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

[virtuoso@slind.org (Alexander Shishkin)]

On Sun, Aug 21, 2010 at 10:44:28 +0300, Alexey Dobriyan wrote:
> 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.

I don't get it, do you suggest to call it BUILD_BUG_ON_SMART_PANTS() and have
people look it up every time to see what it does and do a git-blame to see
who's sense of taste interferes with coding or do you suggest to paste
the "BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))" every time so that
people learn how to write C?

Regards,
--
Alex