[project @ 2001-10-11 16:44:10 by simonpj]

[ghc-hetmet.git] / ghc / compiler / stgSyn / CoreToStg.lhs

%
% (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
%
\section[CoreToStg]{Converts Core to STG Syntax}

And, as we have the info in hand, we may convert some lets to
let-no-escapes.

\begin{code}
module CoreToStg ( coreToStg, coreExprToStg ) where

#include "HsVersions.h"

import CoreSyn
import CoreUtils
import StgSyn

import Type
import TyCon            ( isAlgTyCon )
import Literal
import Id
import Var              ( Var, globalIdDetails, varType )
#ifdef ILX
import MkId             ( unsafeCoerceId )
#endif
import IdInfo
import DataCon
import CostCentre       ( noCCS )
import VarSet
import VarEnv
import DataCon          ( dataConWrapId )
import Maybes           ( maybeToBool )
import Name             ( getOccName, isExternallyVisibleName, isDllName )
import OccName          ( occNameUserString )
import BasicTypes       ( TopLevelFlag(..), isNotTopLevel, Arity )
import CmdLineOpts      ( DynFlags, opt_RuntimeTypes )
import FastTypes        hiding ( fastOr )
import Outputable

infixr 9 `thenLne`
\end{code}

%************************************************************************
%*                                                                      *
\subsection[live-vs-free-doc]{Documentation}
%*                                                                      *
%************************************************************************

(There is other relevant documentation in codeGen/CgLetNoEscape.)

The actual Stg datatype is decorated with {\em live variable}
information, as well as {\em free variable} information.  The two are
{\em not} the same.  Liveness is an operational property rather than a
semantic one.  A variable is live at a particular execution point if
it can be referred to {\em directly} again.  In particular, a dead
variable's stack slot (if it has one):
\begin{enumerate}
\item
should be stubbed to avoid space leaks, and
\item
may be reused for something else.
\end{enumerate}

There ought to be a better way to say this.  Here are some examples:
\begin{verbatim}
        let v = [q] \[x] -> e
        in
        ...v...  (but no q's)
\end{verbatim}

Just after the `in', v is live, but q is dead.  If the whole of that
let expression was enclosed in a case expression, thus:
\begin{verbatim}
        case (let v = [q] \[x] -> e in ...v...) of
                alts[...q...]
\end{verbatim}
(ie @alts@ mention @q@), then @q@ is live even after the `in'; because
we'll return later to the @alts@ and need it.

Let-no-escapes make this a bit more interesting:
\begin{verbatim}
        let-no-escape v = [q] \ [x] -> e
        in
        ...v...
\end{verbatim}
Here, @q@ is still live at the `in', because @v@ is represented not by
a closure but by the current stack state.  In other words, if @v@ is
live then so is @q@.  Furthermore, if @e@ mentions an enclosing
let-no-escaped variable, then {\em its} free variables are also live
if @v@ is.

%************************************************************************
%*                                                                      *
\subsection[caf-info]{Collecting live CAF info}
%*                                                                      *
%************************************************************************

In this pass we also collect information on which CAFs are live for 
constructing SRTs (see SRT.lhs).  

A top-level Id has CafInfo, which is

        - MayHaveCafRefs, if it may refer indirectly to
          one or more CAFs, or
        - NoCafRefs if it definitely doesn't

we collect the CafInfo first by analysing the original Core expression, and
also place this information in the environment.

During CoreToStg, we then pin onto each binding and case expression, a
list of Ids which represents the "live" CAFs at that point.  The meaning
of "live" here is the same as for live variables, see above (which is
why it's convenient to collect CAF information here rather than elsewhere).

The later SRT pass takes these lists of Ids and uses them to construct
the actual nested SRTs, and replaces the lists of Ids with (offset,length)
pairs.


Interaction of let-no-escape with SRTs   [Sept 01]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider

        let-no-escape x = ...caf1...caf2...
        in
        ...x...x...x...

where caf1,caf2 are CAFs.  Since x doesn't have a closure, we 
build SRTs just as if x's defn was inlined at each call site, and
that means that x's CAF refs get duplicated in the overall SRT.

This is unlike ordinary lets, in which the CAF refs are not duplicated.

We could fix this loss of (static) sharing by making a sort of pseudo-closure
for x, solely to put in the SRTs lower down.


%************************************************************************
%*                                                                      *
\subsection[binds-StgVarInfo]{Setting variable info: top-level, binds, RHSs}
%*                                                                      *
%************************************************************************

\begin{code}
coreToStg :: DynFlags -> [CoreBind] -> IO [StgBinding]
coreToStg dflags pgm
  = return pgm'
  where (_, _, pgm') = coreTopBindsToStg emptyVarEnv pgm

coreExprToStg :: CoreExpr -> StgExpr
coreExprToStg expr 
  = new_expr where (new_expr,_,_) = initLne emptyVarEnv (coreToStgExpr expr)


coreTopBindsToStg
    :: IdEnv HowBound           -- environment for the bindings
    -> [CoreBind]
    -> (IdEnv HowBound, FreeVarsInfo, [StgBinding])

coreTopBindsToStg env [] = (env, emptyFVInfo, [])
coreTopBindsToStg env (b:bs)
  = (env2, fvs2, b':bs')
  where
        -- env accumulates down the list of binds, fvs accumulates upwards
        (env1, fvs2, b' ) = coreTopBindToStg env fvs1 b
        (env2, fvs1, bs') = coreTopBindsToStg env1 bs


coreTopBindToStg
        :: IdEnv HowBound
        -> FreeVarsInfo         -- Info about the body
        -> CoreBind
        -> (IdEnv HowBound, FreeVarsInfo, StgBinding)

coreTopBindToStg env body_fvs (NonRec id rhs)
  = let 
        caf_info  = hasCafRefs env rhs
        env'      = extendVarEnv env id how_bound
        how_bound = LetBound (TopLet caf_info) (predictArity rhs)

        (stg_rhs, fvs', lv_info) = 
            initLne env (
              coreToStgRhs body_fvs TopLevel (id,rhs)   `thenLne` \ (stg_rhs, fvs', _) ->
              freeVarsToLiveVars fvs'                   `thenLne` \ lv_info ->
              returnLne (stg_rhs, fvs', lv_info)
           )
        
        bind = StgNonRec (mkSRT lv_info) id stg_rhs
    in
    ASSERT2(predictArity rhs == stgRhsArity stg_rhs, ppr id)
    ASSERT2(consistent caf_info bind, ppr id)
--    WARN(not (consistent caf_info bind), ppr id <+> ppr cafs <+> ppCafInfo caf_info)
    (env', fvs' `unionFVInfo` body_fvs, bind)

coreTopBindToStg env body_fvs (Rec pairs)
  = let 
        (binders, rhss) = unzip pairs

        -- To calculate caf_info, we initially map 
        -- all the binders to NoCafRefs
        env1 = extendVarEnvList env 
                [ (b, LetBound (TopLet NoCafRefs) (error "no arity"))
                | b <- binders ]

        caf_info = hasCafRefss env1{-NB: not env'-} rhss

        env' = extendVarEnvList env 
                [ (b, LetBound (TopLet caf_info) (predictArity rhs)) 
                | (b,rhs) <- pairs ]

        (stg_rhss, fvs', lv_info)
          = initLne env' (
               mapAndUnzip3Lne (coreToStgRhs body_fvs TopLevel) pairs
                        `thenLne` \ (stg_rhss, fvss', _) ->
               let fvs' = unionFVInfos fvss' in
               freeVarsToLiveVars fvs'  `thenLne` \ lv_info ->
               returnLne (stg_rhss, fvs', lv_info)
           )

        bind = StgRec (mkSRT lv_info) (zip binders stg_rhss)
    in
    ASSERT2(and [predictArity rhs == stgRhsArity stg_rhs | (rhs,stg_rhs) <- rhss `zip` stg_rhss], ppr binders)
    ASSERT2(consistent caf_info bind, ppr binders)
--    WARN(not (consistent caf_info bind), ppr binders <+> ppr cafs <+> ppCafInfo caf_info)
    (env', fvs' `unionFVInfo` body_fvs, bind)

-- assertion helper
consistent caf_info bind = mayHaveCafRefs caf_info == stgBindHasCafRefs bind
\end{code}

\begin{code}
coreToStgRhs
        :: FreeVarsInfo         -- Free var info for the scope of the binding
        -> TopLevelFlag
        -> (Id,CoreExpr)
        -> LneM (StgRhs, FreeVarsInfo, EscVarsSet)

coreToStgRhs scope_fv_info top (binder, rhs)
  = coreToStgExpr rhs  `thenLne` \ (new_rhs, rhs_fvs, rhs_escs) ->
    returnLne (mkStgRhs top rhs_fvs binder_info new_rhs, 
               rhs_fvs, rhs_escs)
  where
    binder_info = lookupFVInfo scope_fv_info binder

mkStgRhs :: TopLevelFlag -> FreeVarsInfo -> StgBinderInfo
         -> StgExpr -> StgRhs

mkStgRhs top rhs_fvs binder_info (StgLam _ bndrs body)
  = StgRhsClosure noCCS binder_info
                  (getFVs rhs_fvs)               
                  ReEntrant
                  bndrs body
        
mkStgRhs top rhs_fvs binder_info (StgConApp con args)
  | isNotTopLevel top || not (isDllConApp con args)
  = StgRhsCon noCCS con args

mkStgRhs top rhs_fvs binder_info rhs
  = StgRhsClosure noCCS binder_info
                  (getFVs rhs_fvs)               
                  (updatable [] rhs)
                  [] rhs
  where
    updatable args body | null args && isPAP body  = ReEntrant
                        | otherwise                = Updatable
{- ToDo:
          upd = if isOnceDem dem
                    then (if isNotTop toplev 
                            then SingleEntry    -- HA!  Paydirt for "dem"
                            else 
#ifdef DEBUG
                     trace "WARNING: SE CAFs unsupported, forcing UPD instead" $
#endif
                     Updatable)
                else Updatable
        -- For now we forbid SingleEntry CAFs; they tickle the
        -- ASSERT in rts/Storage.c line 215 at newCAF() re mut_link,
        -- and I don't understand why.  There's only one SE_CAF (well,
        -- only one that tickled a great gaping bug in an earlier attempt
        -- at ClosureInfo.getEntryConvention) in the whole of nofib, 
        -- specifically Main.lvl6 in spectral/cryptarithm2.
        -- So no great loss.  KSW 2000-07.
-}
\end{code}

Detect thunks which will reduce immediately to PAPs, and make them
non-updatable.  This has several advantages:

        - the non-updatable thunk behaves exactly like the PAP,

        - the thunk is more efficient to enter, because it is
          specialised to the task.

        - we save one update frame, one stg_update_PAP, one update
          and lots of PAP_enters.

        - in the case where the thunk is top-level, we save building
          a black hole and futhermore the thunk isn't considered to
          be a CAF any more, so it doesn't appear in any SRTs.

We do it here, because the arity information is accurate, and we need
to do it before the SRT pass to save the SRT entries associated with
any top-level PAPs.

\begin{code}
isPAP (StgApp f args) = idArity f > length args
isPAP _               = False
\end{code}


-- ---------------------------------------------------------------------------
-- Expressions
-- ---------------------------------------------------------------------------

\begin{code}
coreToStgExpr
        :: CoreExpr
        -> LneM (StgExpr,       -- Decorated STG expr
                 FreeVarsInfo,  -- Its free vars (NB free, not live)
                 EscVarsSet)    -- Its escapees, a subset of its free vars;
                                -- also a subset of the domain of the envt
                                -- because we are only interested in the escapees
                                -- for vars which might be turned into
                                -- let-no-escaped ones.
\end{code}

The second and third components can be derived in a simple bottom up pass, not
dependent on any decisions about which variables will be let-no-escaped or
not.  The first component, that is, the decorated expression, may then depend
on these components, but it in turn is not scrutinised as the basis for any
decisions.  Hence no black holes.

\begin{code}
coreToStgExpr (Lit l) = returnLne (StgLit l, emptyFVInfo, emptyVarSet)
coreToStgExpr (Var v) = coreToStgApp Nothing v []

coreToStgExpr expr@(App _ _)
  = coreToStgApp Nothing f args
  where
    (f, args) = myCollectArgs expr

coreToStgExpr expr@(Lam _ _)
  = let
        (args, body) = myCollectBinders expr 
        args'        = filterStgBinders args
    in
    extendVarEnvLne [ (a, LambdaBound) | a <- args' ] $
    coreToStgExpr body  `thenLne` \ (body, body_fvs, body_escs) ->
    let
        fvs             = args' `minusFVBinders` body_fvs
        escs            = body_escs `delVarSetList` args'
        result_expr | null args' = body
                    | otherwise  = StgLam (exprType expr) args' body
    in
    returnLne (result_expr, fvs, escs)

coreToStgExpr (Note (SCC cc) expr)
  = coreToStgExpr expr          `thenLne` ( \ (expr2, fvs, escs) ->
    returnLne (StgSCC cc expr2, fvs, escs) )

#ifdef ILX
-- For ILX, convert (__coerce__ to_ty from_ty e) 
--          into    (coerce to_ty from_ty e)
-- where coerce is real function
coreToStgExpr (Note (Coerce to_ty from_ty) expr)
  = coreToStgExpr (mkApps (Var unsafeCoerceId) 
                          [Type from_ty, Type to_ty, expr])
#endif

coreToStgExpr (Note other_note expr)
  = coreToStgExpr expr

-- Cases require a little more real work.

coreToStgExpr (Case scrut bndr alts)
  = extendVarEnvLne [(bndr, LambdaBound)]       (
         mapAndUnzip3Lne vars_alt alts  `thenLne` \ (alts2, fvs_s, escs_s) ->
         returnLne ( mkStgAlts (idType bndr) alts2,
                     unionFVInfos fvs_s,
                     unionVarSets escs_s )
    )                                   `thenLne` \ (alts2, alts_fvs, alts_escs) ->
    let
        -- Determine whether the default binder is dead or not
        -- This helps the code generator to avoid generating an assignment
        -- for the case binder (is extremely rare cases) ToDo: remove.
        bndr' | bndr `elementOfFVInfo` alts_fvs = bndr
              | otherwise                       = bndr `setIdOccInfo` IAmDead

        -- Don't consider the default binder as being 'live in alts',
        -- since this is from the point of view of the case expr, where
        -- the default binder is not free.
        alts_fvs_wo_bndr  = bndr `minusFVBinder` alts_fvs
        alts_escs_wo_bndr = alts_escs `delVarSet` bndr
    in

    freeVarsToLiveVars alts_fvs_wo_bndr         `thenLne` \ alts_lv_info ->

        -- We tell the scrutinee that everything 
        -- live in the alts is live in it, too.
    setVarsLiveInCont alts_lv_info (
        coreToStgExpr scrut       `thenLne` \ (scrut2, scrut_fvs, scrut_escs) ->
        freeVarsToLiveVars scrut_fvs `thenLne` \ scrut_lv_info ->
        returnLne (scrut2, scrut_fvs, scrut_escs, scrut_lv_info)
      )    
                `thenLne` \ (scrut2, scrut_fvs, scrut_escs, scrut_lv_info) ->

    returnLne (
      StgCase scrut2 (getLiveVars scrut_lv_info)
                     (getLiveVars alts_lv_info)
                     bndr'
                     (mkSRT alts_lv_info)
                     alts2,
      scrut_fvs `unionFVInfo` alts_fvs_wo_bndr,
      alts_escs_wo_bndr `unionVarSet` getFVSet scrut_fvs
                -- You might think we should have scrut_escs, not 
                -- (getFVSet scrut_fvs), but actually we can't call, and 
                -- then return from, a let-no-escape thing.
      )
  where
    vars_alt (con, binders, rhs)
      = let     -- Remove type variables
            binders' = filterStgBinders binders
        in      
        extendVarEnvLne [(b, LambdaBound) | b <- binders']      $
        coreToStgExpr rhs       `thenLne` \ (rhs2, rhs_fvs, rhs_escs) ->
        let
                -- Records whether each param is used in the RHS
            good_use_mask = [ b `elementOfFVInfo` rhs_fvs | b <- binders' ]
        in
        returnLne ( (con, binders', good_use_mask, rhs2),
                    binders' `minusFVBinders` rhs_fvs,
                    rhs_escs `delVarSetList` binders' )
                -- ToDo: remove the delVarSet;
                -- since escs won't include any of these binders
\end{code}

Lets not only take quite a bit of work, but this is where we convert
then to let-no-escapes, if we wish.

(Meanwhile, we don't expect to see let-no-escapes...)
\begin{code}
coreToStgExpr (Let bind body)
  = fixLne (\ ~(_, _, _, no_binder_escapes) ->
        coreToStgLet no_binder_escapes bind body
    )                           `thenLne` \ (new_let, fvs, escs, _) ->

    returnLne (new_let, fvs, escs)
\end{code}

\begin{code}
mkStgAlts scrut_ty orig_alts
 | is_prim_case = StgPrimAlts (tyConAppTyCon scrut_ty) prim_alts deflt
 | otherwise    = StgAlgAlts  maybe_tycon              alg_alts  deflt
  where
    is_prim_case = isUnLiftedType scrut_ty && not (isUnboxedTupleType scrut_ty)

    prim_alts    = [(lit, rhs)             | (LitAlt lit, _, _, rhs)        <- other_alts]
    alg_alts     = [(con, bndrs, use, rhs) | (DataAlt con, bndrs, use, rhs) <- other_alts]

    (other_alts, deflt) 
        = case orig_alts of     -- DEFAULT is always first if it's there at all
            (DEFAULT, _, _, rhs) : other_alts -> (other_alts, StgBindDefault rhs)
            other                             -> (orig_alts,  StgNoDefault)

    maybe_tycon = case alg_alts of 
                        -- Get the tycon from the data con
                        (dc, _, _, _) : _rest -> Just (dataConTyCon dc)

                        -- Otherwise just do your best
                        [] -> case splitTyConApp_maybe (repType scrut_ty) of
                                Just (tc,_) | isAlgTyCon tc -> Just tc
                                _other                      -> Nothing
\end{code}


-- ---------------------------------------------------------------------------
-- Applications
-- ---------------------------------------------------------------------------

\begin{code}
coreToStgApp
         :: Maybe UpdateFlag            -- Just upd <=> this application is
                                        -- the rhs of a thunk binding
                                        --      x = [...] \upd [] -> the_app
                                        -- with specified update flag
        -> Id                           -- Function
        -> [CoreArg]                    -- Arguments
        -> LneM (StgExpr, FreeVarsInfo, EscVarsSet)

coreToStgApp maybe_thunk_body f args
  = coreToStgArgs args          `thenLne` \ (args', args_fvs) ->
    lookupVarLne f              `thenLne` \ how_bound ->

    let
        n_val_args       = valArgCount args
        not_letrec_bound = not (isLetBound how_bound)
        fun_fvs          
          = let fvs = singletonFVInfo f how_bound fun_occ in
            -- e.g. (f :: a -> int) (x :: a) 
            -- Here the free variables are "f", "x" AND the type variable "a"
            -- coreToStgArgs will deal with the arguments recursively
            if opt_RuntimeTypes then
              fvs `unionFVInfo` tyvarFVInfo (tyVarsOfType (varType f))
            else fvs

        -- Mostly, the arity info of a function is in the fn's IdInfo
        -- But new bindings introduced by CoreSat may not have no
        -- arity info; it would do us no good anyway.  For example:
        --      let f = \ab -> e in f
        -- No point in having correct arity info for f!
        -- Hence the hasArity stuff below.
        -- NB: f_arity is only consulted for LetBound things
        f_arity = case how_bound of 
                        LetBound _ arity -> arity
                        ImportBound      -> idArity f

        saturated = f_arity <= n_val_args

        fun_occ 
         | not_letrec_bound         = noBinderInfo      -- Uninteresting variable
         | f_arity > 0 && saturated = stgSatOcc -- Saturated or over-saturated function call
         | otherwise                = stgUnsatOcc       -- Unsaturated function or thunk

        fun_escs
         | not_letrec_bound      = emptyVarSet  -- Only letrec-bound escapees are interesting
         | f_arity == n_val_args = emptyVarSet  -- A function *or thunk* with an exactly
                                                -- saturated call doesn't escape
                                                -- (let-no-escape applies to 'thunks' too)

         | otherwise         = unitVarSet f     -- Inexact application; it does escape

        -- At the moment of the call:

        --  either the function is *not* let-no-escaped, in which case
        --         nothing is live except live_in_cont
        --      or the function *is* let-no-escaped in which case the
        --         variables it uses are live, but still the function
        --         itself is not.  PS.  In this case, the function's
        --         live vars should already include those of the
        --         continuation, but it does no harm to just union the
        --         two regardless.

        res_ty = exprType (mkApps (Var f) args)
        app = case globalIdDetails f of
                DataConId dc | saturated -> StgConApp dc args'
                PrimOpId op              -> ASSERT( saturated )
                                            StgOpApp (StgPrimOp op) args' res_ty
                FCallId call             -> ASSERT( saturated )
                                            StgOpApp (StgFCallOp call (idUnique f)) args' res_ty
                _other                   -> StgApp f args'

    in
    returnLne (
        app,
        fun_fvs  `unionFVInfo` args_fvs,
        fun_escs `unionVarSet` (getFVSet args_fvs)
                                -- All the free vars of the args are disqualified
                                -- from being let-no-escaped.
    )



-- ---------------------------------------------------------------------------
-- Argument lists
-- This is the guy that turns applications into A-normal form
-- ---------------------------------------------------------------------------

coreToStgArgs :: [CoreArg] -> LneM ([StgArg], FreeVarsInfo)
coreToStgArgs []
  = returnLne ([], emptyFVInfo)

coreToStgArgs (Type ty : args)  -- Type argument
  = coreToStgArgs args  `thenLne` \ (args', fvs) ->
    if opt_RuntimeTypes then
        returnLne (StgTypeArg ty : args', fvs `unionFVInfo` tyvarFVInfo (tyVarsOfType ty))
    else
    returnLne (args', fvs)

coreToStgArgs (arg : args)      -- Non-type argument
  = coreToStgArgs args  `thenLne` \ (stg_args, args_fvs) ->
    coreToStgExpr arg   `thenLne` \ (arg', arg_fvs, escs) ->
    let
        fvs = args_fvs `unionFVInfo` arg_fvs
        stg_arg = case arg' of
                       StgApp v []      -> StgVarArg v
                       StgConApp con [] -> StgVarArg (dataConWrapId con)
                       StgLit lit       -> StgLitArg lit
                       _                -> pprPanic "coreToStgArgs" (ppr arg)
    in
    returnLne (stg_arg : stg_args, fvs)


-- ---------------------------------------------------------------------------
-- The magic for lets:
-- ---------------------------------------------------------------------------

coreToStgLet
         :: Bool        -- True <=> yes, we are let-no-escaping this let
         -> CoreBind    -- bindings
         -> CoreExpr    -- body
         -> LneM (StgExpr,      -- new let
                  FreeVarsInfo, -- variables free in the whole let
                  EscVarsSet,   -- variables that escape from the whole let
                  Bool)         -- True <=> none of the binders in the bindings
                                -- is among the escaping vars

coreToStgLet let_no_escape bind body
  = fixLne (\ ~(_, _, _, _, _, rec_body_fvs, _, _) ->

        -- Do the bindings, setting live_in_cont to empty if
        -- we ain't in a let-no-escape world
        getVarsLiveInCont               `thenLne` \ live_in_cont ->
        setVarsLiveInCont (if let_no_escape 
                                then live_in_cont 
                                else emptyLiveInfo)
                          (vars_bind rec_body_fvs bind)
            `thenLne` \ ( bind2, bind_fvs, bind_escs, bind_lv_info, env_ext) ->

        -- Do the body
        extendVarEnvLne env_ext (
          coreToStgExpr body          `thenLne` \(body2, body_fvs, body_escs) ->
          freeVarsToLiveVars body_fvs `thenLne` \ body_lv_info ->

          returnLne (bind2, bind_fvs, bind_escs, getLiveVars bind_lv_info,
                     body2, body_fvs, body_escs, getLiveVars body_lv_info)
        )

    ) `thenLne` (\ (bind2, bind_fvs, bind_escs, bind_lvs, 
                    body2, body_fvs, body_escs, body_lvs) ->


        -- Compute the new let-expression
    let
        new_let | let_no_escape = StgLetNoEscape live_in_whole_let bind_lvs bind2 body2
                | otherwise     = StgLet bind2 body2

        free_in_whole_let
          = binders `minusFVBinders` (bind_fvs `unionFVInfo` body_fvs)

        live_in_whole_let
          = bind_lvs `unionVarSet` (body_lvs `delVarSetList` binders)

        real_bind_escs = if let_no_escape then
                            bind_escs
                         else
                            getFVSet bind_fvs
                            -- Everything escapes which is free in the bindings

        let_escs = (real_bind_escs `unionVarSet` body_escs) `delVarSetList` binders

        all_escs = bind_escs `unionVarSet` body_escs    -- Still includes binders of
                                                        -- this let(rec)

        no_binder_escapes = isEmptyVarSet (set_of_binders `intersectVarSet` all_escs)

#ifdef DEBUG
        -- Debugging code as requested by Andrew Kennedy
        checked_no_binder_escapes
                | not no_binder_escapes && any is_join_var binders
                = pprTrace "Interesting!  A join var that isn't let-no-escaped" (ppr binders)
                  False
                | otherwise = no_binder_escapes
#else
        checked_no_binder_escapes = no_binder_escapes
#endif
                            
                -- Mustn't depend on the passed-in let_no_escape flag, since
                -- no_binder_escapes is used by the caller to derive the flag!
    in
    returnLne (
        new_let,
        free_in_whole_let,
        let_escs,
        checked_no_binder_escapes
    ))
  where
    set_of_binders = mkVarSet binders
    binders        = bindersOf bind

    mk_binding bind_lv_info binder rhs
        = (binder, LetBound (NestedLet live_vars) (predictArity rhs))
        where
           live_vars | let_no_escape = addLiveVar bind_lv_info binder
                     | otherwise     = unitLiveVar binder
                -- c.f. the invariant on NestedLet

    vars_bind :: FreeVarsInfo           -- Free var info for body of binding
              -> CoreBind
              -> LneM (StgBinding,
                       FreeVarsInfo, 
                       EscVarsSet,        -- free vars; escapee vars
                       LiveInfo,          -- Vars and CAFs live in binding
                       [(Id, HowBound)])  -- extension to environment
                                         

    vars_bind body_fvs (NonRec binder rhs)
      = coreToStgRhs body_fvs NotTopLevel (binder,rhs)
                                `thenLne` \ (rhs2, bind_fvs, escs) ->

        freeVarsToLiveVars bind_fvs `thenLne` \ bind_lv_info ->
        let
            env_ext_item = mk_binding bind_lv_info binder rhs
        in
        returnLne (StgNonRec (mkSRT bind_lv_info) binder rhs2, 
                   bind_fvs, escs, bind_lv_info, [env_ext_item])


    vars_bind body_fvs (Rec pairs)
      = fixLne (\ ~(_, rec_rhs_fvs, _, bind_lv_info, _) ->
           let
                rec_scope_fvs = unionFVInfo body_fvs rec_rhs_fvs
                binders = map fst pairs
                env_ext = [ mk_binding bind_lv_info b rhs 
                          | (b,rhs) <- pairs ]
           in
           extendVarEnvLne env_ext (
              mapAndUnzip3Lne (coreToStgRhs rec_scope_fvs NotTopLevel) pairs 
                                        `thenLne` \ (rhss2, fvss, escss) ->
              let
                        bind_fvs = unionFVInfos fvss
                        escs     = unionVarSets escss
              in
              freeVarsToLiveVars (binders `minusFVBinders` bind_fvs)
                                        `thenLne` \ bind_lv_info ->

              returnLne (StgRec (mkSRT bind_lv_info) (binders `zip` rhss2), 
                         bind_fvs, escs, bind_lv_info, env_ext)
           )
        )

is_join_var :: Id -> Bool
-- A hack (used only for compiler debuggging) to tell if
-- a variable started life as a join point ($j)
is_join_var j = occNameUserString (getOccName j) == "$j"
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Arity prediction}
%*                                                                      *
%************************************************************************

To avoid yet another knot, we predict the arity of each function from
its Core form, based on the number of visible top-level lambdas.  
It should be the same as the arity of the STG RHS!

\begin{code}
predictArity :: CoreExpr -> Int
predictArity (Lam x e)
  | isTyVar x = predictArity e
  | otherwise = 1 + predictArity e
predictArity (Note _ e)
  -- Ignore coercions.   Top level sccs are removed by the final 
  -- profiling pass, so we ignore those too.
  = predictArity e
predictArity _ = 0
\end{code}


%************************************************************************
%*                                                                      *
\subsection[LNE-monad]{A little monad for this let-no-escaping pass}
%*                                                                      *
%************************************************************************

There's a lot of stuff to pass around, so we use this @LneM@ monad to
help.  All the stuff here is only passed *down*.

\begin{code}
type LneM a =  IdEnv HowBound
            -> LiveInfo         -- Vars and CAFs live in continuation
            -> a

type LiveInfo = (StgLiveVars,   -- Dynamic live variables; 
                                -- i.e. ones with a nested (non-top-level) binding
                 CafSet)        -- Static live variables;
                                -- i.e. top-level variables that are CAFs or refer to them

type EscVarsSet = IdSet
type CafSet     = IdSet

data HowBound
  = ImportBound         -- Used only as a response to lookupBinding; never
                        -- exists in the range of the (IdEnv HowBound)

  | LetBound            -- A let(rec) in this module
        LetInfo         -- Whether top level or nested
        Arity           -- Its arity (local Ids don't have arity info at this point)

  | LambdaBound         -- Used for both lambda and case

data LetInfo = NestedLet LiveInfo       -- For nested things, what is live if this thing is live?
                                        -- Invariant: the binder itself is always a member of
                                        --            the dynamic set of its own LiveInfo
             | TopLet CafInfo           -- For top level things, is it a CAF, or can it refer to one?

isLetBound (LetBound _ _) = True
isLetBound other          = False

topLevelBound ImportBound             = True
topLevelBound (LetBound (TopLet _) _) = True
topLevelBound other                   = False
\end{code}

For a let(rec)-bound variable, x, we record LiveInfo, the set of
variables that are live if x is live.  This LiveInfo comprises
        (a) dynamic live variables (ones with a non-top-level binding)
        (b) static live variabes (CAFs or things that refer to CAFs)

For "normal" variables (a) is just x alone.  If x is a let-no-escaped
variable then x is represented by a code pointer and a stack pointer
(well, one for each stack).  So all of the variables needed in the
execution of x are live if x is, and are therefore recorded in the
LetBound constructor; x itself *is* included.

The set of dynamic live variables is guaranteed ot have no further let-no-escaped
variables in it.

\begin{code}
emptyLiveInfo :: LiveInfo
emptyLiveInfo = (emptyVarSet,emptyVarSet)

unitLiveVar :: Id -> LiveInfo
unitLiveVar lv = (unitVarSet lv, emptyVarSet)

unitLiveCaf :: Id -> LiveInfo
unitLiveCaf caf = (emptyVarSet, unitVarSet caf)

addLiveVar :: LiveInfo -> Id -> LiveInfo
addLiveVar (lvs, cafs) id = (lvs `extendVarSet` id, cafs)

unionLiveInfo :: LiveInfo -> LiveInfo -> LiveInfo
unionLiveInfo (lv1,caf1) (lv2,caf2) = (lv1 `unionVarSet` lv2, caf1 `unionVarSet` caf2)

mkSRT :: LiveInfo -> SRT
mkSRT (_, cafs) = SRTEntries cafs

getLiveVars :: LiveInfo -> StgLiveVars
getLiveVars (lvs, _) = lvs
\end{code}


The std monad functions:
\begin{code}
initLne :: IdEnv HowBound -> LneM a -> a
initLne env m = m env emptyLiveInfo



{-# INLINE thenLne #-}
{-# INLINE returnLne #-}

returnLne :: a -> LneM a
returnLne e env lvs_cont = e

thenLne :: LneM a -> (a -> LneM b) -> LneM b
thenLne m k env lvs_cont 
  = k (m env lvs_cont) env lvs_cont

mapLne  :: (a -> LneM b)   -> [a] -> LneM [b]
mapLne f [] = returnLne []
mapLne f (x:xs)
  = f x         `thenLne` \ r  ->
    mapLne f xs `thenLne` \ rs ->
    returnLne (r:rs)

mapAndUnzipLne  :: (a -> LneM (b,c))   -> [a] -> LneM ([b],[c])

mapAndUnzipLne f [] = returnLne ([],[])
mapAndUnzipLne f (x:xs)
  = f x                 `thenLne` \ (r1,  r2)  ->
    mapAndUnzipLne f xs `thenLne` \ (rs1, rs2) ->
    returnLne (r1:rs1, r2:rs2)

mapAndUnzip3Lne :: (a -> LneM (b,c,d)) -> [a] -> LneM ([b],[c],[d])

mapAndUnzip3Lne f []    = returnLne ([],[],[])
mapAndUnzip3Lne f (x:xs)
  = f x                  `thenLne` \ (r1,  r2,  r3)  ->
    mapAndUnzip3Lne f xs `thenLne` \ (rs1, rs2, rs3) ->
    returnLne (r1:rs1, r2:rs2, r3:rs3)

fixLne :: (a -> LneM a) -> LneM a
fixLne expr env lvs_cont
  = result
  where
    result = expr result env lvs_cont
\end{code}

Functions specific to this monad:

\begin{code}
getVarsLiveInCont :: LneM LiveInfo
getVarsLiveInCont env lvs_cont = lvs_cont

setVarsLiveInCont :: LiveInfo -> LneM a -> LneM a
setVarsLiveInCont new_lvs_cont expr env lvs_cont
  = expr env new_lvs_cont

extendVarEnvLne :: [(Id, HowBound)] -> LneM a -> LneM a
extendVarEnvLne ids_w_howbound expr env lvs_cont
  = expr (extendVarEnvList env ids_w_howbound) lvs_cont

lookupVarLne :: Id -> LneM HowBound
lookupVarLne v env lvs_cont = returnLne (lookupBinding env v) env lvs_cont

lookupBinding :: IdEnv HowBound -> Id -> HowBound
lookupBinding env v = case lookupVarEnv env v of
                        Just xx -> xx
                        Nothing -> ASSERT2( isGlobalId v, ppr v ) ImportBound


-- The result of lookupLiveVarsForSet, a set of live variables, is
-- only ever tacked onto a decorated expression. It is never used as
-- the basis of a control decision, which might give a black hole.

freeVarsToLiveVars :: FreeVarsInfo -> LneM LiveInfo
freeVarsToLiveVars fvs env live_in_cont
  = returnLne live_info env live_in_cont
  where
    live_info    = foldr unionLiveInfo live_in_cont lvs_from_fvs
    lvs_from_fvs = map do_one (allFreeIds fvs)

    do_one (v, how_bound)
      = case how_bound of
          ImportBound                     -> unitLiveCaf v      -- Only CAF imports are 
                                                                -- recorded in fvs
          LetBound (TopLet caf_info) _ 
                | mayHaveCafRefs caf_info -> unitLiveCaf v
                | otherwise               -> emptyLiveInfo

          LetBound (NestedLet lvs) _      -> lvs        -- lvs already contains v
                                                        -- (see the invariant on NestedLet)

          _lambda_or_case_binding         -> unitLiveVar v      -- Bound by lambda or case
\end{code}

%************************************************************************
%*                                                                      *
\subsection[Free-var info]{Free variable information}
%*                                                                      *
%************************************************************************

\begin{code}
type FreeVarsInfo = VarEnv (Var, HowBound, StgBinderInfo)
        -- The Var is so we can gather up the free variables
        -- as a set.
        --
        -- The HowBound info just saves repeated lookups;
        -- we look up just once when we encounter the occurrence.
        -- INVARIANT: Any ImportBound Ids are HaveCafRef Ids
        --            Imported Ids without CAF refs are simply
        --            not put in the FreeVarsInfo for an expression.
        --            See singletonFVInfo and freeVarsToLiveVars
        --
        -- StgBinderInfo records how it occurs; notably, we
        -- are interested in whether it only occurs in saturated 
        -- applications, because then we don't need to build a
        -- curried version.
        -- If f is mapped to noBinderInfo, that means
        -- that f *is* mentioned (else it wouldn't be in the
        -- IdEnv at all), but perhaps in an unsaturated applications.
        --
        -- All case/lambda-bound things are also mapped to
        -- noBinderInfo, since we aren't interested in their
        -- occurence info.
        --
        -- For ILX we track free var info for type variables too;
        -- hence VarEnv not IdEnv
\end{code}

\begin{code}
emptyFVInfo :: FreeVarsInfo
emptyFVInfo = emptyVarEnv

singletonFVInfo :: Id -> HowBound -> StgBinderInfo -> FreeVarsInfo
-- Don't record non-CAF imports at all, to keep free-var sets small
singletonFVInfo id ImportBound info
   | mayHaveCafRefs (idCafInfo id) = unitVarEnv id (id, ImportBound, info)
   | otherwise                     = emptyVarEnv
singletonFVInfo id how_bound info  = unitVarEnv id (id, how_bound, info)

tyvarFVInfo :: TyVarSet -> FreeVarsInfo
tyvarFVInfo tvs = foldVarSet add emptyFVInfo tvs
        where
          add tv fvs = extendVarEnv fvs tv (tv, LambdaBound, noBinderInfo)
                -- Type variables must be lambda-bound

unionFVInfo :: FreeVarsInfo -> FreeVarsInfo -> FreeVarsInfo
unionFVInfo fv1 fv2 = plusVarEnv_C plusFVInfo fv1 fv2

unionFVInfos :: [FreeVarsInfo] -> FreeVarsInfo
unionFVInfos fvs = foldr unionFVInfo emptyFVInfo fvs

minusFVBinders :: [Id] -> FreeVarsInfo -> FreeVarsInfo
minusFVBinders vs fv = foldr minusFVBinder fv vs

minusFVBinder :: Id -> FreeVarsInfo -> FreeVarsInfo
minusFVBinder v fv | isId v && opt_RuntimeTypes
                   = (fv `delVarEnv` v) `unionFVInfo` 
                     tyvarFVInfo (tyVarsOfType (idType v))
                   | otherwise = fv `delVarEnv` v
        -- When removing a binder, remember to add its type variables
        -- c.f. CoreFVs.delBinderFV

elementOfFVInfo :: Id -> FreeVarsInfo -> Bool
elementOfFVInfo id fvs = maybeToBool (lookupVarEnv fvs id)

lookupFVInfo :: FreeVarsInfo -> Id -> StgBinderInfo
-- Find how the given Id is used.
-- Externally visible things may be used any old how
lookupFVInfo fvs id 
  | isExternallyVisibleName (idName id) = noBinderInfo
  | otherwise = case lookupVarEnv fvs id of
                        Nothing         -> noBinderInfo
                        Just (_,_,info) -> info

allFreeIds :: FreeVarsInfo -> [(Id,HowBound)]   -- Both top level and non-top-level Ids
allFreeIds fvs = [(id,how_bound) | (id,how_bound,_) <- rngVarEnv fvs, isId id]

-- Non-top-level things only, both type variables and ids
-- (type variables only if opt_RuntimeTypes)
getFVs :: FreeVarsInfo -> [Var] 
getFVs fvs = [id | (id, how_bound, _) <- rngVarEnv fvs, 
                    not (topLevelBound how_bound) ]

getFVSet :: FreeVarsInfo -> VarSet
getFVSet fvs = mkVarSet (getFVs fvs)

plusFVInfo (id1,hb1,info1) (id2,hb2,info2)
  = ASSERT (id1 == id2 && hb1 `check_eq_how_bound` hb2)
    (id1, hb1, combineStgBinderInfo info1 info2)

#ifdef DEBUG
-- The HowBound info for a variable in the FVInfo should be consistent
check_eq_how_bound ImportBound        ImportBound        = True
check_eq_how_bound LambdaBound        LambdaBound        = True
check_eq_how_bound (LetBound li1 ar1) (LetBound li2 ar2) = ar1 == ar2 && check_eq_li li1 li2
check_eq_how_bound hb1                hb2                = False

check_eq_li (NestedLet _) (NestedLet _) = True
check_eq_li (TopLet _)    (TopLet _)    = True
check_eq_li li1           li2           = False
#endif
\end{code}

Misc.
\begin{code}
filterStgBinders :: [Var] -> [Var]
filterStgBinders bndrs
  | opt_RuntimeTypes = bndrs
  | otherwise        = filter isId bndrs
\end{code}


\begin{code}
        -- Ignore all notes except SCC
myCollectBinders expr
  = go [] expr
  where
    go bs (Lam b e)          = go (b:bs) e
    go bs e@(Note (SCC _) _) = (reverse bs, e) 
    go bs (Note _ e)         = go bs e
    go bs e                  = (reverse bs, e)

myCollectArgs :: CoreExpr -> (Id, [CoreArg])
        -- We assume that we only have variables
        -- in the function position by now
myCollectArgs expr
  = go expr []
  where
    go (Var v)          as = (v, as)
    go (App f a) as        = go f (a:as)
    go (Note (SCC _) e) as = pprPanic "CoreToStg.myCollectArgs" (ppr expr)
    go (Note n e)       as = go e as
    go _                as = pprPanic "CoreToStg.myCollectArgs" (ppr expr)
\end{code}

%************************************************************************
%*                                                                      *
\subsection{Figuring out CafInfo for an expression}
%*                                                                      *
%************************************************************************

hasCafRefs decides whether a top-level closure can point into the dynamic heap.
We mark such things as `MayHaveCafRefs' because this information is
used to decide whether a particular closure needs to be referenced
in an SRT or not.

There are two reasons for setting MayHaveCafRefs:
        a) The RHS is a CAF: a top-level updatable thunk.
        b) The RHS refers to something that MayHaveCafRefs

Possible improvement: In an effort to keep the number of CAFs (and 
hence the size of the SRTs) down, we could also look at the expression and 
decide whether it requires a small bounded amount of heap, so we can ignore 
it as a CAF.  In these cases however, we would need to use an additional
CAF list to keep track of non-collectable CAFs.  

\begin{code}
hasCafRefs  :: IdEnv HowBound -> CoreExpr -> CafInfo
-- Only called for the RHS of top-level lets
hasCafRefss :: IdEnv HowBound -> [CoreExpr] -> CafInfo
        -- predicate returns True for a given Id if we look at this Id when
        -- calculating the result.  Used to *avoid* looking at the CafInfo
        -- field for an Id that is part of the current recursive group.

hasCafRefs p expr 
  | isCAF expr || isFastTrue (cafRefs p expr) =  MayHaveCafRefs
  | otherwise = NoCafRefs

        -- used for recursive groups.  The whole group is set to
        -- "MayHaveCafRefs" if at least one of the group is a CAF or
        -- refers to any CAFs.
hasCafRefss p exprs
  | any isCAF exprs || isFastTrue (cafRefss p exprs) = MayHaveCafRefs
  | otherwise = NoCafRefs

-- The environment that cafRefs uses has top-level bindings *only*.
-- We don't bother to add local bindings as cafRefs traverses the expression
-- because they will all be for LocalIds (all nested things are LocalIds)
-- However, we must look in the env first, because some top level things
-- might be local Ids

cafRefs p (Var id)
  = case lookupVarEnv p id of
        Just (LetBound (TopLet caf_info) _) -> fastBool (mayHaveCafRefs caf_info)
        Nothing | isGlobalId id             -> fastBool (mayHaveCafRefs (idCafInfo id)) -- Imported
                | otherwise                 -> fastBool False                           -- Nested binder
        _other                              -> error ("cafRefs " ++ showSDoc (ppr id))  -- No nested things in env

cafRefs p (Lit l)            = fastBool False
cafRefs p (App f a)          = fastOr (cafRefs p f) (cafRefs p) a
cafRefs p (Lam x e)          = cafRefs p e
cafRefs p (Let b e)          = fastOr (cafRefss p (rhssOfBind b)) (cafRefs p) e
cafRefs p (Case e bndr alts) = fastOr (cafRefs p e) (cafRefss p) (rhssOfAlts alts)
cafRefs p (Note n e)         = cafRefs p e
cafRefs p (Type t)           = fastBool False

cafRefss p []     = fastBool False
cafRefss p (e:es) = fastOr (cafRefs p e) (cafRefss p) es

-- hack for lazy-or over FastBool.
fastOr a f x = fastBool (isFastTrue a || isFastTrue (f x))

isCAF :: CoreExpr -> Bool
-- Only called for the RHS of top-level lets
isCAF e = not (rhsIsNonUpd e)
  {- ToDo: check type for onceness, i.e. non-updatable thunks? -}


rhsIsNonUpd :: CoreExpr -> Bool
  -- True => Value-lambda, constructor, PAP
  -- This is a bit like CoreUtils.exprIsValue, with the following differences:
  --    a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
  --
  --    b) (C x xs), where C is a contructors is updatable if the application is
  --       dynamic: see isDynConApp
  -- 
  --    c) don't look through unfolding of f in (f x).  I'm suspicious of this one

-- This function has to line up with what the update flag
-- for the StgRhs gets set to in mkStgRhs (above)
--
-- When opt_RuntimeTypes is on, we keep type lambdas and treat
-- them as making the RHS re-entrant (non-updatable).
rhsIsNonUpd (Lam b e)          = isRuntimeVar b || rhsIsNonUpd e
rhsIsNonUpd (Note (SCC _) e)   = False
rhsIsNonUpd (Note _ e)         = rhsIsNonUpd e
rhsIsNonUpd other_expr
  = go other_expr 0 []
  where
    go (Var f) n_args args = idAppIsNonUpd f n_args args
        
    go (App f a) n_args args
        | isTypeArg a = go f n_args args
        | otherwise   = go f (n_args + 1) (a:args)

    go (Note (SCC _) f) n_args args = False
    go (Note _ f) n_args args       = go f n_args args

    go other n_args args = False

idAppIsNonUpd :: Id -> Int -> [CoreExpr] -> Bool
idAppIsNonUpd id n_val_args args
  | Just con <- isDataConId_maybe id = not (isCrossDllConApp con args)
  | otherwise                        = n_val_args < idArity id

isCrossDllConApp :: DataCon -> [CoreExpr] -> Bool
isCrossDllConApp con args = isDllName (dataConName con) || any isCrossDllArg args
-- Top-level constructor applications can usually be allocated 
-- statically, but they can't if 
--      a) the constructor, or any of the arguments, come from another DLL
--      b) any of the arguments are LitLits
-- (because we can't refer to static labels in other DLLs).
-- If this happens we simply make the RHS into an updatable thunk, 
-- and 'exectute' it rather than allocating it statically.
-- All this should match the decision in (see CoreToStg.coreToStgRhs)


isCrossDllArg :: CoreExpr -> Bool
-- True if somewhere in the expression there's a cross-DLL reference
isCrossDllArg (Type _)    = False
isCrossDllArg (Var v)     = isDllName (idName v)
isCrossDllArg (Note _ e)  = isCrossDllArg e
isCrossDllArg (Lit lit)   = isLitLitLit lit
isCrossDllArg (App e1 e2) = isCrossDllArg e1 || isCrossDllArg e2        -- must be a type app
isCrossDllArg (Lam v e)   = isCrossDllArg e     -- must be a type lam
\end{code}