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

1 files changed, 27 insertions, 1 deletions

diff --git a/compiler/basicTypes/BasicTypes.hs b/compiler/basicTypes/BasicTypes.hs
index 7fe4cb9c54..9711edb75a 100644
--- a/compiler/basicTypes/BasicTypes.hs
+++ b/compiler/basicTypes/BasicTypes.hs
@@ -19,7 +19,7 @@ types that
 module BasicTypes(
         Version, bumpVersion, initialVersion,
 
-        ConTag, fIRST_TAG,
+        ConTag, ConTagZ, fIRST_TAG,
 
         Arity, RepArity,
 
@@ -49,6 +49,8 @@ module BasicTypes(
         TupleSort(..), tupleSortBoxity, boxityTupleSort,
         tupleParens,
 
+        sumParens, pprAlternative,
+
         -- ** The OneShotInfo type
         OneShotInfo(..),
         noOneShotInfo, hasNoOneShotInfo, isOneShotInfo,
@@ -132,6 +134,9 @@ type RepArity = Int
 --   or superclass selector
 type ConTag = Int
 
+-- | A *zero-indexed* constructor tag
+type ConTagZ = Int
+
 fIRST_TAG :: ConTag
 -- ^ Tags are allocated from here for real constructors
 --   or for superclass selectors
@@ -619,6 +624,27 @@ tupleParens ConstraintTuple p   -- In debug-style write (% Eq a, Ord b %)
 {-
 ************************************************************************
 *                                                                      *
+                Sums
+*                                                                      *
+************************************************************************
+-}
+
+sumParens :: SDoc -> SDoc
+sumParens p = ptext (sLit "(#") <+> p <+> ptext (sLit "#)")
+
+-- | Pretty print an alternative in an unboxed sum e.g. "| a | |".
+pprAlternative :: (a -> SDoc) -- ^ The pretty printing function to use
+               -> a           -- ^ The things to be pretty printed
+               -> ConTag      -- ^ Alternative (one-based)
+               -> Arity       -- ^ Arity
+               -> SDoc        -- ^ 'SDoc' where the alternative havs been pretty
+                              -- printed and finally packed into a paragraph.
+pprAlternative pp x alt arity =
+    fsep (replicate (alt - 1) vbar ++ [pp x] ++ replicate (arity - alt - 1) vbar)
+
+{-
+************************************************************************
+*                                                                      *
 \subsection[Generic]{Generic flag}
 *                                                                      *
 ************************************************************************