2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 %************************************************************************
6 \section[OccurAnal]{Occurrence analysis pass}
8 %************************************************************************
10 The occurrence analyser re-typechecks a core expression, returning a new
11 core expression with (hopefully) improved usage information.
15 occurAnalyseBinds, occurAnalyseGlobalExpr, occurAnalyseRule
18 #include "HsVersions.h"
21 import CoreFVs ( idRuleVars )
22 import CoreUtils ( exprIsTrivial )
23 import Id ( isDataConId, isOneShotLambda, setOneShotLambda,
24 idOccInfo, setIdOccInfo,
25 isExportedId, modifyIdInfo, idInfo, idArity,
26 idSpecialisation, isLocalId,
29 import IdInfo ( OccInfo(..), shortableIdInfo, copyIdInfo )
34 import Type ( splitFunTy_maybe, splitForAllTys )
35 import Maybes ( maybeToBool, orElse )
36 import Digraph ( stronglyConnCompR, SCC(..) )
37 import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
38 import Unique ( Unique )
39 import UniqFM ( keysUFM )
40 import Util ( zipWithEqual, mapAndUnzip )
45 %************************************************************************
47 \subsection[OccurAnal-main]{Counting occurrences: main function}
49 %************************************************************************
51 Here's the externally-callable interface:
54 occurAnalyseGlobalExpr :: CoreExpr -> CoreExpr
55 occurAnalyseGlobalExpr expr
56 = -- Top level expr, so no interesting free vars, and
57 -- discard occurence info returned
58 snd (occAnal (initOccEnv emptyVarSet) expr)
60 occurAnalyseRule :: CoreRule -> CoreRule
61 occurAnalyseRule rule@(BuiltinRule _ _) = rule
62 occurAnalyseRule (Rule str act tpl_vars tpl_args rhs)
63 -- Add occ info to tpl_vars, rhs
64 = Rule str act tpl_vars' tpl_args rhs'
66 (rhs_uds, rhs') = occAnal (initOccEnv (mkVarSet tpl_vars)) rhs
67 (_, tpl_vars') = tagBinders rhs_uds tpl_vars
71 %************************************************************************
73 \subsection{Top level stuff}
75 %************************************************************************
77 In @occAnalTop@ we do indirection-shorting. That is, if we have this:
79 x_local = <expression>
83 where exp is exported, and loc is not, then we replace it with this:
86 x_exported = <expression>
89 Without this we never get rid of the x_exported = x_local thing. This
90 save a gratuitous jump (from \tr{x_exported} to \tr{x_local}), and
91 makes strictness information propagate better. This used to happen in
92 the final phase, but it's tidier to do it here.
94 If more than one exported thing is equal to a local thing (i.e., the
95 local thing really is shared), then we do one only:
103 x_exported2 = x_exported1
106 We rely on prior eta reduction to simplify things like
108 x_exported = /\ tyvars -> x_local tyvars
112 Hence,there's a possibility of leaving unchanged something like this:
115 x_exported1 = x_local Int
117 By the time we've thrown away the types in STG land this
118 could be eliminated. But I don't think it's very common
119 and it's dangerous to do this fiddling in STG land
120 because we might elminate a binding that's mentioned in the
121 unfolding for something.
124 occurAnalyseBinds :: [CoreBind] -> [CoreBind]
126 occurAnalyseBinds binds
129 (_, _, binds') = go (initOccEnv emptyVarSet) binds
131 go :: OccEnv -> [CoreBind]
132 -> (UsageDetails, -- Occurrence info
133 IdEnv Id, -- Indirection elimination info
134 -- Maps local-id -> exported-id, but it embodies
135 -- bindings of the form exported-id = local-id in
136 -- the argument to go
137 [CoreBind]) -- Occ-analysed bindings, less the exported-id=local-id ones
139 go env [] = (emptyDetails, emptyVarEnv, [])
141 go env (bind : binds)
143 new_env = env `addNewCands` (bindersOf bind)
144 (scope_usage, ind_env, binds') = go new_env binds
145 (final_usage, new_binds) = occAnalBind env (zapBind ind_env bind) scope_usage
146 -- NB: I zap before occur-analysing, so
147 -- I don't need to worry about getting the
148 -- occ info on the new bindings right.
151 NonRec exported_id (Var local_id)
152 | shortMeOut ind_env exported_id local_id
153 -- Special case for eliminating indirections
154 -- Note: it's a shortcoming that this only works for
155 -- non-recursive bindings. Elminating indirections
156 -- makes perfect sense for recursive bindings too, but
157 -- it's more complicated to implement, so I haven't done so
158 -> (scope_usage, ind_env', binds')
160 ind_env' = extendVarEnv ind_env local_id exported_id
162 other -> -- Ho ho! The normal case
163 (final_usage, ind_env, new_binds ++ binds')
166 -- Deal with any indirections
167 zapBind ind_env (NonRec bndr rhs)
168 | bndr `elemVarEnv` ind_env = Rec (zap ind_env (bndr,rhs))
169 -- The Rec isn't strictly necessary, but it's convenient
170 zapBind ind_env (Rec pairs)
171 | or [id `elemVarEnv` ind_env | (id,_) <- pairs] = Rec (concat (map (zap ind_env) pairs))
173 zapBind ind_env bind = bind
175 zap ind_env pair@(local_id,rhs)
176 = case lookupVarEnv ind_env local_id of
178 Just exported_id -> [(local_id, Var exported_id),
181 exported_id' = modifyIdInfo (copyIdInfo (idInfo local_id)) exported_id
183 shortMeOut ind_env exported_id local_id
184 -- The if-then-else stuff is just so I can get a pprTrace to see
185 -- how often I don't get shorting out becuase of IdInfo stuff
186 = if isExportedId exported_id && -- Only if this is exported
188 isLocalId local_id && -- Only if this one is defined in this
189 -- module, so that we *can* change its
190 -- binding to be the exported thing!
192 not (isExportedId local_id) && -- Only if this one is not itself exported,
193 -- since the transformation will nuke it
195 not (local_id `elemVarEnv` ind_env) -- Only if not already substituted for
197 if shortableIdInfo (idInfo exported_id) -- Only if its IdInfo is 'shortable'
198 -- (see the defn of IdInfo.shortableIdInfo)
202 pprTrace "shortMeOut:" (ppr exported_id)
210 %************************************************************************
212 \subsection[OccurAnal-main]{Counting occurrences: main function}
214 %************************************************************************
220 type IdWithOccInfo = Id -- An Id with fresh PragmaInfo attached
222 type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
223 -- which is gotten from the Id.
224 type Details1 = (Id, UsageDetails, CoreExpr)
225 type Details2 = (IdWithOccInfo, CoreExpr)
228 occAnalBind :: OccEnv
230 -> UsageDetails -- Usage details of scope
231 -> (UsageDetails, -- Of the whole let(rec)
234 occAnalBind env (NonRec binder rhs) body_usage
235 | not (binder `usedIn` body_usage) -- It's not mentioned
238 | otherwise -- It's mentioned in the body
239 = (final_body_usage `combineUsageDetails` rhs_usage,
240 [NonRec tagged_binder rhs'])
243 (final_body_usage, tagged_binder) = tagBinder body_usage binder
244 (rhs_usage, rhs') = occAnalRhs env binder rhs
247 Dropping dead code for recursive bindings is done in a very simple way:
249 the entire set of bindings is dropped if none of its binders are
250 mentioned in its body; otherwise none are.
252 This seems to miss an obvious improvement.
267 Now @f@ is unused. But dependency analysis will sort this out into a
268 @letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
269 It isn't easy to do a perfect job in one blow. Consider
283 occAnalBind env (Rec pairs) body_usage
284 = foldr (_scc_ "occAnalBind.dofinal" do_final_bind) (body_usage, []) sccs
286 binders = map fst pairs
287 rhs_env = env `addNewCands` binders
289 analysed_pairs :: [Details1]
290 analysed_pairs = [ (bndr, rhs_usage, rhs')
291 | (bndr, rhs) <- pairs,
292 let (rhs_usage, rhs') = occAnalRhs rhs_env bndr rhs
295 sccs :: [SCC (Node Details1)]
296 sccs = _scc_ "occAnalBind.scc" stronglyConnCompR edges
299 ---- stuff for dependency analysis of binds -------------------------------
300 edges :: [Node Details1]
301 edges = _scc_ "occAnalBind.assoc"
302 [ (details, idUnique id, edges_from rhs_usage)
303 | details@(id, rhs_usage, rhs) <- analysed_pairs
306 -- (a -> b) means a mentions b
307 -- Given the usage details (a UFM that gives occ info for each free var of
308 -- the RHS) we can get the list of free vars -- or rather their Int keys --
309 -- by just extracting the keys from the finite map. Grimy, but fast.
310 -- Previously we had this:
311 -- [ bndr | bndr <- bndrs,
312 -- maybeToBool (lookupVarEnv rhs_usage bndr)]
313 -- which has n**2 cost, and this meant that edges_from alone
314 -- consumed 10% of total runtime!
315 edges_from :: UsageDetails -> [Unique]
316 edges_from rhs_usage = _scc_ "occAnalBind.edges_from"
319 ---- stuff to "re-constitute" bindings from dependency-analysis info ------
322 do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
323 | not (bndr `usedIn` body_usage)
324 = (body_usage, binds_so_far) -- Dead code
326 = (combined_usage, new_bind : binds_so_far)
328 total_usage = combineUsageDetails body_usage rhs_usage
329 (combined_usage, tagged_bndr) = tagBinder total_usage bndr
330 new_bind = NonRec tagged_bndr rhs'
333 do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
334 | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
335 = (body_usage, binds_so_far) -- Dead code
337 = (combined_usage, final_bind:binds_so_far)
339 details = [details | (details, _, _) <- cycle]
340 bndrs = [bndr | (bndr, _, _) <- details]
341 rhs_usages = [rhs_usage | (_, rhs_usage, _) <- details]
342 total_usage = foldr combineUsageDetails body_usage rhs_usages
343 (combined_usage, tagged_bndrs) = tagBinders total_usage bndrs
344 final_bind = Rec (reOrderRec env new_cycle)
346 new_cycle = CyclicSCC (zipWithEqual "occAnalBind" mk_new_bind tagged_bndrs cycle)
347 mk_new_bind tagged_bndr ((_, _, rhs'), key, keys) = ((tagged_bndr, rhs'), key, keys)
350 @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
351 strongly connected component (there's guaranteed to be a cycle). It returns the
353 a) in a better order,
354 b) with some of the Ids having a IMustNotBeINLINEd pragma
356 The "no-inline" Ids are sufficient to break all cycles in the SCC. This means
357 that the simplifier can guarantee not to loop provided it never records an inlining
358 for these no-inline guys.
360 Furthermore, the order of the binds is such that if we neglect dependencies
361 on the no-inline Ids then the binds are topologically sorted. This means
362 that the simplifier will generally do a good job if it works from top bottom,
363 recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
366 [June 98: I don't understand the following paragraphs, and I've
367 changed the a=b case again so that it isn't a special case any more.]
369 Here's a case that bit me:
377 Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
379 My solution was to make a=b bindings record b as Many, rather like INLINE bindings.
380 Perhaps something cleverer would suffice.
383 You might think that you can prevent non-termination simply by making
384 sure that we simplify a recursive binding's RHS in an environment that
385 simply clones the recursive Id. But no. Consider
387 letrec f = \x -> let z = f x' in ...
394 We bind n to its *simplified* RHS, we then *re-simplify* it when
395 we inline n. Then we may well inline f; and then the same thing
398 I don't think it's possible to prevent non-termination by environment
399 manipulation in this way. Apart from anything else, successive
400 iterations of the simplifier may unroll recursive loops in cases like
401 that above. The idea of beaking every recursive loop with an
402 IMustNotBeINLINEd pragma is much much better.
408 -> SCC (Node Details2)
410 -- Sorted into a plausible order. Enough of the Ids have
411 -- dontINLINE pragmas that there are no loops left.
413 -- Non-recursive case
414 reOrderRec env (AcyclicSCC (bind, _, _)) = [bind]
416 -- Common case of simple self-recursion
417 reOrderRec env (CyclicSCC [bind])
418 = [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
420 ((tagged_bndr, rhs), _, _) = bind
422 reOrderRec env (CyclicSCC (bind : binds))
423 = -- Choose a loop breaker, mark it no-inline,
424 -- do SCC analysis on the rest, and recursively sort them out
425 concat (map (reOrderRec env) (stronglyConnCompR unchosen))
427 [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
430 (chosen_pair, unchosen) = choose_loop_breaker bind (score bind) [] binds
431 (tagged_bndr, rhs) = chosen_pair
433 -- This loop looks for the bind with the lowest score
434 -- to pick as the loop breaker. The rest accumulate in
435 choose_loop_breaker (details,_,_) loop_sc acc []
436 = (details, acc) -- Done
438 choose_loop_breaker loop_bind loop_sc acc (bind : binds)
439 | sc < loop_sc -- Lower score so pick this new one
440 = choose_loop_breaker bind sc (loop_bind : acc) binds
442 | otherwise -- No lower so don't pick it
443 = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
447 score :: Node Details2 -> Int -- Higher score => less likely to be picked as loop breaker
448 score ((bndr, rhs), _, _)
449 | exprIsTrivial rhs = 4 -- Practically certain to be inlined
450 -- Used to have also: && not (isExportedId bndr)
451 -- But I found this sometimes cost an extra iteration when we have
452 -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
453 -- where df is the exported dictionary. Then df makes a really
454 -- bad choice for loop breaker
456 | not_fun_ty (idType bndr) = 3 -- Data types help with cases
457 -- This used to have a lower score than inlineCandidate, but
458 -- it's *really* helpful if dictionaries get inlined fast,
459 -- so I'm experimenting with giving higher priority to data-typed things
461 | inlineCandidate bndr rhs = 2 -- Likely to be inlined
463 | not (isEmptyCoreRules (idSpecialisation bndr)) = 1
464 -- Avoid things with specialisations; we'd like
465 -- to take advantage of them in the subsequent bindings
469 inlineCandidate :: Id -> CoreExpr -> Bool
470 inlineCandidate id (Note InlineMe _) = True
471 inlineCandidate id rhs = case idOccInfo id of
475 -- Real example (the Enum Ordering instance from PrelBase):
476 -- rec f = \ x -> case d of (p,q,r) -> p x
477 -- g = \ x -> case d of (p,q,r) -> q x
480 -- Here, f and g occur just once; but we can't inline them into d.
481 -- On the other hand we *could* simplify those case expressions if
482 -- we didn't stupidly choose d as the loop breaker.
483 -- But we won't because constructor args are marked "Many".
485 not_fun_ty ty = not (maybeToBool (splitFunTy_maybe rho_ty))
487 (_, rho_ty) = splitForAllTys ty
490 @occAnalRhs@ deals with the question of bindings where the Id is marked
491 by an INLINE pragma. For these we record that anything which occurs
492 in its RHS occurs many times. This pessimistically assumes that ths
493 inlined binder also occurs many times in its scope, but if it doesn't
494 we'll catch it next time round. At worst this costs an extra simplifier pass.
495 ToDo: try using the occurrence info for the inline'd binder.
497 [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec.
498 [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec.
503 -> Id -> CoreExpr -- Binder and rhs
504 -> (UsageDetails, CoreExpr)
506 occAnalRhs env id rhs
507 = (final_usage, rhs')
509 (rhs_usage, rhs') = occAnal (rhsCtxt env) rhs
510 -- Note that we use an rhsCtxt. This tells the occ anal that it's
511 -- looking at an RHS, which has an effect in occAnalApp
513 -- But there's a problem. Consider
518 -- First time round, it looks as if x1 and x2 occur as an arg of a
519 -- let-bound constructor ==> give them a many-occurrence.
520 -- But then x3 is inlined (unconditionally as it happens) and
521 -- next time round, x2 will be, and the next time round x1 will be
522 -- Result: multiple simplifier iterations. Sigh.
523 -- Possible solution: use rhsCtxt for things that occur just once...
525 -- [March 98] A new wrinkle is that if the binder has specialisations inside
526 -- it then we count the specialised Ids as "extra rhs's". That way
527 -- the "parent" keeps the specialised "children" alive. If the parent
528 -- dies (because it isn't referenced any more), then the children will
529 -- die too unless they are already referenced directly.
531 final_usage = foldVarSet add rhs_usage (idRuleVars id)
532 add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
533 -- (i.e manyOcc) because many copies
534 -- of the specialised thing can appear
542 -> (UsageDetails, -- Gives info only about the "interesting" Ids
545 occAnal env (Type t) = (emptyDetails, Type t)
550 var_uds | isCandidate env v = unitVarEnv v oneOcc
551 | otherwise = emptyDetails
553 -- At one stage, I gathered the idRuleVars for v here too,
554 -- which in a way is the right thing to do.
555 -- But that went wrong right after specialisation, when
556 -- the *occurrences* of the overloaded function didn't have any
557 -- rules in them, so the *specialised* versions looked as if they
558 -- weren't used at all.
562 We regard variables that occur as constructor arguments as "dangerousToDup":
566 f x = let y = expensive x in
568 (case z of {(p,q)->q}, case z of {(p,q)->q})
571 We feel free to duplicate the WHNF (True,y), but that means
572 that y may be duplicated thereby.
574 If we aren't careful we duplicate the (expensive x) call!
575 Constructors are rather like lambdas in this way.
578 occAnal env expr@(Lit lit) = (emptyDetails, expr)
582 occAnal env (Note InlineMe body)
583 = case occAnal env body of { (usage, body') ->
584 (mapVarEnv markMany usage, Note InlineMe body')
587 occAnal env (Note note@(SCC cc) body)
588 = case occAnal env body of { (usage, body') ->
589 (mapVarEnv markInsideSCC usage, Note note body')
592 occAnal env (Note note body)
593 = case occAnal env body of { (usage, body') ->
594 (usage, Note note body')
599 occAnal env app@(App fun arg)
600 = occAnalApp env (collectArgs app) False
602 -- Ignore type variables altogether
603 -- (a) occurrences inside type lambdas only not marked as InsideLam
604 -- (b) type variables not in environment
606 occAnal env expr@(Lam x body) | isTyVar x
607 = case occAnal env body of { (body_usage, body') ->
608 (body_usage, Lam x body')
611 -- For value lambdas we do a special hack. Consider
613 -- If we did nothing, x is used inside the \y, so would be marked
614 -- as dangerous to dup. But in the common case where the abstraction
615 -- is applied to two arguments this is over-pessimistic.
616 -- So instead, we just mark each binder with its occurrence
617 -- info in the *body* of the multiple lambda.
618 -- Then, the simplifier is careful when partially applying lambdas.
620 occAnal env expr@(Lam _ _)
621 = case occAnal env_body body of { (body_usage, body') ->
623 (final_usage, tagged_binders) = tagBinders body_usage binders
624 -- URGH! Sept 99: we don't seem to be able to use binders' here, because
625 -- we get linear-typed things in the resulting program that we can't handle yet.
626 -- (e.g. PrelShow) TODO
628 really_final_usage = if linear then
631 mapVarEnv markInsideLam final_usage
634 mkLams tagged_binders body') }
636 (binders, body) = collectBinders expr
637 (linear, env1, _) = oneShotGroup env binders
638 env2 = env1 `addNewCands` binders -- Add in-scope binders
639 env_body = vanillaCtxt env2 -- Body is (no longer) an RhsContext
641 occAnal env (Case scrut bndr alts)
642 = case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
643 case occAnal (vanillaCtxt env) scrut of { (scrut_usage, scrut') ->
644 -- No need for rhsCtxt
646 alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
647 alts_usage' = addCaseBndrUsage alts_usage
648 (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
649 total_usage = scrut_usage `combineUsageDetails` alts_usage1
651 total_usage `seq` (total_usage, Case scrut' tagged_bndr alts') }}
653 alt_env = env `addNewCand` bndr
655 -- The case binder gets a usage of either "many" or "dead", never "one".
656 -- Reason: we like to inline single occurrences, to eliminate a binding,
657 -- but inlining a case binder *doesn't* eliminate a binding.
658 -- We *don't* want to transform
659 -- case x of w { (p,q) -> f w }
661 -- case x of w { (p,q) -> f (p,q) }
662 addCaseBndrUsage usage = case lookupVarEnv usage bndr of
664 Just occ -> extendVarEnv usage bndr (markMany occ)
666 occAnal env (Let bind body)
667 = case occAnal new_env body of { (body_usage, body') ->
668 case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
669 (final_usage, mkLets new_binds body') }}
671 new_env = env `addNewCands` (bindersOf bind)
674 = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
675 (foldr combineUsageDetails emptyDetails arg_uds_s, args')}
677 arg_env = vanillaCtxt env
680 Applications are dealt with specially because we want
681 the "build hack" to work.
684 -- Hack for build, fold, runST
685 occAnalApp env (Var fun, args) is_rhs
686 = case args_stuff of { (args_uds, args') ->
688 -- We mark the free vars of the argument of a constructor or PAP
689 -- as "many", if it is the RHS of a let(rec).
690 -- This means that nothing gets inlined into a constructor argument
691 -- position, which is what we want. Typically those constructor
692 -- arguments are just variables, or trivial expressions.
694 -- This is the *whole point* of the isRhsEnv predicate
697 isDataConId fun || valArgCount args < idArity fun
698 = mapVarEnv markMany args_uds
699 | otherwise = args_uds
701 (fun_uds `combineUsageDetails` final_args_uds, mkApps (Var fun) args') }
703 fun_uniq = idUnique fun
705 fun_uds | isCandidate env fun = unitVarEnv fun oneOcc
706 | otherwise = emptyDetails
708 args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
709 | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
710 | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
711 | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
712 -- (foldr k z xs) may call k many times, but it never
713 -- shares a partial application of k; hence [False,True]
714 -- This means we can optimise
715 -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
716 -- by floating in the v
718 | otherwise = occAnalArgs env args
721 occAnalApp env (fun, args) is_rhs
722 = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
723 -- The addAppCtxt is a bit cunning. One iteration of the simplifier
724 -- often leaves behind beta redexs like
726 -- Here we would like to mark x,y as one-shot, and treat the whole
727 -- thing much like a let. We do this by pushing some True items
728 -- onto the context stack.
730 case occAnalArgs env args of { (args_uds, args') ->
732 final_uds = fun_uds `combineUsageDetails` args_uds
734 (final_uds, mkApps fun' args') }}
737 -> Int -> CtxtTy -- Argument number, and context to use for it
739 -> (UsageDetails, [CoreExpr])
740 appSpecial env n ctxt args
743 arg_env = vanillaCtxt env
745 go n [] = (emptyDetails, []) -- Too few args
747 go 1 (arg:args) -- The magic arg
748 = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
749 case occAnalArgs env args of { (args_uds, args') ->
750 (combineUsageDetails arg_uds args_uds, arg':args') }}
753 = case occAnal arg_env arg of { (arg_uds, arg') ->
754 case go (n-1) args of { (args_uds, args') ->
755 (combineUsageDetails arg_uds args_uds, arg':args') }}
761 If the case binder occurs at all, the other binders effectively do too.
763 case e of x { (a,b) -> rhs }
766 If e turns out to be (e1,e2) we indeed get something like
767 let a = e1; b = e2; x = (a,b) in rhs
770 occAnalAlt env case_bndr (con, bndrs, rhs)
771 = case occAnal (env `addNewCands` bndrs) rhs of { (rhs_usage, rhs') ->
773 (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
774 final_bndrs | case_bndr `elemVarEnv` final_usage = bndrs
775 | otherwise = tagged_bndrs
776 -- Leave the binders untagged if the case
777 -- binder occurs at all; see note above
779 (final_usage, (con, final_bndrs, rhs')) }
783 %************************************************************************
785 \subsection[OccurAnal-types]{OccEnv}
787 %************************************************************************
791 = OccEnv IdSet -- In-scope Ids; we gather info about these only
792 OccEncl -- Enclosing context information
793 CtxtTy -- Tells about linearity
795 -- OccEncl is used to control whether to inline into constructor arguments
797 -- x = (p,q) -- Don't inline p or q
798 -- y = /\a -> (p a, q a) -- Still don't inline p or q
799 -- z = f (p,q) -- Do inline p,q; it may make a rule fire
800 -- So OccEncl tells enought about the context to know what to do when
801 -- we encounter a contructor application or PAP.
804 = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
805 -- Don't inline into constructor args here
806 | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
807 -- Do inline into constructor args here
812 -- True:ctxt Analysing a function-valued expression that will be
815 -- False:ctxt Analysing a function-valued expression that may
816 -- be applied many times; but when it is,
817 -- the CtxtTy inside applies
819 initOccEnv :: VarSet -> OccEnv
820 initOccEnv vars = OccEnv vars OccRhs []
822 isRhsEnv (OccEnv _ OccRhs _) = True
823 isRhsEnv (OccEnv _ OccVanilla _) = False
825 isCandidate :: OccEnv -> Id -> Bool
826 isCandidate (OccEnv cands encl _) id = id `elemVarSet` cands
828 addNewCands :: OccEnv -> [Id] -> OccEnv
829 addNewCands (OccEnv cands encl ctxt) ids
830 = OccEnv (cands `unionVarSet` mkVarSet ids) encl ctxt
832 addNewCand :: OccEnv -> Id -> OccEnv
833 addNewCand (OccEnv cands encl ctxt) id
834 = OccEnv (extendVarSet cands id) encl ctxt
836 setCtxt :: OccEnv -> CtxtTy -> OccEnv
837 setCtxt (OccEnv cands encl _) ctxt = OccEnv cands encl ctxt
839 oneShotGroup :: OccEnv -> [CoreBndr] -> (Bool, OccEnv, [CoreBndr])
840 -- True <=> this is a one-shot linear lambda group
841 -- The [CoreBndr] are the binders.
843 -- The result binders have one-shot-ness set that they might not have had originally.
844 -- This happens in (build (\cn -> e)). Here the occurrence analyser
845 -- linearity context knows that c,n are one-shot, and it records that fact in
846 -- the binder. This is useful to guide subsequent float-in/float-out tranformations
848 oneShotGroup (OccEnv cands encl ctxt) bndrs
849 = case go ctxt bndrs [] of
850 (new_ctxt, new_bndrs) -> (all is_one_shot new_bndrs, OccEnv cands encl new_ctxt, new_bndrs)
852 is_one_shot b = isId b && isOneShotLambda b
854 go ctxt [] rev_bndrs = (ctxt, reverse rev_bndrs)
856 go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
857 | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
859 bndr' | lin_ctxt = setOneShotLambda bndr
862 go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
865 vanillaCtxt (OccEnv cands _ _) = OccEnv cands OccVanilla []
866 rhsCtxt (OccEnv cands _ _) = OccEnv cands OccRhs []
868 addAppCtxt (OccEnv cands encl ctxt) args
869 = OccEnv cands encl (replicate (valArgCount args) True ++ ctxt)
872 %************************************************************************
874 \subsection[OccurAnal-types]{OccEnv}
876 %************************************************************************
879 type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
881 combineUsageDetails, combineAltsUsageDetails
882 :: UsageDetails -> UsageDetails -> UsageDetails
884 combineUsageDetails usage1 usage2
885 = plusVarEnv_C addOccInfo usage1 usage2
887 combineAltsUsageDetails usage1 usage2
888 = plusVarEnv_C orOccInfo usage1 usage2
890 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
891 addOneOcc usage id info
892 = plusVarEnv_C addOccInfo usage (unitVarEnv id info)
893 -- ToDo: make this more efficient
895 emptyDetails = (emptyVarEnv :: UsageDetails)
897 usedIn :: Id -> UsageDetails -> Bool
898 v `usedIn` details = isExportedId v || v `elemVarEnv` details
900 tagBinders :: UsageDetails -- Of scope
902 -> (UsageDetails, -- Details with binders removed
903 [IdWithOccInfo]) -- Tagged binders
905 tagBinders usage binders
907 usage' = usage `delVarEnvList` binders
908 uss = map (setBinderOcc usage) binders
910 usage' `seq` (usage', uss)
912 tagBinder :: UsageDetails -- Of scope
914 -> (UsageDetails, -- Details with binders removed
915 IdWithOccInfo) -- Tagged binders
917 tagBinder usage binder
919 usage' = usage `delVarEnv` binder
920 binder' = setBinderOcc usage binder
922 usage' `seq` (usage', binder')
924 setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
925 setBinderOcc usage bndr
926 | isTyVar bndr = bndr
927 | isExportedId bndr = case idOccInfo bndr of
929 other -> setIdOccInfo bndr NoOccInfo
930 -- Don't use local usage info for visible-elsewhere things
931 -- BUT *do* erase any IAmALoopBreaker annotation, because we're
932 -- about to re-generate it and it shouldn't be "sticky"
934 | otherwise = setIdOccInfo bndr occ_info
936 occ_info = lookupVarEnv usage bndr `orElse` IAmDead
940 %************************************************************************
942 \subsection{Operations over OccInfo}
944 %************************************************************************
948 oneOcc = OneOcc False True
950 markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
952 markMany IAmDead = IAmDead
953 markMany other = NoOccInfo
955 markInsideSCC occ = markMany occ
957 markInsideLam (OneOcc _ one_br) = OneOcc True one_br
958 markInsideLam occ = occ
960 addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
962 addOccInfo IAmDead info2 = info2
963 addOccInfo info1 IAmDead = info1
964 addOccInfo info1 info2 = NoOccInfo
966 -- (orOccInfo orig new) is used
967 -- when combining occurrence info from branches of a case
969 orOccInfo IAmDead info2 = info2
970 orOccInfo info1 IAmDead = info1
971 orOccInfo (OneOcc in_lam1 one_branch1)
972 (OneOcc in_lam2 one_branch2)
973 = OneOcc (in_lam1 || in_lam2)
974 False -- False, because it occurs in both branches
976 orOccInfo info1 info2 = NoOccInfo