[GHC] #1600: Optimisation: CPR the results of IO

[GHC]

#1600: Optimisation: CPR the results of IO
-------------------------------------+-------------------------------------
        Reporter:  simonmar          |            Owner:
            Type:  task              |           Status:  new
        Priority:  lowest            |        Milestone:  7.6.2
       Component:  Compiler          |          Version:  6.6.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  Runtime           |       Difficulty:  Moderate (less
  performance bug                    |  than a day)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+-------------------------------------

Comment (by nomeata):

 Maybe this code is a slightly better example for something where we want
 to avoid the allocations:
 {{{
 facIO :: Int -> IO Int
 facIO n = if n < 2 then return 1 else do n' <- facIO (n-1); return $! n*n'
 }}}
 The worker Core currently (7.6.3) looks as
 {{{
 FactIO.$wa [Occ=LoopBreaker]
   :: GHC.Prim.Int#
      -> GHC.Prim.State# GHC.Prim.RealWorld
      -> (# GHC.Prim.State# GHC.Prim.RealWorld, GHC.Types.Int #)
 [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType LL]
 FactIO.$wa =
   \ (ww_snv :: GHC.Prim.Int#)
     (w_snx :: GHC.Prim.State# GHC.Prim.RealWorld) ->
     case GHC.Prim.<# ww_snv 2 of _ {
       GHC.Types.False ->
         case FactIO.$wa (GHC.Prim.-# ww_snv 1) w_snx
         of _ { (# ipv_amL, ipv1_amM #) ->
         case ipv1_amM of _ { GHC.Types.I# y_amd ->
         (# ipv_amL, GHC.Types.I# (GHC.Prim.*# ww_snv y_amd) #)
         }
         };
       GHC.Types.True -> (# w_snx, lvl_roe #)
     }
 }}}
 and there is a very obvious packing/unpacking going on.

 A nested CPR should probably first be able to fix this case; and then
 venture into detecting cheap-and-total-code (as required for the variant
 without `$!`) beyond constructors.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1600#comment:18>
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