simplCore/SimplUtils.hs

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

    \section[SimplUtils]{The simplifier utilities}
    -}

    {-# LANGUAGE CPP #-}

    module SimplUtils (
            -- Rebuilding
            mkLam, mkCase, prepareAlts, tryEtaExpandRhs,

            -- Inlining,
            preInlineUnconditionally, postInlineUnconditionally,
            activeUnfolding, activeRule,
            getUnfoldingInRuleMatch,
            simplEnvForGHCi, updModeForStableUnfoldings, updModeForRules,

            -- The continuation type
            SimplCont(..), DupFlag(..),
            isSimplified,
            contIsDupable, contResultType, contHoleType, applyContToJoinType,
            contIsTrivial, contArgs,
            countArgs,
            mkBoringStop, mkRhsStop, mkLazyArgStop, contIsRhsOrArg,
            interestingCallContext,

            -- ArgInfo
            ArgInfo(..), ArgSpec(..), mkArgInfo,
            addValArgTo, addCastTo, addTyArgTo,
            argInfoExpr, argInfoAppArgs, pushSimplifiedArgs,

            abstractFloats
        ) where

    #include "HsVersions.h"

    import SimplEnv
    import CoreMonad        ( SimplifierMode(..), Tick(..) )
    import DynFlags
    import CoreSyn
    import qualified CoreSubst
    import PprCore
    import CoreFVs
    import CoreUtils
    import CoreArity
    import CoreUnfold
    import Name
    import Id
    import IdInfo
    import Var
    import Demand
    import SimplMonad
    import Type     hiding( substTy )
    import Coercion hiding( substCo )
    import DataCon          ( dataConWorkId )
    import VarEnv
    import VarSet
    import BasicTypes
    import Util
    import MonadUtils
    import Outputable
    import Pair
    import PrelRules
    import Literal

    import Control.Monad    ( when )
    import Data.List        ( sortBy )

    {-
    ************************************************************************
    *                                                                      *
                    The SimplCont and DupFlag types
    *                                                                      *
    ************************************************************************

    A SimplCont allows the simplifier to traverse the expression in a
    zipper-like fashion.  The SimplCont represents the rest of the expression,
    "above" the point of interest.

    You can also think of a SimplCont as an "evaluation context", using
    that term in the way it is used for operational semantics. This is the
    way I usually think of it, For example you'll often see a syntax for
    evaluation context looking like
            C ::= []  |  C e   |  case C of alts  |  C `cast` co
    That's the kind of thing we are doing here, and I use that syntax in
    the comments.

    Key points:
      * A SimplCont describes a *strict* context (just like
        evaluation contexts do).  E.g. Just [] is not a SimplCont

      * A SimplCont describes a context that *does not* bind
        any variables.  E.g. \x. [] is not a SimplCont
    -}

    data SimplCont
      = Stop                -- An empty context, or <hole>
            OutType         -- Type of the <hole>
            CallCtxt        -- Tells if there is something interesting about
                            --          the context, and hence the inliner
                            --          should be a bit keener (see interestingCallContext)
                            -- Specifically:
                            --     This is an argument of a function that has RULES
                            --     Inlining the call might allow the rule to fire
                            -- Never ValAppCxt (use ApplyToVal instead)
                            -- or CaseCtxt (use Select instead)

      | CastIt            -- <hole> `cast` co
            OutCoercion             -- The coercion simplified
                                    -- Invariant: never an identity coercion
            SimplCont

      | ApplyToVal {        -- <hole> arg
            sc_dup  :: DupFlag,          -- See Note [DupFlag invariants]
            sc_arg  :: InExpr,           -- The argument,
            sc_env  :: StaticEnv,        --     and its static env
            sc_cont :: SimplCont }

      | ApplyToTy {         -- <hole> ty
            sc_arg_ty  :: OutType,     -- Argument type
            sc_hole_ty :: OutType,     -- Type of the function, presumably (forall a. blah)
                                       -- See Note [The hole type in ApplyToTy]
            sc_cont    :: SimplCont }

      | Select {           -- case <hole> of alts
            sc_dup  :: DupFlag,                 -- See Note [DupFlag invariants]
            sc_bndr :: InId,                    -- case binder
            sc_alts :: [InAlt],                 -- Alternatives
            sc_env  ::  StaticEnv,              --   and their static environment
            sc_cont :: SimplCont }

      -- The two strict forms have no DupFlag, because we never duplicate them
      | StrictBind                  -- (\x* \xs. e) <hole>
            InId [InBndr]           -- let x* = <hole> in e
            InExpr StaticEnv        --      is a special case
            SimplCont

      | StrictArg           -- f e1 ..en <hole>
            ArgInfo         -- Specifies f, e1..en, Whether f has rules, etc
                            --     plus strictness flags for *further* args
            CallCtxt        -- Whether *this* argument position is interesting
            SimplCont

      | TickIt
            (Tickish Id)    -- Tick tickish <hole>
            SimplCont

    data DupFlag = NoDup       -- Unsimplified, might be big
                 | Simplified  -- Simplified
                 | OkToDup     -- Simplified and small

    isSimplified :: DupFlag -> Bool
    isSimplified NoDup = False
    isSimplified _     = True       -- Invariant: the subst-env is empty

    perhapsSubstTy :: DupFlag -> StaticEnv -> Type -> Type
    perhapsSubstTy dup env ty
      | isSimplified dup = ty
      | otherwise        = substTy env ty

    {-
    Note [DupFlag invariants]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    In both (ApplyToVal dup _ env k)
       and  (Select dup _ _ env k)
    the following invariants hold

      (a) if dup = OkToDup, then continuation k is also ok-to-dup
      (b) if dup = OkToDup or Simplified, the subst-env is empty
          (and and hence no need to re-simplify)
    -}

    instance Outputable DupFlag where
      ppr OkToDup    = text "ok"
      ppr NoDup      = text "nodup"
      ppr Simplified = text "simpl"

    instance Outputable SimplCont where
      ppr (Stop ty interesting) = text "Stop" <> brackets (ppr interesting) <+> ppr ty
      ppr (CastIt co cont  )    = (text "CastIt" <+> pprOptCo co) $$ ppr cont
      ppr (TickIt t cont)       = (text "TickIt" <+> ppr t) $$ ppr cont
      ppr (ApplyToTy  { sc_arg_ty = ty, sc_cont = cont })
        = (text "ApplyToTy" <+> pprParendType ty) $$ ppr cont
      ppr (ApplyToVal { sc_arg = arg, sc_dup = dup, sc_cont = cont })
        = (text "ApplyToVal" <+> ppr dup <+> pprParendExpr arg)
                                            $$ ppr cont
      ppr (StrictBind b _ _ _ cont)       = (text "StrictBind" <+> ppr b) $$ ppr cont
      ppr (StrictArg ai _ cont)           = (text "StrictArg" <+> ppr (ai_fun ai)) $$ ppr cont
      ppr (Select { sc_dup = dup, sc_bndr = bndr, sc_alts = alts, sc_env = se, sc_cont = cont })
        = (text "Select" <+> ppr dup <+> ppr bndr) $$
           ifPprDebug (nest 2 $ vcat [ppr (seTvSubst se), ppr alts]) $$ ppr cont

    {- Note [The hole type in ApplyToTy]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The sc_hole_ty field of ApplyToTy records the type of the "hole" in the
    continuation.  It is absolutely necessary to compute contHoleType, but it is
    not used for anything else (and hence may not be evaluated).

    Why is it necessary for contHoleType?  Consider the continuation
         ApplyToType Int (Stop Int)
    corresponding to
         (<hole> @Int) :: Int
    What is the type of <hole>?  It could be (forall a. Int) or (forall a. a),
    and there is no way to know which, so we must record it.

    In a chain of applications  (f @t1 @t2 @t3) we'll lazily compute exprType
    for (f @t1) and (f @t1 @t2), which is potentially non-linear; but it probably
    doesn't matter because we'll never compute them all.

    ************************************************************************
    *                                                                      *
                    ArgInfo and ArgSpec
    *                                                                      *
    ************************************************************************
    -}

    data ArgInfo
      = ArgInfo {
            ai_fun   :: OutId,      -- The function
            ai_args  :: [ArgSpec],  -- ...applied to these args (which are in *reverse* order)
            ai_type  :: OutType,    -- Type of (f a1 ... an)

            ai_rules :: [CoreRule], -- Rules for this function

            ai_encl :: Bool,        -- Flag saying whether this function
                                    -- or an enclosing one has rules (recursively)
                                    --      True => be keener to inline in all args

            ai_strs :: [Bool],      -- Strictness of remaining arguments
                                    --   Usually infinite, but if it is finite it guarantees
                                    --   that the function diverges after being given
                                    --   that number of args
            ai_discs :: [Int]       -- Discounts for remaining arguments; non-zero => be keener to inline
                                    --   Always infinite
        }

    data ArgSpec
      = ValArg OutExpr                    -- Apply to this (coercion or value); c.f. ApplyToVal
      | TyArg { as_arg_ty  :: OutType     -- Apply to this type; c.f. ApplyToTy
              , as_hole_ty :: OutType }   -- Type of the function (presumably forall a. blah)
      | CastBy OutCoercion                -- Cast by this; c.f. CastIt

    instance Outputable ArgSpec where
      ppr (ValArg e)                 = text "ValArg" <+> ppr e
      ppr (TyArg { as_arg_ty = ty }) = text "TyArg" <+> ppr ty
      ppr (CastBy c)                 = text "CastBy" <+> ppr c

    addValArgTo :: ArgInfo -> OutExpr -> ArgInfo
    addValArgTo ai arg = ai { ai_args = ValArg arg : ai_args ai
                            , ai_type = applyTypeToArg (ai_type ai) arg }

    addTyArgTo :: ArgInfo -> OutType -> ArgInfo
    addTyArgTo ai arg_ty = ai { ai_args = arg_spec : ai_args ai
                              , ai_type = piResultTy poly_fun_ty arg_ty }
      where
        poly_fun_ty = ai_type ai
        arg_spec    = TyArg { as_arg_ty = arg_ty, as_hole_ty = poly_fun_ty }

    addCastTo :: ArgInfo -> OutCoercion -> ArgInfo
    addCastTo ai co = ai { ai_args = CastBy co : ai_args ai
                         , ai_type = pSnd (coercionKind co) }

    argInfoAppArgs :: [ArgSpec] -> [OutExpr]
    argInfoAppArgs []                              = []
    argInfoAppArgs (CastBy {}                : _)  = []  -- Stop at a cast
    argInfoAppArgs (ValArg e                 : as) = e       : argInfoAppArgs as
    argInfoAppArgs (TyArg { as_arg_ty = ty } : as) = Type ty : argInfoAppArgs as

    pushSimplifiedArgs :: SimplEnv -> [ArgSpec] -> SimplCont -> SimplCont
    pushSimplifiedArgs _env []           k = k
    pushSimplifiedArgs env  (arg : args) k
      = case arg of
          TyArg { as_arg_ty = arg_ty, as_hole_ty = hole_ty }
                   -> ApplyToTy  { sc_arg_ty = arg_ty, sc_hole_ty = hole_ty, sc_cont = rest }
          ValArg e -> ApplyToVal { sc_arg = e, sc_env = env, sc_dup = Simplified, sc_cont = rest }
          CastBy c -> CastIt c rest
      where
        rest = pushSimplifiedArgs env args k
               -- The env has an empty SubstEnv

    argInfoExpr :: OutId -> [ArgSpec] -> OutExpr
    -- NB: the [ArgSpec] is reversed so that the first arg
    -- in the list is the last one in the application
    argInfoExpr fun rev_args
      = go rev_args
      where
        go []                              = Var fun
        go (ValArg a                 : as) = go as `App` a
        go (TyArg { as_arg_ty = ty } : as) = go as `App` Type ty
        go (CastBy co                : as) = mkCast (go as) co

    {-
    ************************************************************************
    *                                                                      *
                    Functions on SimplCont
    *                                                                      *
    ************************************************************************
    -}

    mkBoringStop :: OutType -> SimplCont
    mkBoringStop ty = Stop ty BoringCtxt

    mkRhsStop :: OutType -> SimplCont       -- See Note [RHS of lets] in CoreUnfold
    mkRhsStop ty = Stop ty RhsCtxt

    mkLazyArgStop :: OutType -> CallCtxt -> SimplCont
    mkLazyArgStop ty cci = Stop ty cci

    -------------------
    contIsRhsOrArg :: SimplCont -> Bool
    contIsRhsOrArg (Stop {})       = True
    contIsRhsOrArg (StrictBind {}) = True
    contIsRhsOrArg (StrictArg {})  = True
    contIsRhsOrArg _               = False

    contIsRhs :: SimplCont -> Bool
    contIsRhs (Stop _ RhsCtxt) = True
    contIsRhs _                = False

    -------------------
    contIsDupable :: SimplCont -> Bool
    contIsDupable (Stop {})                         = True
    contIsDupable (ApplyToTy  { sc_cont = k })      = contIsDupable k
    contIsDupable (ApplyToVal { sc_dup = OkToDup }) = True -- See Note [DupFlag invariants]
    contIsDupable (Select { sc_dup = OkToDup })     = True -- ...ditto...
    contIsDupable (CastIt _ k)                      = contIsDupable k
    contIsDupable _                                 = False

    -------------------
    contIsTrivial :: SimplCont -> Bool
    contIsTrivial (Stop {})                                         = True
    contIsTrivial (ApplyToTy { sc_cont = k })                       = contIsTrivial k
    contIsTrivial (ApplyToVal { sc_arg = Coercion _, sc_cont = k }) = contIsTrivial k
    contIsTrivial (CastIt _ k)                                      = contIsTrivial k
    contIsTrivial _                                                 = False

    -------------------
    contResultType :: SimplCont -> OutType
    contResultType (Stop ty _)                  = ty
    contResultType (CastIt _ k)                 = contResultType k
    contResultType (StrictBind _ _ _ _ k)       = contResultType k
    contResultType (StrictArg _ _ k)            = contResultType k
    contResultType (Select { sc_cont = k })     = contResultType k
    contResultType (ApplyToTy  { sc_cont = k }) = contResultType k
    contResultType (ApplyToVal { sc_cont = k }) = contResultType k
    contResultType (TickIt _ k)                 = contResultType k

    contHoleType :: SimplCont -> OutType
    contHoleType (Stop ty _)                      = ty
    contHoleType (TickIt _ k)                     = contHoleType k
    contHoleType (CastIt co _)                    = pFst (coercionKind co)
    contHoleType (StrictBind b _ _ se _)          = substTy se (idType b)
    contHoleType (StrictArg ai _ _)               = funArgTy (ai_type ai)
    contHoleType (ApplyToTy  { sc_hole_ty = ty }) = ty  -- See Note [The hole type in ApplyToTy]
    contHoleType (ApplyToVal { sc_arg = e, sc_env = se, sc_dup = dup, sc_cont = k })
      = mkFunTy (perhapsSubstTy dup se (exprType e))
                (contHoleType k)
    contHoleType (Select { sc_dup = d, sc_bndr =  b, sc_env = se })
      = perhapsSubstTy d se (idType b)

    applyContToJoinType :: JoinArity -> SimplCont -> OutType -> OutType
    applyContToJoinType ar cont ty
      = setJoinResTy ar (contResultType cont) ty

    -------------------
    countArgs :: SimplCont -> Int
    -- Count all arguments, including types, coercions, and other values
    countArgs (ApplyToTy  { sc_cont = cont }) = 1 + countArgs cont
    countArgs (ApplyToVal { sc_cont = cont }) = 1 + countArgs cont
    countArgs _                               = 0

    contArgs :: SimplCont -> (Bool, [ArgSummary], SimplCont)
    -- Summarises value args, discards type args and coercions
    -- The returned continuation of the call is only used to
    -- answer questions like "are you interesting?"
    contArgs cont
      | lone cont = (True, [], cont)
      | otherwise = go [] cont
      where
        lone (ApplyToTy  {}) = False  -- See Note [Lone variables] in CoreUnfold
        lone (ApplyToVal {}) = False
        lone (CastIt {})     = False
        lone _               = True

        go args (ApplyToVal { sc_arg = arg, sc_env = se, sc_cont = k })
                                            = go (is_interesting arg se : args) k
        go args (ApplyToTy { sc_cont = k }) = go args k
        go args (CastIt _ k)                = go args k
        go args k                           = (False, reverse args, k)

        is_interesting arg se = interestingArg se arg
                       -- Do *not* use short-cutting substitution here
                       -- because we want to get as much IdInfo as possible

    -------------------
    mkArgInfo :: Id
              -> [CoreRule] -- Rules for function
              -> Int        -- Number of value args
              -> SimplCont  -- Context of the call
              -> ArgInfo

    mkArgInfo fun rules n_val_args call_cont
      | n_val_args < idArity fun            -- Note [Unsaturated functions]
      = ArgInfo { ai_fun = fun, ai_args = [], ai_type = fun_ty
                , ai_rules = rules, ai_encl = False
                , ai_strs = vanilla_stricts
                , ai_discs = vanilla_discounts }
      | otherwise
      = ArgInfo { ai_fun = fun, ai_args = [], ai_type = fun_ty
                , ai_rules = rules
                , ai_encl = interestingArgContext rules call_cont
                , ai_strs  = add_type_str fun_ty arg_stricts
                , ai_discs = arg_discounts }
      where
        fun_ty = idType fun

        vanilla_discounts, arg_discounts :: [Int]
        vanilla_discounts = repeat 0
        arg_discounts = case idUnfolding fun of
                            CoreUnfolding {uf_guidance = UnfIfGoodArgs {ug_args = discounts}}
                                  -> discounts ++ vanilla_discounts
                            _     -> vanilla_discounts

        vanilla_stricts, arg_stricts :: [Bool]
        vanilla_stricts  = repeat False

        arg_stricts
          = case splitStrictSig (idStrictness fun) of
              (demands, result_info)
                    | not (demands `lengthExceeds` n_val_args)
                    ->      -- Enough args, use the strictness given.
                            -- For bottoming functions we used to pretend that the arg
                            -- is lazy, so that we don't treat the arg as an
                            -- interesting context.  This avoids substituting
                            -- top-level bindings for (say) strings into
                            -- calls to error.  But now we are more careful about
                            -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
                       if isBotRes result_info then
                            map isStrictDmd demands         -- Finite => result is bottom
                       else
                            map isStrictDmd demands ++ vanilla_stricts
                   | otherwise
                   -> WARN( True, text "More demands than arity" <+> ppr fun <+> ppr (idArity fun)
                                    <+> ppr n_val_args <+> ppr demands )
                       vanilla_stricts      -- Not enough args, or no strictness

        add_type_str :: Type -> [Bool] -> [Bool]
        -- If the function arg types are strict, record that in the 'strictness bits'
        -- No need to instantiate because unboxed types (which dominate the strict
        -- types) can't instantiate type variables.
        -- add_type_str is done repeatedly (for each call); might be better
        -- once-for-all in the function
        -- But beware primops/datacons with no strictness

        add_type_str
          = go
          where
            go _ [] = []
            go fun_ty strs            -- Look through foralls
                | Just (_, fun_ty') <- splitForAllTy_maybe fun_ty       -- Includes coercions
                = go fun_ty' strs
            go fun_ty (str:strs)      -- Add strict-type info
                | Just (arg_ty, fun_ty') <- splitFunTy_maybe fun_ty
                = (str || Just False == isLiftedType_maybe arg_ty) : go fun_ty' strs
                   -- If the type is levity-polymorphic, we can't know whether it's
                   -- strict. isLiftedType_maybe will return Just False only when
                   -- we're sure the type is unlifted.
            go _ strs
                = strs

    {- Note [Unsaturated functions]
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider (test eyeball/inline4)
            x = a:as
            y = f x
    where f has arity 2.  Then we do not want to inline 'x', because
    it'll just be floated out again.  Even if f has lots of discounts
    on its first argument -- it must be saturated for these to kick in
    -}

    {-
    ************************************************************************
    *                                                                      *
            Interesting arguments
    *                                                                      *
    ************************************************************************

    Note [Interesting call context]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want to avoid inlining an expression where there can't possibly be
    any gain, such as in an argument position.  Hence, if the continuation
    is interesting (eg. a case scrutinee, application etc.) then we
    inline, otherwise we don't.

    Previously some_benefit used to return True only if the variable was
    applied to some value arguments.  This didn't work:

            let x = _coerce_ (T Int) Int (I# 3) in
            case _coerce_ Int (T Int) x of
                    I# y -> ....

    we want to inline x, but can't see that it's a constructor in a case
    scrutinee position, and some_benefit is False.

    Another example:

    dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)

    ....  case dMonadST _@_ x0 of (a,b,c) -> ....

    we'd really like to inline dMonadST here, but we *don't* want to
    inline if the case expression is just

            case x of y { DEFAULT -> ... }

    since we can just eliminate this case instead (x is in WHNF).  Similar
    applies when x is bound to a lambda expression.  Hence
    contIsInteresting looks for case expressions with just a single
    default case.
    -}

    interestingCallContext :: SimplCont -> CallCtxt
    -- See Note [Interesting call context]
    interestingCallContext cont
      = interesting cont
      where
        interesting (Select {})     = CaseCtxt
        interesting (ApplyToVal {}) = ValAppCtxt
            -- Can happen if we have (f Int |> co) y
            -- If f has an INLINE prag we need to give it some
            -- motivation to inline. See Note [Cast then apply]
            -- in CoreUnfold
        interesting (StrictArg _ cci _)         = cci
        interesting (StrictBind {})             = BoringCtxt
        interesting (Stop _ cci)                = cci
        interesting (TickIt _ k)                = interesting k
        interesting (ApplyToTy { sc_cont = k }) = interesting k
        interesting (CastIt _ k)                = interesting k
            -- If this call is the arg of a strict function, the context
            -- is a bit interesting.  If we inline here, we may get useful
            -- evaluation information to avoid repeated evals: e.g.
            --      x + (y * z)
            -- Here the contIsInteresting makes the '*' keener to inline,
            -- which in turn exposes a constructor which makes the '+' inline.
            -- Assuming that +,* aren't small enough to inline regardless.
            --
            -- It's also very important to inline in a strict context for things
            -- like
            --              foldr k z (f x)
            -- Here, the context of (f x) is strict, and if f's unfolding is
            -- a build it's *great* to inline it here.  So we must ensure that
            -- the context for (f x) is not totally uninteresting.

    interestingArgContext :: [CoreRule] -> SimplCont -> Bool
    -- If the argument has form (f x y), where x,y are boring,
    -- and f is marked INLINE, then we don't want to inline f.
    -- But if the context of the argument is
    --      g (f x y)
    -- where g has rules, then we *do* want to inline f, in case it
    -- exposes a rule that might fire.  Similarly, if the context is
    --      h (g (f x x))
    -- where h has rules, then we do want to inline f; hence the
    -- call_cont argument to interestingArgContext
    --
    -- The ai-rules flag makes this happen; if it's
    -- set, the inliner gets just enough keener to inline f
    -- regardless of how boring f's arguments are, if it's marked INLINE
    --
    -- The alternative would be to *always* inline an INLINE function,
    -- regardless of how boring its context is; but that seems overkill
    -- For example, it'd mean that wrapper functions were always inlined
    --
    -- The call_cont passed to interestingArgContext is the context of
    -- the call itself, e.g. g <hole> in the example above
    interestingArgContext rules call_cont
      = notNull rules || enclosing_fn_has_rules
      where
        enclosing_fn_has_rules = go call_cont

        go (Select {})         = False
        go (ApplyToVal {})     = False  -- Shouldn't really happen
        go (ApplyToTy  {})     = False  -- Ditto
        go (StrictArg _ cci _) = interesting cci
        go (StrictBind {})     = False      -- ??
        go (CastIt _ c)        = go c
        go (Stop _ cci)        = interesting cci
        go (TickIt _ c)        = go c

        interesting RuleArgCtxt = True
        interesting _           = False

    {- Note [Interesting arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    An argument is interesting if it deserves a discount for unfoldings
    with a discount in that argument position.  The idea is to avoid
    unfolding a function that is applied only to variables that have no
    unfolding (i.e. they are probably lambda bound): f x y z There is
    little point in inlining f here.

    Generally, *values* (like (C a b) and (\x.e)) deserve discounts.  But
    we must look through lets, eg (let x = e in C a b), because the let will
    float, exposing the value, if we inline.  That makes it different to
    exprIsHNF.

    Before 2009 we said it was interesting if the argument had *any* structure
    at all; i.e. (hasSomeUnfolding v).  But does too much inlining; see Trac #3016.

    But we don't regard (f x y) as interesting, unless f is unsaturated.
    If it's saturated and f hasn't inlined, then it's probably not going
    to now!

    Note [Conlike is interesting]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
            f d = ...((*) d x y)...
            ... f (df d')...
    where df is con-like. Then we'd really like to inline 'f' so that the
    rule for (*) (df d) can fire.  To do this
      a) we give a discount for being an argument of a class-op (eg (*) d)
      b) we say that a con-like argument (eg (df d)) is interesting
    -}

    interestingArg :: SimplEnv -> CoreExpr -> ArgSummary
    -- See Note [Interesting arguments]
    interestingArg env e = go env 0 e
      where
        -- n is # value args to which the expression is applied
        go env n (Var v)
           | SimplEnv { seIdSubst = ids, seInScope = in_scope } <- env
           = case snd <$> lookupVarEnv ids v of
               Nothing                     -> go_var n (refineFromInScope in_scope v)
               Just (DoneId v')            -> go_var n (refineFromInScope in_scope v')
               Just (DoneEx e)             -> go (zapSubstEnv env)             n e
               Just (ContEx tvs cvs ids e) -> go (setSubstEnv env tvs cvs ids) n e

        go _   _ (Lit {})          = ValueArg
        go _   _ (Type _)          = TrivArg
        go _   _ (Coercion _)      = TrivArg
        go env n (App fn (Type _)) = go env n fn
        go env n (App fn _)        = go env (n+1) fn
        go env n (Tick _ a)        = go env n a
        go env n (Cast e _)        = go env n e
        go env n (Lam v e)
           | isTyVar v             = go env n e
           | n>0                   = NonTrivArg     -- (\x.b) e   is NonTriv
           | otherwise             = ValueArg
        go _ _ (Case {})           = NonTrivArg
        go env n (Let b e)         = case go env' n e of
                                       ValueArg -> ValueArg
                                       _        -> NonTrivArg
                                   where
                                     env' = env `addNewInScopeIds` bindersOf b

        go_var n v
           | isConLikeId v     = ValueArg   -- Experimenting with 'conlike' rather that
                                            --    data constructors here
           | idArity v > n     = ValueArg   -- Catches (eg) primops with arity but no unfolding
           | n > 0             = NonTrivArg -- Saturated or unknown call
           | conlike_unfolding = ValueArg   -- n==0; look for an interesting unfolding
                                            -- See Note [Conlike is interesting]
           | otherwise         = TrivArg    -- n==0, no useful unfolding
           where
             conlike_unfolding = isConLikeUnfolding (idUnfolding v)

    {-
    ************************************************************************
    *                                                                      *
                      SimplifierMode
    *                                                                      *
    ************************************************************************

    The SimplifierMode controls several switches; see its definition in
    CoreMonad
            sm_rules      :: Bool     -- Whether RULES are enabled
            sm_inline     :: Bool     -- Whether inlining is enabled
            sm_case_case  :: Bool     -- Whether case-of-case is enabled
            sm_eta_expand :: Bool     -- Whether eta-expansion is enabled
    -}

    simplEnvForGHCi :: DynFlags -> SimplEnv
    simplEnvForGHCi dflags
      = mkSimplEnv $ SimplMode { sm_names = ["GHCi"]
                               , sm_phase = InitialPhase
                               , sm_rules = rules_on
                               , sm_inline = False
                               , sm_eta_expand = eta_expand_on
                               , sm_case_case = True }
      where
        rules_on      = gopt Opt_EnableRewriteRules   dflags
        eta_expand_on = gopt Opt_DoLambdaEtaExpansion dflags
       -- Do not do any inlining, in case we expose some unboxed
       -- tuple stuff that confuses the bytecode interpreter

    updModeForStableUnfoldings :: Activation -> SimplifierMode -> SimplifierMode
    -- See Note [Simplifying inside stable unfoldings]
    updModeForStableUnfoldings inline_rule_act current_mode
      = current_mode { sm_phase      = phaseFromActivation inline_rule_act
                     , sm_inline     = True
                     , sm_eta_expand = False }
                         -- sm_eta_expand: see Note [No eta expansion in stable unfoldings]
           -- For sm_rules, just inherit; sm_rules might be "off"
           -- because of -fno-enable-rewrite-rules
      where
        phaseFromActivation (ActiveAfter _ n) = Phase n
        phaseFromActivation _                 = InitialPhase

    updModeForRules :: SimplifierMode -> SimplifierMode
    -- See Note [Simplifying rules]
    updModeForRules current_mode
      = current_mode { sm_phase  = InitialPhase
                     , sm_inline = False
                     , sm_rules  = False
                     , sm_eta_expand = False }

    {- Note [Simplifying rules]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When simplifying a rule LHS, refrain from /any/ inlining or applying
    of other RULES.

    Doing anything to the LHS is plain confusing, because it means that what the
    rule matches is not what the user wrote. c.f. Trac #10595, and #10528.
    Moreover, inlining (or applying rules) on rule LHSs risks introducing
    Ticks into the LHS, which makes matching trickier. Trac #10665, #10745.

    Doing this to either side confounds tools like HERMIT, which seek to reason
    about and apply the RULES as originally written. See Trac #10829.

    Note [No eta expansion in stable unfoldings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have a stable unfolding

      f :: Ord a => a -> IO ()
      -- Unfolding template
      --    = /\a \(d:Ord a) (x:a). bla

    we do not want to eta-expand to

      f :: Ord a => a -> IO ()
      -- Unfolding template
      --    = (/\a \(d:Ord a) (x:a) (eta:State#). bla eta) |> co

    because not specialisation of the overloading doesn't work properly
    (see Note [Specialisation shape] in Specialise), Trac #9509.

    So we disable eta-expansion in stable unfoldings.

    Note [Inlining in gentle mode]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Something is inlined if
       (i)   the sm_inline flag is on, AND
       (ii)  the thing has an INLINE pragma, AND
       (iii) the thing is inlinable in the earliest phase.

    Example of why (iii) is important:
      {-# INLINE [~1] g #-}
      g = ...

      {-# INLINE f #-}
      f x = g (g x)

    If we were to inline g into f's inlining, then an importing module would
    never be able to do
            f e --> g (g e) ---> RULE fires
    because the stable unfolding for f has had g inlined into it.

    On the other hand, it is bad not to do ANY inlining into an
    stable unfolding, because then recursive knots in instance declarations
    don't get unravelled.

    However, *sometimes* SimplGently must do no call-site inlining at all
    (hence sm_inline = False).  Before full laziness we must be careful
    not to inline wrappers, because doing so inhibits floating
        e.g. ...(case f x of ...)...
        ==> ...(case (case x of I# x# -> fw x#) of ...)...
        ==> ...(case x of I# x# -> case fw x# of ...)...
    and now the redex (f x) isn't floatable any more.

    The no-inlining thing is also important for Template Haskell.  You might be
    compiling in one-shot mode with -O2; but when TH compiles a splice before
    running it, we don't want to use -O2.  Indeed, we don't want to inline
    anything, because the byte-code interpreter might get confused about
    unboxed tuples and suchlike.

    Note [Simplifying inside stable unfoldings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We must take care with simplification inside stable unfoldings (which come from
    INLINE pragmas).

    First, consider the following example
            let f = \pq -> BIG
            in
            let g = \y -> f y y
                {-# INLINE g #-}
            in ...g...g...g...g...g...
    Now, if that's the ONLY occurrence of f, it might be inlined inside g,
    and thence copied multiple times when g is inlined. HENCE we treat
    any occurrence in a stable unfolding as a multiple occurrence, not a single
    one; see OccurAnal.addRuleUsage.

    Second, we do want *do* to some modest rules/inlining stuff in stable
    unfoldings, partly to eliminate senseless crap, and partly to break
    the recursive knots generated by instance declarations.

    However, suppose we have
            {-# INLINE <act> f #-}
            f = <rhs>
    meaning "inline f in phases p where activation <act>(p) holds".
    Then what inlinings/rules can we apply to the copy of <rhs> captured in
    f's stable unfolding?  Our model is that literally <rhs> is substituted for
    f when it is inlined.  So our conservative plan (implemented by
    updModeForStableUnfoldings) is this:

      -------------------------------------------------------------
      When simplifying the RHS of an stable unfolding, set the phase
      to the phase in which the stable unfolding first becomes active
      -------------------------------------------------------------

    That ensures that

      a) Rules/inlinings that *cease* being active before p will
         not apply to the stable unfolding, consistent with it being
         inlined in its *original* form in phase p.

      b) Rules/inlinings that only become active *after* p will
         not apply to the stable unfolding, again to be consistent with
         inlining the *original* rhs in phase p.

    For example,
            {-# INLINE f #-}
            f x = ...g...

            {-# NOINLINE [1] g #-}
            g y = ...

            {-# RULE h g = ... #-}
    Here we must not inline g into f's RHS, even when we get to phase 0,
    because when f is later inlined into some other module we want the
    rule for h to fire.

    Similarly, consider
            {-# INLINE f #-}
            f x = ...g...

            g y = ...
    and suppose that there are auto-generated specialisations and a strictness
    wrapper for g.  The specialisations get activation AlwaysActive, and the
    strictness wrapper get activation (ActiveAfter 0).  So the strictness
    wrepper fails the test and won't be inlined into f's stable unfolding. That
    means f can inline, expose the specialised call to g, so the specialisation
    rules can fire.

    A note about wrappers
    ~~~~~~~~~~~~~~~~~~~~~
    It's also important not to inline a worker back into a wrapper.
    A wrapper looks like
            wraper = inline_me (\x -> ...worker... )
    Normally, the inline_me prevents the worker getting inlined into
    the wrapper (initially, the worker's only call site!).  But,
    if the wrapper is sure to be called, the strictness analyser will
    mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
    continuation.
    -}

    activeUnfolding :: SimplEnv -> Id -> Bool
    activeUnfolding env id
      | isCompulsoryUnfolding (realIdUnfolding id)
      = True   -- Even sm_inline can't override compulsory unfoldings
      | otherwise
      = isActive (sm_phase mode) (idInlineActivation id)
      && sm_inline mode
          -- `or` isStableUnfolding (realIdUnfolding id)
          -- Inline things when
          --  (a) they are active
          --  (b) sm_inline says so, except that for stable unfoldings
          --                         (ie pragmas) we inline anyway
      where
        mode = getMode env

    getUnfoldingInRuleMatch :: SimplEnv -> InScopeEnv
    -- When matching in RULE, we want to "look through" an unfolding
    -- (to see a constructor) if *rules* are on, even if *inlinings*
    -- are not.  A notable example is DFuns, which really we want to
    -- match in rules like (op dfun) in gentle mode. Another example
    -- is 'otherwise' which we want exprIsConApp_maybe to be able to
    -- see very early on
    getUnfoldingInRuleMatch env
      = (in_scope, id_unf)
      where
        in_scope = seInScope env
        mode = getMode env
        id_unf id | unf_is_active id = idUnfolding id
                  | otherwise        = NoUnfolding
        unf_is_active id
         | not (sm_rules mode) = -- active_unfolding_minimal id
                                 isStableUnfolding (realIdUnfolding id)
            -- Do we even need to test this?  I think this InScopeEnv
            -- is only consulted if activeRule returns True, which
            -- never happens if sm_rules is False
         | otherwise           = isActive (sm_phase mode) (idInlineActivation id)

    ----------------------
    activeRule :: SimplEnv -> Activation -> Bool
    -- Nothing => No rules at all
    activeRule env
      | not (sm_rules mode) = \_ -> False     -- Rewriting is off
      | otherwise           = isActive (sm_phase mode)
      where
        mode = getMode env

    {-
    ************************************************************************
    *                                                                      *
                      preInlineUnconditionally
    *                                                                      *
    ************************************************************************

    preInlineUnconditionally
    ~~~~~~~~~~~~~~~~~~~~~~~~
    @preInlineUnconditionally@ examines a bndr to see if it is used just
    once in a completely safe way, so that it is safe to discard the
    binding inline its RHS at the (unique) usage site, REGARDLESS of how
    big the RHS might be.  If this is the case we don't simplify the RHS
    first, but just inline it un-simplified.

    This is much better than first simplifying a perhaps-huge RHS and then
    inlining and re-simplifying it.  Indeed, it can be at least quadratically
    better.  Consider

            x1 = e1
            x2 = e2[x1]
            x3 = e3[x2]
            ...etc...
            xN = eN[xN-1]

    We may end up simplifying e1 N times, e2 N-1 times, e3 N-3 times etc.
    This can happen with cascades of functions too:

            f1 = \x1.e1
            f2 = \xs.e2[f1]
            f3 = \xs.e3[f3]
            ...etc...

    THE MAIN INVARIANT is this:

            ----  preInlineUnconditionally invariant -----
       IF preInlineUnconditionally chooses to inline x = <rhs>
       THEN doing the inlining should not change the occurrence
            info for the free vars of <rhs>
            ----------------------------------------------

    For example, it's tempting to look at trivial binding like
            x = y
    and inline it unconditionally.  But suppose x is used many times,
    but this is the unique occurrence of y.  Then inlining x would change
    y's occurrence info, which breaks the invariant.  It matters: y
    might have a BIG rhs, which will now be dup'd at every occurrenc of x.

    Even RHSs labelled InlineMe aren't caught here, because there might be
    no benefit from inlining at the call site.

    [Sept 01] Don't unconditionally inline a top-level thing, because that
    can simply make a static thing into something built dynamically.  E.g.
            x = (a,b)
            main = \s -> h x

    [Remember that we treat \s as a one-shot lambda.]  No point in
    inlining x unless there is something interesting about the call site.

    But watch out: if you aren't careful, some useful foldr/build fusion
    can be lost (most notably in spectral/hartel/parstof) because the
    foldr didn't see the build.  Doing the dynamic allocation isn't a big
    deal, in fact, but losing the fusion can be.  But the right thing here
    seems to be to do a callSiteInline based on the fact that there is
    something interesting about the call site (it's strict).  Hmm.  That
    seems a bit fragile.

    Conclusion: inline top level things gaily until Phase 0 (the last
    phase), at which point don't.

    Note [pre/postInlineUnconditionally in gentle mode]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Even in gentle mode we want to do preInlineUnconditionally.  The
    reason is that too little clean-up happens if you don't inline
    use-once things.  Also a bit of inlining is *good* for full laziness;
    it can expose constant sub-expressions.  Example in
    spectral/mandel/Mandel.hs, where the mandelset function gets a useful
    let-float if you inline windowToViewport

    However, as usual for Gentle mode, do not inline things that are
    inactive in the intial stages.  See Note [Gentle mode].

    Note [Stable unfoldings and preInlineUnconditionally]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Surprisingly, do not pre-inline-unconditionally Ids with INLINE pragmas!
    Example

       {-# INLINE f #-}
       f :: Eq a => a -> a
       f x = ...

       fInt :: Int -> Int
       fInt = f Int dEqInt

       ...fInt...fInt...fInt...

    Here f occurs just once, in the RHS of fInt. But if we inline it there
    it might make fInt look big, and we'll lose the opportunity to inline f
    at each of fInt's call sites.  The INLINE pragma will only inline when
    the application is saturated for exactly this reason; and we don't
    want PreInlineUnconditionally to second-guess it.  A live example is
    Trac #3736.
        c.f. Note [Stable unfoldings and postInlineUnconditionally]

    Note [Top-level bottoming Ids]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Don't inline top-level Ids that are bottoming, even if they are used just
    once, because FloatOut has gone to some trouble to extract them out.
    Inlining them won't make the program run faster!

    Note [Do not inline CoVars unconditionally]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Coercion variables appear inside coercions, and the RHS of a let-binding
    is a term (not a coercion) so we can't necessarily inline the latter in
    the former.
    -}

    preInlineUnconditionally :: DynFlags -> SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
    -- Precondition: rhs satisfies the let/app invariant
    -- See Note [CoreSyn let/app invariant] in CoreSyn
    -- Reason: we don't want to inline single uses, or discard dead bindings,
    --         for unlifted, side-effect-ful bindings
    preInlineUnconditionally dflags env top_lvl bndr rhs
      | not active                               = False
      | isStableUnfolding (idUnfolding bndr)     = False -- Note [Stable unfoldings and preInlineUnconditionally]
      | isTopLevel top_lvl && isBottomingId bndr = False -- Note [Top-level bottoming Ids]
      | not (gopt Opt_SimplPreInlining dflags)   = False
      | isCoVar bndr                             = False -- Note [Do not inline CoVars unconditionally]
      | otherwise = case idOccInfo bndr of
                      IAmDead                    -> True -- Happens in ((\x.1) v)
                      occ@OneOcc { occ_one_br = True }
                                                 -> try_once (occ_in_lam occ)
                                                             (occ_int_cxt occ)
                      _                          -> False
      where
        mode = getMode env
        active = isActive (sm_phase mode) act
                 -- See Note [pre/postInlineUnconditionally in gentle mode]
        act = idInlineActivation bndr
        try_once in_lam int_cxt     -- There's one textual occurrence
            | not in_lam = isNotTopLevel top_lvl || early_phase
            | otherwise  = int_cxt && canInlineInLam rhs

    -- Be very careful before inlining inside a lambda, because (a) we must not
    -- invalidate occurrence information, and (b) we want to avoid pushing a
    -- single allocation (here) into multiple allocations (inside lambda).
    -- Inlining a *function* with a single *saturated* call would be ok, mind you.
    --      || (if is_cheap && not (canInlineInLam rhs) then pprTrace "preinline" (ppr bndr <+> ppr rhs) ok else ok)
    --      where
    --              is_cheap = exprIsCheap rhs
    --              ok = is_cheap && int_cxt

            --      int_cxt         The context isn't totally boring
            -- E.g. let f = \ab.BIG in \y. map f xs
            --      Don't want to substitute for f, because then we allocate
            --      its closure every time the \y is called
            -- But: let f = \ab.BIG in \y. map (f y) xs
            --      Now we do want to substitute for f, even though it's not
            --      saturated, because we're going to allocate a closure for
            --      (f y) every time round the loop anyhow.

            -- canInlineInLam => free vars of rhs are (Once in_lam) or Many,
            -- so substituting rhs inside a lambda doesn't change the occ info.
            -- Sadly, not quite the same as exprIsHNF.
        canInlineInLam (Lit _)    = True
        canInlineInLam (Lam b e)  = isRuntimeVar b || canInlineInLam e
        canInlineInLam (Tick t e) = not (tickishIsCode t) && canInlineInLam e
        canInlineInLam _          = False
          -- not ticks.  Counting ticks cannot be duplicated, and non-counting
          -- ticks around a Lam will disappear anyway.

        early_phase = case sm_phase mode of
                        Phase 0 -> False
                        _       -> True
    -- If we don't have this early_phase test, consider
    --      x = length [1,2,3]
    -- The full laziness pass carefully floats all the cons cells to
    -- top level, and preInlineUnconditionally floats them all back in.
    -- Result is (a) static allocation replaced by dynamic allocation
    --           (b) many simplifier iterations because this tickles
    --               a related problem; only one inlining per pass
    --
    -- On the other hand, I have seen cases where top-level fusion is
    -- lost if we don't inline top level thing (e.g. string constants)
    -- Hence the test for phase zero (which is the phase for all the final
    -- simplifications).  Until phase zero we take no special notice of
    -- top level things, but then we become more leery about inlining
    -- them.

    {-
    ************************************************************************
    *                                                                      *
                      postInlineUnconditionally
    *                                                                      *
    ************************************************************************

    postInlineUnconditionally
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    @postInlineUnconditionally@ decides whether to unconditionally inline
    a thing based on the form of its RHS; in particular if it has a
    trivial RHS.  If so, we can inline and discard the binding altogether.

    NB: a loop breaker has must_keep_binding = True and non-loop-breakers
    only have *forward* references. Hence, it's safe to discard the binding

    NOTE: This isn't our last opportunity to inline.  We're at the binding
    site right now, and we'll get another opportunity when we get to the
    occurrence(s)

    Note that we do this unconditional inlining only for trival RHSs.
    Don't inline even WHNFs inside lambdas; doing so may simply increase
    allocation when the function is called. This isn't the last chance; see
    NOTE above.

    NB: Even inline pragmas (e.g. IMustBeINLINEd) are ignored here Why?
    Because we don't even want to inline them into the RHS of constructor
    arguments. See NOTE above

    NB: At one time even NOINLINE was ignored here: if the rhs is trivial
    it's best to inline it anyway.  We often get a=E; b=a from desugaring,
    with both a and b marked NOINLINE.  But that seems incompatible with
    our new view that inlining is like a RULE, so I'm sticking to the 'active'
    story for now.
    -}

    postInlineUnconditionally
        :: DynFlags -> SimplEnv -> TopLevelFlag
        -> OutId            -- The binder (an InId would be fine too)
                            --            (*not* a CoVar)
        -> OccInfo          -- From the InId
        -> OutExpr
        -> Unfolding
        -> Bool
    -- Precondition: rhs satisfies the let/app invariant
    -- See Note [CoreSyn let/app invariant] in CoreSyn
    -- Reason: we don't want to inline single uses, or discard dead bindings,
    --         for unlifted, side-effect-ful bindings
    postInlineUnconditionally dflags env top_lvl bndr occ_info rhs unfolding
      | not active                  = False
      | isWeakLoopBreaker occ_info  = False -- If it's a loop-breaker of any kind, don't inline
                                            -- because it might be referred to "earlier"
      | isStableUnfolding unfolding = False -- Note [Stable unfoldings and postInlineUnconditionally]
      | isTopLevel top_lvl          = False -- Note [Top level and postInlineUnconditionally]
      | exprIsTrivial rhs           = True
      | otherwise
      = case occ_info of
            -- The point of examining occ_info here is that for *non-values*
            -- that occur outside a lambda, the call-site inliner won't have
            -- a chance (because it doesn't know that the thing
            -- only occurs once).   The pre-inliner won't have gotten
            -- it either, if the thing occurs in more than one branch
            -- So the main target is things like
            --      let x = f y in
            --      case v of
            --         True  -> case x of ...
            --         False -> case x of ...
            -- This is very important in practice; e.g. wheel-seive1 doubles
            -- in allocation if you miss this out
          OneOcc { occ_in_lam = in_lam, occ_int_cxt = int_cxt }
                   -- OneOcc => no code-duplication issue
            ->     smallEnoughToInline dflags unfolding     -- Small enough to dup
                            -- ToDo: consider discount on smallEnoughToInline if int_cxt is true
                            --
                            -- NB: Do NOT inline arbitrarily big things, even if one_br is True
                            -- Reason: doing so risks exponential behaviour.  We simplify a big
                            --         expression, inline it, and simplify it again.  But if the
                            --         very same thing happens in the big expression, we get
                            --         exponential cost!
                            -- PRINCIPLE: when we've already simplified an expression once,
                            -- make sure that we only inline it if it's reasonably small.

               && (not in_lam ||
                            -- Outside a lambda, we want to be reasonably aggressive
                            -- about inlining into multiple branches of case
                            -- e.g. let x = <non-value>
                            --      in case y of { C1 -> ..x..; C2 -> ..x..; C3 -> ... }
                            -- Inlining can be a big win if C3 is the hot-spot, even if
                            -- the uses in C1, C2 are not 'interesting'
                            -- An example that gets worse if you add int_cxt here is 'clausify'

                    (isCheapUnfolding unfolding && int_cxt))
                            -- isCheap => acceptable work duplication; in_lam may be true
                            -- int_cxt to prevent us inlining inside a lambda without some
                            -- good reason.  See the notes on int_cxt in preInlineUnconditionally

          IAmDead -> True   -- This happens; for example, the case_bndr during case of
                            -- known constructor:  case (a,b) of x { (p,q) -> ... }
                            -- Here x isn't mentioned in the RHS, so we don't want to
                            -- create the (dead) let-binding  let x = (a,b) in ...

          _ -> False

    -- Here's an example that we don't handle well:
    --      let f = if b then Left (\x.BIG) else Right (\y.BIG)
    --      in \y. ....case f of {...} ....
    -- Here f is used just once, and duplicating the case work is fine (exprIsCheap).
    -- But
    --  - We can't preInlineUnconditionally because that woud invalidate
    --    the occ info for b.
    --  - We can't postInlineUnconditionally because the RHS is big, and
    --    that risks exponential behaviour
    --  - We can't call-site inline, because the rhs is big
    -- Alas!

      where
        active = isActive (sm_phase (getMode env)) (idInlineActivation bndr)
            -- See Note [pre/postInlineUnconditionally in gentle mode]

    {-
    Note [Top level and postInlineUnconditionally]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We don't do postInlineUnconditionally for top-level things (even for
    ones that are trivial):

      * Doing so will inline top-level error expressions that have been
        carefully floated out by FloatOut.  More generally, it might
        replace static allocation with dynamic.

      * Even for trivial expressions there's a problem.  Consider
          {-# RULE "foo" forall (xs::[T]). reverse xs = ruggle xs #-}
          blah xs = reverse xs
          ruggle = sort
        In one simplifier pass we might fire the rule, getting
          blah xs = ruggle xs
        but in *that* simplifier pass we must not do postInlineUnconditionally
        on 'ruggle' because then we'll have an unbound occurrence of 'ruggle'

        If the rhs is trivial it'll be inlined by callSiteInline, and then
        the binding will be dead and discarded by the next use of OccurAnal

      * There is less point, because the main goal is to get rid of local
        bindings used in multiple case branches.

      * The inliner should inline trivial things at call sites anyway.

      * The Id might be exported.  We could check for that separately,
        but since we aren't going to postInlineUnconditionally /any/
        top-level bindings, we don't need to test.

    Note [Stable unfoldings and postInlineUnconditionally]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Do not do postInlineUnconditionally if the Id has an stable unfolding,
    otherwise we lose the unfolding.  Example

         -- f has stable unfolding with rhs (e |> co)
         --   where 'e' is big
         f = e |> co

    Then there's a danger we'll optimise to

         f' = e
         f = f' |> co

    and now postInlineUnconditionally, losing the stable unfolding on f.  Now f'
    won't inline because 'e' is too big.

        c.f. Note [Stable unfoldings and preInlineUnconditionally]

    ************************************************************************
    *                                                                      *
            Rebuilding a lambda
    *                                                                      *
    ************************************************************************
    -}

    mkLam :: SimplEnv -> [OutBndr] -> OutExpr -> SimplCont -> SimplM OutExpr
    -- mkLam tries three things
    --      a) eta reduction, if that gives a trivial expression
    --      b) eta expansion [only if there are some value lambdas]

    mkLam _env [] body _cont
      = return body
    mkLam env bndrs body cont
      = do { dflags <- getDynFlags
           ; mkLam' dflags bndrs body }
      where
        mkLam' :: DynFlags -> [OutBndr] -> OutExpr -> SimplM OutExpr
        mkLam' dflags bndrs (Cast body co)
          | not (any bad bndrs)
            -- Note [Casts and lambdas]
          = do { lam <- mkLam' dflags bndrs body
               ; return (mkCast lam (mkPiCos Representational bndrs co)) }
          where
            co_vars  = tyCoVarsOfCo co
            bad bndr = isCoVar bndr && bndr `elemVarSet` co_vars

        mkLam' dflags bndrs body@(Lam {})
          = mkLam' dflags (bndrs ++ bndrs1) body1
          where
            (bndrs1, body1) = collectBinders body

        mkLam' dflags bndrs (Tick t expr)
          | tickishFloatable t
          = mkTick t <$> mkLam' dflags bndrs expr

        mkLam' dflags bndrs body
          | gopt Opt_DoEtaReduction dflags
          , Just etad_lam <- tryEtaReduce bndrs body
          = do { tick (EtaReduction (head bndrs))
               ; return etad_lam }

          | not (contIsRhs cont)   -- See Note [Eta-expanding lambdas]
          , sm_eta_expand (getMode env)
          , any isRuntimeVar bndrs
          , let body_arity = exprEtaExpandArity dflags body
          , body_arity > 0
          = do { tick (EtaExpansion (head bndrs))
               ; let res = mkLams bndrs (etaExpand body_arity body)
               ; traceSmpl "eta expand" (vcat [text "before" <+> ppr (mkLams bndrs body)
                                              , text "after" <+> ppr res])
               ; return res }

          | otherwise
          = return (mkLams bndrs body)

    {-
    Note [Eta expanding lambdas]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general we *do* want to eta-expand lambdas. Consider
       f (\x -> case x of (a,b) -> \s -> blah)
    where 's' is a state token, and hence can be eta expanded.  This
    showed up in the code for GHc.IO.Handle.Text.hPutChar, a rather
    important function!

    The eta-expansion will never happen unless we do it now.  (Well, it's
    possible that CorePrep will do it, but CorePrep only has a half-baked
    eta-expander that can't deal with casts.  So it's much better to do it
    here.)

    However, when the lambda is let-bound, as the RHS of a let, we have a
    better eta-expander (in the form of tryEtaExpandRhs), so we don't
    bother to try expansion in mkLam in that case; hence the contIsRhs
    guard.

    NB: We check the SimplEnv (sm_eta_expand), not DynFlags.
        See Note [No eta expansion in stable unfoldings]

    Note [Casts and lambdas]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
            (\x. (\y. e) `cast` g1) `cast` g2
    There is a danger here that the two lambdas look separated, and the
    full laziness pass might float an expression to between the two.

    So this equation in mkLam' floats the g1 out, thus:
            (\x. e `cast` g1)  -->  (\x.e) `cast` (tx -> g1)
    where x:tx.

    In general, this floats casts outside lambdas, where (I hope) they
    might meet and cancel with some other cast:
            \x. e `cast` co   ===>   (\x. e) `cast` (tx -> co)
            /\a. e `cast` co  ===>   (/\a. e) `cast` (/\a. co)
            /\g. e `cast` co  ===>   (/\g. e) `cast` (/\g. co)
                              (if not (g `in` co))

    Notice that it works regardless of 'e'.  Originally it worked only
    if 'e' was itself a lambda, but in some cases that resulted in
    fruitless iteration in the simplifier.  A good example was when
    compiling Text.ParserCombinators.ReadPrec, where we had a definition
    like    (\x. Get `cast` g)
    where Get is a constructor with nonzero arity.  Then mkLam eta-expanded
    the Get, and the next iteration eta-reduced it, and then eta-expanded
    it again.

    Note also the side condition for the case of coercion binders.
    It does not make sense to transform
            /\g. e `cast` g  ==>  (/\g.e) `cast` (/\g.g)
    because the latter is not well-kinded.

    ************************************************************************
    *                                                                      *
                  Eta expansion
    *                                                                      *
    ************************************************************************
    -}

    tryEtaExpandRhs :: SimplEnv -> RecFlag -> OutId -> OutExpr
                    -> SimplM (Arity, OutExpr)
    -- See Note [Eta-expanding at let bindings]
    tryEtaExpandRhs env is_rec bndr rhs
      = do { dflags <- getDynFlags
           ; (new_arity, new_rhs) <- try_expand dflags

           ; WARN( new_arity < old_id_arity,
                   (text "Arity decrease:" <+> (ppr bndr <+> ppr old_id_arity
                    <+> ppr old_arity <+> ppr new_arity) $$ ppr new_rhs) )
                            -- Note [Arity decrease] in Simplify
             return (new_arity, new_rhs) }
      where
        try_expand dflags
          | exprIsTrivial rhs
          = return (exprArity rhs, rhs)

          | sm_eta_expand (getMode env)      -- Provided eta-expansion is on
          , let new_arity1 = findRhsArity dflags bndr rhs old_arity
                new_arity2 = idCallArity bndr
                new_arity  = max new_arity1 new_arity2
          , new_arity > old_arity      -- And the current manifest arity isn't enough
          = if is_rec == Recursive && isJoinId bndr
               then WARN(True, text "Can't eta-expand recursive join point:" <+>
                                 ppr bndr)
                    return (old_arity, rhs)
               else do { tick (EtaExpansion bndr)
                       ; return (new_arity, etaExpand new_arity rhs) }
          | otherwise
          = return (old_arity, rhs)

        old_arity    = exprArity rhs -- See Note [Do not expand eta-expand PAPs]
        old_id_arity = idArity bndr

    {-
    Note [Eta-expanding at let bindings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We now eta expand at let-bindings, which is where the payoff comes.
    The most significant thing is that we can do a simple arity analysis
    (in CoreArity.findRhsArity), which we can't do for free-floating lambdas

    One useful consequence of not eta-expanding lambdas is this example:
       genMap :: C a => ...
       {-# INLINE genMap #-}
       genMap f xs = ...

       myMap :: D a => ...
       {-# INLINE myMap #-}
       myMap = genMap

    Notice that 'genMap' should only inline if applied to two arguments.
    In the stable unfolding for myMap we'll have the unfolding
        (\d -> genMap Int (..d..))
    We do not want to eta-expand to
        (\d f xs -> genMap Int (..d..) f xs)
    because then 'genMap' will inline, and it really shouldn't: at least
    as far as the programmer is concerned, it's not applied to two
    arguments!

    Note [Do not eta-expand PAPs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We used to have old_arity = manifestArity rhs, which meant that we
    would eta-expand even PAPs.  But this gives no particular advantage,
    and can lead to a massive blow-up in code size, exhibited by Trac #9020.
    Suppose we have a PAP
        foo :: IO ()
        foo = returnIO ()
    Then we can eta-expand do
        foo = (\eta. (returnIO () |> sym g) eta) |> g
    where
        g :: IO () ~ State# RealWorld -> (# State# RealWorld, () #)

    But there is really no point in doing this, and it generates masses of
    coercions and whatnot that eventually disappear again. For T9020, GHC
    allocated 6.6G beore, and 0.8G afterwards; and residency dropped from
    1.8G to 45M.

    But note that this won't eta-expand, say
      f = \g -> map g
    Does it matter not eta-expanding such functions?  I'm not sure.  Perhaps
    strictness analysis will have less to bite on?

    ************************************************************************
    *                                                                      *
    \subsection{Floating lets out of big lambdas}
    *                                                                      *
    ************************************************************************

    Note [Floating and type abstraction]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:
            x = /\a. C e1 e2
    We'd like to float this to
            y1 = /\a. e1
            y2 = /\a. e2
            x  = /\a. C (y1 a) (y2 a)
    for the usual reasons: we want to inline x rather vigorously.

    You may think that this kind of thing is rare.  But in some programs it is
    common.  For example, if you do closure conversion you might get:

            data a :-> b = forall e. (e -> a -> b) :$ e

            f_cc :: forall a. a :-> a
            f_cc = /\a. (\e. id a) :$ ()

    Now we really want to inline that f_cc thing so that the
    construction of the closure goes away.

    So I have elaborated simplLazyBind to understand right-hand sides that look
    like
            /\ a1..an. body

    and treat them specially. The real work is done in SimplUtils.abstractFloats,
    but there is quite a bit of plumbing in simplLazyBind as well.

    The same transformation is good when there are lets in the body:

            /\abc -> let(rec) x = e in b
       ==>
            let(rec) x' = /\abc -> let x = x' a b c in e
            in
            /\abc -> let x = x' a b c in b

    This is good because it can turn things like:

            let f = /\a -> letrec g = ... g ... in g
    into
            letrec g' = /\a -> ... g' a ...
            in
            let f = /\ a -> g' a

    which is better.  In effect, it means that big lambdas don't impede
    let-floating.

    This optimisation is CRUCIAL in eliminating the junk introduced by
    desugaring mutually recursive definitions.  Don't eliminate it lightly!

    [May 1999]  If we do this transformation *regardless* then we can
    end up with some pretty silly stuff.  For example,

            let
                st = /\ s -> let { x1=r1 ; x2=r2 } in ...
            in ..
    becomes
            let y1 = /\s -> r1
                y2 = /\s -> r2
                st = /\s -> ...[y1 s/x1, y2 s/x2]
            in ..

    Unless the "..." is a WHNF there is really no point in doing this.
    Indeed it can make things worse.  Suppose x1 is used strictly,
    and is of the form

            x1* = case f y of { (a,b) -> e }

    If we abstract this wrt the tyvar we then can't do the case inline
    as we would normally do.

    That's why the whole transformation is part of the same process that
    floats let-bindings and constructor arguments out of RHSs.  In particular,
    it is guarded by the doFloatFromRhs call in simplLazyBind.

    Note [Which type variables to abstract over]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Abstract only over the type variables free in the rhs wrt which the
    new binding is abstracted.  Note that

      * The naive approach of abstracting wrt the
        tyvars free in the Id's /type/ fails. Consider:
            /\ a b -> let t :: (a,b) = (e1, e2)
                          x :: a     = fst t
                      in ...
        Here, b isn't free in x's type, but we must nevertheless
        abstract wrt b as well, because t's type mentions b.
        Since t is floated too, we'd end up with the bogus:
             poly_t = /\ a b -> (e1, e2)
             poly_x = /\ a   -> fst (poly_t a *b*)

      * We must do closeOverKinds.  Example (Trac #10934):
           f = /\k (f:k->*) (a:k). let t = AccFailure @ (f a) in ...
        Here we want to float 't', but we must remember to abstract over
        'k' as well, even though it is not explicitly mentioned in the RHS,
        otherwise we get
           t = /\ (f:k->*) (a:k). AccFailure @ (f a)
        which is obviously bogus.
    -}

    abstractFloats :: [OutTyVar] -> SimplEnv -> OutExpr -> SimplM ([OutBind], OutExpr)
    abstractFloats main_tvs body_env body
      = ASSERT( notNull body_floats )
        do  { (subst, float_binds) <- mapAccumLM abstract empty_subst body_floats
            ; return (float_binds, CoreSubst.substExpr (text "abstract_floats1") subst body) }
      where
        main_tv_set = mkVarSet main_tvs
        body_floats = getFloatBinds body_env
        empty_subst = CoreSubst.mkEmptySubst (seInScope body_env)

        abstract :: CoreSubst.Subst -> OutBind -> SimplM (CoreSubst.Subst, OutBind)
        abstract subst (NonRec id rhs)
          = do { (poly_id, poly_app) <- mk_poly tvs_here id
               ; let poly_rhs = mkLams tvs_here rhs'
                     subst'   = CoreSubst.extendIdSubst subst id poly_app
               ; return (subst', (NonRec poly_id poly_rhs)) }
          where
            rhs' = CoreSubst.substExpr (text "abstract_floats2") subst rhs

            -- tvs_here: see Note [Which type variables to abstract over]
            tvs_here = toposortTyVars $
                       filter (`elemVarSet` main_tv_set) $
                       closeOverKindsList $
                       exprSomeFreeVarsList isTyVar rhs'

        abstract subst (Rec prs)
           = do { (poly_ids, poly_apps) <- mapAndUnzipM (mk_poly tvs_here) ids
                ; let subst' = CoreSubst.extendSubstList subst (ids `zip` poly_apps)
                      poly_rhss = [mkLams tvs_here (CoreSubst.substExpr (text "abstract_floats3") subst' rhs)
                                  | rhs <- rhss]
                ; return (subst', Rec (poly_ids `zip` poly_rhss)) }
           where
             (ids,rhss) = unzip prs
                    -- For a recursive group, it's a bit of a pain to work out the minimal
                    -- set of tyvars over which to abstract:
                    --      /\ a b c.  let x = ...a... in
                    --                 letrec { p = ...x...q...
                    --                          q = .....p...b... } in
                    --                 ...
                    -- Since 'x' is abstracted over 'a', the {p,q} group must be abstracted
                    -- over 'a' (because x is replaced by (poly_x a)) as well as 'b'.
                    -- Since it's a pain, we just use the whole set, which is always safe
                    --
                    -- If you ever want to be more selective, remember this bizarre case too:
                    --      x::a = x
                    -- Here, we must abstract 'x' over 'a'.
             tvs_here = toposortTyVars main_tvs

        mk_poly tvs_here var
          = do { uniq <- getUniqueM
               ; let  poly_name = setNameUnique (idName var) uniq           -- Keep same name
                      poly_ty   = mkInvForAllTys tvs_here (idType var) -- But new type of course
                      poly_id   = transferPolyIdInfo var tvs_here $ -- Note [transferPolyIdInfo] in Id.hs
                                  mkLocalIdOrCoVar poly_name poly_ty
               ; return (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tvs_here)) }
                    -- In the olden days, it was crucial to copy the occInfo of the original var,
                    -- because we were looking at occurrence-analysed but as yet unsimplified code!
                    -- In particular, we mustn't lose the loop breakers.  BUT NOW we are looking
                    -- at already simplified code, so it doesn't matter
                    --
                    -- It's even right to retain single-occurrence or dead-var info:
                    -- Suppose we started with  /\a -> let x = E in B
                    -- where x occurs once in B. Then we transform to:
                    --      let x' = /\a -> E in /\a -> let x* = x' a in B
                    -- where x* has an INLINE prag on it.  Now, once x* is inlined,
                    -- the occurrences of x' will be just the occurrences originally
                    -- pinned on x.

    {-
    Note [Abstract over coercions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If a coercion variable (g :: a ~ Int) is free in the RHS, then so is the
    type variable a.  Rather than sort this mess out, we simply bale out and abstract
    wrt all the type variables if any of them are coercion variables.

    Historical note: if you use let-bindings instead of a substitution, beware of this:

                    -- Suppose we start with:
                    --
                    --      x = /\ a -> let g = G in E
                    --
                    -- Then we'll float to get
                    --
                    --      x = let poly_g = /\ a -> G
                    --          in /\ a -> let g = poly_g a in E
                    --
                    -- But now the occurrence analyser will see just one occurrence
                    -- of poly_g, not inside a lambda, so the simplifier will
                    -- PreInlineUnconditionally poly_g back into g!  Badk to square 1!
                    -- (I used to think that the "don't inline lone occurrences" stuff
                    --  would stop this happening, but since it's the *only* occurrence,
                    --  PreInlineUnconditionally kicks in first!)
                    --
                    -- Solution: put an INLINE note on g's RHS, so that poly_g seems
                    --           to appear many times.  (NB: mkInlineMe eliminates
                    --           such notes on trivial RHSs, so do it manually.)

    ************************************************************************
    *                                                                      *
                    prepareAlts
    *                                                                      *
    ************************************************************************

    prepareAlts tries these things:

    1.  Eliminate alternatives that cannot match, including the
        DEFAULT alternative.

    2.  If the DEFAULT alternative can match only one possible constructor,
        then make that constructor explicit.
        e.g.
            case e of x { DEFAULT -> rhs }
         ===>
            case e of x { (a,b) -> rhs }
        where the type is a single constructor type.  This gives better code
        when rhs also scrutinises x or e.

    3. Returns a list of the constructors that cannot holds in the
       DEFAULT alternative (if there is one)

    Here "cannot match" includes knowledge from GADTs

    It's a good idea to do this stuff before simplifying the alternatives, to
    avoid simplifying alternatives we know can't happen, and to come up with
    the list of constructors that are handled, to put into the IdInfo of the
    case binder, for use when simplifying the alternatives.

    Eliminating the default alternative in (1) isn't so obvious, but it can
    happen:

    data Colour = Red | Green | Blue

    f x = case x of
            Red -> ..
            Green -> ..
            DEFAULT -> h x

    h y = case y of
            Blue -> ..
            DEFAULT -> [ case y of ... ]

    If we inline h into f, the default case of the inlined h can't happen.
    If we don't notice this, we may end up filtering out *all* the cases
    of the inner case y, which give us nowhere to go!
    -}

    prepareAlts :: OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
    -- The returned alternatives can be empty, none are possible
    prepareAlts scrut case_bndr' alts
      | Just (tc, tys) <- splitTyConApp_maybe (varType case_bndr')
               -- Case binder is needed just for its type. Note that as an
               --   OutId, it has maximum information; this is important.
               --   Test simpl013 is an example
      = do { us <- getUniquesM
           ; let (idcs1, alts1)       = filterAlts tc tys imposs_cons alts
                 (yes2,  alts2)       = refineDefaultAlt us tc tys idcs1 alts1
                 (yes3, idcs3, alts3) = combineIdenticalAlts idcs1 alts2
                 -- "idcs" stands for "impossible default data constructors"
                 -- i.e. the constructors that can't match the default case
           ; when yes2 $ tick (FillInCaseDefault case_bndr')
           ; when yes3 $ tick (AltMerge case_bndr')
           ; return (idcs3, alts3) }

      | otherwise  -- Not a data type, so nothing interesting happens
      = return ([], alts)
      where
        imposs_cons = case scrut of
                        Var v -> otherCons (idUnfolding v)
                        _     -> []

    {-
    ************************************************************************
    *                                                                      *
                    mkCase
    *                                                                      *
    ************************************************************************

    mkCase tries these things

    1.  Merge Nested Cases

           case e of b {             ==>   case e of b {
             p1 -> rhs1                      p1 -> rhs1
             ...                             ...
             pm -> rhsm                      pm -> rhsm
             _  -> case b of b' {            pn -> let b'=b in rhsn
                         pn -> rhsn          ...
                         ...                 po -> let b'=b in rhso
                         po -> rhso          _  -> let b'=b in rhsd
                         _  -> rhsd
           }

        which merges two cases in one case when -- the default alternative of
        the outer case scrutises the same variable as the outer case. This
        transformation is called Case Merging.  It avoids that the same
        variable is scrutinised multiple times.

    2.  Eliminate Identity Case

            case e of               ===> e
                    True  -> True;
                    False -> False

        and similar friends.

    3.  Scrutinee Constant Folding

         case x op# k# of _ {  ===> case x of _ {
            a1# -> e1                  (a1# inv_op# k#) -> e1
            a2# -> e2                  (a2# inv_op# k#) -> e2
            ...                        ...
            DEFAULT -> ed              DEFAULT -> ed

         where (x op# k#) inv_op# k# == x

        And similarly for commuted arguments and for some unary operations.

        The purpose of this transformation is not only to avoid an arithmetic
        operation at runtime but to allow other transformations to apply in cascade.

        Example with the "Merge Nested Cases" optimization (from #12877):

              main = case t of t0
                 0##     -> ...
                 DEFAULT -> case t0 `minusWord#` 1## of t1
                    0##    -> ...
                    DEFAUT -> case t1 `minusWord#` 1## of t2
                       0##     -> ...
                       DEFAULT -> case t2 `minusWord#` 1## of _
                          0##     -> ...
                          DEFAULT -> ...

          becomes:

              main = case t of _
              0##     -> ...
              1##     -> ...
              2##     -> ...
              3##     -> ...
              DEFAULT -> ...

    -}

    mkCase, mkCase1, mkCase2, mkCase3
       :: DynFlags
       -> OutExpr -> OutId
       -> OutType -> [OutAlt]               -- Alternatives in standard (increasing) order
       -> SimplM OutExpr

    --------------------------------------------------
    --      1. Merge Nested Cases
    --------------------------------------------------

    mkCase dflags scrut outer_bndr alts_ty ((DEFAULT, _, deflt_rhs) : outer_alts)
      | gopt Opt_CaseMerge dflags
      , (ticks, Case (Var inner_scrut_var) inner_bndr _ inner_alts)
           <- stripTicksTop tickishFloatable deflt_rhs
      , inner_scrut_var == outer_bndr
      = do  { tick (CaseMerge outer_bndr)

            ; let wrap_alt (con, args, rhs) = ASSERT( outer_bndr `notElem` args )
                                              (con, args, wrap_rhs rhs)
                    -- Simplifier's no-shadowing invariant should ensure
                    -- that outer_bndr is not shadowed by the inner patterns
                  wrap_rhs rhs = Let (NonRec inner_bndr (Var outer_bndr)) rhs
                    -- The let is OK even for unboxed binders,

                  wrapped_alts | isDeadBinder inner_bndr = inner_alts
                               | otherwise               = map wrap_alt inner_alts

                  merged_alts = mergeAlts outer_alts wrapped_alts
                    -- NB: mergeAlts gives priority to the left
                    --      case x of
                    --        A -> e1
                    --        DEFAULT -> case x of
                    --                      A -> e2
                    --                      B -> e3
                    -- When we merge, we must ensure that e1 takes
                    -- precedence over e2 as the value for A!

            ; fmap (mkTicks ticks) $
              mkCase1 dflags scrut outer_bndr alts_ty merged_alts
            }
            -- Warning: don't call mkCase recursively!
            -- Firstly, there's no point, because inner alts have already had
            -- mkCase applied to them, so they won't have a case in their default
            -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
            -- in munge_rhs may put a case into the DEFAULT branch!

    mkCase dflags scrut bndr alts_ty alts = mkCase1 dflags scrut bndr alts_ty alts

    --------------------------------------------------
    --      2. Eliminate Identity Case
    --------------------------------------------------

    mkCase1 _dflags scrut case_bndr _ alts@((_,_,rhs1) : _)      -- Identity case
      | all identity_alt alts
      = do { tick (CaseIdentity case_bndr)
           ; return (mkTicks ticks $ re_cast scrut rhs1) }
      where
        ticks = concatMap (stripTicksT tickishFloatable . thdOf3) (tail alts)
        identity_alt (con, args, rhs) = check_eq rhs con args

        check_eq (Cast rhs co) con args        -- See Note [RHS casts]
          = not (any (`elemVarSet` tyCoVarsOfCo co) args) && check_eq rhs con args
        check_eq (Tick t e) alt args
          = tickishFloatable t && check_eq e alt args

        check_eq (Lit lit) (LitAlt lit') _     = lit == lit'
        check_eq (Var v) _ _  | v == case_bndr = True
        check_eq (Var v)   (DataAlt con) args
          | null arg_tys, null args            = v == dataConWorkId con
                                                 -- Optimisation only
        check_eq rhs        (DataAlt con) args = cheapEqExpr' tickishFloatable rhs $
                                                 mkConApp2 con arg_tys args
        check_eq _          _             _    = False

        arg_tys = tyConAppArgs (idType case_bndr)

            -- Note [RHS casts]
            -- ~~~~~~~~~~~~~~~~
            -- We've seen this:
            --      case e of x { _ -> x `cast` c }
            -- And we definitely want to eliminate this case, to give
            --      e `cast` c
            -- So we throw away the cast from the RHS, and reconstruct
            -- it at the other end.  All the RHS casts must be the same
            -- if (all identity_alt alts) holds.
            --
            -- Don't worry about nested casts, because the simplifier combines them

        re_cast scrut (Cast rhs co) = Cast (re_cast scrut rhs) co
        re_cast scrut _             = scrut

    mkCase1 dflags scrut bndr alts_ty alts = mkCase2 dflags scrut bndr alts_ty alts

    --------------------------------------------------
    --      2. Scrutinee Constant Folding
    --------------------------------------------------

    mkCase2 dflags scrut bndr alts_ty alts
      | gopt Opt_CaseFolding dflags
      , Just (scrut',f) <- caseRules dflags scrut
      = mkCase3 dflags scrut' bndr alts_ty (new_alts f)
      | otherwise
      = mkCase3 dflags scrut bndr alts_ty alts
      where
        -- We need to keep the correct association between the scrutinee and its
        -- binder if the latter isn't dead. Hence we wrap rhs of alternatives with
        -- "let bndr = ... in":
        --
        --     case v + 10 of y        =====> case v of y
        --        20      -> e1                 10      -> let y = 20     in e1
        --        DEFAULT -> e2                 DEFAULT -> let y = v + 10 in e2
        --
        -- Other transformations give: =====> case v of y'
        --                                      10      -> let y = 20      in e1
        --                                      DEFAULT -> let y = y' + 10 in e2
        --
        wrap_rhs l rhs
          | isDeadBinder bndr = rhs
          | otherwise         = Let (NonRec bndr l) rhs

        -- We need to re-sort the alternatives to preserve the #case_invariants#
        new_alts f = sortBy cmpAlt (map (mapAlt f) alts)

        mapAlt f alt@(c,bs,e) = case c of
          DEFAULT          -> (c, bs, wrap_rhs scrut e)
          LitAlt l
            | isLitValue l -> (LitAlt (mapLitValue dflags f l),
                               bs, wrap_rhs (Lit l) e)
          _ -> pprPanic "Unexpected alternative (mkCase2)" (ppr alt)

    --------------------------------------------------
    --      Catch-all
    --------------------------------------------------
    mkCase3 _dflags scrut bndr alts_ty alts
      = return (Case scrut bndr alts_ty alts)

    {-
    Note [Dead binders]
    ~~~~~~~~~~~~~~~~~~~~
    Note that dead-ness is maintained by the simplifier, so that it is
    accurate after simplification as well as before.

    Note [Cascading case merge]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Case merging should cascade in one sweep, because it
    happens bottom-up

          case e of a {
            DEFAULT -> case a of b
                          DEFAULT -> case b of c {
                                         DEFAULT -> e
                                         A -> ea
                          B -> eb
            C -> ec
    ==>
          case e of a {
            DEFAULT -> case a of b
                          DEFAULT -> let c = b in e
                          A -> let c = b in ea
                          B -> eb
            C -> ec
    ==>
          case e of a {
            DEFAULT -> let b = a in let c = b in e
            A -> let b = a in let c = b in ea
            B -> let b = a in eb
            C -> ec

    However here's a tricky case that we still don't catch, and I don't
    see how to catch it in one pass:

      case x of c1 { I# a1 ->
      case a1 of c2 ->
        0 -> ...
        DEFAULT -> case x of c3 { I# a2 ->
                   case a2 of ...

    After occurrence analysis (and its binder-swap) we get this

      case x of c1 { I# a1 ->
      let x = c1 in         -- Binder-swap addition
      case a1 of c2 ->
        0 -> ...
        DEFAULT -> case x of c3 { I# a2 ->
                   case a2 of ...

    When we simplify the inner case x, we'll see that
    x=c1=I# a1.  So we'll bind a2 to a1, and get

      case x of c1 { I# a1 ->
      case a1 of c2 ->
        0 -> ...
        DEFAULT -> case a1 of ...

    This is corect, but we can't do a case merge in this sweep
    because c2 /= a1.  Reason: the binding c1=I# a1 went inwards
    without getting changed to c1=I# c2.

    I don't think this is worth fixing, even if I knew how. It'll
    all come out in the next pass anyway.
    -}