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 MkId ( rUNTIME_ERROR_ID )
17 import FamInstEnv ( FamInstEnv )
22 import FamInstEnv ( topNormaliseType )
23 import DataCon ( dataConRepStrictness, dataConUnivTyVars )
25 import NewDemand ( isStrictDmd, splitStrictSig )
26 import PprCore ( pprParendExpr, pprCoreExpr )
27 import CoreUnfold ( mkUnfolding, callSiteInline, CallCtxt(..) )
29 import Rules ( lookupRule, getRules )
30 import BasicTypes ( isMarkedStrict )
31 import CostCentre ( currentCCS )
32 import TysPrim ( realWorldStatePrimTy )
33 import PrelInfo ( realWorldPrimId )
34 import BasicTypes ( TopLevelFlag(..), isTopLevel,
35 RecFlag(..), isNonRuleLoopBreaker )
36 import Maybes ( orElse )
37 import Data.List ( mapAccumL )
38 import MonadUtils ( foldlM )
39 import StaticFlags ( opt_PassCaseBndrToJoinPoints )
45 The guts of the simplifier is in this module, but the driver loop for
46 the simplifier is in SimplCore.lhs.
49 -----------------------------------------
50 *** IMPORTANT NOTE ***
51 -----------------------------------------
52 The simplifier used to guarantee that the output had no shadowing, but
53 it does not do so any more. (Actually, it never did!) The reason is
54 documented with simplifyArgs.
57 -----------------------------------------
58 *** IMPORTANT NOTE ***
59 -----------------------------------------
60 Many parts of the simplifier return a bunch of "floats" as well as an
61 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
63 All "floats" are let-binds, not case-binds, but some non-rec lets may
64 be unlifted (with RHS ok-for-speculation).
68 -----------------------------------------
69 ORGANISATION OF FUNCTIONS
70 -----------------------------------------
72 - simplify all top-level binders
73 - for NonRec, call simplRecOrTopPair
74 - for Rec, call simplRecBind
77 ------------------------------
78 simplExpr (applied lambda) ==> simplNonRecBind
79 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
80 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
82 ------------------------------
83 simplRecBind [binders already simplfied]
84 - use simplRecOrTopPair on each pair in turn
86 simplRecOrTopPair [binder already simplified]
87 Used for: recursive bindings (top level and nested)
88 top-level non-recursive bindings
90 - check for PreInlineUnconditionally
94 Used for: non-top-level non-recursive bindings
95 beta reductions (which amount to the same thing)
96 Because it can deal with strict arts, it takes a
97 "thing-inside" and returns an expression
99 - check for PreInlineUnconditionally
100 - simplify binder, including its IdInfo
109 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
110 Used for: binding case-binder and constr args in a known-constructor case
111 - check for PreInLineUnconditionally
115 ------------------------------
116 simplLazyBind: [binder already simplified, RHS not]
117 Used for: recursive bindings (top level and nested)
118 top-level non-recursive bindings
119 non-top-level, but *lazy* non-recursive bindings
120 [must not be strict or unboxed]
121 Returns floats + an augmented environment, not an expression
122 - substituteIdInfo and add result to in-scope
123 [so that rules are available in rec rhs]
126 - float if exposes constructor or PAP
130 completeNonRecX: [binder and rhs both simplified]
131 - if the the thing needs case binding (unlifted and not ok-for-spec)
137 completeBind: [given a simplified RHS]
138 [used for both rec and non-rec bindings, top level and not]
139 - try PostInlineUnconditionally
140 - add unfolding [this is the only place we add an unfolding]
145 Right hand sides and arguments
146 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
147 In many ways we want to treat
148 (a) the right hand side of a let(rec), and
149 (b) a function argument
150 in the same way. But not always! In particular, we would
151 like to leave these arguments exactly as they are, so they
152 will match a RULE more easily.
157 It's harder to make the rule match if we ANF-ise the constructor,
158 or eta-expand the PAP:
160 f (let { a = g x; b = h x } in (a,b))
163 On the other hand if we see the let-defns
168 then we *do* want to ANF-ise and eta-expand, so that p and q
169 can be safely inlined.
171 Even floating lets out is a bit dubious. For let RHS's we float lets
172 out if that exposes a value, so that the value can be inlined more vigorously.
175 r = let x = e in (x,x)
177 Here, if we float the let out we'll expose a nice constructor. We did experiments
178 that showed this to be a generally good thing. But it was a bad thing to float
179 lets out unconditionally, because that meant they got allocated more often.
181 For function arguments, there's less reason to expose a constructor (it won't
182 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
183 So for the moment we don't float lets out of function arguments either.
188 For eta expansion, we want to catch things like
190 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
192 If the \x was on the RHS of a let, we'd eta expand to bring the two
193 lambdas together. And in general that's a good thing to do. Perhaps
194 we should eta expand wherever we find a (value) lambda? Then the eta
195 expansion at a let RHS can concentrate solely on the PAP case.
198 %************************************************************************
200 \subsection{Bindings}
202 %************************************************************************
205 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
207 simplTopBinds env0 binds0
208 = do { -- Put all the top-level binders into scope at the start
209 -- so that if a transformation rule has unexpectedly brought
210 -- anything into scope, then we don't get a complaint about that.
211 -- It's rather as if the top-level binders were imported.
212 ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
213 ; dflags <- getDOptsSmpl
214 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
215 dopt Opt_D_dump_rule_firings dflags
216 ; env2 <- simpl_binds dump_flag env1 binds0
217 ; freeTick SimplifierDone
218 ; return (getFloats env2) }
220 -- We need to track the zapped top-level binders, because
221 -- they should have their fragile IdInfo zapped (notably occurrence info)
222 -- That's why we run down binds and bndrs' simultaneously.
224 -- The dump-flag emits a trace for each top-level binding, which
225 -- helps to locate the tracing for inlining and rule firing
226 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
227 simpl_binds _ env [] = return env
228 simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
230 ; simpl_binds dump env' binds }
232 trace_bind True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
233 trace_bind False _ = \x -> x
235 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
236 simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel b b' r
238 (env', b') = addBndrRules env b (lookupRecBndr env b)
242 %************************************************************************
244 \subsection{Lazy bindings}
246 %************************************************************************
248 simplRecBind is used for
249 * recursive bindings only
252 simplRecBind :: SimplEnv -> TopLevelFlag
255 simplRecBind env0 top_lvl pairs0
256 = do { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
257 ; env1 <- go (zapFloats env_with_info) triples
258 ; return (env0 `addRecFloats` env1) }
259 -- addFloats adds the floats from env1,
260 -- _and_ updates env0 with the in-scope set from env1
262 add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
263 -- Add the (substituted) rules to the binder
264 add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
266 (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
268 go env [] = return env
270 go env ((old_bndr, new_bndr, rhs) : pairs)
271 = do { env' <- simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
275 simplOrTopPair is used for
276 * recursive bindings (whether top level or not)
277 * top-level non-recursive bindings
279 It assumes the binder has already been simplified, but not its IdInfo.
282 simplRecOrTopPair :: SimplEnv
284 -> InId -> OutBndr -> InExpr -- Binder and rhs
285 -> SimplM SimplEnv -- Returns an env that includes the binding
287 simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
288 | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
289 = do { tick (PreInlineUnconditionally old_bndr)
290 ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
293 = simplLazyBind env top_lvl Recursive old_bndr new_bndr rhs env
294 -- May not actually be recursive, but it doesn't matter
298 simplLazyBind is used for
299 * [simplRecOrTopPair] recursive bindings (whether top level or not)
300 * [simplRecOrTopPair] top-level non-recursive bindings
301 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
304 1. It assumes that the binder is *already* simplified,
305 and is in scope, and its IdInfo too, except unfolding
307 2. It assumes that the binder type is lifted.
309 3. It does not check for pre-inline-unconditionallly;
310 that should have been done already.
313 simplLazyBind :: SimplEnv
314 -> TopLevelFlag -> RecFlag
315 -> InId -> OutId -- Binder, both pre-and post simpl
316 -- The OutId has IdInfo, except arity, unfolding
317 -> InExpr -> SimplEnv -- The RHS and its environment
320 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
321 = do { let rhs_env = rhs_se `setInScope` env
322 (tvs, body) = case collectTyBinders rhs of
323 (tvs, body) | not_lam body -> (tvs,body)
324 | otherwise -> ([], rhs)
325 not_lam (Lam _ _) = False
327 -- Do not do the "abstract tyyvar" thing if there's
328 -- a lambda inside, becuase it defeats eta-reduction
329 -- f = /\a. \x. g a x
332 ; (body_env, tvs') <- simplBinders rhs_env tvs
333 -- See Note [Floating and type abstraction] in SimplUtils
336 ; (body_env1, body1) <- simplExprF body_env body mkBoringStop
338 -- ANF-ise a constructor or PAP rhs
339 ; (body_env2, body2) <- prepareRhs body_env1 body1
342 <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
343 then -- No floating, just wrap up!
344 do { rhs' <- mkLam tvs' (wrapFloats body_env2 body2)
345 ; return (env, rhs') }
347 else if null tvs then -- Simple floating
348 do { tick LetFloatFromLet
349 ; return (addFloats env body_env2, body2) }
351 else -- Do type-abstraction first
352 do { tick LetFloatFromLet
353 ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
354 ; rhs' <- mkLam tvs' body3
355 ; let env' = foldl (addPolyBind top_lvl) env poly_binds
356 ; return (env', rhs') }
358 ; completeBind env' top_lvl bndr bndr1 rhs' }
361 A specialised variant of simplNonRec used when the RHS is already simplified,
362 notably in knownCon. It uses case-binding where necessary.
365 simplNonRecX :: SimplEnv
366 -> InId -- Old binder
367 -> OutExpr -- Simplified RHS
370 simplNonRecX env bndr new_rhs
371 | isDeadBinder bndr -- Not uncommon; e.g. case (a,b) of b { (p,q) -> p }
372 = return env -- Here b is dead, and we avoid creating
373 | otherwise -- the binding b = (a,b)
374 = do { (env', bndr') <- simplBinder env bndr
375 ; completeNonRecX env' (isStrictId bndr) bndr bndr' new_rhs }
377 completeNonRecX :: SimplEnv
379 -> InId -- Old binder
380 -> OutId -- New binder
381 -> OutExpr -- Simplified RHS
384 completeNonRecX env is_strict old_bndr new_bndr new_rhs
385 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
387 if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
388 then do { tick LetFloatFromLet
389 ; return (addFloats env env1, rhs1) } -- Add the floats to the main env
390 else return (env, wrapFloats env1 rhs1) -- Wrap the floats around the RHS
391 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
394 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
395 Doing so risks exponential behaviour, because new_rhs has been simplified once already
396 In the cases described by the folowing commment, postInlineUnconditionally will
397 catch many of the relevant cases.
398 -- This happens; for example, the case_bndr during case of
399 -- known constructor: case (a,b) of x { (p,q) -> ... }
400 -- Here x isn't mentioned in the RHS, so we don't want to
401 -- create the (dead) let-binding let x = (a,b) in ...
403 -- Similarly, single occurrences can be inlined vigourously
404 -- e.g. case (f x, g y) of (a,b) -> ....
405 -- If a,b occur once we can avoid constructing the let binding for them.
407 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
408 -- Consider case I# (quotInt# x y) of
409 -- I# v -> let w = J# v in ...
410 -- If we gaily inline (quotInt# x y) for v, we end up building an
412 -- let w = J# (quotInt# x y) in ...
413 -- because quotInt# can fail.
415 | preInlineUnconditionally env NotTopLevel bndr new_rhs
416 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
419 ----------------------------------
420 prepareRhs takes a putative RHS, checks whether it's a PAP or
421 constructor application and, if so, converts it to ANF, so that the
422 resulting thing can be inlined more easily. Thus
429 We also want to deal well cases like this
430 v = (f e1 `cast` co) e2
431 Here we want to make e1,e2 trivial and get
432 x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
433 That's what the 'go' loop in prepareRhs does
436 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
437 -- Adds new floats to the env iff that allows us to return a good RHS
438 prepareRhs env (Cast rhs co) -- Note [Float coercions]
439 | (ty1, _ty2) <- coercionKind co -- Do *not* do this if rhs has an unlifted type
440 , not (isUnLiftedType ty1) -- see Note [Float coercions (unlifted)]
441 = do { (env', rhs') <- makeTrivial env rhs
442 ; return (env', Cast rhs' co) }
445 = do { (_is_val, env1, rhs1) <- go 0 env0 rhs0
446 ; return (env1, rhs1) }
448 go n_val_args env (Cast rhs co)
449 = do { (is_val, env', rhs') <- go n_val_args env rhs
450 ; return (is_val, env', Cast rhs' co) }
451 go n_val_args env (App fun (Type ty))
452 = do { (is_val, env', rhs') <- go n_val_args env fun
453 ; return (is_val, env', App rhs' (Type ty)) }
454 go n_val_args env (App fun arg)
455 = do { (is_val, env', fun') <- go (n_val_args+1) env fun
457 True -> do { (env'', arg') <- makeTrivial env' arg
458 ; return (True, env'', App fun' arg') }
459 False -> return (False, env, App fun arg) }
460 go n_val_args env (Var fun)
461 = return (is_val, env, Var fun)
463 is_val = n_val_args > 0 -- There is at least one arg
464 -- ...and the fun a constructor or PAP
465 && (isDataConWorkId fun || n_val_args < idArity fun)
467 = return (False, env, other)
471 Note [Float coercions]
472 ~~~~~~~~~~~~~~~~~~~~~~
473 When we find the binding
475 we'd like to transform it to
477 x = x `cast` co -- A trivial binding
478 There's a chance that e will be a constructor application or function, or something
479 like that, so moving the coerion to the usage site may well cancel the coersions
480 and lead to further optimisation. Example:
483 data instance T Int = T Int
485 foo :: Int -> Int -> Int
490 go n = case x of { T m -> go (n-m) }
491 -- This case should optimise
493 Note [Float coercions (unlifted)]
494 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
495 BUT don't do [Float coercions] if 'e' has an unlifted type.
498 foo :: Int = (error (# Int,Int #) "urk")
499 `cast` CoUnsafe (# Int,Int #) Int
501 If do the makeTrivial thing to the error call, we'll get
502 foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
503 But 'v' isn't in scope!
505 These strange casts can happen as a result of case-of-case
506 bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
511 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
512 -- Binds the expression to a variable, if it's not trivial, returning the variable
516 | otherwise -- See Note [Take care] below
517 = do { var <- newId (fsLit "a") (exprType expr)
518 ; env' <- completeNonRecX env False var var expr
519 -- pprTrace "makeTrivial" (vcat [ppr var <+> ppr (exprArity (substExpr env' (Var var)))
521 -- , ppr (substExpr env' (Var var))
522 -- , ppr (idArity (fromJust (lookupInScope (seInScope env') var))) ]) $
523 ; return (env', substExpr env' (Var var)) }
524 -- The substitution is needed becase we're constructing a new binding
526 -- And if rhs is of form (rhs1 |> co), then we might get
529 -- and now a's RHS is trivial and can be substituted out, and that
530 -- is what completeNonRecX will do
534 %************************************************************************
536 \subsection{Completing a lazy binding}
538 %************************************************************************
541 * deals only with Ids, not TyVars
542 * takes an already-simplified binder and RHS
543 * is used for both recursive and non-recursive bindings
544 * is used for both top-level and non-top-level bindings
546 It does the following:
547 - tries discarding a dead binding
548 - tries PostInlineUnconditionally
549 - add unfolding [this is the only place we add an unfolding]
552 It does *not* attempt to do let-to-case. Why? Because it is used for
553 - top-level bindings (when let-to-case is impossible)
554 - many situations where the "rhs" is known to be a WHNF
555 (so let-to-case is inappropriate).
557 Nor does it do the atomic-argument thing
560 completeBind :: SimplEnv
561 -> TopLevelFlag -- Flag stuck into unfolding
562 -> InId -- Old binder
563 -> OutId -> OutExpr -- New binder and RHS
565 -- completeBind may choose to do its work
566 -- * by extending the substitution (e.g. let x = y in ...)
567 -- * or by adding to the floats in the envt
569 completeBind env top_lvl old_bndr new_bndr new_rhs
570 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
571 -- Inline and discard the binding
572 = do { tick (PostInlineUnconditionally old_bndr)
573 ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
574 return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
575 -- Use the substitution to make quite, quite sure that the
576 -- substitution will happen, since we are going to discard the binding
579 = return (addNonRecWithUnf env new_bndr new_rhs unfolding wkr)
581 unfolding | omit_unfolding = NoUnfolding
582 | otherwise = mkUnfolding (isTopLevel top_lvl) new_rhs
583 old_info = idInfo old_bndr
584 occ_info = occInfo old_info
585 wkr = substWorker env (workerInfo old_info)
586 omit_unfolding = isNonRuleLoopBreaker occ_info
587 -- or not (activeInline env old_bndr)
588 -- Do *not* trim the unfolding in SimplGently, else
589 -- the specialiser can't see it!
592 addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplEnv
593 -- Add a new binding to the environment, complete with its unfolding
594 -- but *do not* do postInlineUnconditionally, because we have already
595 -- processed some of the scope of the binding
596 -- We still want the unfolding though. Consider
598 -- x = /\a. let y = ... in Just y
600 -- Then we float the y-binding out (via abstractFloats and addPolyBind)
601 -- but 'x' may well then be inlined in 'body' in which case we'd like the
602 -- opportunity to inline 'y' too.
604 addPolyBind top_lvl env (NonRec poly_id rhs)
605 = addNonRecWithUnf env poly_id rhs unfolding NoWorker
607 unfolding | not (activeInline env poly_id) = NoUnfolding
608 | otherwise = mkUnfolding (isTopLevel top_lvl) rhs
609 -- addNonRecWithInfo adds the new binding in the
610 -- proper way (ie complete with unfolding etc),
611 -- and extends the in-scope set
613 addPolyBind _ env bind@(Rec _) = extendFloats env bind
614 -- Hack: letrecs are more awkward, so we extend "by steam"
615 -- without adding unfoldings etc. At worst this leads to
616 -- more simplifier iterations
619 addNonRecWithUnf :: SimplEnv
620 -> OutId -> OutExpr -- New binder and RHS
621 -> Unfolding -> WorkerInfo -- and unfolding
623 -- Add suitable IdInfo to the Id, add the binding to the floats, and extend the in-scope set
624 addNonRecWithUnf env new_bndr rhs unfolding wkr
625 = ASSERT( isId new_bndr )
626 WARN( new_arity < old_arity || new_arity < dmd_arity,
627 (ppr final_id <+> ppr old_arity <+> ppr new_arity <+> ppr dmd_arity) $$ ppr rhs )
628 final_id `seq` -- This seq forces the Id, and hence its IdInfo,
629 -- and hence any inner substitutions
630 addNonRec env final_id rhs
631 -- The addNonRec adds it to the in-scope set too
633 dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr
634 old_arity = idArity new_bndr
637 new_arity = exprArity rhs
638 new_bndr_info = idInfo new_bndr `setArityInfo` new_arity
641 -- Add the unfolding *only* for non-loop-breakers
642 -- Making loop breakers not have an unfolding at all
643 -- means that we can avoid tests in exprIsConApp, for example.
644 -- This is important: if exprIsConApp says 'yes' for a recursive
645 -- thing, then we can get into an infinite loop
648 -- If the unfolding is a value, the demand info may
649 -- go pear-shaped, so we nuke it. Example:
651 -- case x of (p,q) -> h p q x
652 -- Here x is certainly demanded. But after we've nuked
653 -- the case, we'll get just
654 -- let x = (a,b) in h a b x
655 -- and now x is not demanded (I'm assuming h is lazy)
656 -- This really happens. Similarly
657 -- let f = \x -> e in ...f..f...
658 -- After inlining f at some of its call sites the original binding may
659 -- (for example) be no longer strictly demanded.
660 -- The solution here is a bit ad hoc...
661 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
664 final_info | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
665 | otherwise = info_w_unf
667 final_id = new_bndr `setIdInfo` final_info
672 %************************************************************************
674 \subsection[Simplify-simplExpr]{The main function: simplExpr}
676 %************************************************************************
678 The reason for this OutExprStuff stuff is that we want to float *after*
679 simplifying a RHS, not before. If we do so naively we get quadratic
680 behaviour as things float out.
682 To see why it's important to do it after, consider this (real) example:
696 a -- Can't inline a this round, cos it appears twice
700 Each of the ==> steps is a round of simplification. We'd save a
701 whole round if we float first. This can cascade. Consider
706 let f = let d1 = ..d.. in \y -> e
710 in \x -> ...(\y ->e)...
712 Only in this second round can the \y be applied, and it
713 might do the same again.
717 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
718 simplExpr env expr = simplExprC env expr mkBoringStop
720 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
721 -- Simplify an expression, given a continuation
722 simplExprC env expr cont
723 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
724 do { (env', expr') <- simplExprF (zapFloats env) expr cont
725 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
726 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
727 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
728 return (wrapFloats env' expr') }
730 --------------------------------------------------
731 simplExprF :: SimplEnv -> InExpr -> SimplCont
732 -> SimplM (SimplEnv, OutExpr)
734 simplExprF env e cont
735 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
736 simplExprF' env e cont
738 simplExprF' :: SimplEnv -> InExpr -> SimplCont
739 -> SimplM (SimplEnv, OutExpr)
740 simplExprF' env (Var v) cont = simplVar env v cont
741 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
742 simplExprF' env (Note n expr) cont = simplNote env n expr cont
743 simplExprF' env (Cast body co) cont = simplCast env body co cont
744 simplExprF' env (App fun arg) cont = simplExprF env fun $
745 ApplyTo NoDup arg env cont
747 simplExprF' env expr@(Lam _ _) cont
748 = simplLam env (map zap bndrs) body cont
749 -- The main issue here is under-saturated lambdas
750 -- (\x1. \x2. e) arg1
751 -- Here x1 might have "occurs-once" occ-info, because occ-info
752 -- is computed assuming that a group of lambdas is applied
753 -- all at once. If there are too few args, we must zap the
756 n_args = countArgs cont
757 n_params = length bndrs
758 (bndrs, body) = collectBinders expr
759 zap | n_args >= n_params = \b -> b
760 | otherwise = \b -> if isTyVar b then b
762 -- NB: we count all the args incl type args
763 -- so we must count all the binders (incl type lambdas)
765 simplExprF' env (Type ty) cont
766 = ASSERT( contIsRhsOrArg cont )
767 do { ty' <- simplType env ty
768 ; rebuild env (Type ty') cont }
770 simplExprF' env (Case scrut bndr _ alts) cont
771 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
772 = -- Simplify the scrutinee with a Select continuation
773 simplExprF env scrut (Select NoDup bndr alts env cont)
776 = -- If case-of-case is off, simply simplify the case expression
777 -- in a vanilla Stop context, and rebuild the result around it
778 do { case_expr' <- simplExprC env scrut case_cont
779 ; rebuild env case_expr' cont }
781 case_cont = Select NoDup bndr alts env mkBoringStop
783 simplExprF' env (Let (Rec pairs) body) cont
784 = do { env' <- simplRecBndrs env (map fst pairs)
785 -- NB: bndrs' don't have unfoldings or rules
786 -- We add them as we go down
788 ; env'' <- simplRecBind env' NotTopLevel pairs
789 ; simplExprF env'' body cont }
791 simplExprF' env (Let (NonRec bndr rhs) body) cont
792 = simplNonRecE env bndr (rhs, env) ([], body) cont
794 ---------------------------------
795 simplType :: SimplEnv -> InType -> SimplM OutType
796 -- Kept monadic just so we can do the seqType
798 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
799 seqType new_ty `seq` return new_ty
801 new_ty = substTy env ty
805 %************************************************************************
807 \subsection{The main rebuilder}
809 %************************************************************************
812 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
813 -- At this point the substitution in the SimplEnv should be irrelevant
814 -- only the in-scope set and floats should matter
815 rebuild env expr cont0
816 = -- pprTrace "rebuild" (ppr expr $$ ppr cont0 $$ ppr (seFloats env)) $
818 Stop {} -> return (env, expr)
819 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
820 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
821 StrictArg fun _ info cont -> rebuildCall env (fun `App` expr) info cont
822 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
823 ; simplLam env' bs body cont }
824 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
825 ; rebuild env (App expr arg') cont }
829 %************************************************************************
833 %************************************************************************
836 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
837 -> SimplM (SimplEnv, OutExpr)
838 simplCast env body co0 cont0
839 = do { co1 <- simplType env co0
840 ; simplExprF env body (addCoerce co1 cont0) }
842 addCoerce co cont = add_coerce co (coercionKind co) cont
844 add_coerce _co (s1, k1) cont -- co :: ty~ty
845 | s1 `coreEqType` k1 = cont -- is a no-op
847 add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
848 | (_l1, t1) <- coercionKind co2
849 -- e |> (g1 :: S1~L) |> (g2 :: L~T1)
852 -- e |> (g1 . g2 :: T1~T2) otherwise
854 -- For example, in the initial form of a worker
855 -- we may find (coerce T (coerce S (\x.e))) y
856 -- and we'd like it to simplify to e[y/x] in one round
858 , s1 `coreEqType` t1 = cont -- The coerces cancel out
859 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
861 add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
862 -- (f |> g) ty ---> (f ty) |> (g @ ty)
863 -- This implements the PushT rule from the paper
864 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
865 , not (isCoVar tyvar)
866 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
868 ty' = substTy (arg_se `setInScope` env) arg_ty
870 -- ToDo: the PushC rule is not implemented at all
872 add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
873 | not (isTypeArg arg) -- This implements the Push rule from the paper
874 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
875 -- (e |> (g :: s1s2 ~ t1->t2)) f
877 -- (e (f |> (arg g :: t1~s1))
878 -- |> (res g :: s2->t2)
880 -- t1t2 must be a function type, t1->t2, because it's applied
881 -- to something but s1s2 might conceivably not be
883 -- When we build the ApplyTo we can't mix the out-types
884 -- with the InExpr in the argument, so we simply substitute
885 -- to make it all consistent. It's a bit messy.
886 -- But it isn't a common case.
888 -- Example of use: Trac #995
889 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
891 -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
892 -- t2 ~ s2 with left and right on the curried form:
893 -- (->) t1 t2 ~ (->) s1 s2
894 [co1, co2] = decomposeCo 2 co
895 new_arg = mkCoerce (mkSymCoercion co1) arg'
896 arg' = substExpr (arg_se `setInScope` env) arg
898 add_coerce co _ cont = CoerceIt co cont
902 %************************************************************************
906 %************************************************************************
909 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
910 -> SimplM (SimplEnv, OutExpr)
912 simplLam env [] body cont = simplExprF env body cont
915 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
916 = do { tick (BetaReduction bndr)
917 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
919 -- Not enough args, so there are real lambdas left to put in the result
920 simplLam env bndrs body cont
921 = do { (env', bndrs') <- simplLamBndrs env bndrs
922 ; body' <- simplExpr env' body
923 ; new_lam <- mkLam bndrs' body'
924 ; rebuild env' new_lam cont }
927 simplNonRecE :: SimplEnv
928 -> InId -- The binder
929 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
930 -> ([InBndr], InExpr) -- Body of the let/lambda
933 -> SimplM (SimplEnv, OutExpr)
935 -- simplNonRecE is used for
936 -- * non-top-level non-recursive lets in expressions
939 -- It deals with strict bindings, via the StrictBind continuation,
940 -- which may abort the whole process
942 -- The "body" of the binding comes as a pair of ([InId],InExpr)
943 -- representing a lambda; so we recurse back to simplLam
944 -- Why? Because of the binder-occ-info-zapping done before
945 -- the call to simplLam in simplExprF (Lam ...)
947 -- First deal with type applications and type lets
948 -- (/\a. e) (Type ty) and (let a = Type ty in e)
949 simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
950 = ASSERT( isTyVar bndr )
951 do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
952 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
954 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
955 | preInlineUnconditionally env NotTopLevel bndr rhs
956 = do { tick (PreInlineUnconditionally bndr)
957 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
960 = do { simplExprF (rhs_se `setFloats` env) rhs
961 (StrictBind bndr bndrs body env cont) }
964 = ASSERT( not (isTyVar bndr) )
965 do { (env1, bndr1) <- simplNonRecBndr env bndr
966 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
967 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
968 ; simplLam env3 bndrs body cont }
972 %************************************************************************
976 %************************************************************************
979 -- Hack alert: we only distinguish subsumed cost centre stacks for the
980 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
981 simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
982 -> SimplM (SimplEnv, OutExpr)
983 simplNote env (SCC cc) e cont
984 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
985 ; rebuild env (mkSCC cc e') cont }
987 -- See notes with SimplMonad.inlineMode
988 simplNote env InlineMe e cont
989 | Just (inside, outside) <- splitInlineCont cont -- Boring boring continuation; see notes above
990 = do { -- Don't inline inside an INLINE expression
991 e' <- simplExprC (setMode inlineMode env) e inside
992 ; rebuild env (mkInlineMe e') outside }
994 | otherwise -- Dissolve the InlineMe note if there's
995 -- an interesting context of any kind to combine with
996 -- (even a type application -- anything except Stop)
997 = simplExprF env e cont
999 simplNote env (CoreNote s) e cont = do
1000 e' <- simplExpr env e
1001 rebuild env (Note (CoreNote s) e') cont
1005 %************************************************************************
1007 \subsection{Dealing with calls}
1009 %************************************************************************
1012 simplVar :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1013 simplVar env var cont
1014 = case substId env var of
1015 DoneEx e -> simplExprF (zapSubstEnv env) e cont
1016 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
1017 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
1018 -- Note [zapSubstEnv]
1019 -- The template is already simplified, so don't re-substitute.
1020 -- This is VITAL. Consider
1022 -- let y = \z -> ...x... in
1024 -- We'll clone the inner \x, adding x->x' in the id_subst
1025 -- Then when we inline y, we must *not* replace x by x' in
1026 -- the inlined copy!!
1028 ---------------------------------------------------------
1029 -- Dealing with a call site
1031 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1032 completeCall env var cont
1033 = do { dflags <- getDOptsSmpl
1034 ; let (args,call_cont) = contArgs cont
1035 -- The args are OutExprs, obtained by *lazily* substituting
1036 -- in the args found in cont. These args are only examined
1037 -- to limited depth (unless a rule fires). But we must do
1038 -- the substitution; rule matching on un-simplified args would
1041 ------------- First try rules ----------------
1042 -- Do this before trying inlining. Some functions have
1043 -- rules *and* are strict; in this case, we don't want to
1044 -- inline the wrapper of the non-specialised thing; better
1045 -- to call the specialised thing instead.
1047 -- We used to use the black-listing mechanism to ensure that inlining of
1048 -- the wrapper didn't occur for things that have specialisations till a
1049 -- later phase, so but now we just try RULES first
1051 -- Note [Rules for recursive functions]
1052 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1053 -- You might think that we shouldn't apply rules for a loop breaker:
1054 -- doing so might give rise to an infinite loop, because a RULE is
1055 -- rather like an extra equation for the function:
1056 -- RULE: f (g x) y = x+y
1059 -- But it's too drastic to disable rules for loop breakers.
1060 -- Even the foldr/build rule would be disabled, because foldr
1061 -- is recursive, and hence a loop breaker:
1062 -- foldr k z (build g) = g k z
1063 -- So it's up to the programmer: rules can cause divergence
1064 ; rule_base <- getSimplRules
1065 ; let in_scope = getInScope env
1066 rules = getRules rule_base var
1067 maybe_rule = case activeRule dflags env of
1068 Nothing -> Nothing -- No rules apply
1069 Just act_fn -> lookupRule act_fn in_scope
1071 ; case maybe_rule of {
1072 Just (rule, rule_rhs) -> do
1073 tick (RuleFired (ru_name rule))
1074 (if dopt Opt_D_dump_rule_firings dflags then
1075 pprTrace "Rule fired" (vcat [
1076 text "Rule:" <+> ftext (ru_name rule),
1077 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1078 text "After: " <+> pprCoreExpr rule_rhs,
1079 text "Cont: " <+> ppr call_cont])
1082 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
1083 -- The ruleArity says how many args the rule consumed
1085 ; Nothing -> do -- No rules
1087 ------------- Next try inlining ----------------
1088 { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
1089 n_val_args = length arg_infos
1090 interesting_cont = interestingCallContext call_cont
1091 active_inline = activeInline env var
1092 maybe_inline = callSiteInline dflags active_inline var
1093 (null args) arg_infos interesting_cont
1094 ; case maybe_inline of {
1095 Just unfolding -- There is an inlining!
1096 -> do { tick (UnfoldingDone var)
1097 ; (if dopt Opt_D_dump_inlinings dflags then
1098 pprTrace ("Inlining done" ++ showSDoc (ppr var)) (vcat [
1099 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1100 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1101 text "Cont: " <+> ppr call_cont])
1104 simplExprF env unfolding cont }
1106 ; Nothing -> -- No inlining!
1108 ------------- No inlining! ----------------
1109 -- Next, look for rules or specialisations that match
1111 rebuildCall env (Var var)
1112 (mkArgInfo var n_val_args call_cont) cont
1115 rebuildCall :: SimplEnv
1116 -> OutExpr -- Function
1119 -> SimplM (SimplEnv, OutExpr)
1120 rebuildCall env fun (ArgInfo { ai_strs = [] }) cont
1121 -- When we run out of strictness args, it means
1122 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1123 -- Then we want to discard the entire strict continuation. E.g.
1124 -- * case (error "hello") of { ... }
1125 -- * (error "Hello") arg
1126 -- * f (error "Hello") where f is strict
1128 -- Then, especially in the first of these cases, we'd like to discard
1129 -- the continuation, leaving just the bottoming expression. But the
1130 -- type might not be right, so we may have to add a coerce.
1131 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1132 = return (env, mk_coerce fun) -- contination to discard, else we do it
1133 where -- again and again!
1134 fun_ty = exprType fun
1135 cont_ty = contResultType env fun_ty cont
1136 co = mkUnsafeCoercion fun_ty cont_ty
1137 mk_coerce expr | cont_ty `coreEqType` fun_ty = expr
1138 | otherwise = mkCoerce co expr
1140 rebuildCall env fun info (ApplyTo _ (Type arg_ty) se cont)
1141 = do { ty' <- simplType (se `setInScope` env) arg_ty
1142 ; rebuildCall env (fun `App` Type ty') info cont }
1145 (ArgInfo { ai_rules = has_rules, ai_strs = str:strs, ai_discs = disc:discs })
1146 (ApplyTo _ arg arg_se cont)
1147 | str -- Strict argument
1148 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1149 simplExprF (arg_se `setFloats` env) arg
1150 (StrictArg fun cci arg_info' cont)
1153 | otherwise -- Lazy argument
1154 -- DO NOT float anything outside, hence simplExprC
1155 -- There is no benefit (unlike in a let-binding), and we'd
1156 -- have to be very careful about bogus strictness through
1157 -- floating a demanded let.
1158 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1160 ; rebuildCall env (fun `App` arg') arg_info' cont }
1162 arg_info' = ArgInfo { ai_rules = has_rules, ai_strs = strs, ai_discs = discs }
1163 cci | has_rules || disc > 0 = ArgCtxt has_rules disc -- Be keener here
1164 | otherwise = BoringCtxt -- Nothing interesting
1166 rebuildCall env fun _ cont
1167 = rebuild env fun cont
1172 This part of the simplifier may break the no-shadowing invariant
1174 f (...(\a -> e)...) (case y of (a,b) -> e')
1175 where f is strict in its second arg
1176 If we simplify the innermost one first we get (...(\a -> e)...)
1177 Simplifying the second arg makes us float the case out, so we end up with
1178 case y of (a,b) -> f (...(\a -> e)...) e'
1179 So the output does not have the no-shadowing invariant. However, there is
1180 no danger of getting name-capture, because when the first arg was simplified
1181 we used an in-scope set that at least mentioned all the variables free in its
1182 static environment, and that is enough.
1184 We can't just do innermost first, or we'd end up with a dual problem:
1185 case x of (a,b) -> f e (...(\a -> e')...)
1187 I spent hours trying to recover the no-shadowing invariant, but I just could
1188 not think of an elegant way to do it. The simplifier is already knee-deep in
1189 continuations. We have to keep the right in-scope set around; AND we have
1190 to get the effect that finding (error "foo") in a strict arg position will
1191 discard the entire application and replace it with (error "foo"). Getting
1192 all this at once is TOO HARD!
1194 %************************************************************************
1196 Rebuilding a cse expression
1198 %************************************************************************
1200 Note [Case elimination]
1201 ~~~~~~~~~~~~~~~~~~~~~~~
1202 The case-elimination transformation discards redundant case expressions.
1203 Start with a simple situation:
1205 case x# of ===> e[x#/y#]
1208 (when x#, y# are of primitive type, of course). We can't (in general)
1209 do this for algebraic cases, because we might turn bottom into
1212 The code in SimplUtils.prepareAlts has the effect of generalise this
1213 idea to look for a case where we're scrutinising a variable, and we
1214 know that only the default case can match. For example:
1218 DEFAULT -> ...(case x of
1222 Here the inner case is first trimmed to have only one alternative, the
1223 DEFAULT, after which it's an instance of the previous case. This
1224 really only shows up in eliminating error-checking code.
1226 We also make sure that we deal with this very common case:
1231 Here we are using the case as a strict let; if x is used only once
1232 then we want to inline it. We have to be careful that this doesn't
1233 make the program terminate when it would have diverged before, so we
1235 - e is already evaluated (it may so if e is a variable)
1236 - x is used strictly, or
1238 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1240 case e of ===> case e of DEFAULT -> r
1244 Now again the case may be elminated by the CaseElim transformation.
1247 Further notes about case elimination
1248 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1249 Consider: test :: Integer -> IO ()
1252 Turns out that this compiles to:
1255 eta1 :: State# RealWorld ->
1256 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1258 (PrelNum.jtos eta ($w[] @ Char))
1260 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1262 Notice the strange '<' which has no effect at all. This is a funny one.
1263 It started like this:
1265 f x y = if x < 0 then jtos x
1266 else if y==0 then "" else jtos x
1268 At a particular call site we have (f v 1). So we inline to get
1270 if v < 0 then jtos x
1271 else if 1==0 then "" else jtos x
1273 Now simplify the 1==0 conditional:
1275 if v<0 then jtos v else jtos v
1277 Now common-up the two branches of the case:
1279 case (v<0) of DEFAULT -> jtos v
1281 Why don't we drop the case? Because it's strict in v. It's technically
1282 wrong to drop even unnecessary evaluations, and in practice they
1283 may be a result of 'seq' so we *definitely* don't want to drop those.
1284 I don't really know how to improve this situation.
1287 ---------------------------------------------------------
1288 -- Eliminate the case if possible
1290 rebuildCase :: SimplEnv
1291 -> OutExpr -- Scrutinee
1292 -> InId -- Case binder
1293 -> [InAlt] -- Alternatives (inceasing order)
1295 -> SimplM (SimplEnv, OutExpr)
1297 --------------------------------------------------
1298 -- 1. Eliminate the case if there's a known constructor
1299 --------------------------------------------------
1301 rebuildCase env scrut case_bndr alts cont
1302 | Just (con,args) <- exprIsConApp_maybe scrut
1303 -- Works when the scrutinee is a variable with a known unfolding
1304 -- as well as when it's an explicit constructor application
1305 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1307 | Lit lit <- scrut -- No need for same treatment as constructors
1308 -- because literals are inlined more vigorously
1309 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1312 --------------------------------------------------
1313 -- 2. Eliminate the case if scrutinee is evaluated
1314 --------------------------------------------------
1316 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
1317 -- See if we can get rid of the case altogether
1318 -- See Note [Case eliminiation]
1319 -- mkCase made sure that if all the alternatives are equal,
1320 -- then there is now only one (DEFAULT) rhs
1321 | all isDeadBinder bndrs -- bndrs are [InId]
1323 -- Check that the scrutinee can be let-bound instead of case-bound
1324 , exprOkForSpeculation scrut
1325 -- OK not to evaluate it
1326 -- This includes things like (==# a# b#)::Bool
1327 -- so that we simplify
1328 -- case ==# a# b# of { True -> x; False -> x }
1331 -- This particular example shows up in default methods for
1332 -- comparision operations (e.g. in (>=) for Int.Int32)
1333 || exprIsHNF scrut -- It's already evaluated
1334 || var_demanded_later scrut -- It'll be demanded later
1336 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1337 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1338 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1339 -- its argument: case x of { y -> dataToTag# y }
1340 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1341 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1343 -- Also we don't want to discard 'seq's
1344 = do { tick (CaseElim case_bndr)
1345 ; env' <- simplNonRecX env case_bndr scrut
1346 ; simplExprF env' rhs cont }
1348 -- The case binder is going to be evaluated later,
1349 -- and the scrutinee is a simple variable
1350 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1351 && not (isTickBoxOp v)
1352 -- ugly hack; covering this case is what
1353 -- exprOkForSpeculation was intended for.
1354 var_demanded_later _ = False
1357 --------------------------------------------------
1358 -- 3. Catch-all case
1359 --------------------------------------------------
1361 rebuildCase env scrut case_bndr alts cont
1362 = do { -- Prepare the continuation;
1363 -- The new subst_env is in place
1364 (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1366 -- Simplify the alternatives
1367 ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
1369 -- Check for empty alternatives
1370 ; if null alts' then
1371 -- This isn't strictly an error, although it is unusual.
1372 -- It's possible that the simplifer might "see" that
1373 -- an inner case has no accessible alternatives before
1374 -- it "sees" that the entire branch of an outer case is
1375 -- inaccessible. So we simply put an error case here instead.
1376 pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $
1377 let res_ty' = contResultType env' (substTy env' (coreAltsType alts)) dup_cont
1378 lit = mkStringLit "Impossible alternative"
1379 in return (env', mkApps (Var rUNTIME_ERROR_ID) [Type res_ty', lit])
1382 { case_expr <- mkCase scrut' case_bndr' alts'
1384 -- Notice that rebuild gets the in-scope set from env, not alt_env
1385 -- The case binder *not* scope over the whole returned case-expression
1386 ; rebuild env' case_expr nodup_cont } }
1389 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1390 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1391 way, there's a chance that v will now only be used once, and hence
1394 Historical note: we use to do the "case binder swap" in the Simplifier
1395 so there were additional complications if the scrutinee was a variable.
1396 Now the binder-swap stuff is done in the occurrence analyer; see
1397 OccurAnal Note [Binder swap].
1401 If the case binder is not dead, then neither are the pattern bound
1403 case <any> of x { (a,b) ->
1404 case x of { (p,q) -> p } }
1405 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1406 The point is that we bring into the envt a binding
1408 after the outer case, and that makes (a,b) alive. At least we do unless
1409 the case binder is guaranteed dead.
1411 Note [Improving seq]
1414 type family F :: * -> *
1415 type instance F Int = Int
1417 ... case e of x { DEFAULT -> rhs } ...
1419 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1421 case e `cast` co of x'::Int
1422 I# x# -> let x = x' `cast` sym co
1425 so that 'rhs' can take advantage of the form of x'. Notice that Note
1426 [Case of cast] may then apply to the result.
1428 This showed up in Roman's experiments. Example:
1429 foo :: F Int -> Int -> Int
1430 foo t n = t `seq` bar n
1433 bar n = bar (n - case t of TI i -> i)
1434 Here we'd like to avoid repeated evaluating t inside the loop, by
1435 taking advantage of the `seq`.
1437 At one point I did transformation in LiberateCase, but it's more robust here.
1438 (Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
1439 LiberateCase gets to see it.)
1442 Historical note [no-case-of-case]
1443 ~~~~~~~~~~~~~~~~~~~~~~
1444 We *used* to suppress the binder-swap in case expressoins when
1445 -fno-case-of-case is on. Old remarks:
1446 "This happens in the first simplifier pass,
1447 and enhances full laziness. Here's the bad case:
1448 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1449 If we eliminate the inner case, we trap it inside the I# v -> arm,
1450 which might prevent some full laziness happening. I've seen this
1451 in action in spectral/cichelli/Prog.hs:
1452 [(m,n) | m <- [1..max], n <- [1..max]]
1453 Hence the check for NoCaseOfCase."
1454 However, now the full-laziness pass itself reverses the binder-swap, so this
1455 check is no longer necessary.
1457 Historical note [Suppressing the case binder-swap]
1458 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1459 There is another situation when it might make sense to suppress the
1460 case-expression binde-swap. If we have
1462 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1463 ...other cases .... }
1465 We'll perform the binder-swap for the outer case, giving
1467 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1468 ...other cases .... }
1470 But there is no point in doing it for the inner case, because w1 can't
1471 be inlined anyway. Furthermore, doing the case-swapping involves
1472 zapping w2's occurrence info (see paragraphs that follow), and that
1473 forces us to bind w2 when doing case merging. So we get
1475 case x of w1 { A -> let w2 = w1 in e1
1476 B -> let w2 = w1 in e2
1477 ...other cases .... }
1479 This is plain silly in the common case where w2 is dead.
1481 Even so, I can't see a good way to implement this idea. I tried
1482 not doing the binder-swap if the scrutinee was already evaluated
1483 but that failed big-time:
1487 case v of w { MkT x ->
1488 case x of x1 { I# y1 ->
1489 case x of x2 { I# y2 -> ...
1491 Notice that because MkT is strict, x is marked "evaluated". But to
1492 eliminate the last case, we must either make sure that x (as well as
1493 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1494 the binder-swap. So this whole note is a no-op.
1498 improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
1499 -> OutExpr -> InId -> OutId -> [InAlt]
1500 -> SimplM (SimplEnv, OutExpr, OutId)
1501 -- Note [Improving seq]
1502 improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1503 | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1504 = do { case_bndr2 <- newId (fsLit "nt") ty2
1505 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
1506 env2 = extendIdSubst env case_bndr rhs
1507 ; return (env2, scrut `Cast` co, case_bndr2) }
1509 improveSeq _ env scrut _ case_bndr1 _
1510 = return (env, scrut, case_bndr1)
1513 improve_case_bndr env scrut case_bndr
1514 -- See Note [no-case-of-case]
1515 -- | switchIsOn (getSwitchChecker env) NoCaseOfCase
1516 -- = (env, case_bndr)
1518 | otherwise -- Failed try; see Note [Suppressing the case binder-swap]
1519 -- not (isEvaldUnfolding (idUnfolding v))
1521 Var v -> (modifyInScope env1 v case_bndr', case_bndr')
1522 -- Note about using modifyInScope for v here
1523 -- We could extend the substitution instead, but it would be
1524 -- a hack because then the substitution wouldn't be idempotent
1525 -- any more (v is an OutId). And this does just as well.
1527 Cast (Var v) co -> (addBinderUnfolding env1 v rhs, case_bndr')
1529 rhs = Cast (Var case_bndr') (mkSymCoercion co)
1531 _ -> (env, case_bndr)
1533 case_bndr' = zapIdOccInfo case_bndr
1534 env1 = modifyInScope env case_bndr case_bndr'
1539 simplAlts does two things:
1541 1. Eliminate alternatives that cannot match, including the
1542 DEFAULT alternative.
1544 2. If the DEFAULT alternative can match only one possible constructor,
1545 then make that constructor explicit.
1547 case e of x { DEFAULT -> rhs }
1549 case e of x { (a,b) -> rhs }
1550 where the type is a single constructor type. This gives better code
1551 when rhs also scrutinises x or e.
1553 Here "cannot match" includes knowledge from GADTs
1555 It's a good idea do do this stuff before simplifying the alternatives, to
1556 avoid simplifying alternatives we know can't happen, and to come up with
1557 the list of constructors that are handled, to put into the IdInfo of the
1558 case binder, for use when simplifying the alternatives.
1560 Eliminating the default alternative in (1) isn't so obvious, but it can
1563 data Colour = Red | Green | Blue
1572 DEFAULT -> [ case y of ... ]
1574 If we inline h into f, the default case of the inlined h can't happen.
1575 If we don't notice this, we may end up filtering out *all* the cases
1576 of the inner case y, which give us nowhere to go!
1580 simplAlts :: SimplEnv
1582 -> InId -- Case binder
1583 -> [InAlt] -- Non-empty
1585 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1586 -- Like simplExpr, this just returns the simplified alternatives;
1587 -- it not return an environment
1589 simplAlts env scrut case_bndr alts cont'
1590 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1591 do { let env0 = zapFloats env
1593 ; (env1, case_bndr1) <- simplBinder env0 case_bndr
1595 ; fam_envs <- getFamEnvs
1596 ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
1597 case_bndr case_bndr1 alts
1599 ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env' scrut' case_bndr' alts
1601 ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
1602 ; return (scrut', case_bndr', alts') }
1604 ------------------------------------
1605 simplAlt :: SimplEnv
1606 -> [AltCon] -- These constructors can't be present when
1607 -- matching the DEFAULT alternative
1608 -> OutId -- The case binder
1613 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1614 = ASSERT( null bndrs )
1615 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1616 -- Record the constructors that the case-binder *can't* be.
1617 ; rhs' <- simplExprC env' rhs cont'
1618 ; return (DEFAULT, [], rhs') }
1620 simplAlt env _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
1621 = ASSERT( null bndrs )
1622 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1623 ; rhs' <- simplExprC env' rhs cont'
1624 ; return (LitAlt lit, [], rhs') }
1626 simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
1627 = do { -- Deal with the pattern-bound variables
1628 -- Mark the ones that are in ! positions in the
1629 -- data constructor as certainly-evaluated.
1630 -- NB: simplLamBinders preserves this eval info
1631 let vs_with_evals = add_evals (dataConRepStrictness con)
1632 ; (env', vs') <- simplLamBndrs env vs_with_evals
1634 -- Bind the case-binder to (con args)
1635 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1636 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1637 env'' = addBinderUnfolding env' case_bndr'
1638 (mkConApp con con_args)
1640 ; rhs' <- simplExprC env'' rhs cont'
1641 ; return (DataAlt con, vs', rhs') }
1643 -- add_evals records the evaluated-ness of the bound variables of
1644 -- a case pattern. This is *important*. Consider
1645 -- data T = T !Int !Int
1647 -- case x of { T a b -> T (a+1) b }
1649 -- We really must record that b is already evaluated so that we don't
1650 -- go and re-evaluate it when constructing the result.
1651 -- See Note [Data-con worker strictness] in MkId.lhs
1656 go (v:vs') strs | isTyVar v = v : go vs' strs
1657 go (v:vs') (str:strs)
1658 | isMarkedStrict str = evald_v : go vs' strs
1659 | otherwise = zapped_v : go vs' strs
1661 zapped_v = zap_occ_info v
1662 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1663 go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
1665 -- See Note [zapOccInfo]
1666 -- zap_occ_info: if the case binder is alive, then we add the unfolding
1668 -- to the envt; so vs are now very much alive
1669 -- Note [Aug06] I can't see why this actually matters, but it's neater
1670 -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
1671 -- ==> case e of t { (a,b) -> ...(a)... }
1672 -- Look, Ma, a is alive now.
1673 zap_occ_info = zapCasePatIdOcc case_bndr'
1675 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1676 addBinderUnfolding env bndr rhs
1677 = modifyInScope env (bndr `setIdUnfolding` mkUnfolding False rhs)
1679 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1680 addBinderOtherCon env bndr cons
1681 = modifyInScope env (bndr `setIdUnfolding` mkOtherCon cons)
1683 zapCasePatIdOcc :: Id -> Id -> Id
1684 -- Consider case e of b { (a,b) -> ... }
1685 -- Then if we bind b to (a,b) in "...", and b is not dead,
1686 -- then we must zap the deadness info on a,b
1687 zapCasePatIdOcc case_bndr
1688 | isDeadBinder case_bndr = \ pat_id -> pat_id
1689 | otherwise = \ pat_id -> zapIdOccInfo pat_id
1693 %************************************************************************
1695 \subsection{Known constructor}
1697 %************************************************************************
1699 We are a bit careful with occurrence info. Here's an example
1701 (\x* -> case x of (a*, b) -> f a) (h v, e)
1703 where the * means "occurs once". This effectively becomes
1704 case (h v, e) of (a*, b) -> f a)
1706 let a* = h v; b = e in f a
1710 All this should happen in one sweep.
1713 knownCon :: SimplEnv -> OutExpr -> AltCon
1714 -> [OutExpr] -- Args *including* the universal args
1715 -> InId -> [InAlt] -> SimplCont
1716 -> SimplM (SimplEnv, OutExpr)
1718 knownCon env scrut con args bndr alts cont
1719 = do { tick (KnownBranch bndr)
1720 ; knownAlt env scrut args bndr (findAlt con alts) cont }
1722 knownAlt :: SimplEnv -> OutExpr -> [OutExpr]
1723 -> InId -> (AltCon, [CoreBndr], InExpr) -> SimplCont
1724 -> SimplM (SimplEnv, OutExpr)
1725 knownAlt env scrut _ bndr (DEFAULT, bs, rhs) cont
1727 do { env' <- simplNonRecX env bndr scrut
1728 -- This might give rise to a binding with non-atomic args
1729 -- like x = Node (f x) (g x)
1730 -- but simplNonRecX will atomic-ify it
1731 ; simplExprF env' rhs cont }
1733 knownAlt env scrut _ bndr (LitAlt _, bs, rhs) cont
1735 do { env' <- simplNonRecX env bndr scrut
1736 ; simplExprF env' rhs cont }
1738 knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
1739 = do { let n_drop_tys = length (dataConUnivTyVars dc)
1740 ; env' <- bind_args env bs (drop n_drop_tys the_args)
1742 -- It's useful to bind bndr to scrut, rather than to a fresh
1743 -- binding x = Con arg1 .. argn
1744 -- because very often the scrut is a variable, so we avoid
1745 -- creating, and then subsequently eliminating, a let-binding
1746 -- BUT, if scrut is a not a variable, we must be careful
1747 -- about duplicating the arg redexes; in that case, make
1748 -- a new con-app from the args
1749 bndr_rhs = case scrut of
1752 con_app = mkConApp dc (take n_drop_tys the_args ++ con_args)
1753 con_args = [substExpr env' (varToCoreExpr b) | b <- bs]
1754 -- args are aready OutExprs, but bs are InIds
1756 ; env'' <- simplNonRecX env' bndr bndr_rhs
1757 ; simplExprF env'' rhs cont }
1759 zap_occ = zapCasePatIdOcc bndr -- bndr is an InId
1762 bind_args env' [] _ = return env'
1764 bind_args env' (b:bs') (Type ty : args)
1765 = ASSERT( isTyVar b )
1766 bind_args (extendTvSubst env' b ty) bs' args
1768 bind_args env' (b:bs') (arg : args)
1770 do { let b' = zap_occ b
1771 -- Note that the binder might be "dead", because it doesn't
1772 -- occur in the RHS; and simplNonRecX may therefore discard
1773 -- it via postInlineUnconditionally.
1774 -- Nevertheless we must keep it if the case-binder is alive,
1775 -- because it may be used in the con_app. See Note [zapOccInfo]
1776 ; env'' <- simplNonRecX env' b' arg
1777 ; bind_args env'' bs' args }
1780 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
1781 text "scrut:" <+> ppr scrut
1785 %************************************************************************
1787 \subsection{Duplicating continuations}
1789 %************************************************************************
1792 prepareCaseCont :: SimplEnv
1793 -> [InAlt] -> SimplCont
1794 -> SimplM (SimplEnv, SimplCont,SimplCont)
1795 -- Return a duplicatable continuation, a non-duplicable part
1796 -- plus some extra bindings (that scope over the entire
1799 -- No need to make it duplicatable if there's only one alternative
1800 prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
1801 prepareCaseCont env _ cont = mkDupableCont env cont
1805 mkDupableCont :: SimplEnv -> SimplCont
1806 -> SimplM (SimplEnv, SimplCont, SimplCont)
1808 mkDupableCont env cont
1809 | contIsDupable cont
1810 = return (env, cont, mkBoringStop)
1812 mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1814 mkDupableCont env (CoerceIt ty cont)
1815 = do { (env', dup, nodup) <- mkDupableCont env cont
1816 ; return (env', CoerceIt ty dup, nodup) }
1818 mkDupableCont env cont@(StrictBind {})
1819 = return (env, mkBoringStop, cont)
1820 -- See Note [Duplicating strict continuations]
1822 mkDupableCont env cont@(StrictArg {})
1823 = return (env, mkBoringStop, cont)
1824 -- See Note [Duplicating strict continuations]
1826 mkDupableCont env (ApplyTo _ arg se cont)
1827 = -- e.g. [...hole...] (...arg...)
1829 -- let a = ...arg...
1830 -- in [...hole...] a
1831 do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1832 ; arg' <- simplExpr (se `setInScope` env') arg
1833 ; (env'', arg'') <- makeTrivial env' arg'
1834 ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
1835 ; return (env'', app_cont, nodup_cont) }
1837 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
1838 -- See Note [Single-alternative case]
1839 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1840 -- | not (isDeadBinder case_bndr)
1841 | all isDeadBinder bs -- InIds
1842 && not (isUnLiftedType (idType case_bndr))
1843 -- Note [Single-alternative-unlifted]
1844 = return (env, mkBoringStop, cont)
1846 mkDupableCont env (Select _ case_bndr alts se cont)
1847 = -- e.g. (case [...hole...] of { pi -> ei })
1849 -- let ji = \xij -> ei
1850 -- in case [...hole...] of { pi -> ji xij }
1851 do { tick (CaseOfCase case_bndr)
1852 ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1853 -- NB: call mkDupableCont here, *not* prepareCaseCont
1854 -- We must make a duplicable continuation, whereas prepareCaseCont
1855 -- doesn't when there is a single case branch
1857 ; let alt_env = se `setInScope` env'
1858 ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
1859 ; alts' <- mapM (simplAlt alt_env' [] case_bndr' dup_cont) alts
1860 -- Safe to say that there are no handled-cons for the DEFAULT case
1861 -- NB: simplBinder does not zap deadness occ-info, so
1862 -- a dead case_bndr' will still advertise its deadness
1863 -- This is really important because in
1864 -- case e of b { (# p,q #) -> ... }
1865 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1866 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1867 -- In the new alts we build, we have the new case binder, so it must retain
1869 -- NB: we don't use alt_env further; it has the substEnv for
1870 -- the alternatives, and we don't want that
1872 ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
1873 ; return (env'', -- Note [Duplicated env]
1874 Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
1878 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1879 -> SimplM (SimplEnv, [InAlt])
1880 -- Absorbs the continuation into the new alternatives
1882 mkDupableAlts env case_bndr' the_alts
1885 go env0 [] = return (env0, [])
1887 = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
1888 ; (env2, alts') <- go env1 alts
1889 ; return (env2, alt' : alts' ) }
1891 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
1892 -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
1893 mkDupableAlt env case_bndr1 (con, bndrs1, rhs1)
1894 | exprIsDupable rhs1 -- Note [Small alternative rhs]
1895 = return (env, (con, bndrs1, rhs1))
1897 = do { let abstract_over bndr
1898 | isTyVar bndr = True -- Abstract over all type variables just in case
1899 | otherwise = not (isDeadBinder bndr)
1900 -- The deadness info on the new Ids is preserved by simplBinders
1902 inst_tys1 = tyConAppArgs (idType case_bndr1)
1903 con_app dc = mkConApp dc (map Type inst_tys1 ++ varsToCoreExprs bndrs1)
1905 (rhs2, final_bndrs) -- See Note [Passing the case binder to join points]
1906 | isDeadBinder case_bndr1
1907 = (rhs1, filter abstract_over bndrs1)
1908 | opt_PassCaseBndrToJoinPoints, not (null bndrs1)
1909 = (rhs1, (case_bndr1 : filter abstract_over bndrs1))
1912 DataAlt dc -> (Let (NonRec case_bndr1 (con_app dc)) rhs1, bndrs1)
1913 LitAlt lit -> ASSERT( null bndrs1 ) (Let (NonRec case_bndr1 (Lit lit)) rhs1, [])
1914 DEFAULT -> ASSERT( null bndrs1 ) (rhs1, [case_bndr1])
1916 ; (final_bndrs1, final_args) -- Note [Join point abstraction]
1917 <- if (any isId final_bndrs)
1918 then return (final_bndrs, varsToCoreExprs final_bndrs)
1919 else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
1920 ; return (rw_id : final_bndrs,
1921 Var realWorldPrimId : varsToCoreExprs final_bndrs) }
1923 ; let rhs_ty1 = exprType rhs1
1924 ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs1 rhs_ty1)
1925 -- Note [Funky mkPiTypes]
1927 ; let -- We make the lambdas into one-shot-lambdas. The
1928 -- join point is sure to be applied at most once, and doing so
1929 -- prevents the body of the join point being floated out by
1930 -- the full laziness pass
1931 really_final_bndrs = map one_shot final_bndrs1
1932 one_shot v | isId v = setOneShotLambda v
1934 join_rhs = mkLams really_final_bndrs rhs2
1935 join_call = mkApps (Var join_bndr) final_args
1937 ; env1 <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
1938 ; return (env1, (con, bndrs1, join_call)) }
1939 -- See Note [Duplicated env]
1942 Note [Passing the case binder to join points]
1943 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1945 case e of cb { C1 -> r1[cb]; C2 x y z -> r2[cb,x] }
1946 and we want to make join points for the two alternatives,
1947 which mention the case binder 'cb'. Should we pass 'cb' to
1948 the join point, or reconstruct it? Here are the two alternatives
1949 for the C2 alternative:
1951 Plan A(pass cb): j2 cb x = r2[cb,x]
1953 Plan B(reconstruct cb): j2 x y z = let cb = C2 x y z in r2[cb,x]
1955 The advantge of Plan B is that we can "see" the definition of cb
1956 in r2, and that may be important when we inline stuff in r2. The
1957 disadvantage is that if this optimisation doesn't happen, we end up
1958 re-allocating C2, when it already exists. This does happen occasionally;
1959 an example is the function nofib/spectral/cichelli/Auxil.$whinsert.
1961 Plan B is always better if the constructor is nullary.
1963 In both cases we don't have liveness info for cb on a branch-by-branch
1964 basis, and it's possible that 'cb' is used in some branches but not
1965 others. Well, the absence analyser will find that out later, so it's
1968 Sadly, at the time of writing, neither choice seems an unequivocal
1969 win. Here are nofib results, for adding -fpass-case-bndr-to-join-points
1970 (all others are zero effect):
1972 Program Size Allocs Runtime Elapsed
1973 --------------------------------------------------------------------------------
1974 cichelli +0.0% -4.4% 0.13 0.13
1975 pic +0.0% -0.7% 0.01 0.04
1976 transform -0.0% +2.8% -0.4% -9.1%
1977 wave4main +0.0% +10.5% +3.1% +3.4%
1978 --------------------------------------------------------------------------------
1979 Min -0.0% -4.4% -7.0% -31.9%
1980 Max +0.1% +10.5% +3.1% +15.0%
1981 Geometric Mean +0.0% +0.1% -1.7% -6.1%
1984 Note [Duplicated env]
1985 ~~~~~~~~~~~~~~~~~~~~~
1986 Some of the alternatives are simplified, but have not been turned into a join point
1987 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
1988 bind the join point, because it might to do PostInlineUnconditionally, and
1989 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
1990 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
1991 at worst delays the join-point inlining.
1993 Note [Small alterantive rhs]
1994 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1995 It is worth checking for a small RHS because otherwise we
1996 get extra let bindings that may cause an extra iteration of the simplifier to
1997 inline back in place. Quite often the rhs is just a variable or constructor.
1998 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1999 iterations because the version with the let bindings looked big, and so wasn't
2000 inlined, but after the join points had been inlined it looked smaller, and so
2003 NB: we have to check the size of rhs', not rhs.
2004 Duplicating a small InAlt might invalidate occurrence information
2005 However, if it *is* dupable, we return the *un* simplified alternative,
2006 because otherwise we'd need to pair it up with an empty subst-env....
2007 but we only have one env shared between all the alts.
2008 (Remember we must zap the subst-env before re-simplifying something).
2009 Rather than do this we simply agree to re-simplify the original (small) thing later.
2011 Note [Funky mkPiTypes]
2012 ~~~~~~~~~~~~~~~~~~~~~~
2013 Notice the funky mkPiTypes. If the contructor has existentials
2014 it's possible that the join point will be abstracted over
2015 type varaibles as well as term variables.
2016 Example: Suppose we have
2017 data T = forall t. C [t]
2019 case (case e of ...) of
2021 We get the join point
2022 let j :: forall t. [t] -> ...
2023 j = /\t \xs::[t] -> rhs
2025 case (case e of ...) of
2026 C t xs::[t] -> j t xs
2028 Note [Join point abstaction]
2029 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2030 If we try to lift a primitive-typed something out
2031 for let-binding-purposes, we will *caseify* it (!),
2032 with potentially-disastrous strictness results. So
2033 instead we turn it into a function: \v -> e
2034 where v::State# RealWorld#. The value passed to this function
2035 is realworld#, which generates (almost) no code.
2037 There's a slight infelicity here: we pass the overall
2038 case_bndr to all the join points if it's used in *any* RHS,
2039 because we don't know its usage in each RHS separately
2041 We used to say "&& isUnLiftedType rhs_ty'" here, but now
2042 we make the join point into a function whenever used_bndrs'
2043 is empty. This makes the join-point more CPR friendly.
2044 Consider: let j = if .. then I# 3 else I# 4
2045 in case .. of { A -> j; B -> j; C -> ... }
2047 Now CPR doesn't w/w j because it's a thunk, so
2048 that means that the enclosing function can't w/w either,
2049 which is a lose. Here's the example that happened in practice:
2050 kgmod :: Int -> Int -> Int
2051 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
2055 I have seen a case alternative like this:
2057 It's a bit silly to add the realWorld dummy arg in this case, making
2060 (the \v alone is enough to make CPR happy) but I think it's rare
2062 Note [Duplicating strict continuations]
2063 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2064 Do *not* duplicate StrictBind and StritArg continuations. We gain
2065 nothing by propagating them into the expressions, and we do lose a
2066 lot. Here's an example:
2067 && (case x of { T -> F; F -> T }) E
2068 Now, && is strict so we end up simplifying the case with
2069 an ArgOf continuation. If we let-bind it, we get
2071 let $j = \v -> && v E
2072 in simplExpr (case x of { T -> F; F -> T })
2074 And after simplifying more we get
2076 let $j = \v -> && v E
2077 in case x of { T -> $j F; F -> $j T }
2078 Which is a Very Bad Thing
2080 The desire not to duplicate is the entire reason that
2081 mkDupableCont returns a pair of continuations.
2083 The original plan had:
2084 e.g. (...strict-fn...) [...hole...]
2086 let $j = \a -> ...strict-fn...
2089 Note [Single-alternative cases]
2090 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2091 This case is just like the ArgOf case. Here's an example:
2095 case (case x of I# x' ->
2097 True -> I# (negate# x')
2098 False -> I# x') of y {
2100 Because the (case x) has only one alternative, we'll transform to
2102 case (case x' <# 0# of
2103 True -> I# (negate# x')
2104 False -> I# x') of y {
2106 But now we do *NOT* want to make a join point etc, giving
2108 let $j = \y -> MkT y
2110 True -> $j (I# (negate# x'))
2112 In this case the $j will inline again, but suppose there was a big
2113 strict computation enclosing the orginal call to MkT. Then, it won't
2114 "see" the MkT any more, because it's big and won't get duplicated.
2115 And, what is worse, nothing was gained by the case-of-case transform.
2117 When should use this case of mkDupableCont?
2118 However, matching on *any* single-alternative case is a *disaster*;
2119 e.g. case (case ....) of (a,b) -> (# a,b #)
2120 We must push the outer case into the inner one!
2123 * Match [(DEFAULT,_,_)], but in the common case of Int,
2124 the alternative-filling-in code turned the outer case into
2125 case (...) of y { I# _ -> MkT y }
2127 * Match on single alternative plus (not (isDeadBinder case_bndr))
2128 Rationale: pushing the case inwards won't eliminate the construction.
2129 But there's a risk of
2130 case (...) of y { (a,b) -> let z=(a,b) in ... }
2131 Now y looks dead, but it'll come alive again. Still, this
2132 seems like the best option at the moment.
2134 * Match on single alternative plus (all (isDeadBinder bndrs))
2135 Rationale: this is essentially seq.
2137 * Match when the rhs is *not* duplicable, and hence would lead to a
2138 join point. This catches the disaster-case above. We can test
2139 the *un-simplified* rhs, which is fine. It might get bigger or
2140 smaller after simplification; if it gets smaller, this case might
2141 fire next time round. NB also that we must test contIsDupable
2142 case_cont *btoo, because case_cont might be big!
2144 HOWEVER: I found that this version doesn't work well, because
2145 we can get let x = case (...) of { small } in ...case x...
2146 When x is inlined into its full context, we find that it was a bad
2147 idea to have pushed the outer case inside the (...) case.
2149 Note [Single-alternative-unlifted]
2150 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2151 Here's another single-alternative where we really want to do case-of-case:
2159 case y_s6X of tpl_s7m {
2160 M1.Mk1 ipv_s70 -> ipv_s70;
2161 M1.Mk2 ipv_s72 -> ipv_s72;
2167 case x_s74 of tpl_s7n {
2168 M1.Mk1 ipv_s77 -> ipv_s77;
2169 M1.Mk2 ipv_s79 -> ipv_s79;
2173 { __DEFAULT -> ==# [wild1_s7b wild_s7c];
2177 So the outer case is doing *nothing at all*, other than serving as a
2178 join-point. In this case we really want to do case-of-case and decide
2179 whether to use a real join point or just duplicate the continuation.
2181 Hence: check whether the case binder's type is unlifted, because then
2182 the outer case is *not* a seq.