Messages in this thread
 From Kyle Moffett <> Subject Re: [PATCH] Add a rounddown_pow_of_two() macro to log2.h. Date Fri, 26 Jan 2007 10:57:30 -0500
`On Jan 26, 2007, at 02:24:35, Robert P. J. Day wrote:> On Thu, 25 Jan 2007, Andrew Morton wrote:>> On Thu, 25 Jan 2007 04:32:12 -0500 (EST)>> "Robert P. J. Day" <rpjday@mindspring.com> wrote:>>> +/*>>> + * round down to nearest power of two>>> + */>>> +static inline __attribute__((const))>>> +unsigned long __rounddown_pow_of_two(unsigned long n)>>> +{>>> +	return 1UL << (fls_long(n) - 1);>>> +}>>>> So __rounddown_pow_of_two(16) returns 8?>> it does?  but if that was true, so would 17, and 18, and 19 ...  i  > didn't actually test this since it seemed so straightforward.  > doesn't fls_long() return the most significant bit?  oh, wait ...  > reading further ...The way that "ilog2" is defined this should be really  straightforward, I dunno why those functions seem sooverly complicated:   roundup_pow_of_two(x) := 1 << ilog2(2*x - 1)   rounddown_pow_of_two(x) := 1 << ilog2(x)Where ilog2(x) simply returns the first bit set in the word:   ilog2(0) => undefined or -1 or something   ilog2(1) => 0   ilog2(2) => 1   ilog2(3) => 1   ilog2(4) => 2   [...]The results:   roundup_pow_of_two(1) = 1    rounddown_pow_of_two(1) = 1   roundup_pow_of_two(2) = 2    rounddown_pow_of_two(2) = 2   roundup_pow_of_two(3) = 4    rounddown_pow_of_two(3) = 2   roundup_pow_of_two(4) = 4    rounddown_pow_of_two(4) = 4   roundup_pow_of_two(5) = 8    rounddown_pow_of_two(5) = 4   [...]Cheers,Kyle Moffett-To unsubscribe from this list: send the line "unsubscribe linux-kernel" inthe body of a message to majordomo@vger.kernel.orgMore majordomo info at  http://vger.kernel.org/majordomo-info.htmlPlease read the FAQ at  http://www.tux.org/lkml/`

Last update: 2007-01-26 17:01    [from the cache]