2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
8 simplBinder, simplBinders, simplIds,
9 tryRhsTyLam, tryEtaExpansion,
10 mkCase, findAlt, findDefault,
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, opt_DictsStrict,
26 import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, etaExpand, exprEtaExpandArity, bindNonRec )
27 import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, substExpr )
28 import Id ( idType, idName,
29 idUnfolding, idStrictness,
32 import IdInfo ( StrictnessInfo(..) )
33 import Maybes ( maybeToBool, catMaybes )
34 import Name ( setNameUnique )
35 import Demand ( isStrict )
37 import Type ( Type, mkForAllTys, seqType, repType,
38 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
39 isDictTy, isDataType, isUnLiftedType,
42 import TyCon ( tyConDataConsIfAvailable )
43 import DataCon ( dataConRepArity )
44 import VarEnv ( SubstEnv )
45 import Util ( lengthExceeds )
50 %************************************************************************
52 \subsection{The continuation data type}
54 %************************************************************************
57 data SimplCont -- Strict contexts
58 = Stop OutType -- Type of the result
59 Bool -- True => This is the RHS of a thunk whose type suggests
60 -- that update-in-place would be possible
61 -- (This makes the inliner a little keener.)
63 | CoerceIt OutType -- The To-type, simplified
66 | InlinePlease -- This continuation makes a function very
67 SimplCont -- keen to inline itelf
70 InExpr SubstEnv -- The argument, as yet unsimplified,
71 SimplCont -- and its subst-env
74 InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
77 | ArgOf DupFlag -- An arbitrary strict context: the argument
78 -- of a strict function, or a primitive-arg fn
80 OutType -- cont_ty: the type of the expression being sought by the context
81 -- f (error "foo") ==> coerce t (error "foo")
83 -- We need to know the type t, to which to coerce.
84 (OutExpr -> SimplM OutExprStuff) -- What to do with the result
85 -- The result expression in the OutExprStuff has type cont_ty
87 instance Outputable SimplCont where
88 ppr (Stop _ _) = ptext SLIT("Stop")
89 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
90 ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
91 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
92 (nest 4 (ppr alts)) $$ ppr cont
93 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
94 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
96 data DupFlag = OkToDup | NoDup
98 instance Outputable DupFlag where
99 ppr OkToDup = ptext SLIT("ok")
100 ppr NoDup = ptext SLIT("nodup")
104 mkRhsStop, mkStop :: OutType -> SimplCont
105 mkStop ty = Stop ty False
106 mkRhsStop ty = Stop ty (canUpdateInPlace ty)
110 contIsDupable :: SimplCont -> Bool
111 contIsDupable (Stop _ _) = True
112 contIsDupable (ApplyTo OkToDup _ _ _) = True
113 contIsDupable (ArgOf OkToDup _ _) = True
114 contIsDupable (Select OkToDup _ _ _ _) = True
115 contIsDupable (CoerceIt _ cont) = contIsDupable cont
116 contIsDupable (InlinePlease cont) = contIsDupable cont
117 contIsDupable other = False
120 discardInline :: SimplCont -> SimplCont
121 discardInline (InlinePlease cont) = cont
122 discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
123 discardInline cont = cont
126 discardableCont :: SimplCont -> Bool
127 discardableCont (Stop _ _) = False
128 discardableCont (CoerceIt _ cont) = discardableCont cont
129 discardableCont (InlinePlease cont) = discardableCont cont
130 discardableCont other = True
132 discardCont :: SimplCont -- A continuation, expecting
133 -> SimplCont -- Replace the continuation with a suitable coerce
134 discardCont cont = case cont of
136 other -> CoerceIt to_ty (mkStop to_ty)
138 to_ty = contResultType cont
141 contResultType :: SimplCont -> OutType
142 contResultType (Stop to_ty _) = to_ty
143 contResultType (ArgOf _ to_ty _) = to_ty
144 contResultType (ApplyTo _ _ _ cont) = contResultType cont
145 contResultType (CoerceIt _ cont) = contResultType cont
146 contResultType (InlinePlease cont) = contResultType cont
147 contResultType (Select _ _ _ _ cont) = contResultType cont
150 countValArgs :: SimplCont -> Int
151 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
152 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
153 countValArgs other = 0
155 countArgs :: SimplCont -> Int
156 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
162 getContArgs :: OutId -> SimplCont
163 -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args
164 SimplCont, -- Remaining continuation
165 Bool) -- Whether we came across an InlineCall
166 -- getContArgs id k = (args, k', inl)
167 -- args are the leading ApplyTo items in k
168 -- (i.e. outermost comes first)
169 -- augmented with demand info from the functionn
170 getContArgs fun orig_cont
171 = getSwitchChecker `thenSmpl` \ chkr ->
173 -- Ignore strictness info if the no-case-of-case
174 -- flag is on. Strictness changes evaluation order
175 -- and that can change full laziness
176 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
177 | otherwise = computed_stricts
179 go [] stricts False orig_cont
181 ----------------------------
184 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
185 = go ((arg,se,False) : acc) ss inl cont
186 -- NB: don't bother to instantiate the function type
189 go acc (s:ss) inl (ApplyTo _ arg se cont)
190 = go ((arg,se,s) : acc) ss inl cont
192 -- An Inline continuation
193 go acc ss inl (InlinePlease cont)
194 = go acc ss True cont
196 -- We're run out of arguments, or else we've run out of demands
197 -- The latter only happens if the result is guaranteed bottom
198 -- This is the case for
199 -- * case (error "hello") of { ... }
200 -- * (error "Hello") arg
201 -- * f (error "Hello") where f is strict
204 | null ss && discardableCont cont = tick BottomFound `thenSmpl_`
205 returnSmpl (reverse acc, discardCont cont, inl)
206 | otherwise = returnSmpl (reverse acc, cont, inl)
208 ----------------------------
209 vanilla_stricts, computed_stricts :: [Bool]
210 vanilla_stricts = repeat False
211 computed_stricts = zipWith (||) fun_stricts arg_stricts
213 ----------------------------
214 (val_arg_tys, _) = splitRepFunTys (idType fun)
215 arg_stricts = map isStrictType val_arg_tys ++ repeat False
216 -- These argument types are used as a cheap and cheerful way to find
217 -- unboxed arguments, which must be strict. But it's an InType
218 -- and so there might be a type variable where we expect a function
219 -- type (the substitution hasn't happened yet). And we don't bother
220 -- doing the type applications for a polymorphic function.
221 -- Hence the split*Rep*FunTys
223 ----------------------------
224 -- If fun_stricts is finite, it means the function returns bottom
225 -- after that number of value args have been consumed
226 -- Otherwise it's infinite, extended with False
228 = case idStrictness fun of
229 StrictnessInfo demands result_bot
230 | not (demands `lengthExceeds` countValArgs orig_cont)
231 -> -- Enough args, use the strictness given.
232 -- For bottoming functions we used to pretend that the arg
233 -- is lazy, so that we don't treat the arg as an
234 -- interesting context. This avoids substituting
235 -- top-level bindings for (say) strings into
236 -- calls to error. But now we are more careful about
237 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
239 map isStrict demands -- Finite => result is bottom
241 map isStrict demands ++ vanilla_stricts
243 other -> vanilla_stricts -- Not enough args, or no strictness
247 isStrictType :: Type -> Bool
248 -- isStrictType computes whether an argument (or let RHS) should
249 -- be computed strictly or lazily, based only on its type
251 | isUnLiftedType ty = True
252 | opt_DictsStrict && isDictTy ty && isDataType ty = True
254 -- Return true only for dictionary types where the dictionary
255 -- has more than one component (else we risk poking on the component
256 -- of a newtype dictionary)
259 interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool
260 -- An argument is interesting if it has *some* structure
261 -- We are here trying to avoid unfolding a function that
262 -- is applied only to variables that have no unfolding
263 -- (i.e. they are probably lambda bound): f x y z
264 -- There is little point in inlining f here.
265 interestingArg in_scope arg subst
266 = analyse (substExpr (mkSubst in_scope subst) arg)
267 -- 'analyse' only looks at the top part of the result
268 -- and substExpr is lazy, so this isn't nearly as brutal
271 analyse (Var v) = hasSomeUnfolding (idUnfolding v)
272 -- Was: isValueUnfolding (idUnfolding v')
273 -- But that seems over-pessimistic
274 analyse (Type _) = False
275 analyse (App fn (Type _)) = analyse fn
276 analyse (Note _ a) = analyse a
278 -- Consider let x = 3 in f x
279 -- The substitution will contain (x -> ContEx 3), and we want to
280 -- to say that x is an interesting argument.
281 -- But consider also (\x. f x y) y
282 -- The substitution will contain (x -> ContEx y), and we want to say
283 -- that x is not interesting (assuming y has no unfolding)
286 Comment about interestingCallContext
287 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
288 We want to avoid inlining an expression where there can't possibly be
289 any gain, such as in an argument position. Hence, if the continuation
290 is interesting (eg. a case scrutinee, application etc.) then we
291 inline, otherwise we don't.
293 Previously some_benefit used to return True only if the variable was
294 applied to some value arguments. This didn't work:
296 let x = _coerce_ (T Int) Int (I# 3) in
297 case _coerce_ Int (T Int) x of
300 we want to inline x, but can't see that it's a constructor in a case
301 scrutinee position, and some_benefit is False.
305 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
307 .... case dMonadST _@_ x0 of (a,b,c) -> ....
309 we'd really like to inline dMonadST here, but we *don't* want to
310 inline if the case expression is just
312 case x of y { DEFAULT -> ... }
314 since we can just eliminate this case instead (x is in WHNF). Similar
315 applies when x is bound to a lambda expression. Hence
316 contIsInteresting looks for case expressions with just a single
320 interestingCallContext :: Bool -- False <=> no args at all
321 -> Bool -- False <=> no value args
323 -- The "lone-variable" case is important. I spent ages
324 -- messing about with unsatisfactory varaints, but this is nice.
325 -- The idea is that if a variable appear all alone
326 -- as an arg of lazy fn, or rhs Stop
327 -- as scrutinee of a case Select
328 -- as arg of a strict fn ArgOf
329 -- then we should not inline it (unless there is some other reason,
330 -- e.g. is is the sole occurrence). We achieve this by making
331 -- interestingCallContext return False for a lone variable.
333 -- Why? At least in the case-scrutinee situation, turning
334 -- let x = (a,b) in case x of y -> ...
336 -- let x = (a,b) in case (a,b) of y -> ...
338 -- let x = (a,b) in let y = (a,b) in ...
339 -- is bad if the binding for x will remain.
341 -- Another example: I discovered that strings
342 -- were getting inlined straight back into applications of 'error'
343 -- because the latter is strict.
345 -- f = \x -> ...(error s)...
347 -- Fundamentally such contexts should not ecourage inlining becuase
348 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
349 -- so there's no gain.
351 -- However, even a type application or coercion isn't a lone variable.
353 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
354 -- We had better inline that sucker! The case won't see through it.
356 -- For now, I'm treating treating a variable applied to types
357 -- in a *lazy* context "lone". The motivating example was
359 -- g = /\a. \y. h (f a)
360 -- There's no advantage in inlining f here, and perhaps
361 -- a significant disadvantage. Hence some_val_args in the Stop case
363 interestingCallContext some_args some_val_args cont
366 interesting (InlinePlease _) = True
367 interesting (Select _ _ _ _ _) = some_args
368 interesting (ApplyTo _ _ _ _) = some_args -- Can happen if we have (coerce t (f x)) y
369 interesting (ArgOf _ _ _) = some_val_args
370 interesting (Stop ty upd_in_place) = some_val_args && upd_in_place
371 interesting (CoerceIt _ cont) = interesting cont
372 -- If this call is the arg of a strict function, the context
373 -- is a bit interesting. If we inline here, we may get useful
374 -- evaluation information to avoid repeated evals: e.g.
376 -- Here the contIsInteresting makes the '*' keener to inline,
377 -- which in turn exposes a constructor which makes the '+' inline.
378 -- Assuming that +,* aren't small enough to inline regardless.
380 -- It's also very important to inline in a strict context for things
383 -- Here, the context of (f x) is strict, and if f's unfolding is
384 -- a build it's *great* to inline it here. So we must ensure that
385 -- the context for (f x) is not totally uninteresting.
389 canUpdateInPlace :: Type -> Bool
390 -- Consider let x = <wurble> in ...
391 -- If <wurble> returns an explicit constructor, we might be able
392 -- to do update in place. So we treat even a thunk RHS context
393 -- as interesting if update in place is possible. We approximate
394 -- this by seeing if the type has a single constructor with a
395 -- small arity. But arity zero isn't good -- we share the single copy
396 -- for that case, so no point in sharing.
398 -- Note the repType: we want to look through newtypes for this purpose
401 | not opt_UF_UpdateInPlace = False
403 = case splitTyConApp_maybe (repType ty) of {
407 case tyConDataConsIfAvailable tycon of
408 [dc] -> arity == 1 || arity == 2
410 arity = dataConRepArity dc
417 %************************************************************************
419 \section{Dealing with a single binder}
421 %************************************************************************
424 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
425 simplBinders bndrs thing_inside
426 = getSubst `thenSmpl` \ subst ->
428 (subst', bndrs') = substBndrs subst bndrs
430 seqBndrs bndrs' `seq`
431 setSubst subst' (thing_inside bndrs')
433 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
434 simplBinder bndr thing_inside
435 = getSubst `thenSmpl` \ subst ->
437 (subst', bndr') = substBndr subst bndr
440 setSubst subst' (thing_inside bndr')
443 -- Same semantics as simplBinders, but a little less
444 -- plumbing and hence a little more efficient.
445 -- Maybe not worth the candle?
446 simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
447 simplIds ids thing_inside
448 = getSubst `thenSmpl` \ subst ->
450 (subst', bndrs') = substIds subst ids
452 seqBndrs bndrs' `seq`
453 setSubst subst' (thing_inside bndrs')
456 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
458 seqBndr b | isTyVar b = b `seq` ()
459 | otherwise = seqType (idType b) `seq`
465 %************************************************************************
467 \subsection{Local tyvar-lifting}
469 %************************************************************************
471 mkRhsTyLam tries this transformation, when the big lambda appears as
472 the RHS of a let(rec) binding:
474 /\abc -> let(rec) x = e in b
476 let(rec) x' = /\abc -> let x = x' a b c in e
478 /\abc -> let x = x' a b c in b
480 This is good because it can turn things like:
482 let f = /\a -> letrec g = ... g ... in g
484 letrec g' = /\a -> ... g' a ...
488 which is better. In effect, it means that big lambdas don't impede
491 This optimisation is CRUCIAL in eliminating the junk introduced by
492 desugaring mutually recursive definitions. Don't eliminate it lightly!
494 So far as the implementation is concerned:
496 Invariant: go F e = /\tvs -> F e
500 = Let x' = /\tvs -> F e
504 G = F . Let x = x' tvs
506 go F (Letrec xi=ei in b)
507 = Letrec {xi' = /\tvs -> G ei}
511 G = F . Let {xi = xi' tvs}
513 [May 1999] If we do this transformation *regardless* then we can
514 end up with some pretty silly stuff. For example,
517 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
522 st = /\s -> ...[y1 s/x1, y2 s/x2]
525 Unless the "..." is a WHNF there is really no point in doing this.
526 Indeed it can make things worse. Suppose x1 is used strictly,
529 x1* = case f y of { (a,b) -> e }
531 If we abstract this wrt the tyvar we then can't do the case inline
532 as we would normally do.
536 tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
538 tryRhsTyLam rhs -- Only does something if there's a let
539 | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that
540 = returnSmpl ([], rhs)
543 = go (\x -> x) body `thenSmpl` \ (binds, body') ->
544 returnSmpl (binds, mkLams tyvars body')
547 (tyvars, body) = collectTyBinders rhs
549 worth_it (Let _ e) = whnf_in_middle e
550 worth_it other = False
551 whnf_in_middle (Let _ e) = whnf_in_middle e
552 whnf_in_middle e = exprIsCheap e
554 go fn (Let bind@(NonRec var rhs) body)
556 = go (fn . Let bind) body
558 go fn (Let (NonRec var rhs) body)
559 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
560 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
561 returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
565 -- main_tyvar_set = mkVarSet tyvars
566 -- var_ty = idType var
567 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
568 -- tyvars_here was an attempt to reduce the number of tyvars
569 -- wrt which the new binding is abstracted. But the naive
570 -- approach of abstract wrt the tyvars free in the Id's type
572 -- /\ a b -> let t :: (a,b) = (e1, e2)
575 -- Here, b isn't free in x's type, but we must nevertheless
576 -- abstract wrt b as well, because t's type mentions b.
577 -- Since t is floated too, we'd end up with the bogus:
578 -- poly_t = /\ a b -> (e1, e2)
579 -- poly_x = /\ a -> fst (poly_t a *b*)
580 -- So for now we adopt the even more naive approach of
581 -- abstracting wrt *all* the tyvars. We'll see if that
582 -- gives rise to problems. SLPJ June 98
584 go fn (Let (Rec prs) body)
585 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
587 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
588 new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])
590 go gn body `thenSmpl` \ (binds, body') ->
591 returnSmpl (new_bind : binds, body')
593 (vars,rhss) = unzip prs
595 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
596 -- var_tys = map idType vars
597 -- See notes with tyvars_here above
599 go fn body = returnSmpl ([], fn body)
601 mk_poly tyvars_here var
602 = getUniqueSmpl `thenSmpl` \ uniq ->
604 poly_name = setNameUnique (idName var) uniq -- Keep same name
605 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
606 poly_id = mkVanillaId poly_name poly_ty
608 -- In the olden days, it was crucial to copy the occInfo of the original var,
609 -- because we were looking at occurrence-analysed but as yet unsimplified code!
610 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
611 -- at already simplified code, so it doesn't matter
613 -- It's even right to retain single-occurrence or dead-var info:
614 -- Suppose we started with /\a -> let x = E in B
615 -- where x occurs once in B. Then we transform to:
616 -- let x' = /\a -> E in /\a -> let x* = x' a in B
617 -- where x* has an INLINE prag on it. Now, once x* is inlined,
618 -- the occurrences of x' will be just the occurrences originally
621 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
623 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
624 -- Suppose we start with:
626 -- x = /\ a -> let g = G in E
628 -- Then we'll float to get
630 -- x = let poly_g = /\ a -> G
631 -- in /\ a -> let g = poly_g a in E
633 -- But now the occurrence analyser will see just one occurrence
634 -- of poly_g, not inside a lambda, so the simplifier will
635 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
636 -- (I used to think that the "don't inline lone occurrences" stuff
637 -- would stop this happening, but since it's the *only* occurrence,
638 -- PreInlineUnconditionally kicks in first!)
640 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
641 -- to appear many times. (NB: mkInlineMe eliminates
642 -- such notes on trivial RHSs, so do it manually.)
646 %************************************************************************
648 \subsection{Eta expansion}
650 %************************************************************************
652 Try eta expansion for RHSs
655 Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
658 Case 2 f = N E1..En ==> z1=E1
661 f = \y1..ym -> N z1..zn y1..ym
663 where (in both cases)
665 * The xi can include type variables
667 * The yi are all value variables
669 * N is a NORMAL FORM (i.e. no redexes anywhere)
670 wanting a suitable number of extra args.
672 * the Ei must not have unlifted type
674 There is no point in looking for a combination of the two, because
675 that would leave use with some lets sandwiched between lambdas; that's
676 what the final test in the first equation is for.
679 tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
680 tryEtaExpansion rhs rhs_ty
681 | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
682 || exprIsTrivial rhs -- Not if RHS is trivial
683 || final_arity == 0 -- Not if arity is zero
684 = returnSmpl ([], rhs)
686 | n_val_args == 0 && not arity_is_manifest
687 = -- Some lambdas but not enough: case 1
688 getUniqSupplySmpl `thenSmpl` \ us ->
689 returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
691 | n_val_args > 0 && not (any cant_bind arg_infos)
692 = -- Partial application: case 2
693 mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
694 getUniqSupplySmpl `thenSmpl` \ us ->
695 returnSmpl (catMaybes maybe_z_binds,
696 etaExpand final_arity us (mkApps fun z_args) rhs_ty)
699 = returnSmpl ([], rhs)
701 (fun, args) = collectArgs rhs
702 n_val_args = valArgCount args
703 (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
704 final_arity = 0 `max` (fun_arity - n_val_args)
705 arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
706 cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
708 bind_z_arg (arg, arg_ty, trivial_arg)
709 | trivial_arg = returnSmpl (Nothing, arg)
710 | otherwise = newId SLIT("z") arg_ty $ \ z ->
711 returnSmpl (Just (NonRec z arg), Var z)
715 %************************************************************************
717 \subsection{Case absorption and identity-case elimination}
719 %************************************************************************
722 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
725 @mkCase@ tries the following transformation (if possible):
727 case e of b { ==> case e of b {
728 p1 -> rhs1 p1 -> rhs1
730 pm -> rhsm pm -> rhsm
731 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
732 {or (prim) case b of b' { _ -> rhsn}}
735 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
739 which merges two cases in one case when -- the default alternative of
740 the outer case scrutises the same variable as the outer case This
741 transformation is called Case Merging. It avoids that the same
742 variable is scrutinised multiple times.
745 mkCase scrut outer_bndr outer_alts
747 && maybeToBool maybe_case_in_default
749 = tick (CaseMerge outer_bndr) `thenSmpl_`
750 returnSmpl (Case scrut outer_bndr new_alts)
751 -- Warning: don't call mkCase recursively!
752 -- Firstly, there's no point, because inner alts have already had
753 -- mkCase applied to them, so they won't have a case in their default
754 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
755 -- in munge_rhs puts a case into the DEFAULT branch!
757 new_alts = outer_alts_without_deflt ++ munged_inner_alts
758 maybe_case_in_default = case findDefault outer_alts of
759 (outer_alts_without_default,
760 Just (Case (Var scrut_var) inner_bndr inner_alts))
762 | outer_bndr == scrut_var
763 -> Just (outer_alts_without_default, inner_bndr, inner_alts)
766 Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
768 -- Eliminate any inner alts which are shadowed by the outer ones
769 outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
771 munged_inner_alts = [ (con, args, munge_rhs rhs)
772 | (con, args, rhs) <- inner_alts,
773 not (con `elem` outer_cons) -- Eliminate shadowed inner alts
775 munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
778 Now the identity-case transformation:
787 mkCase scrut case_bndr alts
788 | all identity_alt alts
789 = tick (CaseIdentity case_bndr) `thenSmpl_`
792 identity_alt (DEFAULT, [], Var v) = v == case_bndr
793 identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs
794 (mkConApp con (map Type arg_tys ++ map varToCoreExpr args))
795 identity_alt other = False
797 arg_tys = tyConAppArgs (idType case_bndr)
803 mkCase other_scrut case_bndr other_alts
804 = returnSmpl (Case other_scrut case_bndr other_alts)
809 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
810 findDefault [] = ([], Nothing)
811 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args )
813 findDefault (alt : alts) = case findDefault alts of
814 (alts', deflt) -> (alt : alts', deflt)
816 findAlt :: AltCon -> [CoreAlt] -> CoreAlt
820 go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
821 go (alt : alts) | matches alt = alt
822 | otherwise = go alts
824 matches (DEFAULT, _, _) = True
825 matches (con1, _, _) = con == con1