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 ( copyIdInfo )
30 import BasicTypes ( OccInfo(..), isOneOcc )
35 import Type ( isFunTy, dropForAlls )
36 import Maybes ( 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
201 if shortableIdInfo (idInfo exported_id) -- Only if its IdInfo is 'shortable'
202 -- (see the defn of IdInfo.shortableIdInfo)
206 pprTrace "shortMeOut:" (ppr exported_id)
215 %************************************************************************
217 \subsection[OccurAnal-main]{Counting occurrences: main function}
219 %************************************************************************
225 type IdWithOccInfo = Id -- An Id with fresh PragmaInfo attached
227 type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
228 -- which is gotten from the Id.
229 type Details1 = (Id, UsageDetails, CoreExpr)
230 type Details2 = (IdWithOccInfo, CoreExpr)
233 occAnalBind :: OccEnv
235 -> UsageDetails -- Usage details of scope
236 -> (UsageDetails, -- Of the whole let(rec)
239 occAnalBind env (NonRec binder rhs) body_usage
240 | not (binder `usedIn` body_usage) -- It's not mentioned
243 | otherwise -- It's mentioned in the body
244 = (final_body_usage `combineUsageDetails` rhs_usage,
245 [NonRec tagged_binder rhs'])
248 (final_body_usage, tagged_binder) = tagBinder body_usage binder
249 (rhs_usage, rhs') = occAnalRhs env binder rhs
252 Dropping dead code for recursive bindings is done in a very simple way:
254 the entire set of bindings is dropped if none of its binders are
255 mentioned in its body; otherwise none are.
257 This seems to miss an obvious improvement.
272 Now @f@ is unused. But dependency analysis will sort this out into a
273 @letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
274 It isn't easy to do a perfect job in one blow. Consider
288 occAnalBind env (Rec pairs) body_usage
289 = foldr (_scc_ "occAnalBind.dofinal" do_final_bind) (body_usage, []) sccs
291 binders = map fst pairs
292 rhs_env = env `addNewCands` binders
294 analysed_pairs :: [Details1]
295 analysed_pairs = [ (bndr, rhs_usage, rhs')
296 | (bndr, rhs) <- pairs,
297 let (rhs_usage, rhs') = occAnalRhs rhs_env bndr rhs
300 sccs :: [SCC (Node Details1)]
301 sccs = _scc_ "occAnalBind.scc" stronglyConnCompR edges
304 ---- stuff for dependency analysis of binds -------------------------------
305 edges :: [Node Details1]
306 edges = _scc_ "occAnalBind.assoc"
307 [ (details, idUnique id, edges_from rhs_usage)
308 | details@(id, rhs_usage, rhs) <- analysed_pairs
311 -- (a -> b) means a mentions b
312 -- Given the usage details (a UFM that gives occ info for each free var of
313 -- the RHS) we can get the list of free vars -- or rather their Int keys --
314 -- by just extracting the keys from the finite map. Grimy, but fast.
315 -- Previously we had this:
316 -- [ bndr | bndr <- bndrs,
317 -- maybeToBool (lookupVarEnv rhs_usage bndr)]
318 -- which has n**2 cost, and this meant that edges_from alone
319 -- consumed 10% of total runtime!
320 edges_from :: UsageDetails -> [Unique]
321 edges_from rhs_usage = _scc_ "occAnalBind.edges_from"
324 ---- stuff to "re-constitute" bindings from dependency-analysis info ------
327 do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
328 | not (bndr `usedIn` body_usage)
329 = (body_usage, binds_so_far) -- Dead code
331 = (combined_usage, new_bind : binds_so_far)
333 total_usage = combineUsageDetails body_usage rhs_usage
334 (combined_usage, tagged_bndr) = tagBinder total_usage bndr
335 new_bind = NonRec tagged_bndr rhs'
338 do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
339 | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
340 = (body_usage, binds_so_far) -- Dead code
342 = (combined_usage, final_bind:binds_so_far)
344 details = [details | (details, _, _) <- cycle]
345 bndrs = [bndr | (bndr, _, _) <- details]
346 rhs_usages = [rhs_usage | (_, rhs_usage, _) <- details]
347 total_usage = foldr combineUsageDetails body_usage rhs_usages
348 (combined_usage, tagged_bndrs) = tagBinders total_usage bndrs
349 final_bind = Rec (reOrderRec env new_cycle)
351 new_cycle = CyclicSCC (zipWithEqual "occAnalBind" mk_new_bind tagged_bndrs cycle)
352 mk_new_bind tagged_bndr ((_, _, rhs'), key, keys) = ((tagged_bndr, rhs'), key, keys)
355 @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
356 strongly connected component (there's guaranteed to be a cycle). It returns the
358 a) in a better order,
359 b) with some of the Ids having a IMustNotBeINLINEd pragma
361 The "no-inline" Ids are sufficient to break all cycles in the SCC. This means
362 that the simplifier can guarantee not to loop provided it never records an inlining
363 for these no-inline guys.
365 Furthermore, the order of the binds is such that if we neglect dependencies
366 on the no-inline Ids then the binds are topologically sorted. This means
367 that the simplifier will generally do a good job if it works from top bottom,
368 recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
371 [June 98: I don't understand the following paragraphs, and I've
372 changed the a=b case again so that it isn't a special case any more.]
374 Here's a case that bit me:
382 Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
384 My solution was to make a=b bindings record b as Many, rather like INLINE bindings.
385 Perhaps something cleverer would suffice.
388 You might think that you can prevent non-termination simply by making
389 sure that we simplify a recursive binding's RHS in an environment that
390 simply clones the recursive Id. But no. Consider
392 letrec f = \x -> let z = f x' in ...
399 We bind n to its *simplified* RHS, we then *re-simplify* it when
400 we inline n. Then we may well inline f; and then the same thing
403 I don't think it's possible to prevent non-termination by environment
404 manipulation in this way. Apart from anything else, successive
405 iterations of the simplifier may unroll recursive loops in cases like
406 that above. The idea of beaking every recursive loop with an
407 IMustNotBeINLINEd pragma is much much better.
413 -> SCC (Node Details2)
415 -- Sorted into a plausible order. Enough of the Ids have
416 -- dontINLINE pragmas that there are no loops left.
418 -- Non-recursive case
419 reOrderRec env (AcyclicSCC (bind, _, _)) = [bind]
421 -- Common case of simple self-recursion
422 reOrderRec env (CyclicSCC [bind])
423 = [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
425 ((tagged_bndr, rhs), _, _) = bind
427 reOrderRec env (CyclicSCC (bind : binds))
428 = -- Choose a loop breaker, mark it no-inline,
429 -- do SCC analysis on the rest, and recursively sort them out
430 concat (map (reOrderRec env) (stronglyConnCompR unchosen))
432 [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
435 (chosen_pair, unchosen) = choose_loop_breaker bind (score bind) [] binds
436 (tagged_bndr, rhs) = chosen_pair
438 -- This loop looks for the bind with the lowest score
439 -- to pick as the loop breaker. The rest accumulate in
440 choose_loop_breaker (details,_,_) loop_sc acc []
441 = (details, acc) -- Done
443 choose_loop_breaker loop_bind loop_sc acc (bind : binds)
444 | sc < loop_sc -- Lower score so pick this new one
445 = choose_loop_breaker bind sc (loop_bind : acc) binds
447 | otherwise -- No lower so don't pick it
448 = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
452 score :: Node Details2 -> Int -- Higher score => less likely to be picked as loop breaker
453 score ((bndr, rhs), _, _)
454 | exprIsTrivial rhs = 4 -- Practically certain to be inlined
455 -- Used to have also: && not (isExportedId bndr)
456 -- But I found this sometimes cost an extra iteration when we have
457 -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
458 -- where df is the exported dictionary. Then df makes a really
459 -- bad choice for loop breaker
461 | not_fun_ty (idType bndr) = 3 -- Data types help with cases
462 -- This used to have a lower score than inlineCandidate, but
463 -- it's *really* helpful if dictionaries get inlined fast,
464 -- so I'm experimenting with giving higher priority to data-typed things
466 | inlineCandidate bndr rhs = 2 -- Likely to be inlined
468 | not (isEmptyCoreRules (idSpecialisation bndr)) = 1
469 -- Avoid things with specialisations; we'd like
470 -- to take advantage of them in the subsequent bindings
474 inlineCandidate :: Id -> CoreExpr -> Bool
475 inlineCandidate id (Note InlineMe _) = True
476 inlineCandidate id rhs = isOneOcc (idOccInfo id)
478 -- Real example (the Enum Ordering instance from PrelBase):
479 -- rec f = \ x -> case d of (p,q,r) -> p x
480 -- g = \ x -> case d of (p,q,r) -> q x
483 -- Here, f and g occur just once; but we can't inline them into d.
484 -- On the other hand we *could* simplify those case expressions if
485 -- we didn't stupidly choose d as the loop breaker.
486 -- But we won't because constructor args are marked "Many".
488 not_fun_ty ty = not (isFunTy (dropForAlls ty))
491 @occAnalRhs@ deals with the question of bindings where the Id is marked
492 by an INLINE pragma. For these we record that anything which occurs
493 in its RHS occurs many times. This pessimistically assumes that ths
494 inlined binder also occurs many times in its scope, but if it doesn't
495 we'll catch it next time round. At worst this costs an extra simplifier pass.
496 ToDo: try using the occurrence info for the inline'd binder.
498 [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec.
499 [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec.
504 -> Id -> CoreExpr -- Binder and rhs
505 -> (UsageDetails, CoreExpr)
507 occAnalRhs env id rhs
508 = (final_usage, rhs')
510 (rhs_usage, rhs') = occAnal (rhsCtxt env) rhs
511 -- Note that we use an rhsCtxt. This tells the occ anal that it's
512 -- looking at an RHS, which has an effect in occAnalApp
514 -- But there's a problem. Consider
519 -- First time round, it looks as if x1 and x2 occur as an arg of a
520 -- let-bound constructor ==> give them a many-occurrence.
521 -- But then x3 is inlined (unconditionally as it happens) and
522 -- next time round, x2 will be, and the next time round x1 will be
523 -- Result: multiple simplifier iterations. Sigh.
524 -- Possible solution: use rhsCtxt for things that occur just once...
526 -- [March 98] A new wrinkle is that if the binder has specialisations inside
527 -- it then we count the specialised Ids as "extra rhs's". That way
528 -- the "parent" keeps the specialised "children" alive. If the parent
529 -- dies (because it isn't referenced any more), then the children will
530 -- die too unless they are already referenced directly.
532 final_usage = foldVarSet add rhs_usage (idRuleVars id)
533 add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
534 -- (i.e manyOcc) because many copies
535 -- of the specialised thing can appear
543 -> (UsageDetails, -- Gives info only about the "interesting" Ids
546 occAnal env (Type t) = (emptyDetails, Type t)
551 var_uds | isCandidate env v = unitVarEnv v oneOcc
552 | otherwise = emptyDetails
554 -- At one stage, I gathered the idRuleVars for v here too,
555 -- which in a way is the right thing to do.
556 -- But that went wrong right after specialisation, when
557 -- the *occurrences* of the overloaded function didn't have any
558 -- rules in them, so the *specialised* versions looked as if they
559 -- weren't used at all.
563 We regard variables that occur as constructor arguments as "dangerousToDup":
567 f x = let y = expensive x in
569 (case z of {(p,q)->q}, case z of {(p,q)->q})
572 We feel free to duplicate the WHNF (True,y), but that means
573 that y may be duplicated thereby.
575 If we aren't careful we duplicate the (expensive x) call!
576 Constructors are rather like lambdas in this way.
579 occAnal env expr@(Lit lit) = (emptyDetails, expr)
583 occAnal env (Note InlineMe body)
584 = case occAnal env body of { (usage, body') ->
585 (mapVarEnv markMany usage, Note InlineMe body')
588 occAnal env (Note note@(SCC cc) body)
589 = case occAnal env body of { (usage, body') ->
590 (mapVarEnv markInsideSCC usage, Note note body')
593 occAnal env (Note note body)
594 = case occAnal env body of { (usage, body') ->
595 (usage, Note note body')
600 occAnal env app@(App fun arg)
601 = occAnalApp env (collectArgs app) False
603 -- Ignore type variables altogether
604 -- (a) occurrences inside type lambdas only not marked as InsideLam
605 -- (b) type variables not in environment
607 occAnal env expr@(Lam x body) | isTyVar x
608 = case occAnal env body of { (body_usage, body') ->
609 (body_usage, Lam x body')
612 -- For value lambdas we do a special hack. Consider
614 -- If we did nothing, x is used inside the \y, so would be marked
615 -- as dangerous to dup. But in the common case where the abstraction
616 -- is applied to two arguments this is over-pessimistic.
617 -- So instead, we just mark each binder with its occurrence
618 -- info in the *body* of the multiple lambda.
619 -- Then, the simplifier is careful when partially applying lambdas.
621 occAnal env expr@(Lam _ _)
622 = case occAnal env_body body of { (body_usage, body') ->
624 (final_usage, tagged_binders) = tagBinders body_usage binders
625 -- URGH! Sept 99: we don't seem to be able to use binders' here, because
626 -- we get linear-typed things in the resulting program that we can't handle yet.
627 -- (e.g. PrelShow) TODO
629 really_final_usage = if linear then
632 mapVarEnv markInsideLam final_usage
635 mkLams tagged_binders body') }
637 (binders, body) = collectBinders expr
638 (linear, env1, _) = oneShotGroup env binders
639 env2 = env1 `addNewCands` binders -- Add in-scope binders
640 env_body = vanillaCtxt env2 -- Body is (no longer) an RhsContext
642 occAnal env (Case scrut bndr alts)
643 = case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
644 case occAnal (vanillaCtxt env) scrut of { (scrut_usage, scrut') ->
645 -- No need for rhsCtxt
647 alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
648 alts_usage' = addCaseBndrUsage alts_usage
649 (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
650 total_usage = scrut_usage `combineUsageDetails` alts_usage1
652 total_usage `seq` (total_usage, Case scrut' tagged_bndr alts') }}
654 alt_env = env `addNewCand` bndr
656 -- The case binder gets a usage of either "many" or "dead", never "one".
657 -- Reason: we like to inline single occurrences, to eliminate a binding,
658 -- but inlining a case binder *doesn't* eliminate a binding.
659 -- We *don't* want to transform
660 -- case x of w { (p,q) -> f w }
662 -- case x of w { (p,q) -> f (p,q) }
663 addCaseBndrUsage usage = case lookupVarEnv usage bndr of
665 Just occ -> extendVarEnv usage bndr (markMany occ)
667 occAnal env (Let bind body)
668 = case occAnal new_env body of { (body_usage, body') ->
669 case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
670 (final_usage, mkLets new_binds body') }}
672 new_env = env `addNewCands` (bindersOf bind)
675 = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
676 (foldr combineUsageDetails emptyDetails arg_uds_s, args')}
678 arg_env = vanillaCtxt env
681 Applications are dealt with specially because we want
682 the "build hack" to work.
685 -- Hack for build, fold, runST
686 occAnalApp env (Var fun, args) is_rhs
687 = case args_stuff of { (args_uds, args') ->
689 -- We mark the free vars of the argument of a constructor or PAP
690 -- as "many", if it is the RHS of a let(rec).
691 -- This means that nothing gets inlined into a constructor argument
692 -- position, which is what we want. Typically those constructor
693 -- arguments are just variables, or trivial expressions.
695 -- This is the *whole point* of the isRhsEnv predicate
698 isDataConId fun || valArgCount args < idArity fun
699 = mapVarEnv markMany args_uds
700 | otherwise = args_uds
702 (fun_uds `combineUsageDetails` final_args_uds, mkApps (Var fun) args') }
704 fun_uniq = idUnique fun
706 fun_uds | isCandidate env fun = unitVarEnv fun oneOcc
707 | otherwise = emptyDetails
709 args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
710 | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
711 | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
712 | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
713 -- (foldr k z xs) may call k many times, but it never
714 -- shares a partial application of k; hence [False,True]
715 -- This means we can optimise
716 -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
717 -- by floating in the v
719 | otherwise = occAnalArgs env args
722 occAnalApp env (fun, args) is_rhs
723 = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
724 -- The addAppCtxt is a bit cunning. One iteration of the simplifier
725 -- often leaves behind beta redexs like
727 -- Here we would like to mark x,y as one-shot, and treat the whole
728 -- thing much like a let. We do this by pushing some True items
729 -- onto the context stack.
731 case occAnalArgs env args of { (args_uds, args') ->
733 final_uds = fun_uds `combineUsageDetails` args_uds
735 (final_uds, mkApps fun' args') }}
738 -> Int -> CtxtTy -- Argument number, and context to use for it
740 -> (UsageDetails, [CoreExpr])
741 appSpecial env n ctxt args
744 arg_env = vanillaCtxt env
746 go n [] = (emptyDetails, []) -- Too few args
748 go 1 (arg:args) -- The magic arg
749 = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
750 case occAnalArgs env args of { (args_uds, args') ->
751 (combineUsageDetails arg_uds args_uds, arg':args') }}
754 = case occAnal arg_env arg of { (arg_uds, arg') ->
755 case go (n-1) args of { (args_uds, args') ->
756 (combineUsageDetails arg_uds args_uds, arg':args') }}
762 If the case binder occurs at all, the other binders effectively do too.
764 case e of x { (a,b) -> rhs }
767 If e turns out to be (e1,e2) we indeed get something like
768 let a = e1; b = e2; x = (a,b) in rhs
771 occAnalAlt env case_bndr (con, bndrs, rhs)
772 = case occAnal (env `addNewCands` bndrs) rhs of { (rhs_usage, rhs') ->
774 (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
775 final_bndrs | case_bndr `elemVarEnv` final_usage = bndrs
776 | otherwise = tagged_bndrs
777 -- Leave the binders untagged if the case
778 -- binder occurs at all; see note above
780 (final_usage, (con, final_bndrs, rhs')) }
784 %************************************************************************
786 \subsection[OccurAnal-types]{OccEnv}
788 %************************************************************************
792 = OccEnv IdSet -- In-scope Ids; we gather info about these only
793 OccEncl -- Enclosing context information
794 CtxtTy -- Tells about linearity
796 -- OccEncl is used to control whether to inline into constructor arguments
798 -- x = (p,q) -- Don't inline p or q
799 -- y = /\a -> (p a, q a) -- Still don't inline p or q
800 -- z = f (p,q) -- Do inline p,q; it may make a rule fire
801 -- So OccEncl tells enought about the context to know what to do when
802 -- we encounter a contructor application or PAP.
805 = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
806 -- Don't inline into constructor args here
807 | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
808 -- Do inline into constructor args here
813 -- True:ctxt Analysing a function-valued expression that will be
816 -- False:ctxt Analysing a function-valued expression that may
817 -- be applied many times; but when it is,
818 -- the CtxtTy inside applies
820 initOccEnv :: VarSet -> OccEnv
821 initOccEnv vars = OccEnv vars OccRhs []
823 isRhsEnv (OccEnv _ OccRhs _) = True
824 isRhsEnv (OccEnv _ OccVanilla _) = False
826 isCandidate :: OccEnv -> Id -> Bool
827 isCandidate (OccEnv cands encl _) id = id `elemVarSet` cands
829 addNewCands :: OccEnv -> [Id] -> OccEnv
830 addNewCands (OccEnv cands encl ctxt) ids
831 = OccEnv (cands `unionVarSet` mkVarSet ids) encl ctxt
833 addNewCand :: OccEnv -> Id -> OccEnv
834 addNewCand (OccEnv cands encl ctxt) id
835 = OccEnv (extendVarSet cands id) encl ctxt
837 setCtxt :: OccEnv -> CtxtTy -> OccEnv
838 setCtxt (OccEnv cands encl _) ctxt = OccEnv cands encl ctxt
840 oneShotGroup :: OccEnv -> [CoreBndr] -> (Bool, OccEnv, [CoreBndr])
841 -- True <=> this is a one-shot linear lambda group
842 -- The [CoreBndr] are the binders.
844 -- The result binders have one-shot-ness set that they might not have had originally.
845 -- This happens in (build (\cn -> e)). Here the occurrence analyser
846 -- linearity context knows that c,n are one-shot, and it records that fact in
847 -- the binder. This is useful to guide subsequent float-in/float-out tranformations
849 oneShotGroup (OccEnv cands encl ctxt) bndrs
850 = case go ctxt bndrs [] of
851 (new_ctxt, new_bndrs) -> (all is_one_shot new_bndrs, OccEnv cands encl new_ctxt, new_bndrs)
853 is_one_shot b = isId b && isOneShotLambda b
855 go ctxt [] rev_bndrs = (ctxt, reverse rev_bndrs)
857 go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
858 | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
860 bndr' | lin_ctxt = setOneShotLambda bndr
863 go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
866 vanillaCtxt (OccEnv cands _ _) = OccEnv cands OccVanilla []
867 rhsCtxt (OccEnv cands _ _) = OccEnv cands OccRhs []
869 addAppCtxt (OccEnv cands encl ctxt) args
870 = OccEnv cands encl (replicate (valArgCount args) True ++ ctxt)
873 %************************************************************************
875 \subsection[OccurAnal-types]{OccEnv}
877 %************************************************************************
880 type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
882 combineUsageDetails, combineAltsUsageDetails
883 :: UsageDetails -> UsageDetails -> UsageDetails
885 combineUsageDetails usage1 usage2
886 = plusVarEnv_C addOccInfo usage1 usage2
888 combineAltsUsageDetails usage1 usage2
889 = plusVarEnv_C orOccInfo usage1 usage2
891 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
892 addOneOcc usage id info
893 = plusVarEnv_C addOccInfo usage (unitVarEnv id info)
894 -- ToDo: make this more efficient
896 emptyDetails = (emptyVarEnv :: UsageDetails)
898 usedIn :: Id -> UsageDetails -> Bool
899 v `usedIn` details = isExportedId v || v `elemVarEnv` details
901 tagBinders :: UsageDetails -- Of scope
903 -> (UsageDetails, -- Details with binders removed
904 [IdWithOccInfo]) -- Tagged binders
906 tagBinders usage binders
908 usage' = usage `delVarEnvList` binders
909 uss = map (setBinderOcc usage) binders
911 usage' `seq` (usage', uss)
913 tagBinder :: UsageDetails -- Of scope
915 -> (UsageDetails, -- Details with binders removed
916 IdWithOccInfo) -- Tagged binders
918 tagBinder usage binder
920 usage' = usage `delVarEnv` binder
921 binder' = setBinderOcc usage binder
923 usage' `seq` (usage', binder')
925 setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
926 setBinderOcc usage bndr
927 | isTyVar bndr = bndr
928 | isExportedId bndr = case idOccInfo bndr of
930 other -> setIdOccInfo bndr NoOccInfo
931 -- Don't use local usage info for visible-elsewhere things
932 -- BUT *do* erase any IAmALoopBreaker annotation, because we're
933 -- about to re-generate it and it shouldn't be "sticky"
935 | otherwise = setIdOccInfo bndr occ_info
937 occ_info = lookupVarEnv usage bndr `orElse` IAmDead
941 %************************************************************************
943 \subsection{Operations over OccInfo}
945 %************************************************************************
949 oneOcc = OneOcc False True
951 markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
953 markMany IAmDead = IAmDead
954 markMany other = NoOccInfo
956 markInsideSCC occ = markMany occ
958 markInsideLam (OneOcc _ one_br) = OneOcc True one_br
959 markInsideLam occ = occ
961 addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
963 addOccInfo IAmDead info2 = info2
964 addOccInfo info1 IAmDead = info1
965 addOccInfo info1 info2 = NoOccInfo
967 -- (orOccInfo orig new) is used
968 -- when combining occurrence info from branches of a case
970 orOccInfo IAmDead info2 = info2
971 orOccInfo info1 IAmDead = info1
972 orOccInfo (OneOcc in_lam1 one_branch1)
973 (OneOcc in_lam2 one_branch2)
974 = OneOcc (in_lam1 || in_lam2)
975 False -- False, because it occurs in both branches
977 orOccInfo info1 info2 = NoOccInfo