Search Linux Wireless

Re: sparse using insane amounts of memory

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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

[Index of Archives]     [Linux Host AP]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Linux Kernel]     [IDE]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux ATA RAID]     [Samba]     [Device Mapper]
  Powered by Linux