On 3/9/07, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
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)))
There is already an inline for this in log2.h
/*
* Determine whether some value is a power of two, where zero is
* *not* considered a power of two.
*/
static inline __attribute__((const))
bool is_power_of_2(unsigned long n)
{
return (n != 0 && ((n & (n - 1)) == 0));
}
- 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.
I guess the person who wrote it wasn't thinking discrete maths at the time.
Darren J.
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