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

12 Feb 2017

      #8635: GHC optimisation flag ignored when importing a local module with derived
type classes
-------------------------------------+-------------------------------------
        Reporter:  Neil Mitchell     |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9370             |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Ben Gamari:

Old description:
...
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.
New description:

 Given Foo.hs and Bar.hs:

 {{{#!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#comment:6
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