[clash-language] Avoiding `OtherCon []` unfoldings, restoring definitions from unfoldings

[ÉRDI Gergő]

This doesn't quite match my experience. For example, the following 
toplevel definition gets an `OtherCon []` unfolding:

nonEmptySubsequences :: [a] -> [[a]]
nonEmptySubsequences [] = []
nonEmptySubsequences (x:xs) = [x] : foldr f [] (nonEmptySubsequences xs)
   where
     f ys r = ys : (x:ys) : r

as can be seen with:

$ ghc -fforce-recomp -fexpose-all-unfoldings -ddump-prep -dsuppress-uniques A.hs

-- RHS size: {terms: 37, types: 55, coercions: 0, joins: 0/6}
A.nonEmptySubsequences [Occ=LoopBreaker] :: forall a. [a] -> [[a]]
[GblId, Arity=1, Unf=OtherCon []]
A.nonEmptySubsequences
   = \ (@ a) (ds [Occ=Once1!] :: [a]) -> ...


So this is not a lifted `case`-bound variable, but a bonafide 
user-originating toplevel definition. And its value also isn't bottom.


On Fri, 1 Apr 2022, Christiaan Baaij wrote:

> So if I understand correctly, OtherCon is only created here:https://gitlab.haskell.org/ghc/ghc/-/blob/a952dd80d40bf6b67194a44ff71d7bf75957d29e/co
> mpiler/GHC/Core/Opt/Simplify.hs#L3071-3077
> 
> simplAlt env _ imposs_deflt_cons case_bndr' cont' (Alt DEFAULT bndrs rhs)
>   = assert (null bndrs) $
>     do  { let env' = addBinderUnfolding env case_bndr'
>                                         (mkOtherCon imposs_deflt_cons)
>                 -- Record the constructors that the case-binder *can't* be.
>         ; rhs' <- simplExprC env' rhs cont'
>         ; return (Alt DEFAULT [] rhs') }
> 
> What you should know is that in Core case-expressions are actually more like:
> 
> case scrut as b of alts
> 
> where `b` binds the evaluated result of `scrut.
> 
> So if I am to understand the `simplAlt` code correctly, `case_bndr'` is the binder for
> the evaluated result of `scrut`.
> And what is recorded in the unfolding is that once we get to the DEFAULT pattern, we
> know that `case_bndr'` cannot be the constructors in `imposs_deflt_cons` (probably the
> constructor matched by the other alternatives).
> 
> Now... there's also a FloutOut pass, which might have floated that `case_bndr'` to the
> TopLevel.
> And I think that is what you're seeing, and I think you can simply ignore them.
> 
> 
> Also... another thing that you should know is that -fexpose-all-unfoldings doesn't
> actually expose *all* unfoldings.
> Bottoming bindings are never exposed.
> That's why in the Clash compiler we have the following code when loading
> core-expressions from .hi fileshttps://github.com/clash-lang/clash-compiler/blob/cb93b418865e244da50e1d2bc85fbc01bf7
> 61f3f/clash-ghc/src-ghc/Clash/GHC/LoadInterfaceFiles.hs#L473-L481
> 
> loadExprFromTyThing :: CoreSyn.CoreBndr -> GHC.TyThing -> Maybe CoreSyn.CoreExpr
> loadExprFromTyThing bndr tyThing = case tyThing of
>   GHC.AnId _id | Var.isId _id ->
>     let _idInfo    = Var.idInfo _id
>         unfolding  = IdInfo.unfoldingInfo _idInfo
>     in case unfolding of
>       CoreSyn.CoreUnfolding {} ->
>         Just (CoreSyn.unfoldingTemplate unfolding)
>       CoreSyn.DFunUnfolding dfbndrs dc es ->
>         Just (MkCore.mkCoreLams dfbndrs (MkCore.mkCoreConApps dc es))
>       CoreSyn.NoUnfolding
> #if MIN_VERSION_ghc(9,0,0)
>         | Demand.isDeadEndSig $ IdInfo.strictnessInfo _idInfo
> #else
>         | Demand.isBottomingSig $ IdInfo.strictnessInfo _idInfo
> #endif
>         -> do
>           let noUnfoldingErr = "no_unfolding " ++ showPpr unsafeGlobalDynFlags bndr
>           Just (MkCore.mkAbsentErrorApp (Var.varType _id) noUnfoldingErr)
>       _ -> Nothing
>   _ -> Nothing
> 
> i.e. when we encounter a NoUnfolding with a bottoming demand signature, we conjure an
> absentError out of thin air.
> 
> 
> On Fri, 1 Apr 2022 at 10:05, ÉRDI Gergő <gergo at erdi.hu> wrote:
>       Hi,
>
>       I'm CC-ing the Clash mailing list because I believe they should have
>       encountered the same problem (and perhaps have found a solution to it
>       already!).
>
>       I'm trying to use `.hi` files compiled with `ExposeAllUnfoldings` set to
>       reconstruct full Core bindings for further processing. By and large, this
>       works, but I get tripped up on identifiers whose unfolding is only given
>       as `OtherCon []`. It is unclear to me what is causing this -- some of them
>       are recursive bindings while others are not.
>
>       The problem, of course, is that if all I know about an identifier is that
>       it is `OtherCon []`, that doesn't allow me to restore its definition. So
>       is there a way to tell GHC to put "full" unfoldings everywhere in
>       `ExposeAllUnfoldings` mode?
>
>       Thanks,
>               Gergo
>
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>       83073%40galaxy.
> 
> 
>

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