[PATCH for -tip 1/2] kernel.h: add MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2

[Anca Emanuel]

I think about some test at the last bit if it is on, the number is not
a power of 2

On Sun, Aug 22, 2010 at 10:38 PM, Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
> * Andi Kleen (andi at firstfloor.org) wrote:
>> > +/* Force a compilation error if condition is constant and not a power of 2 */
>> > +#define MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2(n)               \
>> > +   MAYBE_BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))
>>
>> Looks super-ugly. IMHO just writing MAYBE_BUILD_BUG_ON(!n || n & (n - 1)) directly
>> would be clear enough. If you really think that's unclear define a generic
>> is_power_of_two() macro.
>
> There is already a is_power_of_two macro in log2.h, but I fear it might
> incorrectly interact with "MAYBE_BUILD_BUG_ON" (for some reason passing the
> constant result of a static inline is not treated as a constant by the macro,
> and thus it always "passes" the test).
>
> So if everyone object to this new macro, I'd be tempted to just go with your
> suggestion. However the fact that we already have BUILD_BUG_ON_NOT_POWER_OF_2(n)
> made me think that some people prefer to have it done as a macro.
>
> Other opinions ?
>
> Thanks,
>
> Mathieu
>
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
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