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SubjectRe: [PATCH] log2.h: Macro-ize is_power_of_2() for use in BUILD_BUG_ON
On Wed, 06 Jan 2010 12:21:13 -0800
Roland Dreier <rdreier@cisco.com> wrote:

> 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
>
> We can fix this by changing is_power_of_2() to a macro; we leave the
> inline function for the non-constant case, to avoid evaluating the
> parameter more than once. (gcc does not accept a multi-statement
> expression like "({ unsigned long __n = n; ... })" as a compile-time
> constant so that solution doesn't work)
>
> Signed-off-by: Roland Dreier <rolandd@cisco.com>
> ---
> This is somewhat of an RFC -- I'm a bit undecided whether it's really
> worth making this change. It's prompted by
> http://www.mail-archive.com/linux-rdma@vger.kernel.org/msg01941.html and
> http://www.mail-archive.com/linux-rdma@vger.kernel.org/msg01950.html
> which do ugly things to work around is_power_of_2 not being usable in
> BUILD_BUG_ON().
>
> On the other hand maybe just
>
> /* FOO must be a power of 2 */
> #define FOO_SHIFT 9
> #define FOO (1 << FOO_SHIFT)
>
> is good enough.
>
> include/linux/log2.h | 9 ++++++++-
> 1 files changed, 8 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/log2.h b/include/linux/log2.h
> index 25b8086..248f69c 100644
> --- a/include/linux/log2.h
> +++ b/include/linux/log2.h
> @@ -49,11 +49,18 @@ int __ilog2_u64(u64 n)
> */
>
> static inline __attribute__((const))
> -bool is_power_of_2(unsigned long n)
> +bool __is_power_of_2(unsigned long n)
> {
> return (n != 0 && ((n & (n - 1)) == 0));
> }
>
> +#define is_power_of_2(n) \
> +( \
> + __builtin_constant_p(n) ? \
> + (((n) != 0) && (((n) & ((n) - 1)) == 0)) : \
> + __is_power_of_2(n) \
> +)
> +

We've had recurring struggles with various versions of gcc screwing up
constructs of this form and trying to emit the non-constant code when
the arg was clearly a compile-time constant. One episode which comes
to mind was when we made changes to kmalloc().

Of course, that might not bite us in this case - it would need a lot of
coverage testing to find out.


Perhaps we can avoid worrying about that via

#define BUILD_BUG_ON_NOT_POWER_OF_2(n) \
BUILD_BUG_ON((n != 0 && ((n & (n - 1)) == 0)))

?


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