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, simplRules ) where

    #include "HsVersions.h"

    import GhcPrelude

    import DynFlags
    import SimplMonad
    import Type hiding      ( substTy, substTyVar, extendTvSubst, extendCvSubst )
    import SimplEnv
    import SimplUtils
    import OccurAnal        ( occurAnalyseExpr )
    import FamInstEnv       ( FamInstEnv )
    import Literal          ( litIsLifted ) --, mkMachInt ) -- temporalily commented out. See #8326
    import Id
    import MkId             ( seqId )
    import MkCore           ( mkImpossibleExpr, castBottomExpr )
    import IdInfo
    import Name             ( Name, mkSystemVarName, isExternalName, getOccFS )
    import Coercion hiding  ( substCo, substCoVar )
    import OptCoercion      ( optCoercion )
    import FamInstEnv       ( topNormaliseType_maybe )
    import DataCon          ( DataCon, dataConWorkId, dataConRepStrictness, dataConRepArgTys )
    import CoreMonad        ( Tick(..), SimplMode(..) )
    import CoreSyn
    import Demand           ( StrictSig(..), dmdTypeDepth, isStrictDmd )
    import PprCore          ( pprCoreExpr )
    import CoreUnfold
    import CoreUtils
    import CoreOpt          ( pushCoTyArg, pushCoValArg
                            , joinPointBinding_maybe, joinPointBindings_maybe )
    import Rules            ( mkRuleInfo, lookupRule, getRules )
    import Demand           ( mkClosedStrictSig, topDmd, exnRes )
    import BasicTypes       ( TopLevelFlag(..), isNotTopLevel, isTopLevel,
                              RecFlag(..), Arity )
    import MonadUtils       ( mapAccumLM, liftIO )
    import Maybes           (  orElse )
    import Control.Monad
    import Outputable
    import FastString
    import Pair
    import Util
    import ErrUtils
    import Module          ( moduleName, pprModuleName )

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

    Note [The big picture]
    ~~~~~~~~~~~~~~~~~~~~~~
    The general shape of the simplifier is this:

      simplExpr :: SimplEnv -> InExpr -> SimplCont -> SimplM (SimplFloats, OutExpr)
      simplBind :: SimplEnv -> InBind -> SimplM (SimplFloats, SimplEnv)

     * SimplEnv contains
         - Simplifier mode (which includes DynFlags for convenience)
         - Ambient substitution
         - InScopeSet

     * SimplFloats contains
         - Let-floats (which includes ok-for-spec case-floats)
         - Join floats
         - InScopeSet (including all the floats)

     * Expressions
          simplExpr :: SimplEnv -> InExpr -> SimplCont
                    -> SimplM (SimplFloats, OutExpr)
       The result of simplifying an /expression/ is (floats, expr)
          - A bunch of floats (let bindings, join bindings)
          - A simplified expression.
       The overall result is effectively (let floats in expr)

     * Bindings
          simplBind :: SimplEnv -> InBind -> SimplM (SimplFloats, SimplEnv)
       The result of simplifying a binding is
         - A bunch of floats, the last of which is the simplified binding
           There may be auxiliary bindings too; see prepareRhs
         - An environment suitable for simplifying the scope of the binding

       The floats may also be empty, if the binding is inlined unconditionally;
       in that case the returned SimplEnv will have an augmented substitution.

       The returned floats and env both have an in-scope set, and they are
       guaranteed to be the same.

    Note [Shadowing]
    ~~~~~~~~~~~~~~~~
    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.

    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 (SimplFloats, SimplEnv)
    -- See Note [The big picture]
    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)
            ; (floats, env2) <- simpl_binds env1 binds0
            ; freeTick SimplifierDone
            ; return (floats, 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 (SimplFloats, SimplEnv)
        simpl_binds env []           = return (emptyFloats env, env)
        simpl_binds env (bind:binds) = do { (float,  env1) <- simpl_bind env bind
                                          ; (floats, env2) <- simpl_binds env1 binds
                                          ; return (float `addFloats` floats, env2) }

        simpl_bind env (Rec pairs)  = simplRecBind env TopLevel Nothing pairs
        simpl_bind env (NonRec b r) = do { (env', b') <- addBndrRules env b (lookupRecBndr env b)
                                         ; simplRecOrTopPair env' TopLevel
                                                             NonRecursive Nothing
                                                             b b' r }

    {-
    ************************************************************************
    *                                                                      *
            Lazy bindings
    *                                                                      *
    ************************************************************************

    simplRecBind is used for
            * recursive bindings only
    -}

    simplRecBind :: SimplEnv -> TopLevelFlag -> Maybe SimplCont
                 -> [(InId, InExpr)]
                 -> SimplM (SimplFloats, SimplEnv)
    simplRecBind env0 top_lvl mb_cont pairs0
      = do  { (env_with_info, triples) <- mapAccumLM add_rules env0 pairs0
            ; (rec_floats, env1) <- go env_with_info triples
            ; return (mkRecFloats rec_floats, 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 (emptyFloats env, env)

        go env ((old_bndr, new_bndr, rhs) : pairs)
            = do { (float, env1) <- simplRecOrTopPair env top_lvl Recursive mb_cont
                                                      old_bndr new_bndr rhs
                 ; (floats, env2) <- go env1 pairs
                 ; return (float `addFloats` floats, env2) }

    {-
    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 -> Maybe SimplCont
                      -> InId -> OutBndr -> InExpr  -- Binder and rhs
                      -> SimplM (SimplFloats, SimplEnv)

    simplRecOrTopPair env top_lvl is_rec mb_cont old_bndr new_bndr rhs
      | preInlineUnconditionally env top_lvl old_bndr rhs
      = trace_bind "pre-inline-uncond" $
        do { tick (PreInlineUnconditionally old_bndr)
           ; return ( emptyFloats env
                    , extendIdSubst env old_bndr (mkContEx env rhs)) }

      | Just cont <- mb_cont
      = ASSERT( isNotTopLevel top_lvl && isJoinId new_bndr )
        trace_bind "join" $
        simplJoinBind env cont old_bndr new_bndr rhs env

      | otherwise
      = trace_bind "normal" $
        simplLazyBind env top_lvl is_rec old_bndr new_bndr rhs env

      where
        dflags = seDynFlags env

        -- trace_bind emits a trace for each top-level binding, which
        -- helps to locate the tracing for inlining and rule firing
        trace_bind what thing_inside
          | not (dopt Opt_D_verbose_core2core dflags)
          = thing_inside
          | otherwise
          = pprTrace ("SimplBind " ++ what) (ppr old_bndr) thing_inside

    --------------------------
    simplLazyBind :: SimplEnv
                  -> TopLevelFlag -> RecFlag
                  -> InId -> OutId          -- Binder, both pre-and post simpl
                                            -- Not a JoinId
                                            -- The OutId has IdInfo, except arity, unfolding
                                            -- Ids only, no TyVars
                  -> InExpr -> SimplEnv     -- The RHS and its environment
                  -> SimplM (SimplFloats, SimplEnv)
    -- Precondition: not a JoinId
    -- Precondition: rhs obeys the let/app invariant
    -- NOT used for JoinIds
    simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
      = ASSERT( isId bndr )
        ASSERT2( not (isJoinId bndr), ppr bndr )
        -- pprTrace "simplLazyBind" ((ppr bndr <+> ppr bndr1) $$ ppr rhs $$ ppr (seIdSubst rhs_se)) $
        do  { let   rhs_env     = rhs_se `setInScopeFromE` env
                    (tvs, body) = case collectTyAndValBinders rhs of
                                    (tvs, [], body)
                                      | surely_not_lam body -> (tvs, body)
                                    _                       -> ([], rhs)

                    surely_not_lam (Lam {})     = False
                    surely_not_lam (Tick t e)
                      | not (tickishFloatable t) = surely_not_lam e
                       -- eta-reduction could float
                    surely_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_floats0, body0) <- simplExprF body_env body rhs_cont

                  -- Never float join-floats out of a non-join let-binding
                  -- So wrap the body in the join-floats right now
                  -- Henc: body_floats1 consists only of let-floats
            ; let (body_floats1, body1) = wrapJoinFloatsX body_floats0 body0

            -- ANF-ise a constructor or PAP rhs
            -- We get at most one float per argument here
            ; (let_floats, body2) <- prepareRhs (getMode env) top_lvl
                                                (getOccFS bndr1) (idInfo bndr1) body1
            ; let body_floats2 = body_floats1 `addLetFloats` let_floats

            ; (rhs_floats, rhs')
                <-  if not (doFloatFromRhs top_lvl is_rec False body_floats2 body2)
                    then                    -- No floating, revert to body1
                         do { rhs' <- mkLam env tvs' (wrapFloats body_floats2 body1) rhs_cont
                            ; return (emptyFloats env, rhs') }

                    else if null tvs then   -- Simple floating
                         do { tick LetFloatFromLet
                            ; return (body_floats2, body2) }

                    else                    -- Do type-abstraction first
                         do { tick LetFloatFromLet
                            ; (poly_binds, body3) <- abstractFloats (seDynFlags env) top_lvl
                                                                    tvs' body_floats2 body2
                            ; let floats = foldl extendFloats (emptyFloats env) poly_binds
                            ; rhs' <- mkLam env tvs' body3 rhs_cont
                            ; return (floats, rhs') }

            ; (bind_float, env2) <- completeBind (env `setInScopeFromF` rhs_floats)
                                                 top_lvl Nothing bndr bndr1 rhs'
            ; return (rhs_floats `addFloats` bind_float, env2) }

    --------------------------
    simplJoinBind :: SimplEnv
                  -> SimplCont
                  -> InId -> OutId          -- Binder, both pre-and post simpl
                                            -- The OutId has IdInfo, except arity,
                                            --   unfolding
                  -> InExpr -> SimplEnv     -- The right hand side and its env
                  -> SimplM (SimplFloats, SimplEnv)
    simplJoinBind env cont old_bndr new_bndr rhs rhs_se
      = do  { let rhs_env = rhs_se `setInScopeFromE` env
            ; rhs' <- simplJoinRhs rhs_env old_bndr rhs cont
            ; completeBind env NotTopLevel (Just cont) old_bndr new_bndr rhs' }

    --------------------------
    simplNonRecX :: SimplEnv
                 -> InId            -- Old binder; not a JoinId
                 -> OutExpr         -- Simplified RHS
                 -> SimplM (SimplFloats, SimplEnv)
    -- A specialised variant of simplNonRec used when the RHS is already
    -- simplified, notably in knownCon.  It uses case-binding where necessary.
    --
    -- Precondition: rhs satisfies the let/app invariant

    simplNonRecX env bndr new_rhs
      | ASSERT2( not (isJoinId bndr), ppr bndr )
        isDeadBinder bndr   -- Not uncommon; e.g. case (a,b) of c { (p,q) -> p }
      = return (emptyFloats env, env)    --  Here c is dead, and we avoid
                                             --  creating the binding c = (a,b)

      | Coercion co <- new_rhs
      = return (emptyFloats env, 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; not a JoinId
                    -> OutId                -- New binder
                    -> OutExpr              -- Simplified RHS
                    -> SimplM (SimplFloats, SimplEnv)    -- The new binding is in the floats
    -- 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
      = ASSERT2( not (isJoinId new_bndr), ppr new_bndr )
        do  { (prepd_floats, rhs1) <- prepareRhs (getMode env) top_lvl (getOccFS new_bndr)
                                                 (idInfo new_bndr) new_rhs
            ; let floats = emptyFloats env `addLetFloats` prepd_floats
            ; (rhs_floats, rhs2) <-
                    if doFloatFromRhs NotTopLevel NonRecursive is_strict floats rhs1
                    then    -- Add the floats to the main env
                         do { tick LetFloatFromLet
                            ; return (floats, rhs1) }
                    else    -- Do not float; wrap the floats around the RHS
                         return (emptyFloats env, wrapFloats floats rhs1)

            ; (bind_float, env2) <- completeBind (env `setInScopeFromF` rhs_floats)
                                                 NotTopLevel Nothing
                                                 old_bndr new_bndr rhs2
            ; return (rhs_floats `addFloats` bind_float, env2) }

    {- *********************************************************************
    *                                                                      *
               prepareRhs, makeTrivial
    *                                                                      *
    ************************************************************************

    Note [prepareRhs]
    ~~~~~~~~~~~~~~~~~
    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 :: SimplMode -> TopLevelFlag
               -> FastString   -- Base for any new variables
               -> IdInfo       -- IdInfo for the LHS of this binding
               -> OutExpr
               -> SimplM (LetFloats, OutExpr)
    -- Transforms a RHS into a better RHS by adding floats
    -- e.g        x = Just e
    -- becomes    a = e
    --            x = Just a
    -- See Note [prepareRhs]
    prepareRhs mode top_lvl occ info (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  { (floats, rhs') <- makeTrivialWithInfo mode top_lvl occ sanitised_info rhs
            ; return (floats, Cast rhs' co) }
      where
        sanitised_info = vanillaIdInfo `setStrictnessInfo` strictnessInfo info
                                       `setDemandInfo`     demandInfo info

    prepareRhs mode top_lvl occ _ rhs0
      = do  { (_is_exp, floats, rhs1) <- go 0 rhs0
            ; return (floats, rhs1) }
      where
        go :: Int -> OutExpr -> SimplM (Bool, LetFloats, OutExpr)
        go n_val_args (Cast rhs co)
            = do { (is_exp, floats, rhs') <- go n_val_args rhs
                 ; return (is_exp, floats, Cast rhs' co) }
        go n_val_args (App fun (Type ty))
            = do { (is_exp, floats, rhs') <- go n_val_args fun
                 ; return (is_exp, floats, App rhs' (Type ty)) }
        go n_val_args (App fun arg)
            = do { (is_exp, floats1, fun') <- go (n_val_args+1) fun
                 ; case is_exp of
                    False -> return (False, emptyLetFloats, App fun arg)
                    True  -> do { (floats2, arg') <- makeTrivial mode top_lvl occ arg
                                ; return (True, floats1 `addLetFlts` floats2, App fun' arg') } }
        go n_val_args (Var fun)
            = return (is_exp, emptyLetFloats, 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 (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, floats, rhs') <- go n_val_args rhs
                 ; return (is_exp, floats, 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, floats, rhs') <- go n_val_args rhs
                 ; let tickIt (id, expr)
                           -- we have to take care not to tick top-level literal
                           -- strings. See Note [CoreSyn top-level string literals].
                         | isTopLevel top_lvl && exprIsLiteralString expr
                         = (id, expr)
                         | otherwise
                         = (id, mkTick (mkNoCount t) expr)
                       floats' = mapLetFloats floats tickIt
                 ; return (is_exp, floats', Tick t rhs') }

        go _ other
            = return (False, emptyLetFloats, 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 coercion to the usage site may well cancel the coercions
    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 :: SimplMode -> ArgSpec -> SimplM (LetFloats, ArgSpec)
    makeTrivialArg mode (ValArg e)
      = do { (floats, e') <- makeTrivial mode NotTopLevel (fsLit "arg") e
           ; return (floats, ValArg e') }
    makeTrivialArg _ arg
      = return (emptyLetFloats, arg)  -- CastBy, TyArg

    makeTrivial :: SimplMode -> TopLevelFlag
                -> FastString  -- ^ A "friendly name" to build the new binder from
                -> OutExpr     -- ^ This expression satisfies the let/app invariant
                -> SimplM (LetFloats, OutExpr)
    -- Binds the expression to a variable, if it's not trivial, returning the variable
    makeTrivial mode top_lvl context expr
     = makeTrivialWithInfo mode top_lvl context vanillaIdInfo expr

    makeTrivialWithInfo :: SimplMode -> TopLevelFlag
                        -> FastString  -- ^ a "friendly name" to build the new binder from
                        -> IdInfo
                        -> OutExpr     -- ^ This expression satisfies the let/app invariant
                        -> SimplM (LetFloats, 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 mode top_lvl occ_fs info expr
      | exprIsTrivial expr                          -- Already trivial
      || not (bindingOk top_lvl expr expr_ty)       -- Cannot trivialise
                                                    --   See Note [Cannot trivialise]
      = return (emptyLetFloats, expr)

      | otherwise
      = do  { (floats, expr1) <- prepareRhs mode top_lvl occ_fs info expr
            ; if   exprIsTrivial expr1  -- See Note [Trivial after prepareRhs]
              then return (floats, expr1)
              else do
            { uniq <- getUniqueM
            ; let name = mkSystemVarName uniq occ_fs
                  var  = mkLocalIdOrCoVarWithInfo name expr_ty info

            -- Now something very like completeBind,
            -- but without the postInlineUnconditinoally part
            ; (arity, is_bot, expr2) <- tryEtaExpandRhs mode var expr1
            ; unf <- mkLetUnfolding (sm_dflags mode) top_lvl InlineRhs var expr2

            ; let final_id = addLetBndrInfo var arity is_bot unf
                  bind     = NonRec final_id expr2

            ; return ( floats `addLetFlts` unitLetFloat bind, Var final_id ) }}
       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 expr_ty
      | isTopLevel top_lvl = exprIsTopLevelBindable expr expr_ty
      | otherwise          = True

    {- Note [Trivial after prepareRhs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we call makeTrival on (e |> co), the recursive use of prepareRhs
    may leave us with
       { a1 = e }  and   (a1 |> co)
    Now the latter is trivial, so we don't want to let-bind it.

    Note [Cannot trivialise]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Consider:
       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.

    Literal strings are an exception.

       foo = Ptr "blob"#

    We want to turn this into:

       foo1 = "blob"#
       foo = Ptr foo1

    See Note [CoreSyn top-level string literals] in CoreSyn.

    ************************************************************************
    *                                                                      *
              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
                 -> Maybe SimplCont         -- Required only for join point
                 -> InId                    -- Old binder
                 -> OutId -> OutExpr        -- New binder and RHS
                 -> SimplM (SimplFloats, 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
    --
    -- Binder /can/ be a JoinId
    -- Precondition: rhs obeys the let/app invariant
    completeBind env top_lvl mb_cont old_bndr new_bndr new_rhs
     | isCoVar old_bndr
     = case new_rhs of
         Coercion co -> return (emptyFloats env, extendCvSubst env old_bndr co)
         _           -> return (mkFloatBind env (NonRec 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, is_bot, final_rhs) <- tryEtaExpandRhs (getMode env)
                                                              new_bndr new_rhs

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

          ; let final_bndr = addLetBndrInfo new_bndr new_arity is_bot new_unfolding

          ; if postInlineUnconditionally env top_lvl final_bndr occ_info final_rhs

            then -- Inline and discard the binding
                 do  { tick (PostInlineUnconditionally old_bndr)
                     ; return ( emptyFloats env
                              , extendIdSubst env old_bndr $
                                DoneEx final_rhs (isJoinId_maybe new_bndr)) }
                    -- Use the substitution to make quite, quite sure that the
                    -- substitution will happen, since we are going to discard the binding

            else -- Keep the binding
                 -- pprTrace "Binding" (ppr final_bndr <+> ppr new_unfolding) $
                 return (mkFloatBind env (NonRec final_bndr final_rhs)) }

    addLetBndrInfo :: OutId -> Arity -> Bool -> Unfolding -> OutId
    addLetBndrInfo new_bndr new_arity is_bot new_unf
      = new_bndr `setIdInfo` info5
      where
        info1 = idInfo new_bndr `setArityInfo` new_arity

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

        -- 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_unf
                || (case strictnessInfo info2 of
                      StrictSig dmd_ty -> new_arity < dmdTypeDepth dmd_ty)
              = zapDemandInfo info2 `orElse` info2
              | otherwise
              = info2

        -- Bottoming bindings: see Note [Bottoming bindings]
        info4 | is_bot    = info3 `setStrictnessInfo`
                            mkClosedStrictSig (replicate new_arity topDmd) exnRes
              | otherwise = info3

         -- Zap call arity info. We have used it by now (via
         -- `tryEtaExpandRhs`), and the simplifier can invalidate this
         -- information, leading to broken code later (e.g. #13479)
        info5 = zapCallArityInfo info4

    {- 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 [Bottoming bindings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have
       let x = error "urk"
       in ...(case x of <alts>)...
    or
       let f = \x. error (x ++ "urk")
       in ...(case f "foo" of <alts>)...

    Then we'd like to drop the dead <alts> immediately.  So it's good to
    propagate the info that x's RHS is bottom to x's IdInfo as rapidly as
    possible.

    We use tryEtaExpandRhs on every binding, and it turns ou that the
    arity computation it performs (via CoreArity.findRhsArity) already
    does a simple bottoming-expression analysis.  So all we need to do
    is propagate that info to the binder's IdInfo.

    This showed up in Trac #12150; see comment:16.

    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 (Type ty)
      = do { ty' <- simplType env ty  -- See Note [Avoiding space leaks in OutType]
           ; return (Type ty') }

    simplExpr env expr
      = simplExprC env expr (mkBoringStop expr_out_ty)
      where
        expr_out_ty :: OutType
        expr_out_ty = substTy env (exprType expr)
        -- NB: Since 'expr' is term-valued, not (Type ty), this call
        --     to exprType will succeed.  exprType fails on (Type ty).

    simplExprC :: SimplEnv
               -> InExpr     -- A term-valued expression, never (Type ty)
               -> SimplCont
               -> SimplM OutExpr
            -- Simplify an expression, given a continuation
    simplExprC env expr cont
      = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seLetFloats env) ) $
        do  { (floats, expr') <- simplExprF env expr cont
            ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
              -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
              -- pprTrace "simplExprC ret4" (ppr (seLetFloats env')) $
              return (wrapFloats floats expr') }

    --------------------------------------------------
    simplExprF :: SimplEnv
               -> InExpr     -- A term-valued expression, never (Type ty)
               -> SimplCont
               -> SimplM (SimplFloats, 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)
          ]) $ -}
        simplExprF1 env e cont

    simplExprF1 :: SimplEnv -> InExpr -> SimplCont
                -> SimplM (SimplFloats, OutExpr)

    simplExprF1 _ (Type ty) _
      = pprPanic "simplExprF: type" (ppr ty)
        -- simplExprF does only with term-valued expressions
        -- The (Type ty) case is handled separately by simplExpr
        -- and by the other callers of simplExprF

    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 (App fun arg) cont
      = case arg of
          Type ty -> do { -- The argument type will (almost) certainly be used
                          -- in the output program, so just force it now.
                          -- See Note [Avoiding space leaks in OutType]
                          arg' <- simplType env ty

                          -- But use substTy, not simplType, to avoid forcing
                          -- the hole type; it will likely not be needed.
                          -- See Note [The hole type in ApplyToTy]
                        ; let hole' = substTy env (exprType fun)

                        ; simplExprF env fun $
                          ApplyToTy { sc_arg_ty  = arg'
                                    , sc_hole_ty = hole'
                                    , sc_cont    = cont } }
          _       -> simplExprF env fun $
                     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 { sc_dup = NoDup, sc_bndr = bndr
                                     , sc_alts = alts
                                     , sc_env = env, sc_cont = cont })

    simplExprF1 env (Let (Rec pairs) body) cont
      | Just pairs' <- joinPointBindings_maybe pairs
      = simplRecJoinPoint env pairs' body cont

      | otherwise
      = simplRecE env pairs body cont

    simplExprF1 env (Let (NonRec bndr rhs) body) cont
      | Type ty <- rhs    -- First deal with type lets (let a = Type ty in e)
      = ASSERT( isTyVar bndr )
        do { ty' <- simplType env ty
           ; simplExprF (extendTvSubst env bndr ty') body cont }

      | Just (bndr', rhs') <- joinPointBinding_maybe bndr rhs
      = simplNonRecJoinPoint env bndr' rhs' body cont

      | otherwise
      = simplNonRecE env bndr (rhs, env) ([], body) cont

    {- Note [Avoiding space leaks in OutType]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Since the simplifier is run for multiple iterations, we need to ensure
    that any thunks in the output of one simplifier iteration are forced
    by the evaluation of the next simplifier iteration. Otherwise we may
    retain multiple copies of the Core program and leak a terrible amount
    of memory (as in #13426).

    The simplifier is naturally strict in the entire "Expr part" of the
    input Core program, because any expression may contain binders, which
    we must find in order to extend the SimplEnv accordingly. But types
    do not contain binders and so it is tempting to write things like

        simplExpr env (Type ty) = return (Type (substTy env ty))   -- Bad!

    This is Bad because the result includes a thunk (substTy env ty) which
    retains a reference to the whole simplifier environment; and the next
    simplifier iteration will not force this thunk either, because the
    line above is not strict in ty.

    So instead our strategy is for the simplifier to fully evaluate
    OutTypes when it emits them into the output Core program, for example

        simplExpr env (Type ty) = do { ty' <- simplType env ty     -- Good
                                     ; return (Type ty') }

    where the only difference from above is that simplType calls seqType
    on the result of substTy.

    However, SimplCont can also contain OutTypes and it's not necessarily
    a good idea to force types on the way in to SimplCont, because they
    may end up not being used and forcing them could be a lot of wasted
    work. T5631 is a good example of this.

    - For ApplyToTy's sc_arg_ty, we force the type on the way in because
      the type will almost certainly appear as a type argument in the
      output program.

    - For the hole types in Stop and ApplyToTy, we force the type when we
      emit it into the output program, after obtaining it from
      contResultType. (The hole type in ApplyToTy is only directly used
      to form the result type in a new Stop continuation.)
    -}

    ---------------------------------
    -- Simplify a join point, adding the context.
    -- Context goes *inside* the lambdas. IOW, if the join point has arity n, we do:
    --   \x1 .. xn -> e => \x1 .. xn -> E[e]
    -- Note that we need the arity of the join point, since e may be a lambda
    -- (though this is unlikely). See Note [Case-of-case and join points].
    simplJoinRhs :: SimplEnv -> InId -> InExpr -> SimplCont
                 -> SimplM OutExpr
    simplJoinRhs env bndr expr cont
      | Just arity <- isJoinId_maybe bndr
      =  do { let (join_bndrs, join_body) = collectNBinders arity expr
            ; (env', join_bndrs') <- simplLamBndrs env join_bndrs
            ; join_body' <- simplExprC env' join_body cont
            ; return $ mkLams join_bndrs' join_body' }

      | otherwise
      = pprPanic "simplJoinRhs" (ppr bndr)

    ---------------------------------
    simplType :: SimplEnv -> InType -> SimplM OutType
            -- Kept monadic just so we can do the seqType
            -- See Note [Avoiding space leaks in OutType]
    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 (SimplFloats, 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 (getTCvSubst 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 (SimplFloats, 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 { (floats, expr') <- simplExprF env expr cont
           ; return (floats, 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
           ; (floats, expr1) <- simplExprF env expr inc
           ; let expr2    = wrapFloats floats expr1
                 tickish' = simplTickish env tickish
           ; rebuild env (mkTick tickish' expr2) outc
           }

    -- 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 (SimplFloats, OutExpr)
    -- At this point the substitution in the SimplEnv should be irrelevant;
    -- only the in-scope set matters
    rebuild env expr cont
      = case cont of
          Stop {}          -> return (emptyFloats 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 { sc_bndr = bndr, sc_alts = alts, sc_env = se, sc_cont = cont }
            -> rebuildCase (se `setInScopeFromE` env) expr bndr alts cont

          StrictArg { sc_fun = fun, sc_cont = cont }
            -> rebuildCall env (fun `addValArgTo` expr) cont
          StrictBind { sc_bndr = b, sc_bndrs = bs, sc_body = body
                     , sc_env = se, sc_cont = cont }
            -> do { (floats1, env') <- simplNonRecX (se `setInScopeFromE` env) b expr
                                      -- expr satisfies let/app since it started life
                                      -- in a call to simplNonRecE
                  ; (floats2, expr') <- simplLam env' bs body cont
                  ; return (floats1 `addFloats` floats2, expr') }

          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]
            -> do { (_, _, arg') <- simplArg env dup_flag se arg
                  ; rebuild env (App expr arg') cont }

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

    simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
              -> SimplM (SimplFloats, OutExpr)
    simplCast env body co0 cont0
      = do  { co1   <- simplCoercion env co0
            ; cont1 <- addCoerce co1 cont0
            ; simplExprF env body cont1 }
      where
           addCoerce :: OutCoercion -> SimplCont -> SimplM SimplCont
           addCoerce co1 (CastIt co2 cont)
             = addCoerce (mkTransCo co1 co2) cont

           addCoerce co cont@(ApplyToTy { sc_arg_ty = arg_ty, sc_cont = tail })
             | Just (arg_ty', co') <- pushCoTyArg co arg_ty
             = do { tail' <- addCoerce co' tail
                  ; return (cont { sc_arg_ty = arg_ty', sc_cont = tail' }) }

           addCoerce co cont@(ApplyToVal { sc_arg = arg, sc_env = arg_se
                                    , sc_dup = dup, sc_cont = tail })
             | Just (co1, co2) <- pushCoValArg co
             , Pair _ new_ty <- coercionKind co1
             , not (isTypeLevPoly new_ty)  -- without this check, we get a lev-poly arg
                                           -- See Note [Levity polymorphism invariants] in CoreSyn
                                           -- test: typecheck/should_run/EtaExpandLevPoly
             = do { tail' <- addCoerce co2 tail
                  ; if isReflCo co1
                    then return (cont { sc_cont = tail' })
                         -- Avoid simplifying if possible;
                         -- See Note [Avoiding exponential behaviour]
                    else do
                  { (dup', arg_se', arg') <- simplArg env dup arg_se arg
                       -- When we build the ApplyTo we can't mix the OutCoercion
                       -- 'co' with the InExpr 'arg', so we simplify
                       -- to make it all consistent.  It's a bit messy.
                       -- But it isn't a common case.
                       -- Example of use: Trac #995
                  ; return (ApplyToVal { sc_arg  = mkCast arg' co1
                                       , sc_env  = arg_se'
                                       , sc_dup  = dup'
                                       , sc_cont = tail' }) } }

           addCoerce co cont
             | isReflexiveCo co = return cont
             | otherwise        = return (CastIt co cont)
                    -- It's worth checking isReflexiveCo.
                    -- 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

    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 `setInScopeFromE` env) arg
           ; return (Simplified, zapSubstEnv arg_env, arg') }

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

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

    simplLam env [] body cont
      = simplExprF env body cont

    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, sc_dup = dup })
      | isSimplified dup  -- Don't re-simplify if we've simplified it once
                          -- See Note [Avoiding exponential behaviour]
      = do  { tick (BetaReduction bndr)
            ; (floats1, env') <- simplNonRecX env zapped_bndr arg
            ; (floats2, expr') <- simplLam env' bndrs body cont
            ; return (floats1 `addFloats` floats2, expr') }

      | otherwise
      = do  { tick (BetaReduction bndr)
            ; simplNonRecE env zapped_bndr (arg, arg_se) (bndrs, body) cont }
      where
        zapped_bndr  -- See Note [Zap unfolding when beta-reducing]
          | isId bndr = zapStableUnfolding bndr
          | 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 env bndrs' body' cont
            ; rebuild env' new_lam cont }

    -------------
    simplLamBndr :: SimplEnv -> InBndr -> SimplM (SimplEnv, OutBndr)
    -- 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 && isFragileUnfolding old_unf   -- Special case
      = do { (env1, bndr1) <- simplBinder env bndr
           ; unf'          <- simplStableUnfolding env1 NotTopLevel Nothing bndr old_unf
           ; let bndr2 = bndr1 `setIdUnfolding` unf'
           ; return (modifyInScope env1 bndr2, bndr2) }

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

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

    ------------------
    simplNonRecE :: SimplEnv
                 -> InId                    -- The binder, always an Id
                                            -- Never a join point
                 -> (InExpr, SimplEnv)      -- Rhs of binding (or arg of lambda)
                 -> ([InBndr], InExpr)      -- Body of the let/lambda
                                            --      \xs.e
                 -> SimplCont
                 -> SimplM (SimplFloats, OutExpr)

    -- simplNonRecE is used for
    --  * non-top-level non-recursive non-join-point lets in expressions
    --  * beta reduction
    --
    -- simplNonRec env b (rhs, rhs_se) (bs, body) k
    --   = let env in
    --     cont< let b = rhs_se(rhs) in \bs.body >
    --
    -- 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 ...)

    simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
      | ASSERT( isId bndr && not (isJoinId bndr) )
        preInlineUnconditionally 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 }

      -- Deal with strict bindings
      | isStrictId bndr          -- Includes coercions
      , sm_case_case (getMode env)
      = simplExprF (rhs_se `setInScopeFromE` env) rhs
                   (StrictBind { sc_bndr = bndr, sc_bndrs = bndrs, sc_body = body
                               , sc_env = env, sc_cont = cont, sc_dup = NoDup })

      -- Deal with lazy bindings
      | otherwise
      = ASSERT( not (isTyVar bndr) )
        do { (env1, bndr1) <- simplNonRecBndr env bndr
           ; (env2, bndr2) <- addBndrRules env1 bndr bndr1
           ; (floats1, env3) <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
           ; (floats2, expr') <- simplLam env3 bndrs body cont
           ; return (floats1 `addFloats` floats2, expr') }

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

    -- simplRecE is used for
    --  * non-top-level recursive lets in expressions
    simplRecE env pairs body cont
      = do  { let bndrs = map fst pairs
            ; MASSERT(all (not . isJoinId) bndrs)
            ; env1 <- simplRecBndrs env bndrs
                    -- NB: bndrs' don't have unfoldings or rules
                    -- We add them as we go down
            ; (floats1, env2) <- simplRecBind env1 NotTopLevel Nothing pairs
            ; (floats2, expr') <- simplExprF env2 body cont
            ; return (floats1 `addFloats` floats2, expr') }

    {- Note [Avoiding exponential behaviour]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    One way in which we can get exponential behaviour is if we simplify a
    big expression, and the re-simplify it -- and then this happens in a
    deeply-nested way.  So we must be jolly careful about re-simplifying
    an expression.  That is why completeNonRecX does not try
    preInlineUnconditionally.

    Example:
      f BIG, where f has a RULE
    Then
     * We simplify BIG before trying the rule; but the rule does not fire
     * We inline f = \x. x True
     * So if we did preInlineUnconditionally we'd re-simplify (BIG True)

    However, if BIG has /not/ already been simplified, we'd /like/ to
    simplify BIG True; maybe good things happen.  That is why

    * simplLam has
        - a case for (isSimplified dup), which goes via simplNonRecX, and
        - a case for the un-simplified case, which goes via simplNonRecE

    * We go to some efforts to avoid unnecessarily simplifying ApplyToVal,
      in at least two places
        - In simplCast/addCoerce, where we check for isReflCo
        - In rebuildCall we avoid simplifying arguments before we have to
          (see Note [Trying rewrite rules])

    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.

    ************************************************************************
    *                                                                      *
                         Join points
    *                                                                      *
    ********************************************************************* -}

    simplNonRecJoinPoint :: SimplEnv -> InId -> InExpr
                         -> InExpr -> SimplCont
                         -> SimplM (SimplFloats, OutExpr)
    simplNonRecJoinPoint env bndr rhs body cont
      | ASSERT( isJoinId bndr )
        preInlineUnconditionally env NotTopLevel bndr rhs
      = do { tick (PreInlineUnconditionally bndr)
           ; simplExprF (extendIdSubst env bndr (mkContEx env rhs)) body cont }

       | otherwise
       = wrapJoinCont env cont $ \ env cont ->
         do { -- We push join_cont into the join RHS and the body;
              -- and wrap wrap_cont around the whole thing
            ; let res_ty = contResultType cont
            ; (env1, bndr1)    <- simplNonRecJoinBndr env res_ty bndr
            ; (env2, bndr2)    <- addBndrRules env1 bndr bndr1
            ; (floats1, env3)  <- simplJoinBind env2 cont bndr bndr2 rhs env
            ; (floats2, body') <- simplExprF env3 body cont
            ; return (floats1 `addFloats` floats2, body') }

    ------------------
    simplRecJoinPoint :: SimplEnv -> [(InId, InExpr)]
                      -> InExpr -> SimplCont
                      -> SimplM (SimplFloats, OutExpr)
    simplRecJoinPoint env pairs body cont
      = wrapJoinCont env cont $ \ env cont ->
        do { let bndrs = map fst pairs
                 res_ty = contResultType cont
           ; env1 <- simplRecJoinBndrs env res_ty bndrs
                   -- NB: bndrs' don't have unfoldings or rules
                   -- We add them as we go down
           ; (floats1, env2)  <- simplRecBind env1 NotTopLevel (Just cont) pairs
           ; (floats2, body') <- simplExprF env2 body cont
           ; return (floats1 `addFloats` floats2, body') }

    --------------------
    wrapJoinCont :: SimplEnv -> SimplCont
                 -> (SimplEnv -> SimplCont -> SimplM (SimplFloats, OutExpr))
                 -> SimplM (SimplFloats, OutExpr)
    -- Deal with making the continuation duplicable if necessary,
    -- and with the no-case-of-case situation.
    wrapJoinCont env cont thing_inside
      | contIsStop cont        -- Common case; no need for fancy footwork
      = thing_inside env cont

      | not (sm_case_case (getMode env))
        -- See Note [Join points wih -fno-case-of-case]
      = do { (floats1, expr1) <- thing_inside env (mkBoringStop (contHoleType cont))
           ; let (floats2, expr2) = wrapJoinFloatsX floats1 expr1
           ; (floats3, expr3) <- rebuild (env `setInScopeFromF` floats2) expr2 cont
           ; return (floats2 `addFloats` floats3, expr3) }

      | otherwise
        -- Normal case; see Note [Join points and case-of-case]
      = do { (floats1, cont')  <- mkDupableCont env cont
           ; (floats2, result) <- thing_inside (env `setInScopeFromF` floats1) cont'
           ; return (floats1 `addFloats` floats2, result) }

    --------------------
    trimJoinCont :: Id -> Maybe JoinArity -> SimplCont -> SimplCont
    -- Drop outer context from join point invocation (jump)
    -- See Note [Join points and case-of-case]

    trimJoinCont _ Nothing cont
      = cont -- Not a jump
    trimJoinCont var (Just arity) cont
      = trim arity cont
      where
        trim 0 cont@(Stop {})
          = cont
        trim 0 cont
          = mkBoringStop (contResultType cont)
        trim n cont@(ApplyToVal { sc_cont = k })
          = cont { sc_cont = trim (n-1) k }
        trim n cont@(ApplyToTy { sc_cont = k })
          = cont { sc_cont = trim (n-1) k } -- join arity counts types!
        trim _ cont
          = pprPanic "completeCall" $ ppr var $$ ppr cont

    {- Note [Join points and case-of-case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we perform the case-of-case transform (or otherwise push continuations
    inward), we want to treat join points specially. Since they're always
    tail-called and we want to maintain this invariant, we can do this (for any
    evaluation context E):

      E[join j = e
        in case ... of
             A -> jump j 1
             B -> jump j 2
             C -> f 3]

        -->

      join j = E[e]
      in case ... of
           A -> jump j 1
           B -> jump j 2
           C -> E[f 3]

    As is evident from the example, there are two components to this behavior:

      1. When entering the RHS of a join point, copy the context inside.
      2. When a join point is invoked, discard the outer context.

    We need to be very careful here to remain consistent---neither part is
    optional!

    We need do make the continuation E duplicable (since we are duplicating it)
    with mkDuableCont.

    Note [Join points wih -fno-case-of-case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Supose case-of-case is switched off, and we are simplifying

        case (join j x = <j-rhs> in
              case y of
                 A -> j 1
                 B -> j 2
                 C -> e) of <outer-alts>

    Usually, we'd push the outer continuation (case . of <outer-alts>) into
    both the RHS and the body of the join point j.  But since we aren't doing
    case-of-case we may then end up with this totally bogus result

        join x = case <j-rhs> of <outer-alts> in
        case (case y of
                 A -> j 1
                 B -> j 2
                 C -> e) of <outer-alts>

    This would be OK in the language of the paper, but not in GHC: j is no longer
    a join point.  We can only do the "push contination into the RHS of the
    join point j" if we also push the contination right down to the /jumps/ to
    j, so that it can evaporate there.  If we are doing case-of-case, we'll get to

        join x = case <j-rhs> of <outer-alts> in
        case y of
          A -> j 1
          B -> j 2
          C -> case e of <outer-alts>

    which is great.

    Bottom line: if case-of-case is off, we must stop pushing the continuation
    inwards altogether at any join point.  Instead simplify the (join ... in ...)
    with a Stop continuation, and wrap the original continuation around the
    outside.  Surprisingly tricky!

    ************************************************************************
    *                                                                      *
                         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
            ContEx tvs cvs ids e -> simplExpr (setSubstEnv env tvs cvs ids) e
            DoneId var1          -> return (Var var1)
            DoneEx e _           -> return e

    simplIdF :: SimplEnv -> InId -> SimplCont -> SimplM (SimplFloats, OutExpr)
    simplIdF env var cont
      = case substId env var of
          ContEx tvs cvs ids e -> simplExprF (setSubstEnv env tvs cvs ids) e cont
                                    -- Don't trim; haven't already simplified e,
                                    -- so the cont is not embodied in e

          DoneId var1 -> completeCall env var1 (trimJoinCont var (isJoinId_maybe var1) cont)

          DoneEx e mb_join -> simplExprF (zapSubstEnv env) e (trimJoinCont var mb_join 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 (SimplFloats, OutExpr)
    completeCall env var cont
      | Just expr <- callSiteInline dflags var unfolding
                                    lone_variable arg_infos interesting_cont
      -- Inline the variable's RHS
      = do { checkedTick (UnfoldingDone var)
           ; dump_inline expr cont
           ; simplExprF (zapSubstEnv env) expr cont }

      | otherwise
      -- Don't inline; instead rebuild the call
      = do { rule_base <- getSimplRules
           ; let info = mkArgInfo var (getRules rule_base var)
                                  n_val_args call_cont
           ; rebuildCall env info cont }

      where
        dflags = seDynFlags env
        (lone_variable, arg_infos, call_cont) = contArgs cont
        n_val_args       = length arg_infos
        interesting_cont = interestingCallContext env call_cont
        unfolding        = activeUnfolding env var

        dump_inline unfolding cont
          | not (dopt Opt_D_dump_inlinings dflags) = return ()
          | not (dopt Opt_D_verbose_core2core dflags)
          = when (isExternalName (idName var)) $
                liftIO $ printOutputForUser dflags alwaysQualify $
                    sep [text "Inlining done:", nest 4 (ppr var)]
          | otherwise
          = liftIO $ printOutputForUser 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 (SimplFloats, OutExpr)
    -- We decided not to inline, so
    --    - simplify the arguments
    --    - try rewrite rules
    --    - and rebuild

    ---------- Bottoming applications --------------
    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
                                     -- continuation to discard, else we do it
                                     -- again and again!
      = seqType cont_ty `seq`        -- See Note [Avoiding space leaks in OutType]
        return (emptyFloats env, castBottomExpr res cont_ty)
      where
        res     = argInfoExpr fun rev_args
        cont_ty = contResultType cont

    ---------- Try rewrite RULES --------------
    -- See Note [Trying rewrite rules]
    rebuildCall env info@(ArgInfo { ai_fun = fun, ai_args = rev_args
                                  , ai_rules = Just (nr_wanted, rules) }) cont
      | nr_wanted == 0 || no_more_args
      , let info' = info { ai_rules = Nothing }
      = -- 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]
        do { mb_match <- tryRules env rules fun (reverse rev_args) cont
           ; case mb_match of
                 Just (env', rhs, cont') -> simplExprF env' rhs cont'
                 Nothing                 -> rebuildCall env info' cont }
      where
        no_more_args = case cont of
                          ApplyToTy  {} -> False
                          ApplyToVal {} -> False
                          _             -> True

    ---------- Simplify applications and casts --------------
    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 (addTyArgTo info 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
      , sm_case_case (getMode env)
      = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
        simplExprF (arg_se `setInScopeFromE` env) arg
                   (StrictArg { sc_fun = info', sc_cci = cci_strict
                              , sc_dup = Simplified, sc_cont = 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 `setInScopeFromE` env) arg
                                 (mkLazyArgStop arg_ty cci_lazy)
            ; rebuildCall env (addValArgTo info' arg') cont }
      where
        info'  = info { ai_strs = strs, ai_discs = discs }
        arg_ty = funArgTy fun_ty

        -- Use this for lazy arguments
        cci_lazy | encl_rules = RuleArgCtxt
                 | disc > 0   = DiscArgCtxt  -- Be keener here
                 | otherwise  = BoringCtxt   -- Nothing interesting

        -- ..and this for strict arguments
        cci_strict | encl_rules = RuleArgCtxt
                   | disc > 0   = DiscArgCtxt
                   | otherwise  = RhsCtxt
          -- Why RhsCtxt?  if we see f (g x) (h x), and f is strict, we
          -- want to be a bit more eager to inline g, because it may
          -- expose an eval (on x perhaps) that can be eliminated or
          -- shared. I saw this in nofib 'boyer2', RewriteFuns.onewayunify1
          -- It's worth an 18% improvement in allocation for this
          -- particular benchmark; 5% on 'mate' and 1.3% on 'multiplier'

    ---------- No further useful info, revert to generic rebuild ------------
    rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args }) cont
      = rebuild env (argInfoExpr fun rev_args) cont

    {- Note [Trying rewrite rules]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider an application (f e1 e2 e3) where the e1,e2,e3 are not yet
    simplified.  We want to simplify enough arguments to allow the rules
    to apply, but it's more efficient to avoid simplifying e2,e3 if e1 alone
    is sufficient.  Example: class ops
       (+) dNumInt e2 e3
    If we rewrite ((+) dNumInt) to plusInt, we can take advantage of the
    latter's strictness when simplifying e2, e3.  Moreover, suppose we have
      RULE  f Int = \x. x True

    Then given (f Int e1) we rewrite to
       (\x. x True) e1
    without simplifying e1.  Now we can inline x into its unique call site,
    and absorb the True into it all in the same pass.  If we simplified
    e1 first, we couldn't do that; see Note [Avoiding exponential behaviour].

    So we try to apply rules if either
      (a) no_more_args: we've run out of argument that the rules can "see"
      (b) nr_wanted: none of the rules wants any more arguments

    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 sweep.

    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 (SimplEnv, CoreExpr, SimplCont))

    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 { 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)) }
    -}

      | Just (rule, rule_rhs) <- lookupRule dflags (getUnfoldingInRuleMatch env)
                                            (activeRule env) fn
                                            (argInfoAppArgs args) rules
      -- Fire a rule for the function
      = do { checkedTick (RuleFired (ruleName rule))
           ; let cont' = pushSimplifiedArgs zapped_env
                                            (drop (ruleArity rule) args)
                                            call_cont
                         -- (ruleArity rule) says how
                         -- many args the rule consumed

                 occ_anald_rhs = occurAnalyseExpr rule_rhs
                     -- See Note [Occurrence-analyse after rule firing]
           ; dump rule rule_rhs
           ; return (Just (zapped_env, occ_anald_rhs, cont')) }
                -- The occ_anald_rhs and cont' are all Out things
                -- hence zapping the environment

      | otherwise  -- No rule fires
      = do { nodump  -- This ensures that an empty file is written
           ; return Nothing }

      where
        dflags     = seDynFlags env
        zapped_env = zapSubstEnv env  -- See Note [zapSubstEnv]

        printRuleModule rule
          = parens (maybe (text "BUILTIN")
                          (pprModuleName . moduleName)
                          (ruleModule rule))

        dump rule rule_rhs
          | dopt Opt_D_dump_rule_rewrites dflags
          = log_rule dflags Opt_D_dump_rule_rewrites "Rule fired" $ vcat
              [ text "Rule:" <+> ftext (ruleName rule)
              , text "Module:" <+>  printRuleModule 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 (ruleName rule)
                <+> printRuleModule rule

          | otherwise
          = return ()

        nodump
          | dopt Opt_D_dump_rule_rewrites dflags
          = liftIO $ dumpSDoc dflags alwaysQualify Opt_D_dump_rule_rewrites "" empty

          | dopt Opt_D_dump_rule_firings dflags
          = liftIO $ dumpSDoc dflags alwaysQualify Opt_D_dump_rule_firings "" empty

          | otherwise
          = return ()

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

    trySeqRules :: SimplEnv
                -> OutExpr -> InExpr   -- Scrutinee and RHS
                -> SimplCont
                -> SimplM (Maybe (SimplEnv, CoreExpr, SimplCont))
    -- See Note [User-defined RULES for seq]
    trySeqRules in_env scrut rhs cont
      = do { rule_base <- getSimplRules
           ; tryRules in_env (getRules rule_base seqId) seqId out_args rule_cont }
      where
        no_cast_scrut = drop_casts scrut
        scrut_ty  = exprType no_cast_scrut
        seq_id_ty = idType seqId
        rhs_ty    = substTy in_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  = piResultTy seq_id_ty scrut_ty }
                    , ValArg no_cast_scrut]
        rule_cont = ApplyToVal { sc_dup = NoDup, sc_arg = rhs
                               , sc_env = in_env, sc_cont = cont }
        -- Lazily evaluated, so we don't do most of this

        drop_casts (Cast e _) = drop_casts e
        drop_casts e          = e

    {- Note [User-defined RULES for seq]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Given
       case (scrut |> co) of _ -> rhs
    look for rules that match the expression
       seq @t1 @t2 scrut
    where scrut :: t1
          rhs   :: t2

    If you find a match, rewrite it, and apply to 'rhs'.

    Notice that we can simply drop casts on the fly here, which
    makes it more likely that a rule will match.

    See Note [User-defined RULES for seq] in MkId.

    Note [Occurrence-analyse after rule firing]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    After firing a rule, we occurrence-analyse the instantiated RHS before
    simplifying it.  Usually this doesn't make much difference, but it can
    be huge.  Here's an example (simplCore/should_compile/T7785)

      map f (map f (map f xs)

    = -- Use build/fold form of map, twice
      map f (build (\cn. foldr (mapFB c f) n
                               (build (\cn. foldr (mapFB c f) n xs))))

    = -- Apply fold/build rule
      map f (build (\cn. (\cn. foldr (mapFB c f) n xs) (mapFB c f) n))

    = -- Beta-reduce
      -- Alas we have no occurrence-analysed, so we don't know
      -- that c is used exactly once
      map f (build (\cn. let c1 = mapFB c f in
                         foldr (mapFB c1 f) n xs))

    = -- Use mapFB rule:   mapFB (mapFB c f) g = mapFB c (f.g)
      -- We can do this because (mapFB c n) is a PAP and hence expandable
      map f (build (\cn. let c1 = mapFB c n in
                         foldr (mapFB c (f.f)) n x))

    This is not too bad.  But now do the same with the outer map, and
    we get another use of mapFB, and t can interact with /both/ remaining
    mapFB calls in the above expression.  This is stupid because actually
    that 'c1' binding is dead.  The outer map introduces another c2. If
    there is a deep stack of maps we get lots of dead bindings, and lots
    of redundant work as we repeatedly simplify the result of firing rules.

    The easy thing to do is simply to occurrence analyse the result of
    the rule firing.  Note that this occ-anals not only the RHS of the
    rule, but also the function arguments, which by now are OutExprs.
    E.g.
          RULE f (g x) = x+1

    Call   f (g BIG)  -->   (\x. x+1) BIG

    The rule binders are lambda-bound and applied to the OutExpr arguments
    (here BIG) which lack all internal occurrence info.

    Is this inefficient?  Not really: we are about to walk over the result
    of the rule firing to simplify it, so occurrence analysis is at most
    a constant factor.

    Possible improvement: occ-anal the rules when putting them in the
    database; and in the simplifier just occ-anal the OutExpr arguments.
    But that's more complicated and the rule RHS is usually tiny; so I'm
    just doing the simple thing.

    Historical note: previously we did occ-anal the rules in Rule.hs,
    but failed to occ-anal the OutExpr arguments, which led to the
    nasty performance problem described above.

    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 to let transformation]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    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...

    We treat the unlifted and lifted cases separately:

    * Unlifted case: 'e' satisfies exprOkForSpeculation
      (ok-for-spec is needed to satisfy the let/app invariant).
      This turns     case a +# b of r -> ...r...
      into           let r = a +# b in ...r...
      and thence     .....(a +# b)....

      However, 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.  Annoying.

    * Lifted case: we need to be sure that the expression is already
      evaluated (exprIsHNF).  If it's not already evaluated
          - we risk losing exceptions, divergence or
            user-specified thunk-forcing
          - even if 'e' is guaranteed to converge, we don't want to
            create a thunk (call by need) instead of evaluating it
            right away (call by value)

      However, we can turn the case into a /strict/ let if the 'r' is
      used strictly in the body.  Then we won't lose divergence; and
      we won't build a thunk because the let is strict.
      See also Note [Eliminating redundant seqs]

      NB: absentError satisfies exprIsHNF: see Note [aBSENT_ERROR_ID] in MkCore.
      We want to turn
         case (absentError "foo") of r -> ...MkT r...
      into
         let r = absentError "foo" in ...MkT r...

    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.)

    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 (SimplFloats, 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 { (floats1, env') <- simplNonRecX env case_bndr scrut
                                   -- scrut is a constructor application,
                                   -- hence satisfies let/app invariant
                               ; (floats2, expr') <- simplExprF env' rhs cont
                               ; return (floats1 `addFloats` floats2, expr') }

    --------------------------------------------------
    --      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)
      -- See Note [Case to let transformation]
      | all_dead_bndrs
      , if isUnliftedType (idType case_bndr)
        then exprOkForSpeculation scrut
        else exprIsHNF scrut || scrut_is_demanded_var scrut
      = do { tick (CaseElim case_bndr)
           ; (floats1, env') <- simplNonRecX env case_bndr scrut
           ; (floats2, expr') <- simplExprF env' rhs cont
           ; return (floats1 `addFloats` floats2, expr') }

      -- 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 { mb_rule <- trySeqRules env scrut rhs cont
           ; case mb_rule of
               Just (env', rule_rhs, cont') -> simplExprF env' rule_rhs cont'
               Nothing                      -> reallyRebuildCase env scrut case_bndr alts cont }
      where
        all_dead_bndrs = all isDeadBinder bndrs       -- bndrs are [InId]
        is_plain_seq   = all_dead_bndrs && isDeadBinder case_bndr -- Evaluation *only* for effect

        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
      | not (sm_case_case (getMode env))
      = do { case_expr <- simplAlts env scrut case_bndr alts
                                    (mkBoringStop (contHoleType cont))
           ; rebuild env case_expr cont }

      | otherwise
      = do { (floats, cont') <- mkDupableCaseCont env alts cont
           ; case_expr <- simplAlts (env `setInScopeFromF` floats)
                                    scrut case_bndr alts cont'
           ; return (floats, case_expr) }

    {-
    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 analyser; 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

    We'd like to transform
            case e of (x :: F Int) { DEFAULT -> rhs }
    ===>
            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.

    We'd also like to eliminate empty types (Trac #13468). So if

        data Void
        type instance F Bool = Void

    then we'd like to transform
            case (x :: F Bool) of { _ -> error "urk" }
    ===>
            case (x |> co) of (x' :: Void) of {}

    Nota Bene: we used to have a built-in rule for 'seq' that dropped
    casts, so that
        case (x |> co) of { _ -> blah }
    dropped the cast; in order to improve the chances of trySeqRules
    firing.  But that works in the /opposite/ direction to Note [Improving
    seq] so there's a danger of flip/flopping.  Better to make trySeqRules
    insensitive to the cast, which is now is.

    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         -- Scrutinee
              -> InId            -- Case binder
              -> [InAlt]         -- Non-empty
              -> SimplCont
              -> SimplM OutExpr  -- Returns the complete simplified case expression

    simplAlts env0 scrut case_bndr alts cont'
      = do  { traceSmpl "simplAlts" (vcat [ ppr case_bndr
                                          , text "cont':" <+> ppr cont'
                                          , text "in_scope" <+> ppr (seInScope env0) ])
            ; (env1, case_bndr1) <- simplBinder env0 case_bndr
            ; let case_bndr2 = case_bndr1 `setIdUnfolding` evaldUnfolding
                  env2       = modifyInScope env1 case_bndr2
                  -- See Note [Case binder evaluated-ness]

            ; fam_envs <- getFamEnvs
            ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env2 scrut
                                                           case_bndr case_bndr2 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') $

            ; let alts_ty' = contResultType cont'
            -- See Note [Avoiding space leaks in OutType]
            ; seqType alts_ty' `seq`
              mkCase (seDynFlags env0) scrut' case_bndr' alts_ty' 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,_,_)]
      | 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) Nothing
                  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.hs
        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) = zap str v : go vs' strs
              go _ _ = pprPanic "cat_evals"
                        (ppr con $$
                         ppr vs  $$
                         ppr_with_length the_strs $$
                         ppr_with_length (dataConRepArgTys con) $$
                         ppr_with_length (dataConRepStrictness con))
                where
                  ppr_with_length list
                    = ppr list <+> parens (text "length =" <+> ppr (length list))
                                        -- NB: If this panic triggers, note that
                                        -- NoStrictnessMark doesn't print!

              zap str v = setCaseBndrEvald str $ -- Add eval'dness info
                          zapIdOccInfo v         -- And kill occ info;
                                                 -- see Note [Case alternative occ info]

    addAltUnfoldings :: SimplEnv -> Maybe OutExpr -> OutId -> OutExpr -> SimplM SimplEnv
    addAltUnfoldings env scrut case_bndr con_app
      = do { let con_app_unf = mk_simple_unf 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 $
                                                    mk_simple_unf (Cast con_app (mkSymCo co))
                          _                      -> env1

           ; traceSmpl "addAltUnf" (vcat [ppr case_bndr <+> ppr scrut, ppr con_app])
           ; return env2 }
      where
        mk_simple_unf = mkSimpleUnfolding (seDynFlags env)

    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 [Case binder evaluated-ness]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We pin on a (OtherCon []) unfolding to the case-binder of a Case,
    even though it'll be over-ridden in every case alternative with a more
    informative unfolding.  Why?  Because suppose a later, less clever, pass
    simply replaces all occurrences of the case binder with the binder itself;
    then Lint may complain about the let/app invariant.  Example
        case e of b { DEFAULT -> let v = reallyUnsafePtrEq# b y in ....
                    ; K       -> blah }

    The let/app invariant requires that y is evaluated in the call to
    reallyUnsafePtrEq#, which it is.  But we still want that to be true if we
    propagate binders to occurrences.

    This showed up in Trac #13027.

    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 occurrences
    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 (SimplFloats, OutExpr)

    knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
      = do  { (floats1, env1)  <- bind_args env bs dc_args
            ; (floats2, env2) <- bind_case_bndr env1
            ; (floats3, expr') <- simplExprF env2 rhs cont
            ; return (floats1 `addFloats` floats2 `addFloats` floats3, expr') }
      where
        zap_occ = zapBndrOccInfo (isDeadBinder bndr)    -- bndr is an InId

                      -- Ugh!
        bind_args env' [] _  = return (emptyFloats env', 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') (Coercion co : args)
          = ASSERT( isCoVar b )
            bind_args (extendCvSubst env' b co) 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]
               ; (floats1, env2) <- simplNonRecX env' b' arg  -- arg satisfies let/app invariant
               ; (floats2, env3)  <- bind_args env2 bs' args
               ; return (floats1 `addFloats` floats2, env3) }

        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 (emptyFloats env, env)
          | exprIsTrivial scrut = return (emptyFloats env
                                         , extendIdSubst env bndr (DoneEx scrut Nothing))
          | 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 (SimplFloats, OutExpr)
                    -- This isn't strictly an error, although it is unusual.
                    -- It's possible that the simplifier 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, text "missingAlt" <+> ppr case_bndr )
        -- See Note [Avoiding space leaks in OutType]
        let cont_ty = contResultType cont
        in seqType cont_ty `seq`
           return (emptyFloats env, mkImpossibleExpr cont_ty)

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

    Consider
      let x* = case e of { True -> e1; False -> e2 }
      in b
    where x* is a strict binding.  Then mkDupableCont will be given
    the continuation
       case [] of { True -> e1; False -> e2 } ; let x* = [] in b ; stop
    and will split it into
       dupable:      case [] of { True -> $j1; False -> $j2 } ; stop
       join floats:  $j1 = e1, $j2 = e2
       non_dupable:  let x* = [] in b; stop

    Putting this back together would give
       let x* = let { $j1 = e1; $j2 = e2 } in
                case e of { True -> $j1; False -> $j2 }
       in b
    (Of course we only do this if 'e' wants to duplicate that continuation.)
    Note how important it is that the new join points wrap around the
    inner expression, and not around the whole thing.

    In contrast, any let-bindings introduced by mkDupableCont can wrap
    around the entire thing.

    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.  See Trac #4930.
    -}

    --------------------
    mkDupableCaseCont :: SimplEnv -> [InAlt] -> SimplCont
                      -> SimplM (SimplFloats, SimplCont)
    mkDupableCaseCont env alts cont
      | altsWouldDup alts = mkDupableCont env cont
      | otherwise         = return (emptyFloats env, cont)

    altsWouldDup :: [InAlt] -> Bool -- True iff strictly > 1 non-bottom alternative
    altsWouldDup []  = False        -- See Note [Bottom alternatives]
    altsWouldDup [_] = False
    altsWouldDup (alt:alts)
      | is_bot_alt alt = altsWouldDup alts
      | otherwise      = not (all is_bot_alt alts)
      where
        is_bot_alt (_,_,rhs) = exprIsBottom rhs

    -------------------------
    mkDupableCont :: SimplEnv -> SimplCont
                  -> SimplM ( SimplFloats  -- Incoming SimplEnv augmented with
                                           --   extra let/join-floats and in-scope variables
                            , SimplCont)   -- dup_cont: duplicable continuation

    mkDupableCont env cont
      | contIsDupable cont
      = return (emptyFloats env, cont)

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

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

    -- Duplicating ticks for now, not sure if this is good or not
    mkDupableCont env (TickIt t cont)
      = do  { (floats, cont') <- mkDupableCont env cont
            ; return (floats, TickIt t cont') }

    mkDupableCont env (StrictBind { sc_bndr = bndr, sc_bndrs = bndrs
                                  , sc_body = body, sc_env = se, sc_cont = cont})
      -- See Note [Duplicating StrictBind]
      = do { let sb_env = se `setInScopeFromE` env
           ; (sb_env1, bndr') <- simplBinder sb_env bndr
           ; (floats1, join_inner) <- simplLam sb_env1 bndrs body cont
              -- No need to use mkDupableCont before simplLam; we
              -- use cont once here, and then share the result if necessary

           ; let join_body = wrapFloats floats1 join_inner
                 res_ty    = contResultType cont

           ; (floats2, body2)
                <- if exprIsDupable (seDynFlags env) join_body
                   then return (emptyFloats env, join_body)
                   else do { join_bndr <- newJoinId [bndr'] res_ty
                           ; let join_call = App (Var join_bndr) (Var bndr')
                                 join_rhs  = Lam (setOneShotLambda bndr') join_body
                                 join_bind = NonRec join_bndr join_rhs
                                 floats    = emptyFloats env `extendFloats` join_bind
                           ; return (floats, join_call) }
           ; return ( floats2
                    , StrictBind { sc_bndr = bndr', sc_bndrs = []
                                 , sc_body = body2
                                 , sc_env  = zapSubstEnv se `setInScopeFromF` floats2
                                             -- See Note [StaticEnv invariant] in SimplUtils
                                 , sc_dup  = OkToDup
                                 , sc_cont = mkBoringStop res_ty } ) }

    mkDupableCont env (StrictArg { sc_fun = info, sc_cci = cci, sc_cont = cont })
            -- See Note [Duplicating StrictArg]
            -- NB: sc_dup /= OkToDup; that is caught earlier by contIsDupable
      = do { (floats1, cont') <- mkDupableCont env cont
           ; (floats_s, args') <- mapAndUnzipM (makeTrivialArg (getMode env))
                                               (ai_args info)
           ; return ( foldl addLetFloats floats1 floats_s
                    , StrictArg { sc_fun = info { ai_args = args' }
                                , sc_cci = cci
                                , sc_cont = cont'
                                , sc_dup = OkToDup} ) }

    mkDupableCont env (ApplyToTy { sc_cont = cont
                                 , sc_arg_ty = arg_ty, sc_hole_ty = hole_ty })
      = do  { (floats, cont') <- mkDupableCont env cont
            ; return (floats, ApplyToTy { sc_cont = cont'
                                        , sc_arg_ty = arg_ty, sc_hole_ty = hole_ty }) }

    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
            -- NB: sc_dup /= OkToDup; that is caught earlier by contIsDupable
        do  { (floats1, cont') <- mkDupableCont env cont
            ; let env' = env `setInScopeFromF` floats1
            ; (_, se', arg') <- simplArg env' dup se arg
            ; (let_floats2, arg'') <- makeTrivial (getMode env) NotTopLevel (fsLit "karg") arg'
            ; let all_floats = floats1 `addLetFloats` let_floats2
            ; return ( all_floats
                     , ApplyToVal { sc_arg = arg''
                                  , sc_env = se' `setInScopeFromF` all_floats
                                             -- Ensure that sc_env includes the free vars of
                                             -- arg'' in its in-scope set, even if makeTrivial
                                             -- has turned arg'' into a fresh variable
                                             -- See Note [StaticEnv invariant] in SimplUtils
                                  , sc_dup = OkToDup, sc_cont = cont' }) }

    mkDupableCont env (Select { sc_bndr = case_bndr, sc_alts = alts
                              , sc_env = se, sc_cont = cont })
      =     -- e.g.         (case [...hole...] of { pi -> ei })
            --      ===>
            --              let ji = \xij -> ei
            --              in case [...hole...] of { pi -> ji xij }
            -- NB: sc_dup /= OkToDup; that is caught earlier by contIsDupable
        do  { tick (CaseOfCase case_bndr)
            ; (floats, alt_cont) <- mkDupableCaseCont env alts cont
                    -- NB: We call mkDupableCaseCont here to make cont duplicable
                    --     (if necessary, depending on the number of alts)
                    -- And this is important: see Note [Fusing case continuations]

            ; let alt_env = se `setInScopeFromF` floats
            ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
            ; alts' <- mapM (simplAlt alt_env' Nothing [] case_bndr' alt_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

            ; (join_floats, alts'') <- mapAccumLM (mkDupableAlt (seDynFlags env) case_bndr')
                                                  emptyJoinFloats alts'

            ; let all_floats = floats `addJoinFloats` join_floats
                               -- Note [Duplicated env]
            ; return (all_floats
                     , Select { sc_dup  = OkToDup
                              , sc_bndr = case_bndr'
                              , sc_alts = alts''
                              , sc_env  = zapSubstEnv se `setInScopeFromF` all_floats
                                          -- See Note [StaticEnv invariant] in SimplUtils
                              , sc_cont = mkBoringStop (contResultType cont) } ) }

    mkDupableAlt :: DynFlags -> OutId
                 -> JoinFloats -> OutAlt
                 -> SimplM (JoinFloats, OutAlt)
    mkDupableAlt dflags case_bndr jfloats (con, bndrs', rhs')
      | exprIsDupable dflags rhs'  -- Note [Small alternative rhs]
      = return (jfloats, (con, bndrs', rhs'))

      | otherwise
      = 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 rhs
                                 rhs = mkConApp2 dc (tyConAppArgs scrut_ty) bndrs'

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

                  final_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_args = varsToCoreExprs final_bndrs'
                               -- Note [Join point abstraction]

                    -- 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_bndr <- newJoinId final_bndrs' rhs_ty'

            ; let join_call = mkApps (Var join_bndr) final_args
                  alt'      = (con, bndrs', join_call)

            ; return ( jfloats `addJoinFlts` unitJoinFloat (NonRec join_bndr join_rhs)
                     , alt') }
                    -- 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 (StrictBind 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 scrutinises 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 [Lambda-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 a 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.hs, 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 mkLamTypes]
    ~~~~~~~~~~~~~~~~~~~~~~
    Notice the funky mkLamTypes.  If the constructor 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 [Duplicating StrictArg]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We make a StrictArg duplicable simply by making all its
    stored-up arguments (in sc_fun) trivial, by let-binding
    them.  Thus:
            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.

    Historical aide: previously we did this (where E is a
    big argument:
            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

    Note [Duplicating StrictBind]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We make a StrictBind duplicable in a very similar way to
    that for case expressions.  After all,
       let x* = e in b   is similar to    case e of x -> b

    So we potentially make a join-point for the body, thus:
       let x = [] in b   ==>   join j x = b
                               in let x = [] in j x

    Note [Join point abstraction]  Historical note
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NB: This note is now historical, describing how (in the past) we used
    to add a void argument to nullary join points.  But now that "join
    point" is not a fuzzy concept but a formal syntactic construct (as
    distinguished by the JoinId constructor of IdDetails), each of these
    concerns is handled separately, with no need for a vestigial extra
    argument.

    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

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

    simplLetUnfolding :: SimplEnv-> TopLevelFlag
                      -> Maybe SimplCont
                      -> InId
                      -> OutExpr
                      -> Unfolding -> SimplM Unfolding
    simplLetUnfolding env top_lvl cont_mb id new_rhs unf
      | isStableUnfolding unf
      = simplStableUnfolding env top_lvl cont_mb id unf
      | isExitJoinId id
      = return noUnfolding -- see Note [Do not inline exit join points]
      | otherwise
      = mkLetUnfolding (seDynFlags env) top_lvl InlineRhs id new_rhs

    -------------------
    mkLetUnfolding :: DynFlags -> TopLevelFlag -> UnfoldingSource
                   -> InId -> OutExpr -> SimplM Unfolding
    mkLetUnfolding dflags top_lvl src id new_rhs
      = is_bottoming `seq`  -- See Note [Force bottoming field]
        return (mkUnfolding dflags src is_top_lvl is_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
        is_top_lvl   = isTopLevel top_lvl
        is_bottoming = isBottomingId id

    -------------------
    simplStableUnfolding :: SimplEnv -> TopLevelFlag
                         -> Maybe SimplCont  -- Just k => a join point with continuation k
                         -> InId
                         -> Unfolding -> SimplM Unfolding
    -- Note [Setting the new unfolding]
    simplStableUnfolding env top_lvl mb_cont id unf
      = case unf of
          NoUnfolding   -> return unf
          BootUnfolding -> 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' <- case mb_cont of
                               Just cont -> simplJoinRhs rule_env id expr cont
                               Nothing   -> 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
                         -> mkLetUnfolding dflags top_lvl src id 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
        dflags     = seDynFlags env
        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 a stable unfolding on a loop breaker (which happens for
    INLINABLE), 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 [Forming Rec groups] in OccurAnal
    -}

    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 <- simplRules env (Just (idName out_id)) old_rules
           ; let final_id  = out_id `setIdSpecialisation` mkRuleInfo new_rules
           ; return (modifyInScope env final_id, final_id) }
      where
        old_rules = ruleInfoRules (idSpecialisation in_id)

    simplRules :: SimplEnv -> Maybe Name -> [CoreRule] -> SimplM [CoreRule]
    simplRules env mb_new_nm rules
      = mapM simpl_rule rules
      where
        simpl_rule rule@(BuiltinRule {})
          = return rule

        simpl_rule rule@(Rule { ru_bndrs = bndrs, ru_args = args
                              , ru_fn = fn_name, ru_rhs = rhs })
          = do { (env', bndrs') <- simplBinders env bndrs
               ; let rhs_ty = substTy env' (exprType rhs)
                     rule_cont = mkBoringStop rhs_ty
                     rule_env  = updMode updModeForRules env'
               ; args' <- mapM (simplExpr rule_env) args
               ; rhs'  <- simplExprC rule_env rhs rule_cont
               ; return (rule { ru_bndrs = bndrs'
                              , ru_fn    = mb_new_nm `orElse` fn_name
                              , ru_args  = args'
                              , ru_rhs   = rhs' }) }