#2251: Missing conversion rules for realToFrac causing slowdowns in Int->Double
conversions
--------------------------------------------+-------------------------------
Reporter: dons | Owner: dons
Type: bug | Status: new
Priority: normal | Component: Runtime System
Version: 6.8.2 | Severity: normal
Keywords: floating point, performance | Testcase:
Architecture: Unknown | Os: Unknown
--------------------------------------------+-------------------------------
GHC.Real and GHC.Float currently have:
{{{
-- | general coercion from integral types
fromIntegral :: (Integral a, Num b) => a -> b
fromIntegral = fromInteger . toInteger
{-# RULES
"fromIntegral/Int->Int" fromIntegral = id :: Int -> Int
#-}
-- | general coercion to fractional types
realToFrac :: (Real a, Fractional b) => a -> b
realToFrac = fromRational . toRational
{-# RULES
"realToFrac/Int->Int" realToFrac = id :: Int -> Int
#-}
}}}
But these avoid some primops that can make these conversions
more efficient.
{{{
{-# RULES
"realToFrac/Int->Double" realToFrac = i2d :: Int -> Double
#-}
int2Double :: Int -> Double
int2Double (I# x) = D# (int2Double# x)
}}}
A rule to use the in2Double# primop over realToFrac directly, improves
the running time of this program:
{{{
import Data.Array.Parallel.Unlifted
main = do
let c = replicateU n (2::Double)
a = mapU realToFrac (enumFromToU 0 (n-1) ) :: UArr Double
print (sumU (zipWithU (*) c a))
}}}
From 113 seconds, to 0.194s! A massive speedup.
We should fill out the rules here with at least those for which GHC
has primitives:
{{{
{-# RULES
"realToFrac/Int->Double" realToFrac = int2Double :: Int -> Double
"realToFrac/Int->Float" realToFrac = int2Float :: Int -> Float
#-}
}}}
to the realToFrac and fromIntegral rules in GHC.Float
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2251>
GHC <http://www.haskell.org/ghc/>
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