Convincing GHC not to produce unfoldings during simplification

[Simon Peyton Jones]

Can you tell me how to repro this?  I can't.  I tried
	module Foo( foo ) where
	{-# NOINLINE foo #-}
	foo = ("fred" , True, ('c', 'e'))

And just as expected, I see foo in the interface file, but none of its components.

Simon

| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ben
| Gamari
| Sent: 12 February 2017 17:26
| To: ghc-devs at haskell.org
| Subject: Convincing GHC not to produce unfoldings during simplification
| 
| I'm currently trying to get the interface file bulk imposed by TTypeable
| down. The branch will currently produce two top-level bindings for each
| TyCon.
| For instance, given the type,
| 
|     data Proxy (a :: k) = Proxy
| 
| the branch will produce (ignoring the promoted datacon for a moment),
| 
|     $tcProxy :: TyCon
|     $tcProxy
|       = GHC.Types.TyCon
|           fpr1 fpr2           -- fingerprints
|           $trModule
|           (GHC.Types.TrNameS "Proxy"#)
|           1
|           krep_aebr
| 
|     krep_aebr :: KindRep
|     krep_aebr [InlPrag=NOINLINE CONLIKE]
|       = GHC.Types.KindRepFun
|           (GHC.Types.KindRepVar 0)
|           (GHC.Types.KindRepTYPE GHC.Types.LiftedRep)
| 
| Note that the KindRep has been marked as NOINLINE; I do this
| intentionally as these things can grow quite large (consider, for
| instance, the kind of a promoted tuple constructor). This is especially
| true as the simplifier tends to float out the constituents of the
| KindRep. After the first simplifier phase we end up with,
| 
| 
|     krep_sebQ :: GHC.Types.KindRep
|     [LclId,
|     Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
|             WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 30 0}]
|     krep_sebQ = GHC.Types.$WKindRepVar (GHC.Types.I# 0#)
| 
|     krep_sebR :: GHC.Types.KindRep
|     [LclId,
|     Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
|             WorkFree=False, Expandable=False, Guidance=IF_ARGS [] 20 0}]
|     krep_sebR = GHC.Types.$WKindRepTYPE GHC.Types.LiftedRep
| 
|     krep_aebr [InlPrag=NOINLINE CONLIKE] :: GHC.Types.KindRep
|     [LclId,
|     Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
|             WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 10 30}]
|     krep_aebr = GHC.Types.KindRepFun krep_sebQ krep_sebR
| 
| This means that GHC will now include each of the bindings floated out
| from krep_aebr in the interface file, despite the NOINLINE pragma, as
| they are all depended upon by the exported $tcProxy through their
| unfoldings. This is Quite Bad.
| 
| I have observed that the floated bindings are not included in the
| interface file with -O0. Moreover, the core2core output clearly shows the
| unfoldings being added after the first simplifier phase. I haven't yet
| determined where in the simplifier these unfoldings are being introduced,
| but if anyone has ideas I'd love to hear them.
| 
| Cheers,
| 
| - Ben