Re: [PATCH] Add "is_power_of_2" checking to log2.h.

[Nick Piggin <nickpiggin@yahoo.com.au>]

Robert P. J. Day wrote:
> On Tue, 30 Jan 2007, Nick Piggin wrote:
>
>
> > Robert P. J. Day wrote:
> >
> > >  Add the inline function "is_power_of_2()" to log2.h, where the value
> > > zero is *not* considered to be a power of two.
> >
> > > Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
> > >
> > > /*
> > > + *  Determine whether some value is a power of two, where zero is
> > > + * *not* considered a power of two.
> > > + */
> >
> > Why the qualifier? Zero *is* not a power of 2, is it?
>
>
> no, but it bears repeating since some developers might think it *is*.
> if you peruse the current kernel code, you'll find some tests of the
> simpler form:
>
> 	((n & (n - 1)) == 0))
>
> which is clearly testing for "power of twoness" but which will return
> true for a value of zero.  that's wrong, and it's why it's emphasized
> in the comment.

I would have thought you'd comment the broken ones, but that's just me.

--
SUSE Labs, Novell Inc.
Send instant messages to your online friends http://au.messenger.yahoo.com 
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/