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 )
16 import FamInstEnv ( FamInstEnv )
18 import MkId ( mkImpossibleExpr, seqId )
22 import FamInstEnv ( topNormaliseType )
23 import DataCon ( DataCon, dataConWorkId, dataConRepStrictness )
25 import NewDemand ( isStrictDmd, splitStrictSig )
26 import PprCore ( pprParendExpr, pprCoreExpr )
27 import CoreUnfold ( mkUnfolding, mkCoreUnfolding, mkInlineRule,
28 exprIsConApp_maybe, callSiteInline, CallCtxt(..) )
30 import qualified CoreSubst
31 import CoreArity ( exprArity )
32 import Rules ( lookupRule, getRules )
33 import BasicTypes ( isMarkedStrict, Arity )
34 import CostCentre ( currentCCS, pushCCisNop )
35 import TysPrim ( realWorldStatePrimTy )
36 import PrelInfo ( realWorldPrimId )
37 import BasicTypes ( TopLevelFlag(..), isTopLevel,
38 RecFlag(..), isNonRuleLoopBreaker )
39 import MonadUtils ( foldlM )
40 import Maybes ( orElse )
41 import Data.List ( mapAccumL )
47 The guts of the simplifier is in this module, but the driver loop for
48 the simplifier is in SimplCore.lhs.
51 -----------------------------------------
52 *** IMPORTANT NOTE ***
53 -----------------------------------------
54 The simplifier used to guarantee that the output had no shadowing, but
55 it does not do so any more. (Actually, it never did!) The reason is
56 documented with simplifyArgs.
59 -----------------------------------------
60 *** IMPORTANT NOTE ***
61 -----------------------------------------
62 Many parts of the simplifier return a bunch of "floats" as well as an
63 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
65 All "floats" are let-binds, not case-binds, but some non-rec lets may
66 be unlifted (with RHS ok-for-speculation).
70 -----------------------------------------
71 ORGANISATION OF FUNCTIONS
72 -----------------------------------------
74 - simplify all top-level binders
75 - for NonRec, call simplRecOrTopPair
76 - for Rec, call simplRecBind
79 ------------------------------
80 simplExpr (applied lambda) ==> simplNonRecBind
81 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
82 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
84 ------------------------------
85 simplRecBind [binders already simplfied]
86 - use simplRecOrTopPair on each pair in turn
88 simplRecOrTopPair [binder already simplified]
89 Used for: recursive bindings (top level and nested)
90 top-level non-recursive bindings
92 - check for PreInlineUnconditionally
96 Used for: non-top-level non-recursive bindings
97 beta reductions (which amount to the same thing)
98 Because it can deal with strict arts, it takes a
99 "thing-inside" and returns an expression
101 - check for PreInlineUnconditionally
102 - simplify binder, including its IdInfo
111 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
112 Used for: binding case-binder and constr args in a known-constructor case
113 - check for PreInLineUnconditionally
117 ------------------------------
118 simplLazyBind: [binder already simplified, RHS not]
119 Used for: recursive bindings (top level and nested)
120 top-level non-recursive bindings
121 non-top-level, but *lazy* non-recursive bindings
122 [must not be strict or unboxed]
123 Returns floats + an augmented environment, not an expression
124 - substituteIdInfo and add result to in-scope
125 [so that rules are available in rec rhs]
128 - float if exposes constructor or PAP
132 completeNonRecX: [binder and rhs both simplified]
133 - if the the thing needs case binding (unlifted and not ok-for-spec)
139 completeBind: [given a simplified RHS]
140 [used for both rec and non-rec bindings, top level and not]
141 - try PostInlineUnconditionally
142 - add unfolding [this is the only place we add an unfolding]
147 Right hand sides and arguments
148 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
149 In many ways we want to treat
150 (a) the right hand side of a let(rec), and
151 (b) a function argument
152 in the same way. But not always! In particular, we would
153 like to leave these arguments exactly as they are, so they
154 will match a RULE more easily.
159 It's harder to make the rule match if we ANF-ise the constructor,
160 or eta-expand the PAP:
162 f (let { a = g x; b = h x } in (a,b))
165 On the other hand if we see the let-defns
170 then we *do* want to ANF-ise and eta-expand, so that p and q
171 can be safely inlined.
173 Even floating lets out is a bit dubious. For let RHS's we float lets
174 out if that exposes a value, so that the value can be inlined more vigorously.
177 r = let x = e in (x,x)
179 Here, if we float the let out we'll expose a nice constructor. We did experiments
180 that showed this to be a generally good thing. But it was a bad thing to float
181 lets out unconditionally, because that meant they got allocated more often.
183 For function arguments, there's less reason to expose a constructor (it won't
184 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
185 So for the moment we don't float lets out of function arguments either.
190 For eta expansion, we want to catch things like
192 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
194 If the \x was on the RHS of a let, we'd eta expand to bring the two
195 lambdas together. And in general that's a good thing to do. Perhaps
196 we should eta expand wherever we find a (value) lambda? Then the eta
197 expansion at a let RHS can concentrate solely on the PAP case.
200 %************************************************************************
202 \subsection{Bindings}
204 %************************************************************************
207 simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv
209 simplTopBinds env0 binds0
210 = do { -- Put all the top-level binders into scope at the start
211 -- so that if a transformation rule has unexpectedly brought
212 -- anything into scope, then we don't get a complaint about that.
213 -- It's rather as if the top-level binders were imported.
214 ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
215 ; dflags <- getDOptsSmpl
216 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
217 dopt Opt_D_dump_rule_firings 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 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 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
286 -> InId -> OutBndr -> InExpr -- Binder and rhs
287 -> SimplM SimplEnv -- Returns an env that includes the binding
289 simplRecOrTopPair env top_lvl 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 Recursive old_bndr new_bndr rhs env
296 -- May not actually be recursive, but it doesn't matter
300 simplLazyBind is used for
301 * [simplRecOrTopPair] recursive bindings (whether top level or not)
302 * [simplRecOrTopPair] top-level non-recursive bindings
303 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
306 1. It assumes that the binder is *already* simplified,
307 and is in scope, and its IdInfo too, except unfolding
309 2. It assumes that the binder type is lifted.
311 3. It does not check for pre-inline-unconditionallly;
312 that should have been done already.
315 simplLazyBind :: SimplEnv
316 -> TopLevelFlag -> RecFlag
317 -> InId -> OutId -- Binder, both pre-and post simpl
318 -- The OutId has IdInfo, except arity, unfolding
319 -> InExpr -> SimplEnv -- The RHS and its environment
322 simplLazyBind env top_lvl is_rec bndr bndr1 rhs 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 mkBoringStop
340 -- ANF-ise a constructor or PAP rhs
341 ; (body_env2, body2) <- prepareRhs body_env1 body1
344 <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
345 then -- No floating, just wrap up!
346 do { rhs' <- mkLam env tvs' (wrapFloats body_env2 body2)
347 ; return (env, rhs') }
349 else if null tvs then -- Simple floating
350 do { tick LetFloatFromLet
351 ; return (addFloats env body_env2, body2) }
353 else -- Do type-abstraction first
354 do { tick LetFloatFromLet
355 ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
356 ; rhs' <- mkLam env tvs' body3
357 ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
358 ; return (env', rhs') }
360 ; completeBind env' top_lvl bndr bndr1 rhs' }
363 A specialised variant of simplNonRec used when the RHS is already simplified,
364 notably in knownCon. It uses case-binding where necessary.
367 simplNonRecX :: SimplEnv
368 -> InId -- Old binder
369 -> OutExpr -- Simplified RHS
372 simplNonRecX env bndr new_rhs
373 | isDeadBinder bndr -- Not uncommon; e.g. case (a,b) of b { (p,q) -> p }
374 = return env -- Here b is dead, and we avoid creating
375 | otherwise -- the binding b = (a,b)
376 = do { (env', bndr') <- simplBinder env bndr
377 ; completeNonRecX env' (isStrictId bndr) bndr bndr' new_rhs }
379 completeNonRecX :: SimplEnv
381 -> InId -- Old binder
382 -> OutId -- New binder
383 -> OutExpr -- Simplified RHS
386 completeNonRecX env is_strict old_bndr new_bndr new_rhs
387 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
389 if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
390 then do { tick LetFloatFromLet
391 ; return (addFloats env env1, rhs1) } -- Add the floats to the main env
392 else return (env, wrapFloats env1 rhs1) -- Wrap the floats around the RHS
393 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
396 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
397 Doing so risks exponential behaviour, because new_rhs has been simplified once already
398 In the cases described by the folowing commment, postInlineUnconditionally will
399 catch many of the relevant cases.
400 -- This happens; for example, the case_bndr during case of
401 -- known constructor: case (a,b) of x { (p,q) -> ... }
402 -- Here x isn't mentioned in the RHS, so we don't want to
403 -- create the (dead) let-binding let x = (a,b) in ...
405 -- Similarly, single occurrences can be inlined vigourously
406 -- e.g. case (f x, g y) of (a,b) -> ....
407 -- If a,b occur once we can avoid constructing the let binding for them.
409 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
410 -- Consider case I# (quotInt# x y) of
411 -- I# v -> let w = J# v in ...
412 -- If we gaily inline (quotInt# x y) for v, we end up building an
414 -- let w = J# (quotInt# x y) in ...
415 -- because quotInt# can fail.
417 | preInlineUnconditionally env NotTopLevel bndr new_rhs
418 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
421 ----------------------------------
422 prepareRhs takes a putative RHS, checks whether it's a PAP or
423 constructor application and, if so, converts it to ANF, so that the
424 resulting thing can be inlined more easily. Thus
431 We also want to deal well cases like this
432 v = (f e1 `cast` co) e2
433 Here we want to make e1,e2 trivial and get
434 x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
435 That's what the 'go' loop in prepareRhs does
438 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
439 -- Adds new floats to the env iff that allows us to return a good RHS
440 prepareRhs env (Cast rhs co) -- Note [Float coercions]
441 | (ty1, _ty2) <- coercionKind co -- Do *not* do this if rhs has an unlifted type
442 , not (isUnLiftedType ty1) -- see Note [Float coercions (unlifted)]
443 = do { (env', rhs') <- makeTrivial env rhs
444 ; return (env', Cast rhs' co) }
447 = do { (_is_val, env1, rhs1) <- go 0 env0 rhs0
448 ; return (env1, rhs1) }
450 go n_val_args env (Cast rhs co)
451 = do { (is_val, env', rhs') <- go n_val_args env rhs
452 ; return (is_val, env', Cast rhs' co) }
453 go n_val_args env (App fun (Type ty))
454 = do { (is_val, env', rhs') <- go n_val_args env fun
455 ; return (is_val, env', App rhs' (Type ty)) }
456 go n_val_args env (App fun arg)
457 = do { (is_val, env', fun') <- go (n_val_args+1) env fun
459 True -> do { (env'', arg') <- makeTrivial env' arg
460 ; return (True, env'', App fun' arg') }
461 False -> return (False, env, App fun arg) }
462 go n_val_args env (Var fun)
463 = return (is_val, env, Var fun)
465 is_val = n_val_args > 0 -- There is at least one arg
466 -- ...and the fun a constructor or PAP
467 && (isConLikeId fun || n_val_args < idArity fun)
468 -- See Note [CONLIKE pragma] in BasicTypes
470 = return (False, env, other)
474 Note [Float coercions]
475 ~~~~~~~~~~~~~~~~~~~~~~
476 When we find the binding
478 we'd like to transform it to
480 x = x `cast` co -- A trivial binding
481 There's a chance that e will be a constructor application or function, or something
482 like that, so moving the coerion to the usage site may well cancel the coersions
483 and lead to further optimisation. Example:
486 data instance T Int = T Int
488 foo :: Int -> Int -> Int
493 go n = case x of { T m -> go (n-m) }
494 -- This case should optimise
496 Note [Float coercions (unlifted)]
497 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
498 BUT don't do [Float coercions] if 'e' has an unlifted type.
501 foo :: Int = (error (# Int,Int #) "urk")
502 `cast` CoUnsafe (# Int,Int #) Int
504 If do the makeTrivial thing to the error call, we'll get
505 foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
506 But 'v' isn't in scope!
508 These strange casts can happen as a result of case-of-case
509 bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
514 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
515 -- Binds the expression to a variable, if it's not trivial, returning the variable
519 | otherwise -- See Note [Take care] below
520 = do { var <- newId (fsLit "a") (exprType expr)
521 ; env' <- completeNonRecX env False var var expr
522 -- pprTrace "makeTrivial" (vcat [ppr var <+> ppr (exprArity (substExpr env' (Var var)))
524 -- , ppr (substExpr env' (Var var))
525 -- , ppr (idArity (fromJust (lookupInScope (seInScope env') var))) ]) $
526 ; return (env', substExpr env' (Var var)) }
527 -- The substitution is needed becase we're constructing a new binding
529 -- And if rhs is of form (rhs1 |> co), then we might get
532 -- and now a's RHS is trivial and can be substituted out, and that
533 -- is what completeNonRecX will do
537 %************************************************************************
539 \subsection{Completing a lazy binding}
541 %************************************************************************
544 * deals only with Ids, not TyVars
545 * takes an already-simplified binder and RHS
546 * is used for both recursive and non-recursive bindings
547 * is used for both top-level and non-top-level bindings
549 It does the following:
550 - tries discarding a dead binding
551 - tries PostInlineUnconditionally
552 - add unfolding [this is the only place we add an unfolding]
555 It does *not* attempt to do let-to-case. Why? Because it is used for
556 - top-level bindings (when let-to-case is impossible)
557 - many situations where the "rhs" is known to be a WHNF
558 (so let-to-case is inappropriate).
560 Nor does it do the atomic-argument thing
563 completeBind :: SimplEnv
564 -> TopLevelFlag -- Flag stuck into unfolding
565 -> InId -- Old binder
566 -> OutId -> OutExpr -- New binder and RHS
568 -- completeBind may choose to do its work
569 -- * by extending the substitution (e.g. let x = y in ...)
570 -- * or by adding to the floats in the envt
572 completeBind env top_lvl old_bndr new_bndr new_rhs
573 = do { let old_info = idInfo old_bndr
574 old_unf = unfoldingInfo old_info
575 occ_info = occInfo old_info
577 ; new_unfolding <- simplUnfolding env top_lvl old_bndr occ_info new_rhs old_unf
579 ; if postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs new_unfolding
580 -- Inline and discard the binding
581 then do { tick (PostInlineUnconditionally old_bndr)
582 ; return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
583 -- Use the substitution to make quite, quite sure that the
584 -- substitution will happen, since we are going to discard the binding
586 else return (addNonRecWithUnf env new_bndr new_rhs new_unfolding) }
588 ------------------------------
589 addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv
590 -- Add a new binding to the environment, complete with its unfolding
591 -- but *do not* do postInlineUnconditionally, because we have already
592 -- processed some of the scope of the binding
593 -- We still want the unfolding though. Consider
595 -- x = /\a. let y = ... in Just y
597 -- Then we float the y-binding out (via abstractFloats and addPolyBind)
598 -- but 'x' may well then be inlined in 'body' in which case we'd like the
599 -- opportunity to inline 'y' too.
601 addPolyBind top_lvl env (NonRec poly_id rhs)
602 = do { unfolding <- simplUnfolding env top_lvl poly_id NoOccInfo rhs noUnfolding
603 -- Assumes that poly_id did not have an INLINE prag
604 -- which is perhaps wrong. ToDo: think about this
605 ; return (addNonRecWithUnf env poly_id rhs unfolding) }
607 addPolyBind _ env bind@(Rec _) = return (extendFloats env bind)
608 -- Hack: letrecs are more awkward, so we extend "by steam"
609 -- without adding unfoldings etc. At worst this leads to
610 -- more simplifier iterations
612 ------------------------------
613 addNonRecWithUnf :: SimplEnv
614 -> OutId -> OutExpr -- New binder and RHS
615 -> Unfolding -- New unfolding
617 addNonRecWithUnf env new_bndr new_rhs new_unfolding
618 = let new_arity = exprArity new_rhs
619 old_arity = idArity new_bndr
620 info1 = idInfo new_bndr `setArityInfo` new_arity
622 -- Unfolding info: Note [Setting the new unfolding]
623 info2 = info1 `setUnfoldingInfo` new_unfolding
625 -- Demand info: Note [Setting the demand info]
626 info3 | isEvaldUnfolding new_unfolding = zapDemandInfo info2 `orElse` info2
629 final_id = new_bndr `setIdInfo` info3
630 dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr
632 ASSERT( isId new_bndr )
633 WARN( new_arity < old_arity || new_arity < dmd_arity,
634 (ptext (sLit "Arity decrease:") <+> ppr final_id <+> ppr old_arity
635 <+> ppr new_arity <+> ppr dmd_arity) $$ ppr new_rhs )
636 -- Note [Arity decrease]
638 final_id `seq` -- This seq forces the Id, and hence its IdInfo,
639 -- and hence any inner substitutions
640 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
641 addNonRec env final_id new_rhs
642 -- The addNonRec adds it to the in-scope set too
644 ------------------------------
645 simplUnfolding :: SimplEnv-> TopLevelFlag
646 -> Id -- Debug output only
647 -> OccInfo -> OutExpr
648 -> Unfolding -> SimplM Unfolding
649 -- Note [Setting the new unfolding]
650 simplUnfolding env _ _ _ _ (DFunUnfolding con ops)
651 = return (DFunUnfolding con ops')
653 ops' = map (CoreSubst.substExpr (mkCoreSubst env)) ops
655 simplUnfolding env top_lvl _ _ _
656 (CoreUnfolding { uf_tmpl = expr, uf_arity = arity
657 , uf_guidance = guide@(InlineRule {}) })
658 = do { expr' <- simplExpr (setMode SimplGently env) expr
659 ; let mb_wkr' = CoreSubst.substInlineRuleGuidance (mkCoreSubst env) (ug_ir_info guide)
660 ; return (mkCoreUnfolding (isTopLevel top_lvl) expr' arity
661 (guide { ug_ir_info = mb_wkr' })) }
662 -- See Note [Top-level flag on inline rules] in CoreUnfold
664 simplUnfolding _ top_lvl _ occ_info new_rhs _
665 | omit_unfolding = return NoUnfolding
666 | otherwise = return (mkUnfolding (isTopLevel top_lvl) new_rhs)
668 omit_unfolding = isNonRuleLoopBreaker occ_info
671 Note [Arity decrease]
672 ~~~~~~~~~~~~~~~~~~~~~
673 Generally speaking the arity of a binding should not decrease. But it *can*
674 legitimately happen becuase of RULES. Eg
676 where g has arity 2, will have arity 2. But if there's a rewrite rule
678 where h has arity 1, then f's arity will decrease. Here's a real-life example,
679 which is in the output of Specialise:
682 $dm {Arity 2} = \d.\x. op d
683 {-# RULES forall d. $dm Int d = $s$dm #-}
685 dInt = MkD .... opInt ...
686 opInt {Arity 1} = $dm dInt
688 $s$dm {Arity 0} = \x. op dInt }
690 Here opInt has arity 1; but when we apply the rule its arity drops to 0.
691 That's why Specialise goes to a little trouble to pin the right arity
692 on specialised functions too.
694 Note [Setting the new unfolding]
695 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
696 * If there's an INLINE pragma, we simplify the RHS gently. Maybe we
697 should do nothing at all, but simplifying gently might get rid of
700 * If not, we make an unfolding from the new RHS. But *only* for
701 non-loop-breakers. Making loop breakers not have an unfolding at all
702 means that we can avoid tests in exprIsConApp, for example. This is
703 important: if exprIsConApp says 'yes' for a recursive thing, then we
704 can get into an infinite loop
706 If there's an InlineRule on a loop breaker, we hang on to the inlining.
707 It's pretty dodgy, but the user did say 'INLINE'. May need to revisit
710 Note [Setting the demand info]
711 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
712 If the unfolding is a value, the demand info may
713 go pear-shaped, so we nuke it. Example:
715 case x of (p,q) -> h p q x
716 Here x is certainly demanded. But after we've nuked
717 the case, we'll get just
718 let x = (a,b) in h a b x
719 and now x is not demanded (I'm assuming h is lazy)
720 This really happens. Similarly
721 let f = \x -> e in ...f..f...
722 After inlining f at some of its call sites the original binding may
723 (for example) be no longer strictly demanded.
724 The solution here is a bit ad hoc...
727 %************************************************************************
729 \subsection[Simplify-simplExpr]{The main function: simplExpr}
731 %************************************************************************
733 The reason for this OutExprStuff stuff is that we want to float *after*
734 simplifying a RHS, not before. If we do so naively we get quadratic
735 behaviour as things float out.
737 To see why it's important to do it after, consider this (real) example:
751 a -- Can't inline a this round, cos it appears twice
755 Each of the ==> steps is a round of simplification. We'd save a
756 whole round if we float first. This can cascade. Consider
761 let f = let d1 = ..d.. in \y -> e
765 in \x -> ...(\y ->e)...
767 Only in this second round can the \y be applied, and it
768 might do the same again.
772 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
773 simplExpr env expr = simplExprC env expr mkBoringStop
775 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
776 -- Simplify an expression, given a continuation
777 simplExprC env expr cont
778 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
779 do { (env', expr') <- simplExprF (zapFloats env) expr cont
780 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
781 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
782 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
783 return (wrapFloats env' expr') }
785 --------------------------------------------------
786 simplExprF :: SimplEnv -> InExpr -> SimplCont
787 -> SimplM (SimplEnv, OutExpr)
789 simplExprF env e cont
790 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
791 simplExprF' env e cont
793 simplExprF' :: SimplEnv -> InExpr -> SimplCont
794 -> SimplM (SimplEnv, OutExpr)
795 simplExprF' env (Var v) cont = simplVar env v cont
796 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
797 simplExprF' env (Note n expr) cont = simplNote env n expr cont
798 simplExprF' env (Cast body co) cont = simplCast env body co cont
799 simplExprF' env (App fun arg) cont = simplExprF env fun $
800 ApplyTo NoDup arg env cont
802 simplExprF' env expr@(Lam _ _) cont
803 = simplLam env (map zap bndrs) body cont
804 -- The main issue here is under-saturated lambdas
805 -- (\x1. \x2. e) arg1
806 -- Here x1 might have "occurs-once" occ-info, because occ-info
807 -- is computed assuming that a group of lambdas is applied
808 -- all at once. If there are too few args, we must zap the
811 n_args = countArgs cont
812 n_params = length bndrs
813 (bndrs, body) = collectBinders expr
814 zap | n_args >= n_params = \b -> b
815 | otherwise = \b -> if isTyVar b then b
817 -- NB: we count all the args incl type args
818 -- so we must count all the binders (incl type lambdas)
820 simplExprF' env (Type ty) cont
821 = ASSERT( contIsRhsOrArg cont )
822 do { ty' <- simplType env ty
823 ; rebuild env (Type ty') cont }
825 simplExprF' env (Case scrut bndr _ alts) cont
826 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
827 = -- Simplify the scrutinee with a Select continuation
828 simplExprF env scrut (Select NoDup bndr alts env cont)
831 = -- If case-of-case is off, simply simplify the case expression
832 -- in a vanilla Stop context, and rebuild the result around it
833 do { case_expr' <- simplExprC env scrut case_cont
834 ; rebuild env case_expr' cont }
836 case_cont = Select NoDup bndr alts env mkBoringStop
838 simplExprF' env (Let (Rec pairs) body) cont
839 = do { env' <- simplRecBndrs env (map fst pairs)
840 -- NB: bndrs' don't have unfoldings or rules
841 -- We add them as we go down
843 ; env'' <- simplRecBind env' NotTopLevel pairs
844 ; simplExprF env'' body cont }
846 simplExprF' env (Let (NonRec bndr rhs) body) cont
847 = simplNonRecE env bndr (rhs, env) ([], body) cont
849 ---------------------------------
850 simplType :: SimplEnv -> InType -> SimplM OutType
851 -- Kept monadic just so we can do the seqType
853 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
854 seqType new_ty `seq` return new_ty
856 new_ty = substTy env ty
858 ---------------------------------
859 simplCoercion :: SimplEnv -> InType -> SimplM OutType
861 = do { co' <- simplType env co
862 ; return (optCoercion co') }
866 %************************************************************************
868 \subsection{The main rebuilder}
870 %************************************************************************
873 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
874 -- At this point the substitution in the SimplEnv should be irrelevant
875 -- only the in-scope set and floats should matter
876 rebuild env expr cont0
877 = -- pprTrace "rebuild" (ppr expr $$ ppr cont0 $$ ppr (seFloats env)) $
879 Stop {} -> return (env, expr)
880 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
881 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
882 StrictArg fun _ info cont -> rebuildCall env (fun `App` expr) info cont
883 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
884 ; simplLam env' bs body cont }
885 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
886 ; rebuild env (App expr arg') cont }
890 %************************************************************************
894 %************************************************************************
897 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
898 -> SimplM (SimplEnv, OutExpr)
899 simplCast env body co0 cont0
900 = do { co1 <- simplCoercion env co0
901 ; simplExprF env body (addCoerce co1 cont0) }
903 addCoerce co cont = add_coerce co (coercionKind co) cont
905 add_coerce _co (s1, k1) cont -- co :: ty~ty
906 | s1 `coreEqType` k1 = cont -- is a no-op
908 add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
909 | (_l1, t1) <- coercionKind co2
910 -- e |> (g1 :: S1~L) |> (g2 :: L~T1)
913 -- e |> (g1 . g2 :: S1~T1) otherwise
915 -- For example, in the initial form of a worker
916 -- we may find (coerce T (coerce S (\x.e))) y
917 -- and we'd like it to simplify to e[y/x] in one round
919 , s1 `coreEqType` t1 = cont -- The coerces cancel out
920 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
922 add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
923 -- (f |> g) ty ---> (f ty) |> (g @ ty)
924 -- This implements the PushT rule from the paper
925 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
926 , not (isCoVar tyvar)
927 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
929 ty' = substTy (arg_se `setInScope` env) arg_ty
931 -- ToDo: the PushC rule is not implemented at all
933 add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
934 | not (isTypeArg arg) -- This implements the Push rule from the paper
935 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
936 -- (e |> (g :: s1s2 ~ t1->t2)) f
938 -- (e (f |> (arg g :: t1~s1))
939 -- |> (res g :: s2->t2)
941 -- t1t2 must be a function type, t1->t2, because it's applied
942 -- to something but s1s2 might conceivably not be
944 -- When we build the ApplyTo we can't mix the out-types
945 -- with the InExpr in the argument, so we simply substitute
946 -- to make it all consistent. It's a bit messy.
947 -- But it isn't a common case.
949 -- Example of use: Trac #995
950 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
952 -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
953 -- t2 ~ s2 with left and right on the curried form:
954 -- (->) t1 t2 ~ (->) s1 s2
955 [co1, co2] = decomposeCo 2 co
956 new_arg = mkCoerce (mkSymCoercion co1) arg'
957 arg' = substExpr (arg_se `setInScope` env) arg
959 add_coerce co _ cont = CoerceIt co cont
963 %************************************************************************
967 %************************************************************************
970 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
971 -> SimplM (SimplEnv, OutExpr)
973 simplLam env [] body cont = simplExprF env body cont
976 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
977 = do { tick (BetaReduction bndr)
978 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
980 -- Not enough args, so there are real lambdas left to put in the result
981 simplLam env bndrs body cont
982 = do { (env', bndrs') <- simplLamBndrs env bndrs
983 ; body' <- simplExpr env' body
984 ; new_lam <- mkLam env' bndrs' body'
985 ; rebuild env' new_lam cont }
988 simplNonRecE :: SimplEnv
989 -> InBndr -- The binder
990 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
991 -> ([InBndr], InExpr) -- Body of the let/lambda
994 -> SimplM (SimplEnv, OutExpr)
996 -- simplNonRecE is used for
997 -- * non-top-level non-recursive lets in expressions
1000 -- It deals with strict bindings, via the StrictBind continuation,
1001 -- which may abort the whole process
1003 -- The "body" of the binding comes as a pair of ([InId],InExpr)
1004 -- representing a lambda; so we recurse back to simplLam
1005 -- Why? Because of the binder-occ-info-zapping done before
1006 -- the call to simplLam in simplExprF (Lam ...)
1008 -- First deal with type applications and type lets
1009 -- (/\a. e) (Type ty) and (let a = Type ty in e)
1010 simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
1011 = ASSERT( isTyVar bndr )
1012 do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
1013 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
1015 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
1016 | preInlineUnconditionally env NotTopLevel bndr rhs
1017 = do { tick (PreInlineUnconditionally bndr)
1018 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
1021 = do { simplExprF (rhs_se `setFloats` env) rhs
1022 (StrictBind bndr bndrs body env cont) }
1025 = ASSERT( not (isTyVar bndr) )
1026 do { (env1, bndr1) <- simplNonRecBndr env bndr
1027 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
1028 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
1029 ; simplLam env3 bndrs body cont }
1033 %************************************************************************
1037 %************************************************************************
1040 -- Hack alert: we only distinguish subsumed cost centre stacks for the
1041 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
1042 simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
1043 -> SimplM (SimplEnv, OutExpr)
1044 simplNote env (SCC cc) e cont
1045 | pushCCisNop cc (getEnclosingCC env) -- scc "f" (...(scc "f" e)...)
1046 = simplExprF env e cont -- ==> scc "f" (...e...)
1048 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
1049 ; rebuild env (mkSCC cc e') cont }
1051 simplNote env (CoreNote s) e cont
1052 = do { e' <- simplExpr env e
1053 ; rebuild env (Note (CoreNote s) e') cont }
1057 %************************************************************************
1059 \subsection{Dealing with calls}
1061 %************************************************************************
1064 simplVar :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1065 simplVar env var cont
1066 = case substId env var of
1067 DoneEx e -> simplExprF (zapSubstEnv env) e cont
1068 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
1069 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
1070 -- Note [zapSubstEnv]
1071 -- The template is already simplified, so don't re-substitute.
1072 -- This is VITAL. Consider
1074 -- let y = \z -> ...x... in
1076 -- We'll clone the inner \x, adding x->x' in the id_subst
1077 -- Then when we inline y, we must *not* replace x by x' in
1078 -- the inlined copy!!
1080 ---------------------------------------------------------
1081 -- Dealing with a call site
1083 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1084 completeCall env var cont
1085 = do { let (args,call_cont) = contArgs cont
1086 -- The args are OutExprs, obtained by *lazily* substituting
1087 -- in the args found in cont. These args are only examined
1088 -- to limited depth (unless a rule fires). But we must do
1089 -- the substitution; rule matching on un-simplified args would
1092 ------------- First try rules ----------------
1093 -- Do this before trying inlining. Some functions have
1094 -- rules *and* are strict; in this case, we don't want to
1095 -- inline the wrapper of the non-specialised thing; better
1096 -- to call the specialised thing instead.
1098 -- We used to use the black-listing mechanism to ensure that inlining of
1099 -- the wrapper didn't occur for things that have specialisations till a
1100 -- later phase, so but now we just try RULES first
1102 -- See also Note [Rules for recursive functions]
1103 ; rule_base <- getSimplRules
1104 ; let rules = getRules rule_base var
1105 ; mb_rule <- tryRules env var rules args call_cont
1107 Just (n_args, rule_rhs) -> simplExprF env rule_rhs (dropArgs n_args cont) ;
1108 -- The ruleArity says how many args the rule consumed
1109 ; Nothing -> do -- No rules
1112 ------------- Next try inlining ----------------
1113 { dflags <- getDOptsSmpl
1114 ; let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
1115 n_val_args = length arg_infos
1116 interesting_cont = interestingCallContext call_cont
1117 active_inline = activeInline env var
1118 maybe_inline = callSiteInline dflags active_inline var
1119 (null args) arg_infos interesting_cont
1120 ; case maybe_inline of {
1121 Just unfolding -- There is an inlining!
1122 -> do { tick (UnfoldingDone var)
1123 ; (if dopt Opt_D_dump_inlinings dflags then
1124 pprTrace ("Inlining done: " ++ showSDoc (ppr var)) (vcat [
1125 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1126 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1127 text "Cont: " <+> ppr call_cont])
1130 simplExprF env unfolding cont }
1132 ; Nothing -> -- No inlining!
1134 ------------- No inlining! ----------------
1135 -- Next, look for rules or specialisations that match
1137 rebuildCall env (Var var)
1138 (mkArgInfo var rules n_val_args call_cont)
1142 rebuildCall :: SimplEnv
1143 -> OutExpr -- Function
1146 -> SimplM (SimplEnv, OutExpr)
1147 rebuildCall env fun (ArgInfo { ai_strs = [] }) cont
1148 -- When we run out of strictness args, it means
1149 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1150 -- Then we want to discard the entire strict continuation. E.g.
1151 -- * case (error "hello") of { ... }
1152 -- * (error "Hello") arg
1153 -- * f (error "Hello") where f is strict
1155 -- Then, especially in the first of these cases, we'd like to discard
1156 -- the continuation, leaving just the bottoming expression. But the
1157 -- type might not be right, so we may have to add a coerce.
1158 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1159 = return (env, mk_coerce fun) -- contination to discard, else we do it
1160 where -- again and again!
1161 fun_ty = exprType fun
1162 cont_ty = contResultType env fun_ty cont
1163 co = mkUnsafeCoercion fun_ty cont_ty
1164 mk_coerce expr | cont_ty `coreEqType` fun_ty = expr
1165 | otherwise = mkCoerce co expr
1167 rebuildCall env fun info (ApplyTo _ (Type arg_ty) se cont)
1168 = do { ty' <- simplType (se `setInScope` env) arg_ty
1169 ; rebuildCall env (fun `App` Type ty') info cont }
1172 (ArgInfo { ai_rules = has_rules, ai_strs = str:strs, ai_discs = disc:discs })
1173 (ApplyTo _ arg arg_se cont)
1174 | str -- Strict argument
1175 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1176 simplExprF (arg_se `setFloats` env) arg
1177 (StrictArg fun cci arg_info' cont)
1180 | otherwise -- Lazy argument
1181 -- DO NOT float anything outside, hence simplExprC
1182 -- There is no benefit (unlike in a let-binding), and we'd
1183 -- have to be very careful about bogus strictness through
1184 -- floating a demanded let.
1185 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1187 ; rebuildCall env (fun `App` arg') arg_info' cont }
1189 arg_info' = ArgInfo { ai_rules = has_rules, ai_strs = strs, ai_discs = discs }
1190 cci | has_rules || disc > 0 = ArgCtxt has_rules disc -- Be keener here
1191 | otherwise = BoringCtxt -- Nothing interesting
1193 rebuildCall env fun _ cont
1194 = rebuild env fun cont
1199 This part of the simplifier may break the no-shadowing invariant
1201 f (...(\a -> e)...) (case y of (a,b) -> e')
1202 where f is strict in its second arg
1203 If we simplify the innermost one first we get (...(\a -> e)...)
1204 Simplifying the second arg makes us float the case out, so we end up with
1205 case y of (a,b) -> f (...(\a -> e)...) e'
1206 So the output does not have the no-shadowing invariant. However, there is
1207 no danger of getting name-capture, because when the first arg was simplified
1208 we used an in-scope set that at least mentioned all the variables free in its
1209 static environment, and that is enough.
1211 We can't just do innermost first, or we'd end up with a dual problem:
1212 case x of (a,b) -> f e (...(\a -> e')...)
1214 I spent hours trying to recover the no-shadowing invariant, but I just could
1215 not think of an elegant way to do it. The simplifier is already knee-deep in
1216 continuations. We have to keep the right in-scope set around; AND we have
1217 to get the effect that finding (error "foo") in a strict arg position will
1218 discard the entire application and replace it with (error "foo"). Getting
1219 all this at once is TOO HARD!
1222 %************************************************************************
1226 %************************************************************************
1229 tryRules :: SimplEnv
1230 -> Id -> [CoreRule] -> [OutExpr] -> SimplCont
1231 -> SimplM (Maybe (Arity, CoreExpr)) -- The arity is the number of
1232 -- args consumed by the rule
1233 tryRules env fn rules args call_cont
1237 = do { dflags <- getDOptsSmpl
1238 ; case activeRule dflags env of {
1239 Nothing -> return Nothing ; -- No rules apply
1242 case lookupRule act_fn (getInScope env) fn args rules of {
1243 Nothing -> return Nothing ; -- No rule matches
1244 Just (rule, rule_rhs) ->
1246 do { tick (RuleFired (ru_name rule))
1247 ; (if dopt Opt_D_dump_rule_firings dflags then
1248 pprTrace "Rule fired" (vcat [
1249 text "Rule:" <+> ftext (ru_name rule),
1250 text "Before:" <+> ppr fn <+> sep (map pprParendExpr args),
1251 text "After: " <+> pprCoreExpr rule_rhs,
1252 text "Cont: " <+> ppr call_cont])
1255 return (Just (ruleArity rule, rule_rhs)) }}}}
1258 Note [Rules for recursive functions]
1259 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1260 You might think that we shouldn't apply rules for a loop breaker:
1261 doing so might give rise to an infinite loop, because a RULE is
1262 rather like an extra equation for the function:
1263 RULE: f (g x) y = x+y
1266 But it's too drastic to disable rules for loop breakers.
1267 Even the foldr/build rule would be disabled, because foldr
1268 is recursive, and hence a loop breaker:
1269 foldr k z (build g) = g k z
1270 So it's up to the programmer: rules can cause divergence
1273 %************************************************************************
1275 Rebuilding a cse expression
1277 %************************************************************************
1279 Note [Case elimination]
1280 ~~~~~~~~~~~~~~~~~~~~~~~
1281 The case-elimination transformation discards redundant case expressions.
1282 Start with a simple situation:
1284 case x# of ===> e[x#/y#]
1287 (when x#, y# are of primitive type, of course). We can't (in general)
1288 do this for algebraic cases, because we might turn bottom into
1291 The code in SimplUtils.prepareAlts has the effect of generalise this
1292 idea to look for a case where we're scrutinising a variable, and we
1293 know that only the default case can match. For example:
1297 DEFAULT -> ...(case x of
1301 Here the inner case is first trimmed to have only one alternative, the
1302 DEFAULT, after which it's an instance of the previous case. This
1303 really only shows up in eliminating error-checking code.
1305 We also make sure that we deal with this very common case:
1310 Here we are using the case as a strict let; if x is used only once
1311 then we want to inline it. We have to be careful that this doesn't
1312 make the program terminate when it would have diverged before, so we
1314 - e is already evaluated (it may so if e is a variable)
1315 - x is used strictly, or
1317 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1319 case e of ===> case e of DEFAULT -> r
1323 Now again the case may be elminated by the CaseElim transformation.
1326 Further notes about case elimination
1327 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1328 Consider: test :: Integer -> IO ()
1331 Turns out that this compiles to:
1334 eta1 :: State# RealWorld ->
1335 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1337 (PrelNum.jtos eta ($w[] @ Char))
1339 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1341 Notice the strange '<' which has no effect at all. This is a funny one.
1342 It started like this:
1344 f x y = if x < 0 then jtos x
1345 else if y==0 then "" else jtos x
1347 At a particular call site we have (f v 1). So we inline to get
1349 if v < 0 then jtos x
1350 else if 1==0 then "" else jtos x
1352 Now simplify the 1==0 conditional:
1354 if v<0 then jtos v else jtos v
1356 Now common-up the two branches of the case:
1358 case (v<0) of DEFAULT -> jtos v
1360 Why don't we drop the case? Because it's strict in v. It's technically
1361 wrong to drop even unnecessary evaluations, and in practice they
1362 may be a result of 'seq' so we *definitely* don't want to drop those.
1363 I don't really know how to improve this situation.
1366 ---------------------------------------------------------
1367 -- Eliminate the case if possible
1369 rebuildCase, reallyRebuildCase
1371 -> OutExpr -- Scrutinee
1372 -> InId -- Case binder
1373 -> [InAlt] -- Alternatives (inceasing order)
1375 -> SimplM (SimplEnv, OutExpr)
1377 --------------------------------------------------
1378 -- 1. Eliminate the case if there's a known constructor
1379 --------------------------------------------------
1381 rebuildCase env scrut case_bndr alts cont
1382 | Lit lit <- scrut -- No need for same treatment as constructors
1383 -- because literals are inlined more vigorously
1384 = do { tick (KnownBranch case_bndr)
1385 ; case findAlt (LitAlt lit) alts of
1386 Nothing -> missingAlt env case_bndr alts cont
1387 Just (_, bs, rhs) -> simple_rhs bs rhs }
1389 | Just (con, ty_args, other_args) <- exprIsConApp_maybe scrut
1390 -- Works when the scrutinee is a variable with a known unfolding
1391 -- as well as when it's an explicit constructor application
1392 = do { tick (KnownBranch case_bndr)
1393 ; case findAlt (DataAlt con) alts of
1394 Nothing -> missingAlt env case_bndr alts cont
1395 Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs
1396 Just (_, bs, rhs) -> knownCon env scrut con ty_args other_args
1397 case_bndr bs rhs cont
1400 simple_rhs bs rhs = ASSERT( null bs )
1401 do { env' <- simplNonRecX env case_bndr scrut
1402 ; simplExprF env' rhs cont }
1405 --------------------------------------------------
1406 -- 2. Eliminate the case if scrutinee is evaluated
1407 --------------------------------------------------
1409 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
1410 -- See if we can get rid of the case altogether
1411 -- See Note [Case eliminiation]
1412 -- mkCase made sure that if all the alternatives are equal,
1413 -- then there is now only one (DEFAULT) rhs
1414 | all isDeadBinder bndrs -- bndrs are [InId]
1416 -- Check that the scrutinee can be let-bound instead of case-bound
1417 , exprOkForSpeculation scrut
1418 -- OK not to evaluate it
1419 -- This includes things like (==# a# b#)::Bool
1420 -- so that we simplify
1421 -- case ==# a# b# of { True -> x; False -> x }
1424 -- This particular example shows up in default methods for
1425 -- comparision operations (e.g. in (>=) for Int.Int32)
1426 || exprIsHNF scrut -- It's already evaluated
1427 || var_demanded_later scrut -- It'll be demanded later
1429 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1430 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1431 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1432 -- its argument: case x of { y -> dataToTag# y }
1433 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1434 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1436 -- Also we don't want to discard 'seq's
1437 = do { tick (CaseElim case_bndr)
1438 ; env' <- simplNonRecX env case_bndr scrut
1439 ; simplExprF env' rhs cont }
1441 -- The case binder is going to be evaluated later,
1442 -- and the scrutinee is a simple variable
1443 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1444 && not (isTickBoxOp v)
1445 -- ugly hack; covering this case is what
1446 -- exprOkForSpeculation was intended for.
1447 var_demanded_later _ = False
1449 rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
1450 | all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq'
1451 = -- For this case, see Note [Rules for seq] in MkId
1452 do { let rhs' = substExpr env rhs
1453 out_args = [Type (substTy env (idType case_bndr)),
1454 Type (exprType rhs'), scrut, rhs']
1455 -- Lazily evaluated, so we don't do most of this
1457 ; rule_base <- getSimplRules
1458 ; let rules = getRules rule_base seqId
1459 ; mb_rule <- tryRules env seqId rules out_args cont
1461 Just (n_args, res) -> simplExprF (zapSubstEnv env)
1462 (mkApps res (drop n_args out_args))
1464 Nothing -> reallyRebuildCase env scrut case_bndr alts cont }
1466 rebuildCase env scrut case_bndr alts cont
1467 = reallyRebuildCase env scrut case_bndr alts cont
1469 --------------------------------------------------
1470 -- 3. Catch-all case
1471 --------------------------------------------------
1473 reallyRebuildCase env scrut case_bndr alts cont
1474 = do { -- Prepare the continuation;
1475 -- The new subst_env is in place
1476 (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1478 -- Simplify the alternatives
1479 ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
1481 -- Check for empty alternatives
1482 ; if null alts' then missingAlt env case_bndr alts cont
1484 { case_expr <- mkCase scrut' case_bndr' alts'
1486 -- Notice that rebuild gets the in-scope set from env, not alt_env
1487 -- The case binder *not* scope over the whole returned case-expression
1488 ; rebuild env' case_expr nodup_cont } }
1491 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1492 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1493 way, there's a chance that v will now only be used once, and hence
1496 Historical note: we use to do the "case binder swap" in the Simplifier
1497 so there were additional complications if the scrutinee was a variable.
1498 Now the binder-swap stuff is done in the occurrence analyer; see
1499 OccurAnal Note [Binder swap].
1503 If the case binder is not dead, then neither are the pattern bound
1505 case <any> of x { (a,b) ->
1506 case x of { (p,q) -> p } }
1507 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1508 The point is that we bring into the envt a binding
1510 after the outer case, and that makes (a,b) alive. At least we do unless
1511 the case binder is guaranteed dead.
1513 In practice, the scrutinee is almost always a variable, so we pretty
1514 much always zap the OccInfo of the binders. It doesn't matter much though.
1519 Consider case (v `cast` co) of x { I# ->
1520 ... (case (v `cast` co) of {...}) ...
1521 We'd like to eliminate the inner case. We can get this neatly by
1522 arranging that inside the outer case we add the unfolding
1523 v |-> x `cast` (sym co)
1524 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1526 Note [Improving seq]
1529 type family F :: * -> *
1530 type instance F Int = Int
1532 ... case e of x { DEFAULT -> rhs } ...
1534 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1536 case e `cast` co of x'::Int
1537 I# x# -> let x = x' `cast` sym co
1540 so that 'rhs' can take advantage of the form of x'. Notice that Note
1541 [Case of cast] may then apply to the result.
1543 This showed up in Roman's experiments. Example:
1544 foo :: F Int -> Int -> Int
1545 foo t n = t `seq` bar n
1548 bar n = bar (n - case t of TI i -> i)
1549 Here we'd like to avoid repeated evaluating t inside the loop, by
1550 taking advantage of the `seq`.
1552 At one point I did transformation in LiberateCase, but it's more robust here.
1553 (Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
1554 LiberateCase gets to see it.)
1560 improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
1561 -> OutExpr -> InId -> OutId -> [InAlt]
1562 -> SimplM (SimplEnv, OutExpr, OutId)
1563 -- Note [Improving seq]
1564 improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1565 | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1566 = do { case_bndr2 <- newId (fsLit "nt") ty2
1567 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
1568 env2 = extendIdSubst env case_bndr rhs
1569 ; return (env2, scrut `Cast` co, case_bndr2) }
1571 improveSeq _ env scrut _ case_bndr1 _
1572 = return (env, scrut, case_bndr1)
1575 improve_case_bndr env scrut case_bndr
1576 -- See Note [no-case-of-case]
1577 -- | switchIsOn (getSwitchChecker env) NoCaseOfCase
1578 -- = (env, case_bndr)
1580 | otherwise -- Failed try; see Note [Suppressing the case binder-swap]
1581 -- not (isEvaldUnfolding (idUnfolding v))
1583 Var v -> (modifyInScope env1 v case_bndr', case_bndr')
1584 -- Note about using modifyInScope for v here
1585 -- We could extend the substitution instead, but it would be
1586 -- a hack because then the substitution wouldn't be idempotent
1587 -- any more (v is an OutId). And this does just as well.
1589 Cast (Var v) co -> (addBinderUnfolding env1 v rhs, case_bndr')
1591 rhs = Cast (Var case_bndr') (mkSymCoercion co)
1593 _ -> (env, case_bndr)
1595 case_bndr' = zapIdOccInfo case_bndr
1596 env1 = modifyInScope env case_bndr case_bndr'
1601 simplAlts does two things:
1603 1. Eliminate alternatives that cannot match, including the
1604 DEFAULT alternative.
1606 2. If the DEFAULT alternative can match only one possible constructor,
1607 then make that constructor explicit.
1609 case e of x { DEFAULT -> rhs }
1611 case e of x { (a,b) -> rhs }
1612 where the type is a single constructor type. This gives better code
1613 when rhs also scrutinises x or e.
1615 Here "cannot match" includes knowledge from GADTs
1617 It's a good idea do do this stuff before simplifying the alternatives, to
1618 avoid simplifying alternatives we know can't happen, and to come up with
1619 the list of constructors that are handled, to put into the IdInfo of the
1620 case binder, for use when simplifying the alternatives.
1622 Eliminating the default alternative in (1) isn't so obvious, but it can
1625 data Colour = Red | Green | Blue
1634 DEFAULT -> [ case y of ... ]
1636 If we inline h into f, the default case of the inlined h can't happen.
1637 If we don't notice this, we may end up filtering out *all* the cases
1638 of the inner case y, which give us nowhere to go!
1642 simplAlts :: SimplEnv
1644 -> InId -- Case binder
1645 -> [InAlt] -- Non-empty
1647 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1648 -- Like simplExpr, this just returns the simplified alternatives;
1649 -- it not return an environment
1651 simplAlts env scrut case_bndr alts cont'
1652 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1653 do { let env0 = zapFloats env
1655 ; (env1, case_bndr1) <- simplBinder env0 case_bndr
1657 ; fam_envs <- getFamEnvs
1658 ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
1659 case_bndr case_bndr1 alts
1661 ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env' scrut' case_bndr' alts
1663 ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
1664 ; return (scrut', case_bndr', alts') }
1666 ------------------------------------
1667 simplAlt :: SimplEnv
1668 -> [AltCon] -- These constructors can't be present when
1669 -- matching the DEFAULT alternative
1670 -> OutId -- The case binder
1675 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1676 = ASSERT( null bndrs )
1677 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1678 -- Record the constructors that the case-binder *can't* be.
1679 ; rhs' <- simplExprC env' rhs cont'
1680 ; return (DEFAULT, [], rhs') }
1682 simplAlt env _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
1683 = ASSERT( null bndrs )
1684 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1685 ; rhs' <- simplExprC env' rhs cont'
1686 ; return (LitAlt lit, [], rhs') }
1688 simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
1689 = do { -- Deal with the pattern-bound variables
1690 -- Mark the ones that are in ! positions in the
1691 -- data constructor as certainly-evaluated.
1692 -- NB: simplLamBinders preserves this eval info
1693 let vs_with_evals = add_evals (dataConRepStrictness con)
1694 ; (env', vs') <- simplLamBndrs env vs_with_evals
1696 -- Bind the case-binder to (con args)
1697 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1698 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1699 env'' = addBinderUnfolding env' case_bndr'
1700 (mkConApp con con_args)
1702 ; rhs' <- simplExprC env'' rhs cont'
1703 ; return (DataAlt con, vs', rhs') }
1705 -- add_evals records the evaluated-ness of the bound variables of
1706 -- a case pattern. This is *important*. Consider
1707 -- data T = T !Int !Int
1709 -- case x of { T a b -> T (a+1) b }
1711 -- We really must record that b is already evaluated so that we don't
1712 -- go and re-evaluate it when constructing the result.
1713 -- See Note [Data-con worker strictness] in MkId.lhs
1718 go (v:vs') strs | isTyVar v = v : go vs' strs
1719 go (v:vs') (str:strs)
1720 | isMarkedStrict str = evald_v : go vs' strs
1721 | otherwise = zapped_v : go vs' strs
1723 zapped_v = zap_occ_info v
1724 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1725 go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
1727 -- See Note [zapOccInfo]
1728 -- zap_occ_info: if the case binder is alive, then we add the unfolding
1730 -- to the envt; so vs are now very much alive
1731 -- Note [Aug06] I can't see why this actually matters, but it's neater
1732 -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
1733 -- ==> case e of t { (a,b) -> ...(a)... }
1734 -- Look, Ma, a is alive now.
1735 zap_occ_info = zapCasePatIdOcc case_bndr'
1737 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1738 addBinderUnfolding env bndr rhs
1739 = modifyInScope env (bndr `setIdUnfolding` mkUnfolding False rhs)
1741 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1742 addBinderOtherCon env bndr cons
1743 = modifyInScope env (bndr `setIdUnfolding` mkOtherCon cons)
1745 zapCasePatIdOcc :: Id -> Id -> Id
1746 -- Consider case e of b { (a,b) -> ... }
1747 -- Then if we bind b to (a,b) in "...", and b is not dead,
1748 -- then we must zap the deadness info on a,b
1749 zapCasePatIdOcc case_bndr
1750 | isDeadBinder case_bndr = \ pat_id -> pat_id
1751 | otherwise = \ pat_id -> zapIdOccInfo pat_id
1755 %************************************************************************
1757 \subsection{Known constructor}
1759 %************************************************************************
1761 We are a bit careful with occurrence info. Here's an example
1763 (\x* -> case x of (a*, b) -> f a) (h v, e)
1765 where the * means "occurs once". This effectively becomes
1766 case (h v, e) of (a*, b) -> f a)
1768 let a* = h v; b = e in f a
1772 All this should happen in one sweep.
1775 knownCon :: SimplEnv
1776 -> OutExpr -- The scrutinee
1777 -> DataCon -> [OutType] -> [OutExpr] -- The scrutinee (in pieces)
1778 -> InId -> [InBndr] -> InExpr -- The alternative
1780 -> SimplM (SimplEnv, OutExpr)
1782 knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
1783 = do { env' <- bind_args env bs dc_args
1785 -- It's useful to bind bndr to scrut, rather than to a fresh
1786 -- binding x = Con arg1 .. argn
1787 -- because very often the scrut is a variable, so we avoid
1788 -- creating, and then subsequently eliminating, a let-binding
1789 -- BUT, if scrut is a not a variable, we must be careful
1790 -- about duplicating the arg redexes; in that case, make
1791 -- a new con-app from the args
1792 bndr_rhs | exprIsTrivial scrut = scrut
1793 | otherwise = con_app
1794 con_app = Var (dataConWorkId dc)
1795 `mkTyApps` dc_ty_args
1796 `mkApps` [substExpr env' (varToCoreExpr b) | b <- bs]
1797 -- dc_ty_args are aready OutTypes, but bs are InBndrs
1799 ; env'' <- simplNonRecX env' bndr bndr_rhs
1800 ; simplExprF env'' rhs cont }
1802 zap_occ = zapCasePatIdOcc bndr -- bndr is an InId
1805 bind_args env' [] _ = return env'
1807 bind_args env' (b:bs') (Type ty : args)
1808 = ASSERT( isTyVar b )
1809 bind_args (extendTvSubst env' b ty) bs' args
1811 bind_args env' (b:bs') (arg : args)
1813 do { let b' = zap_occ b
1814 -- Note that the binder might be "dead", because it doesn't
1815 -- occur in the RHS; and simplNonRecX may therefore discard
1816 -- it via postInlineUnconditionally.
1817 -- Nevertheless we must keep it if the case-binder is alive,
1818 -- because it may be used in the con_app. See Note [zapOccInfo]
1819 ; env'' <- simplNonRecX env' b' arg
1820 ; bind_args env'' bs' args }
1823 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$
1824 text "scrut:" <+> ppr scrut
1827 missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr)
1828 -- This isn't strictly an error, although it is unusual.
1829 -- It's possible that the simplifer might "see" that
1830 -- an inner case has no accessible alternatives before
1831 -- it "sees" that the entire branch of an outer case is
1832 -- inaccessible. So we simply put an error case here instead.
1833 missingAlt env case_bndr alts cont
1834 = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr )
1835 return (env, mkImpossibleExpr res_ty)
1837 res_ty = contResultType env (substTy env (coreAltsType alts)) cont
1841 %************************************************************************
1843 \subsection{Duplicating continuations}
1845 %************************************************************************
1848 prepareCaseCont :: SimplEnv
1849 -> [InAlt] -> SimplCont
1850 -> SimplM (SimplEnv, SimplCont,SimplCont)
1851 -- Return a duplicatable continuation, a non-duplicable part
1852 -- plus some extra bindings (that scope over the entire
1855 -- No need to make it duplicatable if there's only one alternative
1856 prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
1857 prepareCaseCont env _ cont = mkDupableCont env cont
1861 mkDupableCont :: SimplEnv -> SimplCont
1862 -> SimplM (SimplEnv, SimplCont, SimplCont)
1864 mkDupableCont env cont
1865 | contIsDupable cont
1866 = return (env, cont, mkBoringStop)
1868 mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1870 mkDupableCont env (CoerceIt ty cont)
1871 = do { (env', dup, nodup) <- mkDupableCont env cont
1872 ; return (env', CoerceIt ty dup, nodup) }
1874 mkDupableCont env cont@(StrictBind {})
1875 = return (env, mkBoringStop, cont)
1876 -- See Note [Duplicating StrictBind]
1878 mkDupableCont env (StrictArg fun cci ai cont)
1879 -- See Note [Duplicating StrictArg]
1880 = do { (env', dup, nodup) <- mkDupableCont env cont
1881 ; (env'', fun') <- mk_dupable_call env' fun
1882 ; return (env'', StrictArg fun' cci ai dup, nodup) }
1884 mk_dupable_call env (Var v) = return (env, Var v)
1885 mk_dupable_call env (App fun arg) = do { (env', fun') <- mk_dupable_call env fun
1886 ; (env'', arg') <- makeTrivial env' arg
1887 ; return (env'', fun' `App` arg') }
1888 mk_dupable_call _ other = pprPanic "mk_dupable_call" (ppr other)
1889 -- The invariant of StrictArg is that the first arg is always an App chain
1891 mkDupableCont env (ApplyTo _ arg se cont)
1892 = -- e.g. [...hole...] (...arg...)
1894 -- let a = ...arg...
1895 -- in [...hole...] a
1896 do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1897 ; arg' <- simplExpr (se `setInScope` env') arg
1898 ; (env'', arg'') <- makeTrivial env' arg'
1899 ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
1900 ; return (env'', app_cont, nodup_cont) }
1902 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
1903 -- See Note [Single-alternative case]
1904 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1905 -- | not (isDeadBinder case_bndr)
1906 | all isDeadBinder bs -- InIds
1907 && not (isUnLiftedType (idType case_bndr))
1908 -- Note [Single-alternative-unlifted]
1909 = return (env, mkBoringStop, cont)
1911 mkDupableCont env (Select _ case_bndr alts se cont)
1912 = -- e.g. (case [...hole...] of { pi -> ei })
1914 -- let ji = \xij -> ei
1915 -- in case [...hole...] of { pi -> ji xij }
1916 do { tick (CaseOfCase case_bndr)
1917 ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1918 -- NB: call mkDupableCont here, *not* prepareCaseCont
1919 -- We must make a duplicable continuation, whereas prepareCaseCont
1920 -- doesn't when there is a single case branch
1922 ; let alt_env = se `setInScope` env'
1923 ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
1924 ; alts' <- mapM (simplAlt alt_env' [] case_bndr' dup_cont) alts
1925 -- Safe to say that there are no handled-cons for the DEFAULT case
1926 -- NB: simplBinder does not zap deadness occ-info, so
1927 -- a dead case_bndr' will still advertise its deadness
1928 -- This is really important because in
1929 -- case e of b { (# p,q #) -> ... }
1930 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1931 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1932 -- In the new alts we build, we have the new case binder, so it must retain
1934 -- NB: we don't use alt_env further; it has the substEnv for
1935 -- the alternatives, and we don't want that
1937 ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
1938 ; return (env'', -- Note [Duplicated env]
1939 Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
1943 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1944 -> SimplM (SimplEnv, [InAlt])
1945 -- Absorbs the continuation into the new alternatives
1947 mkDupableAlts env case_bndr' the_alts
1950 go env0 [] = return (env0, [])
1952 = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
1953 ; (env2, alts') <- go env1 alts
1954 ; return (env2, alt' : alts' ) }
1956 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
1957 -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
1958 mkDupableAlt env case_bndr (con, bndrs', rhs')
1959 | exprIsDupable rhs' -- Note [Small alternative rhs]
1960 = return (env, (con, bndrs', rhs'))
1962 = do { let rhs_ty' = exprType rhs'
1963 scrut_ty = idType case_bndr
1966 DEFAULT -> case_bndr
1967 DataAlt dc -> setIdUnfolding case_bndr unf
1969 -- See Note [Case binders and join points]
1970 unf = mkInlineRule InlSat rhs 0
1971 rhs = mkConApp dc (map Type (tyConAppArgs scrut_ty)
1972 ++ varsToCoreExprs bndrs')
1974 LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt")
1975 <+> ppr case_bndr <+> ppr con )
1977 -- The case binder is alive but trivial, so why has
1978 -- it not been substituted away?
1980 used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs'
1981 | otherwise = bndrs' ++ [case_bndr_w_unf]
1984 | isTyVar bndr = True -- Abstract over all type variables just in case
1985 | otherwise = not (isDeadBinder bndr)
1986 -- The deadness info on the new Ids is preserved by simplBinders
1988 ; (final_bndrs', final_args) -- Note [Join point abstraction]
1989 <- if (any isId used_bndrs')
1990 then return (used_bndrs', varsToCoreExprs used_bndrs')
1991 else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
1992 ; return ([rw_id], [Var realWorldPrimId]) }
1994 ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
1995 -- Note [Funky mkPiTypes]
1997 ; let -- We make the lambdas into one-shot-lambdas. The
1998 -- join point is sure to be applied at most once, and doing so
1999 -- prevents the body of the join point being floated out by
2000 -- the full laziness pass
2001 really_final_bndrs = map one_shot final_bndrs'
2002 one_shot v | isId v = setOneShotLambda v
2004 join_rhs = mkLams really_final_bndrs rhs'
2005 join_call = mkApps (Var join_bndr) final_args
2007 ; env' <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
2008 ; return (env', (con, bndrs', join_call)) }
2009 -- See Note [Duplicated env]
2012 Note [Case binders and join points]
2013 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2015 case (case .. ) of c {
2018 If we make a join point with c but not c# we get
2019 $j = \c -> ....c....
2021 But if later inlining scrutines the c, thus
2023 $j = \c -> ... case c of { I# y -> ... } ...
2025 we won't see that 'c' has already been scrutinised. This actually
2026 happens in the 'tabulate' function in wave4main, and makes a significant
2027 difference to allocation.
2029 An alternative plan is this:
2031 $j = \c# -> let c = I# c# in ...c....
2033 but that is bad if 'c' is *not* later scrutinised.
2035 So instead we do both: we pass 'c' and 'c#' , and record in c's inlining
2036 that it's really I# c#, thus
2038 $j = \c# -> \c[=I# c#] -> ...c....
2040 Absence analysis may later discard 'c'.
2043 Note [Duplicated env]
2044 ~~~~~~~~~~~~~~~~~~~~~
2045 Some of the alternatives are simplified, but have not been turned into a join point
2046 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
2047 bind the join point, because it might to do PostInlineUnconditionally, and
2048 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
2049 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
2050 at worst delays the join-point inlining.
2052 Note [Small alternative rhs]
2053 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2054 It is worth checking for a small RHS because otherwise we
2055 get extra let bindings that may cause an extra iteration of the simplifier to
2056 inline back in place. Quite often the rhs is just a variable or constructor.
2057 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
2058 iterations because the version with the let bindings looked big, and so wasn't
2059 inlined, but after the join points had been inlined it looked smaller, and so
2062 NB: we have to check the size of rhs', not rhs.
2063 Duplicating a small InAlt might invalidate occurrence information
2064 However, if it *is* dupable, we return the *un* simplified alternative,
2065 because otherwise we'd need to pair it up with an empty subst-env....
2066 but we only have one env shared between all the alts.
2067 (Remember we must zap the subst-env before re-simplifying something).
2068 Rather than do this we simply agree to re-simplify the original (small) thing later.
2070 Note [Funky mkPiTypes]
2071 ~~~~~~~~~~~~~~~~~~~~~~
2072 Notice the funky mkPiTypes. If the contructor has existentials
2073 it's possible that the join point will be abstracted over
2074 type varaibles as well as term variables.
2075 Example: Suppose we have
2076 data T = forall t. C [t]
2078 case (case e of ...) of
2080 We get the join point
2081 let j :: forall t. [t] -> ...
2082 j = /\t \xs::[t] -> rhs
2084 case (case e of ...) of
2085 C t xs::[t] -> j t xs
2087 Note [Join point abstaction]
2088 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2089 If we try to lift a primitive-typed something out
2090 for let-binding-purposes, we will *caseify* it (!),
2091 with potentially-disastrous strictness results. So
2092 instead we turn it into a function: \v -> e
2093 where v::State# RealWorld#. The value passed to this function
2094 is realworld#, which generates (almost) no code.
2096 There's a slight infelicity here: we pass the overall
2097 case_bndr to all the join points if it's used in *any* RHS,
2098 because we don't know its usage in each RHS separately
2100 We used to say "&& isUnLiftedType rhs_ty'" here, but now
2101 we make the join point into a function whenever used_bndrs'
2102 is empty. This makes the join-point more CPR friendly.
2103 Consider: let j = if .. then I# 3 else I# 4
2104 in case .. of { A -> j; B -> j; C -> ... }
2106 Now CPR doesn't w/w j because it's a thunk, so
2107 that means that the enclosing function can't w/w either,
2108 which is a lose. Here's the example that happened in practice:
2109 kgmod :: Int -> Int -> Int
2110 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2114 I have seen a case alternative like this:
2116 It's a bit silly to add the realWorld dummy arg in this case, making
2119 (the \v alone is enough to make CPR happy) but I think it's rare
2121 Note [Duplicating StrictArg]
2122 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2123 The original plan had (where E is a big argument)
2125 ==> let $j = \a -> f E a
2128 But this is terrible! Here's an example:
2129 && E (case x of { T -> F; F -> T })
2130 Now, && is strict so we end up simplifying the case with
2131 an ArgOf continuation. If we let-bind it, we get
2132 let $j = \v -> && E v
2133 in simplExpr (case x of { T -> F; F -> T })
2135 And after simplifying more we get
2136 let $j = \v -> && E v
2137 in case x of { T -> $j F; F -> $j T }
2138 Which is a Very Bad Thing
2140 What we do now is this
2144 Now if the thing in the hole is a case expression (which is when
2145 we'll call mkDupableCont), we'll push the function call into the
2146 branches, which is what we want. Now RULES for f may fire, and
2147 call-pattern specialisation. Here's an example from Trac #3116
2150 _ -> Chunk p fpc (o+1) (l-1) bs')
2151 If we can push the call for 'go' inside the case, we get
2152 call-pattern specialisation for 'go', which is *crucial* for
2155 Here is the (&&) example:
2156 && E (case x of { T -> F; F -> T })
2158 case x of { T -> && a F; F -> && a T }
2162 * Arguments to f *after* the strict one are handled by
2163 the ApplyTo case of mkDupableCont. Eg
2166 * We can only do the let-binding of E because the function
2167 part of a StrictArg continuation is an explicit syntax
2168 tree. In earlier versions we represented it as a function
2169 (CoreExpr -> CoreEpxr) which we couldn't take apart.
2171 Do *not* duplicate StrictBind and StritArg continuations. We gain
2172 nothing by propagating them into the expressions, and we do lose a
2175 The desire not to duplicate is the entire reason that
2176 mkDupableCont returns a pair of continuations.
2178 Note [Duplicating StrictBind]
2179 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2180 Unlike StrictArg, there doesn't seem anything to gain from
2181 duplicating a StrictBind continuation, so we don't.
2183 The desire not to duplicate is the entire reason that
2184 mkDupableCont returns a pair of continuations.
2187 Note [Single-alternative cases]
2188 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2189 This case is just like the ArgOf case. Here's an example:
2193 case (case x of I# x' ->
2195 True -> I# (negate# x')
2196 False -> I# x') of y {
2198 Because the (case x) has only one alternative, we'll transform to
2200 case (case x' <# 0# of
2201 True -> I# (negate# x')
2202 False -> I# x') of y {
2204 But now we do *NOT* want to make a join point etc, giving
2206 let $j = \y -> MkT y
2208 True -> $j (I# (negate# x'))
2210 In this case the $j will inline again, but suppose there was a big
2211 strict computation enclosing the orginal call to MkT. Then, it won't
2212 "see" the MkT any more, because it's big and won't get duplicated.
2213 And, what is worse, nothing was gained by the case-of-case transform.
2215 When should use this case of mkDupableCont?
2216 However, matching on *any* single-alternative case is a *disaster*;
2217 e.g. case (case ....) of (a,b) -> (# a,b #)
2218 We must push the outer case into the inner one!
2221 * Match [(DEFAULT,_,_)], but in the common case of Int,
2222 the alternative-filling-in code turned the outer case into
2223 case (...) of y { I# _ -> MkT y }
2225 * Match on single alternative plus (not (isDeadBinder case_bndr))
2226 Rationale: pushing the case inwards won't eliminate the construction.
2227 But there's a risk of
2228 case (...) of y { (a,b) -> let z=(a,b) in ... }
2229 Now y looks dead, but it'll come alive again. Still, this
2230 seems like the best option at the moment.
2232 * Match on single alternative plus (all (isDeadBinder bndrs))
2233 Rationale: this is essentially seq.
2235 * Match when the rhs is *not* duplicable, and hence would lead to a
2236 join point. This catches the disaster-case above. We can test
2237 the *un-simplified* rhs, which is fine. It might get bigger or
2238 smaller after simplification; if it gets smaller, this case might
2239 fire next time round. NB also that we must test contIsDupable
2240 case_cont *btoo, because case_cont might be big!
2242 HOWEVER: I found that this version doesn't work well, because
2243 we can get let x = case (...) of { small } in ...case x...
2244 When x is inlined into its full context, we find that it was a bad
2245 idea to have pushed the outer case inside the (...) case.
2247 Note [Single-alternative-unlifted]
2248 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2249 Here's another single-alternative where we really want to do case-of-case:
2257 case y_s6X of tpl_s7m {
2258 M1.Mk1 ipv_s70 -> ipv_s70;
2259 M1.Mk2 ipv_s72 -> ipv_s72;
2265 case x_s74 of tpl_s7n {
2266 M1.Mk1 ipv_s77 -> ipv_s77;
2267 M1.Mk2 ipv_s79 -> ipv_s79;
2271 { __DEFAULT -> ==# [wild1_s7b wild_s7c];
2275 So the outer case is doing *nothing at all*, other than serving as a
2276 join-point. In this case we really want to do case-of-case and decide
2277 whether to use a real join point or just duplicate the continuation.
2279 Hence: check whether the case binder's type is unlifted, because then
2280 the outer case is *not* a seq.