[GHC] #4957: Simplifier failes to eliminate redundant boxing/unboxing

[GHC]

#4957: Simplifier failes to eliminate redundant boxing/unboxing
---------------------------------+------------------------------------------
    Reporter:  simonpj           |        Owner:              
        Type:  bug               |       Status:  new         
    Priority:  normal            |    Milestone:              
   Component:  Compiler          |      Version:  7.0.1       
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------
 Consider this
 {{{
 f :: Bool -> Int -> Int
 f b 0 = 0
 f b x = let y = case b of
                  True -> case f b (x-1) of
                             I# v -> I# (v -# 1#)
                  False -> case f b (x-1) of
                             I# v -> I# (v +# 1#)
       in
       case b of
          True -> case y of
                    I# w -> I# (w -# 1#)

          False -> case y of
                    I# w -> I# (w +# 1#)
 }}}
 GHC 6.12 -O generates this:
 {{{
 T4941.$wf =
   \ (w_shC :: GHC.Bool.Bool) (ww_shF :: GHC.Prim.Int#) ->
     case ww_shF of ds_Xhm {
       __DEFAULT ->
         let {
           y_shQ :: GHC.Types.Int
           LclId
           [Str: DmdType m]
           y_shQ =
             case w_shC of _ {
               GHC.Bool.False ->
                 case T4941.$wf GHC.Bool.False (GHC.Prim.-# ds_Xhm 1)
                 of ww1_shJ { __DEFAULT ->
                 GHC.Types.I# (GHC.Prim.+# ww1_shJ 1)
                 };
               GHC.Bool.True ->
                 case T4941.$wf GHC.Bool.True (GHC.Prim.-# ds_Xhm 1)
                 of ww1_shJ { __DEFAULT ->
                 GHC.Types.I# (GHC.Prim.-# ww1_shJ 1)
                 }
             } } in
         case w_shC of _ {
           GHC.Bool.False ->
             case y_shQ of _ { GHC.Types.I# w1_adk -> GHC.Prim.+# w1_adk 1
 };
           GHC.Bool.True ->
             case y_shQ of _ { GHC.Types.I# w1_adj -> GHC.Prim.-# w1_adj 1
 }
         };
       0 -> 0
     }
 }}}
 Here y is used strictly, and boxes its value in every branch, and those
 uses immediately unbox it.  We'd prefer to generate an ''unboxed'' value
 for 'y'.

 This ticket is derived from #4941.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4957>
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