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From: Roland Dreier <rdreier@cisco.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, Bart Van Assche <bart.vanassche@gmail.com>,
       David Dillow <dave@thedillows.org>
Subject: [PATCH] Add BUILD_BUG_ON_NOT_POWER_OF_2()
References: <ada3a2jugqu.fsf@roland-alpha.cisco.com>
	<20100106123306.ac85e557.akpm@linux-foundation.org>
	<aday6kbt14a.fsf@roland-alpha.cisco.com>
	<20100106134232.0025e818.akpm@linux-foundation.org>
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Date: Wed, 06 Jan 2010 15:02:40 -0800
In-Reply-To: <20100106134232.0025e818.akpm@linux-foundation.org> (Andrew
	Morton's message of "Wed, 6 Jan 2010 13:42:32 -0800")
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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@cisco.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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