2 % (c) The AQUA Project, Glasgow University, 1994-1996
4 \section[SimplCase]{Simplification of `case' expression}
6 Support code for @Simplify@.
9 #include "HsVersions.h"
11 module SimplCase ( simplCase, bindLargeRhs ) where
14 IMPORT_DELOOPER(SmplLoop) ( simplBind, simplExpr, MagicUnfoldingFun )
16 import BinderInfo -- too boring to try to select things...
17 import CmdLineOpts ( SimplifierSwitch(..) )
19 import CoreUnfold ( Unfolding(..), UnfoldingGuidance(..), SimpleUnfolding
21 import CoreUtils ( coreAltsType, nonErrorRHSs, maybeErrorApp,
24 import Id ( idType, isDataCon, getIdDemandInfo,
25 SYN_IE(DataCon), GenId{-instance Eq-}
27 import IdInfo ( willBeDemanded, DemandInfo )
28 import Literal ( isNoRepLit, Literal{-instance Eq-} )
29 import Maybes ( maybeToBool )
30 import PrelVals ( voidId )
31 import PrimOp ( primOpOkForSpeculation, PrimOp{-instance Eq-} )
34 import SimplUtils ( mkValLamTryingEta )
35 import Type ( isPrimType, getAppDataTyConExpandingDicts, mkFunTy, mkFunTys, eqTy )
36 import TysPrim ( voidTy )
37 import Unique ( Unique{-instance Eq-} )
38 import Usage ( GenUsage{-instance Eq-} )
39 import Util ( isIn, isSingleton, zipEqual, panic, assertPanic )
42 Float let out of case.
46 -> InExpr -- Scrutinee
47 -> InAlts -- Alternatives
48 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
49 -> OutType -- Type of result expression
52 simplCase env (Let bind body) alts rhs_c result_ty
53 | not (switchIsSet env SimplNoLetFromCase)
54 = -- Float the let outside the case scrutinee (if not disabled by flag)
55 tick LetFloatFromCase `thenSmpl_`
56 simplBind env bind (\env -> simplCase env body alts rhs_c result_ty) result_ty
59 OK to do case-of-case if
61 * we allow arbitrary code duplication
65 * the inner case has one alternative
66 case (case e of (a,b) -> rhs) of
77 IF neither of these two things are the case, we avoid code-duplication
78 by abstracting the outer rhss wrt the pattern variables. For example
80 case (case e of { p1->rhs1; ...; pn -> rhsn }) of
86 p1 -> case rhs1 of (x,y) -> b x y
88 pn -> case rhsn of (x,y) -> b x y
91 OK, so outer case expression gets duplicated, but that's all. Furthermore,
92 (a) the binding for "b" will be let-no-escaped, so no heap allocation
93 will take place; the "call" to b will simply be a stack adjustment
95 (b) very commonly, at least some of the rhsi's will be constructors, which
96 makes life even simpler.
98 All of this works equally well if the outer case has multiple rhss.
102 simplCase env (Case inner_scrut inner_alts) outer_alts rhs_c result_ty
103 | switchIsSet env SimplCaseOfCase
104 = -- Ha! Do case-of-case
105 tick CaseOfCase `thenSmpl_`
107 if no_need_to_bind_large_alts
109 simplCase env inner_scrut inner_alts
110 (\env rhs -> simplCase env rhs outer_alts rhs_c result_ty) result_ty
112 bindLargeAlts env outer_alts rhs_c result_ty `thenSmpl` \ (extra_bindings, outer_alts') ->
114 rhs_c' = \env rhs -> simplExpr env rhs []
116 simplCase env inner_scrut inner_alts
117 (\env rhs -> simplCase env rhs outer_alts' rhs_c' result_ty)
119 `thenSmpl` \ case_expr ->
120 returnSmpl (mkCoLetsNoUnboxed extra_bindings case_expr)
123 no_need_to_bind_large_alts = switchIsSet env SimplOkToDupCode ||
124 isSingleton (nonErrorRHSs inner_alts)
127 Case of an application of error.
130 simplCase env scrut alts rhs_c result_ty
131 | maybeToBool maybe_error_app
132 = -- Look for an application of an error id
133 tick CaseOfError `thenSmpl_`
134 rhs_c env retyped_error_app
136 alts_ty = coreAltsType (unTagBindersAlts alts)
137 maybe_error_app = maybeErrorApp scrut (Just alts_ty)
138 Just retyped_error_app = maybe_error_app
141 Finally the default case
144 simplCase env other_scrut alts rhs_c result_ty
145 = -- Float the let outside the case scrutinee
146 simplExpr env other_scrut [] `thenSmpl` \ scrut' ->
147 completeCase env scrut' alts rhs_c
151 %************************************************************************
153 \subsection[Simplify-case]{Completing case-expression simplification}
155 %************************************************************************
160 -> OutExpr -- The already-simplified scrutinee
161 -> InAlts -- The un-simplified alternatives
162 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
163 -> SmplM OutExpr -- The whole case expression
166 Scrutinising a literal or constructor.
167 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
168 It's an obvious win to do:
170 case (C a b) of {...; C p q -> rhs; ...} ===> rhs[a/p,b/q]
172 and the similar thing for primitive case. If we have
176 and x is known to be of constructor form, then we'll already have
177 inlined the constructor to give (case (C a b) of ...), so we don't
178 need to check for the variable case separately.
180 Sanity check: we don't have a good
181 story to tell about case analysis on NoRep things. ToDo.
184 completeCase env (Lit lit) alts rhs_c
185 | not (isNoRepLit lit)
186 = -- Ha! Select the appropriate alternative
187 tick KnownBranch `thenSmpl_`
188 completePrimCaseWithKnownLit env lit alts rhs_c
190 completeCase env expr@(Con con con_args) alts rhs_c
191 = -- Ha! Staring us in the face -- select the appropriate alternative
192 tick KnownBranch `thenSmpl_`
193 completeAlgCaseWithKnownCon env con con_args alts rhs_c
198 Start with a simple situation:
200 case x# of ===> e[x#/y#]
203 (when x#, y# are of primitive type, of course).
204 We can't (in general) do this for algebraic cases, because we might
205 turn bottom into non-bottom!
207 Actually, we generalise this idea to look for a case where we're
208 scrutinising a variable, and we know that only the default case can
213 other -> ...(case x of
217 Here the inner case can be eliminated. This really only shows up in
218 eliminating error-checking code.
220 Lastly, we generalise the transformation to handle this:
226 We only do this for very cheaply compared r's (constructors, literals
227 and variables). If pedantic bottoms is on, we only do it when the
228 scrutinee is a PrimOp which can't fail.
230 We do it *here*, looking at un-simplified alternatives, because we
231 have to check that r doesn't mention the variables bound by the
232 pattern in each alternative, so the binder-info is rather useful.
234 So the case-elimination algorithm is:
236 1. Eliminate alternatives which can't match
238 2. Check whether all the remaining alternatives
239 (a) do not mention in their rhs any of the variables bound in their pattern
240 and (b) have equal rhss
242 3. Check we can safely ditch the case:
243 * PedanticBottoms is off,
244 or * the scrutinee is an already-evaluated variable
245 or * the scrutinee is a primop which is ok for speculation
246 -- ie we want to preserve divide-by-zero errors, and
247 -- calls to error itself!
249 or * [Prim cases] the scrutinee is a primitive variable
251 or * [Alg cases] the scrutinee is a variable and
252 either * the rhs is the same variable
253 (eg case x of C a b -> x ===> x)
254 or * there is only one alternative, the default alternative,
255 and the binder is used strictly in its scope.
256 [NB this is helped by the "use default binder where
257 possible" transformation; see below.]
260 If so, then we can replace the case with one of the rhss.
263 completeCase env scrut alts rhs_c
264 | switchIsSet env SimplDoCaseElim &&
270 (not (switchIsSet env SimplPedanticBottoms) ||
272 scrut_is_eliminable_primitive ||
274 scrut_is_var_and_single_strict_default
277 = tick CaseElim `thenSmpl_`
280 -- Find the non-excluded rhss of the case; always at least one
281 (rhs1:rhss) = possible_rhss
282 all_rhss_same = all (cheap_eq rhs1) rhss
284 -- Find the reduced set of possible rhss, along with an indication of
285 -- whether none of their binders are used
286 (binders_unused, possible_rhss, new_env)
288 PrimAlts alts deflt -> (deflt_binder_unused, -- No binders other than deflt
292 (deflt_binder_unused, deflt_rhs, new_env) = elim_deflt_binder deflt
294 -- Eliminate unused rhss if poss
295 rhss = case scrut_form of
296 OtherLit not_these -> [rhs | (alt_lit,rhs) <- alts,
297 not (alt_lit `is_elem` not_these)
299 other -> [rhs | (_,rhs) <- alts]
301 AlgAlts alts deflt -> (deflt_binder_unused && all alt_binders_unused possible_alts,
302 deflt_rhs ++ [rhs | (_,_,rhs) <- possible_alts],
305 (deflt_binder_unused, deflt_rhs, new_env) = elim_deflt_binder deflt
307 -- Eliminate unused alts if poss
308 possible_alts = case scrut_form of
309 OtherCon not_these ->
310 -- Remove alts which can't match
311 [alt | alt@(alt_con,_,_) <- alts,
312 not (alt_con `is_elem` not_these)]
316 alt_binders_unused (con, args, rhs) = all is_dead args
317 is_dead (_, DeadCode) = True
318 is_dead other_arg = False
320 -- If the scrutinee is a variable, look it up to see what we know about it
321 scrut_form = case scrut of
322 Var v -> lookupRhsInfo env v
325 -- If the scrut is already eval'd then there's no worry about
326 -- eliminating the case
327 scrut_is_evald = isEvaluated scrut_form
329 scrut_is_eliminable_primitive
331 Prim op _ -> primOpOkForSpeculation op
332 Var _ -> case alts of
333 PrimAlts _ _ -> True -- Primitive, hence non-bottom
334 AlgAlts _ _ -> False -- Not primitive
337 -- case v of w -> e{strict in w} ===> e[v/w]
338 scrut_is_var_and_single_strict_default
340 Var _ -> case alts of
341 AlgAlts [] (BindDefault (v,_) _) -> willBeDemanded (getIdDemandInfo v)
345 elim_deflt_binder NoDefault -- No Binder
347 elim_deflt_binder (BindDefault (id, DeadCode) rhs) -- Binder unused
349 elim_deflt_binder (BindDefault used_binder rhs) -- Binder used
351 Var v -> -- Binder used, but can be eliminated in favour of scrut
352 (True, [rhs], extendIdEnvWithAtom env used_binder (VarArg v))
353 non_var -> -- Binder used, and can't be elimd
356 -- Check whether the chosen unique rhs (ie rhs1) is the same as
357 -- the scrutinee. Remember that the rhs is as yet unsimplified.
358 rhs1_is_scrutinee = case (scrut, rhs1) of
359 (Var scrut_var, Var rhs_var)
360 -> case lookupId env rhs_var of
361 VarArg rhs_var' -> rhs_var' == scrut_var
365 is_elem x ys = isIn "completeCase" x ys
368 Scrutinising anything else. If it's a variable, it can't be bound to a
369 constructor or literal, because that would have been inlined
372 completeCase env scrut alts rhs_c
373 = simplAlts env scrut alts rhs_c `thenSmpl` \ alts' ->
381 bindLargeAlts :: SimplEnv
383 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Old rhs handler
384 -> OutType -- Result type
385 -> SmplM ([OutBinding], -- Extra bindings
386 InAlts) -- Modified alts
388 bindLargeAlts env the_lot@(AlgAlts alts deflt) rhs_c rhs_ty
389 = mapAndUnzipSmpl do_alt alts `thenSmpl` \ (alt_bindings, alts') ->
390 bindLargeDefault env deflt rhs_ty rhs_c `thenSmpl` \ (deflt_bindings, deflt') ->
391 returnSmpl (deflt_bindings ++ alt_bindings, AlgAlts alts' deflt')
393 do_alt (con,args,rhs) = bindLargeRhs env args rhs_ty
394 (\env -> rhs_c env rhs) `thenSmpl` \ (bind,rhs') ->
395 returnSmpl (bind, (con,args,rhs'))
397 bindLargeAlts env the_lot@(PrimAlts alts deflt) rhs_c rhs_ty
398 = mapAndUnzipSmpl do_alt alts `thenSmpl` \ (alt_bindings, alts') ->
399 bindLargeDefault env deflt rhs_ty rhs_c `thenSmpl` \ (deflt_bindings, deflt') ->
400 returnSmpl (deflt_bindings ++ alt_bindings, PrimAlts alts' deflt')
402 do_alt (lit,rhs) = bindLargeRhs env [] rhs_ty
403 (\env -> rhs_c env rhs) `thenSmpl` \ (bind,rhs') ->
404 returnSmpl (bind, (lit,rhs'))
406 bindLargeDefault env NoDefault rhs_ty rhs_c
407 = returnSmpl ([], NoDefault)
408 bindLargeDefault env (BindDefault binder rhs) rhs_ty rhs_c
409 = bindLargeRhs env [binder] rhs_ty
410 (\env -> rhs_c env rhs) `thenSmpl` \ (bind,rhs') ->
411 returnSmpl ([bind], BindDefault binder rhs')
414 bindLargeRhs env [x1,..,xn] rhs rhs_ty rhs_c
415 | otherwise = (rhs_id = \x1..xn -> rhs_c rhs,
419 bindLargeRhs :: SimplEnv
420 -> [InBinder] -- The args wrt which the rhs should be abstracted
422 -> (SimplEnv -> SmplM OutExpr) -- Rhs handler
423 -> SmplM (OutBinding, -- New bindings (singleton or empty)
424 InExpr) -- Modified rhs
426 bindLargeRhs env args rhs_ty rhs_c
427 | null used_args && isPrimType rhs_ty
428 -- If we try to lift a primitive-typed something out
429 -- for let-binding-purposes, we will *caseify* it (!),
430 -- with potentially-disastrous strictness results. So
431 -- instead we turn it into a function: \v -> e
432 -- where v::Void. Since arguments of type
433 -- VoidPrim don't generate any code, this gives the
436 -- The general structure is just the same as for the common "otherwise~ case
437 = newId prim_rhs_fun_ty `thenSmpl` \ prim_rhs_fun_id ->
438 newId voidTy `thenSmpl` \ void_arg_id ->
439 rhs_c env `thenSmpl` \ prim_new_body ->
441 returnSmpl (NonRec prim_rhs_fun_id (mkValLam [void_arg_id] prim_new_body),
442 App (Var prim_rhs_fun_id) (VarArg voidId))
445 = -- Make the new binding Id. NB: it's an OutId
446 newId rhs_fun_ty `thenSmpl` \ rhs_fun_id ->
449 cloneIds env used_args `thenSmpl` \ used_args' ->
451 new_env = extendIdEnvWithClones env used_args used_args'
453 rhs_c new_env `thenSmpl` \ rhs' ->
456 = (if switchIsSet new_env SimplDoEtaReduction
457 then mkValLamTryingEta
458 else mkValLam) used_args' rhs'
460 returnSmpl (NonRec rhs_fun_id final_rhs,
461 foldl App (Var rhs_fun_id) used_arg_atoms)
462 -- This is slightly wierd. We're retuning an OutId as part of the
463 -- modified rhs, which is meant to be an InExpr. However, that's ok, because when
464 -- it's processed the OutId won't be found in the environment, so it
465 -- will be left unmodified.
467 rhs_fun_ty :: OutType
468 rhs_fun_ty = mkFunTys [simplTy env (idType id) | (id,_) <- used_args] rhs_ty
470 used_args = [arg | arg@(_,usage) <- args, not (dead usage)]
471 used_arg_atoms = [VarArg arg_id | (arg_id,_) <- used_args]
475 prim_rhs_fun_ty = mkFunTy voidTy rhs_ty
478 Case alternatives when we don't know the scrutinee
479 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
481 A special case for case default. If we have
487 it is best to make sure that \tr{default_e} mentions \tr{x} in
488 preference to \tr{y}. The code generator can do a cheaper job if it
489 doesn't have to come up with a binding for \tr{y}.
492 simplAlts :: SimplEnv
493 -> OutExpr -- Simplified scrutinee;
494 -- only of interest if its a var,
495 -- in which case we record its form
497 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
500 simplAlts env scrut (AlgAlts alts deflt) rhs_c
501 = mapSmpl do_alt alts `thenSmpl` \ alts' ->
502 simplDefault env scrut deflt deflt_form rhs_c `thenSmpl` \ deflt' ->
503 returnSmpl (AlgAlts alts' deflt')
505 deflt_form = OtherCon [con | (con,_,_) <- alts]
506 do_alt (con, con_args, rhs)
507 = cloneIds env con_args `thenSmpl` \ con_args' ->
509 env1 = extendIdEnvWithClones env con_args con_args'
510 new_env = case scrut of
511 Var v -> extendEnvGivenNewRhs env1 v (Con con args)
513 (_, ty_args, _) = getAppDataTyConExpandingDicts (idType v)
514 args = map TyArg ty_args ++ map VarArg con_args'
518 rhs_c new_env rhs `thenSmpl` \ rhs' ->
519 returnSmpl (con, con_args', rhs')
521 simplAlts env scrut (PrimAlts alts deflt) rhs_c
522 = mapSmpl do_alt alts `thenSmpl` \ alts' ->
523 simplDefault env scrut deflt deflt_form rhs_c `thenSmpl` \ deflt' ->
524 returnSmpl (PrimAlts alts' deflt')
526 deflt_form = OtherLit [lit | (lit,_) <- alts]
529 new_env = case scrut of
530 Var v -> extendEnvGivenNewRhs env v (Lit lit)
533 rhs_c new_env rhs `thenSmpl` \ rhs' ->
534 returnSmpl (lit, rhs')
537 Use default binder where possible
538 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
539 There's one complication when simplifying the default clause of
540 a case expression. If we see
545 we'd like to convert it to
550 Reason 1: then there might be just one occurrence of x, and it can be
551 inlined as the case scrutinee. So we spot this case when dealing with
552 the default clause, and add a binding to the environment mapping x to
555 Reason 2: if the body is strict in x' then we can eliminate the
556 case altogether. By using x' in preference to x we give the max chance
557 of the strictness analyser finding that the body is strict in x'.
559 On the other hand, if x does *not* get inlined, then we'll actually
560 get somewhat better code from the former expression. So when
561 doing Core -> STG we convert back!
566 -> OutExpr -- Simplified scrutinee
567 -> InDefault -- Default alternative to be completed
568 -> RhsInfo -- Gives form of scrutinee
569 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Old rhs handler
572 simplDefault env scrut NoDefault form rhs_c
573 = returnSmpl NoDefault
575 -- Special case for variable scrutinee; see notes above.
576 simplDefault env (Var scrut_var) (BindDefault binder@(_,occ_info) rhs)
577 info_from_this_case rhs_c
578 = cloneId env binder `thenSmpl` \ binder' ->
580 env1 = extendIdEnvWithClone env binder binder'
581 env2 = extendEnvGivenRhsInfo env1 binder' occ_info info_from_this_case
583 -- Add form details for the default binder
584 scrut_info = lookupRhsInfo env scrut_var
585 env3 = extendEnvGivenRhsInfo env2 binder' occ_info scrut_info
586 new_env = extendEnvGivenNewRhs env3 scrut_var (Var binder')
588 rhs_c new_env rhs `thenSmpl` \ rhs' ->
589 returnSmpl (BindDefault binder' rhs')
591 simplDefault env scrut (BindDefault binder@(_,occ_info) rhs)
592 info_from_this_case rhs_c
593 = cloneId env binder `thenSmpl` \ binder' ->
595 env1 = extendIdEnvWithClone env binder binder'
596 new_env = extendEnvGivenRhsInfo env1 binder' occ_info info_from_this_case
598 rhs_c new_env rhs `thenSmpl` \ rhs' ->
599 returnSmpl (BindDefault binder' rhs')
602 Case alternatives when we know what the scrutinee is
603 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
606 completePrimCaseWithKnownLit
610 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
613 completePrimCaseWithKnownLit env lit (PrimAlts alts deflt) rhs_c
616 search_alts :: [(Literal, InExpr)] -> SmplM OutExpr
618 search_alts ((alt_lit, rhs) : _)
620 = -- Matching alternative!
623 search_alts (_ : other_alts)
624 = -- This alternative doesn't match; keep looking
625 search_alts other_alts
629 NoDefault -> -- Blargh!
630 panic "completePrimCaseWithKnownLit: No matching alternative and no default"
632 BindDefault binder rhs -> -- OK, there's a default case
633 -- Just bind the Id to the atom and continue
635 new_env = extendIdEnvWithAtom env binder (LitArg lit)
640 @completeAlgCaseWithKnownCon@: We know the constructor, so we can
641 select one case alternative (or default). If we choose the default:
642 we do different things depending on whether the constructor was
643 staring us in the face (e.g., \tr{case (p:ps) of {y -> ...}})
644 [let-bind it] or we just know the \tr{y} is now the same as some other
645 var [substitute \tr{y} out of existence].
648 completeAlgCaseWithKnownCon
650 -> DataCon -> [InArg]
651 -- Scrutinee is (con, type, value arguments)
653 -> (SimplEnv -> InExpr -> SmplM OutExpr) -- Rhs handler
656 completeAlgCaseWithKnownCon env con con_args (AlgAlts alts deflt) rhs_c
657 = ASSERT(isDataCon con)
660 search_alts :: [(Id, [InBinder], InExpr)] -> SmplM OutExpr
662 search_alts ((alt_con, alt_args, rhs) : _)
664 = -- Matching alternative!
666 new_env = extendIdEnvWithAtoms env
667 (zipEqual "SimplCase" alt_args (filter isValArg con_args))
671 search_alts (_ : other_alts)
672 = -- This alternative doesn't match; keep looking
673 search_alts other_alts
676 = -- No matching alternative
678 NoDefault -> -- Blargh!
679 panic "completeAlgCaseWithKnownCon: No matching alternative and no default"
681 BindDefault binder@(_,occ_info) rhs -> -- OK, there's a default case
682 -- let-bind the binder to the constructor
683 cloneId env binder `thenSmpl` \ id' ->
685 new_env = extendEnvGivenBinding env occ_info id' (Con con con_args)
687 rhs_c new_env rhs `thenSmpl` \ rhs' ->
688 returnSmpl (Let (NonRec id' (Con con con_args)) rhs')
691 Case absorption and identity-case elimination
692 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
695 mkCoCase :: OutExpr -> OutAlts -> SmplM OutExpr
698 @mkCoCase@ tries the following transformation (if possible):
700 case v of ==> case v of
701 p1 -> rhs1 p1 -> rhs1
703 pm -> rhsm pm -> rhsm
704 d -> case v of pn -> rhsn[v/d] {or (alg) let d=v in rhsn}
705 {or (prim) case v of d -> rhsn}
708 po -> rhso d -> rhsd[d/d'] {or let d'=d in rhsd}
711 which merges two cases in one case when -- the default alternative of
712 the outer case scrutises the same variable as the outer case This
713 transformation is called Case Merging. It avoids that the same
714 variable is scrutinised multiple times.
716 There's a closely-related transformation:
718 case e of ==> case e of
719 p1 -> rhs1 p1 -> rhs1
721 pm -> rhsm pm -> rhsm
722 d -> case d of pn -> let d = pn in rhsn
724 ... po -> let d = po in rhso
725 po -> rhso d -> rhsd[d/d'] {or let d'=d in rhsd}
728 Here, the let's are essential, because d isn't in scope any more.
729 Sigh. Of course, they may be unused, in which case they'll be
730 eliminated on the next round. Unfortunately, we can't figure out
731 whether or not they are used at this juncture.
733 NB: The binder in a BindDefault USED TO BE guaranteed unused if the
734 scrutinee is a variable, because it'll be mapped to the scrutinised
735 variable. Hence the [v/d] substitions can be omitted.
737 ALAS, now the default binder is used by preference, so we have to
738 generate trivial lets to express the substitutions, which will be
739 eliminated on the next pass.
741 The following code handles *both* these transformations (one
742 equation for AlgAlts, one for PrimAlts):
745 mkCoCase scrut (AlgAlts outer_alts
746 (BindDefault deflt_var
747 (Case (Var scrut_var')
748 (AlgAlts inner_alts inner_deflt))))
749 | (scrut_is_var && scrut_var == scrut_var') -- First transformation
750 || deflt_var == scrut_var' -- Second transformation
751 = -- Aha! The default-absorption rule applies
752 tick CaseMerge `thenSmpl_`
753 returnSmpl (Case scrut (AlgAlts (outer_alts ++ munged_reduced_inner_alts)
754 (munge_alg_deflt deflt_var inner_deflt)))
755 -- NB: see comment in this location for the PrimAlts case
758 scrut_is_var = case scrut of {Var v -> True; other -> False}
759 scrut_var = case scrut of Var v -> v
761 -- Eliminate any inner alts which are shadowed by the outer ones
762 reduced_inner_alts = [alt | alt@(con,_,_) <- inner_alts,
763 not (con `is_elem` outer_cons)]
764 outer_cons = [con | (con,_,_) <- outer_alts]
765 is_elem = isIn "mkAlgAlts"
767 -- Add the lets if necessary
768 munged_reduced_inner_alts = map munge_alt reduced_inner_alts
770 munge_alt (con, args, rhs) = (con, args, Let (NonRec deflt_var v) rhs)
772 v | scrut_is_var = Var scrut_var
773 | otherwise = Con con (map TyArg arg_tys ++ map VarArg args)
775 arg_tys = case (getAppDataTyConExpandingDicts (idType deflt_var)) of
776 (_, arg_tys, _) -> arg_tys
778 mkCoCase scrut (PrimAlts
780 (BindDefault deflt_var (Case
782 (PrimAlts inner_alts inner_deflt))))
783 | (scrut_is_var && scrut_var == scrut_var') ||
784 deflt_var == scrut_var'
785 = -- Aha! The default-absorption rule applies
786 tick CaseMerge `thenSmpl_`
787 returnSmpl (Case scrut (PrimAlts (outer_alts ++ munged_reduced_inner_alts)
788 (munge_prim_deflt deflt_var inner_deflt)))
790 -- Nota Bene: we don't recurse to mkCoCase again, because the
791 -- default will now have a binding in it that prevents
792 -- mkCoCase doing anything useful. Much worse, in this
793 -- PrimAlts case the binding in the default branch is another
794 -- Case, so if we recurse to mkCoCase we will get into an
797 -- ToDo: think of a better way to do this. At the moment
798 -- there is at most one case merge per round. That's probably
799 -- plenty but it seems unclean somehow.
802 scrut_is_var = case scrut of {Var v -> True; other -> False}
803 scrut_var = case scrut of Var v -> v
805 -- Eliminate any inner alts which are shadowed by the outer ones
806 reduced_inner_alts = [alt | alt@(lit,_) <- inner_alts,
807 not (lit `is_elem` outer_lits)]
808 outer_lits = [lit | (lit,_) <- outer_alts]
809 is_elem = isIn "mkPrimAlts"
811 -- Add the lets (well cases actually) if necessary
812 -- The munged alternative looks like
813 -- lit -> case lit of d -> rhs
814 -- The next pass will certainly eliminate the inner case, but
815 -- it isn't easy to do so right away.
816 munged_reduced_inner_alts = map munge_alt reduced_inner_alts
819 | scrut_is_var = (lit, Case (Var scrut_var)
820 (PrimAlts [] (BindDefault deflt_var rhs)))
821 | otherwise = (lit, Case (Lit lit)
822 (PrimAlts [] (BindDefault deflt_var rhs)))
825 Now the identity-case transformation:
836 = tick CaseIdentity `thenSmpl_`
839 identity_alts (AlgAlts alts deflt) = all identity_alg_alt alts && identity_deflt deflt
840 identity_alts (PrimAlts alts deflt) = all identity_prim_alt alts && identity_deflt deflt
842 identity_alg_alt (con, args, Con con' args')
844 && and (zipWith eq_arg args args')
845 && length args == length args'
846 identity_alg_alt other
849 identity_prim_alt (lit, Lit lit') = lit == lit'
850 identity_prim_alt other = False
852 -- For the default case we want to spot both
855 -- case y of { ... ; x -> y }
856 -- as "identity" defaults
857 identity_deflt NoDefault = True
858 identity_deflt (BindDefault binder (Var x)) = x == binder ||
862 identity_deflt _ = False
864 eq_arg binder (VarArg x) = binder == x
871 mkCoCase other_scrut other_alts = returnSmpl (Case other_scrut other_alts)
874 Boring local functions used above. They simply introduce a trivial binding
875 for the binder, d', in an inner default; either
876 let d' = deflt_var in rhs
878 case deflt_var of d' -> rhs
879 depending on whether it's an algebraic or primitive case.
882 munge_prim_deflt _ NoDefault = NoDefault
884 munge_prim_deflt deflt_var (BindDefault d' rhs)
885 = BindDefault deflt_var (Case (Var deflt_var)
886 (PrimAlts [] (BindDefault d' rhs)))
888 munge_alg_deflt _ NoDefault = NoDefault
890 munge_alg_deflt deflt_var (BindDefault d' rhs)
891 = BindDefault deflt_var (Let (NonRec d' (Var deflt_var)) rhs)
893 -- This line caused a generic version of munge_deflt (ie one used for
894 -- both alg and prim) to space leak massively. No idea why.
895 -- = BindDefault deflt_var (mkCoLetUnboxedToCase (NonRec d' (Var deflt_var)) rhs)
899 cheap_eq :: InExpr -> InExpr -> Bool
900 -- A cheap equality test which bales out fast!
902 cheap_eq (Var v1) (Var v2) = v1==v2
903 cheap_eq (Lit l1) (Lit l2) = l1==l2
904 cheap_eq (Con con1 args1) (Con con2 args2)
905 = con1 == con2 && args1 `eq_args` args2
907 cheap_eq (Prim op1 args1) (Prim op2 args2)
908 = op1 ==op2 && args1 `eq_args` args2
910 cheap_eq (App f1 a1) (App f2 a2)
911 = f1 `cheap_eq` f2 && a1 `eq_arg` a2
915 -- ToDo: make CoreArg an instance of Eq
916 eq_args (a1:as1) (a2:as2) = a1 `eq_arg` a2 && as1 `eq_args` as2
920 eq_arg (LitArg l1) (LitArg l2) = l1 == l2
921 eq_arg (VarArg v1) (VarArg v2) = v1 == v2
922 eq_arg (TyArg t1) (TyArg t2) = t1 `eqTy` t2
923 eq_arg (UsageArg u1) (UsageArg u2) = u1 == u2