simplCore/LiberateCase.hs

    {-
    (c) The AQUA Project, Glasgow University, 1994-1998

    \section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop}
    -}

    {-# LANGUAGE CPP #-}
    module LiberateCase ( liberateCase ) where

    #include "HsVersions.h"

    import GhcPrelude

    import DynFlags
    import CoreSyn
    import CoreUnfold       ( couldBeSmallEnoughToInline )
    import TysWiredIn       ( unitDataConId )
    import Id
    import VarEnv
    import Util             ( notNull )

    {-
    The liberate-case transformation
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This module walks over @Core@, and looks for @case@ on free variables.
    The criterion is:
            if there is case on a free on the route to the recursive call,
            then the recursive call is replaced with an unfolding.

    Example

       f = \ t -> case v of
                     V a b -> a : f t

    => the inner f is replaced.

       f = \ t -> case v of
                     V a b -> a : (letrec
                                    f =  \ t -> case v of
                                                   V a b -> a : f t
                                   in f) t
    (note the NEED for shadowing)

    => Simplify

      f = \ t -> case v of
                     V a b -> a : (letrec
                                    f = \ t -> a : f t
                                   in f t)

    Better code, because 'a' is  free inside the inner letrec, rather
    than needing projection from v.

    Note that this deals with *free variables*.  SpecConstr deals with
    *arguments* that are of known form.  E.g.

            last []     = error
            last (x:[]) = x
            last (x:xs) = last xs

    Note [Scrutinee with cast]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this:
        f = \ t -> case (v `cast` co) of
                     V a b -> a : f t

    Exactly the same optimisation (unrolling one call to f) will work here,
    despite the cast.  See mk_alt_env in the Case branch of libCase.

    To think about (Apr 94)
    ~~~~~~~~~~~~~~
    Main worry: duplicating code excessively.  At the moment we duplicate
    the entire binding group once at each recursive call.  But there may
    be a group of recursive calls which share a common set of evaluated
    free variables, in which case the duplication is a plain waste.

    Another thing we could consider adding is some unfold-threshold thing,
    so that we'll only duplicate if the size of the group rhss isn't too
    big.

    Data types
    ~~~~~~~~~~
    The ``level'' of a binder tells how many
    recursive defns lexically enclose the binding
    A recursive defn "encloses" its RHS, not its
    scope.  For example:
    \begin{verbatim}
            letrec f = let g = ... in ...
            in
            let h = ...
            in ...
    \end{verbatim}
    Here, the level of @f@ is zero, the level of @g@ is one,
    and the level of @h@ is zero (NB not one).

    ************************************************************************
    *                                                                      *
             Top-level code
    *                                                                      *
    ************************************************************************
    -}

    liberateCase :: DynFlags -> CoreProgram -> CoreProgram
    liberateCase dflags binds = do_prog (initEnv dflags) binds
      where
        do_prog _   [] = []
        do_prog env (bind:binds) = bind' : do_prog env' binds
                                 where
                                   (env', bind') = libCaseBind env bind

    {-
    ************************************************************************
    *                                                                      *
             Main payload
    *                                                                      *
    ************************************************************************

    Bindings
    ~~~~~~~~
    -}

    libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)

    libCaseBind env (NonRec binder rhs)
      = (addBinders env [binder], NonRec binder (libCase env rhs))

    libCaseBind env (Rec pairs)
      = (env_body, Rec pairs')
      where
        binders = map fst pairs

        env_body = addBinders env binders

        pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]

            -- We extend the rec-env by binding each Id to its rhs, first
            -- processing the rhs with an *un-extended* environment, so
            -- that the same process doesn't occur for ever!
        env_rhs | is_dupable_bind = addRecBinds env dup_pairs
                | otherwise       = env

        dup_pairs = [ (localiseId binder, libCase env_body rhs)
                    | (binder, rhs) <- pairs ]
            -- localiseID : see Note [Need to localiseId in libCaseBind]

        is_dupable_bind = small_enough && all ok_pair pairs

        -- Size: we are going to duplicate dup_pairs; to find their
        --       size, build a fake binding (let { dup_pairs } in (),
        --       and find the size of that
        -- See Note [Small enough]
        small_enough = case bombOutSize env of
                          Nothing   -> True   -- Infinity
                          Just size -> couldBeSmallEnoughToInline (lc_dflags env) size $
                                       Let (Rec dup_pairs) (Var unitDataConId)

        ok_pair (id,_)
            =  idArity id > 0          -- Note [Only functions!]
            && not (isBottomingId id)  -- Note [Not bottoming ids]

    {- Note [Not bottoming Ids]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Do not specialise error-functions (this is unusual, but I once saw it,
    (acually in Data.Typable.Internal)

    Note [Only functions!]
    ~~~~~~~~~~~~~~~~~~~~~~
    Consider the following code

           f = g (case v of V a b -> a : t f)

    where g is expensive. If we aren't careful, liberate case will turn this into

           f = g (case v of
                   V a b -> a : t (letrec f = g (case v of V a b -> a : f t)
                                    in f)
                 )

    Yikes! We evaluate g twice. This leads to a O(2^n) explosion
    if g calls back to the same code recursively.

    Solution: make sure that we only do the liberate-case thing on *functions*

    Note [Small enough]
    ~~~~~~~~~~~~~~~~~~~
    Consider
      \fv. letrec
             f = \x. BIG...(case fv of { (a,b) -> ...g.. })...
             g = \y. SMALL...f...

    Then we *can* in principle do liberate-case on 'g' (small RHS) but not
    for 'f' (too big).  But doing so is not profitable, because duplicating
    'g' at its call site in 'f' doesn't get rid of any cases.  So we just
    ask for the whole group to be small enough.

    Note [Need to localiseId in libCaseBind]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The call to localiseId is needed for two subtle reasons
    (a)  Reset the export flags on the binders so
            that we don't get name clashes on exported things if the
            local binding floats out to top level.  This is most unlikely
            to happen, since the whole point concerns free variables.
            But resetting the export flag is right regardless.

    (b)  Make the name an Internal one.  External Names should never be
            nested; if it were floated to the top level, we'd get a name
            clash at code generation time.

    Expressions
    ~~~~~~~~~~~
    -}

    libCase :: LibCaseEnv
            -> CoreExpr
            -> CoreExpr

    libCase env (Var v)             = libCaseApp env v []
    libCase _   (Lit lit)           = Lit lit
    libCase _   (Type ty)           = Type ty
    libCase _   (Coercion co)       = Coercion co
    libCase env e@(App {})          | let (fun, args) = collectArgs e
                                    , Var v <- fun
                                    = libCaseApp env v args
    libCase env (App fun arg)       = App (libCase env fun) (libCase env arg)
    libCase env (Tick tickish body) = Tick tickish (libCase env body)
    libCase env (Cast e co)         = Cast (libCase env e) co

    libCase env (Lam binder body)
      = Lam binder (libCase (addBinders env [binder]) body)

    libCase env (Let bind body)
      = Let bind' (libCase env_body body)
      where
        (env_body, bind') = libCaseBind env bind

    libCase env (Case scrut bndr ty alts)
      = Case (libCase env scrut) bndr ty (map (libCaseAlt env_alts) alts)
      where
        env_alts = addBinders (mk_alt_env scrut) [bndr]
        mk_alt_env (Var scrut_var) = addScrutedVar env scrut_var
        mk_alt_env (Cast scrut _)  = mk_alt_env scrut       -- Note [Scrutinee with cast]
        mk_alt_env _               = env

    libCaseAlt :: LibCaseEnv -> (AltCon, [CoreBndr], CoreExpr)
                             -> (AltCon, [CoreBndr], CoreExpr)
    libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)

    {-
    Ids
    ~~~

    To unfold, we can't just wrap the id itself in its binding if it's a join point:

      jump j a b c  =>  (joinrec j x y z = ... in jump j) a b c -- wrong!!!

    Every jump must provide all arguments, so we have to be careful to wrap the
    whole jump instead:

      jump j a b c  =>  joinrec j x y z = ... in jump j a b c -- right

    -}

    libCaseApp :: LibCaseEnv -> Id -> [CoreExpr] -> CoreExpr
    libCaseApp env v args
      | Just the_bind <- lookupRecId env v  -- It's a use of a recursive thing
      , notNull free_scruts                 -- with free vars scrutinised in RHS
      = Let the_bind expr'

      | otherwise
      = expr'

      where
        rec_id_level = lookupLevel env v
        free_scruts  = freeScruts env rec_id_level
        expr'        = mkApps (Var v) (map (libCase env) args)

    freeScruts :: LibCaseEnv
               -> LibCaseLevel      -- Level of the recursive Id
               -> [Id]              -- Ids that are scrutinised between the binding
                                    -- of the recursive Id and here
    freeScruts env rec_bind_lvl
      = [v | (v, scrut_bind_lvl, scrut_at_lvl) <- lc_scruts env
           , scrut_bind_lvl <= rec_bind_lvl
           , scrut_at_lvl > rec_bind_lvl]
            -- Note [When to specialise]
            -- Note [Avoiding fruitless liberate-case]

    {-
    Note [When to specialise]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
      f = \x. letrec g = \y. case x of
                               True  -> ... (f a) ...
                               False -> ... (g b) ...

    We get the following levels
              f  0
              x  1
              g  1
              y  2

    Then 'x' is being scrutinised at a deeper level than its binding, so
    it's added to lc_sruts:  [(x,1)]

    We do *not* want to specialise the call to 'f', because 'x' is not free
    in 'f'.  So here the bind-level of 'x' (=1) is not <= the bind-level of 'f' (=0).

    We *do* want to specialise the call to 'g', because 'x' is free in g.
    Here the bind-level of 'x' (=1) is <= the bind-level of 'g' (=1).

    Note [Avoiding fruitless liberate-case]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider also:
      f = \x. case top_lvl_thing of
                    I# _ -> let g = \y. ... g ...
                            in ...

    Here, top_lvl_thing is scrutinised at a level (1) deeper than its
    binding site (0).  Nevertheless, we do NOT want to specialise the call
    to 'g' because all the structure in its free variables is already
    visible at the definition site for g.  Hence, when considering specialising
    an occurrence of 'g', we want to check that there's a scruted-var v st

       a) v's binding site is *outside* g
       b) v's scrutinisation site is *inside* g

    ************************************************************************
    *                                                                      *
            Utility functions
    *                                                                      *
    ************************************************************************
    -}

    addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
    addBinders env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env }) binders
      = env { lc_lvl_env = lvl_env' }
      where
        lvl_env' = extendVarEnvList lvl_env (binders `zip` repeat lvl)

    addRecBinds :: LibCaseEnv -> [(Id,CoreExpr)] -> LibCaseEnv
    addRecBinds env@(LibCaseEnv {lc_lvl = lvl, lc_lvl_env = lvl_env,
                                 lc_rec_env = rec_env}) pairs
      = env { lc_lvl = lvl', lc_lvl_env = lvl_env', lc_rec_env = rec_env' }
      where
        lvl'     = lvl + 1
        lvl_env' = extendVarEnvList lvl_env [(binder,lvl) | (binder,_) <- pairs]
        rec_env' = extendVarEnvList rec_env [(binder, Rec pairs) | (binder,_) <- pairs]

    addScrutedVar :: LibCaseEnv
                  -> Id             -- This Id is being scrutinised by a case expression
                  -> LibCaseEnv

    addScrutedVar env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env,
                                    lc_scruts = scruts }) scrut_var
      | bind_lvl < lvl
      = env { lc_scruts = scruts' }
            -- Add to scruts iff the scrut_var is being scrutinised at
            -- a deeper level than its defn

      | otherwise = env
      where
        scruts'  = (scrut_var, bind_lvl, lvl) : scruts
        bind_lvl = case lookupVarEnv lvl_env scrut_var of
                     Just lvl -> lvl
                     Nothing  -> topLevel

    lookupRecId :: LibCaseEnv -> Id -> Maybe CoreBind
    lookupRecId env id = lookupVarEnv (lc_rec_env env) id

    lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
    lookupLevel env id
      = case lookupVarEnv (lc_lvl_env env) id of
          Just lvl -> lvl
          Nothing  -> topLevel

    {-
    ************************************************************************
    *                                                                      *
             The environment
    *                                                                      *
    ************************************************************************
    -}

    type LibCaseLevel = Int

    topLevel :: LibCaseLevel
    topLevel = 0

    data LibCaseEnv
      = LibCaseEnv {
            lc_dflags :: DynFlags,

            lc_lvl :: LibCaseLevel, -- Current level
                    -- The level is incremented when (and only when) going
                    -- inside the RHS of a (sufficiently small) recursive
                    -- function.

            lc_lvl_env :: IdEnv LibCaseLevel,
                    -- Binds all non-top-level in-scope Ids (top-level and
                    -- imported things have a level of zero)

            lc_rec_env :: IdEnv CoreBind,
                    -- Binds *only* recursively defined ids, to their own
                    -- binding group, and *only* in their own RHSs

            lc_scruts :: [(Id, LibCaseLevel, LibCaseLevel)]
                    -- Each of these Ids was scrutinised by an enclosing
                    -- case expression, at a level deeper than its binding
                    -- level.
                    --
                    -- The first LibCaseLevel is the *binding level* of
                    --   the scrutinised Id,
                    -- The second is the level *at which it was scrutinised*.
                    --   (see Note [Avoiding fruitless liberate-case])
                    -- The former is a bit redundant, since you could always
                    -- look it up in lc_lvl_env, but it's just cached here
                    --
                    -- The order is insignificant; it's a bag really
                    --
                    -- There's one element per scrutinisation;
                    --    in principle the same Id may appear multiple times,
                    --    although that'd be unusual:
                    --       case x of { (a,b) -> ....(case x of ...) .. }
            }

    initEnv :: DynFlags -> LibCaseEnv
    initEnv dflags
      = LibCaseEnv { lc_dflags = dflags,
                     lc_lvl = 0,
                     lc_lvl_env = emptyVarEnv,
                     lc_rec_env = emptyVarEnv,
                     lc_scruts = [] }

    -- Bomb-out size for deciding if
    -- potential liberatees are too big.
    -- (passed in from cmd-line args)
    bombOutSize :: LibCaseEnv -> Maybe Int
    bombOutSize = liberateCaseThreshold . lc_dflags