Re: [GHC] #9630: compile-time performance regression (probably due to Generics)

2 Jun 2017

      #9630: compile-time performance regression (probably due to Generics)
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.9
      Resolution:                    |             Keywords:  deriving-
                                     |  perf, Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9583, #10293,    |  Differential Rev(s):
  #13059, #10818                     |
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 Here's a truly bare-bones, Haskell 98 reproduction:

 {{{#!hs
 module Duh where

 class Duh a where
   duh :: Integer -> a
   noDuh :: a -> Integer

 class GenDuh a where
   gduh :: Integer -> a
   gnoDuh :: a -> Integer

 instance GenDuh () where
   gduh = const ()
   gnoDuh = const 1

 instance GenDuh a => GenDuh (Maybe a) where
   gduh 0 = Nothing
   gduh n = Just (gduh (n - 1))

   gnoDuh Nothing = 0
   gnoDuh (Just x) = 1 + gnoDuh x

 data T = T (Maybe (Maybe ()))

 instance Duh T where
   duh i = T (gduh i)
   noDuh (T m) = gnoDuh m
 }}}

 GHC 8.2.1rc2 with `-O2 -ddump-ds` produces

 {{{
 -- RHS size: {terms: 13, types: 18, coercions: 0, joins: 0/2}
 $cnoDuh_aRo :: T -> Integer
 [LclId]
 $cnoDuh_aRo
   = let {
       $dGenDuh_a2nn :: GenDuh (Maybe ())
       [LclId]
       $dGenDuh_a2nn = Duh.$fGenDuhMaybe @ () Duh.$fGenDuh() } in
     let {
       $dGenDuh_aRs :: GenDuh (Maybe (Maybe ()))
       [LclId]
       $dGenDuh_aRs = Duh.$fGenDuhMaybe @ (Maybe ()) $dGenDuh_a2nn } in
     \ (ds_d2ow :: T) ->
       case ds_d2ow of { T m_azN ->
       gnoDuh @ (Maybe (Maybe ())) $dGenDuh_aRs m_azN
       }

 -- RHS size: {terms: 11, types: 14, coercions: 0, joins: 0/2}
 $cduh_aRi :: Integer -> T
 [LclId]
 $cduh_aRi
   = let {
       $dGenDuh_a2nl :: GenDuh (Maybe ())
       [LclId]
       $dGenDuh_a2nl = Duh.$fGenDuhMaybe @ () Duh.$fGenDuh() } in
     let {
       $dGenDuh_aRm :: GenDuh (Maybe (Maybe ()))
       [LclId]
       $dGenDuh_aRm = Duh.$fGenDuhMaybe @ (Maybe ()) $dGenDuh_a2nl } in
     \ (i_azM :: Integer) ->
       Duh.T (gduh @ (Maybe (Maybe ())) $dGenDuh_aRm i_azM)
 }}}

 We solve the `GenDuh (Maybe (Maybe ()))` constraint twice, and build its
 dictionary twice. I'll attach `-ddump-tc-trace`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9630#comment:56
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