#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler