2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 %************************************************************************
6 \section[OccurAnal]{Occurrence analysis pass}
8 %************************************************************************
10 The occurrence analyser re-typechecks a core expression, returning a new
11 core expression with (hopefully) improved usage information.
15 occurAnalyseBinds, occurAnalyseGlobalExpr, occurAnalyseRule
18 #include "HsVersions.h"
21 import CoreFVs ( idRuleVars )
22 import CoreUtils ( exprIsTrivial )
23 import Id ( isDataConId, isOneShotLambda, setOneShotLambda,
24 idOccInfo, setIdOccInfo,
25 isExportedId, modifyIdInfo, idInfo, idArity,
26 idSpecialisation, isLocalId,
29 import IdInfo ( OccInfo(..), shortableIdInfo, copyIdInfo )
34 import Type ( splitFunTy_maybe, splitForAllTys )
35 import Maybes ( maybeToBool, orElse )
36 import Digraph ( stronglyConnCompR, SCC(..) )
37 import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
38 import Unique ( Unique )
39 import UniqFM ( keysUFM )
40 import Util ( zipWithEqual, mapAndUnzip )
45 %************************************************************************
47 \subsection[OccurAnal-main]{Counting occurrences: main function}
49 %************************************************************************
51 Here's the externally-callable interface:
54 occurAnalyseGlobalExpr :: CoreExpr -> CoreExpr
55 occurAnalyseGlobalExpr expr
56 = -- Top level expr, so no interesting free vars, and
57 -- discard occurence info returned
58 snd (occAnal (initOccEnv emptyVarSet) expr)
60 occurAnalyseRule :: CoreRule -> CoreRule
61 occurAnalyseRule rule@(BuiltinRule _ _) = rule
62 occurAnalyseRule (Rule str act tpl_vars tpl_args rhs)
63 -- Add occ info to tpl_vars, rhs
64 = Rule str act tpl_vars' tpl_args rhs'
66 (rhs_uds, rhs') = occAnal (initOccEnv (mkVarSet tpl_vars)) rhs
67 (_, tpl_vars') = tagBinders rhs_uds tpl_vars
71 %************************************************************************
73 \subsection{Top level stuff}
75 %************************************************************************
77 In @occAnalTop@ we do indirection-shorting. That is, if we have this:
79 x_local = <expression>
83 where exp is exported, and loc is not, then we replace it with this:
86 x_exported = <expression>
89 Without this we never get rid of the x_exported = x_local thing. This
90 save a gratuitous jump (from \tr{x_exported} to \tr{x_local}), and
91 makes strictness information propagate better. This used to happen in
92 the final phase, but it's tidier to do it here.
94 If more than one exported thing is equal to a local thing (i.e., the
95 local thing really is shared), then we do one only:
103 x_exported2 = x_exported1
106 We rely on prior eta reduction to simplify things like
108 x_exported = /\ tyvars -> x_local tyvars
112 Hence,there's a possibility of leaving unchanged something like this:
115 x_exported1 = x_local Int
117 By the time we've thrown away the types in STG land this
118 could be eliminated. But I don't think it's very common
119 and it's dangerous to do this fiddling in STG land
120 because we might elminate a binding that's mentioned in the
121 unfolding for something.
124 occurAnalyseBinds :: [CoreBind] -> [CoreBind]
126 occurAnalyseBinds binds
129 (_, _, binds') = go (initOccEnv emptyVarSet) binds
131 go :: OccEnv -> [CoreBind]
132 -> (UsageDetails, -- Occurrence info
133 IdEnv Id, -- Indirection elimination info
134 -- Maps local-id -> exported-id, but it embodies
135 -- bindings of the form exported-id = local-id in
136 -- the argument to go
137 [CoreBind]) -- Occ-analysed bindings, less the exported-id=local-id ones
139 go env [] = (emptyDetails, emptyVarEnv, [])
141 go env (bind : binds)
143 new_env = env `addNewCands` (bindersOf bind)
144 (scope_usage, ind_env, binds') = go new_env binds
145 (final_usage, new_binds) = occAnalBind env (zapBind ind_env bind) scope_usage
146 -- NB: I zap before occur-analysing, so
147 -- I don't need to worry about getting the
148 -- occ info on the new bindings right.
151 NonRec exported_id (Var local_id)
152 | shortMeOut ind_env exported_id local_id
153 -- Special case for eliminating indirections
154 -- Note: it's a shortcoming that this only works for
155 -- non-recursive bindings. Elminating indirections
156 -- makes perfect sense for recursive bindings too, but
157 -- it's more complicated to implement, so I haven't done so
158 -> (scope_usage, ind_env', binds')
160 ind_env' = extendVarEnv ind_env local_id exported_id
162 other -> -- Ho ho! The normal case
163 (final_usage, ind_env, new_binds ++ binds')
166 -- Deal with any indirections
167 zapBind ind_env (NonRec bndr rhs)
168 | bndr `elemVarEnv` ind_env = Rec (zap ind_env (bndr,rhs))
169 -- The Rec isn't strictly necessary, but it's convenient
170 zapBind ind_env (Rec pairs)
171 | or [id `elemVarEnv` ind_env | (id,_) <- pairs] = Rec (concat (map (zap ind_env) pairs))
173 zapBind ind_env bind = bind
175 zap ind_env pair@(local_id,rhs)
176 = case lookupVarEnv ind_env local_id of
178 Just exported_id -> [(local_id, Var exported_id),
181 exported_id' = modifyIdInfo (copyIdInfo (idInfo local_id)) exported_id
183 shortMeOut ind_env exported_id local_id
184 -- The if-then-else stuff is just so I can get a pprTrace to see
185 -- how often I don't get shorting out becuase of IdInfo stuff
186 = if isExportedId exported_id && -- Only if this is exported
188 isLocalId local_id && -- Only if this one is defined in this
189 -- module, so that we *can* change its
190 -- binding to be the exported thing!
192 not (isExportedId local_id) && -- Only if this one is not itself exported,
193 -- since the transformation will nuke it
195 not (local_id `elemVarEnv` ind_env) -- Only if not already substituted for
197 if shortableIdInfo (idInfo exported_id) -- Only if its IdInfo is 'shortable'
198 -- (see the defn of IdInfo.shortableIdInfo)
202 pprTrace "shortMeOut:" (ppr exported_id)
210 %************************************************************************
212 \subsection[OccurAnal-main]{Counting occurrences: main function}
214 %************************************************************************
220 type IdWithOccInfo = Id -- An Id with fresh PragmaInfo attached
222 type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
223 -- which is gotten from the Id.
224 type Details1 = (Id, UsageDetails, CoreExpr)
225 type Details2 = (IdWithOccInfo, CoreExpr)
228 occAnalBind :: OccEnv
230 -> UsageDetails -- Usage details of scope
231 -> (UsageDetails, -- Of the whole let(rec)
234 occAnalBind env (NonRec binder rhs) body_usage
235 | not (binder `usedIn` body_usage) -- It's not mentioned
238 | otherwise -- It's mentioned in the body
239 = (final_body_usage `combineUsageDetails` rhs_usage,
240 [NonRec tagged_binder rhs'])
243 (final_body_usage, tagged_binder) = tagBinder body_usage binder
244 (rhs_usage, rhs') = occAnalRhs env binder rhs
247 Dropping dead code for recursive bindings is done in a very simple way:
249 the entire set of bindings is dropped if none of its binders are
250 mentioned in its body; otherwise none are.
252 This seems to miss an obvious improvement.
267 Now @f@ is unused. But dependency analysis will sort this out into a
268 @letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
269 It isn't easy to do a perfect job in one blow. Consider
283 occAnalBind env (Rec pairs) body_usage
284 = foldr (_scc_ "occAnalBind.dofinal" do_final_bind) (body_usage, []) sccs
286 binders = map fst pairs
287 rhs_env = env `addNewCands` binders
289 analysed_pairs :: [Details1]
290 analysed_pairs = [ (bndr, rhs_usage, rhs')
291 | (bndr, rhs) <- pairs,
292 let (rhs_usage, rhs') = occAnalRhs rhs_env bndr rhs
295 sccs :: [SCC (Node Details1)]
296 sccs = _scc_ "occAnalBind.scc" stronglyConnCompR edges
299 ---- stuff for dependency analysis of binds -------------------------------
300 edges :: [Node Details1]
301 edges = _scc_ "occAnalBind.assoc"
302 [ (details, idUnique id, edges_from rhs_usage)
303 | details@(id, rhs_usage, rhs) <- analysed_pairs
306 -- (a -> b) means a mentions b
307 -- Given the usage details (a UFM that gives occ info for each free var of
308 -- the RHS) we can get the list of free vars -- or rather their Int keys --
309 -- by just extracting the keys from the finite map. Grimy, but fast.
310 -- Previously we had this:
311 -- [ bndr | bndr <- bndrs,
312 -- maybeToBool (lookupVarEnv rhs_usage bndr)]
313 -- which has n**2 cost, and this meant that edges_from alone
314 -- consumed 10% of total runtime!
315 edges_from :: UsageDetails -> [Unique]
316 edges_from rhs_usage = _scc_ "occAnalBind.edges_from"
319 ---- stuff to "re-constitute" bindings from dependency-analysis info ------
322 do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
323 | not (bndr `usedIn` body_usage)
324 = (body_usage, binds_so_far) -- Dead code
326 = (combined_usage, new_bind : binds_so_far)
328 total_usage = combineUsageDetails body_usage rhs_usage
329 (combined_usage, tagged_bndr) = tagBinder total_usage bndr
330 new_bind = NonRec tagged_bndr rhs'
333 do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
334 | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
335 = (body_usage, binds_so_far) -- Dead code
337 = (combined_usage, final_bind:binds_so_far)
339 details = [details | (details, _, _) <- cycle]
340 bndrs = [bndr | (bndr, _, _) <- details]
341 rhs_usages = [rhs_usage | (_, rhs_usage, _) <- details]
342 total_usage = foldr combineUsageDetails body_usage rhs_usages
343 (combined_usage, tagged_bndrs) = tagBinders total_usage bndrs
344 final_bind = Rec (reOrderRec env new_cycle)
346 new_cycle = CyclicSCC (zipWithEqual "occAnalBind" mk_new_bind tagged_bndrs cycle)
347 mk_new_bind tagged_bndr ((_, _, rhs'), key, keys) = ((tagged_bndr, rhs'), key, keys)
350 @reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
351 strongly connected component (there's guaranteed to be a cycle). It returns the
353 a) in a better order,
354 b) with some of the Ids having a IMustNotBeINLINEd pragma
356 The "no-inline" Ids are sufficient to break all cycles in the SCC. This means
357 that the simplifier can guarantee not to loop provided it never records an inlining
358 for these no-inline guys.
360 Furthermore, the order of the binds is such that if we neglect dependencies
361 on the no-inline Ids then the binds are topologically sorted. This means
362 that the simplifier will generally do a good job if it works from top bottom,
363 recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
366 [June 98: I don't understand the following paragraphs, and I've
367 changed the a=b case again so that it isn't a special case any more.]
369 Here's a case that bit me:
377 Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
379 My solution was to make a=b bindings record b as Many, rather like INLINE bindings.
380 Perhaps something cleverer would suffice.
383 You might think that you can prevent non-termination simply by making
384 sure that we simplify a recursive binding's RHS in an environment that
385 simply clones the recursive Id. But no. Consider
387 letrec f = \x -> let z = f x' in ...
394 We bind n to its *simplified* RHS, we then *re-simplify* it when
395 we inline n. Then we may well inline f; and then the same thing
398 I don't think it's possible to prevent non-termination by environment
399 manipulation in this way. Apart from anything else, successive
400 iterations of the simplifier may unroll recursive loops in cases like
401 that above. The idea of beaking every recursive loop with an
402 IMustNotBeINLINEd pragma is much much better.
408 -> SCC (Node Details2)
410 -- Sorted into a plausible order. Enough of the Ids have
411 -- dontINLINE pragmas that there are no loops left.
413 -- Non-recursive case
414 reOrderRec env (AcyclicSCC (bind, _, _)) = [bind]
416 -- Common case of simple self-recursion
417 reOrderRec env (CyclicSCC [bind])
418 = [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
420 ((tagged_bndr, rhs), _, _) = bind
422 reOrderRec env (CyclicSCC (bind : binds))
423 = -- Choose a loop breaker, mark it no-inline,
424 -- do SCC analysis on the rest, and recursively sort them out
425 concat (map (reOrderRec env) (stronglyConnCompR unchosen))
427 [(setIdOccInfo tagged_bndr IAmALoopBreaker, rhs)]
430 (chosen_pair, unchosen) = choose_loop_breaker bind (score bind) [] binds
431 (tagged_bndr, rhs) = chosen_pair
433 -- This loop looks for the bind with the lowest score
434 -- to pick as the loop breaker. The rest accumulate in
435 choose_loop_breaker (details,_,_) loop_sc acc []
436 = (details, acc) -- Done
438 choose_loop_breaker loop_bind loop_sc acc (bind : binds)
439 | sc < loop_sc -- Lower score so pick this new one
440 = choose_loop_breaker bind sc (loop_bind : acc) binds
442 | otherwise -- No lower so don't pick it
443 = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
447 score :: Node Details2 -> Int -- Higher score => less likely to be picked as loop breaker
448 score ((bndr, rhs), _, _)
449 | exprIsTrivial rhs = 4 -- Practically certain to be inlined
450 -- Used to have also: && not (isExportedId bndr)
451 -- But I found this sometimes cost an extra iteration when we have
452 -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
453 -- where df is the exported dictionary. Then df makes a really
454 -- bad choice for loop breaker
456 | not_fun_ty (idType bndr) = 3 -- Data types help with cases
457 -- This used to have a lower score than inlineCandidate, but
458 -- it's *really* helpful if dictionaries get inlined fast,
459 -- so I'm experimenting with giving higher priority to data-typed things
461 | inlineCandidate bndr rhs = 2 -- Likely to be inlined
463 | not (isEmptyCoreRules (idSpecialisation bndr)) = 1
464 -- Avoid things with specialisations; we'd like
465 -- to take advantage of them in the subsequent bindings
469 inlineCandidate :: Id -> CoreExpr -> Bool
470 inlineCandidate id (Note InlineMe _) = True
471 inlineCandidate id rhs = case idOccInfo id of
475 -- Real example (the Enum Ordering instance from PrelBase):
476 -- rec f = \ x -> case d of (p,q,r) -> p x
477 -- g = \ x -> case d of (p,q,r) -> q x
480 -- Here, f and g occur just once; but we can't inline them into d.
481 -- On the other hand we *could* simplify those case expressions if
482 -- we didn't stupidly choose d as the loop breaker.
483 -- But we won't because constructor args are marked "Many".
485 not_fun_ty ty = not (maybeToBool (splitFunTy_maybe rho_ty))
487 (_, rho_ty) = splitForAllTys ty
490 @occAnalRhs@ deals with the question of bindings where the Id is marked
491 by an INLINE pragma. For these we record that anything which occurs
492 in its RHS occurs many times. This pessimistically assumes that ths
493 inlined binder also occurs many times in its scope, but if it doesn't
494 we'll catch it next time round. At worst this costs an extra simplifier pass.
495 ToDo: try using the occurrence info for the inline'd binder.
497 [March 97] We do the same for atomic RHSs. Reason: see notes with reOrderRec.
498 [June 98, SLPJ] I've undone this change; I don't understand it. See notes with reOrderRec.
503 -> Id -> CoreExpr -- Binder and rhs
504 -> (UsageDetails, CoreExpr)
506 occAnalRhs env id rhs
507 = (final_usage, rhs')
509 (rhs_usage, rhs') = occAnal (rhsCtxt env) rhs
511 -- [March 98] A new wrinkle is that if the binder has specialisations inside
512 -- it then we count the specialised Ids as "extra rhs's". That way
513 -- the "parent" keeps the specialised "children" alive. If the parent
514 -- dies (because it isn't referenced any more), then the children will
515 -- die too unless they are already referenced directly.
517 final_usage = foldVarSet add rhs_usage (idRuleVars id)
518 add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
519 -- (i.e manyOcc) because many copies
520 -- of the specialised thing can appear
528 -> (UsageDetails, -- Gives info only about the "interesting" Ids
531 occAnal env (Type t) = (emptyDetails, Type t)
536 var_uds | isCandidate env v = unitVarEnv v oneOcc
537 | otherwise = emptyDetails
539 -- At one stage, I gathered the idRuleVars for v here too,
540 -- which in a way is the right thing to do.
541 -- But that went wrong right after specialisation, when
542 -- the *occurrences* of the overloaded function didn't have any
543 -- rules in them, so the *specialised* versions looked as if they
544 -- weren't used at all.
548 We regard variables that occur as constructor arguments as "dangerousToDup":
552 f x = let y = expensive x in
554 (case z of {(p,q)->q}, case z of {(p,q)->q})
557 We feel free to duplicate the WHNF (True,y), but that means
558 that y may be duplicated thereby.
560 If we aren't careful we duplicate the (expensive x) call!
561 Constructors are rather like lambdas in this way.
564 occAnal env expr@(Lit lit) = (emptyDetails, expr)
568 occAnal env (Note InlineMe body)
569 = case occAnal env body of { (usage, body') ->
570 (mapVarEnv markMany usage, Note InlineMe body')
573 occAnal env (Note note@(SCC cc) body)
574 = case occAnal env body of { (usage, body') ->
575 (mapVarEnv markInsideSCC usage, Note note body')
578 occAnal env (Note note body)
579 = case occAnal env body of { (usage, body') ->
580 (usage, Note note body')
585 occAnal env app@(App fun arg)
586 = occAnalApp env (collectArgs app) False
588 -- Ignore type variables altogether
589 -- (a) occurrences inside type lambdas only not marked as InsideLam
590 -- (b) type variables not in environment
592 occAnal env expr@(Lam x body) | isTyVar x
593 = case occAnal env body of { (body_usage, body') ->
594 (body_usage, Lam x body')
597 -- For value lambdas we do a special hack. Consider
599 -- If we did nothing, x is used inside the \y, so would be marked
600 -- as dangerous to dup. But in the common case where the abstraction
601 -- is applied to two arguments this is over-pessimistic.
602 -- So instead, we just mark each binder with its occurrence
603 -- info in the *body* of the multiple lambda.
604 -- Then, the simplifier is careful when partially applying lambdas.
606 occAnal env expr@(Lam _ _)
607 = case occAnal env_body body of { (body_usage, body') ->
609 (final_usage, tagged_binders) = tagBinders body_usage binders
610 -- URGH! Sept 99: we don't seem to be able to use binders' here, because
611 -- we get linear-typed things in the resulting program that we can't handle yet.
612 -- (e.g. PrelShow) TODO
614 really_final_usage = if linear then
617 mapVarEnv markInsideLam final_usage
620 mkLams tagged_binders body') }
622 (binders, body) = collectBinders expr
623 (linear, env1, _) = oneShotGroup env binders
624 env2 = env1 `addNewCands` binders -- Add in-scope binders
625 env_body = vanillaCtxt env2 -- Body is (no longer) an RhsContext
627 occAnal env (Case scrut bndr alts)
628 = case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
629 case occAnal (vanillaCtxt env) scrut of { (scrut_usage, scrut') ->
630 -- No need for rhsCtxt
632 alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
633 alts_usage' = addCaseBndrUsage alts_usage
634 (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
635 total_usage = scrut_usage `combineUsageDetails` alts_usage1
637 total_usage `seq` (total_usage, Case scrut' tagged_bndr alts') }}
639 alt_env = env `addNewCand` bndr
641 -- The case binder gets a usage of either "many" or "dead", never "one".
642 -- Reason: we like to inline single occurrences, to eliminate a binding,
643 -- but inlining a case binder *doesn't* eliminate a binding.
644 -- We *don't* want to transform
645 -- case x of w { (p,q) -> f w }
647 -- case x of w { (p,q) -> f (p,q) }
648 addCaseBndrUsage usage = case lookupVarEnv usage bndr of
650 Just occ -> extendVarEnv usage bndr (markMany occ)
652 occAnal env (Let bind body)
653 = case occAnal new_env body of { (body_usage, body') ->
654 case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
655 (final_usage, mkLets new_binds body') }}
657 new_env = env `addNewCands` (bindersOf bind)
660 = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
661 (foldr combineUsageDetails emptyDetails arg_uds_s, args')}
663 arg_env = vanillaCtxt env
666 Applications are dealt with specially because we want
667 the "build hack" to work.
670 -- Hack for build, fold, runST
671 occAnalApp env (Var fun, args) is_rhs
672 = case args_stuff of { (args_uds, args') ->
674 final_uds = fun_uds `combineUsageDetails` args_uds
676 (final_uds, mkApps (Var fun) args') }
678 fun_uniq = idUnique fun
680 fun_uds | isCandidate env fun = unitVarEnv fun oneOcc
681 | otherwise = emptyDetails
683 args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
684 | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
685 | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
686 | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
687 -- (foldr k z xs) may call k many times, but it never
688 -- shares a partial application of k; hence [False,True]
689 -- This means we can optimise
690 -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
691 -- by floating in the v
694 isDataConId fun || valArgCount args < idArity fun
695 = case occAnalArgs env args of
696 (arg_uds, args') -> (mapVarEnv markMany arg_uds, args')
697 -- We mark the free vars of the argument of a constructor or PAP
698 -- as "many", if it is the RHS of a let(rec).
699 -- This means that nothing gets inlined into a constructor argument
700 -- position, which is what we want. Typically those constructor
701 -- arguments are just variables, or trivial expressions.
703 | otherwise = occAnalArgs env args
706 occAnalApp env (fun, args) is_rhs
707 = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
708 -- The addAppCtxt is a bit cunning. One iteration of the simplifier
709 -- often leaves behind beta redexs like
711 -- Here we would like to mark x,y as one-shot, and treat the whole
712 -- thing much like a let. We do this by pushing some True items
713 -- onto the context stack.
715 case occAnalArgs env args of { (args_uds, args') ->
717 final_uds = fun_uds `combineUsageDetails` args_uds
719 (final_uds, mkApps fun' args') }}
722 -> Int -> CtxtTy -- Argument number, and context to use for it
724 -> (UsageDetails, [CoreExpr])
725 appSpecial env n ctxt args
728 arg_env = vanillaCtxt env
730 go n [] = (emptyDetails, []) -- Too few args
732 go 1 (arg:args) -- The magic arg
733 = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
734 case occAnalArgs env args of { (args_uds, args') ->
735 (combineUsageDetails arg_uds args_uds, arg':args') }}
738 = case occAnal arg_env arg of { (arg_uds, arg') ->
739 case go (n-1) args of { (args_uds, args') ->
740 (combineUsageDetails arg_uds args_uds, arg':args') }}
746 If the case binder occurs at all, the other binders effectively do too.
748 case e of x { (a,b) -> rhs }
751 If e turns out to be (e1,e2) we indeed get something like
752 let a = e1; b = e2; x = (a,b) in rhs
755 occAnalAlt env case_bndr (con, bndrs, rhs)
756 = case occAnal (env `addNewCands` bndrs) rhs of { (rhs_usage, rhs') ->
758 (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
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
764 (final_usage, (con, final_bndrs, rhs')) }
768 %************************************************************************
770 \subsection[OccurAnal-types]{OccEnv}
772 %************************************************************************
776 = OccEnv IdSet -- In-scope Ids; we gather info about these only
777 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
804 initOccEnv :: VarSet -> OccEnv
805 initOccEnv vars = OccEnv vars OccRhs []
807 isRhsEnv (OccEnv _ OccRhs _) = True
808 isRhsEnv (OccEnv _ OccVanilla _) = False
810 isCandidate :: OccEnv -> Id -> Bool
811 isCandidate (OccEnv cands encl _) id = id `elemVarSet` cands
813 addNewCands :: OccEnv -> [Id] -> OccEnv
814 addNewCands (OccEnv cands encl ctxt) ids
815 = OccEnv (cands `unionVarSet` mkVarSet ids) encl ctxt
817 addNewCand :: OccEnv -> Id -> OccEnv
818 addNewCand (OccEnv cands encl ctxt) id
819 = OccEnv (extendVarSet cands id) encl ctxt
821 setCtxt :: OccEnv -> CtxtTy -> OccEnv
822 setCtxt (OccEnv cands encl _) ctxt = OccEnv cands encl ctxt
824 oneShotGroup :: OccEnv -> [CoreBndr] -> (Bool, OccEnv, [CoreBndr])
825 -- True <=> this is a one-shot linear lambda group
826 -- The [CoreBndr] are the binders.
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 cands encl ctxt) bndrs
834 = case go ctxt bndrs [] of
835 (new_ctxt, new_bndrs) -> (all is_one_shot new_bndrs, OccEnv cands encl new_ctxt, new_bndrs)
837 is_one_shot b = isId b && isOneShotLambda b
839 go ctxt [] rev_bndrs = (ctxt, reverse rev_bndrs)
841 go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
842 | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
844 bndr' | lin_ctxt = setOneShotLambda bndr
847 go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
850 vanillaCtxt (OccEnv cands _ _) = OccEnv cands OccVanilla []
851 rhsCtxt (OccEnv cands _ _) = OccEnv cands OccRhs []
853 addAppCtxt (OccEnv cands encl ctxt) args
854 = OccEnv cands encl (replicate (valArgCount args) True ++ ctxt)
857 %************************************************************************
859 \subsection[OccurAnal-types]{OccEnv}
861 %************************************************************************
864 type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
866 combineUsageDetails, combineAltsUsageDetails
867 :: UsageDetails -> UsageDetails -> UsageDetails
869 combineUsageDetails usage1 usage2
870 = plusVarEnv_C addOccInfo usage1 usage2
872 combineAltsUsageDetails usage1 usage2
873 = plusVarEnv_C orOccInfo usage1 usage2
875 addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
876 addOneOcc usage id info
877 = plusVarEnv_C addOccInfo usage (unitVarEnv id info)
878 -- ToDo: make this more efficient
880 emptyDetails = (emptyVarEnv :: UsageDetails)
882 usedIn :: Id -> UsageDetails -> Bool
883 v `usedIn` details = isExportedId v || v `elemVarEnv` details
885 tagBinders :: UsageDetails -- Of scope
887 -> (UsageDetails, -- Details with binders removed
888 [IdWithOccInfo]) -- Tagged binders
890 tagBinders usage binders
892 usage' = usage `delVarEnvList` binders
893 uss = map (setBinderOcc usage) binders
895 usage' `seq` (usage', uss)
897 tagBinder :: UsageDetails -- Of scope
899 -> (UsageDetails, -- Details with binders removed
900 IdWithOccInfo) -- Tagged binders
902 tagBinder usage binder
904 usage' = usage `delVarEnv` binder
905 binder' = setBinderOcc usage binder
907 usage' `seq` (usage', binder')
909 setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
910 setBinderOcc usage bndr
911 | isTyVar bndr = bndr
912 | isExportedId bndr = case idOccInfo bndr of
914 other -> setIdOccInfo bndr NoOccInfo
915 -- Don't use local usage info for visible-elsewhere things
916 -- BUT *do* erase any IAmALoopBreaker annotation, because we're
917 -- about to re-generate it and it shouldn't be "sticky"
919 | otherwise = setIdOccInfo bndr occ_info
921 occ_info = lookupVarEnv usage bndr `orElse` IAmDead
925 %************************************************************************
927 \subsection{Operations over OccInfo}
929 %************************************************************************
933 oneOcc = OneOcc False True
935 markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
937 markMany IAmDead = IAmDead
938 markMany other = NoOccInfo
940 markInsideSCC occ = markMany occ
942 markInsideLam (OneOcc _ one_br) = OneOcc True one_br
943 markInsideLam occ = occ
945 addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
947 addOccInfo IAmDead info2 = info2
948 addOccInfo info1 IAmDead = info1
949 addOccInfo info1 info2 = NoOccInfo
951 -- (orOccInfo orig new) is used
952 -- when combining occurrence info from branches of a case
954 orOccInfo IAmDead info2 = info2
955 orOccInfo info1 IAmDead = info1
956 orOccInfo (OneOcc in_lam1 one_branch1)
957 (OneOcc in_lam2 one_branch2)
958 = OneOcc (in_lam1 || in_lam2)
959 False -- False, because it occurs in both branches
961 orOccInfo info1 info2 = NoOccInfo