[GHC] #8774: Transitivity of Auto-Specialization

[GHC]

#8774: Transitivity of Auto-Specialization
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:  Inlining
Operating System:  Linux             |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5928, #8668,     |  Differential Rev(s):
  #8099                              |
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by crockeea):

 I finally got 7.6 installed, and can reproduce the issue. The problem
 isn't that a `Foo a` appears, it's that without the SPECIALIZE pragma,
 there is a function called `Main.$splus` in core that takes/uses
 dictionaries, even though the type is monomorphic:


 {{{
 Main.$splus
   :: Foo.Foo (Foo.Qux GHC.Types.Int)
      -> Foo.Foo (Foo.Qux GHC.Types.Int)
      -> Foo.Foo (Foo.Qux GHC.Types.Int)
 [GblId,
  Arity=2,
  Str=DmdType SS,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=2, Value=True,
          ConLike=True, WorkFree=True, Expandable=True,
          Guidance=IF_ARGS [30 30] 110 20}]
 Main.$splus =
   \ (ds_dJ0 :: Foo.Foo (Foo.Qux GHC.Types.Int))
     (ds1_dJ1 :: Foo.Foo (Foo.Qux GHC.Types.Int)) ->
     case ds_dJ0 of _ {
       Foo.Bar v1_aH6 ->
         case ds1_dJ1 of _ {
           Foo.Bar v2_aH7 ->
             case (Foo.$fNumQux_$c+
                     @ GHC.Types.Int
                     GHC.Num.$fNumInt
                     Data.Vector.Unboxed.Base.$fUnboxInt
                     v1_aH6
                     v2_aH7)
 }}}


 Replying to [comment:10 erikd]:
 > Even with ghc 7.6, this seems to specialize correctly:
 >
 > {{{
 > cabal exec -- ghc -fforce-recomp -ddump-simpl -dsuppress-idinfo main.hs
 2>&1 | grep Foo
 > ...
 >     @ (Foo.Qux GHC.Types.Int)
 >     @ (Foo.Foo (Foo.Qux GHC.Types.Int))
 >     (Foo.$WBar @ (Foo.Qux GHC.Types.Int))
 > ...
 > }}}
 >

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8774#comment:11>
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