simplCore/SAT.hs

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

    ************************************************************************

                   Static Argument Transformation pass

    ************************************************************************

    May be seen as removing invariants from loops:
    Arguments of recursive functions that do not change in recursive
    calls are removed from the recursion, which is done locally
    and only passes the arguments which effectively change.

    Example:
    map = /\ ab -> \f -> \xs -> case xs of
                     []       -> []
                     (a:b) -> f a : map f b

    as map is recursively called with the same argument f (unmodified)
    we transform it to

    map = /\ ab -> \f -> \xs -> let map' ys = case ys of
                           []     -> []
                           (a:b) -> f a : map' b
                    in map' xs

    Notice that for a compiler that uses lambda lifting this is
    useless as map' will be transformed back to what map was.

    We could possibly do the same for big lambdas, but we don't as
    they will eventually be removed in later stages of the compiler,
    therefore there is no penalty in keeping them.

    We only apply the SAT when the number of static args is > 2. This
    produces few bad cases.  See
                    should_transform
    in saTransform.

    Here are the headline nofib results:
                      Size    Allocs   Runtime
    Min             +0.0%    -13.7%    -21.4%
    Max             +0.1%     +0.0%     +5.4%
    Geometric Mean  +0.0%     -0.2%     -6.9%

    The previous patch, to fix polymorphic floatout demand signatures, is
    essential to make this work well!
    -}

    {-# LANGUAGE CPP #-}
    module SAT ( doStaticArgs ) where

    import Var
    import CoreSyn
    import CoreUtils
    import Type
    import Coercion
    import Id
    import Name
    import VarEnv
    import UniqSupply
    import Util
    import UniqFM
    import VarSet
    import Unique
    import UniqSet
    import Outputable

    import Data.List
    import FastString

    #include "HsVersions.h"

    doStaticArgs :: UniqSupply -> CoreProgram -> CoreProgram
    doStaticArgs us binds = snd $ mapAccumL sat_bind_threaded_us us binds
      where
        sat_bind_threaded_us us bind =
            let (us1, us2) = splitUniqSupply us
            in (us1, fst $ runSAT us2 (satBind bind emptyUniqSet))

    -- We don't bother to SAT recursive groups since it can lead
    -- to massive code expansion: see Andre Santos' thesis for details.
    -- This means we only apply the actual SAT to Rec groups of one element,
    -- but we want to recurse into the others anyway to discover other binds
    satBind :: CoreBind -> IdSet -> SatM (CoreBind, IdSATInfo)
    satBind (NonRec binder expr) interesting_ids = do
        (expr', sat_info_expr, expr_app) <- satExpr expr interesting_ids
        return (NonRec binder expr', finalizeApp expr_app sat_info_expr)
    satBind (Rec [(binder, rhs)]) interesting_ids = do
        let interesting_ids' = interesting_ids `addOneToUniqSet` binder
            (rhs_binders, rhs_body) = collectBinders rhs
        (rhs_body', sat_info_rhs_body) <- satTopLevelExpr rhs_body interesting_ids'
        let sat_info_rhs_from_args = unitVarEnv binder (bindersToSATInfo rhs_binders)
            sat_info_rhs' = mergeIdSATInfo sat_info_rhs_from_args sat_info_rhs_body

            shadowing = binder `elementOfUniqSet` interesting_ids
            sat_info_rhs'' = if shadowing
                            then sat_info_rhs' `delFromUFM` binder -- For safety
                            else sat_info_rhs'

        bind' <- saTransformMaybe binder (lookupUFM sat_info_rhs' binder)
                                  rhs_binders rhs_body'
        return (bind', sat_info_rhs'')
    satBind (Rec pairs) interesting_ids = do
        let (binders, rhss) = unzip pairs
        rhss_SATed <- mapM (\e -> satTopLevelExpr e interesting_ids) rhss
        let (rhss', sat_info_rhss') = unzip rhss_SATed
        return (Rec (zipEqual "satBind" binders rhss'), mergeIdSATInfos sat_info_rhss')

    data App = VarApp Id | TypeApp Type | CoApp Coercion
    data Staticness a = Static a | NotStatic

    type IdAppInfo = (Id, SATInfo)

    type SATInfo = [Staticness App]
    type IdSATInfo = IdEnv SATInfo
    emptyIdSATInfo :: IdSATInfo
    emptyIdSATInfo = emptyUFM

    {-
    pprIdSATInfo id_sat_info = vcat (map pprIdAndSATInfo (Map.toList id_sat_info))
      where pprIdAndSATInfo (v, sat_info) = hang (ppr v <> colon) 4 (pprSATInfo sat_info)
    -}

    pprSATInfo :: SATInfo -> SDoc
    pprSATInfo staticness = hcat $ map pprStaticness staticness

    pprStaticness :: Staticness App -> SDoc
    pprStaticness (Static (VarApp _))  = text "SV"
    pprStaticness (Static (TypeApp _)) = text "ST"
    pprStaticness (Static (CoApp _))   = text "SC"
    pprStaticness NotStatic            = text "NS"

    mergeSATInfo :: SATInfo -> SATInfo -> SATInfo
    mergeSATInfo l r = zipWith mergeSA l r
      where
        mergeSA NotStatic _ = NotStatic
        mergeSA _ NotStatic = NotStatic
        mergeSA (Static (VarApp v)) (Static (VarApp v'))
          | v == v'   = Static (VarApp v)
          | otherwise = NotStatic
        mergeSA (Static (TypeApp t)) (Static (TypeApp t'))
          | t `eqType` t' = Static (TypeApp t)
          | otherwise     = NotStatic
        mergeSA (Static (CoApp c)) (Static (CoApp c'))
          | c `eqCoercion` c' = Static (CoApp c)
          | otherwise             = NotStatic
        mergeSA _ _  = pprPanic "mergeSATInfo" $
                              text "Left:"
                           <> pprSATInfo l <> text ", "
                           <> text "Right:"
                           <> pprSATInfo r

    mergeIdSATInfo :: IdSATInfo -> IdSATInfo -> IdSATInfo
    mergeIdSATInfo = plusUFM_C mergeSATInfo

    mergeIdSATInfos :: [IdSATInfo] -> IdSATInfo
    mergeIdSATInfos = foldl' mergeIdSATInfo emptyIdSATInfo

    bindersToSATInfo :: [Id] -> SATInfo
    bindersToSATInfo vs = map (Static . binderToApp) vs
        where binderToApp v | isId v    = VarApp v
                            | isTyVar v = TypeApp $ mkTyVarTy v
                            | otherwise = CoApp $ mkCoVarCo v

    finalizeApp :: Maybe IdAppInfo -> IdSATInfo -> IdSATInfo
    finalizeApp Nothing id_sat_info = id_sat_info
    finalizeApp (Just (v, sat_info')) id_sat_info =
        let sat_info'' = case lookupUFM id_sat_info v of
                            Nothing -> sat_info'
                            Just sat_info -> mergeSATInfo sat_info sat_info'
        in extendVarEnv id_sat_info v sat_info''

    satTopLevelExpr :: CoreExpr -> IdSet -> SatM (CoreExpr, IdSATInfo)
    satTopLevelExpr expr interesting_ids = do
        (expr', sat_info_expr, expr_app) <- satExpr expr interesting_ids
        return (expr', finalizeApp expr_app sat_info_expr)

    satExpr :: CoreExpr -> IdSet -> SatM (CoreExpr, IdSATInfo, Maybe IdAppInfo)
    satExpr var@(Var v) interesting_ids = do
        let app_info = if v `elementOfUniqSet` interesting_ids
                       then Just (v, [])
                       else Nothing
        return (var, emptyIdSATInfo, app_info)

    satExpr lit@(Lit _) _ = do
        return (lit, emptyIdSATInfo, Nothing)

    satExpr (Lam binders body) interesting_ids = do
        (body', sat_info, this_app) <- satExpr body interesting_ids
        return (Lam binders body', finalizeApp this_app sat_info, Nothing)

    satExpr (App fn arg) interesting_ids = do
        (fn', sat_info_fn, fn_app) <- satExpr fn interesting_ids
        let satRemainder = boring fn' sat_info_fn
        case fn_app of
            Nothing -> satRemainder Nothing
            Just (fn_id, fn_app_info) ->
                -- TODO: remove this use of append somehow (use a data structure with O(1) append but a left-to-right kind of interface)
                let satRemainderWithStaticness arg_staticness = satRemainder $ Just (fn_id, fn_app_info ++ [arg_staticness])
                in case arg of
                    Type t     -> satRemainderWithStaticness $ Static (TypeApp t)
                    Coercion c -> satRemainderWithStaticness $ Static (CoApp c)
                    Var v      -> satRemainderWithStaticness $ Static (VarApp v)
                    _          -> satRemainderWithStaticness $ NotStatic
      where
        boring :: CoreExpr -> IdSATInfo -> Maybe IdAppInfo -> SatM (CoreExpr, IdSATInfo, Maybe IdAppInfo)
        boring fn' sat_info_fn app_info =
            do (arg', sat_info_arg, arg_app) <- satExpr arg interesting_ids
               let sat_info_arg' = finalizeApp arg_app sat_info_arg
                   sat_info = mergeIdSATInfo sat_info_fn sat_info_arg'
               return (App fn' arg', sat_info, app_info)

    satExpr (Case expr bndr ty alts) interesting_ids = do
        (expr', sat_info_expr, expr_app) <- satExpr expr interesting_ids
        let sat_info_expr' = finalizeApp expr_app sat_info_expr

        zipped_alts' <- mapM satAlt alts
        let (alts', sat_infos_alts) = unzip zipped_alts'
        return (Case expr' bndr ty alts', mergeIdSATInfo sat_info_expr' (mergeIdSATInfos sat_infos_alts), Nothing)
      where
        satAlt (con, bndrs, expr) = do
            (expr', sat_info_expr) <- satTopLevelExpr expr interesting_ids
            return ((con, bndrs, expr'), sat_info_expr)

    satExpr (Let bind body) interesting_ids = do
        (body', sat_info_body, body_app) <- satExpr body interesting_ids
        (bind', sat_info_bind) <- satBind bind interesting_ids
        return (Let bind' body', mergeIdSATInfo sat_info_body sat_info_bind, body_app)

    satExpr (Tick tickish expr) interesting_ids = do
        (expr', sat_info_expr, expr_app) <- satExpr expr interesting_ids
        return (Tick tickish expr', sat_info_expr, expr_app)

    satExpr ty@(Type _) _ = do
        return (ty, emptyIdSATInfo, Nothing)

    satExpr co@(Coercion _) _ = do
        return (co, emptyIdSATInfo, Nothing)

    satExpr (Cast expr coercion) interesting_ids = do
        (expr', sat_info_expr, expr_app) <- satExpr expr interesting_ids
        return (Cast expr' coercion, sat_info_expr, expr_app)

    {-
    ************************************************************************

                    Static Argument Transformation Monad

    ************************************************************************
    -}

    type SatM result = UniqSM result

    runSAT :: UniqSupply -> SatM a -> a
    runSAT = initUs_

    newUnique :: SatM Unique
    newUnique = getUniqueM

    {-
    ************************************************************************

                    Static Argument Transformation Monad

    ************************************************************************

    To do the transformation, the game plan is to:

    1. Create a small nonrecursive RHS that takes the
       original arguments to the function but discards
       the ones that are static and makes a call to the
       SATed version with the remainder. We intend that
       this will be inlined later, removing the overhead

    2. Bind this nonrecursive RHS over the original body
       WITH THE SAME UNIQUE as the original body so that
       any recursive calls to the original now go via
       the small wrapper

    3. Rebind the original function to a new one which contains
       our SATed function and just makes a call to it:
       we call the thing making this call the local body

    Example: transform this

        map :: forall a b. (a->b) -> [a] -> [b]
        map = /\ab. \(f:a->b) (as:[a]) -> body[map]
    to
        map :: forall a b. (a->b) -> [a] -> [b]
        map = /\ab. \(f:a->b) (as:[a]) ->
             letrec map' :: [a] -> [b]
                        -- The "worker function
                    map' = \(as:[a]) ->
                             let map :: forall a' b'. (a -> b) -> [a] -> [b]
                                    -- The "shadow function
                                 map = /\a'b'. \(f':(a->b) (as:[a]).
                                       map' as
                             in body[map]
             in map' as

    Note [Shadow binding]
    ~~~~~~~~~~~~~~~~~~~~~
    The calls to the inner map inside body[map] should get inlined
    by the local re-binding of 'map'.  We call this the "shadow binding".

    But we can't use the original binder 'map' unchanged, because
    it might be exported, in which case the shadow binding won't be
    discarded as dead code after it is inlined.

    So we use a hack: we make a new SysLocal binder with the *same* unique
    as binder.  (Another alternative would be to reset the export flag.)

    Note [Binder type capture]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    Notice that in the inner map (the "shadow function"), the static arguments
    are discarded -- it's as if they were underscores.  Instead, mentions
    of these arguments (notably in the types of dynamic arguments) are bound
    by the *outer* lambdas of the main function.  So we must make up fresh
    names for the static arguments so that they do not capture variables
    mentioned in the types of dynamic args.

    In the map example, the shadow function must clone the static type
    argument a,b, giving a',b', to ensure that in the \(as:[a]), the 'a'
    is bound by the outer forall.  We clone f' too for consistency, but
    that doesn't matter either way because static Id arguments aren't
    mentioned in the shadow binding at all.

    If we don't we get something like this:

    [Exported]
    [Arity 3]
    GHC.Base.until =
      \ (@ a_aiK)
        (p_a6T :: a_aiK -> GHC.Types.Bool)
        (f_a6V :: a_aiK -> a_aiK)
        (x_a6X :: a_aiK) ->
        letrec {
          sat_worker_s1aU :: a_aiK -> a_aiK
          []
          sat_worker_s1aU =
            \ (x_a6X :: a_aiK) ->
              let {
                sat_shadow_r17 :: forall a_a3O.
                                  (a_a3O -> GHC.Types.Bool) -> (a_a3O -> a_a3O) -> a_a3O -> a_a3O
                []
                sat_shadow_r17 =
                  \ (@ a_aiK)
                    (p_a6T :: a_aiK -> GHC.Types.Bool)
                    (f_a6V :: a_aiK -> a_aiK)
                    (x_a6X :: a_aiK) ->
                    sat_worker_s1aU x_a6X } in
              case p_a6T x_a6X of wild_X3y [ALWAYS Dead Nothing] {
                GHC.Types.False -> GHC.Base.until @ a_aiK p_a6T f_a6V (f_a6V x_a6X);
                GHC.Types.True -> x_a6X
              }; } in
        sat_worker_s1aU x_a6X

    Where sat_shadow has captured the type variables of x_a6X etc as it has a a_aiK
    type argument. This is bad because it means the application sat_worker_s1aU x_a6X
    is not well typed.
    -}

    saTransformMaybe :: Id -> Maybe SATInfo -> [Id] -> CoreExpr -> SatM CoreBind
    saTransformMaybe binder maybe_arg_staticness rhs_binders rhs_body
      | Just arg_staticness <- maybe_arg_staticness
      , should_transform arg_staticness
      = saTransform binder arg_staticness rhs_binders rhs_body
      | otherwise
      = return (Rec [(binder, mkLams rhs_binders rhs_body)])
      where
        should_transform staticness = n_static_args > 1 -- THIS IS THE DECISION POINT
          where
            n_static_args = count isStaticValue staticness

    saTransform :: Id -> SATInfo -> [Id] -> CoreExpr -> SatM CoreBind
    saTransform binder arg_staticness rhs_binders rhs_body
      = do  { shadow_lam_bndrs <- mapM clone binders_w_staticness
            ; uniq             <- newUnique
            ; return (NonRec binder (mk_new_rhs uniq shadow_lam_bndrs)) }
      where
        -- Running example: foldr
        -- foldr \alpha \beta c n xs = e, for some e
        -- arg_staticness = [Static TypeApp, Static TypeApp, Static VarApp, Static VarApp, NonStatic]
        -- rhs_binders = [\alpha, \beta, c, n, xs]
        -- rhs_body = e

        binders_w_staticness = rhs_binders `zip` (arg_staticness ++ repeat NotStatic)
                                            -- Any extra args are assumed NotStatic

        non_static_args :: [Var]
                -- non_static_args = [xs]
                -- rhs_binders_without_type_capture = [\alpha', \beta', c, n, xs]
        non_static_args = [v | (v, NotStatic) <- binders_w_staticness]

        clone (bndr, NotStatic) = return bndr
        clone (bndr, _        ) = do { uniq <- newUnique
                                     ; return (setVarUnique bndr uniq) }

        -- new_rhs = \alpha beta c n xs ->
        --           let sat_worker = \xs -> let sat_shadow = \alpha' beta' c n xs ->
        --                                       sat_worker xs
        --                                   in e
        --           in sat_worker xs
        mk_new_rhs uniq shadow_lam_bndrs
            = mkLams rhs_binders $
              Let (Rec [(rec_body_bndr, rec_body)])
              local_body
            where
              local_body = mkVarApps (Var rec_body_bndr) non_static_args

              rec_body = mkLams non_static_args $
                         Let (NonRec shadow_bndr shadow_rhs) rhs_body

                -- See Note [Binder type capture]
              shadow_rhs = mkLams shadow_lam_bndrs local_body
                -- nonrec_rhs = \alpha' beta' c n xs -> sat_worker xs

              rec_body_bndr = mkSysLocal (fsLit "sat_worker") uniq (exprType rec_body)
                -- rec_body_bndr = sat_worker

                -- See Note [Shadow binding]; make a SysLocal
              shadow_bndr = mkSysLocal (occNameFS (getOccName binder))
                                       (idUnique binder)
                                       (exprType shadow_rhs)

    isStaticValue :: Staticness App -> Bool
    isStaticValue (Static (VarApp _)) = True
    isStaticValue _                   = False