SpecConstr for join points

The attached file Spec.hs has a case (from Roman's fusion code) where SpecConstr is not doing the right thing. Let's look at one of the mutually recursive loops that SpecConstr generates for foo:

lvl_rzY :: GHC.Types.Int
lvl_rzY = GHC.Types.I# 2147483647

lvl1_rA0 :: Data.Either.Either GHC.Types.Int GHC.Types.Int
lvl1_rA0 = Data.Either.Left @ GHC.Types.Int @ GHC.Types.Int lvl_rzY

$s$wgo_szT :: GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int#
$s$wgo_szT =
  \ (sc_sz6 :: GHC.Prim.Int#) (sc1_sz7 :: GHC.Prim.Int#) ->
    let {
      $w$j_syG
        :: GHC.Prim.Int#
           -> Data.Either.Either GHC.Types.Int GHC.Types.Int
           -> GHC.Prim.Int#
      [LclId, Arity=2, Str=DmdType LS]
      $w$j_syG =
        \ (ww_sy6 :: GHC.Prim.Int#)
          (w2_sy8 :: Data.Either.Either GHC.Types.Int GHC.Types.Int) ->
          case GHC.Prim.<=# ww_sy6 0 of _ {
            GHC.Bool.False -> $wgo_syE (GHC.Prim.+# sc_sz6 ww_sy6) w2_sy8;
            GHC.Bool.True -> $wgo_syE sc_sz6 w2_sy8
          } } in
    case GHC.Prim.># sc1_sz7 0 of _ {
      GHC.Bool.False -> $s$wgo1_szS sc_sz6 ipv_swo;
      GHC.Bool.True ->
        case sc1_sz7 of wild1_awb {
          __DEFAULT ->
            case GHC.Prim.remInt# wild1_awb 2 of _ {
              __DEFAULT -> $s$wgo_szT sc_sz6 (GHC.Prim.-# wild1_awb 1);
              0 ->
                case w1_syr of _ { GHC.Types.I# ww_sy6 ->
                $w$j_syG
                  ww_sy6
                  (Data.Either.Left
                     @ GHC.Types.Int
                     @ GHC.Types.Int
                     (GHC.Types.I# (GHC.Prim.-# wild1_awb 1)))
                }
            };
          (-2147483648) ->
            case w1_syr of _ { GHC.Types.I# ww_sy6 ->
            $w$j_syG ww_sy6 lvl1_rA0
            }
        }
    };

Note that the join point has an argument of type (Either Int Int) but it is always called with (Left (I# <n>)). This means that the recursive call in the join point is always of the form ($wgo_syE <m> (Left (I# <n>))) and we have a specialisation for that. However the join point itself doesn't scrutinse its argument, and that means that GHC ignores the potential specialisation.

Simon

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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