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 -- The above warning supression flag is a temporary kludge.
16 -- While working on this module you are encouraged to remove it and fix
17 -- any warnings in the module. See
18 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
22 occurAnalysePgm, occurAnalyseExpr
25 #include "HsVersions.h"
28 import CoreFVs ( idRuleVars )
29 import CoreUtils ( exprIsTrivial, isDefaultAlt )
30 import Id ( isDataConWorkId, isOneShotBndr, setOneShotLambda,
31 idOccInfo, setIdOccInfo, isLocalId,
32 isExportedId, idArity, idHasRules,
35 import BasicTypes ( OccInfo(..), isOneOcc, InterestingCxt )
40 import Maybes ( orElse )
41 import Digraph ( stronglyConnCompR, SCC(..) )
42 import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
43 import Unique ( Unique )
44 import UniqFM ( keysUFM, intersectsUFM )
45 import Util ( mapAndUnzip )
52 %************************************************************************
54 \subsection[OccurAnal-main]{Counting occurrences: main function}
56 %************************************************************************
58 Here's the externally-callable interface:
61 occurAnalysePgm :: [CoreBind] -> [CoreBind]
63 = snd (go initOccEnv binds)
65 go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind])
69 = (final_usage, bind' ++ binds')
71 (bs_usage, binds') = go env binds
72 (final_usage, bind') = occAnalBind env bind bs_usage
74 occurAnalyseExpr :: CoreExpr -> CoreExpr
75 -- Do occurrence analysis, and discard occurence info returned
76 occurAnalyseExpr expr = snd (occAnal initOccEnv expr)
80 %************************************************************************
82 \subsection[OccurAnal-main]{Counting occurrences: main function}
84 %************************************************************************
92 -> UsageDetails -- Usage details of scope
93 -> (UsageDetails, -- Of the whole let(rec)
96 occAnalBind env (NonRec binder rhs) body_usage
97 | not (binder `usedIn` body_usage) -- It's not mentioned
100 | otherwise -- It's mentioned in the body
101 = (body_usage' +++ addRuleUsage rhs_usage binder, -- Note [RulesOnly]
102 [NonRec tagged_binder rhs'])
104 (body_usage', tagged_binder) = tagBinder body_usage binder
105 (rhs_usage, rhs') = occAnalRhs env tagged_binder rhs
108 Dropping dead code for recursive bindings is done in a very simple way:
110 the entire set of bindings is dropped if none of its binders are
111 mentioned in its body; otherwise none are.
113 This seems to miss an obvious improvement.
128 Now @f@ is unused. But dependency analysis will sort this out into a
129 @letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
130 It isn't easy to do a perfect job in one blow. Consider
144 occAnalBind env (Rec pairs) body_usage
145 = foldr ({-# SCC "occAnalBind.dofinal" #-} do_final_bind) (body_usage, []) sccs
147 analysed_pairs :: [Details]
148 analysed_pairs = [ (bndr, rhs_usage, rhs')
149 | (bndr, rhs) <- pairs,
150 let (rhs_usage, rhs') = occAnalRhs env bndr rhs
153 sccs :: [SCC (Node Details)]
154 sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompR edges
157 ---- stuff for dependency analysis of binds -------------------------------
158 edges :: [Node Details]
159 edges = {-# SCC "occAnalBind.assoc" #-}
160 [ (details, idUnique id, edges_from id rhs_usage)
161 | details@(id, rhs_usage, rhs) <- analysed_pairs
164 -- (a -> b) means a mentions b
165 -- Given the usage details (a UFM that gives occ info for each free var of
166 -- the RHS) we can get the list of free vars -- or rather their Int keys --
167 -- by just extracting the keys from the finite map. Grimy, but fast.
168 -- Previously we had this:
169 -- [ bndr | bndr <- bndrs,
170 -- maybeToBool (lookupVarEnv rhs_usage bndr)]
171 -- which has n**2 cost, and this meant that edges_from alone
172 -- consumed 10% of total runtime!
173 edges_from :: Id -> UsageDetails -> [Unique]
174 edges_from bndr rhs_usage = {-# SCC "occAnalBind.edges_from" #-}
175 keysUFM (addRuleUsage rhs_usage bndr)
177 ---- Stuff to "re-constitute" bindings from dependency-analysis info ------
180 do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
181 | not (bndr `usedIn` body_usage)
182 = (body_usage, binds_so_far) -- Dead code
184 = (body_usage' +++ addRuleUsage rhs_usage bndr, new_bind : binds_so_far)
186 (body_usage', tagged_bndr) = tagBinder body_usage bndr
187 new_bind = NonRec tagged_bndr rhs'
190 do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
191 | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
192 = (body_usage, binds_so_far) -- Dead code
193 | otherwise -- If any is used, they all are
194 = (final_usage, final_bind : binds_so_far)
196 details = [details | (details, _, _) <- cycle]
197 bndrs = [bndr | (bndr, _, _) <- details]
198 bndr_usages = [addRuleUsage rhs_usage bndr | (bndr, rhs_usage, _) <- details]
199 total_usage = foldr (+++) body_usage bndr_usages
200 (final_usage, tagged_cycle) = mapAccumL tag_bind total_usage cycle
201 tag_bind usg ((bndr,rhs_usg,rhs),k,ks) = (usg', ((bndr',rhs_usg,rhs),k,ks))
203 (usg', bndr') = tagBinder usg bndr
204 final_bind = Rec (reOrderCycle (mkVarSet bndrs) tagged_cycle)
206 {- An alternative; rebuild the edges. No semantic difference, but perf might change
208 -- Hopefully 'bndrs' is a relatively small group now
209 -- Now get ready for the loop-breaking phase
210 -- We've done dead-code elimination already, so no worries about un-referenced binders
211 keys = map idUnique bndrs
212 mk_node tagged_bndr (_, rhs_usage, rhs')
213 = ((tagged_bndr, rhs'), idUnique tagged_bndr, used)
215 used = [key | key <- keys, used_outside_rule rhs_usage key ]
217 used_outside_rule usage uniq = case lookupUFM_Directly usage uniq of
219 Just RulesOnly -> False -- Ignore rules
224 @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
225 strongly connected component (there's guaranteed to be a cycle). It returns the
227 a) in a better order,
228 b) with some of the Ids having a IAmALoopBreaker pragma
230 The "loop-breaker" Ids are sufficient to break all cycles in the SCC. This means
231 that the simplifier can guarantee not to loop provided it never records an inlining
232 for these no-inline guys.
234 Furthermore, the order of the binds is such that if we neglect dependencies
235 on the no-inline Ids then the binds are topologically sorted. This means
236 that the simplifier will generally do a good job if it works from top bottom,
237 recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
240 [June 98: I don't understand the following paragraphs, and I've
241 changed the a=b case again so that it isn't a special case any more.]
243 Here's a case that bit me:
251 Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
253 My solution was to make a=b bindings record b as Many, rather like INLINE bindings.
254 Perhaps something cleverer would suffice.
259 type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
260 -- which is gotten from the Id.
261 type Details = (Id, UsageDetails, CoreExpr)
263 reOrderRec :: IdSet -- Binders of this group
264 -> SCC (Node Details)
266 -- Sorted into a plausible order. Enough of the Ids have
267 -- IAmALoopBreaker pragmas that there are no loops left.
268 reOrderRec bndrs (AcyclicSCC ((bndr, _, rhs), _, _)) = [(bndr, rhs)]
269 reOrderRec bndrs (CyclicSCC cycle) = reOrderCycle bndrs cycle
271 reOrderCycle :: IdSet -> [Node Details] -> [(Id,CoreExpr)]
272 reOrderCycle bndrs []
273 = panic "reOrderCycle"
274 reOrderCycle bndrs [bind] -- Common case of simple self-recursion
275 = [(makeLoopBreaker bndrs rhs_usg bndr, rhs)]
277 ((bndr, rhs_usg, rhs), _, _) = bind
279 reOrderCycle bndrs (bind : binds)
280 = -- Choose a loop breaker, mark it no-inline,
281 -- do SCC analysis on the rest, and recursively sort them out
282 concatMap (reOrderRec bndrs) (stronglyConnCompR unchosen) ++
283 [(makeLoopBreaker bndrs rhs_usg bndr, rhs)]
286 (chosen_bind, unchosen) = choose_loop_breaker bind (score bind) [] binds
287 (bndr, rhs_usg, rhs) = chosen_bind
289 -- This loop looks for the bind with the lowest score
290 -- to pick as the loop breaker. The rest accumulate in
291 choose_loop_breaker (details,_,_) loop_sc acc []
292 = (details, acc) -- Done
294 choose_loop_breaker loop_bind loop_sc acc (bind : binds)
295 | sc < loop_sc -- Lower score so pick this new one
296 = choose_loop_breaker bind sc (loop_bind : acc) binds
298 | otherwise -- No lower so don't pick it
299 = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
303 score :: Node Details -> Int -- Higher score => less likely to be picked as loop breaker
304 score ((bndr, _, rhs), _, _)
305 | exprIsTrivial rhs = 4 -- Practically certain to be inlined
306 -- Used to have also: && not (isExportedId bndr)
307 -- But I found this sometimes cost an extra iteration when we have
308 -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
309 -- where df is the exported dictionary. Then df makes a really
310 -- bad choice for loop breaker
312 | idHasRules bndr = 3
313 -- Avoid things with specialisations; we'd like
314 -- to take advantage of them in the subsequent bindings
315 -- Also vital to avoid risk of divergence:
316 -- Note [Recursive rules]
318 | inlineCandidate bndr rhs = 2 -- Likely to be inlined
319 -- Note [Inline candidates]
321 | is_con_app rhs = 1 -- Data types help with cases
325 inlineCandidate :: Id -> CoreExpr -> Bool
326 inlineCandidate id (Note InlineMe _) = True
327 inlineCandidate id rhs = isOneOcc (idOccInfo id)
329 -- Real example (the Enum Ordering instance from PrelBase):
330 -- rec f = \ x -> case d of (p,q,r) -> p x
331 -- g = \ x -> case d of (p,q,r) -> q x
334 -- Here, f and g occur just once; but we can't inline them into d.
335 -- On the other hand we *could* simplify those case expressions if
336 -- we didn't stupidly choose d as the loop breaker.
337 -- But we won't because constructor args are marked "Many".
339 -- Cheap and cheerful; the simplifer moves casts out of the way
340 -- The lambda case is important to spot x = /\a. C (f a)
341 -- which comes up when C is a dictionary constructor and
342 -- f is a default method.
343 -- Example: the instance for Show (ST s a) in GHC.ST
345 -- However we *also* treat (\x. C p q) as a con-app-like thing,
346 -- Note [Closure conversion]
347 is_con_app (Var v) = isDataConWorkId v
348 is_con_app (App f _) = is_con_app f
349 is_con_app (Lam b e) = is_con_app e
350 is_con_app (Note _ e) = is_con_app e
351 is_con_app other = False
353 makeLoopBreaker :: VarSet -- Binders of this group
354 -> UsageDetails -- Usage of this rhs (neglecting rules)
356 -- Set the loop-breaker flag, recording whether the thing occurs only in
357 -- the RHS of a RULE (in this recursive group)
358 makeLoopBreaker bndrs rhs_usg bndr
359 = setIdOccInfo bndr (IAmALoopBreaker rules_only)
361 rules_only = bndrs `intersectsUFM` rhs_usg
364 Note [Inline candidates]
365 ~~~~~~~~~~~~~~~~~~~~~~~~
366 At one point I gave is_con_app a higher score than inline-candidate,
367 on the grounds that "it's *really* helpful if dictionaries get inlined fast".
368 However a nofib run revealed no change if they were swapped so that
369 inline-candidate has the higher score. And it's important that it does,
370 else you can get a bad worker-wrapper split thus:
372 $wfoo x = ....foo x....
374 {-loop brk-} foo x = ...$wfoo x...
376 But we *want* the wrapper to be inlined! If it isn't, the interface
377 file sees the unfolding for $wfoo, and sees that foo is strict (and
378 hence it gets an auto-generated wrapper. Result: an infinite inlining
379 in the importing scope. So be a bit careful if you change this. A
380 good example is Tree.repTree in nofib/spectral/minimax. If is_con_app
381 has the higher score, then compiling Game.hs goes into an infinite loop.
383 Note [Recursive rules]
384 ~~~~~~~~~~~~~~~~~~~~~~
385 Consider this group, which is typical of what SpecConstr builds:
387 fs a = ....f (C a)....
388 f x = ....f (C a)....
389 {-# RULE f (C a) = fs a #-}
391 So 'f' and 'fs' are mutually recursive. If we choose 'fs' as the loop breaker,
392 all is well; the RULE is applied, and 'fs' becomes self-recursive.
394 But if we choose 'f' as the loop breaker, we may get an infinite loop:
395 - the RULE is applied in f's RHS (see Note [Self-recursive rules] in Simplify
396 - fs is inlined (say it's small)
397 - now there's another opportunity to apply the RULE
399 So it's very important to choose the RULE-variable as the loop breaker.
400 This showed up when compiling Control.Concurrent.Chan.getChanContents.
402 Note [Closure conversion]
403 ~~~~~~~~~~~~~~~~~~~~~~~~~
404 We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm.
405 The immediate motivation came from the result of a closure-conversion transformation
406 which generated code like this:
408 data Clo a b = forall c. Clo (c -> a -> b) c
410 ($:) :: Clo a b -> a -> b
411 Clo f env $: x = f env x
413 rec { plus = Clo plus1 ()
415 ; plus1 _ n = Clo plus2 n
418 ; plus2 (Succ m) n = Succ (plus $: m $: n) }
420 If we inline 'plus' and 'plus1', everything unravels nicely. But if
421 we choose 'plus1' as the loop breaker (which is entirely possible
422 otherwise), the loop does not unravel nicely.
425 @occAnalRhs@ deals with the question of bindings where the Id is marked
426 by an INLINE pragma. For these we record that anything which occurs
427 in its RHS occurs many times. This pessimistically assumes that ths
428 inlined binder also occurs many times in its scope, but if it doesn't
429 we'll catch it next time round. At worst this costs an extra simplifier pass.
430 ToDo: try using the occurrence info for the inline'd binder.
432 [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec.
433 [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec.
438 -> Id -> CoreExpr -- Binder and rhs
439 -- For non-recs the binder is alrady tagged
440 -- with occurrence info
441 -> (UsageDetails, CoreExpr)
443 occAnalRhs env id rhs
446 ctxt | certainly_inline id = env
447 | otherwise = rhsCtxt
448 -- Note that we generally use an rhsCtxt. This tells the occ anal n
449 -- that it's looking at an RHS, which has an effect in occAnalApp
451 -- But there's a problem. Consider
456 -- First time round, it looks as if x1 and x2 occur as an arg of a
457 -- let-bound constructor ==> give them a many-occurrence.
458 -- But then x3 is inlined (unconditionally as it happens) and
459 -- next time round, x2 will be, and the next time round x1 will be
460 -- Result: multiple simplifier iterations. Sigh.
461 -- Crude solution: use rhsCtxt for things that occur just once...
463 certainly_inline id = case idOccInfo id of
464 OneOcc in_lam one_br _ -> not in_lam && one_br
470 If the binder has RULES inside it then we count the specialised Ids as
471 "extra rhs's". That way the "parent" keeps the specialised "children"
472 alive. If the parent dies (because it isn't referenced any more),
473 then the children will die too unless they are already referenced
476 That's the basic idea. However in a recursive situation we want to be a bit
477 cleverer. Example (from GHC.Enum):
479 eftInt :: Int# -> Int# -> [Int]
480 eftInt x y = ...(non-recursive)...
482 {-# INLINE [0] eftIntFB #-}
483 eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
484 eftIntFB c n x y = ...(non-recursive)...
487 "eftInt" [~1] forall x y. eftInt x y = build (\ c n -> eftIntFB c n x y)
488 "eftIntList" [1] eftIntFB (:) [] = eftInt
491 The two look mutually recursive only because of their RULES; we don't want
492 that to inhibit inlining!
494 So when we identify a LoopBreaker, we mark it to say whether it only mentions
495 the other binders in its recursive group in a RULE. If so, we can inline it,
496 because doing so will not expose new occurrences of binders in its group.
501 addRuleUsage :: UsageDetails -> Id -> UsageDetails
502 -- Add the usage from RULES in Id to the usage
503 addRuleUsage usage id
504 = foldVarSet add usage (idRuleVars id)
506 add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
507 -- (i.e manyOcc) because many copies
508 -- of the specialised thing can appear
516 -> (UsageDetails, -- Gives info only about the "interesting" Ids
519 occAnal env (Type t) = (emptyDetails, Type t)
520 occAnal env (Var v) = (mkOneOcc env v False, Var v)
521 -- At one stage, I gathered the idRuleVars for v here too,
522 -- which in a way is the right thing to do.
523 -- Btu that went wrong right after specialisation, when
524 -- the *occurrences* of the overloaded function didn't have any
525 -- rules in them, so the *specialised* versions looked as if they
526 -- weren't used at all.
529 We regard variables that occur as constructor arguments as "dangerousToDup":
533 f x = let y = expensive x in
535 (case z of {(p,q)->q}, case z of {(p,q)->q})
538 We feel free to duplicate the WHNF (True,y), but that means
539 that y may be duplicated thereby.
541 If we aren't careful we duplicate the (expensive x) call!
542 Constructors are rather like lambdas in this way.
545 occAnal env expr@(Lit lit) = (emptyDetails, expr)
549 occAnal env (Note InlineMe body)
550 = case occAnal env body of { (usage, body') ->
551 (mapVarEnv markMany usage, Note InlineMe body')
554 occAnal env (Note note@(SCC cc) body)
555 = case occAnal env body of { (usage, body') ->
556 (mapVarEnv markInsideSCC usage, Note note body')
559 occAnal env (Note note body)
560 = case occAnal env body of { (usage, body') ->
561 (usage, Note note body')
564 occAnal env (Cast expr co)
565 = case occAnal env expr of { (usage, expr') ->
566 (markRhsUds env True usage, Cast expr' co)
567 -- If we see let x = y `cast` co
568 -- then mark y as 'Many' so that we don't
569 -- immediately inline y again.
574 occAnal env app@(App fun arg)
575 = occAnalApp env (collectArgs app) False
577 -- Ignore type variables altogether
578 -- (a) occurrences inside type lambdas only not marked as InsideLam
579 -- (b) type variables not in environment
581 occAnal env expr@(Lam x body) | isTyVar x
582 = case occAnal env body of { (body_usage, body') ->
583 (body_usage, Lam x body')
586 -- For value lambdas we do a special hack. Consider
588 -- If we did nothing, x is used inside the \y, so would be marked
589 -- as dangerous to dup. But in the common case where the abstraction
590 -- is applied to two arguments this is over-pessimistic.
591 -- So instead, we just mark each binder with its occurrence
592 -- info in the *body* of the multiple lambda.
593 -- Then, the simplifier is careful when partially applying lambdas.
595 occAnal env expr@(Lam _ _)
596 = case occAnal env_body body of { (body_usage, body') ->
598 (final_usage, tagged_binders) = tagBinders body_usage binders
599 -- URGH! Sept 99: we don't seem to be able to use binders' here, because
600 -- we get linear-typed things in the resulting program that we can't handle yet.
601 -- (e.g. PrelShow) TODO
603 really_final_usage = if linear then
606 mapVarEnv markInsideLam final_usage
609 mkLams tagged_binders body') }
611 env_body = vanillaCtxt -- Body is (no longer) an RhsContext
612 (binders, body) = collectBinders expr
613 binders' = oneShotGroup env binders
614 linear = all is_one_shot binders'
615 is_one_shot b = isId b && isOneShotBndr b
617 occAnal env (Case scrut bndr ty alts)
618 = case occ_anal_scrut scrut alts of { (scrut_usage, scrut') ->
619 case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
621 alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
622 alts_usage' = addCaseBndrUsage alts_usage
623 (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
624 total_usage = scrut_usage +++ alts_usage1
626 total_usage `seq` (total_usage, Case scrut' tagged_bndr ty alts') }}
628 -- The case binder gets a usage of either "many" or "dead", never "one".
629 -- Reason: we like to inline single occurrences, to eliminate a binding,
630 -- but inlining a case binder *doesn't* eliminate a binding.
631 -- We *don't* want to transform
632 -- case x of w { (p,q) -> f w }
634 -- case x of w { (p,q) -> f (p,q) }
635 addCaseBndrUsage usage = case lookupVarEnv usage bndr of
637 Just occ -> extendVarEnv usage bndr (markMany occ)
639 alt_env = setVanillaCtxt env
640 -- Consider x = case v of { True -> (p,q); ... }
641 -- Then it's fine to inline p and q
643 occ_anal_scrut (Var v) (alt1 : other_alts)
644 | not (null other_alts) || not (isDefaultAlt alt1)
645 = (mkOneOcc env v True, Var v)
646 occ_anal_scrut scrut alts = occAnal vanillaCtxt scrut
647 -- No need for rhsCtxt
649 occAnal env (Let bind body)
650 = case occAnal env body of { (body_usage, body') ->
651 case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
652 (final_usage, mkLets new_binds body') }}
655 = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
656 (foldr (+++) emptyDetails arg_uds_s, args')}
658 arg_env = vanillaCtxt
661 Applications are dealt with specially because we want
662 the "build hack" to work.
665 occAnalApp env (Var fun, args) is_rhs
666 = case args_stuff of { (args_uds, args') ->
668 final_args_uds = markRhsUds env is_pap args_uds
670 (fun_uds +++ final_args_uds, mkApps (Var fun) args') }
672 fun_uniq = idUnique fun
673 fun_uds = mkOneOcc env fun (valArgCount args > 0)
674 is_pap = isDataConWorkId fun || valArgCount args < idArity fun
676 -- Hack for build, fold, runST
677 args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
678 | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
679 | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
680 | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
681 -- (foldr k z xs) may call k many times, but it never
682 -- shares a partial application of k; hence [False,True]
683 -- This means we can optimise
684 -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
685 -- by floating in the v
687 | otherwise = occAnalArgs env args
690 occAnalApp env (fun, args) is_rhs
691 = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
692 -- The addAppCtxt is a bit cunning. One iteration of the simplifier
693 -- often leaves behind beta redexs like
695 -- Here we would like to mark x,y as one-shot, and treat the whole
696 -- thing much like a let. We do this by pushing some True items
697 -- onto the context stack.
699 case occAnalArgs env args of { (args_uds, args') ->
701 final_uds = fun_uds +++ args_uds
703 (final_uds, mkApps fun' args') }}
706 markRhsUds :: OccEnv -- Check if this is a RhsEnv
707 -> Bool -- and this is true
708 -> UsageDetails -- The do markMany on this
710 -- We mark the free vars of the argument of a constructor or PAP
711 -- as "many", if it is the RHS of a let(rec).
712 -- This means that nothing gets inlined into a constructor argument
713 -- position, which is what we want. Typically those constructor
714 -- arguments are just variables, or trivial expressions.
716 -- This is the *whole point* of the isRhsEnv predicate
717 markRhsUds env is_pap arg_uds
718 | isRhsEnv env && is_pap = mapVarEnv markMany arg_uds
719 | otherwise = arg_uds
723 -> Int -> CtxtTy -- Argument number, and context to use for it
725 -> (UsageDetails, [CoreExpr])
726 appSpecial env n ctxt args
729 arg_env = vanillaCtxt
731 go n [] = (emptyDetails, []) -- Too few args
733 go 1 (arg:args) -- The magic arg
734 = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
735 case occAnalArgs env args of { (args_uds, args') ->
736 (arg_uds +++ args_uds, arg':args') }}
739 = case occAnal arg_env arg of { (arg_uds, arg') ->
740 case go (n-1) args of { (args_uds, args') ->
741 (arg_uds +++ args_uds, arg':args') }}
747 If the case binder occurs at all, the other binders effectively do too.
749 case e of x { (a,b) -> rhs }
752 If e turns out to be (e1,e2) we indeed get something like
753 let a = e1; b = e2; x = (a,b) in rhs
755 Note [Aug 06]: I don't think this is necessary any more, and it helpe
756 to know when binders are unused. See esp the call to
757 isDeadBinder in Simplify.mkDupableAlt
760 occAnalAlt env case_bndr (con, bndrs, rhs)
761 = case occAnal env rhs of { (rhs_usage, rhs') ->
763 (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
764 final_bndrs = tagged_bndrs -- See Note [Aug06] above
766 final_bndrs | case_bndr `elemVarEnv` final_usage = bndrs
767 | otherwise = tagged_bndrs
768 -- Leave the binders untagged if the case
769 -- binder occurs at all; see note above
772 (final_usage, (con, final_bndrs, rhs')) }
776 %************************************************************************
778 \subsection[OccurAnal-types]{OccEnv}
780 %************************************************************************
784 = OccEnv OccEncl -- Enclosing context information
785 CtxtTy -- Tells about linearity
787 -- OccEncl is used to control whether to inline into constructor arguments
789 -- x = (p,q) -- Don't inline p or q
790 -- y = /\a -> (p a, q a) -- Still don't inline p or q
791 -- z = f (p,q) -- Do inline p,q; it may make a rule fire
792 -- So OccEncl tells enought about the context to know what to do when
793 -- we encounter a contructor application or PAP.
796 = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
797 -- Don't inline into constructor args here
798 | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
799 -- Do inline into constructor args here
804 -- True:ctxt Analysing a function-valued expression that will be
807 -- False:ctxt Analysing a function-valued expression that may
808 -- be applied many times; but when it is,
809 -- the CtxtTy inside applies
812 initOccEnv = OccEnv OccRhs []
814 vanillaCtxt = OccEnv OccVanilla []
815 rhsCtxt = OccEnv OccRhs []
817 isRhsEnv (OccEnv OccRhs _) = True
818 isRhsEnv (OccEnv OccVanilla _) = False
820 setVanillaCtxt :: OccEnv -> OccEnv
821 setVanillaCtxt (OccEnv OccRhs ctxt_ty) = OccEnv OccVanilla ctxt_ty
822 setVanillaCtxt other_env = other_env
824 setCtxt :: OccEnv -> CtxtTy -> OccEnv
825 setCtxt (OccEnv encl _) ctxt = OccEnv encl ctxt
827 oneShotGroup :: OccEnv -> [CoreBndr] -> [CoreBndr]
828 -- The result binders have one-shot-ness set that they might not have had originally.
829 -- This happens in (build (\cn -> e)). Here the occurrence analyser
830 -- linearity context knows that c,n are one-shot, and it records that fact in
831 -- the binder. This is useful to guide subsequent float-in/float-out tranformations
833 oneShotGroup (OccEnv encl ctxt) bndrs
836 go ctxt [] rev_bndrs = reverse rev_bndrs
838 go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
839 | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
841 bndr' | lin_ctxt = setOneShotLambda bndr
844 go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
846 addAppCtxt (OccEnv encl ctxt) args
847 = OccEnv encl (replicate (valArgCount args) True ++ ctxt)
850 %************************************************************************
852 \subsection[OccurAnal-types]{OccEnv}
854 %************************************************************************
857 type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
859 (+++), combineAltsUsageDetails
860 :: UsageDetails -> UsageDetails -> UsageDetails
863 = plusVarEnv_C addOccInfo usage1 usage2
865 combineAltsUsageDetails usage1 usage2
866 = plusVarEnv_C orOccInfo usage1 usage2
868 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
869 addOneOcc usage id info
870 = plusVarEnv_C addOccInfo usage (unitVarEnv id info)
871 -- ToDo: make this more efficient
873 emptyDetails = (emptyVarEnv :: UsageDetails)
875 usedIn :: Id -> UsageDetails -> Bool
876 v `usedIn` details = isExportedId v || v `elemVarEnv` details
878 type IdWithOccInfo = Id
880 tagBinders :: UsageDetails -- Of scope
882 -> (UsageDetails, -- Details with binders removed
883 [IdWithOccInfo]) -- Tagged binders
885 tagBinders usage binders
887 usage' = usage `delVarEnvList` binders
888 uss = map (setBinderOcc usage) binders
890 usage' `seq` (usage', uss)
892 tagBinder :: UsageDetails -- Of scope
894 -> (UsageDetails, -- Details with binders removed
895 IdWithOccInfo) -- Tagged binders
897 tagBinder usage binder
899 usage' = usage `delVarEnv` binder
900 binder' = setBinderOcc usage binder
902 usage' `seq` (usage', binder')
904 setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
905 setBinderOcc usage bndr
906 | isTyVar bndr = bndr
907 | isExportedId bndr = case idOccInfo bndr of
909 other -> setIdOccInfo bndr NoOccInfo
910 -- Don't use local usage info for visible-elsewhere things
911 -- BUT *do* erase any IAmALoopBreaker annotation, because we're
912 -- about to re-generate it and it shouldn't be "sticky"
914 | otherwise = setIdOccInfo bndr occ_info
916 occ_info = lookupVarEnv usage bndr `orElse` IAmDead
920 %************************************************************************
922 \subsection{Operations over OccInfo}
924 %************************************************************************
927 mkOneOcc :: OccEnv -> Id -> InterestingCxt -> UsageDetails
928 mkOneOcc env id int_cxt
929 | isLocalId id = unitVarEnv id (OneOcc False True int_cxt)
930 | otherwise = emptyDetails
932 markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
934 markMany IAmDead = IAmDead
935 markMany other = NoOccInfo
937 markInsideSCC occ = markMany occ
939 markInsideLam (OneOcc _ one_br int_cxt) = OneOcc True one_br int_cxt
940 markInsideLam occ = occ
942 addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
944 addOccInfo IAmDead info2 = info2
945 addOccInfo info1 IAmDead = info1
946 addOccInfo info1 info2 = NoOccInfo
948 -- (orOccInfo orig new) is used
949 -- when combining occurrence info from branches of a case
951 orOccInfo IAmDead info2 = info2
952 orOccInfo info1 IAmDead = info1
953 orOccInfo (OneOcc in_lam1 one_branch1 int_cxt1)
954 (OneOcc in_lam2 one_branch2 int_cxt2)
955 = OneOcc (in_lam1 || in_lam2)
956 False -- False, because it occurs in both branches
957 (int_cxt1 && int_cxt2)
958 orOccInfo info1 info2 = NoOccInfo