% % (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! \begin{code}

    module SAT ( doStaticArgs ) where

    import Var
    import CoreSyn
    import CoreUtils
    import Type
    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"

\end{code} \begin{code}

    doStaticArgs :: UniqSupply -> [CoreBind] -> [CoreBind]
    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))

\end{code} \begin{code}

    -- 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')

\end{code} \begin{code}

    data App = VarApp Id | TypeApp Type
    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 _))  = ptext (sLit "SV")
    pprStaticness (Static (TypeApp _)) = ptext (sLit "ST")
    pprStaticness NotStatic            = ptext (sLit "NS")

    mergeSATInfo :: SATInfo -> SATInfo -> SATInfo
    mergeSATInfo [] _  = []
    mergeSATInfo _  [] = []
    mergeSATInfo (NotStatic:statics) (_:apps) = NotStatic : mergeSATInfo statics apps
    mergeSATInfo (_:statics) (NotStatic:apps) = NotStatic : mergeSATInfo statics apps
    mergeSATInfo ((Static (VarApp v)):statics)  ((Static (VarApp v')):apps)  = (if v == v' then Static (VarApp v) else NotStatic) : mergeSATInfo statics apps
    mergeSATInfo ((Static (TypeApp t)):statics) ((Static (TypeApp t')):apps) = (if t `coreEqType` t' then Static (TypeApp t) else NotStatic) : mergeSATInfo statics apps
    mergeSATInfo l  r  = pprPanic "mergeSATInfo" $ ptext (sLit "Left:") <> pprSATInfo l <> ptext (sLit ", ")
                                                <> ptext (sLit "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 = if isId v
                              then VarApp v
                              else TypeApp $ mkTyVarTy 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''

\end{code} \begin{code}

    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)
                    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 (Note note expr) interesting_ids = do
        (expr', sat_info_expr, expr_app) <- satExpr expr interesting_ids
        return (Note note expr', sat_info_expr, expr_app)

    satExpr ty@(Type _) _ = do
        return (ty, 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)

\end{code} %************************************************************************ Static Argument Transformation Monad %************************************************************************ \begin{code}

    type SatM result = UniqSM result

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

    newUnique :: SatM Unique
    newUnique = getUniqueUs

\end{code} %************************************************************************ 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.Bool.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.Bool.Bool) -> (a_a3O -> a_a3O) -> a_a3O -> a_a3O [] sat_shadow_r17 = \ (@ a_aiK) (p_a6T :: a_aiK -> GHC.Bool.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.Bool.False -> GHC.Base.until @ a_aiK p_a6T f_a6V (f_a6V x_a6X); GHC.Bool.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. \begin{code}

    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 = length (filter 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

\end{code}