[GHC] #5000: Eliminate absent arguments in non-strict positions

[GHC]

#5000: Eliminate absent arguments in non-strict positions
---------------------------------+------------------------------------------
    Reporter:  batterseapower    |       Owner:                         
        Type:  feature request   |      Status:  new                    
    Priority:  normal            |   Component:  Compiler               
     Version:  7.0.2             |    Keywords:                         
    Testcase:                    |   Blockedby:                         
          Os:  Unknown/Multiple  |    Blocking:                         
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
---------------------------------+------------------------------------------
Description changed by batterseapower:

Old description:

> One of my programs has this is the output of GHC -O2:
>
> {{{
> Rec {
> h28_s14N
>   :: forall t_aMK t_aML.
>      Main.Nat -> t_aMK -> t_aML -> Main.Nat -> Main.Nat
> [LclId,
>  Arity=4,
>  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=4, Value=True,
>          ConLike=True, Cheap=True, Expandable=True,
>          Guidance=IF_ARGS [0 0 0 0] 6 2}]
> h28_s14N =
>   \ (@ t_aMK)
>     (@ t_aML)
>     (az94z94z941_u439_amh :: Main.Nat)
>     (awS1_u462_ami :: t_aMK)
>     (awS1_u466_amj :: t_aML)
>     (x_u590_amk :: Main.Nat) ->
>     Main.S
>       (h29_s14O
>          @ t_aMK
>          @ t_aML
>          az94z94z941_u439_amh
>          awS1_u462_ami
>          awS1_u466_amj
>          x_u590_amk)
>
> h29_s14O [Occ=LoopBreaker]
>   :: forall t_aMK t_aML.
>      Main.Nat -> t_aMK -> t_aML -> Main.Nat -> Main.Nat
> [LclId,
>  Arity=4,
>  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=4, Value=True,
>          ConLike=True, Cheap=True, Expandable=True,
>          Guidance=IF_ARGS [0 0 0 5] 9 0}]
> h29_s14O =
>   \ (@ t_aMK)
>     (@ t_aML)
>     (az94z94z941_u439_amm :: Main.Nat)
>     (awS1_u462_amn :: t_aMK)
>     (awS1_u466_amo :: t_aML)
>     (x_u590_amp :: Main.Nat) ->
>     case x_u590_amp of _ {
>       Main.Z -> h21'_al8 @ Main.Nat az94z94z941_u439_amm;
>       Main.S x_u595_amq ->
>         h28_s14N
>           @ t_aMK
>           @ t_aML
>           az94z94z941_u439_amm
>           awS1_u462_amn
>           awS1_u466_amo
>           x_u595_amq
>     }
> end Rec }
> }}}
>
> This is ridiculous because the two awS1 arguments are totally dead. This
> greatly increases allocations made by the program.
>
> I speculate that this occurs because the recursive call to h29 is guarded
> by a use of the S constructor, making it non-strict.

New description:

 One of my programs has this is the output of the post-worker/wrapper
 simplification:

 {{{
 Rec {
 h28_s14N
   :: forall t_aMK t_aML.
      Main.Nat -> t_aMK -> t_aML -> Main.Nat -> Main.Nat
 [LclId,
  Arity=4,
  Str=DmdType LLLL,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=4, Value=True,
          ConLike=True, Cheap=True, Expandable=True,
          Guidance=IF_ARGS [0 0 0 0] 6 2}]
 h28_s14N =
   \ (@ t_aMK)
     (@ t_aML)
     (az94z94z941_u439_amh :: Main.Nat)
     (awS1_u462_ami :: t_aMK)
     (awS1_u466_amj :: t_aML)
     (x_u590_amk :: Main.Nat) ->
     Main.S
       (h29_s14O
          @ t_aMK
          @ t_aML
          az94z94z941_u439_amh
          awS1_u462_ami
          awS1_u466_amj
          x_u590_amk)

 h29_s14O [Occ=LoopBreaker]
   :: forall t_aMK t_aML.
      Main.Nat -> t_aMK -> t_aML -> Main.Nat -> Main.Nat
 [LclId,
  Arity=4,
  Str=DmdType LLLS,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=4, Value=True,
          ConLike=True, Cheap=True, Expandable=True,
          Guidance=IF_ARGS [0 0 0 3] 7 0}]
 h29_s14O =
   \ (@ t_aMK)
     (@ t_aML)
     (az94z94z941_u439_amm :: Main.Nat)
     (awS1_u462_amn :: t_aMK)
     (awS1_u466_amo :: t_aML)
     (x_u590_amp :: Main.Nat) ->
     case x_u590_amp of _ {
       Main.Z -> az94z94z941_u439_amm;
       Main.S x_u595_amq [Dmd=Just L] ->
         h28_s14N
           @ t_aMK
           @ t_aML
           az94z94z941_u439_amm
           awS1_u462_amn
           awS1_u466_amo
           x_u595_amq
     }
 end Rec }
 }}}

 This is ridiculous because the two awS1 arguments are totally dead. This
 greatly increases allocations made by the program.

 These absent arguments should be removed by the strictness analyser,
 However I speculate that this occurs because the recursive call to h29 is
 guarded by a use of the S constructor, making it non-strict.

--

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5000#comment:1>
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