb.fsf_-_@roland-alpha.cisco.com>
Date: Wed, 06 Jan 2010 15:02:40 -0800
From: Roland Dreier <rdreier@...co.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
Bart Van Assche <bart.vanassche@...il.com>,
David Dillow <dave@...dillows.org>
Subject: [PATCH] Add BUILD_BUG_ON_NOT_POWER_OF_2()
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
As suggested by akpm, rather than monkeying around with is_power_of_2()
and risking gcc warts about constant expressions, just create a macro
BUILD_BUG_ON_NOT_POWER_OF_2() to encapsulate this common requirement.
Signed-off-by: Roland Dreier <rolandd@...co.com>
---
> mm.. I think _something_ is worth it. The requirement that a constant
> be a power of two is a very common one in the kernel.
Fair enough... here's your suggestion of BUILD_BUG_ON_NOT_POWER_OF_2().
<linux/kernel.h> does seem to be going the way of a gazillion very
specific helper macros, so I guess this fits right in.
include/linux/kernel.h | 4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fc9f5a..a1b6652 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -734,6 +734,10 @@ struct sysinfo {
/* Force a compilation error if condition is constant and true */
#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
+/* Force a compilation error if expression is not a power of 2 */
+#define BUILD_BUG_ON_NOT_POWER_OF_2(n) \
+ BUILD_BUG_ON(((n) != 0 && (((n) & ((n) - 1)) == 0)))
+
/* Force a compilation error if condition is true, but also produce a
result (of value 0 and type size_t), so the expression can be used
e.g. in a structure initializer (or where-ever else comma expressions
--
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