codeGen/StgCmmBind.hs

    {-# LANGUAGE CPP #-}

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

    module StgCmmBind (
            cgTopRhsClosure,
            cgBind,
            emitBlackHoleCode,
            pushUpdateFrame, emitUpdateFrame
      ) where

    #include "HsVersions.h"

    import StgCmmExpr
    import StgCmmMonad
    import StgCmmEnv
    import StgCmmCon
    import StgCmmHeap
    import StgCmmProf (curCCS, ldvEnterClosure, enterCostCentreFun, enterCostCentreThunk,
                       initUpdFrameProf)
    import StgCmmTicky
    import StgCmmLayout
    import StgCmmUtils
    import StgCmmClosure
    import StgCmmForeign    (emitPrimCall)

    import MkGraph
    import CoreSyn          ( AltCon(..), tickishIsCode )
    import SMRep
    import Cmm
    import CmmInfo
    import CmmUtils
    import CLabel
    import StgSyn
    import CostCentre
    import Id
    import IdInfo
    import Name
    import Module
    import ListSetOps
    import Util
    import BasicTypes
    import Outputable
    import FastString
    import DynFlags

    import Control.Monad

    #if __GLASGOW_HASKELL__ >= 709
    import Prelude hiding ((<*>))
    #endif

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

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

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

    cgTopRhsClosure dflags rec id ccs _ upd_flag args body =
      let closure_label = mkLocalClosureLabel (idName id) (idCafInfo id)
          cg_id_info    = litIdInfo dflags id lf_info (CmmLabel closure_label)
          lf_info       = mkClosureLFInfo dflags id TopLevel [] upd_flag args
      in (cg_id_info, gen_code dflags 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)
      --
      -- 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 dflags _ closure_label
        | StgApp f [] <- body, null args, isNonRec rec
        = do
             cg_info <- getCgIdInfo f
             let closure_rep   = mkStaticClosureFields dflags
                                        indStaticInfoTable ccs MayHaveCafRefs
                                        [unLit (idInfoToAmode cg_info)]
             emitDataLits closure_label closure_rep
             return ()

      gen_code dflags lf_info closure_label
       = do {     -- LAY OUT THE OBJECT
              let name = idName id
            ; mod_name <- getModuleName
            ; let descr         = closureDescription dflags mod_name name
                  closure_info  = mkClosureInfo dflags True id lf_info 0 0 descr

                  caffy         = idCafInfo id
                  info_tbl      = mkCmmInfo closure_info -- XXX short-cut
                  closure_rep   = mkStaticClosureFields dflags info_tbl ccs caffy []

                     -- BUILD THE OBJECT, AND GENERATE INFO TABLE (IF NECESSARY)
            ; emitDataLits closure_label closure_rep
            ; let fv_details :: [(NonVoid Id, VirtualHpOffset)]
                  (_, _, fv_details) = mkVirtHeapOffsets dflags (isLFThunk lf_info)
                                                   (addIdReps [])
            -- Don't drop the non-void args until the closure info has been made
            ; forkClosureBody (closureCodeBody True id closure_info ccs
                                    (nonVoidIds args) (length args) body fv_details)

            ; return () }

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

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

    cgBind :: StgBinding -> 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 pratice 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
          -> StgRhs
          -> FCode (
                     CgIdInfo         -- The info for this binding
                   , FCode CmmAGraph  -- A computation which will generate the
                                      -- code for the binding, and return an
                                      -- assignent of the form "x = Hp - n"
                                      -- (see above)
                   )

    cgRhs id (StgRhsCon cc con args)
      = withNewTickyCounterThunk False (idName id) $ -- False for "not static"
        buildDynCon id True cc con args

    {- See Note [GC recovery] in compiler/codeGen/StgCmmClosure.hs -}
    cgRhs name (StgRhsClosure cc bi fvs upd_flag _srt args body)
      = do dflags <- getDynFlags
           mkRhsClosure dflags name cc bi (nonVoidIds fvs) upd_flag args body

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

    mkRhsClosure :: DynFlags -> Id -> CostCentreStack -> StgBinderInfo
                 -> [NonVoid Id]                    -- Free vars
                 -> UpdateFlag
                 -> [Id]                            -- Args
                 -> StgExpr
                 -> 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    dflags bndr _cc _bi
                    [NonVoid the_fv]                -- Just one free var
                    upd_flag                -- Updatable thunk
                    []                      -- A thunk
                    expr
      | let strip = snd . stripStgTicksTop (not . tickishIsCode)
      , StgCase (StgApp scrutinee [{-no args-}])
             _ _ _ _   -- ignore uniq, etc.
             (AlgAlt _)
             [(DataAlt _, params, _use_mask, 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 dflags (addIdReps params)
                                       -- Just want the layout
      , Just the_offset <- assocMaybe params_w_offsets (NonVoid selectee)

      , let offset_into_int = bytesToWordsRoundUp dflags the_offset
                              - fixedHdrSizeW dflags
      , offset_into_int <= mAX_SPEC_SELECTEE_SIZE dflags -- 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    dflags bndr _cc _bi
                    fvs
                    upd_flag
                    []                      -- No args; a thunk
                    (StgApp fun_id args)

      | args `lengthIs` (arity-1)
            && all (isGcPtrRep . idPrimRep . unsafe_stripNV) fvs
            && isUpdatable upd_flag
            && arity <= mAX_SPEC_AP_SIZE dflags
            && not (gopt Opt_SccProfilingOn dflags)
                                      -- not when profiling: we don't want to
                                      -- lose information about this particular
                                      -- thunk (e.g. its type) (#949)

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

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

    ---------- Default case ------------------
    mkRhsClosure dflags bndr cc _ fvs upd_flag args body
      = do  { let lf_info = mkClosureLFInfo dflags 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
                    is_elem      = isIn "cgRhsClosure"
                    bndr_is_a_fv = (NonVoid bndr) `is_elem` fvs
                    reduced_fvs | bndr_is_a_fv = fvs `minusList` [NonVoid bndr]
                                | otherwise    = fvs

            -- 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)]
                    (tot_wds, ptr_wds, fv_details)
                       = mkVirtHeapOffsets dflags (isLFThunk lf_info)
                                           (addIdReps (map unsafe_stripNV reduced_fvs))
                    closure_info = mkClosureInfo dflags 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 (nonVoidIds args)
                                    (length args) body fv_details

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

            -- RETURN
            ; return (mkRhsInit dflags 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 False (idName bndr) $ -- False for "not static"
        do
      {     -- LAY OUT THE OBJECT
        mod_name <- getModuleName
      ; dflags <- getDynFlags
      ; let (tot_wds, ptr_wds, payload_w_offsets)
                = mkVirtHeapOffsets dflags (isLFThunk lf_info) (addArgReps payload)

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

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

      ; tickyEnterStdThunk closure_info

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

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

    mkClosureLFInfo :: DynFlags
                    -> Id           -- The binder
                    -> TopLevelFlag -- True of top level
                    -> [NonVoid Id] -- Free vars
                    -> UpdateFlag   -- Update flag
                    -> [Id]         -- Args
                    -> LambdaFormInfo
    mkClosureLFInfo dflags bndr top fvs upd_flag args
      | null args =
            mkLFThunk (idType bndr) top (map unsafe_stripNV fvs) upd_flag
      | otherwise =
            mkLFReEntrant top (map unsafe_stripNV fvs) args (mkArgDescr dflags 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
                    -> [NonVoid Id]    -- incoming args to the closure
                    -> Int             -- arity, including void args
                    -> StgExpr
                    -> [(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.

      The Macros for GrAnSim are produced at the beginning of the
      argSatisfactionCheck (by calling fetchAndReschedule).
      There info if Node points to closure is available. -- HWL -}

    closureCodeBody top_lvl bndr cl_info cc _args arity body fv_details
      | arity == 0 -- No args i.e. thunk
      = withNewTickyCounterThunk (isStaticClosure cl_info) (closureName cl_info) $
        emitClosureProcAndInfoTable top_lvl bndr lf_info info_tbl [] $
          \(_, node, _) -> thunkCode cl_info fv_details cc node arity body
       where
         lf_info  = closureLFInfo cl_info
         info_tbl = mkCmmInfo cl_info

    closureCodeBody top_lvl bndr cl_info cc args arity body fv_details
      = -- Note: args may be [], if all args are Void
        withNewTickyCounterFun (closureName cl_info) args $ do {

            ; let
                 lf_info  = closureLFInfo cl_info
                 info_tbl = mkCmmInfo cl_info

            -- Emit the main entry code
            ; emitClosureProcAndInfoTable top_lvl bndr lf_info info_tbl args $
                \(_offset, node, arg_regs) -> do
                    -- Emit slow-entry code (for entering a closure through a PAP)
                    { mkSlowEntryCode bndr cl_info arg_regs
                    ; dflags <- getDynFlags
                    ; let node_points = nodeMustPointToIt dflags lf_info
                          node' = if node_points then Just node else Nothing
                    ; loop_header_id <- newLabelC
                    -- Extend reader monad with information that
                    -- self-recursive tail calls can be optimized into local
                    -- jumps. See Note [Self-recursive tail calls] in StgCmmExpr.
                    ; 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 dflags)
                             [ CmmReg (CmmLocal node) -- See [NodeReg clobbered with loopification]
                             , mkIntExpr dflags (funTag dflags 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 dflags <- getDynFlags
          let tag = lfDynTag dflags lf_info
          emit $ mkTaggedObjectLoad dflags 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 dflags <- getDynFlags
           let node = idToReg dflags (NonVoid bndr)
               slow_lbl = closureSlowEntryLabel  cl_info
               fast_lbl = closureLocalEntryLabel dflags cl_info
               -- mkDirectJump does not clobber `Node' containing function closure
               jump = mkJump dflags NativeNodeCall
                                    (mkLblExpr fast_lbl)
                                    (map (CmmReg . CmmLocal) (node : arg_regs))
                                    (initUpdFrameOff dflags)
           tscope <- getTickScope
           emitProcWithConvention Slow Nothing slow_lbl
             (node : arg_regs) (jump, tscope)
      | otherwise = return ()

    -----------------------------------------
    thunkCode :: ClosureInfo -> [(NonVoid Id, ByteOff)] -> CostCentreStack
              -> LocalReg -> Int -> StgExpr -> FCode ()
    thunkCode cl_info fv_details _cc node arity body
      = do { dflags <- getDynFlags
           ; let node_points = nodeMustPointToIt dflags (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' arity [] $ 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 { tickyEnterThunk cl_info
                   ; 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

      -- 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 CmmParse.

      let  eager_blackholing =  not (gopt Opt_SccProfilingOn dflags)
                             && 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
        emitStore (cmmOffsetW dflags node (fixedHdrSizeW dflags))
                      (CmmReg (CmmGlobal CurrentTSO))
        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 (gopt Opt_SccProfilingOn 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 True
                    ; 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
           dflags <- getDynFlags
           let
               hdr         = fixedHdrSize dflags
               frame       = updfr + hdr + sIZEOF_StgUpdateFrame_NoHdr dflags
           --
           emitUpdateFrame dflags (CmmStackSlot Old frame) lbl updatee
           withUpdFrameOff frame body

    emitUpdateFrame :: DynFlags -> CmmExpr -> CLabel -> CmmExpr -> FCode ()
    emitUpdateFrame dflags frame lbl updatee = do
      let
               hdr         = fixedHdrSize dflags
               off_updatee = hdr + oFFSET_StgUpdateFrame_updatee dflags
      --
      emitStore frame (mkLblExpr lbl)
      emitStore (cmmOffset dflags 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
             -> Bool               -- True <=> updatable, False <=> single-entry
             -> 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 _is_upd = do
      { dflags <- getDynFlags
            -- Call the RTS function newCAF, returning the newly-allocated
            -- blackhole indirection closure
      ; let newCAF_lbl = mkForeignLabel (fsLit "newCAF") Nothing
                                        ForeignLabelInExternalPackage IsFunction
      ; bh <- newTemp (bWord dflags)
      ; emitRtsCallGen [(bh,AddrHint)] newCAF_lbl
          [ (CmmReg (CmmGlobal BaseReg),  AddrHint),
            (CmmReg (CmmLocal node), AddrHint) ]
          False

      -- see Note [atomic CAF entry] in rts/sm/Storage.c
      ; updfr  <- getUpdFrameOff
      ; let target = entryCode dflags (closureInfoPtr dflags (CmmReg (CmmLocal node)))
      ; emit =<< mkCmmIfThen
          (cmmEqWord dflags (CmmReg (CmmLocal bh)) (zeroExpr dflags))
            -- re-enter the CAF
           (mkJump dflags 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.lhs with a description generated from the data constructor
    closureDescription dflags mod_name name
      = showSDocDump dflags (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.