Re: [Haskell-cafe] forLoop + strict State monad is much faster than foldl'

30 Apr 2014

      On Wednesday 30 April 2014, 13:42:36, Joachim Breitner wrote:
...
Hi,
Am Dienstag, den 29.04.2014, 18:16 -0700 schrieb John Lato:
...
So this is interesting.  The forLoop code gets compiled into a nice
loop in core over unboxed Ints.  The foldl' function OTOH still goes
via a list.  I expect it's not foldl' itself that's slow, rather that
it doesn't fuse with the list producers in current GHCs.  This may be
improved in the future.  Especially as the vectorized foldl' does
fuse.
This should have improved in GHC 7.9, where I have fixed
https://ghc.haskell.org/trac/ghc/ticket/7994 (making foldl a good
consumer).
Confirmed, foldl' (+) 0 [1 .. 5000000 :: Int] gets the very nice core

Rec {
Main.$wgo [Occ=LoopBreaker]
  :: GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int#
[GblId, Arity=2, Caf=NoCafRefs, Str=DmdType ]
Main.$wgo =
  \ (w :: GHC.Prim.Int#) (ww :: GHC.Prim.Int#) ->
    case w of wild {
      __DEFAULT -> Main.$wgo (GHC.Prim.+# wild 1) (GHC.Prim.+# ww wild);
      5000000 -> GHC.Prim.+# ww 5000000
    }
end Rec }

with 7.9.20140425. The core for Integer is naturally not quite as nice, but 
still the list is eliminated.

Thanks, Joachim.

Cheers,
Daniel

Re: [Haskell-cafe] forLoop + strict State monad is much faster than foldl'

Daniel Fischer