\begin{code}
-{-# OPTIONS_GHC -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
"realToFrac/Float->Double" realToFrac = float2Double
"realToFrac/Double->Float" realToFrac = double2Float
"realToFrac/Double->Double" realToFrac = id :: Double -> Double
-"realToFrac/Int->Double" realToFrac = int2Double
-"realToFrac/Int->Float" realToFrac = int2Float
+"realToFrac/Int->Double" realToFrac = int2Double -- See Note [realToFrac int-to-float]
+"realToFrac/Int->Float" realToFrac = int2Float -- ..ditto
#-}
\end{code}
+Note [realToFrac int-to-float]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Don found that the RULES for realToFrac/Int->Double and simliarly
+Float made a huge difference to some stream-fusion programs. Here's
+an example
+
+ import Data.Array.Vector
+
+ n = 40000000
+
+ main = do
+ let c = replicateU n (2::Double)
+ a = mapU realToFrac (enumFromToU 0 (n-1) ) :: UArr Double
+ print (sumU (zipWithU (*) c a))
+
+Without the RULE we get this loop body:
+
+ case $wtoRational sc_sY4 of ww_aM7 { (# ww1_aM9, ww2_aMa #) ->
+ case $wfromRat ww1_aM9 ww2_aMa of tpl_X1P { D# ipv_sW3 ->
+ Main.$s$wfold
+ (+# sc_sY4 1)
+ (+# wild_X1i 1)
+ (+## sc2_sY6 (*## 2.0 ipv_sW3))
+
+And with the rule:
+
+ Main.$s$wfold
+ (+# sc_sXT 1)
+ (+# wild_X1h 1)
+ (+## sc2_sXV (*## 2.0 (int2Double# sc_sXT)))
+
+The running time of the program goes from 120 seconds to 0.198 seconds
+with the native backend, and 0.143 seconds with the C backend.
+
+A few more details in Trac #2251, and the patch message
+"Add RULES for realToFrac from Int".
+
%*********************************************************
%* *
\subsection{Utils}