simplCore/Simplify.hs

    {-
    (c) The AQUA Project, Glasgow University, 1993-1998

    \section[Simplify]{The main module of the simplifier}
    -}

    {-# LANGUAGE CPP #-}

    module Simplify ( simplTopBinds, simplExpr, simplRule ) where

    #include "HsVersions.h"

    import DynFlags
    import SimplMonad
    import Type hiding      ( substTy, extendTvSubst, substTyVar )
    import SimplEnv
    import SimplUtils
    import FamInstEnv       ( FamInstEnv )
    import Literal          ( litIsLifted ) --, mkMachInt ) -- temporalily commented out. See #8326
    import Id
    import MkId             ( seqId, voidPrimId )
    import MkCore           ( mkImpossibleExpr, castBottomExpr )
    import IdInfo
    import Name             ( Name, mkSystemVarName, isExternalName )
    import Coercion hiding  ( substCo, substTy, substCoVar, extendTvSubst )
    import OptCoercion      ( optCoercion )
    import FamInstEnv       ( topNormaliseType_maybe )
    import DataCon          ( DataCon, dataConWorkId, dataConRepStrictness
                            , isMarkedStrict ) --, dataConTyCon, dataConTag, fIRST_TAG )
    --import TyCon            ( isEnumerationTyCon ) -- temporalily commented out. See #8326
    import CoreMonad        ( Tick(..), SimplifierMode(..) )
    import CoreSyn
    import Demand           ( StrictSig(..), dmdTypeDepth, isStrictDmd )
    import PprCore          ( pprCoreExpr )
    import CoreUnfold
    import CoreUtils
    import CoreArity
    --import PrimOp           ( tagToEnumKey ) -- temporalily commented out. See #8326
    import Rules            ( mkSpecInfo, lookupRule, getRules )
    import TysPrim          ( voidPrimTy ) --, intPrimTy ) -- temporalily commented out. See #8326
    import BasicTypes       ( TopLevelFlag(..), isTopLevel, RecFlag(..) )
    import MonadUtils       ( foldlM, mapAccumLM, liftIO )
    import Maybes           ( orElse )
    --import Unique           ( hasKey ) -- temporalily commented out. See #8326
    import Control.Monad
    import Outputable
    import FastString
    import Pair
    import Util
    import ErrUtils

    {-
    The guts of the simplifier is in this module, but the driver loop for
    the simplifier is in SimplCore.lhs.

    -----------------------------------------
            *** IMPORTANT NOTE ***
    -----------------------------------------
    The simplifier used to guarantee that the output had no shadowing, but
    it does not do so any more.   (Actually, it never did!)  The reason is
    documented with simplifyArgs.

    -----------------------------------------
            *** IMPORTANT NOTE ***
    -----------------------------------------
    Many parts of the simplifier return a bunch of "floats" as well as an
    expression. This is wrapped as a datatype SimplUtils.FloatsWith.

    All "floats" are let-binds, not case-binds, but some non-rec lets may
    be unlifted (with RHS ok-for-speculation).

    -----------------------------------------
            ORGANISATION OF FUNCTIONS
    -----------------------------------------
    simplTopBinds
      - simplify all top-level binders
      - for NonRec, call simplRecOrTopPair
      - for Rec,    call simplRecBind

            ------------------------------
    simplExpr (applied lambda)      ==> simplNonRecBind
    simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
    simplExpr (Let (Rec ...)    ..) ==> simplify binders; simplRecBind

            ------------------------------
    simplRecBind    [binders already simplfied]
      - use simplRecOrTopPair on each pair in turn

    simplRecOrTopPair [binder already simplified]
      Used for: recursive bindings (top level and nested)
                top-level non-recursive bindings
      Returns:
      - check for PreInlineUnconditionally
      - simplLazyBind

    simplNonRecBind
      Used for: non-top-level non-recursive bindings
                beta reductions (which amount to the same thing)
      Because it can deal with strict arts, it takes a
            "thing-inside" and returns an expression

      - check for PreInlineUnconditionally
      - simplify binder, including its IdInfo
      - if strict binding
            simplStrictArg
            mkAtomicArgs
            completeNonRecX
        else
            simplLazyBind
            addFloats

    simplNonRecX:   [given a *simplified* RHS, but an *unsimplified* binder]
      Used for: binding case-binder and constr args in a known-constructor case
      - check for PreInLineUnconditionally
      - simplify binder
      - completeNonRecX

            ------------------------------
    simplLazyBind:  [binder already simplified, RHS not]
      Used for: recursive bindings (top level and nested)
                top-level non-recursive bindings
                non-top-level, but *lazy* non-recursive bindings
            [must not be strict or unboxed]
      Returns floats + an augmented environment, not an expression
      - substituteIdInfo and add result to in-scope
            [so that rules are available in rec rhs]
      - simplify rhs
      - mkAtomicArgs
      - float if exposes constructor or PAP
      - completeBind

    completeNonRecX:        [binder and rhs both simplified]
      - if the the thing needs case binding (unlifted and not ok-for-spec)
            build a Case
       else
            completeBind
            addFloats

    completeBind:   [given a simplified RHS]
            [used for both rec and non-rec bindings, top level and not]
      - try PostInlineUnconditionally
      - add unfolding [this is the only place we add an unfolding]
      - add arity

    Right hand sides and arguments
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In many ways we want to treat
            (a) the right hand side of a let(rec), and
            (b) a function argument
    in the same way.  But not always!  In particular, we would
    like to leave these arguments exactly as they are, so they
    will match a RULE more easily.

            f (g x, h x)
            g (+ x)

    It's harder to make the rule match if we ANF-ise the constructor,
    or eta-expand the PAP:

            f (let { a = g x; b = h x } in (a,b))
            g (\y. + x y)

    On the other hand if we see the let-defns

            p = (g x, h x)
            q = + x

    then we *do* want to ANF-ise and eta-expand, so that p and q
    can be safely inlined.

    Even floating lets out is a bit dubious.  For let RHS's we float lets
    out if that exposes a value, so that the value can be inlined more vigorously.
    For example

            r = let x = e in (x,x)

    Here, if we float the let out we'll expose a nice constructor. We did experiments
    that showed this to be a generally good thing.  But it was a bad thing to float
    lets out unconditionally, because that meant they got allocated more often.

    For function arguments, there's less reason to expose a constructor (it won't
    get inlined).  Just possibly it might make a rule match, but I'm pretty skeptical.
    So for the moment we don't float lets out of function arguments either.

    Eta expansion
    ~~~~~~~~~~~~~~
    For eta expansion, we want to catch things like

            case e of (a,b) -> \x -> case a of (p,q) -> \y -> r

    If the \x was on the RHS of a let, we'd eta expand to bring the two
    lambdas together.  And in general that's a good thing to do.  Perhaps
    we should eta expand wherever we find a (value) lambda?  Then the eta
    expansion at a let RHS can concentrate solely on the PAP case.

    ************************************************************************
    *                                                                      *
    \subsection{Bindings}
    *                                                                      *
    ************************************************************************
    -}

    simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv

    simplTopBinds env0 binds0
      = do  {       -- Put all the top-level binders into scope at the start
                    -- so that if a transformation rule has unexpectedly brought
                    -- anything into scope, then we don't get a complaint about that.
                    -- It's rather as if the top-level binders were imported.
                    -- See note [Glomming] in OccurAnal.
            ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
            ; env2 <- simpl_binds env1 binds0
            ; freeTick SimplifierDone
            ; return env2 }
      where
            -- We need to track the zapped top-level binders, because
            -- they should have their fragile IdInfo zapped (notably occurrence info)
            -- That's why we run down binds and bndrs' simultaneously.
            --
        simpl_binds :: SimplEnv -> [InBind] -> SimplM SimplEnv
        simpl_binds env []           = return env
        simpl_binds env (bind:binds) = do { env' <- simpl_bind env bind
                                          ; simpl_binds env' binds }

        simpl_bind env (Rec pairs)  = simplRecBind      env  TopLevel pairs
        simpl_bind env (NonRec b r) = do { (env', b') <- addBndrRules env b (lookupRecBndr env b)
                                         ; simplRecOrTopPair env' TopLevel NonRecursive b b' r }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Lazy bindings}
    *                                                                      *
    ************************************************************************

    simplRecBind is used for
            * recursive bindings only
    -}

    simplRecBind :: SimplEnv -> TopLevelFlag
                 -> [(InId, InExpr)]
                 -> SimplM SimplEnv
    simplRecBind env0 top_lvl pairs0
      = do  { (env_with_info, triples) <- mapAccumLM add_rules env0 pairs0
            ; env1 <- go (zapFloats env_with_info) triples
            ; return (env0 `addRecFloats` env1) }
            -- addFloats adds the floats from env1,
            -- _and_ updates env0 with the in-scope set from env1
      where
        add_rules :: SimplEnv -> (InBndr,InExpr) -> SimplM (SimplEnv, (InBndr, OutBndr, InExpr))
            -- Add the (substituted) rules to the binder
        add_rules env (bndr, rhs)
            = do { (env', bndr') <- addBndrRules env bndr (lookupRecBndr env bndr)
                 ; return (env', (bndr, bndr', rhs)) }

        go env [] = return env

        go env ((old_bndr, new_bndr, rhs) : pairs)
            = do { env' <- simplRecOrTopPair env top_lvl Recursive old_bndr new_bndr rhs
                 ; go env' pairs }

    {-
    simplOrTopPair is used for
            * recursive bindings (whether top level or not)
            * top-level non-recursive bindings

    It assumes the binder has already been simplified, but not its IdInfo.
    -}

    simplRecOrTopPair :: SimplEnv
                      -> TopLevelFlag -> RecFlag
                      -> InId -> OutBndr -> InExpr  -- Binder and rhs
                      -> SimplM SimplEnv    -- Returns an env that includes the binding

    simplRecOrTopPair env top_lvl is_rec old_bndr new_bndr rhs
      = do { dflags <- getDynFlags
           ; trace_bind dflags $
               if preInlineUnconditionally dflags env top_lvl old_bndr rhs
                        -- Check for unconditional inline
               then do tick (PreInlineUnconditionally old_bndr)
                       return (extendIdSubst env old_bndr (mkContEx env rhs))
               else simplLazyBind env top_lvl is_rec old_bndr new_bndr rhs env }
      where
        trace_bind dflags thing_inside
          | not (dopt Opt_D_verbose_core2core dflags)
          = thing_inside
          | otherwise
          = pprTrace "SimplBind" (ppr old_bndr) thing_inside
            -- trace_bind emits a trace for each top-level binding, which
            -- helps to locate the tracing for inlining and rule firing

    {-
    simplLazyBind is used for
      * [simplRecOrTopPair] recursive bindings (whether top level or not)
      * [simplRecOrTopPair] top-level non-recursive bindings
      * [simplNonRecE]      non-top-level *lazy* non-recursive bindings

    Nota bene:
        1. It assumes that the binder is *already* simplified,
           and is in scope, and its IdInfo too, except unfolding

        2. It assumes that the binder type is lifted.

        3. It does not check for pre-inline-unconditionally;
           that should have been done already.
    -}

    simplLazyBind :: SimplEnv
                  -> TopLevelFlag -> RecFlag
                  -> InId -> OutId          -- Binder, both pre-and post simpl
                                            -- The OutId has IdInfo, except arity, unfolding
                  -> InExpr -> SimplEnv     -- The RHS and its environment
                  -> SimplM SimplEnv
    -- Precondition: rhs obeys the let/app invariant
    simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
      = -- pprTrace "simplLazyBind" ((ppr bndr <+> ppr bndr1) $$ ppr rhs $$ ppr (seIdSubst rhs_se)) $
        do  { let   rhs_env     = rhs_se `setInScope` env
                    (tvs, body) = case collectTyBinders rhs of
                                    (tvs, body) | not_lam body -> (tvs,body)
                                                | otherwise    -> ([], rhs)
                    not_lam (Lam _ _)  = False
                    not_lam (Tick t e) | not (tickishFloatable t)
                                       = not_lam e -- eta-reduction could float
                    not_lam _          = True
                            -- Do not do the "abstract tyyvar" thing if there's
                            -- a lambda inside, because it defeats eta-reduction
                            --    f = /\a. \x. g a x
                            -- should eta-reduce.

            ; (body_env, tvs') <- simplBinders rhs_env tvs
                    -- See Note [Floating and type abstraction] in SimplUtils

            -- Simplify the RHS
            ; let   rhs_cont = mkRhsStop (substTy body_env (exprType body))
            ; (body_env1, body1) <- simplExprF body_env body rhs_cont
            -- ANF-ise a constructor or PAP rhs
            ; (body_env2, body2) <- prepareRhs top_lvl body_env1 bndr1 body1

            ; (env', rhs')
                <-  if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
                    then                            -- No floating, revert to body1
                         do { rhs' <- mkLam tvs' (wrapFloats body_env1 body1) rhs_cont
                            ; return (env, rhs') }

                    else if null tvs then           -- Simple floating
                         do { tick LetFloatFromLet
                            ; return (addFloats env body_env2, body2) }

                    else                            -- Do type-abstraction first
                         do { tick LetFloatFromLet
                            ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
                            ; rhs' <- mkLam tvs' body3 rhs_cont
                            ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
                            ; return (env', rhs') }

            ; completeBind env' top_lvl bndr bndr1 rhs' }

    {-
    A specialised variant of simplNonRec used when the RHS is already simplified,
    notably in knownCon.  It uses case-binding where necessary.
    -}

    simplNonRecX :: SimplEnv
                 -> InId            -- Old binder
                 -> OutExpr         -- Simplified RHS
                 -> SimplM SimplEnv
    -- Precondition: rhs satisfies the let/app invariant
    simplNonRecX env bndr new_rhs
      | isDeadBinder bndr   -- Not uncommon; e.g. case (a,b) of c { (p,q) -> p }
      = return env    --  Here c is dead, and we avoid creating
                      --   the binding c = (a,b)

      | Coercion co <- new_rhs
      = return (extendCvSubst env bndr co)

      | otherwise
      = do  { (env', bndr') <- simplBinder env bndr
            ; completeNonRecX NotTopLevel env' (isStrictId bndr) bndr bndr' new_rhs }
                    -- simplNonRecX is only used for NotTopLevel things

    completeNonRecX :: TopLevelFlag -> SimplEnv
                    -> Bool
                    -> InId                 -- Old binder
                    -> OutId                -- New binder
                    -> OutExpr              -- Simplified RHS
                    -> SimplM SimplEnv
    -- Precondition: rhs satisfies the let/app invariant
    --               See Note [CoreSyn let/app invariant] in CoreSyn

    completeNonRecX top_lvl env is_strict old_bndr new_bndr new_rhs
      = do  { (env1, rhs1) <- prepareRhs top_lvl (zapFloats env) new_bndr new_rhs
            ; (env2, rhs2) <-
                    if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
                    then do { tick LetFloatFromLet
                            ; return (addFloats env env1, rhs1) }   -- Add the floats to the main env
                    else return (env, wrapFloats env1 rhs1)         -- Wrap the floats around the RHS
            ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }

    {-
    {- No, no, no!  Do not try preInlineUnconditionally in completeNonRecX
       Doing so risks exponential behaviour, because new_rhs has been simplified once already
       In the cases described by the folowing commment, postInlineUnconditionally will
       catch many of the relevant cases.
            -- This happens; for example, the case_bndr during case of
            -- known constructor:  case (a,b) of x { (p,q) -> ... }
            -- Here x isn't mentioned in the RHS, so we don't want to
            -- create the (dead) let-binding  let x = (a,b) in ...
            --
            -- Similarly, single occurrences can be inlined vigourously
            -- e.g.  case (f x, g y) of (a,b) -> ....
            -- If a,b occur once we can avoid constructing the let binding for them.

       Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
            -- Consider     case I# (quotInt# x y) of
            --                I# v -> let w = J# v in ...
            -- If we gaily inline (quotInt# x y) for v, we end up building an
            -- extra thunk:
            --                let w = J# (quotInt# x y) in ...
            -- because quotInt# can fail.

      | preInlineUnconditionally env NotTopLevel bndr new_rhs
      = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
    -}

    ----------------------------------
    prepareRhs takes a putative RHS, checks whether it's a PAP or
    constructor application and, if so, converts it to ANF, so that the
    resulting thing can be inlined more easily.  Thus
            x = (f a, g b)
    becomes
            t1 = f a
            t2 = g b
            x = (t1,t2)

    We also want to deal well cases like this
            v = (f e1 `cast` co) e2
    Here we want to make e1,e2 trivial and get
            x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
    That's what the 'go' loop in prepareRhs does
    -}

    prepareRhs :: TopLevelFlag -> SimplEnv -> OutId -> OutExpr -> SimplM (SimplEnv, OutExpr)
    -- Adds new floats to the env iff that allows us to return a good RHS
    prepareRhs top_lvl env id (Cast rhs co)    -- Note [Float coercions]
      | Pair ty1 _ty2 <- coercionKind co       -- Do *not* do this if rhs has an unlifted type
      , not (isUnLiftedType ty1)            -- see Note [Float coercions (unlifted)]
      = do  { (env', rhs') <- makeTrivialWithInfo top_lvl env sanitised_info rhs
            ; return (env', Cast rhs' co) }
      where
        sanitised_info = vanillaIdInfo `setStrictnessInfo` strictnessInfo info
                                       `setDemandInfo` demandInfo info
        info = idInfo id

    prepareRhs top_lvl env0 _ rhs0
      = do  { (_is_exp, env1, rhs1) <- go 0 env0 rhs0
            ; return (env1, rhs1) }
      where
        go n_val_args env (Cast rhs co)
            = do { (is_exp, env', rhs') <- go n_val_args env rhs
                 ; return (is_exp, env', Cast rhs' co) }
        go n_val_args env (App fun (Type ty))
            = do { (is_exp, env', rhs') <- go n_val_args env fun
                 ; return (is_exp, env', App rhs' (Type ty)) }
        go n_val_args env (App fun arg)
            = do { (is_exp, env', fun') <- go (n_val_args+1) env fun
                 ; case is_exp of
                    True -> do { (env'', arg') <- makeTrivial top_lvl env' arg
                               ; return (True, env'', App fun' arg') }
                    False -> return (False, env, App fun arg) }
        go n_val_args env (Var fun)
            = return (is_exp, env, Var fun)
            where
              is_exp = isExpandableApp fun n_val_args   -- The fun a constructor or PAP
                            -- See Note [CONLIKE pragma] in BasicTypes
                            -- The definition of is_exp should match that in
                            -- OccurAnal.occAnalApp

        go n_val_args env (Tick t rhs)
            -- We want to be able to float bindings past this
            -- tick. Non-scoping ticks don't care.
            | tickishScoped t == NoScope
            = do { (is_exp, env', rhs') <- go n_val_args env rhs
                 ; return (is_exp, env', Tick t rhs') }
            -- On the other hand, for scoping ticks we need to be able to
            -- copy them on the floats, which in turn is only allowed if
            -- we can obtain non-counting ticks.
            | not (tickishCounts t) || tickishCanSplit t
            = do { (is_exp, env', rhs') <- go n_val_args (zapFloats env) rhs
                 ; let tickIt (id, expr) = (id, mkTick (mkNoCount t) expr)
                       floats' = seFloats $ env `addFloats` mapFloats env' tickIt
                 ; return (is_exp, env' { seFloats = floats' }, Tick t rhs') }

        go _ env other
            = return (False, env, other)

    {-
    Note [Float coercions]
    ~~~~~~~~~~~~~~~~~~~~~~
    When we find the binding
            x = e `cast` co
    we'd like to transform it to
            x' = e
            x = x `cast` co         -- A trivial binding
    There's a chance that e will be a constructor application or function, or something
    like that, so moving the coerion to the usage site may well cancel the coersions
    and lead to further optimisation.  Example:

         data family T a :: *
         data instance T Int = T Int

         foo :: Int -> Int -> Int
         foo m n = ...
            where
              x = T m
              go 0 = 0
              go n = case x of { T m -> go (n-m) }
                    -- This case should optimise

    Note [Preserve strictness when floating coercions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In the Note [Float coercions] transformation, keep the strictness info.
    Eg
            f = e `cast` co    -- f has strictness SSL
    When we transform to
            f' = e             -- f' also has strictness SSL
            f = f' `cast` co   -- f still has strictness SSL

    Its not wrong to drop it on the floor, but better to keep it.

    Note [Float coercions (unlifted)]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    BUT don't do [Float coercions] if 'e' has an unlifted type.
    This *can* happen:

         foo :: Int = (error (# Int,Int #) "urk")
                      `cast` CoUnsafe (# Int,Int #) Int

    If do the makeTrivial thing to the error call, we'll get
        foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
    But 'v' isn't in scope!

    These strange casts can happen as a result of case-of-case
            bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
                    (# p,q #) -> p+q
    -}

    makeTrivialArg :: SimplEnv -> ArgSpec -> SimplM (SimplEnv, ArgSpec)
    makeTrivialArg env (ValArg e) = do { (env', e') <- makeTrivial NotTopLevel env e
                                       ; return (env', ValArg e') }
    makeTrivialArg env arg        = return (env, arg)  -- CastBy, TyArg

    makeTrivial :: TopLevelFlag -> SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
    -- Binds the expression to a variable, if it's not trivial, returning the variable
    makeTrivial top_lvl env expr = makeTrivialWithInfo top_lvl env vanillaIdInfo expr

    makeTrivialWithInfo :: TopLevelFlag -> SimplEnv -> IdInfo
                        -> OutExpr -> SimplM (SimplEnv, OutExpr)
    -- Propagate strictness and demand info to the new binder
    -- Note [Preserve strictness when floating coercions]
    -- Returned SimplEnv has same substitution as incoming one
    makeTrivialWithInfo top_lvl env info expr
      | exprIsTrivial expr                          -- Already trivial
      || not (bindingOk top_lvl expr expr_ty)       -- Cannot trivialise
                                                    --   See Note [Cannot trivialise]
      = return (env, expr)
      | otherwise           -- See Note [Take care] below
      = do  { uniq <- getUniqueM
            ; let name = mkSystemVarName uniq (fsLit "a")
                  var = mkLocalIdWithInfo name expr_ty info
            ; env'  <- completeNonRecX top_lvl env False var var expr
            ; expr' <- simplVar env' var
            ; return (env', expr') }
            -- The simplVar is needed becase we're constructing a new binding
            --     a = rhs
            -- And if rhs is of form (rhs1 |> co), then we might get
            --     a1 = rhs1
            --     a = a1 |> co
            -- and now a's RHS is trivial and can be substituted out, and that
            -- is what completeNonRecX will do
            -- To put it another way, it's as if we'd simplified
            --    let var = e in var
      where
        expr_ty = exprType expr

    bindingOk :: TopLevelFlag -> CoreExpr -> Type -> Bool
    -- True iff we can have a binding of this expression at this level
    -- Precondition: the type is the type of the expression
    bindingOk top_lvl _ expr_ty
      | isTopLevel top_lvl = not (isUnLiftedType expr_ty)
      | otherwise          = True

    {-
    Note [Cannot trivialise]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Consider tih
       f :: Int -> Addr#

       foo :: Bar
       foo = Bar (f 3)

    Then we can't ANF-ise foo, even though we'd like to, because
    we can't make a top-level binding for the Addr# (f 3). And if
    so we don't want to turn it into
       foo = let x = f 3 in Bar x
    because we'll just end up inlining x back, and that makes the
    simplifier loop.  Better not to ANF-ise it at all.

    A case in point is literal strings (a MachStr is not regarded as
    trivial):

       foo = Ptr "blob"#

    We don't want to ANF-ise this.

    ************************************************************************
    *                                                                      *
    \subsection{Completing a lazy binding}
    *                                                                      *
    ************************************************************************

    completeBind
      * deals only with Ids, not TyVars
      * takes an already-simplified binder and RHS
      * is used for both recursive and non-recursive bindings
      * is used for both top-level and non-top-level bindings

    It does the following:
      - tries discarding a dead binding
      - tries PostInlineUnconditionally
      - add unfolding [this is the only place we add an unfolding]
      - add arity

    It does *not* attempt to do let-to-case.  Why?  Because it is used for
      - top-level bindings (when let-to-case is impossible)
      - many situations where the "rhs" is known to be a WHNF
                    (so let-to-case is inappropriate).

    Nor does it do the atomic-argument thing
    -}

    completeBind :: SimplEnv
                 -> TopLevelFlag            -- Flag stuck into unfolding
                 -> InId                    -- Old binder
                 -> OutId -> OutExpr        -- New binder and RHS
                 -> SimplM SimplEnv
    -- completeBind may choose to do its work
    --      * by extending the substitution (e.g. let x = y in ...)
    --      * or by adding to the floats in the envt
    --
    -- Precondition: rhs obeys the let/app invariant
    completeBind env top_lvl old_bndr new_bndr new_rhs
     | isCoVar old_bndr
     = case new_rhs of
         Coercion co -> return (extendCvSubst env old_bndr co)
         _           -> return (addNonRec env new_bndr new_rhs)

     | otherwise
     = ASSERT( isId new_bndr )
       do { let old_info = idInfo old_bndr
                old_unf  = unfoldingInfo old_info
                occ_info = occInfo old_info

            -- Do eta-expansion on the RHS of the binding
            -- See Note [Eta-expanding at let bindings] in SimplUtils
          ; (new_arity, final_rhs) <- tryEtaExpandRhs env new_bndr new_rhs

            -- Simplify the unfolding
          ; new_unfolding <- simplLetUnfolding env top_lvl old_bndr final_rhs old_unf

          ; dflags <- getDynFlags
          ; if postInlineUnconditionally dflags env top_lvl new_bndr occ_info
                                         final_rhs new_unfolding

                            -- Inline and discard the binding
            then do  { tick (PostInlineUnconditionally old_bndr)
                     ; return (extendIdSubst env old_bndr (DoneEx final_rhs)) }
                    -- Use the substitution to make quite, quite sure that the
                    -- substitution will happen, since we are going to discard the binding
            else
       do { let info1 = idInfo new_bndr `setArityInfo` new_arity

                  -- Unfolding info: Note [Setting the new unfolding]
                info2 = info1 `setUnfoldingInfo` new_unfolding

                  -- Demand info: Note [Setting the demand info]
                  --
                  -- We also have to nuke demand info if for some reason
                  -- eta-expansion *reduces* the arity of the binding to less
                  -- than that of the strictness sig. This can happen: see Note [Arity decrease].
                info3 | isEvaldUnfolding new_unfolding
                        || (case strictnessInfo info2 of
                              StrictSig dmd_ty -> new_arity < dmdTypeDepth dmd_ty)
                      = zapDemandInfo info2 `orElse` info2
                      | otherwise
                      = info2

                final_id = new_bndr `setIdInfo` info3

          ; -- pprTrace "Binding" (ppr final_id <+> ppr new_unfolding) $
            return (addNonRec env final_id final_rhs) } }
                    -- The addNonRec adds it to the in-scope set too

    ------------------------------
    addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv
    -- Add a new binding to the environment, complete with its unfolding
    -- but *do not* do postInlineUnconditionally, because we have already
    -- processed some of the scope of the binding
    -- We still want the unfolding though.  Consider
    --      let
    --            x = /\a. let y = ... in Just y
    --      in body
    -- Then we float the y-binding out (via abstractFloats and addPolyBind)
    -- but 'x' may well then be inlined in 'body' in which case we'd like the
    -- opportunity to inline 'y' too.
    --
    -- INVARIANT: the arity is correct on the incoming binders

    addPolyBind top_lvl env (NonRec poly_id rhs)
      = do  { unfolding <- simplLetUnfolding env top_lvl poly_id rhs noUnfolding
                            -- Assumes that poly_id did not have an INLINE prag
                            -- which is perhaps wrong.  ToDo: think about this
            ; let final_id = setIdInfo poly_id $
                             idInfo poly_id `setUnfoldingInfo` unfolding

            ; return (addNonRec env final_id rhs) }

    addPolyBind _ env bind@(Rec _)
      = return (extendFloats env bind)
            -- Hack: letrecs are more awkward, so we extend "by steam"
            -- without adding unfoldings etc.  At worst this leads to
            -- more simplifier iterations

    {- Note [Arity decrease]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Generally speaking the arity of a binding should not decrease.  But it *can*
    legitimately happen because of RULES.  Eg
            f = g Int
    where g has arity 2, will have arity 2.  But if there's a rewrite rule
            g Int --> h
    where h has arity 1, then f's arity will decrease.  Here's a real-life example,
    which is in the output of Specialise:

         Rec {
            $dm {Arity 2} = \d.\x. op d
            {-# RULES forall d. $dm Int d = $s$dm #-}

            dInt = MkD .... opInt ...
            opInt {Arity 1} = $dm dInt

            $s$dm {Arity 0} = \x. op dInt }

    Here opInt has arity 1; but when we apply the rule its arity drops to 0.
    That's why Specialise goes to a little trouble to pin the right arity
    on specialised functions too.

    Note [Setting the demand info]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If the unfolding is a value, the demand info may
    go pear-shaped, so we nuke it.  Example:
         let x = (a,b) in
         case x of (p,q) -> h p q x
    Here x is certainly demanded. But after we've nuked
    the case, we'll get just
         let x = (a,b) in h a b x
    and now x is not demanded (I'm assuming h is lazy)
    This really happens.  Similarly
         let f = \x -> e in ...f..f...
    After inlining f at some of its call sites the original binding may
    (for example) be no longer strictly demanded.
    The solution here is a bit ad hoc...

    ************************************************************************
    *                                                                      *
    \subsection[Simplify-simplExpr]{The main function: simplExpr}
    *                                                                      *
    ************************************************************************

    The reason for this OutExprStuff stuff is that we want to float *after*
    simplifying a RHS, not before.  If we do so naively we get quadratic
    behaviour as things float out.

    To see why it's important to do it after, consider this (real) example:

            let t = f x
            in fst t
    ==>
            let t = let a = e1
                        b = e2
                    in (a,b)
            in fst t
    ==>
            let a = e1
                b = e2
                t = (a,b)
            in
            a       -- Can't inline a this round, cos it appears twice
    ==>
            e1

    Each of the ==> steps is a round of simplification.  We'd save a
    whole round if we float first.  This can cascade.  Consider

            let f = g d
            in \x -> ...f...
    ==>
            let f = let d1 = ..d.. in \y -> e
            in \x -> ...f...
    ==>
            let d1 = ..d..
            in \x -> ...(\y ->e)...

    Only in this second round can the \y be applied, and it
    might do the same again.
    -}

    simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
    simplExpr env expr = simplExprC env expr (mkBoringStop expr_out_ty)
      where
        expr_out_ty :: OutType
        expr_out_ty = substTy env (exprType expr)

    simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
            -- Simplify an expression, given a continuation
    simplExprC env expr cont
      = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
        do  { (env', expr') <- simplExprF (zapFloats env) expr cont
            ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
              -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
              -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
              return (wrapFloats env' expr') }

    --------------------------------------------------
    simplExprF :: SimplEnv -> InExpr -> SimplCont
               -> SimplM (SimplEnv, OutExpr)

    simplExprF env e cont
      = {- pprTrace "simplExprF" (vcat
          [ ppr e
          , text "cont =" <+> ppr cont
          , text "inscope =" <+> ppr (seInScope env)
          , text "tvsubst =" <+> ppr (seTvSubst env)
          , text "idsubst =" <+> ppr (seIdSubst env)
          , text "cvsubst =" <+> ppr (seCvSubst env)
          {- , ppr (seFloats env) -}
          ]) $ -}
        simplExprF1 env e cont

    simplExprF1 :: SimplEnv -> InExpr -> SimplCont
                -> SimplM (SimplEnv, OutExpr)
    simplExprF1 env (Var v)        cont = simplIdF env v cont
    simplExprF1 env (Lit lit)      cont = rebuild env (Lit lit) cont
    simplExprF1 env (Tick t expr)  cont = simplTick env t expr cont
    simplExprF1 env (Cast body co) cont = simplCast env body co cont
    simplExprF1 env (Coercion co)  cont = simplCoercionF env co cont
    simplExprF1 env (Type ty)      cont = ASSERT( contIsRhsOrArg cont )
                                          rebuild env (Type (substTy env ty)) cont

    simplExprF1 env (App fun arg) cont
      = simplExprF env fun $
        case arg of
          Type ty -> ApplyToTy  { sc_arg_ty  = substTy env ty
                                , sc_hole_ty = substTy env (exprType fun)
                                , sc_cont    = cont }
          _       -> ApplyToVal { sc_arg = arg, sc_env = env
                                , sc_dup = NoDup, sc_cont = cont }

    simplExprF1 env expr@(Lam {}) cont
      = simplLam env zapped_bndrs body cont
            -- The main issue here is under-saturated lambdas
            --   (\x1. \x2. e) arg1
            -- Here x1 might have "occurs-once" occ-info, because occ-info
            -- is computed assuming that a group of lambdas is applied
            -- all at once.  If there are too few args, we must zap the
            -- occ-info, UNLESS the remaining binders are one-shot
      where
        (bndrs, body) = collectBinders expr
        zapped_bndrs | need_to_zap = map zap bndrs
                     | otherwise   = bndrs

        need_to_zap = any zappable_bndr (drop n_args bndrs)
        n_args = countArgs cont
            -- NB: countArgs counts all the args (incl type args)
            -- and likewise drop counts all binders (incl type lambdas)

        zappable_bndr b = isId b && not (isOneShotBndr b)
        zap b | isTyVar b = b
              | otherwise = zapLamIdInfo b

    simplExprF1 env (Case scrut bndr _ alts) cont
      = simplExprF env scrut (Select NoDup bndr alts env cont)

    simplExprF1 env (Let (Rec pairs) body) cont
      = do  { env' <- simplRecBndrs env (map fst pairs)
                    -- NB: bndrs' don't have unfoldings or rules
                    -- We add them as we go down

            ; env'' <- simplRecBind env' NotTopLevel pairs
            ; simplExprF env'' body cont }

    simplExprF1 env (Let (NonRec bndr rhs) body) cont
      = simplNonRecE env bndr (rhs, env) ([], body) cont

    ---------------------------------
    simplType :: SimplEnv -> InType -> SimplM OutType
            -- Kept monadic just so we can do the seqType
    simplType env ty
      = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
        seqType new_ty `seq` return new_ty
      where
        new_ty = substTy env ty

    ---------------------------------
    simplCoercionF :: SimplEnv -> InCoercion -> SimplCont
                   -> SimplM (SimplEnv, OutExpr)
    simplCoercionF env co cont
      = do { co' <- simplCoercion env co
           ; rebuild env (Coercion co') cont }

    simplCoercion :: SimplEnv -> InCoercion -> SimplM OutCoercion
    simplCoercion env co
      = let opt_co = optCoercion (getCvSubst env) co
        in seqCo opt_co `seq` return opt_co

    -----------------------------------
    -- | Push a TickIt context outwards past applications and cases, as
    -- long as this is a non-scoping tick, to let case and application
    -- optimisations apply.

    simplTick :: SimplEnv -> Tickish Id -> InExpr -> SimplCont
              -> SimplM (SimplEnv, OutExpr)
    simplTick env tickish expr cont
      -- A scoped tick turns into a continuation, so that we can spot
      -- (scc t (\x . e)) in simplLam and eliminate the scc.  If we didn't do
      -- it this way, then it would take two passes of the simplifier to
      -- reduce ((scc t (\x . e)) e').
      -- NB, don't do this with counting ticks, because if the expr is
      -- bottom, then rebuildCall will discard the continuation.

    -- XXX: we cannot do this, because the simplifier assumes that
    -- the context can be pushed into a case with a single branch. e.g.
    --    scc<f>  case expensive of p -> e
    -- becomes
    --    case expensive of p -> scc<f> e
    --
    -- So I'm disabling this for now.  It just means we will do more
    -- simplifier iterations that necessary in some cases.

    --  | tickishScoped tickish && not (tickishCounts tickish)
    --  = simplExprF env expr (TickIt tickish cont)

      -- For unscoped or soft-scoped ticks, we are allowed to float in new
      -- cost, so we simply push the continuation inside the tick.  This
      -- has the effect of moving the tick to the outside of a case or
      -- application context, allowing the normal case and application
      -- optimisations to fire.
      | tickish `tickishScopesLike` SoftScope
      = do { (env', expr') <- simplExprF env expr cont
           ; return (env', mkTick tickish expr')
           }

      -- Push tick inside if the context looks like this will allow us to
      -- do a case-of-case - see Note [case-of-scc-of-case]
      | Select {} <- cont, Just expr' <- push_tick_inside
      = simplExprF env expr' cont

      -- We don't want to move the tick, but we might still want to allow
      -- floats to pass through with appropriate wrapping (or not, see
      -- wrap_floats below)
      --- | not (tickishCounts tickish) || tickishCanSplit tickish
      -- = wrap_floats

      | otherwise
      = no_floating_past_tick

     where

      -- Try to push tick inside a case, see Note [case-of-scc-of-case].
      push_tick_inside =
        case expr0 of
          Case scrut bndr ty alts
                 -> Just $ Case (tickScrut scrut) bndr ty (map tickAlt alts)
          _other -> Nothing
       where (ticks, expr0) = stripTicksTop movable (Tick tickish expr)
             movable t      = not (tickishCounts t) ||
                              t `tickishScopesLike` NoScope ||
                              tickishCanSplit t
             tickScrut e    = foldr mkTick e ticks
             -- Alternatives get annotated with all ticks that scope in some way,
             -- but we don't want to count entries.
             tickAlt (c,bs,e) = (c,bs, foldr mkTick e ts_scope)
             ts_scope         = map mkNoCount $
                                filter (not . (`tickishScopesLike` NoScope)) ticks

      no_floating_past_tick =
        do { let (inc,outc) = splitCont cont
           ; (env', expr') <- simplExprF (zapFloats env) expr inc
           ; let tickish' = simplTickish env tickish
           ; (env'', expr'') <- rebuild (zapFloats env')
                                        (wrapFloats env' expr')
                                        (TickIt tickish' outc)
           ; return (addFloats env env'', expr'')
           }

    -- Alternative version that wraps outgoing floats with the tick.  This
    -- results in ticks being duplicated, as we don't make any attempt to
    -- eliminate the tick if we re-inline the binding (because the tick
    -- semantics allows unrestricted inlining of HNFs), so I'm not doing
    -- this any more.  FloatOut will catch any real opportunities for
    -- floating.
    --
    --  wrap_floats =
    --    do { let (inc,outc) = splitCont cont
    --       ; (env', expr') <- simplExprF (zapFloats env) expr inc
    --       ; let tickish' = simplTickish env tickish
    --       ; let wrap_float (b,rhs) = (zapIdStrictness (setIdArity b 0),
    --                                   mkTick (mkNoCount tickish') rhs)
    --              -- when wrapping a float with mkTick, we better zap the Id's
    --              -- strictness info and arity, because it might be wrong now.
    --       ; let env'' = addFloats env (mapFloats env' wrap_float)
    --       ; rebuild env'' expr' (TickIt tickish' outc)
    --       }

      simplTickish env tickish
        | Breakpoint n ids <- tickish
              = Breakpoint n (map (getDoneId . substId env) ids)
        | otherwise = tickish

      -- Push type application and coercion inside a tick
      splitCont :: SimplCont -> (SimplCont, SimplCont)
      splitCont cont@(ApplyToTy { sc_cont = tail }) = (cont { sc_cont = inc }, outc)
        where (inc,outc) = splitCont tail
      splitCont (CastIt co c) = (CastIt co inc, outc)
        where (inc,outc) = splitCont c
      splitCont other = (mkBoringStop (contHoleType other), other)

      getDoneId (DoneId id) = id
      getDoneId (DoneEx e)  = getIdFromTrivialExpr e -- Note [substTickish] in CoreSubst
      getDoneId other = pprPanic "getDoneId" (ppr other)

    -- Note [case-of-scc-of-case]
    -- It's pretty important to be able to transform case-of-case when
    -- there's an SCC in the way.  For example, the following comes up
    -- in nofib/real/compress/Encode.hs:
    --
    --        case scctick<code_string.r1>
    --             case $wcode_string_r13s wild_XC w1_s137 w2_s138 l_aje
    --             of _ { (# ww1_s13f, ww2_s13g, ww3_s13h #) ->
    --             (ww1_s13f, ww2_s13g, ww3_s13h)
    --             }
    --        of _ { (ww_s12Y, ww1_s12Z, ww2_s130) ->
    --        tick<code_string.f1>
    --        (ww_s12Y,
    --         ww1_s12Z,
    --         PTTrees.PT
    --           @ GHC.Types.Char @ GHC.Types.Int wild2_Xj ww2_s130 r_ajf)
    --        }
    --
    -- We really want this case-of-case to fire, because then the 3-tuple
    -- will go away (indeed, the CPR optimisation is relying on this
    -- happening).  But the scctick is in the way - we need to push it
    -- inside to expose the case-of-case.  So we perform this
    -- transformation on the inner case:
    --
    --   scctick c (case e of { p1 -> e1; ...; pn -> en })
    --    ==>
    --   case (scctick c e) of { p1 -> scc c e1; ...; pn -> scc c en }
    --
    -- So we've moved a constant amount of work out of the scc to expose
    -- the case.  We only do this when the continuation is interesting: in
    -- for now, it has to be another Case (maybe generalise this later).

    {-
    ************************************************************************
    *                                                                      *
    \subsection{The main rebuilder}
    *                                                                      *
    ************************************************************************
    -}

    rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
    -- At this point the substitution in the SimplEnv should be irrelevant
    -- only the in-scope set and floats should matter
    rebuild env expr cont
      = case cont of
          Stop {}                       -> return (env, expr)
          TickIt t cont                 -> rebuild env (mkTick t expr) cont
          CastIt co cont                -> rebuild env (mkCast expr co) cont
                                        -- NB: mkCast implements the (Coercion co |> g) optimisation

          Select _ bndr alts se cont    -> rebuildCase (se `setFloats` env) expr bndr alts cont
          StrictArg info _ cont         -> rebuildCall env (info `addValArgTo` expr) cont

          StrictBind b bs body se cont  -> do { env' <- simplNonRecX (se `setFloats` env) b expr
                                                   -- expr satisfies let/app since it started life
                                                   -- in a call to simplNonRecE
                                              ; simplLam env' bs body cont }

          ApplyToTy  { sc_arg_ty = ty, sc_cont = cont}
            -> rebuild env (App expr (Type ty)) cont
          ApplyToVal { sc_arg = arg, sc_env = se, sc_dup = dup_flag, sc_cont = cont}
            -- See Note [Avoid redundant simplification]
            | isSimplified dup_flag     -> rebuild env (App expr arg) cont
            | otherwise                 -> do { arg' <- simplExpr (se `setInScope` env) arg
                                              ; rebuild env (App expr arg') cont }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Lambdas}
    *                                                                      *
    ************************************************************************
    -}

    simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
              -> SimplM (SimplEnv, OutExpr)
    simplCast env body co0 cont0
      = do  { co1   <- simplCoercion env co0
            ; cont1 <- addCoerce co1 cont0
            ; simplExprF env body cont1 }
      where
           addCoerce co cont = add_coerce co (coercionKind co) cont

           add_coerce _co (Pair s1 k1) cont     -- co :: ty~ty
             | s1 `eqType` k1 = return cont    -- is a no-op

           add_coerce co1 (Pair s1 _k2) (CastIt co2 cont)
             | (Pair _l1 t1) <- coercionKind co2
                    --      e |> (g1 :: S1~L) |> (g2 :: L~T1)
                    -- ==>
                    --      e,                       if S1=T1
                    --      e |> (g1 . g2 :: S1~T1)  otherwise
                    --
                    -- For example, in the initial form of a worker
                    -- we may find  (coerce T (coerce S (\x.e))) y
                    -- and we'd like it to simplify to e[y/x] in one round
                    -- of simplification
             , s1 `eqType` t1  = return cont     -- The coerces cancel out
             | otherwise       = return (CastIt (mkTransCo co1 co2) cont)

           add_coerce co (Pair s1s2 _t1t2) cont@(ApplyToTy { sc_arg_ty = arg_ty, sc_cont = tail })
                    -- (f |> g) ty  --->   (f ty) |> (g @ ty)
                    -- This implements the PushT rule from the paper
             | Just (tyvar,_) <- splitForAllTy_maybe s1s2
             = ASSERT( isTyVar tyvar )
               do { cont' <- addCoerce new_cast tail
                  ; return (cont { sc_cont = cont' }) }
             where
               new_cast = mkInstCo co arg_ty

           add_coerce co (Pair s1s2 t1t2) (ApplyToVal { sc_arg = arg, sc_env = arg_se
                                                      , sc_dup = dup, sc_cont = cont })
             | isFunTy s1s2   -- This implements the Push rule from the paper
             , isFunTy t1t2   -- Check t1t2 to ensure 'arg' is a value arg
                    --      (e |> (g :: s1s2 ~ t1->t2)) f
                    -- ===>
                    --      (e (f |> (arg g :: t1~s1))
                    --      |> (res g :: s2->t2)
                    --
                    -- t1t2 must be a function type, t1->t2, because it's applied
                    -- to something but s1s2 might conceivably not be
                    --
                    -- When we build the ApplyTo we can't mix the out-types
                    -- with the InExpr in the argument, so we simply substitute
                    -- to make it all consistent.  It's a bit messy.
                    -- But it isn't a common case.
                    --
                    -- Example of use: Trac #995
             = do { let arg' = substExpr arg_se arg
                               -- It's important that this is lazy, because this argument
                               -- may be disarded if turns out to be the argument of
                               -- (\_ -> e)     This can make a huge difference;
                               -- see Trac #10527
                  ; cont' <- addCoerce co2 cont
                  ; return (ApplyToVal { sc_arg  = mkCast arg' (mkSymCo co1)
                                       , sc_env  = zapSubstEnv arg_se
                                       , sc_dup  = dup
                                       , sc_cont = cont' }) }
             where
               -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
               -- t2 ~ s2 with left and right on the curried form:
               --    (->) t1 t2 ~ (->) s1 s2
               [co1, co2] = decomposeCo 2 co

           add_coerce co _ cont = return (CastIt co cont)

    simplArg :: SimplEnv -> DupFlag -> StaticEnv -> CoreExpr
             -> SimplM (DupFlag, StaticEnv, OutExpr)
    simplArg env dup_flag arg_env arg
      | isSimplified dup_flag
      = return (dup_flag, arg_env, arg)
      | otherwise
      = do { arg' <- simplExpr (arg_env `setInScope` env) arg
           ; return (Simplified, zapSubstEnv arg_env, arg') }

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Lambdas}
    *                                                                      *
    ************************************************************************

    Note [Zap unfolding when beta-reducing]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Lambda-bound variables can have stable unfoldings, such as
       $j = \x. \b{Unf=Just x}. e
    See Note [Case binders and join points] below; the unfolding for lets
    us optimise e better.  However when we beta-reduce it we want to
    revert to using the actual value, otherwise we can end up in the
    stupid situation of
              let x = blah in
              let b{Unf=Just x} = y
              in ...b...
    Here it'd be far better to drop the unfolding and use the actual RHS.
    -}

    simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
             -> SimplM (SimplEnv, OutExpr)

    simplLam env [] body cont = simplExprF env body cont

            -- Beta reduction

    simplLam env (bndr:bndrs) body (ApplyToTy { sc_arg_ty = arg_ty, sc_cont = cont })
      = do { tick (BetaReduction bndr)
           ; simplLam (extendTvSubst env bndr arg_ty) bndrs body cont }

    simplLam env (bndr:bndrs) body (ApplyToVal { sc_arg = arg, sc_env = arg_se
                                               , sc_cont = cont })
      = do  { tick (BetaReduction bndr)
            ; simplNonRecE env (zap_unfolding bndr) (arg, arg_se) (bndrs, body) cont }
      where
        zap_unfolding bndr  -- See Note [Zap unfolding when beta-reducing]
          | isId bndr, isStableUnfolding (realIdUnfolding bndr)
          = setIdUnfolding bndr NoUnfolding
          | otherwise = bndr

          -- discard a non-counting tick on a lambda.  This may change the
          -- cost attribution slightly (moving the allocation of the
          -- lambda elsewhere), but we don't care: optimisation changes
          -- cost attribution all the time.
    simplLam env bndrs body (TickIt tickish cont)
      | not (tickishCounts tickish)
      = simplLam env bndrs body cont

            -- Not enough args, so there are real lambdas left to put in the result
    simplLam env bndrs body cont
      = do  { (env', bndrs') <- simplLamBndrs env bndrs
            ; body' <- simplExpr env' body
            ; new_lam <- mkLam bndrs' body' cont
            ; rebuild env' new_lam cont }

    simplLamBndrs :: SimplEnv -> [InBndr] -> SimplM (SimplEnv, [OutBndr])
    simplLamBndrs env bndrs = mapAccumLM simplLamBndr env bndrs

    -------------
    simplLamBndr :: SimplEnv -> Var -> SimplM (SimplEnv, Var)
    -- Used for lambda binders.  These sometimes have unfoldings added by
    -- the worker/wrapper pass that must be preserved, because they can't
    -- be reconstructed from context.  For example:
    --      f x = case x of (a,b) -> fw a b x
    --      fw a b x{=(a,b)} = ...
    -- The "{=(a,b)}" is an unfolding we can't reconstruct otherwise.
    simplLamBndr env bndr
      | isId bndr && hasSomeUnfolding old_unf   -- Special case
      = do { (env1, bndr1) <- simplBinder env bndr
           ; unf'          <- simplUnfolding env1 NotTopLevel bndr old_unf
           ; let bndr2 = bndr1 `setIdUnfolding` unf'
           ; return (modifyInScope env1 bndr2, bndr2) }

      | otherwise
      = simplBinder env bndr                -- Normal case
      where
        old_unf = idUnfolding bndr

    ------------------
    simplNonRecE :: SimplEnv
                 -> InBndr                  -- The binder
                 -> (InExpr, SimplEnv)      -- Rhs of binding (or arg of lambda)
                 -> ([InBndr], InExpr)      -- Body of the let/lambda
                                            --      \xs.e
                 -> SimplCont
                 -> SimplM (SimplEnv, OutExpr)

    -- simplNonRecE is used for
    --  * non-top-level non-recursive lets in expressions
    --  * beta reduction
    --
    -- It deals with strict bindings, via the StrictBind continuation,
    -- which may abort the whole process
    --
    -- Precondition: rhs satisfies the let/app invariant
    --               Note [CoreSyn let/app invariant] in CoreSyn
    --
    -- The "body" of the binding comes as a pair of ([InId],InExpr)
    -- representing a lambda; so we recurse back to simplLam
    -- Why?  Because of the binder-occ-info-zapping done before
    --       the call to simplLam in simplExprF (Lam ...)

            -- First deal with type applications and type lets
            --   (/\a. e) (Type ty)   and   (let a = Type ty in e)
    simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
      = ASSERT( isTyVar bndr )
        do  { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
            ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }

    simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
      = do dflags <- getDynFlags
           case () of
             _ | preInlineUnconditionally dflags env NotTopLevel bndr rhs
               -> do { tick (PreInlineUnconditionally bndr)
                     ; -- pprTrace "preInlineUncond" (ppr bndr <+> ppr rhs) $
                      simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }

               | isStrictId bndr          -- Includes coercions
               -> simplExprF (rhs_se `setFloats` env) rhs
                             (StrictBind bndr bndrs body env cont)

               | otherwise
               -> ASSERT( not (isTyVar bndr) )
                  do { (env1, bndr1) <- simplNonRecBndr env bndr
                     ; (env2, bndr2) <- addBndrRules env1 bndr bndr1
                     ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
                     ; simplLam env3 bndrs body cont }

    {-
    ************************************************************************
    *                                                                      *
                         Variables
    *                                                                      *
    ************************************************************************
    -}

    simplVar :: SimplEnv -> InVar -> SimplM OutExpr
    -- Look up an InVar in the environment
    simplVar env var
      | isTyVar var = return (Type (substTyVar env var))
      | isCoVar var = return (Coercion (substCoVar env var))
      | otherwise
      = case substId env var of
            DoneId var1          -> return (Var var1)
            DoneEx e             -> return e
            ContEx tvs cvs ids e -> simplExpr (setSubstEnv env tvs cvs ids) e

    simplIdF :: SimplEnv -> InId -> SimplCont -> SimplM (SimplEnv, OutExpr)
    simplIdF env var cont
      = case substId env var of
            DoneEx e             -> simplExprF (zapSubstEnv env) e cont
            ContEx tvs cvs ids e -> simplExprF (setSubstEnv env tvs cvs ids) e cont
            DoneId var1          -> completeCall env var1 cont
                    -- Note [zapSubstEnv]
                    -- The template is already simplified, so don't re-substitute.
                    -- This is VITAL.  Consider
                    --      let x = e in
                    --      let y = \z -> ...x... in
                    --      \ x -> ...y...
                    -- We'll clone the inner \x, adding x->x' in the id_subst
                    -- Then when we inline y, we must *not* replace x by x' in
                    -- the inlined copy!!

    ---------------------------------------------------------
    --      Dealing with a call site

    completeCall :: SimplEnv -> OutId -> SimplCont -> SimplM (SimplEnv, OutExpr)
    completeCall env var cont
      = do  {   ------------- Try inlining ----------------
              dflags <- getDynFlags
            ; let  (lone_variable, arg_infos, call_cont) = contArgs cont
                   n_val_args = length arg_infos
                   interesting_cont = interestingCallContext call_cont
                   unfolding    = activeUnfolding env var
                   maybe_inline = callSiteInline dflags var unfolding
                                                 lone_variable arg_infos interesting_cont
            ; case maybe_inline of {
                Just expr      -- There is an inlining!
                  ->  do { checkedTick (UnfoldingDone var)
                         ; dump_inline dflags expr cont
                         ; simplExprF (zapSubstEnv env) expr cont }

                ; Nothing -> do               -- No inlining!

            { rule_base <- getSimplRules
            ; let info = mkArgInfo var (getRules rule_base var) n_val_args call_cont
            ; rebuildCall env info cont
        }}}
      where
        dump_inline dflags unfolding cont
          | not (dopt Opt_D_dump_inlinings dflags) = return ()
          | not (dopt Opt_D_verbose_core2core dflags)
          = when (isExternalName (idName var)) $
                liftIO $ printInfoForUser dflags alwaysQualify $
                    sep [text "Inlining done:", nest 4 (ppr var)]
          | otherwise
          = liftIO $ printInfoForUser dflags alwaysQualify $
               sep [text "Inlining done: " <> ppr var,
                    nest 4 (vcat [text "Inlined fn: " <+> nest 2 (ppr unfolding),
                                  text "Cont:  " <+> ppr cont])]

    rebuildCall :: SimplEnv
                -> ArgInfo
                -> SimplCont
                -> SimplM (SimplEnv, OutExpr)
    rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_strs = [] }) cont
      -- When we run out of strictness args, it means
      -- that the call is definitely bottom; see SimplUtils.mkArgInfo
      -- Then we want to discard the entire strict continuation.  E.g.
      --    * case (error "hello") of { ... }
      --    * (error "Hello") arg
      --    * f (error "Hello") where f is strict
      --    etc
      -- Then, especially in the first of these cases, we'd like to discard
      -- the continuation, leaving just the bottoming expression.  But the
      -- type might not be right, so we may have to add a coerce.
      | not (contIsTrivial cont)     -- Only do this if there is a non-trivial
      = return (env, castBottomExpr res cont_ty)  -- contination to discard, else we do it
      where                                       -- again and again!
        res     = argInfoExpr fun rev_args
        cont_ty = contResultType cont

    rebuildCall env info (CastIt co cont)
      = rebuildCall env (addCastTo info co) cont

    rebuildCall env info (ApplyToTy { sc_arg_ty = arg_ty, sc_cont = cont })
      = rebuildCall env (info `addTyArgTo` arg_ty) cont

    rebuildCall env info@(ArgInfo { ai_encl = encl_rules, ai_type = fun_ty
                                  , ai_strs = str:strs, ai_discs = disc:discs })
                (ApplyToVal { sc_arg = arg, sc_env = arg_se
                            , sc_dup = dup_flag, sc_cont = cont })
      | isSimplified dup_flag     -- See Note [Avoid redundant simplification]
      = rebuildCall env (addValArgTo info' arg) cont

      | str                 -- Strict argument
      = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
        simplExprF (arg_se `setFloats` env) arg
                   (StrictArg info' cci cont)
                    -- Note [Shadowing]

      | otherwise                           -- Lazy argument
            -- DO NOT float anything outside, hence simplExprC
            -- There is no benefit (unlike in a let-binding), and we'd
            -- have to be very careful about bogus strictness through
            -- floating a demanded let.
      = do  { arg' <- simplExprC (arg_se `setInScope` env) arg
                                 (mkLazyArgStop (funArgTy fun_ty) cci)
            ; rebuildCall env (addValArgTo info' arg') cont }
      where
        info' = info { ai_strs = strs, ai_discs = discs }
        cci | encl_rules = RuleArgCtxt
            | disc > 0   = DiscArgCtxt  -- Be keener here
            | otherwise  = BoringCtxt   -- Nothing interesting

    rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_rules = rules }) cont
      | null rules
      = rebuild env (argInfoExpr fun rev_args) cont      -- No rules, common case

      | otherwise
      = do {  -- We've accumulated a simplified call in <fun,rev_args>
              -- so try rewrite rules; see Note [RULEs apply to simplified arguments]
              -- See also Note [Rules for recursive functions]
            ; let env' = zapSubstEnv env  -- See Note [zapSubstEnv];
                                          -- and NB that 'rev_args' are all fully simplified
            ; mb_rule <- tryRules env' rules fun (reverse rev_args) cont
            ; case mb_rule of {
                 Just (rule_rhs, cont') -> simplExprF env' rule_rhs cont'

                     -- Rules don't match
               ; Nothing -> rebuild env (argInfoExpr fun rev_args) cont      -- No rules
        } }

    {-
    Note [RULES apply to simplified arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's very desirable to try RULES once the arguments have been simplified, because
    doing so ensures that rule cascades work in one pass.  Consider
       {-# RULES g (h x) = k x
                 f (k x) = x #-}
       ...f (g (h x))...
    Then we want to rewrite (g (h x)) to (k x) and only then try f's rules. If
    we match f's rules against the un-simplified RHS, it won't match.  This
    makes a particularly big difference when superclass selectors are involved:
            op ($p1 ($p2 (df d)))
    We want all this to unravel in one sweeep.

    Note [Avoid redundant simplification]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Because RULES apply to simplified arguments, there's a danger of repeatedly
    simplifying already-simplified arguments.  An important example is that of
            (>>=) d e1 e2
    Here e1, e2 are simplified before the rule is applied, but don't really
    participate in the rule firing. So we mark them as Simplified to avoid
    re-simplifying them.

    Note [Shadowing]
    ~~~~~~~~~~~~~~~~
    This part of the simplifier may break the no-shadowing invariant
    Consider
            f (...(\a -> e)...) (case y of (a,b) -> e')
    where f is strict in its second arg
    If we simplify the innermost one first we get (...(\a -> e)...)
    Simplifying the second arg makes us float the case out, so we end up with
            case y of (a,b) -> f (...(\a -> e)...) e'
    So the output does not have the no-shadowing invariant.  However, there is
    no danger of getting name-capture, because when the first arg was simplified
    we used an in-scope set that at least mentioned all the variables free in its
    static environment, and that is enough.

    We can't just do innermost first, or we'd end up with a dual problem:
            case x of (a,b) -> f e (...(\a -> e')...)

    I spent hours trying to recover the no-shadowing invariant, but I just could
    not think of an elegant way to do it.  The simplifier is already knee-deep in
    continuations.  We have to keep the right in-scope set around; AND we have
    to get the effect that finding (error "foo") in a strict arg position will
    discard the entire application and replace it with (error "foo").  Getting
    all this at once is TOO HARD!

    ************************************************************************
    *                                                                      *
                    Rewrite rules
    *                                                                      *
    ************************************************************************
    -}

    tryRules :: SimplEnv -> [CoreRule]
             -> Id -> [ArgSpec] -> SimplCont
             -> SimplM (Maybe (CoreExpr, SimplCont))
    -- The SimplEnv already has zapSubstEnv applied to it

    tryRules env rules fn args call_cont
      | null rules
      = return Nothing
    {- Disabled until we fix #8326
      | fn `hasKey` tagToEnumKey   -- See Note [Optimising tagToEnum#]
      , [_type_arg, val_arg] <- args
      , Select dup bndr ((_,[],rhs1) : rest_alts) se cont <- call_cont
      , isDeadBinder bndr
      = do { dflags <- getDynFlags
           ; let enum_to_tag :: CoreAlt -> CoreAlt
                    -- Takes   K -> e  into   tagK# -> e
                    -- where tagK# is the tag of constructor K
                 enum_to_tag (DataAlt con, [], rhs)
                   = ASSERT( isEnumerationTyCon (dataConTyCon con) )
                    (LitAlt tag, [], rhs)
                  where
                    tag = mkMachInt dflags (toInteger (dataConTag con - fIRST_TAG))
                 enum_to_tag alt = pprPanic "tryRules: tagToEnum" (ppr alt)

                 new_alts = (DEFAULT, [], rhs1) : map enum_to_tag rest_alts
                 new_bndr = setIdType bndr intPrimTy
                     -- The binder is dead, but should have the right type
          ; return (Just (val_arg, Select dup new_bndr new_alts se cont)) }
    -}
      | otherwise
      = do { dflags <- getDynFlags
           ; case lookupRule dflags (getUnfoldingInRuleMatch env) (activeRule env)
                             fn (argInfoAppArgs args) rules of {
               Nothing               -> return Nothing ;   -- No rule matches
               Just (rule, rule_rhs) ->
                 do { checkedTick (RuleFired (ru_name rule))
                    ; let cont' = pushSimplifiedArgs env
                                                     (drop (ruleArity rule) args)
                                                     call_cont
                          -- (ruleArity rule) says how many args the rule consumed
                    ; dump dflags rule rule_rhs
                    ; return (Just (rule_rhs, cont')) }}}
      where
        dump dflags rule rule_rhs
          | dopt Opt_D_dump_rule_rewrites dflags
          = log_rule dflags Opt_D_dump_rule_rewrites "Rule fired" $ vcat
              [ text "Rule:" <+> ftext (ru_name rule)
              , text "Before:" <+> hang (ppr fn) 2 (sep (map ppr args))
              , text "After: " <+> pprCoreExpr rule_rhs
              , text "Cont:  " <+> ppr call_cont ]

          | dopt Opt_D_dump_rule_firings dflags
          = log_rule dflags Opt_D_dump_rule_firings "Rule fired:" $
              ftext (ru_name rule)

          | otherwise
          = return ()

        log_rule dflags flag hdr details
          = liftIO . dumpSDoc dflags alwaysQualify flag "" $
                       sep [text hdr, nest 4 details]

    {-
    Note [Optimising tagToEnum#]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have an enumeration data type:

      data Foo = A | B | C

    Then we want to transform

       case tagToEnum# x of   ==>    case x of
         A -> e1                       DEFAULT -> e1
         B -> e2                       1#      -> e2
         C -> e3                       2#      -> e3

    thereby getting rid of the tagToEnum# altogether.  If there was a DEFAULT
    alternative we retain it (remember it comes first).  If not the case must
    be exhaustive, and we reflect that in the transformed version by adding
    a DEFAULT.  Otherwise Lint complains that the new case is not exhaustive.
    See #8317.

    Note [Rules for recursive functions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    You might think that we shouldn't apply rules for a loop breaker:
    doing so might give rise to an infinite loop, because a RULE is
    rather like an extra equation for the function:
         RULE:           f (g x) y = x+y
         Eqn:            f a     y = a-y

    But it's too drastic to disable rules for loop breakers.
    Even the foldr/build rule would be disabled, because foldr
    is recursive, and hence a loop breaker:
         foldr k z (build g) = g k z
    So it's up to the programmer: rules can cause divergence

    ************************************************************************
    *                                                                      *
                    Rebuilding a case expression
    *                                                                      *
    ************************************************************************

    Note [Case elimination]
    ~~~~~~~~~~~~~~~~~~~~~~~
    The case-elimination transformation discards redundant case expressions.
    Start with a simple situation:

            case x# of      ===>   let y# = x# in e
              y# -> e

    (when x#, y# are of primitive type, of course).  We can't (in general)
    do this for algebraic cases, because we might turn bottom into
    non-bottom!

    The code in SimplUtils.prepareAlts has the effect of generalise this
    idea to look for a case where we're scrutinising a variable, and we
    know that only the default case can match.  For example:

            case x of
              0#      -> ...
              DEFAULT -> ...(case x of
                             0#      -> ...
                             DEFAULT -> ...) ...

    Here the inner case is first trimmed to have only one alternative, the
    DEFAULT, after which it's an instance of the previous case.  This
    really only shows up in eliminating error-checking code.

    Note that SimplUtils.mkCase combines identical RHSs.  So

            case e of       ===> case e of DEFAULT -> r
               True  -> r
               False -> r

    Now again the case may be elminated by the CaseElim transformation.
    This includes things like (==# a# b#)::Bool so that we simplify
          case ==# a# b# of { True -> x; False -> x }
    to just
          x
    This particular example shows up in default methods for
    comparison operations (e.g. in (>=) for Int.Int32)

    Note [Case elimination: lifted case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If a case over a lifted type has a single alternative, and is being used
    as a strict 'let' (all isDeadBinder bndrs), we may want to do this
    transformation:

        case e of r       ===>   let r = e in ...r...
          _ -> ...r...

            (a) 'e' is already evaluated (it may so if e is a variable)
                Specifically we check (exprIsHNF e).  In this case
                we can just allocate the WHNF directly with a let.
    or
            (b) 'x' is not used at all and e is ok-for-speculation
                 The ok-for-spec bit checks that we don't lose any
                 exceptions or divergence.

                 NB: it'd be *sound* to switch from case to let if the
                 scrutinee was not yet WHNF but was guaranteed to
                 converge; but sticking with case means we won't build a
                 thunk

    or
            (c) 'x' is used strictly in the body, and 'e' is a variable
                Then we can just substitute 'e' for 'x' in the body.
                See Note [Eliminating redundant seqs]

    For (b), the "not used at all" test is important.  Consider
       case (case a ># b of { True -> (p,q); False -> (q,p) }) of
         r -> blah
    The scrutinee is ok-for-speculation (it looks inside cases), but we do
    not want to transform to
       let r = case a ># b of { True -> (p,q); False -> (q,p) }
       in blah
    because that builds an unnecessary thunk.

    Note [Eliminating redundant seqs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have this:
       case x of r { _ -> ..r.. }
    where 'r' is used strictly in (..r..), the case is effectively a 'seq'
    on 'x', but since 'r' is used strictly anyway, we can safely transform to
       (...x...)

    Note that this can change the error behaviour.  For example, we might
    transform
        case x of { _ -> error "bad" }
        --> error "bad"
    which is might be puzzling if 'x' currently lambda-bound, but later gets
    let-bound to (error "good").

    Nevertheless, the paper "A semantics for imprecise exceptions" allows
    this transformation. If you want to fix the evaluation order, use
    'pseq'.  See Trac #8900 for an example where the loss of this
    transformation bit us in practice.

    See also Note [Empty case alternatives] in CoreSyn.

    Just for reference, the original code (added Jan 13) looked like this:
         || case_bndr_evald_next rhs

        case_bndr_evald_next :: CoreExpr -> Bool
          -- See Note [Case binder next]
        case_bndr_evald_next (Var v)         = v == case_bndr
        case_bndr_evald_next (Cast e _)      = case_bndr_evald_next e
        case_bndr_evald_next (App e _)       = case_bndr_evald_next e
        case_bndr_evald_next (Case e _ _ _)  = case_bndr_evald_next e
        case_bndr_evald_next _               = False

    (This came up when fixing Trac #7542. See also Note [Eta reduction of
    an eval'd function] in CoreUtils.)

    Note [Case elimination: unlifted case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       case a +# b of r -> ...r...
    Then we do case-elimination (to make a let) followed by inlining,
    to get
            .....(a +# b)....
    If we have
       case indexArray# a i of r -> ...r...
    we might like to do the same, and inline the (indexArray# a i).
    But indexArray# is not okForSpeculation, so we don't build a let
    in rebuildCase (lest it get floated *out*), so the inlining doesn't
    happen either.

    This really isn't a big deal I think. The let can be

    Further notes about case elimination
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider:       test :: Integer -> IO ()
                    test = print

    Turns out that this compiles to:
        Print.test
          = \ eta :: Integer
              eta1 :: Void# ->
              case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
              case hPutStr stdout
                     (PrelNum.jtos eta ($w[] @ Char))
                     eta1
              of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s  }}

    Notice the strange '<' which has no effect at all. This is a funny one.
    It started like this:

    f x y = if x < 0 then jtos x
              else if y==0 then "" else jtos x

    At a particular call site we have (f v 1).  So we inline to get

            if v < 0 then jtos x
            else if 1==0 then "" else jtos x

    Now simplify the 1==0 conditional:

            if v<0 then jtos v else jtos v

    Now common-up the two branches of the case:

            case (v<0) of DEFAULT -> jtos v

    Why don't we drop the case?  Because it's strict in v.  It's technically
    wrong to drop even unnecessary evaluations, and in practice they
    may be a result of 'seq' so we *definitely* don't want to drop those.
    I don't really know how to improve this situation.
    -}

    ---------------------------------------------------------
    --      Eliminate the case if possible

    rebuildCase, reallyRebuildCase
       :: SimplEnv
       -> OutExpr          -- Scrutinee
       -> InId             -- Case binder
       -> [InAlt]          -- Alternatives (inceasing order)
       -> SimplCont
       -> SimplM (SimplEnv, OutExpr)

    --------------------------------------------------
    --      1. Eliminate the case if there's a known constructor
    --------------------------------------------------

    rebuildCase env scrut case_bndr alts cont
      | Lit lit <- scrut    -- No need for same treatment as constructors
                            -- because literals are inlined more vigorously
      , not (litIsLifted lit)
      = do  { tick (KnownBranch case_bndr)
            ; case findAlt (LitAlt lit) alts of
                Nothing           -> missingAlt env case_bndr alts cont
                Just (_, bs, rhs) -> simple_rhs bs rhs }

      | Just (con, ty_args, other_args) <- exprIsConApp_maybe (getUnfoldingInRuleMatch env) scrut
            -- Works when the scrutinee is a variable with a known unfolding
            -- as well as when it's an explicit constructor application
      = do  { tick (KnownBranch case_bndr)
            ; case findAlt (DataAlt con) alts of
                Nothing  -> missingAlt env case_bndr alts cont
                Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs
                Just (_, bs, rhs)       -> knownCon env scrut con ty_args other_args
                                                    case_bndr bs rhs cont
            }
      where
        simple_rhs bs rhs = ASSERT( null bs )
                            do { env' <- simplNonRecX env case_bndr scrut
                                   -- scrut is a constructor application,
                                   -- hence satisfies let/app invariant
                               ; simplExprF env' rhs cont }

    --------------------------------------------------
    --      2. Eliminate the case if scrutinee is evaluated
    --------------------------------------------------

    rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
      -- See if we can get rid of the case altogether
      -- See Note [Case elimination]
      -- mkCase made sure that if all the alternatives are equal,
      -- then there is now only one (DEFAULT) rhs

      -- 2a.  Dropping the case altogether, if
      --      a) it binds nothing (so it's really just a 'seq')
      --      b) evaluating the scrutinee has no side effects
      | is_plain_seq
      , exprOkForSideEffects scrut
              -- The entire case is dead, so we can drop it
              -- if the scrutinee converges without having imperative
              -- side effects or raising a Haskell exception
              -- See Note [PrimOp can_fail and has_side_effects] in PrimOp
       = simplExprF env rhs cont

      -- 2b.  Turn the case into a let, if
      --      a) it binds only the case-binder
      --      b) unlifted case: the scrutinee is ok-for-speculation
      --           lifted case: the scrutinee is in HNF (or will later be demanded)
      | all_dead_bndrs
      , if is_unlifted
        then exprOkForSpeculation scrut  -- See Note [Case elimination: unlifted case]
        else exprIsHNF scrut             -- See Note [Case elimination: lifted case]
          || scrut_is_demanded_var scrut
      = do { tick (CaseElim case_bndr)
           ; env' <- simplNonRecX env case_bndr scrut
           ; simplExprF env' rhs cont }

      -- 2c. Try the seq rules if
      --     a) it binds only the case binder
      --     b) a rule for seq applies
      -- See Note [User-defined RULES for seq] in MkId
      | is_plain_seq
      = do { let scrut_ty = exprType scrut
                 rhs_ty   = substTy env (exprType rhs)
                 out_args = [ TyArg { as_arg_ty  = scrut_ty
                                    , as_hole_ty = seq_id_ty }
                            , TyArg { as_arg_ty  = rhs_ty
                                    , as_hole_ty = applyTy seq_id_ty scrut_ty }
                            , ValArg scrut]
                 rule_cont = ApplyToVal { sc_dup = NoDup, sc_arg = rhs
                                        , sc_env = env, sc_cont = cont }
                 env' = zapSubstEnv env
                 -- Lazily evaluated, so we don't do most of this

           ; rule_base <- getSimplRules
           ; mb_rule <- tryRules env' (getRules rule_base seqId) seqId out_args rule_cont
           ; case mb_rule of
               Just (rule_rhs, cont') -> simplExprF env' rule_rhs cont'
               Nothing                -> reallyRebuildCase env scrut case_bndr alts cont }
      where
        is_unlifted        = isUnLiftedType (idType case_bndr)
        all_dead_bndrs     = all isDeadBinder bndrs       -- bndrs are [InId]
        is_plain_seq       = all_dead_bndrs && isDeadBinder case_bndr -- Evaluation *only* for effect
        seq_id_ty          = idType seqId

        scrut_is_demanded_var :: CoreExpr -> Bool
                -- See Note [Eliminating redundant seqs]
        scrut_is_demanded_var (Cast s _) = scrut_is_demanded_var s
        scrut_is_demanded_var (Var _)    = isStrictDmd (idDemandInfo case_bndr)
        scrut_is_demanded_var _          = False

    rebuildCase env scrut case_bndr alts cont
      = reallyRebuildCase env scrut case_bndr alts cont

    --------------------------------------------------
    --      3. Catch-all case
    --------------------------------------------------

    reallyRebuildCase env scrut case_bndr alts cont
      = do  {       -- Prepare the continuation;
                    -- The new subst_env is in place
              (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont

            -- Simplify the alternatives
            ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont

            ; dflags <- getDynFlags
            ; let alts_ty' = contResultType dup_cont
            ; case_expr <- mkCase dflags scrut' case_bndr' alts_ty' alts'

            -- Notice that rebuild gets the in-scope set from env', not alt_env
            -- (which in any case is only build in simplAlts)
            -- The case binder *not* scope over the whole returned case-expression
            ; rebuild env' case_expr nodup_cont }

    {-
    simplCaseBinder checks whether the scrutinee is a variable, v.  If so,
    try to eliminate uses of v in the RHSs in favour of case_bndr; that
    way, there's a chance that v will now only be used once, and hence
    inlined.

    Historical note: we use to do the "case binder swap" in the Simplifier
    so there were additional complications if the scrutinee was a variable.
    Now the binder-swap stuff is done in the occurrence analyer; see
    OccurAnal Note [Binder swap].

    Note [knownCon occ info]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    If the case binder is not dead, then neither are the pattern bound
    variables:
            case <any> of x { (a,b) ->
            case x of { (p,q) -> p } }
    Here (a,b) both look dead, but come alive after the inner case is eliminated.
    The point is that we bring into the envt a binding
            let x = (a,b)
    after the outer case, and that makes (a,b) alive.  At least we do unless
    the case binder is guaranteed dead.

    Note [Case alternative occ info]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we are simply reconstructing a case (the common case), we always
    zap the occurrence info on the binders in the alternatives.  Even
    if the case binder is dead, the scrutinee is usually a variable, and *that*
    can bring the case-alternative binders back to life.
    See Note [Add unfolding for scrutinee]

    Note [Improving seq]
    ~~~~~~~~~~~~~~~~~~~
    Consider
            type family F :: * -> *
            type instance F Int = Int

            ... case e of x { DEFAULT -> rhs } ...

    where x::F Int.  Then we'd like to rewrite (F Int) to Int, getting

            case e `cast` co of x'::Int
               I# x# -> let x = x' `cast` sym co
                        in rhs

    so that 'rhs' can take advantage of the form of x'.

    Notice that Note [Case of cast] (in OccurAnal) may then apply to the result.

    Nota Bene: We only do the [Improving seq] transformation if the
    case binder 'x' is actually used in the rhs; that is, if the case
    is *not* a *pure* seq.
      a) There is no point in adding the cast to a pure seq.
      b) There is a good reason not to: doing so would interfere
         with seq rules (Note [Built-in RULES for seq] in MkId).
         In particular, this [Improving seq] thing *adds* a cast
         while [Built-in RULES for seq] *removes* one, so they
         just flip-flop.

    You might worry about
       case v of x { __DEFAULT ->
          ... case (v `cast` co) of y { I# -> ... }}
    This is a pure seq (since x is unused), so [Improving seq] won't happen.
    But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get
       case v of x { __DEFAULT ->
          ... case (x `cast` co) of y { I# -> ... }}
    Now the outer case is not a pure seq, so [Improving seq] will happen,
    and then the inner case will disappear.

    The need for [Improving seq] showed up in Roman's experiments.  Example:
      foo :: F Int -> Int -> Int
      foo t n = t `seq` bar n
         where
           bar 0 = 0
           bar n = bar (n - case t of TI i -> i)
    Here we'd like to avoid repeated evaluating t inside the loop, by
    taking advantage of the `seq`.

    At one point I did transformation in LiberateCase, but it's more
    robust here.  (Otherwise, there's a danger that we'll simply drop the
    'seq' altogether, before LiberateCase gets to see it.)
    -}

    simplAlts :: SimplEnv
              -> OutExpr
              -> InId                       -- Case binder
              -> [InAlt]                    -- Non-empty
              -> SimplCont
              -> SimplM (OutExpr, OutId, [OutAlt])  -- Includes the continuation
    -- Like simplExpr, this just returns the simplified alternatives;
    -- it does not return an environment
    -- The returned alternatives can be empty, none are possible

    simplAlts env scrut case_bndr alts cont'
      = do  { let env0 = zapFloats env

            ; (env1, case_bndr1) <- simplBinder env0 case_bndr

            ; fam_envs <- getFamEnvs
            ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
                                                           case_bndr case_bndr1 alts

            ; (imposs_deflt_cons, in_alts) <- prepareAlts scrut' case_bndr' alts
              -- NB: it's possible that the returned in_alts is empty: this is handled
              -- by the caller (rebuildCase) in the missingAlt function

            ; alts' <- mapM (simplAlt alt_env' (Just scrut') imposs_deflt_cons case_bndr' cont') in_alts
            ; -- pprTrace "simplAlts" (ppr case_bndr $$ ppr alts_ty $$ ppr alts_ty' $$ ppr alts $$ ppr cont') $
              return (scrut', case_bndr', alts') }

    ------------------------------------
    improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
               -> OutExpr -> InId -> OutId -> [InAlt]
               -> SimplM (SimplEnv, OutExpr, OutId)
    -- Note [Improving seq]
    improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
      | not (isDeadBinder case_bndr) -- Not a pure seq!  See Note [Improving seq]
      , Just (co, ty2) <- topNormaliseType_maybe fam_envs (idType case_bndr1)
      = do { case_bndr2 <- newId (fsLit "nt") ty2
            ; let rhs  = DoneEx (Var case_bndr2 `Cast` mkSymCo co)
                  env2 = extendIdSubst env case_bndr rhs
            ; return (env2, scrut `Cast` co, case_bndr2) }

    improveSeq _ env scrut _ case_bndr1 _
      = return (env, scrut, case_bndr1)

    ------------------------------------
    simplAlt :: SimplEnv
             -> Maybe OutExpr  -- The scrutinee
             -> [AltCon]       -- These constructors can't be present when
                               -- matching the DEFAULT alternative
             -> OutId          -- The case binder
             -> SimplCont
             -> InAlt
             -> SimplM OutAlt

    simplAlt env _ imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
      = ASSERT( null bndrs )
        do  { let env' = addBinderUnfolding env case_bndr'
                                            (mkOtherCon imposs_deflt_cons)
                    -- Record the constructors that the case-binder *can't* be.
            ; rhs' <- simplExprC env' rhs cont'
            ; return (DEFAULT, [], rhs') }

    simplAlt env scrut' _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
      = ASSERT( null bndrs )
        do  { env' <- addAltUnfoldings env scrut' case_bndr' (Lit lit)
            ; rhs' <- simplExprC env' rhs cont'
            ; return (LitAlt lit, [], rhs') }

    simplAlt env scrut' _ case_bndr' cont' (DataAlt con, vs, rhs)
      = do  {       -- Deal with the pattern-bound variables
                    -- Mark the ones that are in ! positions in the
                    -- data constructor as certainly-evaluated.
                    -- NB: simplLamBinders preserves this eval info
            ; let vs_with_evals = add_evals (dataConRepStrictness con)
            ; (env', vs') <- simplLamBndrs env vs_with_evals

                    -- Bind the case-binder to (con args)
            ; let inst_tys' = tyConAppArgs (idType case_bndr')
                  con_app :: OutExpr
                  con_app   = mkConApp2 con inst_tys' vs'

            ; env'' <- addAltUnfoldings env' scrut' case_bndr' con_app
            ; rhs' <- simplExprC env'' rhs cont'
            ; return (DataAlt con, vs', rhs') }
      where
            -- add_evals records the evaluated-ness of the bound variables of
            -- a case pattern.  This is *important*.  Consider
            --      data T = T !Int !Int
            --
            --      case x of { T a b -> T (a+1) b }
            --
            -- We really must record that b is already evaluated so that we don't
            -- go and re-evaluate it when constructing the result.
            -- See Note [Data-con worker strictness] in MkId.lhs
        add_evals the_strs
            = go vs the_strs
            where
              go [] [] = []
              go (v:vs') strs | isTyVar v = v : go vs' strs
              go (v:vs') (str:strs)
                | isMarkedStrict str = evald_v  : go vs' strs
                | otherwise          = zapped_v : go vs' strs
                where
                  zapped_v = zapIdOccInfo v   -- See Note [Case alternative occ info]
                  evald_v  = zapped_v `setIdUnfolding` evaldUnfolding
              go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)

    addAltUnfoldings :: SimplEnv -> Maybe OutExpr -> OutId -> OutExpr -> SimplM SimplEnv
    addAltUnfoldings env scrut case_bndr con_app
      = do { dflags <- getDynFlags
           ; let con_app_unf = mkSimpleUnfolding dflags con_app
                 env1 = addBinderUnfolding env case_bndr con_app_unf

                 -- See Note [Add unfolding for scrutinee]
                 env2 = case scrut of
                          Just (Var v)           -> addBinderUnfolding env1 v con_app_unf
                          Just (Cast (Var v) co) -> addBinderUnfolding env1 v $
                                                    mkSimpleUnfolding dflags (Cast con_app (mkSymCo co))
                          _                      -> env1

           ; traceSmpl "addAltUnf" (vcat [ppr case_bndr <+> ppr scrut, ppr con_app])
           ; return env2 }

    addBinderUnfolding :: SimplEnv -> Id -> Unfolding -> SimplEnv
    addBinderUnfolding env bndr unf
      | debugIsOn, Just tmpl <- maybeUnfoldingTemplate unf
      = WARN( not (eqType (idType bndr) (exprType tmpl)),
              ppr bndr $$ ppr (idType bndr) $$ ppr tmpl $$ ppr (exprType tmpl) )
        modifyInScope env (bndr `setIdUnfolding` unf)

      | otherwise
      = modifyInScope env (bndr `setIdUnfolding` unf)

    zapBndrOccInfo :: Bool -> Id -> Id
    -- Consider  case e of b { (a,b) -> ... }
    -- Then if we bind b to (a,b) in "...", and b is not dead,
    -- then we must zap the deadness info on a,b
    zapBndrOccInfo keep_occ_info pat_id
      | keep_occ_info = pat_id
      | otherwise     = zapIdOccInfo pat_id

    {-
    Note [Add unfolding for scrutinee]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general it's unlikely that a variable scrutinee will appear
    in the case alternatives   case x of { ...x unlikely to appear... }
    because the binder-swap in OccAnal has got rid of all such occcurrences
    See Note [Binder swap] in OccAnal.

    BUT it is still VERY IMPORTANT to add a suitable unfolding for a
    variable scrutinee, in simplAlt.  Here's why
       case x of y
         (a,b) -> case b of c
                    I# v -> ...(f y)...
    There is no occurrence of 'b' in the (...(f y)...).  But y gets
    the unfolding (a,b), and *that* mentions b.  If f has a RULE
        RULE f (p, I# q) = ...
    we want that rule to match, so we must extend the in-scope env with a
    suitable unfolding for 'y'.  It's *essential* for rule matching; but
    it's also good for case-elimintation -- suppose that 'f' was inlined
    and did multi-level case analysis, then we'd solve it in one
    simplifier sweep instead of two.

    Exactly the same issue arises in SpecConstr;
    see Note [Add scrutinee to ValueEnv too] in SpecConstr

    HOWEVER, given
      case x of y { Just a -> r1; Nothing -> r2 }
    we do not want to add the unfolding x -> y to 'x', which might seem cool,
    since 'y' itself has different unfoldings in r1 and r2.  Reason: if we
    did that, we'd have to zap y's deadness info and that is a very useful
    piece of information.

    So instead we add the unfolding x -> Just a, and x -> Nothing in the
    respective RHSs.

    ************************************************************************
    *                                                                      *
    \subsection{Known constructor}
    *                                                                      *
    ************************************************************************

    We are a bit careful with occurrence info.  Here's an example

            (\x* -> case x of (a*, b) -> f a) (h v, e)

    where the * means "occurs once".  This effectively becomes
            case (h v, e) of (a*, b) -> f a)
    and then
            let a* = h v; b = e in f a
    and then
            f (h v)

    All this should happen in one sweep.
    -}

    knownCon :: SimplEnv
             -> OutExpr                             -- The scrutinee
             -> DataCon -> [OutType] -> [OutExpr]   -- The scrutinee (in pieces)
             -> InId -> [InBndr] -> InExpr          -- The alternative
             -> SimplCont
             -> SimplM (SimplEnv, OutExpr)

    knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
      = do  { env'  <- bind_args env bs dc_args
            ; env'' <- bind_case_bndr env'
            ; simplExprF env'' rhs cont }
      where
        zap_occ = zapBndrOccInfo (isDeadBinder bndr)    -- bndr is an InId

                      -- Ugh!
        bind_args env' [] _  = return env'

        bind_args env' (b:bs') (Type ty : args)
          = ASSERT( isTyVar b )
            bind_args (extendTvSubst env' b ty) bs' args

        bind_args env' (b:bs') (arg : args)
          = ASSERT( isId b )
            do { let b' = zap_occ b
                 -- Note that the binder might be "dead", because it doesn't
                 -- occur in the RHS; and simplNonRecX may therefore discard
                 -- it via postInlineUnconditionally.
                 -- Nevertheless we must keep it if the case-binder is alive,
                 -- because it may be used in the con_app.  See Note [knownCon occ info]
               ; env'' <- simplNonRecX env' b' arg  -- arg satisfies let/app invariant
               ; bind_args env'' bs' args }

        bind_args _ _ _ =
          pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$
                                 text "scrut:" <+> ppr scrut

           -- It's useful to bind bndr to scrut, rather than to a fresh
           -- binding      x = Con arg1 .. argn
           -- because very often the scrut is a variable, so we avoid
           -- creating, and then subsequently eliminating, a let-binding
           -- BUT, if scrut is a not a variable, we must be careful
           -- about duplicating the arg redexes; in that case, make
           -- a new con-app from the args
        bind_case_bndr env
          | isDeadBinder bndr   = return env
          | exprIsTrivial scrut = return (extendIdSubst env bndr (DoneEx scrut))
          | otherwise           = do { dc_args <- mapM (simplVar env) bs
                                             -- dc_ty_args are aready OutTypes,
                                             -- but bs are InBndrs
                                     ; let con_app = Var (dataConWorkId dc)
                                                     `mkTyApps` dc_ty_args
                                                     `mkApps`   dc_args
                                     ; simplNonRecX env bndr con_app }

    -------------------
    missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr)
                    -- This isn't strictly an error, although it is unusual.
                    -- It's possible that the simplifer might "see" that
                    -- an inner case has no accessible alternatives before
                    -- it "sees" that the entire branch of an outer case is
                    -- inaccessible.  So we simply put an error case here instead.
    missingAlt env case_bndr _ cont
      = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr )
        return (env, mkImpossibleExpr (contResultType cont))

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Duplicating continuations}
    *                                                                      *
    ************************************************************************
    -}

    prepareCaseCont :: SimplEnv
                    -> [InAlt] -> SimplCont
                    -> SimplM (SimplEnv,
                               SimplCont,   -- Dupable part
                               SimplCont)   -- Non-dupable part
    -- We are considering
    --     K[case _ of { p1 -> r1; ...; pn -> rn }]
    -- where K is some enclosing continuation for the case
    -- Goal: split K into two pieces Kdup,Knodup so that
    --       a) Kdup can be duplicated
    --       b) Knodup[Kdup[e]] = K[e]
    -- The idea is that we'll transform thus:
    --          Knodup[ (case _ of { p1 -> Kdup[r1]; ...; pn -> Kdup[rn] }
    --
    -- We may also return some extra bindings in SimplEnv (that scope over
    -- the entire continuation)
    --
    -- When case-of-case is off, just make the entire continuation non-dupable

    prepareCaseCont env alts cont
      | not (sm_case_case (getMode env)) = return (env, mkBoringStop (contHoleType cont), cont)
      | not (many_alts alts)             = return (env, cont, mkBoringStop (contResultType cont))
      | otherwise                        = mkDupableCont env cont
      where
        many_alts :: [InAlt] -> Bool  -- True iff strictly > 1 non-bottom alternative
        many_alts []  = False         -- See Note [Bottom alternatives]
        many_alts [_] = False
        many_alts (alt:alts)
          | is_bot_alt alt = many_alts alts
          | otherwise      = not (all is_bot_alt alts)

        is_bot_alt (_,_,rhs) = exprIsBottom rhs

    {-
    Note [Bottom alternatives]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we have
         case (case x of { A -> error .. ; B -> e; C -> error ..)
           of alts
    then we can just duplicate those alts because the A and C cases
    will disappear immediately.  This is more direct than creating
    join points and inlining them away; and in some cases we would
    not even create the join points (see Note [Single-alternative case])
    and we would keep the case-of-case which is silly.  See Trac #4930.
    -}

    mkDupableCont :: SimplEnv -> SimplCont
                  -> SimplM (SimplEnv, SimplCont, SimplCont)

    mkDupableCont env cont
      | contIsDupable cont
      = return (env, cont, mkBoringStop (contResultType cont))

    mkDupableCont _   (Stop {}) = panic "mkDupableCont"     -- Handled by previous eqn

    mkDupableCont env (CastIt ty cont)
      = do  { (env', dup, nodup) <- mkDupableCont env cont
            ; return (env', CastIt ty dup, nodup) }

    -- Duplicating ticks for now, not sure if this is good or not
    mkDupableCont env cont@(TickIt{})
      = return (env, mkBoringStop (contHoleType cont), cont)

    mkDupableCont env cont@(StrictBind {})
      =  return (env, mkBoringStop (contHoleType cont), cont)
            -- See Note [Duplicating StrictBind]

    mkDupableCont env (StrictArg info cci cont)
            -- See Note [Duplicating StrictArg]
      = do { (env', dup, nodup) <- mkDupableCont env cont
           ; (env'', args')     <- mapAccumLM makeTrivialArg env' (ai_args info)
           ; return (env'', StrictArg (info { ai_args = args' }) cci dup, nodup) }

    mkDupableCont env cont@(ApplyToTy { sc_cont = tail })
      = do  { (env', dup_cont, nodup_cont) <- mkDupableCont env tail
            ; return (env', cont { sc_cont = dup_cont }, nodup_cont ) }

    mkDupableCont env (ApplyToVal { sc_arg = arg, sc_dup = dup, sc_env = se, sc_cont = cont })
      =     -- e.g.         [...hole...] (...arg...)
            --      ==>
            --              let a = ...arg...
            --              in [...hole...] a
        do  { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
            ; (_, se', arg') <- simplArg env' dup se arg
            ; (env'', arg'') <- makeTrivial NotTopLevel env' arg'
            ; let app_cont = ApplyToVal { sc_arg = arg'', sc_env = se'
                                        , sc_dup = OkToDup, sc_cont = dup_cont }
            ; return (env'', app_cont, nodup_cont) }

    mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
    --  See Note [Single-alternative case]
    --  | not (exprIsDupable rhs && contIsDupable case_cont)
    --  | not (isDeadBinder case_bndr)
      | all isDeadBinder bs  -- InIds
        && not (isUnLiftedType (idType case_bndr))
        -- Note [Single-alternative-unlifted]
      = return (env, mkBoringStop (contHoleType cont), cont)

    mkDupableCont env (Select _ case_bndr alts se cont)
      =     -- e.g.         (case [...hole...] of { pi -> ei })
            --      ===>
            --              let ji = \xij -> ei
            --              in case [...hole...] of { pi -> ji xij }
        do  { tick (CaseOfCase case_bndr)
            ; (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
                    -- NB: We call prepareCaseCont here.  If there is only one
                    -- alternative, then dup_cont may be big, but that's ok
                    -- because we push it into the single alternative, and then
                    -- use mkDupableAlt to turn that simplified alternative into
                    -- a join point if it's too big to duplicate.
                    -- And this is important: see Note [Fusing case continuations]

            ; let alt_env = se `setInScope` env'

            ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
            ; alts' <- mapM (simplAlt alt_env' Nothing [] case_bndr' dup_cont) alts
            -- Safe to say that there are no handled-cons for the DEFAULT case
                    -- NB: simplBinder does not zap deadness occ-info, so
                    -- a dead case_bndr' will still advertise its deadness
                    -- This is really important because in
                    --      case e of b { (# p,q #) -> ... }
                    -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
                    -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
                    -- In the new alts we build, we have the new case binder, so it must retain
                    -- its deadness.
            -- NB: we don't use alt_env further; it has the substEnv for
            --     the alternatives, and we don't want that

            ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
            ; return (env'',  -- Note [Duplicated env]
                      Select OkToDup case_bndr' alts'' (zapSubstEnv env'')
                             (mkBoringStop (contHoleType nodup_cont)),
                      nodup_cont) }

    mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
                  -> SimplM (SimplEnv, [InAlt])
    -- Absorbs the continuation into the new alternatives

    mkDupableAlts env case_bndr' the_alts
      = go env the_alts
      where
        go env0 [] = return (env0, [])
        go env0 (alt:alts)
            = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
                 ; (env2, alts') <- go env1 alts
                 ; return (env2, alt' : alts' ) }

    mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
                  -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
    mkDupableAlt env case_bndr (con, bndrs', rhs') = do
      dflags <- getDynFlags
      if exprIsDupable dflags rhs'  -- Note [Small alternative rhs]
       then return (env, (con, bndrs', rhs'))
       else
        do  { let rhs_ty'  = exprType rhs'
                  scrut_ty = idType case_bndr
                  case_bndr_w_unf
                    = case con of
                          DEFAULT    -> case_bndr
                          DataAlt dc -> setIdUnfolding case_bndr unf
                              where
                                     -- See Note [Case binders and join points]
                                 unf = mkInlineUnfolding Nothing rhs
                                 rhs = mkConApp2 dc (tyConAppArgs scrut_ty) bndrs'

                          LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt")
                                                    <+> ppr case_bndr <+> ppr con )
                                       case_bndr
                               -- The case binder is alive but trivial, so why has
                               -- it not been substituted away?

                  used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs'
                              | otherwise              = bndrs' ++ [case_bndr_w_unf]

                  abstract_over bndr
                      | isTyVar bndr = True -- Abstract over all type variables just in case
                      | otherwise    = not (isDeadBinder bndr)
                            -- The deadness info on the new Ids is preserved by simplBinders

            ; (final_bndrs', final_args)    -- Note [Join point abstraction]
                    <- if (any isId used_bndrs')
                       then return (used_bndrs', varsToCoreExprs used_bndrs')
                        else do { rw_id <- newId (fsLit "w") voidPrimTy
                                ; return ([setOneShotLambda rw_id], [Var voidPrimId]) }

            ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
                    -- Note [Funky mkPiTypes]

            ; let   -- We make the lambdas into one-shot-lambdas.  The
                    -- join point is sure to be applied at most once, and doing so
                    -- prevents the body of the join point being floated out by
                    -- the full laziness pass
                    really_final_bndrs     = map one_shot final_bndrs'
                    one_shot v | isId v    = setOneShotLambda v
                               | otherwise = v
                    join_rhs   = mkLams really_final_bndrs rhs'
                    join_arity = exprArity join_rhs
                    join_call  = mkApps (Var join_bndr) final_args

            ; env' <- addPolyBind NotTopLevel env (NonRec (join_bndr `setIdArity` join_arity) join_rhs)
            ; return (env', (con, bndrs', join_call)) }
                    -- See Note [Duplicated env]

    {-
    Note [Fusing case continuations]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's important to fuse two successive case continuations when the
    first has one alternative.  That's why we call prepareCaseCont here.
    Consider this, which arises from thunk splitting (see Note [Thunk
    splitting] in WorkWrap):

          let
            x* = case (case v of {pn -> rn}) of
                   I# a -> I# a
          in body

    The simplifier will find
        (Var v) with continuation
                Select (pn -> rn) (
                Select [I# a -> I# a] (
                StrictBind body Stop

    So we'll call mkDupableCont on
       Select [I# a -> I# a] (StrictBind body Stop)
    There is just one alternative in the first Select, so we want to
    simplify the rhs (I# a) with continuation (StricgtBind body Stop)
    Supposing that body is big, we end up with
              let $j a = <let x = I# a in body>
              in case v of { pn -> case rn of
                                     I# a -> $j a }
    This is just what we want because the rn produces a box that
    the case rn cancels with.

    See Trac #4957 a fuller example.

    Note [Case binders and join points]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this
       case (case .. ) of c {
         I# c# -> ....c....

    If we make a join point with c but not c# we get
      $j = \c -> ....c....

    But if later inlining scrutines the c, thus

      $j = \c -> ... case c of { I# y -> ... } ...

    we won't see that 'c' has already been scrutinised.  This actually
    happens in the 'tabulate' function in wave4main, and makes a significant
    difference to allocation.

    An alternative plan is this:

       $j = \c# -> let c = I# c# in ...c....

    but that is bad if 'c' is *not* later scrutinised.

    So instead we do both: we pass 'c' and 'c#' , and record in c's inlining
    (a stable unfolding) that it's really I# c#, thus

       $j = \c# -> \c[=I# c#] -> ...c....

    Absence analysis may later discard 'c'.

    NB: take great care when doing strictness analysis;
        see Note [Lamba-bound unfoldings] in DmdAnal.

    Also note that we can still end up passing stuff that isn't used.  Before
    strictness analysis we have
       let $j x y c{=(x,y)} = (h c, ...)
       in ...
    After strictness analysis we see that h is strict, we end up with
       let $j x y c{=(x,y)} = ($wh x y, ...)
    and c is unused.

    Note [Duplicated env]
    ~~~~~~~~~~~~~~~~~~~~~
    Some of the alternatives are simplified, but have not been turned into a join point
    So they *must* have an zapped subst-env.  So we can't use completeNonRecX to
    bind the join point, because it might to do PostInlineUnconditionally, and
    we'd lose that when zapping the subst-env.  We could have a per-alt subst-env,
    but zapping it (as we do in mkDupableCont, the Select case) is safe, and
    at worst delays the join-point inlining.

    Note [Small alternative rhs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It is worth checking for a small RHS because otherwise we
    get extra let bindings that may cause an extra iteration of the simplifier to
    inline back in place.  Quite often the rhs is just a variable or constructor.
    The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
    iterations because the version with the let bindings looked big, and so wasn't
    inlined, but after the join points had been inlined it looked smaller, and so
    was inlined.

    NB: we have to check the size of rhs', not rhs.
    Duplicating a small InAlt might invalidate occurrence information
    However, if it *is* dupable, we return the *un* simplified alternative,
    because otherwise we'd need to pair it up with an empty subst-env....
    but we only have one env shared between all the alts.
    (Remember we must zap the subst-env before re-simplifying something).
    Rather than do this we simply agree to re-simplify the original (small) thing later.

    Note [Funky mkPiTypes]
    ~~~~~~~~~~~~~~~~~~~~~~
    Notice the funky mkPiTypes.  If the contructor has existentials
    it's possible that the join point will be abstracted over
    type variables as well as term variables.
     Example:  Suppose we have
            data T = forall t.  C [t]
     Then faced with
            case (case e of ...) of
                C t xs::[t] -> rhs
     We get the join point
            let j :: forall t. [t] -> ...
                j = /\t \xs::[t] -> rhs
            in
            case (case e of ...) of
                C t xs::[t] -> j t xs

    Note [Join point abstraction]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Join points always have at least one value argument,
    for several reasons

    * If we try to lift a primitive-typed something out
      for let-binding-purposes, we will *caseify* it (!),
      with potentially-disastrous strictness results.  So
      instead we turn it into a function: \v -> e
      where v::Void#.  The value passed to this function is void,
      which generates (almost) no code.

    * CPR.  We used to say "&& isUnLiftedType rhs_ty'" here, but now
      we make the join point into a function whenever used_bndrs'
      is empty.  This makes the join-point more CPR friendly.
      Consider:       let j = if .. then I# 3 else I# 4
                      in case .. of { A -> j; B -> j; C -> ... }

      Now CPR doesn't w/w j because it's a thunk, so
      that means that the enclosing function can't w/w either,
      which is a lose.  Here's the example that happened in practice:
              kgmod :: Int -> Int -> Int
              kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
                          then 78
                          else 5

    * Let-no-escape.  We want a join point to turn into a let-no-escape
      so that it is implemented as a jump, and one of the conditions
      for LNE is that it's not updatable.  In CoreToStg, see
      Note [What is a non-escaping let]

    * Floating.  Since a join point will be entered once, no sharing is
      gained by floating out, but something might be lost by doing
      so because it might be allocated.

    I have seen a case alternative like this:
            True -> \v -> ...
    It's a bit silly to add the realWorld dummy arg in this case, making
            $j = \s v -> ...
               True -> $j s
    (the \v alone is enough to make CPR happy) but I think it's rare

    There's a slight infelicity here: we pass the overall
    case_bndr to all the join points if it's used in *any* RHS,
    because we don't know its usage in each RHS separately

    Note [Duplicating StrictArg]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The original plan had (where E is a big argument)
    e.g.    f E [..hole..]
            ==>     let $j = \a -> f E a
                    in $j [..hole..]

    But this is terrible! Here's an example:
            && E (case x of { T -> F; F -> T })
    Now, && is strict so we end up simplifying the case with

    an ArgOf continuation.  If we let-bind it, we get
            let $j = \v -> && E v
            in simplExpr (case x of { T -> F; F -> T })
                         (ArgOf (\r -> $j r)
    And after simplifying more we get
            let $j = \v -> && E v
            in case x of { T -> $j F; F -> $j T }
    Which is a Very Bad Thing

    What we do now is this
            f E [..hole..]
            ==>     let a = E
                    in f a [..hole..]
    Now if the thing in the hole is a case expression (which is when
    we'll call mkDupableCont), we'll push the function call into the
    branches, which is what we want.  Now RULES for f may fire, and
    call-pattern specialisation.  Here's an example from Trac #3116
         go (n+1) (case l of
                     1  -> bs'
                     _  -> Chunk p fpc (o+1) (l-1) bs')
    If we can push the call for 'go' inside the case, we get
    call-pattern specialisation for 'go', which is *crucial* for
    this program.

    Here is the (&&) example:
            && E (case x of { T -> F; F -> T })
      ==>   let a = E in
            case x of { T -> && a F; F -> && a T }
    Much better!

    Notice that
      * Arguments to f *after* the strict one are handled by
        the ApplyToVal case of mkDupableCont.  Eg
            f [..hole..] E

      * We can only do the let-binding of E because the function
        part of a StrictArg continuation is an explicit syntax
        tree.  In earlier versions we represented it as a function
        (CoreExpr -> CoreEpxr) which we couldn't take apart.

    Do *not* duplicate StrictBind and StritArg continuations.  We gain
    nothing by propagating them into the expressions, and we do lose a
    lot.

    The desire not to duplicate is the entire reason that
    mkDupableCont returns a pair of continuations.

    Note [Duplicating StrictBind]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Unlike StrictArg, there doesn't seem anything to gain from
    duplicating a StrictBind continuation, so we don't.

    Note [Single-alternative cases]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This case is just like the ArgOf case.  Here's an example:
            data T a = MkT !a
            ...(MkT (abs x))...
    Then we get
            case (case x of I# x' ->
                  case x' <# 0# of
                    True  -> I# (negate# x')
                    False -> I# x') of y {
              DEFAULT -> MkT y
    Because the (case x) has only one alternative, we'll transform to
            case x of I# x' ->
            case (case x' <# 0# of
                    True  -> I# (negate# x')
                    False -> I# x') of y {
              DEFAULT -> MkT y
    But now we do *NOT* want to make a join point etc, giving
            case x of I# x' ->
            let $j = \y -> MkT y
            in case x' <# 0# of
                    True  -> $j (I# (negate# x'))
                    False -> $j (I# x')
    In this case the $j will inline again, but suppose there was a big
    strict computation enclosing the orginal call to MkT.  Then, it won't
    "see" the MkT any more, because it's big and won't get duplicated.
    And, what is worse, nothing was gained by the case-of-case transform.

    So, in circumstances like these, we don't want to build join points
    and push the outer case into the branches of the inner one. Instead,
    don't duplicate the continuation.

    When should we use this strategy?  We should not use it on *every*
    single-alternative case:
      e.g.  case (case ....) of (a,b) -> (# a,b #)
    Here we must push the outer case into the inner one!
    Other choices:

       * Match [(DEFAULT,_,_)], but in the common case of Int,
         the alternative-filling-in code turned the outer case into
                    case (...) of y { I# _ -> MkT y }

       * Match on single alternative plus (not (isDeadBinder case_bndr))
         Rationale: pushing the case inwards won't eliminate the construction.
         But there's a risk of
                    case (...) of y { (a,b) -> let z=(a,b) in ... }
         Now y looks dead, but it'll come alive again.  Still, this
         seems like the best option at the moment.

       * Match on single alternative plus (all (isDeadBinder bndrs))
         Rationale: this is essentially  seq.

       * Match when the rhs is *not* duplicable, and hence would lead to a
         join point.  This catches the disaster-case above.  We can test
         the *un-simplified* rhs, which is fine.  It might get bigger or
         smaller after simplification; if it gets smaller, this case might
         fire next time round.  NB also that we must test contIsDupable
         case_cont *too, because case_cont might be big!

         HOWEVER: I found that this version doesn't work well, because
         we can get         let x = case (...) of { small } in ...case x...
         When x is inlined into its full context, we find that it was a bad
         idea to have pushed the outer case inside the (...) case.

    Note [Single-alternative-unlifted]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Here's another single-alternative where we really want to do case-of-case:

    data Mk1 = Mk1 Int# | Mk2 Int#

    M1.f =
        \r [x_s74 y_s6X]
            case
                case y_s6X of tpl_s7m {
                  M1.Mk1 ipv_s70 -> ipv_s70;
                  M1.Mk2 ipv_s72 -> ipv_s72;
                }
            of
            wild_s7c
            { __DEFAULT ->
                  case
                      case x_s74 of tpl_s7n {
                        M1.Mk1 ipv_s77 -> ipv_s77;
                        M1.Mk2 ipv_s79 -> ipv_s79;
                      }
                  of
                  wild1_s7b
                  { __DEFAULT -> ==# [wild1_s7b wild_s7c];
                  };
            };

    So the outer case is doing *nothing at all*, other than serving as a
    join-point.  In this case we really want to do case-of-case and decide
    whether to use a real join point or just duplicate the continuation:

        let $j s7c = case x of
                       Mk1 ipv77 -> (==) s7c ipv77
                       Mk1 ipv79 -> (==) s7c ipv79
        in
        case y of
          Mk1 ipv70 -> $j ipv70
          Mk2 ipv72 -> $j ipv72

    Hence: check whether the case binder's type is unlifted, because then
    the outer case is *not* a seq.

    ************************************************************************
    *                                                                      *
                        Unfoldings
    *                                                                      *
    ************************************************************************
    -}

    simplLetUnfolding :: SimplEnv-> TopLevelFlag
                      -> InId
                      -> OutExpr
                      -> Unfolding -> SimplM Unfolding
    simplLetUnfolding env top_lvl id new_rhs unf
      | isStableUnfolding unf
      = simplUnfolding env top_lvl id unf
      | otherwise
      = bottoming `seq`  -- See Note [Force bottoming field]
        do { dflags <- getDynFlags
           ; return (mkUnfolding dflags InlineRhs (isTopLevel top_lvl) bottoming new_rhs) }
                -- We make an  unfolding *even for loop-breakers*.
                -- Reason: (a) It might be useful to know that they are WHNF
                --         (b) In TidyPgm we currently assume that, if we want to
                --             expose the unfolding then indeed we *have* an unfolding
                --             to expose.  (We could instead use the RHS, but currently
                --             we don't.)  The simple thing is always to have one.
      where
        bottoming = isBottomingId id

    simplUnfolding :: SimplEnv-> TopLevelFlag -> InId -> Unfolding -> SimplM Unfolding
    -- Note [Setting the new unfolding]
    simplUnfolding env top_lvl id unf
      = case unf of
          NoUnfolding -> return unf
          OtherCon {} -> return unf

          DFunUnfolding { df_bndrs = bndrs, df_con = con, df_args = args }
            -> do { (env', bndrs') <- simplBinders rule_env bndrs
                  ; args' <- mapM (simplExpr env') args
                  ; return (mkDFunUnfolding bndrs' con args') }

          CoreUnfolding { uf_tmpl = expr, uf_src = src, uf_guidance = guide }
            | isStableSource src
            -> do { expr' <- simplExpr rule_env expr
                  ; case guide of
                      UnfWhen { ug_arity = arity, ug_unsat_ok = sat_ok }  -- Happens for INLINE things
                         -> let guide' = UnfWhen { ug_arity = arity, ug_unsat_ok = sat_ok
                                                 , ug_boring_ok = inlineBoringOk expr' }
                            -- Refresh the boring-ok flag, in case expr'
                            -- has got small. This happens, notably in the inlinings
                            -- for dfuns for single-method classes; see
                            -- Note [Single-method classes] in TcInstDcls.
                            -- A test case is Trac #4138
                            in return (mkCoreUnfolding src is_top_lvl expr' guide')
                                -- See Note [Top-level flag on inline rules] in CoreUnfold

                      _other              -- Happens for INLINABLE things
                         -> bottoming `seq` -- See Note [Force bottoming field]
                            do { dflags <- getDynFlags
                               ; return (mkUnfolding dflags src is_top_lvl bottoming expr') } }
                    -- If the guidance is UnfIfGoodArgs, this is an INLINABLE
                    -- unfolding, and we need to make sure the guidance is kept up
                    -- to date with respect to any changes in the unfolding.

            | otherwise -> return noUnfolding   -- Discard unstable unfoldings
      where
        bottoming = isBottomingId id
        is_top_lvl = isTopLevel top_lvl
        act        = idInlineActivation id
        rule_env   = updMode (updModeForStableUnfoldings act) env
                   -- See Note [Simplifying inside stable unfoldings] in SimplUtils

    {-
    Note [Force bottoming field]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We need to force bottoming, or the new unfolding holds
    on to the old unfolding (which is part of the id).

    Note [Setting the new unfolding]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    * If there's an INLINE pragma, we simplify the RHS gently.  Maybe we
      should do nothing at all, but simplifying gently might get rid of
      more crap.

    * If not, we make an unfolding from the new RHS.  But *only* for
      non-loop-breakers. Making loop breakers not have an unfolding at all
      means that we can avoid tests in exprIsConApp, for example.  This is
      important: if exprIsConApp says 'yes' for a recursive thing, then we
      can get into an infinite loop

    If there's an stable unfolding on a loop breaker (which happens for
    INLINEABLE), we hang on to the inlining.  It's pretty dodgy, but the
    user did say 'INLINE'.  May need to revisit this choice.

    ************************************************************************
    *                                                                      *
                        Rules
    *                                                                      *
    ************************************************************************

    Note [Rules in a letrec]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    After creating fresh binders for the binders of a letrec, we
    substitute the RULES and add them back onto the binders; this is done
    *before* processing any of the RHSs.  This is important.  Manuel found
    cases where he really, really wanted a RULE for a recursive function
    to apply in that function's own right-hand side.

    See Note [Loop breaking and RULES] in OccAnal.
    -}

    addBndrRules :: SimplEnv -> InBndr -> OutBndr -> SimplM (SimplEnv, OutBndr)
    -- Rules are added back into the bin
    addBndrRules env in_id out_id
      | null old_rules
      = return (env, out_id)
      | otherwise
      = do { new_rules <- mapM (simplRule env (Just (idName out_id))) old_rules
           ; let final_id  = out_id `setIdSpecialisation` mkSpecInfo new_rules
           ; return (modifyInScope env final_id, final_id) }
      where
        old_rules = specInfoRules (idSpecialisation in_id)

    simplRule :: SimplEnv -> Maybe Name -> CoreRule -> SimplM CoreRule
    simplRule _   _         rule@(BuiltinRule {}) = return rule
    simplRule env mb_new_nm rule@(Rule { ru_bndrs = bndrs, ru_args = args
                                       , ru_fn = fn_name, ru_rhs = rhs
                                       , ru_act = act })
      = do { (env, bndrs') <- simplBinders env bndrs
           ; let rule_env = updMode (updModeForStableUnfoldings act) env
           ; args' <- mapM (simplExpr rule_env) args
           ; rhs'  <- simplExpr rule_env rhs
           ; return (rule { ru_bndrs = bndrs'
                          , ru_fn    = mb_new_nm `orElse` fn_name
                          , ru_args  = args'
                          , ru_rhs   = rhs' }) }