2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[Simplify]{The main module of the simplifier}
7 module Simplify ( simplTopBinds, simplExpr ) where
9 #include "HsVersions.h"
13 import Type hiding ( substTy, extendTvSubst, substTyVar )
16 import FamInstEnv ( FamInstEnv )
18 import MkId ( seqId, realWorldPrimId )
19 import MkCore ( mkImpossibleExpr )
21 import Name ( mkSystemVarName, isExternalName )
22 import Coercion hiding ( substCo, substTy, substCoVar, extendTvSubst )
23 import OptCoercion ( optCoercion )
24 import FamInstEnv ( topNormaliseType )
25 import DataCon ( DataCon, dataConWorkId, dataConRepStrictness )
26 import CoreMonad ( Tick(..), SimplifierMode(..) )
28 import Demand ( isStrictDmd )
29 import PprCore ( pprParendExpr, pprCoreExpr )
32 import qualified CoreSubst
34 import Rules ( lookupRule, getRules )
35 import BasicTypes ( isMarkedStrict, Arity )
36 import CostCentre ( currentCCS, pushCCisNop )
37 import TysPrim ( realWorldStatePrimTy )
38 import BasicTypes ( TopLevelFlag(..), isTopLevel, RecFlag(..) )
39 import MonadUtils ( foldlM, mapAccumLM )
40 import Maybes ( orElse, isNothing )
41 import Data.List ( mapAccumL )
48 The guts of the simplifier is in this module, but the driver loop for
49 the simplifier is in SimplCore.lhs.
52 -----------------------------------------
53 *** IMPORTANT NOTE ***
54 -----------------------------------------
55 The simplifier used to guarantee that the output had no shadowing, but
56 it does not do so any more. (Actually, it never did!) The reason is
57 documented with simplifyArgs.
60 -----------------------------------------
61 *** IMPORTANT NOTE ***
62 -----------------------------------------
63 Many parts of the simplifier return a bunch of "floats" as well as an
64 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
66 All "floats" are let-binds, not case-binds, but some non-rec lets may
67 be unlifted (with RHS ok-for-speculation).
71 -----------------------------------------
72 ORGANISATION OF FUNCTIONS
73 -----------------------------------------
75 - simplify all top-level binders
76 - for NonRec, call simplRecOrTopPair
77 - for Rec, call simplRecBind
80 ------------------------------
81 simplExpr (applied lambda) ==> simplNonRecBind
82 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
83 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
85 ------------------------------
86 simplRecBind [binders already simplfied]
87 - use simplRecOrTopPair on each pair in turn
89 simplRecOrTopPair [binder already simplified]
90 Used for: recursive bindings (top level and nested)
91 top-level non-recursive bindings
93 - check for PreInlineUnconditionally
97 Used for: non-top-level non-recursive bindings
98 beta reductions (which amount to the same thing)
99 Because it can deal with strict arts, it takes a
100 "thing-inside" and returns an expression
102 - check for PreInlineUnconditionally
103 - simplify binder, including its IdInfo
112 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
113 Used for: binding case-binder and constr args in a known-constructor case
114 - check for PreInLineUnconditionally
118 ------------------------------
119 simplLazyBind: [binder already simplified, RHS not]
120 Used for: recursive bindings (top level and nested)
121 top-level non-recursive bindings
122 non-top-level, but *lazy* non-recursive bindings
123 [must not be strict or unboxed]
124 Returns floats + an augmented environment, not an expression
125 - substituteIdInfo and add result to in-scope
126 [so that rules are available in rec rhs]
129 - float if exposes constructor or PAP
133 completeNonRecX: [binder and rhs both simplified]
134 - if the the thing needs case binding (unlifted and not ok-for-spec)
140 completeBind: [given a simplified RHS]
141 [used for both rec and non-rec bindings, top level and not]
142 - try PostInlineUnconditionally
143 - add unfolding [this is the only place we add an unfolding]
148 Right hand sides and arguments
149 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
150 In many ways we want to treat
151 (a) the right hand side of a let(rec), and
152 (b) a function argument
153 in the same way. But not always! In particular, we would
154 like to leave these arguments exactly as they are, so they
155 will match a RULE more easily.
160 It's harder to make the rule match if we ANF-ise the constructor,
161 or eta-expand the PAP:
163 f (let { a = g x; b = h x } in (a,b))
166 On the other hand if we see the let-defns
171 then we *do* want to ANF-ise and eta-expand, so that p and q
172 can be safely inlined.
174 Even floating lets out is a bit dubious. For let RHS's we float lets
175 out if that exposes a value, so that the value can be inlined more vigorously.
178 r = let x = e in (x,x)
180 Here, if we float the let out we'll expose a nice constructor. We did experiments
181 that showed this to be a generally good thing. But it was a bad thing to float
182 lets out unconditionally, because that meant they got allocated more often.
184 For function arguments, there's less reason to expose a constructor (it won't
185 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
186 So for the moment we don't float lets out of function arguments either.
191 For eta expansion, we want to catch things like
193 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
195 If the \x was on the RHS of a let, we'd eta expand to bring the two
196 lambdas together. And in general that's a good thing to do. Perhaps
197 we should eta expand wherever we find a (value) lambda? Then the eta
198 expansion at a let RHS can concentrate solely on the PAP case.
201 %************************************************************************
203 \subsection{Bindings}
205 %************************************************************************
208 simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv
210 simplTopBinds env0 binds0
211 = do { -- Put all the top-level binders into scope at the start
212 -- so that if a transformation rule has unexpectedly brought
213 -- anything into scope, then we don't get a complaint about that.
214 -- It's rather as if the top-level binders were imported.
215 ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
216 ; dflags <- getDOptsSmpl
217 ; let dump_flag = dopt Opt_D_verbose_core2core dflags
218 ; env2 <- simpl_binds dump_flag env1 binds0
219 ; freeTick SimplifierDone
222 -- We need to track the zapped top-level binders, because
223 -- they should have their fragile IdInfo zapped (notably occurrence info)
224 -- That's why we run down binds and bndrs' simultaneously.
226 -- The dump-flag emits a trace for each top-level binding, which
227 -- helps to locate the tracing for inlining and rule firing
228 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
229 simpl_binds _ env [] = return env
230 simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
232 ; simpl_binds dump env' binds }
234 trace_bind True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
235 trace_bind False _ = \x -> x
237 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
238 simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel NonRecursive b b' r
240 (env', b') = addBndrRules env b (lookupRecBndr env b)
244 %************************************************************************
246 \subsection{Lazy bindings}
248 %************************************************************************
250 simplRecBind is used for
251 * recursive bindings only
254 simplRecBind :: SimplEnv -> TopLevelFlag
257 simplRecBind env0 top_lvl pairs0
258 = do { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
259 ; env1 <- go (zapFloats env_with_info) triples
260 ; return (env0 `addRecFloats` env1) }
261 -- addFloats adds the floats from env1,
262 -- _and_ updates env0 with the in-scope set from env1
264 add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
265 -- Add the (substituted) rules to the binder
266 add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
268 (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
270 go env [] = return env
272 go env ((old_bndr, new_bndr, rhs) : pairs)
273 = do { env' <- simplRecOrTopPair env top_lvl Recursive old_bndr new_bndr rhs
277 simplOrTopPair is used for
278 * recursive bindings (whether top level or not)
279 * top-level non-recursive bindings
281 It assumes the binder has already been simplified, but not its IdInfo.
284 simplRecOrTopPair :: SimplEnv
285 -> TopLevelFlag -> RecFlag
286 -> InId -> OutBndr -> InExpr -- Binder and rhs
287 -> SimplM SimplEnv -- Returns an env that includes the binding
289 simplRecOrTopPair env top_lvl is_rec old_bndr new_bndr rhs
290 | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
291 = do { tick (PreInlineUnconditionally old_bndr)
292 ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
295 = simplLazyBind env top_lvl is_rec old_bndr new_bndr rhs env
299 simplLazyBind is used for
300 * [simplRecOrTopPair] recursive bindings (whether top level or not)
301 * [simplRecOrTopPair] top-level non-recursive bindings
302 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
305 1. It assumes that the binder is *already* simplified,
306 and is in scope, and its IdInfo too, except unfolding
308 2. It assumes that the binder type is lifted.
310 3. It does not check for pre-inline-unconditionallly;
311 that should have been done already.
314 simplLazyBind :: SimplEnv
315 -> TopLevelFlag -> RecFlag
316 -> InId -> OutId -- Binder, both pre-and post simpl
317 -- The OutId has IdInfo, except arity, unfolding
318 -> InExpr -> SimplEnv -- The RHS and its environment
321 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
322 = -- pprTrace "simplLazyBind" ((ppr bndr <+> ppr bndr1) $$ ppr rhs $$ ppr (seIdSubst rhs_se)) $
323 do { let rhs_env = rhs_se `setInScope` env
324 (tvs, body) = case collectTyBinders rhs of
325 (tvs, body) | not_lam body -> (tvs,body)
326 | otherwise -> ([], rhs)
327 not_lam (Lam _ _) = False
329 -- Do not do the "abstract tyyvar" thing if there's
330 -- a lambda inside, becuase it defeats eta-reduction
331 -- f = /\a. \x. g a x
334 ; (body_env, tvs') <- simplBinders rhs_env tvs
335 -- See Note [Floating and type abstraction] in SimplUtils
338 ; (body_env1, body1) <- simplExprF body_env body mkRhsStop
339 -- ANF-ise a constructor or PAP rhs
340 ; (body_env2, body2) <- prepareRhs top_lvl body_env1 bndr1 body1
343 <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
344 then -- No floating, revert to body1
345 do { rhs' <- mkLam env tvs' (wrapFloats body_env1 body1)
346 ; return (env, rhs') }
348 else if null tvs then -- Simple floating
349 do { tick LetFloatFromLet
350 ; return (addFloats env body_env2, body2) }
352 else -- Do type-abstraction first
353 do { tick LetFloatFromLet
354 ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
355 ; rhs' <- mkLam env tvs' body3
356 ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
357 ; return (env', rhs') }
359 ; completeBind env' top_lvl bndr bndr1 rhs' }
362 A specialised variant of simplNonRec used when the RHS is already simplified,
363 notably in knownCon. It uses case-binding where necessary.
366 simplNonRecX :: SimplEnv
367 -> InId -- Old binder
368 -> OutExpr -- Simplified RHS
371 simplNonRecX env bndr new_rhs
372 | isDeadBinder bndr -- Not uncommon; e.g. case (a,b) of b { (p,q) -> p }
373 = return env -- Here b is dead, and we avoid creating
374 | Coercion co <- new_rhs
375 = return (extendCvSubst env bndr co)
376 | otherwise -- the binding b = (a,b)
377 = do { (env', bndr') <- simplBinder env bndr
378 ; completeNonRecX NotTopLevel env' (isStrictId bndr) bndr bndr' new_rhs }
379 -- simplNonRecX is only used for NotTopLevel things
381 completeNonRecX :: TopLevelFlag -> SimplEnv
383 -> InId -- Old binder
384 -> OutId -- New binder
385 -> OutExpr -- Simplified RHS
388 completeNonRecX top_lvl env is_strict old_bndr new_bndr new_rhs
389 = do { (env1, rhs1) <- prepareRhs top_lvl (zapFloats env) new_bndr new_rhs
391 if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
392 then do { tick LetFloatFromLet
393 ; return (addFloats env env1, rhs1) } -- Add the floats to the main env
394 else return (env, wrapFloats env1 rhs1) -- Wrap the floats around the RHS
395 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
398 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
399 Doing so risks exponential behaviour, because new_rhs has been simplified once already
400 In the cases described by the folowing commment, postInlineUnconditionally will
401 catch many of the relevant cases.
402 -- This happens; for example, the case_bndr during case of
403 -- known constructor: case (a,b) of x { (p,q) -> ... }
404 -- Here x isn't mentioned in the RHS, so we don't want to
405 -- create the (dead) let-binding let x = (a,b) in ...
407 -- Similarly, single occurrences can be inlined vigourously
408 -- e.g. case (f x, g y) of (a,b) -> ....
409 -- If a,b occur once we can avoid constructing the let binding for them.
411 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
412 -- Consider case I# (quotInt# x y) of
413 -- I# v -> let w = J# v in ...
414 -- If we gaily inline (quotInt# x y) for v, we end up building an
416 -- let w = J# (quotInt# x y) in ...
417 -- because quotInt# can fail.
419 | preInlineUnconditionally env NotTopLevel bndr new_rhs
420 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
423 ----------------------------------
424 prepareRhs takes a putative RHS, checks whether it's a PAP or
425 constructor application and, if so, converts it to ANF, so that the
426 resulting thing can be inlined more easily. Thus
433 We also want to deal well cases like this
434 v = (f e1 `cast` co) e2
435 Here we want to make e1,e2 trivial and get
436 x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
437 That's what the 'go' loop in prepareRhs does
440 prepareRhs :: TopLevelFlag -> SimplEnv -> OutId -> OutExpr -> SimplM (SimplEnv, OutExpr)
441 -- Adds new floats to the env iff that allows us to return a good RHS
442 prepareRhs top_lvl env id (Cast rhs co) -- Note [Float coercions]
443 | Pair ty1 _ty2 <- coercionKind co -- Do *not* do this if rhs has an unlifted type
444 , not (isUnLiftedType ty1) -- see Note [Float coercions (unlifted)]
445 = do { (env', rhs') <- makeTrivialWithInfo top_lvl env sanitised_info rhs
446 ; return (env', Cast rhs' co) }
448 sanitised_info = vanillaIdInfo `setStrictnessInfo` strictnessInfo info
449 `setDemandInfo` demandInfo info
452 prepareRhs top_lvl env0 _ rhs0
453 = do { (_is_exp, env1, rhs1) <- go 0 env0 rhs0
454 ; return (env1, rhs1) }
456 go n_val_args env (Cast rhs co)
457 = do { (is_exp, env', rhs') <- go n_val_args env rhs
458 ; return (is_exp, env', Cast rhs' co) }
459 go n_val_args env (App fun (Type ty))
460 = do { (is_exp, env', rhs') <- go n_val_args env fun
461 ; return (is_exp, env', App rhs' (Type ty)) }
462 go n_val_args env (App fun arg)
463 = do { (is_exp, env', fun') <- go (n_val_args+1) env fun
465 True -> do { (env'', arg') <- makeTrivial top_lvl env' arg
466 ; return (True, env'', App fun' arg') }
467 False -> return (False, env, App fun arg) }
468 go n_val_args env (Var fun)
469 = return (is_exp, env, Var fun)
471 is_exp = isExpandableApp fun n_val_args -- The fun a constructor or PAP
472 -- See Note [CONLIKE pragma] in BasicTypes
473 -- The definition of is_exp should match that in
474 -- OccurAnal.occAnalApp
477 = return (False, env, other)
481 Note [Float coercions]
482 ~~~~~~~~~~~~~~~~~~~~~~
483 When we find the binding
485 we'd like to transform it to
487 x = x `cast` co -- A trivial binding
488 There's a chance that e will be a constructor application or function, or something
489 like that, so moving the coerion to the usage site may well cancel the coersions
490 and lead to further optimisation. Example:
493 data instance T Int = T Int
495 foo :: Int -> Int -> Int
500 go n = case x of { T m -> go (n-m) }
501 -- This case should optimise
503 Note [Preserve strictness when floating coercions]
504 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
505 In the Note [Float coercions] transformation, keep the strictness info.
507 f = e `cast` co -- f has strictness SSL
509 f' = e -- f' also has strictness SSL
510 f = f' `cast` co -- f still has strictness SSL
512 Its not wrong to drop it on the floor, but better to keep it.
514 Note [Float coercions (unlifted)]
515 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
516 BUT don't do [Float coercions] if 'e' has an unlifted type.
519 foo :: Int = (error (# Int,Int #) "urk")
520 `cast` CoUnsafe (# Int,Int #) Int
522 If do the makeTrivial thing to the error call, we'll get
523 foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
524 But 'v' isn't in scope!
526 These strange casts can happen as a result of case-of-case
527 bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
532 makeTrivial :: TopLevelFlag -> SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
533 -- Binds the expression to a variable, if it's not trivial, returning the variable
534 makeTrivial top_lvl env expr = makeTrivialWithInfo top_lvl env vanillaIdInfo expr
536 makeTrivialWithInfo :: TopLevelFlag -> SimplEnv -> IdInfo
537 -> OutExpr -> SimplM (SimplEnv, OutExpr)
538 -- Propagate strictness and demand info to the new binder
539 -- Note [Preserve strictness when floating coercions]
540 -- Returned SimplEnv has same substitution as incoming one
541 makeTrivialWithInfo top_lvl env info expr
542 | exprIsTrivial expr -- Already trivial
543 || not (bindingOk top_lvl expr expr_ty) -- Cannot trivialise
544 -- See Note [Cannot trivialise]
546 | otherwise -- See Note [Take care] below
547 = do { uniq <- getUniqueM
548 ; let name = mkSystemVarName uniq (fsLit "a")
549 var = mkLocalIdWithInfo name expr_ty info
550 ; env' <- completeNonRecX top_lvl env False var var expr
551 ; expr' <- simplVar env' var
552 ; return (env', expr') }
553 -- The simplVar is needed becase we're constructing a new binding
555 -- And if rhs is of form (rhs1 |> co), then we might get
558 -- and now a's RHS is trivial and can be substituted out, and that
559 -- is what completeNonRecX will do
560 -- To put it another way, it's as if we'd simplified
561 -- let var = e in var
563 expr_ty = exprType expr
565 bindingOk :: TopLevelFlag -> CoreExpr -> Type -> Bool
566 -- True iff we can have a binding of this expression at this level
567 -- Precondition: the type is the type of the expression
568 bindingOk top_lvl _ expr_ty
569 | isTopLevel top_lvl = not (isUnLiftedType expr_ty)
573 Note [Cannot trivialise]
574 ~~~~~~~~~~~~~~~~~~~~~~~~
581 Then we can't ANF-ise foo, even though we'd like to, because
582 we can't make a top-level binding for the Addr# (f 3). And if
583 so we don't want to turn it into
584 foo = let x = f 3 in Bar x
585 because we'll just end up inlining x back, and that makes the
586 simplifier loop. Better not to ANF-ise it at all.
588 A case in point is literal strings (a MachStr is not regarded as
593 We don't want to ANF-ise this.
595 %************************************************************************
597 \subsection{Completing a lazy binding}
599 %************************************************************************
602 * deals only with Ids, not TyVars
603 * takes an already-simplified binder and RHS
604 * is used for both recursive and non-recursive bindings
605 * is used for both top-level and non-top-level bindings
607 It does the following:
608 - tries discarding a dead binding
609 - tries PostInlineUnconditionally
610 - add unfolding [this is the only place we add an unfolding]
613 It does *not* attempt to do let-to-case. Why? Because it is used for
614 - top-level bindings (when let-to-case is impossible)
615 - many situations where the "rhs" is known to be a WHNF
616 (so let-to-case is inappropriate).
618 Nor does it do the atomic-argument thing
621 completeBind :: SimplEnv
622 -> TopLevelFlag -- Flag stuck into unfolding
623 -> InId -- Old binder
624 -> OutId -> OutExpr -- New binder and RHS
626 -- completeBind may choose to do its work
627 -- * by extending the substitution (e.g. let x = y in ...)
628 -- * or by adding to the floats in the envt
630 completeBind env top_lvl old_bndr new_bndr new_rhs
631 = ASSERT( isId new_bndr )
632 do { let old_info = idInfo old_bndr
633 old_unf = unfoldingInfo old_info
634 occ_info = occInfo old_info
636 -- Do eta-expansion on the RHS of the binding
637 -- See Note [Eta-expanding at let bindings] in SimplUtils
638 ; (new_arity, final_rhs) <- tryEtaExpand env new_bndr new_rhs
640 -- Simplify the unfolding
641 ; new_unfolding <- simplUnfolding env top_lvl old_bndr final_rhs old_unf
643 ; if postInlineUnconditionally env top_lvl new_bndr occ_info final_rhs new_unfolding
644 -- Inline and discard the binding
645 then do { tick (PostInlineUnconditionally old_bndr)
646 ; -- pprTrace "postInlineUnconditionally"
647 -- (ppr old_bndr <+> equals <+> ppr final_rhs $$ ppr occ_info) $
648 return (extendIdSubst env old_bndr (DoneEx final_rhs)) }
649 -- Use the substitution to make quite, quite sure that the
650 -- substitution will happen, since we are going to discard the binding
652 do { let info1 = idInfo new_bndr `setArityInfo` new_arity
654 -- Unfolding info: Note [Setting the new unfolding]
655 info2 = info1 `setUnfoldingInfo` new_unfolding
657 -- Demand info: Note [Setting the demand info]
658 info3 | isEvaldUnfolding new_unfolding = zapDemandInfo info2 `orElse` info2
661 final_id = new_bndr `setIdInfo` info3
663 ; -- pprTrace "Binding" (ppr final_id <+> ppr new_unfolding) $
664 return (addNonRec env final_id final_rhs) } }
665 -- The addNonRec adds it to the in-scope set too
667 ------------------------------
668 addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv
669 -- Add a new binding to the environment, complete with its unfolding
670 -- but *do not* do postInlineUnconditionally, because we have already
671 -- processed some of the scope of the binding
672 -- We still want the unfolding though. Consider
674 -- x = /\a. let y = ... in Just y
676 -- Then we float the y-binding out (via abstractFloats and addPolyBind)
677 -- but 'x' may well then be inlined in 'body' in which case we'd like the
678 -- opportunity to inline 'y' too.
680 addPolyBind top_lvl env (NonRec poly_id rhs)
681 = do { unfolding <- simplUnfolding env top_lvl poly_id rhs noUnfolding
682 -- Assumes that poly_id did not have an INLINE prag
683 -- which is perhaps wrong. ToDo: think about this
684 ; let final_id = setIdInfo poly_id $
685 idInfo poly_id `setUnfoldingInfo` unfolding
686 `setArityInfo` exprArity rhs
688 ; return (addNonRec env final_id rhs) }
690 addPolyBind _ env bind@(Rec _)
691 = return (extendFloats env bind)
692 -- Hack: letrecs are more awkward, so we extend "by steam"
693 -- without adding unfoldings etc. At worst this leads to
694 -- more simplifier iterations
696 ------------------------------
697 simplUnfolding :: SimplEnv-> TopLevelFlag
700 -> Unfolding -> SimplM Unfolding
701 -- Note [Setting the new unfolding]
702 simplUnfolding env _ _ _ (DFunUnfolding ar con ops)
703 = return (DFunUnfolding ar con ops')
705 ops' = map (fmap (substExpr (text "simplUnfolding") env)) ops
707 simplUnfolding env top_lvl id _
708 (CoreUnfolding { uf_tmpl = expr, uf_arity = arity
709 , uf_src = src, uf_guidance = guide })
711 = do { expr' <- simplExpr rule_env expr
712 ; let src' = CoreSubst.substUnfoldingSource (mkCoreSubst (text "inline-unf") env) src
713 is_top_lvl = isTopLevel top_lvl
715 UnfWhen sat_ok _ -- Happens for INLINE things
716 -> let guide' = UnfWhen sat_ok (inlineBoringOk expr')
717 -- Refresh the boring-ok flag, in case expr'
718 -- has got small. This happens, notably in the inlinings
719 -- for dfuns for single-method classes; see
720 -- Note [Single-method classes] in TcInstDcls.
721 -- A test case is Trac #4138
722 in return (mkCoreUnfolding src' is_top_lvl expr' arity guide')
723 -- See Note [Top-level flag on inline rules] in CoreUnfold
725 _other -- Happens for INLINABLE things
726 -> let bottoming = isBottomingId id
727 in bottoming `seq` -- See Note [Force bottoming field]
728 return (mkUnfolding src' is_top_lvl bottoming expr')
729 -- If the guidance is UnfIfGoodArgs, this is an INLINABLE
730 -- unfolding, and we need to make sure the guidance is kept up
731 -- to date with respect to any changes in the unfolding.
734 act = idInlineActivation id
735 rule_env = updMode (updModeForInlineRules act) env
736 -- See Note [Simplifying inside InlineRules] in SimplUtils
738 simplUnfolding _ top_lvl id new_rhs _
739 = let bottoming = isBottomingId id
740 in bottoming `seq` -- See Note [Force bottoming field]
741 return (mkUnfolding InlineRhs (isTopLevel top_lvl) bottoming new_rhs)
742 -- We make an unfolding *even for loop-breakers*.
743 -- Reason: (a) It might be useful to know that they are WHNF
744 -- (b) In TidyPgm we currently assume that, if we want to
745 -- expose the unfolding then indeed we *have* an unfolding
746 -- to expose. (We could instead use the RHS, but currently
747 -- we don't.) The simple thing is always to have one.
750 Note [Force bottoming field]
751 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
752 We need to force bottoming, or the new unfolding holds
753 on to the old unfolding (which is part of the id).
755 Note [Arity decrease]
756 ~~~~~~~~~~~~~~~~~~~~~
757 Generally speaking the arity of a binding should not decrease. But it *can*
758 legitimately happen becuase of RULES. Eg
760 where g has arity 2, will have arity 2. But if there's a rewrite rule
762 where h has arity 1, then f's arity will decrease. Here's a real-life example,
763 which is in the output of Specialise:
766 $dm {Arity 2} = \d.\x. op d
767 {-# RULES forall d. $dm Int d = $s$dm #-}
769 dInt = MkD .... opInt ...
770 opInt {Arity 1} = $dm dInt
772 $s$dm {Arity 0} = \x. op dInt }
774 Here opInt has arity 1; but when we apply the rule its arity drops to 0.
775 That's why Specialise goes to a little trouble to pin the right arity
776 on specialised functions too.
778 Note [Setting the new unfolding]
779 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
780 * If there's an INLINE pragma, we simplify the RHS gently. Maybe we
781 should do nothing at all, but simplifying gently might get rid of
784 * If not, we make an unfolding from the new RHS. But *only* for
785 non-loop-breakers. Making loop breakers not have an unfolding at all
786 means that we can avoid tests in exprIsConApp, for example. This is
787 important: if exprIsConApp says 'yes' for a recursive thing, then we
788 can get into an infinite loop
790 If there's an InlineRule on a loop breaker, we hang on to the inlining.
791 It's pretty dodgy, but the user did say 'INLINE'. May need to revisit
794 Note [Setting the demand info]
795 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
796 If the unfolding is a value, the demand info may
797 go pear-shaped, so we nuke it. Example:
799 case x of (p,q) -> h p q x
800 Here x is certainly demanded. But after we've nuked
801 the case, we'll get just
802 let x = (a,b) in h a b x
803 and now x is not demanded (I'm assuming h is lazy)
804 This really happens. Similarly
805 let f = \x -> e in ...f..f...
806 After inlining f at some of its call sites the original binding may
807 (for example) be no longer strictly demanded.
808 The solution here is a bit ad hoc...
811 %************************************************************************
813 \subsection[Simplify-simplExpr]{The main function: simplExpr}
815 %************************************************************************
817 The reason for this OutExprStuff stuff is that we want to float *after*
818 simplifying a RHS, not before. If we do so naively we get quadratic
819 behaviour as things float out.
821 To see why it's important to do it after, consider this (real) example:
835 a -- Can't inline a this round, cos it appears twice
839 Each of the ==> steps is a round of simplification. We'd save a
840 whole round if we float first. This can cascade. Consider
845 let f = let d1 = ..d.. in \y -> e
849 in \x -> ...(\y ->e)...
851 Only in this second round can the \y be applied, and it
852 might do the same again.
856 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
857 simplExpr env expr = simplExprC env expr mkBoringStop
859 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
860 -- Simplify an expression, given a continuation
861 simplExprC env expr cont
862 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
863 do { (env', expr') <- simplExprF (zapFloats env) expr cont
864 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
865 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
866 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
867 return (wrapFloats env' expr') }
869 --------------------------------------------------
870 simplExprF :: SimplEnv -> InExpr -> SimplCont
871 -> SimplM (SimplEnv, OutExpr)
873 simplExprF env e cont
874 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
875 simplExprF' env e cont
877 simplExprF' :: SimplEnv -> InExpr -> SimplCont
878 -> SimplM (SimplEnv, OutExpr)
879 simplExprF' env (Var v) cont = simplIdF env v cont
880 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
881 simplExprF' env (Note n expr) cont = simplNote env n expr cont
882 simplExprF' env (Cast body co) cont = simplCast env body co cont
883 simplExprF' env (App fun arg) cont = simplExprF env fun $
884 ApplyTo NoDup arg env cont
886 simplExprF' env expr@(Lam {}) cont
887 = simplLam env zapped_bndrs body cont
888 -- The main issue here is under-saturated lambdas
889 -- (\x1. \x2. e) arg1
890 -- Here x1 might have "occurs-once" occ-info, because occ-info
891 -- is computed assuming that a group of lambdas is applied
892 -- all at once. If there are too few args, we must zap the
893 -- occ-info, UNLESS the remaining binders are one-shot
895 (bndrs, body) = collectBinders expr
896 zapped_bndrs | need_to_zap = map zap bndrs
899 need_to_zap = any zappable_bndr (drop n_args bndrs)
900 n_args = countArgs cont
901 -- NB: countArgs counts all the args (incl type args)
902 -- and likewise drop counts all binders (incl type lambdas)
904 zappable_bndr b = isId b && not (isOneShotBndr b)
905 zap b | isTyVar b = b
906 | otherwise = zapLamIdInfo b
908 simplExprF' env (Type ty) cont
909 = ASSERT( contIsRhsOrArg cont )
910 rebuild env (Type (substTy env ty)) cont
912 simplExprF' env (Coercion co) cont
913 = ASSERT( contIsRhsOrArg cont )
914 do { co' <- simplCoercion env co
915 ; rebuild env (Coercion co') cont }
917 simplExprF' env (Case scrut bndr _ alts) cont
918 | sm_case_case (getMode env)
919 = -- Simplify the scrutinee with a Select continuation
920 simplExprF env scrut (Select NoDup bndr alts env cont)
923 = -- If case-of-case is off, simply simplify the case expression
924 -- in a vanilla Stop context, and rebuild the result around it
925 do { case_expr' <- simplExprC env scrut
926 (Select NoDup bndr alts env mkBoringStop)
927 ; rebuild env case_expr' cont }
929 simplExprF' env (Let (Rec pairs) body) cont
930 = do { env' <- simplRecBndrs env (map fst pairs)
931 -- NB: bndrs' don't have unfoldings or rules
932 -- We add them as we go down
934 ; env'' <- simplRecBind env' NotTopLevel pairs
935 ; simplExprF env'' body cont }
937 simplExprF' env (Let (NonRec bndr rhs) body) cont
938 = simplNonRecE env bndr (rhs, env) ([], body) cont
940 ---------------------------------
941 simplType :: SimplEnv -> InType -> SimplM OutType
942 -- Kept monadic just so we can do the seqType
944 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
945 seqType new_ty `seq` return new_ty
947 new_ty = substTy env ty
949 ---------------------------------
950 simplCoercion :: SimplEnv -> InCoercion -> SimplM OutCoercion
952 = -- pprTrace "simplCoercion" (ppr co $$ ppr (getCvSubst env)) $
953 seqCo new_co `seq` return new_co
955 new_co = optCoercion (getCvSubst env) co
959 %************************************************************************
961 \subsection{The main rebuilder}
963 %************************************************************************
966 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
967 -- At this point the substitution in the SimplEnv should be irrelevant
968 -- only the in-scope set and floats should matter
969 rebuild env expr cont
971 Stop {} -> return (env, expr)
972 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
973 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
974 StrictArg info _ cont -> rebuildCall env (info `addArgTo` expr) cont
975 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
976 ; simplLam env' bs body cont }
977 ApplyTo dup_flag arg se cont -- See Note [Avoid redundant simplification]
978 | isSimplified dup_flag -> rebuild env (App expr arg) cont
979 | otherwise -> do { arg' <- simplExpr (se `setInScope` env) arg
980 ; rebuild env (App expr arg') cont }
984 %************************************************************************
988 %************************************************************************
991 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
992 -> SimplM (SimplEnv, OutExpr)
993 simplCast env body co0 cont0
994 = do { co1 <- simplCoercion env co0
995 ; simplExprF env body (addCoerce co1 cont0) }
997 addCoerce co cont = add_coerce co (coercionKind co) cont
999 add_coerce _co (Pair s1 k1) cont -- co :: ty~ty
1000 | s1 `eqType` k1 = cont -- is a no-op
1002 add_coerce co1 (Pair s1 _k2) (CoerceIt co2 cont)
1003 | (Pair _l1 t1) <- coercionKind co2
1004 -- e |> (g1 :: S1~L) |> (g2 :: L~T1)
1007 -- e |> (g1 . g2 :: S1~T1) otherwise
1009 -- For example, in the initial form of a worker
1010 -- we may find (coerce T (coerce S (\x.e))) y
1011 -- and we'd like it to simplify to e[y/x] in one round
1012 -- of simplification
1013 , s1 `eqType` t1 = cont -- The coerces cancel out
1014 | otherwise = CoerceIt (mkTransCo co1 co2) cont
1016 add_coerce co (Pair s1s2 _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
1017 -- (f |> g) ty ---> (f ty) |> (g @ ty)
1018 -- This implements the PushT rule from the paper
1019 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
1020 = ASSERT( isTyVar tyvar )
1021 ApplyTo Simplified (Type arg_ty') (zapSubstEnv arg_se) (addCoerce new_cast cont)
1023 new_cast = mkInstCo co arg_ty'
1024 arg_ty' | isSimplified dup = arg_ty
1025 | otherwise = substTy (arg_se `setInScope` env) arg_ty
1028 add_coerce co (Pair s1s2 _t1t2) (ApplyTo dup (Coercion arg_co) arg_se cont)
1029 -- This implements the PushC rule from the paper
1030 | Just (covar,_) <- splitForAllTy_maybe s1s2
1031 = ASSERT( isCoVar covar )
1032 ApplyTo Simplified (Coercion new_arg_co) (zapSubstEnv arg_se) (addCoerce co1 cont)
1034 [co0, co1] = decomposeCo 2 co
1035 [co00, co01] = decomposeCo 2 co0
1037 arg_co' | isSimplified dup = arg_co
1038 | otherwise = substCo (arg_se `setInScope` env) arg_co
1039 new_arg_co = co00 `mkTransCo`
1044 add_coerce co (Pair s1s2 t1t2) (ApplyTo dup arg arg_se cont)
1045 | isFunTy s1s2 -- This implements the Push rule from the paper
1046 , isFunTy t1t2 -- Check t1t2 to ensure 'arg' is a value arg
1047 -- (e |> (g :: s1s2 ~ t1->t2)) f
1049 -- (e (f |> (arg g :: t1~s1))
1050 -- |> (res g :: s2->t2)
1052 -- t1t2 must be a function type, t1->t2, because it's applied
1053 -- to something but s1s2 might conceivably not be
1055 -- When we build the ApplyTo we can't mix the out-types
1056 -- with the InExpr in the argument, so we simply substitute
1057 -- to make it all consistent. It's a bit messy.
1058 -- But it isn't a common case.
1060 -- Example of use: Trac #995
1061 = ApplyTo dup new_arg (zapSubstEnv arg_se) (addCoerce co2 cont)
1063 -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
1064 -- t2 ~ s2 with left and right on the curried form:
1065 -- (->) t1 t2 ~ (->) s1 s2
1066 [co1, co2] = decomposeCo 2 co
1067 new_arg = mkCoerce (mkSymCo co1) arg'
1068 arg' = substExpr (text "move-cast") (arg_se `setInScope` env) arg
1070 add_coerce co _ cont = CoerceIt co cont
1074 %************************************************************************
1076 \subsection{Lambdas}
1078 %************************************************************************
1080 Note [Zap unfolding when beta-reducing]
1081 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1082 Lambda-bound variables can have stable unfoldings, such as
1083 $j = \x. \b{Unf=Just x}. e
1084 See Note [Case binders and join points] below; the unfolding for lets
1085 us optimise e better. However when we beta-reduce it we want to
1086 revert to using the actual value, otherwise we can end up in the
1089 let b{Unf=Just x} = y
1091 Here it'd be far better to drop the unfolding and use the actual RHS.
1094 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
1095 -> SimplM (SimplEnv, OutExpr)
1097 simplLam env [] body cont = simplExprF env body cont
1100 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
1101 = do { tick (BetaReduction bndr)
1102 ; simplNonRecE env (zap_unfolding bndr) (arg, arg_se) (bndrs, body) cont }
1104 zap_unfolding bndr -- See Note [Zap unfolding when beta-reducing]
1105 | isId bndr, isStableUnfolding (realIdUnfolding bndr)
1106 = setIdUnfolding bndr NoUnfolding
1109 -- Not enough args, so there are real lambdas left to put in the result
1110 simplLam env bndrs body cont
1111 = do { (env', bndrs') <- simplLamBndrs env bndrs
1112 ; body' <- simplExpr env' body
1113 ; new_lam <- mkLam env' bndrs' body'
1114 ; rebuild env' new_lam cont }
1117 simplNonRecE :: SimplEnv
1118 -> InBndr -- The binder
1119 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
1120 -> ([InBndr], InExpr) -- Body of the let/lambda
1123 -> SimplM (SimplEnv, OutExpr)
1125 -- simplNonRecE is used for
1126 -- * non-top-level non-recursive lets in expressions
1129 -- It deals with strict bindings, via the StrictBind continuation,
1130 -- which may abort the whole process
1132 -- The "body" of the binding comes as a pair of ([InId],InExpr)
1133 -- representing a lambda; so we recurse back to simplLam
1134 -- Why? Because of the binder-occ-info-zapping done before
1135 -- the call to simplLam in simplExprF (Lam ...)
1137 -- First deal with type applications and type lets
1138 -- (/\a. e) (Type ty) and (let a = Type ty in e)
1139 simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
1140 = ASSERT( isTyVar bndr )
1141 do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
1142 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
1144 simplNonRecE env bndr (Coercion co_arg, rhs_se) (bndrs, body) cont
1145 = ASSERT( isCoVar bndr )
1146 do { co_arg' <- simplCoercion (rhs_se `setInScope` env) co_arg
1147 ; simplLam (extendCvSubst env bndr co_arg') bndrs body cont }
1149 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
1150 | preInlineUnconditionally env NotTopLevel bndr rhs
1151 = do { tick (PreInlineUnconditionally bndr)
1152 ; -- pprTrace "preInlineUncond" (ppr bndr <+> ppr rhs) $
1153 simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
1156 = do { simplExprF (rhs_se `setFloats` env) rhs
1157 (StrictBind bndr bndrs body env cont) }
1160 = ASSERT( not (isTyVar bndr) )
1161 do { (env1, bndr1) <- simplNonRecBndr env bndr
1162 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
1163 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
1164 ; simplLam env3 bndrs body cont }
1168 %************************************************************************
1172 %************************************************************************
1175 -- Hack alert: we only distinguish subsumed cost centre stacks for the
1176 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
1177 simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
1178 -> SimplM (SimplEnv, OutExpr)
1179 simplNote env (SCC cc) e cont
1180 | pushCCisNop cc (getEnclosingCC env) -- scc "f" (...(scc "f" e)...)
1181 = simplExprF env e cont -- ==> scc "f" (...e...)
1183 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
1184 ; rebuild env (mkSCC cc e') cont }
1186 simplNote env (CoreNote s) e cont
1187 = do { e' <- simplExpr env e
1188 ; rebuild env (Note (CoreNote s) e') cont }
1192 %************************************************************************
1196 %************************************************************************
1199 simplVar :: SimplEnv -> InVar -> SimplM OutExpr
1200 -- Look up an InVar in the environment
1202 | isTyVar var = return (Type (substTyVar env var))
1203 | isCoVar var = return (Coercion (substCoVar env var))
1205 = case substId env var of
1206 DoneId var1 -> return (Var var1)
1207 DoneEx e -> return e
1208 ContEx tvs cvs ids e -> simplExpr (setSubstEnv env tvs cvs ids) e
1210 simplIdF :: SimplEnv -> InId -> SimplCont -> SimplM (SimplEnv, OutExpr)
1211 simplIdF env var cont
1212 = case substId env var of
1213 DoneEx e -> simplExprF (zapSubstEnv env) e cont
1214 ContEx tvs cvs ids e -> simplExprF (setSubstEnv env tvs cvs ids) e cont
1215 DoneId var1 -> completeCall env var1 cont
1216 -- Note [zapSubstEnv]
1217 -- The template is already simplified, so don't re-substitute.
1218 -- This is VITAL. Consider
1220 -- let y = \z -> ...x... in
1222 -- We'll clone the inner \x, adding x->x' in the id_subst
1223 -- Then when we inline y, we must *not* replace x by x' in
1224 -- the inlined copy!!
1226 ---------------------------------------------------------
1227 -- Dealing with a call site
1229 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1230 completeCall env var cont
1231 = do { ------------- Try inlining ----------------
1232 dflags <- getDOptsSmpl
1233 ; let (lone_variable, arg_infos, call_cont) = contArgs cont
1234 -- The args are OutExprs, obtained by *lazily* substituting
1235 -- in the args found in cont. These args are only examined
1236 -- to limited depth (unless a rule fires). But we must do
1237 -- the substitution; rule matching on un-simplified args would
1240 n_val_args = length arg_infos
1241 interesting_cont = interestingCallContext call_cont
1242 unfolding = activeUnfolding env var
1243 maybe_inline = callSiteInline dflags var unfolding
1244 lone_variable arg_infos interesting_cont
1245 ; case maybe_inline of {
1246 Just expr -- There is an inlining!
1247 -> do { tick (UnfoldingDone var)
1248 ; trace_inline dflags expr cont $
1249 simplExprF (zapSubstEnv env) expr cont }
1251 ; Nothing -> do -- No inlining!
1253 { rule_base <- getSimplRules
1254 ; let info = mkArgInfo var (getRules rule_base var) n_val_args call_cont
1255 ; rebuildCall env info cont
1258 trace_inline dflags unfolding cont stuff
1259 | not (dopt Opt_D_dump_inlinings dflags) = stuff
1260 | not (dopt Opt_D_verbose_core2core dflags)
1261 = if isExternalName (idName var) then
1262 pprDefiniteTrace "Inlining done:" (ppr var) stuff
1265 = pprDefiniteTrace ("Inlining done: " ++ showSDoc (ppr var))
1266 (vcat [text "Inlined fn: " <+> nest 2 (ppr unfolding),
1267 text "Cont: " <+> ppr cont])
1270 rebuildCall :: SimplEnv
1273 -> SimplM (SimplEnv, OutExpr)
1274 rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_strs = [] }) cont
1275 -- When we run out of strictness args, it means
1276 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1277 -- Then we want to discard the entire strict continuation. E.g.
1278 -- * case (error "hello") of { ... }
1279 -- * (error "Hello") arg
1280 -- * f (error "Hello") where f is strict
1282 -- Then, especially in the first of these cases, we'd like to discard
1283 -- the continuation, leaving just the bottoming expression. But the
1284 -- type might not be right, so we may have to add a coerce.
1285 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1286 = return (env, mk_coerce res) -- contination to discard, else we do it
1287 where -- again and again!
1288 res = mkApps (Var fun) (reverse rev_args)
1289 res_ty = exprType res
1290 cont_ty = contResultType env res_ty cont
1291 co = mkUnsafeCo res_ty cont_ty
1292 mk_coerce expr | cont_ty `eqType` res_ty = expr
1293 | otherwise = mkCoerce co expr
1295 rebuildCall env info (ApplyTo dup_flag (Type arg_ty) se cont)
1296 = do { arg_ty' <- if isSimplified dup_flag then return arg_ty
1297 else simplType (se `setInScope` env) arg_ty
1298 ; rebuildCall env (info `addArgTo` Type arg_ty') cont }
1300 rebuildCall env info (ApplyTo dup_flag (Coercion arg_co) se cont)
1301 = do { arg_co' <- if isSimplified dup_flag then return arg_co
1302 else simplCoercion (se `setInScope` env) arg_co
1303 ; rebuildCall env (info `addArgTo` Coercion arg_co') cont }
1305 rebuildCall env info@(ArgInfo { ai_encl = encl_rules
1306 , ai_strs = str:strs, ai_discs = disc:discs })
1307 (ApplyTo dup_flag arg arg_se cont)
1308 | isSimplified dup_flag -- See Note [Avoid redundant simplification]
1309 = rebuildCall env (addArgTo info' arg) cont
1311 | str -- Strict argument
1312 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1313 simplExprF (arg_se `setFloats` env) arg
1314 (StrictArg info' cci cont)
1317 | otherwise -- Lazy argument
1318 -- DO NOT float anything outside, hence simplExprC
1319 -- There is no benefit (unlike in a let-binding), and we'd
1320 -- have to be very careful about bogus strictness through
1321 -- floating a demanded let.
1322 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1324 ; rebuildCall env (addArgTo info' arg') cont }
1326 info' = info { ai_strs = strs, ai_discs = discs }
1327 cci | encl_rules || disc > 0 = ArgCtxt encl_rules -- Be keener here
1328 | otherwise = BoringCtxt -- Nothing interesting
1330 rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_rules = rules }) cont
1331 = do { -- We've accumulated a simplified call in <fun,rev_args>
1332 -- so try rewrite rules; see Note [RULEs apply to simplified arguments]
1333 -- See also Note [Rules for recursive functions]
1334 ; let args = reverse rev_args
1335 env' = zapSubstEnv env
1336 ; mb_rule <- tryRules env rules fun args cont
1338 Just (n_args, rule_rhs) -> simplExprF env' rule_rhs $
1339 pushSimplifiedArgs env' (drop n_args args) cont ;
1340 -- n_args says how many args the rule consumed
1341 ; Nothing -> rebuild env (mkApps (Var fun) args) cont -- No rules
1345 Note [RULES apply to simplified arguments]
1346 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1347 It's very desirable to try RULES once the arguments have been simplified, because
1348 doing so ensures that rule cascades work in one pass. Consider
1349 {-# RULES g (h x) = k x
1352 Then we want to rewrite (g (h x)) to (k x) and only then try f's rules. If
1353 we match f's rules against the un-simplified RHS, it won't match. This
1354 makes a particularly big difference when superclass selectors are involved:
1355 op ($p1 ($p2 (df d)))
1356 We want all this to unravel in one sweeep.
1358 Note [Avoid redundant simplification]
1359 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1360 Because RULES apply to simplified arguments, there's a danger of repeatedly
1361 simplifying already-simplified arguments. An important example is that of
1363 Here e1, e2 are simplified before the rule is applied, but don't really
1364 participate in the rule firing. So we mark them as Simplified to avoid
1365 re-simplifying them.
1369 This part of the simplifier may break the no-shadowing invariant
1371 f (...(\a -> e)...) (case y of (a,b) -> e')
1372 where f is strict in its second arg
1373 If we simplify the innermost one first we get (...(\a -> e)...)
1374 Simplifying the second arg makes us float the case out, so we end up with
1375 case y of (a,b) -> f (...(\a -> e)...) e'
1376 So the output does not have the no-shadowing invariant. However, there is
1377 no danger of getting name-capture, because when the first arg was simplified
1378 we used an in-scope set that at least mentioned all the variables free in its
1379 static environment, and that is enough.
1381 We can't just do innermost first, or we'd end up with a dual problem:
1382 case x of (a,b) -> f e (...(\a -> e')...)
1384 I spent hours trying to recover the no-shadowing invariant, but I just could
1385 not think of an elegant way to do it. The simplifier is already knee-deep in
1386 continuations. We have to keep the right in-scope set around; AND we have
1387 to get the effect that finding (error "foo") in a strict arg position will
1388 discard the entire application and replace it with (error "foo"). Getting
1389 all this at once is TOO HARD!
1392 %************************************************************************
1396 %************************************************************************
1399 tryRules :: SimplEnv -> [CoreRule]
1400 -> Id -> [OutExpr] -> SimplCont
1401 -> SimplM (Maybe (Arity, CoreExpr)) -- The arity is the number of
1402 -- args consumed by the rule
1403 tryRules env rules fn args call_cont
1407 = do { dflags <- getDOptsSmpl
1408 ; case activeRule dflags env of {
1409 Nothing -> return Nothing ; -- No rules apply
1411 case lookupRule act_fn (getUnfoldingInRuleMatch env) (getInScope env) fn args rules of {
1412 Nothing -> return Nothing ; -- No rule matches
1413 Just (rule, rule_rhs) ->
1415 do { tick (RuleFired (ru_name rule))
1416 ; trace_dump dflags rule rule_rhs $
1417 return (Just (ruleArity rule, rule_rhs)) }}}}
1419 trace_dump dflags rule rule_rhs stuff
1420 | not (dopt Opt_D_dump_rule_firings dflags)
1421 , not (dopt Opt_D_dump_rule_rewrites dflags) = stuff
1423 | not (dopt Opt_D_dump_rule_rewrites dflags)
1424 = pprDefiniteTrace "Rule fired:" (ftext (ru_name rule)) stuff
1427 = pprDefiniteTrace "Rule fired"
1428 (vcat [text "Rule:" <+> ftext (ru_name rule),
1429 text "Before:" <+> hang (ppr fn) 2 (sep (map pprParendExpr args)),
1430 text "After: " <+> pprCoreExpr rule_rhs,
1431 text "Cont: " <+> ppr call_cont])
1435 Note [Rules for recursive functions]
1436 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1437 You might think that we shouldn't apply rules for a loop breaker:
1438 doing so might give rise to an infinite loop, because a RULE is
1439 rather like an extra equation for the function:
1440 RULE: f (g x) y = x+y
1443 But it's too drastic to disable rules for loop breakers.
1444 Even the foldr/build rule would be disabled, because foldr
1445 is recursive, and hence a loop breaker:
1446 foldr k z (build g) = g k z
1447 So it's up to the programmer: rules can cause divergence
1450 %************************************************************************
1452 Rebuilding a case expression
1454 %************************************************************************
1456 Note [Case elimination]
1457 ~~~~~~~~~~~~~~~~~~~~~~~
1458 The case-elimination transformation discards redundant case expressions.
1459 Start with a simple situation:
1461 case x# of ===> let y# = x# in e
1464 (when x#, y# are of primitive type, of course). We can't (in general)
1465 do this for algebraic cases, because we might turn bottom into
1468 The code in SimplUtils.prepareAlts has the effect of generalise this
1469 idea to look for a case where we're scrutinising a variable, and we
1470 know that only the default case can match. For example:
1474 DEFAULT -> ...(case x of
1478 Here the inner case is first trimmed to have only one alternative, the
1479 DEFAULT, after which it's an instance of the previous case. This
1480 really only shows up in eliminating error-checking code.
1482 Note that SimplUtils.mkCase combines identical RHSs. So
1484 case e of ===> case e of DEFAULT -> r
1488 Now again the case may be elminated by the CaseElim transformation.
1489 This includes things like (==# a# b#)::Bool so that we simplify
1490 case ==# a# b# of { True -> x; False -> x }
1493 This particular example shows up in default methods for
1494 comparision operations (e.g. in (>=) for Int.Int32)
1496 Note [CaseElimination: lifted case]
1497 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1498 We also make sure that we deal with this very common case,
1499 where x has a lifted type:
1504 Here we are using the case as a strict let; if x is used only once
1505 then we want to inline it. We have to be careful that this doesn't
1506 make the program terminate when it would have diverged before, so we
1508 (a) 'e' is already evaluated (it may so if e is a variable)
1509 Specifically we check (exprIsHNF e)
1511 (b) the scrutinee is a variable and 'x' is used strictly
1513 (c) 'x' is not used at all and e is ok-for-speculation
1515 For the (c), consider
1516 case (case a ># b of { True -> (p,q); False -> (q,p) }) of
1518 The scrutinee is ok-for-speculation (it looks inside cases), but we do
1519 not want to transform to
1520 let r = case a ># b of { True -> (p,q); False -> (q,p) }
1522 because that builds an unnecessary thunk.
1525 Further notes about case elimination
1526 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1527 Consider: test :: Integer -> IO ()
1530 Turns out that this compiles to:
1533 eta1 :: State# RealWorld ->
1534 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1536 (PrelNum.jtos eta ($w[] @ Char))
1538 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1540 Notice the strange '<' which has no effect at all. This is a funny one.
1541 It started like this:
1543 f x y = if x < 0 then jtos x
1544 else if y==0 then "" else jtos x
1546 At a particular call site we have (f v 1). So we inline to get
1548 if v < 0 then jtos x
1549 else if 1==0 then "" else jtos x
1551 Now simplify the 1==0 conditional:
1553 if v<0 then jtos v else jtos v
1555 Now common-up the two branches of the case:
1557 case (v<0) of DEFAULT -> jtos v
1559 Why don't we drop the case? Because it's strict in v. It's technically
1560 wrong to drop even unnecessary evaluations, and in practice they
1561 may be a result of 'seq' so we *definitely* don't want to drop those.
1562 I don't really know how to improve this situation.
1565 ---------------------------------------------------------
1566 -- Eliminate the case if possible
1568 rebuildCase, reallyRebuildCase
1570 -> OutExpr -- Scrutinee
1571 -> InId -- Case binder
1572 -> [InAlt] -- Alternatives (inceasing order)
1574 -> SimplM (SimplEnv, OutExpr)
1576 --------------------------------------------------
1577 -- 1. Eliminate the case if there's a known constructor
1578 --------------------------------------------------
1580 rebuildCase env scrut case_bndr alts cont
1581 | Lit lit <- scrut -- No need for same treatment as constructors
1582 -- because literals are inlined more vigorously
1583 = do { tick (KnownBranch case_bndr)
1584 ; case findAlt (LitAlt lit) alts of
1585 Nothing -> missingAlt env case_bndr alts cont
1586 Just (_, bs, rhs) -> simple_rhs bs rhs }
1588 | Just (con, ty_args, other_args) <- exprIsConApp_maybe (getUnfoldingInRuleMatch env) scrut
1589 -- Works when the scrutinee is a variable with a known unfolding
1590 -- as well as when it's an explicit constructor application
1591 = do { tick (KnownBranch case_bndr)
1592 ; case findAlt (DataAlt con) alts of
1593 Nothing -> missingAlt env case_bndr alts cont
1594 Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs
1595 Just (_, bs, rhs) -> knownCon env scrut con ty_args other_args
1596 case_bndr bs rhs cont
1599 simple_rhs bs rhs = ASSERT( null bs )
1600 do { env' <- simplNonRecX env case_bndr scrut
1601 ; simplExprF env' rhs cont }
1604 --------------------------------------------------
1605 -- 2. Eliminate the case if scrutinee is evaluated
1606 --------------------------------------------------
1608 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
1609 -- See if we can get rid of the case altogether
1610 -- See Note [Case elimination]
1611 -- mkCase made sure that if all the alternatives are equal,
1612 -- then there is now only one (DEFAULT) rhs
1613 | all isDeadBinder bndrs -- bndrs are [InId]
1615 , if isUnLiftedType (idType case_bndr)
1616 then ok_for_spec -- Satisfy the let-binding invariant
1618 = do { tick (CaseElim case_bndr)
1619 ; env' <- simplNonRecX env case_bndr scrut
1620 -- If case_bndr is deads, simplNonRecX will discard
1621 ; simplExprF env' rhs cont }
1623 elim_lifted -- See Note [Case elimination: lifted case]
1625 || (strict_case_bndr && scrut_is_var scrut)
1626 -- The case binder is going to be evaluated later,
1627 -- and the scrutinee is a simple variable
1629 || (is_plain_seq && ok_for_spec)
1630 -- Note: not the same as exprIsHNF
1632 ok_for_spec = exprOkForSpeculation scrut
1633 is_plain_seq = isDeadBinder case_bndr -- Evaluation *only* for effect
1634 strict_case_bndr = isStrictDmd (idDemandInfo case_bndr)
1636 scrut_is_var (Cast s _) = scrut_is_var s
1637 scrut_is_var (Var v) = not (isTickBoxOp v)
1638 -- ugly hack; covering this case is what
1639 -- exprOkForSpeculation was intended for.
1640 scrut_is_var _ = False
1643 --------------------------------------------------
1644 -- 3. Try seq rules; see Note [User-defined RULES for seq] in MkId
1645 --------------------------------------------------
1647 rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
1648 | all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq'
1649 = do { let rhs' = substExpr (text "rebuild-case") env rhs
1650 out_args = [Type (substTy env (idType case_bndr)),
1651 Type (exprType rhs'), scrut, rhs']
1652 -- Lazily evaluated, so we don't do most of this
1654 ; rule_base <- getSimplRules
1655 ; mb_rule <- tryRules env (getRules rule_base seqId) seqId out_args cont
1657 Just (n_args, res) -> simplExprF (zapSubstEnv env)
1658 (mkApps res (drop n_args out_args))
1660 Nothing -> reallyRebuildCase env scrut case_bndr alts cont }
1662 rebuildCase env scrut case_bndr alts cont
1663 = reallyRebuildCase env scrut case_bndr alts cont
1665 --------------------------------------------------
1666 -- 3. Catch-all case
1667 --------------------------------------------------
1669 reallyRebuildCase env scrut case_bndr alts cont
1670 = do { -- Prepare the continuation;
1671 -- The new subst_env is in place
1672 (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1674 -- Simplify the alternatives
1675 ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
1677 -- Check for empty alternatives
1678 ; if null alts' then missingAlt env case_bndr alts cont
1680 { dflags <- getDOptsSmpl
1681 ; case_expr <- mkCase dflags scrut' case_bndr' alts'
1683 -- Notice that rebuild gets the in-scope set from env', not alt_env
1684 -- (which in any case is only build in simplAlts)
1685 -- The case binder *not* scope over the whole returned case-expression
1686 ; rebuild env' case_expr nodup_cont } }
1689 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1690 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1691 way, there's a chance that v will now only be used once, and hence
1694 Historical note: we use to do the "case binder swap" in the Simplifier
1695 so there were additional complications if the scrutinee was a variable.
1696 Now the binder-swap stuff is done in the occurrence analyer; see
1697 OccurAnal Note [Binder swap].
1701 If the case binder is not dead, then neither are the pattern bound
1703 case <any> of x { (a,b) ->
1704 case x of { (p,q) -> p } }
1705 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1706 The point is that we bring into the envt a binding
1708 after the outer case, and that makes (a,b) alive. At least we do unless
1709 the case binder is guaranteed dead.
1711 In practice, the scrutinee is almost always a variable, so we pretty
1712 much always zap the OccInfo of the binders. It doesn't matter much though.
1714 Note [Improving seq]
1717 type family F :: * -> *
1718 type instance F Int = Int
1720 ... case e of x { DEFAULT -> rhs } ...
1722 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1724 case e `cast` co of x'::Int
1725 I# x# -> let x = x' `cast` sym co
1728 so that 'rhs' can take advantage of the form of x'.
1730 Notice that Note [Case of cast] (in OccurAnal) may then apply to the result.
1732 Nota Bene: We only do the [Improving seq] transformation if the
1733 case binder 'x' is actually used in the rhs; that is, if the case
1734 is *not* a *pure* seq.
1735 a) There is no point in adding the cast to a pure seq.
1736 b) There is a good reason not to: doing so would interfere
1737 with seq rules (Note [Built-in RULES for seq] in MkId).
1738 In particular, this [Improving seq] thing *adds* a cast
1739 while [Built-in RULES for seq] *removes* one, so they
1742 You might worry about
1743 case v of x { __DEFAULT ->
1744 ... case (v `cast` co) of y { I# -> ... }}
1745 This is a pure seq (since x is unused), so [Improving seq] won't happen.
1746 But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get
1747 case v of x { __DEFAULT ->
1748 ... case (x `cast` co) of y { I# -> ... }}
1749 Now the outer case is not a pure seq, so [Improving seq] will happen,
1750 and then the inner case will disappear.
1752 The need for [Improving seq] showed up in Roman's experiments. Example:
1753 foo :: F Int -> Int -> Int
1754 foo t n = t `seq` bar n
1757 bar n = bar (n - case t of TI i -> i)
1758 Here we'd like to avoid repeated evaluating t inside the loop, by
1759 taking advantage of the `seq`.
1761 At one point I did transformation in LiberateCase, but it's more
1762 robust here. (Otherwise, there's a danger that we'll simply drop the
1763 'seq' altogether, before LiberateCase gets to see it.)
1766 simplAlts :: SimplEnv
1768 -> InId -- Case binder
1769 -> [InAlt] -- Non-empty
1771 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1772 -- Like simplExpr, this just returns the simplified alternatives;
1773 -- it does not return an environment
1775 simplAlts env scrut case_bndr alts cont'
1776 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seTvSubst env)) $
1777 do { let env0 = zapFloats env
1779 ; (env1, case_bndr1) <- simplBinder env0 case_bndr
1781 ; fam_envs <- getFamEnvs
1782 ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
1783 case_bndr case_bndr1 alts
1785 ; (imposs_deflt_cons, in_alts) <- prepareAlts scrut' case_bndr' alts
1787 ; let mb_var_scrut = case scrut' of { Var v -> Just v; _ -> Nothing }
1788 ; alts' <- mapM (simplAlt alt_env' mb_var_scrut
1789 imposs_deflt_cons case_bndr' cont') in_alts
1790 ; return (scrut', case_bndr', alts') }
1793 ------------------------------------
1794 improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
1795 -> OutExpr -> InId -> OutId -> [InAlt]
1796 -> SimplM (SimplEnv, OutExpr, OutId)
1797 -- Note [Improving seq]
1798 improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1799 | not (isDeadBinder case_bndr) -- Not a pure seq! See Note [Improving seq]
1800 , Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1801 = do { case_bndr2 <- newId (fsLit "nt") ty2
1802 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCo co)
1803 env2 = extendIdSubst env case_bndr rhs
1804 ; return (env2, scrut `Cast` co, case_bndr2) }
1806 improveSeq _ env scrut _ case_bndr1 _
1807 = return (env, scrut, case_bndr1)
1810 ------------------------------------
1811 simplAlt :: SimplEnv
1812 -> Maybe OutId -- Scrutinee
1813 -> [AltCon] -- These constructors can't be present when
1814 -- matching the DEFAULT alternative
1815 -> OutId -- The case binder
1820 simplAlt env scrut imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1821 = ASSERT( null bndrs )
1822 do { let env' = addBinderUnfolding env scrut case_bndr'
1823 (mkOtherCon imposs_deflt_cons)
1824 -- Record the constructors that the case-binder *can't* be.
1825 ; rhs' <- simplExprC env' rhs cont'
1826 ; return (DEFAULT, [], rhs') }
1828 simplAlt env scrut _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
1829 = ASSERT( null bndrs )
1830 do { let env' = addBinderUnfolding env scrut case_bndr'
1831 (mkSimpleUnfolding (Lit lit))
1832 ; rhs' <- simplExprC env' rhs cont'
1833 ; return (LitAlt lit, [], rhs') }
1835 simplAlt env scrut _ case_bndr' cont' (DataAlt con, vs, rhs)
1836 = do { -- Deal with the pattern-bound variables
1837 -- Mark the ones that are in ! positions in the
1838 -- data constructor as certainly-evaluated.
1839 -- NB: simplLamBinders preserves this eval info
1840 let vs_with_evals = add_evals (dataConRepStrictness con)
1841 ; (env', vs') <- simplLamBndrs env vs_with_evals
1843 -- Bind the case-binder to (con args)
1844 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1845 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1846 unf = mkSimpleUnfolding (mkConApp con con_args)
1847 env'' = addBinderUnfolding env' scrut case_bndr' unf
1849 ; rhs' <- simplExprC env'' rhs cont'
1850 ; return (DataAlt con, vs', rhs') }
1852 -- add_evals records the evaluated-ness of the bound variables of
1853 -- a case pattern. This is *important*. Consider
1854 -- data T = T !Int !Int
1856 -- case x of { T a b -> T (a+1) b }
1858 -- We really must record that b is already evaluated so that we don't
1859 -- go and re-evaluate it when constructing the result.
1860 -- See Note [Data-con worker strictness] in MkId.lhs
1865 go (v:vs') strs | isTyVar v = v : go vs' strs
1866 go (v:vs') (str:strs)
1867 | isMarkedStrict str = evald_v : go vs' strs
1868 | otherwise = zapped_v : go vs' strs
1870 zapped_v = zapBndrOccInfo keep_occ_info v
1871 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1872 go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
1874 -- See Note [zapOccInfo]
1875 -- zap_occ_info: if the case binder is alive, then we add the unfolding
1877 -- to the envt; so vs are now very much alive
1878 -- Note [Aug06] I can't see why this actually matters, but it's neater
1879 -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
1880 -- ==> case e of t { (a,b) -> ...(a)... }
1881 -- Look, Ma, a is alive now.
1882 keep_occ_info = isDeadBinder case_bndr' && isNothing scrut
1884 addBinderUnfolding :: SimplEnv -> Maybe OutId -> Id -> Unfolding -> SimplEnv
1885 addBinderUnfolding env scrut bndr unf
1887 Just v -> modifyInScope env1 (v `setIdUnfolding` unf)
1890 env1 = modifyInScope env bndr_w_unf
1891 bndr_w_unf = bndr `setIdUnfolding` unf
1893 zapBndrOccInfo :: Bool -> Id -> Id
1894 -- Consider case e of b { (a,b) -> ... }
1895 -- Then if we bind b to (a,b) in "...", and b is not dead,
1896 -- then we must zap the deadness info on a,b
1897 zapBndrOccInfo keep_occ_info pat_id
1898 | keep_occ_info = pat_id
1899 | otherwise = zapIdOccInfo pat_id
1902 Note [Add unfolding for scrutinee]
1903 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1904 In general it's unlikely that a variable scrutinee will appear
1905 in the case alternatives case x of { ...x unlikely to appear... }
1906 because the binder-swap in OccAnal has got rid of all such occcurrences
1907 See Note [Binder swap] in OccAnal.
1909 BUT it is still VERY IMPORTANT to add a suitable unfolding for a
1910 variable scrutinee, in simplAlt. Here's why
1912 (a,b) -> case b of c
1914 There is no occurrence of 'b' in the (...(f y)...). But y gets
1915 the unfolding (a,b), and *that* mentions b. If f has a RULE
1916 RULE f (p, I# q) = ...
1917 we want that rule to match, so we must extend the in-scope env with a
1918 suitable unfolding for 'y'. It's *essential* for rule matching; but
1919 it's also good for case-elimintation -- suppose that 'f' was inlined
1920 and did multi-level case analysis, then we'd solve it in one
1921 simplifier sweep instead of two.
1923 Exactly the same issue arises in SpecConstr;
1924 see Note [Add scrutinee to ValueEnv too] in SpecConstr
1926 %************************************************************************
1928 \subsection{Known constructor}
1930 %************************************************************************
1932 We are a bit careful with occurrence info. Here's an example
1934 (\x* -> case x of (a*, b) -> f a) (h v, e)
1936 where the * means "occurs once". This effectively becomes
1937 case (h v, e) of (a*, b) -> f a)
1939 let a* = h v; b = e in f a
1943 All this should happen in one sweep.
1946 knownCon :: SimplEnv
1947 -> OutExpr -- The scrutinee
1948 -> DataCon -> [OutType] -> [OutExpr] -- The scrutinee (in pieces)
1949 -> InId -> [InBndr] -> InExpr -- The alternative
1951 -> SimplM (SimplEnv, OutExpr)
1953 knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
1954 = do { env' <- bind_args env bs dc_args
1955 ; env'' <- bind_case_bndr env'
1956 ; simplExprF env'' rhs cont }
1958 zap_occ = zapBndrOccInfo (isDeadBinder bndr) -- bndr is an InId
1961 bind_args env' [] _ = return env'
1963 bind_args env' (b:bs') (Type ty : args)
1964 = ASSERT( isTyVar b )
1965 bind_args (extendTvSubst env' b ty) bs' args
1967 bind_args env' (b:bs') (arg : args)
1969 do { let b' = zap_occ b
1970 -- Note that the binder might be "dead", because it doesn't
1971 -- occur in the RHS; and simplNonRecX may therefore discard
1972 -- it via postInlineUnconditionally.
1973 -- Nevertheless we must keep it if the case-binder is alive,
1974 -- because it may be used in the con_app. See Note [zapOccInfo]
1975 ; env'' <- simplNonRecX env' b' arg
1976 ; bind_args env'' bs' args }
1979 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$
1980 text "scrut:" <+> ppr scrut
1982 -- It's useful to bind bndr to scrut, rather than to a fresh
1983 -- binding x = Con arg1 .. argn
1984 -- because very often the scrut is a variable, so we avoid
1985 -- creating, and then subsequently eliminating, a let-binding
1986 -- BUT, if scrut is a not a variable, we must be careful
1987 -- about duplicating the arg redexes; in that case, make
1988 -- a new con-app from the args
1990 | isDeadBinder bndr = return env
1991 | exprIsTrivial scrut = return (extendIdSubst env bndr (DoneEx scrut))
1992 | otherwise = do { dc_args <- mapM (simplVar env) bs
1993 -- dc_ty_args are aready OutTypes,
1994 -- but bs are InBndrs
1995 ; let con_app = Var (dataConWorkId dc)
1996 `mkTyApps` dc_ty_args
1998 ; simplNonRecX env bndr con_app }
2001 missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr)
2002 -- This isn't strictly an error, although it is unusual.
2003 -- It's possible that the simplifer might "see" that
2004 -- an inner case has no accessible alternatives before
2005 -- it "sees" that the entire branch of an outer case is
2006 -- inaccessible. So we simply put an error case here instead.
2007 missingAlt env case_bndr alts cont
2008 = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr )
2009 return (env, mkImpossibleExpr res_ty)
2011 res_ty = contResultType env (substTy env (coreAltsType alts)) cont
2015 %************************************************************************
2017 \subsection{Duplicating continuations}
2019 %************************************************************************
2022 prepareCaseCont :: SimplEnv
2023 -> [InAlt] -> SimplCont
2024 -> SimplM (SimplEnv, SimplCont, SimplCont)
2025 -- We are considering
2026 -- K[case _ of { p1 -> r1; ...; pn -> rn }]
2027 -- where K is some enclosing continuation for the case
2028 -- Goal: split K into two pieces Kdup,Knodup so that
2029 -- a) Kdup can be duplicated
2030 -- b) Knodup[Kdup[e]] = K[e]
2031 -- The idea is that we'll transform thus:
2032 -- Knodup[ (case _ of { p1 -> Kdup[r1]; ...; pn -> Kdup[rn] }
2034 -- We also return some extra bindings in SimplEnv (that scope over
2035 -- the entire continuation)
2037 prepareCaseCont env alts cont
2038 | many_alts alts = mkDupableCont env cont
2039 | otherwise = return (env, cont, mkBoringStop)
2041 many_alts :: [InAlt] -> Bool -- True iff strictly > 1 non-bottom alternative
2042 many_alts [] = False -- See Note [Bottom alternatives]
2043 many_alts [_] = False
2044 many_alts (alt:alts)
2045 | is_bot_alt alt = many_alts alts
2046 | otherwise = not (all is_bot_alt alts)
2048 is_bot_alt (_,_,rhs) = exprIsBottom rhs
2051 Note [Bottom alternatives]
2052 ~~~~~~~~~~~~~~~~~~~~~~~~~~
2054 case (case x of { A -> error .. ; B -> e; C -> error ..)
2056 then we can just duplicate those alts because the A and C cases
2057 will disappear immediately. This is more direct than creating
2058 join points and inlining them away; and in some cases we would
2059 not even create the join points (see Note [Single-alternative case])
2060 and we would keep the case-of-case which is silly. See Trac #4930.
2063 mkDupableCont :: SimplEnv -> SimplCont
2064 -> SimplM (SimplEnv, SimplCont, SimplCont)
2066 mkDupableCont env cont
2067 | contIsDupable cont
2068 = return (env, cont, mkBoringStop)
2070 mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
2072 mkDupableCont env (CoerceIt ty cont)
2073 = do { (env', dup, nodup) <- mkDupableCont env cont
2074 ; return (env', CoerceIt ty dup, nodup) }
2076 mkDupableCont env cont@(StrictBind {})
2077 = return (env, mkBoringStop, cont)
2078 -- See Note [Duplicating StrictBind]
2080 mkDupableCont env (StrictArg info cci cont)
2081 -- See Note [Duplicating StrictArg]
2082 = do { (env', dup, nodup) <- mkDupableCont env cont
2083 ; (env'', args') <- mapAccumLM (makeTrivial NotTopLevel) env' (ai_args info)
2084 ; return (env'', StrictArg (info { ai_args = args' }) cci dup, nodup) }
2086 mkDupableCont env (ApplyTo _ arg se cont)
2087 = -- e.g. [...hole...] (...arg...)
2089 -- let a = ...arg...
2090 -- in [...hole...] a
2091 do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
2092 ; arg' <- simplExpr (se `setInScope` env') arg
2093 ; (env'', arg'') <- makeTrivial NotTopLevel env' arg'
2094 ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
2095 ; return (env'', app_cont, nodup_cont) }
2097 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
2098 -- See Note [Single-alternative case]
2099 -- | not (exprIsDupable rhs && contIsDupable case_cont)
2100 -- | not (isDeadBinder case_bndr)
2101 | all isDeadBinder bs -- InIds
2102 && not (isUnLiftedType (idType case_bndr))
2103 -- Note [Single-alternative-unlifted]
2104 = return (env, mkBoringStop, cont)
2106 mkDupableCont env (Select _ case_bndr alts se cont)
2107 = -- e.g. (case [...hole...] of { pi -> ei })
2109 -- let ji = \xij -> ei
2110 -- in case [...hole...] of { pi -> ji xij }
2111 do { tick (CaseOfCase case_bndr)
2112 ; (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
2113 -- NB: We call prepareCaseCont here. If there is only one
2114 -- alternative, then dup_cont may be big, but that's ok
2115 -- becuase we push it into the single alternative, and then
2116 -- use mkDupableAlt to turn that simplified alternative into
2117 -- a join point if it's too big to duplicate.
2118 -- And this is important: see Note [Fusing case continuations]
2120 ; let alt_env = se `setInScope` env'
2121 ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
2122 ; alts' <- mapM (simplAlt alt_env' Nothing [] case_bndr' dup_cont) alts
2123 -- Safe to say that there are no handled-cons for the DEFAULT case
2124 -- NB: simplBinder does not zap deadness occ-info, so
2125 -- a dead case_bndr' will still advertise its deadness
2126 -- This is really important because in
2127 -- case e of b { (# p,q #) -> ... }
2128 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
2129 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
2130 -- In the new alts we build, we have the new case binder, so it must retain
2132 -- NB: we don't use alt_env further; it has the substEnv for
2133 -- the alternatives, and we don't want that
2135 ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
2136 ; return (env'', -- Note [Duplicated env]
2137 Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
2141 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
2142 -> SimplM (SimplEnv, [InAlt])
2143 -- Absorbs the continuation into the new alternatives
2145 mkDupableAlts env case_bndr' the_alts
2148 go env0 [] = return (env0, [])
2150 = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
2151 ; (env2, alts') <- go env1 alts
2152 ; return (env2, alt' : alts' ) }
2154 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
2155 -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
2156 mkDupableAlt env case_bndr (con, bndrs', rhs')
2157 | exprIsDupable rhs' -- Note [Small alternative rhs]
2158 = return (env, (con, bndrs', rhs'))
2160 = do { let rhs_ty' = exprType rhs'
2161 scrut_ty = idType case_bndr
2164 DEFAULT -> case_bndr
2165 DataAlt dc -> setIdUnfolding case_bndr unf
2167 -- See Note [Case binders and join points]
2168 unf = mkInlineUnfolding Nothing rhs
2169 rhs = mkConApp dc (map Type (tyConAppArgs scrut_ty)
2170 ++ varsToCoreExprs bndrs')
2172 LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt")
2173 <+> ppr case_bndr <+> ppr con )
2175 -- The case binder is alive but trivial, so why has
2176 -- it not been substituted away?
2178 used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs'
2179 | otherwise = bndrs' ++ [case_bndr_w_unf]
2182 | isTyVar bndr = True -- Abstract over all type variables just in case
2183 | otherwise = not (isDeadBinder bndr)
2184 -- The deadness info on the new Ids is preserved by simplBinders
2186 ; (final_bndrs', final_args) -- Note [Join point abstraction]
2187 <- if (any isId used_bndrs')
2188 then return (used_bndrs', varsToCoreExprs used_bndrs')
2189 else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
2190 ; return ([rw_id], [Var realWorldPrimId]) }
2192 ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
2193 -- Note [Funky mkPiTypes]
2195 ; let -- We make the lambdas into one-shot-lambdas. The
2196 -- join point is sure to be applied at most once, and doing so
2197 -- prevents the body of the join point being floated out by
2198 -- the full laziness pass
2199 really_final_bndrs = map one_shot final_bndrs'
2200 one_shot v | isId v = setOneShotLambda v
2202 join_rhs = mkLams really_final_bndrs rhs'
2203 join_call = mkApps (Var join_bndr) final_args
2205 ; env' <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
2206 ; return (env', (con, bndrs', join_call)) }
2207 -- See Note [Duplicated env]
2210 Note [Fusing case continuations]
2211 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2212 It's important to fuse two successive case continuations when the
2213 first has one alternative. That's why we call prepareCaseCont here.
2214 Consider this, which arises from thunk splitting (see Note [Thunk
2215 splitting] in WorkWrap):
2218 x* = case (case v of {pn -> rn}) of
2222 The simplifier will find
2223 (Var v) with continuation
2225 Select [I# a -> I# a] (
2226 StrictBind body Stop
2228 So we'll call mkDupableCont on
2229 Select [I# a -> I# a] (StrictBind body Stop)
2230 There is just one alternative in the first Select, so we want to
2231 simplify the rhs (I# a) with continuation (StricgtBind body Stop)
2232 Supposing that body is big, we end up with
2233 let $j a = <let x = I# a in body>
2234 in case v of { pn -> case rn of
2236 This is just what we want because the rn produces a box that
2237 the case rn cancels with.
2239 See Trac #4957 a fuller example.
2241 Note [Case binders and join points]
2242 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2244 case (case .. ) of c {
2247 If we make a join point with c but not c# we get
2248 $j = \c -> ....c....
2250 But if later inlining scrutines the c, thus
2252 $j = \c -> ... case c of { I# y -> ... } ...
2254 we won't see that 'c' has already been scrutinised. This actually
2255 happens in the 'tabulate' function in wave4main, and makes a significant
2256 difference to allocation.
2258 An alternative plan is this:
2260 $j = \c# -> let c = I# c# in ...c....
2262 but that is bad if 'c' is *not* later scrutinised.
2264 So instead we do both: we pass 'c' and 'c#' , and record in c's inlining
2265 (an InlineRule) that it's really I# c#, thus
2267 $j = \c# -> \c[=I# c#] -> ...c....
2269 Absence analysis may later discard 'c'.
2271 NB: take great care when doing strictness analysis;
2272 see Note [Lamba-bound unfoldings] in DmdAnal.
2274 Also note that we can still end up passing stuff that isn't used. Before
2275 strictness analysis we have
2276 let $j x y c{=(x,y)} = (h c, ...)
2278 After strictness analysis we see that h is strict, we end up with
2279 let $j x y c{=(x,y)} = ($wh x y, ...)
2282 Note [Duplicated env]
2283 ~~~~~~~~~~~~~~~~~~~~~
2284 Some of the alternatives are simplified, but have not been turned into a join point
2285 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
2286 bind the join point, because it might to do PostInlineUnconditionally, and
2287 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
2288 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
2289 at worst delays the join-point inlining.
2291 Note [Small alternative rhs]
2292 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2293 It is worth checking for a small RHS because otherwise we
2294 get extra let bindings that may cause an extra iteration of the simplifier to
2295 inline back in place. Quite often the rhs is just a variable or constructor.
2296 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
2297 iterations because the version with the let bindings looked big, and so wasn't
2298 inlined, but after the join points had been inlined it looked smaller, and so
2301 NB: we have to check the size of rhs', not rhs.
2302 Duplicating a small InAlt might invalidate occurrence information
2303 However, if it *is* dupable, we return the *un* simplified alternative,
2304 because otherwise we'd need to pair it up with an empty subst-env....
2305 but we only have one env shared between all the alts.
2306 (Remember we must zap the subst-env before re-simplifying something).
2307 Rather than do this we simply agree to re-simplify the original (small) thing later.
2309 Note [Funky mkPiTypes]
2310 ~~~~~~~~~~~~~~~~~~~~~~
2311 Notice the funky mkPiTypes. If the contructor has existentials
2312 it's possible that the join point will be abstracted over
2313 type varaibles as well as term variables.
2314 Example: Suppose we have
2315 data T = forall t. C [t]
2317 case (case e of ...) of
2319 We get the join point
2320 let j :: forall t. [t] -> ...
2321 j = /\t \xs::[t] -> rhs
2323 case (case e of ...) of
2324 C t xs::[t] -> j t xs
2326 Note [Join point abstaction]
2327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2328 If we try to lift a primitive-typed something out
2329 for let-binding-purposes, we will *caseify* it (!),
2330 with potentially-disastrous strictness results. So
2331 instead we turn it into a function: \v -> e
2332 where v::State# RealWorld#. The value passed to this function
2333 is realworld#, which generates (almost) no code.
2335 There's a slight infelicity here: we pass the overall
2336 case_bndr to all the join points if it's used in *any* RHS,
2337 because we don't know its usage in each RHS separately
2339 We used to say "&& isUnLiftedType rhs_ty'" here, but now
2340 we make the join point into a function whenever used_bndrs'
2341 is empty. This makes the join-point more CPR friendly.
2342 Consider: let j = if .. then I# 3 else I# 4
2343 in case .. of { A -> j; B -> j; C -> ... }
2345 Now CPR doesn't w/w j because it's a thunk, so
2346 that means that the enclosing function can't w/w either,
2347 which is a lose. Here's the example that happened in practice:
2348 kgmod :: Int -> Int -> Int
2349 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2353 I have seen a case alternative like this:
2355 It's a bit silly to add the realWorld dummy arg in this case, making
2358 (the \v alone is enough to make CPR happy) but I think it's rare
2360 Note [Duplicating StrictArg]
2361 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2362 The original plan had (where E is a big argument)
2364 ==> let $j = \a -> f E a
2367 But this is terrible! Here's an example:
2368 && E (case x of { T -> F; F -> T })
2369 Now, && is strict so we end up simplifying the case with
2370 an ArgOf continuation. If we let-bind it, we get
2371 let $j = \v -> && E v
2372 in simplExpr (case x of { T -> F; F -> T })
2374 And after simplifying more we get
2375 let $j = \v -> && E v
2376 in case x of { T -> $j F; F -> $j T }
2377 Which is a Very Bad Thing
2379 What we do now is this
2383 Now if the thing in the hole is a case expression (which is when
2384 we'll call mkDupableCont), we'll push the function call into the
2385 branches, which is what we want. Now RULES for f may fire, and
2386 call-pattern specialisation. Here's an example from Trac #3116
2389 _ -> Chunk p fpc (o+1) (l-1) bs')
2390 If we can push the call for 'go' inside the case, we get
2391 call-pattern specialisation for 'go', which is *crucial* for
2394 Here is the (&&) example:
2395 && E (case x of { T -> F; F -> T })
2397 case x of { T -> && a F; F -> && a T }
2401 * Arguments to f *after* the strict one are handled by
2402 the ApplyTo case of mkDupableCont. Eg
2405 * We can only do the let-binding of E because the function
2406 part of a StrictArg continuation is an explicit syntax
2407 tree. In earlier versions we represented it as a function
2408 (CoreExpr -> CoreEpxr) which we couldn't take apart.
2410 Do *not* duplicate StrictBind and StritArg continuations. We gain
2411 nothing by propagating them into the expressions, and we do lose a
2414 The desire not to duplicate is the entire reason that
2415 mkDupableCont returns a pair of continuations.
2417 Note [Duplicating StrictBind]
2418 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2419 Unlike StrictArg, there doesn't seem anything to gain from
2420 duplicating a StrictBind continuation, so we don't.
2423 Note [Single-alternative cases]
2424 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2425 This case is just like the ArgOf case. Here's an example:
2429 case (case x of I# x' ->
2431 True -> I# (negate# x')
2432 False -> I# x') of y {
2434 Because the (case x) has only one alternative, we'll transform to
2436 case (case x' <# 0# of
2437 True -> I# (negate# x')
2438 False -> I# x') of y {
2440 But now we do *NOT* want to make a join point etc, giving
2442 let $j = \y -> MkT y
2444 True -> $j (I# (negate# x'))
2446 In this case the $j will inline again, but suppose there was a big
2447 strict computation enclosing the orginal call to MkT. Then, it won't
2448 "see" the MkT any more, because it's big and won't get duplicated.
2449 And, what is worse, nothing was gained by the case-of-case transform.
2451 So, in circumstances like these, we don't want to build join points
2452 and push the outer case into the branches of the inner one. Instead,
2453 don't duplicate the continuation.
2455 When should we use this strategy? We should not use it on *every*
2456 single-alternative case:
2457 e.g. case (case ....) of (a,b) -> (# a,b #)
2458 Here we must push the outer case into the inner one!
2461 * Match [(DEFAULT,_,_)], but in the common case of Int,
2462 the alternative-filling-in code turned the outer case into
2463 case (...) of y { I# _ -> MkT y }
2465 * Match on single alternative plus (not (isDeadBinder case_bndr))
2466 Rationale: pushing the case inwards won't eliminate the construction.
2467 But there's a risk of
2468 case (...) of y { (a,b) -> let z=(a,b) in ... }
2469 Now y looks dead, but it'll come alive again. Still, this
2470 seems like the best option at the moment.
2472 * Match on single alternative plus (all (isDeadBinder bndrs))
2473 Rationale: this is essentially seq.
2475 * Match when the rhs is *not* duplicable, and hence would lead to a
2476 join point. This catches the disaster-case above. We can test
2477 the *un-simplified* rhs, which is fine. It might get bigger or
2478 smaller after simplification; if it gets smaller, this case might
2479 fire next time round. NB also that we must test contIsDupable
2480 case_cont *too, because case_cont might be big!
2482 HOWEVER: I found that this version doesn't work well, because
2483 we can get let x = case (...) of { small } in ...case x...
2484 When x is inlined into its full context, we find that it was a bad
2485 idea to have pushed the outer case inside the (...) case.
2487 Note [Single-alternative-unlifted]
2488 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2489 Here's another single-alternative where we really want to do case-of-case:
2491 data Mk1 = Mk1 Int# | Mk2 Int#
2496 case y_s6X of tpl_s7m {
2497 M1.Mk1 ipv_s70 -> ipv_s70;
2498 M1.Mk2 ipv_s72 -> ipv_s72;
2504 case x_s74 of tpl_s7n {
2505 M1.Mk1 ipv_s77 -> ipv_s77;
2506 M1.Mk2 ipv_s79 -> ipv_s79;
2510 { __DEFAULT -> ==# [wild1_s7b wild_s7c];
2514 So the outer case is doing *nothing at all*, other than serving as a
2515 join-point. In this case we really want to do case-of-case and decide
2516 whether to use a real join point or just duplicate the continuation:
2518 let $j s7c = case x of
2519 Mk1 ipv77 -> (==) s7c ipv77
2520 Mk1 ipv79 -> (==) s7c ipv79
2523 Mk1 ipv70 -> $j ipv70
2524 Mk2 ipv72 -> $j ipv72
2526 Hence: check whether the case binder's type is unlifted, because then
2527 the outer case is *not* a seq.