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Opened by Neil Mitchell@ndmitchellReporter

GHC optimisation flag ignored when importing a local module with derived type classes

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.

Edited by Ben Gamari