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 )
22 import Name ( mkSystemVarName, isExternalName )
24 import OptCoercion ( optCoercion )
25 import FamInstEnv ( topNormaliseType )
26 import DataCon ( DataCon, dataConWorkId, dataConRepStrictness )
27 import CoreMonad ( Tick(..), SimplifierMode(..) )
29 import Demand ( isStrictDmd )
30 import PprCore ( pprParendExpr, pprCoreExpr )
31 import CoreUnfold ( mkUnfolding, mkCoreUnfolding
32 , mkInlineUnfolding, mkSimpleUnfolding
33 , exprIsConApp_maybe, callSiteInline, CallCtxt(..) )
35 import qualified CoreSubst
37 import Rules ( lookupRule, getRules )
38 import BasicTypes ( isMarkedStrict, Arity )
39 import CostCentre ( currentCCS, pushCCisNop )
40 import TysPrim ( realWorldStatePrimTy )
41 import BasicTypes ( TopLevelFlag(..), isTopLevel, RecFlag(..) )
42 import MonadUtils ( foldlM, mapAccumLM )
43 import Maybes ( orElse )
44 import Data.List ( mapAccumL )
50 The guts of the simplifier is in this module, but the driver loop for
51 the simplifier is in SimplCore.lhs.
54 -----------------------------------------
55 *** IMPORTANT NOTE ***
56 -----------------------------------------
57 The simplifier used to guarantee that the output had no shadowing, but
58 it does not do so any more. (Actually, it never did!) The reason is
59 documented with simplifyArgs.
62 -----------------------------------------
63 *** IMPORTANT NOTE ***
64 -----------------------------------------
65 Many parts of the simplifier return a bunch of "floats" as well as an
66 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
68 All "floats" are let-binds, not case-binds, but some non-rec lets may
69 be unlifted (with RHS ok-for-speculation).
73 -----------------------------------------
74 ORGANISATION OF FUNCTIONS
75 -----------------------------------------
77 - simplify all top-level binders
78 - for NonRec, call simplRecOrTopPair
79 - for Rec, call simplRecBind
82 ------------------------------
83 simplExpr (applied lambda) ==> simplNonRecBind
84 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
85 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
87 ------------------------------
88 simplRecBind [binders already simplfied]
89 - use simplRecOrTopPair on each pair in turn
91 simplRecOrTopPair [binder already simplified]
92 Used for: recursive bindings (top level and nested)
93 top-level non-recursive bindings
95 - check for PreInlineUnconditionally
99 Used for: non-top-level non-recursive bindings
100 beta reductions (which amount to the same thing)
101 Because it can deal with strict arts, it takes a
102 "thing-inside" and returns an expression
104 - check for PreInlineUnconditionally
105 - simplify binder, including its IdInfo
114 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
115 Used for: binding case-binder and constr args in a known-constructor case
116 - check for PreInLineUnconditionally
120 ------------------------------
121 simplLazyBind: [binder already simplified, RHS not]
122 Used for: recursive bindings (top level and nested)
123 top-level non-recursive bindings
124 non-top-level, but *lazy* non-recursive bindings
125 [must not be strict or unboxed]
126 Returns floats + an augmented environment, not an expression
127 - substituteIdInfo and add result to in-scope
128 [so that rules are available in rec rhs]
131 - float if exposes constructor or PAP
135 completeNonRecX: [binder and rhs both simplified]
136 - if the the thing needs case binding (unlifted and not ok-for-spec)
142 completeBind: [given a simplified RHS]
143 [used for both rec and non-rec bindings, top level and not]
144 - try PostInlineUnconditionally
145 - add unfolding [this is the only place we add an unfolding]
150 Right hand sides and arguments
151 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
152 In many ways we want to treat
153 (a) the right hand side of a let(rec), and
154 (b) a function argument
155 in the same way. But not always! In particular, we would
156 like to leave these arguments exactly as they are, so they
157 will match a RULE more easily.
162 It's harder to make the rule match if we ANF-ise the constructor,
163 or eta-expand the PAP:
165 f (let { a = g x; b = h x } in (a,b))
168 On the other hand if we see the let-defns
173 then we *do* want to ANF-ise and eta-expand, so that p and q
174 can be safely inlined.
176 Even floating lets out is a bit dubious. For let RHS's we float lets
177 out if that exposes a value, so that the value can be inlined more vigorously.
180 r = let x = e in (x,x)
182 Here, if we float the let out we'll expose a nice constructor. We did experiments
183 that showed this to be a generally good thing. But it was a bad thing to float
184 lets out unconditionally, because that meant they got allocated more often.
186 For function arguments, there's less reason to expose a constructor (it won't
187 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
188 So for the moment we don't float lets out of function arguments either.
193 For eta expansion, we want to catch things like
195 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
197 If the \x was on the RHS of a let, we'd eta expand to bring the two
198 lambdas together. And in general that's a good thing to do. Perhaps
199 we should eta expand wherever we find a (value) lambda? Then the eta
200 expansion at a let RHS can concentrate solely on the PAP case.
203 %************************************************************************
205 \subsection{Bindings}
207 %************************************************************************
210 simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv
212 simplTopBinds env0 binds0
213 = do { -- Put all the top-level binders into scope at the start
214 -- so that if a transformation rule has unexpectedly brought
215 -- anything into scope, then we don't get a complaint about that.
216 -- It's rather as if the top-level binders were imported.
217 ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
218 ; dflags <- getDOptsSmpl
219 ; let dump_flag = dopt Opt_D_verbose_core2core dflags
220 ; env2 <- simpl_binds dump_flag env1 binds0
221 ; freeTick SimplifierDone
224 -- We need to track the zapped top-level binders, because
225 -- they should have their fragile IdInfo zapped (notably occurrence info)
226 -- That's why we run down binds and bndrs' simultaneously.
228 -- The dump-flag emits a trace for each top-level binding, which
229 -- helps to locate the tracing for inlining and rule firing
230 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
231 simpl_binds _ env [] = return env
232 simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
234 ; simpl_binds dump env' binds }
236 trace_bind True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
237 trace_bind False _ = \x -> x
239 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
240 simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel NonRecursive b b' r
242 (env', b') = addBndrRules env b (lookupRecBndr env b)
246 %************************************************************************
248 \subsection{Lazy bindings}
250 %************************************************************************
252 simplRecBind is used for
253 * recursive bindings only
256 simplRecBind :: SimplEnv -> TopLevelFlag
259 simplRecBind env0 top_lvl pairs0
260 = do { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
261 ; env1 <- go (zapFloats env_with_info) triples
262 ; return (env0 `addRecFloats` env1) }
263 -- addFloats adds the floats from env1,
264 -- _and_ updates env0 with the in-scope set from env1
266 add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
267 -- Add the (substituted) rules to the binder
268 add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
270 (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
272 go env [] = return env
274 go env ((old_bndr, new_bndr, rhs) : pairs)
275 = do { env' <- simplRecOrTopPair env top_lvl Recursive old_bndr new_bndr rhs
279 simplOrTopPair is used for
280 * recursive bindings (whether top level or not)
281 * top-level non-recursive bindings
283 It assumes the binder has already been simplified, but not its IdInfo.
286 simplRecOrTopPair :: SimplEnv
287 -> TopLevelFlag -> RecFlag
288 -> InId -> OutBndr -> InExpr -- Binder and rhs
289 -> SimplM SimplEnv -- Returns an env that includes the binding
291 simplRecOrTopPair env top_lvl is_rec old_bndr new_bndr rhs
292 | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
293 = do { tick (PreInlineUnconditionally old_bndr)
294 ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
297 = simplLazyBind env top_lvl is_rec old_bndr new_bndr rhs env
301 simplLazyBind is used for
302 * [simplRecOrTopPair] recursive bindings (whether top level or not)
303 * [simplRecOrTopPair] top-level non-recursive bindings
304 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
307 1. It assumes that the binder is *already* simplified,
308 and is in scope, and its IdInfo too, except unfolding
310 2. It assumes that the binder type is lifted.
312 3. It does not check for pre-inline-unconditionallly;
313 that should have been done already.
316 simplLazyBind :: SimplEnv
317 -> TopLevelFlag -> RecFlag
318 -> InId -> OutId -- Binder, both pre-and post simpl
319 -- The OutId has IdInfo, except arity, unfolding
320 -> InExpr -> SimplEnv -- The RHS and its environment
323 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
324 = -- pprTrace "simplLazyBind" ((ppr bndr <+> ppr bndr1) $$ ppr rhs $$ ppr (seIdSubst rhs_se)) $
325 do { let rhs_env = rhs_se `setInScope` env
326 (tvs, body) = case collectTyBinders rhs of
327 (tvs, body) | not_lam body -> (tvs,body)
328 | otherwise -> ([], rhs)
329 not_lam (Lam _ _) = False
331 -- Do not do the "abstract tyyvar" thing if there's
332 -- a lambda inside, becuase it defeats eta-reduction
333 -- f = /\a. \x. g a x
336 ; (body_env, tvs') <- simplBinders rhs_env tvs
337 -- See Note [Floating and type abstraction] in SimplUtils
340 ; (body_env1, body1) <- simplExprF body_env body mkRhsStop
341 -- ANF-ise a constructor or PAP rhs
342 ; (body_env2, body2) <- prepareRhs top_lvl body_env1 bndr1 body1
345 <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
346 then -- No floating, revert to body1
347 do { rhs' <- mkLam env tvs' (wrapFloats body_env1 body1)
348 ; return (env, rhs') }
350 else if null tvs then -- Simple floating
351 do { tick LetFloatFromLet
352 ; return (addFloats env body_env2, body2) }
354 else -- Do type-abstraction first
355 do { tick LetFloatFromLet
356 ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
357 ; rhs' <- mkLam env tvs' body3
358 ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
359 ; return (env', rhs') }
361 ; completeBind env' top_lvl bndr bndr1 rhs' }
364 A specialised variant of simplNonRec used when the RHS is already simplified,
365 notably in knownCon. It uses case-binding where necessary.
368 simplNonRecX :: SimplEnv
369 -> InId -- Old binder
370 -> OutExpr -- Simplified RHS
373 simplNonRecX env bndr new_rhs
374 | isDeadBinder bndr -- Not uncommon; e.g. case (a,b) of b { (p,q) -> p }
375 = return env -- Here b is dead, and we avoid creating
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 | (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 occ_info 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 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 NoOccInfo 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
699 -> OccInfo -> OutExpr
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 (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
716 -- We need to force bottoming, or the new unfolding holds
717 -- on to the old unfolding (which is part of the id).
718 let bottoming = isBottomingId id
719 in bottoming `seq` return (mkUnfolding src' is_top_lvl bottoming expr')
720 -- If the guidance is UnfIfGoodArgs, this is an INLINABLE
721 -- unfolding, and we need to make sure the guidance is kept up
722 -- to date with respect to any changes in the unfolding.
724 return (mkCoreUnfolding src' is_top_lvl expr' arity guide)
725 -- See Note [Top-level flag on inline rules] in CoreUnfold
728 act = idInlineActivation id
729 rule_env = updMode (updModeForInlineRules act) env
730 -- See Note [Simplifying inside InlineRules] in SimplUtils
732 simplUnfolding _ top_lvl id _occ_info new_rhs _
733 = -- We need to force bottoming, or the new unfolding holds
734 -- on to the old unfolding (which is part of the id).
735 let bottoming = isBottomingId id
736 in bottoming `seq` return (mkUnfolding InlineRhs (isTopLevel top_lvl) bottoming new_rhs)
737 -- We make an unfolding *even for loop-breakers*.
738 -- Reason: (a) It might be useful to know that they are WHNF
739 -- (b) In TidyPgm we currently assume that, if we want to
740 -- expose the unfolding then indeed we *have* an unfolding
741 -- to expose. (We could instead use the RHS, but currently
742 -- we don't.) The simple thing is always to have one.
745 Note [Arity decrease]
746 ~~~~~~~~~~~~~~~~~~~~~
747 Generally speaking the arity of a binding should not decrease. But it *can*
748 legitimately happen becuase of RULES. Eg
750 where g has arity 2, will have arity 2. But if there's a rewrite rule
752 where h has arity 1, then f's arity will decrease. Here's a real-life example,
753 which is in the output of Specialise:
756 $dm {Arity 2} = \d.\x. op d
757 {-# RULES forall d. $dm Int d = $s$dm #-}
759 dInt = MkD .... opInt ...
760 opInt {Arity 1} = $dm dInt
762 $s$dm {Arity 0} = \x. op dInt }
764 Here opInt has arity 1; but when we apply the rule its arity drops to 0.
765 That's why Specialise goes to a little trouble to pin the right arity
766 on specialised functions too.
768 Note [Setting the new unfolding]
769 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
770 * If there's an INLINE pragma, we simplify the RHS gently. Maybe we
771 should do nothing at all, but simplifying gently might get rid of
774 * If not, we make an unfolding from the new RHS. But *only* for
775 non-loop-breakers. Making loop breakers not have an unfolding at all
776 means that we can avoid tests in exprIsConApp, for example. This is
777 important: if exprIsConApp says 'yes' for a recursive thing, then we
778 can get into an infinite loop
780 If there's an InlineRule on a loop breaker, we hang on to the inlining.
781 It's pretty dodgy, but the user did say 'INLINE'. May need to revisit
784 Note [Setting the demand info]
785 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
786 If the unfolding is a value, the demand info may
787 go pear-shaped, so we nuke it. Example:
789 case x of (p,q) -> h p q x
790 Here x is certainly demanded. But after we've nuked
791 the case, we'll get just
792 let x = (a,b) in h a b x
793 and now x is not demanded (I'm assuming h is lazy)
794 This really happens. Similarly
795 let f = \x -> e in ...f..f...
796 After inlining f at some of its call sites the original binding may
797 (for example) be no longer strictly demanded.
798 The solution here is a bit ad hoc...
801 %************************************************************************
803 \subsection[Simplify-simplExpr]{The main function: simplExpr}
805 %************************************************************************
807 The reason for this OutExprStuff stuff is that we want to float *after*
808 simplifying a RHS, not before. If we do so naively we get quadratic
809 behaviour as things float out.
811 To see why it's important to do it after, consider this (real) example:
825 a -- Can't inline a this round, cos it appears twice
829 Each of the ==> steps is a round of simplification. We'd save a
830 whole round if we float first. This can cascade. Consider
835 let f = let d1 = ..d.. in \y -> e
839 in \x -> ...(\y ->e)...
841 Only in this second round can the \y be applied, and it
842 might do the same again.
846 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
847 simplExpr env expr = simplExprC env expr mkBoringStop
849 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
850 -- Simplify an expression, given a continuation
851 simplExprC env expr cont
852 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
853 do { (env', expr') <- simplExprF (zapFloats env) expr cont
854 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
855 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
856 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
857 return (wrapFloats env' expr') }
859 --------------------------------------------------
860 simplExprF :: SimplEnv -> InExpr -> SimplCont
861 -> SimplM (SimplEnv, OutExpr)
863 simplExprF env e cont
864 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
865 simplExprF' env e cont
867 simplExprF' :: SimplEnv -> InExpr -> SimplCont
868 -> SimplM (SimplEnv, OutExpr)
869 simplExprF' env (Var v) cont = simplVarF env v cont
870 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
871 simplExprF' env (Note n expr) cont = simplNote env n expr cont
872 simplExprF' env (Cast body co) cont = simplCast env body co cont
873 simplExprF' env (App fun arg) cont = simplExprF env fun $
874 ApplyTo NoDup arg env cont
876 simplExprF' env expr@(Lam _ _) cont
877 = simplLam env zapped_bndrs body cont
878 -- The main issue here is under-saturated lambdas
879 -- (\x1. \x2. e) arg1
880 -- Here x1 might have "occurs-once" occ-info, because occ-info
881 -- is computed assuming that a group of lambdas is applied
882 -- all at once. If there are too few args, we must zap the
883 -- occ-info, UNLESS the remaining binders are one-shot
885 (bndrs, body) = collectBinders expr
886 zapped_bndrs | need_to_zap = map zap bndrs
889 need_to_zap = any zappable_bndr (drop n_args bndrs)
890 n_args = countArgs cont
891 -- NB: countArgs counts all the args (incl type args)
892 -- and likewise drop counts all binders (incl type lambdas)
894 zappable_bndr b = isId b && not (isOneShotBndr b)
895 zap b | isTyCoVar b = b
896 | otherwise = zapLamIdInfo b
898 simplExprF' env (Type ty) cont
899 = ASSERT( contIsRhsOrArg cont )
900 do { ty' <- simplCoercion env ty
901 ; rebuild env (Type ty') cont }
903 simplExprF' env (Case scrut bndr _ alts) cont
904 | sm_case_case (getMode env)
905 = -- Simplify the scrutinee with a Select continuation
906 simplExprF env scrut (Select NoDup bndr alts env cont)
909 = -- If case-of-case is off, simply simplify the case expression
910 -- in a vanilla Stop context, and rebuild the result around it
911 do { case_expr' <- simplExprC env scrut
912 (Select NoDup bndr alts env mkBoringStop)
913 ; rebuild env case_expr' cont }
915 simplExprF' env (Let (Rec pairs) body) cont
916 = do { env' <- simplRecBndrs env (map fst pairs)
917 -- NB: bndrs' don't have unfoldings or rules
918 -- We add them as we go down
920 ; env'' <- simplRecBind env' NotTopLevel pairs
921 ; simplExprF env'' body cont }
923 simplExprF' env (Let (NonRec bndr rhs) body) cont
924 = simplNonRecE env bndr (rhs, env) ([], body) cont
926 ---------------------------------
927 simplType :: SimplEnv -> InType -> SimplM OutType
928 -- Kept monadic just so we can do the seqType
930 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
931 seqType new_ty `seq` return new_ty
933 new_ty = substTy env ty
935 ---------------------------------
936 simplCoercion :: SimplEnv -> InType -> SimplM OutType
937 -- The InType isn't *necessarily* a coercion, but it might be
938 -- (in a type application, say) and optCoercion is a no-op on types
940 = seqType new_co `seq` return new_co
942 new_co = optCoercion (getTvSubst env) co
946 %************************************************************************
948 \subsection{The main rebuilder}
950 %************************************************************************
953 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
954 -- At this point the substitution in the SimplEnv should be irrelevant
955 -- only the in-scope set and floats should matter
956 rebuild env expr cont
958 Stop {} -> return (env, expr)
959 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
960 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
961 StrictArg info _ cont -> rebuildCall env (info `addArgTo` expr) cont
962 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
963 ; simplLam env' bs body cont }
964 ApplyTo dup_flag arg se cont -- See Note [Avoid redundant simplification]
965 | isSimplified dup_flag -> rebuild env (App expr arg) cont
966 | otherwise -> do { arg' <- simplExpr (se `setInScope` env) arg
967 ; rebuild env (App expr arg') cont }
971 %************************************************************************
975 %************************************************************************
978 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
979 -> SimplM (SimplEnv, OutExpr)
980 simplCast env body co0 cont0
981 = do { co1 <- simplCoercion env co0
982 ; simplExprF env body (addCoerce co1 cont0) }
984 addCoerce co cont = add_coerce co (coercionKind co) cont
986 add_coerce _co (s1, k1) cont -- co :: ty~ty
987 | s1 `coreEqType` k1 = cont -- is a no-op
989 add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
990 | (_l1, t1) <- coercionKind co2
991 -- e |> (g1 :: S1~L) |> (g2 :: L~T1)
994 -- e |> (g1 . g2 :: S1~T1) otherwise
996 -- For example, in the initial form of a worker
997 -- we may find (coerce T (coerce S (\x.e))) y
998 -- and we'd like it to simplify to e[y/x] in one round
1000 , s1 `coreEqType` t1 = cont -- The coerces cancel out
1001 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
1003 add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
1004 -- (f |> g) ty ---> (f ty) |> (g @ ty)
1005 -- This implements the PushT and PushC rules from the paper
1006 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
1008 (new_arg_ty, new_cast)
1009 | isCoVar tyvar = (new_arg_co, mkCselRCoercion co) -- PushC rule
1010 | otherwise = (ty', mkInstCoercion co ty') -- PushT rule
1012 ApplyTo dup (Type new_arg_ty) (zapSubstEnv arg_se) (addCoerce new_cast cont)
1014 ty' = substTy (arg_se `setInScope` env) arg_ty
1015 new_arg_co = mkCsel1Coercion co `mkTransCoercion`
1016 ty' `mkTransCoercion`
1017 mkSymCoercion (mkCsel2Coercion co)
1019 add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
1020 | not (isTypeArg arg) -- This implements the Push rule from the paper
1021 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
1022 -- (e |> (g :: s1s2 ~ t1->t2)) f
1024 -- (e (f |> (arg g :: t1~s1))
1025 -- |> (res g :: s2->t2)
1027 -- t1t2 must be a function type, t1->t2, because it's applied
1028 -- to something but s1s2 might conceivably not be
1030 -- When we build the ApplyTo we can't mix the out-types
1031 -- with the InExpr in the argument, so we simply substitute
1032 -- to make it all consistent. It's a bit messy.
1033 -- But it isn't a common case.
1035 -- Example of use: Trac #995
1036 = ApplyTo dup new_arg (zapSubstEnv arg_se) (addCoerce co2 cont)
1038 -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
1039 -- t2 ~ s2 with left and right on the curried form:
1040 -- (->) t1 t2 ~ (->) s1 s2
1041 [co1, co2] = decomposeCo 2 co
1042 new_arg = mkCoerce (mkSymCoercion co1) arg'
1043 arg' = substExpr (text "move-cast") (arg_se `setInScope` env) arg
1045 add_coerce co _ cont = CoerceIt co cont
1049 %************************************************************************
1051 \subsection{Lambdas}
1053 %************************************************************************
1056 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
1057 -> SimplM (SimplEnv, OutExpr)
1059 simplLam env [] body cont = simplExprF env body cont
1062 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
1063 = do { tick (BetaReduction bndr)
1064 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
1066 -- Not enough args, so there are real lambdas left to put in the result
1067 simplLam env bndrs body cont
1068 = do { (env', bndrs') <- simplLamBndrs env bndrs
1069 ; body' <- simplExpr env' body
1070 ; new_lam <- mkLam env' bndrs' body'
1071 ; rebuild env' new_lam cont }
1074 simplNonRecE :: SimplEnv
1075 -> InBndr -- The binder
1076 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
1077 -> ([InBndr], InExpr) -- Body of the let/lambda
1080 -> SimplM (SimplEnv, OutExpr)
1082 -- simplNonRecE is used for
1083 -- * non-top-level non-recursive lets in expressions
1086 -- It deals with strict bindings, via the StrictBind continuation,
1087 -- which may abort the whole process
1089 -- The "body" of the binding comes as a pair of ([InId],InExpr)
1090 -- representing a lambda; so we recurse back to simplLam
1091 -- Why? Because of the binder-occ-info-zapping done before
1092 -- the call to simplLam in simplExprF (Lam ...)
1094 -- First deal with type applications and type lets
1095 -- (/\a. e) (Type ty) and (let a = Type ty in e)
1096 simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
1097 = ASSERT( isTyCoVar bndr )
1098 do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
1099 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
1101 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
1102 | preInlineUnconditionally env NotTopLevel bndr rhs
1103 = do { tick (PreInlineUnconditionally bndr)
1104 ; -- pprTrace "preInlineUncond" (ppr bndr <+> ppr rhs) $
1105 simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
1108 = do { simplExprF (rhs_se `setFloats` env) rhs
1109 (StrictBind bndr bndrs body env cont) }
1112 = ASSERT( not (isTyCoVar bndr) )
1113 do { (env1, bndr1) <- simplNonRecBndr env bndr
1114 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
1115 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
1116 ; simplLam env3 bndrs body cont }
1120 %************************************************************************
1124 %************************************************************************
1127 -- Hack alert: we only distinguish subsumed cost centre stacks for the
1128 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
1129 simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
1130 -> SimplM (SimplEnv, OutExpr)
1131 simplNote env (SCC cc) e cont
1132 | pushCCisNop cc (getEnclosingCC env) -- scc "f" (...(scc "f" e)...)
1133 = simplExprF env e cont -- ==> scc "f" (...e...)
1135 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
1136 ; rebuild env (mkSCC cc e') cont }
1138 simplNote env (CoreNote s) e cont
1139 = do { e' <- simplExpr env e
1140 ; rebuild env (Note (CoreNote s) e') cont }
1144 %************************************************************************
1148 %************************************************************************
1151 simplVar :: SimplEnv -> InVar -> SimplM OutExpr
1152 -- Look up an InVar in the environment
1155 = return (Type (substTyVar env var))
1157 = case substId env var of
1158 DoneId var1 -> return (Var var1)
1159 DoneEx e -> return e
1160 ContEx tvs ids e -> simplExpr (setSubstEnv env tvs ids) e
1162 simplVarF :: SimplEnv -> InId -> SimplCont -> SimplM (SimplEnv, OutExpr)
1163 simplVarF env var cont
1164 = case substId env var of
1165 DoneEx e -> simplExprF (zapSubstEnv env) e cont
1166 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
1167 DoneId var1 -> completeCall env var1 cont
1168 -- Note [zapSubstEnv]
1169 -- The template is already simplified, so don't re-substitute.
1170 -- This is VITAL. Consider
1172 -- let y = \z -> ...x... in
1174 -- We'll clone the inner \x, adding x->x' in the id_subst
1175 -- Then when we inline y, we must *not* replace x by x' in
1176 -- the inlined copy!!
1178 ---------------------------------------------------------
1179 -- Dealing with a call site
1181 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1182 completeCall env var cont
1183 = do { ------------- Try inlining ----------------
1184 dflags <- getDOptsSmpl
1185 ; let (lone_variable, arg_infos, call_cont) = contArgs cont
1186 -- The args are OutExprs, obtained by *lazily* substituting
1187 -- in the args found in cont. These args are only examined
1188 -- to limited depth (unless a rule fires). But we must do
1189 -- the substitution; rule matching on un-simplified args would
1192 n_val_args = length arg_infos
1193 interesting_cont = interestingCallContext call_cont
1194 unfolding = activeUnfolding env var
1195 maybe_inline = callSiteInline dflags var unfolding
1196 lone_variable arg_infos interesting_cont
1197 ; case maybe_inline of {
1198 Just expr -- There is an inlining!
1199 -> do { tick (UnfoldingDone var)
1200 ; trace_inline dflags expr cont $
1201 simplExprF (zapSubstEnv env) expr cont }
1203 ; Nothing -> do -- No inlining!
1205 { rule_base <- getSimplRules
1206 ; let info = mkArgInfo var (getRules rule_base var) n_val_args call_cont
1207 ; rebuildCall env info cont
1210 trace_inline dflags unfolding cont stuff
1211 | not (dopt Opt_D_dump_inlinings dflags) = stuff
1212 | not (dopt Opt_D_verbose_core2core dflags)
1213 = if isExternalName (idName var) then
1214 pprTrace "Inlining done:" (ppr var) stuff
1217 = pprTrace ("Inlining done: " ++ showSDoc (ppr var))
1218 (vcat [text "Inlined fn: " <+> nest 2 (ppr unfolding),
1219 text "Cont: " <+> ppr cont])
1222 rebuildCall :: SimplEnv
1225 -> SimplM (SimplEnv, OutExpr)
1226 rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_strs = [] }) cont
1227 -- When we run out of strictness args, it means
1228 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1229 -- Then we want to discard the entire strict continuation. E.g.
1230 -- * case (error "hello") of { ... }
1231 -- * (error "Hello") arg
1232 -- * f (error "Hello") where f is strict
1234 -- Then, especially in the first of these cases, we'd like to discard
1235 -- the continuation, leaving just the bottoming expression. But the
1236 -- type might not be right, so we may have to add a coerce.
1237 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1238 = return (env, mk_coerce res) -- contination to discard, else we do it
1239 where -- again and again!
1240 res = mkApps (Var fun) (reverse rev_args)
1241 res_ty = exprType res
1242 cont_ty = contResultType env res_ty cont
1243 co = mkUnsafeCoercion res_ty cont_ty
1244 mk_coerce expr | cont_ty `coreEqType` res_ty = expr
1245 | otherwise = mkCoerce co expr
1247 rebuildCall env info (ApplyTo _ (Type arg_ty) se cont)
1248 = do { ty' <- simplCoercion (se `setInScope` env) arg_ty
1249 ; rebuildCall env (info `addArgTo` Type ty') cont }
1251 rebuildCall env info@(ArgInfo { ai_encl = encl_rules
1252 , ai_strs = str:strs, ai_discs = disc:discs })
1253 (ApplyTo dup_flag arg arg_se cont)
1254 | isSimplified dup_flag -- See Note [Avoid redundant simplification]
1255 = rebuildCall env (addArgTo info' arg) cont
1257 | str -- Strict argument
1258 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1259 simplExprF (arg_se `setFloats` env) arg
1260 (StrictArg info' cci cont)
1263 | otherwise -- Lazy argument
1264 -- DO NOT float anything outside, hence simplExprC
1265 -- There is no benefit (unlike in a let-binding), and we'd
1266 -- have to be very careful about bogus strictness through
1267 -- floating a demanded let.
1268 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1270 ; rebuildCall env (addArgTo info' arg') cont }
1272 info' = info { ai_strs = strs, ai_discs = discs }
1273 cci | encl_rules || disc > 0 = ArgCtxt encl_rules -- Be keener here
1274 | otherwise = BoringCtxt -- Nothing interesting
1276 rebuildCall env (ArgInfo { ai_fun = fun, ai_args = rev_args, ai_rules = rules }) cont
1277 = do { -- We've accumulated a simplified call in <fun,rev_args>
1278 -- so try rewrite rules; see Note [RULEs apply to simplified arguments]
1279 -- See also Note [Rules for recursive functions]
1280 ; let args = reverse rev_args
1281 env' = zapSubstEnv env
1282 ; mb_rule <- tryRules env rules fun args cont
1284 Just (n_args, rule_rhs) -> simplExprF env' rule_rhs $
1285 pushSimplifiedArgs env' (drop n_args args) cont ;
1286 -- n_args says how many args the rule consumed
1287 ; Nothing -> rebuild env (mkApps (Var fun) args) cont -- No rules
1291 Note [RULES apply to simplified arguments]
1292 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1293 It's very desirable to try RULES once the arguments have been simplified, because
1294 doing so ensures that rule cascades work in one pass. Consider
1295 {-# RULES g (h x) = k x
1298 Then we want to rewrite (g (h x)) to (k x) and only then try f's rules. If
1299 we match f's rules against the un-simplified RHS, it won't match. This
1300 makes a particularly big difference when superclass selectors are involved:
1301 op ($p1 ($p2 (df d)))
1302 We want all this to unravel in one sweeep.
1304 Note [Avoid redundant simplification]
1305 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1306 Because RULES apply to simplified arguments, there's a danger of repeatedly
1307 simplifying already-simplified arguments. An important example is that of
1309 Here e1, e2 are simplified before the rule is applied, but don't really
1310 participate in the rule firing. So we mark them as Simplified to avoid
1311 re-simplifying them.
1315 This part of the simplifier may break the no-shadowing invariant
1317 f (...(\a -> e)...) (case y of (a,b) -> e')
1318 where f is strict in its second arg
1319 If we simplify the innermost one first we get (...(\a -> e)...)
1320 Simplifying the second arg makes us float the case out, so we end up with
1321 case y of (a,b) -> f (...(\a -> e)...) e'
1322 So the output does not have the no-shadowing invariant. However, there is
1323 no danger of getting name-capture, because when the first arg was simplified
1324 we used an in-scope set that at least mentioned all the variables free in its
1325 static environment, and that is enough.
1327 We can't just do innermost first, or we'd end up with a dual problem:
1328 case x of (a,b) -> f e (...(\a -> e')...)
1330 I spent hours trying to recover the no-shadowing invariant, but I just could
1331 not think of an elegant way to do it. The simplifier is already knee-deep in
1332 continuations. We have to keep the right in-scope set around; AND we have
1333 to get the effect that finding (error "foo") in a strict arg position will
1334 discard the entire application and replace it with (error "foo"). Getting
1335 all this at once is TOO HARD!
1338 %************************************************************************
1342 %************************************************************************
1345 tryRules :: SimplEnv -> [CoreRule]
1346 -> Id -> [OutExpr] -> SimplCont
1347 -> SimplM (Maybe (Arity, CoreExpr)) -- The arity is the number of
1348 -- args consumed by the rule
1349 tryRules env rules fn args call_cont
1353 = do { dflags <- getDOptsSmpl
1354 ; case activeRule dflags env of {
1355 Nothing -> return Nothing ; -- No rules apply
1357 case lookupRule act_fn (getUnfoldingInRuleMatch env) (getInScope env) fn args rules of {
1358 Nothing -> return Nothing ; -- No rule matches
1359 Just (rule, rule_rhs) ->
1361 do { tick (RuleFired (ru_name rule))
1362 ; trace_dump dflags rule rule_rhs $
1363 return (Just (ruleArity rule, rule_rhs)) }}}}
1365 trace_dump dflags rule rule_rhs stuff
1366 | not (dopt Opt_D_dump_rule_firings dflags)
1367 , not (dopt Opt_D_dump_rule_rewrites dflags) = stuff
1368 | not (dopt Opt_D_dump_rule_rewrites dflags)
1370 = pprTrace "Rule fired:" (ftext (ru_name rule)) stuff
1372 = pprTrace "Rule fired"
1373 (vcat [text "Rule:" <+> ftext (ru_name rule),
1374 text "Before:" <+> hang (ppr fn) 2 (sep (map pprParendExpr args)),
1375 text "After: " <+> pprCoreExpr rule_rhs,
1376 text "Cont: " <+> ppr call_cont])
1380 Note [Rules for recursive functions]
1381 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1382 You might think that we shouldn't apply rules for a loop breaker:
1383 doing so might give rise to an infinite loop, because a RULE is
1384 rather like an extra equation for the function:
1385 RULE: f (g x) y = x+y
1388 But it's too drastic to disable rules for loop breakers.
1389 Even the foldr/build rule would be disabled, because foldr
1390 is recursive, and hence a loop breaker:
1391 foldr k z (build g) = g k z
1392 So it's up to the programmer: rules can cause divergence
1395 %************************************************************************
1397 Rebuilding a cse expression
1399 %************************************************************************
1401 Note [Case elimination]
1402 ~~~~~~~~~~~~~~~~~~~~~~~
1403 The case-elimination transformation discards redundant case expressions.
1404 Start with a simple situation:
1406 case x# of ===> e[x#/y#]
1409 (when x#, y# are of primitive type, of course). We can't (in general)
1410 do this for algebraic cases, because we might turn bottom into
1413 The code in SimplUtils.prepareAlts has the effect of generalise this
1414 idea to look for a case where we're scrutinising a variable, and we
1415 know that only the default case can match. For example:
1419 DEFAULT -> ...(case x of
1423 Here the inner case is first trimmed to have only one alternative, the
1424 DEFAULT, after which it's an instance of the previous case. This
1425 really only shows up in eliminating error-checking code.
1427 We also make sure that we deal with this very common case:
1432 Here we are using the case as a strict let; if x is used only once
1433 then we want to inline it. We have to be careful that this doesn't
1434 make the program terminate when it would have diverged before, so we
1436 - e is already evaluated (it may so if e is a variable)
1437 - x is used strictly, or
1439 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1441 case e of ===> case e of DEFAULT -> r
1445 Now again the case may be elminated by the CaseElim transformation.
1448 Further notes about case elimination
1449 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1450 Consider: test :: Integer -> IO ()
1453 Turns out that this compiles to:
1456 eta1 :: State# RealWorld ->
1457 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1459 (PrelNum.jtos eta ($w[] @ Char))
1461 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1463 Notice the strange '<' which has no effect at all. This is a funny one.
1464 It started like this:
1466 f x y = if x < 0 then jtos x
1467 else if y==0 then "" else jtos x
1469 At a particular call site we have (f v 1). So we inline to get
1471 if v < 0 then jtos x
1472 else if 1==0 then "" else jtos x
1474 Now simplify the 1==0 conditional:
1476 if v<0 then jtos v else jtos v
1478 Now common-up the two branches of the case:
1480 case (v<0) of DEFAULT -> jtos v
1482 Why don't we drop the case? Because it's strict in v. It's technically
1483 wrong to drop even unnecessary evaluations, and in practice they
1484 may be a result of 'seq' so we *definitely* don't want to drop those.
1485 I don't really know how to improve this situation.
1488 ---------------------------------------------------------
1489 -- Eliminate the case if possible
1491 rebuildCase, reallyRebuildCase
1493 -> OutExpr -- Scrutinee
1494 -> InId -- Case binder
1495 -> [InAlt] -- Alternatives (inceasing order)
1497 -> SimplM (SimplEnv, OutExpr)
1499 --------------------------------------------------
1500 -- 1. Eliminate the case if there's a known constructor
1501 --------------------------------------------------
1503 rebuildCase env scrut case_bndr alts cont
1504 | Lit lit <- scrut -- No need for same treatment as constructors
1505 -- because literals are inlined more vigorously
1506 = do { tick (KnownBranch case_bndr)
1507 ; case findAlt (LitAlt lit) alts of
1508 Nothing -> missingAlt env case_bndr alts cont
1509 Just (_, bs, rhs) -> simple_rhs bs rhs }
1511 | Just (con, ty_args, other_args) <- exprIsConApp_maybe (getUnfoldingInRuleMatch env) scrut
1512 -- Works when the scrutinee is a variable with a known unfolding
1513 -- as well as when it's an explicit constructor application
1514 = do { tick (KnownBranch case_bndr)
1515 ; case findAlt (DataAlt con) alts of
1516 Nothing -> missingAlt env case_bndr alts cont
1517 Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs
1518 Just (_, bs, rhs) -> knownCon env scrut con ty_args other_args
1519 case_bndr bs rhs cont
1522 simple_rhs bs rhs = ASSERT( null bs )
1523 do { env' <- simplNonRecX env case_bndr scrut
1524 ; simplExprF env' rhs cont }
1527 --------------------------------------------------
1528 -- 2. Eliminate the case if scrutinee is evaluated
1529 --------------------------------------------------
1531 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
1532 -- See if we can get rid of the case altogether
1533 -- See Note [Case elimination]
1534 -- mkCase made sure that if all the alternatives are equal,
1535 -- then there is now only one (DEFAULT) rhs
1536 | all isDeadBinder bndrs -- bndrs are [InId]
1538 -- Check that the scrutinee can be let-bound instead of case-bound
1539 , exprOkForSpeculation scrut
1540 -- OK not to evaluate it
1541 -- This includes things like (==# a# b#)::Bool
1542 -- so that we simplify
1543 -- case ==# a# b# of { True -> x; False -> x }
1546 -- This particular example shows up in default methods for
1547 -- comparision operations (e.g. in (>=) for Int.Int32)
1548 || exprIsHNF scrut -- It's already evaluated
1549 || var_demanded_later scrut -- It'll be demanded later
1551 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1552 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1553 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1554 -- its argument: case x of { y -> dataToTag# y }
1555 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1556 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1558 -- Also we don't want to discard 'seq's
1559 = do { tick (CaseElim case_bndr)
1560 ; env' <- simplNonRecX env case_bndr scrut
1561 ; simplExprF env' rhs cont }
1563 -- The case binder is going to be evaluated later,
1564 -- and the scrutinee is a simple variable
1565 var_demanded_later (Var v) = isStrictDmd (idDemandInfo case_bndr)
1566 && not (isTickBoxOp v)
1567 -- ugly hack; covering this case is what
1568 -- exprOkForSpeculation was intended for.
1569 var_demanded_later _ = False
1571 --------------------------------------------------
1572 -- 3. Try seq rules; see Note [User-defined RULES for seq] in MkId
1573 --------------------------------------------------
1575 rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
1576 | all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq'
1577 = do { let rhs' = substExpr (text "rebuild-case") env rhs
1578 out_args = [Type (substTy env (idType case_bndr)),
1579 Type (exprType rhs'), scrut, rhs']
1580 -- Lazily evaluated, so we don't do most of this
1582 ; rule_base <- getSimplRules
1583 ; mb_rule <- tryRules env (getRules rule_base seqId) seqId out_args cont
1585 Just (n_args, res) -> simplExprF (zapSubstEnv env)
1586 (mkApps res (drop n_args out_args))
1588 Nothing -> reallyRebuildCase env scrut case_bndr alts cont }
1590 rebuildCase env scrut case_bndr alts cont
1591 = reallyRebuildCase env scrut case_bndr alts cont
1593 --------------------------------------------------
1594 -- 3. Catch-all case
1595 --------------------------------------------------
1597 reallyRebuildCase env scrut case_bndr alts cont
1598 = do { -- Prepare the continuation;
1599 -- The new subst_env is in place
1600 (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1602 -- Simplify the alternatives
1603 ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
1605 -- Check for empty alternatives
1606 ; if null alts' then missingAlt env case_bndr alts cont
1608 { dflags <- getDOptsSmpl
1609 ; case_expr <- mkCase dflags scrut' case_bndr' alts'
1611 -- Notice that rebuild gets the in-scope set from env', not alt_env
1612 -- (which in any case is only build in simplAlts)
1613 -- The case binder *not* scope over the whole returned case-expression
1614 ; rebuild env' case_expr nodup_cont } }
1617 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1618 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1619 way, there's a chance that v will now only be used once, and hence
1622 Historical note: we use to do the "case binder swap" in the Simplifier
1623 so there were additional complications if the scrutinee was a variable.
1624 Now the binder-swap stuff is done in the occurrence analyer; see
1625 OccurAnal Note [Binder swap].
1629 If the case binder is not dead, then neither are the pattern bound
1631 case <any> of x { (a,b) ->
1632 case x of { (p,q) -> p } }
1633 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1634 The point is that we bring into the envt a binding
1636 after the outer case, and that makes (a,b) alive. At least we do unless
1637 the case binder is guaranteed dead.
1639 In practice, the scrutinee is almost always a variable, so we pretty
1640 much always zap the OccInfo of the binders. It doesn't matter much though.
1645 Consider case (v `cast` co) of x { I# y ->
1646 ... (case (v `cast` co) of {...}) ...
1647 We'd like to eliminate the inner case. We can get this neatly by
1648 arranging that inside the outer case we add the unfolding
1649 v |-> x `cast` (sym co)
1650 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1652 Note [Improving seq]
1655 type family F :: * -> *
1656 type instance F Int = Int
1658 ... case e of x { DEFAULT -> rhs } ...
1660 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1662 case e `cast` co of x'::Int
1663 I# x# -> let x = x' `cast` sym co
1666 so that 'rhs' can take advantage of the form of x'.
1668 Notice that Note [Case of cast] may then apply to the result.
1670 Nota Bene: We only do the [Improving seq] transformation if the
1671 case binder 'x' is actually used in the rhs; that is, if the case
1672 is *not* a *pure* seq.
1673 a) There is no point in adding the cast to a pure seq.
1674 b) There is a good reason not to: doing so would interfere
1675 with seq rules (Note [Built-in RULES for seq] in MkId).
1676 In particular, this [Improving seq] thing *adds* a cast
1677 while [Built-in RULES for seq] *removes* one, so they
1680 You might worry about
1681 case v of x { __DEFAULT ->
1682 ... case (v `cast` co) of y { I# -> ... }}
1683 This is a pure seq (since x is unused), so [Improving seq] won't happen.
1684 But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get
1685 case v of x { __DEFAULT ->
1686 ... case (x `cast` co) of y { I# -> ... }}
1687 Now the outer case is not a pure seq, so [Improving seq] will happen,
1688 and then the inner case will disappear.
1690 The need for [Improving seq] showed up in Roman's experiments. Example:
1691 foo :: F Int -> Int -> Int
1692 foo t n = t `seq` bar n
1695 bar n = bar (n - case t of TI i -> i)
1696 Here we'd like to avoid repeated evaluating t inside the loop, by
1697 taking advantage of the `seq`.
1699 At one point I did transformation in LiberateCase, but it's more
1700 robust here. (Otherwise, there's a danger that we'll simply drop the
1701 'seq' altogether, before LiberateCase gets to see it.)
1704 simplAlts :: SimplEnv
1706 -> InId -- Case binder
1707 -> [InAlt] -- Non-empty
1709 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1710 -- Like simplExpr, this just returns the simplified alternatives;
1711 -- it does not return an environment
1713 simplAlts env scrut case_bndr alts cont'
1714 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seTvSubst env)) $
1715 do { let env0 = zapFloats env
1717 ; (env1, case_bndr1) <- simplBinder env0 case_bndr
1719 ; fam_envs <- getFamEnvs
1720 ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
1721 case_bndr case_bndr1 alts
1723 ; (imposs_deflt_cons, in_alts) <- prepareAlts scrut' case_bndr' alts
1725 ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
1726 ; return (scrut', case_bndr', alts') }
1729 ------------------------------------
1730 improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
1731 -> OutExpr -> InId -> OutId -> [InAlt]
1732 -> SimplM (SimplEnv, OutExpr, OutId)
1733 -- Note [Improving seq]
1734 improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1735 | not (isDeadBinder case_bndr) -- Not a pure seq! See the Note!
1736 , Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1737 = do { case_bndr2 <- newId (fsLit "nt") ty2
1738 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
1739 env2 = extendIdSubst env case_bndr rhs
1740 ; return (env2, scrut `Cast` co, case_bndr2) }
1742 improveSeq _ env scrut _ case_bndr1 _
1743 = return (env, scrut, case_bndr1)
1746 ------------------------------------
1747 simplAlt :: SimplEnv
1748 -> [AltCon] -- These constructors can't be present when
1749 -- matching the DEFAULT alternative
1750 -> OutId -- The case binder
1755 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1756 = ASSERT( null bndrs )
1757 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1758 -- Record the constructors that the case-binder *can't* be.
1759 ; rhs' <- simplExprC env' rhs cont'
1760 ; return (DEFAULT, [], rhs') }
1762 simplAlt env _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
1763 = ASSERT( null bndrs )
1764 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1765 ; rhs' <- simplExprC env' rhs cont'
1766 ; return (LitAlt lit, [], rhs') }
1768 simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
1769 = do { -- Deal with the pattern-bound variables
1770 -- Mark the ones that are in ! positions in the
1771 -- data constructor as certainly-evaluated.
1772 -- NB: simplLamBinders preserves this eval info
1773 let vs_with_evals = add_evals (dataConRepStrictness con)
1774 ; (env', vs') <- simplLamBndrs env vs_with_evals
1776 -- Bind the case-binder to (con args)
1777 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1778 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1779 env'' = addBinderUnfolding env' case_bndr'
1780 (mkConApp con con_args)
1782 ; rhs' <- simplExprC env'' rhs cont'
1783 ; return (DataAlt con, vs', rhs') }
1785 -- add_evals records the evaluated-ness of the bound variables of
1786 -- a case pattern. This is *important*. Consider
1787 -- data T = T !Int !Int
1789 -- case x of { T a b -> T (a+1) b }
1791 -- We really must record that b is already evaluated so that we don't
1792 -- go and re-evaluate it when constructing the result.
1793 -- See Note [Data-con worker strictness] in MkId.lhs
1798 go (v:vs') strs | isTyCoVar v = v : go vs' strs
1799 go (v:vs') (str:strs)
1800 | isMarkedStrict str = evald_v : go vs' strs
1801 | otherwise = zapped_v : go vs' strs
1803 zapped_v = zap_occ_info v
1804 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1805 go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
1807 -- See Note [zapOccInfo]
1808 -- zap_occ_info: if the case binder is alive, then we add the unfolding
1810 -- to the envt; so vs are now very much alive
1811 -- Note [Aug06] I can't see why this actually matters, but it's neater
1812 -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
1813 -- ==> case e of t { (a,b) -> ...(a)... }
1814 -- Look, Ma, a is alive now.
1815 zap_occ_info = zapCasePatIdOcc case_bndr'
1817 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1818 addBinderUnfolding env bndr rhs
1819 = modifyInScope env (bndr `setIdUnfolding` mkSimpleUnfolding rhs)
1821 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1822 addBinderOtherCon env bndr cons
1823 = modifyInScope env (bndr `setIdUnfolding` mkOtherCon cons)
1825 zapCasePatIdOcc :: Id -> Id -> Id
1826 -- Consider case e of b { (a,b) -> ... }
1827 -- Then if we bind b to (a,b) in "...", and b is not dead,
1828 -- then we must zap the deadness info on a,b
1829 zapCasePatIdOcc case_bndr
1830 | isDeadBinder case_bndr = \ pat_id -> pat_id
1831 | otherwise = \ pat_id -> zapIdOccInfo pat_id
1835 %************************************************************************
1837 \subsection{Known constructor}
1839 %************************************************************************
1841 We are a bit careful with occurrence info. Here's an example
1843 (\x* -> case x of (a*, b) -> f a) (h v, e)
1845 where the * means "occurs once". This effectively becomes
1846 case (h v, e) of (a*, b) -> f a)
1848 let a* = h v; b = e in f a
1852 All this should happen in one sweep.
1855 knownCon :: SimplEnv
1856 -> OutExpr -- The scrutinee
1857 -> DataCon -> [OutType] -> [OutExpr] -- The scrutinee (in pieces)
1858 -> InId -> [InBndr] -> InExpr -- The alternative
1860 -> SimplM (SimplEnv, OutExpr)
1862 knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
1863 = do { env' <- bind_args env bs dc_args
1864 ; env'' <- bind_case_bndr env'
1865 ; simplExprF env'' rhs cont }
1867 zap_occ = zapCasePatIdOcc bndr -- bndr is an InId
1870 bind_args env' [] _ = return env'
1872 bind_args env' (b:bs') (Type ty : args)
1873 = ASSERT( isTyCoVar b )
1874 bind_args (extendTvSubst env' b ty) bs' args
1876 bind_args env' (b:bs') (arg : args)
1878 do { let b' = zap_occ b
1879 -- Note that the binder might be "dead", because it doesn't
1880 -- occur in the RHS; and simplNonRecX may therefore discard
1881 -- it via postInlineUnconditionally.
1882 -- Nevertheless we must keep it if the case-binder is alive,
1883 -- because it may be used in the con_app. See Note [zapOccInfo]
1884 ; env'' <- simplNonRecX env' b' arg
1885 ; bind_args env'' bs' args }
1888 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$
1889 text "scrut:" <+> ppr scrut
1891 -- It's useful to bind bndr to scrut, rather than to a fresh
1892 -- binding x = Con arg1 .. argn
1893 -- because very often the scrut is a variable, so we avoid
1894 -- creating, and then subsequently eliminating, a let-binding
1895 -- BUT, if scrut is a not a variable, we must be careful
1896 -- about duplicating the arg redexes; in that case, make
1897 -- a new con-app from the args
1899 | isDeadBinder bndr = return env
1900 | exprIsTrivial scrut = return (extendIdSubst env bndr (DoneEx scrut))
1901 | otherwise = do { dc_args <- mapM (simplVar env) bs
1902 -- dc_ty_args are aready OutTypes,
1903 -- but bs are InBndrs
1904 ; let con_app = Var (dataConWorkId dc)
1905 `mkTyApps` dc_ty_args
1907 ; simplNonRecX env bndr con_app }
1910 missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr)
1911 -- This isn't strictly an error, although it is unusual.
1912 -- It's possible that the simplifer might "see" that
1913 -- an inner case has no accessible alternatives before
1914 -- it "sees" that the entire branch of an outer case is
1915 -- inaccessible. So we simply put an error case here instead.
1916 missingAlt env case_bndr alts cont
1917 = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr )
1918 return (env, mkImpossibleExpr res_ty)
1920 res_ty = contResultType env (substTy env (coreAltsType alts)) cont
1924 %************************************************************************
1926 \subsection{Duplicating continuations}
1928 %************************************************************************
1931 prepareCaseCont :: SimplEnv
1932 -> [InAlt] -> SimplCont
1933 -> SimplM (SimplEnv, SimplCont,SimplCont)
1934 -- Return a duplicatable continuation, a non-duplicable part
1935 -- plus some extra bindings (that scope over the entire
1938 -- No need to make it duplicatable if there's only one alternative
1939 prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
1940 prepareCaseCont env _ cont = mkDupableCont env cont
1944 mkDupableCont :: SimplEnv -> SimplCont
1945 -> SimplM (SimplEnv, SimplCont, SimplCont)
1947 mkDupableCont env cont
1948 | contIsDupable cont
1949 = return (env, cont, mkBoringStop)
1951 mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1953 mkDupableCont env (CoerceIt ty cont)
1954 = do { (env', dup, nodup) <- mkDupableCont env cont
1955 ; return (env', CoerceIt ty dup, nodup) }
1957 mkDupableCont env cont@(StrictBind {})
1958 = return (env, mkBoringStop, cont)
1959 -- See Note [Duplicating StrictBind]
1961 mkDupableCont env (StrictArg info cci cont)
1962 -- See Note [Duplicating StrictArg]
1963 = do { (env', dup, nodup) <- mkDupableCont env cont
1964 ; (env'', args') <- mapAccumLM (makeTrivial NotTopLevel) env' (ai_args info)
1965 ; return (env'', StrictArg (info { ai_args = args' }) cci dup, nodup) }
1967 mkDupableCont env (ApplyTo _ arg se cont)
1968 = -- e.g. [...hole...] (...arg...)
1970 -- let a = ...arg...
1971 -- in [...hole...] a
1972 do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1973 ; arg' <- simplExpr (se `setInScope` env') arg
1974 ; (env'', arg'') <- makeTrivial NotTopLevel env' arg'
1975 ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
1976 ; return (env'', app_cont, nodup_cont) }
1978 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
1979 -- See Note [Single-alternative case]
1980 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1981 -- | not (isDeadBinder case_bndr)
1982 | all isDeadBinder bs -- InIds
1983 && not (isUnLiftedType (idType case_bndr))
1984 -- Note [Single-alternative-unlifted]
1985 = return (env, mkBoringStop, cont)
1987 mkDupableCont env (Select _ case_bndr alts se cont)
1988 = -- e.g. (case [...hole...] of { pi -> ei })
1990 -- let ji = \xij -> ei
1991 -- in case [...hole...] of { pi -> ji xij }
1992 do { tick (CaseOfCase case_bndr)
1993 ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1994 -- NB: call mkDupableCont here, *not* prepareCaseCont
1995 -- We must make a duplicable continuation, whereas prepareCaseCont
1996 -- doesn't when there is a single case branch
1998 ; let alt_env = se `setInScope` env'
1999 ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
2000 ; alts' <- mapM (simplAlt alt_env' [] case_bndr' dup_cont) alts
2001 -- Safe to say that there are no handled-cons for the DEFAULT case
2002 -- NB: simplBinder does not zap deadness occ-info, so
2003 -- a dead case_bndr' will still advertise its deadness
2004 -- This is really important because in
2005 -- case e of b { (# p,q #) -> ... }
2006 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
2007 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
2008 -- In the new alts we build, we have the new case binder, so it must retain
2010 -- NB: we don't use alt_env further; it has the substEnv for
2011 -- the alternatives, and we don't want that
2013 ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
2014 ; return (env'', -- Note [Duplicated env]
2015 Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
2019 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
2020 -> SimplM (SimplEnv, [InAlt])
2021 -- Absorbs the continuation into the new alternatives
2023 mkDupableAlts env case_bndr' the_alts
2026 go env0 [] = return (env0, [])
2028 = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
2029 ; (env2, alts') <- go env1 alts
2030 ; return (env2, alt' : alts' ) }
2032 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
2033 -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
2034 mkDupableAlt env case_bndr (con, bndrs', rhs')
2035 | exprIsDupable rhs' -- Note [Small alternative rhs]
2036 = return (env, (con, bndrs', rhs'))
2038 = do { let rhs_ty' = exprType rhs'
2039 scrut_ty = idType case_bndr
2042 DEFAULT -> case_bndr
2043 DataAlt dc -> setIdUnfolding case_bndr unf
2045 -- See Note [Case binders and join points]
2046 unf = mkInlineUnfolding Nothing rhs
2047 rhs = mkConApp dc (map Type (tyConAppArgs scrut_ty)
2048 ++ varsToCoreExprs bndrs')
2050 LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt")
2051 <+> ppr case_bndr <+> ppr con )
2053 -- The case binder is alive but trivial, so why has
2054 -- it not been substituted away?
2056 used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs'
2057 | otherwise = bndrs' ++ [case_bndr_w_unf]
2060 | isTyCoVar bndr = True -- Abstract over all type variables just in case
2061 | otherwise = not (isDeadBinder bndr)
2062 -- The deadness info on the new Ids is preserved by simplBinders
2064 ; (final_bndrs', final_args) -- Note [Join point abstraction]
2065 <- if (any isId used_bndrs')
2066 then return (used_bndrs', varsToCoreExprs used_bndrs')
2067 else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
2068 ; return ([rw_id], [Var realWorldPrimId]) }
2070 ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
2071 -- Note [Funky mkPiTypes]
2073 ; let -- We make the lambdas into one-shot-lambdas. The
2074 -- join point is sure to be applied at most once, and doing so
2075 -- prevents the body of the join point being floated out by
2076 -- the full laziness pass
2077 really_final_bndrs = map one_shot final_bndrs'
2078 one_shot v | isId v = setOneShotLambda v
2080 join_rhs = mkLams really_final_bndrs rhs'
2081 join_call = mkApps (Var join_bndr) final_args
2083 ; env' <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
2084 ; return (env', (con, bndrs', join_call)) }
2085 -- See Note [Duplicated env]
2088 Note [Case binders and join points]
2089 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2091 case (case .. ) of c {
2094 If we make a join point with c but not c# we get
2095 $j = \c -> ....c....
2097 But if later inlining scrutines the c, thus
2099 $j = \c -> ... case c of { I# y -> ... } ...
2101 we won't see that 'c' has already been scrutinised. This actually
2102 happens in the 'tabulate' function in wave4main, and makes a significant
2103 difference to allocation.
2105 An alternative plan is this:
2107 $j = \c# -> let c = I# c# in ...c....
2109 but that is bad if 'c' is *not* later scrutinised.
2111 So instead we do both: we pass 'c' and 'c#' , and record in c's inlining
2112 (an InlineRule) that it's really I# c#, thus
2114 $j = \c# -> \c[=I# c#] -> ...c....
2116 Absence analysis may later discard 'c'.
2118 NB: take great care when doing strictness analysis;
2119 see Note [Lamba-bound unfoldings] in DmdAnal.
2121 Also note that we can still end up passing stuff that isn't used. Before
2122 strictness analysis we have
2123 let $j x y c{=(x,y)} = (h c, ...)
2125 After strictness analysis we see that h is strict, we end up with
2126 let $j x y c{=(x,y)} = ($wh x y, ...)
2129 Note [Duplicated env]
2130 ~~~~~~~~~~~~~~~~~~~~~
2131 Some of the alternatives are simplified, but have not been turned into a join point
2132 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
2133 bind the join point, because it might to do PostInlineUnconditionally, and
2134 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
2135 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
2136 at worst delays the join-point inlining.
2138 Note [Small alternative rhs]
2139 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2140 It is worth checking for a small RHS because otherwise we
2141 get extra let bindings that may cause an extra iteration of the simplifier to
2142 inline back in place. Quite often the rhs is just a variable or constructor.
2143 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
2144 iterations because the version with the let bindings looked big, and so wasn't
2145 inlined, but after the join points had been inlined it looked smaller, and so
2148 NB: we have to check the size of rhs', not rhs.
2149 Duplicating a small InAlt might invalidate occurrence information
2150 However, if it *is* dupable, we return the *un* simplified alternative,
2151 because otherwise we'd need to pair it up with an empty subst-env....
2152 but we only have one env shared between all the alts.
2153 (Remember we must zap the subst-env before re-simplifying something).
2154 Rather than do this we simply agree to re-simplify the original (small) thing later.
2156 Note [Funky mkPiTypes]
2157 ~~~~~~~~~~~~~~~~~~~~~~
2158 Notice the funky mkPiTypes. If the contructor has existentials
2159 it's possible that the join point will be abstracted over
2160 type varaibles as well as term variables.
2161 Example: Suppose we have
2162 data T = forall t. C [t]
2164 case (case e of ...) of
2166 We get the join point
2167 let j :: forall t. [t] -> ...
2168 j = /\t \xs::[t] -> rhs
2170 case (case e of ...) of
2171 C t xs::[t] -> j t xs
2173 Note [Join point abstaction]
2174 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2175 If we try to lift a primitive-typed something out
2176 for let-binding-purposes, we will *caseify* it (!),
2177 with potentially-disastrous strictness results. So
2178 instead we turn it into a function: \v -> e
2179 where v::State# RealWorld#. The value passed to this function
2180 is realworld#, which generates (almost) no code.
2182 There's a slight infelicity here: we pass the overall
2183 case_bndr to all the join points if it's used in *any* RHS,
2184 because we don't know its usage in each RHS separately
2186 We used to say "&& isUnLiftedType rhs_ty'" here, but now
2187 we make the join point into a function whenever used_bndrs'
2188 is empty. This makes the join-point more CPR friendly.
2189 Consider: let j = if .. then I# 3 else I# 4
2190 in case .. of { A -> j; B -> j; C -> ... }
2192 Now CPR doesn't w/w j because it's a thunk, so
2193 that means that the enclosing function can't w/w either,
2194 which is a lose. Here's the example that happened in practice:
2195 kgmod :: Int -> Int -> Int
2196 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2200 I have seen a case alternative like this:
2202 It's a bit silly to add the realWorld dummy arg in this case, making
2205 (the \v alone is enough to make CPR happy) but I think it's rare
2207 Note [Duplicating StrictArg]
2208 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2209 The original plan had (where E is a big argument)
2211 ==> let $j = \a -> f E a
2214 But this is terrible! Here's an example:
2215 && E (case x of { T -> F; F -> T })
2216 Now, && is strict so we end up simplifying the case with
2217 an ArgOf continuation. If we let-bind it, we get
2218 let $j = \v -> && E v
2219 in simplExpr (case x of { T -> F; F -> T })
2221 And after simplifying more we get
2222 let $j = \v -> && E v
2223 in case x of { T -> $j F; F -> $j T }
2224 Which is a Very Bad Thing
2226 What we do now is this
2230 Now if the thing in the hole is a case expression (which is when
2231 we'll call mkDupableCont), we'll push the function call into the
2232 branches, which is what we want. Now RULES for f may fire, and
2233 call-pattern specialisation. Here's an example from Trac #3116
2236 _ -> Chunk p fpc (o+1) (l-1) bs')
2237 If we can push the call for 'go' inside the case, we get
2238 call-pattern specialisation for 'go', which is *crucial* for
2241 Here is the (&&) example:
2242 && E (case x of { T -> F; F -> T })
2244 case x of { T -> && a F; F -> && a T }
2248 * Arguments to f *after* the strict one are handled by
2249 the ApplyTo case of mkDupableCont. Eg
2252 * We can only do the let-binding of E because the function
2253 part of a StrictArg continuation is an explicit syntax
2254 tree. In earlier versions we represented it as a function
2255 (CoreExpr -> CoreEpxr) which we couldn't take apart.
2257 Do *not* duplicate StrictBind and StritArg continuations. We gain
2258 nothing by propagating them into the expressions, and we do lose a
2261 The desire not to duplicate is the entire reason that
2262 mkDupableCont returns a pair of continuations.
2264 Note [Duplicating StrictBind]
2265 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2266 Unlike StrictArg, there doesn't seem anything to gain from
2267 duplicating a StrictBind continuation, so we don't.
2269 The desire not to duplicate is the entire reason that
2270 mkDupableCont returns a pair of continuations.
2273 Note [Single-alternative cases]
2274 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2275 This case is just like the ArgOf case. Here's an example:
2279 case (case x of I# x' ->
2281 True -> I# (negate# x')
2282 False -> I# x') of y {
2284 Because the (case x) has only one alternative, we'll transform to
2286 case (case x' <# 0# of
2287 True -> I# (negate# x')
2288 False -> I# x') of y {
2290 But now we do *NOT* want to make a join point etc, giving
2292 let $j = \y -> MkT y
2294 True -> $j (I# (negate# x'))
2296 In this case the $j will inline again, but suppose there was a big
2297 strict computation enclosing the orginal call to MkT. Then, it won't
2298 "see" the MkT any more, because it's big and won't get duplicated.
2299 And, what is worse, nothing was gained by the case-of-case transform.
2301 So, in circumstances like these, we don't want to build join points
2302 and push the outer case into the branches of the inner one. Instead,
2303 don't duplicate the continuation.
2305 When should we use this strategy? We should not use it on *every*
2306 single-alternative case:
2307 e.g. case (case ....) of (a,b) -> (# a,b #)
2308 Here we must push the outer case into the inner one!
2311 * Match [(DEFAULT,_,_)], but in the common case of Int,
2312 the alternative-filling-in code turned the outer case into
2313 case (...) of y { I# _ -> MkT y }
2315 * Match on single alternative plus (not (isDeadBinder case_bndr))
2316 Rationale: pushing the case inwards won't eliminate the construction.
2317 But there's a risk of
2318 case (...) of y { (a,b) -> let z=(a,b) in ... }
2319 Now y looks dead, but it'll come alive again. Still, this
2320 seems like the best option at the moment.
2322 * Match on single alternative plus (all (isDeadBinder bndrs))
2323 Rationale: this is essentially seq.
2325 * Match when the rhs is *not* duplicable, and hence would lead to a
2326 join point. This catches the disaster-case above. We can test
2327 the *un-simplified* rhs, which is fine. It might get bigger or
2328 smaller after simplification; if it gets smaller, this case might
2329 fire next time round. NB also that we must test contIsDupable
2330 case_cont *too, because case_cont might be big!
2332 HOWEVER: I found that this version doesn't work well, because
2333 we can get let x = case (...) of { small } in ...case x...
2334 When x is inlined into its full context, we find that it was a bad
2335 idea to have pushed the outer case inside the (...) case.
2337 Note [Single-alternative-unlifted]
2338 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2339 Here's another single-alternative where we really want to do case-of-case:
2347 case y_s6X of tpl_s7m {
2348 M1.Mk1 ipv_s70 -> ipv_s70;
2349 M1.Mk2 ipv_s72 -> ipv_s72;
2355 case x_s74 of tpl_s7n {
2356 M1.Mk1 ipv_s77 -> ipv_s77;
2357 M1.Mk2 ipv_s79 -> ipv_s79;
2361 { __DEFAULT -> ==# [wild1_s7b wild_s7c];
2365 So the outer case is doing *nothing at all*, other than serving as a
2366 join-point. In this case we really want to do case-of-case and decide
2367 whether to use a real join point or just duplicate the continuation.
2369 Hence: check whether the case binder's type is unlifted, because then
2370 the outer case is *not* a seq.