% % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % %************************************************************************ %* * \section[OccurAnal]{Occurrence analysis pass} %* * %************************************************************************ The occurrence analyser re-typechecks a core expression, returning a new core expression with (hopefully) improved usage information. \begin{code} module OccurAnal ( occurAnalysePgm, occurAnalyseExpr ) where #include "HsVersions.h" import CoreSyn import CoreFVs import CoreUtils ( exprIsTrivial, isDefaultAlt ) import Coercion ( mkSymCoercion ) import Id import IdInfo import BasicTypes import VarSet import VarEnv import Maybes ( orElse ) import Digraph ( SCC(..), stronglyConnCompFromEdgedVerticesR ) import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey ) import Unique ( Unique ) import UniqFM ( keysUFM, intersectUFM_C, foldUFM_Directly ) import Util ( mapAndUnzip ) import Outputable import Data.List \end{code} %************************************************************************ %* * \subsection[OccurAnal-main]{Counting occurrences: main function} %* * %************************************************************************ Here's the externally-callable interface: \begin{code} occurAnalysePgm :: [CoreBind] -> [CoreBind] occurAnalysePgm binds = snd (go initOccEnv binds) where go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind]) go _ [] = (emptyDetails, []) go env (bind:binds) = (final_usage, bind' ++ binds') where (bs_usage, binds') = go env binds (final_usage, bind') = occAnalBind env bind bs_usage occurAnalyseExpr :: CoreExpr -> CoreExpr -- Do occurrence analysis, and discard occurence info returned occurAnalyseExpr expr = snd (occAnal initOccEnv expr) \end{code} %************************************************************************ %* * \subsection[OccurAnal-main]{Counting occurrences: main function} %* * %************************************************************************ Bindings ~~~~~~~~ \begin{code} occAnalBind :: OccEnv -> CoreBind -> UsageDetails -- Usage details of scope -> (UsageDetails, -- Of the whole let(rec) [CoreBind]) occAnalBind env (NonRec binder rhs) body_usage | isTyVar binder -- A type let; we don't gather usage info = (body_usage, [NonRec binder rhs]) | not (binder `usedIn` body_usage) -- It's not mentioned = (body_usage, []) | otherwise -- It's mentioned in the body = (body_usage' +++ addRuleUsage rhs_usage binder, -- Note [Rules are extra RHSs] [NonRec tagged_binder rhs']) where (body_usage', tagged_binder) = tagBinder body_usage binder (rhs_usage, rhs') = occAnalRhs env tagged_binder rhs \end{code} Note [Dead code] ~~~~~~~~~~~~~~~~ Dropping dead code for recursive bindings is done in a very simple way: the entire set of bindings is dropped if none of its binders are mentioned in its body; otherwise none are. This seems to miss an obvious improvement. letrec f = ...g... g = ...f... in ...g... ===> letrec f = ...g... g = ...(...g...)... in ...g... Now 'f' is unused! But it's OK! Dependency analysis will sort this out into a letrec for 'g' and a 'let' for 'f', and then 'f' will get dropped. It isn't easy to do a perfect job in one blow. Consider letrec f = ...g... g = ...h... h = ...k... k = ...m... m = ...m... in ...m... Note [Loop breaking and RULES] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Loop breaking is surprisingly subtle. First read the section 4 of "Secrets of the GHC inliner". This describes our basic plan. However things are made quite a bit more complicated by RULES. Remember * Note [Rules are extra RHSs] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ A RULE for 'f' is like an extra RHS for 'f'. That way the "parent" keeps the specialised "children" alive. If the parent dies (because it isn't referenced any more), then the children will die too (unless they are already referenced directly). To that end, we build a Rec group for each cyclic strongly connected component, *treating f's rules as extra RHSs for 'f'*. When we make the Rec groups we include variables free in *either* LHS *or* RHS of the rule. The former might seems silly, but see Note [Rule dependency info]. So in Example [eftInt], eftInt and eftIntFB will be put in the same Rec, even though their 'main' RHSs are both non-recursive. * Note [Rules are visible in their own rec group] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want the rules for 'f' to be visible in f's right-hand side. And we'd like them to be visible in other functions in f's Rec group. E.g. in Example [Specialisation rules] we want f' rule to be visible in both f's RHS, and fs's RHS. This means that we must simplify the RULEs first, before looking at any of the definitions. This is done by Simplify.simplRecBind, when it calls addLetIdInfo. * Note [Choosing loop breakers] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We avoid infinite inlinings by choosing loop breakers, and ensuring that a loop breaker cuts each loop. But what is a "loop"? In particular, a RULES is like an equation for 'f' that is *always* inlined if it are applicable. We do *not* disable rules for loop-breakers. It's up to whoever makes the rules to make sure that the rules themselves alwasys terminate. See Note [Rules for recursive functions] in Simplify.lhs Hence, if f's RHS mentions g, and g has a RULE that mentions h, and h has a RULE that mentions f then we *must* choose f to be a loop breaker. In general, take the free variables of f's RHS, and augment it with all the variables reachable by RULES from those starting points. That is the whole reason for computing rule_fv_env in occAnalBind. (Of course we only consider free vars that are also binders in this Rec group.) Note that when we compute this rule_fv_env, we only consider variables free in the *RHS* of the rule, in contrast to the way we build the Rec group in the first place (Note [Rule dependency info]) Note that in Example [eftInt], *neither* eftInt *nor* eftIntFB is chosen as a loop breaker, because their RHSs don't mention each other. And indeed both can be inlined safely. Note that the edges of the graph we use for computing loop breakers are not the same as the edges we use for computing the Rec blocks. That's why we compute rec_edges for the Rec block analysis loop_breaker_edges for the loop breaker analysis * Note [Weak loop breakers] ~~~~~~~~~~~~~~~~~~~~~~~~~ There is a last nasty wrinkle. Suppose we have Rec { f = f_rhs RULE f [] = g h = h_rhs g = h ...more... } Remmber that we simplify the RULES before any RHS (see Note [Rules are visible in their own rec group] above). So we must *not* postInlineUnconditionally 'g', even though its RHS turns out to be trivial. (I'm assuming that 'g' is not choosen as a loop breaker.) We "solve" this by making g a "weak" or "rules-only" loop breaker, with OccInfo = IAmLoopBreaker True. A normal "strong" loop breaker has IAmLoopBreaker False. So Inline postInlineUnconditinoally IAmLoopBreaker False no no IAmLoopBreaker True yes no other yes yes The **sole** reason for this kind of loop breaker is so that postInlineUnconditionally does not fire. Ugh. * Note [Rule dependency info] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ The VarSet in a SpecInfo is used for dependency analysis in the occurrence analyser. We must track free vars in *both* lhs and rhs. Why both? Consider x = y RULE f x = 4 Then if we substitute y for x, we'd better do so in the rule's LHS too, so we'd better ensure the dependency is respected Example [eftInt] ~~~~~~~~~~~~~~~ Example (from GHC.Enum): eftInt :: Int# -> Int# -> [Int] eftInt x y = ...(non-recursive)... {-# INLINE [0] eftIntFB #-} eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r eftIntFB c n x y = ...(non-recursive)... {-# RULES "eftInt" [~1] forall x y. eftInt x y = build (\ c n -> eftIntFB c n x y) "eftIntList" [1] eftIntFB (:) [] = eftInt #-} Example [Specialisation rules] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this group, which is typical of what SpecConstr builds: fs a = ....f (C a).... f x = ....f (C a).... {-# RULE f (C a) = fs a #-} So 'f' and 'fs' are in the same Rec group (since f refers to fs via its RULE). But watch out! If 'fs' is not chosen as a loop breaker, we may get an infinite loop: - the RULE is applied in f's RHS (see Note [Self-recursive rules] in Simplify - fs is inlined (say it's small) - now there's another opportunity to apply the RULE This showed up when compiling Control.Concurrent.Chan.getChanContents. \begin{code} occAnalBind env (Rec pairs) body_usage = foldr occAnalRec (body_usage, []) sccs -- For a recursive group, we -- * occ-analyse all the RHSs -- * compute strongly-connected components -- * feed those components to occAnalRec where -------------Dependency analysis ------------------------------ bndr_set = mkVarSet (map fst pairs) sccs :: [SCC (Node Details)] sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompFromEdgedVerticesR rec_edges rec_edges :: [Node Details] rec_edges = {-# SCC "occAnalBind.assoc" #-} map make_node pairs make_node (bndr, rhs) = (ND bndr rhs' rhs_usage rhs_fvs, idUnique bndr, out_edges) where (rhs_usage, rhs') = occAnalRhs env bndr rhs rhs_fvs = intersectUFM_C (\b _ -> b) bndr_set rhs_usage out_edges = keysUFM (rhs_fvs `unionVarSet` idRuleVars bndr) -- (a -> b) means a mentions b -- Given the usage details (a UFM that gives occ info for each free var of -- the RHS) we can get the list of free vars -- or rather their Int keys -- -- by just extracting the keys from the finite map. Grimy, but fast. -- Previously we had this: -- [ bndr | bndr <- bndrs, -- maybeToBool (lookupVarEnv rhs_usage bndr)] -- which has n**2 cost, and this meant that edges_from alone -- consumed 10% of total runtime! ----------------------------- occAnalRec :: SCC (Node Details) -> (UsageDetails, [CoreBind]) -> (UsageDetails, [CoreBind]) -- The NonRec case is just like a Let (NonRec ...) above occAnalRec (AcyclicSCC (ND bndr rhs rhs_usage _, _, _)) (body_usage, binds) | not (bndr `usedIn` body_usage) = (body_usage, binds) | otherwise -- It's mentioned in the body = (body_usage' +++ addRuleUsage rhs_usage bndr, -- Note [Rules are extra RHSs] NonRec tagged_bndr rhs : binds) where (body_usage', tagged_bndr) = tagBinder body_usage bndr -- The Rec case is the interesting one -- See Note [Loop breaking] occAnalRec (CyclicSCC nodes) (body_usage, binds) | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage = (body_usage, binds) -- Dead code | otherwise -- At this point we always build a single Rec = (final_usage, Rec pairs : binds) where bndrs = [b | (ND b _ _ _, _, _) <- nodes] bndr_set = mkVarSet bndrs ---------------------------- -- Tag the binders with their occurrence info total_usage = foldl add_usage body_usage nodes add_usage body_usage (ND bndr _ rhs_usage _, _, _) = body_usage +++ addRuleUsage rhs_usage bndr (final_usage, tagged_nodes) = mapAccumL tag_node total_usage nodes tag_node :: UsageDetails -> Node Details -> (UsageDetails, Node Details) -- (a) Tag the binders in the details with occ info -- (b) Mark the binder with "weak loop-breaker" OccInfo -- saying "no preInlineUnconditionally" if it is used -- in any rule (lhs or rhs) of the recursive group -- See Note [Weak loop breakers] tag_node usage (ND bndr rhs rhs_usage rhs_fvs, k, ks) = (usage `delVarEnv` bndr, (ND bndr2 rhs rhs_usage rhs_fvs, k, ks)) where bndr2 | bndr `elemVarSet` all_rule_fvs = makeLoopBreaker True bndr1 | otherwise = bndr1 bndr1 = setBinderOcc usage bndr all_rule_fvs = bndr_set `intersectVarSet` foldr (unionVarSet . idRuleVars) emptyVarSet bndrs ---------------------------- -- Now reconstruct the cycle pairs | no_rules = reOrderCycle tagged_nodes | otherwise = concatMap reOrderRec (stronglyConnCompFromEdgedVerticesR loop_breaker_edges) -- See Note [Choosing loop breakers] for looop_breaker_edges loop_breaker_edges = map mk_node tagged_nodes mk_node (details@(ND _ _ _ rhs_fvs), k, _) = (details, k, new_ks) where new_ks = keysUFM (extendFvs rule_fv_env rhs_fvs rhs_fvs) ------------------------------------ rule_fv_env :: IdEnv IdSet -- Variables from this group mentioned in RHS of rules -- Domain is *subset* of bound vars (others have no rule fvs) rule_fv_env = rule_loop init_rule_fvs no_rules = null init_rule_fvs init_rule_fvs = [(b, rule_fvs) | b <- bndrs , let rule_fvs = idRuleRhsVars b `intersectVarSet` bndr_set , not (isEmptyVarSet rule_fvs)] rule_loop :: [(Id,IdSet)] -> IdEnv IdSet -- Finds fixpoint rule_loop fv_list | no_change = env | otherwise = rule_loop new_fv_list where env = mkVarEnv init_rule_fvs (no_change, new_fv_list) = mapAccumL bump True fv_list bump no_change (b,fvs) | new_fvs `subVarSet` fvs = (no_change, (b,fvs)) | otherwise = (False, (b,new_fvs `unionVarSet` fvs)) where new_fvs = extendFvs env emptyVarSet fvs idRuleRhsVars :: Id -> VarSet -- Just the variables free on the *rhs* of a rule -- See Note [Choosing loop breakers] idRuleRhsVars id = foldr (unionVarSet . ruleRhsFreeVars) emptyVarSet (idCoreRules id) extendFvs :: IdEnv IdSet -> IdSet -> IdSet -> IdSet -- (extendFVs env fvs s) returns (fvs `union` env(s)) extendFvs env fvs id_set = foldUFM_Directly add fvs id_set where add uniq _ fvs = case lookupVarEnv_Directly env uniq of Just fvs' -> fvs' `unionVarSet` fvs Nothing -> fvs \end{code} @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic strongly connected component (there's guaranteed to be a cycle). It returns the same pairs, but a) in a better order, b) with some of the Ids having a IAmALoopBreaker pragma The "loop-breaker" Ids are sufficient to break all cycles in the SCC. This means that the simplifier can guarantee not to loop provided it never records an inlining for these no-inline guys. Furthermore, the order of the binds is such that if we neglect dependencies on the no-inline Ids then the binds are topologically sorted. This means that the simplifier will generally do a good job if it works from top bottom, recording inlinings for any Ids which aren't marked as "no-inline" as it goes. ============== [June 98: I don't understand the following paragraphs, and I've changed the a=b case again so that it isn't a special case any more.] Here's a case that bit me: letrec a = b b = \x. BIG in ...a...a...a.... Re-ordering doesn't change the order of bindings, but there was no loop-breaker. My solution was to make a=b bindings record b as Many, rather like INLINE bindings. Perhaps something cleverer would suffice. =============== \begin{code} type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique, -- which is gotten from the Id. data Details = ND Id -- Binder CoreExpr -- RHS UsageDetails -- Full usage from RHS (*not* including rules) IdSet -- Other binders from this Rec group mentioned on RHS -- (derivable from UsageDetails but cached here) reOrderRec :: SCC (Node Details) -> [(Id,CoreExpr)] -- Sorted into a plausible order. Enough of the Ids have -- IAmALoopBreaker pragmas that there are no loops left. reOrderRec (AcyclicSCC (ND bndr rhs _ _, _, _)) = [(bndr, rhs)] reOrderRec (CyclicSCC cycle) = reOrderCycle cycle reOrderCycle :: [Node Details] -> [(Id,CoreExpr)] reOrderCycle [] = panic "reOrderCycle" reOrderCycle [bind] -- Common case of simple self-recursion = [(makeLoopBreaker False bndr, rhs)] where (ND bndr rhs _ _, _, _) = bind reOrderCycle (bind : binds) = -- Choose a loop breaker, mark it no-inline, -- do SCC analysis on the rest, and recursively sort them out concatMap reOrderRec (stronglyConnCompFromEdgedVerticesR unchosen) ++ [(makeLoopBreaker False bndr, rhs)] where (chosen_bind, unchosen) = choose_loop_breaker bind (score bind) [] binds ND bndr rhs _ _ = chosen_bind -- This loop looks for the bind with the lowest score -- to pick as the loop breaker. The rest accumulate in choose_loop_breaker (details,_,_) _loop_sc acc [] = (details, acc) -- Done choose_loop_breaker loop_bind loop_sc acc (bind : binds) | sc < loop_sc -- Lower score so pick this new one = choose_loop_breaker bind sc (loop_bind : acc) binds | otherwise -- No lower so don't pick it = choose_loop_breaker loop_bind loop_sc (bind : acc) binds where sc = score bind score :: Node Details -> Int -- Higher score => less likely to be picked as loop breaker score (ND bndr rhs _ _, _, _) | workerExists (idWorkerInfo bndr) = 10 -- Note [Worker inline loop] | exprIsTrivial rhs = 5 -- Practically certain to be inlined -- Used to have also: && not (isExportedId bndr) -- But I found this sometimes cost an extra iteration when we have -- rec { d = (a,b); a = ...df...; b = ...df...; df = d } -- where df is the exported dictionary. Then df makes a really -- bad choice for loop breaker | is_con_app rhs = 3 -- Data types help with cases -- Note [conapp] -- If an Id is marked "never inline" then it makes a great loop breaker -- The only reason for not checking that here is that it is rare -- and I've never seen a situation where it makes a difference, -- so it probably isn't worth the time to test on every binder -- | isNeverActive (idInlinePragma bndr) = -10 | inlineCandidate bndr rhs = 2 -- Likely to be inlined -- Note [Inline candidates] | not (neverUnfold (idUnfolding bndr)) = 1 -- the Id has some kind of unfolding | otherwise = 0 inlineCandidate :: Id -> CoreExpr -> Bool inlineCandidate _ (Note InlineMe _) = True inlineCandidate id _ = isOneOcc (idOccInfo id) -- Note [conapp] -- -- It's really really important to inline dictionaries. Real -- example (the Enum Ordering instance from GHC.Base): -- -- rec f = \ x -> case d of (p,q,r) -> p x -- g = \ x -> case d of (p,q,r) -> q x -- d = (v, f, g) -- -- Here, f and g occur just once; but we can't inline them into d. -- On the other hand we *could* simplify those case expressions if -- we didn't stupidly choose d as the loop breaker. -- But we won't because constructor args are marked "Many". -- Inlining dictionaries is really essential to unravelling -- the loops in static numeric dictionaries, see GHC.Float. -- Cheap and cheerful; the simplifer moves casts out of the way -- The lambda case is important to spot x = /\a. C (f a) -- which comes up when C is a dictionary constructor and -- f is a default method. -- Example: the instance for Show (ST s a) in GHC.ST -- -- However we *also* treat (\x. C p q) as a con-app-like thing, -- Note [Closure conversion] is_con_app (Var v) = isDataConWorkId v is_con_app (App f _) = is_con_app f is_con_app (Lam _ e) = is_con_app e is_con_app (Note _ e) = is_con_app e is_con_app _ = False makeLoopBreaker :: Bool -> Id -> Id -- Set the loop-breaker flag -- See Note [Weak loop breakers] makeLoopBreaker weak bndr = setIdOccInfo bndr (IAmALoopBreaker weak) \end{code} Note [Worker inline loop] ~~~~~~~~~~~~~~~~~~~~~~~~ Never choose a wrapper as the loop breaker! Because wrappers get auto-generated inlinings when importing, and that can lead to an infinite inlining loop. For example: rec { $wfoo x = ....foo x.... {-loop brk-} foo x = ...$wfoo x... } The interface file sees the unfolding for $wfoo, and sees that foo is strict (and hence it gets an auto-generated wrapper). Result: an infinite inlining in the importing scope. So be a bit careful if you change this. A good example is Tree.repTree in nofib/spectral/minimax. If the repTree wrapper is chosen as the loop breaker then compiling Game.hs goes into an infinite loop (this happened when we gave is_con_app a lower score than inline candidates). Note [Closure conversion] ~~~~~~~~~~~~~~~~~~~~~~~~~ We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm. The immediate motivation came from the result of a closure-conversion transformation which generated code like this: data Clo a b = forall c. Clo (c -> a -> b) c ($:) :: Clo a b -> a -> b Clo f env $: x = f env x rec { plus = Clo plus1 () ; plus1 _ n = Clo plus2 n ; plus2 Zero n = n ; plus2 (Succ m) n = Succ (plus $: m $: n) } If we inline 'plus' and 'plus1', everything unravels nicely. But if we choose 'plus1' as the loop breaker (which is entirely possible otherwise), the loop does not unravel nicely. @occAnalRhs@ deals with the question of bindings where the Id is marked by an INLINE pragma. For these we record that anything which occurs in its RHS occurs many times. This pessimistically assumes that ths inlined binder also occurs many times in its scope, but if it doesn't we'll catch it next time round. At worst this costs an extra simplifier pass. ToDo: try using the occurrence info for the inline'd binder. [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec. [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec. \begin{code} occAnalRhs :: OccEnv -> Id -> CoreExpr -- Binder and rhs -- For non-recs the binder is alrady tagged -- with occurrence info -> (UsageDetails, CoreExpr) occAnalRhs env id rhs = occAnal ctxt rhs where ctxt | certainly_inline id = env | otherwise = rhsCtxt -- Note that we generally use an rhsCtxt. This tells the occ anal n -- that it's looking at an RHS, which has an effect in occAnalApp -- -- But there's a problem. Consider -- x1 = a0 : [] -- x2 = a1 : x1 -- x3 = a2 : x2 -- g = f x3 -- First time round, it looks as if x1 and x2 occur as an arg of a -- let-bound constructor ==> give them a many-occurrence. -- But then x3 is inlined (unconditionally as it happens) and -- next time round, x2 will be, and the next time round x1 will be -- Result: multiple simplifier iterations. Sigh. -- Crude solution: use rhsCtxt for things that occur just once... certainly_inline id = case idOccInfo id of OneOcc in_lam one_br _ -> not in_lam && one_br _ -> False \end{code} \begin{code} addRuleUsage :: UsageDetails -> Id -> UsageDetails -- Add the usage from RULES in Id to the usage addRuleUsage usage id = foldVarSet add usage (idRuleVars id) where add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info -- (i.e manyOcc) because many copies -- of the specialised thing can appear \end{code} Expressions ~~~~~~~~~~~ \begin{code} occAnal :: OccEnv -> CoreExpr -> (UsageDetails, -- Gives info only about the "interesting" Ids CoreExpr) occAnal _ (Type t) = (emptyDetails, Type t) occAnal env (Var v) = (mkOneOcc env v False, Var v) -- At one stage, I gathered the idRuleVars for v here too, -- which in a way is the right thing to do. -- But that went wrong right after specialisation, when -- the *occurrences* of the overloaded function didn't have any -- rules in them, so the *specialised* versions looked as if they -- weren't used at all. \end{code} We regard variables that occur as constructor arguments as "dangerousToDup": \begin{verbatim} module A where f x = let y = expensive x in let z = (True,y) in (case z of {(p,q)->q}, case z of {(p,q)->q}) \end{verbatim} We feel free to duplicate the WHNF (True,y), but that means that y may be duplicated thereby. If we aren't careful we duplicate the (expensive x) call! Constructors are rather like lambdas in this way. \begin{code} occAnal _ expr@(Lit _) = (emptyDetails, expr) \end{code} \begin{code} occAnal env (Note InlineMe body) = case occAnal env body of { (usage, body') -> (mapVarEnv markMany usage, Note InlineMe body') } occAnal env (Note note@(SCC _) body) = case occAnal env body of { (usage, body') -> (mapVarEnv markInsideSCC usage, Note note body') } occAnal env (Note note body) = case occAnal env body of { (usage, body') -> (usage, Note note body') } occAnal env (Cast expr co) = case occAnal env expr of { (usage, expr') -> (markRhsUds env True usage, Cast expr' co) -- If we see let x = y `cast` co -- then mark y as 'Many' so that we don't -- immediately inline y again. } \end{code} \begin{code} occAnal env app@(App _ _) = occAnalApp env (collectArgs app) -- Ignore type variables altogether -- (a) occurrences inside type lambdas only not marked as InsideLam -- (b) type variables not in environment occAnal env (Lam x body) | isTyVar x = case occAnal env body of { (body_usage, body') -> (body_usage, Lam x body') } -- For value lambdas we do a special hack. Consider -- (\x. \y. ...x...) -- If we did nothing, x is used inside the \y, so would be marked -- as dangerous to dup. But in the common case where the abstraction -- is applied to two arguments this is over-pessimistic. -- So instead, we just mark each binder with its occurrence -- info in the *body* of the multiple lambda. -- Then, the simplifier is careful when partially applying lambdas. occAnal env expr@(Lam _ _) = case occAnal env_body body of { (body_usage, body') -> let (final_usage, tagged_binders) = tagBinders body_usage binders -- URGH! Sept 99: we don't seem to be able to use binders' here, because -- we get linear-typed things in the resulting program that we can't handle yet. -- (e.g. PrelShow) TODO really_final_usage = if linear then final_usage else mapVarEnv markInsideLam final_usage in (really_final_usage, mkLams tagged_binders body') } where env_body = vanillaCtxt -- Body is (no longer) an RhsContext (binders, body) = collectBinders expr binders' = oneShotGroup env binders linear = all is_one_shot binders' is_one_shot b = isId b && isOneShotBndr b occAnal env (Case scrut bndr ty alts) = case occ_anal_scrut scrut alts of { (scrut_usage, scrut') -> case mapAndUnzip occ_anal_alt alts of { (alts_usage_s, alts') -> let alts_usage = foldr1 combineAltsUsageDetails alts_usage_s alts_usage' = addCaseBndrUsage alts_usage (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr total_usage = scrut_usage +++ alts_usage1 in total_usage `seq` (total_usage, Case scrut' tagged_bndr ty alts') }} where -- Note [Case binder usage] -- ~~~~~~~~~~~~~~~~~~~~~~~~ -- The case binder gets a usage of either "many" or "dead", never "one". -- Reason: we like to inline single occurrences, to eliminate a binding, -- but inlining a case binder *doesn't* eliminate a binding. -- We *don't* want to transform -- case x of w { (p,q) -> f w } -- into -- case x of w { (p,q) -> f (p,q) } addCaseBndrUsage usage = case lookupVarEnv usage bndr of Nothing -> usage Just _ -> extendVarEnv usage bndr NoOccInfo alt_env = setVanillaCtxt env -- Consider x = case v of { True -> (p,q); ... } -- Then it's fine to inline p and q bndr_swap = case scrut of Var v -> Just (v, Var bndr) Cast (Var v) co -> Just (v, Cast (Var bndr) (mkSymCoercion co)) _other -> Nothing occ_anal_alt = occAnalAlt alt_env bndr bndr_swap occ_anal_scrut (Var v) (alt1 : other_alts) | not (null other_alts) || not (isDefaultAlt alt1) = (mkOneOcc env v True, Var v) -- The 'True' says that the variable occurs -- in an interesting context; the case has -- at least one non-default alternative occ_anal_scrut scrut _alts = occAnal vanillaCtxt scrut -- No need for rhsCtxt occAnal env (Let bind body) = case occAnal env body of { (body_usage, body') -> case occAnalBind env bind body_usage of { (final_usage, new_binds) -> (final_usage, mkLets new_binds body') }} occAnalArgs :: OccEnv -> [CoreExpr] -> (UsageDetails, [CoreExpr]) occAnalArgs _env args = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') -> (foldr (+++) emptyDetails arg_uds_s, args')} where arg_env = vanillaCtxt \end{code} Applications are dealt with specially because we want the "build hack" to work. \begin{code} occAnalApp :: OccEnv -> (Expr CoreBndr, [Arg CoreBndr]) -> (UsageDetails, Expr CoreBndr) occAnalApp env (Var fun, args) = case args_stuff of { (args_uds, args') -> let final_args_uds = markRhsUds env is_pap args_uds in (fun_uds +++ final_args_uds, mkApps (Var fun) args') } where fun_uniq = idUnique fun fun_uds = mkOneOcc env fun (valArgCount args > 0) is_pap = isDataConWorkId fun || valArgCount args < idArity fun -- Hack for build, fold, runST args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args -- (foldr k z xs) may call k many times, but it never -- shares a partial application of k; hence [False,True] -- This means we can optimise -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs -- by floating in the v | otherwise = occAnalArgs env args occAnalApp env (fun, args) = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') -> -- The addAppCtxt is a bit cunning. One iteration of the simplifier -- often leaves behind beta redexs like -- (\x y -> e) a1 a2 -- Here we would like to mark x,y as one-shot, and treat the whole -- thing much like a let. We do this by pushing some True items -- onto the context stack. case occAnalArgs env args of { (args_uds, args') -> let final_uds = fun_uds +++ args_uds in (final_uds, mkApps fun' args') }} markRhsUds :: OccEnv -- Check if this is a RhsEnv -> Bool -- and this is true -> UsageDetails -- The do markMany on this -> UsageDetails -- We mark the free vars of the argument of a constructor or PAP -- as "many", if it is the RHS of a let(rec). -- This means that nothing gets inlined into a constructor argument -- position, which is what we want. Typically those constructor -- arguments are just variables, or trivial expressions. -- -- This is the *whole point* of the isRhsEnv predicate markRhsUds env is_pap arg_uds | isRhsEnv env && is_pap = mapVarEnv markMany arg_uds | otherwise = arg_uds appSpecial :: OccEnv -> Int -> CtxtTy -- Argument number, and context to use for it -> [CoreExpr] -> (UsageDetails, [CoreExpr]) appSpecial env n ctxt args = go n args where arg_env = vanillaCtxt go _ [] = (emptyDetails, []) -- Too few args go 1 (arg:args) -- The magic arg = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') -> case occAnalArgs env args of { (args_uds, args') -> (arg_uds +++ args_uds, arg':args') }} go n (arg:args) = case occAnal arg_env arg of { (arg_uds, arg') -> case go (n-1) args of { (args_uds, args') -> (arg_uds +++ args_uds, arg':args') }} \end{code} Note [Binder swap] ~~~~~~~~~~~~~~~~~~ We do these two transformations right here: (1) case x of b { pi -> ri } ==> case x of b { pi -> let x=b in ri } (2) case (x |> co) of b { pi -> ri } ==> case (x |> co) of b { pi -> let x = b |> sym co in ri } Why (2)? See Note [Ccase of cast] In both cases, in a particular alternative (pi -> ri), we only add the binding if (a) x occurs free in (pi -> ri) (ie it occurs in ri, but is not bound in pi) (b) the pi does not bind b (or the free vars of co) (c) x is not a We need (a) and (b) for the inserted binding to be correct. Notice that (a) rapidly becomes false, so no bindings are injected. Notice the deliberate shadowing of 'x'. But we must call localiseId on 'x' first, in case it's a GlobalId, or has an External Name. See, for example, SimplEnv Note [Global Ids in the substitution]. For the alternatives where we inject the binding, we can transfer all x's OccInfo to b. And that is the point. The reason for doing these transformations here is because it allows us to adjust the OccInfo for 'x' and 'b' as we go. * Suppose the only occurrences of 'x' are the scrutinee and in the ri; then this transformation makes it occur just once, and hence get inlined right away. * If we do this in the Simplifier, we don't know whether 'x' is used in ri, so we are forced to pessimistically zap b's OccInfo even though it is typically dead (ie neither it nor x appear in the ri). There's nothing actually wrong with zapping it, except that it's kind of nice to know which variables are dead. My nose tells me to keep this information as robustly as possible. The Maybe (Id,CoreExpr) passed to occAnalAlt is the extra let-binding {x=b}; it's Nothing if the binder-swap doesn't happen. Note [Case of cast] ~~~~~~~~~~~~~~~~~~~ Consider case (x `cast` co) of b { I# -> ... (case (x `cast` co) of {...}) ... We'd like to eliminate the inner case. That is the motivation for equation (2) in Note [Binder swap]. When we get to the inner case, we inline x, cancel the casts, and away we go. Note [Binders in case alternatives] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider case x of y { (a,b) -> f y } We treat 'a', 'b' as dead, because they don't physically occur in the case alternative. (Indeed, a variable is dead iff it doesn't occur in its scope in the output of OccAnal.) This invariant is It really helpe to know when binders are unused. See esp the call to isDeadBinder in Simplify.mkDupableAlt In this example, though, the Simplifier will bring 'a' and 'b' back to life, beause it binds 'y' to (a,b) (imagine got inlined and scrutinised y). \begin{code} occAnalAlt :: OccEnv -> CoreBndr -> Maybe (Id, CoreExpr) -- Note [Binder swap] -> CoreAlt -> (UsageDetails, Alt IdWithOccInfo) occAnalAlt env case_bndr mb_scrut_var (con, bndrs, rhs) = case occAnal env rhs of { (rhs_usage, rhs') -> let (alt_usg, tagged_bndrs) = tagBinders rhs_usage bndrs bndrs' = tagged_bndrs -- See Note [Binders in case alternatives] in case mb_scrut_var of Just (scrut_var, scrut_rhs) -- See Note [Binder swap] | scrut_var `localUsedIn` alt_usg -- (a) Fast path, usually false , not (any shadowing bndrs) -- (b) -> (addOneOcc usg_wo_scrut case_bndr NoOccInfo, -- See Note [Case binder usage] for the NoOccInfo (con, bndrs', Let (NonRec scrut_var' scrut_rhs) rhs')) where (usg_wo_scrut, scrut_var') = tagBinder alt_usg (localiseId scrut_var) -- Note the localiseId; we're making a new binding -- for it, and it might have an External Name, or -- even be a GlobalId shadowing bndr = bndr `elemVarSet` rhs_fvs rhs_fvs = exprFreeVars scrut_rhs _other -> (alt_usg, (con, bndrs', rhs')) } \end{code} %************************************************************************ %* * \subsection[OccurAnal-types]{OccEnv} %* * %************************************************************************ \begin{code} data OccEnv = OccEnv OccEncl -- Enclosing context information CtxtTy -- Tells about linearity -- OccEncl is used to control whether to inline into constructor arguments -- For example: -- x = (p,q) -- Don't inline p or q -- y = /\a -> (p a, q a) -- Still don't inline p or q -- z = f (p,q) -- Do inline p,q; it may make a rule fire -- So OccEncl tells enought about the context to know what to do when -- we encounter a contructor application or PAP. data OccEncl = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda -- Don't inline into constructor args here | OccVanilla -- Argument of function, body of lambda, scruintee of case etc. -- Do inline into constructor args here type CtxtTy = [Bool] -- [] No info -- -- True:ctxt Analysing a function-valued expression that will be -- applied just once -- -- False:ctxt Analysing a function-valued expression that may -- be applied many times; but when it is, -- the CtxtTy inside applies initOccEnv :: OccEnv initOccEnv = OccEnv OccRhs [] vanillaCtxt :: OccEnv vanillaCtxt = OccEnv OccVanilla [] rhsCtxt :: OccEnv rhsCtxt = OccEnv OccRhs [] isRhsEnv :: OccEnv -> Bool isRhsEnv (OccEnv OccRhs _) = True isRhsEnv (OccEnv OccVanilla _) = False setVanillaCtxt :: OccEnv -> OccEnv setVanillaCtxt (OccEnv OccRhs ctxt_ty) = OccEnv OccVanilla ctxt_ty setVanillaCtxt other_env = other_env setCtxt :: OccEnv -> CtxtTy -> OccEnv setCtxt (OccEnv encl _) ctxt = OccEnv encl ctxt oneShotGroup :: OccEnv -> [CoreBndr] -> [CoreBndr] -- The result binders have one-shot-ness set that they might not have had originally. -- This happens in (build (\cn -> e)). Here the occurrence analyser -- linearity context knows that c,n are one-shot, and it records that fact in -- the binder. This is useful to guide subsequent float-in/float-out tranformations oneShotGroup (OccEnv _encl ctxt) bndrs = go ctxt bndrs [] where go _ [] rev_bndrs = reverse rev_bndrs go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs | isId bndr = go ctxt bndrs (bndr':rev_bndrs) where bndr' | lin_ctxt = setOneShotLambda bndr | otherwise = bndr go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs) addAppCtxt :: OccEnv -> [Arg CoreBndr] -> OccEnv addAppCtxt (OccEnv encl ctxt) args = OccEnv encl (replicate (valArgCount args) True ++ ctxt) \end{code} %************************************************************************ %* * \subsection[OccurAnal-types]{OccEnv} %* * %************************************************************************ \begin{code} type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage -- INVARIANT: never IAmDead -- (Deadness is signalled by not being in the map at all) (+++), combineAltsUsageDetails :: UsageDetails -> UsageDetails -> UsageDetails (+++) usage1 usage2 = plusVarEnv_C addOccInfo usage1 usage2 combineAltsUsageDetails usage1 usage2 = plusVarEnv_C orOccInfo usage1 usage2 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails addOneOcc usage id info = plusVarEnv_C addOccInfo usage (unitVarEnv id info) -- ToDo: make this more efficient emptyDetails :: UsageDetails emptyDetails = (emptyVarEnv :: UsageDetails) localUsedIn, usedIn :: Id -> UsageDetails -> Bool v `localUsedIn` details = v `elemVarEnv` details v `usedIn` details = isExportedId v || v `localUsedIn` details type IdWithOccInfo = Id tagBinders :: UsageDetails -- Of scope -> [Id] -- Binders -> (UsageDetails, -- Details with binders removed [IdWithOccInfo]) -- Tagged binders tagBinders usage binders = let usage' = usage `delVarEnvList` binders uss = map (setBinderOcc usage) binders in usage' `seq` (usage', uss) tagBinder :: UsageDetails -- Of scope -> Id -- Binders -> (UsageDetails, -- Details with binders removed IdWithOccInfo) -- Tagged binders tagBinder usage binder = let usage' = usage `delVarEnv` binder binder' = setBinderOcc usage binder in usage' `seq` (usage', binder') setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr setBinderOcc usage bndr | isTyVar bndr = bndr | isExportedId bndr = case idOccInfo bndr of NoOccInfo -> bndr _ -> setIdOccInfo bndr NoOccInfo -- Don't use local usage info for visible-elsewhere things -- BUT *do* erase any IAmALoopBreaker annotation, because we're -- about to re-generate it and it shouldn't be "sticky" | otherwise = setIdOccInfo bndr occ_info where occ_info = lookupVarEnv usage bndr `orElse` IAmDead \end{code} %************************************************************************ %* * \subsection{Operations over OccInfo} %* * %************************************************************************ \begin{code} mkOneOcc :: OccEnv -> Id -> InterestingCxt -> UsageDetails mkOneOcc _env id int_cxt | isLocalId id = unitVarEnv id (OneOcc False True int_cxt) | otherwise = emptyDetails markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo markMany _ = NoOccInfo markInsideSCC occ = markMany occ markInsideLam (OneOcc _ one_br int_cxt) = OneOcc True one_br int_cxt markInsideLam occ = occ addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo addOccInfo a1 a2 = ASSERT( not (isDeadOcc a1 || isDeadOcc a2) ) NoOccInfo -- Both branches are at least One -- (Argument is never IAmDead) -- (orOccInfo orig new) is used -- when combining occurrence info from branches of a case orOccInfo (OneOcc in_lam1 _ int_cxt1) (OneOcc in_lam2 _ int_cxt2) = OneOcc (in_lam1 || in_lam2) False -- False, because it occurs in both branches (int_cxt1 && int_cxt2) orOccInfo a1 a2 = ASSERT( not (isDeadOcc a1 || isDeadOcc a2) ) NoOccInfo \end{code}