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

Mathieu Desnoyers
2010-08-21T18:40:21Z

* Alexey Dobriyan (adobriyan@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 ? Or perharps you have
other suggestions ? Commit cc8ef6eb21e964b1c5eb97b2d0e8ac9893e1bf86 introduced
"BUILD_BUG_ON_NOT_POWER_OF_2()" btw.