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, 5 insertions, 2 deletions

diff --git a/compiler/codeGen/StgCmmForeign.hs b/compiler/codeGen/StgCmmForeign.hs
index c8db8644db..eb14e8cce6 100644
--- a/compiler/codeGen/StgCmmForeign.hs
+++ b/compiler/codeGen/StgCmmForeign.hs
@@ -34,6 +34,7 @@ import Cmm
 import CmmUtils
 import MkGraph
 import Type
+import RepType
 import TysPrim
 import CLabel
 import SMRep
@@ -110,7 +111,7 @@ cgForeignCall (CCall (CCallSpec target cconv safety)) stg_args res_ty
 
             _something_else ->
                 do { _ <- emitForeignCall safety res_regs call_target call_args
-                   ; emitReturn (map (CmmReg . CmmLocal) res_regs)
+                   ; emitReturn (map (CmmExprArg . CmmReg . CmmLocal) res_regs)
                    }
          }
 
@@ -523,10 +524,12 @@ getFCallArgs args
   = do  { mb_cmms <- mapM get args
         ; return (catMaybes mb_cmms) }
   where
+    get arg@(StgRubbishArg{})
+            = pprPanic "getFCallArgs" (text "Rubbish arg in foreign call:" <+> ppr arg)
     get arg | isVoidRep arg_rep
             = return Nothing
             | otherwise
-            = do { cmm <- getArgAmode (NonVoid arg)
+            = do { cmm <- getArgAmode_no_rubbish (NonVoid arg)
                  ; dflags <- getDynFlags
                  ; return (Just (add_shim dflags arg_ty cmm, hint)) }
             where