content: {-# LANGUAGE ConstraintKinds      #-}
  {-# LANGUAGE DataKinds            #-}
  {-# LANGUAGE DeriveDataTypeable   #-}
  {-# LANGUAGE FlexibleContexts     #-}
  {-# LANGUAGE LambdaCase           #-}
  {-# LANGUAGE TypeFamilies         #-}
  {-# LANGUAGE NamedFieldPuns       #-}
  {-# LANGUAGE UndecidableInstances #-}
  
  {-
  (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
  
  Shared term graph (STG) syntax for spineless-tagless code generation
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  This data type represents programs just before code generation (conversion to
  @Cmm@): basically, what we have is a stylised form of Core syntax, the style
  being one that happens to be ideally suited to spineless tagless code
  generation.
  -}
  
  module GHC.Stg.Syntax (
          StgArg(..),
  
          GenStgTopBinding(..), GenStgBinding(..), GenStgExpr(..), GenStgRhs(..),
          GenStgAlt(..), AltType(..),
  
          StgPass(..), BinderP, XRhsClosure, XLet, XLetNoEscape,
          NoExtFieldSilent, noExtFieldSilent,
          OutputablePass,
  
          UpdateFlag(..), isUpdatable,
  
          ConstructorNumber(..),
  
          -- a set of synonyms for the vanilla parameterisation
          StgTopBinding, StgBinding, StgExpr, StgRhs, StgAlt,
  
          -- a set of synonyms for the code gen parameterisation
          CgStgTopBinding, CgStgBinding, CgStgExpr, CgStgRhs, CgStgAlt,
  
          -- Same for taggedness
          TgStgTopBinding, TgStgBinding, TgStgExpr, TgStgRhs, TgStgAlt,
  
          -- a set of synonyms for the lambda lifting parameterisation
          LlStgTopBinding, LlStgBinding, LlStgExpr, LlStgRhs, LlStgAlt,
  
          -- a set of synonyms to distinguish in- and out variants
          InStgArg,  InStgTopBinding,  InStgBinding,  InStgExpr,  InStgRhs,  InStgAlt,
          OutStgArg, OutStgTopBinding, OutStgBinding, OutStgExpr, OutStgRhs, OutStgAlt,
  
          -- StgOp
          StgOp(..),
  
          -- utils
          stgRhsArity, freeVarsOfRhs,
          stgArgType,
          stgArgRep,
          stgArgRep1,
          stgArgRepU,
          stgArgRep_maybe,
  
          stgCaseBndrInScope,
  
          -- ppr
          StgPprOpts(..),
          panicStgPprOpts, shortStgPprOpts,
          pprStgArg, pprStgExpr, pprStgRhs, pprStgBinding, pprStgAlt,
          pprGenStgTopBinding, pprStgTopBinding,
          pprGenStgTopBindings, pprStgTopBindings
      ) where
  
  import GHC.Prelude
  
  import GHC.Stg.EnforceEpt.TagSig( TagSig )
  import GHC.Stg.Lift.Types
    -- To avoid having an orphan instances for BinderP, XLet etc
  
  import GHC.Types.CostCentre ( CostCentreStack )
  
  import GHC.Core     ( AltCon )
  import GHC.Core.DataCon
  import GHC.Core.TyCon    ( PrimRep(..), PrimOrVoidRep(..), TyCon )
  import GHC.Core.Type     ( Type )
  import GHC.Core.Ppr( {- instances -} )
  
  import GHC.Types.ForeignCall ( ForeignCall )
  import GHC.Types.Id
  import GHC.Types.Tickish     ( StgTickish )
  import GHC.Types.Var.Set
  import GHC.Types.Literal     ( Literal, literalType )
  import GHC.Types.RepType ( typePrimRep, typePrimRep1, typePrimRepU, typePrimRep_maybe )
  
  import GHC.Utils.Outputable
  import GHC.Utils.Panic.Plain
  
  import GHC.Builtin.PrimOps ( PrimOp, PrimCall )
  
  import Data.ByteString ( ByteString )
  import Data.Data   ( Data )
  import Data.List   ( intersperse )
  
  {-
  ************************************************************************
  *                                                                      *
  GenStgBinding
  *                                                                      *
  ************************************************************************
  
  As usual, expressions are interesting; other things are boring. Here are the
  boring things (except note the @GenStgRhs@), parameterised with respect to
  binder and occurrence information (just as in @GHC.Core@):
  -}
  
  -- | A top-level binding.
  data GenStgTopBinding pass
  -- See Note [Core top-level string literals]
    = StgTopLifted (GenStgBinding pass)
    | StgTopStringLit Id ByteString
  
  data GenStgBinding pass
    = StgNonRec (BinderP pass) (GenStgRhs pass)
    | StgRec    [(BinderP pass, GenStgRhs pass)]
  
  {-
  ************************************************************************
  *                                                                      *
  StgArg
  *                                                                      *
  ************************************************************************
  -}
  
  data StgArg
    = StgVarArg  Id
    | StgLitArg  Literal
  
  -- | Type of an @StgArg@
  --
  -- Very half baked because we have lost the type arguments.
  --
  -- This function should be avoided: in STG we aren't supposed to
  -- look at types, but only PrimReps.
  -- Use 'stgArgRep', 'stgArgRep_maybe', 'stgArgRep1' instaed.
  stgArgType :: StgArg -> Type
  stgArgType (StgVarArg v)   = idType v
  stgArgType (StgLitArg lit) = literalType lit
  
  stgArgRep :: StgArg -> [PrimRep]
  stgArgRep ty = typePrimRep (stgArgType ty)
  
  stgArgRep_maybe :: StgArg -> Maybe [PrimRep]
  stgArgRep_maybe ty = typePrimRep_maybe (stgArgType ty)
  
  -- | Assumes that the argument has at most one PrimRep, which holds after unarisation.
  -- See Note [Post-unarisation invariants] in GHC.Stg.Unarise.
  -- See Note [VoidRep] in GHC.Types.RepType.
  stgArgRep1 :: StgArg -> PrimOrVoidRep
  stgArgRep1 ty = typePrimRep1 (stgArgType ty)
  
  -- | Assumes that the argument has exactly one PrimRep.
  -- See Note [VoidRep] in GHC.Types.RepType.
  stgArgRepU :: StgArg -> PrimRep
  stgArgRepU ty = typePrimRepU (stgArgType ty)
  
  -- | Given an alt type and whether the program is unarised, return whether the
  -- case binder is in scope.
  --
  -- Case binders of unboxed tuple or unboxed sum type always dead after the
  -- unariser has run. See Note [Post-unarisation invariants] in GHC.Stg.Unarise.
  stgCaseBndrInScope :: AltType -> Bool {- ^ unarised? -} -> Bool
  stgCaseBndrInScope alt_ty unarised =
      case alt_ty of
        AlgAlt _      -> True
        PrimAlt _     -> True
        MultiValAlt _ -> not unarised
        PolyAlt       -> True
  
  {-
  ************************************************************************
  *                                                                      *
  STG expressions
  *                                                                      *
  ************************************************************************
  
  The @GenStgExpr@ data type is parameterised on binder and occurrence info, as
  before.
  
  ************************************************************************
  *                                                                      *
  GenStgExpr
  *                                                                      *
  ************************************************************************
  
  An application is of a function to a list of atoms (not expressions).
  Operationally, we want to push the arguments on the stack and call the function.
  (If the arguments were expressions, we would have to build their closures
  first.)
  
  There is no constructor for a lone variable; it would appear as @StgApp var []@.
  -}
  
  data GenStgExpr pass
    = StgApp
          Id       -- function
          [StgArg] -- arguments; may be empty
  
  {-
  ************************************************************************
  *                                                                      *
  StgConApp and StgPrimApp --- saturated applications
  *                                                                      *
  ************************************************************************
  
  There are specialised forms of application, for constructors, primitives, and
  literals.
  
  Note [Constructor applications in STG]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  After the unarisation pass:
  * In `StgConApp` and `StgRhsCon` and `StgAlt` we filter out the void arguments,
    leaving only non-void ones.
  * In `StgApp` and `StgOpApp` we retain void arguments.
  
  We can do this because we know that `StgConApp` and `StgRhsCon` are saturated applications,
  so we lose no information by dropping those void args.  In contrast, in `StgApp` we need the
   void argument to compare the number of args in the call with the arity of the function.
  
  This is an open design choice.  We could instead choose to treat all these applications
  consistently (keeping the void args).  But for some reason we don't, and this Note simply
  documents that design choice.
  
  As an example, consider:
  
          data T a = MkT !Int a Void#
  
  The wrapper's representation and the worker's representation (i.e. the
  datacon's Core representation) are respectively:
  
          $WMkT :: Int  -> a -> Void# -> T a
          MkT   :: Int# -> a -> Void# -> T a
  
  T would end up being used in STG post-unarise as:
  
    let x = MkT 1# y
    in ...
        case x of
          MkT int a -> ...
  
  The Void# argument is dropped. In essence we only generate binders for runtime
  relevant values.
  
  We also flatten out unboxed tuples in this process. See the unarise
  pass for details on how this is done. But as an example consider
  `data S = MkS Bool (# Bool | Char #)` which when matched on would
  result in an alternative with three binders like this
  
      MkS bool tag tpl_field ->
  
  See Note [Translating unboxed sums to unboxed tuples] and Note [Unarisation]
  for the details of this transformation.
  
  -}
  
    | StgLit      Literal
  
          -- StgConApp is vital for returning unboxed tuples or sums
          -- which can't be let-bound
    | StgConApp   DataCon
                  ConstructorNumber
                  [StgArg] -- Saturated. See Note [Constructor applications in STG]
                  [[PrimRep]]   -- See Note [Representations in StgConApp] in GHC.Stg.Unarise
  
    | StgOpApp    StgOp    -- Primitive op or foreign call
                  [StgArg] -- Saturated.
                  Type     -- Result type
                           -- We need to know this so that we can
                           -- assign result registers
  
  {-
  ************************************************************************
  *                                                                      *
  GenStgExpr: case-expressions
  *                                                                      *
  ************************************************************************
  
  This has the same boxed/unboxed business as Core case expressions.
  -}
  
    | StgCase
          (GenStgExpr pass) -- the thing to examine
          (BinderP pass) -- binds the result of evaluating the scrutinee
          AltType
          [GenStgAlt pass]
                      -- The DEFAULT case is always *first*
                      -- if it is there at all
  
  {-
  ************************************************************************
  *                                                                      *
  GenStgExpr: let(rec)-expressions
  *                                                                      *
  ************************************************************************
  
  The various forms of let(rec)-expression encode most of the interesting things
  we want to do.
  
  -   let-closure x = [free-vars] [args] expr in e
  
    is equivalent to
  
      let x = (\free-vars -> \args -> expr) free-vars
  
    @args@ may be empty (and is for most closures). It isn't under circumstances
    like this:
  
      let x = (\y -> y+z)
  
    This gets mangled to
  
      let-closure x = [z] [y] (y+z)
  
    The idea is that we compile code for @(y+z)@ in an environment in which @z@ is
    bound to an offset from Node, and `y` is bound to an offset from the stack
    pointer.
  
    (A let-closure is an @StgLet@ with a @StgRhsClosure@ RHS.)
  
  -   let-constructor x = Constructor [args] in e
  
    (A let-constructor is an @StgLet@ with a @StgRhsCon@ RHS.)
  
  - Letrec-expressions are essentially the same deal as let-closure/
    let-constructor, so we use a common structure and distinguish between them
    with an @is_recursive@ boolean flag.
  
  -   let-unboxed u = <an arbitrary arithmetic expression in unboxed values> in e
  
    All the stuff on the RHS must be fully evaluated. No function calls either!
  
    (We've backed away from this toward case-expressions with suitably-magical
    alts ...)
  
  - Advanced stuff here! Not to start with, but makes pattern matching generate
    more efficient code.
  
      let-escapes-not fail = expr
      in e'
  
    Here the idea is that @e'@ guarantees not to put @fail@ in a data structure,
    or pass it to another function. All @e'@ will ever do is tail-call @fail@.
    Rather than build a closure for @fail@, all we need do is to record the stack
    level at the moment of the @let-escapes-not@; then entering @fail@ is just a
    matter of adjusting the stack pointer back down to that point and entering the
    code for it.
  
    Another example:
  
      f x y = let z = huge-expression in
              if y==1 then z else
              if y==2 then z else
              1
  
    (A let-escapes-not is an @StgLetNoEscape@.)
  
  - We may eventually want:
  
      let-literal x = Literal in e
  
  And so the code for let(rec)-things:
  -}
  
    | StgLet
          (XLet pass)
          (GenStgBinding pass)    -- right hand sides (see below)
          (GenStgExpr pass)       -- body
  
    | StgLetNoEscape
          (XLetNoEscape pass)
          (GenStgBinding pass)    -- right hand sides (see below)
          (GenStgExpr pass)       -- body
  
  {-
  *************************************************************************
  *                                                                      *
  GenStgExpr: hpc, scc and other debug annotations
  *                                                                      *
  *************************************************************************
  
  Finally for @hpc@ expressions we introduce a new STG construct.
  -}
  
    | StgTick
      StgTickish
      (GenStgExpr pass)       -- sub expression
  
  -- END of GenStgExpr
  
  {-
  ************************************************************************
  *                                                                      *
  STG right-hand sides
  *                                                                      *
  ************************************************************************
  
  Here's the rest of the interesting stuff for @StgLet@s; the first flavour is for
  closures:
  -}
  
  data GenStgRhs pass
    = StgRhsClosure
          (XRhsClosure pass) -- ^ Extension point for non-global free var
                             --   list just before 'CodeGen'.
          CostCentreStack    -- ^ CCS to be attached (default is CurrentCCS)
          !UpdateFlag        -- ^ 'ReEntrant' | 'Updatable' | 'SingleEntry'
          [BinderP pass]     -- ^ arguments; if empty, then not a function;
                             --   as above, order is important.
          (GenStgExpr pass)  -- ^ body
          Type               -- ^ result type
  
  {-
  An example may be in order.  Consider:
  
    let t = \x -> \y -> ... x ... y ... p ... q in e
  
  Pulling out the free vars and stylising somewhat, we get the equivalent:
  
    let t = (\[p,q] -> \[x,y] -> ... x ... y ... p ...q) p q
  
  Stg-operationally, the @[x,y]@ are on the stack, the @[p,q]@ are offsets from
  @Node@ into the closure, and the code ptr for the closure will be exactly that
  in parentheses above.
  
  The second flavour of right-hand-side is for constructors (simple but
  important):
  -}
  
    | StgRhsCon
          CostCentreStack -- CCS to be attached (default is CurrentCCS).
                          -- Top-level (static) ones will end up with
                          -- DontCareCCS, because we don't count static
                          -- data in heap profiles, and we don't set CCCS
                          -- from static closure.
          DataCon         -- Constructor. Never an unboxed tuple or sum, as those
                          -- are not allocated.
          ConstructorNumber
          [StgTickish]
          [StgArg]        -- Saturated Args. See Note [Constructor applications in STG]
          Type            -- Type, for rewriting to an StgRhsClosure
  
  -- | Like 'GHC.Hs.Extension.NoExtField', but with an 'Outputable' instance that
  -- returns 'empty'.
  data NoExtFieldSilent = NoExtFieldSilent
    deriving (Data, Eq, Ord)
  
  instance Outputable NoExtFieldSilent where
    ppr _ = empty
  
  -- | Used when constructing a term with an unused extension point that should
  -- not appear in pretty-printed output at all.
  noExtFieldSilent :: NoExtFieldSilent
  noExtFieldSilent = NoExtFieldSilent
  -- TODO: Maybe move this to GHC.Hs.Extension? I'm not sure about the
  -- implications on build time...
  
  stgRhsArity :: StgRhs -> Int
  stgRhsArity (StgRhsClosure _ _ _ bndrs _ _)
    = assert (all isId bndrs) $ length bndrs
    -- The arity never includes type parameters, but they should have gone by now
  stgRhsArity (StgRhsCon {}) = 0
  
  freeVarsOfRhs :: (XRhsClosure pass ~ DIdSet) => GenStgRhs pass -> DIdSet
  freeVarsOfRhs (StgRhsCon _ _ _ _ args _) = mkDVarSet [ id | StgVarArg id <- args ]
  freeVarsOfRhs (StgRhsClosure fvs _ _ _ _ _) = fvs
  
  {-
  ************************************************************************
  *                                                                      *
  STG case alternatives
  *                                                                      *
  ************************************************************************
  
  Very like in Core syntax (except no type-world stuff).
  
  The type constructor is guaranteed not to be abstract; that is, we can see its
  representation. This is important because the code generator uses it to
  determine return conventions etc. But it's not trivial where there's a module
  loop involved, because some versions of a type constructor might not have all
  the constructors visible. So mkStgAlgAlts (in CoreToStg) ensures that it gets
  the TyCon from the constructors or literals (which are guaranteed to have the
  Real McCoy) rather than from the scrutinee type.
  -}
  
  data GenStgAlt pass = GenStgAlt
    { alt_con          :: !AltCon            -- alts: data constructor,
    , alt_bndrs        :: ![BinderP pass]    -- constructor's parameters,
    , alt_rhs          :: !(GenStgExpr pass) -- right-hand side.
    }
  
  data AltType
    = PolyAlt             -- Polymorphic (a boxed type variable, lifted or unlifted)
    | MultiValAlt Int     -- Multi value of this arity (unboxed tuple or sum)
                          -- the arity could indeed be 1 for unary unboxed tuple
                          -- or enum-like unboxed sums
    | AlgAlt      TyCon   -- Algebraic data type; the AltCons will be DataAlts
    | PrimAlt     PrimRep -- Primitive data type; the AltCons (if any) will be LitAlts
  
  {-
  ************************************************************************
  *                                                                      *
  The Plain STG parameterisation
  *                                                                      *
  ************************************************************************
  
    Note [STG Extension points]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We now make use of extension points in STG for different passes which want
    to associate information with AST nodes.
  
    Currently the pipeline is roughly:
  
    CoreToStg: Core -> Stg
    StgSimpl: Stg -> Stg
    CodeGen: Stg -> Cmm
  
      As part of StgSimpl we run late lambda lifting (Ll).
      Late lambda lift:
      Stg -> FvStg -> LlStg -> Stg
  
    CodeGen:
      As part of CodeGen we run tag inference.
      Tag Inference:
        Stg -> Stg 'InferTaggedBinders` -> Stg
  
      And at a last step we add the free Variables:
        Stg -> CgStg
  
    Which finally CgStg being used to generate Cmm.
  
  -}
  
  type StgTopBinding = GenStgTopBinding 'Vanilla
  type StgBinding    = GenStgBinding    'Vanilla
  type StgExpr       = GenStgExpr       'Vanilla
  type StgRhs        = GenStgRhs        'Vanilla
  type StgAlt        = GenStgAlt        'Vanilla
  
  type LlStgTopBinding = GenStgTopBinding 'LiftLams
  type LlStgBinding    = GenStgBinding    'LiftLams
  type LlStgExpr       = GenStgExpr       'LiftLams
  type LlStgRhs        = GenStgRhs        'LiftLams
  type LlStgAlt        = GenStgAlt        'LiftLams
  
  type CgStgTopBinding = GenStgTopBinding 'CodeGen
  type CgStgBinding    = GenStgBinding    'CodeGen
  type CgStgExpr       = GenStgExpr       'CodeGen
  type CgStgRhs        = GenStgRhs        'CodeGen
  type CgStgAlt        = GenStgAlt        'CodeGen
  
  type TgStgTopBinding = GenStgTopBinding 'CodeGen
  type TgStgBinding    = GenStgBinding    'CodeGen
  type TgStgExpr       = GenStgExpr       'CodeGen
  type TgStgRhs        = GenStgRhs        'CodeGen
  type TgStgAlt        = GenStgAlt        'CodeGen
  
  {- Many passes apply a substitution, and it's very handy to have type
     synonyms to remind us whether or not the substitution has been applied.
     See GHC.Core for precedence in Core land
  -}
  
  type InStgTopBinding  = StgTopBinding
  type InStgBinding     = StgBinding
  type InStgArg         = StgArg
  type InStgExpr        = StgExpr
  type InStgRhs         = StgRhs
  type InStgAlt         = StgAlt
  type OutStgTopBinding = StgTopBinding
  type OutStgBinding    = StgBinding
  type OutStgArg        = StgArg
  type OutStgExpr       = StgExpr
  type OutStgRhs        = StgRhs
  type OutStgAlt        = StgAlt
  
  -- | When `-fdistinct-constructor-tables` is turned on then
  -- each usage of a constructor is given an unique number and
  -- an info table is generated for each different constructor.
  data ConstructorNumber =
        NoNumber | Numbered Int
  
  instance Outputable ConstructorNumber where
    ppr NoNumber = empty
    ppr (Numbered n) = text "#" <> ppr n
  
  {-
  Note Stg Passes
  ~~~~~~~~~~~~~~~
  Here is a short summary of the STG pipeline and where we use the different
  StgPass data type indexes:
  
    1. CoreToStg.Prep performs several transformations that prepare the desugared
       and simplified core to be converted to STG. One of these transformations is
       making it so that value lambdas only exist as the RHS of a binding.
       See Note [CorePrep Overview].
  
    2. CoreToStg converts the prepared core to STG, specifically GenStg*
       parameterised by 'Vanilla. See the GHC.CoreToStg Module.
  
    3. Stg.Pipeline does a number of passes on the generated STG. One of these is
       the lambda-lifting pass, which internally uses the 'LiftLams
       parameterisation to store information for deciding whether or not to lift
       each binding.
       See Note [Late lambda lifting in STG].
  
    4. EPT enforcement takes in 'Vanilla and produces 'InferTagged STG, while using
       the InferTaggedBinders annotated AST internally.
       See Note [EPT enforcement].
  
    5. Stg.FVs annotates closures with their free variables. To store these
       annotations we use the 'CodeGen parameterisation.
       See the GHC.Stg.FVs module.
  
    6. The Module Stg.StgToCmm generates Cmm from the CodeGen annotated STG.
  -}
  
  
  -- | Used as a data type index for the stgSyn AST
  data StgPass
    = Vanilla
    | LiftLams -- ^ Use internally by the lambda lifting pass
    | InferTaggedBinders -- ^ Tag inference information on binders.
                         -- See Note [EPT enforcement] in GHC.Stg.EnforceEpt
    | InferTagged -- ^ Tag inference information put on relevant StgApp nodes
                  -- See Note [EPT enforcement] in GHC.Stg.EnforceEpt
    | CodeGen
  
  type family BinderP (pass :: StgPass)
  type instance BinderP 'Vanilla            = Id
  type instance BinderP 'CodeGen            = Id
  type instance BinderP 'InferTagged        = Id
  type instance BinderP 'InferTaggedBinders = (Id, TagSig)
  type instance BinderP 'LiftLams           = BinderInfo
  
  type family XRhsClosure (pass :: StgPass)
  type instance XRhsClosure 'Vanilla            = NoExtFieldSilent
  type instance XRhsClosure  'LiftLams          = DIdSet
  type instance XRhsClosure 'InferTagged        = NoExtFieldSilent
  type instance XRhsClosure 'InferTaggedBinders = XRhsClosure  'CodeGen
  -- | Code gen needs to track non-global free vars
  type instance XRhsClosure 'CodeGen = DIdSet
  
  
  type family XLet (pass :: StgPass)
  type instance XLet 'Vanilla            = NoExtFieldSilent
  type instance XLet 'LiftLams           = Skeleton
  type instance XLet 'InferTagged        = NoExtFieldSilent
  type instance XLet 'InferTaggedBinders = XLet 'CodeGen
  type instance XLet 'CodeGen            = NoExtFieldSilent
  
  type family XLetNoEscape (pass :: StgPass)
  type instance XLetNoEscape 'Vanilla            = NoExtFieldSilent
  type instance XLetNoEscape 'LiftLams           = Skeleton
  type instance XLetNoEscape 'InferTagged        = NoExtFieldSilent
  type instance XLetNoEscape 'InferTaggedBinders = XLetNoEscape 'CodeGen
  type instance XLetNoEscape 'CodeGen            = NoExtFieldSilent
  
  {-
  
  ************************************************************************
  *                                                                      *
  UpdateFlag
  *                                                                      *
  ************************************************************************
  
  This is also used in @LambdaFormInfo@ in the @ClosureInfo@ module.
  
  -}
  
  data UpdateFlag
    = ReEntrant
        -- ^ A @ReEntrant@ closure may be entered multiple times, but should not
        -- be updated or blackholed.
    | Updatable
        -- ^ An @Updatable@ closure should be updated after evaluation (and may be
        -- blackholed during evaluation).
    | SingleEntry
        -- ^ A @SingleEntry@ closure will only be entered once, and so need not be
        -- updated but may safely be blackholed.
    | JumpedTo
        -- ^ A @JumpedTo@ (join-point) closure is entered once or multiple times
        -- but has no heap-allocated associated closure.
    deriving (Show,Eq)
  
  instance Outputable UpdateFlag where
      ppr u = char $ case u of
                         ReEntrant   -> 'r'
                         Updatable   -> 'u'
                         SingleEntry -> 's'
                         JumpedTo    -> 'j'
  
  isUpdatable :: UpdateFlag -> Bool
  isUpdatable ReEntrant   = False
  isUpdatable SingleEntry = False
  isUpdatable Updatable   = True
  isUpdatable JumpedTo    = False
  
  {-
  ************************************************************************
  *                                                                      *
  StgOp
  *                                                                      *
  ************************************************************************
  
  An StgOp allows us to group together PrimOps and ForeignCalls. It's quite useful
  to move these around together, notably in StgOpApp and COpStmt.
  -}
  
  data StgOp
    = StgPrimOp  PrimOp
  
    | StgPrimCallOp PrimCall
  
    | StgFCallOp ForeignCall Type
          -- The Type, which is obtained from the foreign import declaration
          -- itself, is needed by the stg-to-cmm pass to determine the offset to
          -- apply to unlifted boxed arguments in GHC.StgToCmm.Foreign. See Note
          -- [Unlifted boxed arguments to foreign calls]
  
  {-
  ************************************************************************
  *                                                                      *
  Pretty-printing
  *                                                                      *
  ************************************************************************
  
  Robin Popplestone asked for semi-colon separators on STG binds; here's hoping he
  likes terminators instead...  Ditto for case alternatives.
  -}
  
  type OutputablePass pass =
    ( Outputable (XLet pass)
    , Outputable (XLetNoEscape pass)
    , Outputable (XRhsClosure pass)
    , OutputableBndr (BinderP pass)
    )
  
  -- | STG pretty-printing options
  data StgPprOpts = StgPprOpts
     { stgSccEnabled :: !Bool -- ^ Enable cost-centres
     }
  
  -- | STG pretty-printing options used for panic messages
  panicStgPprOpts :: StgPprOpts
  panicStgPprOpts = StgPprOpts
     { stgSccEnabled = True
     }
  
  -- | STG pretty-printing options used for short messages
  shortStgPprOpts :: StgPprOpts
  shortStgPprOpts = StgPprOpts
     { stgSccEnabled = False
     }
  
  
  pprGenStgTopBinding
    :: OutputablePass pass => StgPprOpts -> GenStgTopBinding pass -> SDoc
  pprGenStgTopBinding opts b = case b of
     StgTopStringLit bndr str -> hang (hsep [pprBndr LetBind bndr, equals]) 4 (pprHsBytes str <> semi)
     StgTopLifted bind        -> pprGenStgBinding opts bind
  
  pprGenStgBinding :: OutputablePass pass => StgPprOpts -> GenStgBinding pass -> SDoc
  pprGenStgBinding opts b = case b of
     StgNonRec bndr rhs -> hang (hsep [pprBndr LetBind bndr, equals]) 4 (pprStgRhs opts rhs <> semi)
     StgRec pairs       -> vcat [ text "Rec {"
                                , vcat (intersperse blankLine (map ppr_bind pairs))
                                , text "end Rec }" ]
                           where
                             ppr_bind (bndr, expr)
                               = hang (hsep [pprBndr LetBind bndr, equals])
                                      4 (pprStgRhs opts expr <> semi)
  
  instance OutputablePass pass => Outputable  (GenStgBinding pass) where
    ppr = pprGenStgBinding panicStgPprOpts
  
  pprGenStgTopBindings :: (OutputablePass pass) => StgPprOpts -> [GenStgTopBinding pass] -> SDoc
  pprGenStgTopBindings opts binds
    = vcat $ intersperse blankLine (map (pprGenStgTopBinding opts) binds)
  
  pprStgBinding :: OutputablePass pass => StgPprOpts -> GenStgBinding pass -> SDoc
  pprStgBinding = pprGenStgBinding
  
  pprStgTopBinding :: OutputablePass pass => StgPprOpts -> GenStgTopBinding pass -> SDoc
  pprStgTopBinding = pprGenStgTopBinding
  
  pprStgTopBindings :: OutputablePass pass => StgPprOpts -> [GenStgTopBinding pass] -> SDoc
  pprStgTopBindings = pprGenStgTopBindings
  
  pprIdWithRep :: Id -> SDoc
  pprIdWithRep v = ppr v <> pprTypeRep (idType v)
  
  pprTypeRep :: Type -> SDoc
  pprTypeRep ty =
      ppUnlessOption sdocSuppressStgReps $
      char ':' <> case typePrimRep ty of
                    [r] -> ppr r
                    r -> ppr r
  
  
  instance Outputable StgArg where
    ppr = pprStgArg
  
  pprStgArg :: StgArg -> SDoc
  pprStgArg (StgVarArg var) = pprIdWithRep var
  pprStgArg (StgLitArg con) = ppr con <> pprTypeRep (literalType con)
  
  instance OutputablePass pass => Outputable  (GenStgExpr pass) where
    ppr = pprStgExpr panicStgPprOpts
  
  pprStgExpr :: OutputablePass pass => StgPprOpts -> GenStgExpr pass -> SDoc
  pprStgExpr opts e = case e of
                             -- special case
     StgLit lit           -> ppr lit
                             -- general case
     StgApp func args
        | null args
        , Just sig <- idTagSig_maybe func
        -> ppr func <> ppr sig
        | otherwise -> hang (ppr func) 4 (interppSP args) -- TODO: Print taggedness
     StgConApp con n args _ -> hsep [ ppr con, ppr n, brackets (interppSP args) ]
     StgOpApp op args _   -> hsep [ pprStgOp op, brackets (interppSP args)]
  
  -- special case: let v = <very specific thing>
  --               in
  --               let ...
  --               in
  --               ...
  --
  -- Very special!  Suspicious! (SLPJ)
  
  {-
     StgLet srt (StgNonRec bndr (StgRhsClosure cc bi free_vars upd_flag args rhs))
                          expr@(StgLet _ _))
     -> ($$)
        (hang (hcat [text "let { ", ppr bndr, text " = ",
                            ppr cc,
                            pp_binder_info bi,
                            text " [", whenPprDebug (interppSP free_vars), text "] \\",
                            ppr upd_flag, text " [",
                            interppSP args, char ']'])
              8 (sep [hsep [ppr rhs, text "} in"]]))
        (ppr expr)
  -}
  
     -- special case: let ... in let ...
     StgLet ext bind expr@StgLet{} -> ($$)
        (sep [hang (text "let" <+> ppr ext <+> text "{")
                  2 (hsep [pprGenStgBinding opts bind, text "} in"])])
        (pprStgExpr opts expr)
  
     -- general case
     StgLet ext bind expr
        -> sep [ hang (text "let" <+> ppr ext <+> text "{")
                      2 (pprGenStgBinding opts bind)
               , hang (text "} in ") 2 (pprStgExpr opts expr)
               ]
  
     StgLetNoEscape ext bind expr
        -> sep [ hang (text "let-no-escape" <+> ppr ext <+> text "{")
                      2 (pprGenStgBinding opts bind)
               , hang (text "} in ") 2 (pprStgExpr opts expr)
               ]
  
     StgTick tickish expr -> sdocOption sdocSuppressTicks $ \case
        True  -> pprStgExpr opts expr
        False -> sep [ ppr tickish, pprStgExpr opts expr ]
  
     -- Don't indent for a single case alternative.
     StgCase expr bndr alt_type [alt]
        -> sep [ sep [ text "case"
                     , nest 4 (hsep [ pprStgExpr opts expr
                                    , whenPprDebug (dcolon <+> ppr alt_type)
                                    ])
                     , text "of"
                     , pprBndr CaseBind bndr
                     , char '{'
                     ]
               , pprStgAlt opts False alt
               , char '}'
               ]
  
     StgCase expr bndr alt_type alts
        -> sep [ sep [ text "case"
                     , nest 4 (hsep [ pprStgExpr opts expr
                                    , whenPprDebug (dcolon <+> ppr alt_type)
                                    ])
                     , text "of"
                     , pprBndr CaseBind bndr, char '{'
                     ]
               , nest 2 (vcat (map (pprStgAlt opts True) alts))
               , char '}'
               ]
  
  
  pprStgAlt :: OutputablePass pass => StgPprOpts -> Bool -> GenStgAlt pass -> SDoc
  pprStgAlt opts indent GenStgAlt{alt_con, alt_bndrs, alt_rhs}
    | indent    = hang altPattern 4 (pprStgExpr opts alt_rhs <> semi)
    | otherwise = sep [altPattern, pprStgExpr opts alt_rhs <> semi]
      where
        altPattern = hsep [ ppr alt_con
                          , sep (map (pprBndr CasePatBind) alt_bndrs)
                          , text "->"
                          ]
  
  
  pprStgOp :: StgOp -> SDoc
  pprStgOp (StgPrimOp  op)   = ppr op
  pprStgOp (StgPrimCallOp op)= ppr op
  pprStgOp (StgFCallOp op _) = ppr op
  
  instance Outputable StgOp where
    ppr = pprStgOp
  
  instance Outputable AltType where
    ppr PolyAlt         = text "Polymorphic"
    ppr (MultiValAlt n) = text "MultiAlt" <+> ppr n
    ppr (AlgAlt tc)     = text "Alg"    <+> ppr tc
    ppr (PrimAlt tc)    = text "Prim"   <+> ppr tc
  
  pprStgRhs :: OutputablePass pass => StgPprOpts -> GenStgRhs pass -> SDoc
  pprStgRhs opts rhs = case rhs of
     StgRhsClosure ext cc upd_flag args body _
        -> hang (hsep [ if stgSccEnabled opts then ppr cc else empty
                      , ppUnlessOption sdocSuppressStgExts (ppr ext)
                      , char '\\' <> ppr upd_flag, brackets (interppSP args)
                      ])
                4 (pprStgExpr opts body)
  
     StgRhsCon cc con mid _ticks args _
        -> hcat [ if stgSccEnabled opts then ppr cc <> space else empty
                , case mid of
                    NoNumber -> empty
                    Numbered n -> hcat [ppr n, space]
                -- The bang indicates this is an StgRhsCon instead of an StgConApp.
                , ppr con, text "! ", brackets (sep (map pprStgArg args))]
  
  instance OutputablePass pass => Outputable  (GenStgRhs pass) where
    ppr = pprStgRhs panicStgPprOpts
  