[GHC] #14899: Significant compilation time regression between 8.4 and HEAD due to coverage checking

[GHC]

#14899: Significant compilation time regression between 8.4 and HEAD due to
coverage checking
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Curiously, data family instances seem to play a role in this. If I replace
 the data family formulation of `Sing` with a normal datatype:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 class SEq k where
   (%==) :: forall (a :: k) (b :: k). Sing a -> Sing b -> ()
   infix 4 %==

 data Foo a b c d
   = A a b c d |
     B a b c d |
     C a b c d |
     D a b c d |
     E a b c d |
     F a b c d

 data Sing (z_awDE :: k) where
     SA :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('A a b c d)
     SB :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('B a b c d)
     SC :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('C a b c d)
     SD :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('D a b c d)
     SE :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('E a b c d)
     SF :: Sing a -> Sing b -> Sing c -> Sing d -> Sing ('F a b c d)

 $([d| instance (SEq a, SEq b, SEq c, SEq d) => SEq (Foo a b c d) where
         (%==) (SA _ _ _ _) (SA _ _ _ _) = ()
         (%==) (SA _ _ _ _) (SB _ _ _ _) = ()
         (%==) (SA _ _ _ _) (SC _ _ _ _) = ()
         (%==) (SA _ _ _ _) (SD _ _ _ _) = ()
         (%==) (SA _ _ _ _) (SE _ _ _ _) = ()
         (%==) (SA _ _ _ _) (SF _ _ _ _) = ()
         (%==) (SB _ _ _ _) (SA _ _ _ _) = ()
         (%==) (SB _ _ _ _) (SB _ _ _ _) = ()
         (%==) (SB _ _ _ _) (SC _ _ _ _) = ()
         (%==) (SB _ _ _ _) (SD _ _ _ _) = ()
         (%==) (SB _ _ _ _) (SE _ _ _ _) = ()
         (%==) (SB _ _ _ _) (SF _ _ _ _) = ()
         (%==) (SC _ _ _ _) (SA _ _ _ _) = ()
         (%==) (SC _ _ _ _) (SB _ _ _ _) = ()
         (%==) (SC _ _ _ _) (SC _ _ _ _) = ()
         (%==) (SC _ _ _ _) (SD _ _ _ _) = ()
         (%==) (SC _ _ _ _) (SE _ _ _ _) = ()
         (%==) (SC _ _ _ _) (SF _ _ _ _) = ()
         (%==) (SD _ _ _ _) (SA _ _ _ _) = ()
         (%==) (SD _ _ _ _) (SB _ _ _ _) = ()
         (%==) (SD _ _ _ _) (SC _ _ _ _) = ()
         (%==) (SD _ _ _ _) (SD _ _ _ _) = ()
         (%==) (SD _ _ _ _) (SE _ _ _ _) = ()
         (%==) (SD _ _ _ _) (SF _ _ _ _) = ()
         (%==) (SE _ _ _ _) (SA _ _ _ _) = ()
         (%==) (SE _ _ _ _) (SB _ _ _ _) = ()
         (%==) (SE _ _ _ _) (SC _ _ _ _) = ()
         (%==) (SE _ _ _ _) (SD _ _ _ _) = ()
         (%==) (SE _ _ _ _) (SE _ _ _ _) = ()
         (%==) (SE _ _ _ _) (SF _ _ _ _) = ()
         (%==) (SF _ _ _ _) (SA _ _ _ _) = ()
         (%==) (SF _ _ _ _) (SB _ _ _ _) = ()
         (%==) (SF _ _ _ _) (SC _ _ _ _) = ()
         (%==) (SF _ _ _ _) (SD _ _ _ _) = ()
         (%==) (SF _ _ _ _) (SE _ _ _ _) = ()
         (%==) (SF _ _ _ _) (SF _ _ _ _) = () |])
 }}}

 Then the compilation time for GHC HEAD goes back to being the same as in
 8.4.1.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14899#comment:1>
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