[GHC] #8635: GHC optimisation flag ignored when importing a local module with derived type classes

[GHC]

#8635: GHC optimisation flag ignored when importing a local module with derived
type classes
------------------------------------+-------------------------------------
       Reporter:  NeilMitchell      |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Given Foo.hs and Bar.hs:

 {{{
 module Foo where
 data Food = Food -- deriving Eq

 {-# OPTIONS_GHC -O2 -ddump-simpl #-}
 module Bar where
 import Foo
 bar :: Int -> Bool
 bar x = x == 72
 }}}

 If I run:

 {{{
 ghc --make Bar -fforce-recomp
 }}}

 I get (snipped):

 {{{
 Bar.bar =
   \ (x_afk :: GHC.Types.Int) ->
     case x_afk of _ { GHC.Types.I# x1_alo ->
     case x1_alo of _ {
       __DEFAULT -> GHC.Types.False;
       72 -> GHC.Types.True
     }
     }
 }}}

 `bar` now looks pretty well optimised. However, if I uncomment the
 `deriving Eq` I get:

 {{{
 Bar.bar1 = GHC.Types.I# 72
 Bar.bar2 = GHC.Classes.== @ GHC.Types.Int GHC.Classes.$fEqInt
 Bar.bar = \ (x_amD :: GHC.Types.Int) -> Bar.bar2 x_amD Bar.bar1
 }}}

 Now `bar` looks like terrible code, using dictionaries, boxing etc. It
 seems adding `deriving` in the imported and unused module makes it ignore
 the optimisation level.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8635>
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