2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
8 simplBinder, simplBinders, simplRecIds, simplLetId,
9 tryRhsTyLam, tryEtaExpansion,
12 -- The continuation type
13 SimplCont(..), DupFlag(..), contIsDupable, contResultType,
14 countValArgs, countArgs, mkRhsStop, mkStop,
15 getContArgs, interestingCallContext, interestingArg, isStrictType, discardInline
19 #include "HsVersions.h"
21 import CmdLineOpts ( switchIsOn, SimplifierSwitch(..),
22 opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge,
26 import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap,
27 etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce,
30 import Subst ( InScopeSet, mkSubst, substExpr )
31 import qualified Subst ( simplBndrs, simplBndr, simplLetId )
32 import Id ( idType, idName,
33 idUnfolding, idNewStrictness,
36 import IdInfo ( StrictnessInfo(..) )
37 import Maybes ( maybeToBool, catMaybes )
38 import Name ( setNameUnique )
39 import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
41 import Type ( Type, mkForAllTys, seqType,
42 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
43 isUnLiftedType, isStrictType,
46 import TyCon ( tyConDataConsIfAvailable )
47 import DataCon ( dataConRepArity )
48 import VarEnv ( SubstEnv )
49 import Util ( lengthExceeds, mapAccumL )
54 %************************************************************************
56 \subsection{The continuation data type}
58 %************************************************************************
61 data SimplCont -- Strict contexts
62 = Stop OutType -- Type of the result
63 Bool -- True => This is the RHS of a thunk whose type suggests
64 -- that update-in-place would be possible
65 -- (This makes the inliner a little keener.)
67 | CoerceIt OutType -- The To-type, simplified
70 | InlinePlease -- This continuation makes a function very
71 SimplCont -- keen to inline itelf
74 InExpr SubstEnv -- The argument, as yet unsimplified,
75 SimplCont -- and its subst-env
78 InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
81 | ArgOf DupFlag -- An arbitrary strict context: the argument
82 -- of a strict function, or a primitive-arg fn
84 OutType -- cont_ty: the type of the expression being sought by the context
85 -- f (error "foo") ==> coerce t (error "foo")
87 -- We need to know the type t, to which to coerce.
88 (OutExpr -> SimplM OutExprStuff) -- What to do with the result
89 -- The result expression in the OutExprStuff has type cont_ty
91 instance Outputable SimplCont where
92 ppr (Stop _ _) = ptext SLIT("Stop")
93 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
94 ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
95 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
96 (nest 4 (ppr alts)) $$ ppr cont
97 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
98 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
100 data DupFlag = OkToDup | NoDup
102 instance Outputable DupFlag where
103 ppr OkToDup = ptext SLIT("ok")
104 ppr NoDup = ptext SLIT("nodup")
108 mkRhsStop, mkStop :: OutType -> SimplCont
109 mkStop ty = Stop ty False
110 mkRhsStop ty = Stop ty (canUpdateInPlace ty)
114 contIsDupable :: SimplCont -> Bool
115 contIsDupable (Stop _ _) = True
116 contIsDupable (ApplyTo OkToDup _ _ _) = True
117 contIsDupable (ArgOf OkToDup _ _) = True
118 contIsDupable (Select OkToDup _ _ _ _) = True
119 contIsDupable (CoerceIt _ cont) = contIsDupable cont
120 contIsDupable (InlinePlease cont) = contIsDupable cont
121 contIsDupable other = False
124 discardInline :: SimplCont -> SimplCont
125 discardInline (InlinePlease cont) = cont
126 discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
127 discardInline cont = cont
130 discardableCont :: SimplCont -> Bool
131 discardableCont (Stop _ _) = False
132 discardableCont (CoerceIt _ cont) = discardableCont cont
133 discardableCont (InlinePlease cont) = discardableCont cont
134 discardableCont other = True
136 discardCont :: SimplCont -- A continuation, expecting
137 -> SimplCont -- Replace the continuation with a suitable coerce
138 discardCont cont = case cont of
140 other -> CoerceIt to_ty (mkStop to_ty)
142 to_ty = contResultType cont
145 contResultType :: SimplCont -> OutType
146 contResultType (Stop to_ty _) = to_ty
147 contResultType (ArgOf _ to_ty _) = to_ty
148 contResultType (ApplyTo _ _ _ cont) = contResultType cont
149 contResultType (CoerceIt _ cont) = contResultType cont
150 contResultType (InlinePlease cont) = contResultType cont
151 contResultType (Select _ _ _ _ cont) = contResultType cont
154 countValArgs :: SimplCont -> Int
155 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
156 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
157 countValArgs other = 0
159 countArgs :: SimplCont -> Int
160 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
166 getContArgs :: OutId -> SimplCont
167 -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args
168 SimplCont, -- Remaining continuation
169 Bool) -- Whether we came across an InlineCall
170 -- getContArgs id k = (args, k', inl)
171 -- args are the leading ApplyTo items in k
172 -- (i.e. outermost comes first)
173 -- augmented with demand info from the functionn
174 getContArgs fun orig_cont
175 = getSwitchChecker `thenSmpl` \ chkr ->
177 -- Ignore strictness info if the no-case-of-case
178 -- flag is on. Strictness changes evaluation order
179 -- and that can change full laziness
180 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
181 | otherwise = computed_stricts
183 go [] stricts False orig_cont
185 ----------------------------
188 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
189 = go ((arg,se,False) : acc) ss inl cont
190 -- NB: don't bother to instantiate the function type
193 go acc (s:ss) inl (ApplyTo _ arg se cont)
194 = go ((arg,se,s) : acc) ss inl cont
196 -- An Inline continuation
197 go acc ss inl (InlinePlease cont)
198 = go acc ss True cont
200 -- We're run out of arguments, or else we've run out of demands
201 -- The latter only happens if the result is guaranteed bottom
202 -- This is the case for
203 -- * case (error "hello") of { ... }
204 -- * (error "Hello") arg
205 -- * f (error "Hello") where f is strict
208 | null ss && discardableCont cont = tick BottomFound `thenSmpl_`
209 returnSmpl (reverse acc, discardCont cont, inl)
210 | otherwise = returnSmpl (reverse acc, cont, inl)
212 ----------------------------
213 vanilla_stricts, computed_stricts :: [Bool]
214 vanilla_stricts = repeat False
215 computed_stricts = zipWith (||) fun_stricts arg_stricts
217 ----------------------------
218 (val_arg_tys, _) = splitRepFunTys (idType fun)
219 arg_stricts = map isStrictType val_arg_tys ++ repeat False
220 -- These argument types are used as a cheap and cheerful way to find
221 -- unboxed arguments, which must be strict. But it's an InType
222 -- and so there might be a type variable where we expect a function
223 -- type (the substitution hasn't happened yet). And we don't bother
224 -- doing the type applications for a polymorphic function.
225 -- Hence the split*Rep*FunTys
227 ----------------------------
228 -- If fun_stricts is finite, it means the function returns bottom
229 -- after that number of value args have been consumed
230 -- Otherwise it's infinite, extended with False
232 = case splitStrictSig (idNewStrictness fun) of
233 (demands, result_info)
234 | not (demands `lengthExceeds` countValArgs orig_cont)
235 -> -- Enough args, use the strictness given.
236 -- For bottoming functions we used to pretend that the arg
237 -- is lazy, so that we don't treat the arg as an
238 -- interesting context. This avoids substituting
239 -- top-level bindings for (say) strings into
240 -- calls to error. But now we are more careful about
241 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
242 if isBotRes result_info then
243 map isStrictDmd demands -- Finite => result is bottom
245 map isStrictDmd demands ++ vanilla_stricts
247 other -> vanilla_stricts -- Not enough args, or no strictness
250 interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool
251 -- An argument is interesting if it has *some* structure
252 -- We are here trying to avoid unfolding a function that
253 -- is applied only to variables that have no unfolding
254 -- (i.e. they are probably lambda bound): f x y z
255 -- There is little point in inlining f here.
256 interestingArg in_scope arg subst
257 = analyse (substExpr (mkSubst in_scope subst) arg)
258 -- 'analyse' only looks at the top part of the result
259 -- and substExpr is lazy, so this isn't nearly as brutal
262 analyse (Var v) = hasSomeUnfolding (idUnfolding v)
263 -- Was: isValueUnfolding (idUnfolding v')
264 -- But that seems over-pessimistic
265 analyse (Type _) = False
266 analyse (App fn (Type _)) = analyse fn
267 analyse (Note _ a) = analyse a
269 -- Consider let x = 3 in f x
270 -- The substitution will contain (x -> ContEx 3), and we want to
271 -- to say that x is an interesting argument.
272 -- But consider also (\x. f x y) y
273 -- The substitution will contain (x -> ContEx y), and we want to say
274 -- that x is not interesting (assuming y has no unfolding)
277 Comment about interestingCallContext
278 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
279 We want to avoid inlining an expression where there can't possibly be
280 any gain, such as in an argument position. Hence, if the continuation
281 is interesting (eg. a case scrutinee, application etc.) then we
282 inline, otherwise we don't.
284 Previously some_benefit used to return True only if the variable was
285 applied to some value arguments. This didn't work:
287 let x = _coerce_ (T Int) Int (I# 3) in
288 case _coerce_ Int (T Int) x of
291 we want to inline x, but can't see that it's a constructor in a case
292 scrutinee position, and some_benefit is False.
296 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
298 .... case dMonadST _@_ x0 of (a,b,c) -> ....
300 we'd really like to inline dMonadST here, but we *don't* want to
301 inline if the case expression is just
303 case x of y { DEFAULT -> ... }
305 since we can just eliminate this case instead (x is in WHNF). Similar
306 applies when x is bound to a lambda expression. Hence
307 contIsInteresting looks for case expressions with just a single
311 interestingCallContext :: Bool -- False <=> no args at all
312 -> Bool -- False <=> no value args
314 -- The "lone-variable" case is important. I spent ages
315 -- messing about with unsatisfactory varaints, but this is nice.
316 -- The idea is that if a variable appear all alone
317 -- as an arg of lazy fn, or rhs Stop
318 -- as scrutinee of a case Select
319 -- as arg of a strict fn ArgOf
320 -- then we should not inline it (unless there is some other reason,
321 -- e.g. is is the sole occurrence). We achieve this by making
322 -- interestingCallContext return False for a lone variable.
324 -- Why? At least in the case-scrutinee situation, turning
325 -- let x = (a,b) in case x of y -> ...
327 -- let x = (a,b) in case (a,b) of y -> ...
329 -- let x = (a,b) in let y = (a,b) in ...
330 -- is bad if the binding for x will remain.
332 -- Another example: I discovered that strings
333 -- were getting inlined straight back into applications of 'error'
334 -- because the latter is strict.
336 -- f = \x -> ...(error s)...
338 -- Fundamentally such contexts should not ecourage inlining becuase
339 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
340 -- so there's no gain.
342 -- However, even a type application or coercion isn't a lone variable.
344 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
345 -- We had better inline that sucker! The case won't see through it.
347 -- For now, I'm treating treating a variable applied to types
348 -- in a *lazy* context "lone". The motivating example was
350 -- g = /\a. \y. h (f a)
351 -- There's no advantage in inlining f here, and perhaps
352 -- a significant disadvantage. Hence some_val_args in the Stop case
354 interestingCallContext some_args some_val_args cont
357 interesting (InlinePlease _) = True
358 interesting (Select _ _ _ _ _) = some_args
359 interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
360 -- Perhaps True is a bit over-keen, but I've
361 -- seen (coerce f) x, where f has an INLINE prag,
362 -- So we have to give some motivaiton for inlining it
363 interesting (ArgOf _ _ _) = some_val_args
364 interesting (Stop ty upd_in_place) = some_val_args && upd_in_place
365 interesting (CoerceIt _ cont) = interesting cont
366 -- If this call is the arg of a strict function, the context
367 -- is a bit interesting. If we inline here, we may get useful
368 -- evaluation information to avoid repeated evals: e.g.
370 -- Here the contIsInteresting makes the '*' keener to inline,
371 -- which in turn exposes a constructor which makes the '+' inline.
372 -- Assuming that +,* aren't small enough to inline regardless.
374 -- It's also very important to inline in a strict context for things
377 -- Here, the context of (f x) is strict, and if f's unfolding is
378 -- a build it's *great* to inline it here. So we must ensure that
379 -- the context for (f x) is not totally uninteresting.
383 canUpdateInPlace :: Type -> Bool
384 -- Consider let x = <wurble> in ...
385 -- If <wurble> returns an explicit constructor, we might be able
386 -- to do update in place. So we treat even a thunk RHS context
387 -- as interesting if update in place is possible. We approximate
388 -- this by seeing if the type has a single constructor with a
389 -- small arity. But arity zero isn't good -- we share the single copy
390 -- for that case, so no point in sharing.
393 | not opt_UF_UpdateInPlace = False
395 = case splitTyConApp_maybe ty of
397 Just (tycon, _) -> case tyConDataConsIfAvailable tycon of
398 [dc] -> arity == 1 || arity == 2
400 arity = dataConRepArity dc
406 %************************************************************************
408 \section{Dealing with a single binder}
410 %************************************************************************
413 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
414 simplBinders bndrs thing_inside
415 = getSubst `thenSmpl` \ subst ->
417 (subst', bndrs') = Subst.simplBndrs subst bndrs
419 seqBndrs bndrs' `seq`
420 setSubst subst' (thing_inside bndrs')
422 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
423 simplBinder bndr thing_inside
424 = getSubst `thenSmpl` \ subst ->
426 (subst', bndr') = Subst.simplBndr subst bndr
429 setSubst subst' (thing_inside bndr')
432 simplRecIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
433 simplRecIds ids thing_inside
434 = getSubst `thenSmpl` \ subst ->
436 (subst', ids') = mapAccumL Subst.simplLetId subst ids
439 setSubst subst' (thing_inside ids')
441 simplLetId :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
442 simplLetId id thing_inside
443 = getSubst `thenSmpl` \ subst ->
445 (subst', id') = Subst.simplLetId subst id
448 setSubst subst' (thing_inside id')
451 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
453 seqBndr b | isTyVar b = b `seq` ()
454 | otherwise = seqType (idType b) `seq`
460 %************************************************************************
462 \subsection{Local tyvar-lifting}
464 %************************************************************************
466 mkRhsTyLam tries this transformation, when the big lambda appears as
467 the RHS of a let(rec) binding:
469 /\abc -> let(rec) x = e in b
471 let(rec) x' = /\abc -> let x = x' a b c in e
473 /\abc -> let x = x' a b c in b
475 This is good because it can turn things like:
477 let f = /\a -> letrec g = ... g ... in g
479 letrec g' = /\a -> ... g' a ...
483 which is better. In effect, it means that big lambdas don't impede
486 This optimisation is CRUCIAL in eliminating the junk introduced by
487 desugaring mutually recursive definitions. Don't eliminate it lightly!
489 So far as the implementation is concerned:
491 Invariant: go F e = /\tvs -> F e
495 = Let x' = /\tvs -> F e
499 G = F . Let x = x' tvs
501 go F (Letrec xi=ei in b)
502 = Letrec {xi' = /\tvs -> G ei}
506 G = F . Let {xi = xi' tvs}
508 [May 1999] If we do this transformation *regardless* then we can
509 end up with some pretty silly stuff. For example,
512 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
517 st = /\s -> ...[y1 s/x1, y2 s/x2]
520 Unless the "..." is a WHNF there is really no point in doing this.
521 Indeed it can make things worse. Suppose x1 is used strictly,
524 x1* = case f y of { (a,b) -> e }
526 If we abstract this wrt the tyvar we then can't do the case inline
527 as we would normally do.
531 tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
533 tryRhsTyLam rhs -- Only does something if there's a let
534 | null tyvars || not (worth_it body) -- inside a type lambda,
535 = returnSmpl ([], rhs) -- and a WHNF inside that
538 = go (\x -> x) body `thenSmpl` \ (binds, body') ->
539 returnSmpl (binds, mkLams tyvars body')
542 (tyvars, body) = collectTyBinders rhs
544 worth_it e@(Let _ _) = whnf_in_middle e
547 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
548 whnf_in_middle (Let _ e) = whnf_in_middle e
549 whnf_in_middle e = exprIsCheap e
551 go fn (Let bind@(NonRec var rhs) body)
553 = go (fn . Let bind) body
555 go fn (Let (NonRec var rhs) body)
556 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
557 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
558 returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
562 -- main_tyvar_set = mkVarSet tyvars
563 -- var_ty = idType var
564 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
565 -- tyvars_here was an attempt to reduce the number of tyvars
566 -- wrt which the new binding is abstracted. But the naive
567 -- approach of abstract wrt the tyvars free in the Id's type
569 -- /\ a b -> let t :: (a,b) = (e1, e2)
572 -- Here, b isn't free in x's type, but we must nevertheless
573 -- abstract wrt b as well, because t's type mentions b.
574 -- Since t is floated too, we'd end up with the bogus:
575 -- poly_t = /\ a b -> (e1, e2)
576 -- poly_x = /\ a -> fst (poly_t a *b*)
577 -- So for now we adopt the even more naive approach of
578 -- abstracting wrt *all* the tyvars. We'll see if that
579 -- gives rise to problems. SLPJ June 98
581 go fn (Let (Rec prs) body)
582 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
584 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
585 new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])
587 go gn body `thenSmpl` \ (binds, body') ->
588 returnSmpl (new_bind : binds, body')
590 (vars,rhss) = unzip prs
592 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
593 -- var_tys = map idType vars
594 -- See notes with tyvars_here above
596 go fn body = returnSmpl ([], fn body)
598 mk_poly tyvars_here var
599 = getUniqueSmpl `thenSmpl` \ uniq ->
601 poly_name = setNameUnique (idName var) uniq -- Keep same name
602 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
603 poly_id = mkLocalId poly_name poly_ty
605 -- In the olden days, it was crucial to copy the occInfo of the original var,
606 -- because we were looking at occurrence-analysed but as yet unsimplified code!
607 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
608 -- at already simplified code, so it doesn't matter
610 -- It's even right to retain single-occurrence or dead-var info:
611 -- Suppose we started with /\a -> let x = E in B
612 -- where x occurs once in B. Then we transform to:
613 -- let x' = /\a -> E in /\a -> let x* = x' a in B
614 -- where x* has an INLINE prag on it. Now, once x* is inlined,
615 -- the occurrences of x' will be just the occurrences originally
618 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
620 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
621 -- Suppose we start with:
623 -- x = /\ a -> let g = G in E
625 -- Then we'll float to get
627 -- x = let poly_g = /\ a -> G
628 -- in /\ a -> let g = poly_g a in E
630 -- But now the occurrence analyser will see just one occurrence
631 -- of poly_g, not inside a lambda, so the simplifier will
632 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
633 -- (I used to think that the "don't inline lone occurrences" stuff
634 -- would stop this happening, but since it's the *only* occurrence,
635 -- PreInlineUnconditionally kicks in first!)
637 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
638 -- to appear many times. (NB: mkInlineMe eliminates
639 -- such notes on trivial RHSs, so do it manually.)
643 %************************************************************************
645 \subsection{Eta expansion}
647 %************************************************************************
649 Try eta expansion for RHSs
652 Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
655 Case 2 f = N E1..En ==> z1=E1
658 f = \y1..ym -> N z1..zn y1..ym
660 where (in both cases)
662 * The xi can include type variables
664 * The yi are all value variables
666 * N is a NORMAL FORM (i.e. no redexes anywhere)
667 wanting a suitable number of extra args.
669 * the Ei must not have unlifted type
671 There is no point in looking for a combination of the two, because
672 that would leave use with some lets sandwiched between lambdas; that's
673 what the final test in the first equation is for.
675 In Case 1, we may have to sandwich some coerces between the lambdas
676 to make the types work. exprEtaExpandArity looks through coerces
677 when computing arity; and etaExpand adds the coerces as necessary when
678 actually computing the expansion.
681 tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
682 tryEtaExpansion rhs rhs_ty
683 | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
684 || exprIsTrivial rhs -- Not if RHS is trivial
685 || final_arity == 0 -- Not if arity is zero
686 = returnSmpl ([], rhs)
688 | n_val_args == 0 && not arity_is_manifest
689 = -- Some lambdas but not enough: case 1
690 getUniqSupplySmpl `thenSmpl` \ us ->
691 returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
693 | n_val_args > 0 && not (any cant_bind arg_infos)
694 = -- Partial application: case 2
695 mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
696 getUniqSupplySmpl `thenSmpl` \ us ->
697 returnSmpl (catMaybes maybe_z_binds,
698 etaExpand final_arity us (mkApps fun z_args) rhs_ty)
701 = returnSmpl ([], rhs)
703 (fun, args) = collectArgs rhs
704 n_val_args = valArgCount args
705 (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
706 final_arity = 0 `max` (fun_arity - n_val_args)
707 arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
708 cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
710 bind_z_arg (arg, arg_ty, trivial_arg)
711 | trivial_arg = returnSmpl (Nothing, arg)
712 | otherwise = newId SLIT("z") arg_ty $ \ z ->
713 returnSmpl (Just (NonRec z arg), Var z)
717 %************************************************************************
719 \subsection{Case absorption and identity-case elimination}
721 %************************************************************************
724 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
727 @mkCase@ tries the following transformation (if possible):
729 case e of b { ==> case e of b {
730 p1 -> rhs1 p1 -> rhs1
732 pm -> rhsm pm -> rhsm
733 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
734 {or (prim) case b of b' { _ -> rhsn}}
737 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
741 which merges two cases in one case when -- the default alternative of
742 the outer case scrutises the same variable as the outer case This
743 transformation is called Case Merging. It avoids that the same
744 variable is scrutinised multiple times.
747 mkCase scrut outer_bndr outer_alts
749 && maybeToBool maybe_case_in_default
751 = tick (CaseMerge outer_bndr) `thenSmpl_`
752 returnSmpl (Case scrut outer_bndr new_alts)
753 -- Warning: don't call mkCase recursively!
754 -- Firstly, there's no point, because inner alts have already had
755 -- mkCase applied to them, so they won't have a case in their default
756 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
757 -- in munge_rhs puts a case into the DEFAULT branch!
759 new_alts = add_default maybe_inner_default
760 (outer_alts_without_deflt ++ inner_con_alts)
762 maybe_case_in_default = case findDefault outer_alts of
763 (outer_alts_without_default,
764 Just (Case (Var scrut_var) inner_bndr inner_alts))
765 | outer_bndr == scrut_var
766 -> Just (outer_alts_without_default, inner_bndr, inner_alts)
769 Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
771 -- Eliminate any inner alts which are shadowed by the outer ones
772 outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
774 munged_inner_alts = [ (con, args, munge_rhs rhs)
775 | (con, args, rhs) <- inner_alts,
776 not (con `elem` outer_cons) -- Eliminate shadowed inner alts
778 munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
780 (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
782 add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
783 add_default Nothing alts = alts
786 Now the identity-case transformation:
795 mkCase scrut case_bndr alts
796 | all identity_alt alts
797 = tick (CaseIdentity case_bndr) `thenSmpl_`
798 returnSmpl (re_note scrut)
800 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
802 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
803 identity_rhs (LitAlt lit) _ = Lit lit
804 identity_rhs DEFAULT _ = Var case_bndr
806 arg_tys = map Type (tyConAppArgs (idType case_bndr))
809 -- case coerce T e of x { _ -> coerce T' x }
810 -- And we definitely want to eliminate this case!
811 -- So we throw away notes from the RHS, and reconstruct
812 -- (at least an approximation) at the other end
813 de_note (Note _ e) = de_note e
816 -- re_note wraps a coerce if it might be necessary
817 re_note scrut = case head alts of
818 (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
822 The catch-all case. We do a final transformation that I've
823 occasionally seen making a big difference:
825 case e of =====> case e of
826 C _ -> f x D v -> ....v....
827 D v -> ....v.... DEFAULT -> f x
830 The point is that we merge common RHSs, at least for the DEFAULT case.
831 [One could do something more elaborate but I've never seen it needed.]
832 The case where this came up was like this (lib/std/PrelCError.lhs):
838 where @is@ was something like
840 p `is` n = p /= (-1) && p == n
842 This gave rise to a horrible sequence of cases
849 and similarly in cascade for all the join points!
852 mkCase other_scrut case_bndr other_alts
853 = returnSmpl (Case other_scrut case_bndr (mergeDefault other_alts))
855 mergeDefault (deflt_alt@(DEFAULT,_,deflt_rhs) : con_alts)
856 = deflt_alt : [alt | alt@(con,_,rhs) <- con_alts, not (rhs `cheapEqExpr` deflt_rhs)]
857 -- NB: we can neglect the binders because we won't get equality if the
858 -- binders are mentioned in rhs (no shadowing)
859 mergeDefault other_alts