Re: [GHC] #14594: 2 modules / 2500LOC takes nearly 3 minutes to build

5 Jan 2018

      #14594: 2 modules / 2500LOC takes nearly 3 minutes to build
-------------------------------------+-------------------------------------
        Reporter:  schyler           |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 I've looked in a little more detail. The thing I've found most surprising
 and suspicious is that we end up with absolutely ''enormous'' unfoldings
 for some dictionaries. For `Data (Maybe Int)`, we get this mouthful:

 {{{#!hs
 $s$fDataMaybe_s57H [InlPrag=NOUSERINLINE CONLIKE]
   :: Data (Maybe Int)
 [LclId,
  Unf=DFun: \ ->
        Data.Data.C:Data TYPE: Maybe Int
                         (base-4.11.0.0:Data.Typeable.Internal.mkTrApp
                            @ * @ * @ Maybe @ Int Data.Data.$fDataMaybe5
 Data.Data.$fDataInt4)
                         `cast` (Sym
 (base-4.11.0.0:Data.Typeable.Internal.N:Typeable[0]) <*>_N <Maybe
 Int>_N
                                 :: (Type.Reflection.TypeRep (Maybe Int) ::
 *)
                                    ~R# (Typeable (Maybe Int) ::
 Constraint))
                         \ (@ (c_a5c3 :: * -> *))
                           (k_a5c4 [Occ=Once!]
                              :: forall d b. Data d => c_a5c3 (d -> b) -> d
 -> c_a5c3 b)
                           (z_a5c7 [Occ=Once*!] :: forall g. g -> c_a5c3 g)
                           (ds_a5c9 [Occ=Once!] :: Maybe Int) ->
                           case ds_a5c9 of {
                             Nothing -> z_a5c7 @ (Maybe Int)
 (GHC.Base.Nothing @ Int);
                             Just a1_a5ce [Occ=Once] ->
                               k_a5c4
                                 @ Int
                                 @ (Maybe Int)
                                 Data.Data.$fDataInt
                                 (z_a5c7 @ (Int -> Maybe Int)
 (GHC.Base.Just @ Int))
                                 a1_a5ce
                           }
                         Data.Data.$fDataMaybe_$cgunfold @ Int
 Data.Data.$fDataInt
                         \ (ds_a51o [Occ=Once!] :: Maybe Int) ->
                           case ds_a51o of {
                             Nothing -> Data.Data.$cNothing;
                             Just _ [Occ=Dead] -> Data.Data.$cJust
                           }
                         \ _ [Occ=Dead] -> Data.Data.$tMaybe

 ...
 ...
 }}}

 Why are we inlining method definitions into the dictionary unfolding at
 all? I'd expect us to wait for someone to pluck a method from the
 dictionary unfolding and then consider whether to inline the method.

 Now I don't know if this has anything to do with the problem, but it looks
 weird.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14594#comment:11
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