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 occurAnalysePgm, occurAnalyseExpr
18 #include "HsVersions.h"
21 import CoreFVs ( idRuleVars )
22 import CoreUtils ( exprIsTrivial, isDefaultAlt )
23 import Id ( isDataConWorkId, isOneShotBndr, setOneShotLambda,
24 idOccInfo, setIdOccInfo, isLocalId,
25 isExportedId, idArity, idHasRules,
28 import BasicTypes ( OccInfo(..), isOneOcc, InterestingCxt )
33 import Maybes ( orElse )
34 import Digraph ( stronglyConnCompR, SCC(..) )
35 import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
36 import Unique ( Unique )
37 import UniqFM ( keysUFM, intersectsUFM )
38 import Util ( mapAndUnzip )
45 %************************************************************************
47 \subsection[OccurAnal-main]{Counting occurrences: main function}
49 %************************************************************************
51 Here's the externally-callable interface:
54 occurAnalysePgm :: [CoreBind] -> [CoreBind]
56 = snd (go initOccEnv binds)
58 go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind])
62 = (final_usage, bind' ++ binds')
64 (bs_usage, binds') = go env binds
65 (final_usage, bind') = occAnalBind env bind bs_usage
67 occurAnalyseExpr :: CoreExpr -> CoreExpr
68 -- Do occurrence analysis, and discard occurence info returned
69 occurAnalyseExpr expr = snd (occAnal initOccEnv expr)
73 %************************************************************************
75 \subsection[OccurAnal-main]{Counting occurrences: main function}
77 %************************************************************************
85 -> UsageDetails -- Usage details of scope
86 -> (UsageDetails, -- Of the whole let(rec)
89 occAnalBind env (NonRec binder rhs) body_usage
90 | not (binder `usedIn` body_usage) -- It's not mentioned
93 | otherwise -- It's mentioned in the body
94 = (body_usage' +++ addRuleUsage rhs_usage binder, -- Note [RulesOnly]
95 [NonRec tagged_binder rhs'])
97 (body_usage', tagged_binder) = tagBinder body_usage binder
98 (rhs_usage, rhs') = occAnalRhs env tagged_binder rhs
101 Dropping dead code for recursive bindings is done in a very simple way:
103 the entire set of bindings is dropped if none of its binders are
104 mentioned in its body; otherwise none are.
106 This seems to miss an obvious improvement.
121 Now @f@ is unused. But dependency analysis will sort this out into a
122 @letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
123 It isn't easy to do a perfect job in one blow. Consider
137 occAnalBind env (Rec pairs) body_usage
138 = foldr (_scc_ "occAnalBind.dofinal" do_final_bind) (body_usage, []) sccs
140 analysed_pairs :: [Details]
141 analysed_pairs = [ (bndr, rhs_usage, rhs')
142 | (bndr, rhs) <- pairs,
143 let (rhs_usage, rhs') = occAnalRhs env bndr rhs
146 sccs :: [SCC (Node Details)]
147 sccs = _scc_ "occAnalBind.scc" stronglyConnCompR edges
150 ---- stuff for dependency analysis of binds -------------------------------
151 edges :: [Node Details]
152 edges = _scc_ "occAnalBind.assoc"
153 [ (details, idUnique id, edges_from id rhs_usage)
154 | details@(id, rhs_usage, rhs) <- analysed_pairs
157 -- (a -> b) means a mentions b
158 -- Given the usage details (a UFM that gives occ info for each free var of
159 -- the RHS) we can get the list of free vars -- or rather their Int keys --
160 -- by just extracting the keys from the finite map. Grimy, but fast.
161 -- Previously we had this:
162 -- [ bndr | bndr <- bndrs,
163 -- maybeToBool (lookupVarEnv rhs_usage bndr)]
164 -- which has n**2 cost, and this meant that edges_from alone
165 -- consumed 10% of total runtime!
166 edges_from :: Id -> UsageDetails -> [Unique]
167 edges_from bndr rhs_usage = _scc_ "occAnalBind.edges_from"
168 keysUFM (addRuleUsage rhs_usage bndr)
170 ---- Stuff to "re-constitute" bindings from dependency-analysis info ------
173 do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
174 | not (bndr `usedIn` body_usage)
175 = (body_usage, binds_so_far) -- Dead code
177 = (body_usage' +++ addRuleUsage rhs_usage bndr, new_bind : binds_so_far)
179 (body_usage', tagged_bndr) = tagBinder body_usage bndr
180 new_bind = NonRec tagged_bndr rhs'
183 do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
184 | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
185 = (body_usage, binds_so_far) -- Dead code
186 | otherwise -- If any is used, they all are
187 = (final_usage, final_bind : binds_so_far)
189 details = [details | (details, _, _) <- cycle]
190 bndrs = [bndr | (bndr, _, _) <- details]
191 bndr_usages = [addRuleUsage rhs_usage bndr | (bndr, rhs_usage, _) <- details]
192 total_usage = foldr (+++) body_usage bndr_usages
193 (final_usage, tagged_cycle) = mapAccumL tag_bind total_usage cycle
194 tag_bind usg ((bndr,rhs_usg,rhs),k,ks) = (usg', ((bndr',rhs_usg,rhs),k,ks))
196 (usg', bndr') = tagBinder usg bndr
197 final_bind = Rec (reOrderCycle (mkVarSet bndrs) tagged_cycle)
199 {- An alternative; rebuild the edges. No semantic difference, but perf might change
201 -- Hopefully 'bndrs' is a relatively small group now
202 -- Now get ready for the loop-breaking phase
203 -- We've done dead-code elimination already, so no worries about un-referenced binders
204 keys = map idUnique bndrs
205 mk_node tagged_bndr (_, rhs_usage, rhs')
206 = ((tagged_bndr, rhs'), idUnique tagged_bndr, used)
208 used = [key | key <- keys, used_outside_rule rhs_usage key ]
210 used_outside_rule usage uniq = case lookupUFM_Directly usage uniq of
212 Just RulesOnly -> False -- Ignore rules
217 @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
218 strongly connected component (there's guaranteed to be a cycle). It returns the
220 a) in a better order,
221 b) with some of the Ids having a IAmALoopBreaker pragma
223 The "loop-breaker" Ids are sufficient to break all cycles in the SCC. This means
224 that the simplifier can guarantee not to loop provided it never records an inlining
225 for these no-inline guys.
227 Furthermore, the order of the binds is such that if we neglect dependencies
228 on the no-inline Ids then the binds are topologically sorted. This means
229 that the simplifier will generally do a good job if it works from top bottom,
230 recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
233 [June 98: I don't understand the following paragraphs, and I've
234 changed the a=b case again so that it isn't a special case any more.]
236 Here's a case that bit me:
244 Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
246 My solution was to make a=b bindings record b as Many, rather like INLINE bindings.
247 Perhaps something cleverer would suffice.
252 type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
253 -- which is gotten from the Id.
254 type Details = (Id, UsageDetails, CoreExpr)
256 reOrderRec :: IdSet -- Binders of this group
257 -> SCC (Node Details)
259 -- Sorted into a plausible order. Enough of the Ids have
260 -- IAmALoopBreaker pragmas that there are no loops left.
261 reOrderRec bndrs (AcyclicSCC ((bndr, _, rhs), _, _)) = [(bndr, rhs)]
262 reOrderRec bndrs (CyclicSCC cycle) = reOrderCycle bndrs cycle
264 reOrderCycle :: IdSet -> [Node Details] -> [(Id,CoreExpr)]
265 reOrderCycle bndrs []
266 = panic "reOrderCycle"
267 reOrderCycle bndrs [bind] -- Common case of simple self-recursion
268 = [(makeLoopBreaker bndrs rhs_usg bndr, rhs)]
270 ((bndr, rhs_usg, rhs), _, _) = bind
272 reOrderCycle bndrs (bind : binds)
273 = -- Choose a loop breaker, mark it no-inline,
274 -- do SCC analysis on the rest, and recursively sort them out
275 concatMap (reOrderRec bndrs) (stronglyConnCompR unchosen) ++
276 [(makeLoopBreaker bndrs rhs_usg bndr, rhs)]
279 (chosen_bind, unchosen) = choose_loop_breaker bind (score bind) [] binds
280 (bndr, rhs_usg, rhs) = chosen_bind
282 -- This loop looks for the bind with the lowest score
283 -- to pick as the loop breaker. The rest accumulate in
284 choose_loop_breaker (details,_,_) loop_sc acc []
285 = (details, acc) -- Done
287 choose_loop_breaker loop_bind loop_sc acc (bind : binds)
288 | sc < loop_sc -- Lower score so pick this new one
289 = choose_loop_breaker bind sc (loop_bind : acc) binds
291 | otherwise -- No lower so don't pick it
292 = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
296 score :: Node Details -> Int -- Higher score => less likely to be picked as loop breaker
297 score ((bndr, _, rhs), _, _)
298 | exprIsTrivial rhs = 4 -- Practically certain to be inlined
299 -- Used to have also: && not (isExportedId bndr)
300 -- But I found this sometimes cost an extra iteration when we have
301 -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
302 -- where df is the exported dictionary. Then df makes a really
303 -- bad choice for loop breaker
305 | idHasRules bndr = 3
306 -- Avoid things with specialisations; we'd like
307 -- to take advantage of them in the subsequent bindings
308 -- Also vital to avoid risk of divergence:
309 -- Note [Recursive rules]
311 | inlineCandidate bndr rhs = 2 -- Likely to be inlined
312 -- Note [Inline candidates]
314 | is_con_app rhs = 1 -- Data types help with cases
318 inlineCandidate :: Id -> CoreExpr -> Bool
319 inlineCandidate id (Note InlineMe _) = True
320 inlineCandidate id rhs = isOneOcc (idOccInfo id)
322 -- Real example (the Enum Ordering instance from PrelBase):
323 -- rec f = \ x -> case d of (p,q,r) -> p x
324 -- g = \ x -> case d of (p,q,r) -> q x
327 -- Here, f and g occur just once; but we can't inline them into d.
328 -- On the other hand we *could* simplify those case expressions if
329 -- we didn't stupidly choose d as the loop breaker.
330 -- But we won't because constructor args are marked "Many".
332 -- Cheap and cheerful; the simplifer moves casts out of the way
333 -- The lambda case is important to spot x = /\a. C (f a)
334 -- which comes up when C is a dictionary constructor and
335 -- f is a default method.
336 -- Example: the instance for Show (ST s a) in GHC.ST
338 -- However we *also* treat (\x. C p q) as a con-app-like thing,
339 -- Note [Closure conversion]
340 is_con_app (Var v) = isDataConWorkId v
341 is_con_app (App f _) = is_con_app f
342 is_con_app (Lam b e) = is_con_app e
343 is_con_app (Note _ e) = is_con_app e
344 is_con_app other = False
346 makeLoopBreaker :: VarSet -- Binders of this group
347 -> UsageDetails -- Usage of this rhs (neglecting rules)
349 -- Set the loop-breaker flag, recording whether the thing occurs only in
350 -- the RHS of a RULE (in this recursive group)
351 makeLoopBreaker bndrs rhs_usg bndr
352 = setIdOccInfo bndr (IAmALoopBreaker rules_only)
354 rules_only = bndrs `intersectsUFM` rhs_usg
357 Note [Inline candidates]
358 ~~~~~~~~~~~~~~~~~~~~~~~~
359 At one point I gave is_con_app a higher score than inline-candidate,
360 on the grounds that "it's *really* helpful if dictionaries get inlined fast".
361 However a nofib run revealed no change if they were swapped so that
362 inline-candidate has the higher score. And it's important that it does,
363 else you can get a bad worker-wrapper split thus:
365 $wfoo x = ....foo x....
367 {-loop brk-} foo x = ...$wfoo x...
369 But we *want* the wrapper to be inlined! If it isn't, the interface
370 file sees the unfolding for $wfoo, and sees that foo is strict (and
371 hence it gets an auto-generated wrapper. Result: an infinite inlining
372 in the importing scope. So be a bit careful if you change this. A
373 good example is Tree.repTree in nofib/spectral/minimax. If is_con_app
374 has the higher score, then compiling Game.hs goes into an infinite loop.
376 Note [Recursive rules]
377 ~~~~~~~~~~~~~~~~~~~~~~
378 Consider this group, which is typical of what SpecConstr builds:
380 fs a = ....f (C a)....
381 f x = ....f (C a)....
382 {-# RULE f (C a) = fs a #-}
384 So 'f' and 'fs' are mutually recursive. If we choose 'fs' as the loop breaker,
385 all is well; the RULE is applied, and 'fs' becomes self-recursive.
387 But if we choose 'f' as the loop breaker, we may get an infinite loop:
388 - the RULE is applied in f's RHS (see Note [Self-recursive rules] in Simplify
389 - fs is inlined (say it's small)
390 - now there's another opportunity to apply the RULE
392 So it's very important to choose the RULE-variable as the loop breaker.
393 This showed up when compiling Control.Concurrent.Chan.getChanContents.
395 Note [Closure conversion]
396 ~~~~~~~~~~~~~~~~~~~~~~~~~
397 We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm.
398 The immediate motivation came from the result of a closure-conversion transformation
399 which generated code like this:
401 data Clo a b = forall c. Clo (c -> a -> b) c
403 ($:) :: Clo a b -> a -> b
404 Clo f env $: x = f env x
406 rec { plus = Clo plus1 ()
408 ; plus1 _ n = Clo plus2 n
411 ; plus2 (Succ m) n = Succ (plus $: m $: n) }
413 If we inline 'plus' and 'plus1', everything unravels nicely. But if
414 we choose 'plus1' as the loop breaker (which is entirely possible
415 otherwise), the loop does not unravel nicely.
418 @occAnalRhs@ deals with the question of bindings where the Id is marked
419 by an INLINE pragma. For these we record that anything which occurs
420 in its RHS occurs many times. This pessimistically assumes that ths
421 inlined binder also occurs many times in its scope, but if it doesn't
422 we'll catch it next time round. At worst this costs an extra simplifier pass.
423 ToDo: try using the occurrence info for the inline'd binder.
425 [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec.
426 [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec.
431 -> Id -> CoreExpr -- Binder and rhs
432 -- For non-recs the binder is alrady tagged
433 -- with occurrence info
434 -> (UsageDetails, CoreExpr)
436 occAnalRhs env id rhs
439 ctxt | certainly_inline id = env
440 | otherwise = rhsCtxt
441 -- Note that we generally use an rhsCtxt. This tells the occ anal n
442 -- that it's looking at an RHS, which has an effect in occAnalApp
444 -- But there's a problem. Consider
449 -- First time round, it looks as if x1 and x2 occur as an arg of a
450 -- let-bound constructor ==> give them a many-occurrence.
451 -- But then x3 is inlined (unconditionally as it happens) and
452 -- next time round, x2 will be, and the next time round x1 will be
453 -- Result: multiple simplifier iterations. Sigh.
454 -- Crude solution: use rhsCtxt for things that occur just once...
456 certainly_inline id = case idOccInfo id of
457 OneOcc in_lam one_br _ -> not in_lam && one_br
463 If the binder has RULES inside it then we count the specialised Ids as
464 "extra rhs's". That way the "parent" keeps the specialised "children"
465 alive. If the parent dies (because it isn't referenced any more),
466 then the children will die too unless they are already referenced
469 That's the basic idea. However in a recursive situation we want to be a bit
470 cleverer. Example (from GHC.Enum):
472 eftInt :: Int# -> Int# -> [Int]
473 eftInt x y = ...(non-recursive)...
475 {-# INLINE [0] eftIntFB #-}
476 eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
477 eftIntFB c n x y = ...(non-recursive)...
480 "eftInt" [~1] forall x y. eftInt x y = build (\ c n -> eftIntFB c n x y)
481 "eftIntList" [1] eftIntFB (:) [] = eftInt
484 The two look mutually recursive only because of their RULES; we don't want
485 that to inhibit inlining!
487 So when we identify a LoopBreaker, we mark it to say whether it only mentions
488 the other binders in its recursive group in a RULE. If so, we can inline it,
489 because doing so will not expose new occurrences of binders in its group.
494 addRuleUsage :: UsageDetails -> Id -> UsageDetails
495 -- Add the usage from RULES in Id to the usage
496 addRuleUsage usage id
497 = foldVarSet add usage (idRuleVars id)
499 add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
500 -- (i.e manyOcc) because many copies
501 -- of the specialised thing can appear
509 -> (UsageDetails, -- Gives info only about the "interesting" Ids
512 occAnal env (Type t) = (emptyDetails, Type t)
513 occAnal env (Var v) = (mkOneOcc env v False, Var v)
514 -- At one stage, I gathered the idRuleVars for v here too,
515 -- which in a way is the right thing to do.
516 -- Btu that went wrong right after specialisation, when
517 -- the *occurrences* of the overloaded function didn't have any
518 -- rules in them, so the *specialised* versions looked as if they
519 -- weren't used at all.
522 We regard variables that occur as constructor arguments as "dangerousToDup":
526 f x = let y = expensive x in
528 (case z of {(p,q)->q}, case z of {(p,q)->q})
531 We feel free to duplicate the WHNF (True,y), but that means
532 that y may be duplicated thereby.
534 If we aren't careful we duplicate the (expensive x) call!
535 Constructors are rather like lambdas in this way.
538 occAnal env expr@(Lit lit) = (emptyDetails, expr)
542 occAnal env (Note InlineMe body)
543 = case occAnal env body of { (usage, body') ->
544 (mapVarEnv markMany usage, Note InlineMe body')
547 occAnal env (Note note@(SCC cc) body)
548 = case occAnal env body of { (usage, body') ->
549 (mapVarEnv markInsideSCC usage, Note note body')
552 occAnal env (Note note body)
553 = case occAnal env body of { (usage, body') ->
554 (usage, Note note body')
557 occAnal env (Cast expr co)
558 = case occAnal env expr of { (usage, expr') ->
559 (markRhsUds env True usage, Cast expr' co)
560 -- If we see let x = y `cast` co
561 -- then mark y as 'Many' so that we don't
562 -- immediately inline y again.
567 occAnal env app@(App fun arg)
568 = occAnalApp env (collectArgs app) False
570 -- Ignore type variables altogether
571 -- (a) occurrences inside type lambdas only not marked as InsideLam
572 -- (b) type variables not in environment
574 occAnal env expr@(Lam x body) | isTyVar x
575 = case occAnal env body of { (body_usage, body') ->
576 (body_usage, Lam x body')
579 -- For value lambdas we do a special hack. Consider
581 -- If we did nothing, x is used inside the \y, so would be marked
582 -- as dangerous to dup. But in the common case where the abstraction
583 -- is applied to two arguments this is over-pessimistic.
584 -- So instead, we just mark each binder with its occurrence
585 -- info in the *body* of the multiple lambda.
586 -- Then, the simplifier is careful when partially applying lambdas.
588 occAnal env expr@(Lam _ _)
589 = case occAnal env_body body of { (body_usage, body') ->
591 (final_usage, tagged_binders) = tagBinders body_usage binders
592 -- URGH! Sept 99: we don't seem to be able to use binders' here, because
593 -- we get linear-typed things in the resulting program that we can't handle yet.
594 -- (e.g. PrelShow) TODO
596 really_final_usage = if linear then
599 mapVarEnv markInsideLam final_usage
602 mkLams tagged_binders body') }
604 env_body = vanillaCtxt -- Body is (no longer) an RhsContext
605 (binders, body) = collectBinders expr
606 binders' = oneShotGroup env binders
607 linear = all is_one_shot binders'
608 is_one_shot b = isId b && isOneShotBndr b
610 occAnal env (Case scrut bndr ty alts)
611 = case occ_anal_scrut scrut alts of { (scrut_usage, scrut') ->
612 case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
614 alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
615 alts_usage' = addCaseBndrUsage alts_usage
616 (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
617 total_usage = scrut_usage +++ alts_usage1
619 total_usage `seq` (total_usage, Case scrut' tagged_bndr ty alts') }}
621 -- The case binder gets a usage of either "many" or "dead", never "one".
622 -- Reason: we like to inline single occurrences, to eliminate a binding,
623 -- but inlining a case binder *doesn't* eliminate a binding.
624 -- We *don't* want to transform
625 -- case x of w { (p,q) -> f w }
627 -- case x of w { (p,q) -> f (p,q) }
628 addCaseBndrUsage usage = case lookupVarEnv usage bndr of
630 Just occ -> extendVarEnv usage bndr (markMany occ)
632 alt_env = setVanillaCtxt env
633 -- Consider x = case v of { True -> (p,q); ... }
634 -- Then it's fine to inline p and q
636 occ_anal_scrut (Var v) (alt1 : other_alts)
637 | not (null other_alts) || not (isDefaultAlt alt1)
638 = (mkOneOcc env v True, Var v)
639 occ_anal_scrut scrut alts = occAnal vanillaCtxt scrut
640 -- No need for rhsCtxt
642 occAnal env (Let bind body)
643 = case occAnal env body of { (body_usage, body') ->
644 case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
645 (final_usage, mkLets new_binds body') }}
648 = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
649 (foldr (+++) emptyDetails arg_uds_s, args')}
651 arg_env = vanillaCtxt
654 Applications are dealt with specially because we want
655 the "build hack" to work.
658 occAnalApp env (Var fun, args) is_rhs
659 = case args_stuff of { (args_uds, args') ->
661 final_args_uds = markRhsUds env is_pap args_uds
663 (fun_uds +++ final_args_uds, mkApps (Var fun) args') }
665 fun_uniq = idUnique fun
666 fun_uds = mkOneOcc env fun (valArgCount args > 0)
667 is_pap = isDataConWorkId fun || valArgCount args < idArity fun
669 -- Hack for build, fold, runST
670 args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
671 | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
672 | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
673 | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
674 -- (foldr k z xs) may call k many times, but it never
675 -- shares a partial application of k; hence [False,True]
676 -- This means we can optimise
677 -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
678 -- by floating in the v
680 | otherwise = occAnalArgs env args
683 occAnalApp env (fun, args) is_rhs
684 = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
685 -- The addAppCtxt is a bit cunning. One iteration of the simplifier
686 -- often leaves behind beta redexs like
688 -- Here we would like to mark x,y as one-shot, and treat the whole
689 -- thing much like a let. We do this by pushing some True items
690 -- onto the context stack.
692 case occAnalArgs env args of { (args_uds, args') ->
694 final_uds = fun_uds +++ args_uds
696 (final_uds, mkApps fun' args') }}
699 markRhsUds :: OccEnv -- Check if this is a RhsEnv
700 -> Bool -- and this is true
701 -> UsageDetails -- The do markMany on this
703 -- We mark the free vars of the argument of a constructor or PAP
704 -- as "many", if it is the RHS of a let(rec).
705 -- This means that nothing gets inlined into a constructor argument
706 -- position, which is what we want. Typically those constructor
707 -- arguments are just variables, or trivial expressions.
709 -- This is the *whole point* of the isRhsEnv predicate
710 markRhsUds env is_pap arg_uds
711 | isRhsEnv env && is_pap = mapVarEnv markMany arg_uds
712 | otherwise = arg_uds
716 -> Int -> CtxtTy -- Argument number, and context to use for it
718 -> (UsageDetails, [CoreExpr])
719 appSpecial env n ctxt args
722 arg_env = vanillaCtxt
724 go n [] = (emptyDetails, []) -- Too few args
726 go 1 (arg:args) -- The magic arg
727 = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
728 case occAnalArgs env args of { (args_uds, args') ->
729 (arg_uds +++ args_uds, arg':args') }}
732 = case occAnal arg_env arg of { (arg_uds, arg') ->
733 case go (n-1) args of { (args_uds, args') ->
734 (arg_uds +++ args_uds, arg':args') }}
740 If the case binder occurs at all, the other binders effectively do too.
742 case e of x { (a,b) -> rhs }
745 If e turns out to be (e1,e2) we indeed get something like
746 let a = e1; b = e2; x = (a,b) in rhs
748 Note [Aug 06]: I don't think this is necessary any more, and it helpe
749 to know when binders are unused. See esp the call to
750 isDeadBinder in Simplify.mkDupableAlt
753 occAnalAlt env case_bndr (con, bndrs, rhs)
754 = case occAnal env rhs of { (rhs_usage, rhs') ->
756 (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
757 final_bndrs = tagged_bndrs -- See Note [Aug06] above
759 final_bndrs | case_bndr `elemVarEnv` final_usage = bndrs
760 | otherwise = tagged_bndrs
761 -- Leave the binders untagged if the case
762 -- binder occurs at all; see note above
765 (final_usage, (con, final_bndrs, rhs')) }
769 %************************************************************************
771 \subsection[OccurAnal-types]{OccEnv}
773 %************************************************************************
777 = OccEnv OccEncl -- Enclosing context information
778 CtxtTy -- Tells about linearity
780 -- OccEncl is used to control whether to inline into constructor arguments
782 -- x = (p,q) -- Don't inline p or q
783 -- y = /\a -> (p a, q a) -- Still don't inline p or q
784 -- z = f (p,q) -- Do inline p,q; it may make a rule fire
785 -- So OccEncl tells enought about the context to know what to do when
786 -- we encounter a contructor application or PAP.
789 = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
790 -- Don't inline into constructor args here
791 | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
792 -- Do inline into constructor args here
797 -- True:ctxt Analysing a function-valued expression that will be
800 -- False:ctxt Analysing a function-valued expression that may
801 -- be applied many times; but when it is,
802 -- the CtxtTy inside applies
805 initOccEnv = OccEnv OccRhs []
807 vanillaCtxt = OccEnv OccVanilla []
808 rhsCtxt = OccEnv OccRhs []
810 isRhsEnv (OccEnv OccRhs _) = True
811 isRhsEnv (OccEnv OccVanilla _) = False
813 setVanillaCtxt :: OccEnv -> OccEnv
814 setVanillaCtxt (OccEnv OccRhs ctxt_ty) = OccEnv OccVanilla ctxt_ty
815 setVanillaCtxt other_env = other_env
817 setCtxt :: OccEnv -> CtxtTy -> OccEnv
818 setCtxt (OccEnv encl _) ctxt = OccEnv encl ctxt
820 oneShotGroup :: OccEnv -> [CoreBndr] -> [CoreBndr]
821 -- The result binders have one-shot-ness set that they might not have had originally.
822 -- This happens in (build (\cn -> e)). Here the occurrence analyser
823 -- linearity context knows that c,n are one-shot, and it records that fact in
824 -- the binder. This is useful to guide subsequent float-in/float-out tranformations
826 oneShotGroup (OccEnv encl ctxt) bndrs
829 go ctxt [] rev_bndrs = reverse rev_bndrs
831 go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
832 | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
834 bndr' | lin_ctxt = setOneShotLambda bndr
837 go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
839 addAppCtxt (OccEnv encl ctxt) args
840 = OccEnv encl (replicate (valArgCount args) True ++ ctxt)
843 %************************************************************************
845 \subsection[OccurAnal-types]{OccEnv}
847 %************************************************************************
850 type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
852 (+++), combineAltsUsageDetails
853 :: UsageDetails -> UsageDetails -> UsageDetails
856 = plusVarEnv_C addOccInfo usage1 usage2
858 combineAltsUsageDetails usage1 usage2
859 = plusVarEnv_C orOccInfo usage1 usage2
861 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
862 addOneOcc usage id info
863 = plusVarEnv_C addOccInfo usage (unitVarEnv id info)
864 -- ToDo: make this more efficient
866 emptyDetails = (emptyVarEnv :: UsageDetails)
868 usedIn :: Id -> UsageDetails -> Bool
869 v `usedIn` details = isExportedId v || v `elemVarEnv` details
871 type IdWithOccInfo = Id
873 tagBinders :: UsageDetails -- Of scope
875 -> (UsageDetails, -- Details with binders removed
876 [IdWithOccInfo]) -- Tagged binders
878 tagBinders usage binders
880 usage' = usage `delVarEnvList` binders
881 uss = map (setBinderOcc usage) binders
883 usage' `seq` (usage', uss)
885 tagBinder :: UsageDetails -- Of scope
887 -> (UsageDetails, -- Details with binders removed
888 IdWithOccInfo) -- Tagged binders
890 tagBinder usage binder
892 usage' = usage `delVarEnv` binder
893 binder' = setBinderOcc usage binder
895 usage' `seq` (usage', binder')
897 setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
898 setBinderOcc usage bndr
899 | isTyVar bndr = bndr
900 | isExportedId bndr = case idOccInfo bndr of
902 other -> setIdOccInfo bndr NoOccInfo
903 -- Don't use local usage info for visible-elsewhere things
904 -- BUT *do* erase any IAmALoopBreaker annotation, because we're
905 -- about to re-generate it and it shouldn't be "sticky"
907 | otherwise = setIdOccInfo bndr occ_info
909 occ_info = lookupVarEnv usage bndr `orElse` IAmDead
913 %************************************************************************
915 \subsection{Operations over OccInfo}
917 %************************************************************************
920 mkOneOcc :: OccEnv -> Id -> InterestingCxt -> UsageDetails
921 mkOneOcc env id int_cxt
922 | isLocalId id = unitVarEnv id (OneOcc False True int_cxt)
923 | otherwise = emptyDetails
925 markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
927 markMany IAmDead = IAmDead
928 markMany other = NoOccInfo
930 markInsideSCC occ = markMany occ
932 markInsideLam (OneOcc _ one_br int_cxt) = OneOcc True one_br int_cxt
933 markInsideLam occ = occ
935 addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
937 addOccInfo IAmDead info2 = info2
938 addOccInfo info1 IAmDead = info1
939 addOccInfo info1 info2 = NoOccInfo
941 -- (orOccInfo orig new) is used
942 -- when combining occurrence info from branches of a case
944 orOccInfo IAmDead info2 = info2
945 orOccInfo info1 IAmDead = info1
946 orOccInfo (OneOcc in_lam1 one_branch1 int_cxt1)
947 (OneOcc in_lam2 one_branch2 int_cxt2)
948 = OneOcc (in_lam1 || in_lam2)
949 False -- False, because it occurs in both branches
950 (int_cxt1 && int_cxt2)
951 orOccInfo info1 info2 = NoOccInfo