codeGen/StgCmmUtils.hs

    {-# LANGUAGE CPP #-}

    -----------------------------------------------------------------------------
    --
    -- Code generator utilities; mostly monadic
    --
    -- (c) The University of Glasgow 2004-2006
    --
    -----------------------------------------------------------------------------

    module StgCmmUtils (
            cgLit, mkSimpleLit,
            emitDataLits, mkDataLits,
            emitRODataLits, mkRODataLits,
            emitRtsCall, emitRtsCallWithResult, emitRtsCallGen,
            assignTemp, newTemp,

            newUnboxedTupleRegs,

            emitMultiAssign, emitCmmLitSwitch, emitSwitch,

            tagToClosure, mkTaggedObjectLoad,

            callerSaves, callerSaveVolatileRegs, get_GlobalReg_addr,

            cmmAndWord, cmmOrWord, cmmNegate, cmmEqWord, cmmNeWord,
            cmmUGtWord, cmmSubWord, cmmMulWord, cmmAddWord, cmmUShrWord,
            cmmOffsetExprW, cmmOffsetExprB,
            cmmRegOffW, cmmRegOffB,
            cmmLabelOffW, cmmLabelOffB,
            cmmOffsetW, cmmOffsetB,
            cmmOffsetLitW, cmmOffsetLitB,
            cmmLoadIndexW,
            cmmConstrTag1,

            cmmUntag, cmmIsTagged,

            addToMem, addToMemE, addToMemLblE, addToMemLbl,
            mkWordCLit,
            newStringCLit, newByteStringCLit,
            blankWord,
      ) where

    #include "HsVersions.h"

    import StgCmmMonad
    import StgCmmClosure
    import Cmm
    import BlockId
    import MkGraph
    import CodeGen.Platform
    import CLabel
    import CmmUtils
    import CmmSwitch

    import ForeignCall
    import IdInfo
    import Type
    import TyCon
    import SMRep
    import Module
    import Literal
    import Digraph
    import Util
    import Unique
    import UniqSupply (MonadUnique(..))
    import DynFlags
    import FastString
    import Outputable
    import RepType

    import qualified Data.ByteString as BS
    import qualified Data.Map as M
    import Data.Char
    import Data.List
    import Data.Ord
    import Data.Word

    -------------------------------------------------------------------------
    --
    --      Literals
    --
    -------------------------------------------------------------------------

    cgLit :: Literal -> FCode CmmLit
    cgLit (MachStr s) = newByteStringCLit (BS.unpack s)
     -- not unpackFS; we want the UTF-8 byte stream.
    cgLit other_lit   = do dflags <- getDynFlags
                           return (mkSimpleLit dflags other_lit)

    mkSimpleLit :: DynFlags -> Literal -> CmmLit
    mkSimpleLit dflags (MachChar   c)    = CmmInt (fromIntegral (ord c)) (wordWidth dflags)
    mkSimpleLit dflags MachNullAddr      = zeroCLit dflags
    mkSimpleLit dflags (MachInt i)       = CmmInt i (wordWidth dflags)
    mkSimpleLit _      (MachInt64 i)     = CmmInt i W64
    mkSimpleLit dflags (MachWord i)      = CmmInt i (wordWidth dflags)
    mkSimpleLit _      (MachWord64 i)    = CmmInt i W64
    mkSimpleLit _      (MachFloat r)     = CmmFloat r W32
    mkSimpleLit _      (MachDouble r)    = CmmFloat r W64
    mkSimpleLit _      (MachLabel fs ms fod)
            = CmmLabel (mkForeignLabel fs ms labelSrc fod)
            where
                    -- TODO: Literal labels might not actually be in the current package...
                    labelSrc = ForeignLabelInThisPackage
    mkSimpleLit _ other             = pprPanic "mkSimpleLit" (ppr other)

    --------------------------------------------------------------------------
    --
    -- Incrementing a memory location
    --
    --------------------------------------------------------------------------

    addToMemLbl :: CmmType -> CLabel -> Int -> CmmAGraph
    addToMemLbl rep lbl n = addToMem rep (CmmLit (CmmLabel lbl)) n

    addToMemLblE :: CmmType -> CLabel -> CmmExpr -> CmmAGraph
    addToMemLblE rep lbl = addToMemE rep (CmmLit (CmmLabel lbl))

    addToMem :: CmmType     -- rep of the counter
             -> CmmExpr     -- Address
             -> Int         -- What to add (a word)
             -> CmmAGraph
    addToMem rep ptr n = addToMemE rep ptr (CmmLit (CmmInt (toInteger n) (typeWidth rep)))

    addToMemE :: CmmType    -- rep of the counter
              -> CmmExpr    -- Address
              -> CmmExpr    -- What to add (a word-typed expression)
              -> CmmAGraph
    addToMemE rep ptr n
      = mkStore ptr (CmmMachOp (MO_Add (typeWidth rep)) [CmmLoad ptr rep, n])

    -------------------------------------------------------------------------
    --
    --      Loading a field from an object,
    --      where the object pointer is itself tagged
    --
    -------------------------------------------------------------------------

    mkTaggedObjectLoad
      :: DynFlags -> LocalReg -> LocalReg -> ByteOff -> DynTag -> CmmAGraph
    -- (loadTaggedObjectField reg base off tag) generates assignment
    --      reg = bitsK[ base + off - tag ]
    -- where K is fixed by 'reg'
    mkTaggedObjectLoad dflags reg base offset tag
      = mkAssign (CmmLocal reg)
                 (CmmLoad (cmmOffsetB dflags
                                      (CmmReg (CmmLocal base))
                                      (offset - tag))
                          (localRegType reg))

    -------------------------------------------------------------------------
    --
    --      Converting a closure tag to a closure for enumeration types
    --      (this is the implementation of tagToEnum#).
    --
    -------------------------------------------------------------------------

    tagToClosure :: DynFlags -> TyCon -> CmmExpr -> CmmExpr
    tagToClosure dflags tycon tag
      = CmmLoad (cmmOffsetExprW dflags closure_tbl tag) (bWord dflags)
      where closure_tbl = CmmLit (CmmLabel lbl)
            lbl = mkClosureTableLabel (tyConName tycon) NoCafRefs

    -------------------------------------------------------------------------
    --
    --      Conditionals and rts calls
    --
    -------------------------------------------------------------------------

    emitRtsCall :: UnitId -> FastString -> [(CmmExpr,ForeignHint)] -> Bool -> FCode ()
    emitRtsCall pkg fun args safe = emitRtsCallGen [] (mkCmmCodeLabel pkg fun) args safe

    emitRtsCallWithResult :: LocalReg -> ForeignHint -> UnitId -> FastString
            -> [(CmmExpr,ForeignHint)] -> Bool -> FCode ()
    emitRtsCallWithResult res hint pkg fun args safe
       = emitRtsCallGen [(res,hint)] (mkCmmCodeLabel pkg fun) args safe

    -- Make a call to an RTS C procedure
    emitRtsCallGen
       :: [(LocalReg,ForeignHint)]
       -> CLabel
       -> [(CmmExpr,ForeignHint)]
       -> Bool -- True <=> CmmSafe call
       -> FCode ()
    emitRtsCallGen res lbl args safe
      = do { dflags <- getDynFlags
           ; updfr_off <- getUpdFrameOff
           ; let (caller_save, caller_load) = callerSaveVolatileRegs dflags
           ; emit caller_save
           ; call updfr_off
           ; emit caller_load }
      where
        call updfr_off =
          if safe then
            emit =<< mkCmmCall fun_expr res' args' updfr_off
          else do
            let conv = ForeignConvention CCallConv arg_hints res_hints CmmMayReturn
            emit $ mkUnsafeCall (ForeignTarget fun_expr conv) res' args'
        (args', arg_hints) = unzip args
        (res',  res_hints) = unzip res
        fun_expr = mkLblExpr lbl

    -----------------------------------------------------------------------------
    --
    --      Caller-Save Registers
    --
    -----------------------------------------------------------------------------

    -- Here we generate the sequence of saves/restores required around a
    -- foreign call instruction.

    -- TODO: reconcile with includes/Regs.h
    --  * Regs.h claims that BaseReg should be saved last and loaded first
    --    * This might not have been tickled before since BaseReg is callee save
    --  * Regs.h saves SparkHd, ParkT1, SparkBase and SparkLim
    --
    -- This code isn't actually used right now, because callerSaves
    -- only ever returns true in the current universe for registers NOT in
    -- system_regs (just do a grep for CALLER_SAVES in
    -- includes/stg/MachRegs.h).  It's all one giant no-op, and for
    -- good reason: having to save system registers on every foreign call
    -- would be very expensive, so we avoid assigning them to those
    -- registers when we add support for an architecture.
    --
    -- Note that the old code generator actually does more work here: it
    -- also saves other global registers.  We can't (nor want) to do that
    -- here, as we don't have liveness information.  And really, we
    -- shouldn't be doing the workaround at this point in the pipeline, see
    -- Note [Register parameter passing] and the ToDo on CmmCall in
    -- cmm/CmmNode.hs.  Right now the workaround is to avoid inlining across
    -- unsafe foreign calls in rewriteAssignments, but this is strictly
    -- temporary.
    callerSaveVolatileRegs :: DynFlags -> (CmmAGraph, CmmAGraph)
    callerSaveVolatileRegs dflags = (caller_save, caller_load)
      where
        platform = targetPlatform dflags

        caller_save = catAGraphs (map callerSaveGlobalReg    regs_to_save)
        caller_load = catAGraphs (map callerRestoreGlobalReg regs_to_save)

        system_regs = [ Sp,SpLim,Hp,HpLim,CCCS,CurrentTSO,CurrentNursery
                        {- ,SparkHd,SparkTl,SparkBase,SparkLim -}
                      , BaseReg ]

        regs_to_save = filter (callerSaves platform) system_regs

        callerSaveGlobalReg reg
            = mkStore (get_GlobalReg_addr dflags reg) (CmmReg (CmmGlobal reg))

        callerRestoreGlobalReg reg
            = mkAssign (CmmGlobal reg)
                       (CmmLoad (get_GlobalReg_addr dflags reg) (globalRegType dflags reg))

    -- -----------------------------------------------------------------------------
    -- Global registers

    -- We map STG registers onto appropriate CmmExprs.  Either they map
    -- to real machine registers or stored as offsets from BaseReg.  Given
    -- a GlobalReg, get_GlobalReg_addr always produces the
    -- register table address for it.
    -- (See also get_GlobalReg_reg_or_addr in MachRegs)

    get_GlobalReg_addr :: DynFlags -> GlobalReg -> CmmExpr
    get_GlobalReg_addr dflags BaseReg = regTableOffset dflags 0
    get_GlobalReg_addr dflags mid
        = get_Regtable_addr_from_offset dflags
                                        (globalRegType dflags mid) (baseRegOffset dflags mid)

    -- Calculate a literal representing an offset into the register table.
    -- Used when we don't have an actual BaseReg to offset from.
    regTableOffset :: DynFlags -> Int -> CmmExpr
    regTableOffset dflags n =
      CmmLit (CmmLabelOff mkMainCapabilityLabel (oFFSET_Capability_r dflags + n))

    get_Regtable_addr_from_offset :: DynFlags -> CmmType -> Int -> CmmExpr
    get_Regtable_addr_from_offset dflags _rep offset =
        if haveRegBase (targetPlatform dflags)
        then CmmRegOff (CmmGlobal BaseReg) offset
        else regTableOffset dflags offset

    -- -----------------------------------------------------------------------------
    -- Information about global registers

    baseRegOffset :: DynFlags -> GlobalReg -> Int

    baseRegOffset dflags Sp             = oFFSET_StgRegTable_rSp dflags
    baseRegOffset dflags SpLim          = oFFSET_StgRegTable_rSpLim dflags
    baseRegOffset dflags (LongReg 1)    = oFFSET_StgRegTable_rL1 dflags
    baseRegOffset dflags Hp             = oFFSET_StgRegTable_rHp dflags
    baseRegOffset dflags HpLim          = oFFSET_StgRegTable_rHpLim dflags
    baseRegOffset dflags CCCS           = oFFSET_StgRegTable_rCCCS dflags
    baseRegOffset dflags CurrentTSO     = oFFSET_StgRegTable_rCurrentTSO dflags
    baseRegOffset dflags CurrentNursery = oFFSET_StgRegTable_rCurrentNursery dflags
    baseRegOffset dflags HpAlloc        = oFFSET_StgRegTable_rHpAlloc dflags
    baseRegOffset dflags GCEnter1       = oFFSET_stgGCEnter1 dflags
    baseRegOffset dflags GCFun          = oFFSET_stgGCFun dflags
    baseRegOffset _      reg            = pprPanic "StgCmmUtils.baseRegOffset:" (ppr reg)

    -------------------------------------------------------------------------
    --
    --      Strings generate a top-level data block
    --
    -------------------------------------------------------------------------

    emitDataLits :: CLabel -> [CmmLit] -> FCode ()
    -- Emit a data-segment data block
    emitDataLits lbl lits = emitDecl (mkDataLits (Section Data lbl) lbl lits)

    emitRODataLits :: CLabel -> [CmmLit] -> FCode ()
    -- Emit a read-only data block
    emitRODataLits lbl lits = emitDecl (mkRODataLits lbl lits)

    newStringCLit :: String -> FCode CmmLit
    -- Make a global definition for the string,
    -- and return its label
    newStringCLit str = newByteStringCLit (map (fromIntegral . ord) str)

    newByteStringCLit :: [Word8] -> FCode CmmLit
    newByteStringCLit bytes
      = do  { uniq <- newUnique
            ; let (lit, decl) = mkByteStringCLit (mkStringLitLabel uniq) bytes
            ; emitDecl decl
            ; return lit }

    -------------------------------------------------------------------------
    --
    --      Assigning expressions to temporaries
    --
    -------------------------------------------------------------------------

    assignTemp :: CmmExpr -> FCode LocalReg
    -- Make sure the argument is in a local register.
    -- We don't bother being particularly aggressive with avoiding
    -- unnecessary local registers, since we can rely on a later
    -- optimization pass to inline as necessary (and skipping out
    -- on things like global registers can be a little dangerous
    -- due to them being trashed on foreign calls--though it means
    -- the optimization pass doesn't have to do as much work)
    assignTemp (CmmReg (CmmLocal reg)) = return reg
    assignTemp e = do { dflags <- getDynFlags
                      ; uniq <- newUnique
                      ; let reg = LocalReg uniq (cmmExprType dflags e)
                      ; emitAssign (CmmLocal reg) e
                      ; return reg }

    newTemp :: MonadUnique m => CmmType -> m LocalReg
    newTemp rep = do { uniq <- getUniqueM
                     ; return (LocalReg uniq rep) }

    newUnboxedTupleRegs :: Type -> FCode ([LocalReg], [ForeignHint])
    -- Choose suitable local regs to use for the components
    -- of an unboxed tuple that we are about to return to
    -- the Sequel.  If the Sequel is a join point, using the
    -- regs it wants will save later assignments.
    newUnboxedTupleRegs res_ty
      = ASSERT( isUnboxedTupleType res_ty )
        do  { dflags <- getDynFlags
            ; sequel <- getSequel
            ; regs <- choose_regs dflags sequel
            ; ASSERT( regs `equalLength` reps )
              return (regs, map primRepForeignHint reps) }
      where
        reps = typePrimRep res_ty
        choose_regs _ (AssignTo regs _) = return regs
        choose_regs dflags _            = mapM (newTemp . primRepCmmType dflags) reps

    -------------------------------------------------------------------------
    --      emitMultiAssign
    -------------------------------------------------------------------------

    emitMultiAssign :: [LocalReg] -> [CmmExpr] -> FCode ()
    -- Emit code to perform the assignments in the
    -- input simultaneously, using temporary variables when necessary.

    type Key  = Int
    type Vrtx = (Key, Stmt) -- Give each vertex a unique number,
                            -- for fast comparison
    type Stmt = (LocalReg, CmmExpr) -- r := e

    -- We use the strongly-connected component algorithm, in which
    --      * the vertices are the statements
    --      * an edge goes from s1 to s2 iff
    --              s1 assigns to something s2 uses
    --        that is, if s1 should *follow* s2 in the final order

    emitMultiAssign []    []    = return ()
    emitMultiAssign [reg] [rhs] = emitAssign (CmmLocal reg) rhs
    emitMultiAssign regs rhss   = do
      dflags <- getDynFlags
      ASSERT2( equalLength regs rhss, ppr regs $$ ppr rhss )
        unscramble dflags ([1..] `zip` (regs `zip` rhss))

    unscramble :: DynFlags -> [Vrtx] -> FCode ()
    unscramble dflags vertices = mapM_ do_component components
      where
            edges :: [ (Vrtx, Key, [Key]) ]
            edges = [ (vertex, key1, edges_from stmt1)
                    | vertex@(key1, stmt1) <- vertices ]

            edges_from :: Stmt -> [Key]
            edges_from stmt1 = [ key2 | (key2, stmt2) <- vertices,
                                        stmt1 `mustFollow` stmt2 ]

            components :: [SCC Vrtx]
            components = stronglyConnCompFromEdgedVerticesUniq edges

            -- do_components deal with one strongly-connected component
            -- Not cyclic, or singleton?  Just do it
            do_component :: SCC Vrtx -> FCode ()
            do_component (AcyclicSCC (_,stmt))  = mk_graph stmt
            do_component (CyclicSCC [])         = panic "do_component"
            do_component (CyclicSCC [(_,stmt)]) = mk_graph stmt

                    -- Cyclic?  Then go via temporaries.  Pick one to
                    -- break the loop and try again with the rest.
            do_component (CyclicSCC ((_,first_stmt) : rest)) = do
                dflags <- getDynFlags
                u <- newUnique
                let (to_tmp, from_tmp) = split dflags u first_stmt
                mk_graph to_tmp
                unscramble dflags rest
                mk_graph from_tmp

            split :: DynFlags -> Unique -> Stmt -> (Stmt, Stmt)
            split dflags uniq (reg, rhs)
              = ((tmp, rhs), (reg, CmmReg (CmmLocal tmp)))
              where
                rep = cmmExprType dflags rhs
                tmp = LocalReg uniq rep

            mk_graph :: Stmt -> FCode ()
            mk_graph (reg, rhs) = emitAssign (CmmLocal reg) rhs

            mustFollow :: Stmt -> Stmt -> Bool
            (reg, _) `mustFollow` (_, rhs) = regUsedIn dflags (CmmLocal reg) rhs

    -------------------------------------------------------------------------
    --      mkSwitch
    -------------------------------------------------------------------------

    emitSwitch :: CmmExpr                      -- Tag to switch on
               -> [(ConTagZ, CmmAGraphScoped)] -- Tagged branches
               -> Maybe CmmAGraphScoped        -- Default branch (if any)
               -> ConTagZ -> ConTagZ           -- Min and Max possible values;
                                               -- behaviour outside this range is
                                               -- undefined
               -> FCode ()

    -- First, two rather common cases in which there is no work to do
    emitSwitch _ []         (Just code) _ _ = emit (fst code)
    emitSwitch _ [(_,code)] Nothing     _ _ = emit (fst code)

    -- Right, off we go
    emitSwitch tag_expr branches mb_deflt lo_tag hi_tag = do
        join_lbl      <- newBlockId
        mb_deflt_lbl  <- label_default join_lbl mb_deflt
        branches_lbls <- label_branches join_lbl branches
        tag_expr'     <- assignTemp' tag_expr

        -- Sort the branches before calling mk_discrete_switch
        let branches_lbls' = [ (fromIntegral i, l) | (i,l) <- sortBy (comparing fst) branches_lbls ]
        let range = (fromIntegral lo_tag, fromIntegral hi_tag)

        emit $ mk_discrete_switch False tag_expr' branches_lbls' mb_deflt_lbl range

        emitLabel join_lbl

    mk_discrete_switch :: Bool -- ^ Use signed comparisons
              -> CmmExpr
              -> [(Integer, BlockId)]
              -> Maybe BlockId
              -> (Integer, Integer)
              -> CmmAGraph

    -- SINGLETON TAG RANGE: no case analysis to do
    mk_discrete_switch _ _tag_expr [(tag, lbl)] _ (lo_tag, hi_tag)
      | lo_tag == hi_tag
      = ASSERT( tag == lo_tag )
        mkBranch lbl

    -- SINGLETON BRANCH, NO DEFAULT: no case analysis to do
    mk_discrete_switch _ _tag_expr [(_tag,lbl)] Nothing _
      = mkBranch lbl
            -- The simplifier might have eliminated a case
            --       so we may have e.g. case xs of
            --                               [] -> e
            -- In that situation we can be sure the (:) case
            -- can't happen, so no need to test

    -- SOMETHING MORE COMPLICATED: defer to CmmImplementSwitchPlans
    -- See Note [Cmm Switches, the general plan] in CmmSwitch
    mk_discrete_switch signed tag_expr branches mb_deflt range
      = mkSwitch tag_expr $ mkSwitchTargets signed range mb_deflt (M.fromList branches)

    divideBranches :: Ord a => [(a,b)] -> ([(a,b)], a, [(a,b)])
    divideBranches branches = (lo_branches, mid, hi_branches)
      where
        -- 2 branches => n_branches `div` 2 = 1
        --            => branches !! 1 give the *second* tag
        -- There are always at least 2 branches here
        (mid,_) = branches !! (length branches `div` 2)
        (lo_branches, hi_branches) = span is_lo branches
        is_lo (t,_) = t < mid

    --------------
    emitCmmLitSwitch :: CmmExpr                    -- Tag to switch on
                   -> [(Literal, CmmAGraphScoped)] -- Tagged branches
                   -> CmmAGraphScoped              -- Default branch (always)
                   -> FCode ()                     -- Emit the code
    emitCmmLitSwitch _scrut []       deflt = emit $ fst deflt
    emitCmmLitSwitch scrut  branches deflt = do
        scrut' <- assignTemp' scrut
        join_lbl <- newBlockId
        deflt_lbl <- label_code join_lbl deflt
        branches_lbls <- label_branches join_lbl branches

        dflags <- getDynFlags
        let cmm_ty = cmmExprType dflags scrut
            rep = typeWidth cmm_ty

        -- We find the necessary type information in the literals in the branches
        let signed = case head branches of
                        (MachInt _, _) ->   True
                        (MachInt64 _, _) -> True
                        _ -> False

        let range | signed    = (tARGET_MIN_INT dflags, tARGET_MAX_INT dflags)
                  | otherwise = (0, tARGET_MAX_WORD dflags)

        if isFloatType cmm_ty
        then emit =<< mk_float_switch rep scrut' deflt_lbl noBound branches_lbls
        else emit $ mk_discrete_switch
            signed
            scrut'
            [(litValue lit,l) | (lit,l) <- branches_lbls]
            (Just deflt_lbl)
            range
        emitLabel join_lbl

    -- | lower bound (inclusive), upper bound (exclusive)
    type LitBound = (Maybe Literal, Maybe Literal)

    noBound :: LitBound
    noBound = (Nothing, Nothing)

    mk_float_switch :: Width -> CmmExpr -> BlockId
                  -> LitBound
                  -> [(Literal,BlockId)]
                  -> FCode CmmAGraph
    mk_float_switch rep scrut deflt _bounds [(lit,blk)]
      = do dflags <- getDynFlags
           return $ mkCbranch (cond dflags) deflt blk Nothing
      where
        cond dflags = CmmMachOp ne [scrut, CmmLit cmm_lit]
          where
            cmm_lit = mkSimpleLit dflags lit
            ne      = MO_F_Ne rep

    mk_float_switch rep scrut deflt_blk_id (lo_bound, hi_bound) branches
      = do dflags <- getDynFlags
           lo_blk <- mk_float_switch rep scrut deflt_blk_id bounds_lo lo_branches
           hi_blk <- mk_float_switch rep scrut deflt_blk_id bounds_hi hi_branches
           mkCmmIfThenElse (cond dflags) lo_blk hi_blk
      where
        (lo_branches, mid_lit, hi_branches) = divideBranches branches

        bounds_lo = (lo_bound, Just mid_lit)
        bounds_hi = (Just mid_lit, hi_bound)

        cond dflags = CmmMachOp lt [scrut, CmmLit cmm_lit]
          where
            cmm_lit = mkSimpleLit dflags mid_lit
            lt      = MO_F_Lt rep

    --------------
    label_default :: BlockId -> Maybe CmmAGraphScoped -> FCode (Maybe BlockId)
    label_default _ Nothing
      = return  Nothing
    label_default join_lbl (Just code)
      = do lbl <- label_code join_lbl code
           return (Just lbl)

    --------------
    label_branches :: BlockId -> [(a,CmmAGraphScoped)] -> FCode [(a,BlockId)]
    label_branches _join_lbl []
      = return []
    label_branches join_lbl ((tag,code):branches)
      = do lbl <- label_code join_lbl code
           branches' <- label_branches join_lbl branches
           return ((tag,lbl):branches')

    --------------
    label_code :: BlockId -> CmmAGraphScoped -> FCode BlockId
    --  label_code J code
    --      generates
    --  [L: code; goto J]
    -- and returns L
    label_code join_lbl (code,tsc) = do
        lbl <- newBlockId
        emitOutOfLine lbl (code MkGraph.<*> mkBranch join_lbl, tsc)
        return lbl

    --------------
    assignTemp' :: CmmExpr -> FCode CmmExpr
    assignTemp' e
      | isTrivialCmmExpr e = return e
      | otherwise = do
           dflags <- getDynFlags
           lreg <- newTemp (cmmExprType dflags e)
           let reg = CmmLocal lreg
           emitAssign reg e
           return (CmmReg reg)