2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
8 simplBinder, simplBinders, simplRecIds, simplLetId, simplLamBinder,
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, simplLamBndr )
32 import Id ( idType, idName,
33 idUnfolding, idNewStrictness,
36 import Maybes ( maybeToBool, catMaybes )
37 import Name ( setNameUnique )
38 import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
40 import Type ( Type, mkForAllTys, seqType,
41 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
42 isUnLiftedType, isStrictType,
45 import TyCon ( tyConDataConsIfAvailable )
46 import DataCon ( dataConRepArity )
47 import VarEnv ( SubstEnv )
48 import Util ( lengthExceeds, mapAccumL )
53 %************************************************************************
55 \subsection{The continuation data type}
57 %************************************************************************
60 data SimplCont -- Strict contexts
61 = Stop OutType -- Type of the result
62 Bool -- True => This is the RHS of a thunk whose type suggests
63 -- that update-in-place would be possible
64 -- (This makes the inliner a little keener.)
66 | CoerceIt OutType -- The To-type, simplified
69 | InlinePlease -- This continuation makes a function very
70 SimplCont -- keen to inline itelf
73 InExpr SubstEnv -- The argument, as yet unsimplified,
74 SimplCont -- and its subst-env
77 InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
80 | ArgOf DupFlag -- An arbitrary strict context: the argument
81 -- of a strict function, or a primitive-arg fn
83 OutType -- cont_ty: the type of the expression being sought by the context
84 -- f (error "foo") ==> coerce t (error "foo")
86 -- We need to know the type t, to which to coerce.
87 (OutExpr -> SimplM OutExprStuff) -- What to do with the result
88 -- The result expression in the OutExprStuff has type cont_ty
90 instance Outputable SimplCont where
91 ppr (Stop _ _) = ptext SLIT("Stop")
92 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
93 ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
94 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
95 (nest 4 (ppr alts)) $$ ppr cont
96 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
97 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
99 data DupFlag = OkToDup | NoDup
101 instance Outputable DupFlag where
102 ppr OkToDup = ptext SLIT("ok")
103 ppr NoDup = ptext SLIT("nodup")
107 mkRhsStop, mkStop :: OutType -> SimplCont
108 mkStop ty = Stop ty False
109 mkRhsStop ty = Stop ty (canUpdateInPlace ty)
113 contIsDupable :: SimplCont -> Bool
114 contIsDupable (Stop _ _) = True
115 contIsDupable (ApplyTo OkToDup _ _ _) = True
116 contIsDupable (ArgOf OkToDup _ _) = True
117 contIsDupable (Select OkToDup _ _ _ _) = True
118 contIsDupable (CoerceIt _ cont) = contIsDupable cont
119 contIsDupable (InlinePlease cont) = contIsDupable cont
120 contIsDupable other = False
123 discardInline :: SimplCont -> SimplCont
124 discardInline (InlinePlease cont) = cont
125 discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
126 discardInline cont = cont
129 discardableCont :: SimplCont -> Bool
130 discardableCont (Stop _ _) = False
131 discardableCont (CoerceIt _ cont) = discardableCont cont
132 discardableCont (InlinePlease cont) = discardableCont cont
133 discardableCont other = True
135 discardCont :: SimplCont -- A continuation, expecting
136 -> SimplCont -- Replace the continuation with a suitable coerce
137 discardCont cont = case cont of
139 other -> CoerceIt to_ty (mkStop to_ty)
141 to_ty = contResultType cont
144 contResultType :: SimplCont -> OutType
145 contResultType (Stop to_ty _) = to_ty
146 contResultType (ArgOf _ to_ty _) = to_ty
147 contResultType (ApplyTo _ _ _ cont) = contResultType cont
148 contResultType (CoerceIt _ cont) = contResultType cont
149 contResultType (InlinePlease cont) = contResultType cont
150 contResultType (Select _ _ _ _ cont) = contResultType cont
153 countValArgs :: SimplCont -> Int
154 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
155 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
156 countValArgs other = 0
158 countArgs :: SimplCont -> Int
159 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
165 getContArgs :: OutId -> SimplCont
166 -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args
167 SimplCont, -- Remaining continuation
168 Bool) -- Whether we came across an InlineCall
169 -- getContArgs id k = (args, k', inl)
170 -- args are the leading ApplyTo items in k
171 -- (i.e. outermost comes first)
172 -- augmented with demand info from the functionn
173 getContArgs fun orig_cont
174 = getSwitchChecker `thenSmpl` \ chkr ->
176 -- Ignore strictness info if the no-case-of-case
177 -- flag is on. Strictness changes evaluation order
178 -- and that can change full laziness
179 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
180 | otherwise = computed_stricts
182 go [] stricts False orig_cont
184 ----------------------------
187 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
188 = go ((arg,se,False) : acc) ss inl cont
189 -- NB: don't bother to instantiate the function type
192 go acc (s:ss) inl (ApplyTo _ arg se cont)
193 = go ((arg,se,s) : acc) ss inl cont
195 -- An Inline continuation
196 go acc ss inl (InlinePlease cont)
197 = go acc ss True cont
199 -- We're run out of arguments, or else we've run out of demands
200 -- The latter only happens if the result is guaranteed bottom
201 -- This is the case for
202 -- * case (error "hello") of { ... }
203 -- * (error "Hello") arg
204 -- * f (error "Hello") where f is strict
207 | null ss && discardableCont cont = tick BottomFound `thenSmpl_`
208 returnSmpl (reverse acc, discardCont cont, inl)
209 | otherwise = returnSmpl (reverse acc, cont, inl)
211 ----------------------------
212 vanilla_stricts, computed_stricts :: [Bool]
213 vanilla_stricts = repeat False
214 computed_stricts = zipWith (||) fun_stricts arg_stricts
216 ----------------------------
217 (val_arg_tys, _) = splitRepFunTys (idType fun)
218 arg_stricts = map isStrictType val_arg_tys ++ repeat False
219 -- These argument types are used as a cheap and cheerful way to find
220 -- unboxed arguments, which must be strict. But it's an InType
221 -- and so there might be a type variable where we expect a function
222 -- type (the substitution hasn't happened yet). And we don't bother
223 -- doing the type applications for a polymorphic function.
224 -- Hence the split*Rep*FunTys
226 ----------------------------
227 -- If fun_stricts is finite, it means the function returns bottom
228 -- after that number of value args have been consumed
229 -- Otherwise it's infinite, extended with False
231 = case splitStrictSig (idNewStrictness fun) of
232 (demands, result_info)
233 | not (demands `lengthExceeds` countValArgs orig_cont)
234 -> -- Enough args, use the strictness given.
235 -- For bottoming functions we used to pretend that the arg
236 -- is lazy, so that we don't treat the arg as an
237 -- interesting context. This avoids substituting
238 -- top-level bindings for (say) strings into
239 -- calls to error. But now we are more careful about
240 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
241 if isBotRes result_info then
242 map isStrictDmd demands -- Finite => result is bottom
244 map isStrictDmd demands ++ vanilla_stricts
246 other -> vanilla_stricts -- Not enough args, or no strictness
249 interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool
250 -- An argument is interesting if it has *some* structure
251 -- We are here trying to avoid unfolding a function that
252 -- is applied only to variables that have no unfolding
253 -- (i.e. they are probably lambda bound): f x y z
254 -- There is little point in inlining f here.
255 interestingArg in_scope arg subst
256 = analyse (substExpr (mkSubst in_scope subst) arg)
257 -- 'analyse' only looks at the top part of the result
258 -- and substExpr is lazy, so this isn't nearly as brutal
261 analyse (Var v) = hasSomeUnfolding (idUnfolding v)
262 -- Was: isValueUnfolding (idUnfolding v')
263 -- But that seems over-pessimistic
264 analyse (Type _) = False
265 analyse (App fn (Type _)) = analyse fn
266 analyse (Note _ a) = analyse a
268 -- Consider let x = 3 in f x
269 -- The substitution will contain (x -> ContEx 3), and we want to
270 -- to say that x is an interesting argument.
271 -- But consider also (\x. f x y) y
272 -- The substitution will contain (x -> ContEx y), and we want to say
273 -- that x is not interesting (assuming y has no unfolding)
276 Comment about interestingCallContext
277 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
278 We want to avoid inlining an expression where there can't possibly be
279 any gain, such as in an argument position. Hence, if the continuation
280 is interesting (eg. a case scrutinee, application etc.) then we
281 inline, otherwise we don't.
283 Previously some_benefit used to return True only if the variable was
284 applied to some value arguments. This didn't work:
286 let x = _coerce_ (T Int) Int (I# 3) in
287 case _coerce_ Int (T Int) x of
290 we want to inline x, but can't see that it's a constructor in a case
291 scrutinee position, and some_benefit is False.
295 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
297 .... case dMonadST _@_ x0 of (a,b,c) -> ....
299 we'd really like to inline dMonadST here, but we *don't* want to
300 inline if the case expression is just
302 case x of y { DEFAULT -> ... }
304 since we can just eliminate this case instead (x is in WHNF). Similar
305 applies when x is bound to a lambda expression. Hence
306 contIsInteresting looks for case expressions with just a single
310 interestingCallContext :: Bool -- False <=> no args at all
311 -> Bool -- False <=> no value args
313 -- The "lone-variable" case is important. I spent ages
314 -- messing about with unsatisfactory varaints, but this is nice.
315 -- The idea is that if a variable appear all alone
316 -- as an arg of lazy fn, or rhs Stop
317 -- as scrutinee of a case Select
318 -- as arg of a strict fn ArgOf
319 -- then we should not inline it (unless there is some other reason,
320 -- e.g. is is the sole occurrence). We achieve this by making
321 -- interestingCallContext return False for a lone variable.
323 -- Why? At least in the case-scrutinee situation, turning
324 -- let x = (a,b) in case x of y -> ...
326 -- let x = (a,b) in case (a,b) of y -> ...
328 -- let x = (a,b) in let y = (a,b) in ...
329 -- is bad if the binding for x will remain.
331 -- Another example: I discovered that strings
332 -- were getting inlined straight back into applications of 'error'
333 -- because the latter is strict.
335 -- f = \x -> ...(error s)...
337 -- Fundamentally such contexts should not ecourage inlining becuase
338 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
339 -- so there's no gain.
341 -- However, even a type application or coercion isn't a lone variable.
343 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
344 -- We had better inline that sucker! The case won't see through it.
346 -- For now, I'm treating treating a variable applied to types
347 -- in a *lazy* context "lone". The motivating example was
349 -- g = /\a. \y. h (f a)
350 -- There's no advantage in inlining f here, and perhaps
351 -- a significant disadvantage. Hence some_val_args in the Stop case
353 interestingCallContext some_args some_val_args cont
356 interesting (InlinePlease _) = True
357 interesting (Select _ _ _ _ _) = some_args
358 interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
359 -- Perhaps True is a bit over-keen, but I've
360 -- seen (coerce f) x, where f has an INLINE prag,
361 -- So we have to give some motivaiton for inlining it
362 interesting (ArgOf _ _ _) = some_val_args
363 interesting (Stop ty upd_in_place) = some_val_args && upd_in_place
364 interesting (CoerceIt _ cont) = interesting cont
365 -- If this call is the arg of a strict function, the context
366 -- is a bit interesting. If we inline here, we may get useful
367 -- evaluation information to avoid repeated evals: e.g.
369 -- Here the contIsInteresting makes the '*' keener to inline,
370 -- which in turn exposes a constructor which makes the '+' inline.
371 -- Assuming that +,* aren't small enough to inline regardless.
373 -- It's also very important to inline in a strict context for things
376 -- Here, the context of (f x) is strict, and if f's unfolding is
377 -- a build it's *great* to inline it here. So we must ensure that
378 -- the context for (f x) is not totally uninteresting.
382 canUpdateInPlace :: Type -> Bool
383 -- Consider let x = <wurble> in ...
384 -- If <wurble> returns an explicit constructor, we might be able
385 -- to do update in place. So we treat even a thunk RHS context
386 -- as interesting if update in place is possible. We approximate
387 -- this by seeing if the type has a single constructor with a
388 -- small arity. But arity zero isn't good -- we share the single copy
389 -- for that case, so no point in sharing.
392 | not opt_UF_UpdateInPlace = False
394 = case splitTyConApp_maybe ty of
396 Just (tycon, _) -> case tyConDataConsIfAvailable tycon of
397 [dc] -> arity == 1 || arity == 2
399 arity = dataConRepArity dc
405 %************************************************************************
407 \section{Dealing with a single binder}
409 %************************************************************************
412 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
413 simplBinders bndrs thing_inside
414 = getSubst `thenSmpl` \ subst ->
416 (subst', bndrs') = Subst.simplBndrs subst bndrs
418 seqBndrs bndrs' `seq`
419 setSubst subst' (thing_inside bndrs')
421 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
422 simplBinder bndr thing_inside
423 = getSubst `thenSmpl` \ subst ->
425 (subst', bndr') = Subst.simplBndr subst bndr
428 setSubst subst' (thing_inside bndr')
431 simplLamBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
432 simplLamBinder bndr thing_inside
433 = getSubst `thenSmpl` \ subst ->
435 (subst', bndr') = Subst.simplLamBndr subst bndr
438 setSubst subst' (thing_inside bndr')
441 simplRecIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
442 simplRecIds ids thing_inside
443 = getSubst `thenSmpl` \ subst ->
445 (subst', ids') = mapAccumL Subst.simplLetId subst ids
448 setSubst subst' (thing_inside ids')
450 simplLetId :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
451 simplLetId id thing_inside
452 = getSubst `thenSmpl` \ subst ->
454 (subst', id') = Subst.simplLetId subst id
457 setSubst subst' (thing_inside id')
460 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
462 seqBndr b | isTyVar b = b `seq` ()
463 | otherwise = seqType (idType b) `seq`
469 %************************************************************************
471 \subsection{Local tyvar-lifting}
473 %************************************************************************
475 mkRhsTyLam tries this transformation, when the big lambda appears as
476 the RHS of a let(rec) binding:
478 /\abc -> let(rec) x = e in b
480 let(rec) x' = /\abc -> let x = x' a b c in e
482 /\abc -> let x = x' a b c in b
484 This is good because it can turn things like:
486 let f = /\a -> letrec g = ... g ... in g
488 letrec g' = /\a -> ... g' a ...
492 which is better. In effect, it means that big lambdas don't impede
495 This optimisation is CRUCIAL in eliminating the junk introduced by
496 desugaring mutually recursive definitions. Don't eliminate it lightly!
498 So far as the implementation is concerned:
500 Invariant: go F e = /\tvs -> F e
504 = Let x' = /\tvs -> F e
508 G = F . Let x = x' tvs
510 go F (Letrec xi=ei in b)
511 = Letrec {xi' = /\tvs -> G ei}
515 G = F . Let {xi = xi' tvs}
517 [May 1999] If we do this transformation *regardless* then we can
518 end up with some pretty silly stuff. For example,
521 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
526 st = /\s -> ...[y1 s/x1, y2 s/x2]
529 Unless the "..." is a WHNF there is really no point in doing this.
530 Indeed it can make things worse. Suppose x1 is used strictly,
533 x1* = case f y of { (a,b) -> e }
535 If we abstract this wrt the tyvar we then can't do the case inline
536 as we would normally do.
540 tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
542 tryRhsTyLam rhs -- Only does something if there's a let
543 | null tyvars || not (worth_it body) -- inside a type lambda,
544 = returnSmpl ([], rhs) -- and a WHNF inside that
547 = go (\x -> x) body `thenSmpl` \ (binds, body') ->
548 returnSmpl (binds, mkLams tyvars body')
551 (tyvars, body) = collectTyBinders rhs
553 worth_it e@(Let _ _) = whnf_in_middle e
556 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
557 whnf_in_middle (Let _ e) = whnf_in_middle e
558 whnf_in_middle e = exprIsCheap e
560 go fn (Let bind@(NonRec var rhs) body)
562 = go (fn . Let bind) body
564 go fn (Let (NonRec var rhs) body)
565 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
566 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
567 returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
571 -- main_tyvar_set = mkVarSet tyvars
572 -- var_ty = idType var
573 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
574 -- tyvars_here was an attempt to reduce the number of tyvars
575 -- wrt which the new binding is abstracted. But the naive
576 -- approach of abstract wrt the tyvars free in the Id's type
578 -- /\ a b -> let t :: (a,b) = (e1, e2)
581 -- Here, b isn't free in x's type, but we must nevertheless
582 -- abstract wrt b as well, because t's type mentions b.
583 -- Since t is floated too, we'd end up with the bogus:
584 -- poly_t = /\ a b -> (e1, e2)
585 -- poly_x = /\ a -> fst (poly_t a *b*)
586 -- So for now we adopt the even more naive approach of
587 -- abstracting wrt *all* the tyvars. We'll see if that
588 -- gives rise to problems. SLPJ June 98
590 go fn (Let (Rec prs) body)
591 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
593 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
594 new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])
596 go gn body `thenSmpl` \ (binds, body') ->
597 returnSmpl (new_bind : binds, body')
599 (vars,rhss) = unzip prs
601 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
602 -- var_tys = map idType vars
603 -- See notes with tyvars_here above
605 go fn body = returnSmpl ([], fn body)
607 mk_poly tyvars_here var
608 = getUniqueSmpl `thenSmpl` \ uniq ->
610 poly_name = setNameUnique (idName var) uniq -- Keep same name
611 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
612 poly_id = mkLocalId poly_name poly_ty
614 -- In the olden days, it was crucial to copy the occInfo of the original var,
615 -- because we were looking at occurrence-analysed but as yet unsimplified code!
616 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
617 -- at already simplified code, so it doesn't matter
619 -- It's even right to retain single-occurrence or dead-var info:
620 -- Suppose we started with /\a -> let x = E in B
621 -- where x occurs once in B. Then we transform to:
622 -- let x' = /\a -> E in /\a -> let x* = x' a in B
623 -- where x* has an INLINE prag on it. Now, once x* is inlined,
624 -- the occurrences of x' will be just the occurrences originally
627 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
629 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
630 -- Suppose we start with:
632 -- x = /\ a -> let g = G in E
634 -- Then we'll float to get
636 -- x = let poly_g = /\ a -> G
637 -- in /\ a -> let g = poly_g a in E
639 -- But now the occurrence analyser will see just one occurrence
640 -- of poly_g, not inside a lambda, so the simplifier will
641 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
642 -- (I used to think that the "don't inline lone occurrences" stuff
643 -- would stop this happening, but since it's the *only* occurrence,
644 -- PreInlineUnconditionally kicks in first!)
646 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
647 -- to appear many times. (NB: mkInlineMe eliminates
648 -- such notes on trivial RHSs, so do it manually.)
652 %************************************************************************
654 \subsection{Eta expansion}
656 %************************************************************************
658 Try eta expansion for RHSs
661 Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
664 Case 2 f = N E1..En ==> z1=E1
667 f = \y1..ym -> N z1..zn y1..ym
669 where (in both cases)
671 * The xi can include type variables
673 * The yi are all value variables
675 * N is a NORMAL FORM (i.e. no redexes anywhere)
676 wanting a suitable number of extra args.
678 * the Ei must not have unlifted type
680 There is no point in looking for a combination of the two, because
681 that would leave use with some lets sandwiched between lambdas; that's
682 what the final test in the first equation is for.
684 In Case 1, we may have to sandwich some coerces between the lambdas
685 to make the types work. exprEtaExpandArity looks through coerces
686 when computing arity; and etaExpand adds the coerces as necessary when
687 actually computing the expansion.
690 tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
691 tryEtaExpansion rhs rhs_ty
692 | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
693 || exprIsTrivial rhs -- Not if RHS is trivial
694 || final_arity == 0 -- Not if arity is zero
695 = returnSmpl ([], rhs)
697 | n_val_args == 0 && not arity_is_manifest
698 = -- Some lambdas but not enough: case 1
699 getUniqSupplySmpl `thenSmpl` \ us ->
700 returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
702 | n_val_args > 0 && not (any cant_bind arg_infos)
703 = -- Partial application: case 2
704 mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
705 getUniqSupplySmpl `thenSmpl` \ us ->
706 returnSmpl (catMaybes maybe_z_binds,
707 etaExpand final_arity us (mkApps fun z_args) rhs_ty)
710 = returnSmpl ([], rhs)
712 (fun, args) = collectArgs rhs
713 n_val_args = valArgCount args
714 (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
715 final_arity = 0 `max` (fun_arity - n_val_args)
716 arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
717 cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
719 bind_z_arg (arg, arg_ty, trivial_arg)
720 | trivial_arg = returnSmpl (Nothing, arg)
721 | otherwise = newId SLIT("z") arg_ty $ \ z ->
722 returnSmpl (Just (NonRec z arg), Var z)
726 %************************************************************************
728 \subsection{Case absorption and identity-case elimination}
730 %************************************************************************
733 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
736 @mkCase@ tries the following transformation (if possible):
738 case e of b { ==> case e of b {
739 p1 -> rhs1 p1 -> rhs1
741 pm -> rhsm pm -> rhsm
742 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
743 {or (prim) case b of b' { _ -> rhsn}}
746 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
750 which merges two cases in one case when -- the default alternative of
751 the outer case scrutises the same variable as the outer case This
752 transformation is called Case Merging. It avoids that the same
753 variable is scrutinised multiple times.
756 mkCase scrut outer_bndr outer_alts
758 && maybeToBool maybe_case_in_default
760 = tick (CaseMerge outer_bndr) `thenSmpl_`
761 returnSmpl (Case scrut outer_bndr new_alts)
762 -- Warning: don't call mkCase recursively!
763 -- Firstly, there's no point, because inner alts have already had
764 -- mkCase applied to them, so they won't have a case in their default
765 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
766 -- in munge_rhs puts a case into the DEFAULT branch!
768 new_alts = add_default maybe_inner_default
769 (outer_alts_without_deflt ++ inner_con_alts)
771 maybe_case_in_default = case findDefault outer_alts of
772 (outer_alts_without_default,
773 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
789 (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
791 add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
792 add_default Nothing alts = alts
795 Now the identity-case transformation:
804 mkCase scrut case_bndr alts
805 | all identity_alt alts
806 = tick (CaseIdentity case_bndr) `thenSmpl_`
807 returnSmpl (re_note scrut)
809 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
811 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
812 identity_rhs (LitAlt lit) _ = Lit lit
813 identity_rhs DEFAULT _ = Var case_bndr
815 arg_tys = map Type (tyConAppArgs (idType case_bndr))
818 -- case coerce T e of x { _ -> coerce T' x }
819 -- And we definitely want to eliminate this case!
820 -- So we throw away notes from the RHS, and reconstruct
821 -- (at least an approximation) at the other end
822 de_note (Note _ e) = de_note e
825 -- re_note wraps a coerce if it might be necessary
826 re_note scrut = case head alts of
827 (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
831 The catch-all case. We do a final transformation that I've
832 occasionally seen making a big difference:
834 case e of =====> case e of
835 C _ -> f x D v -> ....v....
836 D v -> ....v.... DEFAULT -> f x
839 The point is that we merge common RHSs, at least for the DEFAULT case.
840 [One could do something more elaborate but I've never seen it needed.]
841 The case where this came up was like this (lib/std/PrelCError.lhs):
847 where @is@ was something like
849 p `is` n = p /= (-1) && p == n
851 This gave rise to a horrible sequence of cases
858 and similarly in cascade for all the join points!
861 mkCase other_scrut case_bndr other_alts
862 = returnSmpl (Case other_scrut case_bndr (mergeDefault other_alts))
864 mergeDefault (deflt_alt@(DEFAULT,_,deflt_rhs) : con_alts)
865 = deflt_alt : [alt | alt@(con,_,rhs) <- con_alts, not (rhs `cheapEqExpr` deflt_rhs)]
866 -- NB: we can neglect the binders because we won't get equality if the
867 -- binders are mentioned in rhs (no shadowing)
868 mergeDefault other_alts