Missing conversion rules for realToFrac causing slowdowns in Int->Double conversions

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

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	dons@galois.com
Operating system	Unknown
Architecture	Unknown