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 )
21 import Name ( mkSystemVarName )
23 import FamInstEnv ( topNormaliseType )
24 import DataCon ( DataCon, dataConWorkId, dataConRepStrictness )
26 import NewDemand ( isStrictDmd, splitStrictSig )
27 import PprCore ( pprParendExpr, pprCoreExpr )
28 import CoreUnfold ( mkUnfolding, mkCoreUnfolding, mkInlineRule,
29 exprIsConApp_maybe, callSiteInline, CallCtxt(..) )
31 import qualified CoreSubst
32 import CoreArity ( exprArity )
33 import Rules ( lookupRule, getRules )
34 import BasicTypes ( isMarkedStrict, Arity )
35 import CostCentre ( currentCCS, pushCCisNop )
36 import TysPrim ( realWorldStatePrimTy )
37 import PrelInfo ( realWorldPrimId )
38 import BasicTypes ( TopLevelFlag(..), isTopLevel,
39 RecFlag(..), isNonRuleLoopBreaker )
40 import MonadUtils ( foldlM )
41 import Maybes ( orElse )
42 import Data.List ( mapAccumL )
48 The guts of the simplifier is in this module, but the driver loop for
49 the simplifier is in SimplCore.lhs.
52 -----------------------------------------
53 *** IMPORTANT NOTE ***
54 -----------------------------------------
55 The simplifier used to guarantee that the output had no shadowing, but
56 it does not do so any more. (Actually, it never did!) The reason is
57 documented with simplifyArgs.
60 -----------------------------------------
61 *** IMPORTANT NOTE ***
62 -----------------------------------------
63 Many parts of the simplifier return a bunch of "floats" as well as an
64 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
66 All "floats" are let-binds, not case-binds, but some non-rec lets may
67 be unlifted (with RHS ok-for-speculation).
71 -----------------------------------------
72 ORGANISATION OF FUNCTIONS
73 -----------------------------------------
75 - simplify all top-level binders
76 - for NonRec, call simplRecOrTopPair
77 - for Rec, call simplRecBind
80 ------------------------------
81 simplExpr (applied lambda) ==> simplNonRecBind
82 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
83 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
85 ------------------------------
86 simplRecBind [binders already simplfied]
87 - use simplRecOrTopPair on each pair in turn
89 simplRecOrTopPair [binder already simplified]
90 Used for: recursive bindings (top level and nested)
91 top-level non-recursive bindings
93 - check for PreInlineUnconditionally
97 Used for: non-top-level non-recursive bindings
98 beta reductions (which amount to the same thing)
99 Because it can deal with strict arts, it takes a
100 "thing-inside" and returns an expression
102 - check for PreInlineUnconditionally
103 - simplify binder, including its IdInfo
112 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
113 Used for: binding case-binder and constr args in a known-constructor case
114 - check for PreInLineUnconditionally
118 ------------------------------
119 simplLazyBind: [binder already simplified, RHS not]
120 Used for: recursive bindings (top level and nested)
121 top-level non-recursive bindings
122 non-top-level, but *lazy* non-recursive bindings
123 [must not be strict or unboxed]
124 Returns floats + an augmented environment, not an expression
125 - substituteIdInfo and add result to in-scope
126 [so that rules are available in rec rhs]
129 - float if exposes constructor or PAP
133 completeNonRecX: [binder and rhs both simplified]
134 - if the the thing needs case binding (unlifted and not ok-for-spec)
140 completeBind: [given a simplified RHS]
141 [used for both rec and non-rec bindings, top level and not]
142 - try PostInlineUnconditionally
143 - add unfolding [this is the only place we add an unfolding]
148 Right hand sides and arguments
149 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
150 In many ways we want to treat
151 (a) the right hand side of a let(rec), and
152 (b) a function argument
153 in the same way. But not always! In particular, we would
154 like to leave these arguments exactly as they are, so they
155 will match a RULE more easily.
160 It's harder to make the rule match if we ANF-ise the constructor,
161 or eta-expand the PAP:
163 f (let { a = g x; b = h x } in (a,b))
166 On the other hand if we see the let-defns
171 then we *do* want to ANF-ise and eta-expand, so that p and q
172 can be safely inlined.
174 Even floating lets out is a bit dubious. For let RHS's we float lets
175 out if that exposes a value, so that the value can be inlined more vigorously.
178 r = let x = e in (x,x)
180 Here, if we float the let out we'll expose a nice constructor. We did experiments
181 that showed this to be a generally good thing. But it was a bad thing to float
182 lets out unconditionally, because that meant they got allocated more often.
184 For function arguments, there's less reason to expose a constructor (it won't
185 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
186 So for the moment we don't float lets out of function arguments either.
191 For eta expansion, we want to catch things like
193 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
195 If the \x was on the RHS of a let, we'd eta expand to bring the two
196 lambdas together. And in general that's a good thing to do. Perhaps
197 we should eta expand wherever we find a (value) lambda? Then the eta
198 expansion at a let RHS can concentrate solely on the PAP case.
201 %************************************************************************
203 \subsection{Bindings}
205 %************************************************************************
208 simplTopBinds :: SimplEnv -> [InBind] -> SimplM SimplEnv
210 simplTopBinds env0 binds0
211 = do { -- Put all the top-level binders into scope at the start
212 -- so that if a transformation rule has unexpectedly brought
213 -- anything into scope, then we don't get a complaint about that.
214 -- It's rather as if the top-level binders were imported.
215 ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
216 ; dflags <- getDOptsSmpl
217 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
218 dopt Opt_D_dump_rule_firings dflags
219 ; env2 <- simpl_binds dump_flag env1 binds0
220 ; freeTick SimplifierDone
223 -- We need to track the zapped top-level binders, because
224 -- they should have their fragile IdInfo zapped (notably occurrence info)
225 -- That's why we run down binds and bndrs' simultaneously.
227 -- The dump-flag emits a trace for each top-level binding, which
228 -- helps to locate the tracing for inlining and rule firing
229 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
230 simpl_binds _ env [] = return env
231 simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
233 ; simpl_binds dump env' binds }
235 trace_bind True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
236 trace_bind False _ = \x -> x
238 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
239 simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel b b' r
241 (env', b') = addBndrRules env b (lookupRecBndr env b)
245 %************************************************************************
247 \subsection{Lazy bindings}
249 %************************************************************************
251 simplRecBind is used for
252 * recursive bindings only
255 simplRecBind :: SimplEnv -> TopLevelFlag
258 simplRecBind env0 top_lvl pairs0
259 = do { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
260 ; env1 <- go (zapFloats env_with_info) triples
261 ; return (env0 `addRecFloats` env1) }
262 -- addFloats adds the floats from env1,
263 -- _and_ updates env0 with the in-scope set from env1
265 add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
266 -- Add the (substituted) rules to the binder
267 add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
269 (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
271 go env [] = return env
273 go env ((old_bndr, new_bndr, rhs) : pairs)
274 = do { env' <- simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
278 simplOrTopPair is used for
279 * recursive bindings (whether top level or not)
280 * top-level non-recursive bindings
282 It assumes the binder has already been simplified, but not its IdInfo.
285 simplRecOrTopPair :: SimplEnv
287 -> InId -> OutBndr -> InExpr -- Binder and rhs
288 -> SimplM SimplEnv -- Returns an env that includes the binding
290 simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
291 | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
292 = do { tick (PreInlineUnconditionally old_bndr)
293 ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
296 = simplLazyBind env top_lvl Recursive old_bndr new_bndr rhs env
297 -- May not actually be recursive, but it doesn't matter
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 = do { let rhs_env = rhs_se `setInScope` env
325 (tvs, body) = case collectTyBinders rhs of
326 (tvs, body) | not_lam body -> (tvs,body)
327 | otherwise -> ([], rhs)
328 not_lam (Lam _ _) = False
330 -- Do not do the "abstract tyyvar" thing if there's
331 -- a lambda inside, becuase it defeats eta-reduction
332 -- f = /\a. \x. g a x
335 ; (body_env, tvs') <- simplBinders rhs_env tvs
336 -- See Note [Floating and type abstraction] in SimplUtils
339 ; (body_env1, body1) <- simplExprF body_env body mkRhsStop
340 -- ANF-ise a constructor or PAP rhs
341 ; (body_env2, body2) <- prepareRhs body_env1 bndr1 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_bndr 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 -> OutId -> OutExpr -> SimplM (SimplEnv, OutExpr)
439 -- Adds new floats to the env iff that allows us to return a good RHS
440 prepareRhs env id (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') <- makeTrivialWithInfo env sanitised_info rhs
444 ; return (env', Cast rhs' co) }
446 sanitised_info = vanillaIdInfo `setNewStrictnessInfo` newStrictnessInfo info
447 `setNewDemandInfo` newDemandInfo info
450 prepareRhs env0 _ rhs0
451 = do { (_is_val, env1, rhs1) <- go 0 env0 rhs0
452 ; return (env1, rhs1) }
454 go n_val_args env (Cast rhs co)
455 = do { (is_val, env', rhs') <- go n_val_args env rhs
456 ; return (is_val, env', Cast rhs' co) }
457 go n_val_args env (App fun (Type ty))
458 = do { (is_val, env', rhs') <- go n_val_args env fun
459 ; return (is_val, env', App rhs' (Type ty)) }
460 go n_val_args env (App fun arg)
461 = do { (is_val, env', fun') <- go (n_val_args+1) env fun
463 True -> do { (env'', arg') <- makeTrivial env' arg
464 ; return (True, env'', App fun' arg') }
465 False -> return (False, env, App fun arg) }
466 go n_val_args env (Var fun)
467 = return (is_val, env, Var fun)
469 is_val = n_val_args > 0 -- There is at least one arg
470 -- ...and the fun a constructor or PAP
471 && (isConLikeId fun || n_val_args < idArity fun)
472 -- See Note [CONLIKE pragma] in BasicTypes
474 = return (False, env, other)
478 Note [Float coercions]
479 ~~~~~~~~~~~~~~~~~~~~~~
480 When we find the binding
482 we'd like to transform it to
484 x = x `cast` co -- A trivial binding
485 There's a chance that e will be a constructor application or function, or something
486 like that, so moving the coerion to the usage site may well cancel the coersions
487 and lead to further optimisation. Example:
490 data instance T Int = T Int
492 foo :: Int -> Int -> Int
497 go n = case x of { T m -> go (n-m) }
498 -- This case should optimise
500 Note [Preserve strictness when floating coercions]
501 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
502 In the Note [Float coercions] transformation, keep the strictness info.
504 f = e `cast` co -- f has strictness SSL
506 f' = e -- f' also has strictness SSL
507 f = f' `cast` co -- f still has strictness SSL
509 Its not wrong to drop it on the floor, but better to keep it.
511 Note [Float coercions (unlifted)]
512 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
513 BUT don't do [Float coercions] if 'e' has an unlifted type.
516 foo :: Int = (error (# Int,Int #) "urk")
517 `cast` CoUnsafe (# Int,Int #) Int
519 If do the makeTrivial thing to the error call, we'll get
520 foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
521 But 'v' isn't in scope!
523 These strange casts can happen as a result of case-of-case
524 bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
529 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
530 -- Binds the expression to a variable, if it's not trivial, returning the variable
531 makeTrivial env expr = makeTrivialWithInfo env vanillaIdInfo expr
533 makeTrivialWithInfo :: SimplEnv -> IdInfo -> OutExpr -> SimplM (SimplEnv, OutExpr)
534 -- Propagate strictness and demand info to the new binder
535 -- Note [Preserve strictness when floating coercions]
536 makeTrivialWithInfo env info expr
539 | otherwise -- See Note [Take care] below
540 = do { uniq <- getUniqueM
541 ; let name = mkSystemVarName uniq (fsLit "a")
542 var = mkLocalIdWithInfo name (exprType expr) info
543 ; env' <- completeNonRecX env False var var expr
544 ; return (env', substExpr env' (Var var)) }
545 -- The substitution is needed becase we're constructing a new binding
547 -- And if rhs is of form (rhs1 |> co), then we might get
550 -- and now a's RHS is trivial and can be substituted out, and that
551 -- is what completeNonRecX will do
555 %************************************************************************
557 \subsection{Completing a lazy binding}
559 %************************************************************************
562 * deals only with Ids, not TyVars
563 * takes an already-simplified binder and RHS
564 * is used for both recursive and non-recursive bindings
565 * is used for both top-level and non-top-level bindings
567 It does the following:
568 - tries discarding a dead binding
569 - tries PostInlineUnconditionally
570 - add unfolding [this is the only place we add an unfolding]
573 It does *not* attempt to do let-to-case. Why? Because it is used for
574 - top-level bindings (when let-to-case is impossible)
575 - many situations where the "rhs" is known to be a WHNF
576 (so let-to-case is inappropriate).
578 Nor does it do the atomic-argument thing
581 completeBind :: SimplEnv
582 -> TopLevelFlag -- Flag stuck into unfolding
583 -> InId -- Old binder
584 -> OutId -> OutExpr -- New binder and RHS
586 -- completeBind may choose to do its work
587 -- * by extending the substitution (e.g. let x = y in ...)
588 -- * or by adding to the floats in the envt
590 completeBind env top_lvl old_bndr new_bndr new_rhs
591 = do { let old_info = idInfo old_bndr
592 old_unf = unfoldingInfo old_info
593 occ_info = occInfo old_info
595 ; new_unfolding <- simplUnfolding env top_lvl old_bndr occ_info new_rhs old_unf
597 ; if postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs new_unfolding
598 -- Inline and discard the binding
599 then do { tick (PostInlineUnconditionally old_bndr)
600 ; return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
601 -- Use the substitution to make quite, quite sure that the
602 -- substitution will happen, since we are going to discard the binding
604 else return (addNonRecWithUnf env new_bndr new_rhs new_unfolding) }
606 ------------------------------
607 addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv
608 -- Add a new binding to the environment, complete with its unfolding
609 -- but *do not* do postInlineUnconditionally, because we have already
610 -- processed some of the scope of the binding
611 -- We still want the unfolding though. Consider
613 -- x = /\a. let y = ... in Just y
615 -- Then we float the y-binding out (via abstractFloats and addPolyBind)
616 -- but 'x' may well then be inlined in 'body' in which case we'd like the
617 -- opportunity to inline 'y' too.
619 addPolyBind top_lvl env (NonRec poly_id rhs)
620 = do { unfolding <- simplUnfolding env top_lvl poly_id NoOccInfo rhs noUnfolding
621 -- Assumes that poly_id did not have an INLINE prag
622 -- which is perhaps wrong. ToDo: think about this
623 ; return (addNonRecWithUnf env poly_id rhs unfolding) }
625 addPolyBind _ env bind@(Rec _) = return (extendFloats env bind)
626 -- Hack: letrecs are more awkward, so we extend "by steam"
627 -- without adding unfoldings etc. At worst this leads to
628 -- more simplifier iterations
630 ------------------------------
631 addNonRecWithUnf :: SimplEnv
632 -> OutId -> OutExpr -- New binder and RHS
633 -> Unfolding -- New unfolding
635 addNonRecWithUnf env new_bndr new_rhs new_unfolding
636 = let new_arity = exprArity new_rhs
637 old_arity = idArity new_bndr
638 info1 = idInfo new_bndr `setArityInfo` new_arity
640 -- Unfolding info: Note [Setting the new unfolding]
641 info2 = info1 `setUnfoldingInfo` new_unfolding
643 -- Demand info: Note [Setting the demand info]
644 info3 | isEvaldUnfolding new_unfolding = zapDemandInfo info2 `orElse` info2
647 final_id = new_bndr `setIdInfo` info3
648 dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr
650 ASSERT( isId new_bndr )
651 WARN( new_arity < old_arity || new_arity < dmd_arity,
652 (ptext (sLit "Arity decrease:") <+> ppr final_id <+> ppr old_arity
653 <+> ppr new_arity <+> ppr dmd_arity) )
654 -- Note [Arity decrease]
656 final_id `seq` -- This seq forces the Id, and hence its IdInfo,
657 -- and hence any inner substitutions
658 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
659 addNonRec env final_id new_rhs
660 -- The addNonRec adds it to the in-scope set too
662 ------------------------------
663 simplUnfolding :: SimplEnv-> TopLevelFlag
664 -> Id -- Debug output only
665 -> OccInfo -> OutExpr
666 -> Unfolding -> SimplM Unfolding
667 -- Note [Setting the new unfolding]
668 simplUnfolding env _ _ _ _ (DFunUnfolding con ops)
669 = return (DFunUnfolding con ops')
671 ops' = map (CoreSubst.substExpr (mkCoreSubst env)) ops
673 simplUnfolding env top_lvl _ _ _
674 (CoreUnfolding { uf_tmpl = expr, uf_arity = arity
675 , uf_guidance = guide@(InlineRule {}) })
676 = do { expr' <- simplExpr (setMode simplGentlyForInlineRules env) expr
677 -- See Note [Simplifying gently inside InlineRules] in SimplUtils
678 ; let mb_wkr' = CoreSubst.substInlineRuleInfo (mkCoreSubst env) (ir_info guide)
679 ; return (mkCoreUnfolding (isTopLevel top_lvl) expr' arity
680 (guide { ir_info = mb_wkr' })) }
681 -- See Note [Top-level flag on inline rules] in CoreUnfold
683 simplUnfolding _ top_lvl _ occ_info new_rhs _
684 | omit_unfolding = return NoUnfolding
685 | otherwise = return (mkUnfolding (isTopLevel top_lvl) new_rhs)
687 omit_unfolding = isNonRuleLoopBreaker occ_info
690 Note [Arity decrease]
691 ~~~~~~~~~~~~~~~~~~~~~
692 Generally speaking the arity of a binding should not decrease. But it *can*
693 legitimately happen becuase of RULES. Eg
695 where g has arity 2, will have arity 2. But if there's a rewrite rule
697 where h has arity 1, then f's arity will decrease. Here's a real-life example,
698 which is in the output of Specialise:
701 $dm {Arity 2} = \d.\x. op d
702 {-# RULES forall d. $dm Int d = $s$dm #-}
704 dInt = MkD .... opInt ...
705 opInt {Arity 1} = $dm dInt
707 $s$dm {Arity 0} = \x. op dInt }
709 Here opInt has arity 1; but when we apply the rule its arity drops to 0.
710 That's why Specialise goes to a little trouble to pin the right arity
711 on specialised functions too.
713 Note [Setting the new unfolding]
714 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
715 * If there's an INLINE pragma, we simplify the RHS gently. Maybe we
716 should do nothing at all, but simplifying gently might get rid of
719 * If not, we make an unfolding from the new RHS. But *only* for
720 non-loop-breakers. Making loop breakers not have an unfolding at all
721 means that we can avoid tests in exprIsConApp, for example. This is
722 important: if exprIsConApp says 'yes' for a recursive thing, then we
723 can get into an infinite loop
725 If there's an InlineRule on a loop breaker, we hang on to the inlining.
726 It's pretty dodgy, but the user did say 'INLINE'. May need to revisit
729 Note [Setting the demand info]
730 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
731 If the unfolding is a value, the demand info may
732 go pear-shaped, so we nuke it. Example:
734 case x of (p,q) -> h p q x
735 Here x is certainly demanded. But after we've nuked
736 the case, we'll get just
737 let x = (a,b) in h a b x
738 and now x is not demanded (I'm assuming h is lazy)
739 This really happens. Similarly
740 let f = \x -> e in ...f..f...
741 After inlining f at some of its call sites the original binding may
742 (for example) be no longer strictly demanded.
743 The solution here is a bit ad hoc...
746 %************************************************************************
748 \subsection[Simplify-simplExpr]{The main function: simplExpr}
750 %************************************************************************
752 The reason for this OutExprStuff stuff is that we want to float *after*
753 simplifying a RHS, not before. If we do so naively we get quadratic
754 behaviour as things float out.
756 To see why it's important to do it after, consider this (real) example:
770 a -- Can't inline a this round, cos it appears twice
774 Each of the ==> steps is a round of simplification. We'd save a
775 whole round if we float first. This can cascade. Consider
780 let f = let d1 = ..d.. in \y -> e
784 in \x -> ...(\y ->e)...
786 Only in this second round can the \y be applied, and it
787 might do the same again.
791 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
792 simplExpr env expr = simplExprC env expr mkBoringStop
794 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
795 -- Simplify an expression, given a continuation
796 simplExprC env expr cont
797 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
798 do { (env', expr') <- simplExprF (zapFloats env) expr cont
799 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
800 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
801 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
802 return (wrapFloats env' expr') }
804 --------------------------------------------------
805 simplExprF :: SimplEnv -> InExpr -> SimplCont
806 -> SimplM (SimplEnv, OutExpr)
808 simplExprF env e cont
809 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
810 simplExprF' env e cont
812 simplExprF' :: SimplEnv -> InExpr -> SimplCont
813 -> SimplM (SimplEnv, OutExpr)
814 simplExprF' env (Var v) cont = simplVar env v cont
815 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
816 simplExprF' env (Note n expr) cont = simplNote env n expr cont
817 simplExprF' env (Cast body co) cont = simplCast env body co cont
818 simplExprF' env (App fun arg) cont = simplExprF env fun $
819 ApplyTo NoDup arg env cont
821 simplExprF' env expr@(Lam _ _) cont
822 = simplLam env (map zap bndrs) body cont
823 -- The main issue here is under-saturated lambdas
824 -- (\x1. \x2. e) arg1
825 -- Here x1 might have "occurs-once" occ-info, because occ-info
826 -- is computed assuming that a group of lambdas is applied
827 -- all at once. If there are too few args, we must zap the
830 n_args = countArgs cont
831 n_params = length bndrs
832 (bndrs, body) = collectBinders expr
833 zap | n_args >= n_params = \b -> b
834 | otherwise = \b -> if isTyVar b then b
836 -- NB: we count all the args incl type args
837 -- so we must count all the binders (incl type lambdas)
839 simplExprF' env (Type ty) cont
840 = ASSERT( contIsRhsOrArg cont )
841 do { ty' <- simplCoercion env ty
842 ; rebuild env (Type ty') cont }
844 simplExprF' env (Case scrut bndr _ alts) cont
845 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
846 = -- Simplify the scrutinee with a Select continuation
847 simplExprF env scrut (Select NoDup bndr alts env cont)
850 = -- If case-of-case is off, simply simplify the case expression
851 -- in a vanilla Stop context, and rebuild the result around it
852 do { case_expr' <- simplExprC env scrut case_cont
853 ; rebuild env case_expr' cont }
855 case_cont = Select NoDup bndr alts env mkBoringStop
857 simplExprF' env (Let (Rec pairs) body) cont
858 = do { env' <- simplRecBndrs env (map fst pairs)
859 -- NB: bndrs' don't have unfoldings or rules
860 -- We add them as we go down
862 ; env'' <- simplRecBind env' NotTopLevel pairs
863 ; simplExprF env'' body cont }
865 simplExprF' env (Let (NonRec bndr rhs) body) cont
866 = simplNonRecE env bndr (rhs, env) ([], body) cont
868 ---------------------------------
869 simplType :: SimplEnv -> InType -> SimplM OutType
870 -- Kept monadic just so we can do the seqType
872 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
873 seqType new_ty `seq` return new_ty
875 new_ty = substTy env ty
877 ---------------------------------
878 simplCoercion :: SimplEnv -> InType -> SimplM OutType
879 -- The InType isn't *necessarily* a coercion, but it might be
880 -- (in a type application, say) and optCoercion is a no-op on types
882 = do { co' <- simplType env co
883 ; return (optCoercion co') }
887 %************************************************************************
889 \subsection{The main rebuilder}
891 %************************************************************************
894 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
895 -- At this point the substitution in the SimplEnv should be irrelevant
896 -- only the in-scope set and floats should matter
897 rebuild env expr cont0
898 = -- pprTrace "rebuild" (ppr expr $$ ppr cont0 $$ ppr (seFloats env)) $
900 Stop {} -> return (env, expr)
901 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
902 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
903 StrictArg fun _ info cont -> rebuildCall env (fun `App` expr) info cont
904 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
905 ; simplLam env' bs body cont }
906 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
907 ; rebuild env (App expr arg') cont }
911 %************************************************************************
915 %************************************************************************
918 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
919 -> SimplM (SimplEnv, OutExpr)
920 simplCast env body co0 cont0
921 = do { co1 <- simplCoercion env co0
922 ; simplExprF env body (addCoerce co1 cont0) }
924 addCoerce co cont = add_coerce co (coercionKind co) cont
926 add_coerce _co (s1, k1) cont -- co :: ty~ty
927 | s1 `coreEqType` k1 = cont -- is a no-op
929 add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
930 | (_l1, t1) <- coercionKind co2
931 -- e |> (g1 :: S1~L) |> (g2 :: L~T1)
934 -- e |> (g1 . g2 :: S1~T1) otherwise
936 -- For example, in the initial form of a worker
937 -- we may find (coerce T (coerce S (\x.e))) y
938 -- and we'd like it to simplify to e[y/x] in one round
940 , s1 `coreEqType` t1 = cont -- The coerces cancel out
941 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
943 add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
944 -- (f |> g) ty ---> (f ty) |> (g @ ty)
945 -- This implements the PushT rule from the paper
946 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
947 , not (isCoVar tyvar)
948 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
950 ty' = substTy (arg_se `setInScope` env) arg_ty
952 -- ToDo: the PushC rule is not implemented at all
954 add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
955 | not (isTypeArg arg) -- This implements the Push rule from the paper
956 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
957 -- (e |> (g :: s1s2 ~ t1->t2)) f
959 -- (e (f |> (arg g :: t1~s1))
960 -- |> (res g :: s2->t2)
962 -- t1t2 must be a function type, t1->t2, because it's applied
963 -- to something but s1s2 might conceivably not be
965 -- When we build the ApplyTo we can't mix the out-types
966 -- with the InExpr in the argument, so we simply substitute
967 -- to make it all consistent. It's a bit messy.
968 -- But it isn't a common case.
970 -- Example of use: Trac #995
971 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
973 -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
974 -- t2 ~ s2 with left and right on the curried form:
975 -- (->) t1 t2 ~ (->) s1 s2
976 [co1, co2] = decomposeCo 2 co
977 new_arg = mkCoerce (mkSymCoercion co1) arg'
978 arg' = substExpr (arg_se `setInScope` env) arg
980 add_coerce co _ cont = CoerceIt co cont
984 %************************************************************************
988 %************************************************************************
991 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
992 -> SimplM (SimplEnv, OutExpr)
994 simplLam env [] body cont = simplExprF env body cont
997 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
998 = do { tick (BetaReduction bndr)
999 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
1001 -- Not enough args, so there are real lambdas left to put in the result
1002 simplLam env bndrs body cont
1003 = do { (env', bndrs') <- simplLamBndrs env bndrs
1004 ; body' <- simplExpr env' body
1005 ; new_lam <- mkLam env' bndrs' body'
1006 ; rebuild env' new_lam cont }
1009 simplNonRecE :: SimplEnv
1010 -> InBndr -- The binder
1011 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
1012 -> ([InBndr], InExpr) -- Body of the let/lambda
1015 -> SimplM (SimplEnv, OutExpr)
1017 -- simplNonRecE is used for
1018 -- * non-top-level non-recursive lets in expressions
1021 -- It deals with strict bindings, via the StrictBind continuation,
1022 -- which may abort the whole process
1024 -- The "body" of the binding comes as a pair of ([InId],InExpr)
1025 -- representing a lambda; so we recurse back to simplLam
1026 -- Why? Because of the binder-occ-info-zapping done before
1027 -- the call to simplLam in simplExprF (Lam ...)
1029 -- First deal with type applications and type lets
1030 -- (/\a. e) (Type ty) and (let a = Type ty in e)
1031 simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
1032 = ASSERT( isTyVar bndr )
1033 do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
1034 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
1036 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
1037 | preInlineUnconditionally env NotTopLevel bndr rhs
1038 = do { tick (PreInlineUnconditionally bndr)
1039 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
1042 = do { simplExprF (rhs_se `setFloats` env) rhs
1043 (StrictBind bndr bndrs body env cont) }
1046 = ASSERT( not (isTyVar bndr) )
1047 do { (env1, bndr1) <- simplNonRecBndr env bndr
1048 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
1049 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
1050 ; simplLam env3 bndrs body cont }
1054 %************************************************************************
1058 %************************************************************************
1061 -- Hack alert: we only distinguish subsumed cost centre stacks for the
1062 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
1063 simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
1064 -> SimplM (SimplEnv, OutExpr)
1065 simplNote env (SCC cc) e cont
1066 | pushCCisNop cc (getEnclosingCC env) -- scc "f" (...(scc "f" e)...)
1067 = simplExprF env e cont -- ==> scc "f" (...e...)
1069 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
1070 ; rebuild env (mkSCC cc e') cont }
1072 simplNote env (CoreNote s) e cont
1073 = do { e' <- simplExpr env e
1074 ; rebuild env (Note (CoreNote s) e') cont }
1078 %************************************************************************
1080 \subsection{Dealing with calls}
1082 %************************************************************************
1085 simplVar :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1086 simplVar env var cont
1087 = case substId env var of
1088 DoneEx e -> simplExprF (zapSubstEnv env) e cont
1089 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
1090 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
1091 -- Note [zapSubstEnv]
1092 -- The template is already simplified, so don't re-substitute.
1093 -- This is VITAL. Consider
1095 -- let y = \z -> ...x... in
1097 -- We'll clone the inner \x, adding x->x' in the id_subst
1098 -- Then when we inline y, we must *not* replace x by x' in
1099 -- the inlined copy!!
1101 ---------------------------------------------------------
1102 -- Dealing with a call site
1104 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1105 completeCall env var cont
1106 = do { let (args,call_cont) = contArgs cont
1107 -- The args are OutExprs, obtained by *lazily* substituting
1108 -- in the args found in cont. These args are only examined
1109 -- to limited depth (unless a rule fires). But we must do
1110 -- the substitution; rule matching on un-simplified args would
1113 ------------- First try rules ----------------
1114 -- Do this before trying inlining. Some functions have
1115 -- rules *and* are strict; in this case, we don't want to
1116 -- inline the wrapper of the non-specialised thing; better
1117 -- to call the specialised thing instead.
1119 -- We used to use the black-listing mechanism to ensure that inlining of
1120 -- the wrapper didn't occur for things that have specialisations till a
1121 -- later phase, so but now we just try RULES first
1123 -- See also Note [Rules for recursive functions]
1124 ; rule_base <- getSimplRules
1125 ; let rules = getRules rule_base var
1126 ; mb_rule <- tryRules env var rules args call_cont
1128 Just (n_args, rule_rhs) -> simplExprF env rule_rhs (dropArgs n_args cont) ;
1129 -- The ruleArity says how many args the rule consumed
1130 ; Nothing -> do -- No rules
1133 ------------- Next try inlining ----------------
1134 { dflags <- getDOptsSmpl
1135 ; let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
1136 n_val_args = length arg_infos
1137 interesting_cont = interestingCallContext call_cont
1138 active_inline = activeInline env var
1139 maybe_inline = callSiteInline dflags active_inline var
1140 (null args) arg_infos interesting_cont
1141 ; case maybe_inline of {
1142 Just unfolding -- There is an inlining!
1143 -> do { tick (UnfoldingDone var)
1144 ; (if dopt Opt_D_dump_inlinings dflags then
1145 pprTrace ("Inlining done: " ++ showSDoc (ppr var)) (vcat [
1146 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1147 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1148 text "Cont: " <+> ppr call_cont])
1151 simplExprF env unfolding cont }
1153 ; Nothing -> -- No inlining!
1155 ------------- No inlining! ----------------
1156 -- Next, look for rules or specialisations that match
1158 rebuildCall env (Var var)
1159 (mkArgInfo var rules n_val_args call_cont)
1163 rebuildCall :: SimplEnv
1164 -> OutExpr -- Function
1167 -> SimplM (SimplEnv, OutExpr)
1168 rebuildCall env fun (ArgInfo { ai_strs = [] }) cont
1169 -- When we run out of strictness args, it means
1170 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1171 -- Then we want to discard the entire strict continuation. E.g.
1172 -- * case (error "hello") of { ... }
1173 -- * (error "Hello") arg
1174 -- * f (error "Hello") where f is strict
1176 -- Then, especially in the first of these cases, we'd like to discard
1177 -- the continuation, leaving just the bottoming expression. But the
1178 -- type might not be right, so we may have to add a coerce.
1179 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1180 = return (env, mk_coerce fun) -- contination to discard, else we do it
1181 where -- again and again!
1182 fun_ty = exprType fun
1183 cont_ty = contResultType env fun_ty cont
1184 co = mkUnsafeCoercion fun_ty cont_ty
1185 mk_coerce expr | cont_ty `coreEqType` fun_ty = expr
1186 | otherwise = mkCoerce co expr
1188 rebuildCall env fun info (ApplyTo _ (Type arg_ty) se cont)
1189 = do { ty' <- simplCoercion (se `setInScope` env) arg_ty
1190 ; rebuildCall env (fun `App` Type ty') info cont }
1193 (ArgInfo { ai_rules = has_rules, ai_strs = str:strs, ai_discs = disc:discs })
1194 (ApplyTo _ arg arg_se cont)
1195 | str -- Strict argument
1196 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1197 simplExprF (arg_se `setFloats` env) arg
1198 (StrictArg fun cci arg_info' cont)
1201 | otherwise -- Lazy argument
1202 -- DO NOT float anything outside, hence simplExprC
1203 -- There is no benefit (unlike in a let-binding), and we'd
1204 -- have to be very careful about bogus strictness through
1205 -- floating a demanded let.
1206 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1208 ; rebuildCall env (fun `App` arg') arg_info' cont }
1210 arg_info' = ArgInfo { ai_rules = has_rules, ai_strs = strs, ai_discs = discs }
1211 cci | has_rules || disc > 0 = ArgCtxt has_rules -- Be keener here
1212 | otherwise = BoringCtxt -- Nothing interesting
1214 rebuildCall env fun _ cont
1215 = rebuild env fun cont
1220 This part of the simplifier may break the no-shadowing invariant
1222 f (...(\a -> e)...) (case y of (a,b) -> e')
1223 where f is strict in its second arg
1224 If we simplify the innermost one first we get (...(\a -> e)...)
1225 Simplifying the second arg makes us float the case out, so we end up with
1226 case y of (a,b) -> f (...(\a -> e)...) e'
1227 So the output does not have the no-shadowing invariant. However, there is
1228 no danger of getting name-capture, because when the first arg was simplified
1229 we used an in-scope set that at least mentioned all the variables free in its
1230 static environment, and that is enough.
1232 We can't just do innermost first, or we'd end up with a dual problem:
1233 case x of (a,b) -> f e (...(\a -> e')...)
1235 I spent hours trying to recover the no-shadowing invariant, but I just could
1236 not think of an elegant way to do it. The simplifier is already knee-deep in
1237 continuations. We have to keep the right in-scope set around; AND we have
1238 to get the effect that finding (error "foo") in a strict arg position will
1239 discard the entire application and replace it with (error "foo"). Getting
1240 all this at once is TOO HARD!
1243 %************************************************************************
1247 %************************************************************************
1250 tryRules :: SimplEnv
1251 -> Id -> [CoreRule] -> [OutExpr] -> SimplCont
1252 -> SimplM (Maybe (Arity, CoreExpr)) -- The arity is the number of
1253 -- args consumed by the rule
1254 tryRules env fn rules args call_cont
1258 = do { dflags <- getDOptsSmpl
1259 ; case activeRule dflags env of {
1260 Nothing -> return Nothing ; -- No rules apply
1263 case lookupRule act_fn (getInScope env) fn args rules of {
1264 Nothing -> return Nothing ; -- No rule matches
1265 Just (rule, rule_rhs) ->
1267 do { tick (RuleFired (ru_name rule))
1268 ; (if dopt Opt_D_dump_rule_firings dflags then
1269 pprTrace "Rule fired" (vcat [
1270 text "Rule:" <+> ftext (ru_name rule),
1271 text "Before:" <+> ppr fn <+> sep (map pprParendExpr args),
1272 text "After: " <+> pprCoreExpr rule_rhs,
1273 text "Cont: " <+> ppr call_cont])
1276 return (Just (ruleArity rule, rule_rhs)) }}}}
1279 Note [Rules for recursive functions]
1280 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1281 You might think that we shouldn't apply rules for a loop breaker:
1282 doing so might give rise to an infinite loop, because a RULE is
1283 rather like an extra equation for the function:
1284 RULE: f (g x) y = x+y
1287 But it's too drastic to disable rules for loop breakers.
1288 Even the foldr/build rule would be disabled, because foldr
1289 is recursive, and hence a loop breaker:
1290 foldr k z (build g) = g k z
1291 So it's up to the programmer: rules can cause divergence
1294 %************************************************************************
1296 Rebuilding a cse expression
1298 %************************************************************************
1300 Note [Case elimination]
1301 ~~~~~~~~~~~~~~~~~~~~~~~
1302 The case-elimination transformation discards redundant case expressions.
1303 Start with a simple situation:
1305 case x# of ===> e[x#/y#]
1308 (when x#, y# are of primitive type, of course). We can't (in general)
1309 do this for algebraic cases, because we might turn bottom into
1312 The code in SimplUtils.prepareAlts has the effect of generalise this
1313 idea to look for a case where we're scrutinising a variable, and we
1314 know that only the default case can match. For example:
1318 DEFAULT -> ...(case x of
1322 Here the inner case is first trimmed to have only one alternative, the
1323 DEFAULT, after which it's an instance of the previous case. This
1324 really only shows up in eliminating error-checking code.
1326 We also make sure that we deal with this very common case:
1331 Here we are using the case as a strict let; if x is used only once
1332 then we want to inline it. We have to be careful that this doesn't
1333 make the program terminate when it would have diverged before, so we
1335 - e is already evaluated (it may so if e is a variable)
1336 - x is used strictly, or
1338 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1340 case e of ===> case e of DEFAULT -> r
1344 Now again the case may be elminated by the CaseElim transformation.
1347 Further notes about case elimination
1348 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1349 Consider: test :: Integer -> IO ()
1352 Turns out that this compiles to:
1355 eta1 :: State# RealWorld ->
1356 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1358 (PrelNum.jtos eta ($w[] @ Char))
1360 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1362 Notice the strange '<' which has no effect at all. This is a funny one.
1363 It started like this:
1365 f x y = if x < 0 then jtos x
1366 else if y==0 then "" else jtos x
1368 At a particular call site we have (f v 1). So we inline to get
1370 if v < 0 then jtos x
1371 else if 1==0 then "" else jtos x
1373 Now simplify the 1==0 conditional:
1375 if v<0 then jtos v else jtos v
1377 Now common-up the two branches of the case:
1379 case (v<0) of DEFAULT -> jtos v
1381 Why don't we drop the case? Because it's strict in v. It's technically
1382 wrong to drop even unnecessary evaluations, and in practice they
1383 may be a result of 'seq' so we *definitely* don't want to drop those.
1384 I don't really know how to improve this situation.
1387 ---------------------------------------------------------
1388 -- Eliminate the case if possible
1390 rebuildCase, reallyRebuildCase
1392 -> OutExpr -- Scrutinee
1393 -> InId -- Case binder
1394 -> [InAlt] -- Alternatives (inceasing order)
1396 -> SimplM (SimplEnv, OutExpr)
1398 --------------------------------------------------
1399 -- 1. Eliminate the case if there's a known constructor
1400 --------------------------------------------------
1402 rebuildCase env scrut case_bndr alts cont
1403 | Lit lit <- scrut -- No need for same treatment as constructors
1404 -- because literals are inlined more vigorously
1405 = do { tick (KnownBranch case_bndr)
1406 ; case findAlt (LitAlt lit) alts of
1407 Nothing -> missingAlt env case_bndr alts cont
1408 Just (_, bs, rhs) -> simple_rhs bs rhs }
1410 | Just (con, ty_args, other_args) <- exprIsConApp_maybe scrut
1411 -- Works when the scrutinee is a variable with a known unfolding
1412 -- as well as when it's an explicit constructor application
1413 = do { tick (KnownBranch case_bndr)
1414 ; case findAlt (DataAlt con) alts of
1415 Nothing -> missingAlt env case_bndr alts cont
1416 Just (DEFAULT, bs, rhs) -> simple_rhs bs rhs
1417 Just (_, bs, rhs) -> knownCon env scrut con ty_args other_args
1418 case_bndr bs rhs cont
1421 simple_rhs bs rhs = ASSERT( null bs )
1422 do { env' <- simplNonRecX env case_bndr scrut
1423 ; simplExprF env' rhs cont }
1426 --------------------------------------------------
1427 -- 2. Eliminate the case if scrutinee is evaluated
1428 --------------------------------------------------
1430 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
1431 -- See if we can get rid of the case altogether
1432 -- See Note [Case eliminiation]
1433 -- mkCase made sure that if all the alternatives are equal,
1434 -- then there is now only one (DEFAULT) rhs
1435 | all isDeadBinder bndrs -- bndrs are [InId]
1437 -- Check that the scrutinee can be let-bound instead of case-bound
1438 , exprOkForSpeculation scrut
1439 -- OK not to evaluate it
1440 -- This includes things like (==# a# b#)::Bool
1441 -- so that we simplify
1442 -- case ==# a# b# of { True -> x; False -> x }
1445 -- This particular example shows up in default methods for
1446 -- comparision operations (e.g. in (>=) for Int.Int32)
1447 || exprIsHNF scrut -- It's already evaluated
1448 || var_demanded_later scrut -- It'll be demanded later
1450 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1451 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1452 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1453 -- its argument: case x of { y -> dataToTag# y }
1454 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1455 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1457 -- Also we don't want to discard 'seq's
1458 = do { tick (CaseElim case_bndr)
1459 ; env' <- simplNonRecX env case_bndr scrut
1460 ; simplExprF env' rhs cont }
1462 -- The case binder is going to be evaluated later,
1463 -- and the scrutinee is a simple variable
1464 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1465 && not (isTickBoxOp v)
1466 -- ugly hack; covering this case is what
1467 -- exprOkForSpeculation was intended for.
1468 var_demanded_later _ = False
1470 rebuildCase env scrut case_bndr alts@[(_, bndrs, rhs)] cont
1471 | all isDeadBinder (case_bndr : bndrs) -- So this is just 'seq'
1472 = -- For this case, see Note [User-defined RULES for seq] in MkId
1473 do { let rhs' = substExpr env rhs
1474 out_args = [Type (substTy env (idType case_bndr)),
1475 Type (exprType rhs'), scrut, rhs']
1476 -- Lazily evaluated, so we don't do most of this
1478 ; rule_base <- getSimplRules
1479 ; let rules = getRules rule_base seqId
1480 ; mb_rule <- tryRules env seqId rules out_args cont
1482 Just (n_args, res) -> simplExprF (zapSubstEnv env)
1483 (mkApps res (drop n_args out_args))
1485 Nothing -> reallyRebuildCase env scrut case_bndr alts cont }
1487 rebuildCase env scrut case_bndr alts cont
1488 = reallyRebuildCase env scrut case_bndr alts cont
1490 --------------------------------------------------
1491 -- 3. Catch-all case
1492 --------------------------------------------------
1494 reallyRebuildCase env scrut case_bndr alts cont
1495 = do { -- Prepare the continuation;
1496 -- The new subst_env is in place
1497 (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1499 -- Simplify the alternatives
1500 ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
1502 -- Check for empty alternatives
1503 ; if null alts' then missingAlt env case_bndr alts cont
1505 { case_expr <- mkCase scrut' case_bndr' alts'
1507 -- Notice that rebuild gets the in-scope set from env, not alt_env
1508 -- The case binder *not* scope over the whole returned case-expression
1509 ; rebuild env' case_expr nodup_cont } }
1512 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1513 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1514 way, there's a chance that v will now only be used once, and hence
1517 Historical note: we use to do the "case binder swap" in the Simplifier
1518 so there were additional complications if the scrutinee was a variable.
1519 Now the binder-swap stuff is done in the occurrence analyer; see
1520 OccurAnal Note [Binder swap].
1524 If the case binder is not dead, then neither are the pattern bound
1526 case <any> of x { (a,b) ->
1527 case x of { (p,q) -> p } }
1528 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1529 The point is that we bring into the envt a binding
1531 after the outer case, and that makes (a,b) alive. At least we do unless
1532 the case binder is guaranteed dead.
1534 In practice, the scrutinee is almost always a variable, so we pretty
1535 much always zap the OccInfo of the binders. It doesn't matter much though.
1540 Consider case (v `cast` co) of x { I# y ->
1541 ... (case (v `cast` co) of {...}) ...
1542 We'd like to eliminate the inner case. We can get this neatly by
1543 arranging that inside the outer case we add the unfolding
1544 v |-> x `cast` (sym co)
1545 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1547 Note [Improving seq]
1550 type family F :: * -> *
1551 type instance F Int = Int
1553 ... case e of x { DEFAULT -> rhs } ...
1555 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1557 case e `cast` co of x'::Int
1558 I# x# -> let x = x' `cast` sym co
1561 so that 'rhs' can take advantage of the form of x'.
1563 Notice that Note [Case of cast] may then apply to the result.
1565 Nota Bene: We only do the [Improving seq] transformation if the
1566 case binder 'x' is actually used in the rhs; that is, if the case
1567 is *not* a *pure* seq.
1568 a) There is no point in adding the cast to a pure seq.
1569 b) There is a good reason not to: doing so would interfere
1570 with seq rules (Note [Built-in RULES for seq] in MkId).
1571 In particular, this [Improving seq] thing *adds* a cast
1572 while [Built-in RULES for seq] *removes* one, so they
1575 You might worry about
1576 case v of x { __DEFAULT ->
1577 ... case (v `cast` co) of y { I# -> ... }}
1578 This is a pure seq (since x is unused), so [Improving seq] won't happen.
1579 But it's ok: the simplifier will replace 'v' by 'x' in the rhs to get
1580 case v of x { __DEFAULT ->
1581 ... case (x `cast` co) of y { I# -> ... }}
1582 Now the outer case is not a pure seq, so [Improving seq] will happen,
1583 and then the inner case will disappear.
1585 The need for [Improving seq] showed up in Roman's experiments. Example:
1586 foo :: F Int -> Int -> Int
1587 foo t n = t `seq` bar n
1590 bar n = bar (n - case t of TI i -> i)
1591 Here we'd like to avoid repeated evaluating t inside the loop, by
1592 taking advantage of the `seq`.
1594 At one point I did transformation in LiberateCase, but it's more
1595 robust here. (Otherwise, there's a danger that we'll simply drop the
1596 'seq' altogether, before LiberateCase gets to see it.)
1600 improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
1601 -> OutExpr -> InId -> OutId -> [InAlt]
1602 -> SimplM (SimplEnv, OutExpr, OutId)
1603 -- Note [Improving seq]
1604 improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1605 | not (isDeadBinder case_bndr) -- Not a pure seq! See the Note!
1606 , Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1607 = do { case_bndr2 <- newId (fsLit "nt") ty2
1608 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
1609 env2 = extendIdSubst env case_bndr rhs
1610 ; return (env2, scrut `Cast` co, case_bndr2) }
1612 improveSeq _ env scrut _ case_bndr1 _
1613 = return (env, scrut, case_bndr1)
1617 simplAlts does two things:
1619 1. Eliminate alternatives that cannot match, including the
1620 DEFAULT alternative.
1622 2. If the DEFAULT alternative can match only one possible constructor,
1623 then make that constructor explicit.
1625 case e of x { DEFAULT -> rhs }
1627 case e of x { (a,b) -> rhs }
1628 where the type is a single constructor type. This gives better code
1629 when rhs also scrutinises x or e.
1631 Here "cannot match" includes knowledge from GADTs
1633 It's a good idea do do this stuff before simplifying the alternatives, to
1634 avoid simplifying alternatives we know can't happen, and to come up with
1635 the list of constructors that are handled, to put into the IdInfo of the
1636 case binder, for use when simplifying the alternatives.
1638 Eliminating the default alternative in (1) isn't so obvious, but it can
1641 data Colour = Red | Green | Blue
1650 DEFAULT -> [ case y of ... ]
1652 If we inline h into f, the default case of the inlined h can't happen.
1653 If we don't notice this, we may end up filtering out *all* the cases
1654 of the inner case y, which give us nowhere to go!
1658 simplAlts :: SimplEnv
1660 -> InId -- Case binder
1661 -> [InAlt] -- Non-empty
1663 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1664 -- Like simplExpr, this just returns the simplified alternatives;
1665 -- it not return an environment
1667 simplAlts env scrut case_bndr alts cont'
1668 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1669 do { let env0 = zapFloats env
1671 ; (env1, case_bndr1) <- simplBinder env0 case_bndr
1673 ; fam_envs <- getFamEnvs
1674 ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
1675 case_bndr case_bndr1 alts
1677 ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env' scrut' case_bndr' alts
1679 ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
1680 ; return (scrut', case_bndr', alts') }
1682 ------------------------------------
1683 simplAlt :: SimplEnv
1684 -> [AltCon] -- These constructors can't be present when
1685 -- matching the DEFAULT alternative
1686 -> OutId -- The case binder
1691 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1692 = ASSERT( null bndrs )
1693 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1694 -- Record the constructors that the case-binder *can't* be.
1695 ; rhs' <- simplExprC env' rhs cont'
1696 ; return (DEFAULT, [], rhs') }
1698 simplAlt env _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
1699 = ASSERT( null bndrs )
1700 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1701 ; rhs' <- simplExprC env' rhs cont'
1702 ; return (LitAlt lit, [], rhs') }
1704 simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
1705 = do { -- Deal with the pattern-bound variables
1706 -- Mark the ones that are in ! positions in the
1707 -- data constructor as certainly-evaluated.
1708 -- NB: simplLamBinders preserves this eval info
1709 let vs_with_evals = add_evals (dataConRepStrictness con)
1710 ; (env', vs') <- simplLamBndrs env vs_with_evals
1712 -- Bind the case-binder to (con args)
1713 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1714 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1715 env'' = addBinderUnfolding env' case_bndr'
1716 (mkConApp con con_args)
1718 ; rhs' <- simplExprC env'' rhs cont'
1719 ; return (DataAlt con, vs', rhs') }
1721 -- add_evals records the evaluated-ness of the bound variables of
1722 -- a case pattern. This is *important*. Consider
1723 -- data T = T !Int !Int
1725 -- case x of { T a b -> T (a+1) b }
1727 -- We really must record that b is already evaluated so that we don't
1728 -- go and re-evaluate it when constructing the result.
1729 -- See Note [Data-con worker strictness] in MkId.lhs
1734 go (v:vs') strs | isTyVar v = v : go vs' strs
1735 go (v:vs') (str:strs)
1736 | isMarkedStrict str = evald_v : go vs' strs
1737 | otherwise = zapped_v : go vs' strs
1739 zapped_v = zap_occ_info v
1740 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1741 go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
1743 -- See Note [zapOccInfo]
1744 -- zap_occ_info: if the case binder is alive, then we add the unfolding
1746 -- to the envt; so vs are now very much alive
1747 -- Note [Aug06] I can't see why this actually matters, but it's neater
1748 -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
1749 -- ==> case e of t { (a,b) -> ...(a)... }
1750 -- Look, Ma, a is alive now.
1751 zap_occ_info = zapCasePatIdOcc case_bndr'
1753 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1754 addBinderUnfolding env bndr rhs
1755 = modifyInScope env (bndr `setIdUnfolding` mkUnfolding False rhs)
1757 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1758 addBinderOtherCon env bndr cons
1759 = modifyInScope env (bndr `setIdUnfolding` mkOtherCon cons)
1761 zapCasePatIdOcc :: Id -> Id -> Id
1762 -- Consider case e of b { (a,b) -> ... }
1763 -- Then if we bind b to (a,b) in "...", and b is not dead,
1764 -- then we must zap the deadness info on a,b
1765 zapCasePatIdOcc case_bndr
1766 | isDeadBinder case_bndr = \ pat_id -> pat_id
1767 | otherwise = \ pat_id -> zapIdOccInfo pat_id
1771 %************************************************************************
1773 \subsection{Known constructor}
1775 %************************************************************************
1777 We are a bit careful with occurrence info. Here's an example
1779 (\x* -> case x of (a*, b) -> f a) (h v, e)
1781 where the * means "occurs once". This effectively becomes
1782 case (h v, e) of (a*, b) -> f a)
1784 let a* = h v; b = e in f a
1788 All this should happen in one sweep.
1791 knownCon :: SimplEnv
1792 -> OutExpr -- The scrutinee
1793 -> DataCon -> [OutType] -> [OutExpr] -- The scrutinee (in pieces)
1794 -> InId -> [InBndr] -> InExpr -- The alternative
1796 -> SimplM (SimplEnv, OutExpr)
1798 knownCon env scrut dc dc_ty_args dc_args bndr bs rhs cont
1799 = do { env' <- bind_args env bs dc_args
1801 -- It's useful to bind bndr to scrut, rather than to a fresh
1802 -- binding x = Con arg1 .. argn
1803 -- because very often the scrut is a variable, so we avoid
1804 -- creating, and then subsequently eliminating, a let-binding
1805 -- BUT, if scrut is a not a variable, we must be careful
1806 -- about duplicating the arg redexes; in that case, make
1807 -- a new con-app from the args
1808 bndr_rhs | exprIsTrivial scrut = scrut
1809 | otherwise = con_app
1810 con_app = Var (dataConWorkId dc)
1811 `mkTyApps` dc_ty_args
1812 `mkApps` [substExpr env' (varToCoreExpr b) | b <- bs]
1813 -- dc_ty_args are aready OutTypes, but bs are InBndrs
1815 ; env'' <- simplNonRecX env' bndr bndr_rhs
1816 ; simplExprF env'' rhs cont }
1818 zap_occ = zapCasePatIdOcc bndr -- bndr is an InId
1821 bind_args env' [] _ = return env'
1823 bind_args env' (b:bs') (Type ty : args)
1824 = ASSERT( isTyVar b )
1825 bind_args (extendTvSubst env' b ty) bs' args
1827 bind_args env' (b:bs') (arg : args)
1829 do { let b' = zap_occ b
1830 -- Note that the binder might be "dead", because it doesn't
1831 -- occur in the RHS; and simplNonRecX may therefore discard
1832 -- it via postInlineUnconditionally.
1833 -- Nevertheless we must keep it if the case-binder is alive,
1834 -- because it may be used in the con_app. See Note [zapOccInfo]
1835 ; env'' <- simplNonRecX env' b' arg
1836 ; bind_args env'' bs' args }
1839 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr dc_args $$
1840 text "scrut:" <+> ppr scrut
1843 missingAlt :: SimplEnv -> Id -> [InAlt] -> SimplCont -> SimplM (SimplEnv, OutExpr)
1844 -- This isn't strictly an error, although it is unusual.
1845 -- It's possible that the simplifer might "see" that
1846 -- an inner case has no accessible alternatives before
1847 -- it "sees" that the entire branch of an outer case is
1848 -- inaccessible. So we simply put an error case here instead.
1849 missingAlt env case_bndr alts cont
1850 = WARN( True, ptext (sLit "missingAlt") <+> ppr case_bndr )
1851 return (env, mkImpossibleExpr res_ty)
1853 res_ty = contResultType env (substTy env (coreAltsType alts)) cont
1857 %************************************************************************
1859 \subsection{Duplicating continuations}
1861 %************************************************************************
1864 prepareCaseCont :: SimplEnv
1865 -> [InAlt] -> SimplCont
1866 -> SimplM (SimplEnv, SimplCont,SimplCont)
1867 -- Return a duplicatable continuation, a non-duplicable part
1868 -- plus some extra bindings (that scope over the entire
1871 -- No need to make it duplicatable if there's only one alternative
1872 prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
1873 prepareCaseCont env _ cont = mkDupableCont env cont
1877 mkDupableCont :: SimplEnv -> SimplCont
1878 -> SimplM (SimplEnv, SimplCont, SimplCont)
1880 mkDupableCont env cont
1881 | contIsDupable cont
1882 = return (env, cont, mkBoringStop)
1884 mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1886 mkDupableCont env (CoerceIt ty cont)
1887 = do { (env', dup, nodup) <- mkDupableCont env cont
1888 ; return (env', CoerceIt ty dup, nodup) }
1890 mkDupableCont env cont@(StrictBind {})
1891 = return (env, mkBoringStop, cont)
1892 -- See Note [Duplicating StrictBind]
1894 mkDupableCont env (StrictArg fun cci ai cont)
1895 -- See Note [Duplicating StrictArg]
1896 = do { (env', dup, nodup) <- mkDupableCont env cont
1897 ; (env'', fun') <- mk_dupable_call env' fun
1898 ; return (env'', StrictArg fun' cci ai dup, nodup) }
1900 mk_dupable_call env (Var v) = return (env, Var v)
1901 mk_dupable_call env (App fun arg) = do { (env', fun') <- mk_dupable_call env fun
1902 ; (env'', arg') <- makeTrivial env' arg
1903 ; return (env'', fun' `App` arg') }
1904 mk_dupable_call _ other = pprPanic "mk_dupable_call" (ppr other)
1905 -- The invariant of StrictArg is that the first arg is always an App chain
1907 mkDupableCont env (ApplyTo _ arg se cont)
1908 = -- e.g. [...hole...] (...arg...)
1910 -- let a = ...arg...
1911 -- in [...hole...] a
1912 do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1913 ; arg' <- simplExpr (se `setInScope` env') arg
1914 ; (env'', arg'') <- makeTrivial env' arg'
1915 ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
1916 ; return (env'', app_cont, nodup_cont) }
1918 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
1919 -- See Note [Single-alternative case]
1920 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1921 -- | not (isDeadBinder case_bndr)
1922 | all isDeadBinder bs -- InIds
1923 && not (isUnLiftedType (idType case_bndr))
1924 -- Note [Single-alternative-unlifted]
1925 = return (env, mkBoringStop, cont)
1927 mkDupableCont env (Select _ case_bndr alts se cont)
1928 = -- e.g. (case [...hole...] of { pi -> ei })
1930 -- let ji = \xij -> ei
1931 -- in case [...hole...] of { pi -> ji xij }
1932 do { tick (CaseOfCase case_bndr)
1933 ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1934 -- NB: call mkDupableCont here, *not* prepareCaseCont
1935 -- We must make a duplicable continuation, whereas prepareCaseCont
1936 -- doesn't when there is a single case branch
1938 ; let alt_env = se `setInScope` env'
1939 ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
1940 ; alts' <- mapM (simplAlt alt_env' [] case_bndr' dup_cont) alts
1941 -- Safe to say that there are no handled-cons for the DEFAULT case
1942 -- NB: simplBinder does not zap deadness occ-info, so
1943 -- a dead case_bndr' will still advertise its deadness
1944 -- This is really important because in
1945 -- case e of b { (# p,q #) -> ... }
1946 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1947 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1948 -- In the new alts we build, we have the new case binder, so it must retain
1950 -- NB: we don't use alt_env further; it has the substEnv for
1951 -- the alternatives, and we don't want that
1953 ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
1954 ; return (env'', -- Note [Duplicated env]
1955 Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
1959 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1960 -> SimplM (SimplEnv, [InAlt])
1961 -- Absorbs the continuation into the new alternatives
1963 mkDupableAlts env case_bndr' the_alts
1966 go env0 [] = return (env0, [])
1968 = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
1969 ; (env2, alts') <- go env1 alts
1970 ; return (env2, alt' : alts' ) }
1972 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
1973 -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
1974 mkDupableAlt env case_bndr (con, bndrs', rhs')
1975 | exprIsDupable rhs' -- Note [Small alternative rhs]
1976 = return (env, (con, bndrs', rhs'))
1978 = do { let rhs_ty' = exprType rhs'
1979 scrut_ty = idType case_bndr
1982 DEFAULT -> case_bndr
1983 DataAlt dc -> setIdUnfolding case_bndr unf
1985 -- See Note [Case binders and join points]
1986 unf = mkInlineRule InlSat rhs 0
1987 rhs = mkConApp dc (map Type (tyConAppArgs scrut_ty)
1988 ++ varsToCoreExprs bndrs')
1990 LitAlt {} -> WARN( True, ptext (sLit "mkDupableAlt")
1991 <+> ppr case_bndr <+> ppr con )
1993 -- The case binder is alive but trivial, so why has
1994 -- it not been substituted away?
1996 used_bndrs' | isDeadBinder case_bndr = filter abstract_over bndrs'
1997 | otherwise = bndrs' ++ [case_bndr_w_unf]
2000 | isTyVar bndr = True -- Abstract over all type variables just in case
2001 | otherwise = not (isDeadBinder bndr)
2002 -- The deadness info on the new Ids is preserved by simplBinders
2004 ; (final_bndrs', final_args) -- Note [Join point abstraction]
2005 <- if (any isId used_bndrs')
2006 then return (used_bndrs', varsToCoreExprs used_bndrs')
2007 else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
2008 ; return ([rw_id], [Var realWorldPrimId]) }
2010 ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
2011 -- Note [Funky mkPiTypes]
2013 ; let -- We make the lambdas into one-shot-lambdas. The
2014 -- join point is sure to be applied at most once, and doing so
2015 -- prevents the body of the join point being floated out by
2016 -- the full laziness pass
2017 really_final_bndrs = map one_shot final_bndrs'
2018 one_shot v | isId v = setOneShotLambda v
2020 join_rhs = mkLams really_final_bndrs rhs'
2021 join_call = mkApps (Var join_bndr) final_args
2023 ; env' <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
2024 ; return (env', (con, bndrs', join_call)) }
2025 -- See Note [Duplicated env]
2028 Note [Case binders and join points]
2029 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2031 case (case .. ) of c {
2034 If we make a join point with c but not c# we get
2035 $j = \c -> ....c....
2037 But if later inlining scrutines the c, thus
2039 $j = \c -> ... case c of { I# y -> ... } ...
2041 we won't see that 'c' has already been scrutinised. This actually
2042 happens in the 'tabulate' function in wave4main, and makes a significant
2043 difference to allocation.
2045 An alternative plan is this:
2047 $j = \c# -> let c = I# c# in ...c....
2049 but that is bad if 'c' is *not* later scrutinised.
2051 So instead we do both: we pass 'c' and 'c#' , and record in c's inlining
2052 that it's really I# c#, thus
2054 $j = \c# -> \c[=I# c#] -> ...c....
2056 Absence analysis may later discard 'c'.
2059 Note [Duplicated env]
2060 ~~~~~~~~~~~~~~~~~~~~~
2061 Some of the alternatives are simplified, but have not been turned into a join point
2062 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
2063 bind the join point, because it might to do PostInlineUnconditionally, and
2064 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
2065 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
2066 at worst delays the join-point inlining.
2068 Note [Small alternative rhs]
2069 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2070 It is worth checking for a small RHS because otherwise we
2071 get extra let bindings that may cause an extra iteration of the simplifier to
2072 inline back in place. Quite often the rhs is just a variable or constructor.
2073 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
2074 iterations because the version with the let bindings looked big, and so wasn't
2075 inlined, but after the join points had been inlined it looked smaller, and so
2078 NB: we have to check the size of rhs', not rhs.
2079 Duplicating a small InAlt might invalidate occurrence information
2080 However, if it *is* dupable, we return the *un* simplified alternative,
2081 because otherwise we'd need to pair it up with an empty subst-env....
2082 but we only have one env shared between all the alts.
2083 (Remember we must zap the subst-env before re-simplifying something).
2084 Rather than do this we simply agree to re-simplify the original (small) thing later.
2086 Note [Funky mkPiTypes]
2087 ~~~~~~~~~~~~~~~~~~~~~~
2088 Notice the funky mkPiTypes. If the contructor has existentials
2089 it's possible that the join point will be abstracted over
2090 type varaibles as well as term variables.
2091 Example: Suppose we have
2092 data T = forall t. C [t]
2094 case (case e of ...) of
2096 We get the join point
2097 let j :: forall t. [t] -> ...
2098 j = /\t \xs::[t] -> rhs
2100 case (case e of ...) of
2101 C t xs::[t] -> j t xs
2103 Note [Join point abstaction]
2104 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2105 If we try to lift a primitive-typed something out
2106 for let-binding-purposes, we will *caseify* it (!),
2107 with potentially-disastrous strictness results. So
2108 instead we turn it into a function: \v -> e
2109 where v::State# RealWorld#. The value passed to this function
2110 is realworld#, which generates (almost) no code.
2112 There's a slight infelicity here: we pass the overall
2113 case_bndr to all the join points if it's used in *any* RHS,
2114 because we don't know its usage in each RHS separately
2116 We used to say "&& isUnLiftedType rhs_ty'" here, but now
2117 we make the join point into a function whenever used_bndrs'
2118 is empty. This makes the join-point more CPR friendly.
2119 Consider: let j = if .. then I# 3 else I# 4
2120 in case .. of { A -> j; B -> j; C -> ... }
2122 Now CPR doesn't w/w j because it's a thunk, so
2123 that means that the enclosing function can't w/w either,
2124 which is a lose. Here's the example that happened in practice:
2125 kgmod :: Int -> Int -> Int
2126 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2130 I have seen a case alternative like this:
2132 It's a bit silly to add the realWorld dummy arg in this case, making
2135 (the \v alone is enough to make CPR happy) but I think it's rare
2137 Note [Duplicating StrictArg]
2138 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2139 The original plan had (where E is a big argument)
2141 ==> let $j = \a -> f E a
2144 But this is terrible! Here's an example:
2145 && E (case x of { T -> F; F -> T })
2146 Now, && is strict so we end up simplifying the case with
2147 an ArgOf continuation. If we let-bind it, we get
2148 let $j = \v -> && E v
2149 in simplExpr (case x of { T -> F; F -> T })
2151 And after simplifying more we get
2152 let $j = \v -> && E v
2153 in case x of { T -> $j F; F -> $j T }
2154 Which is a Very Bad Thing
2156 What we do now is this
2160 Now if the thing in the hole is a case expression (which is when
2161 we'll call mkDupableCont), we'll push the function call into the
2162 branches, which is what we want. Now RULES for f may fire, and
2163 call-pattern specialisation. Here's an example from Trac #3116
2166 _ -> Chunk p fpc (o+1) (l-1) bs')
2167 If we can push the call for 'go' inside the case, we get
2168 call-pattern specialisation for 'go', which is *crucial* for
2171 Here is the (&&) example:
2172 && E (case x of { T -> F; F -> T })
2174 case x of { T -> && a F; F -> && a T }
2178 * Arguments to f *after* the strict one are handled by
2179 the ApplyTo case of mkDupableCont. Eg
2182 * We can only do the let-binding of E because the function
2183 part of a StrictArg continuation is an explicit syntax
2184 tree. In earlier versions we represented it as a function
2185 (CoreExpr -> CoreEpxr) which we couldn't take apart.
2187 Do *not* duplicate StrictBind and StritArg continuations. We gain
2188 nothing by propagating them into the expressions, and we do lose a
2191 The desire not to duplicate is the entire reason that
2192 mkDupableCont returns a pair of continuations.
2194 Note [Duplicating StrictBind]
2195 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2196 Unlike StrictArg, there doesn't seem anything to gain from
2197 duplicating a StrictBind continuation, so we don't.
2199 The desire not to duplicate is the entire reason that
2200 mkDupableCont returns a pair of continuations.
2203 Note [Single-alternative cases]
2204 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2205 This case is just like the ArgOf case. Here's an example:
2209 case (case x of I# x' ->
2211 True -> I# (negate# x')
2212 False -> I# x') of y {
2214 Because the (case x) has only one alternative, we'll transform to
2216 case (case x' <# 0# of
2217 True -> I# (negate# x')
2218 False -> I# x') of y {
2220 But now we do *NOT* want to make a join point etc, giving
2222 let $j = \y -> MkT y
2224 True -> $j (I# (negate# x'))
2226 In this case the $j will inline again, but suppose there was a big
2227 strict computation enclosing the orginal call to MkT. Then, it won't
2228 "see" the MkT any more, because it's big and won't get duplicated.
2229 And, what is worse, nothing was gained by the case-of-case transform.
2231 When should use this case of mkDupableCont?
2232 However, matching on *any* single-alternative case is a *disaster*;
2233 e.g. case (case ....) of (a,b) -> (# a,b #)
2234 We must push the outer case into the inner one!
2237 * Match [(DEFAULT,_,_)], but in the common case of Int,
2238 the alternative-filling-in code turned the outer case into
2239 case (...) of y { I# _ -> MkT y }
2241 * Match on single alternative plus (not (isDeadBinder case_bndr))
2242 Rationale: pushing the case inwards won't eliminate the construction.
2243 But there's a risk of
2244 case (...) of y { (a,b) -> let z=(a,b) in ... }
2245 Now y looks dead, but it'll come alive again. Still, this
2246 seems like the best option at the moment.
2248 * Match on single alternative plus (all (isDeadBinder bndrs))
2249 Rationale: this is essentially seq.
2251 * Match when the rhs is *not* duplicable, and hence would lead to a
2252 join point. This catches the disaster-case above. We can test
2253 the *un-simplified* rhs, which is fine. It might get bigger or
2254 smaller after simplification; if it gets smaller, this case might
2255 fire next time round. NB also that we must test contIsDupable
2256 case_cont *btoo, because case_cont might be big!
2258 HOWEVER: I found that this version doesn't work well, because
2259 we can get let x = case (...) of { small } in ...case x...
2260 When x is inlined into its full context, we find that it was a bad
2261 idea to have pushed the outer case inside the (...) case.
2263 Note [Single-alternative-unlifted]
2264 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2265 Here's another single-alternative where we really want to do case-of-case:
2273 case y_s6X of tpl_s7m {
2274 M1.Mk1 ipv_s70 -> ipv_s70;
2275 M1.Mk2 ipv_s72 -> ipv_s72;
2281 case x_s74 of tpl_s7n {
2282 M1.Mk1 ipv_s77 -> ipv_s77;
2283 M1.Mk2 ipv_s79 -> ipv_s79;
2287 { __DEFAULT -> ==# [wild1_s7b wild_s7c];
2291 So the outer case is doing *nothing at all*, other than serving as a
2292 join-point. In this case we really want to do case-of-case and decide
2293 whether to use a real join point or just duplicate the continuation.
2295 Hence: check whether the case binder's type is unlifted, because then
2296 the outer case is *not* a seq.