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

7 Mar 2018

      #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
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternMatchWarnings               |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following program:

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

 data family Sing (z :: k)

 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 instance Sing (z_awDE :: Foo a b c d) 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 _ _ _ _) = () |])
 }}}

 It takes significantly longer to compile this program on 8.4 and HEAD:

 {{{
 $ /opt/ghc/8.4.1/bin/ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.4.1
 $ time /opt/ghc/8.4.1/bin/ghc Bug.hs -fforce-recomp
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 real    0m0.285s
 user    0m0.236s
 sys     0m0.036s
 $ /opt/ghc/head/bin/ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.5.20180306
 $ time /opt/ghc/head/bin/ghc Bug.hs -fforce-recomp
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 real    0m29.684s
 user    0m29.656s
 sys     0m0.060s
 }}}

 The reason for this regression is somewhat incidental—it's due to commit
 ffb2738f86c4e4c3f0eaacf0a95d7326fdd2e383 (`Fix #14838 by marking TH-
 spliced code as FromSource`). Before that commit, we were supressing
 pattern-match coverage checking entirely on TH-quoted code. We no longer
 do this, which means that we coverage-check the TH-quoted instance in that
 program, which appears to be why it takes so long to compile.

 This is a serious issue in practice because a good chunk of
 `singletons`-generated code is of this form, which means that a good
 amount of code is effectively uncompilable on GHC HEAD now. (See, for
 instance, this [https://travis-
 ci.org/goldfirere/singletons/jobs/350483543#L1182 Travis failure] on GHC
 HEAD.)

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14899
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