2 % (c) The AQUA Project, Glasgow University, 1994-1996
4 \section[SimplCase]{Simplification of `case' expression}
6 Support code for @Simplify@.
9 module SimplCase ( simplCase, bindLargeRhs ) where
11 #include "HsVersions.h"
13 import {-# SOURCE #-} Simplify ( simplBind, simplExpr )
15 import BinderInfo -- too boring to try to select things...
16 import CmdLineOpts ( SimplifierSwitch(..) )
18 import CoreUnfold ( Unfolding(..) )
19 import CoreUtils ( coreAltsType, nonErrorRHSs, maybeErrorApp,
20 unTagBindersAlts, unTagBinders, coreExprType
22 import Id ( idType, isDataCon, getIdDemandInfo, dataConArgTys,
23 DataCon, GenId{-instance Eq-},
26 import IdInfo ( willBeDemanded, DemandInfo )
27 import Literal ( isNoRepLit, Literal{-instance Eq-} )
28 import Maybes ( maybeToBool )
29 import PrelVals ( voidId )
30 import PrimOp ( primOpOkForSpeculation, PrimOp{-instance Eq-} )
31 import SimplVar ( simplBinder, simplBinders )
32 import SimplUtils ( newId, newIds )
35 import Type ( isUnpointedType, splitAlgTyConApp_maybe, splitAlgTyConApp, mkFunTy, mkFunTys )
36 import TyCon ( isDataTyCon )
37 import TysPrim ( voidTy )
38 import Util ( Eager, runEager, appEager,
39 isIn, isSingleton, zipEqual, panic, assertPanic )
43 Float let out of case.
47 -> InExpr -- Scrutinee
48 -> (SubstEnvs, InAlts) -- Alternatives, and their static environment
49 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
50 -> OutType -- Type of result expression
53 simplCase env (Let bind body) alts rhs_c result_ty
54 | not (switchIsSet env SimplNoLetFromCase)
55 = -- Float the let outside the case scrutinee (if not disabled by flag)
56 tick LetFloatFromCase `thenSmpl_`
57 simplBind env bind (\env -> simplCase env body alts rhs_c result_ty) result_ty
60 OK to do case-of-case if
62 * we allow arbitrary code duplication
66 * the inner case has one alternative
67 case (case e of (a,b) -> rhs) of
78 IF neither of these two things are the case, we avoid code-duplication
79 by abstracting the outer rhss wrt the pattern variables. For example
81 case (case e of { p1->rhs1; ...; pn -> rhsn }) of
87 p1 -> case rhs1 of (x,y) -> b x y
89 pn -> case rhsn of (x,y) -> b x y
92 OK, so outer case expression gets duplicated, but that's all. Furthermore,
93 (a) the binding for "b" will be let-no-escaped, so no heap allocation
94 will take place; the "call" to b will simply be a stack adjustment
96 (b) very commonly, at least some of the rhsi's will be constructors, which
97 makes life even simpler.
99 All of this works equally well if the outer case has multiple rhss.
103 simplCase env (Case inner_scrut inner_alts) (subst_envs, outer_alts) rhs_c result_ty
104 | switchIsSet env SimplCaseOfCase
105 = -- Ha! Do case-of-case
106 tick CaseOfCase `thenSmpl_`
108 if no_need_to_bind_large_alts
110 simplCase env inner_scrut (getSubstEnvs env, inner_alts)
111 (\env' rhs -> simplCase env' rhs (subst_envs, outer_alts) rhs_c result_ty)
114 bindLargeAlts env_alts outer_alts rhs_c result_ty `thenSmpl` \ (extra_bindings, outer_alts') ->
116 rhs_c' = \env rhs -> simplExpr env rhs [] result_ty
118 simplCase env inner_scrut (getSubstEnvs env, inner_alts)
119 (\env rhs -> simplCase env rhs (emptySubstEnvs, outer_alts') rhs_c' result_ty)
121 `thenSmpl` \ case_expr ->
122 returnSmpl (mkCoLetsNoUnboxed extra_bindings case_expr)
125 env_alts = setSubstEnvs env subst_envs
127 no_need_to_bind_large_alts = switchIsSet env SimplOkToDupCode ||
128 isSingleton (nonErrorRHSs inner_alts)
131 Case of an application of error.
134 simplCase env scrut alts rhs_c result_ty
135 | maybeToBool maybe_error_app
136 = -- Look for an application of an error id
137 tick CaseOfError `thenSmpl_`
138 simplExpr env retyped_error_app [] result_ty
140 -- We must apply simplExpr because "rhs" isn't yet simplified.
141 -- The ice is a little thin because body_ty is an OutType; but it's ok really
143 maybe_error_app = maybeErrorApp scrut (Just result_ty)
144 Just retyped_error_app = maybe_error_app
147 Finally the default case
150 simplCase env other_scrut (subst_envs, alts) rhs_c result_ty
151 = simplTy env scrut_ty `appEager` \ scrut_ty' ->
152 simplExpr env_scrut other_scrut [] scrut_ty' `thenSmpl` \ scrut' ->
153 completeCase env_alts scrut' alts rhs_c
155 -- When simplifying the scrutinee of a complete case that
156 -- has no default alternative
157 env_scrut = case alts of
158 AlgAlts _ NoDefault -> setCaseScrutinee env
159 PrimAlts _ NoDefault -> setCaseScrutinee env
162 env_alts = setSubstEnvs env subst_envs
164 scrut_ty = coreExprType (unTagBinders other_scrut)
168 %************************************************************************
170 \subsection[Simplify-case]{Completing case-expression simplification}
172 %************************************************************************
177 -> OutExpr -- The already-simplified scrutinee
178 -> InAlts -- The un-simplified alternatives
179 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
180 -> SmplM OutExpr -- The whole case expression
183 Scrutinising a literal or constructor.
184 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
185 It's an obvious win to do:
187 case (C a b) of {...; C p q -> rhs; ...} ===> rhs[a/p,b/q]
189 and the similar thing for primitive case. If we have
193 and x is known to be of constructor form, then we'll already have
194 inlined the constructor to give (case (C a b) of ...), so we don't
195 need to check for the variable case separately.
197 Sanity check: we don't have a good
198 story to tell about case analysis on NoRep things. ToDo.
201 completeCase env (Lit lit) alts rhs_c
202 | not (isNoRepLit lit)
203 = -- Ha! Select the appropriate alternative
204 tick KnownBranch `thenSmpl_`
205 completePrimCaseWithKnownLit env lit alts rhs_c
207 completeCase env expr@(Con con con_args) alts rhs_c
208 = -- Ha! Staring us in the face -- select the appropriate alternative
209 tick KnownBranch `thenSmpl_`
210 completeAlgCaseWithKnownCon env con con_args alts rhs_c
215 Start with a simple situation:
217 case x# of ===> e[x#/y#]
220 (when x#, y# are of primitive type, of course).
221 We can't (in general) do this for algebraic cases, because we might
222 turn bottom into non-bottom!
224 Actually, we generalise this idea to look for a case where we're
225 scrutinising a variable, and we know that only the default case can
230 other -> ...(case x of
234 Here the inner case can be eliminated. This really only shows up in
235 eliminating error-checking code.
237 Lastly, we generalise the transformation to handle this:
243 We only do this for very cheaply compared r's (constructors, literals
244 and variables). If pedantic bottoms is on, we only do it when the
245 scrutinee is a PrimOp which can't fail.
247 We do it *here*, looking at un-simplified alternatives, because we
248 have to check that r doesn't mention the variables bound by the
249 pattern in each alternative, so the binder-info is rather useful.
251 So the case-elimination algorithm is:
253 1. Eliminate alternatives which can't match
255 2. Check whether all the remaining alternatives
256 (a) do not mention in their rhs any of the variables bound in their pattern
257 and (b) have equal rhss
259 3. Check we can safely ditch the case:
260 * PedanticBottoms is off,
261 or * the scrutinee is an already-evaluated variable
262 or * the scrutinee is a primop which is ok for speculation
263 -- ie we want to preserve divide-by-zero errors, and
264 -- calls to error itself!
266 or * [Prim cases] the scrutinee is a primitive variable
268 or * [Alg cases] the scrutinee is a variable and
269 either * the rhs is the same variable
270 (eg case x of C a b -> x ===> x)
271 or * there is only one alternative, the default alternative,
272 and the binder is used strictly in its scope.
273 [NB this is helped by the "use default binder where
274 possible" transformation; see below.]
277 If so, then we can replace the case with one of the rhss.
280 completeCase env scrut alts rhs_c
281 | switchIsSet env SimplDoCaseElim &&
287 (not (switchIsSet env SimplPedanticBottoms) ||
289 scrut_is_eliminable_primitive ||
291 scrut_is_var_and_single_strict_default
294 = tick CaseElim `thenSmpl_`
297 -- Find the non-excluded rhss of the case; always at least one
298 (rhs1:rhss) = possible_rhss
299 all_rhss_same = all (cheap_eq rhs1) rhss
301 -- Find the reduced set of possible rhss, along with an indication of
302 -- whether none of their binders are used
303 (binders_unused, possible_rhss, new_env)
305 PrimAlts alts deflt -> (deflt_binder_unused, -- No binders other than deflt
309 (deflt_binder_unused, deflt_rhs, new_env) = elim_deflt_binder deflt
311 -- Eliminate unused rhss if poss
312 rhss = case scrut_form of
313 OtherLit not_these -> [rhs | (alt_lit,rhs) <- alts,
314 not (alt_lit `is_elem` not_these)
316 other -> [rhs | (_,rhs) <- alts]
318 AlgAlts alts deflt -> (deflt_binder_unused && all alt_binders_unused possible_alts,
319 deflt_rhs ++ [rhs | (_,_,rhs) <- possible_alts],
322 (deflt_binder_unused, deflt_rhs, new_env) = elim_deflt_binder deflt
324 -- Eliminate unused alts if poss
325 possible_alts = case scrut_form of
326 OtherCon not_these ->
327 -- Remove alts which can't match
328 [alt | alt@(alt_con,_,_) <- alts,
329 not (alt_con `is_elem` not_these)]
333 alt_binders_unused (con, args, rhs) = all is_dead args
334 is_dead (_, DeadCode) = True
335 is_dead other_arg = False
337 -- If the scrutinee is a variable, look it up to see what we know about it
338 scrut_form = case scrut of
339 Var v -> lookupUnfolding env v
342 -- If the scrut is already eval'd then there's no worry about
343 -- eliminating the case
344 scrut_is_evald = isEvaluated scrut_form
346 scrut_is_eliminable_primitive
348 Prim op _ -> primOpOkForSpeculation op
349 Var _ -> case alts of
350 PrimAlts _ _ -> True -- Primitive, hence non-bottom
351 AlgAlts _ _ -> False -- Not primitive
354 -- case v of w -> e{strict in w} ===> e[v/w]
355 scrut_is_var_and_single_strict_default
357 Var _ -> case alts of
358 AlgAlts [] (BindDefault (v,_) _) -> willBeDemanded (getIdDemandInfo v)
362 elim_deflt_binder NoDefault -- No Binder
364 elim_deflt_binder (BindDefault (id, DeadCode) rhs) -- Binder unused
366 elim_deflt_binder (BindDefault used_binder rhs) -- Binder used
368 Var v -> -- Binder used, but can be eliminated in favour of scrut
369 (True, [rhs], bindIdToAtom env used_binder (VarArg v))
370 non_var -> -- Binder used, and can't be elimd
373 -- Check whether the chosen unique rhs (ie rhs1) is the same as
374 -- the scrutinee. Remember that the rhs is as yet unsimplified.
375 rhs1_is_scrutinee = case (scrut, rhs1) of
376 (Var scrut_var, Var rhs_var)
377 -> case (lookupIdSubst env rhs_var) of
378 Nothing -> rhs_var == scrut_var
379 Just (SubstVar rhs_var') -> rhs_var' == scrut_var
383 is_elem x ys = isIn "completeCase" x ys
386 Scrutinising anything else. If it's a variable, it can't be bound to a
387 constructor or literal, because that would have been inlined
390 completeCase env scrut alts rhs_c
391 = simplAlts env scrut alts rhs_c `thenSmpl` \ alts' ->
392 mkCoCase env scrut alts'
399 bindLargeAlts :: SimplEnv
401 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Old rhs handler
402 -> OutType -- Result type
403 -> SmplM ([OutBinding], -- Extra bindings
404 InAlts) -- Modified alts
406 bindLargeAlts env the_lot@(AlgAlts alts deflt) rhs_c rhs_ty
407 = mapAndUnzipSmpl do_alt alts `thenSmpl` \ (alt_bindings, alts') ->
408 bindLargeDefault env deflt rhs_ty rhs_c `thenSmpl` \ (deflt_bindings, deflt') ->
409 returnSmpl (deflt_bindings ++ alt_bindings, AlgAlts alts' deflt')
411 do_alt (con,args,rhs) = bindLargeRhs env args rhs_ty
412 (\env -> rhs_c env rhs) `thenSmpl` \ (bind,rhs') ->
413 returnSmpl (bind, (con,args,rhs'))
415 bindLargeAlts env the_lot@(PrimAlts alts deflt) rhs_c rhs_ty
416 = mapAndUnzipSmpl do_alt alts `thenSmpl` \ (alt_bindings, alts') ->
417 bindLargeDefault env deflt rhs_ty rhs_c `thenSmpl` \ (deflt_bindings, deflt') ->
418 returnSmpl (deflt_bindings ++ alt_bindings, PrimAlts alts' deflt')
420 do_alt (lit,rhs) = bindLargeRhs env [] rhs_ty
421 (\env -> rhs_c env rhs) `thenSmpl` \ (bind,rhs') ->
422 returnSmpl (bind, (lit,rhs'))
424 bindLargeDefault env NoDefault rhs_ty rhs_c
425 = returnSmpl ([], NoDefault)
426 bindLargeDefault env (BindDefault binder rhs) rhs_ty rhs_c
427 = bindLargeRhs env [binder] rhs_ty
428 (\env -> rhs_c env rhs) `thenSmpl` \ (bind,rhs') ->
429 returnSmpl ([bind], BindDefault binder rhs')
432 bindLargeRhs env [x1,..,xn] rhs rhs_ty rhs_c
433 | otherwise = (rhs_id = \x1..xn -> rhs_c rhs,
437 bindLargeRhs :: SimplEnv
438 -> [InBinder] -- The args wrt which the rhs should be abstracted
440 -> (SimplEnv -> SmplM OutExpr) -- Rhs handler
441 -> SmplM (OutBinding, -- New bindings (singleton or empty)
442 InExpr) -- Modified rhs
444 bindLargeRhs env args rhs_ty rhs_c
445 | null used_args && isUnpointedType rhs_ty
446 -- If we try to lift a primitive-typed something out
447 -- for let-binding-purposes, we will *caseify* it (!),
448 -- with potentially-disastrous strictness results. So
449 -- instead we turn it into a function: \v -> e
450 -- where v::Void. Since arguments of type
451 -- VoidPrim don't generate any code, this gives the
454 -- The general structure is just the same as for the common "otherwise~ case
455 = newId prim_rhs_fun_ty `thenSmpl` \ prim_rhs_fun_id ->
456 newId voidTy `thenSmpl` \ void_arg_id ->
457 rhs_c env `thenSmpl` \ prim_new_body ->
459 returnSmpl (NonRec prim_rhs_fun_id (mkValLam [void_arg_id] prim_new_body),
460 App (Var prim_rhs_fun_id) (VarArg voidId))
463 = -- Generate the rhs
464 simplBinders env used_args `thenSmpl` \ (new_env, used_args') ->
466 rhs_fun_ty :: OutType
467 rhs_fun_ty = mkFunTys (map idType used_args') rhs_ty
470 -- Make the new binding Id. NB: it's an OutId
471 newId rhs_fun_ty `thenSmpl` \ rhs_fun_id ->
472 rhs_c new_env `thenSmpl` \ rhs' ->
474 final_rhs = mkValLam used_args' rhs'
476 returnSmpl (NonRec rhs_fun_id final_rhs,
477 foldl App (Var rhs_fun_id) used_arg_atoms)
478 -- This is slightly wierd. We're retuning an OutId as part of the
479 -- modified rhs, which is meant to be an InExpr. However, that's ok, because when
480 -- it's processed the OutId won't be found in the environment, so it
481 -- will be left unmodified.
484 used_args = [arg | arg@(_,usage) <- args, not (dead usage)]
485 used_arg_atoms = [VarArg arg_id | (arg_id,_) <- used_args]
489 prim_rhs_fun_ty = mkFunTy voidTy rhs_ty
492 Case alternatives when we don't know the scrutinee
493 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
495 A special case for case default. If we have
501 it is best to make sure that \tr{default_e} mentions \tr{x} in
502 preference to \tr{y}. The code generator can do a cheaper job if it
503 doesn't have to come up with a binding for \tr{y}.
506 simplAlts :: SimplEnv
507 -> OutExpr -- Simplified scrutinee;
508 -- only of interest if its a var,
509 -- in which case we record its form
511 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
513 -- For single-constructor types
514 -- case e of y -> b ===> case e of (a,b) -> let y = (a,b) in b
516 simplAlts env scrut (AlgAlts [] (BindDefault bndr@(id,occ_info) rhs)) rhs_c
517 | maybeToBool maybe_data_ty &&
518 not (null cons) && -- Not an abstract type (can arise if we're pruning tydecl imports)
520 isDataTyCon tycon -- doesn't apply to (constructor-less) newtypes
521 = newIds inst_con_arg_tys `thenSmpl` \ new_bindees ->
523 new_args = [ (b, bad_occ_info) | b <- new_bindees ]
524 con_app = mkCon con ty_args (map VarArg new_bindees)
525 new_rhs = Let (NonRec bndr con_app) rhs
527 simplAlts env scrut (AlgAlts [(con,new_args,new_rhs)] NoDefault) rhs_c
529 maybe_data_ty = splitAlgTyConApp_maybe (idType id)
530 Just (tycon, ty_args, cons) = maybe_data_ty
531 (con:other_cons) = cons
532 inst_con_arg_tys = dataConArgTys con ty_args
533 bad_occ_info = ManyOcc 0 -- Non-committal!
535 simplAlts env scrut (AlgAlts alts deflt) rhs_c
536 = mapSmpl do_alt alts `thenSmpl` \ alts' ->
537 simplDefault env scrut deflt deflt_form rhs_c `thenSmpl` \ deflt' ->
538 returnSmpl (AlgAlts alts' deflt')
540 deflt_form = OtherCon [con | (con,_,_) <- alts]
541 do_alt (con, con_args, rhs)
542 = simplBinders env con_args `thenSmpl` \ (env1, con_args') ->
544 new_env = case scrut of
545 Var v -> extendEnvGivenNewRhs env1 v (Con con args)
547 (_, ty_args, _) = splitAlgTyConApp (idType v)
548 args = map TyArg ty_args ++ map VarArg con_args'
552 rhs_c new_env rhs `thenSmpl` \ rhs' ->
553 returnSmpl (con, con_args', rhs')
555 simplAlts env scrut (PrimAlts alts deflt) rhs_c
556 = mapSmpl do_alt alts `thenSmpl` \ alts' ->
557 simplDefault env scrut deflt deflt_form rhs_c `thenSmpl` \ deflt' ->
558 returnSmpl (PrimAlts alts' deflt')
560 deflt_form = OtherLit [lit | (lit,_) <- alts]
563 new_env = case scrut of
564 Var v -> extendEnvGivenNewRhs env v (Lit lit)
567 rhs_c new_env rhs `thenSmpl` \ rhs' ->
568 returnSmpl (lit, rhs')
571 Use default binder where possible
572 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
573 There's one complication when simplifying the default clause of
574 a case expression. If we see
579 we'd like to convert it to
584 Reason 1: then there might be just one occurrence of x, and it can be
585 inlined as the case scrutinee. So we spot this case when dealing with
586 the default clause, and add a binding to the environment mapping x to
589 Reason 2: if the body is strict in x' then we can eliminate the
590 case altogether. By using x' in preference to x we give the max chance
591 of the strictness analyser finding that the body is strict in x'.
593 On the other hand, if x does *not* get inlined, then we'll actually
594 get somewhat better code from the former expression. So when
595 doing Core -> STG we convert back!
600 -> OutExpr -- Simplified scrutinee
601 -> InDefault -- Default alternative to be completed
602 -> Unfolding -- Gives form of scrutinee
603 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Old rhs handler
606 simplDefault env scrut NoDefault form rhs_c
607 = returnSmpl NoDefault
609 -- Special case for variable scrutinee; see notes above.
610 simplDefault env (Var scrut_var) (BindDefault binder@(_,occ_info) rhs)
611 info_from_this_case rhs_c
612 = simplBinder env binder `thenSmpl` \ (env1, binder') ->
614 env2 = extendEnvGivenNewRhs env1 scrut_var (Var binder')
616 -- Add form details for the default binder
617 scrut_unf = lookupUnfolding env scrut_var
618 new_env = extendEnvGivenUnfolding env2 binder' noBinderInfo scrut_unf
619 -- Use noBinderInfo rather than occ_info because we've
620 -- added more occurrences by binding the scrut_var to it
622 rhs_c new_env rhs `thenSmpl` \ rhs' ->
623 returnSmpl (BindDefault binder' rhs')
625 simplDefault env scrut (BindDefault binder@(_,occ_info) rhs)
626 info_from_this_case rhs_c
627 = simplBinder env binder `thenSmpl` \ (env1, binder') ->
629 new_env = extendEnvGivenUnfolding env1 binder' occ_info info_from_this_case
631 rhs_c new_env rhs `thenSmpl` \ rhs' ->
632 returnSmpl (BindDefault binder' rhs')
635 Case alternatives when we know what the scrutinee is
636 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
639 completePrimCaseWithKnownLit
643 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
646 completePrimCaseWithKnownLit env lit (PrimAlts alts deflt) rhs_c
649 search_alts :: [(Literal, InExpr)] -> SmplM OutExpr
651 search_alts ((alt_lit, rhs) : _)
653 = -- Matching alternative!
656 search_alts (_ : other_alts)
657 = -- This alternative doesn't match; keep looking
658 search_alts other_alts
662 NoDefault -> -- Blargh!
663 panic "completePrimCaseWithKnownLit: No matching alternative and no default"
665 BindDefault binder rhs -> -- OK, there's a default case
666 -- Just bind the Id to the atom and continue
668 new_env = bindIdToAtom env binder (LitArg lit)
673 @completeAlgCaseWithKnownCon@: We know the constructor, so we can
674 select one case alternative (or default). If we choose the default:
675 we do different things depending on whether the constructor was
676 staring us in the face (e.g., \tr{case (p:ps) of {y -> ...}})
677 [let-bind it] or we just know the \tr{y} is now the same as some other
678 var [substitute \tr{y} out of existence].
681 completeAlgCaseWithKnownCon
683 -> DataCon -> [InArg]
684 -- Scrutinee is (con, type, value arguments)
686 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
689 completeAlgCaseWithKnownCon env con con_args a@(AlgAlts alts deflt) rhs_c
690 = ASSERT(isDataCon con)
693 search_alts :: [(Id, [InBinder], InExpr)] -> SmplM OutExpr
695 search_alts ((alt_con, alt_args, rhs) : _)
697 = -- Matching alternative!
699 val_args = filter isValArg con_args
700 new_env = foldr bind env (zipEqual "SimplCase" alt_args val_args)
701 bind (bndr, atom) env = bindIdToAtom env bndr atom
705 search_alts (_ : other_alts)
706 = -- This alternative doesn't match; keep looking
707 search_alts other_alts
710 = -- No matching alternative
712 NoDefault -> -- Blargh!
713 pprPanic "completeAlgCaseWithKnownCon: No matching alternative and no default"
714 (ppr con <+> ppr con_args $$ ppr a)
716 BindDefault binder@(_,occ_info) rhs -> -- OK, there's a default case
717 -- let-bind the binder to the constructor
718 simplBinder env binder `thenSmpl` \ (env1, id') ->
720 new_env = extendEnvGivenBinding env1 occ_info id' (Con con con_args)
722 rhs_c new_env rhs `thenSmpl` \ rhs' ->
723 returnSmpl (Let (NonRec id' (Con con con_args)) rhs')
726 Case absorption and identity-case elimination
727 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
730 mkCoCase :: SimplEnv -> OutExpr -> OutAlts -> SmplM OutExpr
733 @mkCoCase@ tries the following transformation (if possible):
735 case v of ==> case v of
736 p1 -> rhs1 p1 -> rhs1
738 pm -> rhsm pm -> rhsm
739 d -> case v of pn -> rhsn[v/d] {or (alg) let d=v in rhsn}
740 {or (prim) case v of d -> rhsn}
743 po -> rhso d -> rhsd[d/d'] {or let d'=d in rhsd}
746 which merges two cases in one case when -- the default alternative of
747 the outer case scrutises the same variable as the outer case This
748 transformation is called Case Merging. It avoids that the same
749 variable is scrutinised multiple times.
751 There's a closely-related transformation:
753 case e of ==> case e of
754 p1 -> rhs1 p1 -> rhs1
756 pm -> rhsm pm -> rhsm
757 d -> case d of pn -> let d = pn in rhsn
759 ... po -> let d = po in rhso
760 po -> rhso d -> rhsd[d/d'] {or let d'=d in rhsd}
763 Here, the let's are essential, because d isn't in scope any more.
764 Sigh. Of course, they may be unused, in which case they'll be
765 eliminated on the next round. Unfortunately, we can't figure out
766 whether or not they are used at this juncture.
768 NB: The binder in a BindDefault USED TO BE guaranteed unused if the
769 scrutinee is a variable, because it'll be mapped to the scrutinised
770 variable. Hence the [v/d] substitions can be omitted.
772 ALAS, now the default binder is used by preference, so we have to
773 generate trivial lets to express the substitutions, which will be
774 eliminated on the next pass.
776 The following code handles *both* these transformations (one
777 equation for AlgAlts, one for PrimAlts):
780 mkCoCase env scrut (AlgAlts outer_alts
781 (BindDefault deflt_var
782 (Case (Var scrut_var')
783 (AlgAlts inner_alts inner_deflt))))
784 | switchIsSet env SimplCaseMerge &&
785 ((scrut_is_var && scrut_var == scrut_var') || -- First transformation
786 deflt_var == scrut_var') -- Second transformation
787 = -- Aha! The default-absorption rule applies
788 tick CaseMerge `thenSmpl_`
789 returnSmpl (Case scrut (AlgAlts (outer_alts ++ munged_reduced_inner_alts)
790 (munge_alg_deflt deflt_var inner_deflt)))
791 -- NB: see comment in this location for the PrimAlts case
794 scrut_is_var = case scrut of {Var v -> True; other -> False}
795 scrut_var = case scrut of Var v -> v
797 -- Eliminate any inner alts which are shadowed by the outer ones
798 reduced_inner_alts = [alt | alt@(con,_,_) <- inner_alts,
799 not (con `is_elem` outer_cons)]
800 outer_cons = [con | (con,_,_) <- outer_alts]
801 is_elem = isIn "mkAlgAlts"
803 -- Add the lets if necessary
804 munged_reduced_inner_alts = map munge_alt reduced_inner_alts
806 munge_alt (con, args, rhs) = (con, args, Let (NonRec deflt_var v) rhs)
808 v | scrut_is_var = Var scrut_var
809 | otherwise = Con con (map TyArg arg_tys ++ map VarArg args)
811 arg_tys = case (splitAlgTyConApp (idType deflt_var)) of
812 (_, arg_tys, _) -> arg_tys
814 mkCoCase env scrut (PrimAlts
816 (BindDefault deflt_var (Case
818 (PrimAlts inner_alts inner_deflt))))
819 | switchIsSet env SimplCaseMerge &&
820 ((scrut_is_var && scrut_var == scrut_var') ||
821 deflt_var == scrut_var')
822 = -- Aha! The default-absorption rule applies
823 tick CaseMerge `thenSmpl_`
824 returnSmpl (Case scrut (PrimAlts (outer_alts ++ munged_reduced_inner_alts)
825 (munge_prim_deflt deflt_var inner_deflt)))
827 -- Nota Bene: we don't recurse to mkCoCase again, because the
828 -- default will now have a binding in it that prevents
829 -- mkCoCase doing anything useful. Much worse, in this
830 -- PrimAlts case the binding in the default branch is another
831 -- Case, so if we recurse to mkCoCase we will get into an
834 -- ToDo: think of a better way to do this. At the moment
835 -- there is at most one case merge per round. That's probably
836 -- plenty but it seems unclean somehow.
839 scrut_is_var = case scrut of {Var v -> True; other -> False}
840 scrut_var = case scrut of Var v -> v
842 -- Eliminate any inner alts which are shadowed by the outer ones
843 reduced_inner_alts = [alt | alt@(lit,_) <- inner_alts,
844 not (lit `is_elem` outer_lits)]
845 outer_lits = [lit | (lit,_) <- outer_alts]
846 is_elem = isIn "mkPrimAlts"
848 -- Add the lets (well cases actually) if necessary
849 -- The munged alternative looks like
850 -- lit -> case lit of d -> rhs
851 -- The next pass will certainly eliminate the inner case, but
852 -- it isn't easy to do so right away.
853 munged_reduced_inner_alts = map munge_alt reduced_inner_alts
856 | scrut_is_var = (lit, Case (Var scrut_var)
857 (PrimAlts [] (BindDefault deflt_var rhs)))
858 | otherwise = (lit, Case (Lit lit)
859 (PrimAlts [] (BindDefault deflt_var rhs)))
862 Now the identity-case transformation:
871 mkCoCase env scrut alts
873 = tick CaseIdentity `thenSmpl_`
876 identity_alts (AlgAlts alts deflt) = all identity_alg_alt alts && identity_deflt deflt
877 identity_alts (PrimAlts alts deflt) = all identity_prim_alt alts && identity_deflt deflt
879 identity_alg_alt (con, args, Con con' args')
881 && and (zipWith eq_arg args args')
882 && length args == length args'
883 identity_alg_alt other
886 identity_prim_alt (lit, Lit lit') = lit == lit'
887 identity_prim_alt other = False
889 -- For the default case we want to spot both
892 -- case y of { ... ; x -> y }
893 -- as "identity" defaults
894 identity_deflt NoDefault = True
895 identity_deflt (BindDefault binder (Var x)) = x == binder ||
899 identity_deflt _ = False
901 eq_arg binder (VarArg x) = binder == x
908 mkCoCase env other_scrut other_alts = returnSmpl (Case other_scrut other_alts)
911 Boring local functions used above. They simply introduce a trivial binding
912 for the binder, d', in an inner default; either
913 let d' = deflt_var in rhs
915 case deflt_var of d' -> rhs
916 depending on whether it's an algebraic or primitive case.
919 munge_prim_deflt _ NoDefault = NoDefault
921 munge_prim_deflt deflt_var (BindDefault d' rhs)
922 = BindDefault deflt_var (Case (Var deflt_var)
923 (PrimAlts [] (BindDefault d' rhs)))
925 munge_alg_deflt _ NoDefault = NoDefault
927 munge_alg_deflt deflt_var (BindDefault d' rhs)
928 = BindDefault deflt_var (Let (NonRec d' (Var deflt_var)) rhs)
930 -- This line caused a generic version of munge_deflt (ie one used for
931 -- both alg and prim) to space leak massively. No idea why.
932 -- = BindDefault deflt_var (mkCoLetUnboxedToCase (NonRec d' (Var deflt_var)) rhs)
936 cheap_eq :: InExpr -> InExpr -> Bool
937 -- A cheap equality test which bales out fast!
939 cheap_eq (Var v1) (Var v2) = v1==v2
940 cheap_eq (Lit l1) (Lit l2) = l1==l2
941 cheap_eq (Con con1 args1) (Con con2 args2)
942 = con1 == con2 && args1 `eq_args` args2
944 cheap_eq (Prim op1 args1) (Prim op2 args2)
945 = op1 ==op2 && args1 `eq_args` args2
947 cheap_eq (App f1 a1) (App f2 a2)
948 = f1 `cheap_eq` f2 && a1 `eq_arg` a2
952 -- ToDo: make CoreArg an instance of Eq
953 eq_args (a1:as1) (a2:as2) = a1 `eq_arg` a2 && as1 `eq_args` as2
957 eq_arg (LitArg l1) (LitArg l2) = l1 == l2
958 eq_arg (VarArg v1) (VarArg v2) = v1 == v2
959 eq_arg (TyArg t1) (TyArg t2) = t1 == t2