Load the interface details for GHC.* even without -O

Neil says: I just tried compiling the following program:

foo = (1 :: Int) == (2 :: Int)

with ghc --ddump-simpl, and no optimisation flags, in GHC 6.6.1

The resultant code is:

Test.foo =
  case GHC.Base.$f2 of tpl_X8 { GHC.Base.:DEq tpl1_B2 tpl2_B3 ->
  tpl1_B2 (GHC.Base.I# 1) (GHC.Base.I# 2)
  }

GHC has introduced dictionaries in this example. In comparison, Yhc and nhc wouldn't introduce dictionaries here, as the type class desugaring knows the type of the item is fixed, and makes a direct call to the appropriate function.

If the desugaring was changed, it would make programs in GHCi run faster, would reduce the size of programs, and would speed up the optimiser, as it would have less to do. I am not sure how much additional work this would require in the simplifier, but if it was minimal, the gains would probably be worth it.

The reason nothing happens is that without -0 GHC does not load the unfoldings for anything from interface files, including GHC.Base. Idea: even without -O, load the unfoldings from GHC.* modules. That would pick up a lot of very fundamental stuff, and might, as Neil says, make compilation faster.

Relatively easy to try. But would need a good nofib run to test the effects. Any takers? I'd point you at what to change.

Simon

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Attached file Tick2207.hs ($824).

Exhibits problems described in #2207

With ghc 7.6.1 and the attached file:

Orac:~/work/ghc $ ghc -ddump-simpl Tick2207.hs
[1 of 1] Compiling Main             ( Tick2207.hs, Tick2207.o )
...
Main.foo :: GHC.Types.Bool
[GblId]
Main.foo =
  GHC.Classes.==
    @ GHC.Types.Int
    GHC.Classes.$fEqInt
    (GHC.Types.I# 1)
    (GHC.Types.I# 2)
...

Output is much prettier, but still exhibits OP's issue.

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For what is worth, this is in 7.8.3:

❯❯❯ ghc -ddump-simpl a.hs
[1 of 1] Compiling Main             ( a.hs, a.o )

==================== Tidy Core ====================
Result size of Tidy Core = {terms: 13, types: 7, coercions: 0}

Main.main :: GHC.Types.IO ()
[GblId, Str=DmdType]
Main.main =
  System.IO.putStrLn
    (GHC.Show.show
       @ GHC.Types.Bool
       GHC.Show.$fShowBool
       (GHC.Classes.==
          @ GHC.Types.Int
          GHC.Classes.$fEqInt
          (GHC.Types.I# 1)
          (GHC.Types.I# 2)))

:Main.main :: GHC.Types.IO ()
[GblId, Str=DmdType]
:Main.main = GHC.TopHandler.runMainIO @ () Main.main

For:

main = putStrLn $ show foo
foo = (1 :: Int) == (2 :: Int)

Might be interesting to try.

added newcomer label

I'm willing to have a go at this issue, is anyone already working on it?

assigned to @trac-Azel

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Still true in HEAD. Passing -fno-ignore-interface-pragmas is enough to get:

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
foo :: Bool
[GblId, Unf=OtherCon []]
foo = GHC.Types.False

The following patch should be enough to implement the change:

--- compiler/GHC/Driver/DynFlags.hs
+++ compiler/GHC/Driver/DynFlags.hs
@@ -1253,7 +1253,7 @@ optLevelFlags -- see Note [Documenting optimisation flags]
     , ([0,1,2], Opt_ProfManualCcs )
     , ([2], Opt_DictsStrict)

-    , ([0],     Opt_IgnoreInterfacePragmas)
     , ([0],     Opt_OmitInterfacePragmas)

     , ([1,2],   Opt_CoreConstantFolding)

With this and -O0 (default) the simplifier still only runs the Final phase, but it does a bit more work. We could enable it for 9.10 and see if anyone complains :-)