I know that ghc performs hardly any optimisation if called without -O or -O2.
However, is it necessary that just adding a single function definition to a
program *without* calling this function increases execution time by 50%?
(ghc-2.08)
Example:
{-
fromStepTo :: Int -> Int -> Int -> [Int]
fromStepTo start step end
| start > end = []
| otherwise = start : fromStepTo (start+step) step end
-}
all2 :: (a -> Bool) -> [a] -> Bool
all2 p [] = True
all2 p (x:xs) = p x && all2 p xs
main = putStr (show (all2 even [2,4..2000000]))
If `fromStepTo' is commented out as above I measure:
64,001,320 bytes allocated in the heap
300 bytes maximum residency (0.0%, 6 sample(s))
52 garbage collections performed (6 major, 46 minor)
INIT time 0.01s ( 0.11s elapsed)
MUT time 8.04s ( 8.51s elapsed)
GC time 2.51s ( 2.70s elapsed)
Total time 10.56s ( 11.32s elapsed)
However, deleting `{-' and `-}' results in
64,001,320 bytes allocated in the heap
300 bytes maximum residency (0.0%, 6 sample(s))
52 garbage collections performed (6 major, 46 minor)
INIT time 0.01s ( 0.14s elapsed)
MUT time 12.17s ( 12.49s elapsed)
GC time 2.44s ( 2.68s elapsed)
Total time 14.62s ( 15.31s elapsed)
With -O both versions give the same result:
24,000,852 bytes allocated in the heap
232 bytes maximum residency (0.0%, 5 sample(s))
21 garbage collections performed (5 major, 16 minor)
INIT time 0.02s ( 0.07s elapsed)
MUT time 2.79s ( 2.99s elapsed)
GC time 2.13s ( 2.31s elapsed)
Total time 4.94s ( 5.37s elapsed)
Olaf
--
OLAF CHITIL, Lehrstuhl fuer Informatik II, RWTH Aachen, 52056 Aachen, Germany
Tel: (+49/0)241/80-21212; Fax: (+49/0)241/8888-217
URL: http://www-i2.informatik.rwth-aachen.de/~chitil/
