#2281: properFraction implemented with modf primitive?
---------------------------------+------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.14.1
Component: libraries/base | Version: 6.8.2
Keywords: | Difficulty: Unknown
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Changes (by igloo):
* owner: igloo =>
* milestone: 6.12.3 => 6.14.1
Comment:
I've confirmed performance is worse with this program:
{{{
{-# LANGUAGE ForeignFunctionInterface #-}
import Foreign
import Foreign.C
main :: IO ()
main = f
count :: Int
count = 100000000
f :: IO ()
f = with 0 $ \dptr ->
do let loop :: Int -> Double -> Double
loop 0 d = d
loop n d = modf (realToFrac d) dptr `seq` loop (n - 1) (d +
0.01)
print $ loop count 100.456
g :: IO ()
g = do let loop :: Int -> Double -> Double
loop 0 d = d
loop n d = case properFraction d :: (Int, Double) of
(_, frac) -> frac `seq` loop (n - 1) (d + 0.01)
print $ loop count 100.456
foreign import ccall "modf" modf :: CDouble -> Ptr CDouble -> CDouble
}}}
{{{
./f 8.22s user 0.00s system 99% cpu 8.231 total
./g 38.35s user 0.03s system 100% cpu 38.382 total
}}}
but I'm a bit nervous about the Integral type used affecting the result:
{{{
properFraction x
= case (decodeFloat x) of { (m,n) ->
let b = floatRadix x in
if n >= 0 then
(fromInteger m * fromInteger b ^ n, 0.0)
else
case (quotRem m (b^(negate n))) of { (w,r) ->
(fromInteger w, encodeFloat r n)
}
}
}}}
I'd suggest the way forward is for someone to make a patch and then turn
this ticket into a [http://www.haskell.org/haskellwiki/Library_submissions
library submission].
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2281#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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