GHC/StgToCmm/Bind.hs

    -----------------------------------------------------------------------------
    --
    -- Stg to C-- code generation: bindings
    --
    -- (c) The University of Glasgow 2004-2006
    --
    -----------------------------------------------------------------------------

    module GHC.StgToCmm.Bind (
            cgTopRhsClosure,
            cgBind,
            emitBlackHoleCode,
            pushUpdateFrame, emitUpdateFrame
      ) where

    import GHC.Prelude hiding ((<*>))

    import GHC.Platform
    import GHC.Platform.Profile

    import GHC.StgToCmm.Expr
    import GHC.StgToCmm.Monad
    import GHC.StgToCmm.Env
    import GHC.StgToCmm.DataCon
    import GHC.StgToCmm.Heap
    import GHC.StgToCmm.Prof (ldvEnterClosure, enterCostCentreFun, enterCostCentreThunk,
                       initUpdFrameProf)
    import GHC.StgToCmm.Ticky
    import GHC.StgToCmm.Layout
    import GHC.StgToCmm.Utils
    import GHC.StgToCmm.Closure
    import GHC.StgToCmm.Foreign    (emitPrimCall)

    import GHC.Cmm.Graph
    import GHC.Core          ( AltCon(..) )
    import GHC.Cmm.BlockId
    import GHC.Runtime.Heap.Layout
    import GHC.Cmm
    import GHC.Cmm.Info
    import GHC.Cmm.Utils
    import GHC.Cmm.CLabel
    import GHC.Stg.Syntax
    import GHC.Types.CostCentre
    import GHC.Types.Id
    import GHC.Types.Id.Info
    import GHC.Types.Name
    import GHC.Unit.Module
    import GHC.Data.List.SetOps
    import GHC.Utils.Misc
    import GHC.Types.Var.Set
    import GHC.Types.Basic
    import GHC.Types.Tickish ( tickishIsCode )
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Data.FastString
    import GHC.Driver.Session
    import GHC.Driver.Ppr

    import Control.Monad

    ------------------------------------------------------------------------
    --              Top-level bindings
    ------------------------------------------------------------------------

    -- For closures bound at top level, allocate in static space.
    -- They should have no free variables.

    cgTopRhsClosure :: Platform
                    -> RecFlag              -- member of a recursive group?
                    -> Id
                    -> CostCentreStack      -- Optional cost centre annotation
                    -> UpdateFlag
                    -> [Id]                 -- Args
                    -> CgStgExpr
                    -> (CgIdInfo, FCode ())

    cgTopRhsClosure platform rec id ccs upd_flag args body =
      let closure_label = mkLocalClosureLabel (idName id) (idCafInfo id)
          cg_id_info    = litIdInfo platform id lf_info (CmmLabel closure_label)
          lf_info       = mkClosureLFInfo platform id TopLevel [] upd_flag args
      in (cg_id_info, gen_code lf_info closure_label)
      where
      -- special case for a indirection (f = g).  We create an IND_STATIC
      -- closure pointing directly to the indirectee.  This is exactly
      -- what the CAF will eventually evaluate to anyway, we're just
      -- shortcutting the whole process, and generating a lot less code
      -- (#7308). Eventually the IND_STATIC closure will be eliminated
      -- by assembly '.equiv' directives, where possible (#15155).
      -- See note [emit-time elimination of static indirections] in "GHC.Cmm.CLabel".
      --
      -- Note: we omit the optimisation when this binding is part of a
      -- recursive group, because the optimisation would inhibit the black
      -- hole detection from working in that case.  Test
      -- concurrent/should_run/4030 fails, for instance.
      --
      gen_code _ closure_label
        | StgApp f [] <- body, null args, isNonRec rec
        = do
             cg_info <- getCgIdInfo f
             emitDataCon closure_label indStaticInfoTable ccs [unLit (idInfoToAmode cg_info)]

      gen_code lf_info _closure_label
       = do { profile <- getProfile
            ; dflags <- getDynFlags
            ; let name = idName id
            ; mod_name <- getModuleName
            ; let descr         = closureDescription dflags mod_name name
                  closure_info  = mkClosureInfo profile True id lf_info 0 0 descr

            -- We don't generate the static closure here, because we might
            -- want to add references to static closures to it later.  The
            -- static closure is generated by GHC.Cmm.Info.Build.updInfoSRTs,
            -- See Note [SRTs], specifically the [FUN] optimisation.

            ; let fv_details :: [(NonVoid Id, ByteOff)]
                  header = if isLFThunk lf_info then ThunkHeader else StdHeader
                  (_, _, fv_details) = mkVirtHeapOffsets profile header []
            -- Don't drop the non-void args until the closure info has been made
            ; forkClosureBody (closureCodeBody True id closure_info ccs
                                    args body fv_details)

            ; return () }

      unLit (CmmLit l) = l
      unLit _ = panic "unLit"

    ------------------------------------------------------------------------
    --              Non-top-level bindings
    ------------------------------------------------------------------------

    cgBind :: CgStgBinding -> FCode ()
    cgBind (StgNonRec name rhs)
      = do  { (info, fcode) <- cgRhs name rhs
            ; addBindC info
            ; init <- fcode
            ; emit init }
            -- init cannot be used in body, so slightly better to sink it eagerly

    cgBind (StgRec pairs)
      = do  {  r <- sequence $ unzipWith cgRhs pairs
            ;  let (id_infos, fcodes) = unzip r
            ;  addBindsC id_infos
            ;  (inits, body) <- getCodeR $ sequence fcodes
            ;  emit (catAGraphs inits <*> body) }

    {- Note [cgBind rec]

       Recursive let-bindings are tricky.
       Consider the following pseudocode:

         let x = \_ ->  ... y ...
             y = \_ ->  ... z ...
             z = \_ ->  ... x ...
         in ...

       For each binding, we need to allocate a closure, and each closure must
       capture the address of the other closures.
       We want to generate the following C-- code:
         // Initialization Code
         x = hp - 24; // heap address of x's closure
         y = hp - 40; // heap address of x's closure
         z = hp - 64; // heap address of x's closure
         // allocate and initialize x
         m[hp-8]   = ...
         m[hp-16]  = y       // the closure for x captures y
         m[hp-24] = x_info;
         // allocate and initialize y
         m[hp-32] = z;       // the closure for y captures z
         m[hp-40] = y_info;
         // allocate and initialize z
         ...

       For each closure, we must generate not only the code to allocate and
       initialize the closure itself, but also some initialization Code that
       sets a variable holding the closure pointer.

       We could generate a pair of the (init code, body code), but since
       the bindings are recursive we also have to initialise the
       environment with the CgIdInfo for all the bindings before compiling
       anything.  So we do this in 3 stages:

         1. collect all the CgIdInfos and initialise the environment
         2. compile each binding into (init, body) code
         3. emit all the inits, and then all the bodies

       We'd rather not have separate functions to do steps 1 and 2 for
       each binding, since in practice they share a lot of code.  So we
       have just one function, cgRhs, that returns a pair of the CgIdInfo
       for step 1, and a monadic computation to generate the code in step
       2.

       The alternative to separating things in this way is to use a
       fixpoint.  That's what we used to do, but it introduces a
       maintenance nightmare because there is a subtle dependency on not
       being too strict everywhere.  Doing things this way means that the
       FCode monad can be strict, for example.
     -}

    cgRhs :: Id
          -> CgStgRhs
          -> FCode (
                     CgIdInfo         -- The info for this binding
                   , FCode CmmAGraph  -- A computation which will generate the
                                      -- code for the binding, and return an
                                      -- assignment of the form "x = Hp - n"
                                      -- (see above)
                   )

    cgRhs id (StgRhsCon cc con mn _ts args)
      = withNewTickyCounterCon (idName id) con $
        buildDynCon id mn True cc con (assertNonVoidStgArgs args)
          -- con args are always non-void,
          -- see Note [Post-unarisation invariants] in GHC.Stg.Unarise

    {- See Note [GC recovery] in "GHC.StgToCmm.Closure" -}
    cgRhs id (StgRhsClosure fvs cc upd_flag args body)
      = do profile <- getProfile
           mkRhsClosure profile id cc (nonVoidIds (dVarSetElems fvs)) upd_flag args body

    ------------------------------------------------------------------------
    --              Non-constructor right hand sides
    ------------------------------------------------------------------------

    mkRhsClosure :: Profile -> Id -> CostCentreStack
                 -> [NonVoid Id]                    -- Free vars
                 -> UpdateFlag
                 -> [Id]                            -- Args
                 -> CgStgExpr
                 -> FCode (CgIdInfo, FCode CmmAGraph)

    {- mkRhsClosure looks for two special forms of the right-hand side:
            a) selector thunks
            b) AP thunks

    If neither happens, it just calls mkClosureLFInfo.  You might think
    that mkClosureLFInfo should do all this, but it seems wrong for the
    latter to look at the structure of an expression

    Note [Selectors]
    ~~~~~~~~~~~~~~~~
    We look at the body of the closure to see if it's a selector---turgid,
    but nothing deep.  We are looking for a closure of {\em exactly} the
    form:

    ...  = [the_fv] \ u [] ->
             case the_fv of
               con a_1 ... a_n -> a_i

    Note [Ap thunks]
    ~~~~~~~~~~~~~~~~
    A more generic AP thunk of the form

            x = [ x_1...x_n ] \.. [] -> x_1 ... x_n

    A set of these is compiled statically into the RTS, so we just use
    those.  We could extend the idea to thunks where some of the x_i are
    global ids (and hence not free variables), but this would entail
    generating a larger thunk.  It might be an option for non-optimising
    compilation, though.

    We only generate an Ap thunk if all the free variables are pointers,
    for semi-obvious reasons.

    -}

    ---------- Note [Selectors] ------------------
    mkRhsClosure    profile bndr _cc
                    [NonVoid the_fv]                -- Just one free var
                    upd_flag                -- Updatable thunk
                    []                      -- A thunk
                    expr
      | let strip = stripStgTicksTopE (not . tickishIsCode)
      , StgCase (StgApp scrutinee [{-no args-}])
             _   -- ignore bndr
             (AlgAlt _)
             [(DataAlt _, params, sel_expr)] <- strip expr
      , StgApp selectee [{-no args-}] <- strip sel_expr
      , the_fv == scrutinee                -- Scrutinee is the only free variable

      , let (_, _, params_w_offsets) = mkVirtConstrOffsets profile (addIdReps (assertNonVoidIds params))
                                       -- pattern binders are always non-void,
                                       -- see Note [Post-unarisation invariants] in GHC.Stg.Unarise
      , Just the_offset <- assocMaybe params_w_offsets (NonVoid selectee)

      , let offset_into_int = bytesToWordsRoundUp (profilePlatform profile) the_offset
                              - fixedHdrSizeW profile
      , offset_into_int <= pc_MAX_SPEC_SELECTEE_SIZE (profileConstants profile) -- Offset is small enough
      = -- NOT TRUE: ASSERT(is_single_constructor)
        -- The simplifier may have statically determined that the single alternative
        -- is the only possible case and eliminated the others, even if there are
        -- other constructors in the datatype.  It's still ok to make a selector
        -- thunk in this case, because we *know* which constructor the scrutinee
        -- will evaluate to.
        --
        -- srt is discarded; it must be empty
        let lf_info = mkSelectorLFInfo bndr offset_into_int (isUpdatable upd_flag)
        in cgRhsStdThunk bndr lf_info [StgVarArg the_fv]

    ---------- Note [Ap thunks] ------------------
    mkRhsClosure    profile bndr _cc
                    fvs
                    upd_flag
                    []                      -- No args; a thunk
                    (StgApp fun_id args)

      -- We are looking for an "ApThunk"; see data con ApThunk in GHC.StgToCmm.Closure
      -- of form (x1 x2 .... xn), where all the xi are locals (not top-level)
      -- So the xi will all be free variables
      | args `lengthIs` (n_fvs-1)  -- This happens only if the fun_id and
                                   -- args are all distinct local variables
                                   -- The "-1" is for fun_id
        -- Missed opportunity:   (f x x) is not detected
      , all (isGcPtrRep . idPrimRep . fromNonVoid) fvs
      , isUpdatable upd_flag
      , n_fvs <= pc_MAX_SPEC_AP_SIZE (profileConstants profile)
      , not (profileIsProfiling profile)
                             -- not when profiling: we don't want to
                             -- lose information about this particular
                             -- thunk (e.g. its type) (#949)
      , idArity fun_id == unknownArity -- don't spoil a known call

              -- Ha! an Ap thunk
      = cgRhsStdThunk bndr lf_info payload

      where
        n_fvs   = length fvs
        lf_info = mkApLFInfo bndr upd_flag n_fvs
        -- the payload has to be in the correct order, hence we can't
        -- just use the fvs.
        payload = StgVarArg fun_id : args

    ---------- Default case ------------------
    mkRhsClosure profile bndr cc fvs upd_flag args body
      = do  { let lf_info = mkClosureLFInfo (profilePlatform profile) bndr NotTopLevel fvs upd_flag args
            ; (id_info, reg) <- rhsIdInfo bndr lf_info
            ; return (id_info, gen_code lf_info reg) }
     where
     gen_code lf_info reg
      = do  {       -- LAY OUT THE OBJECT
            -- If the binder is itself a free variable, then don't store
            -- it in the closure.  Instead, just bind it to Node on entry.
            -- NB we can be sure that Node will point to it, because we
            -- haven't told mkClosureLFInfo about this; so if the binder
            -- _was_ a free var of its RHS, mkClosureLFInfo thinks it *is*
            -- stored in the closure itself, so it will make sure that
            -- Node points to it...
            ; let   reduced_fvs = filter (NonVoid bndr /=) fvs

            ; profile <- getProfile
            ; let platform = profilePlatform profile

            -- MAKE CLOSURE INFO FOR THIS CLOSURE
            ; mod_name <- getModuleName
            ; dflags <- getDynFlags
            ; let   name  = idName bndr
                    descr = closureDescription dflags mod_name name
                    fv_details :: [(NonVoid Id, ByteOff)]
                    header = if isLFThunk lf_info then ThunkHeader else StdHeader
                    (tot_wds, ptr_wds, fv_details)
                       = mkVirtHeapOffsets profile header (addIdReps reduced_fvs)
                    closure_info = mkClosureInfo profile False       -- Not static
                                                 bndr lf_info tot_wds ptr_wds
                                                 descr

            -- BUILD ITS INFO TABLE AND CODE
            ; forkClosureBody $
                    -- forkClosureBody: (a) ensure that bindings in here are not seen elsewhere
                    --                  (b) ignore Sequel from context; use empty Sequel
                    -- And compile the body
                    closureCodeBody False bndr closure_info cc args
                                    body fv_details

            -- BUILD THE OBJECT
    --      ; (use_cc, blame_cc) <- chooseDynCostCentres cc args body
            ; let use_cc = cccsExpr; blame_cc = cccsExpr
            ; emit (mkComment $ mkFastString "calling allocDynClosure")
            ; let toVarArg (NonVoid a, off) = (NonVoid (StgVarArg a), off)
            ; let info_tbl = mkCmmInfo closure_info bndr currentCCS
            ; hp_plus_n <- allocDynClosure (Just bndr) info_tbl lf_info use_cc blame_cc
                                             (map toVarArg fv_details)

            -- RETURN
            ; return (mkRhsInit platform reg lf_info hp_plus_n) }

    -------------------------
    cgRhsStdThunk
            :: Id
            -> LambdaFormInfo
            -> [StgArg]             -- payload
            -> FCode (CgIdInfo, FCode CmmAGraph)

    cgRhsStdThunk bndr lf_info payload
     = do  { (id_info, reg) <- rhsIdInfo bndr lf_info
           ; return (id_info, gen_code reg)
           }
     where
     gen_code reg  -- AHA!  A STANDARD-FORM THUNK
      = withNewTickyCounterStdThunk (lfUpdatable lf_info) (idName bndr) $
        do
      {     -- LAY OUT THE OBJECT
        mod_name <- getModuleName
      ; dflags <- getDynFlags
      ; profile <- getProfile
      ; let platform = profilePlatform profile
            header = if isLFThunk lf_info then ThunkHeader else StdHeader
            (tot_wds, ptr_wds, payload_w_offsets)
                = mkVirtHeapOffsets profile header
                    (addArgReps (nonVoidStgArgs payload))

            descr = closureDescription dflags mod_name (idName bndr)
            closure_info = mkClosureInfo profile False       -- Not static
                                         bndr lf_info tot_wds ptr_wds
                                         descr

    --  ; (use_cc, blame_cc) <- chooseDynCostCentres cc [{- no args-}] body
      ; let use_cc = cccsExpr; blame_cc = cccsExpr

            -- BUILD THE OBJECT
      ; let info_tbl = mkCmmInfo closure_info bndr currentCCS
      ; hp_plus_n <- allocDynClosure (Just bndr) info_tbl lf_info
                                       use_cc blame_cc payload_w_offsets

            -- RETURN
      ; return (mkRhsInit platform reg lf_info hp_plus_n) }

    mkClosureLFInfo :: Platform
                    -> Id           -- The binder
                    -> TopLevelFlag -- True of top level
                    -> [NonVoid Id] -- Free vars
                    -> UpdateFlag   -- Update flag
                    -> [Id]         -- Args
                    -> LambdaFormInfo
    mkClosureLFInfo platform bndr top fvs upd_flag args
      | null args =
            mkLFThunk (idType bndr) top (map fromNonVoid fvs) upd_flag
      | otherwise =
            mkLFReEntrant top (map fromNonVoid fvs) args (mkArgDescr platform args)

    ------------------------------------------------------------------------
    --              The code for closures
    ------------------------------------------------------------------------

    closureCodeBody :: Bool            -- whether this is a top-level binding
                    -> Id              -- the closure's name
                    -> ClosureInfo     -- Lots of information about this closure
                    -> CostCentreStack -- Optional cost centre attached to closure
                    -> [Id]            -- incoming args to the closure
                    -> CgStgExpr
                    -> [(NonVoid Id, ByteOff)] -- the closure's free vars
                    -> FCode ()

    {- There are two main cases for the code for closures.

    * If there are *no arguments*, then the closure is a thunk, and not in
      normal form. So it should set up an update frame (if it is
      shared). NB: Thunks cannot have a primitive type!

    * If there is *at least one* argument, then this closure is in
      normal form, so there is no need to set up an update frame.
    -}

    -- No args i.e. thunk
    closureCodeBody top_lvl bndr cl_info cc [] body fv_details
      = withNewTickyCounterThunk
            (isStaticClosure cl_info)
            (closureUpdReqd cl_info)
            (closureName cl_info) $
        emitClosureProcAndInfoTable top_lvl bndr lf_info info_tbl [] $
          \(_, node, _) -> thunkCode cl_info fv_details cc node body
       where
         lf_info  = closureLFInfo cl_info
         info_tbl = mkCmmInfo cl_info bndr cc

    closureCodeBody top_lvl bndr cl_info cc args@(arg0:_) body fv_details
      = let nv_args = nonVoidIds args
            arity = length args
        in
        -- See Note [OneShotInfo overview] in GHC.Types.Basic.
        withNewTickyCounterFun (isOneShotBndr arg0) (closureName cl_info)
            nv_args $ do {

            ; let
                 lf_info  = closureLFInfo cl_info
                 info_tbl = mkCmmInfo cl_info bndr cc

            -- Emit the main entry code
            ; emitClosureProcAndInfoTable top_lvl bndr lf_info info_tbl nv_args $
                \(_offset, node, arg_regs) -> do
                    -- Emit slow-entry code (for entering a closure through a PAP)
                    { mkSlowEntryCode bndr cl_info arg_regs
                    ; profile <- getProfile
                    ; platform <- getPlatform
                    ; let node_points = nodeMustPointToIt profile lf_info
                          node' = if node_points then Just node else Nothing
                    ; loop_header_id <- newBlockId
                    -- Extend reader monad with information that
                    -- self-recursive tail calls can be optimized into local
                    -- jumps. See Note [Self-recursive tail calls] in GHC.StgToCmm.Expr.
                    ; withSelfLoop (bndr, loop_header_id, arg_regs) $ do
                    {
                    -- Main payload
                    ; entryHeapCheck cl_info node' arity arg_regs $ do
                    { -- emit LDV code when profiling
                      when node_points (ldvEnterClosure cl_info (CmmLocal node))
                    -- ticky after heap check to avoid double counting
                    ; tickyEnterFun cl_info
                    ; enterCostCentreFun cc
                        (CmmMachOp (mo_wordSub platform)
                             [ CmmReg (CmmLocal node) -- See [NodeReg clobbered with loopification]
                             , mkIntExpr platform (funTag platform cl_info) ])
                    ; fv_bindings <- mapM bind_fv fv_details
                    -- Load free vars out of closure *after*
                    -- heap check, to reduce live vars over check
                    ; when node_points $ load_fvs node lf_info fv_bindings
                    ; void $ cgExpr body
                    }}}

      }

    -- Note [NodeReg clobbered with loopification]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- Previously we used to pass nodeReg (aka R1) here. With profiling, upon
    -- entering a closure, enterFunCCS was called with R1 passed to it. But since R1
    -- may get clobbered inside the body of a closure, and since a self-recursive
    -- tail call does not restore R1, a subsequent call to enterFunCCS received a
    -- possibly bogus value from R1. The solution is to not pass nodeReg (aka R1) to
    -- enterFunCCS. Instead, we pass node, the callee-saved temporary that stores
    -- the original value of R1. This way R1 may get modified but loopification will
    -- not care.

    -- A function closure pointer may be tagged, so we
    -- must take it into account when accessing the free variables.
    bind_fv :: (NonVoid Id, ByteOff) -> FCode (LocalReg, ByteOff)
    bind_fv (id, off) = do { reg <- rebindToReg id; return (reg, off) }

    load_fvs :: LocalReg -> LambdaFormInfo -> [(LocalReg, ByteOff)] -> FCode ()
    load_fvs node lf_info = mapM_ (\ (reg, off) ->
       do platform <- getPlatform
          let tag = lfDynTag platform lf_info
          emit $ mkTaggedObjectLoad platform reg node off tag)

    -----------------------------------------
    -- The "slow entry" code for a function.  This entry point takes its
    -- arguments on the stack.  It loads the arguments into registers
    -- according to the calling convention, and jumps to the function's
    -- normal entry point.  The function's closure is assumed to be in
    -- R1/node.
    --
    -- The slow entry point is used for unknown calls: eg. stg_PAP_entry

    mkSlowEntryCode :: Id -> ClosureInfo -> [LocalReg] -> FCode ()
    -- If this function doesn't have a specialised ArgDescr, we need
    -- to generate the function's arg bitmap and slow-entry code.
    -- Here, we emit the slow-entry code.
    mkSlowEntryCode bndr cl_info arg_regs -- function closure is already in `Node'
      | Just (_, ArgGen _) <- closureFunInfo cl_info
      = do profile <- getProfile
           platform <- getPlatform
           let node = idToReg platform (NonVoid bndr)
               slow_lbl = closureSlowEntryLabel  platform cl_info
               fast_lbl = closureLocalEntryLabel platform cl_info
               -- mkDirectJump does not clobber `Node' containing function closure
               jump = mkJump profile NativeNodeCall
                                    (mkLblExpr fast_lbl)
                                    (map (CmmReg . CmmLocal) (node : arg_regs))
                                    (initUpdFrameOff platform)
           tscope <- getTickScope
           emitProcWithConvention Slow Nothing slow_lbl
             (node : arg_regs) (jump, tscope)
      | otherwise = return ()

    -----------------------------------------
    thunkCode :: ClosureInfo -> [(NonVoid Id, ByteOff)] -> CostCentreStack
              -> LocalReg -> CgStgExpr -> FCode ()
    thunkCode cl_info fv_details _cc node body
      = do { profile <- getProfile
           ; let node_points = nodeMustPointToIt profile (closureLFInfo cl_info)
                 node'       = if node_points then Just node else Nothing
            ; ldvEnterClosure cl_info (CmmLocal node) -- NB: Node always points when profiling

            -- Heap overflow check
            ; entryHeapCheck cl_info node' 0 [] $ do
            { -- Overwrite with black hole if necessary
              -- but *after* the heap-overflow check
            ; tickyEnterThunk cl_info
            ; when (blackHoleOnEntry cl_info && node_points)
                    (blackHoleIt node)

              -- Push update frame
            ; setupUpdate cl_info node $
                -- We only enter cc after setting up update so
                -- that cc of enclosing scope will be recorded
                -- in update frame CAF/DICT functions will be
                -- subsumed by this enclosing cc
                do { enterCostCentreThunk (CmmReg nodeReg)
                   ; let lf_info = closureLFInfo cl_info
                   ; fv_bindings <- mapM bind_fv fv_details
                   ; load_fvs node lf_info fv_bindings
                   ; void $ cgExpr body }}}

    ------------------------------------------------------------------------
    --              Update and black-hole wrappers
    ------------------------------------------------------------------------

    blackHoleIt :: LocalReg -> FCode ()
    -- Only called for closures with no args
    -- Node points to the closure
    blackHoleIt node_reg
      = emitBlackHoleCode (CmmReg (CmmLocal node_reg))

    emitBlackHoleCode :: CmmExpr -> FCode ()
    emitBlackHoleCode node = do
      dflags <- getDynFlags
      profile <- getProfile
      let platform = profilePlatform profile

      -- 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

      -- Note the eager-blackholing check is here rather than in blackHoleOnEntry,
      -- because emitBlackHoleCode is called from GHC.Cmm.Parser.

      let  eager_blackholing =  not (profileIsProfiling profile)
                             && gopt Opt_EagerBlackHoling dflags
                 -- Profiling needs slop filling (to support LDV
                 -- profiling), so currently eager blackholing doesn't
                 -- work with profiling.

      when eager_blackholing $ do
        whenUpdRemSetEnabled $ emitUpdRemSetPushThunk node
        emitStore (cmmOffsetW platform node (fixedHdrSizeW profile)) currentTSOExpr
        -- See Note [Heap memory barriers] in SMP.h.
        emitPrimCall [] MO_WriteBarrier []
        emitStore node (CmmReg (CmmGlobal EagerBlackholeInfo))

    setupUpdate :: ClosureInfo -> LocalReg -> FCode () -> FCode ()
            -- Nota Bene: this function does not change Node (even if it's a CAF),
            -- so that the cost centre in the original closure can still be
            -- extracted by a subsequent enterCostCentre
    setupUpdate closure_info node body
      | not (lfUpdatable (closureLFInfo closure_info))
      = body

      | not (isStaticClosure closure_info)
      = if not (closureUpdReqd closure_info)
          then do tickyUpdateFrameOmitted; body
          else do
              tickyPushUpdateFrame
              dflags <- getDynFlags
              let
                  bh = blackHoleOnEntry closure_info &&
                       not (sccProfilingEnabled dflags) &&
                       gopt Opt_EagerBlackHoling dflags

                  lbl | bh        = mkBHUpdInfoLabel
                      | otherwise = mkUpdInfoLabel

              pushUpdateFrame lbl (CmmReg (CmmLocal node)) body

      | otherwise   -- A static closure
      = do  { tickyUpdateBhCaf closure_info

            ; if closureUpdReqd closure_info
              then do       -- Blackhole the (updatable) CAF:
                    { upd_closure <- link_caf node
                    ; pushUpdateFrame mkBHUpdInfoLabel upd_closure body }
              else do {tickyUpdateFrameOmitted; body}
        }

    -----------------------------------------------------------------------------
    -- Setting up update frames

    -- Push the update frame on the stack in the Entry area,
    -- leaving room for the return address that is already
    -- at the old end of the area.
    --
    pushUpdateFrame :: CLabel -> CmmExpr -> FCode () -> FCode ()
    pushUpdateFrame lbl updatee body
      = do
           updfr  <- getUpdFrameOff
           profile <- getProfile
           let
               hdr         = fixedHdrSize profile
               frame       = updfr + hdr + pc_SIZEOF_StgUpdateFrame_NoHdr (profileConstants profile)
           --
           emitUpdateFrame (CmmStackSlot Old frame) lbl updatee
           withUpdFrameOff frame body

    emitUpdateFrame :: CmmExpr -> CLabel -> CmmExpr -> FCode ()
    emitUpdateFrame frame lbl updatee = do
      profile <- getProfile
      let
               hdr         = fixedHdrSize profile
               off_updatee = hdr + pc_OFFSET_StgUpdateFrame_updatee (platformConstants platform)
               platform    = profilePlatform profile
      --
      emitStore frame (mkLblExpr lbl)
      emitStore (cmmOffset platform frame off_updatee) updatee
      initUpdFrameProf frame

    -----------------------------------------------------------------------------
    -- Entering a CAF
    --
    -- See Note [CAF management] in rts/sm/Storage.c

    link_caf :: LocalReg           -- pointer to the closure
             -> FCode CmmExpr      -- Returns amode for closure to be updated
    -- This function returns the address of the black hole, so it can be
    -- updated with the new value when available.
    link_caf node = do
      { profile <- getProfile
            -- Call the RTS function newCAF, returning the newly-allocated
            -- blackhole indirection closure
      ; let newCAF_lbl = mkForeignLabel (fsLit "newCAF") Nothing
                                        ForeignLabelInExternalPackage IsFunction
      ; let platform = profilePlatform profile
      ; bh <- newTemp (bWord platform)
      ; emitRtsCallGen [(bh,AddrHint)] newCAF_lbl
          [ (baseExpr,  AddrHint),
            (CmmReg (CmmLocal node), AddrHint) ]
          False

      -- see Note [atomic CAF entry] in rts/sm/Storage.c
      ; updfr  <- getUpdFrameOff
      ; ptr_opts <- getPtrOpts
      ; let target = entryCode platform (closureInfoPtr ptr_opts (CmmReg (CmmLocal node)))
      ; emit =<< mkCmmIfThen
          (cmmEqWord platform (CmmReg (CmmLocal bh)) (zeroExpr platform))
            -- re-enter the CAF
           (mkJump profile NativeNodeCall target [] updfr)

      ; return (CmmReg (CmmLocal bh)) }

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

    -- For "global" data constructors the description is simply occurrence
    -- name of the data constructor itself.  Otherwise it is determined by
    -- @closureDescription@ from the let binding information.

    closureDescription
       :: DynFlags
       -> Module            -- Module
       -> Name              -- Id of closure binding
       -> String
            -- Not called for StgRhsCon which have global info tables built in
            -- CgConTbls.hs with a description generated from the data constructor
    closureDescription dflags mod_name name
      = showSDocDump (initSDocContext dflags defaultDumpStyle) (char '<' <>
                        (if isExternalName name
                          then ppr name -- ppr will include the module name prefix
                          else pprModule mod_name <> char '.' <> ppr name) <>
                        char '>')
       -- showSDocDump, because we want to see the unique on the Name.