Implement unboxed sum primitive type

Summary:
This patch implements primitive unboxed sum types, as described in
https://ghc.haskell.org/trac/ghc/wiki/UnpackedSumTypes.

Main changes are:

- Add new syntax for unboxed sums types, terms and patterns. Hidden
  behind `-XUnboxedSums`.

- Add unlifted unboxed sum type constructors and data constructors,
  extend type and pattern checkers and desugarer.

- Add new RuntimeRep for unboxed sums.

- Extend unarise pass to translate unboxed sums to unboxed tuples right
  before code generation.

- Add `StgRubbishArg` to `StgArg`, and a new type `CmmArg` for better
  code generation when sum values are involved.

- Add user manual section for unboxed sums.

Some other changes:

- Generalize `UbxTupleRep` to `MultiRep` and `UbxTupAlt` to
  `MultiValAlt` to be able to use those with both sums and tuples.

- Don't use `tyConPrimRep` in `isVoidTy`: `tyConPrimRep` is really
  wrong, given an `Any` `TyCon`, there's no way to tell what its kind
  is, but `kindPrimRep` and in turn `tyConPrimRep` returns `PtrRep`.

- Fix some bugs on the way: #12375.

Not included in this patch:

- Update Haddock for new the new unboxed sum syntax.

- `TemplateHaskell` support is left as future work.

For reviewers:

- Front-end code is mostly trivial and adapted from unboxed tuple code
  for type checking, pattern checking, renaming, desugaring etc.

- Main translation routines are in `RepType` and `UnariseStg`.
  Documentation in `UnariseStg` should be enough for understanding
  what's going on.

Credits:

- Johan Tibell wrote the initial front-end and interface file
  extensions.

- Simon Peyton Jones reviewed this patch many times, wrote some code,
  and helped with debugging.

Reviewers: bgamari, alanz, goldfire, RyanGlScott, simonpj, austin,
           simonmar, hvr, erikd

Reviewed By: simonpj

Subscribers: Iceland_jack, ggreif, ezyang, RyanGlScott, goldfire,
             thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2259

2 files changed, 7 insertions, 1 deletions

diff --git a/compiler/vectorise/Vectorise/Builtins/Base.hs b/compiler/vectorise/Vectorise/Builtins/Base.hs
index 30438f0d1a..4837bde208 100644
--- a/compiler/vectorise/Vectorise/Builtins/Base.hs
+++ b/compiler/vectorise/Vectorise/Builtins/Base.hs
@@ -34,7 +34,7 @@ import TysPrim
 import BasicTypes
 import Class
 import CoreSyn
-import TysWiredIn
+import TysWiredIn hiding (sumTyCon)
 import Type
 import TyCon
 import DataCon
diff --git a/compiler/vectorise/Vectorise/Type/TyConDecl.hs b/compiler/vectorise/Vectorise/Type/TyConDecl.hs
index 7aa79215d5..a9498a5423 100644
--- a/compiler/vectorise/Vectorise/Type/TyConDecl.hs
+++ b/compiler/vectorise/Vectorise/Type/TyConDecl.hs
@@ -155,6 +155,12 @@ vectAlgTyConRhs tc (TupleTyCon { data_con = con })
     -- but it's the behaviour we had before I refactored the
     -- representation of AlgTyConRhs to add tuples
 
+vectAlgTyConRhs tc (SumTyCon { data_cons = cons })
+  = -- FIXME (osa): I'm pretty sure this is broken.. TupleTyCon case is probably
+    -- also broken when the tuple is unboxed.
+    vectAlgTyConRhs tc (DataTyCon { data_cons = cons
+                                  , is_enum = all (((==) 0) . dataConRepArity) cons })
+
 vectAlgTyConRhs tc (NewTyCon {})
   = do dflags <- getDynFlags
        cantVectorise dflags noNewtypeErr (ppr tc)