#7994: Make foldl into a good consumer -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by nomeata): I gave it a shot, analysing the body and the rhs of a let in alternation until a fixed point is found with regard to * demand on the rhs * rhs’s demand type * body’s demand type and as expected, the resulting core looks good: {{{ $wgo_s1oi [Occ=LoopBreaker] :: GHC.Prim.Int# -> GHC.Prim.Double# -> GHC.Prim.Double# -> Data.Complex.Complex GHC.Types.Double [LclId, Arity=3, Str=DmdType <S,U><L,U><L,U>] $wgo_s1oi = \ (w_s1o3 :: GHC.Prim.Int#) (ww1_s1oa :: GHC.Prim.Double#) (ww2_s1of :: GHC.Prim.Double#) -> case Foo.f (GHC.Types.I# w_s1o3) of _ [Occ=Dead] { Data.Complex.:+ ww4_s1nS ww5_s1nX -> case ww4_s1nS of _ [Occ=Dead] { GHC.Types.D# ww7_s1re -> case ww5_s1nX of _ [Occ=Dead] { GHC.Types.D# ww9_s1rg -> case GHC.Prim.tagToEnum# @ GHC.Types.Bool (GHC.Prim.==# w_s1o3 ww_s1om) of _ [Occ=Dead] { GHC.Types.False -> $wgo_s1oi (GHC.Prim.+# w_s1o3 1) (GHC.Prim.+## ww1_s1oa ww7_s1re) (GHC.Prim.+## ww2_s1of ww9_s1rg); GHC.Types.True -> Data.Complex.:+ @ GHC.Types.Double (GHC.Types.D# (GHC.Prim.+## ww1_s1oa ww7_s1re)) (GHC.Types.D# (GHC.Prim.+## ww2_s1of ww9_s1rg)) } } } }; } in $wgo_s1oi 1 0.0 0.0; }}} But also as expected, for other code, with more nested lets, compilation time becomes unacceptably large. Has anyone considered doing the fixed-pointing not locally for every let, but across the whole top-level-binding? I.e. initialize all demands and strictness signatures with their strongest possible value (`absDmd` and `botDmdType`), and then analyse the whole expression in repeated passes over all of it, until nothing changes any more. (Or, a variant: Do that initialisation, but also do local fixed-pointing, starting from the stored values, so that when an inner let is re-analysed, but nothing has changed, then there is only one iteration.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7994#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler