2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
8 simplBinder, simplBinders, simplIds,
10 mkCase, findAlt, findDefault,
12 -- The continuation type
13 SimplCont(..), DupFlag(..), contIsDupable, contResultType,
14 pushArgs, discardCont, countValArgs, countArgs,
15 analyseCont, discardInline
19 #include "HsVersions.h"
22 import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge )
24 import CoreUnfold ( isValueUnfolding )
25 import CoreFVs ( exprFreeVars )
26 import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, exprEtaExpandArity, bindNonRec )
27 import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, lookupIdSubst )
28 import Id ( Id, idType, isId, idName,
29 idOccInfo, idUnfolding,
32 import IdInfo ( arityLowerBound, setOccInfo, vanillaIdInfo )
33 import Maybes ( maybeToBool, catMaybes )
34 import Name ( isLocalName, setNameUnique )
36 import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType, repType,
37 splitTyConApp_maybe, splitAlgTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys
39 import DataCon ( dataConRepArity )
40 import TysPrim ( statePrimTyCon )
41 import Var ( setVarUnique )
43 import VarEnv ( SubstEnv, SubstResult(..) )
44 import UniqSupply ( splitUniqSupply, uniqFromSupply )
45 import Util ( zipWithEqual, mapAccumL )
50 %************************************************************************
52 \subsection{The continuation data type}
54 %************************************************************************
57 data SimplCont -- Strict contexts
58 = Stop OutType -- Type of the result
60 | CoerceIt OutType -- The To-type, simplified
63 | InlinePlease -- This continuation makes a function very
64 SimplCont -- keen to inline itelf
67 InExpr SubstEnv -- The argument, as yet unsimplified,
68 SimplCont -- and its subst-env
71 InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
74 | ArgOf DupFlag -- An arbitrary strict context: the argument
75 -- of a strict function, or a primitive-arg fn
77 OutType -- The type of the expression being sought by the context
78 -- f (error "foo") ==> coerce t (error "foo")
80 -- We need to know the type t, to which to coerce.
81 (OutExpr -> SimplM OutExprStuff) -- What to do with the result
83 instance Outputable SimplCont where
84 ppr (Stop _) = ptext SLIT("Stop")
85 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
86 ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
87 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
88 (nest 4 (ppr alts)) $$ ppr cont
89 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
90 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
92 data DupFlag = OkToDup | NoDup
94 instance Outputable DupFlag where
95 ppr OkToDup = ptext SLIT("ok")
96 ppr NoDup = ptext SLIT("nodup")
98 contIsDupable :: SimplCont -> Bool
99 contIsDupable (Stop _) = True
100 contIsDupable (ApplyTo OkToDup _ _ _) = True
101 contIsDupable (ArgOf OkToDup _ _) = True
102 contIsDupable (Select OkToDup _ _ _ _) = True
103 contIsDupable (CoerceIt _ cont) = contIsDupable cont
104 contIsDupable (InlinePlease cont) = contIsDupable cont
105 contIsDupable other = False
107 pushArgs :: SubstEnv -> [InExpr] -> SimplCont -> SimplCont
108 pushArgs se [] cont = cont
109 pushArgs se (arg:args) cont = ApplyTo NoDup arg se (pushArgs se args cont)
111 discardCont :: SimplCont -- A continuation, expecting
112 -> SimplCont -- Replace the continuation with a suitable coerce
113 discardCont (Stop to_ty) = Stop to_ty
114 discardCont cont = CoerceIt to_ty (Stop to_ty)
116 to_ty = contResultType cont
118 contResultType :: SimplCont -> OutType
119 contResultType (Stop to_ty) = to_ty
120 contResultType (ArgOf _ to_ty _) = to_ty
121 contResultType (ApplyTo _ _ _ cont) = contResultType cont
122 contResultType (CoerceIt _ cont) = contResultType cont
123 contResultType (InlinePlease cont) = contResultType cont
124 contResultType (Select _ _ _ _ cont) = contResultType cont
126 countValArgs :: SimplCont -> Int
127 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
128 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
129 countValArgs other = 0
131 countArgs :: SimplCont -> Int
132 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
137 Comment about analyseCont
138 ~~~~~~~~~~~~~~~~~~~~~~~~~
139 We want to avoid inlining an expression where there can't possibly be
140 any gain, such as in an argument position. Hence, if the continuation
141 is interesting (eg. a case scrutinee, application etc.) then we
142 inline, otherwise we don't.
144 Previously some_benefit used to return True only if the variable was
145 applied to some value arguments. This didn't work:
147 let x = _coerce_ (T Int) Int (I# 3) in
148 case _coerce_ Int (T Int) x of
151 we want to inline x, but can't see that it's a constructor in a case
152 scrutinee position, and some_benefit is False.
156 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
158 .... case dMonadST _@_ x0 of (a,b,c) -> ....
160 we'd really like to inline dMonadST here, but we *don't* want to
161 inline if the case expression is just
163 case x of y { DEFAULT -> ... }
165 since we can just eliminate this case instead (x is in WHNF). Similar
166 applies when x is bound to a lambda expression. Hence
167 contIsInteresting looks for case expressions with just a single
171 analyseCont :: InScopeSet -> SimplCont
172 -> ([Bool], -- Arg-info flags; one for each value argument
173 Bool, -- Context of the result of the call is interesting
174 Bool) -- There was an InlinePlease
176 analyseCont in_scope cont
178 -- The "lone-variable" case is important. I spent ages
179 -- messing about with unsatisfactory varaints, but this is nice.
180 -- The idea is that if a variable appear all alone
181 -- as an arg of lazy fn, or rhs Stop
182 -- as scrutinee of a case Select
183 -- as arg of a strict fn ArgOf
184 -- then we should not inline it (unless there is some other reason,
185 -- e.g. is is the sole occurrence).
186 -- Why not? At least in the case-scrutinee situation, turning
187 -- case x of y -> ...
189 -- let y = (a,b) in ...
190 -- is bad if the binding for x will remain.
192 -- Another example: I discovered that strings
193 -- were getting inlined straight back into applications of 'error'
194 -- because the latter is strict.
196 -- f = \x -> ...(error s)...
198 -- Fundamentally such contexts should not ecourage inlining becuase
199 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
200 -- so there's no gain.
202 -- However, even a type application isn't a lone variable. Consider
203 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
204 -- We had better inline that sucker! The case won't see through it.
206 (Stop _) -> boring_result -- Don't inline a lone variable
207 (Select _ _ _ _ _) -> boring_result -- Ditto
208 (ArgOf _ _ _) -> boring_result -- Ditto
209 (ApplyTo _ (Type _) _ cont) -> analyse_ty_app cont
210 other -> analyse_app cont
212 boring_result = ([], False, False)
214 -- For now, I'm treating not treating a variable applied to types as
215 -- "lone". The motivating example was
217 -- g = /\a. \y. h (f a)
218 -- There's no advantage in inlining f here, and perhaps
219 -- a significant disadvantage.
220 analyse_ty_app (Stop _) = boring_result
221 analyse_ty_app (ArgOf _ _ _) = boring_result
222 analyse_ty_app (Select _ _ _ _ _) = ([], True, False) -- See the $fMonadST example above
223 analyse_ty_app (ApplyTo _ (Type _) _ cont) = analyse_ty_app cont
224 analyse_ty_app cont = analyse_app cont
226 analyse_app (InlinePlease cont)
227 = case analyse_app cont of
228 (infos, icont, inline) -> (infos, icont, True)
230 analyse_app (ApplyTo _ arg subst cont)
231 | isValArg arg = case analyse_app cont of
232 (infos, icont, inline) -> (analyse_arg subst arg : infos, icont, inline)
233 | otherwise = analyse_app cont
235 analyse_app cont = ([], interesting_call_context cont, False)
237 -- An argument is interesting if it has *some* structure
238 -- We are here trying to avoid unfolding a function that
239 -- is applied only to variables that have no unfolding
240 -- (i.e. they are probably lambda bound): f x y z
241 -- There is little point in inlining f here.
242 analyse_arg :: SubstEnv -> InExpr -> Bool
243 analyse_arg subst (Var v) = case lookupIdSubst (mkSubst in_scope subst) v of
244 DoneId v' _ -> isValueUnfolding (idUnfolding v')
246 analyse_arg subst (Type _) = False
247 analyse_arg subst (App fn (Type _)) = analyse_arg subst fn
248 analyse_arg subst (Note _ a) = analyse_arg subst a
249 analyse_arg subst other = True
251 interesting_call_context (Stop ty) = canUpdateInPlace ty
252 interesting_call_context (InlinePlease _) = True
253 interesting_call_context (Select _ _ _ _ _) = True
254 interesting_call_context (CoerceIt _ cont) = interesting_call_context cont
255 interesting_call_context (ApplyTo _ (Type _) _ cont) = interesting_call_context cont
256 interesting_call_context (ApplyTo _ _ _ _) = True
257 interesting_call_context (ArgOf _ _ _) = True
258 -- If this call is the arg of a strict function, the context
259 -- is a bit interesting. If we inline here, we may get useful
260 -- evaluation information to avoid repeated evals: e.g.
262 -- Here the contIsInteresting makes the '*' keener to inline,
263 -- which in turn exposes a constructor which makes the '+' inline.
264 -- Assuming that +,* aren't small enough to inline regardless.
266 -- It's also very important to inline in a strict context for things
269 -- Here, the context of (f x) is strict, and if f's unfolding is
270 -- a build it's *great* to inline it here. So we must ensure that
271 -- the context for (f x) is not totally uninteresting.
274 discardInline :: SimplCont -> SimplCont
275 discardInline (InlinePlease cont) = cont
276 discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
277 discardInline cont = cont
279 -- Consider let x = <wurble> in ...
280 -- If <wurble> returns an explicit constructor, we might be able
281 -- to do update in place. So we treat even a thunk RHS context
282 -- as interesting if update in place is possible. We approximate
283 -- this by seeing if the type has a single constructor with a
284 -- small arity. But arity zero isn't good -- we share the single copy
285 -- for that case, so no point in sharing.
287 -- Note the repType: we want to look through newtypes for this purpose
289 canUpdateInPlace ty = case splitAlgTyConApp_maybe (repType ty) of
290 Just (_, _, [dc]) -> arity == 1 || arity == 2
292 arity = dataConRepArity dc
298 %************************************************************************
300 \section{Dealing with a single binder}
302 %************************************************************************
305 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
306 simplBinders bndrs thing_inside
307 = getSubst `thenSmpl` \ subst ->
309 (subst', bndrs') = substBndrs subst bndrs
311 seqBndrs bndrs' `seq`
312 setSubst subst' (thing_inside bndrs')
314 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
315 simplBinder bndr thing_inside
316 = getSubst `thenSmpl` \ subst ->
318 (subst', bndr') = substBndr subst bndr
321 setSubst subst' (thing_inside bndr')
324 -- Same semantics as simplBinders, but a little less
325 -- plumbing and hence a little more efficient.
326 -- Maybe not worth the candle?
327 simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
328 simplIds ids thing_inside
329 = getSubst `thenSmpl` \ subst ->
331 (subst', bndrs') = substIds subst ids
333 seqBndrs bndrs' `seq`
334 setSubst subst' (thing_inside bndrs')
337 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
339 seqBndr b | isTyVar b = b `seq` ()
340 | otherwise = seqType (idType b) `seq`
346 %************************************************************************
348 \subsection{Transform a RHS}
350 %************************************************************************
352 Try (a) eta expansion
353 (b) type-lambda swizzling
356 transformRhs :: InExpr -> SimplM InExpr
358 = tryEtaExpansion body `thenSmpl` \ body' ->
359 mkRhsTyLam tyvars body'
361 (tyvars, body) = collectTyBinders rhs
365 %************************************************************************
367 \subsection{Local tyvar-lifting}
369 %************************************************************************
371 mkRhsTyLam tries this transformation, when the big lambda appears as
372 the RHS of a let(rec) binding:
374 /\abc -> let(rec) x = e in b
376 let(rec) x' = /\abc -> let x = x' a b c in e
378 /\abc -> let x = x' a b c in b
380 This is good because it can turn things like:
382 let f = /\a -> letrec g = ... g ... in g
384 letrec g' = /\a -> ... g' a ...
388 which is better. In effect, it means that big lambdas don't impede
391 This optimisation is CRUCIAL in eliminating the junk introduced by
392 desugaring mutually recursive definitions. Don't eliminate it lightly!
394 So far as the implemtation is concerned:
396 Invariant: go F e = /\tvs -> F e
400 = Let x' = /\tvs -> F e
404 G = F . Let x = x' tvs
406 go F (Letrec xi=ei in b)
407 = Letrec {xi' = /\tvs -> G ei}
411 G = F . Let {xi = xi' tvs}
413 [May 1999] If we do this transformation *regardless* then we can
414 end up with some pretty silly stuff. For example,
417 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
422 st = /\s -> ...[y1 s/x1, y2 s/x2]
425 Unless the "..." is a WHNF there is really no point in doing this.
426 Indeed it can make things worse. Suppose x1 is used strictly,
429 x1* = case f y of { (a,b) -> e }
431 If we abstract this wrt the tyvar we then can't do the case inline
432 as we would normally do.
436 mkRhsTyLam tyvars body -- Only does something if there's a let
437 | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that
438 = returnSmpl (mkLams tyvars body)
442 worth_it (Let _ e) = whnf_in_middle e
443 worth_it other = False
444 whnf_in_middle (Let _ e) = whnf_in_middle e
445 whnf_in_middle e = exprIsCheap e
447 main_tyvar_set = mkVarSet tyvars
449 go fn (Let bind@(NonRec var rhs) body) | exprIsTrivial rhs
450 = go (fn . Let bind) body
452 go fn (Let bind@(NonRec var rhs) body)
453 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
454 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ body' ->
455 returnSmpl (Let (NonRec var' (mkLams tyvars_here (fn rhs))) body')
458 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
459 -- tyvars_here was an attempt to reduce the number of tyvars
460 -- wrt which the new binding is abstracted. But the naive
461 -- approach of abstract wrt the tyvars free in the Id's type
463 -- /\ a b -> let t :: (a,b) = (e1, e2)
466 -- Here, b isn't free in x's type, but we must nevertheless
467 -- abstract wrt b as well, because t's type mentions b.
468 -- Since t is floated too, we'd end up with the bogus:
469 -- poly_t = /\ a b -> (e1, e2)
470 -- poly_x = /\ a -> fst (poly_t a *b*)
471 -- So for now we adopt the even more naive approach of
472 -- abstracting wrt *all* the tyvars. We'll see if that
473 -- gives rise to problems. SLPJ June 98
477 go fn (Let (Rec prs) body)
478 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
480 gn body = fn $ foldr Let body (zipWith mk_silly_bind vars rhss')
482 go gn body `thenSmpl` \ body' ->
483 returnSmpl (Let (Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])) body')
485 (vars,rhss) = unzip prs
487 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
488 -- See notes with tyvars_here above
490 var_tys = map idType vars
492 go fn body = returnSmpl (mkLams tyvars (fn body))
494 mk_poly tyvars_here var
495 = getUniqueSmpl `thenSmpl` \ uniq ->
497 poly_name = setNameUnique (idName var) uniq -- Keep same name
498 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
500 -- It's crucial to copy the occInfo of the original var, because
501 -- we're looking at occurrence-analysed but as yet unsimplified code!
502 -- In particular, we mustn't lose the loop breakers.
504 -- It's even right to retain single-occurrence or dead-var info:
505 -- Suppose we started with /\a -> let x = E in B
506 -- where x occurs once in E. Then we transform to:
507 -- let x' = /\a -> E in /\a -> let x* = x' a in B
508 -- where x* has an INLINE prag on it. Now, once x* is inlined,
509 -- the occurrences of x' will be just the occurrences originaly
511 poly_info = vanillaIdInfo `setOccInfo` idOccInfo var
513 poly_id = mkId poly_name poly_ty poly_info
515 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
517 mk_silly_bind var rhs = NonRec var rhs
518 -- The Inline note is really important! If we don't say
519 -- INLINE on these silly little bindings then look what happens!
520 -- Suppose we start with:
522 -- x = let g = /\a -> \x -> f x x
524 -- /\ b -> let g* = g b in E
526 -- Then: * the binding for g gets floated out
527 -- * but then it gets inlined into the rhs of g*
528 -- * then the binding for g* is floated out of the /\b
529 -- * so we're back to square one
530 -- The silly binding for g* must be INLINEd, so that
531 -- we simply substitute for g* throughout.
535 %************************************************************************
537 \subsection{Eta expansion}
539 %************************************************************************
541 Try eta expansion for RHSs
544 \x1..xn -> N ==> \x1..xn y1..ym -> N y1..ym
546 N E1..En ==> let z1=E1 .. zn=En in \y1..ym -> N z1..zn y1..ym
548 where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere)
549 wanting a suitable number of extra args.
551 NB: the Ei may have unlifted type, but the simplifier (which is applied
552 to the result) deals OK with this.
554 There is no point in looking for a combination of the two,
555 because that would leave use with some lets sandwiched between lambdas;
556 that's what the final test in the first equation is for.
559 tryEtaExpansion :: InExpr -> SimplM InExpr
561 | not opt_SimplDoLambdaEtaExpansion
562 || exprIsTrivial rhs -- Don't eta-expand a trival RHS
563 || null y_tys -- No useful expansion
564 || not (null x_bndrs || and trivial_args) -- Not (no x-binders or no z-binds)
567 | otherwise -- Consider eta expansion
568 = newIds y_tys $ ( \ y_bndrs ->
569 tick (EtaExpansion (head y_bndrs)) `thenSmpl_`
570 mapAndUnzipSmpl bind_z_arg (args `zip` trivial_args) `thenSmpl` (\ (maybe_z_binds, z_args) ->
571 returnSmpl (mkLams x_bndrs $
572 mkLets (catMaybes maybe_z_binds) $
574 mkApps (mkApps fun z_args) (map Var y_bndrs))))
576 (x_bndrs, body) = collectValBinders rhs
577 (fun, args) = collectArgs body
578 trivial_args = map exprIsTrivial args
579 fun_arity = exprEtaExpandArity fun
581 bind_z_arg (arg, trivial_arg)
582 | trivial_arg = returnSmpl (Nothing, arg)
583 | otherwise = newId (exprType arg) $ \ z ->
584 returnSmpl (Just (NonRec z arg), Var z)
586 -- Note: I used to try to avoid the exprType call by using
587 -- the type of the binder. But this type doesn't necessarily
588 -- belong to the same substitution environment as this rhs;
589 -- and we are going to make extra term binders (y_bndrs) from the type
590 -- which will be processed with the rhs substitution environment.
591 -- This only went wrong in a mind bendingly complicated case.
592 (potential_extra_arg_tys, inner_ty) = splitFunTys (exprType body)
595 y_tys = take no_extras_wanted potential_extra_arg_tys
597 no_extras_wanted :: Int
598 no_extras_wanted = 0 `max`
600 -- We used to expand the arity to the previous arity fo the
601 -- function; but this is pretty dangerous. Consdier
603 -- so that f has arity 2. Now float something into f's RHS:
604 -- f = let z = BIG in \xy -> e
605 -- The last thing we want to do now is to put some lambdas
607 -- f = \xy -> let z = BIG in e
609 -- (bndr_arity - no_of_xs) `max`
611 -- See if the body could obviously do with more args
612 (fun_arity - valArgCount args)
614 -- This case is now deal with by exprEtaExpandArity
615 -- Finally, see if it's a state transformer, and xs is non-null
616 -- (so it's also a function not a thunk) in which
617 -- case we eta-expand on principle! This can waste work,
618 -- but usually doesn't.
619 -- I originally checked for a singleton type [ty] in this case
620 -- but then I found a situation in which I had
621 -- \ x -> let {..} in \ s -> f (...) s
622 -- AND f RETURNED A FUNCTION. That is, 's' wasn't the only
623 -- potential extra arg.
624 -- case (x_bndrs, potential_extra_arg_tys) of
625 -- (_:_, ty:_) -> case splitTyConApp_maybe ty of
626 -- Just (tycon,_) | tycon == statePrimTyCon -> 1
632 %************************************************************************
634 \subsection{Case absorption and identity-case elimination}
636 %************************************************************************
639 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
642 @mkCase@ tries the following transformation (if possible):
644 case e of b { ==> case e of b {
645 p1 -> rhs1 p1 -> rhs1
647 pm -> rhsm pm -> rhsm
648 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
649 {or (prim) case b of b' { _ -> rhsn}}
652 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
656 which merges two cases in one case when -- the default alternative of
657 the outer case scrutises the same variable as the outer case This
658 transformation is called Case Merging. It avoids that the same
659 variable is scrutinised multiple times.
662 mkCase scrut outer_bndr outer_alts
664 && maybeToBool maybe_case_in_default
666 = tick (CaseMerge outer_bndr) `thenSmpl_`
667 returnSmpl (Case scrut outer_bndr new_alts)
668 -- Warning: don't call mkCase recursively!
669 -- Firstly, there's no point, because inner alts have already had
670 -- mkCase applied to them, so they won't have a case in their default
671 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
672 -- in munge_rhs puts a case into the DEFAULT branch!
674 new_alts = outer_alts_without_deflt ++ munged_inner_alts
675 maybe_case_in_default = case findDefault outer_alts of
676 (outer_alts_without_default,
677 Just (Case (Var scrut_var) inner_bndr inner_alts))
679 | outer_bndr == scrut_var
680 -> Just (outer_alts_without_default, inner_bndr, inner_alts)
683 Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
685 -- Eliminate any inner alts which are shadowed by the outer ones
686 outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
688 munged_inner_alts = [ (con, args, munge_rhs rhs)
689 | (con, args, rhs) <- inner_alts,
690 not (con `elem` outer_cons) -- Eliminate shadowed inner alts
692 munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
695 Now the identity-case transformation:
704 mkCase scrut case_bndr alts
705 | all identity_alt alts
706 = tick (CaseIdentity case_bndr) `thenSmpl_`
709 identity_alt (DEFAULT, [], Var v) = v == case_bndr
710 identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs
711 (mkConApp con (map Type arg_tys ++ map varToCoreExpr args))
712 identity_alt other = False
714 arg_tys = case splitTyConApp_maybe (idType case_bndr) of
715 Just (tycon, arg_tys) -> arg_tys
721 mkCase other_scrut case_bndr other_alts
722 = returnSmpl (Case other_scrut case_bndr other_alts)
727 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
728 findDefault [] = ([], Nothing)
729 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args )
731 findDefault (alt : alts) = case findDefault alts of
732 (alts', deflt) -> (alt : alts', deflt)
734 findAlt :: AltCon -> [CoreAlt] -> CoreAlt
738 go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
739 go (alt : alts) | matches alt = alt
740 | otherwise = go alts
742 matches (DEFAULT, _, _) = True
743 matches (con1, _, _) = con == con1