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 PprCore ( {- instance Outputable Expr -} )
25 import CoreUnfold ( isValueUnfolding )
26 import CoreFVs ( exprFreeVars )
27 import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, exprEtaExpandArity, bindNonRec )
28 import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, lookupIdSubst )
29 import Id ( Id, idType, isId, idName,
30 idOccInfo, idUnfolding,
33 import IdInfo ( arityLowerBound, setOccInfo, vanillaIdInfo )
34 import Maybes ( maybeToBool, catMaybes )
35 import Name ( isLocalName, setNameUnique )
37 import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType, repType,
38 splitTyConApp_maybe, splitAlgTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys
40 import PprType ( {- instance Outputable Type -} )
41 import DataCon ( dataConRepArity )
42 import TysPrim ( statePrimTyCon )
43 import Var ( setVarUnique )
45 import VarEnv ( SubstEnv, SubstResult(..) )
46 import UniqSupply ( splitUniqSupply, uniqFromSupply )
47 import Util ( zipWithEqual, mapAccumL )
52 %************************************************************************
54 \subsection{The continuation data type}
56 %************************************************************************
59 data SimplCont -- Strict contexts
60 = Stop OutType -- Type of the result
62 | CoerceIt OutType -- The To-type, simplified
65 | InlinePlease -- This continuation makes a function very
66 SimplCont -- keen to inline itelf
69 InExpr SubstEnv -- The argument, as yet unsimplified,
70 SimplCont -- and its subst-env
73 InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
76 | ArgOf DupFlag -- An arbitrary strict context: the argument
77 -- of a strict function, or a primitive-arg fn
79 OutType -- The type of the expression being sought by the context
80 -- f (error "foo") ==> coerce t (error "foo")
82 -- We need to know the type t, to which to coerce.
83 (OutExpr -> SimplM OutExprStuff) -- What to do with the result
85 instance Outputable SimplCont where
86 ppr (Stop _) = ptext SLIT("Stop")
87 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
88 ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
89 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
90 (nest 4 (ppr alts)) $$ ppr cont
91 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
92 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
94 data DupFlag = OkToDup | NoDup
96 instance Outputable DupFlag where
97 ppr OkToDup = ptext SLIT("ok")
98 ppr NoDup = ptext SLIT("nodup")
100 contIsDupable :: SimplCont -> Bool
101 contIsDupable (Stop _) = True
102 contIsDupable (ApplyTo OkToDup _ _ _) = True
103 contIsDupable (ArgOf OkToDup _ _) = True
104 contIsDupable (Select OkToDup _ _ _ _) = True
105 contIsDupable (CoerceIt _ cont) = contIsDupable cont
106 contIsDupable (InlinePlease cont) = contIsDupable cont
107 contIsDupable other = False
109 pushArgs :: SubstEnv -> [InExpr] -> SimplCont -> SimplCont
110 pushArgs se [] cont = cont
111 pushArgs se (arg:args) cont = ApplyTo NoDup arg se (pushArgs se args cont)
113 discardCont :: SimplCont -- A continuation, expecting
114 -> SimplCont -- Replace the continuation with a suitable coerce
115 discardCont (Stop to_ty) = Stop to_ty
116 discardCont cont = CoerceIt to_ty (Stop to_ty)
118 to_ty = contResultType cont
120 contResultType :: SimplCont -> OutType
121 contResultType (Stop to_ty) = to_ty
122 contResultType (ArgOf _ to_ty _) = to_ty
123 contResultType (ApplyTo _ _ _ cont) = contResultType cont
124 contResultType (CoerceIt _ cont) = contResultType cont
125 contResultType (InlinePlease cont) = contResultType cont
126 contResultType (Select _ _ _ _ cont) = contResultType cont
128 countValArgs :: SimplCont -> Int
129 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
130 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
131 countValArgs other = 0
133 countArgs :: SimplCont -> Int
134 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
139 Comment about analyseCont
140 ~~~~~~~~~~~~~~~~~~~~~~~~~
141 We want to avoid inlining an expression where there can't possibly be
142 any gain, such as in an argument position. Hence, if the continuation
143 is interesting (eg. a case scrutinee, application etc.) then we
144 inline, otherwise we don't.
146 Previously some_benefit used to return True only if the variable was
147 applied to some value arguments. This didn't work:
149 let x = _coerce_ (T Int) Int (I# 3) in
150 case _coerce_ Int (T Int) x of
153 we want to inline x, but can't see that it's a constructor in a case
154 scrutinee position, and some_benefit is False.
158 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
160 .... case dMonadST _@_ x0 of (a,b,c) -> ....
162 we'd really like to inline dMonadST here, but we *don't* want to
163 inline if the case expression is just
165 case x of y { DEFAULT -> ... }
167 since we can just eliminate this case instead (x is in WHNF). Similar
168 applies when x is bound to a lambda expression. Hence
169 contIsInteresting looks for case expressions with just a single
173 analyseCont :: InScopeSet -> SimplCont
174 -> ([Bool], -- Arg-info flags; one for each value argument
175 Bool, -- Context of the result of the call is interesting
176 Bool) -- There was an InlinePlease
178 analyseCont in_scope cont
180 -- The "lone-variable" case is important. I spent ages
181 -- messing about with unsatisfactory varaints, but this is nice.
182 -- The idea is that if a variable appear all alone
183 -- as an arg of lazy fn, or rhs Stop
184 -- as scrutinee of a case Select
185 -- as arg of a strict fn ArgOf
186 -- then we should not inline it (unless there is some other reason,
187 -- e.g. is is the sole occurrence).
188 -- Why not? At least in the case-scrutinee situation, turning
189 -- case x of y -> ...
191 -- let y = (a,b) in ...
192 -- is bad if the binding for x will remain.
194 -- Another example: I discovered that strings
195 -- were getting inlined straight back into applications of 'error'
196 -- because the latter is strict.
198 -- f = \x -> ...(error s)...
200 -- Fundamentally such contexts should not ecourage inlining becuase
201 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
202 -- so there's no gain.
204 -- However, even a type application isn't a lone variable. Consider
205 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
206 -- We had better inline that sucker! The case won't see through it.
208 (Stop _) -> boring_result -- Don't inline a lone variable
209 (Select _ _ _ _ _) -> boring_result -- Ditto
210 (ArgOf _ _ _) -> boring_result -- Ditto
211 (ApplyTo _ (Type _) _ cont) -> analyse_ty_app cont
212 other -> analyse_app cont
214 boring_result = ([], False, False)
216 -- For now, I'm treating not treating a variable applied to types as
217 -- "lone". The motivating example was
219 -- g = /\a. \y. h (f a)
220 -- There's no advantage in inlining f here, and perhaps
221 -- a significant disadvantage.
222 analyse_ty_app (Stop _) = boring_result
223 analyse_ty_app (ArgOf _ _ _) = boring_result
224 analyse_ty_app (Select _ _ _ _ _) = ([], True, False) -- See the $fMonadST example above
225 analyse_ty_app (ApplyTo _ (Type _) _ cont) = analyse_ty_app cont
226 analyse_ty_app cont = analyse_app cont
228 analyse_app (InlinePlease cont)
229 = case analyse_app cont of
230 (infos, icont, inline) -> (infos, icont, True)
232 analyse_app (ApplyTo _ arg subst cont)
233 | isValArg arg = case analyse_app cont of
234 (infos, icont, inline) -> (analyse_arg subst arg : infos, icont, inline)
235 | otherwise = analyse_app cont
237 analyse_app cont = ([], interesting_call_context cont, False)
239 -- An argument is interesting if it has *some* structure
240 -- We are here trying to avoid unfolding a function that
241 -- is applied only to variables that have no unfolding
242 -- (i.e. they are probably lambda bound): f x y z
243 -- There is little point in inlining f here.
244 analyse_arg :: SubstEnv -> InExpr -> Bool
245 analyse_arg subst (Var v) = case lookupIdSubst (mkSubst in_scope subst) v of
246 DoneId v' _ -> isValueUnfolding (idUnfolding v')
248 analyse_arg subst (Type _) = False
249 analyse_arg subst (App fn (Type _)) = analyse_arg subst fn
250 analyse_arg subst (Note _ a) = analyse_arg subst a
251 analyse_arg subst other = True
253 interesting_call_context (Stop ty) = canUpdateInPlace ty
254 interesting_call_context (InlinePlease _) = True
255 interesting_call_context (Select _ _ _ _ _) = True
256 interesting_call_context (CoerceIt _ cont) = interesting_call_context cont
257 interesting_call_context (ApplyTo _ (Type _) _ cont) = interesting_call_context cont
258 interesting_call_context (ApplyTo _ _ _ _) = True
259 interesting_call_context (ArgOf _ _ _) = True
260 -- If this call is the arg of a strict function, the context
261 -- is a bit interesting. If we inline here, we may get useful
262 -- evaluation information to avoid repeated evals: e.g.
264 -- Here the contIsInteresting makes the '*' keener to inline,
265 -- which in turn exposes a constructor which makes the '+' inline.
266 -- Assuming that +,* aren't small enough to inline regardless.
268 -- It's also very important to inline in a strict context for things
271 -- Here, the context of (f x) is strict, and if f's unfolding is
272 -- a build it's *great* to inline it here. So we must ensure that
273 -- the context for (f x) is not totally uninteresting.
276 discardInline :: SimplCont -> SimplCont
277 discardInline (InlinePlease cont) = cont
278 discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
279 discardInline cont = cont
281 -- Consider let x = <wurble> in ...
282 -- If <wurble> returns an explicit constructor, we might be able
283 -- to do update in place. So we treat even a thunk RHS context
284 -- as interesting if update in place is possible. We approximate
285 -- this by seeing if the type has a single constructor with a
286 -- small arity. But arity zero isn't good -- we share the single copy
287 -- for that case, so no point in sharing.
289 -- Note the repType: we want to look through newtypes for this purpose
291 canUpdateInPlace ty = case splitAlgTyConApp_maybe (repType ty) of
292 Just (_, _, [dc]) -> arity == 1 || arity == 2
294 arity = dataConRepArity dc
300 %************************************************************************
302 \section{Dealing with a single binder}
304 %************************************************************************
307 simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
308 simplBinders bndrs thing_inside
309 = getSubst `thenSmpl` \ subst ->
311 (subst', bndrs') = substBndrs subst bndrs
313 seqBndrs bndrs' `seq`
314 setSubst subst' (thing_inside bndrs')
316 simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
317 simplBinder bndr thing_inside
318 = getSubst `thenSmpl` \ subst ->
320 (subst', bndr') = substBndr subst bndr
323 setSubst subst' (thing_inside bndr')
326 -- Same semantics as simplBinders, but a little less
327 -- plumbing and hence a little more efficient.
328 -- Maybe not worth the candle?
329 simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
330 simplIds ids thing_inside
331 = getSubst `thenSmpl` \ subst ->
333 (subst', bndrs') = substIds subst ids
335 seqBndrs bndrs' `seq`
336 setSubst subst' (thing_inside bndrs')
339 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
341 seqBndr b | isTyVar b = b `seq` ()
342 | otherwise = seqType (idType b) `seq`
348 %************************************************************************
350 \subsection{Transform a RHS}
352 %************************************************************************
354 Try (a) eta expansion
355 (b) type-lambda swizzling
358 transformRhs :: InExpr -> SimplM InExpr
360 = tryEtaExpansion body `thenSmpl` \ body' ->
361 mkRhsTyLam tyvars body'
363 (tyvars, body) = collectTyBinders rhs
367 %************************************************************************
369 \subsection{Local tyvar-lifting}
371 %************************************************************************
373 mkRhsTyLam tries this transformation, when the big lambda appears as
374 the RHS of a let(rec) binding:
376 /\abc -> let(rec) x = e in b
378 let(rec) x' = /\abc -> let x = x' a b c in e
380 /\abc -> let x = x' a b c in b
382 This is good because it can turn things like:
384 let f = /\a -> letrec g = ... g ... in g
386 letrec g' = /\a -> ... g' a ...
390 which is better. In effect, it means that big lambdas don't impede
393 This optimisation is CRUCIAL in eliminating the junk introduced by
394 desugaring mutually recursive definitions. Don't eliminate it lightly!
396 So far as the implemtation is concerned:
398 Invariant: go F e = /\tvs -> F e
402 = Let x' = /\tvs -> F e
406 G = F . Let x = x' tvs
408 go F (Letrec xi=ei in b)
409 = Letrec {xi' = /\tvs -> G ei}
413 G = F . Let {xi = xi' tvs}
415 [May 1999] If we do this transformation *regardless* then we can
416 end up with some pretty silly stuff. For example,
419 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
424 st = /\s -> ...[y1 s/x1, y2 s/x2]
427 Unless the "..." is a WHNF there is really no point in doing this.
428 Indeed it can make things worse. Suppose x1 is used strictly,
431 x1* = case f y of { (a,b) -> e }
433 If we abstract this wrt the tyvar we then can't do the case inline
434 as we would normally do.
438 mkRhsTyLam tyvars body -- Only does something if there's a let
439 | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that
440 = returnSmpl (mkLams tyvars body)
444 worth_it (Let _ e) = whnf_in_middle e
445 worth_it other = False
446 whnf_in_middle (Let _ e) = whnf_in_middle e
447 whnf_in_middle e = exprIsCheap e
449 main_tyvar_set = mkVarSet tyvars
451 go fn (Let bind@(NonRec var rhs) body) | exprIsTrivial rhs
452 = go (fn . Let bind) body
454 go fn (Let bind@(NonRec var rhs) body)
455 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
456 go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ body' ->
457 returnSmpl (Let (NonRec var' (mkLams tyvars_here (fn rhs))) body')
460 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
461 -- tyvars_here was an attempt to reduce the number of tyvars
462 -- wrt which the new binding is abstracted. But the naive
463 -- approach of abstract wrt the tyvars free in the Id's type
465 -- /\ a b -> let t :: (a,b) = (e1, e2)
468 -- Here, b isn't free in x's type, but we must nevertheless
469 -- abstract wrt b as well, because t's type mentions b.
470 -- Since t is floated too, we'd end up with the bogus:
471 -- poly_t = /\ a b -> (e1, e2)
472 -- poly_x = /\ a -> fst (poly_t a *b*)
473 -- So for now we adopt the even more naive approach of
474 -- abstracting wrt *all* the tyvars. We'll see if that
475 -- gives rise to problems. SLPJ June 98
479 go fn (Let (Rec prs) body)
480 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
482 gn body = fn $ foldr Let body (zipWith mk_silly_bind vars rhss')
484 go gn body `thenSmpl` \ body' ->
485 returnSmpl (Let (Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])) body')
487 (vars,rhss) = unzip prs
489 -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
490 -- See notes with tyvars_here above
492 var_tys = map idType vars
494 go fn body = returnSmpl (mkLams tyvars (fn body))
496 mk_poly tyvars_here var
497 = getUniqueSmpl `thenSmpl` \ uniq ->
499 poly_name = setNameUnique (idName var) uniq -- Keep same name
500 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
502 -- It's crucial to copy the occInfo of the original var, because
503 -- we're looking at occurrence-analysed but as yet unsimplified code!
504 -- In particular, we mustn't lose the loop breakers.
506 -- It's even right to retain single-occurrence or dead-var info:
507 -- Suppose we started with /\a -> let x = E in B
508 -- where x occurs once in E. Then we transform to:
509 -- let x' = /\a -> E in /\a -> let x* = x' a in B
510 -- where x* has an INLINE prag on it. Now, once x* is inlined,
511 -- the occurrences of x' will be just the occurrences originaly
513 poly_info = vanillaIdInfo `setOccInfo` idOccInfo var
515 poly_id = mkId poly_name poly_ty poly_info
517 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
519 mk_silly_bind var rhs = NonRec var rhs
520 -- The Inline note is really important! If we don't say
521 -- INLINE on these silly little bindings then look what happens!
522 -- Suppose we start with:
524 -- x = let g = /\a -> \x -> f x x
526 -- /\ b -> let g* = g b in E
528 -- Then: * the binding for g gets floated out
529 -- * but then it gets inlined into the rhs of g*
530 -- * then the binding for g* is floated out of the /\b
531 -- * so we're back to square one
532 -- The silly binding for g* must be INLINEd, so that
533 -- we simply substitute for g* throughout.
537 %************************************************************************
539 \subsection{Eta expansion}
541 %************************************************************************
543 Try eta expansion for RHSs
546 \x1..xn -> N ==> \x1..xn y1..ym -> N y1..ym
548 N E1..En ==> let z1=E1 .. zn=En in \y1..ym -> N z1..zn y1..ym
550 where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere)
551 wanting a suitable number of extra args.
553 NB: the Ei may have unlifted type, but the simplifier (which is applied
554 to the result) deals OK with this.
556 There is no point in looking for a combination of the two,
557 because that would leave use with some lets sandwiched between lambdas;
558 that's what the final test in the first equation is for.
561 tryEtaExpansion :: InExpr -> SimplM InExpr
563 | not opt_SimplDoLambdaEtaExpansion
564 || exprIsTrivial rhs -- Don't eta-expand a trival RHS
565 || null y_tys -- No useful expansion
566 || not (null x_bndrs || and trivial_args) -- Not (no x-binders or no z-binds)
569 | otherwise -- Consider eta expansion
570 = newIds SLIT("y") y_tys $ ( \ y_bndrs ->
571 tick (EtaExpansion (head y_bndrs)) `thenSmpl_`
572 mapAndUnzipSmpl bind_z_arg (args `zip` trivial_args) `thenSmpl` (\ (maybe_z_binds, z_args) ->
573 returnSmpl (mkLams x_bndrs $
574 mkLets (catMaybes maybe_z_binds) $
576 mkApps (mkApps fun z_args) (map Var y_bndrs))))
578 (x_bndrs, body) = collectValBinders rhs
579 (fun, args) = collectArgs body
580 trivial_args = map exprIsTrivial args
581 fun_arity = exprEtaExpandArity fun
583 bind_z_arg (arg, trivial_arg)
584 | trivial_arg = returnSmpl (Nothing, arg)
585 | otherwise = newId SLIT("z") (exprType arg) $ \ z ->
586 returnSmpl (Just (NonRec z arg), Var z)
588 -- Note: I used to try to avoid the exprType call by using
589 -- the type of the binder. But this type doesn't necessarily
590 -- belong to the same substitution environment as this rhs;
591 -- and we are going to make extra term binders (y_bndrs) from the type
592 -- which will be processed with the rhs substitution environment.
593 -- This only went wrong in a mind bendingly complicated case.
594 (potential_extra_arg_tys, inner_ty) = splitFunTys (exprType body)
597 y_tys = take no_extras_wanted potential_extra_arg_tys
599 no_extras_wanted :: Int
600 no_extras_wanted = 0 `max`
602 -- We used to expand the arity to the previous arity fo the
603 -- function; but this is pretty dangerous. Consdier
605 -- so that f has arity 2. Now float something into f's RHS:
606 -- f = let z = BIG in \xy -> e
607 -- The last thing we want to do now is to put some lambdas
609 -- f = \xy -> let z = BIG in e
611 -- (bndr_arity - no_of_xs) `max`
613 -- See if the body could obviously do with more args
614 (fun_arity - valArgCount args)
616 -- This case is now deal with by exprEtaExpandArity
617 -- Finally, see if it's a state transformer, and xs is non-null
618 -- (so it's also a function not a thunk) in which
619 -- case we eta-expand on principle! This can waste work,
620 -- but usually doesn't.
621 -- I originally checked for a singleton type [ty] in this case
622 -- but then I found a situation in which I had
623 -- \ x -> let {..} in \ s -> f (...) s
624 -- AND f RETURNED A FUNCTION. That is, 's' wasn't the only
625 -- potential extra arg.
626 -- case (x_bndrs, potential_extra_arg_tys) of
627 -- (_:_, ty:_) -> case splitTyConApp_maybe ty of
628 -- Just (tycon,_) | tycon == statePrimTyCon -> 1
634 %************************************************************************
636 \subsection{Case absorption and identity-case elimination}
638 %************************************************************************
641 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
644 @mkCase@ tries the following transformation (if possible):
646 case e of b { ==> case e of b {
647 p1 -> rhs1 p1 -> rhs1
649 pm -> rhsm pm -> rhsm
650 _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
651 {or (prim) case b of b' { _ -> rhsn}}
654 po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
658 which merges two cases in one case when -- the default alternative of
659 the outer case scrutises the same variable as the outer case This
660 transformation is called Case Merging. It avoids that the same
661 variable is scrutinised multiple times.
664 mkCase scrut outer_bndr outer_alts
666 && maybeToBool maybe_case_in_default
668 = tick (CaseMerge outer_bndr) `thenSmpl_`
669 returnSmpl (Case scrut outer_bndr new_alts)
670 -- Warning: don't call mkCase recursively!
671 -- Firstly, there's no point, because inner alts have already had
672 -- mkCase applied to them, so they won't have a case in their default
673 -- Secondly, if you do, you get an infinite loop, because the bindNonRec
674 -- in munge_rhs puts a case into the DEFAULT branch!
676 new_alts = outer_alts_without_deflt ++ munged_inner_alts
677 maybe_case_in_default = case findDefault outer_alts of
678 (outer_alts_without_default,
679 Just (Case (Var scrut_var) inner_bndr inner_alts))
681 | outer_bndr == scrut_var
682 -> Just (outer_alts_without_default, inner_bndr, inner_alts)
685 Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
687 -- Eliminate any inner alts which are shadowed by the outer ones
688 outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
690 munged_inner_alts = [ (con, args, munge_rhs rhs)
691 | (con, args, rhs) <- inner_alts,
692 not (con `elem` outer_cons) -- Eliminate shadowed inner alts
694 munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
697 Now the identity-case transformation:
706 mkCase scrut case_bndr alts
707 | all identity_alt alts
708 = tick (CaseIdentity case_bndr) `thenSmpl_`
711 identity_alt (DEFAULT, [], Var v) = v == case_bndr
712 identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs
713 (mkConApp con (map Type arg_tys ++ map varToCoreExpr args))
714 identity_alt other = False
716 arg_tys = case splitTyConApp_maybe (idType case_bndr) of
717 Just (tycon, arg_tys) -> arg_tys
723 mkCase other_scrut case_bndr other_alts
724 = returnSmpl (Case other_scrut case_bndr other_alts)
729 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
730 findDefault [] = ([], Nothing)
731 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args )
733 findDefault (alt : alts) = case findDefault alts of
734 (alts', deflt) -> (alt : alts', deflt)
736 findAlt :: AltCon -> [CoreAlt] -> CoreAlt
740 go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
741 go (alt : alts) | matches alt = alt
742 | otherwise = go alts
744 matches (DEFAULT, _, _) = True
745 matches (con1, _, _) = con == con1