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, 14 insertions, 5 deletions

diff --git a/compiler/codeGen/StgCmmMonad.hs b/compiler/codeGen/StgCmmMonad.hs
index 2742acdcdb..8f66cfaa91 100644
--- a/compiler/codeGen/StgCmmMonad.hs
+++ b/compiler/codeGen/StgCmmMonad.hs
@@ -19,8 +19,8 @@ module StgCmmMonad (
 
         emit, emitDecl, emitProc,
         emitProcWithConvention, emitProcWithStackFrame,
-        emitOutOfLine, emitAssign, emitStore, emitComment,
-        emitTick, emitUnwind,
+        emitOutOfLine, emitAssign, emitAssign', emitStore,
+        emitComment, emitTick, emitUnwind,
 
         getCmm, aGraphToGraph,
         getCodeR, getCode, getCodeScoped, getHeapUsage,
@@ -76,6 +76,7 @@ import Unique
 import UniqSupply
 import FastString
 import Outputable
+import RepType (typePrimRep)
 
 import Control.Monad
 import Data.List
@@ -743,6 +744,14 @@ emitUnwind g e = do
 emitAssign :: CmmReg  -> CmmExpr -> FCode ()
 emitAssign l r = emitCgStmt (CgStmt (CmmAssign l r))
 
+emitAssign' :: CmmReg -> CmmArg -> FCode ()
+emitAssign' l (CmmExprArg r) = emitAssign l r
+emitAssign' l (CmmRubbishArg ty)
+  | isGcPtrRep (typePrimRep ty)
+  = emitAssign l rubbishExpr
+  | otherwise
+  = return ()
+
 emitStore :: CmmExpr  -> CmmExpr -> FCode ()
 emitStore l r = emitCgStmt (CgStmt (CmmStore l r))
 
@@ -858,8 +867,8 @@ mkCmmIfThen e tbranch = do
                       , mkLabel tid tscp, tbranch, mkLabel endif tscp ]
 
 
-mkCall :: CmmExpr -> (Convention, Convention) -> [CmmFormal] -> [CmmActual]
-       -> UpdFrameOffset -> [CmmActual] -> FCode CmmAGraph
+mkCall :: CmmExpr -> (Convention, Convention) -> [CmmFormal] -> [CmmArg]
+       -> UpdFrameOffset -> [CmmArg] -> FCode CmmAGraph
 mkCall f (callConv, retConv) results actuals updfr_off extra_stack = do
   dflags <- getDynFlags
   k      <- newLabelC
@@ -869,7 +878,7 @@ mkCall f (callConv, retConv) results actuals updfr_off extra_stack = do
       copyout = mkCallReturnsTo dflags f callConv actuals k off updfr_off extra_stack
   return $ catAGraphs [copyout, mkLabel k tscp, copyin]
 
-mkCmmCall :: CmmExpr -> [CmmFormal] -> [CmmActual] -> UpdFrameOffset
+mkCmmCall :: CmmExpr -> [CmmFormal] -> [CmmArg] -> UpdFrameOffset
           -> FCode CmmAGraph
 mkCmmCall f results actuals updfr_off
    = mkCall f (NativeDirectCall, NativeReturn) results actuals updfr_off []