Re: sparse using insane amounts of memory

[Ivo van Doorn <ivdoorn-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

> > 	#define is_power_of_two(x) (!((x) & ((x)-1)))
> > 	#define low_bit_mask(x) (((x)-1) & ~(x))
> > 	#define is_contiguous_mask(x) is_power_of_two(1 + (x) + low_bit_mask(x))
> 
> Side note: I didn't check this. So if you actually do this, please 
> double-check. The math should all be good, but there's a few caveats:

Checked, and seems to work. Even sparse appears to be very happy. :)
I'll do some more extensive testing later today before submitting a patch.
 
> The "zero is special" thing means that if you don't want to accept zero as 
> a valid mask (it technically *is* a contiguous set of bits set - it's just 
> the empty set) you'd need to check for it specially.

Good point, I'll add a check for that as well.

> Thus endeth Linus' "games with bits" lecture. It was probably more than 
> you really wanted to know. There's a ton of games you can play with simple 
> "x-1" and bitmasking ops like this).

Thanks, for both the macro's and the lecture. ;)

Ivo