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

["Robert P. J. Day" <rpjday@mindspring.com>]

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.

rday

-- 
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Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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