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 - To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
