id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3767,SpecConstr for join points,simonpj,,"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 

",bug,new,low,7.6.2,Compiler,6.12.1,,,dterei,Unknown/Multiple,Unknown/Multiple,None/Unknown,,,,,