linux-kernel - Re: [PATCH] Add BUILD_BUG_ON_NOT_POWER_OF

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Date:	Wed, 06 Jan 2010 23:51:53 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	Bart Van Assche <bvanassche@....org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, David Dillow <dave@...dillows.org>
Subject: Re: [PATCH] Add BUILD_BUG_ON_NOT_POWER_OF_2()


 > Can you please change the above into the following, such that the
 > macro does what its name suggests:

Argh!  Good catch, thanks -- I even tested my patch, but with my brain
in reverse so I sent the bad patch.

Andrew, please replace with this one, which includes your update to the
comment as well:

===

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>
---
 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..328bca6 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 a constant 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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