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 |
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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 so overly 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
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