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 ( shortableIdInfo, copyIdInfo )
30 import BasicTypes ( OccInfo(..), isOneOcc )
35 import Type ( splitFunTy_maybe, splitForAllTys )
36 import Maybes ( maybeToBool, orElse )
37 import Digraph ( stronglyConnCompR, SCC(..) )
38 import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
39 import Unique ( Unique )
40 import UniqFM ( keysUFM )
41 import Util ( zipWithEqual, mapAndUnzip )
46 %************************************************************************
48 \subsection[OccurAnal-main]{Counting occurrences: main function}
50 %************************************************************************
52 Here's the externally-callable interface:
55 occurAnalyseGlobalExpr :: CoreExpr -> CoreExpr
56 occurAnalyseGlobalExpr expr
57 = -- Top level expr, so no interesting free vars, and
58 -- discard occurence info returned
59 snd (occAnal (initOccEnv emptyVarSet) expr)
61 occurAnalyseRule :: CoreRule -> CoreRule
62 occurAnalyseRule rule@(BuiltinRule _ _) = rule
63 occurAnalyseRule (Rule str act tpl_vars tpl_args rhs)
64 -- Add occ info to tpl_vars, rhs
65 = Rule str act tpl_vars' tpl_args rhs'
67 (rhs_uds, rhs') = occAnal (initOccEnv (mkVarSet tpl_vars)) rhs
68 (_, tpl_vars') = tagBinders rhs_uds tpl_vars
72 %************************************************************************
74 \subsection{Top level stuff}
76 %************************************************************************
78 In @occAnalTop@ we do indirection-shorting. That is, if we have this:
80 x_local = <expression>
84 where exp is exported, and loc is not, then we replace it with this:
87 x_exported = <expression>
90 Without this we never get rid of the x_exported = x_local thing. This
91 save a gratuitous jump (from \tr{x_exported} to \tr{x_local}), and
92 makes strictness information propagate better. This used to happen in
93 the final phase, but it's tidier to do it here.
95 If more than one exported thing is equal to a local thing (i.e., the
96 local thing really is shared), then we do one only:
100 x_exported2 = x_local
104 x_exported2 = x_exported1
107 We rely on prior eta reduction to simplify things like
109 x_exported = /\ tyvars -> x_local tyvars
113 Hence,there's a possibility of leaving unchanged something like this:
116 x_exported1 = x_local Int
118 By the time we've thrown away the types in STG land this
119 could be eliminated. But I don't think it's very common
120 and it's dangerous to do this fiddling in STG land
121 because we might elminate a binding that's mentioned in the
122 unfolding for something.
125 occurAnalyseBinds :: [CoreBind] -> [CoreBind]
127 occurAnalyseBinds binds
130 (_, _, binds') = go (initOccEnv emptyVarSet) binds
132 go :: OccEnv -> [CoreBind]
133 -> (UsageDetails, -- Occurrence info
134 IdEnv Id, -- Indirection elimination info
135 -- Maps local-id -> exported-id, but it embodies
136 -- bindings of the form exported-id = local-id in
137 -- the argument to go
138 [CoreBind]) -- Occ-analysed bindings, less the exported-id=local-id ones
140 go env [] = (emptyDetails, emptyVarEnv, [])
142 go env (bind : binds)
144 new_env = env `addNewCands` (bindersOf bind)
145 (scope_usage, ind_env, binds') = go new_env binds
146 (final_usage, new_binds) = occAnalBind env (zapBind ind_env bind) scope_usage
147 -- NB: I zap before occur-analysing, so
148 -- I don't need to worry about getting the
149 -- occ info on the new bindings right.
152 NonRec exported_id (Var local_id)
153 | shortMeOut ind_env exported_id local_id
154 -- Special case for eliminating indirections
155 -- Note: it's a shortcoming that this only works for
156 -- non-recursive bindings. Elminating indirections
157 -- makes perfect sense for recursive bindings too, but
158 -- it's more complicated to implement, so I haven't done so
159 -> (scope_usage, ind_env', binds')
161 ind_env' = extendVarEnv ind_env local_id exported_id
163 other -> -- Ho ho! The normal case
164 (final_usage, ind_env, new_binds ++ binds')
167 -- Deal with any indirections
168 zapBind ind_env (NonRec bndr rhs)
169 | bndr `elemVarEnv` ind_env = Rec (zap ind_env (bndr,rhs))
170 -- The Rec isn't strictly necessary, but it's convenient
171 zapBind ind_env (Rec pairs)
172 | or [id `elemVarEnv` ind_env | (id,_) <- pairs] = Rec (concat (map (zap ind_env) pairs))
174 zapBind ind_env bind = bind
176 zap ind_env pair@(local_id,rhs)
177 = case lookupVarEnv ind_env local_id of
179 Just exported_id -> [(local_id, Var exported_id),
182 exported_id' = modifyIdInfo (copyIdInfo (idInfo local_id)) exported_id
184 shortMeOut ind_env exported_id local_id
185 -- The if-then-else stuff is just so I can get a pprTrace to see
186 -- how often I don't get shorting out becuase of IdInfo stuff
187 = if isExportedId exported_id && -- Only if this is exported
189 isLocalId local_id && -- Only if this one is defined in this
190 -- module, so that we *can* change its
191 -- binding to be the exported thing!
193 not (isExportedId local_id) && -- Only if this one is not itself exported,
194 -- since the transformation will nuke it
196 not (local_id `elemVarEnv` ind_env) -- Only if not already substituted for
198 if shortableIdInfo (idInfo exported_id) -- Only if its IdInfo is 'shortable'
199 -- (see the defn of IdInfo.shortableIdInfo)
203 pprTrace "shortMeOut:" (ppr exported_id)
211 %************************************************************************
213 \subsection[OccurAnal-main]{Counting occurrences: main function}
215 %************************************************************************
221 type IdWithOccInfo = Id -- An Id with fresh PragmaInfo attached
223 type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
224 -- which is gotten from the Id.
225 type Details1 = (Id, UsageDetails, CoreExpr)
226 type Details2 = (IdWithOccInfo, CoreExpr)
229 occAnalBind :: OccEnv
231 -> UsageDetails -- Usage details of scope
232 -> (UsageDetails, -- Of the whole let(rec)
235 occAnalBind env (NonRec binder rhs) body_usage
236 | not (binder `usedIn` body_usage) -- It's not mentioned
239 | otherwise -- It's mentioned in the body
240 = (final_body_usage `combineUsageDetails` rhs_usage,
241 [NonRec tagged_binder rhs'])
244 (final_body_usage, tagged_binder) = tagBinder body_usage binder
245 (rhs_usage, rhs') = occAnalRhs env binder rhs
248 Dropping dead code for recursive bindings is done in a very simple way:
250 the entire set of bindings is dropped if none of its binders are
251 mentioned in its body; otherwise none are.
253 This seems to miss an obvious improvement.
268 Now @f@ is unused. But dependency analysis will sort this out into a
269 @letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
270 It isn't easy to do a perfect job in one blow. Consider
284 occAnalBind env (Rec pairs) body_usage
285 = foldr (_scc_ "occAnalBind.dofinal" do_final_bind) (body_usage, []) sccs
287 binders = map fst pairs
288 rhs_env = env `addNewCands` binders
290 analysed_pairs :: [Details1]
291 analysed_pairs = [ (bndr, rhs_usage, rhs')
292 | (bndr, rhs) <- pairs,
293 let (rhs_usage, rhs') = occAnalRhs rhs_env bndr rhs
296 sccs :: [SCC (Node Details1)]
297 sccs = _scc_ "occAnalBind.scc" stronglyConnCompR edges
300 ---- stuff for dependency analysis of binds -------------------------------
301 edges :: [Node Details1]
302 edges = _scc_ "occAnalBind.assoc"
303 [ (details, idUnique id, edges_from rhs_usage)
304 | details@(id, rhs_usage, rhs) <- analysed_pairs
307 -- (a -> b) means a mentions b
308 -- Given the usage details (a UFM that gives occ info for each free var of
309 -- the RHS) we can get the list of free vars -- or rather their Int keys --
310 -- by just extracting the keys from the finite map. Grimy, but fast.
311 -- Previously we had this:
312 -- [ bndr | bndr <- bndrs,
313 -- maybeToBool (lookupVarEnv rhs_usage bndr)]
314 -- which has n**2 cost, and this meant that edges_from alone
315 -- consumed 10% of total runtime!
316 edges_from :: UsageDetails -> [Unique]
317 edges_from rhs_usage = _scc_ "occAnalBind.edges_from"
320 ---- stuff to "re-constitute" bindings from dependency-analysis info ------
323 do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
324 | not (bndr `usedIn` body_usage)
325 = (body_usage, binds_so_far) -- Dead code
327 = (combined_usage, new_bind : binds_so_far)
329 total_usage = combineUsageDetails body_usage rhs_usage
330 (combined_usage, tagged_bndr) = tagBinder total_usage bndr
331 new_bind = NonRec tagged_bndr rhs'
334 do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
335 | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
336 = (body_usage, binds_so_far) -- Dead code
338 = (combined_usage, final_bind:binds_so_far)
340 details = [details | (details, _, _) <- cycle]
341 bndrs = [bndr | (bndr, _, _) <- details]
342 rhs_usages = [rhs_usage | (_, rhs_usage, _) <- details]
343 total_usage = foldr combineUsageDetails body_usage rhs_usages
344 (combined_usage, tagged_bndrs) = tagBinders total_usage bndrs
345 final_bind = Rec (reOrderRec env new_cycle)
347 new_cycle = CyclicSCC (zipWithEqual "occAnalBind" mk_new_bind tagged_bndrs cycle)
348 mk_new_bind tagged_bndr ((_, _, rhs'), key, keys) = ((tagged_bndr, rhs'), key, keys)
351 @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
352 strongly connected component (there's guaranteed to be a cycle). It returns the
354 a) in a better order,
355 b) with some of the Ids having a IMustNotBeINLINEd pragma
357 The "no-inline" Ids are sufficient to break all cycles in the SCC. This means
358 that the simplifier can guarantee not to loop provided it never records an inlining
359 for these no-inline guys.
361 Furthermore, the order of the binds is such that if we neglect dependencies
362 on the no-inline Ids then the binds are topologically sorted. This means
363 that the simplifier will generally do a good job if it works from top bottom,
364 recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
367 [June 98: I don't understand the following paragraphs, and I've
368 changed the a=b case again so that it isn't a special case any more.]
370 Here's a case that bit me:
378 Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
380 My solution was to make a=b bindings record b as Many, rather like INLINE bindings.
381 Perhaps something cleverer would suffice.
384 You might think that you can prevent non-termination simply by making
385 sure that we simplify a recursive binding's RHS in an environment that
386 simply clones the recursive Id. But no. Consider
388 letrec f = \x -> let z = f x' in ...
395 We bind n to its *simplified* RHS, we then *re-simplify* it when
396 we inline n. Then we may well inline f; and then the same thing
399 I don't think it's possible to prevent non-termination by environment
400 manipulation in this way. Apart from anything else, successive
401 iterations of the simplifier may unroll recursive loops in cases like
402 that above. The idea of beaking every recursive loop with an
403 IMustNotBeINLINEd pragma is much much better.
409 -> SCC (Node Details2)
411 -- Sorted into a plausible order. Enough of the Ids have
412 -- dontINLINE pragmas that there are no loops left.
414 -- Non-recursive case
415 reOrderRec env (AcyclicSCC (bind, _, _)) = [bind]
417 -- Common case of simple self-recursion
418 reOrderRec env (CyclicSCC [bind])
419 = [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
421 ((tagged_bndr, rhs), _, _) = bind
423 reOrderRec env (CyclicSCC (bind : binds))
424 = -- Choose a loop breaker, mark it no-inline,
425 -- do SCC analysis on the rest, and recursively sort them out
426 concat (map (reOrderRec env) (stronglyConnCompR unchosen))
428 [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
431 (chosen_pair, unchosen) = choose_loop_breaker bind (score bind) [] binds
432 (tagged_bndr, rhs) = chosen_pair
434 -- This loop looks for the bind with the lowest score
435 -- to pick as the loop breaker. The rest accumulate in
436 choose_loop_breaker (details,_,_) loop_sc acc []
437 = (details, acc) -- Done
439 choose_loop_breaker loop_bind loop_sc acc (bind : binds)
440 | sc < loop_sc -- Lower score so pick this new one
441 = choose_loop_breaker bind sc (loop_bind : acc) binds
443 | otherwise -- No lower so don't pick it
444 = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
448 score :: Node Details2 -> Int -- Higher score => less likely to be picked as loop breaker
449 score ((bndr, rhs), _, _)
450 | exprIsTrivial rhs = 4 -- Practically certain to be inlined
451 -- Used to have also: && not (isExportedId bndr)
452 -- But I found this sometimes cost an extra iteration when we have
453 -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
454 -- where df is the exported dictionary. Then df makes a really
455 -- bad choice for loop breaker
457 | not_fun_ty (idType bndr) = 3 -- Data types help with cases
458 -- This used to have a lower score than inlineCandidate, but
459 -- it's *really* helpful if dictionaries get inlined fast,
460 -- so I'm experimenting with giving higher priority to data-typed things
462 | inlineCandidate bndr rhs = 2 -- Likely to be inlined
464 | not (isEmptyCoreRules (idSpecialisation bndr)) = 1
465 -- Avoid things with specialisations; we'd like
466 -- to take advantage of them in the subsequent bindings
470 inlineCandidate :: Id -> CoreExpr -> Bool
471 inlineCandidate id (Note InlineMe _) = True
472 inlineCandidate id rhs = isOneOcc (idOccInfo id)
474 -- Real example (the Enum Ordering instance from PrelBase):
475 -- rec f = \ x -> case d of (p,q,r) -> p x
476 -- g = \ x -> case d of (p,q,r) -> q x
479 -- Here, f and g occur just once; but we can't inline them into d.
480 -- On the other hand we *could* simplify those case expressions if
481 -- we didn't stupidly choose d as the loop breaker.
482 -- But we won't because constructor args are marked "Many".
484 not_fun_ty ty = not (maybeToBool (splitFunTy_maybe rho_ty))
486 (_, rho_ty) = splitForAllTys ty
489 @occAnalRhs@ deals with the question of bindings where the Id is marked
490 by an INLINE pragma. For these we record that anything which occurs
491 in its RHS occurs many times. This pessimistically assumes that ths
492 inlined binder also occurs many times in its scope, but if it doesn't
493 we'll catch it next time round. At worst this costs an extra simplifier pass.
494 ToDo: try using the occurrence info for the inline'd binder.
496 [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec.
497 [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec.
502 -> Id -> CoreExpr -- Binder and rhs
503 -> (UsageDetails, CoreExpr)
505 occAnalRhs env id rhs
506 = (final_usage, rhs')
508 (rhs_usage, rhs') = occAnal (rhsCtxt env) rhs
509 -- Note that we use an rhsCtxt. This tells the occ anal that it's
510 -- looking at an RHS, which has an effect in occAnalApp
512 -- But there's a problem. Consider
517 -- First time round, it looks as if x1 and x2 occur as an arg of a
518 -- let-bound constructor ==> give them a many-occurrence.
519 -- But then x3 is inlined (unconditionally as it happens) and
520 -- next time round, x2 will be, and the next time round x1 will be
521 -- Result: multiple simplifier iterations. Sigh.
522 -- Possible solution: use rhsCtxt for things that occur just once...
524 -- [March 98] A new wrinkle is that if the binder has specialisations inside
525 -- it then we count the specialised Ids as "extra rhs's". That way
526 -- the "parent" keeps the specialised "children" alive. If the parent
527 -- dies (because it isn't referenced any more), then the children will
528 -- die too unless they are already referenced directly.
530 final_usage = foldVarSet add rhs_usage (idRuleVars id)
531 add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
532 -- (i.e manyOcc) because many copies
533 -- of the specialised thing can appear
541 -> (UsageDetails, -- Gives info only about the "interesting" Ids
544 occAnal env (Type t) = (emptyDetails, Type t)
549 var_uds | isCandidate env v = unitVarEnv v oneOcc
550 | otherwise = emptyDetails
552 -- At one stage, I gathered the idRuleVars for v here too,
553 -- which in a way is the right thing to do.
554 -- But that went wrong right after specialisation, when
555 -- the *occurrences* of the overloaded function didn't have any
556 -- rules in them, so the *specialised* versions looked as if they
557 -- weren't used at all.
561 We regard variables that occur as constructor arguments as "dangerousToDup":
565 f x = let y = expensive x in
567 (case z of {(p,q)->q}, case z of {(p,q)->q})
570 We feel free to duplicate the WHNF (True,y), but that means
571 that y may be duplicated thereby.
573 If we aren't careful we duplicate the (expensive x) call!
574 Constructors are rather like lambdas in this way.
577 occAnal env expr@(Lit lit) = (emptyDetails, expr)
581 occAnal env (Note InlineMe body)
582 = case occAnal env body of { (usage, body') ->
583 (mapVarEnv markMany usage, Note InlineMe body')
586 occAnal env (Note note@(SCC cc) body)
587 = case occAnal env body of { (usage, body') ->
588 (mapVarEnv markInsideSCC usage, Note note body')
591 occAnal env (Note note body)
592 = case occAnal env body of { (usage, body') ->
593 (usage, Note note body')
598 occAnal env app@(App fun arg)
599 = occAnalApp env (collectArgs app) False
601 -- Ignore type variables altogether
602 -- (a) occurrences inside type lambdas only not marked as InsideLam
603 -- (b) type variables not in environment
605 occAnal env expr@(Lam x body) | isTyVar x
606 = case occAnal env body of { (body_usage, body') ->
607 (body_usage, Lam x body')
610 -- For value lambdas we do a special hack. Consider
612 -- If we did nothing, x is used inside the \y, so would be marked
613 -- as dangerous to dup. But in the common case where the abstraction
614 -- is applied to two arguments this is over-pessimistic.
615 -- So instead, we just mark each binder with its occurrence
616 -- info in the *body* of the multiple lambda.
617 -- Then, the simplifier is careful when partially applying lambdas.
619 occAnal env expr@(Lam _ _)
620 = case occAnal env_body body of { (body_usage, body') ->
622 (final_usage, tagged_binders) = tagBinders body_usage binders
623 -- URGH! Sept 99: we don't seem to be able to use binders' here, because
624 -- we get linear-typed things in the resulting program that we can't handle yet.
625 -- (e.g. PrelShow) TODO
627 really_final_usage = if linear then
630 mapVarEnv markInsideLam final_usage
633 mkLams tagged_binders body') }
635 (binders, body) = collectBinders expr
636 (linear, env1, _) = oneShotGroup env binders
637 env2 = env1 `addNewCands` binders -- Add in-scope binders
638 env_body = vanillaCtxt env2 -- Body is (no longer) an RhsContext
640 occAnal env (Case scrut bndr alts)
641 = case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
642 case occAnal (vanillaCtxt env) scrut of { (scrut_usage, scrut') ->
643 -- No need for rhsCtxt
645 alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
646 alts_usage' = addCaseBndrUsage alts_usage
647 (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
648 total_usage = scrut_usage `combineUsageDetails` alts_usage1
650 total_usage `seq` (total_usage, Case scrut' tagged_bndr alts') }}
652 alt_env = env `addNewCand` bndr
654 -- The case binder gets a usage of either "many" or "dead", never "one".
655 -- Reason: we like to inline single occurrences, to eliminate a binding,
656 -- but inlining a case binder *doesn't* eliminate a binding.
657 -- We *don't* want to transform
658 -- case x of w { (p,q) -> f w }
660 -- case x of w { (p,q) -> f (p,q) }
661 addCaseBndrUsage usage = case lookupVarEnv usage bndr of
663 Just occ -> extendVarEnv usage bndr (markMany occ)
665 occAnal env (Let bind body)
666 = case occAnal new_env body of { (body_usage, body') ->
667 case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
668 (final_usage, mkLets new_binds body') }}
670 new_env = env `addNewCands` (bindersOf bind)
673 = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
674 (foldr combineUsageDetails emptyDetails arg_uds_s, args')}
676 arg_env = vanillaCtxt env
679 Applications are dealt with specially because we want
680 the "build hack" to work.
683 -- Hack for build, fold, runST
684 occAnalApp env (Var fun, args) is_rhs
685 = case args_stuff of { (args_uds, args') ->
687 -- We mark the free vars of the argument of a constructor or PAP
688 -- as "many", if it is the RHS of a let(rec).
689 -- This means that nothing gets inlined into a constructor argument
690 -- position, which is what we want. Typically those constructor
691 -- arguments are just variables, or trivial expressions.
693 -- This is the *whole point* of the isRhsEnv predicate
696 isDataConId fun || valArgCount args < idArity fun
697 = mapVarEnv markMany args_uds
698 | otherwise = args_uds
700 (fun_uds `combineUsageDetails` final_args_uds, mkApps (Var fun) args') }
702 fun_uniq = idUnique fun
704 fun_uds | isCandidate env fun = unitVarEnv fun oneOcc
705 | otherwise = emptyDetails
707 args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
708 | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
709 | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
710 | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
711 -- (foldr k z xs) may call k many times, but it never
712 -- shares a partial application of k; hence [False,True]
713 -- This means we can optimise
714 -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
715 -- by floating in the v
717 | otherwise = occAnalArgs env args
720 occAnalApp env (fun, args) is_rhs
721 = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
722 -- The addAppCtxt is a bit cunning. One iteration of the simplifier
723 -- often leaves behind beta redexs like
725 -- Here we would like to mark x,y as one-shot, and treat the whole
726 -- thing much like a let. We do this by pushing some True items
727 -- onto the context stack.
729 case occAnalArgs env args of { (args_uds, args') ->
731 final_uds = fun_uds `combineUsageDetails` args_uds
733 (final_uds, mkApps fun' args') }}
736 -> Int -> CtxtTy -- Argument number, and context to use for it
738 -> (UsageDetails, [CoreExpr])
739 appSpecial env n ctxt args
742 arg_env = vanillaCtxt env
744 go n [] = (emptyDetails, []) -- Too few args
746 go 1 (arg:args) -- The magic arg
747 = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
748 case occAnalArgs env args of { (args_uds, args') ->
749 (combineUsageDetails arg_uds args_uds, arg':args') }}
752 = case occAnal arg_env arg of { (arg_uds, arg') ->
753 case go (n-1) args of { (args_uds, args') ->
754 (combineUsageDetails arg_uds args_uds, arg':args') }}
760 If the case binder occurs at all, the other binders effectively do too.
762 case e of x { (a,b) -> rhs }
765 If e turns out to be (e1,e2) we indeed get something like
766 let a = e1; b = e2; x = (a,b) in rhs
769 occAnalAlt env case_bndr (con, bndrs, rhs)
770 = case occAnal (env `addNewCands` bndrs) rhs of { (rhs_usage, rhs') ->
772 (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
773 final_bndrs | case_bndr `elemVarEnv` final_usage = bndrs
774 | otherwise = tagged_bndrs
775 -- Leave the binders untagged if the case
776 -- binder occurs at all; see note above
778 (final_usage, (con, final_bndrs, rhs')) }
782 %************************************************************************
784 \subsection[OccurAnal-types]{OccEnv}
786 %************************************************************************
790 = OccEnv IdSet -- In-scope Ids; we gather info about these only
791 OccEncl -- Enclosing context information
792 CtxtTy -- Tells about linearity
794 -- OccEncl is used to control whether to inline into constructor arguments
796 -- x = (p,q) -- Don't inline p or q
797 -- y = /\a -> (p a, q a) -- Still don't inline p or q
798 -- z = f (p,q) -- Do inline p,q; it may make a rule fire
799 -- So OccEncl tells enought about the context to know what to do when
800 -- we encounter a contructor application or PAP.
803 = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
804 -- Don't inline into constructor args here
805 | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
806 -- Do inline into constructor args here
811 -- True:ctxt Analysing a function-valued expression that will be
814 -- False:ctxt Analysing a function-valued expression that may
815 -- be applied many times; but when it is,
816 -- the CtxtTy inside applies
818 initOccEnv :: VarSet -> OccEnv
819 initOccEnv vars = OccEnv vars OccRhs []
821 isRhsEnv (OccEnv _ OccRhs _) = True
822 isRhsEnv (OccEnv _ OccVanilla _) = False
824 isCandidate :: OccEnv -> Id -> Bool
825 isCandidate (OccEnv cands encl _) id = id `elemVarSet` cands
827 addNewCands :: OccEnv -> [Id] -> OccEnv
828 addNewCands (OccEnv cands encl ctxt) ids
829 = OccEnv (cands `unionVarSet` mkVarSet ids) encl ctxt
831 addNewCand :: OccEnv -> Id -> OccEnv
832 addNewCand (OccEnv cands encl ctxt) id
833 = OccEnv (extendVarSet cands id) encl ctxt
835 setCtxt :: OccEnv -> CtxtTy -> OccEnv
836 setCtxt (OccEnv cands encl _) ctxt = OccEnv cands encl ctxt
838 oneShotGroup :: OccEnv -> [CoreBndr] -> (Bool, OccEnv, [CoreBndr])
839 -- True <=> this is a one-shot linear lambda group
840 -- The [CoreBndr] are the binders.
842 -- The result binders have one-shot-ness set that they might not have had originally.
843 -- This happens in (build (\cn -> e)). Here the occurrence analyser
844 -- linearity context knows that c,n are one-shot, and it records that fact in
845 -- the binder. This is useful to guide subsequent float-in/float-out tranformations
847 oneShotGroup (OccEnv cands encl ctxt) bndrs
848 = case go ctxt bndrs [] of
849 (new_ctxt, new_bndrs) -> (all is_one_shot new_bndrs, OccEnv cands encl new_ctxt, new_bndrs)
851 is_one_shot b = isId b && isOneShotLambda b
853 go ctxt [] rev_bndrs = (ctxt, reverse rev_bndrs)
855 go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
856 | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
858 bndr' | lin_ctxt = setOneShotLambda bndr
861 go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
864 vanillaCtxt (OccEnv cands _ _) = OccEnv cands OccVanilla []
865 rhsCtxt (OccEnv cands _ _) = OccEnv cands OccRhs []
867 addAppCtxt (OccEnv cands encl ctxt) args
868 = OccEnv cands encl (replicate (valArgCount args) True ++ ctxt)
871 %************************************************************************
873 \subsection[OccurAnal-types]{OccEnv}
875 %************************************************************************
878 type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
880 combineUsageDetails, combineAltsUsageDetails
881 :: UsageDetails -> UsageDetails -> UsageDetails
883 combineUsageDetails usage1 usage2
884 = plusVarEnv_C addOccInfo usage1 usage2
886 combineAltsUsageDetails usage1 usage2
887 = plusVarEnv_C orOccInfo usage1 usage2
889 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
890 addOneOcc usage id info
891 = plusVarEnv_C addOccInfo usage (unitVarEnv id info)
892 -- ToDo: make this more efficient
894 emptyDetails = (emptyVarEnv :: UsageDetails)
896 usedIn :: Id -> UsageDetails -> Bool
897 v `usedIn` details = isExportedId v || v `elemVarEnv` details
899 tagBinders :: UsageDetails -- Of scope
901 -> (UsageDetails, -- Details with binders removed
902 [IdWithOccInfo]) -- Tagged binders
904 tagBinders usage binders
906 usage' = usage `delVarEnvList` binders
907 uss = map (setBinderOcc usage) binders
909 usage' `seq` (usage', uss)
911 tagBinder :: UsageDetails -- Of scope
913 -> (UsageDetails, -- Details with binders removed
914 IdWithOccInfo) -- Tagged binders
916 tagBinder usage binder
918 usage' = usage `delVarEnv` binder
919 binder' = setBinderOcc usage binder
921 usage' `seq` (usage', binder')
923 setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
924 setBinderOcc usage bndr
925 | isTyVar bndr = bndr
926 | isExportedId bndr = case idOccInfo bndr of
928 other -> setIdOccInfo bndr NoOccInfo
929 -- Don't use local usage info for visible-elsewhere things
930 -- BUT *do* erase any IAmALoopBreaker annotation, because we're
931 -- about to re-generate it and it shouldn't be "sticky"
933 | otherwise = setIdOccInfo bndr occ_info
935 occ_info = lookupVarEnv usage bndr `orElse` IAmDead
939 %************************************************************************
941 \subsection{Operations over OccInfo}
943 %************************************************************************
947 oneOcc = OneOcc False True
949 markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
951 markMany IAmDead = IAmDead
952 markMany other = NoOccInfo
954 markInsideSCC occ = markMany occ
956 markInsideLam (OneOcc _ one_br) = OneOcc True one_br
957 markInsideLam occ = occ
959 addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
961 addOccInfo IAmDead info2 = info2
962 addOccInfo info1 IAmDead = info1
963 addOccInfo info1 info2 = NoOccInfo
965 -- (orOccInfo orig new) is used
966 -- when combining occurrence info from branches of a case
968 orOccInfo IAmDead info2 = info2
969 orOccInfo info1 IAmDead = info1
970 orOccInfo (OneOcc in_lam1 one_branch1)
971 (OneOcc in_lam2 one_branch2)
972 = OneOcc (in_lam1 || in_lam2)
973 False -- False, because it occurs in both branches
975 orOccInfo info1 info2 = NoOccInfo