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

[adobriyan@gmail.com (Alexey Dobriyan)]

On Sat, Aug 21, 2010 at 02:40:15PM -0400, Mathieu Desnoyers wrote:
> * Alexey Dobriyan (adobriyan at gmail.com) wrote:
> > On Sat, Aug 21, 2010 at 10:17:51AM -0400, Mathieu Desnoyers 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))
> > 
> > Sorry, this is tasteless macro.
> 
> Let's look at the surrounding where I added this macro in kernel.h:
> 
> 
> /* Force a compilation error if condition is true */
> #define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
> 
> /* 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
>    aren't permitted). */
> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
> 
> 
> So I am guessing that you plan to rewrite all of these ?

Of course not.

> Or perharps you have other suggestions ?

Learn that n & (n - 1) is idiomatic way to check for power of two in C.
Done that, encoding that information in identifier won't make sense.