Remove StgRubbishArg and CmmArg

The idea behind adding special "rubbish" arguments was in unboxed sum types
depending on the tag some arguments are not used and we don't want to move some
special values (like 0 for literals and some special pointer for boxed slots)
for those arguments (to stack locations or registers). "StgRubbishArg" was an
indicator to the code generator that the value won't be used. During Stg-to-Cmm
we were then not generating any move or store instructions at all.

This caused problems in the register allocator because some variables were only
initialized in some code paths. As an example, suppose we have this STG: (after
unarise)

    Lib.$WT =
        \r [dt_sit]
            case
                case dt_sit of {
                  Lib.F dt_siv [Occ=Once] ->
                      (#,,#) [1# dt_siv StgRubbishArg::GHC.Prim.Int#];
                  Lib.I dt_siw [Occ=Once] ->
                      (#,,#) [2# StgRubbishArg::GHC.Types.Any dt_siw];
                }
            of
            dt_six
            { (#,,#) us_giC us_giD us_giE -> Lib.T [us_giC us_giD us_giE];
            };

This basically unpacks a sum type to an unboxed sum with 3 fields, and then
moves the unboxed sum to a constructor (`Lib.T`).

This is the Cmm for the inner case expression (case expression in the scrutinee
position of the outer case):

    ciN:
        ...
        -- look at dt_sit's tag
        if (_ciT::P64 != 1) goto ciS; else goto ciR;
    ciS: -- Tag is 2, i.e. Lib.F
        _siw::I64 = I64[_siu::P64 + 6];
        _giE::I64 = _siw::I64;
        _giD::P64 = stg_RUBBISH_ENTRY_info;
        _giC::I64 = 2;
        goto ciU;
    ciR: -- Tag is 1, i.e. Lib.I
        _siv::P64 = P64[_siu::P64 + 7];
        _giD::P64 = _siv::P64;
        _giC::I64 = 1;
        goto ciU;

Here one of the blocks `ciS` and `ciR` is executed and then the execution
continues to `ciR`, but only `ciS` initializes `_giE`, in the other branch
`_giE` is not initialized, because it's "rubbish" in the STG and so we don't
generate an assignment during code generator. The code generator then panics
during the register allocations:

    ghc-stage1: panic! (the 'impossible' happened)
      (GHC version 8.1.20160722 for x86_64-unknown-linux):
            LocalReg's live-in to graph ciY {_giE::I64}

(`_giD` is also "rubbish" in `ciS`, but it's still initialized because it's a
pointer slot, we have to initialize it otherwise garbage collector follows the
pointer to some random place. So we only remove assignment if the "rubbish" arg
has unboxed type.)

This patch removes `StgRubbishArg` and `CmmArg`. We now always initialize
rubbish slots. If the slot is for boxed types we use the existing `absentError`,
otherwise we initialize the slot with literal 0.

Reviewers: simonpj, erikd, austin, simonmar, bgamari

Reviewed By: erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2446

2 files changed, 1 insertions, 8 deletions

diff --git a/compiler/stgSyn/StgLint.hs b/compiler/stgSyn/StgLint.hs
index eb07e6b447..0dba8d8359 100644
--- a/compiler/stgSyn/StgLint.hs
+++ b/compiler/stgSyn/StgLint.hs
@@ -82,7 +82,6 @@ lintStgBindings whodunnit binds
 lintStgArg :: StgArg -> LintM (Maybe Type)
 lintStgArg (StgLitArg lit) = return (Just (literalType lit))
 lintStgArg (StgVarArg v)   = lintStgVar v
-lintStgArg (StgRubbishArg ty) = return (Just ty)
 
 lintStgVar :: Id -> LintM (Maybe Kind)
 lintStgVar v = do checkInScope v
diff --git a/compiler/stgSyn/StgSyn.hs b/compiler/stgSyn/StgSyn.hs
index 60147bc8d8..b553cd74dd 100644
--- a/compiler/stgSyn/StgSyn.hs
+++ b/compiler/stgSyn/StgSyn.hs
@@ -92,10 +92,6 @@ data GenStgArg occ
   = StgVarArg  occ
   | StgLitArg  Literal
 
-    -- A rubbish arg is a value that's not supposed to be used by the generated
-    -- code, but it may be a GC root (i.e. used by GC) if the type is boxed.
-  | StgRubbishArg Type
-
 -- | Does this constructor application refer to
 -- anything in a different *Windows* DLL?
 -- If so, we can't allocate it statically
@@ -137,7 +133,6 @@ isAddrRep _       = False
 stgArgType :: StgArg -> Type
 stgArgType (StgVarArg v)   = idType v
 stgArgType (StgLitArg lit) = literalType lit
-stgArgType (StgRubbishArg ty) = ty
 
 
 -- | Strip ticks of a given type from an STG expression
@@ -197,7 +192,7 @@ primitives, and literals.
                 [Type]          -- See Note [Types in StgConApp] in UnariseStg
 
   | StgOpApp    StgOp           -- Primitive op or foreign call
-                [GenStgArg occ] -- Saturated. Not rubbish.
+                [GenStgArg occ] -- Saturated.
                 Type            -- Result type
                                 -- We need to know this so that we can
                                 -- assign result registers
@@ -659,7 +654,6 @@ instance (OutputableBndr bndr, Outputable bdee, Ord bdee)
 pprStgArg :: (Outputable bdee) => GenStgArg bdee -> SDoc
 pprStgArg (StgVarArg var) = ppr var
 pprStgArg (StgLitArg con) = ppr con
-pprStgArg (StgRubbishArg ty) = text "StgRubbishArg" <> dcolon <> ppr ty
 
 pprStgExpr :: (OutputableBndr bndr, Outputable bdee, Ord bdee)
            => GenStgExpr bndr bdee -> SDoc