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" <[email protected]> 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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