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Message-ID: <adaskaiqrmu.fsf@roland-alpha.cisco.com>
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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