Re: hrm...

26 Jan 2007


      On 1/27/07, John Meacham  wrote:
...
so I have this simple bit of code, which should be fast but seems to be
being compiled to something very slow.
...
import Data.Word
import Data.Bits
fhb :: Word -> Word
fhb w = b1 .|. b2  where
    b2 = if 0xFFFF0000 .&. w /= 0 then 0x2 else 0
    b1 = if 0xFF00FF00 .&. w /= 0 then 0x1 else 0
what it compiles to is something involving Integers, lots of coercions
and other nasty stuff when it should consist of a couple of primitive
operations.
Output from an AMD64 box:

$wfhb =
  \ (ww_sIw :: GHC.Prim.Word#) ->
    case GHC.Prim.eqWord# (GHC.Prim.and# __word 4278255360 ww_sIw) __word 0
    of wild2_aHI {
      GHC.Base.False ->
        case GHC.Prim.eqWord# (GHC.Prim.and# __word 4294901760 ww_sIw) __word 0
        of wild21_XHW {
          GHC.Base.False -> __word 3; GHC.Base.True -> __word 1
        };
      GHC.Base.True ->
        case GHC.Prim.eqWord# (GHC.Prim.and# __word 4294901760 ww_sIw) __word 0
        of wild21_XHW {
          GHC.Base.False -> __word 2; GHC.Base.True -> __word 0
        }
    }


-- 
Cheers,
  Lemmih