stranal/WwLib.hs

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

    \section[WwLib]{A library for the ``worker\/wrapper'' back-end to the strictness analyser}
    -}

    {-# LANGUAGE CPP #-}

    module WwLib ( mkWwBodies, mkWWstr, mkWorkerArgs
                 , deepSplitProductType_maybe, findTypeShape
     ) where

    #include "HsVersions.h"

    import CoreSyn
    import CoreUtils        ( exprType, mkCast )
    import Id               ( Id, idType, mkSysLocal, idDemandInfo, setIdDemandInfo,
                              setIdUnfolding,
                              setIdInfo, idOneShotInfo, setIdOneShotInfo
                            )
    import IdInfo           ( vanillaIdInfo )
    import DataCon
    import Demand
    import MkCore           ( mkRuntimeErrorApp, aBSENT_ERROR_ID )
    import MkId             ( voidArgId, voidPrimId )
    import TysPrim          ( voidPrimTy )
    import TysWiredIn       ( tupleCon )
    import Type
    import Coercion hiding  ( substTy, substTyVarBndr )
    import FamInstEnv
    import BasicTypes       ( TupleSort(..), OneShotInfo(..), worstOneShot )
    import Literal          ( absentLiteralOf )
    import TyCon
    import UniqSupply
    import Unique
    import Maybes
    import Util
    import Outputable
    import DynFlags
    import FastString

    {-
    ************************************************************************
    *                                                                      *
    \subsection[mkWrapperAndWorker]{@mkWrapperAndWorker@}
    *                                                                      *
    ************************************************************************

    Here's an example.  The original function is:

    \begin{verbatim}
    g :: forall a . Int -> [a] -> a

    g = \/\ a -> \ x ys ->
            case x of
              0 -> head ys
              _ -> head (tail ys)
    \end{verbatim}

    From this, we want to produce:
    \begin{verbatim}
    -- wrapper (an unfolding)
    g :: forall a . Int -> [a] -> a

    g = \/\ a -> \ x ys ->
            case x of
              I# x# -> $wg a x# ys
                -- call the worker; don't forget the type args!

    -- worker
    $wg :: forall a . Int# -> [a] -> a

    $wg = \/\ a -> \ x# ys ->
            let
                x = I# x#
            in
                case x of               -- note: body of g moved intact
                  0 -> head ys
                  _ -> head (tail ys)
    \end{verbatim}

    Something we have to be careful about:  Here's an example:

    \begin{verbatim}
    -- "f" strictness: U(P)U(P)
    f (I# a) (I# b) = a +# b

    g = f   -- "g" strictness same as "f"
    \end{verbatim}

    \tr{f} will get a worker all nice and friendly-like; that's good.
    {\em But we don't want a worker for \tr{g}}, even though it has the
    same strictness as \tr{f}.  Doing so could break laziness, at best.

    Consequently, we insist that the number of strictness-info items is
    exactly the same as the number of lambda-bound arguments.  (This is
    probably slightly paranoid, but OK in practice.)  If it isn't the
    same, we ``revise'' the strictness info, so that we won't propagate
    the unusable strictness-info into the interfaces.

    ************************************************************************
    *                                                                      *
    \subsection{The worker wrapper core}
    *                                                                      *
    ************************************************************************

    @mkWwBodies@ is called when doing the worker\/wrapper split inside a module.
    -}

    mkWwBodies :: DynFlags
               -> FamInstEnvs
               -> Type                                  -- Type of original function
               -> [Demand]                              -- Strictness of original function
               -> DmdResult                             -- Info about function result
               -> [OneShotInfo]                         -- One-shot-ness of the function, value args only
               -> UniqSM (Maybe ([Demand],              -- Demands for worker (value) args
                                 Id -> CoreExpr,        -- Wrapper body, lacking only the worker Id
                                 CoreExpr -> CoreExpr)) -- Worker body, lacking the original function rhs

    -- wrap_fn_args E       = \x y -> E
    -- work_fn_args E       = E x y

    -- wrap_fn_str E        = case x of { (a,b) ->
    --                        case a of { (a1,a2) ->
    --                        E a1 a2 b y }}
    -- work_fn_str E        = \a2 a2 b y ->
    --                        let a = (a1,a2) in
    --                        let x = (a,b) in
    --                        E

    mkWwBodies dflags fam_envs fun_ty demands res_info one_shots
      = do  { let arg_info = demands `zip` (one_shots ++ repeat NoOneShotInfo)
                  all_one_shots = foldr (worstOneShot . snd) OneShotLam arg_info
            ; (wrap_args, wrap_fn_args, work_fn_args, res_ty) <- mkWWargs emptyTvSubst fun_ty arg_info
            ; (useful1, work_args, wrap_fn_str, work_fn_str) <- mkWWstr dflags fam_envs wrap_args

            -- Do CPR w/w.  See Note [Always do CPR w/w]
            ; (useful2, wrap_fn_cpr, work_fn_cpr,  cpr_res_ty) <- mkWWcpr fam_envs res_ty res_info

            ; let (work_lam_args, work_call_args) = mkWorkerArgs dflags work_args all_one_shots cpr_res_ty
                  worker_args_dmds = [idDemandInfo v | v <- work_call_args, isId v]
                  wrapper_body = wrap_fn_args . wrap_fn_cpr . wrap_fn_str . applyToVars work_call_args . Var
                  worker_body = mkLams work_lam_args. work_fn_str . work_fn_cpr . work_fn_args

            ; if useful1 && not (only_one_void_argument) || useful2
              then return (Just (worker_args_dmds, wrapper_body, worker_body))
              else return Nothing
            }
            -- We use an INLINE unconditionally, even if the wrapper turns out to be
            -- something trivial like
            --      fw = ...
            --      f = __inline__ (coerce T fw)
            -- The point is to propagate the coerce to f's call sites, so even though
            -- f's RHS is now trivial (size 1) we still want the __inline__ to prevent
            -- fw from being inlined into f's RHS
      where
        -- Note [Do not split void functions]
        only_one_void_argument
          | [d] <- demands
          , Just (arg_ty1, _) <- splitFunTy_maybe fun_ty
          , isAbsDmd d && isVoidTy arg_ty1
          = True
          | otherwise
          = False

    {-
    Note [Always do CPR w/w]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    At one time we refrained from doing CPR w/w for thunks, on the grounds that
    we might duplicate work.  But that is already handled by the demand analyser,
    which doesn't give the CPR proprety if w/w might waste work: see
    Note [CPR for thunks] in DmdAnal.

    And if something *has* been given the CPR property and we don't w/w, it's
    a disaster, because then the enclosing function might say it has the CPR
    property, but now doesn't and there a cascade of disaster.  A good example
    is Trac #5920.

    ************************************************************************
    *                                                                      *
    \subsection{Making wrapper args}
    *                                                                      *
    ************************************************************************

    During worker-wrapper stuff we may end up with an unlifted thing
    which we want to let-bind without losing laziness.  So we
    add a void argument.  E.g.

            f = /\a -> \x y z -> E::Int#    -- E does not mention x,y,z
    ==>
            fw = /\ a -> \void -> E
            f  = /\ a -> \x y z -> fw realworld

    We use the state-token type which generates no code.
    -}

    mkWorkerArgs :: DynFlags -> [Var]
                 -> OneShotInfo  -- Whether all arguments are one-shot
                 -> Type    -- Type of body
                 -> ([Var], -- Lambda bound args
                     [Var]) -- Args at call site
    mkWorkerArgs dflags args all_one_shot res_ty
        | any isId args || not needsAValueLambda
        = (args, args)
        | otherwise
        = (args ++ [newArg], args ++ [voidPrimId])
        where
          needsAValueLambda =
            isUnLiftedType res_ty
            || not (gopt Opt_FunToThunk dflags)
               -- see Note [Protecting the last value argument]

          -- see Note [All One-Shot Arguments of a Worker]
          newArg = setIdOneShotInfo voidArgId all_one_shot

    {-
    Note [Protecting the last value argument]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If the user writes (\_ -> E), they might be intentionally disallowing
    the sharing of E. Since absence analysis and worker-wrapper are keen
    to remove such unused arguments, we add in a void argument to prevent
    the function from becoming a thunk.

    The user can avoid adding the void argument with the -ffun-to-thunk
    flag. However, this can create sharing, which may be bad in two ways. 1) It can
    create a space leak. 2) It can prevent inlining *under a lambda*. If w/w
    removes the last argument from a function f, then f now looks like a thunk, and
    so f can't be inlined *under a lambda*.

    Note [All One-Shot Arguments of a Worker]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Sometimes, derived join-points are just lambda-lifted thunks, whose
    only argument is of the unit type and is never used. This might
    interfere with the absence analysis, basing on which results these
    never-used arguments are eliminated in the worker. The additional
    argument `all_one_shot` of `mkWorkerArgs` is to prevent this.

    Example.  Suppose we have
       foo = \p(one-shot) q(one-shot). y + 3
    Then we drop the unused args to give
       foo   = \pq. $wfoo void#
       $wfoo = \void(one-shot). y + 3

    But suppse foo didn't have all one-shot args:
       foo = \p(not-one-shot) q(one-shot). expensive y + 3
    Then we drop the unused args to give
       foo   = \pq. $wfoo void#
       $wfoo = \void(not-one-shot). y + 3

    If we made the void-arg one-shot we might inline an expensive
    computation for y, which would be terrible!

    ************************************************************************
    *                                                                      *
    \subsection{Coercion stuff}
    *                                                                      *
    ************************************************************************

    We really want to "look through" coerces.
    Reason: I've seen this situation:

            let f = coerce T (\s -> E)
            in \x -> case x of
                        p -> coerce T' f
                        q -> \s -> E2
                        r -> coerce T' f

    If only we w/w'd f, we'd get
            let f = coerce T (\s -> fw s)
                fw = \s -> E
            in ...

    Now we'll inline f to get

            let fw = \s -> E
            in \x -> case x of
                        p -> fw
                        q -> \s -> E2
                        r -> fw

    Now we'll see that fw has arity 1, and will arity expand
    the \x to get what we want.
    -}

    -- mkWWargs just does eta expansion
    -- is driven off the function type and arity.
    -- It chomps bites off foralls, arrows, newtypes
    -- and keeps repeating that until it's satisfied the supplied arity

    mkWWargs :: TvSubst             -- Freshening substitution to apply to the type
                                    --   See Note [Freshen type variables]
             -> Type                -- The type of the function
             -> [(Demand,OneShotInfo)]     -- Demands and one-shot info for value arguments
             -> UniqSM  ([Var],            -- Wrapper args
                         CoreExpr -> CoreExpr,      -- Wrapper fn
                         CoreExpr -> CoreExpr,      -- Worker fn
                         Type)                      -- Type of wrapper body

    mkWWargs subst fun_ty arg_info
      | null arg_info
      = return ([], id, id, substTy subst fun_ty)

      | ((dmd,one_shot):arg_info') <- arg_info
      , Just (arg_ty, fun_ty') <- splitFunTy_maybe fun_ty
      = do  { uniq <- getUniqueM
            ; let arg_ty' = substTy subst arg_ty
                  id = mk_wrap_arg uniq arg_ty' dmd one_shot
            ; (wrap_args, wrap_fn_args, work_fn_args, res_ty)
                  <- mkWWargs subst fun_ty' arg_info'
            ; return (id : wrap_args,
                      Lam id . wrap_fn_args,
                      work_fn_args . (`App` varToCoreExpr id),
                      res_ty) }

      | Just (tv, fun_ty') <- splitForAllTy_maybe fun_ty
      = do  { let (subst', tv') = substTyVarBndr subst tv
                    -- This substTyVarBndr clones the type variable when necy
                    -- See Note [Freshen type variables]
            ; (wrap_args, wrap_fn_args, work_fn_args, res_ty)
                 <- mkWWargs subst' fun_ty' arg_info
            ; return (tv' : wrap_args,
                      Lam tv' . wrap_fn_args,
                      work_fn_args . (`App` Type (mkTyVarTy tv')),
                      res_ty) }

      | Just (co, rep_ty) <- topNormaliseNewType_maybe fun_ty
            -- The newtype case is for when the function has
            -- a newtype after the arrow (rare)
            --
            -- It's also important when we have a function returning (say) a pair
            -- wrapped in a  newtype, at least if CPR analysis can look
            -- through such newtypes, which it probably can since they are
            -- simply coerces.

      = do { (wrap_args, wrap_fn_args, work_fn_args, res_ty)
                <-  mkWWargs subst rep_ty arg_info
            ; return (wrap_args,
                      \e -> Cast (wrap_fn_args e) (mkSymCo co),
                      \e -> work_fn_args (Cast e co),
                      res_ty) }

      | otherwise
      = WARN( True, ppr fun_ty )                    -- Should not happen: if there is a demand
        return ([], id, id, substTy subst fun_ty)   -- then there should be a function arrow

    applyToVars :: [Var] -> CoreExpr -> CoreExpr
    applyToVars vars fn = mkVarApps fn vars

    mk_wrap_arg :: Unique -> Type -> Demand -> OneShotInfo -> Id
    mk_wrap_arg uniq ty dmd one_shot
      = mkSysLocal (fsLit "w") uniq ty
           `setIdDemandInfo` dmd
           `setIdOneShotInfo` one_shot

    {-
    Note [Freshen type variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Wen we do a worker/wrapper split, we must not use shadowed names,
    else we'll get
       f = /\ a /\a. fw a a
    which is obviously wrong.  Type variables can can in principle shadow,
    within a type (e.g. forall a. a -> forall a. a->a).  But type
    variables *are* mentioned in <blah>, so we must substitute.

    That's why we carry the TvSubst through mkWWargs

    ************************************************************************
    *                                                                      *
    \subsection{Strictness stuff}
    *                                                                      *
    ************************************************************************
    -}

    mkWWstr :: DynFlags
            -> FamInstEnvs
            -> [Var]                                -- Wrapper args; have their demand info on them
                                                    --  *Includes type variables*
            -> UniqSM (Bool,                        -- Is this useful
                       [Var],                       -- Worker args
                       CoreExpr -> CoreExpr,        -- Wrapper body, lacking the worker call
                                                    -- and without its lambdas
                                                    -- This fn adds the unboxing

                       CoreExpr -> CoreExpr)        -- Worker body, lacking the original body of the function,
                                                    -- and lacking its lambdas.
                                                    -- This fn does the reboxing
    mkWWstr _ _ []
      = return (False, [], nop_fn, nop_fn)

    mkWWstr dflags fam_envs (arg : args) = do
        (useful1, args1, wrap_fn1, work_fn1) <- mkWWstr_one dflags fam_envs arg
        (useful2, args2, wrap_fn2, work_fn2) <- mkWWstr dflags fam_envs args
        return (useful1 || useful2, args1 ++ args2, wrap_fn1 . wrap_fn2, work_fn1 . work_fn2)

    {-
    Note [Unpacking arguments with product and polymorphic demands]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The argument is unpacked in a case if it has a product type and has a
    strict *and* used demand put on it. I.e., arguments, with demands such
    as the following ones:

       <S,U(U, L)>
       <S(L,S),U>

    will be unpacked, but

       <S,U> or <B,U>

    will not, because the pieces aren't used. This is quite important otherwise
    we end up unpacking massive tuples passed to the bottoming function. Example:

            f :: ((Int,Int) -> String) -> (Int,Int) -> a
            f g pr = error (g pr)

            main = print (f fst (1, error "no"))

    Does 'main' print "error 1" or "error no"?  We don't really want 'f'
    to unbox its second argument.  This actually happened in GHC's onwn
    source code, in Packages.applyPackageFlag, which ended up un-boxing
    the enormous DynFlags tuple, and being strict in the
    as-yet-un-filled-in pkgState files.
    -}

    ----------------------
    -- mkWWstr_one wrap_arg = (useful, work_args, wrap_fn, work_fn)
    --   *  wrap_fn assumes wrap_arg is in scope,
    --        brings into scope work_args (via cases)
    --   * work_fn assumes work_args are in scope, a
    --        brings into scope wrap_arg (via lets)
    mkWWstr_one :: DynFlags -> FamInstEnvs -> Var
        -> UniqSM (Bool, [Var], CoreExpr -> CoreExpr, CoreExpr -> CoreExpr)
    mkWWstr_one dflags fam_envs arg
      | isTyVar arg
      = return (False, [arg],  nop_fn, nop_fn)

      -- See Note [Worker-wrapper for bottoming functions]
      | isAbsDmd dmd
      , Just work_fn <- mk_absent_let dflags arg
         -- Absent case.  We can't always handle absence for arbitrary
         -- unlifted types, so we need to choose just the cases we can
         --- (that's what mk_absent_let does)
      = return (True, [], nop_fn, work_fn)

      -- See Note [Worthy functions for Worker-Wrapper split]
      | isSeqDmd dmd  -- `seq` demand; evaluate in wrapper in the hope
                      -- of dropping seqs in the worker
      = let arg_w_unf = arg `setIdUnfolding` evaldUnfolding
              -- Tell the worker arg that it's sure to be evaluated
              -- so that internal seqs can be dropped
        in return (True, [arg_w_unf], mk_seq_case arg, nop_fn)
                    -- Pass the arg, anyway, even if it is in theory discarded
                    -- Consider
                    --      f x y = x `seq` y
                    -- x gets a (Eval (Poly Abs)) demand, but if we fail to pass it to the worker
                    -- we ABSOLUTELY MUST record that x is evaluated in the wrapper.
                    -- Something like:
                    --      f x y = x `seq` fw y
                    --      fw y = let x{Evald} = error "oops" in (x `seq` y)
                    -- If we don't pin on the "Evald" flag, the seq doesn't disappear, and
                    -- we end up evaluating the absent thunk.
                    -- But the Evald flag is pretty weird, and I worry that it might disappear
                    -- during simplification, so for now I've just nuked this whole case

      | isStrictDmd dmd
      , Just cs <- splitProdDmd_maybe dmd
          -- See Note [Unpacking arguments with product and polymorphic demands]
      , Just (data_con, inst_tys, inst_con_arg_tys, co)
                 <- deepSplitProductType_maybe fam_envs (idType arg)
      , cs `equalLength` inst_con_arg_tys
          -- See Note [mkWWstr and unsafeCoerce]
      =  do { (uniq1:uniqs) <- getUniquesM
            ; let   unpk_args      = zipWith mk_ww_local uniqs inst_con_arg_tys
                    unpk_args_w_ds = zipWithEqual "mkWWstr" set_worker_arg_info unpk_args cs
                    unbox_fn       = mkUnpackCase (Var arg) co uniq1
                                                  data_con unpk_args
                    rebox_fn       = Let (NonRec arg con_app)
                    con_app        = mkConApp2 data_con inst_tys unpk_args `mkCast` mkSymCo co
             ; (_, worker_args, wrap_fn, work_fn) <- mkWWstr dflags fam_envs unpk_args_w_ds
             ; return (True, worker_args, unbox_fn . wrap_fn, work_fn . rebox_fn) }
                               -- Don't pass the arg, rebox instead

      | otherwise   -- Other cases
      = return (False, [arg], nop_fn, nop_fn)

      where
        dmd = idDemandInfo arg
        one_shot = idOneShotInfo arg
            -- If the wrapper argument is a one-shot lambda, then
            -- so should (all) the corresponding worker arguments be
            -- This bites when we do w/w on a case join point
        set_worker_arg_info worker_arg demand
          = worker_arg `setIdDemandInfo`  demand
                       `setIdOneShotInfo` one_shot

    ----------------------
    nop_fn :: CoreExpr -> CoreExpr
    nop_fn body = body

    {-
    Note [mkWWstr and unsafeCoerce]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    By using unsafeCoerce, it is possible to make the number of demands fail to
    match the number of constructor arguments; this happened in Trac #8037.
    If so, the worker/wrapper split doesn't work right and we get a Core Lint
    bug.  The fix here is simply to decline to do w/w if that happens.

    ************************************************************************
    *                                                                      *
             Type scrutiny that is specfic to demand analysis
    *                                                                      *
    ************************************************************************

    Note [Do not unpack class dictionaries]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If we have
       f :: Ord a => [a] -> Int -> a
       {-# INLINABLE f #-}
    and we worker/wrapper f, we'll get a worker with an INLINALBE pragma
    (see Note [Worker-wrapper for INLINABLE functions] in WorkWrap), which
    can still be specialised by the type-class specialiser, something like
       fw :: Ord a => [a] -> Int# -> a

    BUT if f is strict in the Ord dictionary, we might unpack it, to get
       fw :: (a->a->Bool) -> [a] -> Int# -> a
    and the type-class specialiser can't specialise that.  An example is
    Trac #6056.

    Moreover, dictinoaries can have a lot of fields, so unpacking them can
    increase closure sizes.

    Conclusion: don't unpack dictionaries.
    -}

    deepSplitProductType_maybe :: FamInstEnvs -> Type -> Maybe (DataCon, [Type], [Type], Coercion)
    -- If    deepSplitProductType_maybe ty = Just (dc, tys, arg_tys, co)
    -- then  dc @ tys (args::arg_tys) :: rep_ty
    --       co :: ty ~ rep_ty
    deepSplitProductType_maybe fam_envs ty
      | let (co, ty1) = topNormaliseType_maybe fam_envs ty
                        `orElse` (mkReflCo Representational ty, ty)
      , Just (tc, tc_args) <- splitTyConApp_maybe ty1
      , Just con <- isDataProductTyCon_maybe tc
      , not (isClassTyCon tc)  -- See Note [Do not unpack class dictionaries]
      = Just (con, tc_args, dataConInstArgTys con tc_args, co)
    deepSplitProductType_maybe _ _ = Nothing

    deepSplitCprType_maybe :: FamInstEnvs -> ConTag -> Type -> Maybe (DataCon, [Type], [Type], Coercion)
    -- If    deepSplitCprType_maybe n ty = Just (dc, tys, arg_tys, co)
    -- then  dc @ tys (args::arg_tys) :: rep_ty
    --       co :: ty ~ rep_ty
    deepSplitCprType_maybe fam_envs con_tag ty
      | let (co, ty1) = topNormaliseType_maybe fam_envs ty
                        `orElse` (mkReflCo Representational ty, ty)
      , Just (tc, tc_args) <- splitTyConApp_maybe ty1
      , isDataTyCon tc
      , let cons = tyConDataCons tc
      , cons `lengthAtLeast` con_tag -- This might not be true if we import the
                                     -- type constructor via a .hs-bool file (#8743)
      , let con  = cons !! (con_tag - fIRST_TAG)
      = Just (con, tc_args, dataConInstArgTys con tc_args, co)
    deepSplitCprType_maybe _ _ _ = Nothing

    findTypeShape :: FamInstEnvs -> Type -> TypeShape
    -- Uncover the arrow and product shape of a type
    -- The data type TypeShape is defined in Demand
    -- See Note [Trimming a demand to a type] in Demand
    findTypeShape fam_envs ty
      | Just (_, ty') <- splitForAllTy_maybe ty
      = findTypeShape fam_envs ty'

      | Just (tc, tc_args)  <- splitTyConApp_maybe ty
      , Just con <- isDataProductTyCon_maybe tc
      = TsProd (map (findTypeShape fam_envs) $ dataConInstArgTys con tc_args)

      | Just (_, res) <- splitFunTy_maybe ty
      = TsFun (findTypeShape fam_envs res)

      | Just (_, ty') <- topNormaliseType_maybe fam_envs ty
      = findTypeShape fam_envs ty'

      | otherwise
      = TsUnk

    {-
    ************************************************************************
    *                                                                      *
    \subsection{CPR stuff}
    *                                                                      *
    ************************************************************************

    @mkWWcpr@ takes the worker/wrapper pair produced from the strictness
    info and adds in the CPR transformation.  The worker returns an
    unboxed tuple containing non-CPR components.  The wrapper takes this
    tuple and re-produces the correct structured output.

    The non-CPR results appear ordered in the unboxed tuple as if by a
    left-to-right traversal of the result structure.
    -}

    mkWWcpr :: FamInstEnvs
            -> Type                              -- function body type
            -> DmdResult                         -- CPR analysis results
            -> UniqSM (Bool,                     -- Is w/w'ing useful?
                       CoreExpr -> CoreExpr,     -- New wrapper
                       CoreExpr -> CoreExpr,     -- New worker
                       Type)                     -- Type of worker's body

    mkWWcpr fam_envs body_ty res
      = case returnsCPR_maybe res of
           Nothing      -> return (False, id, id, body_ty)  -- No CPR info
           Just con_tag | Just stuff <- deepSplitCprType_maybe fam_envs con_tag body_ty
                        -> mkWWcpr_help stuff
                        |  otherwise
                           -- See Note [non-algebraic or open body type warning]
                        -> WARN( True, text "mkWWcpr: non-algebraic or open body type" <+> ppr body_ty )
                           return (False, id, id, body_ty)

    mkWWcpr_help :: (DataCon, [Type], [Type], Coercion)
                 -> UniqSM (Bool, CoreExpr -> CoreExpr, CoreExpr -> CoreExpr, Type)

    mkWWcpr_help (data_con, inst_tys, arg_tys, co)
      | [arg_ty1] <- arg_tys
      , isUnLiftedType arg_ty1
            -- Special case when there is a single result of unlifted type
            --
            -- Wrapper:     case (..call worker..) of x -> C x
            -- Worker:      case (   ..body..    ) of C x -> x
      = do { (work_uniq : arg_uniq : _) <- getUniquesM
           ; let arg       = mk_ww_local arg_uniq  arg_ty1
                 con_app   = mkConApp2 data_con inst_tys [arg] `mkCast` mkSymCo co

           ; return ( True
                    , \ wkr_call -> Case wkr_call arg (exprType con_app) [(DEFAULT, [], con_app)]
                    , \ body     -> mkUnpackCase body co work_uniq data_con [arg] (Var arg)
                    , arg_ty1 ) }

      | otherwise   -- The general case
            -- Wrapper: case (..call worker..) of (# a, b #) -> C a b
            -- Worker:  case (   ...body...  ) of C a b -> (# a, b #)
      = do { (work_uniq : uniqs) <- getUniquesM
           ; let (wrap_wild : args) = zipWith mk_ww_local uniqs (ubx_tup_ty : arg_tys)
                 ubx_tup_con  = tupleCon UnboxedTuple (length arg_tys)
                 ubx_tup_ty   = exprType ubx_tup_app
                 ubx_tup_app  = mkConApp2 ubx_tup_con arg_tys args
                 con_app      = mkConApp2 data_con inst_tys args `mkCast` mkSymCo co

           ; return (True
                    , \ wkr_call -> Case wkr_call wrap_wild (exprType con_app)  [(DataAlt ubx_tup_con, args, con_app)]
                    , \ body     -> mkUnpackCase body co work_uniq data_con args ubx_tup_app
                    , ubx_tup_ty ) }

    mkUnpackCase ::  CoreExpr -> Coercion -> Unique -> DataCon -> [Id] -> CoreExpr -> CoreExpr
    -- (mkUnpackCase e co uniq Con args body)
    --      returns
    -- case e |> co of bndr { Con args -> body }

    mkUnpackCase (Tick tickish e) co uniq con args body   -- See Note [Profiling and unpacking]
      = Tick tickish (mkUnpackCase e co uniq con args body)
    mkUnpackCase scrut co uniq boxing_con unpk_args body
      = Case casted_scrut bndr (exprType body)
             [(DataAlt boxing_con, unpk_args, body)]
      where
        casted_scrut = scrut `mkCast` co
        bndr = mk_ww_local uniq (exprType casted_scrut)

    {-
    Note [non-algebraic or open body type warning]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    There are a few cases where the W/W transformation is told that something
    returns a constructor, but the type at hand doesn't really match this. One
    real-world example involves unsafeCoerce:
      foo = IO a
      foo = unsafeCoerce c_exit
      foreign import ccall "c_exit" c_exit :: IO ()
    Here CPR will tell you that `foo` returns a () constructor for sure, but trying
    to create a worker/wrapper for type `a` obviously fails.
    (This was a real example until ee8e792  in libraries/base.)

    It does not seem feasible to avoid all such cases already in the analyser (and
    after all, the analysis is not really wrong), so we simply do nothing here in
    mkWWcpr. But we still want to emit warning with -DDEBUG, to hopefully catch
    other cases where something went avoidably wrong.

    Note [Profiling and unpacking]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If the original function looked like
            f = \ x -> {-# SCC "foo" #-} E

    then we want the CPR'd worker to look like
            \ x -> {-# SCC "foo" #-} (case E of I# x -> x)
    and definitely not
            \ x -> case ({-# SCC "foo" #-} E) of I# x -> x)

    This transform doesn't move work or allocation
    from one cost centre to another.

    Later [SDM]: presumably this is because we want the simplifier to
    eliminate the case, and the scc would get in the way?  I'm ok with
    including the case itself in the cost centre, since it is morally
    part of the function (post transformation) anyway.

    ************************************************************************
    *                                                                      *
    \subsection{Utilities}
    *                                                                      *
    ************************************************************************

    Note [Absent errors]
    ~~~~~~~~~~~~~~~~~~~~
    We make a new binding for Ids that are marked absent, thus
       let x = absentError "x :: Int"
    The idea is that this binding will never be used; but if it
    buggily is used we'll get a runtime error message.

    Coping with absence for *unlifted* types is important; see, for
    example, Trac #4306.  For these we find a suitable literal,
    using Literal.absentLiteralOf.  We don't have literals for
    every primitive type, so the function is partial.

        [I did try the experiment of using an error thunk for unlifted
        things too, relying on the simplifier to drop it as dead code,
        by making absentError
          (a) *not* be a bottoming Id,
          (b) be "ok for speculation"
        But that relies on the simplifier finding that it really
        is dead code, which is fragile, and indeed failed when
        profiling is on, which disables various optimisations.  So
        using a literal will do.]
    -}

    mk_absent_let :: DynFlags -> Id -> Maybe (CoreExpr -> CoreExpr)
    mk_absent_let dflags arg
      | not (isUnLiftedType arg_ty)
      = Just (Let (NonRec arg abs_rhs))
      | Just tc <- tyConAppTyCon_maybe arg_ty
      , Just lit <- absentLiteralOf tc
      = Just (Let (NonRec arg (Lit lit)))
      | arg_ty `eqType` voidPrimTy
      = Just (Let (NonRec arg (Var voidPrimId)))
      | otherwise
      = WARN( True, ptext (sLit "No absent value for") <+> ppr arg_ty )
        Nothing
      where
        arg_ty  = idType arg
        abs_rhs = mkRuntimeErrorApp aBSENT_ERROR_ID arg_ty msg
        msg     = showSDoc dflags (ppr arg <+> ppr (idType arg))

    mk_seq_case :: Id -> CoreExpr -> CoreExpr
    mk_seq_case arg body = Case (Var arg) (sanitiseCaseBndr arg) (exprType body) [(DEFAULT, [], body)]

    sanitiseCaseBndr :: Id -> Id
    -- The argument we are scrutinising has the right type to be
    -- a case binder, so it's convenient to re-use it for that purpose.
    -- But we *must* throw away all its IdInfo.  In particular, the argument
    -- will have demand info on it, and that demand info may be incorrect for
    -- the case binder.  e.g.       case ww_arg of ww_arg { I# x -> ... }
    -- Quite likely ww_arg isn't used in '...'.  The case may get discarded
    -- if the case binder says "I'm demanded".  This happened in a situation
    -- like         (x+y) `seq` ....
    sanitiseCaseBndr id = id `setIdInfo` vanillaIdInfo

    mk_ww_local :: Unique -> Type -> Id
    mk_ww_local uniq ty = mkSysLocal (fsLit "ww") uniq ty