coreSyn/CoreUtils.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Utility functions on @Core@ syntax
    -}

    {-# LANGUAGE CPP #-}

    -- | Commonly useful utilites for manipulating the Core language
    module CoreUtils (
            -- * Constructing expressions
            mkCast,
            mkTick, mkTicks, mkTickNoHNF, tickHNFArgs,
            bindNonRec, needsCaseBinding,
            mkAltExpr,

            -- * Taking expressions apart
            findDefault, addDefault, findAlt, isDefaultAlt,
            mergeAlts, trimConArgs,
            filterAlts, combineIdenticalAlts, refineDefaultAlt,

            -- * Properties of expressions
            exprType, coreAltType, coreAltsType, isExprLevPoly,
            exprIsDupable, exprIsTrivial, getIdFromTrivialExpr, exprIsBottom,
            getIdFromTrivialExpr_maybe,
            exprIsCheap, exprIsExpandable, exprIsOk, CheapAppFun,
            exprIsHNF, exprOkForSpeculation, exprOkForSideEffects, exprIsWorkFree,
            exprIsBig, exprIsConLike,
            rhsIsStatic, isCheapApp, isExpandableApp,
            exprIsLiteralString, exprIsTopLevelBindable,

            -- * Equality
            cheapEqExpr, cheapEqExpr', eqExpr,
            diffExpr, diffBinds,

            -- * Eta reduction
            tryEtaReduce,

            -- * Manipulating data constructors and types
            exprToType, exprToCoercion_maybe,
            applyTypeToArgs, applyTypeToArg,
            dataConRepInstPat, dataConRepFSInstPat,
            isEmptyTy,

            -- * Working with ticks
            stripTicksTop, stripTicksTopE, stripTicksTopT,
            stripTicksE, stripTicksT,

            -- * StaticPtr
            collectMakeStaticArgs,

            -- * Join points
            isJoinBind
        ) where

    #include "HsVersions.h"

    import CoreSyn
    import PrelNames ( makeStaticName )
    import PprCore
    import CoreFVs( exprFreeVars )
    import Var
    import SrcLoc
    import VarEnv
    import VarSet
    import Name
    import Literal
    import DataCon
    import PrimOp
    import Id
    import IdInfo
    import PrelNames( absentErrorIdKey )
    import Type
    import TyCoRep( TyBinder(..) )
    import Coercion
    import TyCon
    import Unique
    import Outputable
    import TysPrim
    import DynFlags
    import FastString
    import Maybes
    import ListSetOps       ( minusList )
    import BasicTypes       ( Arity )
    import Platform
    import Util
    import Pair
    import Data.Function       ( on )
    import Data.List
    import Data.Ord            ( comparing )
    import OrdList

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Find the type of a Core atom/expression}
    *                                                                      *
    ************************************************************************
    -}

    exprType :: CoreExpr -> Type
    -- ^ Recover the type of a well-typed Core expression. Fails when
    -- applied to the actual 'CoreSyn.Type' expression as it cannot
    -- really be said to have a type
    exprType (Var var)           = idType var
    exprType (Lit lit)           = literalType lit
    exprType (Coercion co)       = coercionType co
    exprType (Let bind body)
      | NonRec tv rhs <- bind    -- See Note [Type bindings]
      , Type ty <- rhs           = substTyWithUnchecked [tv] [ty] (exprType body)
      | otherwise                = exprType body
    exprType (Case _ _ ty _)     = ty
    exprType (Cast _ co)         = pSnd (coercionKind co)
    exprType (Tick _ e)          = exprType e
    exprType (Lam binder expr)   = mkLamType binder (exprType expr)
    exprType e@(App _ _)
      = case collectArgs e of
            (fun, args) -> applyTypeToArgs e (exprType fun) args

    exprType other = pprTrace "exprType" (pprCoreExpr other) alphaTy

    coreAltType :: CoreAlt -> Type
    -- ^ Returns the type of the alternatives right hand side
    coreAltType (_,bs,rhs)
      | any bad_binder bs = expandTypeSynonyms ty
      | otherwise         = ty    -- Note [Existential variables and silly type synonyms]
      where
        ty           = exprType rhs
        free_tvs     = tyCoVarsOfType ty
        bad_binder b = b `elemVarSet` free_tvs

    coreAltsType :: [CoreAlt] -> Type
    -- ^ Returns the type of the first alternative, which should be the same as for all alternatives
    coreAltsType (alt:_) = coreAltType alt
    coreAltsType []      = panic "corAltsType"

    -- | Is this expression levity polymorphic? This should be the
    -- same as saying (isKindLevPoly . typeKind . exprType) but
    -- much faster.
    isExprLevPoly :: CoreExpr -> Bool
    isExprLevPoly = go
      where
       go (Var _)                      = False  -- no levity-polymorphic binders
       go (Lit _)                      = False  -- no levity-polymorphic literals
       go e@(App f _) | not (go_app f) = False
                      | otherwise      = check_type e
       go (Lam _ _)                    = False
       go (Let _ e)                    = go e
       go e@(Case {})                  = check_type e -- checking type is fast
       go e@(Cast {})                  = check_type e
       go (Tick _ e)                   = go e
       go e@(Type {})                  = pprPanic "isExprLevPoly ty" (ppr e)
       go (Coercion {})                = False  -- this case can happen in SetLevels

       check_type = isTypeLevPoly . exprType  -- slow approach

          -- if the function is a variable (common case), check its
          -- levityInfo. This might mean we don't need to look up and compute
          -- on the type. Spec of these functions: return False if there is
          -- no possibility, ever, of this expression becoming levity polymorphic,
          -- no matter what it's applied to; return True otherwise.
          -- returning True is always safe. See also Note [Levity info] in
          -- IdInfo
       go_app (Var id)        = not (isNeverLevPolyId id)
       go_app (Lit _)         = False
       go_app (App f _)       = go_app f
       go_app (Lam _ e)       = go_app e
       go_app (Let _ e)       = go_app e
       go_app (Case _ _ ty _) = resultIsLevPoly ty
       go_app (Cast _ co)     = resultIsLevPoly (pSnd $ coercionKind co)
       go_app (Tick _ e)      = go_app e
       go_app e@(Type {})     = pprPanic "isExprLevPoly app ty" (ppr e)
       go_app e@(Coercion {}) = pprPanic "isExprLevPoly app co" (ppr e)

    {-
    Note [Type bindings]
    ~~~~~~~~~~~~~~~~~~~~
    Core does allow type bindings, although such bindings are
    not much used, except in the output of the desuguarer.
    Example:
         let a = Int in (\x:a. x)
    Given this, exprType must be careful to substitute 'a' in the
    result type (Trac #8522).

    Note [Existential variables and silly type synonyms]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
            data T = forall a. T (Funny a)
            type Funny a = Bool
            f :: T -> Bool
            f (T x) = x

    Now, the type of 'x' is (Funny a), where 'a' is existentially quantified.
    That means that 'exprType' and 'coreAltsType' may give a result that *appears*
    to mention an out-of-scope type variable.  See Trac #3409 for a more real-world
    example.

    Various possibilities suggest themselves:

     - Ignore the problem, and make Lint not complain about such variables

     - Expand all type synonyms (or at least all those that discard arguments)
          This is tricky, because at least for top-level things we want to
          retain the type the user originally specified.

     - Expand synonyms on the fly, when the problem arises. That is what
       we are doing here.  It's not too expensive, I think.

    Note that there might be existentially quantified coercion variables, too.
    -}

    -- Not defined with applyTypeToArg because you can't print from CoreSyn.
    applyTypeToArgs :: CoreExpr -> Type -> [CoreExpr] -> Type
    -- ^ A more efficient version of 'applyTypeToArg' when we have several arguments.
    -- The first argument is just for debugging, and gives some context
    applyTypeToArgs e op_ty args
      = go op_ty args
      where
        go op_ty []                   = op_ty
        go op_ty (Type ty : args)     = go_ty_args op_ty [ty] args
        go op_ty (Coercion co : args) = go_ty_args op_ty [mkCoercionTy co] args
        go op_ty (_ : args)           | Just (_, res_ty) <- splitFunTy_maybe op_ty
                                      = go res_ty args
        go _ _ = pprPanic "applyTypeToArgs" panic_msg

        -- go_ty_args: accumulate type arguments so we can
        -- instantiate all at once with piResultTys
        go_ty_args op_ty rev_tys (Type ty : args)
           = go_ty_args op_ty (ty:rev_tys) args
        go_ty_args op_ty rev_tys (Coercion co : args)
           = go_ty_args op_ty (mkCoercionTy co : rev_tys) args
        go_ty_args op_ty rev_tys args
           = go (piResultTys op_ty (reverse rev_tys)) args

        panic_msg = vcat [ text "Expression:" <+> pprCoreExpr e
                         , text "Type:" <+> ppr op_ty
                         , text "Args:" <+> ppr args ]

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Attaching notes}
    *                                                                      *
    ************************************************************************
    -}

    -- | Wrap the given expression in the coercion safely, dropping
    -- identity coercions and coalescing nested coercions
    mkCast :: CoreExpr -> Coercion -> CoreExpr
    mkCast e co
      | ASSERT2( coercionRole co == Representational
               , text "coercion" <+> ppr co <+> ptext (sLit "passed to mkCast")
                 <+> ppr e <+> text "has wrong role" <+> ppr (coercionRole co) )
        isReflCo co
      = e

    mkCast (Coercion e_co) co
      | isCoercionType (pSnd (coercionKind co))
           -- The guard here checks that g has a (~#) on both sides,
           -- otherwise decomposeCo fails.  Can in principle happen
           -- with unsafeCoerce
      = Coercion (mkCoCast e_co co)

    mkCast (Cast expr co2) co
      = WARN(let { Pair  from_ty  _to_ty  = coercionKind co;
                   Pair _from_ty2  to_ty2 = coercionKind co2} in
                not (from_ty `eqType` to_ty2),
                 vcat ([ text "expr:" <+> ppr expr
                       , text "co2:" <+> ppr co2
                       , text "co:" <+> ppr co ]) )
        mkCast expr (mkTransCo co2 co)

    mkCast (Tick t expr) co
       = Tick t (mkCast expr co)

    mkCast expr co
      = let Pair from_ty _to_ty = coercionKind co in
        WARN( not (from_ty `eqType` exprType expr),
              text "Trying to coerce" <+> text "(" <> ppr expr
              $$ text "::" <+> ppr (exprType expr) <> text ")"
              $$ ppr co $$ ppr (coercionType co) )
        (Cast expr co)

    -- | Wraps the given expression in the source annotation, dropping the
    -- annotation if possible.
    mkTick :: Tickish Id -> CoreExpr -> CoreExpr
    mkTick t orig_expr = mkTick' id id orig_expr
     where
      -- Some ticks (cost-centres) can be split in two, with the
      -- non-counting part having laxer placement properties.
      canSplit = tickishCanSplit t && tickishPlace (mkNoCount t) /= tickishPlace t

      mkTick' :: (CoreExpr -> CoreExpr) -- ^ apply after adding tick (float through)
              -> (CoreExpr -> CoreExpr) -- ^ apply before adding tick (float with)
              -> CoreExpr               -- ^ current expression
              -> CoreExpr
      mkTick' top rest expr = case expr of

        -- Never tick primitive string literals. These should ultimately float up to
        -- the top-level where they must be unadorned. See Note
        -- [CoreSyn top-level string literals] for details.
        _ | exprIsLiteralString expr          -> expr

        -- Cost centre ticks should never be reordered relative to each
        -- other. Therefore we can stop whenever two collide.
        Tick t2 e
          | ProfNote{} <- t2, ProfNote{} <- t -> top $ Tick t $ rest expr

        -- Otherwise we assume that ticks of different placements float
        -- through each other.
          | tickishPlace t2 /= tickishPlace t -> mkTick' (top . Tick t2) rest e

        -- For annotations this is where we make sure to not introduce
        -- redundant ticks.
          | tickishContains t t2              -> mkTick' top rest e
          | tickishContains t2 t              -> orig_expr
          | otherwise                         -> mkTick' top (rest . Tick t2) e

        -- Ticks don't care about types, so we just float all ticks
        -- through them. Note that it's not enough to check for these
        -- cases top-level. While mkTick will never produce Core with type
        -- expressions below ticks, such constructs can be the result of
        -- unfoldings. We therefore make an effort to put everything into
        -- the right place no matter what we start with.
        Cast e co   -> mkTick' (top . flip Cast co) rest e
        Coercion co -> Coercion co

        Lam x e
          -- Always float through type lambdas. Even for non-type lambdas,
          -- floating is allowed for all but the most strict placement rule.
          | not (isRuntimeVar x) || tickishPlace t /= PlaceRuntime
          -> mkTick' (top . Lam x) rest e

          -- If it is both counting and scoped, we split the tick into its
          -- two components, often allowing us to keep the counting tick on
          -- the outside of the lambda and push the scoped tick inside.
          -- The point of this is that the counting tick can probably be
          -- floated, and the lambda may then be in a position to be
          -- beta-reduced.
          | canSplit
          -> top $ Tick (mkNoScope t) $ rest $ Lam x $ mkTick (mkNoCount t) e

        App f arg
          -- Always float through type applications.
          | not (isRuntimeArg arg)
          -> mkTick' (top . flip App arg) rest f

          -- We can also float through constructor applications, placement
          -- permitting. Again we can split.
          | isSaturatedConApp expr && (tickishPlace t==PlaceCostCentre || canSplit)
          -> if tickishPlace t == PlaceCostCentre
             then top $ rest $ tickHNFArgs t expr
             else top $ Tick (mkNoScope t) $ rest $ tickHNFArgs (mkNoCount t) expr

        Var x
          | notFunction && tickishPlace t == PlaceCostCentre
          -> orig_expr
          | notFunction && canSplit
          -> top $ Tick (mkNoScope t) $ rest expr
          where
            -- SCCs can be eliminated on variables provided the variable
            -- is not a function.  In these cases the SCC makes no difference:
            -- the cost of evaluating the variable will be attributed to its
            -- definition site.  When the variable refers to a function, however,
            -- an SCC annotation on the variable affects the cost-centre stack
            -- when the function is called, so we must retain those.
            notFunction = not (isFunTy (idType x))

        Lit{}
          | tickishPlace t == PlaceCostCentre
          -> orig_expr

        -- Catch-all: Annotate where we stand
        _any -> top $ Tick t $ rest expr

    mkTicks :: [Tickish Id] -> CoreExpr -> CoreExpr
    mkTicks ticks expr = foldr mkTick expr ticks

    isSaturatedConApp :: CoreExpr -> Bool
    isSaturatedConApp e = go e []
      where go (App f a) as = go f (a:as)
            go (Var fun) args
               = isConLikeId fun && idArity fun == valArgCount args
            go (Cast f _) as = go f as
            go _ _ = False

    mkTickNoHNF :: Tickish Id -> CoreExpr -> CoreExpr
    mkTickNoHNF t e
      | exprIsHNF e = tickHNFArgs t e
      | otherwise   = mkTick t e

    -- push a tick into the arguments of a HNF (call or constructor app)
    tickHNFArgs :: Tickish Id -> CoreExpr -> CoreExpr
    tickHNFArgs t e = push t e
     where
      push t (App f (Type u)) = App (push t f) (Type u)
      push t (App f arg) = App (push t f) (mkTick t arg)
      push _t e = e

    -- | Strip ticks satisfying a predicate from top of an expression
    stripTicksTop :: (Tickish Id -> Bool) -> Expr b -> ([Tickish Id], Expr b)
    stripTicksTop p = go []
      where go ts (Tick t e) | p t = go (t:ts) e
            go ts other            = (reverse ts, other)

    -- | Strip ticks satisfying a predicate from top of an expression,
    -- returning the remaining expression
    stripTicksTopE :: (Tickish Id -> Bool) -> Expr b -> Expr b
    stripTicksTopE p = go
      where go (Tick t e) | p t = go e
            go other            = other

    -- | Strip ticks satisfying a predicate from top of an expression,
    -- returning the ticks
    stripTicksTopT :: (Tickish Id -> Bool) -> Expr b -> [Tickish Id]
    stripTicksTopT p = go []
      where go ts (Tick t e) | p t = go (t:ts) e
            go ts _                = ts

    -- | Completely strip ticks satisfying a predicate from an
    -- expression. Note this is O(n) in the size of the expression!
    stripTicksE :: (Tickish Id -> Bool) -> Expr b -> Expr b
    stripTicksE p expr = go expr
      where go (App e a)        = App (go e) (go a)
            go (Lam b e)        = Lam b (go e)
            go (Let b e)        = Let (go_bs b) (go e)
            go (Case e b t as)  = Case (go e) b t (map go_a as)
            go (Cast e c)       = Cast (go e) c
            go (Tick t e)
              | p t             = go e
              | otherwise       = Tick t (go e)
            go other            = other
            go_bs (NonRec b e)  = NonRec b (go e)
            go_bs (Rec bs)      = Rec (map go_b bs)
            go_b (b, e)         = (b, go e)
            go_a (c,bs,e)       = (c,bs, go e)

    stripTicksT :: (Tickish Id -> Bool) -> Expr b -> [Tickish Id]
    stripTicksT p expr = fromOL $ go expr
      where go (App e a)        = go e `appOL` go a
            go (Lam _ e)        = go e
            go (Let b e)        = go_bs b `appOL` go e
            go (Case e _ _ as)  = go e `appOL` concatOL (map go_a as)
            go (Cast e _)       = go e
            go (Tick t e)
              | p t             = t `consOL` go e
              | otherwise       = go e
            go _                = nilOL
            go_bs (NonRec _ e)  = go e
            go_bs (Rec bs)      = concatOL (map go_b bs)
            go_b (_, e)         = go e
            go_a (_, _, e)      = go e

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Other expression construction}
    *                                                                      *
    ************************************************************************
    -}

    bindNonRec :: Id -> CoreExpr -> CoreExpr -> CoreExpr
    -- ^ @bindNonRec x r b@ produces either:
    --
    -- > let x = r in b
    --
    -- or:
    --
    -- > case r of x { _DEFAULT_ -> b }
    --
    -- depending on whether we have to use a @case@ or @let@
    -- binding for the expression (see 'needsCaseBinding').
    -- It's used by the desugarer to avoid building bindings
    -- that give Core Lint a heart attack, although actually
    -- the simplifier deals with them perfectly well. See
    -- also 'MkCore.mkCoreLet'
    bindNonRec bndr rhs body
      | needsCaseBinding (idType bndr) rhs = Case rhs bndr (exprType body) [(DEFAULT, [], body)]
      | otherwise                          = Let (NonRec bndr rhs) body

    -- | Tests whether we have to use a @case@ rather than @let@ binding for this expression
    -- as per the invariants of 'CoreExpr': see "CoreSyn#let_app_invariant"
    needsCaseBinding :: Type -> CoreExpr -> Bool
    needsCaseBinding ty rhs = isUnliftedType ty && not (exprOkForSpeculation rhs)
            -- Make a case expression instead of a let
            -- These can arise either from the desugarer,
            -- or from beta reductions: (\x.e) (x +# y)

    mkAltExpr :: AltCon     -- ^ Case alternative constructor
              -> [CoreBndr] -- ^ Things bound by the pattern match
              -> [Type]     -- ^ The type arguments to the case alternative
              -> CoreExpr
    -- ^ This guy constructs the value that the scrutinee must have
    -- given that you are in one particular branch of a case
    mkAltExpr (DataAlt con) args inst_tys
      = mkConApp con (map Type inst_tys ++ varsToCoreExprs args)
    mkAltExpr (LitAlt lit) [] []
      = Lit lit
    mkAltExpr (LitAlt _) _ _ = panic "mkAltExpr LitAlt"
    mkAltExpr DEFAULT _ _ = panic "mkAltExpr DEFAULT"

    {-
    ************************************************************************
    *                                                                      *
                   Operations oer case alternatives
    *                                                                      *
    ************************************************************************

    The default alternative must be first, if it exists at all.
    This makes it easy to find, though it makes matching marginally harder.
    -}

    -- | Extract the default case alternative
    findDefault :: [(AltCon, [a], b)] -> ([(AltCon, [a], b)], Maybe b)
    findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null args ) (alts, Just rhs)
    findDefault alts                        =                     (alts, Nothing)

    addDefault :: [(AltCon, [a], b)] -> Maybe b -> [(AltCon, [a], b)]
    addDefault alts Nothing    = alts
    addDefault alts (Just rhs) = (DEFAULT, [], rhs) : alts

    isDefaultAlt :: (AltCon, a, b) -> Bool
    isDefaultAlt (DEFAULT, _, _) = True
    isDefaultAlt _               = False

    -- | Find the case alternative corresponding to a particular
    -- constructor: panics if no such constructor exists
    findAlt :: AltCon -> [(AltCon, a, b)] -> Maybe (AltCon, a, b)
        -- A "Nothing" result *is* legitmiate
        -- See Note [Unreachable code]
    findAlt con alts
      = case alts of
            (deflt@(DEFAULT,_,_):alts) -> go alts (Just deflt)
            _                          -> go alts Nothing
      where
        go []                     deflt = deflt
        go (alt@(con1,_,_) : alts) deflt
          = case con `cmpAltCon` con1 of
              LT -> deflt   -- Missed it already; the alts are in increasing order
              EQ -> Just alt
              GT -> ASSERT( not (con1 == DEFAULT) ) go alts deflt

    {- Note [Unreachable code]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    It is possible (although unusual) for GHC to find a case expression
    that cannot match.  For example:

         data Col = Red | Green | Blue
         x = Red
         f v = case x of
                  Red -> ...
                  _ -> ...(case x of { Green -> e1; Blue -> e2 })...

    Suppose that for some silly reason, x isn't substituted in the case
    expression.  (Perhaps there's a NOINLINE on it, or profiling SCC stuff
    gets in the way; cf Trac #3118.)  Then the full-lazines pass might produce
    this

         x = Red
         lvl = case x of { Green -> e1; Blue -> e2 })
         f v = case x of
                 Red -> ...
                 _ -> ...lvl...

    Now if x gets inlined, we won't be able to find a matching alternative
    for 'Red'.  That's because 'lvl' is unreachable.  So rather than crashing
    we generate (error "Inaccessible alternative").

    Similar things can happen (augmented by GADTs) when the Simplifier
    filters down the matching alternatives in Simplify.rebuildCase.
    -}

    ---------------------------------
    mergeAlts :: [(AltCon, a, b)] -> [(AltCon, a, b)] -> [(AltCon, a, b)]
    -- ^ Merge alternatives preserving order; alternatives in
    -- the first argument shadow ones in the second
    mergeAlts [] as2 = as2
    mergeAlts as1 [] = as1
    mergeAlts (a1:as1) (a2:as2)
      = case a1 `cmpAlt` a2 of
            LT -> a1 : mergeAlts as1      (a2:as2)
            EQ -> a1 : mergeAlts as1      as2       -- Discard a2
            GT -> a2 : mergeAlts (a1:as1) as2

    ---------------------------------
    trimConArgs :: AltCon -> [CoreArg] -> [CoreArg]
    -- ^ Given:
    --
    -- > case (C a b x y) of
    -- >        C b x y -> ...
    --
    -- We want to drop the leading type argument of the scrutinee
    -- leaving the arguments to match against the pattern

    trimConArgs DEFAULT      args = ASSERT( null args ) []
    trimConArgs (LitAlt _)   args = ASSERT( null args ) []
    trimConArgs (DataAlt dc) args = dropList (dataConUnivTyVars dc) args

    filterAlts :: TyCon                -- ^ Type constructor of scrutinee's type (used to prune possibilities)
               -> [Type]               -- ^ And its type arguments
               -> [AltCon]             -- ^ 'imposs_cons': constructors known to be impossible due to the form of the scrutinee
               -> [(AltCon, [Var], a)] -- ^ Alternatives
               -> ([AltCon], [(AltCon, [Var], a)])
                 -- Returns:
                 --  1. Constructors that will never be encountered by the
                 --     *default* case (if any).  A superset of imposs_cons
                 --  2. The new alternatives, trimmed by
                 --        a) remove imposs_cons
                 --        b) remove constructors which can't match because of GADTs
                 --      and with the DEFAULT expanded to a DataAlt if there is exactly
                 --      remaining constructor that can match
                 --
                 -- NB: the final list of alternatives may be empty:
                 -- This is a tricky corner case.  If the data type has no constructors,
                 -- which GHC allows, or if the imposs_cons covers all constructors (after taking
                 -- account of GADTs), then no alternatives can match.
                 --
                 -- If callers need to preserve the invariant that there is always at least one branch
                 -- in a "case" statement then they will need to manually add a dummy case branch that just
                 -- calls "error" or similar.
    filterAlts _tycon inst_tys imposs_cons alts
      = (imposs_deflt_cons, addDefault trimmed_alts maybe_deflt)
      where
        (alts_wo_default, maybe_deflt) = findDefault alts
        alt_cons = [con | (con,_,_) <- alts_wo_default]

        trimmed_alts = filterOut (impossible_alt inst_tys) alts_wo_default

        imposs_deflt_cons = nub (imposs_cons ++ alt_cons)
             -- "imposs_deflt_cons" are handled
             --   EITHER by the context,
             --   OR by a non-DEFAULT branch in this case expression.

        impossible_alt :: [Type] -> (AltCon, a, b) -> Bool
        impossible_alt _ (con, _, _) | con `elem` imposs_cons = True
        impossible_alt inst_tys (DataAlt con, _, _) = dataConCannotMatch inst_tys con
        impossible_alt _  _                         = False

    refineDefaultAlt :: [Unique] -> TyCon -> [Type]
                     -> [AltCon]  -- Constructors that cannot match the DEFAULT (if any)
                     -> [CoreAlt]
                     -> (Bool, [CoreAlt])
    -- Refine the default alternative to a DataAlt,
    -- if there is a unique way to do so
    refineDefaultAlt us tycon tys imposs_deflt_cons all_alts
      | (DEFAULT,_,rhs) : rest_alts <- all_alts
      , isAlgTyCon tycon            -- It's a data type, tuple, or unboxed tuples.
      , not (isNewTyCon tycon)      -- We can have a newtype, if we are just doing an eval:
                                    --      case x of { DEFAULT -> e }
                                    -- and we don't want to fill in a default for them!
      , Just all_cons <- tyConDataCons_maybe tycon
      , let imposs_data_cons = [con | DataAlt con <- imposs_deflt_cons]   -- We now know it's a data type
            impossible con   = con `elem` imposs_data_cons || dataConCannotMatch tys con
      = case filterOut impossible all_cons of
           -- Eliminate the default alternative
           -- altogether if it can't match:
           []    -> (False, rest_alts)

           -- It matches exactly one constructor, so fill it in:
           [con] -> (True, mergeAlts rest_alts [(DataAlt con, ex_tvs ++ arg_ids, rhs)])
                           -- We need the mergeAlts to keep the alternatives in the right order
                 where
                    (ex_tvs, arg_ids) = dataConRepInstPat us con tys

           -- It matches more than one, so do nothing
           _  -> (False, all_alts)

      | debugIsOn, isAlgTyCon tycon, null (tyConDataCons tycon)
      , not (isFamilyTyCon tycon || isAbstractTyCon tycon)
            -- Check for no data constructors
            -- This can legitimately happen for abstract types and type families,
            -- so don't report that
      = (False, all_alts)

      | otherwise      -- The common case
      = (False, all_alts)

    {- Note [Combine identical alternatives]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If several alternatives are identical, merge them into a single
    DEFAULT alternative.  I've occasionally seen this making a big
    difference:

         case e of               =====>     case e of
           C _ -> f x                         D v -> ....v....
           D v -> ....v....                   DEFAULT -> f x
           DEFAULT -> f x

    The point is that we merge common RHSs, at least for the DEFAULT case.
    [One could do something more elaborate but I've never seen it needed.]
    To avoid an expensive test, we just merge branches equal to the *first*
    alternative; this picks up the common cases
         a) all branches equal
         b) some branches equal to the DEFAULT (which occurs first)

    The case where Combine Identical Alternatives transformation showed up
    was like this (base/Foreign/C/Err/Error.hs):

            x | p `is` 1 -> e1
              | p `is` 2 -> e2
            ...etc...

    where @is@ was something like

            p `is` n = p /= (-1) && p == n

    This gave rise to a horrible sequence of cases

            case p of
              (-1) -> $j p
              1    -> e1
              DEFAULT -> $j p

    and similarly in cascade for all the join points!

    NB: it's important that all this is done in [InAlt], *before* we work
    on the alternatives themselves, because Simpify.simplAlt may zap the
    occurrence info on the binders in the alternatives, which in turn
    defeats combineIdenticalAlts (see Trac #7360).

    Note [Care with impossible-constructors when combining alternatives]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have (Trac #10538)
       data T = A | B | C | D

          case x::T of   (Imposs-default-cons {A,B})
             DEFAULT -> e1
             A -> e2
             B -> e1

    When calling combineIdentialAlts, we'll have computed that the
    "impossible constructors" for the DEFAULT alt is {A,B}, since if x is
    A or B we'll take the other alternatives.  But suppose we combine B
    into the DEFAULT, to get

          case x::T of   (Imposs-default-cons {A})
             DEFAULT -> e1
             A -> e2

    Then we must be careful to trim the impossible constructors to just {A},
    else we risk compiling 'e1' wrong!

    Not only that, but we take care when there is no DEFAULT beforehand,
    because we are introducing one.  Consider

       case x of   (Imposs-default-cons {A,B,C})
         A -> e1
         B -> e2
         C -> e1

    Then when combining the A and C alternatives we get

       case x of   (Imposs-default-cons {B})
         DEFAULT -> e1
         B -> e2

    Note that we have a new DEFAULT branch that we didn't have before.  So
    we need delete from the "impossible-default-constructors" all the
    known-con alternatives that we have eliminated. (In Trac #11172 we
    missed the first one.)

    -}

    combineIdenticalAlts :: [AltCon]    -- Constructors that cannot match DEFAULT
                         -> [CoreAlt]
                         -> (Bool,      -- True <=> something happened
                             [AltCon],  -- New constructors that cannot match DEFAULT
                             [CoreAlt]) -- New alternatives
    -- See Note [Combine identical alternatives]
    -- True <=> we did some combining, result is a single DEFAULT alternative
    combineIdenticalAlts imposs_deflt_cons ((con1,bndrs1,rhs1) : rest_alts)
      | all isDeadBinder bndrs1    -- Remember the default
      , not (null elim_rest) -- alternative comes first
      = (True, imposs_deflt_cons', deflt_alt : filtered_rest)
      where
        (elim_rest, filtered_rest) = partition identical_to_alt1 rest_alts
        deflt_alt = (DEFAULT, [], mkTicks (concat tickss) rhs1)

         -- See Note [Care with impossible-constructors when combining alternatives]
        imposs_deflt_cons' = imposs_deflt_cons `minusList` elim_cons
        elim_cons = elim_con1 ++ map fstOf3 elim_rest
        elim_con1 = case con1 of     -- Don't forget con1!
                      DEFAULT -> []  -- See Note [
                      _       -> [con1]

        cheapEqTicked e1 e2 = cheapEqExpr' tickishFloatable e1 e2
        identical_to_alt1 (_con,bndrs,rhs)
          = all isDeadBinder bndrs && rhs `cheapEqTicked` rhs1
        tickss = map (stripTicksT tickishFloatable . thdOf3) elim_rest

    combineIdenticalAlts imposs_cons alts
      = (False, imposs_cons, alts)

    {- *********************************************************************
    *                                                                      *
                 exprIsTrivial
    *                                                                      *
    ************************************************************************

    Note [exprIsTrivial]
    ~~~~~~~~~~~~~~~~~~~~
    @exprIsTrivial@ is true of expressions we are unconditionally happy to
                    duplicate; simple variables and constants, and type
                    applications.  Note that primop Ids aren't considered
                    trivial unless

    Note [Variables are trivial]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    There used to be a gruesome test for (hasNoBinding v) in the
    Var case:
            exprIsTrivial (Var v) | hasNoBinding v = idArity v == 0
    The idea here is that a constructor worker, like \$wJust, is
    really short for (\x -> \$wJust x), because \$wJust has no binding.
    So it should be treated like a lambda.  Ditto unsaturated primops.
    But now constructor workers are not "have-no-binding" Ids.  And
    completely un-applied primops and foreign-call Ids are sufficiently
    rare that I plan to allow them to be duplicated and put up with
    saturating them.

    Note [Tick trivial]
    ~~~~~~~~~~~~~~~~~~~
    Ticks are only trivial if they are pure annotations. If we treat
    "tick<n> x" as trivial, it will be inlined inside lambdas and the
    entry count will be skewed, for example.  Furthermore "scc<n> x" will
    turn into just "x" in mkTick.

    Note [Empty case is trivial]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The expression (case (x::Int) Bool of {}) is just a type-changing
    case used when we are sure that 'x' will not return.  See
    Note [Empty case alternatives] in CoreSyn.

    If the scrutinee is trivial, then so is the whole expression; and the
    CoreToSTG pass in fact drops the case expression leaving only the
    scrutinee.

    Having more trivial expressions is good.  Moreover, if we don't treat
    it as trivial we may land up with let-bindings like
       let v = case x of {} in ...
    and after CoreToSTG that gives
       let v = x in ...
    and that confuses the code generator (Trac #11155). So best to kill
    it off at source.
    -}

    exprIsTrivial :: CoreExpr -> Bool
    exprIsTrivial (Var _)          = True        -- See Note [Variables are trivial]
    exprIsTrivial (Type _)         = True
    exprIsTrivial (Coercion _)     = True
    exprIsTrivial (Lit lit)        = litIsTrivial lit
    exprIsTrivial (App e arg)      = not (isRuntimeArg arg) && exprIsTrivial e
    exprIsTrivial (Lam b e)        = not (isRuntimeVar b) && exprIsTrivial e
    exprIsTrivial (Tick t e)       = not (tickishIsCode t) && exprIsTrivial e
                                     -- See Note [Tick trivial]
    exprIsTrivial (Cast e _)       = exprIsTrivial e
    exprIsTrivial (Case e _ _ [])  = exprIsTrivial e  -- See Note [Empty case is trivial]
    exprIsTrivial _                = False

    {-
    Note [getIdFromTrivialExpr]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When substituting in a breakpoint we need to strip away the type cruft
    from a trivial expression and get back to the Id.  The invariant is
    that the expression we're substituting was originally trivial
    according to exprIsTrivial, AND the expression is not a literal.
    See Note [substTickish] for how breakpoint substitution preserves
    this extra invariant.

    We also need this functionality in CorePrep to extract out Id of a
    function which we are saturating.  However, in this case we don't know
    if the variable actually refers to a literal; thus we use
    'getIdFromTrivialExpr_maybe' to handle this case.  See test
    T12076lit for an example where this matters.
    -}

    getIdFromTrivialExpr :: CoreExpr -> Id
    getIdFromTrivialExpr e
        = fromMaybe (pprPanic "getIdFromTrivialExpr" (ppr e))
                    (getIdFromTrivialExpr_maybe e)

    getIdFromTrivialExpr_maybe :: CoreExpr -> Maybe Id
    -- See Note [getIdFromTrivialExpr]
    getIdFromTrivialExpr_maybe e = go e
      where go (Var v) = Just v
            go (App f t) | not (isRuntimeArg t) = go f
            go (Tick t e) | not (tickishIsCode t) = go e
            go (Cast e _) = go e
            go (Lam b e) | not (isRuntimeVar b) = go e
            go _ = Nothing

    {-
    exprIsBottom is a very cheap and cheerful function; it may return
    False for bottoming expressions, but it never costs much to ask.  See
    also CoreArity.exprBotStrictness_maybe, but that's a bit more
    expensive.
    -}

    exprIsBottom :: CoreExpr -> Bool
    -- See Note [Bottoming expressions]
    exprIsBottom e
      | isEmptyTy (exprType e)
      = True
      | otherwise
      = go 0 e
      where
        go n (Var v) = isBottomingId v &&  n >= idArity v
        go n (App e a) | isTypeArg a = go n e
                       | otherwise   = go (n+1) e
        go n (Tick _ e)              = go n e
        go n (Cast e _)              = go n e
        go n (Let _ e)               = go n e
        go n (Lam v e) | isTyVar v   = go n e
        go _ (Case _ _ _ alts)       = null alts
           -- See Note [Empty case alternatives] in CoreSyn
        go _ _                       = False

    {- Note [Bottoming expressions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A bottoming expression is guaranteed to diverge, or raise an
    exception.  We can test for it in two different ways, and exprIsBottom
    checks for both of these situations:

    * Visibly-bottom computations.  For example
          (error Int "Hello")
      is visibly bottom.  The strictness analyser also finds out if
      a function diverges or raises an exception, and puts that info
      in its strictness signature.

    * Empty types.  If a type is empty, its only inhabitant is bottom.
      For example:
          data T
          f :: T -> Bool
          f = \(x:t). case x of Bool {}
      Since T has no data constructors, the case alternatives are of course
      empty.  However note that 'x' is not bound to a visibly-bottom value;
      it's the *type* that tells us it's going to diverge.

    A GADT may also be empty even though it has constructors:
            data T a where
              T1 :: a -> T Bool
              T2 :: T Int
            ...(case (x::T Char) of {})...
    Here (T Char) is uninhabited.  A more realistic case is (Int ~ Bool),
    which is likewise uninhabited.

    ************************************************************************
    *                                                                      *
                 exprIsDupable
    *                                                                      *
    ************************************************************************

    Note [exprIsDupable]
    ~~~~~~~~~~~~~~~~~~~~
    @exprIsDupable@ is true of expressions that can be duplicated at a modest
                    cost in code size.  This will only happen in different case
                    branches, so there's no issue about duplicating work.

                    That is, exprIsDupable returns True of (f x) even if
                    f is very very expensive to call.

                    Its only purpose is to avoid fruitless let-binding
                    and then inlining of case join points
    -}

    exprIsDupable :: DynFlags -> CoreExpr -> Bool
    exprIsDupable dflags e
      = isJust (go dupAppSize e)
      where
        go :: Int -> CoreExpr -> Maybe Int
        go n (Type {})     = Just n
        go n (Coercion {}) = Just n
        go n (Var {})      = decrement n
        go n (Tick _ e)    = go n e
        go n (Cast e _)    = go n e
        go n (App f a) | Just n' <- go n a = go n' f
        go n (Lit lit) | litIsDupable dflags lit = decrement n
        go _ _ = Nothing

        decrement :: Int -> Maybe Int
        decrement 0 = Nothing
        decrement n = Just (n-1)

    dupAppSize :: Int
    dupAppSize = 8   -- Size of term we are prepared to duplicate
                     -- This is *just* big enough to make test MethSharing
                     -- inline enough join points.  Really it should be
                     -- smaller, and could be if we fixed Trac #4960.

    {-
    ************************************************************************
    *                                                                      *
                 exprIsCheap, exprIsExpandable
    *                                                                      *
    ************************************************************************

    Note [exprIsWorkFree]
    ~~~~~~~~~~~~~~~~~~~~~
    exprIsWorkFree is used when deciding whether to inline something; we
    don't inline it if doing so might duplicate work, by peeling off a
    complete copy of the expression.  Here we do not want even to
    duplicate a primop (Trac #5623):
       eg   let x = a #+ b in x +# x
       we do not want to inline/duplicate x

    Previously we were a bit more liberal, which led to the primop-duplicating
    problem.  However, being more conservative did lead to a big regression in
    one nofib benchmark, wheel-sieve1.  The situation looks like this:

       let noFactor_sZ3 :: GHC.Types.Int -> GHC.Types.Bool
           noFactor_sZ3 = case s_adJ of _ { GHC.Types.I# x_aRs ->
             case GHC.Prim.<=# x_aRs 2 of _ {
               GHC.Types.False -> notDivBy ps_adM qs_adN;
               GHC.Types.True -> lvl_r2Eb }}
           go = \x. ...(noFactor (I# y))....(go x')...

    The function 'noFactor' is heap-allocated and then called.  Turns out
    that 'notDivBy' is strict in its THIRD arg, but that is invisible to
    the caller of noFactor, which therefore cannot do w/w and
    heap-allocates noFactor's argument.  At the moment (May 12) we are just
    going to put up with this, because the previous more aggressive inlining
    (which treated 'noFactor' as work-free) was duplicating primops, which
    in turn was making inner loops of array calculations runs slow (#5623)

    Note [Case expressions are work-free]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Are case-expressions work-free?  Consider
        let v = case x of (p,q) -> p
            go = \y -> ...case v of ...
    Should we inline 'v' at its use site inside the loop?  At the moment
    we do.  I experimented with saying that case are *not* work-free, but
    that increased allocation slightly.  It's a fairly small effect, and at
    the moment we go for the slightly more aggressive version which treats
    (case x of ....) as work-free if the alternatives are.

    Moreover it improves arities of overloaded functions where
    there is only dictionary selection (no construction) involved

    Note [exprIsCheap]   See also Note [Interaction of exprIsCheap and lone variables]
    ~~~~~~~~~~~~~~~~~~   in CoreUnfold.hs
    @exprIsCheap@ looks at a Core expression and returns \tr{True} if
    it is obviously in weak head normal form, or is cheap to get to WHNF.
    [Note that that's not the same as exprIsDupable; an expression might be
    big, and hence not dupable, but still cheap.]

    By ``cheap'' we mean a computation we're willing to:
            push inside a lambda, or
            inline at more than one place
    That might mean it gets evaluated more than once, instead of being
    shared.  The main examples of things which aren't WHNF but are
    ``cheap'' are:

      *     case e of
              pi -> ei
            (where e, and all the ei are cheap)

      *     let x = e in b
            (where e and b are cheap)

      *     op x1 ... xn
            (where op is a cheap primitive operator)

      *     error "foo"
            (because we are happy to substitute it inside a lambda)

    Notice that a variable is considered 'cheap': we can push it inside a lambda,
    because sharing will make sure it is only evaluated once.

    Note [exprIsCheap and exprIsHNF]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Note that exprIsHNF does not imply exprIsCheap.  Eg
            let x = fac 20 in Just x
    This responds True to exprIsHNF (you can discard a seq), but
    False to exprIsCheap.

    Note [exprIsExpandable]
    ~~~~~~~~~~~~~~~~~~~~~~~
    An expression is "expandable" if we are willing to dupicate it, if doing
    so might make a RULE or case-of-constructor fire.  Mainly this means
    data-constructor applications, but it's a bit more generous than exprIsCheap
    because it is true of "CONLIKE" Ids: see Note [CONLIKE pragma] in BasicTypes.

    It is used to set the uf_expandable field of an Unfolding, and that
    in turn is used
      * In RULE matching
      * In exprIsConApp_maybe, exprIsLiteral_maybe, exprIsLambda_maybe

    But take care: exprIsExpandable should /not/ be true of primops.  I
    found this in test T5623a:
        let q = /\a. Ptr a (a +# b)
        in case q @ Float of Ptr v -> ...q...

    q's inlining should not be expandable, else exprIsConApp_maybe will
    say that (q @ Float) expands to (Ptr a (a +# b)), and that will
    duplicate the (a +# b) primop, which we should not do lightly.
    (It's quite hard to trigger this bug, but T13155 does so for GHC 8.0.)

    Note [Arguments in exprIsOk]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    What predicate should we apply to the argument of an application?  We
    used to say "exprIsTrivial arg" due to concerns about duplicating
    nested constructor applications, but see #4978.  The principle here is
    that
       let x = a +# b in c *# x
    should behave equivalently to
       c *# (a +# b)
    Since lets with cheap RHSs are accepted, so should paps with cheap arguments
    -}

    --------------------
    exprIsCheap :: CoreExpr -> Bool
    exprIsCheap = exprIsOk isCheapApp

    exprIsExpandable :: CoreExpr -> Bool -- See Note [exprIsExpandable]
    exprIsExpandable = exprIsOk isExpandableApp

    exprIsWorkFree :: CoreExpr -> Bool   -- See Note [exprIsWorkFree]
    exprIsWorkFree = exprIsOk isWorkFreeApp

    --------------------
    exprIsOk :: CheapAppFun -> CoreExpr -> Bool
    exprIsOk ok_app e
      = ok e
      where
        ok e = go 0 e

        -- n is the number of value arguments
        go n (Var v)                      = ok_app v n
        go _ (Lit {})                     = True
        go _ (Type {})                    = True
        go _ (Coercion {})                = True
        go n (Cast e _)                   = go n e
        go n (Case scrut _ _ alts)        = foldl (&&) (ok scrut)
                                            [ go n rhs | (_,_,rhs) <- alts ]
        go n (Tick t e) | tickishCounts t = False
                        | otherwise       = go n e
        go n (Lam x e)  | isRuntimeVar x  = n==0 || go (n-1) e
                        | otherwise       = go n e
        go n (App f e)  | isRuntimeArg e  = go (n+1) f && ok e
                        | otherwise       = go n f
        go _ (Let {})                     = False

          -- Case: see Note [Case expressions are work-free]
          -- App:  see Note [Arguments in exprIsOk]
          -- Let:  the old exprIsCheap worked through lets

    -------------------------------------
    type CheapAppFun = Id -> Arity -> Bool
      -- Is an application of this function to n *value* args
      -- always cheap, assuming the arguments are cheap?
      -- True mainly of data constructors, partial applications;
      -- but with minor variations:
      --    isWorkFreeApp
      --    isCheapApp
      --    isExpandableApp

      -- NB: isCheapApp and isExpandableApp are called from outside
      --     this module, so don't be tempted to move the notRedex
      --     stuff into the call site in exprIsOk, and remove it
      --     from the CheapAppFun implementations

    notRedex :: CheapAppFun
    notRedex fn n_val_args
      =  n_val_args == 0           -- No value args
      || n_val_args < idArity fn   -- Partial application
      || isBottomingId fn   -- OK to duplicate calls to bottom;
                            -- it certainly doesn't need to be shared!

    isWorkFreeApp :: CheapAppFun
    isWorkFreeApp fn n_val_args
      | notRedex fn n_val_args
      = True
      | otherwise
      = case idDetails fn of
          DataConWorkId {} -> True
          _                -> False

    isCheapApp :: CheapAppFun
    isCheapApp fn n_val_args
      | notRedex fn n_val_args
      = True
      | otherwise
      = case idDetails fn of
          DataConWorkId {} -> True
          RecSelId {}      -> n_val_args == 1  -- See Note [Record selection]
          ClassOpId {}     -> n_val_args == 1
          PrimOpId op      -> primOpIsCheap op
          _                -> False
            -- In principle we should worry about primops
            -- that return a type variable, since the result
            -- might be applied to something, but I'm not going
            -- to bother to check the number of args

    isExpandableApp :: CheapAppFun
    isExpandableApp fn n_val_args
      | notRedex fn n_val_args
      = True
      | isConLikeId fn
      = True
      | otherwise
      = case idDetails fn of
          DataConWorkId {} -> True
          RecSelId {}      -> n_val_args == 1  -- See Note [Record selection]
          ClassOpId {}     -> n_val_args == 1
          PrimOpId {}      -> False
          _                -> all_pred_args n_val_args (idType fn)

      where
      -- See if all the arguments are PredTys (implicit params or classes)
      -- If so we'll regard it as expandable; see Note [Expandable overloadings]
         all_pred_args n_val_args ty
           | n_val_args == 0
           = True

           | Just (bndr, ty) <- splitPiTy_maybe ty
           = caseBinder bndr
               (\_tv -> all_pred_args n_val_args ty)
               (\bndr_ty -> isPredTy bndr_ty && all_pred_args (n_val_args-1) ty)

           | otherwise
           = False

    {- Note [Record selection]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    I'm experimenting with making record selection
    look cheap, so we will substitute it inside a
    lambda.  Particularly for dictionary field selection.

    BUT: Take care with (sel d x)!  The (sel d) might be cheap, but
    there's no guarantee that (sel d x) will be too.  Hence (n_val_args == 1)

    Note [Expandable overloadings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose the user wrote this
       {-# RULE  forall x. foo (negate x) = h x #-}
       f x = ....(foo (negate x))....
    He'd expect the rule to fire. But since negate is overloaded, we might
    get this:
        f = \d -> let n = negate d in \x -> ...foo (n x)...
    So we treat the application of a function (negate in this case) to a
    *dictionary* as expandable.  In effect, every function is CONLIKE when
    it's applied only to dictionaries.

    ************************************************************************
    *                                                                      *
                 exprOkForSpeculation
    *                                                                      *
    ************************************************************************
    -}

    -----------------------------
    -- | 'exprOkForSpeculation' returns True of an expression that is:
    --
    --  * Safe to evaluate even if normal order eval might not
    --    evaluate the expression at all, or
    --
    --  * Safe /not/ to evaluate even if normal order would do so
    --
    -- It is usually called on arguments of unlifted type, but not always
    -- In particular, Simplify.rebuildCase calls it on lifted types
    -- when a 'case' is a plain 'seq'. See the example in
    -- Note [exprOkForSpeculation: case expressions] below
    --
    -- Precisely, it returns @True@ iff:
    --  a) The expression guarantees to terminate,
    --  b) soon,
    --  c) without causing a write side effect (e.g. writing a mutable variable)
    --  d) without throwing a Haskell exception
    --  e) without risking an unchecked runtime exception (array out of bounds,
    --     divide by zero)
    --
    -- For @exprOkForSideEffects@ the list is the same, but omitting (e).
    --
    -- Note that
    --    exprIsHNF            implies exprOkForSpeculation
    --    exprOkForSpeculation implies exprOkForSideEffects
    --
    -- See Note [PrimOp can_fail and has_side_effects] in PrimOp
    -- and Note [Implementation: how can_fail/has_side_effects affect transformations]
    --
    -- As an example of the considerations in this test, consider:
    --
    -- > let x = case y# +# 1# of { r# -> I# r# }
    -- > in E
    --
    -- being translated to:
    --
    -- > case y# +# 1# of { r# ->
    -- >    let x = I# r#
    -- >    in E
    -- > }
    --
    -- We can only do this if the @y + 1@ is ok for speculation: it has no
    -- side effects, and can't diverge or raise an exception.
    exprOkForSpeculation, exprOkForSideEffects :: CoreExpr -> Bool
    exprOkForSpeculation = expr_ok primOpOkForSpeculation
    exprOkForSideEffects = expr_ok primOpOkForSideEffects
      -- Polymorphic in binder type
      -- There is one call at a non-Id binder type, in SetLevels

    expr_ok :: (PrimOp -> Bool) -> CoreExpr -> Bool
    expr_ok _ (Lit _)      = True
    expr_ok _ (Type _)     = True
    expr_ok _ (Coercion _) = True

    expr_ok primop_ok (Var v)    = app_ok primop_ok v []
    expr_ok primop_ok (Cast e _) = expr_ok primop_ok e
    expr_ok primop_ok (Lam b e)
                     | isTyVar b = expr_ok primop_ok  e
                     | otherwise = True

    -- Tick annotations that *tick* cannot be speculated, because these
    -- are meant to identify whether or not (and how often) the particular
    -- source expression was evaluated at runtime.
    expr_ok primop_ok (Tick tickish e)
       | tickishCounts tickish = False
       | otherwise             = expr_ok primop_ok e

    expr_ok primop_ok (Case e _ _ alts)
      =  expr_ok primop_ok e  -- Note [exprOkForSpeculation: case expressions]
      && all (\(_,_,rhs) -> expr_ok primop_ok rhs) alts
      && altsAreExhaustive alts     -- Note [Exhaustive alts]

    expr_ok primop_ok other_expr
      = case collectArgs other_expr of
            (expr, args) | Var f <- stripTicksTopE (not . tickishCounts) expr
                         -> app_ok primop_ok f args
            _            -> False

    -----------------------------
    app_ok :: (PrimOp -> Bool) -> Id -> [CoreExpr] -> Bool
    app_ok primop_ok fun args
      = case idDetails fun of
          DFunId new_type ->  not new_type
             -- DFuns terminate, unless the dict is implemented
             -- with a newtype in which case they may not

          DataConWorkId {} -> True
                    -- The strictness of the constructor has already
                    -- been expressed by its "wrapper", so we don't need
                    -- to take the arguments into account

          PrimOpId op
            | isDivOp op
            , [arg1, Lit lit] <- args
            -> not (isZeroLit lit) && expr_ok primop_ok arg1
                  -- Special case for dividing operations that fail
                  -- In general they are NOT ok-for-speculation
                  -- (which primop_ok will catch), but they ARE OK
                  -- if the divisor is definitely non-zero.
                  -- Often there is a literal divisor, and this
                  -- can get rid of a thunk in an inner loop

            | otherwise
            -> primop_ok op     -- Check the primop itself
            && and (zipWith arg_ok arg_tys args)  -- Check the arguments

          _other -> isUnliftedType (idType fun)          -- c.f. the Var case of exprIsHNF
                 || idArity fun > n_val_args             -- Partial apps
                 || (n_val_args == 0 &&
                     isEvaldUnfolding (idUnfolding fun)) -- Let-bound values
                 where
                   n_val_args = valArgCount args
      where
        (arg_tys, _) = splitPiTys (idType fun)

        arg_ok :: TyBinder -> CoreExpr -> Bool
        arg_ok (Named _) _ = True   -- A type argument
        arg_ok (Anon ty) arg        -- A term argument
           | isUnliftedType ty = expr_ok primop_ok arg
           | otherwise         = True  -- See Note [Primops with lifted arguments]

    -----------------------------
    altsAreExhaustive :: [Alt b] -> Bool
    -- True  <=> the case alternatives are definiely exhaustive
    -- False <=> they may or may not be
    altsAreExhaustive []
      = False    -- Should not happen
    altsAreExhaustive ((con1,_,_) : alts)
      = case con1 of
          DEFAULT   -> True
          LitAlt {} -> False
          DataAlt c -> 1 + length alts == tyConFamilySize (dataConTyCon c)
          -- It is possible to have an exhaustive case that does not
          -- enumerate all constructors, notably in a GADT match, but
          -- we behave conservatively here -- I don't think it's important
          -- enough to deserve special treatment

    -- | True of dyadic operators that can fail only if the second arg is zero!
    isDivOp :: PrimOp -> Bool
    -- This function probably belongs in PrimOp, or even in
    -- an automagically generated file.. but it's such a
    -- special case I thought I'd leave it here for now.
    isDivOp IntQuotOp        = True
    isDivOp IntRemOp         = True
    isDivOp WordQuotOp       = True
    isDivOp WordRemOp        = True
    isDivOp FloatDivOp       = True
    isDivOp DoubleDivOp      = True
    isDivOp _                = False

    {-
    Note [exprOkForSpeculation: case expressions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    It's always sound for exprOkForSpeculation to return False, and we
    don't want it to take too long, so it bales out on complicated-looking
    terms.  Notably lets, which can be stacked very deeply; and in any
    case the argument of exprOkForSpeculation is usually in a strict context,
    so any lets will have been floated away.

    However, we keep going on case-expressions.  An example like this one
    showed up in DPH code (Trac #3717):
        foo :: Int -> Int
        foo 0 = 0
        foo n = (if n < 5 then 1 else 2) `seq` foo (n-1)

    If exprOkForSpeculation doesn't look through case expressions, you get this:
        T.$wfoo =
          \ (ww :: GHC.Prim.Int#) ->
            case ww of ds {
              __DEFAULT -> case (case <# ds 5 of _ {
                              GHC.Types.False -> lvl1;
                              GHC.Types.True -> lvl})
                           of _ { __DEFAULT ->
                           T.$wfoo (GHC.Prim.-# ds_XkE 1) };
              0 -> 0
            }

    The inner case is redundant, and should be nuked.

    Note [Exhaustive alts]
    ~~~~~~~~~~~~~~~~~~~~~~
    We might have something like
      case x of {
        A -> ...
        _ -> ...(case x of { B -> ...; C -> ... })...
    Here, the inner case is fine, because the A alternative
    can't happen, but it's not ok to float the inner case outside
    the outer one (even if we know x is evaluated outside), because
    then it would be non-exhaustive. See Trac #5453.

    Similarly, this is a valid program (albeit a slightly dodgy one)
       let v = case x of { B -> ...; C -> ... }
       in case x of
             A -> ...
             _ ->  ...v...v....
    But we don't want to speculate the v binding.

    One could try to be clever, but the easy fix is simpy to regard
    a non-exhaustive case as *not* okForSpeculation.

    Note [Primops with lifted arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Is this ok-for-speculation (see Trac #13027)?
       reallyUnsafePtrEq# a b
    Well, yes.  The primop accepts lifted arguments and does not
    evaluate them.  Indeed, in general primops are, well, primitive
    and do not perform evaluation.

    There is one primop, dataToTag#, which does /require/ a lifted
    argument to be evaluted.  To ensure this, CorePrep adds an
    eval if it can't see the the argument is definitely evaluated
    (see [dataToTag magic] in CorePrep).

    We make no attempt to guarantee that dataToTag#'s argument is
    evaluated here.  Main reason: it's very fragile to test for the
    evaluatedness of a lifted argument.  Consider
        case x of y -> let v = dataToTag# y in ...

    where x/y have type Int, say.  'y' looks evaluated (by the enclosing
    case) so all is well.  Now the FloatOut pass does a binder-swap (for
    very good reasons), changing to
       case x of y -> let v = dataToTag# x in ...

    See also Note [dataToTag#] in primops.txt.pp.

    Bottom line:
      * in exprOkForSpeculation we simply ignore all lifted arguments.

    ************************************************************************
    *                                                                      *
                 exprIsHNF, exprIsConLike
    *                                                                      *
    ************************************************************************
    -}

    -- Note [exprIsHNF]             See also Note [exprIsCheap and exprIsHNF]
    -- ~~~~~~~~~~~~~~~~
    -- | exprIsHNF returns true for expressions that are certainly /already/
    -- evaluated to /head/ normal form.  This is used to decide whether it's ok
    -- to change:
    --
    -- > case x of _ -> e
    --
    --    into:
    --
    -- > e
    --
    -- and to decide whether it's safe to discard a 'seq'.
    --
    -- So, it does /not/ treat variables as evaluated, unless they say they are.
    -- However, it /does/ treat partial applications and constructor applications
    -- as values, even if their arguments are non-trivial, provided the argument
    -- type is lifted. For example, both of these are values:
    --
    -- > (:) (f x) (map f xs)
    -- > map (...redex...)
    --
    -- because 'seq' on such things completes immediately.
    --
    -- For unlifted argument types, we have to be careful:
    --
    -- > C (f x :: Int#)
    --
    -- Suppose @f x@ diverges; then @C (f x)@ is not a value. However this can't
    -- happen: see "CoreSyn#let_app_invariant". This invariant states that arguments of
    -- unboxed type must be ok-for-speculation (or trivial).
    exprIsHNF :: CoreExpr -> Bool           -- True => Value-lambda, constructor, PAP
    exprIsHNF = exprIsHNFlike isDataConWorkId isEvaldUnfolding

    -- | Similar to 'exprIsHNF' but includes CONLIKE functions as well as
    -- data constructors. Conlike arguments are considered interesting by the
    -- inliner.
    exprIsConLike :: CoreExpr -> Bool       -- True => lambda, conlike, PAP
    exprIsConLike = exprIsHNFlike isConLikeId isConLikeUnfolding

    -- | Returns true for values or value-like expressions. These are lambdas,
    -- constructors / CONLIKE functions (as determined by the function argument)
    -- or PAPs.
    --
    exprIsHNFlike :: (Var -> Bool) -> (Unfolding -> Bool) -> CoreExpr -> Bool
    exprIsHNFlike is_con is_con_unf = is_hnf_like
      where
        is_hnf_like (Var v) -- NB: There are no value args at this point
          =  is_con v       -- Catches nullary constructors,
                            --      so that [] and () are values, for example
          || idArity v > 0  -- Catches (e.g.) primops that don't have unfoldings
          || is_con_unf (idUnfolding v)
            -- Check the thing's unfolding; it might be bound to a value
            -- We don't look through loop breakers here, which is a bit conservative
            -- but otherwise I worry that if an Id's unfolding is just itself,
            -- we could get an infinite loop

        is_hnf_like (Lit _)          = True
        is_hnf_like (Type _)         = True       -- Types are honorary Values;
                                                  -- we don't mind copying them
        is_hnf_like (Coercion _)     = True       -- Same for coercions
        is_hnf_like (Lam b e)        = isRuntimeVar b || is_hnf_like e
        is_hnf_like (Tick tickish e) = not (tickishCounts tickish)
                                          && is_hnf_like e
                                          -- See Note [exprIsHNF Tick]
        is_hnf_like (Cast e _)       = is_hnf_like e
        is_hnf_like (App e a)
          | isValArg a               = app_is_value e 1
          | otherwise                = is_hnf_like e
        is_hnf_like (Let _ e)        = is_hnf_like e  -- Lazy let(rec)s don't affect us
        is_hnf_like _                = False

        -- There is at least one value argument
        -- 'n' is number of value args to which the expression is applied
        app_is_value :: CoreExpr -> Int -> Bool
        app_is_value (Var f)    nva = id_app_is_value f nva
        app_is_value (Tick _ f) nva = app_is_value f nva
        app_is_value (Cast f _) nva = app_is_value f nva
        app_is_value (App f a)  nva
          | isValArg a              = app_is_value f (nva + 1)
          | otherwise               = app_is_value f nva
        app_is_value _ _ = False

        id_app_is_value id n_val_args
           = is_con id
           || idArity id > n_val_args
           || id `hasKey` absentErrorIdKey  -- See Note [aBSENT_ERROR_ID] in MkCore
                          -- absentError behaves like an honorary data constructor

    {-
    Note [exprIsHNF Tick]

    We can discard source annotations on HNFs as long as they aren't
    tick-like:

      scc c (\x . e)    =>  \x . e
      scc c (C x1..xn)  =>  C x1..xn

    So we regard these as HNFs.  Tick annotations that tick are not
    regarded as HNF if the expression they surround is HNF, because the
    tick is there to tell us that the expression was evaluated, so we
    don't want to discard a seq on it.
    -}

    -- | Can we bind this 'CoreExpr' at the top level?
    exprIsTopLevelBindable :: CoreExpr -> Type -> Bool
    -- See Note [CoreSyn top-level string literals]
    -- Precondition: exprType expr = ty
    exprIsTopLevelBindable expr ty
      = exprIsLiteralString expr
      || not (isUnliftedType ty)

    exprIsLiteralString :: CoreExpr -> Bool
    exprIsLiteralString (Lit (MachStr _)) = True
    exprIsLiteralString _ = False

    {-
    ************************************************************************
    *                                                                      *
                 Instantiating data constructors
    *                                                                      *
    ************************************************************************

    These InstPat functions go here to avoid circularity between DataCon and Id
    -}

    dataConRepInstPat   ::                 [Unique] -> DataCon -> [Type] -> ([TyVar], [Id])
    dataConRepFSInstPat :: [FastString] -> [Unique] -> DataCon -> [Type] -> ([TyVar], [Id])

    dataConRepInstPat   = dataConInstPat (repeat ((fsLit "ipv")))
    dataConRepFSInstPat = dataConInstPat

    dataConInstPat :: [FastString]          -- A long enough list of FSs to use for names
                   -> [Unique]              -- An equally long list of uniques, at least one for each binder
                   -> DataCon
                   -> [Type]                -- Types to instantiate the universally quantified tyvars
                   -> ([TyVar], [Id])       -- Return instantiated variables
    -- dataConInstPat arg_fun fss us con inst_tys returns a tuple
    -- (ex_tvs, arg_ids),
    --
    --   ex_tvs are intended to be used as binders for existential type args
    --
    --   arg_ids are indended to be used as binders for value arguments,
    --     and their types have been instantiated with inst_tys and ex_tys
    --     The arg_ids include both evidence and
    --     programmer-specified arguments (both after rep-ing)
    --
    -- Example.
    --  The following constructor T1
    --
    --  data T a where
    --    T1 :: forall b. Int -> b -> T(a,b)
    --    ...
    --
    --  has representation type
    --   forall a. forall a1. forall b. (a ~ (a1,b)) =>
    --     Int -> b -> T a
    --
    --  dataConInstPat fss us T1 (a1',b') will return
    --
    --  ([a1'', b''], [c :: (a1', b')~(a1'', b''), x :: Int, y :: b''])
    --
    --  where the double-primed variables are created with the FastStrings and
    --  Uniques given as fss and us
    dataConInstPat fss uniqs con inst_tys
      = ASSERT( univ_tvs `equalLength` inst_tys )
        (ex_bndrs, arg_ids)
      where
        univ_tvs = dataConUnivTyVars con
        ex_tvs   = dataConExTyVars con
        arg_tys  = dataConRepArgTys con
        arg_strs = dataConRepStrictness con  -- 1-1 with arg_tys
        n_ex = length ex_tvs

          -- split the Uniques and FastStrings
        (ex_uniqs, id_uniqs) = splitAt n_ex uniqs
        (ex_fss,   id_fss)   = splitAt n_ex fss

          -- Make the instantiating substitution for universals
        univ_subst = zipTvSubst univ_tvs inst_tys

          -- Make existential type variables, applying and extending the substitution
        (full_subst, ex_bndrs) = mapAccumL mk_ex_var univ_subst
                                           (zip3 ex_tvs ex_fss ex_uniqs)

        mk_ex_var :: TCvSubst -> (TyVar, FastString, Unique) -> (TCvSubst, TyVar)
        mk_ex_var subst (tv, fs, uniq) = (Type.extendTvSubstWithClone subst tv
                                           new_tv
                                         , new_tv)
          where
            new_tv = mkTyVar (mkSysTvName uniq fs) kind
            kind   = Type.substTyUnchecked subst (tyVarKind tv)

          -- Make value vars, instantiating types
        arg_ids = zipWith4 mk_id_var id_uniqs id_fss arg_tys arg_strs
        mk_id_var uniq fs ty str
          = setCaseBndrEvald str $  -- See Note [Mark evaluated arguments]
            mkLocalIdOrCoVar name (Type.substTy full_subst ty)
          where
            name = mkInternalName uniq (mkVarOccFS fs) noSrcSpan

    {-
    Note [Mark evaluated arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When pattern matching on a constructor with strict fields, the binder
    can have an 'evaldUnfolding'.  Moreover, it *should* have one, so that
    when loading an interface file unfolding like:
      data T = MkT !Int
      f x = case x of { MkT y -> let v::Int# = case y of I# n -> n+1
                                 in ... }
    we don't want Lint to complain.  The 'y' is evaluated, so the
    case in the RHS of the binding for 'v' is fine.  But only if we
    *know* that 'y' is evaluated.

    c.f. add_evals in Simplify.simplAlt

    ************************************************************************
    *                                                                      *
             Equality
    *                                                                      *
    ************************************************************************
    -}

    -- | A cheap equality test which bales out fast!
    --      If it returns @True@ the arguments are definitely equal,
    --      otherwise, they may or may not be equal.
    --
    -- See also 'exprIsBig'
    cheapEqExpr :: Expr b -> Expr b -> Bool
    cheapEqExpr = cheapEqExpr' (const False)

    -- | Cheap expression equality test, can ignore ticks by type.
    cheapEqExpr' :: (Tickish Id -> Bool) -> Expr b -> Expr b -> Bool
    cheapEqExpr' ignoreTick = go_s
      where go_s = go `on` stripTicksTopE ignoreTick
            go (Var v1)   (Var v2)   = v1 == v2
            go (Lit lit1) (Lit lit2) = lit1 == lit2
            go (Type t1)  (Type t2)  = t1 `eqType` t2
            go (Coercion c1) (Coercion c2) = c1 `eqCoercion` c2

            go (App f1 a1) (App f2 a2)
              = f1 `go_s` f2 && a1 `go_s` a2

            go (Cast e1 t1) (Cast e2 t2)
              = e1 `go_s` e2 && t1 `eqCoercion` t2

            go (Tick t1 e1) (Tick t2 e2)
              = t1 == t2 && e1 `go_s` e2

            go _ _ = False
            {-# INLINE go #-}
    {-# INLINE cheapEqExpr' #-}

    exprIsBig :: Expr b -> Bool
    -- ^ Returns @True@ of expressions that are too big to be compared by 'cheapEqExpr'
    exprIsBig (Lit _)      = False
    exprIsBig (Var _)      = False
    exprIsBig (Type _)     = False
    exprIsBig (Coercion _) = False
    exprIsBig (Lam _ e)    = exprIsBig e
    exprIsBig (App f a)    = exprIsBig f || exprIsBig a
    exprIsBig (Cast e _)   = exprIsBig e    -- Hopefully coercions are not too big!
    exprIsBig (Tick _ e)   = exprIsBig e
    exprIsBig _            = True

    eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
    -- Compares for equality, modulo alpha
    eqExpr in_scope e1 e2
      = go (mkRnEnv2 in_scope) e1 e2
      where
        go env (Var v1) (Var v2)
          | rnOccL env v1 == rnOccR env v2
          = True

        go _   (Lit lit1)    (Lit lit2)      = lit1 == lit2
        go env (Type t1)    (Type t2)        = eqTypeX env t1 t2
        go env (Coercion co1) (Coercion co2) = eqCoercionX env co1 co2
        go env (Cast e1 co1) (Cast e2 co2) = eqCoercionX env co1 co2 && go env e1 e2
        go env (App f1 a1)   (App f2 a2)   = go env f1 f2 && go env a1 a2
        go env (Tick n1 e1)  (Tick n2 e2)  = eqTickish env n1 n2 && go env e1 e2

        go env (Lam b1 e1)  (Lam b2 e2)
          =  eqTypeX env (varType b1) (varType b2)   -- False for Id/TyVar combination
          && go (rnBndr2 env b1 b2) e1 e2

        go env (Let (NonRec v1 r1) e1) (Let (NonRec v2 r2) e2)
          =  go env r1 r2  -- No need to check binder types, since RHSs match
          && go (rnBndr2 env v1 v2) e1 e2

        go env (Let (Rec ps1) e1) (Let (Rec ps2) e2)
          = length ps1 == length ps2
          && all2 (go env') rs1 rs2 && go env' e1 e2
          where
            (bs1,rs1) = unzip ps1
            (bs2,rs2) = unzip ps2
            env' = rnBndrs2 env bs1 bs2

        go env (Case e1 b1 t1 a1) (Case e2 b2 t2 a2)
          | null a1   -- See Note [Empty case alternatives] in TrieMap
          = null a2 && go env e1 e2 && eqTypeX env t1 t2
          | otherwise
          =  go env e1 e2 && all2 (go_alt (rnBndr2 env b1 b2)) a1 a2

        go _ _ _ = False

        -----------
        go_alt env (c1, bs1, e1) (c2, bs2, e2)
          = c1 == c2 && go (rnBndrs2 env bs1 bs2) e1 e2

    eqTickish :: RnEnv2 -> Tickish Id -> Tickish Id -> Bool
    eqTickish env (Breakpoint lid lids) (Breakpoint rid rids)
          = lid == rid  &&  map (rnOccL env) lids == map (rnOccR env) rids
    eqTickish _ l r = l == r

    -- | Finds differences between core expressions, modulo alpha and
    -- renaming. Setting @top@ means that the @IdInfo@ of bindings will be
    -- checked for differences as well.
    diffExpr :: Bool -> RnEnv2 -> CoreExpr -> CoreExpr -> [SDoc]
    diffExpr _   env (Var v1)   (Var v2)   | rnOccL env v1 == rnOccR env v2 = []
    diffExpr _   _   (Lit lit1) (Lit lit2) | lit1 == lit2                   = []
    diffExpr _   env (Type t1)  (Type t2)  | eqTypeX env t1 t2              = []
    diffExpr _   env (Coercion co1) (Coercion co2)
                                           | eqCoercionX env co1 co2        = []
    diffExpr top env (Cast e1 co1)  (Cast e2 co2)
      | eqCoercionX env co1 co2                = diffExpr top env e1 e2
    diffExpr top env (Tick n1 e1)   e2
      | not (tickishIsCode n1)                 = diffExpr top env e1 e2
    diffExpr top env e1             (Tick n2 e2)
      | not (tickishIsCode n2)                 = diffExpr top env e1 e2
    diffExpr top env (Tick n1 e1)   (Tick n2 e2)
      | eqTickish env n1 n2                    = diffExpr top env e1 e2
     -- The error message of failed pattern matches will contain
     -- generated names, which are allowed to differ.
    diffExpr _   _   (App (App (Var absent) _) _)
                     (App (App (Var absent2) _) _)
      | isBottomingId absent && isBottomingId absent2 = []
    diffExpr top env (App f1 a1)    (App f2 a2)
      = diffExpr top env f1 f2 ++ diffExpr top env a1 a2
    diffExpr top env (Lam b1 e1)  (Lam b2 e2)
      | eqTypeX env (varType b1) (varType b2)   -- False for Id/TyVar combination
      = diffExpr top (rnBndr2 env b1 b2) e1 e2
    diffExpr top env (Let bs1 e1) (Let bs2 e2)
      = let (ds, env') = diffBinds top env (flattenBinds [bs1]) (flattenBinds [bs2])
        in ds ++ diffExpr top env' e1 e2
    diffExpr top env (Case e1 b1 t1 a1) (Case e2 b2 t2 a2)
      | length a1 == length a2 && not (null a1) || eqTypeX env t1 t2
        -- See Note [Empty case alternatives] in TrieMap
      = diffExpr top env e1 e2 ++ concat (zipWith diffAlt a1 a2)
      where env' = rnBndr2 env b1 b2
            diffAlt (c1, bs1, e1) (c2, bs2, e2)
              | c1 /= c2  = [text "alt-cons " <> ppr c1 <> text " /= " <> ppr c2]
              | otherwise = diffExpr top (rnBndrs2 env' bs1 bs2) e1 e2
    diffExpr _  _ e1 e2
      = [fsep [ppr e1, text "/=", ppr e2]]

    -- | Finds differences between core bindings, see @diffExpr@.
    --
    -- The main problem here is that while we expect the binds to have the
    -- same order in both lists, this is not guaranteed. To do this
    -- properly we'd either have to do some sort of unification or check
    -- all possible mappings, which would be seriously expensive. So
    -- instead we simply match single bindings as far as we can. This
    -- leaves us just with mutually recursive and/or mismatching bindings,
    -- which we then speculatively match by ordering them. It's by no means
    -- perfect, but gets the job done well enough.
    diffBinds :: Bool -> RnEnv2 -> [(Var, CoreExpr)] -> [(Var, CoreExpr)]
              -> ([SDoc], RnEnv2)
    diffBinds top env binds1 = go (length binds1) env binds1
     where go _    env []     []
              = ([], env)
           go fuel env binds1 binds2
              -- No binds left to compare? Bail out early.
              | null binds1 || null binds2
              = (warn env binds1 binds2, env)
              -- Iterated over all binds without finding a match? Then
              -- try speculatively matching binders by order.
              | fuel == 0
              = if not $ env `inRnEnvL` fst (head binds1)
                then let env' = uncurry (rnBndrs2 env) $ unzip $
                                zip (sort $ map fst binds1) (sort $ map fst binds2)
                     in go (length binds1) env' binds1 binds2
                -- If we have already tried that, give up
                else (warn env binds1 binds2, env)
           go fuel env ((bndr1,expr1):binds1) binds2
              | let matchExpr (bndr,expr) =
                      (not top || null (diffIdInfo env bndr bndr1)) &&
                      null (diffExpr top (rnBndr2 env bndr1 bndr) expr1 expr)
              , (binds2l, (bndr2,_):binds2r) <- break matchExpr binds2
              = go (length binds1) (rnBndr2 env bndr1 bndr2)
                    binds1 (binds2l ++ binds2r)
              | otherwise -- No match, so push back (FIXME O(n^2))
              = go (fuel-1) env (binds1++[(bndr1,expr1)]) binds2
           go _ _ _ _ = panic "diffBinds: impossible" -- GHC isn't smart enough

           -- We have tried everything, but couldn't find a good match. So
           -- now we just return the comparison results when we pair up
           -- the binds in a pseudo-random order.
           warn env binds1 binds2 =
             concatMap (uncurry (diffBind env)) (zip binds1' binds2') ++
             unmatched "unmatched left-hand:" (drop l binds1') ++
             unmatched "unmatched right-hand:" (drop l binds2')
            where binds1' = sortBy (comparing fst) binds1
                  binds2' = sortBy (comparing fst) binds2
                  l = min (length binds1') (length binds2')
           unmatched _   [] = []
           unmatched txt bs = [text txt $$ ppr (Rec bs)]
           diffBind env (bndr1,expr1) (bndr2,expr2)
             | ds@(_:_) <- diffExpr top env expr1 expr2
             = locBind "in binding" bndr1 bndr2 ds
             | otherwise
             = diffIdInfo env bndr1 bndr2

    -- | Find differences in @IdInfo@. We will especially check whether
    -- the unfoldings match, if present (see @diffUnfold@).
    diffIdInfo :: RnEnv2 -> Var -> Var -> [SDoc]
    diffIdInfo env bndr1 bndr2
      | arityInfo info1 == arityInfo info2
        && cafInfo info1 == cafInfo info2
        && oneShotInfo info1 == oneShotInfo info2
        && inlinePragInfo info1 == inlinePragInfo info2
        && occInfo info1 == occInfo info2
        && demandInfo info1 == demandInfo info2
        && callArityInfo info1 == callArityInfo info2
        && levityInfo info1 == levityInfo info2
      = locBind "in unfolding of" bndr1 bndr2 $
        diffUnfold env (unfoldingInfo info1) (unfoldingInfo info2)
      | otherwise
      = locBind "in Id info of" bndr1 bndr2
        [fsep [pprBndr LetBind bndr1, text "/=", pprBndr LetBind bndr2]]
      where info1 = idInfo bndr1; info2 = idInfo bndr2

    -- | Find differences in unfoldings. Note that we will not check for
    -- differences of @IdInfo@ in unfoldings, as this is generally
    -- redundant, and can lead to an exponential blow-up in complexity.
    diffUnfold :: RnEnv2 -> Unfolding -> Unfolding -> [SDoc]
    diffUnfold _   NoUnfolding    NoUnfolding                 = []
    diffUnfold _   BootUnfolding  BootUnfolding               = []
    diffUnfold _   (OtherCon cs1) (OtherCon cs2) | cs1 == cs2 = []
    diffUnfold env (DFunUnfolding bs1 c1 a1)
                   (DFunUnfolding bs2 c2 a2)
      | c1 == c2 && length bs1 == length bs2
      = concatMap (uncurry (diffExpr False env')) (zip a1 a2)
      where env' = rnBndrs2 env bs1 bs2
    diffUnfold env (CoreUnfolding t1 _ _ v1 cl1 wf1 x1 g1)
                   (CoreUnfolding t2 _ _ v2 cl2 wf2 x2 g2)
      | v1 == v2 && cl1 == cl2
        && wf1 == wf2 && x1 == x2 && g1 == g2
      = diffExpr False env t1 t2
    diffUnfold _   uf1 uf2
      = [fsep [ppr uf1, text "/=", ppr uf2]]

    -- | Add location information to diff messages
    locBind :: String -> Var -> Var -> [SDoc] -> [SDoc]
    locBind loc b1 b2 diffs = map addLoc diffs
      where addLoc d            = d $$ nest 2 (parens (text loc <+> bindLoc))
            bindLoc | b1 == b2  = ppr b1
                    | otherwise = ppr b1 <> char '/' <> ppr b2

    {-
    ************************************************************************
    *                                                                      *
                    Eta reduction
    *                                                                      *
    ************************************************************************

    Note [Eta reduction conditions]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We try for eta reduction here, but *only* if we get all the way to an
    trivial expression.  We don't want to remove extra lambdas unless we
    are going to avoid allocating this thing altogether.

    There are some particularly delicate points here:

    * We want to eta-reduce if doing so leaves a trivial expression,
      *including* a cast.  For example
           \x. f |> co  -->  f |> co
      (provided co doesn't mention x)

    * Eta reduction is not valid in general:
            \x. bot  /=  bot
      This matters, partly for old-fashioned correctness reasons but,
      worse, getting it wrong can yield a seg fault. Consider
            f = \x.f x
            h y = case (case y of { True -> f `seq` True; False -> False }) of
                    True -> ...; False -> ...

      If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
      says f=bottom, and replaces the (f `seq` True) with just
      (f `cast` unsafe-co).  BUT, as thing stand, 'f' got arity 1, and it
      *keeps* arity 1 (perhaps also wrongly).  So CorePrep eta-expands
      the definition again, so that it does not termninate after all.
      Result: seg-fault because the boolean case actually gets a function value.
      See Trac #1947.

      So it's important to do the right thing.

    * Note [Arity care]: we need to be careful if we just look at f's
      arity. Currently (Dec07), f's arity is visible in its own RHS (see
      Note [Arity robustness] in SimplEnv) so we must *not* trust the
      arity when checking that 'f' is a value.  Otherwise we will
      eta-reduce
          f = \x. f x
      to
          f = f
      Which might change a terminating program (think (f `seq` e)) to a
      non-terminating one.  So we check for being a loop breaker first.

      However for GlobalIds we can look at the arity; and for primops we
      must, since they have no unfolding.

    * Regardless of whether 'f' is a value, we always want to
      reduce (/\a -> f a) to f
      This came up in a RULE: foldr (build (/\a -> g a))
      did not match           foldr (build (/\b -> ...something complex...))
      The type checker can insert these eta-expanded versions,
      with both type and dictionary lambdas; hence the slightly
      ad-hoc isDictId

    * Never *reduce* arity. For example
          f = \xy. g x y
      Then if h has arity 1 we don't want to eta-reduce because then
      f's arity would decrease, and that is bad

    These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
    Alas.

    Note [Eta reduction with casted arguments]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
        (\(x:t3). f (x |> g)) :: t3 -> t2
      where
        f :: t1 -> t2
        g :: t3 ~ t1
    This should be eta-reduced to

        f |> (sym g -> t2)

    So we need to accumulate a coercion, pushing it inward (past
    variable arguments only) thus:
       f (x |> co_arg) |> co  -->  (f |> (sym co_arg -> co)) x
       f (x:t)         |> co  -->  (f |> (t -> co)) x
       f @ a           |> co  -->  (f |> (forall a.co)) @ a
       f @ (g:t1~t2)   |> co  -->  (f |> (t1~t2 => co)) @ (g:t1~t2)
    These are the equations for ok_arg.

    It's true that we could also hope to eta reduce these:
        (\xy. (f x |> g) y)
        (\xy. (f x y) |> g)
    But the simplifier pushes those casts outwards, so we don't
    need to address that here.
    -}

    tryEtaReduce :: [Var] -> CoreExpr -> Maybe CoreExpr
    tryEtaReduce bndrs body
      = go (reverse bndrs) body (mkRepReflCo (exprType body))
      where
        incoming_arity = count isId bndrs

        go :: [Var]            -- Binders, innermost first, types [a3,a2,a1]
           -> CoreExpr         -- Of type tr
           -> Coercion         -- Of type tr ~ ts
           -> Maybe CoreExpr   -- Of type a1 -> a2 -> a3 -> ts
        -- See Note [Eta reduction with casted arguments]
        -- for why we have an accumulating coercion
        go [] fun co
          | ok_fun fun
          , let used_vars = exprFreeVars fun `unionVarSet` tyCoVarsOfCo co
          , not (any (`elemVarSet` used_vars) bndrs)
          = Just (mkCast fun co)   -- Check for any of the binders free in the result
                                   -- including the accumulated coercion

        go bs (Tick t e) co
          | tickishFloatable t
          = fmap (Tick t) $ go bs e co
          -- Float app ticks: \x -> Tick t (e x) ==> Tick t e

        go (b : bs) (App fun arg) co
          | Just (co', ticks) <- ok_arg b arg co
          = fmap (flip (foldr mkTick) ticks) $ go bs fun co'
                -- Float arg ticks: \x -> e (Tick t x) ==> Tick t e

        go _ _ _  = Nothing         -- Failure!

        ---------------
        -- Note [Eta reduction conditions]
        ok_fun (App fun (Type {})) = ok_fun fun
        ok_fun (Cast fun _)        = ok_fun fun
        ok_fun (Tick _ expr)       = ok_fun expr
        ok_fun (Var fun_id)        = ok_fun_id fun_id || all ok_lam bndrs
        ok_fun _fun                = False

        ---------------
        ok_fun_id fun = fun_arity fun >= incoming_arity

        ---------------
        fun_arity fun             -- See Note [Arity care]
           | isLocalId fun
           , isStrongLoopBreaker (idOccInfo fun) = 0
           | arity > 0                           = arity
           | isEvaldUnfolding (idUnfolding fun)  = 1
                -- See Note [Eta reduction of an eval'd function]
           | otherwise                           = 0
           where
             arity = idArity fun

        ---------------
        ok_lam v = isTyVar v || isEvVar v

        ---------------
        ok_arg :: Var              -- Of type bndr_t
               -> CoreExpr         -- Of type arg_t
               -> Coercion         -- Of kind (t1~t2)
               -> Maybe (Coercion  -- Of type (arg_t -> t1 ~  bndr_t -> t2)
                                   --   (and similarly for tyvars, coercion args)
                        , [Tickish Var])
        -- See Note [Eta reduction with casted arguments]
        ok_arg bndr (Type ty) co
           | Just tv <- getTyVar_maybe ty
           , bndr == tv  = Just (mkHomoForAllCos [tv] co, [])
        ok_arg bndr (Var v) co
           | bndr == v   = let reflCo = mkRepReflCo (idType bndr)
                           in Just (mkFunCo Representational reflCo co, [])
        ok_arg bndr (Cast e co_arg) co
           | (ticks, Var v) <- stripTicksTop tickishFloatable e
           , bndr == v
           = Just (mkFunCo Representational (mkSymCo co_arg) co, ticks)
           -- The simplifier combines multiple casts into one,
           -- so we can have a simple-minded pattern match here
        ok_arg bndr (Tick t arg) co
           | tickishFloatable t, Just (co', ticks) <- ok_arg bndr arg co
           = Just (co', t:ticks)

        ok_arg _ _ _ = Nothing

    {-
    Note [Eta reduction of an eval'd function]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In Haskell it is not true that    f = \x. f x
    because f might be bottom, and 'seq' can distinguish them.

    But it *is* true that   f = f `seq` \x. f x
    and we'd like to simplify the latter to the former.  This amounts
    to the rule that
      * when there is just *one* value argument,
      * f is not bottom
    we can eta-reduce    \x. f x  ===>  f

    This turned up in Trac #7542.

    ************************************************************************
    *                                                                      *
    \subsection{Determining non-updatable right-hand-sides}
    *                                                                      *
    ************************************************************************

    Top-level constructor applications can usually be allocated
    statically, but they can't if the constructor, or any of the
    arguments, come from another DLL (because we can't refer to static
    labels in other DLLs).

    If this happens we simply make the RHS into an updatable thunk,
    and 'execute' it rather than allocating it statically.
    -}

    -- | This function is called only on *top-level* right-hand sides.
    -- Returns @True@ if the RHS can be allocated statically in the output,
    -- with no thunks involved at all.
    rhsIsStatic :: Platform
                -> (Name -> Bool)         -- Which names are dynamic
                -> (Integer -> CoreExpr)  -- Desugaring for integer literals (disgusting)
                                          -- C.f. Note [Disgusting computation of CafRefs]
                                          --      in TidyPgm
                -> CoreExpr -> Bool
    -- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
    -- refers to, CAFs; (ii) in CoreToStg to decide whether to put an
    -- update flag on it and (iii) in DsExpr to decide how to expand
    -- list literals
    --
    -- The basic idea is that rhsIsStatic returns True only if the RHS is
    --      (a) a value lambda
    --      (b) a saturated constructor application with static args
    --
    -- BUT watch out for
    --  (i) Any cross-DLL references kill static-ness completely
    --      because they must be 'executed' not statically allocated
    --      ("DLL" here really only refers to Windows DLLs, on other platforms,
    --      this is not necessary)
    --
    -- (ii) We treat partial applications as redexes, because in fact we
    --      make a thunk for them that runs and builds a PAP
    --      at run-time.  The only applications that are treated as
    --      static are *saturated* applications of constructors.

    -- We used to try to be clever with nested structures like this:
    --              ys = (:) w ((:) w [])
    -- on the grounds that CorePrep will flatten ANF-ise it later.
    -- But supporting this special case made the function much more
    -- complicated, because the special case only applies if there are no
    -- enclosing type lambdas:
    --              ys = /\ a -> Foo (Baz ([] a))
    -- Here the nested (Baz []) won't float out to top level in CorePrep.
    --
    -- But in fact, even without -O, nested structures at top level are
    -- flattened by the simplifier, so we don't need to be super-clever here.
    --
    -- Examples
    --
    --      f = \x::Int. x+7        TRUE
    --      p = (True,False)        TRUE
    --
    --      d = (fst p, False)      FALSE because there's a redex inside
    --                              (this particular one doesn't happen but...)
    --
    --      h = D# (1.0## /## 2.0##)        FALSE (redex again)
    --      n = /\a. Nil a                  TRUE
    --
    --      t = /\a. (:) (case w a of ...) (Nil a)  FALSE (redex)
    --
    --
    -- This is a bit like CoreUtils.exprIsHNF, with the following differences:
    --    a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
    --
    --    b) (C x xs), where C is a constructor is updatable if the application is
    --         dynamic
    --
    --    c) don't look through unfolding of f in (f x).

    rhsIsStatic platform is_dynamic_name cvt_integer rhs = is_static False rhs
      where
      is_static :: Bool     -- True <=> in a constructor argument; must be atomic
                -> CoreExpr -> Bool

      is_static False  (Lam b e)              = isRuntimeVar b || is_static False e
      is_static in_arg (Tick n e)             = not (tickishIsCode n)
                                                  && is_static in_arg e
      is_static in_arg (Cast e _)             = is_static in_arg e
      is_static _      (Coercion {})          = True   -- Behaves just like a literal
      is_static in_arg (Lit (LitInteger i _)) = is_static in_arg (cvt_integer i)
      is_static _      (Lit (MachLabel {}))   = False
      is_static _      (Lit _)                = True
            -- A MachLabel (foreign import "&foo") in an argument
            -- prevents a constructor application from being static.  The
            -- reason is that it might give rise to unresolvable symbols
            -- in the object file: under Linux, references to "weak"
            -- symbols from the data segment give rise to "unresolvable
            -- relocation" errors at link time This might be due to a bug
            -- in the linker, but we'll work around it here anyway.
            -- SDM 24/2/2004

      is_static in_arg other_expr = go other_expr 0
       where
        go (Var f) n_val_args
            | (platformOS platform /= OSMinGW32) ||
              not (is_dynamic_name (idName f))
            =  saturated_data_con f n_val_args
            || (in_arg && n_val_args == 0)
                    -- A naked un-applied variable is *not* deemed a static RHS
                    -- E.g.         f = g
                    -- Reason: better to update so that the indirection gets shorted
                    --         out, and the true value will be seen
                    -- NB: if you change this, you'll break the invariant that THUNK_STATICs
                    --     are always updatable.  If you do so, make sure that non-updatable
                    --     ones have enough space for their static link field!

        go (App f a) n_val_args
            | isTypeArg a                    = go f n_val_args
            | not in_arg && is_static True a = go f (n_val_args + 1)
            -- The (not in_arg) checks that we aren't in a constructor argument;
            -- if we are, we don't allow (value) applications of any sort
            --
            -- NB. In case you wonder, args are sometimes not atomic.  eg.
            --   x = D# (1.0## /## 2.0##)
            -- can't float because /## can fail.

        go (Tick n f) n_val_args = not (tickishIsCode n) && go f n_val_args
        go (Cast e _) n_val_args = go e n_val_args
        go _          _          = False

        saturated_data_con f n_val_args
            = case isDataConWorkId_maybe f of
                Just dc -> n_val_args == dataConRepArity dc
                Nothing -> False

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Type utilities}
    *                                                                      *
    ************************************************************************
    -}

    -- | True if the type has no non-bottom elements, e.g. when it is an empty
    -- datatype, or a GADT with non-satisfiable type parameters, e.g. Int :~: Bool.
    -- See Note [Bottoming expressions]
    --
    -- See Note [No alternatives lint check] for another use of this function.
    isEmptyTy :: Type -> Bool
    isEmptyTy ty
        -- Data types where, given the particular type parameters, no data
        -- constructor matches, are empty.
        -- This includes data types with no constructors, e.g. Data.Void.Void.
        | Just (tc, inst_tys) <- splitTyConApp_maybe ty
        , Just dcs <- tyConDataCons_maybe tc
        , all (dataConCannotMatch inst_tys) dcs
        = True
        | otherwise
        = False

    {-
    *****************************************************
    *
    * StaticPtr
    *
    *****************************************************
    -}

    -- | @collectMakeStaticArgs (makeStatic t srcLoc e)@ yields
    -- @Just (makeStatic, t, srcLoc, e)@.
    --
    -- Returns @Nothing@ for every other expression.
    collectMakeStaticArgs
      :: CoreExpr -> Maybe (CoreExpr, Type, CoreExpr, CoreExpr)
    collectMakeStaticArgs e
        | (fun@(Var b), [Type t, loc, arg], _) <- collectArgsTicks (const True) e
        , idName b == makeStaticName = Just (fun, t, loc, arg)
    collectMakeStaticArgs _          = Nothing

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Join points}
    *                                                                      *
    ************************************************************************
    -}

    -- | Does this binding bind a join point (or a recursive group of join points)?
    isJoinBind :: CoreBind -> Bool
    isJoinBind (NonRec b _)       = isJoinId b
    isJoinBind (Rec ((b, _) : _)) = isJoinId b
    isJoinBind _                  = False