Am Samstag, 17. Mai 2008 22:48 schrieb anton muhin:
Why not -O3?
As far as I know - and Brandon S.Allbery said so, too - -O3 isn't better than -O2.
Using a real list of primes,
What's the size of the real list?
arbitrary, however, since it's [Int], it will actually be at most 105097565 primes long, but since only numbers up to 5000 are checked, only 670 primes will ever be considered - When I check numbers 1 to 50000 (5133 primes, so 5134 primes evaluated), with -O0 / -O2, it's Jeroen 1 : 14.5 s / 2.4 s Jeroen 2 : 52.5 s / 49.7 s (== x) . head . dropWhile (< x) : 8.2 s /4.1 s go primes : 5.5 s / 2.5 s So Jeroen 1 is the best to be optimised :)
but another implementation of isPrime:
isPrime x = (== x) $ head $ dropWhile (< x) primes
With -O2, this is about 20% slower than the Jeroen's first version, without optimisations 50% faster. Strange.
Well, head has its overhead as well. Cf. two variants:
firstNotLess :: Int -> [Int] -> Int firstNotLess s (x:xs) = if x < s then firstNotLess s xs else x
dropLess :: Int -> [Int] -> [Int] dropLess s l@(x:xs) = if x < s then dropLess s xs else l
isPrime :: Int -> Bool isPrime x = x == (firstNotLess x primes)
isPrime' :: Int -> Bool isPrime' x = x == (head $ dropLess x primes)
On my box firstNotLess gives numbers pretty close (if not better) than
for primes up to 50000, that's 6.8 s / 2.1 s with -O0 / -O2 respectively on mine
Jeroen's first variant, while head $ dropLess notably worse.
5.8 s / 2.4 s here.
isPrime :: Int -> Bool isPrime x = go primes where go (p:ps) = case compare x p of LT -> False EQ -> True GT -> go ps
does best (on my box), with and without optimisations (very very slightly with -O2) for a list of real primes, but not for [1 .. ].
And what happens for [1..]?
With -O2, Jeroen 1 was best (1.62), nex firstNotLess (1.63), head . dropLess (1.74), then in close succesion Jeroen 2 (1.92), go primes (1.96) and head . dropWhile (1.99), with -O0, it's head . dropWhile (1.7 s, YES, that actually is faster with -O0 than with -O2!), head . dropLess (2.0), Jeroen 2 and firstNotLess (2.1 s), go primes (2.3 s), Jeroen 1 (3.2 s). Weirder and weirder.
However, more than can be squished out of fiddling with these versions could be gained from a better algorithm.
Definitely.
yours, anton.