On Thu, 8 Mar 2007, Linus Torvalds wrote:
>
> To check a value for being a nice range of consecutive bits, you can
> simply do:
>
> #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:
- I might have made a mistake
- 0 is special, and is generally considered to be a power of two (and
this is more fundamental than you'd think: it's not just fall-out from
the particular expression chosen, it is fundamentally *required* to
handle overflow, and you can think of 0 as 2**x, x > wordsize if that
makes you more comfortable with the notion that zero is a power-of-two
in any finite representation of 2's complement)
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.
But the "I might have made a mistake" part is worth just remembering, and
just double-checking it all.
Linus
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