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,
27 etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce )
28 import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, substExpr )
29 import Id ( idType, idName,
30 idUnfolding, idStrictness,
33 import IdInfo ( StrictnessInfo(..) )
34 import Maybes ( maybeToBool, catMaybes )
35 import Name ( setNameUnique )
36 import Demand ( isStrict )
38 import Type ( Type, mkForAllTys, seqType, repType,
39 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
40 isDictTy, isDataType, isUnLiftedType,
43 import TyCon ( tyConDataConsIfAvailable )
44 import DataCon ( dataConRepArity )
45 import VarEnv ( SubstEnv )
46 import Util ( lengthExceeds )
51 %************************************************************************
53 \subsection{The continuation data type}
55 %************************************************************************
58 data SimplCont -- Strict contexts
59 = Stop OutType -- Type of the result
60 Bool -- True => This is the RHS of a thunk whose type suggests
61 -- that update-in-place would be possible
62 -- (This makes the inliner a little keener.)
64 | CoerceIt OutType -- The To-type, simplified
67 | InlinePlease -- This continuation makes a function very
68 SimplCont -- keen to inline itelf
71 InExpr SubstEnv -- The argument, as yet unsimplified,
72 SimplCont -- and its subst-env
75 InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
78 | ArgOf DupFlag -- An arbitrary strict context: the argument
79 -- of a strict function, or a primitive-arg fn
81 OutType -- cont_ty: the type of the expression being sought by the context
82 -- f (error "foo") ==> coerce t (error "foo")
84 -- We need to know the type t, to which to coerce.
85 (OutExpr -> SimplM OutExprStuff) -- What to do with the result
86 -- The result expression in the OutExprStuff has type cont_ty
88 instance Outputable SimplCont where
89 ppr (Stop _ _) = ptext SLIT("Stop")
90 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
91 ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
92 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
93 (nest 4 (ppr alts)) $$ ppr cont
94 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
95 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
97 data DupFlag = OkToDup | NoDup
99 instance Outputable DupFlag where
100 ppr OkToDup = ptext SLIT("ok")
101 ppr NoDup = ptext SLIT("nodup")
105 mkRhsStop, mkStop :: OutType -> SimplCont
106 mkStop ty = Stop ty False
107 mkRhsStop ty = Stop ty (canUpdateInPlace ty)
111 contIsDupable :: SimplCont -> Bool
112 contIsDupable (Stop _ _) = True
113 contIsDupable (ApplyTo OkToDup _ _ _) = True
114 contIsDupable (ArgOf OkToDup _ _) = True
115 contIsDupable (Select OkToDup _ _ _ _) = True
116 contIsDupable (CoerceIt _ cont) = contIsDupable cont
117 contIsDupable (InlinePlease cont) = contIsDupable cont
118 contIsDupable other = False
121 discardInline :: SimplCont -> SimplCont
122 discardInline (InlinePlease cont) = cont
123 discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
124 discardInline cont = cont
127 discardableCont :: SimplCont -> Bool
128 discardableCont (Stop _ _) = False
129 discardableCont (CoerceIt _ cont) = discardableCont cont
130 discardableCont (InlinePlease cont) = discardableCont cont
131 discardableCont other = True
133 discardCont :: SimplCont -- A continuation, expecting
134 -> SimplCont -- Replace the continuation with a suitable coerce
135 discardCont cont = case cont of
137 other -> CoerceIt to_ty (mkStop to_ty)
139 to_ty = contResultType cont
142 contResultType :: SimplCont -> OutType
143 contResultType (Stop to_ty _) = to_ty
144 contResultType (ArgOf _ to_ty _) = to_ty
145 contResultType (ApplyTo _ _ _ cont) = contResultType cont
146 contResultType (CoerceIt _ cont) = contResultType cont
147 contResultType (InlinePlease cont) = contResultType cont
148 contResultType (Select _ _ _ _ cont) = contResultType cont
151 countValArgs :: SimplCont -> Int
152 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
153 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
154 countValArgs other = 0
156 countArgs :: SimplCont -> Int
157 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
163 getContArgs :: OutId -> SimplCont
164 -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args
165 SimplCont, -- Remaining continuation
166 Bool) -- Whether we came across an InlineCall
167 -- getContArgs id k = (args, k', inl)
168 -- args are the leading ApplyTo items in k
169 -- (i.e. outermost comes first)
170 -- augmented with demand info from the functionn
171 getContArgs fun orig_cont
172 = getSwitchChecker `thenSmpl` \ chkr ->
174 -- Ignore strictness info if the no-case-of-case
175 -- flag is on. Strictness changes evaluation order
176 -- and that can change full laziness
177 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
178 | otherwise = computed_stricts
180 go [] stricts False orig_cont
182 ----------------------------
185 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
186 = go ((arg,se,False) : acc) ss inl cont
187 -- NB: don't bother to instantiate the function type
190 go acc (s:ss) inl (ApplyTo _ arg se cont)
191 = go ((arg,se,s) : acc) ss inl cont
193 -- An Inline continuation
194 go acc ss inl (InlinePlease cont)
195 = go acc ss True cont
197 -- We're run out of arguments, or else we've run out of demands
198 -- The latter only happens if the result is guaranteed bottom
199 -- This is the case for
200 -- * case (error "hello") of { ... }
201 -- * (error "Hello") arg
202 -- * f (error "Hello") where f is strict
205 | null ss && discardableCont cont = tick BottomFound `thenSmpl_`
206 returnSmpl (reverse acc, discardCont cont, inl)
207 | otherwise = returnSmpl (reverse acc, cont, inl)
209 ----------------------------
210 vanilla_stricts, computed_stricts :: [Bool]
211 vanilla_stricts = repeat False
212 computed_stricts = zipWith (||) fun_stricts arg_stricts
214 ----------------------------
215 (val_arg_tys, _) = splitRepFunTys (idType fun)
216 arg_stricts = map isStrictType val_arg_tys ++ repeat False
217 -- These argument types are used as a cheap and cheerful way to find
218 -- unboxed arguments, which must be strict. But it's an InType
219 -- and so there might be a type variable where we expect a function
220 -- type (the substitution hasn't happened yet). And we don't bother
221 -- doing the type applications for a polymorphic function.
222 -- Hence the split*Rep*FunTys
224 ----------------------------
225 -- If fun_stricts is finite, it means the function returns bottom
226 -- after that number of value args have been consumed
227 -- Otherwise it's infinite, extended with False
229 = case idStrictness fun of
230 StrictnessInfo demands result_bot
231 | not (demands `lengthExceeds` countValArgs orig_cont)
232 -> -- Enough args, use the strictness given.
233 -- For bottoming functions we used to pretend that the arg
234 -- is lazy, so that we don't treat the arg as an
235 -- interesting context. This avoids substituting
236 -- top-level bindings for (say) strings into
237 -- calls to error. But now we are more careful about
238 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
240 map isStrict demands -- Finite => result is bottom
242 map isStrict demands ++ vanilla_stricts
244 other -> vanilla_stricts -- Not enough args, or no strictness
248 isStrictType :: Type -> Bool
249 -- isStrictType computes whether an argument (or let RHS) should
250 -- be computed strictly or lazily, based only on its type
252 | isUnLiftedType ty = True
253 | opt_DictsStrict && isDictTy ty && isDataType ty = True
255 -- Return true only for dictionary types where the dictionary
256 -- has more than one component (else we risk poking on the component
257 -- of a newtype dictionary)
260 interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool
261 -- An argument is interesting if it has *some* structure
262 -- We are here trying to avoid unfolding a function that
263 -- is applied only to variables that have no unfolding
264 -- (i.e. they are probably lambda bound): f x y z
265 -- There is little point in inlining f here.
266 interestingArg in_scope arg subst
267 = analyse (substExpr (mkSubst in_scope subst) arg)
268 -- 'analyse' only looks at the top part of the result
269 -- and substExpr is lazy, so this isn't nearly as brutal
272 analyse (Var v) = hasSomeUnfolding (idUnfolding v)
273 -- Was: isValueUnfolding (idUnfolding v')
274 -- But that seems over-pessimistic
275 analyse (Type _) = False
276 analyse (App fn (Type _)) = analyse fn
277 analyse (Note _ a) = analyse a
279 -- Consider let x = 3 in f x
280 -- The substitution will contain (x -> ContEx 3), and we want to
281 -- to say that x is an interesting argument.
282 -- But consider also (\x. f x y) y
283 -- The substitution will contain (x -> ContEx y), and we want to say
284 -- that x is not interesting (assuming y has no unfolding)
287 Comment about interestingCallContext
288 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
289 We want to avoid inlining an expression where there can't possibly be
290 any gain, such as in an argument position. Hence, if the continuation
291 is interesting (eg. a case scrutinee, application etc.) then we
292 inline, otherwise we don't.
294 Previously some_benefit used to return True only if the variable was
295 applied to some value arguments. This didn't work:
297 let x = _coerce_ (T Int) Int (I# 3) in
298 case _coerce_ Int (T Int) x of
301 we want to inline x, but can't see that it's a constructor in a case
302 scrutinee position, and some_benefit is False.
306 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
308 .... case dMonadST _@_ x0 of (a,b,c) -> ....
310 we'd really like to inline dMonadST here, but we *don't* want to
311 inline if the case expression is just
313 case x of y { DEFAULT -> ... }
315 since we can just eliminate this case instead (x is in WHNF). Similar
316 applies when x is bound to a lambda expression. Hence
317 contIsInteresting looks for case expressions with just a single
321 interestingCallContext :: Bool -- False <=> no args at all
322 -> Bool -- False <=> no value args
324 -- The "lone-variable" case is important. I spent ages
325 -- messing about with unsatisfactory varaints, but this is nice.
326 -- The idea is that if a variable appear all alone
327 -- as an arg of lazy fn, or rhs Stop
328 -- as scrutinee of a case Select
329 -- as arg of a strict fn ArgOf
330 -- then we should not inline it (unless there is some other reason,
331 -- e.g. is is the sole occurrence). We achieve this by making
332 -- interestingCallContext return False for a lone variable.
334 -- Why? At least in the case-scrutinee situation, turning
335 -- let x = (a,b) in case x of y -> ...
337 -- let x = (a,b) in case (a,b) of y -> ...
339 -- let x = (a,b) in let y = (a,b) in ...
340 -- is bad if the binding for x will remain.
342 -- Another example: I discovered that strings
343 -- were getting inlined straight back into applications of 'error'
344 -- because the latter is strict.
346 -- f = \x -> ...(error s)...
348 -- Fundamentally such contexts should not ecourage inlining becuase
349 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
350 -- so there's no gain.
352 -- However, even a type application or coercion isn't a lone variable.
354 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
355 -- We had better inline that sucker! The case won't see through it.
357 -- For now, I'm treating treating a variable applied to types
358 -- in a *lazy* context "lone". The motivating example was
360 -- g = /\a. \y. h (f a)
361 -- There's no advantage in inlining f here, and perhaps
362 -- a significant disadvantage. Hence some_val_args in the Stop case
364 interestingCallContext some_args some_val_args cont
367 interesting (InlinePlease _) = True
368 interesting (Select _ _ _ _ _) = some_args
369 interesting (ApplyTo _ _ _ _) = some_args -- Can happen if we have (coerce t (f x)) y
370 interesting (ArgOf _ _ _) = some_val_args
371 interesting (Stop ty upd_in_place) = some_val_args && upd_in_place
372 interesting (CoerceIt _ cont) = interesting cont
373 -- If this call is the arg of a strict function, the context
374 -- is a bit interesting. If we inline here, we may get useful
375 -- evaluation information to avoid repeated evals: e.g.
377 -- Here the contIsInteresting makes the '*' keener to inline,
378 -- which in turn exposes a constructor which makes the '+' inline.
379 -- Assuming that +,* aren't small enough to inline regardless.
381 -- It's also very important to inline in a strict context for things
384 -- Here, the context of (f x) is strict, and if f's unfolding is
385 -- a build it's *great* to inline it here. So we must ensure that
386 -- the context for (f x) is not totally uninteresting.
390 canUpdateInPlace :: Type -> Bool
391 -- Consider let x = <wurble> in ...
392 -- If <wurble> returns an explicit constructor, we might be able
393 -- to do update in place. So we treat even a thunk RHS context
394 -- as interesting if update in place is possible. We approximate
395 -- this by seeing if the type has a single constructor with a
396 -- small arity. But arity zero isn't good -- we share the single copy
397 -- for that case, so no point in sharing.
399 -- Note the repType: we want to look through newtypes for this purpose
402 | not opt_UF_UpdateInPlace = False
404 = case splitTyConApp_maybe (repType ty) of {
408 case tyConDataConsIfAvailable tycon of
409 [dc] -> arity == 1 || arity == 2
411 arity = dataConRepArity dc
418 %************************************************************************
420 \section{Dealing with a single binder}
422 %************************************************************************
425 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
426 simplBinders bndrs thing_inside
427 = getSubst `thenSmpl` \ subst ->
429 (subst', bndrs') = substBndrs subst bndrs
431 seqBndrs bndrs' `seq`
432 setSubst subst' (thing_inside bndrs')
434 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
435 simplBinder bndr thing_inside
436 = getSubst `thenSmpl` \ subst ->
438 (subst', bndr') = substBndr subst bndr
441 setSubst subst' (thing_inside bndr')
444 -- Same semantics as simplBinders, but a little less
445 -- plumbing and hence a little more efficient.
446 -- Maybe not worth the candle?
447 simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
448 simplIds ids thing_inside
449 = getSubst `thenSmpl` \ subst ->
451 (subst', bndrs') = substIds subst ids
453 seqBndrs bndrs' `seq`
454 setSubst subst' (thing_inside bndrs')
457 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
459 seqBndr b | isTyVar b = b `seq` ()
460 | otherwise = seqType (idType b) `seq`
466 %************************************************************************
468 \subsection{Local tyvar-lifting}
470 %************************************************************************
472 mkRhsTyLam tries this transformation, when the big lambda appears as
473 the RHS of a let(rec) binding:
475 /\abc -> let(rec) x = e in b
477 let(rec) x' = /\abc -> let x = x' a b c in e
479 /\abc -> let x = x' a b c in b
481 This is good because it can turn things like:
483 let f = /\a -> letrec g = ... g ... in g
485 letrec g' = /\a -> ... g' a ...
489 which is better. In effect, it means that big lambdas don't impede
492 This optimisation is CRUCIAL in eliminating the junk introduced by
493 desugaring mutually recursive definitions. Don't eliminate it lightly!
495 So far as the implementation is concerned:
497 Invariant: go F e = /\tvs -> F e
501 = Let x' = /\tvs -> F e
505 G = F . Let x = x' tvs
507 go F (Letrec xi=ei in b)
508 = Letrec {xi' = /\tvs -> G ei}
512 G = F . Let {xi = xi' tvs}
514 [May 1999] If we do this transformation *regardless* then we can
515 end up with some pretty silly stuff. For example,
518 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
523 st = /\s -> ...[y1 s/x1, y2 s/x2]
526 Unless the "..." is a WHNF there is really no point in doing this.
527 Indeed it can make things worse. Suppose x1 is used strictly,
530 x1* = case f y of { (a,b) -> e }
532 If we abstract this wrt the tyvar we then can't do the case inline
533 as we would normally do.
537 tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
539 tryRhsTyLam rhs -- Only does something if there's a let
540 | null tyvars || not (worth_it body) -- inside a type lambda,
541 = returnSmpl ([], rhs) -- and a WHNF inside that
544 = go (\x -> x) body `thenSmpl` \ (binds, body') ->
545 returnSmpl (binds, mkLams tyvars body')
548 (tyvars, body) = collectTyBinders rhs
550 worth_it e@(Let _ _) = whnf_in_middle e
553 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
554 whnf_in_middle (Let _ e) = whnf_in_middle e
555 whnf_in_middle e = exprIsCheap e
557 go fn (Let bind@(NonRec var rhs) body)
559 = go (fn . Let bind) body
561 go fn (Let (NonRec var rhs) body)
562 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
563 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
564 returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
568 -- main_tyvar_set = mkVarSet tyvars
569 -- var_ty = idType var
570 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
571 -- tyvars_here was an attempt to reduce the number of tyvars
572 -- wrt which the new binding is abstracted. But the naive
573 -- approach of abstract wrt the tyvars free in the Id's type
575 -- /\ a b -> let t :: (a,b) = (e1, e2)
578 -- Here, b isn't free in x's type, but we must nevertheless
579 -- abstract wrt b as well, because t's type mentions b.
580 -- Since t is floated too, we'd end up with the bogus:
581 -- poly_t = /\ a b -> (e1, e2)
582 -- poly_x = /\ a -> fst (poly_t a *b*)
583 -- So for now we adopt the even more naive approach of
584 -- abstracting wrt *all* the tyvars. We'll see if that
585 -- gives rise to problems. SLPJ June 98
587 go fn (Let (Rec prs) body)
588 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
590 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
591 new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])
593 go gn body `thenSmpl` \ (binds, body') ->
594 returnSmpl (new_bind : binds, body')
596 (vars,rhss) = unzip prs
598 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
599 -- var_tys = map idType vars
600 -- See notes with tyvars_here above
602 go fn body = returnSmpl ([], fn body)
604 mk_poly tyvars_here var
605 = getUniqueSmpl `thenSmpl` \ uniq ->
607 poly_name = setNameUnique (idName var) uniq -- Keep same name
608 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
609 poly_id = mkVanillaId poly_name poly_ty
611 -- In the olden days, it was crucial to copy the occInfo of the original var,
612 -- because we were looking at occurrence-analysed but as yet unsimplified code!
613 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
614 -- at already simplified code, so it doesn't matter
616 -- It's even right to retain single-occurrence or dead-var info:
617 -- Suppose we started with /\a -> let x = E in B
618 -- where x occurs once in B. Then we transform to:
619 -- let x' = /\a -> E in /\a -> let x* = x' a in B
620 -- where x* has an INLINE prag on it. Now, once x* is inlined,
621 -- the occurrences of x' will be just the occurrences originally
624 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
626 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
627 -- Suppose we start with:
629 -- x = /\ a -> let g = G in E
631 -- Then we'll float to get
633 -- x = let poly_g = /\ a -> G
634 -- in /\ a -> let g = poly_g a in E
636 -- But now the occurrence analyser will see just one occurrence
637 -- of poly_g, not inside a lambda, so the simplifier will
638 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
639 -- (I used to think that the "don't inline lone occurrences" stuff
640 -- would stop this happening, but since it's the *only* occurrence,
641 -- PreInlineUnconditionally kicks in first!)
643 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
644 -- to appear many times. (NB: mkInlineMe eliminates
645 -- such notes on trivial RHSs, so do it manually.)
649 %************************************************************************
651 \subsection{Eta expansion}
653 %************************************************************************
655 Try eta expansion for RHSs
658 Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
661 Case 2 f = N E1..En ==> z1=E1
664 f = \y1..ym -> N z1..zn y1..ym
666 where (in both cases)
668 * The xi can include type variables
670 * The yi are all value variables
672 * N is a NORMAL FORM (i.e. no redexes anywhere)
673 wanting a suitable number of extra args.
675 * the Ei must not have unlifted type
677 There is no point in looking for a combination of the two, because
678 that would leave use with some lets sandwiched between lambdas; that's
679 what the final test in the first equation is for.
682 tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
683 tryEtaExpansion rhs rhs_ty
684 | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
685 || exprIsTrivial rhs -- Not if RHS is trivial
686 || final_arity == 0 -- Not if arity is zero
687 = returnSmpl ([], rhs)
689 | n_val_args == 0 && not arity_is_manifest
690 = -- Some lambdas but not enough: case 1
691 getUniqSupplySmpl `thenSmpl` \ us ->
692 returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
694 | n_val_args > 0 && not (any cant_bind arg_infos)
695 = -- Partial application: case 2
696 mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
697 getUniqSupplySmpl `thenSmpl` \ us ->
698 returnSmpl (catMaybes maybe_z_binds,
699 etaExpand final_arity us (mkApps fun z_args) rhs_ty)
702 = returnSmpl ([], rhs)
704 (fun, args) = collectArgs rhs
705 n_val_args = valArgCount args
706 (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
707 final_arity = 0 `max` (fun_arity - n_val_args)
708 arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
709 cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
711 bind_z_arg (arg, arg_ty, trivial_arg)
712 | trivial_arg = returnSmpl (Nothing, arg)
713 | otherwise = newId SLIT("z") arg_ty $ \ z ->
714 returnSmpl (Just (NonRec z arg), Var z)
718 %************************************************************************
720 \subsection{Case absorption and identity-case elimination}
722 %************************************************************************
725 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
728 @mkCase@ tries the following transformation (if possible):
730 case e of b { ==> case e of b {
731 p1 -> rhs1 p1 -> rhs1
733 pm -> rhsm pm -> rhsm
734 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
735 {or (prim) case b of b' { _ -> rhsn}}
738 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
742 which merges two cases in one case when -- the default alternative of
743 the outer case scrutises the same variable as the outer case This
744 transformation is called Case Merging. It avoids that the same
745 variable is scrutinised multiple times.
748 mkCase scrut outer_bndr outer_alts
750 && maybeToBool maybe_case_in_default
752 = tick (CaseMerge outer_bndr) `thenSmpl_`
753 returnSmpl (Case scrut outer_bndr new_alts)
754 -- Warning: don't call mkCase recursively!
755 -- Firstly, there's no point, because inner alts have already had
756 -- mkCase applied to them, so they won't have a case in their default
757 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
758 -- in munge_rhs puts a case into the DEFAULT branch!
760 new_alts = outer_alts_without_deflt ++ munged_inner_alts
761 maybe_case_in_default = case findDefault outer_alts of
762 (outer_alts_without_default,
763 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
781 Now the identity-case transformation:
790 mkCase scrut case_bndr alts
791 | all identity_alt alts
792 = tick (CaseIdentity case_bndr) `thenSmpl_`
793 returnSmpl (re_note scrut)
795 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
797 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
798 identity_rhs (LitAlt lit) _ = Lit lit
799 identity_rhs DEFAULT _ = Var case_bndr
801 arg_tys = map Type (tyConAppArgs (idType case_bndr))
804 -- case coerce T e of x { _ -> coerce T' x }
805 -- And we definitely want to eliminate this case!
806 -- So we throw away notes from the RHS, and reconstruct
807 -- (at least an approximation) at the other end
808 de_note (Note _ e) = de_note e
811 -- re_note wraps a coerce if it might be necessary
812 re_note scrut = case head alts of
813 (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
820 mkCase other_scrut case_bndr other_alts
821 = returnSmpl (Case other_scrut case_bndr other_alts)
826 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
827 findDefault [] = ([], Nothing)
828 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args )
830 findDefault (alt : alts) = case findDefault alts of
831 (alts', deflt) -> (alt : alts', deflt)
833 findAlt :: AltCon -> [CoreAlt] -> CoreAlt
837 go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
838 go (alt : alts) | matches alt = alt
839 | otherwise = go alts
841 matches (DEFAULT, _, _) = True
842 matches (con1, _, _) = con == con1