GHC/StgToCmm/Closure.hs

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE RecordWildCards #-}
    {-# LANGUAGE LambdaCase #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    {-# LANGUAGE FlexibleInstances #-}

    -----------------------------------------------------------------------------
    --
    -- Stg to C-- code generation:
    --
    -- The types   LambdaFormInfo
    --             ClosureInfo
    --
    -- Nothing monadic in here!
    --
    -----------------------------------------------------------------------------

    module GHC.StgToCmm.Closure (
            DynTag,  tagForCon, isSmallFamily,

            idPrimRep, isVoidRep, isGcPtrRep, addIdReps, addArgReps,
            argPrimRep,

            NonVoid(..), fromNonVoid, nonVoidIds, nonVoidStgArgs,
            assertNonVoidIds, assertNonVoidStgArgs,

            -- * LambdaFormInfo
            LambdaFormInfo,         -- Abstract
            StandardFormInfo,        -- ...ditto...
            mkLFThunk, mkLFReEntrant, mkConLFInfo, mkSelectorLFInfo,
            mkApLFInfo, mkLFImported, mkLFArgument, mkLFLetNoEscape,
            mkLFStringLit,
            lfDynTag,
            isLFThunk, isLFReEntrant, lfUpdatable,

            -- * Used by other modules
            CgLoc(..), SelfLoopInfo, CallMethod(..),
            nodeMustPointToIt, isKnownFun, funTag, tagForArity,
            CallOpts(..), getCallMethod,

            -- * ClosureInfo
            ClosureInfo,
            mkClosureInfo,
            mkCmmInfo,

            -- ** Inspection
            closureLFInfo, closureName,

            -- ** Labels
            -- These just need the info table label
            closureInfoLabel, staticClosureLabel,
            closureSlowEntryLabel, closureLocalEntryLabel,

            -- ** Predicates
            -- These are really just functions on LambdaFormInfo
            closureUpdReqd,
            closureReEntrant, closureFunInfo,
            isToplevClosure,

            blackHoleOnEntry,  -- Needs LambdaFormInfo and SMRep
            isStaticClosure,   -- Needs SMPre

            -- * InfoTables
            mkDataConInfoTable,
            cafBlackHoleInfoTable,
            indStaticInfoTable,
            staticClosureNeedsLink,
        ) where

    #include "HsVersions.h"

    import GHC.Prelude
    import GHC.Platform
    import GHC.Platform.Profile

    import GHC.Stg.Syntax
    import GHC.Runtime.Heap.Layout
    import GHC.Cmm
    import GHC.Cmm.Utils
    import GHC.Cmm.Ppr.Expr() -- For Outputable instances
    import GHC.StgToCmm.Types

    import GHC.Types.CostCentre
    import GHC.Cmm.BlockId
    import GHC.Cmm.CLabel
    import GHC.Types.Id
    import GHC.Types.Id.Info
    import GHC.Core.DataCon
    import GHC.Types.Name
    import GHC.Core.Type
    import GHC.Core.TyCo.Rep
    import GHC.Tc.Utils.TcType
    import GHC.Core.TyCon
    import GHC.Types.RepType
    import GHC.Types.Basic
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Utils.Misc

    import Data.Coerce (coerce)
    import qualified Data.ByteString.Char8 as BS8

    -----------------------------------------------------------------------------
    --                Data types and synonyms
    -----------------------------------------------------------------------------

    -- These data types are mostly used by other modules, especially
    -- GHC.StgToCmm.Monad, but we define them here because some functions in this
    -- module need to have access to them as well

    data CgLoc
      = CmmLoc CmmExpr      -- A stable CmmExpr; that is, one not mentioning
                            -- Hp, so that it remains valid across calls

      | LneLoc BlockId [LocalReg]             -- A join point
            -- A join point (= let-no-escape) should only
            -- be tail-called, and in a saturated way.
            -- To tail-call it, assign to these locals,
            -- and branch to the block id

    instance OutputableP Platform CgLoc where
       pdoc = pprCgLoc

    pprCgLoc :: Platform -> CgLoc -> SDoc
    pprCgLoc platform = \case
       CmmLoc e    -> text "cmm" <+> pdoc platform e
       LneLoc b rs -> text "lne" <+> ppr b <+> ppr rs

    type SelfLoopInfo = (Id, BlockId, [LocalReg])

    -- used by ticky profiling
    isKnownFun :: LambdaFormInfo -> Bool
    isKnownFun LFReEntrant{} = True
    isKnownFun LFLetNoEscape = True
    isKnownFun _             = False

    -------------------------------------
    --        Non-void types
    -------------------------------------
    -- We frequently need the invariant that an Id or a an argument
    -- is of a non-void type. This type is a witness to the invariant.

    newtype NonVoid a = NonVoid a
      deriving (Eq, Show)

    fromNonVoid :: NonVoid a -> a
    fromNonVoid (NonVoid a) = a

    instance (Outputable a) => Outputable (NonVoid a) where
      ppr (NonVoid a) = ppr a

    nonVoidIds :: [Id] -> [NonVoid Id]
    nonVoidIds ids = [NonVoid id | id <- ids, not (isVoidTy (idType id))]

    -- | Used in places where some invariant ensures that all these Ids are
    -- non-void; e.g. constructor field binders in case expressions.
    -- See Note [Post-unarisation invariants] in "GHC.Stg.Unarise".
    assertNonVoidIds :: [Id] -> [NonVoid Id]
    assertNonVoidIds ids = ASSERT(not (any (isVoidTy . idType) ids))
                           coerce ids

    nonVoidStgArgs :: [StgArg] -> [NonVoid StgArg]
    nonVoidStgArgs args = [NonVoid arg | arg <- args, not (isVoidTy (stgArgType arg))]

    -- | Used in places where some invariant ensures that all these arguments are
    -- non-void; e.g. constructor arguments.
    -- See Note [Post-unarisation invariants] in "GHC.Stg.Unarise".
    assertNonVoidStgArgs :: [StgArg] -> [NonVoid StgArg]
    assertNonVoidStgArgs args = ASSERT(not (any (isVoidTy . stgArgType) args))
                                coerce args

    -----------------------------------------------------------------------------
    --                Representations
    -----------------------------------------------------------------------------

    -- Why are these here?

    -- | Assumes that there is precisely one 'PrimRep' of the type. This assumption
    -- holds after unarise.
    -- See Note [Post-unarisation invariants]
    idPrimRep :: Id -> PrimRep
    idPrimRep id = typePrimRep1 (idType id)
        -- See also Note [VoidRep] in GHC.Types.RepType

    -- | Assumes that Ids have one PrimRep, which holds after unarisation.
    -- See Note [Post-unarisation invariants]
    addIdReps :: [NonVoid Id] -> [NonVoid (PrimRep, Id)]
    addIdReps = map (\id -> let id' = fromNonVoid id
                             in NonVoid (idPrimRep id', id'))

    -- | Assumes that arguments have one PrimRep, which holds after unarisation.
    -- See Note [Post-unarisation invariants]
    addArgReps :: [NonVoid StgArg] -> [NonVoid (PrimRep, StgArg)]
    addArgReps = map (\arg -> let arg' = fromNonVoid arg
                               in NonVoid (argPrimRep arg', arg'))

    -- | Assumes that the argument has one PrimRep, which holds after unarisation.
    -- See Note [Post-unarisation invariants]
    argPrimRep :: StgArg -> PrimRep
    argPrimRep arg = typePrimRep1 (stgArgType arg)

    ------------------------------------------------------
    --                Building LambdaFormInfo
    ------------------------------------------------------

    mkLFArgument :: Id -> LambdaFormInfo
    mkLFArgument id
      | isUnliftedType ty      = LFUnlifted
      | might_be_a_function ty = LFUnknown True
      | otherwise              = LFUnknown False
      where
        ty = idType id

    -------------
    mkLFLetNoEscape :: LambdaFormInfo
    mkLFLetNoEscape = LFLetNoEscape

    -------------
    mkLFReEntrant :: TopLevelFlag    -- True of top level
                  -> [Id]            -- Free vars
                  -> [Id]            -- Args
                  -> ArgDescr        -- Argument descriptor
                  -> LambdaFormInfo

    mkLFReEntrant _ _ [] _
      = pprPanic "mkLFReEntrant" empty
    mkLFReEntrant top fvs args arg_descr
      = LFReEntrant top (length args) (null fvs) arg_descr

    -------------
    mkLFThunk :: Type -> TopLevelFlag -> [Id] -> UpdateFlag -> LambdaFormInfo
    mkLFThunk thunk_ty top fvs upd_flag
      = ASSERT( not (isUpdatable upd_flag) || not (isUnliftedType thunk_ty) )
        LFThunk top (null fvs)
                (isUpdatable upd_flag)
                NonStandardThunk
                (might_be_a_function thunk_ty)

    --------------
    might_be_a_function :: Type -> Bool
    -- Return False only if we are *sure* it's a data type
    -- Look through newtypes etc as much as poss
    might_be_a_function ty
      | [LiftedRep] <- typePrimRep ty
      , Just tc <- tyConAppTyCon_maybe (unwrapType ty)
      , isDataTyCon tc
      = False
      | otherwise
      = True

    -------------
    mkConLFInfo :: DataCon -> LambdaFormInfo
    mkConLFInfo con = LFCon con

    -------------
    mkSelectorLFInfo :: Id -> Int -> Bool -> LambdaFormInfo
    mkSelectorLFInfo id offset updatable
      = LFThunk NotTopLevel False updatable (SelectorThunk offset)
            (might_be_a_function (idType id))

    -------------
    mkApLFInfo :: Id -> UpdateFlag -> Arity -> LambdaFormInfo
    mkApLFInfo id upd_flag arity
      = LFThunk NotTopLevel (arity == 0) (isUpdatable upd_flag) (ApThunk arity)
            (might_be_a_function (idType id))

    -------------
    mkLFImported :: Id -> LambdaFormInfo
    mkLFImported id =
        -- See Note [Conveying CAF-info and LFInfo between modules] in
        -- GHC.StgToCmm.Types
        case idLFInfo_maybe id of
          Just lf_info ->
            -- Use the LambdaFormInfo from the interface
            lf_info
          Nothing
            -- Interface doesn't have a LambdaFormInfo, make a conservative one from
            -- the type.
            | Just con <- isDataConWorkId_maybe id
            , isNullaryRepDataCon con
            -> LFCon con   -- An imported nullary constructor
                           -- We assume that the constructor is evaluated so that
                           -- the id really does point directly to the constructor

            | arity > 0
            -> LFReEntrant TopLevel arity True ArgUnknown

            | otherwise
            -> mkLFArgument id -- Not sure of exact arity
      where
        arity = idFunRepArity id

    -------------
    mkLFStringLit :: LambdaFormInfo
    mkLFStringLit = LFUnlifted

    -----------------------------------------------------
    --                Dynamic pointer tagging
    -----------------------------------------------------

    type DynTag = Int       -- The tag on a *pointer*
                            -- (from the dynamic-tagging paper)

    -- Note [Data constructor dynamic tags]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- The family size of a data type (the number of constructors
    -- or the arity of a function) can be either:
    --    * small, if the family size < 2**tag_bits
    --    * big, otherwise.
    --
    -- Small families can have the constructor tag in the tag bits.
    -- Big families always use the tag values 1..mAX_PTR_TAG to represent
    -- evaluatedness, the last one lumping together all overflowing ones.
    -- We don't have very many tag bits: for example, we have 2 bits on
    -- x86-32 and 3 bits on x86-64.
    --
    -- Also see Note [Tagging big families] in GHC.StgToCmm.Expr

    isSmallFamily :: Platform -> Int -> Bool
    isSmallFamily platform fam_size = fam_size <= mAX_PTR_TAG platform

    tagForCon :: Platform -> DataCon -> DynTag
    tagForCon platform con = min (dataConTag con) (mAX_PTR_TAG platform)
    -- NB: 1-indexed

    tagForArity :: Platform -> RepArity -> DynTag
    tagForArity platform arity
     | isSmallFamily platform arity = arity
     | otherwise                    = 0

    -- | Return the tag in the low order bits of a variable bound
    -- to this LambdaForm
    lfDynTag :: Platform -> LambdaFormInfo -> DynTag
    lfDynTag platform lf = case lf of
       LFCon con               -> tagForCon   platform con
       LFReEntrant _ arity _ _ -> tagForArity platform arity
       _other                  -> 0

    -----------------------------------------------------------------------------
    --                Observing LambdaFormInfo
    -----------------------------------------------------------------------------

    ------------
    isLFThunk :: LambdaFormInfo -> Bool
    isLFThunk (LFThunk {})  = True
    isLFThunk _ = False

    isLFReEntrant :: LambdaFormInfo -> Bool
    isLFReEntrant (LFReEntrant {}) = True
    isLFReEntrant _                = False

    -----------------------------------------------------------------------------
    --                Choosing SM reps
    -----------------------------------------------------------------------------

    lfClosureType :: LambdaFormInfo -> ClosureTypeInfo
    lfClosureType (LFReEntrant _ arity _ argd) = Fun arity argd
    lfClosureType (LFCon con)                  = Constr (dataConTagZ con)
                                                        (dataConIdentity con)
    lfClosureType (LFThunk _ _ _ is_sel _)     = thunkClosureType is_sel
    lfClosureType _                            = panic "lfClosureType"

    thunkClosureType :: StandardFormInfo -> ClosureTypeInfo
    thunkClosureType (SelectorThunk off) = ThunkSelector off
    thunkClosureType _                   = Thunk

    -- We *do* get non-updatable top-level thunks sometimes.  eg. f = g
    -- gets compiled to a jump to g (if g has non-zero arity), instead of
    -- messing around with update frames and PAPs.  We set the closure type
    -- to FUN_STATIC in this case.

    -----------------------------------------------------------------------------
    --                nodeMustPointToIt
    -----------------------------------------------------------------------------

    nodeMustPointToIt :: Profile -> LambdaFormInfo -> Bool
    -- If nodeMustPointToIt is true, then the entry convention for
    -- this closure has R1 (the "Node" register) pointing to the
    -- closure itself --- the "self" argument

    nodeMustPointToIt _ (LFReEntrant top _ no_fvs _)
      =  not no_fvs          -- Certainly if it has fvs we need to point to it
      || isNotTopLevel top   -- See Note [GC recovery]
            -- For lex_profiling we also access the cost centre for a
            -- non-inherited (i.e. non-top-level) function.
            -- The isNotTopLevel test above ensures this is ok.

    nodeMustPointToIt profile (LFThunk top no_fvs updatable NonStandardThunk _)
      =  not no_fvs            -- Self parameter
      || isNotTopLevel top     -- Note [GC recovery]
      || updatable             -- Need to push update frame
      || profileIsProfiling profile
              -- For the non-updatable (single-entry case):
              --
              -- True if has fvs (in which case we need access to them, and we
              --                    should black-hole it)
              -- or profiling (in which case we need to recover the cost centre
              --                 from inside it)  ToDo: do we need this even for
              --                                    top-level thunks? If not,
              --                                    isNotTopLevel subsumes this

    nodeMustPointToIt _ (LFThunk {})        -- Node must point to a standard-form thunk
      = True

    nodeMustPointToIt _ (LFCon _) = True

            -- Strictly speaking, the above two don't need Node to point
            -- to it if the arity = 0.  But this is a *really* unlikely
            -- situation.  If we know it's nil (say) and we are entering
            -- it. Eg: let x = [] in x then we will certainly have inlined
            -- x, since nil is a simple atom.  So we gain little by not
            -- having Node point to known zero-arity things.  On the other
            -- hand, we do lose something; Patrick's code for figuring out
            -- when something has been updated but not entered relies on
            -- having Node point to the result of an update.  SLPJ
            -- 27/11/92.

    nodeMustPointToIt _ (LFUnknown _)   = True
    nodeMustPointToIt _ LFUnlifted      = False
    nodeMustPointToIt _ LFLetNoEscape   = False

    {- Note [GC recovery]
    ~~~~~~~~~~~~~~~~~~~~~
    If we a have a local let-binding (function or thunk)
       let f = <body> in ...
    AND <body> allocates, then the heap-overflow check needs to know how
    to re-start the evaluation.  It uses the "self" pointer to do this.
    So even if there are no free variables in <body>, we still make
    nodeMustPointToIt be True for non-top-level bindings.

    Why do any such bindings exist?  After all, let-floating should have
    floated them out.  Well, a clever optimiser might leave one there to
    avoid a space leak, deliberately recomputing a thunk.  Also (and this
    really does happen occasionally) let-floating may make a function f smaller
    so it can be inlined, so now (f True) may generate a local no-fv closure.
    This actually happened during bootstrapping GHC itself, with f=mkRdrFunBind
    in GHC.Tc.Deriv.Generate.) -}

    -----------------------------------------------------------------------------
    --                getCallMethod
    -----------------------------------------------------------------------------

    {- The entry conventions depend on the type of closure being entered,
    whether or not it has free variables, and whether we're running
    sequentially or in parallel.

    Closure                           Node   Argument   Enter
    Characteristics              Par   Req'd  Passing    Via
    ---------------------------------------------------------------------------
    Unknown                     & no  & yes & stack     & node
    Known fun (>1 arg), no fvs  & no  & no  & registers & fast entry (enough args)
                                                        & slow entry (otherwise)
    Known fun (>1 arg), fvs     & no  & yes & registers & fast entry (enough args)
    0 arg, no fvs \r,\s         & no  & no  & n/a       & direct entry
    0 arg, no fvs \u            & no  & yes & n/a       & node
    0 arg, fvs \r,\s,selector   & no  & yes & n/a       & node
    0 arg, fvs \r,\s            & no  & yes & n/a       & direct entry
    0 arg, fvs \u               & no  & yes & n/a       & node
    Unknown                     & yes & yes & stack     & node
    Known fun (>1 arg), no fvs  & yes & no  & registers & fast entry (enough args)
                                                        & slow entry (otherwise)
    Known fun (>1 arg), fvs     & yes & yes & registers & node
    0 arg, fvs \r,\s,selector   & yes & yes & n/a       & node
    0 arg, no fvs \r,\s         & yes & no  & n/a       & direct entry
    0 arg, no fvs \u            & yes & yes & n/a       & node
    0 arg, fvs \r,\s            & yes & yes & n/a       & node
    0 arg, fvs \u               & yes & yes & n/a       & node

    When black-holing, single-entry closures could also be entered via node
    (rather than directly) to catch double-entry. -}

    data CallMethod
      = EnterIt             -- No args, not a function

      | JumpToIt BlockId [LocalReg] -- A join point or a header of a local loop

      | ReturnIt            -- It's a value (function, unboxed value,
                            -- or constructor), so just return it.

      | SlowCall                -- Unknown fun, or known fun with
                            -- too few args.

      | DirectEntry         -- Jump directly, with args in regs
            CLabel          --   The code label
            RepArity        --   Its arity

    data CallOpts = CallOpts
       { co_profile       :: !Profile   -- ^ Platform profile
       , co_loopification :: !Bool      -- ^ Loopification enabled (cf @-floopification@)
       , co_ticky         :: !Bool      -- ^ Ticky profiling enabled (cf @-ticky@)
       }

    getCallMethod :: CallOpts
                  -> Name           -- Function being applied
                  -> Id             -- Function Id used to chech if it can refer to
                                    -- CAF's and whether the function is tail-calling
                                    -- itself
                  -> LambdaFormInfo -- Its info
                  -> RepArity       -- Number of available arguments
                  -> RepArity       -- Number of them being void arguments
                  -> CgLoc          -- Passed in from cgIdApp so that we can
                                    -- handle let-no-escape bindings and self-recursive
                                    -- tail calls using the same data constructor,
                                    -- JumpToIt. This saves us one case branch in
                                    -- cgIdApp
                  -> Maybe SelfLoopInfo -- can we perform a self-recursive tail call?
                  -> CallMethod

    getCallMethod opts _ id _ n_args v_args _cg_loc
                  (Just (self_loop_id, block_id, args))
      | co_loopification opts
      , id == self_loop_id
      , args `lengthIs` (n_args - v_args)
      -- If these patterns match then we know that:
      --   * loopification optimisation is turned on
      --   * function is performing a self-recursive call in a tail position
      --   * number of non-void parameters of the function matches functions arity.
      -- See Note [Self-recursive tail calls] and Note [Void arguments in
      -- self-recursive tail calls] in GHC.StgToCmm.Expr for more details
      = JumpToIt block_id args

    getCallMethod opts name id (LFReEntrant _ arity _ _) n_args _v_args _cg_loc
                  _self_loop_info
      | n_args == 0 -- No args at all
      && not (profileIsProfiling (co_profile opts))
         -- See Note [Evaluating functions with profiling] in rts/Apply.cmm
      = ASSERT( arity /= 0 ) ReturnIt
      | n_args < arity = SlowCall        -- Not enough args
      | otherwise      = DirectEntry (enterIdLabel (profilePlatform (co_profile opts)) name (idCafInfo id)) arity

    getCallMethod _ _name _ LFUnlifted n_args _v_args _cg_loc _self_loop_info
      = ASSERT( n_args == 0 ) ReturnIt

    getCallMethod _ _name _ (LFCon _) n_args _v_args _cg_loc _self_loop_info
      = ASSERT( n_args == 0 ) ReturnIt
        -- n_args=0 because it'd be ill-typed to apply a saturated
        --          constructor application to anything

    getCallMethod opts name id (LFThunk _ _ updatable std_form_info is_fun)
                  n_args _v_args _cg_loc _self_loop_info
      | is_fun      -- it *might* be a function, so we must "call" it (which is always safe)
      = SlowCall    -- We cannot just enter it [in eval/apply, the entry code
                    -- is the fast-entry code]

      -- Since is_fun is False, we are *definitely* looking at a data value
      | updatable || co_ticky opts -- to catch double entry
          {- OLD: || opt_SMP
             I decided to remove this, because in SMP mode it doesn't matter
             if we enter the same thunk multiple times, so the optimisation
             of jumping directly to the entry code is still valid.  --SDM
            -}
      = EnterIt

      -- even a non-updatable selector thunk can be updated by the garbage
      -- collector, so we must enter it. (#8817)
      | SelectorThunk{} <- std_form_info
      = EnterIt

        -- We used to have ASSERT( n_args == 0 ), but actually it is
        -- possible for the optimiser to generate
        --   let bot :: Int = error Int "urk"
        --   in (bot `cast` unsafeCoerce Int (Int -> Int)) 3
        -- This happens as a result of the case-of-error transformation
        -- So the right thing to do is just to enter the thing

      | otherwise        -- Jump direct to code for single-entry thunks
      = ASSERT( n_args == 0 )
        DirectEntry (thunkEntryLabel (profilePlatform (co_profile opts)) name (idCafInfo id) std_form_info
                    updatable) 0

    getCallMethod _ _name _ (LFUnknown True) _n_arg _v_args _cg_locs _self_loop_info
      = SlowCall -- might be a function

    getCallMethod _ name _ (LFUnknown False) n_args _v_args _cg_loc _self_loop_info
      = ASSERT2( n_args == 0, ppr name <+> ppr n_args )
        EnterIt -- Not a function

    getCallMethod _ _name _ LFLetNoEscape _n_args _v_args (LneLoc blk_id lne_regs)
                  _self_loop_info
      = JumpToIt blk_id lne_regs

    getCallMethod _ _ _ _ _ _ _ _ = panic "Unknown call method"

    -----------------------------------------------------------------------------
    --              Data types for closure information
    -----------------------------------------------------------------------------

    {- ClosureInfo: information about a binding

       We make a ClosureInfo for each let binding (both top level and not),
       but not bindings for data constructors: for those we build a CmmInfoTable
       directly (see mkDataConInfoTable).

       To a first approximation:
           ClosureInfo = (LambdaFormInfo, CmmInfoTable)

       A ClosureInfo has enough information
         a) to construct the info table itself, and build other things
            related to the binding (e.g. slow entry points for a function)
         b) to allocate a closure containing that info pointer (i.e.
               it knows the info table label)
    -}

    data ClosureInfo
      = ClosureInfo {
            closureName :: !Name,           -- The thing bound to this closure
               -- we don't really need this field: it's only used in generating
               -- code for ticky and profiling, and we could pass the information
               -- around separately, but it doesn't do much harm to keep it here.

            closureLFInfo :: !LambdaFormInfo, -- NOTE: not an LFCon
              -- this tells us about what the closure contains: it's right-hand-side.

              -- the rest is just an unpacked CmmInfoTable.
            closureInfoLabel :: !CLabel,
            closureSMRep     :: !SMRep,          -- representation used by storage mgr
            closureProf      :: !ProfilingInfo
        }

    -- | Convert from 'ClosureInfo' to 'CmmInfoTable'.
    mkCmmInfo :: ClosureInfo -> Id -> CostCentreStack -> CmmInfoTable
    mkCmmInfo ClosureInfo {..} id ccs
      = CmmInfoTable { cit_lbl  = closureInfoLabel
                     , cit_rep  = closureSMRep
                     , cit_prof = closureProf
                     , cit_srt  = Nothing
                     , cit_clo  = if isStaticRep closureSMRep
                                    then Just (id,ccs)
                                    else Nothing }

    --------------------------------------
    --        Building ClosureInfos
    --------------------------------------

    mkClosureInfo :: Profile
                  -> Bool                -- Is static
                  -> Id
                  -> LambdaFormInfo
                  -> Int -> Int        -- Total and pointer words
                  -> String         -- String descriptor
                  -> ClosureInfo
    mkClosureInfo profile is_static id lf_info tot_wds ptr_wds val_descr
      = ClosureInfo { closureName      = name
                    , closureLFInfo    = lf_info
                    , closureInfoLabel = info_lbl   -- These three fields are
                    , closureSMRep     = sm_rep     -- (almost) an info table
                    , closureProf      = prof }     -- (we don't have an SRT yet)
      where
        name       = idName id
        sm_rep     = mkHeapRep profile is_static ptr_wds nonptr_wds (lfClosureType lf_info)
        prof       = mkProfilingInfo profile id val_descr
        nonptr_wds = tot_wds - ptr_wds

        info_lbl = mkClosureInfoTableLabel (profilePlatform profile) id lf_info

    --------------------------------------
    --   Other functions over ClosureInfo
    --------------------------------------

    -- Eager blackholing is normally disabled, but can be turned on with
    -- -feager-blackholing.  When it is on, we replace the info pointer of
    -- the thunk with stg_EAGER_BLACKHOLE_info on entry.

    -- If we wanted to do eager blackholing with slop filling,
    -- we'd need to do it at the *end* of a basic block, otherwise
    -- we overwrite the free variables in the thunk that we still
    -- need.  We have a patch for this from Andy Cheadle, but not
    -- incorporated yet. --SDM [6/2004]
    --
    -- Previously, eager blackholing was enabled when ticky-ticky
    -- was on. But it didn't work, and it wasn't strictly necessary
    -- to bring back minimal ticky-ticky, so now EAGER_BLACKHOLING
    -- is unconditionally disabled. -- krc 1/2007
    --
    -- Static closures are never themselves black-holed.

    blackHoleOnEntry :: ClosureInfo -> Bool
    blackHoleOnEntry cl_info
      | isStaticRep (closureSMRep cl_info)
      = False        -- Never black-hole a static closure

      | otherwise
      = case closureLFInfo cl_info of
          LFReEntrant {}            -> False
          LFLetNoEscape             -> False
          LFThunk _ _no_fvs upd _ _ -> upd   -- See Note [Black-holing non-updatable thunks]
          _other -> panic "blackHoleOnEntry"

    {- Note [Black-holing non-updatable thunks]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We must not black-hole non-updatable (single-entry) thunks otherwise
    we run into issues like #10414. Specifically:

      * There is no reason to black-hole a non-updatable thunk: it should
        not be competed for by multiple threads

      * It could, conceivably, cause a space leak if we don't black-hole
        it, if there was a live but never-followed pointer pointing to it.
        Let's hope that doesn't happen.

      * It is dangerous to black-hole a non-updatable thunk because
         - is not updated (of course)
         - hence, if it is black-holed and another thread tries to evaluate
           it, that thread will block forever
        This actually happened in #10414.  So we do not black-hole
        non-updatable thunks.

      * How could two threads evaluate the same non-updatable (single-entry)
        thunk?  See Reid Barton's example below.

      * Only eager blackholing could possibly black-hole a non-updatable
        thunk, because lazy black-holing only affects thunks with an
        update frame on the stack.

    Here is and example due to Reid Barton (#10414):
        x = \u []  concat [[1], []]
    with the following definitions,

        concat x = case x of
            []       -> []
            (:) x xs -> (++) x (concat xs)

        (++) xs ys = case xs of
            []         -> ys
            (:) x rest -> (:) x ((++) rest ys)

    Where we use the syntax @\u []@ to denote an updatable thunk and @\s []@ to
    denote a single-entry (i.e. non-updatable) thunk. After a thread evaluates @x@
    to WHNF and calls @(++)@ the heap will contain the following thunks,

        x = 1 : y
        y = \u []  (++) [] z
        z = \s []  concat []

    Now that the stage is set, consider the follow evaluations by two racing threads
    A and B,

      1. Both threads enter @y@ before either is able to replace it with an
         indirection

      2. Thread A does the case analysis in @(++)@ and consequently enters @z@,
         replacing it with a black-hole

      3. At some later point thread B does the same case analysis and also attempts
         to enter @z@. However, it finds that it has been replaced with a black-hole
         so it blocks.

      4. Thread A eventually finishes evaluating @z@ (to @[]@) and updates @y@
         accordingly. It does *not* update @z@, however, as it is single-entry. This
         leaves Thread B blocked forever on a black-hole which will never be
         updated.

    To avoid this sort of condition we never black-hole non-updatable thunks.
    -}

    isStaticClosure :: ClosureInfo -> Bool
    isStaticClosure cl_info = isStaticRep (closureSMRep cl_info)

    closureUpdReqd :: ClosureInfo -> Bool
    closureUpdReqd ClosureInfo{ closureLFInfo = lf_info } = lfUpdatable lf_info

    lfUpdatable :: LambdaFormInfo -> Bool
    lfUpdatable (LFThunk _ _ upd _ _)  = upd
    lfUpdatable _ = False

    closureReEntrant :: ClosureInfo -> Bool
    closureReEntrant (ClosureInfo { closureLFInfo = LFReEntrant {} }) = True
    closureReEntrant _ = False

    closureFunInfo :: ClosureInfo -> Maybe (RepArity, ArgDescr)
    closureFunInfo (ClosureInfo { closureLFInfo = lf_info }) = lfFunInfo lf_info

    lfFunInfo :: LambdaFormInfo ->  Maybe (RepArity, ArgDescr)
    lfFunInfo (LFReEntrant _ arity _ arg_desc)  = Just (arity, arg_desc)
    lfFunInfo _                                 = Nothing

    funTag :: Platform -> ClosureInfo -> DynTag
    funTag platform (ClosureInfo { closureLFInfo = lf_info })
        = lfDynTag platform lf_info

    isToplevClosure :: ClosureInfo -> Bool
    isToplevClosure (ClosureInfo { closureLFInfo = lf_info })
      = case lf_info of
          LFReEntrant TopLevel _ _ _ -> True
          LFThunk TopLevel _ _ _ _   -> True
          _other                     -> False

    --------------------------------------
    --   Label generation
    --------------------------------------

    staticClosureLabel :: Platform -> ClosureInfo -> CLabel
    staticClosureLabel platform = toClosureLbl platform .  closureInfoLabel

    closureSlowEntryLabel :: Platform -> ClosureInfo -> CLabel
    closureSlowEntryLabel platform = toSlowEntryLbl platform . closureInfoLabel

    closureLocalEntryLabel :: Platform -> ClosureInfo -> CLabel
    closureLocalEntryLabel platform
      | platformTablesNextToCode platform = toInfoLbl  platform . closureInfoLabel
      | otherwise                         = toEntryLbl platform . closureInfoLabel

    mkClosureInfoTableLabel :: Platform -> Id -> LambdaFormInfo -> CLabel
    mkClosureInfoTableLabel platform id lf_info
      = case lf_info of
            LFThunk _ _ upd_flag (SelectorThunk offset) _
                          -> mkSelectorInfoLabel platform upd_flag offset

            LFThunk _ _ upd_flag (ApThunk arity) _
                          -> mkApInfoTableLabel platform upd_flag arity

            LFThunk{}     -> std_mk_lbl name cafs
            LFReEntrant{} -> std_mk_lbl name cafs
            _other        -> panic "closureInfoTableLabel"

      where
        name = idName id

        std_mk_lbl | is_local  = mkLocalInfoTableLabel
                   | otherwise = mkInfoTableLabel

        cafs     = idCafInfo id
        is_local = isDataConWorkId id
           -- Make the _info pointer for the implicit datacon worker
           -- binding local. The reason we can do this is that importing
           -- code always either uses the _closure or _con_info. By the
           -- invariants in "GHC.CoreToStg.Prep" anything else gets eta expanded.

    -- | thunkEntryLabel is a local help function, not exported.  It's used from
    -- getCallMethod.
    thunkEntryLabel :: Platform -> Name -> CafInfo -> StandardFormInfo -> Bool -> CLabel
    thunkEntryLabel platform thunk_id caf_info sfi upd_flag = case sfi of
       ApThunk arity        -> enterApLabel       platform upd_flag arity
       SelectorThunk offset -> enterSelectorLabel platform upd_flag offset
       _                    -> enterIdLabel       platform thunk_id caf_info

    enterApLabel :: Platform -> Bool -> Arity -> CLabel
    enterApLabel platform is_updatable arity
      | platformTablesNextToCode platform = mkApInfoTableLabel platform is_updatable arity
      | otherwise                         = mkApEntryLabel     platform is_updatable arity

    enterSelectorLabel :: Platform -> Bool -> WordOff -> CLabel
    enterSelectorLabel platform upd_flag offset
      | platformTablesNextToCode platform = mkSelectorInfoLabel  platform upd_flag offset
      | otherwise                         = mkSelectorEntryLabel platform upd_flag offset

    enterIdLabel :: Platform -> Name -> CafInfo -> CLabel
    enterIdLabel platform id c
      | platformTablesNextToCode platform = mkInfoTableLabel id c
      | otherwise                         = mkEntryLabel id c

    --------------------------------------
    --   Profiling
    --------------------------------------

    -- Profiling requires two pieces of information to be determined for
    -- each closure's info table --- description and type.

    -- The description is stored directly in the @CClosureInfoTable@ when the
    -- info table is built.

    -- The type is determined from the type information stored with the @Id@
    -- in the closure info using @closureTypeDescr@.

    mkProfilingInfo :: Profile -> Id -> String -> ProfilingInfo
    mkProfilingInfo profile id val_descr
      | not (profileIsProfiling profile) = NoProfilingInfo
      | otherwise                        = ProfilingInfo ty_descr_w8 (BS8.pack val_descr)
      where
        ty_descr_w8  = BS8.pack (getTyDescription (idType id))

    getTyDescription :: Type -> String
    getTyDescription ty
      = case (tcSplitSigmaTy ty) of { (_, _, tau_ty) ->
        case tau_ty of
          TyVarTy _              -> "*"
          AppTy fun _            -> getTyDescription fun
          TyConApp tycon _       -> getOccString tycon
          FunTy {}              -> '-' : fun_result tau_ty
          ForAllTy _  ty         -> getTyDescription ty
          LitTy n                -> getTyLitDescription n
          CastTy ty _            -> getTyDescription ty
          CoercionTy co          -> pprPanic "getTyDescription" (ppr co)
        }
      where
        fun_result (FunTy { ft_res = res }) = '>' : fun_result res
        fun_result other                    = getTyDescription other

    getTyLitDescription :: TyLit -> String
    getTyLitDescription l =
      case l of
        NumTyLit n -> show n
        StrTyLit n -> show n
        CharTyLit n -> show n

    --------------------------------------
    --   CmmInfoTable-related things
    --------------------------------------

    mkDataConInfoTable :: Profile -> DataCon -> ConInfoTableLocation -> Bool -> Int -> Int -> CmmInfoTable
    mkDataConInfoTable profile data_con mn is_static ptr_wds nonptr_wds
     = CmmInfoTable { cit_lbl  = info_lbl
                    , cit_rep  = sm_rep
                    , cit_prof = prof
                    , cit_srt  = Nothing
                    , cit_clo  = Nothing }
     where
       name = dataConName data_con
       info_lbl = mkConInfoTableLabel name mn -- NoCAFRefs
       sm_rep = mkHeapRep profile is_static ptr_wds nonptr_wds cl_type
       cl_type = Constr (dataConTagZ data_con) (dataConIdentity data_con)
                      -- We keep the *zero-indexed* tag in the srt_len field
                      -- of the info table of a data constructor.

       prof | not (profileIsProfiling profile) = NoProfilingInfo
            | otherwise                        = ProfilingInfo ty_descr val_descr

       ty_descr  = BS8.pack $ occNameString $ getOccName $ dataConTyCon data_con
       val_descr = BS8.pack $ occNameString $ getOccName data_con

    -- We need a black-hole closure info to pass to @allocDynClosure@ when we
    -- want to allocate the black hole on entry to a CAF.

    cafBlackHoleInfoTable :: CmmInfoTable
    cafBlackHoleInfoTable
      = CmmInfoTable { cit_lbl  = mkCAFBlackHoleInfoTableLabel
                     , cit_rep  = blackHoleRep
                     , cit_prof = NoProfilingInfo
                     , cit_srt  = Nothing
                     , cit_clo  = Nothing }

    indStaticInfoTable :: CmmInfoTable
    indStaticInfoTable
      = CmmInfoTable { cit_lbl  = mkIndStaticInfoLabel
                     , cit_rep  = indStaticRep
                     , cit_prof = NoProfilingInfo
                     , cit_srt  = Nothing
                     , cit_clo  = Nothing }

    staticClosureNeedsLink :: Bool -> CmmInfoTable -> Bool
    -- A static closure needs a link field to aid the GC when traversing
    -- the static closure graph.  But it only needs such a field if either
    --        a) it has an SRT
    --        b) it's a constructor with one or more pointer fields
    -- In case (b), the constructor's fields themselves play the role
    -- of the SRT.
    staticClosureNeedsLink has_srt CmmInfoTable{ cit_rep = smrep }
      | isConRep smrep         = not (isStaticNoCafCon smrep)
      | otherwise              = has_srt