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, opt_DictsStrict,
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, idStrictness,
36 import IdInfo ( StrictnessInfo(..) )
37 import Maybes ( maybeToBool, catMaybes )
38 import Name ( setNameUnique )
39 import Demand ( isStrict )
41 import Type ( Type, mkForAllTys, seqType, repType,
42 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
43 isDictTy, isDataType, isUnLiftedType,
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 idStrictness fun of
233 StrictnessInfo demands result_bot
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)
243 map isStrict demands -- Finite => result is bottom
245 map isStrict demands ++ vanilla_stricts
247 other -> vanilla_stricts -- Not enough args, or no strictness
251 isStrictType :: Type -> Bool
252 -- isStrictType computes whether an argument (or let RHS) should
253 -- be computed strictly or lazily, based only on its type
255 | isUnLiftedType ty = True
256 | opt_DictsStrict && isDictTy ty && isDataType ty = True
258 -- Return true only for dictionary types where the dictionary
259 -- has more than one component (else we risk poking on the component
260 -- of a newtype dictionary)
263 interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool
264 -- An argument is interesting if it has *some* structure
265 -- We are here trying to avoid unfolding a function that
266 -- is applied only to variables that have no unfolding
267 -- (i.e. they are probably lambda bound): f x y z
268 -- There is little point in inlining f here.
269 interestingArg in_scope arg subst
270 = analyse (substExpr (mkSubst in_scope subst) arg)
271 -- 'analyse' only looks at the top part of the result
272 -- and substExpr is lazy, so this isn't nearly as brutal
275 analyse (Var v) = hasSomeUnfolding (idUnfolding v)
276 -- Was: isValueUnfolding (idUnfolding v')
277 -- But that seems over-pessimistic
278 analyse (Type _) = False
279 analyse (App fn (Type _)) = analyse fn
280 analyse (Note _ a) = analyse a
282 -- Consider let x = 3 in f x
283 -- The substitution will contain (x -> ContEx 3), and we want to
284 -- to say that x is an interesting argument.
285 -- But consider also (\x. f x y) y
286 -- The substitution will contain (x -> ContEx y), and we want to say
287 -- that x is not interesting (assuming y has no unfolding)
290 Comment about interestingCallContext
291 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
292 We want to avoid inlining an expression where there can't possibly be
293 any gain, such as in an argument position. Hence, if the continuation
294 is interesting (eg. a case scrutinee, application etc.) then we
295 inline, otherwise we don't.
297 Previously some_benefit used to return True only if the variable was
298 applied to some value arguments. This didn't work:
300 let x = _coerce_ (T Int) Int (I# 3) in
301 case _coerce_ Int (T Int) x of
304 we want to inline x, but can't see that it's a constructor in a case
305 scrutinee position, and some_benefit is False.
309 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
311 .... case dMonadST _@_ x0 of (a,b,c) -> ....
313 we'd really like to inline dMonadST here, but we *don't* want to
314 inline if the case expression is just
316 case x of y { DEFAULT -> ... }
318 since we can just eliminate this case instead (x is in WHNF). Similar
319 applies when x is bound to a lambda expression. Hence
320 contIsInteresting looks for case expressions with just a single
324 interestingCallContext :: Bool -- False <=> no args at all
325 -> Bool -- False <=> no value args
327 -- The "lone-variable" case is important. I spent ages
328 -- messing about with unsatisfactory varaints, but this is nice.
329 -- The idea is that if a variable appear all alone
330 -- as an arg of lazy fn, or rhs Stop
331 -- as scrutinee of a case Select
332 -- as arg of a strict fn ArgOf
333 -- then we should not inline it (unless there is some other reason,
334 -- e.g. is is the sole occurrence). We achieve this by making
335 -- interestingCallContext return False for a lone variable.
337 -- Why? At least in the case-scrutinee situation, turning
338 -- let x = (a,b) in case x of y -> ...
340 -- let x = (a,b) in case (a,b) of y -> ...
342 -- let x = (a,b) in let y = (a,b) in ...
343 -- is bad if the binding for x will remain.
345 -- Another example: I discovered that strings
346 -- were getting inlined straight back into applications of 'error'
347 -- because the latter is strict.
349 -- f = \x -> ...(error s)...
351 -- Fundamentally such contexts should not ecourage inlining becuase
352 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
353 -- so there's no gain.
355 -- However, even a type application or coercion isn't a lone variable.
357 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
358 -- We had better inline that sucker! The case won't see through it.
360 -- For now, I'm treating treating a variable applied to types
361 -- in a *lazy* context "lone". The motivating example was
363 -- g = /\a. \y. h (f a)
364 -- There's no advantage in inlining f here, and perhaps
365 -- a significant disadvantage. Hence some_val_args in the Stop case
367 interestingCallContext some_args some_val_args cont
370 interesting (InlinePlease _) = True
371 interesting (Select _ _ _ _ _) = some_args
372 interesting (ApplyTo _ _ _ _) = some_args -- Can happen if we have (coerce t (f x)) y
373 interesting (ArgOf _ _ _) = some_val_args
374 interesting (Stop ty upd_in_place) = some_val_args && upd_in_place
375 interesting (CoerceIt _ cont) = interesting cont
376 -- If this call is the arg of a strict function, the context
377 -- is a bit interesting. If we inline here, we may get useful
378 -- evaluation information to avoid repeated evals: e.g.
380 -- Here the contIsInteresting makes the '*' keener to inline,
381 -- which in turn exposes a constructor which makes the '+' inline.
382 -- Assuming that +,* aren't small enough to inline regardless.
384 -- It's also very important to inline in a strict context for things
387 -- Here, the context of (f x) is strict, and if f's unfolding is
388 -- a build it's *great* to inline it here. So we must ensure that
389 -- the context for (f x) is not totally uninteresting.
393 canUpdateInPlace :: Type -> Bool
394 -- Consider let x = <wurble> in ...
395 -- If <wurble> returns an explicit constructor, we might be able
396 -- to do update in place. So we treat even a thunk RHS context
397 -- as interesting if update in place is possible. We approximate
398 -- this by seeing if the type has a single constructor with a
399 -- small arity. But arity zero isn't good -- we share the single copy
400 -- for that case, so no point in sharing.
402 -- Note the repType: we want to look through newtypes for this purpose
405 | not opt_UF_UpdateInPlace = False
407 = case splitTyConApp_maybe (repType ty) of {
411 case tyConDataConsIfAvailable tycon of
412 [dc] -> arity == 1 || arity == 2
414 arity = dataConRepArity dc
421 %************************************************************************
423 \section{Dealing with a single binder}
425 %************************************************************************
428 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
429 simplBinders bndrs thing_inside
430 = getSubst `thenSmpl` \ subst ->
432 (subst', bndrs') = Subst.simplBndrs subst bndrs
434 seqBndrs bndrs' `seq`
435 setSubst subst' (thing_inside bndrs')
437 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
438 simplBinder bndr thing_inside
439 = getSubst `thenSmpl` \ subst ->
441 (subst', bndr') = Subst.simplBndr subst bndr
444 setSubst subst' (thing_inside bndr')
447 simplRecIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
448 simplRecIds ids thing_inside
449 = getSubst `thenSmpl` \ subst ->
451 (subst', ids') = mapAccumL Subst.simplLetId subst ids
454 setSubst subst' (thing_inside ids')
456 simplLetId :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
457 simplLetId id thing_inside
458 = getSubst `thenSmpl` \ subst ->
460 (subst', id') = Subst.simplLetId subst id
463 setSubst subst' (thing_inside id')
466 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
468 seqBndr b | isTyVar b = b `seq` ()
469 | otherwise = seqType (idType b) `seq`
475 %************************************************************************
477 \subsection{Local tyvar-lifting}
479 %************************************************************************
481 mkRhsTyLam tries this transformation, when the big lambda appears as
482 the RHS of a let(rec) binding:
484 /\abc -> let(rec) x = e in b
486 let(rec) x' = /\abc -> let x = x' a b c in e
488 /\abc -> let x = x' a b c in b
490 This is good because it can turn things like:
492 let f = /\a -> letrec g = ... g ... in g
494 letrec g' = /\a -> ... g' a ...
498 which is better. In effect, it means that big lambdas don't impede
501 This optimisation is CRUCIAL in eliminating the junk introduced by
502 desugaring mutually recursive definitions. Don't eliminate it lightly!
504 So far as the implementation is concerned:
506 Invariant: go F e = /\tvs -> F e
510 = Let x' = /\tvs -> F e
514 G = F . Let x = x' tvs
516 go F (Letrec xi=ei in b)
517 = Letrec {xi' = /\tvs -> G ei}
521 G = F . Let {xi = xi' tvs}
523 [May 1999] If we do this transformation *regardless* then we can
524 end up with some pretty silly stuff. For example,
527 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
532 st = /\s -> ...[y1 s/x1, y2 s/x2]
535 Unless the "..." is a WHNF there is really no point in doing this.
536 Indeed it can make things worse. Suppose x1 is used strictly,
539 x1* = case f y of { (a,b) -> e }
541 If we abstract this wrt the tyvar we then can't do the case inline
542 as we would normally do.
546 tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
548 tryRhsTyLam rhs -- Only does something if there's a let
549 | null tyvars || not (worth_it body) -- inside a type lambda,
550 = returnSmpl ([], rhs) -- and a WHNF inside that
553 = go (\x -> x) body `thenSmpl` \ (binds, body') ->
554 returnSmpl (binds, mkLams tyvars body')
557 (tyvars, body) = collectTyBinders rhs
559 worth_it e@(Let _ _) = whnf_in_middle e
562 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
563 whnf_in_middle (Let _ e) = whnf_in_middle e
564 whnf_in_middle e = exprIsCheap e
566 go fn (Let bind@(NonRec var rhs) body)
568 = go (fn . Let bind) body
570 go fn (Let (NonRec var rhs) body)
571 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
572 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
573 returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
577 -- main_tyvar_set = mkVarSet tyvars
578 -- var_ty = idType var
579 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
580 -- tyvars_here was an attempt to reduce the number of tyvars
581 -- wrt which the new binding is abstracted. But the naive
582 -- approach of abstract wrt the tyvars free in the Id's type
584 -- /\ a b -> let t :: (a,b) = (e1, e2)
587 -- Here, b isn't free in x's type, but we must nevertheless
588 -- abstract wrt b as well, because t's type mentions b.
589 -- Since t is floated too, we'd end up with the bogus:
590 -- poly_t = /\ a b -> (e1, e2)
591 -- poly_x = /\ a -> fst (poly_t a *b*)
592 -- So for now we adopt the even more naive approach of
593 -- abstracting wrt *all* the tyvars. We'll see if that
594 -- gives rise to problems. SLPJ June 98
596 go fn (Let (Rec prs) body)
597 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
599 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
600 new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])
602 go gn body `thenSmpl` \ (binds, body') ->
603 returnSmpl (new_bind : binds, body')
605 (vars,rhss) = unzip prs
607 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
608 -- var_tys = map idType vars
609 -- See notes with tyvars_here above
611 go fn body = returnSmpl ([], fn body)
613 mk_poly tyvars_here var
614 = getUniqueSmpl `thenSmpl` \ uniq ->
616 poly_name = setNameUnique (idName var) uniq -- Keep same name
617 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
618 poly_id = mkVanillaId poly_name poly_ty
620 -- In the olden days, it was crucial to copy the occInfo of the original var,
621 -- because we were looking at occurrence-analysed but as yet unsimplified code!
622 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
623 -- at already simplified code, so it doesn't matter
625 -- It's even right to retain single-occurrence or dead-var info:
626 -- Suppose we started with /\a -> let x = E in B
627 -- where x occurs once in B. Then we transform to:
628 -- let x' = /\a -> E in /\a -> let x* = x' a in B
629 -- where x* has an INLINE prag on it. Now, once x* is inlined,
630 -- the occurrences of x' will be just the occurrences originally
633 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
635 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
636 -- Suppose we start with:
638 -- x = /\ a -> let g = G in E
640 -- Then we'll float to get
642 -- x = let poly_g = /\ a -> G
643 -- in /\ a -> let g = poly_g a in E
645 -- But now the occurrence analyser will see just one occurrence
646 -- of poly_g, not inside a lambda, so the simplifier will
647 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
648 -- (I used to think that the "don't inline lone occurrences" stuff
649 -- would stop this happening, but since it's the *only* occurrence,
650 -- PreInlineUnconditionally kicks in first!)
652 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
653 -- to appear many times. (NB: mkInlineMe eliminates
654 -- such notes on trivial RHSs, so do it manually.)
658 %************************************************************************
660 \subsection{Eta expansion}
662 %************************************************************************
664 Try eta expansion for RHSs
667 Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
670 Case 2 f = N E1..En ==> z1=E1
673 f = \y1..ym -> N z1..zn y1..ym
675 where (in both cases)
677 * The xi can include type variables
679 * The yi are all value variables
681 * N is a NORMAL FORM (i.e. no redexes anywhere)
682 wanting a suitable number of extra args.
684 * the Ei must not have unlifted type
686 There is no point in looking for a combination of the two, because
687 that would leave use with some lets sandwiched between lambdas; that's
688 what the final test in the first equation is for.
691 tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
692 tryEtaExpansion rhs rhs_ty
693 | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
694 || exprIsTrivial rhs -- Not if RHS is trivial
695 || final_arity == 0 -- Not if arity is zero
696 = returnSmpl ([], rhs)
698 | n_val_args == 0 && not arity_is_manifest
699 = -- Some lambdas but not enough: case 1
700 getUniqSupplySmpl `thenSmpl` \ us ->
701 returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
703 | n_val_args > 0 && not (any cant_bind arg_infos)
704 = -- Partial application: case 2
705 mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
706 getUniqSupplySmpl `thenSmpl` \ us ->
707 returnSmpl (catMaybes maybe_z_binds,
708 etaExpand final_arity us (mkApps fun z_args) rhs_ty)
711 = returnSmpl ([], rhs)
713 (fun, args) = collectArgs rhs
714 n_val_args = valArgCount args
715 (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
716 final_arity = 0 `max` (fun_arity - n_val_args)
717 arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
718 cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
720 bind_z_arg (arg, arg_ty, trivial_arg)
721 | trivial_arg = returnSmpl (Nothing, arg)
722 | otherwise = newId SLIT("z") arg_ty $ \ z ->
723 returnSmpl (Just (NonRec z arg), Var z)
727 %************************************************************************
729 \subsection{Case absorption and identity-case elimination}
731 %************************************************************************
734 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
737 @mkCase@ tries the following transformation (if possible):
739 case e of b { ==> case e of b {
740 p1 -> rhs1 p1 -> rhs1
742 pm -> rhsm pm -> rhsm
743 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
744 {or (prim) case b of b' { _ -> rhsn}}
747 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
751 which merges two cases in one case when -- the default alternative of
752 the outer case scrutises the same variable as the outer case This
753 transformation is called Case Merging. It avoids that the same
754 variable is scrutinised multiple times.
757 mkCase scrut outer_bndr outer_alts
759 && maybeToBool maybe_case_in_default
761 = tick (CaseMerge outer_bndr) `thenSmpl_`
762 returnSmpl (Case scrut outer_bndr new_alts)
763 -- Warning: don't call mkCase recursively!
764 -- Firstly, there's no point, because inner alts have already had
765 -- mkCase applied to them, so they won't have a case in their default
766 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
767 -- in munge_rhs puts a case into the DEFAULT branch!
769 new_alts = outer_alts_without_deflt ++ munged_inner_alts
770 maybe_case_in_default = case findDefault outer_alts of
771 (outer_alts_without_default,
772 Just (Case (Var scrut_var) inner_bndr inner_alts))
774 | outer_bndr == scrut_var
775 -> Just (outer_alts_without_default, inner_bndr, inner_alts)
778 Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
780 -- Eliminate any inner alts which are shadowed by the outer ones
781 outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
783 munged_inner_alts = [ (con, args, munge_rhs rhs)
784 | (con, args, rhs) <- inner_alts,
785 not (con `elem` outer_cons) -- Eliminate shadowed inner alts
787 munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
790 Now the identity-case transformation:
799 mkCase scrut case_bndr alts
800 | all identity_alt alts
801 = tick (CaseIdentity case_bndr) `thenSmpl_`
802 returnSmpl (re_note scrut)
804 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
806 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
807 identity_rhs (LitAlt lit) _ = Lit lit
808 identity_rhs DEFAULT _ = Var case_bndr
810 arg_tys = map Type (tyConAppArgs (idType case_bndr))
813 -- case coerce T e of x { _ -> coerce T' x }
814 -- And we definitely want to eliminate this case!
815 -- So we throw away notes from the RHS, and reconstruct
816 -- (at least an approximation) at the other end
817 de_note (Note _ e) = de_note e
820 -- re_note wraps a coerce if it might be necessary
821 re_note scrut = case head alts of
822 (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
829 mkCase other_scrut case_bndr other_alts
830 = returnSmpl (Case other_scrut case_bndr other_alts)