[GHC] #13744: Compile-time regression in 8.2 when compiling bloodhound's test suite

22 May 2017


      #13744: Compile-time regression in 8.2 when compiling bloodhound's test suite
-------------------------------------+-------------------------------------
           Reporter:  refold         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           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:
-------------------------------------+-------------------------------------
 GHC 8.2 takes much longer to compile bloodhound's test suite than GHC
 8.0.2. With 8.0.2, compilation takes approximately a minute on my machine,
 8.2 takes approximately four minutes and uses much more memory.

 How to reproduce:

 {{{
 $ git clone -b ghc-8.2 https://github.com/23Skidoo/bloodhound.git
 $ cd bloodhound
 $ cabal new-build -w ghc-8.0.1 --enable-tests
 $ cabal new-build -w ghc-8.2.1 --enable-tests
 }}}


 GHC version:

 {{{
 $ ghc-8.2.1 --version
 The Glorious Glasgow Haskell Compilation System, version 8.2.0.20170505
 }}}

 These warnings (fixed in my `ghc-8.2` branch of `bloodhound`) may provide
 a clue to what's happening:

 {{{
 tests/V1/tests.hs:1876:17: warning: [-Wsimplifiable-class-constraints]
     • The constraint ‘SOP.All SOP.SListI (SOP.GCode a)’
         matches an instance declaration
       instance forall k (f :: k -> Constraint) (xs :: [k]).
                (Generics.SOP.Constraint.AllF f xs, SOP.SListI xs) =>
                SOP.All f xs
         -- Defined in ‘Generics.SOP.Constraint’
       This makes type inference for inner bindings fragile;
         either use MonoLocalBinds, or simplify it using the instance
     • In the type signature:
         sopArbitrary :: forall a.
                         (Generic a,
                          SOP.GTo a,
                          SOP.All SOP.SListI (SOP.GCode a),
                          SOP.All2 Arbitrary (SOP.GCode a)) =>
                         Gen a
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13744
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