Re: hrm...

26 Jan 2007

      john:
...
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.
looks suspicous!

Ideally I'd want something like this produced:

    fhb_ideal :: Word -> Word
    fhb_ideal (W# w) =
        W# ((int2Word# (case word2Int# (int2Word# 0xFF00FF00# `and#` w) of 0# -> 0#; _ -> 1#))
            `or#`
            (int2Word# (case word2Int# (int2Word# 0xFFFF0000# `and#` w) of 0# -> 0#; _ -> 2#)))

which generates the core:

    M.$wfhb_ideal =
      \ (ww_sP7 :: GHC.Prim.Word#) ->
        GHC.Prim.or#
          (GHC.Prim.int2Word#
             (case GHC.Prim.word2Int# (GHC.Prim.and# __word 4278255360 ww_sP7) of ds_dMF {
                __DEFAULT -> 1; 0 -> 0
              }))
          (GHC.Prim.int2Word#
             (case GHC.Prim.word2Int# (GHC.Prim.and# __word 4294901760 ww_sP7) of ds_dMI {
                __DEFAULT -> 2; 0 -> 0

              }))

Whereas the test example:

    fhb_boxed :: Word -> Word
    fhb_boxed w = b1 .|. b2  where
        b2 = if 0xFFFF0000 .&. w /= 0 then 0x2 else 0
        b1 = if 0xFF00FF00 .&. w /= 0 then 0x1 else 0

Turns into some nasty:

    M.lit =
      case GHC.Prim.addIntC# 2147418113 2147483647
      of wild2_aN5 { (# r_aN7, c_aN8 #) ->
      case case c_aN8 of wild3_aN9 {
             __DEFAULT ->
               case GHC.Prim.int2Integer# 2147418113 of wild4_aNa { (# s_aNc, d_aNd #) ->
               case GHC.Prim.int2Integer# 2147483647 of wild5_aNe { (# s1_aNg, d1_aNh #) ->
               case GHC.Prim.plusInteger# s_aNc d_aNd s1_aNg d1_aNh
               of wild_aO8 { (# s2_aOa, d2_aOb #) ->
               GHC.Prim.integer2Word# s2_aOa d2_aOb
               }
               }
               };
             0 -> GHC.Prim.int2Word# r_aN7
           }
      of ww_aNW { __DEFAULT ->
      GHC.Word.W# ww_aNW
      }
      }

    M.lit1 :: GHC.Word.Word
    [GlobalId]
    [Str: DmdType]
    M.lit1 =
      case GHC.Prim.addIntC# 2130771713 2147483647
      of wild2_aN5 { (# r_aN7, c_aN8 #) ->
      case case c_aN8 of wild3_aN9 {
             __DEFAULT ->
               case GHC.Prim.int2Integer# 2130771713 of wild4_aNa { (# s_aNc, d_aNd #) ->
               case GHC.Prim.int2Integer# 2147483647 of wild5_aNe { (# s1_aNg, d1_aNh #) ->
               case GHC.Prim.plusInteger# s_aNc d_aNd s1_aNg d1_aNh
               of wild_aO8 { (# s2_aOa, d2_aOb #) ->
               GHC.Prim.integer2Word# s2_aOa d2_aOb
               }
               }
               };
             0 -> GHC.Prim.int2Word# r_aN7
           }
      of ww_aNW { __DEFAULT ->
      GHC.Word.W# ww_aNW
      }
      }

    M.$wfhb_boxed :: GHC.Prim.Word# -> GHC.Prim.Word#
    [GlobalId]
    [Arity 1
     Str: DmdType L]
    M.$wfhb_boxed =
      \ (ww_sPh :: GHC.Prim.Word#) ->
        case M.lit1 of wild_aOp { GHC.Word.W# x#_aOr ->
        case GHC.Prim.eqWord# (GHC.Prim.and# x#_aOr ww_sPh) __word 0 of wild2_aOk {
          GHC.Base.False ->
            case M.lit of wild1_XPt { GHC.Word.W# x#1_XPx ->
            case GHC.Prim.eqWord# (GHC.Prim.and# x#1_XPx ww_sPh) __word 0 of wild21_XOP {
              GHC.Base.False -> __word 3; GHC.Base.True -> __word 1
            }
            };
          GHC.Base.True ->
            case M.lit of wild1_XPt { GHC.Word.W# x#1_XPx ->
            case GHC.Prim.eqWord# (GHC.Prim.and# x#1_XPx ww_sPh) __word 0 of wild21_XOP {
              GHC.Base.False -> __word 2; GHC.Base.True -> __word 0
            }
            }
        }
        }

So not sure where those Integer thingies are creeping in.

Here's a little test case, btw, with a QuickCheck property.

-- Don

Re: hrm...

dons＠cse.unsw.edu.au