{- (c) The AQUA Project, Glasgow University, 1994-1998 \section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop} -} {-# LANGUAGE CPP #-} module GHC.Core.Opt.LiberateCase ( liberateCase ) where #include "HsVersions.h" import GHC.Prelude import GHC.Driver.Session import GHC.Core import GHC.Core.Unfold import GHC.Builtin.Types ( unitDataConId ) import GHC.Types.Id import GHC.Types.Var.Env import GHC.Utils.Misc ( 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 (initLiberateCaseEnv dflags) binds where do_prog _ [] = [] do_prog env (bind:binds) = bind' : do_prog env' binds where (env', bind') = libCaseBind env bind initLiberateCaseEnv :: DynFlags -> LibCaseEnv initLiberateCaseEnv dflags = LibCaseEnv { lc_threshold = liberateCaseThreshold dflags , lc_uf_opts = unfoldingOpts dflags , lc_lvl = 0 , lc_lvl_env = emptyVarEnv , lc_rec_env = emptyVarEnv , lc_scruts = [] } {- ************************************************************************ * * 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 lc_threshold env of Nothing -> True -- Infinity Just size -> couldBeSmallEnoughToInline (lc_uf_opts env) size \$ Let (Rec dup_pairs) (Var unitDataConId) ok_pair (id,_) = idArity id > 0 -- Note [Only functions!] && not (isDeadEndId id) -- Note [Not bottoming ids] {- Note [Not bottoming Ids] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Do not specialise error-functions (this is unusual, but I once saw it, (actually 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 -> Alt CoreBndr -> Alt CoreBndr libCaseAlt env (Alt con args rhs) = Alt 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_threshold :: Maybe Int, -- ^ Bomb-out size for deciding if potential liberatees are too -- big. lc_uf_opts :: UnfoldingOpts, -- ^ Unfolding options 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 ...) .. } }