kernel.h: add BUILD_BUG_ON_NOT_POWER_OF_2()

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@cisco.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: David Dillow <dave@thedillows.org>
Cc: "Robert P. J. Day" <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fc9f5a..328bca6 100644 (file)
--- 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