[Haskell-cafe] Optimizing cellular automata & the beauty of unlifted types

Felipe Lessa felipe.lessa at gmail.com
Fri Dec 21 14:22:30 EST 2007

On Dec 21, 2007 3:00 PM, Justin Bailey <jgbailey at gmail.com> wrote:
> It really did help. I started with an implementation that used Ints,
> and this sped the program up by at least 2x. I think that's because of
> the bit-manipulation I'm doing. For example, Data.Bits defines the
> bitwise and operation on Ints as:
>   (I# x#) .&.   (I# y#)  = I# (word2Int# (int2Word# x# `and#` int2Word# y#))
> Which you can see has 3 conversions in it. The I#
> deconstruction/construction is also suspicious to me, but I don't know
> if there are performance costs there or not. Regardless, using Word#
> directly lets me write (assuming w1# and w2# are already words):
>   w1# `and#` w2#
> Maybe better rewrite rules in the Data.Bits library would eliminate
> unnecessary conversions and this wouldn't be necessary

Wouldn't it be sufficient to use Data.Word and rely on GHC unboxing
capabilities? Compiling

import Data.Bits
import Data.Word

main = do
    a <- getLine
    b <- getLine
    print (read a .&. read b :: Word)

with GHC 6.6.1 gives me

	   (case GHC.Read.read @ GHC.Word.Word GHC.Word.$f40 a_afE
	    of wild1_a1tT { GHC.Word.W# x#_a1tV ->
	    case GHC.Read.read @ GHC.Word.Word GHC.Word.$f40 a87_a1ub
	    of wild11_a1tW { GHC.Word.W# y#_a1tY ->
	    GHC.Word.$w$dmshow1 (GHC.Prim.and# x#_a1tV y#_a1tY)

which of course does the right thing.



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