Rubbish literals for all representations (#18983)wip/T18983

This patch cleans up the complexity around WW's `mk_absent_let` by
broadening the scope of `LitRubbish`. Rubbish literals now store the
`PrimRep` they represent and are ultimately lowered in Cmm.
This in turn allows absent literals of `VecRep` or `VoidRep`. The latter
allows absent literals for unlifted coercions, as requested in #18983.

I took the liberty to rewrite and clean up `Note [Absent fillers]` and
`Note [Rubbish values]` to account for the new implementation and to
make them more orthogonal in their description.

I didn't add a new regression test, as `T18982` already contains the
test in the ticket and its test output changes as expected.

Fixes #18983.

27 files changed, 538 insertions, 390 deletions

diff --git a/compiler/GHC/Builtin/Types/Prim.hs b/compiler/GHC/Builtin/Types/Prim.hs
index 29bb386001..115c76516d 100644
--- a/compiler/GHC/Builtin/Types/Prim.hs
+++ b/compiler/GHC/Builtin/Types/Prim.hs
@@ -35,7 +35,7 @@ module GHC.Builtin.Types.Prim(
         tYPETyCon, tYPETyConName,
 
         -- Kinds
-        tYPE, primRepToRuntimeRep,
+        tYPE, primRepToRuntimeRep, primRepsToRuntimeRep,
 
         functionWithMultiplicity,
         funTyCon, funTyConName,
@@ -587,7 +587,7 @@ pcPrimTyCon name roles rep
 -- Defined here to avoid (more) module loops
 primRepToRuntimeRep :: PrimRep -> Type
 primRepToRuntimeRep rep = case rep of
-  VoidRep       -> TyConApp tupleRepDataConTyCon [mkPromotedListTy runtimeRepTy []]
+  VoidRep       -> mkTupleRep []
   LiftedRep     -> liftedRepTy
   UnliftedRep   -> unliftedRepTy
   IntRep        -> intRepDataConTy
@@ -626,6 +626,17 @@ primRepToRuntimeRep rep = case rep of
         FloatElemRep  -> floatElemRepDataConTy
         DoubleElemRep -> doubleElemRepDataConTy
 
+-- | Given a list of types representing 'RuntimeRep's @reps@, construct
+-- @'TupleRep' reps@.
+mkTupleRep :: [Type] -> Type
+mkTupleRep reps = TyConApp tupleRepDataConTyCon [mkPromotedListTy runtimeRepTy reps]
+
+-- | Convert a list of 'PrimRep's to a 'Type' of kind RuntimeRep
+-- Defined here to avoid (more) module loops
+primRepsToRuntimeRep :: [PrimRep] -> Type
+primRepsToRuntimeRep [rep] = primRepToRuntimeRep rep
+primRepsToRuntimeRep reps  = mkTupleRep $ map primRepToRuntimeRep reps
+
 pcPrimTyCon0 :: Name -> PrimRep -> TyCon
 pcPrimTyCon0 name rep
   = pcPrimTyCon name [] rep
diff --git a/compiler/GHC/Cmm/LayoutStack.hs b/compiler/GHC/Cmm/LayoutStack.hs
index 9e86ab58c5..ac46f23f1f 100644
--- a/compiler/GHC/Cmm/LayoutStack.hs
+++ b/compiler/GHC/Cmm/LayoutStack.hs
@@ -8,7 +8,8 @@ import GHC.Prelude hiding ((<*>))
 import GHC.Platform
 import GHC.Platform.Profile
 
-import GHC.StgToCmm.Utils      ( callerSaveVolatileRegs, newTemp  ) -- XXX layering violation
+import GHC.StgToCmm.Monad      ( newTemp  ) -- XXX layering violation
+import GHC.StgToCmm.Utils      ( callerSaveVolatileRegs  ) -- XXX layering violation
 import GHC.StgToCmm.Foreign    ( saveThreadState, loadThreadState ) -- XXX layering violation
 
 import GHC.Types.Basic
diff --git a/compiler/GHC/Cmm/Parser.y b/compiler/GHC/Cmm/Parser.y
index 8a972b91d5..666441a687 100644
--- a/compiler/GHC/Cmm/Parser.y
+++ b/compiler/GHC/Cmm/Parser.y
@@ -218,6 +218,7 @@ import qualified GHC.StgToCmm.Monad as F
 import GHC.StgToCmm.Utils
 import GHC.StgToCmm.Foreign
 import GHC.StgToCmm.Expr
+import GHC.StgToCmm.Lit
 import GHC.StgToCmm.Closure
 import GHC.StgToCmm.Layout     hiding (ArgRep(..))
 import GHC.StgToCmm.Ticky
diff --git a/compiler/GHC/Core/Make.hs b/compiler/GHC/Core/Make.hs
index 42a1b78c0c..60ae13bee7 100644
--- a/compiler/GHC/Core/Make.hs
+++ b/compiler/GHC/Core/Make.hs
@@ -1024,7 +1024,7 @@ aBSENT_ERROR_ID
  where
    absent_ty = mkSpecForAllTys [alphaTyVar] (mkVisFunTyMany addrPrimTy alphaTy)
    -- Not runtime-rep polymorphic. aBSENT_ERROR_ID is only used for
-   -- lifted-type things; see Note [Absent errors] in GHC.Core.Opt.WorkWrap.Utils
+   -- lifted-type things; see Note [Absent fillers] in GHC.Core.Opt.WorkWrap.Utils
    arity_info = vanillaIdInfo `setArityInfo` 1
    -- NB: no bottoming strictness info, unlike other error-ids.
    -- See Note [aBSENT_ERROR_ID]
diff --git a/compiler/GHC/Core/Opt/WorkWrap/Utils.hs b/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
index 5223e66817..f51e716c38 100644
--- a/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
+++ b/compiler/GHC/Core/Opt/WorkWrap/Utils.hs
@@ -20,7 +20,7 @@ import GHC.Prelude
 
 import GHC.Core
 import GHC.Core.Utils   ( exprType, mkCast, mkDefaultCase, mkSingleAltCase
-                        , dataConRepFSInstPat )
+                        , bindNonRec, dataConRepFSInstPat )
 import GHC.Types.Id
 import GHC.Types.Id.Info ( JoinArity )
 import GHC.Core.DataCon
@@ -29,14 +29,14 @@ import GHC.Types.Cpr
 import GHC.Core.Make    ( mkAbsentErrorApp, mkCoreUbxTup
                         , mkCoreApp, mkCoreLet )
 import GHC.Types.Id.Make ( voidArgId, voidPrimId )
-import GHC.Builtin.Types      ( tupleDataCon, unboxedUnitTy )
-import GHC.Types.Literal ( absentLiteralOf, rubbishLit )
+import GHC.Builtin.Types      ( tupleDataCon )
+import GHC.Types.Literal ( mkLitRubbish )
 import GHC.Types.Var.Env ( mkInScopeSet )
 import GHC.Types.Var.Set ( VarSet )
 import GHC.Core.Type
 import GHC.Core.Multiplicity
 import GHC.Core.Predicate ( isClassPred )
-import GHC.Types.RepType  ( isVoidTy, typePrimRep )
+import GHC.Types.RepType  ( isVoidTy, typeMonoPrimRep_maybe )
 import GHC.Core.Coercion
 import GHC.Core.FamInstEnv
 import GHC.Types.Basic       ( Boxity(..) )
@@ -895,9 +895,9 @@ mkWWstr_one dflags fam_envs has_inlineable_prag arg
   = return (False, [arg],  nop_fn, nop_fn)
 
   | isAbsDmd dmd
-  , Just work_fn <- mk_absent_let dflags fam_envs arg dmd
-     -- Absent case.  We can't always handle absence for arbitrary
-     -- unlifted types, so we need to choose just the cases we can
+  , Just work_fn <- mk_absent_let dflags arg dmd
+     -- Absent case.  We can't always handle absence for rep-polymorphic
+     -- types, so we need to choose just the cases we can
      -- (that's what mk_absent_let does)
   = return (True, [], nop_fn, work_fn)
 
@@ -1281,70 +1281,74 @@ part of the function (post transformation) anyway.
 *                                                                      *
 ************************************************************************
 
-Note [Absent errors]
-~~~~~~~~~~~~~~~~~~~~
+Note [Absent fillers]
+~~~~~~~~~~~~~~~~~~~~~
 Consider
-  data T = MkT [Int] [Int] ![Int]
-  f :: T -> Int# -> blah
-  f ps w = case ps of MkT xs _ _ -> <body mentioning xs>
-Then f gets a strictness sig of <S(L,A,A)><A>. We make worker $wf thus:
-
-$wf :: [Int] -> blah
-$wf xs = case ps of MkT xs _ _ -> <body mentioning xs>
-  where
-    ys = absentError "ys :: [Int]"
-    zs = LitRubbish True
-    ps = MkT xs ys zs
-    w  = 0#
-
-We make a let-binding for Absent arguments, such as ys and w, that are not even
-passed to the worker. They should, of course, never be used. We distinguish four
-cases:
-
-1. Ordinary boxed, lifted arguments, like 'ys' We make a new binding for Ids
-   that are marked absent, thus
-      let ys = absentError "ys :: [Int]"
-   The idea is that this binding will never be used; but if it
-   buggily is used we'll get a runtime error message.
-
-2. Boxed, lifted types, with a strict demand, like 'zs'.  You may ask: how the
-   demand be both absent and strict?  That's exactly what happens for 'zs': it
-   is not used, so its demand is Absent, but then during w/w, in
-   addDataConStrictness, we strictify the demand.  So it gets cardinality C_10,
-   the empty interval.
-
-   We don't want to use an error-thunk for 'zs' because MkT's third argument has
-   a bang, and hence should be always evaluated. This turned out to be
-   important when fixing #16970, which establishes the invariant that strict
-   constructor arguments are always evaluated. So we use LitRubbish instead
-   of an error thunk -- see #19133.
-
-   These first two cases are distinguished by isStrictDmd in lifted_rhs.
-
-3. Unboxed types, like 'w', with a type like Float#, Int#. Coping with absence
-   for unboxed types is important; see, for example, #4306 and #15627.  We
-   simply find a suitable literal, using Literal.absentLiteralOf.  We don't have
-   literals for every primitive type, so the function is partial.
-
-4. Boxed, unlifted types, like (Array# t).  We can't use absentError because
-   unlifted bindings ares strict.  So we use LitRubbish, which we need to apply
-   to the required type.
-
-Case (2) and (4) crucially use LitRubbish as the placeholder: see Note [Rubbish
-literals] in GHC.Types.Literal.  We could do that in case (1) as well, but we
-get slightly better self-checking with an error thunk.
 
-Suppose we use LitRubbish and absence analysis is Wrong, so that the "absent"
-value is used after all.  Then in case (2) we could get a seg-fault, because we
-may have replaced, say, a [Either Int Bool] by (), and that will fail if we do
-case analysis on it.  Similarly with boxed unlifted types, case (4).
-
-In case (3), if absence analysis is wrong we could conceivably get an exception,
-from a divide-by-zero with the absent value.  But it's very unlikely.
+  data T = MkT [Int] [Int] ![Int]  -- NB: last field is strict
+  f :: T -> Int# -> blah
+  f ps w = case ps of MkT xs ys zs -> <body mentioning xs>
 
-Only in case (1) can we guarantee a civilised runtime error.  Not much we can do
-about this; we really rely on absence analysis to be correct.
+Then f gets a strictness sig of <S(L,A,A)><A>. We make a worker $wf thus:
 
+  $wf :: [Int] -> blah
+  $wf xs = case ps of MkT xs _ _ -> <body mentioning xs>
+    where
+      ys = absentError "ys :: [Int]"
+      zs = RUBBISH[LiftedRep] @[Int]
+      ps = MkT xs ys zs
+      w  = RUBBISH[IntRep] @Int#
+
+The absent arguments 'ys', 'zs' and 'w' aren't even passed to the worker.
+And neither should they! They are never used, their value is irrelevant (hence
+they are *dead code*) and they are probably discarded after the next run of the
+Simplifier (when they are in fact *unreachable code*). Yet, we have to come up
+with "filler" values that we bind the absent arg Ids to.
+
+That is exactly what Note [Rubbish values] are for: A convenient way to
+conjure filler values at any type (and any representation or levity!).
+
+Needless to say, there are some wrinkles:
+
+  1. In case we have a absent, /lazy/, and /lifted/ arg, we use an error-thunk
+     instead. If absence analysis was wrong (e.g., #11126) and the binding
+     in fact is used, then we get a nice panic message instead of undefined
+     runtime behavior (See Modes of failure from Note [Rubbish values]).
+
+     Obviously, we can't use an error-thunk if the value is of unlifted rep
+     (like 'Int#' or 'MutVar#'), because we'd immediately evaluate the panic.
+
+  2. We also mustn't put an error-thunk (that fills in for an absent value of
+     lifted rep) in a strict field, because #16970 establishes the invariant
+     that strict fields are always evaluated, by (re-)evaluating what is put in
+     a strict field. That's the reason why 'zs' binds a rubbish literal instead
+     of an error-thunk, see #19133.
+
+     How do we detect when we are about to put an error-thunk in a strict field?
+     Ideally, we'd just look at the 'StrictnessMark' of the DataCon's field, but
+     it's quite nasty to thread the marks though 'mkWWstr' and 'mkWWstr_one'.
+     So we rather look out for a necessary condition for strict fields:
+     Note [Add demands for strict constructors] makes it so that the demand on
+     'zs' is absent and /strict/: It will get cardinality 'C_10', the empty
+     interval, rather than 'C_00'. Hence the 'isStrictDmd' check: It guarantees
+     we never fill in an error-thunk for an absent strict field.
+     But that also means we emit a rubbish lit for other args that have
+     cardinality 'C_10' (say, the arg to a bottoming function) where we could've
+     used an error-thunk, but that's a small price to pay for simplicity.
+
+  3. We can only emit a RubbishLit if the arg's type @arg_ty@ is mono-rep, e.g.
+     of the form @TYPE rep@ where @rep@ is not (and doesn't contain) a variable.
+     Why? Because if we don't know its representation (e.g. size in memory,
+     register class), we don't know what or how much rubbish to emit in codegen.
+     'typeMonoPrimRep_maybe' returns 'Nothing' in this case and we simply fall
+     back to passing the original parameter to the worker.
+
+     Note that currently this case should not occur, because binders always
+     have to be representation monomorphic. But in the future, we might allow
+     levity polymorphism, e.g. a polymorphic levity variable in 'BoxedRep'.
+
+While (1) and (2) are simply an optimisation in terms of compiler debugging
+experience, (3) should be irrelevant in most programs, if not all.
 
 Historical note: I did try the experiment of using an error thunk for unlifted
 things too, relying on the simplifier to drop it as dead code.  But this is
@@ -1368,66 +1372,46 @@ fragile
 --
 -- If @mk_absent_let _ id == Just wrap@, then @wrap e@ will wrap a let binding
 -- for @id@ with that RHS around @e@. Otherwise, there could no suitable RHS be
--- found (currently only happens for bindings of 'VecRep' representation).
-mk_absent_let :: DynFlags -> FamInstEnvs -> Id -> Demand -> Maybe (CoreExpr -> CoreExpr)
-mk_absent_let dflags fam_envs arg dmd
-
-  -- The lifted case: Bind 'absentError'
-  -- See Note [Absent errors]
-  | not (isUnliftedType arg_ty)
-  = Just (Let (NonRec lifted_arg lifted_rhs))
-  -- The 'UnliftedRep' (because polymorphic) case: Bind @__RUBBISH \@arg_ty@
-  -- See Note [Absent errors]
-
-  | [UnliftedRep] <- typePrimRep arg_ty
-  = Just (Let (NonRec arg unlifted_rhs))
-
-  -- The monomorphic unlifted cases: Bind to some literal, if possible
-  -- See Note [Absent errors]
-  | Just tc <- tyConAppTyCon_maybe nty
-  , Just lit <- absentLiteralOf tc
-  = Just (Let (NonRec arg (Lit lit `mkCast` mkSymCo co)))
-
-  | nty `eqType` unboxedUnitTy
-  = Just (Let (NonRec arg (Var voidPrimId `mkCast` mkSymCo co)))
-
-  | otherwise
+-- found.
+mk_absent_let :: DynFlags -> Id -> Demand -> Maybe (CoreExpr -> CoreExpr)
+mk_absent_let dflags arg dmd
+  -- The lifted case: Bind 'absentError' for a nice panic message if we are
+  -- wrong (like we were in #11126). See (1) in Note [Absent fillers]
+  | Just [LiftedRep] <- mb_mono_prim_reps
+  , not (isStrictDmd dmd) -- See (2) in Note [Absent fillers]
+  = Just (Let (NonRec arg panic_rhs))
+
+  -- The default case for mono rep: Bind @RUBBISH[prim_reps] \@arg_ty@
+  -- See Note [Absent fillers], the main part
+  | Just prim_reps <- mb_mono_prim_reps
+  = Just (bindNonRec arg (mkTyApps (Lit (mkLitRubbish prim_reps)) [arg_ty]))
+
+  -- Catch all: Either @arg_ty@ wasn't of form @TYPE rep@ or @rep@ wasn't mono rep.
+  -- See (3) in Note [Absent fillers]
+  | Nothing <- mb_mono_prim_reps
   = WARN( True, text "No absent value for" <+> ppr arg_ty )
-    Nothing -- Can happen for 'State#' and things of 'VecRep'
+    Nothing
   where
-    lifted_arg   = arg `setIdStrictness` botSig `setIdCprInfo` mkCprSig 0 botCpr
-              -- Note in strictness signature that this is bottoming
-              -- (for the sake of the "empty case scrutinee not known to
-              -- diverge for sure lint" warning)
-
-    lifted_rhs | isStrictDmd dmd = mkTyApps (Lit (rubbishLit True))  [arg_ty]
-               | otherwise       = mkAbsentErrorApp arg_ty msg
-    unlifted_rhs = mkTyApps (Lit (rubbishLit False)) [arg_ty]
-
-    arg_ty       = idType arg
-
-    -- Normalise the type to have best chance of finding an absent literal
-    -- e.g. (#17852)   data unlifted N = MkN Int#
-    --                 f :: N -> a -> a
-    --                 f _ x = x
-    (co, nty)    = topNormaliseType_maybe fam_envs arg_ty
-                   `orElse` (mkRepReflCo arg_ty, arg_ty)
-
-    msg          = showSDoc (gopt_set dflags Opt_SuppressUniques)
-                            (vcat
-                              [ text "Arg:" <+> ppr arg
-                              , text "Type:" <+> ppr arg_ty
-                              , file_msg
-                              ])
-    file_msg     = case outputFile dflags of
-                     Nothing -> empty
-                     Just f  -> text "In output file " <+> quotes (text f)
+    arg_ty            = idType arg
+    mb_mono_prim_reps = typeMonoPrimRep_maybe arg_ty
+
+    panic_rhs = mkAbsentErrorApp arg_ty msg
+
+    msg       = showSDoc (gopt_set dflags Opt_SuppressUniques)
+                         (vcat
+                           [ text "Arg:" <+> ppr arg
+                           , text "Type:" <+> ppr arg_ty
+                           , file_msg
+                           ])
               -- We need to suppress uniques here because otherwise they'd
               -- end up in the generated code as strings. This is bad for
               -- determinism, because with different uniques the strings
               -- will have different lengths and hence different costs for
               -- the inliner leading to different inlining.
               -- See also Note [Unique Determinism] in GHC.Types.Unique
+    file_msg  = case outputFile dflags of
+                  Nothing -> empty
+                  Just f  -> text "In output file " <+> quotes (text f)
 
 ww_prefix :: FastString
 ww_prefix = fsLit "ww"
diff --git a/compiler/GHC/Core/TyCon.hs b/compiler/GHC/Core/TyCon.hs
index 87b7336a76..a460116c3b 100644
--- a/compiler/GHC/Core/TyCon.hs
+++ b/compiler/GHC/Core/TyCon.hs
@@ -1,6 +1,7 @@
-{-# LANGUAGE CPP               #-}
-{-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE LambdaCase        #-}
+{-# LANGUAGE CPP                #-}
+{-# LANGUAGE FlexibleInstances  #-}
+{-# LANGUAGE LambdaCase         #-}
+{-# LANGUAGE DeriveDataTypeable #-}
 
 {-
 (c) The University of Glasgow 2006
@@ -121,6 +122,7 @@ module GHC.Core.TyCon(
 
         -- * Primitive representations of Types
         PrimRep(..), PrimElemRep(..),
+        primElemRepToPrimRep,
         isVoidRep, isGcPtrRep,
         primRepSizeB,
         primElemRepSizeB,
@@ -1480,7 +1482,7 @@ data PrimRep
   | FloatRep
   | DoubleRep
   | VecRep Int PrimElemRep  -- ^ A vector
-  deriving( Eq, Show )
+  deriving( Data.Data, Eq, Ord, Show )
 
 data PrimElemRep
   = Int8ElemRep
@@ -1493,7 +1495,7 @@ data PrimElemRep
   | Word64ElemRep
   | FloatElemRep
   | DoubleElemRep
-   deriving( Eq, Show )
+   deriving( Data.Data, Eq, Ord, Show, Enum )
 
 instance Outputable PrimRep where
   ppr r = text (show r)
@@ -1501,6 +1503,50 @@ instance Outputable PrimRep where
 instance Outputable PrimElemRep where
   ppr r = text (show r)
 
+instance Binary PrimRep where
+  put_ bh VoidRep        = putByte bh 0
+  put_ bh LiftedRep      = putByte bh 1
+  put_ bh UnliftedRep    = putByte bh 2
+  put_ bh Int8Rep        = putByte bh 3
+  put_ bh Int16Rep       = putByte bh 4
+  put_ bh Int32Rep       = putByte bh 5
+  put_ bh Int64Rep       = putByte bh 6
+  put_ bh IntRep         = putByte bh 7
+  put_ bh Word8Rep       = putByte bh 8
+  put_ bh Word16Rep      = putByte bh 9
+  put_ bh Word32Rep      = putByte bh 10
+  put_ bh Word64Rep      = putByte bh 11
+  put_ bh WordRep        = putByte bh 12
+  put_ bh AddrRep        = putByte bh 13
+  put_ bh FloatRep       = putByte bh 14
+  put_ bh DoubleRep      = putByte bh 15
+  put_ bh (VecRep n per) = putByte bh 16 *> put_ bh n *> put_ bh per
+  get  bh = do
+    h <- getByte bh
+    case h of
+      0  -> pure VoidRep
+      1  -> pure LiftedRep
+      2  -> pure UnliftedRep
+      3  -> pure Int8Rep
+      4  -> pure Int16Rep
+      5  -> pure Int32Rep
+      6  -> pure Int64Rep
+      7  -> pure IntRep
+      8  -> pure Word8Rep
+      9  -> pure Word16Rep
+      10 -> pure Word32Rep
+      11 -> pure Word64Rep
+      12 -> pure WordRep
+      13 -> pure AddrRep
+      14 -> pure FloatRep
+      15 -> pure DoubleRep
+      16 -> VecRep <$> get bh <*> get bh
+      _  -> pprPanic "Binary:PrimRep" (int (fromIntegral h))
+
+instance Binary PrimElemRep where
+  put_ bh per = putByte bh (fromIntegral (fromEnum per))
+  get  bh = toEnum . fromIntegral <$> getByte bh
+
 isVoidRep :: PrimRep -> Bool
 isVoidRep VoidRep = True
 isVoidRep _other  = False
@@ -1552,19 +1598,22 @@ primRepSizeB platform = \case
    LiftedRep        -> platformWordSizeInBytes platform
    UnliftedRep      -> platformWordSizeInBytes platform
    VoidRep          -> 0
-   (VecRep len rep) -> len * primElemRepSizeB rep
-
-primElemRepSizeB :: PrimElemRep -> Int
-primElemRepSizeB Int8ElemRep   = 1
-primElemRepSizeB Int16ElemRep  = 2
-primElemRepSizeB Int32ElemRep  = 4
-primElemRepSizeB Int64ElemRep  = 8
-primElemRepSizeB Word8ElemRep  = 1
-primElemRepSizeB Word16ElemRep = 2
-primElemRepSizeB Word32ElemRep = 4
-primElemRepSizeB Word64ElemRep = 8
-primElemRepSizeB FloatElemRep  = 4
-primElemRepSizeB DoubleElemRep = 8
+   (VecRep len rep) -> len * primElemRepSizeB platform rep
+
+primElemRepSizeB :: Platform -> PrimElemRep -> Int
+primElemRepSizeB platform = primRepSizeB platform . primElemRepToPrimRep
+
+primElemRepToPrimRep :: PrimElemRep -> PrimRep
+primElemRepToPrimRep Int8ElemRep   = Int8Rep
+primElemRepToPrimRep Int16ElemRep  = Int16Rep
+primElemRepToPrimRep Int32ElemRep  = Int32Rep
+primElemRepToPrimRep Int64ElemRep  = Int64Rep
+primElemRepToPrimRep Word8ElemRep  = Word8Rep
+primElemRepToPrimRep Word16ElemRep = Word16Rep
+primElemRepToPrimRep Word32ElemRep = Word32Rep
+primElemRepToPrimRep Word64ElemRep = Word64Rep
+primElemRepToPrimRep FloatElemRep  = FloatRep
+primElemRepToPrimRep DoubleElemRep = DoubleRep
 
 -- | Return if Rep stands for floating type,
 -- returns Nothing for vector types.
@@ -1574,7 +1623,6 @@ primRepIsFloat  DoubleRep    = Just True
 primRepIsFloat  (VecRep _ _) = Nothing
 primRepIsFloat  _            = Just False
 
-
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/GHC/Core/Utils.hs b/compiler/GHC/Core/Utils.hs
index 3f228f747d..6b779ef1aa 100644
--- a/compiler/GHC/Core/Utils.hs
+++ b/compiler/GHC/Core/Utils.hs
@@ -1609,11 +1609,14 @@ expr_ok primop_ok (Case scrut bndr _ alts)
 expr_ok primop_ok other_expr
   | (expr, args) <- collectArgs other_expr
   = case stripTicksTopE (not . tickishCounts) expr of
-        Var f   -> app_ok primop_ok f args
+        Var f            -> app_ok primop_ok f args
         -- 'LitRubbish' is the only literal that can occur in the head of an
         -- application and will not be matched by the above case (Var /= Lit).
-        Lit lit -> ASSERT( isRubbishLit lit ) True
-        _       -> False
+        Lit LitRubbish{} -> True
+#if defined(DEBUG)
+        Lit _            -> pprPanic "Non-rubbish lit in app head" (ppr other_expr)
+#endif
+        _                -> False
 
 -----------------------------
 app_ok :: (PrimOp -> Bool) -> Id -> [CoreExpr] -> Bool
diff --git a/compiler/GHC/CoreToStg.hs b/compiler/GHC/CoreToStg.hs
index d8a6dd0e95..1158fcde39 100644
--- a/compiler/GHC/CoreToStg.hs
+++ b/compiler/GHC/CoreToStg.hs
@@ -39,7 +39,7 @@ import GHC.Types.Var.Env
 import GHC.Unit.Module
 import GHC.Types.Name   ( isExternalName, nameModule_maybe )
 import GHC.Types.Basic  ( Arity )
-import GHC.Builtin.Types ( unboxedUnitDataCon, unitDataConId )
+import GHC.Builtin.Types ( unboxedUnitDataCon )
 import GHC.Types.Literal
 import GHC.Utils.Outputable
 import GHC.Utils.Monad
@@ -388,12 +388,8 @@ coreToStgExpr
 -- CorePrep should have converted them all to a real core representation.
 coreToStgExpr (Lit (LitNumber LitNumInteger _)) = panic "coreToStgExpr: LitInteger"
 coreToStgExpr (Lit (LitNumber LitNumNatural _)) = panic "coreToStgExpr: LitNatural"
-coreToStgExpr (Lit l)      = return (StgLit l)
-coreToStgExpr (App (Lit lit) _some_boxed_type)
-  | isRubbishLit lit
-  -- We lower 'LitRubbish' to @()@ here, which is much easier than doing it in
-  -- a STG to Cmm pass. Doesn't matter whether it is lifted or unlifted
-  = coreToStgExpr (Var unitDataConId)
+coreToStgExpr (Lit l)                           = return (StgLit l)
+coreToStgExpr (App l@(Lit LitRubbish{}) Type{}) = coreToStgExpr l
 coreToStgExpr (Var v) = coreToStgApp v [] []
 coreToStgExpr (Coercion _)
   -- See Note [Coercion tokens]
diff --git a/compiler/GHC/Stg/Unarise.hs b/compiler/GHC/Stg/Unarise.hs
index c9160ff72a..03c2deb03e 100644
--- a/compiler/GHC/Stg/Unarise.hs
+++ b/compiler/GHC/Stg/Unarise.hs
@@ -193,6 +193,9 @@ STG programs after unarisation have these invariants:
     This means that it's safe to wrap `StgArg`s of DataCon applications with
     `GHC.StgToCmm.Env.NonVoid`, for example.
 
+  * Similar to unboxed tuples, Note [Rubbish values] of TupleRep may only
+    appear in return position.
+
   * Alt binders (binders in patterns) are always non-void.
 
   * Binders always have zero (for void arguments) or one PrimRep.
@@ -207,6 +210,7 @@ import GHC.Prelude
 import GHC.Types.Basic
 import GHC.Core
 import GHC.Core.DataCon
+import GHC.Core.TyCon ( isVoidRep )
 import GHC.Data.FastString (FastString, mkFastString)
 import GHC.Types.Id
 import GHC.Types.Literal
@@ -349,6 +353,11 @@ unariseExpr rho (StgCase scrut bndr alt_ty alts)
   , Just args' <- unariseMulti_maybe rho dc args ty_args
   = elimCase rho args' bndr alt_ty alts
 
+  -- See (3) of Note [Rubbish values] in GHC.Types.Literal
+  | StgLit lit <- scrut
+  , Just args' <- unariseRubbish_maybe lit
+  = elimCase rho args' bndr alt_ty alts
+
   -- general case
   | otherwise
   = do scrut' <- unariseExpr rho scrut
@@ -379,6 +388,22 @@ unariseMulti_maybe rho dc args ty_args
   | otherwise
   = Nothing
 
+-- Doesn't return void args.
+unariseRubbish_maybe :: Literal -> Maybe [OutStgArg]
+unariseRubbish_maybe lit
+  | LitRubbish preps <- lit
+  , [prep] <- preps
+  , not (isVoidRep prep)
+  -- Single, non-void PrimRep. Nothing to do!
+  = Nothing
+
+  | LitRubbish preps <- lit
+  -- Multiple reps, possibly with VoidRep. Eliminate!
+  = Just [ StgLitArg (LitRubbish [prep]) | prep <- preps, not (isVoidRep prep) ]
+
+  | otherwise
+  = Nothing
+
 --------------------------------------------------------------------------------
 
 elimCase :: UnariseEnv
@@ -719,8 +744,11 @@ unariseConArg rho (StgVarArg x) =
                                   -- Here realWorld# is not in the envt, but
                                   -- is a void, and so should be eliminated
       | otherwise -> [StgVarArg x]
-unariseConArg _ arg@(StgLitArg lit) =
-    ASSERT(not (isVoidTy (literalType lit)))  -- We have no void literals
+unariseConArg _ arg@(StgLitArg lit)
+  | Just as <- unariseRubbish_maybe lit
+  = as
+  | otherwise
+  = ASSERT(not (isVoidTy (literalType lit))) -- We have no non-rubbish void literals
     [arg]
 
 unariseConArgs :: UnariseEnv -> [InStgArg] -> [OutStgArg]
diff --git a/compiler/GHC/StgToCmm/ArgRep.hs b/compiler/GHC/StgToCmm/ArgRep.hs
index 8fc1796d6f..2ec0e177e8 100644
--- a/compiler/GHC/StgToCmm/ArgRep.hs
+++ b/compiler/GHC/StgToCmm/ArgRep.hs
@@ -87,7 +87,7 @@ toArgRep platform rep = case rep of
                            PW8 -> N
    FloatRep          -> F
    DoubleRep         -> D
-   (VecRep len elem) -> case len*primElemRepSizeB elem of
+   (VecRep len elem) -> case len*primElemRepSizeB platform elem of
                            16 -> V16
                            32 -> V32
                            64 -> V64
diff --git a/compiler/GHC/StgToCmm/Env.hs b/compiler/GHC/StgToCmm/Env.hs
index ebfff0185f..5f4ef641c4 100644
--- a/compiler/GHC/StgToCmm/Env.hs
+++ b/compiler/GHC/StgToCmm/Env.hs
@@ -17,7 +17,6 @@ module GHC.StgToCmm.Env (
 
         bindArgsToRegs, bindToReg, rebindToReg,
         bindArgToReg, idToReg,
-        getArgAmode, getNonVoidArgAmodes,
         getCgIdInfo,
         maybeLetNoEscape,
     ) where
@@ -26,10 +25,8 @@ module GHC.StgToCmm.Env (
 
 import GHC.Prelude
 
-import GHC.Core.TyCon
 import GHC.Platform
 import GHC.StgToCmm.Monad
-import GHC.StgToCmm.Utils
 import GHC.StgToCmm.Closure
 
 import GHC.Cmm.CLabel
@@ -40,7 +37,6 @@ import GHC.Cmm.Utils
 import GHC.Types.Id
 import GHC.Cmm.Graph
 import GHC.Types.Name
-import GHC.Stg.Syntax
 import GHC.Core.Type
 import GHC.Builtin.Types.Prim
 import GHC.Types.Unique.FM
@@ -162,22 +158,6 @@ cgLookupPanic id
               ])
 
 
---------------------
-getArgAmode :: NonVoid StgArg -> FCode CmmExpr
-getArgAmode (NonVoid (StgVarArg var)) = idInfoToAmode <$> getCgIdInfo var
-getArgAmode (NonVoid (StgLitArg lit)) = CmmLit <$> cgLit lit
-
-getNonVoidArgAmodes :: [StgArg] -> FCode [CmmExpr]
--- NB: Filters out void args,
---     so the result list may be shorter than the argument list
-getNonVoidArgAmodes [] = return []
-getNonVoidArgAmodes (arg:args)
-  | isVoidRep (argPrimRep arg) = getNonVoidArgAmodes args
-  | otherwise = do { amode  <- getArgAmode (NonVoid arg)
-                   ; amodes <- getNonVoidArgAmodes args
-                   ; return ( amode : amodes ) }
-
-
 ------------------------------------------------------------------------
 --        Interface functions for binding and re-binding names
 ------------------------------------------------------------------------
diff --git a/compiler/GHC/StgToCmm/Expr.hs b/compiler/GHC/StgToCmm/Expr.hs
index 1b57fc3813..dbc2a9ea06 100644
--- a/compiler/GHC/StgToCmm/Expr.hs
+++ b/compiler/GHC/StgToCmm/Expr.hs
@@ -10,7 +10,7 @@
 --
 -----------------------------------------------------------------------------
 
-module GHC.StgToCmm.Expr ( cgExpr ) where
+module GHC.StgToCmm.Expr ( cgExpr, cgLit ) where
 
 #include "HsVersions.h"
 
@@ -24,6 +24,7 @@ import GHC.StgToCmm.Env
 import GHC.StgToCmm.DataCon
 import GHC.StgToCmm.Prof (saveCurrentCostCentre, restoreCurrentCostCentre, emitSetCCC)
 import GHC.StgToCmm.Layout
+import GHC.StgToCmm.Lit
 import GHC.StgToCmm.Prim
 import GHC.StgToCmm.Hpc
 import GHC.StgToCmm.Ticky
@@ -115,8 +116,8 @@ cgExpr (StgOpApp (StgPrimOp DataToTagOp) [StgVarArg a] _res_ty) = do
 cgExpr (StgOpApp op args ty) = cgOpApp op args ty
 cgExpr (StgConApp con mn args _) = cgConApp con mn args
 cgExpr (StgTick t e)         = cgTick t >> cgExpr e
-cgExpr (StgLit lit)       = do cmm_lit <- cgLit lit
-                               emitReturn [CmmLit cmm_lit]
+cgExpr (StgLit lit)          = do cmm_expr <- cgLit lit
+                                  emitReturn [cmm_expr]
 
 cgExpr (StgLet _ binds expr) = do { cgBind binds;     cgExpr expr }
 cgExpr (StgLetNoEscape _ binds expr) =
diff --git a/compiler/GHC/StgToCmm/Expr.hs-boot b/compiler/GHC/StgToCmm/Expr.hs-boot
new file mode 100644
index 0000000000..5dd63a81dc
--- /dev/null
+++ b/compiler/GHC/StgToCmm/Expr.hs-boot
@@ -0,0 +1,7 @@
+module GHC.StgToCmm.Expr where
+
+import GHC.Cmm.Expr
+import GHC.StgToCmm.Monad
+import GHC.Types.Literal
+
+cgLit :: Literal -> FCode CmmExpr
diff --git a/compiler/GHC/StgToCmm/Foreign.hs b/compiler/GHC/StgToCmm/Foreign.hs
index 21c85d569c..95fa21d648 100644
--- a/compiler/GHC/StgToCmm/Foreign.hs
+++ b/compiler/GHC/StgToCmm/Foreign.hs
@@ -27,7 +27,6 @@ import GHC.Platform.Profile
 
 import GHC.Stg.Syntax
 import GHC.StgToCmm.Prof (storeCurCCS, ccsType)
-import GHC.StgToCmm.Env
 import GHC.StgToCmm.Monad
 import GHC.StgToCmm.Utils
 import GHC.StgToCmm.Closure
diff --git a/compiler/GHC/StgToCmm/Heap.hs b/compiler/GHC/StgToCmm/Heap.hs
index 62b9785ed6..16161cb028 100644
--- a/compiler/GHC/StgToCmm/Heap.hs
+++ b/compiler/GHC/StgToCmm/Heap.hs
@@ -32,7 +32,6 @@ import GHC.StgToCmm.Monad
 import GHC.StgToCmm.Prof (profDynAlloc, dynProfHdr, staticProfHdr)
 import GHC.StgToCmm.Ticky
 import GHC.StgToCmm.Closure
-import GHC.StgToCmm.Env
 
 import GHC.Cmm.Graph
 
diff --git a/compiler/GHC/StgToCmm/Layout.hs b/compiler/GHC/StgToCmm/Layout.hs
index e45955d119..d10d7f6345 100644
--- a/compiler/GHC/StgToCmm/Layout.hs
+++ b/compiler/GHC/StgToCmm/Layout.hs
@@ -26,7 +26,8 @@ module GHC.StgToCmm.Layout (
         mkVirtConstrSizes,
         getHpRelOffset,
 
-        ArgRep(..), toArgRep, argRepSizeW -- re-exported from GHC.StgToCmm.ArgRep
+        ArgRep(..), toArgRep, argRepSizeW, -- re-exported from GHC.StgToCmm.ArgRep
+        getArgAmode, getNonVoidArgAmodes
   ) where
 
 
@@ -42,6 +43,7 @@ import GHC.StgToCmm.Env
 import GHC.StgToCmm.ArgRep -- notably: ( slowCallPattern )
 import GHC.StgToCmm.Ticky
 import GHC.StgToCmm.Monad
+import GHC.StgToCmm.Lit
 import GHC.StgToCmm.Utils
 
 import GHC.Cmm.Graph
@@ -591,6 +593,24 @@ stdPattern reps
         _ -> Nothing
 
 -------------------------------------------------------------------------
+--        Amodes for arguments
+-------------------------------------------------------------------------
+
+getArgAmode :: NonVoid StgArg -> FCode CmmExpr
+getArgAmode (NonVoid (StgVarArg var)) = idInfoToAmode <$> getCgIdInfo var
+getArgAmode (NonVoid (StgLitArg lit)) = cgLit lit
+
+getNonVoidArgAmodes :: [StgArg] -> FCode [CmmExpr]
+-- NB: Filters out void args,
+--     so the result list may be shorter than the argument list
+getNonVoidArgAmodes [] = return []
+getNonVoidArgAmodes (arg:args)
+  | isVoidRep (argPrimRep arg) = getNonVoidArgAmodes args
+  | otherwise = do { amode  <- getArgAmode (NonVoid arg)
+                   ; amodes <- getNonVoidArgAmodes args
+                   ; return ( amode : amodes ) }
+
+-------------------------------------------------------------------------
 --
 --        Generating the info table and code for a closure
 --
diff --git a/compiler/GHC/StgToCmm/Lit.hs b/compiler/GHC/StgToCmm/Lit.hs
new file mode 100644
index 0000000000..244a593f9a
--- /dev/null
+++ b/compiler/GHC/StgToCmm/Lit.hs
@@ -0,0 +1,105 @@
+{-# LANGUAGE CPP, LambdaCase #-}
+
+-----------------------------------------------------------------------------
+--
+-- Stg to C-- code generation: literals
+--
+-- (c) The University of Glasgow 2004-2006
+--
+-----------------------------------------------------------------------------
+
+module GHC.StgToCmm.Lit (
+    cgLit, mkSimpleLit,
+    newStringCLit, newByteStringCLit
+  ) where
+
+#include "HsVersions.h"
+
+import GHC.Prelude
+
+import GHC.Platform
+import GHC.StgToCmm.Monad
+import GHC.StgToCmm.Env
+import GHC.Cmm
+import GHC.Cmm.CLabel
+import GHC.Cmm.Utils
+
+import GHC.Types.Literal
+import GHC.Builtin.Types ( unitDataConId )
+import GHC.Core.TyCon
+import GHC.Utils.Misc
+import GHC.Utils.Outputable
+import GHC.Utils.Panic
+
+import Data.ByteString (ByteString)
+import qualified Data.ByteString.Char8 as BS8
+import Data.Char (ord)
+
+newStringCLit :: String -> FCode CmmLit
+-- ^ Make a global definition for the string,
+-- and return its label
+newStringCLit str = newByteStringCLit (BS8.pack str)
+
+newByteStringCLit :: ByteString -> FCode CmmLit
+newByteStringCLit bytes
+  = do  { uniq <- newUnique
+        ; let (lit, decl) = mkByteStringCLit (mkStringLitLabel uniq) bytes
+        ; emitDecl decl
+        ; return lit }
+
+cgLit :: Literal -> FCode CmmExpr
+cgLit (LitString s) =
+  CmmLit <$> newByteStringCLit s
+ -- not unpackFS; we want the UTF-8 byte stream.
+cgLit (LitRubbish preps) =
+  case expectOnly "cgLit:Rubbish" preps of -- Note [Post-unarisation invariants]
+    VoidRep     -> panic "cgLit:VoidRep"   -- dito
+    LiftedRep   -> idInfoToAmode <$> getCgIdInfo unitDataConId
+    UnliftedRep -> idInfoToAmode <$> getCgIdInfo unitDataConId
+    AddrRep     -> cgLit LitNullAddr
+    VecRep n elem -> do
+      platform <- getPlatform
+      let elem_lit = mkSimpleLit platform (num_rep_lit (primElemRepToPrimRep elem))
+      pure (CmmLit (CmmVec (replicate n elem_lit)))
+    prep        -> cgLit (num_rep_lit prep)
+    where
+      num_rep_lit IntRep    = mkLitIntUnchecked 0
+      num_rep_lit Int8Rep   = mkLitInt8Unchecked 0
+      num_rep_lit Int16Rep  = mkLitInt16Unchecked 0
+      num_rep_lit Int32Rep  = mkLitInt32Unchecked 0
+      num_rep_lit Int64Rep  = mkLitInt64Unchecked 0
+      num_rep_lit WordRep   = mkLitWordUnchecked 0
+      num_rep_lit Word8Rep  = mkLitWord8Unchecked 0
+      num_rep_lit Word16Rep = mkLitWord16Unchecked 0
+      num_rep_lit Word32Rep = mkLitWord32Unchecked 0
+      num_rep_lit Word64Rep = mkLitWord64Unchecked 0
+      num_rep_lit FloatRep  = LitFloat 0
+      num_rep_lit DoubleRep = LitDouble 0
+      num_rep_lit other     = pprPanic "num_rep_lit: Not a num lit" (ppr other)
+cgLit other_lit = do
+  platform <- getPlatform
+  pure (CmmLit (mkSimpleLit platform other_lit))
+
+mkSimpleLit :: Platform -> Literal -> CmmLit
+mkSimpleLit platform = \case
+   (LitChar   c)                -> CmmInt (fromIntegral (ord c))
+                                          (wordWidth platform)
+   LitNullAddr                  -> zeroCLit platform
+   (LitNumber LitNumInt i)      -> CmmInt i (wordWidth platform)
+   (LitNumber LitNumInt8 i)     -> CmmInt i W8
+   (LitNumber LitNumInt16 i)    -> CmmInt i W16
+   (LitNumber LitNumInt32 i)    -> CmmInt i W32
+   (LitNumber LitNumInt64 i)    -> CmmInt i W64
+   (LitNumber LitNumWord i)     -> CmmInt i (wordWidth platform)
+   (LitNumber LitNumWord8 i)    -> CmmInt i W8
+   (LitNumber LitNumWord16 i)   -> CmmInt i W16
+   (LitNumber LitNumWord32 i)   -> CmmInt i W32
+   (LitNumber LitNumWord64 i)   -> CmmInt i W64
+   (LitFloat r)                 -> CmmFloat r W32
+   (LitDouble r)                -> CmmFloat r W64
+   (LitLabel fs ms fod)
+     -> let -- TODO: Literal labels might not actually be in the current package...
+            labelSrc = ForeignLabelInThisPackage
+        in CmmLabel (mkForeignLabel fs ms labelSrc fod)
+   other -> pprPanic "mkSimpleLit" (ppr other)
+
diff --git a/compiler/GHC/StgToCmm/Monad.hs b/compiler/GHC/StgToCmm/Monad.hs
index 98720a2f50..c2c3b93125 100644
--- a/compiler/GHC/StgToCmm/Monad.hs
+++ b/compiler/GHC/StgToCmm/Monad.hs
@@ -24,6 +24,8 @@ module GHC.StgToCmm.Monad (
         emitOutOfLine, emitAssign, emitStore,
         emitComment, emitTick, emitUnwind,
 
+        newTemp,
+
         getCmm, aGraphToGraph, getPlatform, getProfile,
         getCodeR, getCode, getCodeScoped, getHeapUsage,
         getCallOpts, getPtrOpts,
@@ -479,6 +481,10 @@ newUnique = do
         setState $ state { cgs_uniqs = us' }
         return u
 
+newTemp :: MonadUnique m => CmmType -> m LocalReg
+newTemp rep = do { uniq <- getUniqueM
+                 ; return (LocalReg uniq rep) }
+
 ------------------
 getInfoDown :: FCode CgInfoDownwards
 getInfoDown = FCode $ \info_down state -> (info_down,state)
diff --git a/compiler/GHC/StgToCmm/Prim.hs b/compiler/GHC/StgToCmm/Prim.hs
index 484863d37a..fbd08b55a9 100644
--- a/compiler/GHC/StgToCmm/Prim.hs
+++ b/compiler/GHC/StgToCmm/Prim.hs
@@ -25,7 +25,6 @@ import GHC.Platform.Profile
 
 import GHC.StgToCmm.Layout
 import GHC.StgToCmm.Foreign
-import GHC.StgToCmm.Env
 import GHC.StgToCmm.Monad
 import GHC.StgToCmm.Utils
 import GHC.StgToCmm.Ticky
diff --git a/compiler/GHC/StgToCmm/Prof.hs b/compiler/GHC/StgToCmm/Prof.hs
index f0b9b2ae8c..08a06f3242 100644
--- a/compiler/GHC/StgToCmm/Prof.hs
+++ b/compiler/GHC/StgToCmm/Prof.hs
@@ -36,6 +36,7 @@ import GHC.Platform.Profile
 import GHC.StgToCmm.Closure
 import GHC.StgToCmm.Utils
 import GHC.StgToCmm.Monad
+import GHC.StgToCmm.Lit
 import GHC.Runtime.Heap.Layout
 
 import GHC.Cmm.Graph
diff --git a/compiler/GHC/StgToCmm/Ticky.hs b/compiler/GHC/StgToCmm/Ticky.hs
index 44a99a0cae..e9e67f6b83 100644
--- a/compiler/GHC/StgToCmm/Ticky.hs
+++ b/compiler/GHC/StgToCmm/Ticky.hs
@@ -109,6 +109,7 @@ import GHC.StgToCmm.ArgRep    ( slowCallPattern , toArgRep , argRepString )
 import GHC.StgToCmm.Closure
 import GHC.StgToCmm.Utils
 import GHC.StgToCmm.Monad
+import GHC.StgToCmm.Lit       ( newStringCLit )
 
 import GHC.Stg.Syntax
 import GHC.Cmm.Expr
diff --git a/compiler/GHC/StgToCmm/Utils.hs b/compiler/GHC/StgToCmm/Utils.hs
index 86d8a8d842..35af67cc54 100644
--- a/compiler/GHC/StgToCmm/Utils.hs
+++ b/compiler/GHC/StgToCmm/Utils.hs
@@ -1,5 +1,4 @@
 {-# LANGUAGE CPP #-}
-{-# LANGUAGE LambdaCase #-}
 
 -----------------------------------------------------------------------------
 --
@@ -10,11 +9,10 @@
 -----------------------------------------------------------------------------
 
 module GHC.StgToCmm.Utils (
-        cgLit, mkSimpleLit,
         emitDataLits, emitRODataLits,
         emitDataCon,
         emitRtsCall, emitRtsCallWithResult, emitRtsCallGen,
-        assignTemp, newTemp,
+        assignTemp,
 
         newUnboxedTupleRegs,
 
@@ -38,7 +36,6 @@ module GHC.StgToCmm.Utils (
         cmmUntag, cmmIsTagged,
 
         addToMem, addToMemE, addToMemLblE, addToMemLbl,
-        newStringCLit, newByteStringCLit,
 
         -- * Update remembered set operations
         whenUpdRemSetEnabled,
@@ -55,6 +52,7 @@ import GHC.Prelude
 import GHC.Platform
 import GHC.StgToCmm.Monad
 import GHC.StgToCmm.Closure
+import GHC.StgToCmm.Lit (mkSimpleLit)
 import GHC.Cmm
 import GHC.Cmm.BlockId
 import GHC.Cmm.Graph as CmmGraph
@@ -74,7 +72,6 @@ import GHC.Types.Literal
 import GHC.Data.Graph.Directed
 import GHC.Utils.Misc
 import GHC.Types.Unique
-import GHC.Types.Unique.Supply (MonadUnique(..))
 import GHC.Driver.Session
 import GHC.Data.FastString
 import GHC.Utils.Outputable
@@ -83,10 +80,7 @@ import GHC.Types.RepType
 import GHC.Types.CostCentre
 import GHC.Types.IPE
 
-import Data.ByteString (ByteString)
-import qualified Data.ByteString.Char8 as BS8
 import qualified Data.Map as M
-import Data.Char
 import Data.List (sortBy)
 import Data.Ord
 import GHC.Types.Unique.Map
@@ -98,42 +92,6 @@ import GHC.Types.Unique.FM
 import GHC.Data.Maybe
 import Control.Monad
 
--------------------------------------------------------------------------
---
---      Literals
---
--------------------------------------------------------------------------
-
-cgLit :: Literal -> FCode CmmLit
-cgLit (LitString s) = newByteStringCLit s
- -- not unpackFS; we want the UTF-8 byte stream.
-cgLit other_lit     = do platform <- getPlatform
-                         return (mkSimpleLit platform other_lit)
-
-mkSimpleLit :: Platform -> Literal -> CmmLit
-mkSimpleLit platform = \case
-   (LitChar   c)                -> CmmInt (fromIntegral (ord c))
-                                          (wordWidth platform)
-   LitNullAddr                  -> zeroCLit platform
-   (LitNumber LitNumInt i)      -> CmmInt i (wordWidth platform)
-   (LitNumber LitNumInt8 i)     -> CmmInt i W8
-   (LitNumber LitNumInt16 i)    -> CmmInt i W16
-   (LitNumber LitNumInt32 i)    -> CmmInt i W32
-   (LitNumber LitNumInt64 i)    -> CmmInt i W64
-   (LitNumber LitNumWord i)     -> CmmInt i (wordWidth platform)
-   (LitNumber LitNumWord8 i)    -> CmmInt i W8
-   (LitNumber LitNumWord16 i)   -> CmmInt i W16
-   (LitNumber LitNumWord32 i)   -> CmmInt i W32
-   (LitNumber LitNumWord64 i)   -> CmmInt i W64
-   (LitFloat r)                 -> CmmFloat r W32
-   (LitDouble r)                -> CmmFloat r W64
-   (LitLabel fs ms fod)
-     -> let -- TODO: Literal labels might not actually be in the current package...
-            labelSrc = ForeignLabelInThisPackage
-        in CmmLabel (mkForeignLabel fs ms labelSrc fod)
-   -- NB: LitRubbish should have been lowered in "CoreToStg"
-   other -> pprPanic "mkSimpleLit" (ppr other)
-
 --------------------------------------------------------------------------
 --
 -- Incrementing a memory location
@@ -302,18 +260,6 @@ emitDataCon :: CLabel -> CmmInfoTable -> CostCentreStack -> [CmmLit] -> FCode ()
 emitDataCon lbl itbl ccs payload =
   emitDecl (CmmData (Section Data lbl) (CmmStatics lbl itbl ccs payload))
 
-newStringCLit :: String -> FCode CmmLit
--- Make a global definition for the string,
--- and return its label
-newStringCLit str = newByteStringCLit (BS8.pack str)
-
-newByteStringCLit :: ByteString -> FCode CmmLit
-newByteStringCLit bytes
-  = do  { uniq <- newUnique
-        ; let (lit, decl) = mkByteStringCLit (mkStringLitLabel uniq) bytes
-        ; emitDecl decl
-        ; return lit }
-
 -------------------------------------------------------------------------
 --
 --      Assigning expressions to temporaries
@@ -335,10 +281,6 @@ assignTemp e = do { platform <- getPlatform
                   ; emitAssign (CmmLocal reg) e
                   ; return reg }
 
-newTemp :: MonadUnique m => CmmType -> m LocalReg
-newTemp rep = do { uniq <- getUniqueM
-                 ; return (LocalReg uniq rep) }
-
 newUnboxedTupleRegs :: Type -> FCode ([LocalReg], [ForeignHint])
 -- Choose suitable local regs to use for the components
 -- of an unboxed tuple that we are about to return to
@@ -605,7 +547,6 @@ assignTemp' e
        emitAssign reg e
        return (CmmReg reg)
 
-
 ---------------------------------------------------------------------------
 -- Pushing to the update remembered set
 ---------------------------------------------------------------------------
diff --git a/compiler/GHC/Types/Literal.hs b/compiler/GHC/Types/Literal.hs
index be23f2405e..3d41444848 100644
--- a/compiler/GHC/Types/Literal.hs
+++ b/compiler/GHC/Types/Literal.hs
@@ -20,23 +20,23 @@ module GHC.Types.Literal
 
         -- ** Creating Literals
         , mkLitInt, mkLitIntWrap, mkLitIntWrapC, mkLitIntUnchecked
-        , mkLitWord, mkLitWordWrap, mkLitWordWrapC
-        , mkLitInt8, mkLitInt8Wrap
-        , mkLitWord8, mkLitWord8Wrap
-        , mkLitInt16, mkLitInt16Wrap
-        , mkLitWord16, mkLitWord16Wrap
-        , mkLitInt32, mkLitInt32Wrap
-        , mkLitWord32, mkLitWord32Wrap
-        , mkLitInt64, mkLitInt64Wrap
-        , mkLitWord64, mkLitWord64Wrap
+        , mkLitWord, mkLitWordWrap, mkLitWordWrapC, mkLitWordUnchecked
+        , mkLitInt8, mkLitInt8Wrap, mkLitInt8Unchecked
+        , mkLitWord8, mkLitWord8Wrap, mkLitWord8Unchecked
+        , mkLitInt16, mkLitInt16Wrap, mkLitInt16Unchecked
+        , mkLitWord16, mkLitWord16Wrap, mkLitWord16Unchecked
+        , mkLitInt32, mkLitInt32Wrap, mkLitInt32Unchecked
+        , mkLitWord32, mkLitWord32Wrap, mkLitWord32Unchecked
+        , mkLitInt64, mkLitInt64Wrap, mkLitInt64Unchecked
+        , mkLitWord64, mkLitWord64Wrap, mkLitWord64Unchecked
         , mkLitFloat, mkLitDouble
         , mkLitChar, mkLitString
         , mkLitInteger, mkLitNatural
         , mkLitNumber, mkLitNumberWrap
+        , mkLitRubbish
 
         -- ** Operations on Literals
         , literalType
-        , absentLiteralOf
         , pprLiteral
         , litNumIsSigned
         , litNumCheckRange
@@ -61,7 +61,6 @@ module GHC.Types.Literal
         , charToIntLit, intToCharLit
         , floatToIntLit, intToFloatLit, doubleToIntLit, intToDoubleLit
         , nullAddrLit, floatToDoubleLit, doubleToFloatLit
-        , rubbishLit, isRubbishLit
         ) where
 
 #include "HsVersions.h"
@@ -70,7 +69,6 @@ import GHC.Prelude
 
 import GHC.Builtin.Types.Prim
 import {-# SOURCE #-} GHC.Builtin.Types
-import GHC.Builtin.Names
 import GHC.Core.Type
 import GHC.Core.TyCon
 import GHC.Utils.Outputable
@@ -79,7 +77,6 @@ import GHC.Types.Basic
 import GHC.Utils.Binary
 import GHC.Settings.Constants
 import GHC.Platform
-import GHC.Types.Unique.FM
 import GHC.Utils.Misc
 import GHC.Utils.Panic
 
@@ -114,8 +111,7 @@ import Numeric ( fromRat )
 -- * The literal derived from the label mentioned in a \"foreign label\"
 --   declaration ('LitLabel')
 --
--- * A 'LitRubbish' to be used in place of values of 'UnliftedRep'
---   (i.e. 'MutVar#') when the value is never used.
+-- * A 'LitRubbish' to be used in place of values that are never used.
 --
 -- * A character
 -- * A string
@@ -138,10 +134,13 @@ data Literal
                                 -- that can be represented as a Literal. Create
                                 -- with 'nullAddrLit'
 
-  | LitRubbish Bool             -- ^ A nonsense value; always boxed, but
-                                --      True <=> lifted, False <=> unlifted
-                                -- Used when a binding is absent.
-                                -- See Note [Rubbish literals]
+  | LitRubbish [PrimRep]        -- ^ A nonsense value of the given
+                                -- representation. See Note [Rubbish values].
+                                --
+                                -- The @[PrimRep]@ of a 'Type' can be obtained
+                                -- from 'typeMonoPrimRep_maybe'. The field
+                                -- becomes empty or singleton post-unarisation,
+                                -- see Note [Post-unarisation invariants].
 
   | LitFloat   Rational         -- ^ @Float#@. Create with 'mkLitFloat'
   | LitDouble  Rational         -- ^ @Double#@. Create with 'mkLitDouble'
@@ -194,6 +193,12 @@ litNumIsSigned nt = case nt of
   LitNumWord32  -> False
   LitNumWord64  -> False
 
+instance Binary LitNumType where
+   put_ bh numTyp = putByte bh (fromIntegral (fromEnum numTyp))
+   get bh = do
+      h <- getByte bh
+      return (toEnum (fromIntegral h))
+
 {-
 Note [BigNum literals]
 ~~~~~~~~~~~~~~~~~~~~~~
@@ -225,12 +230,6 @@ for more details.
 
 -}
 
-instance Binary LitNumType where
-   put_ bh numTyp = putByte bh (fromIntegral (fromEnum numTyp))
-   get bh = do
-      h <- getByte bh
-      return (toEnum (fromIntegral h))
-
 instance Binary Literal where
     put_ bh (LitChar aa)     = do putByte bh 0; put_ bh aa
     put_ bh (LitString ab)   = do putByte bh 1; put_ bh ab
@@ -272,9 +271,10 @@ instance Binary Literal where
                     nt <- get bh
                     i  <- get bh
                     return (LitNumber nt i)
-              _ -> do
+              7 -> do
                     b <- get bh
                     return (LitRubbish b)
+              _ -> pprPanic "Binary:Literal" (int (fromIntegral h))
 
 instance Outputable Literal where
     ppr = pprLiteral id
@@ -555,6 +555,12 @@ mkLitNatural :: Integer -> Literal
 mkLitNatural x = ASSERT2( inNaturalRange x,  integer x )
                     (LitNumber LitNumNatural x)
 
+-- | Create a rubbish literal of the given representation.
+-- The representation of a 'Type' can be obtained via 'typeMonoPrimRep_maybe'.
+-- See Note [Rubbish values].
+mkLitRubbish :: [PrimRep] -> Literal
+mkLitRubbish = LitRubbish
+
 inNaturalRange :: Integer -> Bool
 inNaturalRange x = x >= 0
 
@@ -694,14 +700,6 @@ doubleToFloatLit l             = pprPanic "doubleToFloatLit" (ppr l)
 nullAddrLit :: Literal
 nullAddrLit = LitNullAddr
 
--- | A rubbish literal; see Note [Rubbish literals]
-rubbishLit :: Bool -> Literal
-rubbishLit is_lifted = LitRubbish is_lifted
-
-isRubbishLit :: Literal -> Bool
-isRubbishLit (LitRubbish {}) = True
-isRubbishLit _               = False
-
 {-
         Predicates
         ~~~~~~~~~~
@@ -797,7 +795,8 @@ litIsLifted (LitNumber nt _) = case nt of
   LitNumWord16  -> False
   LitNumWord32  -> False
   LitNumWord64  -> False
-litIsLifted _                  = False
+litIsLifted _                        = False
+  -- Even RUBBISH[LiftedRep] is unlifted, as rubbish values are always evaluated.
 
 {-
         Types
@@ -825,40 +824,10 @@ literalType (LitNumber lt _)  = case lt of
    LitNumWord16  -> word16PrimTy
    LitNumWord32  -> word32PrimTy
    LitNumWord64  -> word64PrimTy
-literalType (LitRubbish is_lifted) = mkForAllTy a Inferred (mkTyVarTy a)
+literalType (LitRubbish preps) = mkForAllTy a Inferred (mkTyVarTy a)
   where
-    -- See Note [Rubbish literals]
-    a | is_lifted = alphaTyVar
-      | otherwise = alphaTyVarUnliftedRep
-
-absentLiteralOf :: TyCon -> Maybe Literal
--- Return a literal of the appropriate primitive
--- TyCon, to use as a placeholder when it doesn't matter
--- Rubbish literals are handled in GHC.Core.Opt.WorkWrap.Utils, because
---  1. Looking at the TyCon is not enough, we need the actual type
---  2. This would need to return a type application to a literal
-absentLiteralOf tc = lookupUFM absent_lits tc
-
--- We do not use TyConEnv here to avoid import cycles.
-absent_lits :: UniqFM TyCon Literal
-absent_lits = listToUFM_Directly
-                        -- Explicitly construct the mape from the known
-                        -- keys of these tyCons.
-                        [ (addrPrimTyConKey,    LitNullAddr)
-                        , (charPrimTyConKey,    LitChar 'x')
-                        , (intPrimTyConKey,     mkLitIntUnchecked 0)
-                        , (int8PrimTyConKey,    mkLitInt8Unchecked 0)
-                        , (int16PrimTyConKey,   mkLitInt16Unchecked 0)
-                        , (int32PrimTyConKey,   mkLitInt32Unchecked 0)
-                        , (int64PrimTyConKey,   mkLitInt64Unchecked 0)
-                        , (wordPrimTyConKey,    mkLitWordUnchecked 0)
-                        , (word8PrimTyConKey,   mkLitWord8Unchecked 0)
-                        , (word16PrimTyConKey,  mkLitWord16Unchecked 0)
-                        , (word32PrimTyConKey,  mkLitWord32Unchecked 0)
-                        , (word64PrimTyConKey,  mkLitWord64Unchecked 0)
-                        , (floatPrimTyConKey,   LitFloat 0)
-                        , (doublePrimTyConKey,  LitDouble 0)
-                        ]
+    -- See Note [Rubbish values]
+    a = head $ mkTemplateTyVars [tYPE (primRepsToRuntimeRep preps)]
 
 {-
         Comparison
@@ -910,9 +879,8 @@ pprLiteral add_par (LitLabel l mb fod) =
     where b = case mb of
               Nothing -> pprHsString l
               Just x  -> doubleQuotes (text (unpackFS l ++ '@':show x))
-pprLiteral _       (LitRubbish is_lifted)
-  = text "__RUBBISH"
-    <> parens (if is_lifted then text "lifted" else text "unlifted")
+pprLiteral _       (LitRubbish reps)
+  = text "RUBBISH" <> ppr reps
 
 pprIntegerVal :: (SDoc -> SDoc) -> Integer -> SDoc
 -- See Note [Printing of literals in Core].
@@ -954,61 +922,77 @@ LitFloat        -1.0#
 LitDouble       -1.0##
 LitInteger      -1                 (-1)
 LitLabel        "__label" ...      ("__label" ...)
-LitRubbish      "__RUBBISH"
-
-Note [Rubbish literals]
-~~~~~~~~~~~~~~~~~~~~~~~
-During worker/wrapper after demand analysis, where an argument
-is unused (absent) we do the following w/w split (supposing that
-y is absent):
-
-  f x y z = e
-===>
-  f x y z = $wf x z
-  $wf x z = let y = <absent value>
-            in e
-
-Usually the binding for y is ultimately optimised away, and
-even if not it should never be evaluated -- but that's the
-way the w/w split starts off.
-
-What is <absent value>?
-* For lifted values <absent value> can be a call to 'error'.
-* For primitive types like Int# or Word# we can use any random
-  value of that type.
-* But what about /unlifted/ but /boxed/ types like MutVar# or
-  Array#?  Or /lifted/ but /strict/ values, such as a field of
-  a strict data constructor.  For these we use LitRubbish.
-  See Note [Absent errors] in GHC.Core.Opt.WorkWrap.Utils.hs
-
-The literal (LitRubbish is_lifted)
-has type
-  LitRubbish :: forall (a :: TYPE LiftedRep). a     if is_lifted
-  LitRubbish :: forall (a :: TYPE UnliftedRep). a   otherwise
-
-So we might see a w/w split like
-  $wf x z = let y :: Array# Int = (LitRubbish False) @(Array# Int)
-            in e
-
-Here are the moving parts, but see also Note [Absent errors] in
-GHC.Core.Opt.WorkWrap.Utils
-
-* We define LitRubbish as a constructor in GHC.Types.Literal.Literal
-
-* It is given its polymorphic type by Literal.literalType
-
-* GHC.Core.Opt.WorkWrap.Utils.mk_absent_let introduces a LitRubbish for absent
-  arguments of boxed, unlifted type; or boxed, lifted arguments of strict data
-  constructors.
-
-* In CoreToSTG we convert (RubishLit @t) to just ().  STG is untyped, so this
-  will work OK for both lifted and unlifted (but boxed) values. The important
-  thing is that it is a heap pointer, which the garbage collector can follow if
-  it encounters it.
-
-  We considered maintaining LitRubbish in STG, and lowering it in the code
-  generators, but it seems simpler to do it once and for all in CoreToSTG.
-
-  In GHC.ByteCode.Asm we just lower it as a 0 literal, because it's all boxed to
-  the host GC anyway.
--}
+LitRubbish      "RUBBISH[...]"
+
+Note [Rubbish values]
+~~~~~~~~~~~~~~~~~~~~~
+Sometimes, we need to cough up a rubbish value of a certain type that is used
+in place of dead code we thus aim to eliminate. The value of a dead occurrence
+has no effect on the dynamic semantics of the program, so we can pick any value
+of the same representation.
+Exploiting the results of absence analysis in worker/wrapper is a scenario where
+we need such a rubbish value, see Note [Absent fillers] for examples.
+
+It's completely undefined what the *value* of a rubbish value is, e.g., we could
+pick @0#@ for @Int#@ or @42#@; it mustn't matter where it's inserted into a Core
+program. We embed these rubbish values in the 'LitRubbish' case of the 'Literal'
+data type. Here are the moving parts:
+
+  1. Source Haskell: No way to produce rubbish lits in source syntax. Purely
+     an IR feature.
+
+  2. Core: 'LitRubbish' carries a @[PrimRep]@ which represents the monomorphic
+     'RuntimeRep' of the type it is substituting for.
+     We have it that @RUBBISH[IntRep]@ has type @forall (a :: TYPE IntRep). a@,
+     and the type application @RUBBISH[IntRep] \@Int# :: Int#@ represents
+     a rubbish value of type @Int#@. Rubbish lits are completely opaque in Core.
+     In general, @RUBBISH[preps] :: forall (a :: TYPE rep). a@, where @rep@
+     is the 'RuntimeRep' corresponding to @preps :: [PrimRep]@
+     (via 'primRepsToRuntimeRep'). See 'literalType'.
+     Why not encode a 'RuntimeRep' via a @Type@? Thus
+     > data Literal = ... | LitRubbish Type | ...
+     Because
+       * We have to provide an Eq and Ord instance and @Type@ has none
+       * The encoded @Type@ might be polymorphic and we can only emit code for
+         monomorphic 'RuntimeRep's anyway.
+
+  3. STG: The type app in @RUBBISH[IntRep] \@Int# :: Int#@ is erased and we get
+     the (untyped) 'StgLit' @RUBBISH[IntRep] :: Int#@ in STG.
+     It's treated mostly opaque, with the exception of the Unariser, where we
+     take apart a case scrutinisation on, or arg occurrence of, e.g.,
+     @RUBBISH[IntRep,DoubleRep]@ (which may stand in for @(# Int#, Double# #)@)
+     into its sub-parts @RUBBISH[IntRep]@ and @RUBBISH[DoubleRep]@, similar to
+     unboxed tuples. @RUBBISH[VoidRep]@ is erased.
+     See 'unariseRubbish_maybe' and also Note [Post-unarisation invariants].
+
+  4. Cmm: We translate 'LitRubbish' to their actual rubbish value in 'cgLit'.
+     The particulars are boring, and only matter when debugging illicit use of
+     a rubbish value; see Modes of failure below.
+
+  5. Bytecode: In GHC.ByteCode.Asm we just lower it as a 0 literal, because it's
+     all boxed to the host GC anyway.
+
+Why not lower LitRubbish in CoreToStg? Because it enables us to use RubbishLit
+when unarising unboxed sums in the future, and it allows rubbish values of e.g.
+VecRep, for which we can't cough up dummy values in STG.
+
+Modes of failure
+----------------
+Suppose there is a bug in GHC, and a rubbish value is used after all. That is
+undefined behavior, of course, but let us list a few examples for failure modes:
+
+ a) For an value of unboxed numeric type like @Int#@, we just use a silly
+    value like 42#. The error might propoagate indefinitely, hence we better
+    pick a rather unique literal. Same for Word, Floats, Char and VecRep.
+ b) For AddrRep (like String lits), we mit a null pointer, resulting in a
+    definitive segfault when accessed.
+ c) For boxed values, unlifted or not, we use a pointer to a fixed closure,
+    like @()@, so that the GC has a pointer to follow.
+    If we use that pointer as an 'Array#', we will likely access fields of the
+    array that don't exist, and a seg-fault is likely, but not guaranteed.
+    If we use that pointer as @Either Int Bool@, we might try to access the
+    'Int' field of the 'Left' constructor (which has the same ConTag as '()'),
+    which doesn't exists. In the best case, we'll find an invalid pointer in its
+    position and get a seg-fault, in the worst case the error manifests only one
+    or two indirections later.
+ -}
diff --git a/compiler/GHC/Types/RepType.hs b/compiler/GHC/Types/RepType.hs
index 2957dddb5d..017b7cc3da 100644
--- a/compiler/GHC/Types/RepType.hs
+++ b/compiler/GHC/Types/RepType.hs
@@ -11,7 +11,7 @@ module GHC.Types.RepType
     isVoidTy,
 
     -- * Type representation for the code generator
-    typePrimRep, typePrimRep1,
+    typePrimRep, typePrimRep1, typeMonoPrimRep_maybe,
     runtimeRepPrimRep, typePrimRepArgs,
     PrimRep(..), primRepToType,
     countFunRepArgs, countConRepArgs, tyConPrimRep, tyConPrimRep1,
@@ -34,7 +34,7 @@ import GHC.Core.TyCon.RecWalk
 import GHC.Core.TyCo.Rep
 import GHC.Core.Type
 import GHC.Builtin.Types.Prim
-import {-# SOURCE #-} GHC.Builtin.Types ( anyTypeOfKind )
+import {-# SOURCE #-} GHC.Builtin.Types ( anyTypeOfKind, runtimeRepTy )
 
 import GHC.Utils.Misc
 import GHC.Utils.Outputable
@@ -493,6 +493,14 @@ typePrimRep1 ty = case typePrimRep ty of
   [rep] -> rep
   _     -> pprPanic "typePrimRep1" (ppr ty $$ ppr (typePrimRep ty))
 
+-- | Like 'typePrimRep', but returns 'Nothing' instead of panicking, when
+--
+--    * The @ty@ was not of form @TYPE rep@
+--    * @rep@ was not monomorphic
+--
+typeMonoPrimRep_maybe :: Type -> Maybe [PrimRep]
+typeMonoPrimRep_maybe ty = getRuntimeRep_maybe ty >>= runtimeRepMonoPrimRep_maybe
+
 -- | Find the runtime representation of a 'TyCon'. Defined here to
 -- avoid module loops. Returns a list of the register shapes necessary.
 -- See also Note [Getting from RuntimeRep to PrimRep]
@@ -526,6 +534,18 @@ kindPrimRep doc ki
   = pprPanic "kindPrimRep" (ppr ki $$ doc)
 
 -- | Take a type of kind RuntimeRep and extract the list of 'PrimRep' that
+-- it encodes if it's a monomorphic rep. Otherwise returns 'Nothing'.
+-- See also Note [Getting from RuntimeRep to PrimRep]
+runtimeRepMonoPrimRep_maybe :: HasDebugCallStack => Type -> Maybe [PrimRep]
+runtimeRepMonoPrimRep_maybe rr_ty
+  | Just (rr_dc, args) <- splitTyConApp_maybe rr_ty
+  , ASSERT2( runtimeRepTy `eqType` typeKind rr_ty, ppr rr_ty ) True
+  , RuntimeRep fun <- tyConRuntimeRepInfo rr_dc
+  = Just (fun args)
+  | otherwise
+  = Nothing -- not mono rep
+
+-- | Take a type of kind RuntimeRep and extract the list of 'PrimRep' that
 -- it encodes. See also Note [Getting from RuntimeRep to PrimRep]
 runtimeRepPrimRep :: HasDebugCallStack => SDoc -> Type -> [PrimRep]
 runtimeRepPrimRep doc rr_ty
diff --git a/compiler/GHC/Utils/Misc.hs b/compiler/GHC/Utils/Misc.hs
index f7168190e4..abd85b6b66 100644
--- a/compiler/GHC/Utils/Misc.hs
+++ b/compiler/GHC/Utils/Misc.hs
@@ -43,7 +43,7 @@ module GHC.Utils.Misc (
         listLengthCmp, atLength,
         equalLength, compareLength, leLength, ltLength,
 
-        isSingleton, only, GHC.Utils.Misc.singleton,
+        isSingleton, only, expectOnly, GHC.Utils.Misc.singleton,
         notNull, snocView,
 
         isIn, isn'tIn,
@@ -563,6 +563,18 @@ only (a:_) = a
 #endif
 only _ = panic "Util: only"
 
+-- | Extract the single element of a list and panic with the given message if
+-- there are more elements or the list was empty.
+-- Like 'expectJust', but for lists.
+expectOnly :: HasCallStack => String -> [a] -> a
+{-# INLINE expectOnly #-}
+#if defined(DEBUG)
+expectOnly _   [a]   = a
+#else
+expectOnly _   (a:_) = a
+#endif
+expectOnly msg _     = panic ("expectOnly: " ++ msg)
+
 -- Debugging/specialising versions of \tr{elem} and \tr{notElem}
 
 # if !defined(DEBUG)
diff --git a/compiler/ghc.cabal.in b/compiler/ghc.cabal.in
index dd216823d2..3fa4ee2bb3 100644
--- a/compiler/ghc.cabal.in
+++ b/compiler/ghc.cabal.in
@@ -549,6 +549,7 @@ Library
         GHC.StgToCmm.Heap
         GHC.StgToCmm.Hpc
         GHC.StgToCmm.Layout
+        GHC.StgToCmm.Lit
         GHC.StgToCmm.Monad
         GHC.StgToCmm.Prim
         GHC.StgToCmm.Prof
diff --git a/testsuite/tests/stranal/should_compile/T18982.stderr b/testsuite/tests/stranal/should_compile/T18982.stderr
index 3e6074e759..310eed5cc3 100644
--- a/testsuite/tests/stranal/should_compile/T18982.stderr
+++ b/testsuite/tests/stranal/should_compile/T18982.stderr
@@ -1,8 +1,8 @@
 
 ==================== Tidy Core ====================
-Result size of Tidy Core = {terms: 311, types: 249, coercions: 4, joins: 0/0}
+Result size of Tidy Core = {terms: 315, types: 214, coercions: 2, joins: 0/0}
 
--- RHS size: {terms: 8, types: 11, coercions: 1, joins: 0/0}
+-- RHS size: {terms: 8, types: 9, coercions: 1, joins: 0/0}
 T18982.$WExGADT :: forall e. (e ~ Int) => e %1 -> Int %1 -> ExGADT Int
 T18982.$WExGADT = \ (@e) (dt :: e ~ Int) (dt :: e) (dt :: Int) -> T18982.ExGADT @Int @e @~(<Int>_N :: Int GHC.Prim.~# Int) dt dt dt
 
@@ -10,7 +10,7 @@ T18982.$WExGADT = \ (@e) (dt :: e ~ Int) (dt :: e) (dt :: Int) -> T18982.ExGADT
 T18982.$WGADT :: Int %1 -> GADT Int
 T18982.$WGADT = \ (dt :: Int) -> T18982.GADT @Int @~(<Int>_N :: Int GHC.Prim.~# Int) dt
 
--- RHS size: {terms: 7, types: 8, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 7, types: 6, coercions: 0, joins: 0/0}
 T18982.$WEx :: forall e a. e %1 -> a %1 -> Ex a
 T18982.$WEx = \ (@e) (@a) (dt :: e) (dt :: a) -> T18982.Ex @a @e dt dt
 
@@ -210,27 +210,27 @@ T18982.$tc'ExGADT2 = GHC.Types.TrNameS T18982.$tc'ExGADT3
 T18982.$tc'ExGADT :: GHC.Types.TyCon
 T18982.$tc'ExGADT = GHC.Types.TyCon 8468257409157161049## 5503123603717080600## T18982.$trModule T18982.$tc'ExGADT2 1# T18982.$tc'ExGADT1
 
--- RHS size: {terms: 11, types: 14, coercions: 0, joins: 0/0}
-T18982.$wi :: forall {a} {e}. (a GHC.Prim.~# Int) -> e -> GHC.Prim.Int# -> GHC.Prim.Int#
-T18982.$wi = \ (@a) (@e) (ww :: a GHC.Prim.~# Int) (ww1 :: e) (ww2 :: GHC.Prim.Int#) -> case ww1 of { __DEFAULT -> GHC.Prim.+# ww2 1# }
+-- RHS size: {terms: 13, types: 15, coercions: 0, joins: 0/0}
+T18982.$wi :: forall {a} {e}. e -> GHC.Prim.Int# -> GHC.Prim.Int#
+T18982.$wi = \ (@a) (@e) (ww :: e) (ww1 :: GHC.Prim.Int#) -> case RUBBISH[] @(a GHC.Prim.~# Int) of ww2 { __DEFAULT -> case ww of { __DEFAULT -> GHC.Prim.+# ww1 1# } }
 
--- RHS size: {terms: 15, types: 27, coercions: 1, joins: 0/0}
+-- RHS size: {terms: 15, types: 22, coercions: 0, joins: 0/0}
 i :: forall a. ExGADT a -> Int
-i = \ (@a) (w :: ExGADT a) -> case w of { ExGADT @e ww1 ww2 ww3 ww4 -> case ww4 of { GHC.Types.I# ww6 -> case T18982.$wi @a @e @~(ww1 :: a GHC.Prim.~# Int) ww3 ww6 of ww7 { __DEFAULT -> GHC.Types.I# ww7 } } }
+i = \ (@a) (w :: ExGADT a) -> case w of { ExGADT @e ww1 ww2 ww3 ww4 -> case ww4 of { GHC.Types.I# ww6 -> case T18982.$wi @a @e ww3 ww6 of ww7 { __DEFAULT -> GHC.Types.I# ww7 } } }
 
--- RHS size: {terms: 6, types: 10, coercions: 0, joins: 0/0}
-T18982.$wh :: forall {a}. (a GHC.Prim.~# Int) -> GHC.Prim.Int# -> GHC.Prim.Int#
-T18982.$wh = \ (@a) (ww :: a GHC.Prim.~# Int) (ww1 :: GHC.Prim.Int#) -> GHC.Prim.+# ww1 1#
+-- RHS size: {terms: 8, types: 12, coercions: 0, joins: 0/0}
+T18982.$wh :: forall {a}. GHC.Prim.Int# -> GHC.Prim.Int#
+T18982.$wh = \ (@a) (ww :: GHC.Prim.Int#) -> case RUBBISH[] @(a GHC.Prim.~# Int) of ww1 { __DEFAULT -> GHC.Prim.+# ww 1# }
 
--- RHS size: {terms: 14, types: 18, coercions: 1, joins: 0/0}
+-- RHS size: {terms: 14, types: 15, coercions: 0, joins: 0/0}
 h :: forall a. GADT a -> Int
-h = \ (@a) (w :: GADT a) -> case w of { GADT ww1 ww2 -> case ww2 of { GHC.Types.I# ww4 -> case T18982.$wh @a @~(ww1 :: a GHC.Prim.~# Int) ww4 of ww5 { __DEFAULT -> GHC.Types.I# ww5 } } }
+h = \ (@a) (w :: GADT a) -> case w of { GADT ww1 ww2 -> case ww2 of { GHC.Types.I# ww4 -> case T18982.$wh @a ww4 of ww5 { __DEFAULT -> GHC.Types.I# ww5 } } }
 
--- RHS size: {terms: 9, types: 5, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 9, types: 4, coercions: 0, joins: 0/0}
 T18982.$wg :: forall {e}. e -> GHC.Prim.Int# -> GHC.Prim.Int#
 T18982.$wg = \ (@e) (ww :: e) (ww1 :: GHC.Prim.Int#) -> case ww of { __DEFAULT -> GHC.Prim.+# ww1 1# }
 
--- RHS size: {terms: 14, types: 12, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 14, types: 11, coercions: 0, joins: 0/0}
 g :: Ex Int -> Int
 g = \ (w :: Ex Int) -> case w of { Ex @e ww1 ww2 -> case ww2 of { GHC.Types.I# ww4 -> case T18982.$wg @e ww1 ww4 of ww5 { __DEFAULT -> GHC.Types.I# ww5 } } }