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 )
21 import FamInstEnv ( topNormaliseType )
22 import DataCon ( dataConRepStrictness, dataConUnivTyVars )
24 import NewDemand ( isStrictDmd )
25 import PprCore ( pprParendExpr, pprCoreExpr )
26 import CoreUnfold ( mkUnfolding, callSiteInline, CallCtxt(..) )
28 import Rules ( lookupRule )
29 import BasicTypes ( isMarkedStrict )
30 import CostCentre ( currentCCS )
31 import TysPrim ( realWorldStatePrimTy )
32 import PrelInfo ( realWorldPrimId )
33 import BasicTypes ( TopLevelFlag(..), isTopLevel,
34 RecFlag(..), isNonRuleLoopBreaker )
35 import Maybes ( orElse )
36 import Data.List ( mapAccumL )
42 The guts of the simplifier is in this module, but the driver loop for
43 the simplifier is in SimplCore.lhs.
46 -----------------------------------------
47 *** IMPORTANT NOTE ***
48 -----------------------------------------
49 The simplifier used to guarantee that the output had no shadowing, but
50 it does not do so any more. (Actually, it never did!) The reason is
51 documented with simplifyArgs.
54 -----------------------------------------
55 *** IMPORTANT NOTE ***
56 -----------------------------------------
57 Many parts of the simplifier return a bunch of "floats" as well as an
58 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
60 All "floats" are let-binds, not case-binds, but some non-rec lets may
61 be unlifted (with RHS ok-for-speculation).
65 -----------------------------------------
66 ORGANISATION OF FUNCTIONS
67 -----------------------------------------
69 - simplify all top-level binders
70 - for NonRec, call simplRecOrTopPair
71 - for Rec, call simplRecBind
74 ------------------------------
75 simplExpr (applied lambda) ==> simplNonRecBind
76 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
77 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
79 ------------------------------
80 simplRecBind [binders already simplfied]
81 - use simplRecOrTopPair on each pair in turn
83 simplRecOrTopPair [binder already simplified]
84 Used for: recursive bindings (top level and nested)
85 top-level non-recursive bindings
87 - check for PreInlineUnconditionally
91 Used for: non-top-level non-recursive bindings
92 beta reductions (which amount to the same thing)
93 Because it can deal with strict arts, it takes a
94 "thing-inside" and returns an expression
96 - check for PreInlineUnconditionally
97 - simplify binder, including its IdInfo
106 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
107 Used for: binding case-binder and constr args in a known-constructor case
108 - check for PreInLineUnconditionally
112 ------------------------------
113 simplLazyBind: [binder already simplified, RHS not]
114 Used for: recursive bindings (top level and nested)
115 top-level non-recursive bindings
116 non-top-level, but *lazy* non-recursive bindings
117 [must not be strict or unboxed]
118 Returns floats + an augmented environment, not an expression
119 - substituteIdInfo and add result to in-scope
120 [so that rules are available in rec rhs]
123 - float if exposes constructor or PAP
127 completeNonRecX: [binder and rhs both simplified]
128 - if the the thing needs case binding (unlifted and not ok-for-spec)
134 completeBind: [given a simplified RHS]
135 [used for both rec and non-rec bindings, top level and not]
136 - try PostInlineUnconditionally
137 - add unfolding [this is the only place we add an unfolding]
142 Right hand sides and arguments
143 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
144 In many ways we want to treat
145 (a) the right hand side of a let(rec), and
146 (b) a function argument
147 in the same way. But not always! In particular, we would
148 like to leave these arguments exactly as they are, so they
149 will match a RULE more easily.
154 It's harder to make the rule match if we ANF-ise the constructor,
155 or eta-expand the PAP:
157 f (let { a = g x; b = h x } in (a,b))
160 On the other hand if we see the let-defns
165 then we *do* want to ANF-ise and eta-expand, so that p and q
166 can be safely inlined.
168 Even floating lets out is a bit dubious. For let RHS's we float lets
169 out if that exposes a value, so that the value can be inlined more vigorously.
172 r = let x = e in (x,x)
174 Here, if we float the let out we'll expose a nice constructor. We did experiments
175 that showed this to be a generally good thing. But it was a bad thing to float
176 lets out unconditionally, because that meant they got allocated more often.
178 For function arguments, there's less reason to expose a constructor (it won't
179 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
180 So for the moment we don't float lets out of function arguments either.
185 For eta expansion, we want to catch things like
187 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
189 If the \x was on the RHS of a let, we'd eta expand to bring the two
190 lambdas together. And in general that's a good thing to do. Perhaps
191 we should eta expand wherever we find a (value) lambda? Then the eta
192 expansion at a let RHS can concentrate solely on the PAP case.
195 %************************************************************************
197 \subsection{Bindings}
199 %************************************************************************
202 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
204 simplTopBinds env0 binds0
205 = do { -- Put all the top-level binders into scope at the start
206 -- so that if a transformation rule has unexpectedly brought
207 -- anything into scope, then we don't get a complaint about that.
208 -- It's rather as if the top-level binders were imported.
209 ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
210 ; dflags <- getDOptsSmpl
211 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
212 dopt Opt_D_dump_rule_firings dflags
213 ; env2 <- simpl_binds dump_flag env1 binds0
214 ; freeTick SimplifierDone
215 ; return (getFloats env2) }
217 -- We need to track the zapped top-level binders, because
218 -- they should have their fragile IdInfo zapped (notably occurrence info)
219 -- That's why we run down binds and bndrs' simultaneously.
221 -- The dump-flag emits a trace for each top-level binding, which
222 -- helps to locate the tracing for inlining and rule firing
223 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
224 simpl_binds _ env [] = return env
225 simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
227 ; simpl_binds dump env' binds }
229 trace_bind True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
230 trace_bind False _ = \x -> x
232 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
233 simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel b b' r
235 (env', b') = addBndrRules env b (lookupRecBndr env b)
239 %************************************************************************
241 \subsection{Lazy bindings}
243 %************************************************************************
245 simplRecBind is used for
246 * recursive bindings only
249 simplRecBind :: SimplEnv -> TopLevelFlag
252 simplRecBind env0 top_lvl pairs0
253 = do { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
254 ; env1 <- go (zapFloats env_with_info) triples
255 ; return (env0 `addRecFloats` env1) }
256 -- addFloats adds the floats from env1,
257 -- _and_ updates env0 with the in-scope set from env1
259 add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
260 -- Add the (substituted) rules to the binder
261 add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
263 (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
265 go env [] = return env
267 go env ((old_bndr, new_bndr, rhs) : pairs)
268 = do { env' <- simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
272 simplOrTopPair is used for
273 * recursive bindings (whether top level or not)
274 * top-level non-recursive bindings
276 It assumes the binder has already been simplified, but not its IdInfo.
279 simplRecOrTopPair :: SimplEnv
281 -> InId -> OutBndr -> InExpr -- Binder and rhs
282 -> SimplM SimplEnv -- Returns an env that includes the binding
284 simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
285 | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
286 = do { tick (PreInlineUnconditionally old_bndr)
287 ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
290 = simplLazyBind env top_lvl Recursive old_bndr new_bndr rhs env
291 -- May not actually be recursive, but it doesn't matter
295 simplLazyBind is used for
296 * [simplRecOrTopPair] recursive bindings (whether top level or not)
297 * [simplRecOrTopPair] top-level non-recursive bindings
298 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
301 1. It assumes that the binder is *already* simplified,
302 and is in scope, and its IdInfo too, except unfolding
304 2. It assumes that the binder type is lifted.
306 3. It does not check for pre-inline-unconditionallly;
307 that should have been done already.
310 simplLazyBind :: SimplEnv
311 -> TopLevelFlag -> RecFlag
312 -> InId -> OutId -- Binder, both pre-and post simpl
313 -- The OutId has IdInfo, except arity, unfolding
314 -> InExpr -> SimplEnv -- The RHS and its environment
317 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
318 = do { let rhs_env = rhs_se `setInScope` env
319 (tvs, body) = case collectTyBinders rhs of
320 (tvs, body) | not_lam body -> (tvs,body)
321 | otherwise -> ([], rhs)
322 not_lam (Lam _ _) = False
324 -- Do not do the "abstract tyyvar" thing if there's
325 -- a lambda inside, becuase it defeats eta-reduction
326 -- f = /\a. \x. g a x
329 ; (body_env, tvs') <- simplBinders rhs_env tvs
330 -- See Note [Floating and type abstraction] in SimplUtils
333 ; (body_env1, body1) <- simplExprF body_env body mkBoringStop
335 -- ANF-ise a constructor or PAP rhs
336 ; (body_env2, body2) <- prepareRhs body_env1 body1
339 <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
340 then -- No floating, just wrap up!
341 do { rhs' <- mkLam tvs' (wrapFloats body_env2 body2)
342 ; return (env, rhs') }
344 else if null tvs then -- Simple floating
345 do { tick LetFloatFromLet
346 ; return (addFloats env body_env2, body2) }
348 else -- Do type-abstraction first
349 do { tick LetFloatFromLet
350 ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
351 ; rhs' <- mkLam tvs' body3
352 ; let env' = foldl (addPolyBind top_lvl) env poly_binds
353 ; return (env', rhs') }
355 ; completeBind env' top_lvl bndr bndr1 rhs' }
358 A specialised variant of simplNonRec used when the RHS is already simplified,
359 notably in knownCon. It uses case-binding where necessary.
362 simplNonRecX :: SimplEnv
363 -> InId -- Old binder
364 -> OutExpr -- Simplified RHS
367 simplNonRecX env bndr new_rhs
368 = do { (env', bndr') <- simplBinder env bndr
369 ; completeNonRecX env' (isStrictId bndr) bndr bndr' new_rhs }
371 completeNonRecX :: SimplEnv
373 -> InId -- Old binder
374 -> OutId -- New binder
375 -> OutExpr -- Simplified RHS
378 completeNonRecX env is_strict old_bndr new_bndr new_rhs
379 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
381 if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
382 then do { tick LetFloatFromLet
383 ; return (addFloats env env1, rhs1) } -- Add the floats to the main env
384 else return (env, wrapFloats env1 rhs1) -- Wrap the floats around the RHS
385 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
388 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
389 Doing so risks exponential behaviour, because new_rhs has been simplified once already
390 In the cases described by the folowing commment, postInlineUnconditionally will
391 catch many of the relevant cases.
392 -- This happens; for example, the case_bndr during case of
393 -- known constructor: case (a,b) of x { (p,q) -> ... }
394 -- Here x isn't mentioned in the RHS, so we don't want to
395 -- create the (dead) let-binding let x = (a,b) in ...
397 -- Similarly, single occurrences can be inlined vigourously
398 -- e.g. case (f x, g y) of (a,b) -> ....
399 -- If a,b occur once we can avoid constructing the let binding for them.
401 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
402 -- Consider case I# (quotInt# x y) of
403 -- I# v -> let w = J# v in ...
404 -- If we gaily inline (quotInt# x y) for v, we end up building an
406 -- let w = J# (quotInt# x y) in ...
407 -- because quotInt# can fail.
409 | preInlineUnconditionally env NotTopLevel bndr new_rhs
410 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
413 ----------------------------------
414 prepareRhs takes a putative RHS, checks whether it's a PAP or
415 constructor application and, if so, converts it to ANF, so that the
416 resulting thing can be inlined more easily. Thus
423 We also want to deal well cases like this
424 v = (f e1 `cast` co) e2
425 Here we want to make e1,e2 trivial and get
426 x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
427 That's what the 'go' loop in prepareRhs does
430 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
431 -- Adds new floats to the env iff that allows us to return a good RHS
432 prepareRhs env (Cast rhs co) -- Note [Float coercions]
433 | (ty1, _ty2) <- coercionKind co -- Do *not* do this if rhs has an unlifted type
434 , not (isUnLiftedType ty1) -- see Note [Float coercions (unlifted)]
435 = do { (env', rhs') <- makeTrivial env rhs
436 ; return (env', Cast rhs' co) }
439 = do { (_is_val, env1, rhs1) <- go 0 env0 rhs0
440 ; return (env1, rhs1) }
442 go n_val_args env (Cast rhs co)
443 = do { (is_val, env', rhs') <- go n_val_args env rhs
444 ; return (is_val, env', Cast rhs' co) }
445 go n_val_args env (App fun (Type ty))
446 = do { (is_val, env', rhs') <- go n_val_args env fun
447 ; return (is_val, env', App rhs' (Type ty)) }
448 go n_val_args env (App fun arg)
449 = do { (is_val, env', fun') <- go (n_val_args+1) env fun
451 True -> do { (env'', arg') <- makeTrivial env' arg
452 ; return (True, env'', App fun' arg') }
453 False -> return (False, env, App fun arg) }
454 go n_val_args env (Var fun)
455 = return (is_val, env, Var fun)
457 is_val = n_val_args > 0 -- There is at least one arg
458 -- ...and the fun a constructor or PAP
459 && (isDataConWorkId fun || n_val_args < idArity fun)
461 = return (False, env, other)
465 Note [Float coercions]
466 ~~~~~~~~~~~~~~~~~~~~~~
467 When we find the binding
469 we'd like to transform it to
471 x = x `cast` co -- A trivial binding
472 There's a chance that e will be a constructor application or function, or something
473 like that, so moving the coerion to the usage site may well cancel the coersions
474 and lead to further optimisation. Example:
477 data instance T Int = T Int
479 foo :: Int -> Int -> Int
484 go n = case x of { T m -> go (n-m) }
485 -- This case should optimise
487 Note [Float coercions (unlifted)]
488 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
489 BUT don't do [Float coercions] if 'e' has an unlifted type.
492 foo :: Int = (error (# Int,Int #) "urk")
493 `cast` CoUnsafe (# Int,Int #) Int
495 If do the makeTrivial thing to the error call, we'll get
496 foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
497 But 'v' isn't in scope!
499 These strange casts can happen as a result of case-of-case
500 bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
505 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
506 -- Binds the expression to a variable, if it's not trivial, returning the variable
510 | otherwise -- See Note [Take care] below
511 = do { var <- newId (fsLit "a") (exprType expr)
512 ; env' <- completeNonRecX env False var var expr
513 ; return (env', substExpr env' (Var var)) }
517 %************************************************************************
519 \subsection{Completing a lazy binding}
521 %************************************************************************
524 * deals only with Ids, not TyVars
525 * takes an already-simplified binder and RHS
526 * is used for both recursive and non-recursive bindings
527 * is used for both top-level and non-top-level bindings
529 It does the following:
530 - tries discarding a dead binding
531 - tries PostInlineUnconditionally
532 - add unfolding [this is the only place we add an unfolding]
535 It does *not* attempt to do let-to-case. Why? Because it is used for
536 - top-level bindings (when let-to-case is impossible)
537 - many situations where the "rhs" is known to be a WHNF
538 (so let-to-case is inappropriate).
540 Nor does it do the atomic-argument thing
543 completeBind :: SimplEnv
544 -> TopLevelFlag -- Flag stuck into unfolding
545 -> InId -- Old binder
546 -> OutId -> OutExpr -- New binder and RHS
548 -- completeBind may choose to do its work
549 -- * by extending the substitution (e.g. let x = y in ...)
550 -- * or by adding to the floats in the envt
552 completeBind env top_lvl old_bndr new_bndr new_rhs
553 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
554 -- Inline and discard the binding
555 = do { tick (PostInlineUnconditionally old_bndr)
556 ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
557 return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
558 -- Use the substitution to make quite, quite sure that the
559 -- substitution will happen, since we are going to discard the binding
562 = return (addNonRecWithUnf env new_bndr new_rhs unfolding wkr)
564 unfolding | omit_unfolding = NoUnfolding
565 | otherwise = mkUnfolding (isTopLevel top_lvl) new_rhs
566 old_info = idInfo old_bndr
567 occ_info = occInfo old_info
568 wkr = substWorker env (workerInfo old_info)
569 omit_unfolding = isNonRuleLoopBreaker occ_info || not (activeInline env old_bndr)
572 addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplEnv
573 -- Add a new binding to the environment, complete with its unfolding
574 -- but *do not* do postInlineUnconditionally, because we have already
575 -- processed some of the scope of the binding
576 -- We still want the unfolding though. Consider
578 -- x = /\a. let y = ... in Just y
580 -- Then we float the y-binding out (via abstractFloats and addPolyBind)
581 -- but 'x' may well then be inlined in 'body' in which case we'd like the
582 -- opportunity to inline 'y' too.
584 addPolyBind top_lvl env (NonRec poly_id rhs)
585 = addNonRecWithUnf env poly_id rhs unfolding NoWorker
587 unfolding | not (activeInline env poly_id) = NoUnfolding
588 | otherwise = mkUnfolding (isTopLevel top_lvl) rhs
589 -- addNonRecWithInfo adds the new binding in the
590 -- proper way (ie complete with unfolding etc),
591 -- and extends the in-scope set
593 addPolyBind _ env bind@(Rec _) = extendFloats env bind
594 -- Hack: letrecs are more awkward, so we extend "by steam"
595 -- without adding unfoldings etc. At worst this leads to
596 -- more simplifier iterations
599 addNonRecWithUnf :: SimplEnv
600 -> OutId -> OutExpr -- New binder and RHS
601 -> Unfolding -> WorkerInfo -- and unfolding
603 -- Add suitable IdInfo to the Id, add the binding to the floats, and extend the in-scope set
604 addNonRecWithUnf env new_bndr rhs unfolding wkr
605 = final_id `seq` -- This seq forces the Id, and hence its IdInfo,
606 -- and hence any inner substitutions
607 addNonRec env final_id rhs
608 -- The addNonRec adds it to the in-scope set too
611 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity rhs
614 -- Add the unfolding *only* for non-loop-breakers
615 -- Making loop breakers not have an unfolding at all
616 -- means that we can avoid tests in exprIsConApp, for example.
617 -- This is important: if exprIsConApp says 'yes' for a recursive
618 -- thing, then we can get into an infinite loop
621 -- If the unfolding is a value, the demand info may
622 -- go pear-shaped, so we nuke it. Example:
624 -- case x of (p,q) -> h p q x
625 -- Here x is certainly demanded. But after we've nuked
626 -- the case, we'll get just
627 -- let x = (a,b) in h a b x
628 -- and now x is not demanded (I'm assuming h is lazy)
629 -- This really happens. Similarly
630 -- let f = \x -> e in ...f..f...
631 -- After inlining f at some of its call sites the original binding may
632 -- (for example) be no longer strictly demanded.
633 -- The solution here is a bit ad hoc...
634 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
637 final_info | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
638 | otherwise = info_w_unf
640 final_id = new_bndr `setIdInfo` final_info
645 %************************************************************************
647 \subsection[Simplify-simplExpr]{The main function: simplExpr}
649 %************************************************************************
651 The reason for this OutExprStuff stuff is that we want to float *after*
652 simplifying a RHS, not before. If we do so naively we get quadratic
653 behaviour as things float out.
655 To see why it's important to do it after, consider this (real) example:
669 a -- Can't inline a this round, cos it appears twice
673 Each of the ==> steps is a round of simplification. We'd save a
674 whole round if we float first. This can cascade. Consider
679 let f = let d1 = ..d.. in \y -> e
683 in \x -> ...(\y ->e)...
685 Only in this second round can the \y be applied, and it
686 might do the same again.
690 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
691 simplExpr env expr = simplExprC env expr mkBoringStop
693 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
694 -- Simplify an expression, given a continuation
695 simplExprC env expr cont
696 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
697 do { (env', expr') <- simplExprF (zapFloats env) expr cont
698 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
699 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
700 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
701 return (wrapFloats env' expr') }
703 --------------------------------------------------
704 simplExprF :: SimplEnv -> InExpr -> SimplCont
705 -> SimplM (SimplEnv, OutExpr)
707 simplExprF env e cont
708 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
709 simplExprF' env e cont
711 simplExprF' :: SimplEnv -> InExpr -> SimplCont
712 -> SimplM (SimplEnv, OutExpr)
713 simplExprF' env (Var v) cont = simplVar env v cont
714 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
715 simplExprF' env (Note n expr) cont = simplNote env n expr cont
716 simplExprF' env (Cast body co) cont = simplCast env body co cont
717 simplExprF' env (App fun arg) cont = simplExprF env fun $
718 ApplyTo NoDup arg env cont
720 simplExprF' env expr@(Lam _ _) cont
721 = simplLam env (map zap bndrs) body cont
722 -- The main issue here is under-saturated lambdas
723 -- (\x1. \x2. e) arg1
724 -- Here x1 might have "occurs-once" occ-info, because occ-info
725 -- is computed assuming that a group of lambdas is applied
726 -- all at once. If there are too few args, we must zap the
729 n_args = countArgs cont
730 n_params = length bndrs
731 (bndrs, body) = collectBinders expr
732 zap | n_args >= n_params = \b -> b
733 | otherwise = \b -> if isTyVar b then b
735 -- NB: we count all the args incl type args
736 -- so we must count all the binders (incl type lambdas)
738 simplExprF' env (Type ty) cont
739 = ASSERT( contIsRhsOrArg cont )
740 do { ty' <- simplType env ty
741 ; rebuild env (Type ty') cont }
743 simplExprF' env (Case scrut bndr _ alts) cont
744 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
745 = -- Simplify the scrutinee with a Select continuation
746 simplExprF env scrut (Select NoDup bndr alts env cont)
749 = -- If case-of-case is off, simply simplify the case expression
750 -- in a vanilla Stop context, and rebuild the result around it
751 do { case_expr' <- simplExprC env scrut case_cont
752 ; rebuild env case_expr' cont }
754 case_cont = Select NoDup bndr alts env mkBoringStop
756 simplExprF' env (Let (Rec pairs) body) cont
757 = do { env' <- simplRecBndrs env (map fst pairs)
758 -- NB: bndrs' don't have unfoldings or rules
759 -- We add them as we go down
761 ; env'' <- simplRecBind env' NotTopLevel pairs
762 ; simplExprF env'' body cont }
764 simplExprF' env (Let (NonRec bndr rhs) body) cont
765 = simplNonRecE env bndr (rhs, env) ([], body) cont
767 ---------------------------------
768 simplType :: SimplEnv -> InType -> SimplM OutType
769 -- Kept monadic just so we can do the seqType
771 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
772 seqType new_ty `seq` return new_ty
774 new_ty = substTy env ty
778 %************************************************************************
780 \subsection{The main rebuilder}
782 %************************************************************************
785 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
786 -- At this point the substitution in the SimplEnv should be irrelevant
787 -- only the in-scope set and floats should matter
788 rebuild env expr cont0
789 = -- pprTrace "rebuild" (ppr expr $$ ppr cont0 $$ ppr (seFloats env)) $
791 Stop {} -> return (env, expr)
792 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
793 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
794 StrictArg fun _ info cont -> rebuildCall env (fun `App` expr) info cont
795 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
796 ; simplLam env' bs body cont }
797 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
798 ; rebuild env (App expr arg') cont }
802 %************************************************************************
806 %************************************************************************
809 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
810 -> SimplM (SimplEnv, OutExpr)
811 simplCast env body co0 cont0
812 = do { co1 <- simplType env co0
813 ; simplExprF env body (addCoerce co1 cont0) }
815 addCoerce co cont = add_coerce co (coercionKind co) cont
817 add_coerce _co (s1, k1) cont -- co :: ty~ty
818 | s1 `coreEqType` k1 = cont -- is a no-op
820 add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
821 | (_l1, t1) <- coercionKind co2
822 -- coerce T1 S1 (coerce S1 K1 e)
825 -- coerce T1 K1 e, otherwise
827 -- For example, in the initial form of a worker
828 -- we may find (coerce T (coerce S (\x.e))) y
829 -- and we'd like it to simplify to e[y/x] in one round
831 , s1 `coreEqType` t1 = cont -- The coerces cancel out
832 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
834 add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
835 -- (f `cast` g) ty ---> (f ty) `cast` (g @ ty)
836 -- This implements the PushT rule from the paper
837 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
838 , not (isCoVar tyvar)
839 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
841 ty' = substTy (arg_se `setInScope` env) arg_ty
843 -- ToDo: the PushC rule is not implemented at all
845 add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
846 | not (isTypeArg arg) -- This implements the Push rule from the paper
847 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
848 -- co : s1s2 :=: t1t2
849 -- (coerce (T1->T2) (S1->S2) F) E
851 -- coerce T2 S2 (F (coerce S1 T1 E))
853 -- t1t2 must be a function type, T1->T2, because it's applied
854 -- to something but s1s2 might conceivably not be
856 -- When we build the ApplyTo we can't mix the out-types
857 -- with the InExpr in the argument, so we simply substitute
858 -- to make it all consistent. It's a bit messy.
859 -- But it isn't a common case.
861 -- Example of use: Trac #995
862 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
864 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
865 -- t2 :=: s2 with left and right on the curried form:
866 -- (->) t1 t2 :=: (->) s1 s2
867 [co1, co2] = decomposeCo 2 co
868 new_arg = mkCoerce (mkSymCoercion co1) arg'
869 arg' = substExpr (arg_se `setInScope` env) arg
871 add_coerce co _ cont = CoerceIt co cont
875 %************************************************************************
879 %************************************************************************
882 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
883 -> SimplM (SimplEnv, OutExpr)
885 simplLam env [] body cont = simplExprF env body cont
888 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
889 = do { tick (BetaReduction bndr)
890 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
892 -- Not enough args, so there are real lambdas left to put in the result
893 simplLam env bndrs body cont
894 = do { (env', bndrs') <- simplLamBndrs env bndrs
895 ; body' <- simplExpr env' body
896 ; new_lam <- mkLam bndrs' body'
897 ; rebuild env' new_lam cont }
900 simplNonRecE :: SimplEnv
901 -> InId -- The binder
902 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
903 -> ([InBndr], InExpr) -- Body of the let/lambda
906 -> SimplM (SimplEnv, OutExpr)
908 -- simplNonRecE is used for
909 -- * non-top-level non-recursive lets in expressions
912 -- It deals with strict bindings, via the StrictBind continuation,
913 -- which may abort the whole process
915 -- The "body" of the binding comes as a pair of ([InId],InExpr)
916 -- representing a lambda; so we recurse back to simplLam
917 -- Why? Because of the binder-occ-info-zapping done before
918 -- the call to simplLam in simplExprF (Lam ...)
920 -- First deal with type applications and type lets
921 -- (/\a. e) (Type ty) and (let a = Type ty in e)
922 simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
923 = ASSERT( isTyVar bndr )
924 do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
925 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
927 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
928 | preInlineUnconditionally env NotTopLevel bndr rhs
929 = do { tick (PreInlineUnconditionally bndr)
930 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
933 = do { simplExprF (rhs_se `setFloats` env) rhs
934 (StrictBind bndr bndrs body env cont) }
937 = do { (env1, bndr1) <- simplNonRecBndr env bndr
938 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
939 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
940 ; simplLam env3 bndrs body cont }
944 %************************************************************************
948 %************************************************************************
951 -- Hack alert: we only distinguish subsumed cost centre stacks for the
952 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
953 simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
954 -> SimplM (SimplEnv, OutExpr)
955 simplNote env (SCC cc) e cont
956 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
957 ; rebuild env (mkSCC cc e') cont }
959 -- See notes with SimplMonad.inlineMode
960 simplNote env InlineMe e cont
961 | Just (inside, outside) <- splitInlineCont cont -- Boring boring continuation; see notes above
962 = do { -- Don't inline inside an INLINE expression
963 e' <- simplExprC (setMode inlineMode env) e inside
964 ; rebuild env (mkInlineMe e') outside }
966 | otherwise -- Dissolve the InlineMe note if there's
967 -- an interesting context of any kind to combine with
968 -- (even a type application -- anything except Stop)
969 = simplExprF env e cont
971 simplNote env (CoreNote s) e cont = do
972 e' <- simplExpr env e
973 rebuild env (Note (CoreNote s) e') cont
977 %************************************************************************
979 \subsection{Dealing with calls}
981 %************************************************************************
984 simplVar :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
985 simplVar env var cont
986 = case substId env var of
987 DoneEx e -> simplExprF (zapSubstEnv env) e cont
988 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
989 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
990 -- Note [zapSubstEnv]
991 -- The template is already simplified, so don't re-substitute.
992 -- This is VITAL. Consider
994 -- let y = \z -> ...x... in
996 -- We'll clone the inner \x, adding x->x' in the id_subst
997 -- Then when we inline y, we must *not* replace x by x' in
998 -- the inlined copy!!
1000 ---------------------------------------------------------
1001 -- Dealing with a call site
1003 completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
1004 completeCall env var cont
1005 = do { dflags <- getDOptsSmpl
1006 ; let (args,call_cont) = contArgs cont
1007 -- The args are OutExprs, obtained by *lazily* substituting
1008 -- in the args found in cont. These args are only examined
1009 -- to limited depth (unless a rule fires). But we must do
1010 -- the substitution; rule matching on un-simplified args would
1013 ------------- First try rules ----------------
1014 -- Do this before trying inlining. Some functions have
1015 -- rules *and* are strict; in this case, we don't want to
1016 -- inline the wrapper of the non-specialised thing; better
1017 -- to call the specialised thing instead.
1019 -- We used to use the black-listing mechanism to ensure that inlining of
1020 -- the wrapper didn't occur for things that have specialisations till a
1021 -- later phase, so but now we just try RULES first
1023 -- Note [Rules for recursive functions]
1024 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1025 -- You might think that we shouldn't apply rules for a loop breaker:
1026 -- doing so might give rise to an infinite loop, because a RULE is
1027 -- rather like an extra equation for the function:
1028 -- RULE: f (g x) y = x+y
1031 -- But it's too drastic to disable rules for loop breakers.
1032 -- Even the foldr/build rule would be disabled, because foldr
1033 -- is recursive, and hence a loop breaker:
1034 -- foldr k z (build g) = g k z
1035 -- So it's up to the programmer: rules can cause divergence
1037 ; let in_scope = getInScope env
1038 maybe_rule = case activeRule dflags env of
1039 Nothing -> Nothing -- No rules apply
1040 Just act_fn -> lookupRule act_fn in_scope
1042 ; case maybe_rule of {
1043 Just (rule, rule_rhs) -> do
1044 tick (RuleFired (ru_name rule))
1045 (if dopt Opt_D_dump_rule_firings dflags then
1046 pprTrace "Rule fired" (vcat [
1047 text "Rule:" <+> ftext (ru_name rule),
1048 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1049 text "After: " <+> pprCoreExpr rule_rhs,
1050 text "Cont: " <+> ppr call_cont])
1053 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
1054 -- The ruleArity says how many args the rule consumed
1056 ; Nothing -> do -- No rules
1058 ------------- Next try inlining ----------------
1059 { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
1060 n_val_args = length arg_infos
1061 interesting_cont = interestingCallContext call_cont
1062 active_inline = activeInline env var
1063 maybe_inline = callSiteInline dflags active_inline var
1064 (null args) arg_infos interesting_cont
1065 ; case maybe_inline of {
1066 Just unfolding -- There is an inlining!
1067 -> do { tick (UnfoldingDone var)
1068 ; (if dopt Opt_D_dump_inlinings dflags then
1069 pprTrace ("Inlining done" ++ showSDoc (ppr var)) (vcat [
1070 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1071 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1072 text "Cont: " <+> ppr call_cont])
1075 simplExprF env unfolding cont }
1077 ; Nothing -> -- No inlining!
1079 ------------- No inlining! ----------------
1080 -- Next, look for rules or specialisations that match
1082 rebuildCall env (Var var)
1083 (mkArgInfo var n_val_args call_cont) cont
1086 rebuildCall :: SimplEnv
1087 -> OutExpr -- Function
1090 -> SimplM (SimplEnv, OutExpr)
1091 rebuildCall env fun (ArgInfo { ai_strs = [] }) cont
1092 -- When we run out of strictness args, it means
1093 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1094 -- Then we want to discard the entire strict continuation. E.g.
1095 -- * case (error "hello") of { ... }
1096 -- * (error "Hello") arg
1097 -- * f (error "Hello") where f is strict
1099 -- Then, especially in the first of these cases, we'd like to discard
1100 -- the continuation, leaving just the bottoming expression. But the
1101 -- type might not be right, so we may have to add a coerce.
1102 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1103 = return (env, mk_coerce fun) -- contination to discard, else we do it
1104 where -- again and again!
1105 fun_ty = exprType fun
1106 cont_ty = contResultType env fun_ty cont
1107 co = mkUnsafeCoercion fun_ty cont_ty
1108 mk_coerce expr | cont_ty `coreEqType` fun_ty = expr
1109 | otherwise = mkCoerce co expr
1111 rebuildCall env fun info (ApplyTo _ (Type arg_ty) se cont)
1112 = do { ty' <- simplType (se `setInScope` env) arg_ty
1113 ; rebuildCall env (fun `App` Type ty') info cont }
1116 (ArgInfo { ai_rules = has_rules, ai_strs = str:strs, ai_discs = disc:discs })
1117 (ApplyTo _ arg arg_se cont)
1118 | str -- Strict argument
1119 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1120 simplExprF (arg_se `setFloats` env) arg
1121 (StrictArg fun cci arg_info' cont)
1124 | otherwise -- Lazy argument
1125 -- DO NOT float anything outside, hence simplExprC
1126 -- There is no benefit (unlike in a let-binding), and we'd
1127 -- have to be very careful about bogus strictness through
1128 -- floating a demanded let.
1129 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1131 ; rebuildCall env (fun `App` arg') arg_info' cont }
1133 arg_info' = ArgInfo { ai_rules = has_rules, ai_strs = strs, ai_discs = discs }
1134 cci | has_rules || disc > 0 = ArgCtxt has_rules disc -- Be keener here
1135 | otherwise = BoringCtxt -- Nothing interesting
1137 rebuildCall env fun _ cont
1138 = rebuild env fun cont
1143 This part of the simplifier may break the no-shadowing invariant
1145 f (...(\a -> e)...) (case y of (a,b) -> e')
1146 where f is strict in its second arg
1147 If we simplify the innermost one first we get (...(\a -> e)...)
1148 Simplifying the second arg makes us float the case out, so we end up with
1149 case y of (a,b) -> f (...(\a -> e)...) e'
1150 So the output does not have the no-shadowing invariant. However, there is
1151 no danger of getting name-capture, because when the first arg was simplified
1152 we used an in-scope set that at least mentioned all the variables free in its
1153 static environment, and that is enough.
1155 We can't just do innermost first, or we'd end up with a dual problem:
1156 case x of (a,b) -> f e (...(\a -> e')...)
1158 I spent hours trying to recover the no-shadowing invariant, but I just could
1159 not think of an elegant way to do it. The simplifier is already knee-deep in
1160 continuations. We have to keep the right in-scope set around; AND we have
1161 to get the effect that finding (error "foo") in a strict arg position will
1162 discard the entire application and replace it with (error "foo"). Getting
1163 all this at once is TOO HARD!
1165 %************************************************************************
1167 Rebuilding a cse expression
1169 %************************************************************************
1171 Blob of helper functions for the "case-of-something-else" situation.
1174 ---------------------------------------------------------
1175 -- Eliminate the case if possible
1177 rebuildCase :: SimplEnv
1178 -> OutExpr -- Scrutinee
1179 -> InId -- Case binder
1180 -> [InAlt] -- Alternatives (inceasing order)
1182 -> SimplM (SimplEnv, OutExpr)
1184 --------------------------------------------------
1185 -- 1. Eliminate the case if there's a known constructor
1186 --------------------------------------------------
1188 rebuildCase env scrut case_bndr alts cont
1189 | Just (con,args) <- exprIsConApp_maybe scrut
1190 -- Works when the scrutinee is a variable with a known unfolding
1191 -- as well as when it's an explicit constructor application
1192 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1194 | Lit lit <- scrut -- No need for same treatment as constructors
1195 -- because literals are inlined more vigorously
1196 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1199 --------------------------------------------------
1200 -- 2. Eliminate the case if scrutinee is evaluated
1201 --------------------------------------------------
1203 rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
1204 -- See if we can get rid of the case altogether
1205 -- See the extensive notes on case-elimination above
1206 -- mkCase made sure that if all the alternatives are equal,
1207 -- then there is now only one (DEFAULT) rhs
1208 | all isDeadBinder bndrs -- bndrs are [InId]
1210 -- Check that the scrutinee can be let-bound instead of case-bound
1211 , exprOkForSpeculation scrut
1212 -- OK not to evaluate it
1213 -- This includes things like (==# a# b#)::Bool
1214 -- so that we simplify
1215 -- case ==# a# b# of { True -> x; False -> x }
1218 -- This particular example shows up in default methods for
1219 -- comparision operations (e.g. in (>=) for Int.Int32)
1220 || exprIsHNF scrut -- It's already evaluated
1221 || var_demanded_later scrut -- It'll be demanded later
1223 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1224 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1225 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1226 -- its argument: case x of { y -> dataToTag# y }
1227 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1228 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1230 -- Also we don't want to discard 'seq's
1231 = do { tick (CaseElim case_bndr)
1232 ; env' <- simplNonRecX env case_bndr scrut
1233 ; simplExprF env' rhs cont }
1235 -- The case binder is going to be evaluated later,
1236 -- and the scrutinee is a simple variable
1237 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1238 && not (isTickBoxOp v)
1239 -- ugly hack; covering this case is what
1240 -- exprOkForSpeculation was intended for.
1241 var_demanded_later _ = False
1244 --------------------------------------------------
1245 -- 3. Catch-all case
1246 --------------------------------------------------
1248 rebuildCase env scrut case_bndr alts cont
1249 = do { -- Prepare the continuation;
1250 -- The new subst_env is in place
1251 (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1253 -- Simplify the alternatives
1254 ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
1256 -- Check for empty alternatives
1257 ; if null alts' then
1258 -- This isn't strictly an error, although it is unusual.
1259 -- It's possible that the simplifer might "see" that
1260 -- an inner case has no accessible alternatives before
1261 -- it "sees" that the entire branch of an outer case is
1262 -- inaccessible. So we simply put an error case here instead.
1263 pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $
1264 let res_ty' = contResultType env' (substTy env' (coreAltsType alts)) dup_cont
1265 lit = mkStringLit "Impossible alternative"
1266 in return (env', mkApps (Var rUNTIME_ERROR_ID) [Type res_ty', lit])
1269 { case_expr <- mkCase scrut' case_bndr' alts'
1271 -- Notice that rebuild gets the in-scope set from env, not alt_env
1272 -- The case binder *not* scope over the whole returned case-expression
1273 ; rebuild env' case_expr nodup_cont } }
1276 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1277 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1278 way, there's a chance that v will now only be used once, and hence
1281 Note [no-case-of-case]
1282 ~~~~~~~~~~~~~~~~~~~~~~
1283 We *used* to suppress the binder-swap in case expressoins when
1284 -fno-case-of-case is on. Old remarks:
1285 "This happens in the first simplifier pass,
1286 and enhances full laziness. Here's the bad case:
1287 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1288 If we eliminate the inner case, we trap it inside the I# v -> arm,
1289 which might prevent some full laziness happening. I've seen this
1290 in action in spectral/cichelli/Prog.hs:
1291 [(m,n) | m <- [1..max], n <- [1..max]]
1292 Hence the check for NoCaseOfCase."
1293 However, now the full-laziness pass itself reverses the binder-swap, so this
1294 check is no longer necessary.
1296 Note [Suppressing the case binder-swap]
1297 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1298 There is another situation when it might make sense to suppress the
1299 case-expression binde-swap. If we have
1301 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1302 ...other cases .... }
1304 We'll perform the binder-swap for the outer case, giving
1306 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1307 ...other cases .... }
1309 But there is no point in doing it for the inner case, because w1 can't
1310 be inlined anyway. Furthermore, doing the case-swapping involves
1311 zapping w2's occurrence info (see paragraphs that follow), and that
1312 forces us to bind w2 when doing case merging. So we get
1314 case x of w1 { A -> let w2 = w1 in e1
1315 B -> let w2 = w1 in e2
1316 ...other cases .... }
1318 This is plain silly in the common case where w2 is dead.
1320 Even so, I can't see a good way to implement this idea. I tried
1321 not doing the binder-swap if the scrutinee was already evaluated
1322 but that failed big-time:
1326 case v of w { MkT x ->
1327 case x of x1 { I# y1 ->
1328 case x of x2 { I# y2 -> ...
1330 Notice that because MkT is strict, x is marked "evaluated". But to
1331 eliminate the last case, we must either make sure that x (as well as
1332 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1333 the binder-swap. So this whole note is a no-op.
1337 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1338 any occurrence info (eg IAmDead) in the case binder, because the
1339 case-binder now effectively occurs whenever v does. AND we have to do
1340 the same for the pattern-bound variables! Example:
1342 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1344 Here, b and p are dead. But when we move the argment inside the first
1345 case RHS, and eliminate the second case, we get
1347 case x of { (a,b) -> a b }
1349 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1352 Indeed, this can happen anytime the case binder isn't dead:
1353 case <any> of x { (a,b) ->
1354 case x of { (p,q) -> p } }
1355 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1356 The point is that we bring into the envt a binding
1358 after the outer case, and that makes (a,b) alive. At least we do unless
1359 the case binder is guaranteed dead.
1363 Consider case (v `cast` co) of x { I# ->
1364 ... (case (v `cast` co) of {...}) ...
1365 We'd like to eliminate the inner case. We can get this neatly by
1366 arranging that inside the outer case we add the unfolding
1367 v |-> x `cast` (sym co)
1368 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1370 Note [Improving seq]
1373 type family F :: * -> *
1374 type instance F Int = Int
1376 ... case e of x { DEFAULT -> rhs } ...
1378 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1380 case e `cast` co of x'::Int
1381 I# x# -> let x = x' `cast` sym co
1384 so that 'rhs' can take advantage of the form of x'. Notice that Note
1385 [Case of cast] may then apply to the result.
1387 This showed up in Roman's experiments. Example:
1388 foo :: F Int -> Int -> Int
1389 foo t n = t `seq` bar n
1392 bar n = bar (n - case t of TI i -> i)
1393 Here we'd like to avoid repeated evaluating t inside the loop, by
1394 taking advantage of the `seq`.
1396 At one point I did transformation in LiberateCase, but it's more robust here.
1397 (Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
1398 LiberateCase gets to see it.)
1400 Note [Case elimination]
1401 ~~~~~~~~~~~~~~~~~~~~~~~
1402 The case-elimination transformation discards redundant case expressions.
1403 Start with a simple situation:
1405 case x# of ===> e[x#/y#]
1408 (when x#, y# are of primitive type, of course). We can't (in general)
1409 do this for algebraic cases, because we might turn bottom into
1412 The code in SimplUtils.prepareAlts has the effect of generalise this
1413 idea to look for a case where we're scrutinising a variable, and we
1414 know that only the default case can match. For example:
1418 DEFAULT -> ...(case x of
1422 Here the inner case is first trimmed to have only one alternative, the
1423 DEFAULT, after which it's an instance of the previous case. This
1424 really only shows up in eliminating error-checking code.
1426 We also make sure that we deal with this very common case:
1431 Here we are using the case as a strict let; if x is used only once
1432 then we want to inline it. We have to be careful that this doesn't
1433 make the program terminate when it would have diverged before, so we
1435 - e is already evaluated (it may so if e is a variable)
1436 - x is used strictly, or
1438 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1440 case e of ===> case e of DEFAULT -> r
1444 Now again the case may be elminated by the CaseElim transformation.
1447 Further notes about case elimination
1448 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1449 Consider: test :: Integer -> IO ()
1452 Turns out that this compiles to:
1455 eta1 :: State# RealWorld ->
1456 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1458 (PrelNum.jtos eta ($w[] @ Char))
1460 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1462 Notice the strange '<' which has no effect at all. This is a funny one.
1463 It started like this:
1465 f x y = if x < 0 then jtos x
1466 else if y==0 then "" else jtos x
1468 At a particular call site we have (f v 1). So we inline to get
1470 if v < 0 then jtos x
1471 else if 1==0 then "" else jtos x
1473 Now simplify the 1==0 conditional:
1475 if v<0 then jtos v else jtos v
1477 Now common-up the two branches of the case:
1479 case (v<0) of DEFAULT -> jtos v
1481 Why don't we drop the case? Because it's strict in v. It's technically
1482 wrong to drop even unnecessary evaluations, and in practice they
1483 may be a result of 'seq' so we *definitely* don't want to drop those.
1484 I don't really know how to improve this situation.
1488 simplCaseBinder :: SimplEnv -> OutExpr -> OutId -> [InAlt]
1489 -> SimplM (SimplEnv, OutExpr, OutId)
1490 simplCaseBinder env0 scrut0 case_bndr0 alts
1491 = do { (env1, case_bndr1) <- simplBinder env0 case_bndr0
1493 ; fam_envs <- getFamEnvs
1494 ; (env2, scrut2, case_bndr2) <- improve_seq fam_envs env1 scrut0
1495 case_bndr0 case_bndr1 alts
1496 -- Note [Improving seq]
1498 ; let (env3, case_bndr3) = improve_case_bndr env2 scrut2 case_bndr2
1499 -- Note [Case of cast]
1501 ; return (env3, scrut2, case_bndr3) }
1504 improve_seq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1505 | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1506 = do { case_bndr2 <- newId (fsLit "nt") ty2
1507 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
1508 env2 = extendIdSubst env case_bndr rhs
1509 ; return (env2, scrut `Cast` co, case_bndr2) }
1511 improve_seq _ env scrut _ case_bndr1 _
1512 = return (env, scrut, case_bndr1)
1515 improve_case_bndr env scrut case_bndr
1516 -- See Note [no-case-of-case]
1517 -- | switchIsOn (getSwitchChecker env) NoCaseOfCase
1518 -- = (env, case_bndr)
1520 | otherwise -- Failed try; see Note [Suppressing the case binder-swap]
1521 -- not (isEvaldUnfolding (idUnfolding v))
1523 Var v -> (modifyInScope env1 v case_bndr', case_bndr')
1524 -- Note about using modifyInScope for v here
1525 -- We could extend the substitution instead, but it would be
1526 -- a hack because then the substitution wouldn't be idempotent
1527 -- any more (v is an OutId). And this does just as well.
1529 Cast (Var v) co -> (addBinderUnfolding env1 v rhs, case_bndr')
1531 rhs = Cast (Var case_bndr') (mkSymCoercion co)
1533 _ -> (env, case_bndr)
1535 case_bndr' = zapOccInfo case_bndr
1536 env1 = modifyInScope env case_bndr case_bndr'
1539 zapOccInfo :: InId -> InId -- See Note [zapOccInfo]
1540 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1544 simplAlts does two things:
1546 1. Eliminate alternatives that cannot match, including the
1547 DEFAULT alternative.
1549 2. If the DEFAULT alternative can match only one possible constructor,
1550 then make that constructor explicit.
1552 case e of x { DEFAULT -> rhs }
1554 case e of x { (a,b) -> rhs }
1555 where the type is a single constructor type. This gives better code
1556 when rhs also scrutinises x or e.
1558 Here "cannot match" includes knowledge from GADTs
1560 It's a good idea do do this stuff before simplifying the alternatives, to
1561 avoid simplifying alternatives we know can't happen, and to come up with
1562 the list of constructors that are handled, to put into the IdInfo of the
1563 case binder, for use when simplifying the alternatives.
1565 Eliminating the default alternative in (1) isn't so obvious, but it can
1568 data Colour = Red | Green | Blue
1577 DEFAULT -> [ case y of ... ]
1579 If we inline h into f, the default case of the inlined h can't happen.
1580 If we don't notice this, we may end up filtering out *all* the cases
1581 of the inner case y, which give us nowhere to go!
1585 simplAlts :: SimplEnv
1587 -> InId -- Case binder
1588 -> [InAlt] -- Non-empty
1590 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1591 -- Like simplExpr, this just returns the simplified alternatives;
1592 -- it not return an environment
1594 simplAlts env scrut case_bndr alts cont'
1595 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1596 do { let alt_env = zapFloats env
1597 ; (alt_env', scrut', case_bndr') <- simplCaseBinder alt_env scrut case_bndr 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 -- zap_occ_info: if the case binder is alive, then we add the unfolding
1667 -- to the envt; so vs are now very much alive
1668 -- Note [Aug06] I can't see why this actually matters, but it's neater
1669 -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
1670 -- ==> case e of t { (a,b) -> ...(a)... }
1671 -- Look, Ma, a is alive now.
1672 zap_occ_info | isDeadBinder case_bndr' = \ident -> ident
1673 | otherwise = zapOccInfo
1675 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1676 addBinderUnfolding env bndr rhs
1677 = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
1679 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1680 addBinderOtherCon env bndr cons
1681 = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
1685 %************************************************************************
1687 \subsection{Known constructor}
1689 %************************************************************************
1691 We are a bit careful with occurrence info. Here's an example
1693 (\x* -> case x of (a*, b) -> f a) (h v, e)
1695 where the * means "occurs once". This effectively becomes
1696 case (h v, e) of (a*, b) -> f a)
1698 let a* = h v; b = e in f a
1702 All this should happen in one sweep.
1705 knownCon :: SimplEnv -> OutExpr -> AltCon
1706 -> [OutExpr] -- Args *including* the universal args
1707 -> InId -> [InAlt] -> SimplCont
1708 -> SimplM (SimplEnv, OutExpr)
1710 knownCon env scrut con args bndr alts cont
1711 = do { tick (KnownBranch bndr)
1712 ; knownAlt env scrut args bndr (findAlt con alts) cont }
1714 knownAlt :: SimplEnv -> OutExpr -> [OutExpr]
1715 -> InId -> (AltCon, [CoreBndr], InExpr) -> SimplCont
1716 -> SimplM (SimplEnv, OutExpr)
1717 knownAlt env scrut _ bndr (DEFAULT, bs, rhs) cont
1719 do { env' <- simplNonRecX env bndr scrut
1720 -- This might give rise to a binding with non-atomic args
1721 -- like x = Node (f x) (g x)
1722 -- but simplNonRecX will atomic-ify it
1723 ; simplExprF env' rhs cont }
1725 knownAlt env scrut _ bndr (LitAlt _, bs, rhs) cont
1727 do { env' <- simplNonRecX env bndr scrut
1728 ; simplExprF env' rhs cont }
1730 knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
1731 = do { let dead_bndr = isDeadBinder bndr -- bndr is an InId
1732 n_drop_tys = length (dataConUnivTyVars dc)
1733 ; env' <- bind_args env dead_bndr bs (drop n_drop_tys the_args)
1735 -- It's useful to bind bndr to scrut, rather than to a fresh
1736 -- binding x = Con arg1 .. argn
1737 -- because very often the scrut is a variable, so we avoid
1738 -- creating, and then subsequently eliminating, a let-binding
1739 -- BUT, if scrut is a not a variable, we must be careful
1740 -- about duplicating the arg redexes; in that case, make
1741 -- a new con-app from the args
1742 bndr_rhs = case scrut of
1745 con_app = mkConApp dc (take n_drop_tys the_args ++ con_args)
1746 con_args = [substExpr env' (varToCoreExpr b) | b <- bs]
1747 -- args are aready OutExprs, but bs are InIds
1749 ; env'' <- simplNonRecX env' bndr bndr_rhs
1750 ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env'')) $
1751 simplExprF env'' rhs cont }
1754 bind_args env' _ [] _ = return env'
1756 bind_args env' dead_bndr (b:bs') (Type ty : args)
1757 = ASSERT( isTyVar b )
1758 bind_args (extendTvSubst env' b ty) dead_bndr bs' args
1760 bind_args env' dead_bndr (b:bs') (arg : args)
1762 do { let b' = if dead_bndr then b else zapOccInfo b
1763 -- Note that the binder might be "dead", because it doesn't
1764 -- occur in the RHS; and simplNonRecX may therefore discard
1765 -- it via postInlineUnconditionally.
1766 -- Nevertheless we must keep it if the case-binder is alive,
1767 -- because it may be used in the con_app. See Note [zapOccInfo]
1768 ; env'' <- simplNonRecX env' b' arg
1769 ; bind_args env'' dead_bndr bs' args }
1772 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
1773 text "scrut:" <+> ppr scrut
1777 %************************************************************************
1779 \subsection{Duplicating continuations}
1781 %************************************************************************
1784 prepareCaseCont :: SimplEnv
1785 -> [InAlt] -> SimplCont
1786 -> SimplM (SimplEnv, SimplCont,SimplCont)
1787 -- Return a duplicatable continuation, a non-duplicable part
1788 -- plus some extra bindings (that scope over the entire
1791 -- No need to make it duplicatable if there's only one alternative
1792 prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
1793 prepareCaseCont env _ cont = mkDupableCont env cont
1797 mkDupableCont :: SimplEnv -> SimplCont
1798 -> SimplM (SimplEnv, SimplCont, SimplCont)
1800 mkDupableCont env cont
1801 | contIsDupable cont
1802 = return (env, cont, mkBoringStop)
1804 mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1806 mkDupableCont env (CoerceIt ty cont)
1807 = do { (env', dup, nodup) <- mkDupableCont env cont
1808 ; return (env', CoerceIt ty dup, nodup) }
1810 mkDupableCont env cont@(StrictBind {})
1811 = return (env, mkBoringStop, cont)
1812 -- See Note [Duplicating strict continuations]
1814 mkDupableCont env cont@(StrictArg {})
1815 = return (env, mkBoringStop, cont)
1816 -- See Note [Duplicating strict continuations]
1818 mkDupableCont env (ApplyTo _ arg se cont)
1819 = -- e.g. [...hole...] (...arg...)
1821 -- let a = ...arg...
1822 -- in [...hole...] a
1823 do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1824 ; arg' <- simplExpr (se `setInScope` env') arg
1825 ; (env'', arg'') <- makeTrivial env' arg'
1826 ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env') dup_cont
1827 ; return (env'', app_cont, nodup_cont) }
1829 mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
1830 -- See Note [Single-alternative case]
1831 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1832 -- | not (isDeadBinder case_bndr)
1833 | all isDeadBinder bs -- InIds
1834 && not (isUnLiftedType (idType case_bndr))
1835 -- Note [Single-alternative-unlifted]
1836 = return (env, mkBoringStop, cont)
1838 mkDupableCont env (Select _ case_bndr alts se cont)
1839 = -- e.g. (case [...hole...] of { pi -> ei })
1841 -- let ji = \xij -> ei
1842 -- in case [...hole...] of { pi -> ji xij }
1843 do { tick (CaseOfCase case_bndr)
1844 ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
1845 -- NB: call mkDupableCont here, *not* prepareCaseCont
1846 -- We must make a duplicable continuation, whereas prepareCaseCont
1847 -- doesn't when there is a single case branch
1849 ; let alt_env = se `setInScope` env'
1850 ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
1851 ; alts' <- mapM (simplAlt alt_env' [] case_bndr' dup_cont) alts
1852 -- Safe to say that there are no handled-cons for the DEFAULT case
1853 -- NB: simplBinder does not zap deadness occ-info, so
1854 -- a dead case_bndr' will still advertise its deadness
1855 -- This is really important because in
1856 -- case e of b { (# p,q #) -> ... }
1857 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1858 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1859 -- In the new alts we build, we have the new case binder, so it must retain
1861 -- NB: we don't use alt_env further; it has the substEnv for
1862 -- the alternatives, and we don't want that
1864 ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
1865 ; return (env'', -- Note [Duplicated env]
1866 Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
1870 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1871 -> SimplM (SimplEnv, [InAlt])
1872 -- Absorbs the continuation into the new alternatives
1874 mkDupableAlts env case_bndr' the_alts
1877 go env0 [] = return (env0, [])
1879 = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
1880 ; (env2, alts') <- go env1 alts
1881 ; return (env2, alt' : alts' ) }
1883 mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
1884 -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
1885 mkDupableAlt env case_bndr' (con, bndrs', rhs')
1886 | exprIsDupable rhs' -- Note [Small alternative rhs]
1887 = return (env, (con, bndrs', rhs'))
1889 = do { let rhs_ty' = exprType rhs'
1890 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1892 | isTyVar bndr = True -- Abstract over all type variables just in case
1893 | otherwise = not (isDeadBinder bndr)
1894 -- The deadness info on the new Ids is preserved by simplBinders
1896 ; (final_bndrs', final_args) -- Note [Join point abstraction]
1897 <- if (any isId used_bndrs')
1898 then return (used_bndrs', varsToCoreExprs used_bndrs')
1899 else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
1900 ; return ([rw_id], [Var realWorldPrimId]) }
1902 ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs' rhs_ty')
1903 -- Note [Funky mkPiTypes]
1905 ; let -- We make the lambdas into one-shot-lambdas. The
1906 -- join point is sure to be applied at most once, and doing so
1907 -- prevents the body of the join point being floated out by
1908 -- the full laziness pass
1909 really_final_bndrs = map one_shot final_bndrs'
1910 one_shot v | isId v = setOneShotLambda v
1912 join_rhs = mkLams really_final_bndrs rhs'
1913 join_call = mkApps (Var join_bndr) final_args
1915 ; return (addPolyBind NotTopLevel env (NonRec join_bndr join_rhs), (con, bndrs', join_call)) }
1916 -- See Note [Duplicated env]
1919 Note [Duplicated env]
1920 ~~~~~~~~~~~~~~~~~~~~~
1921 Some of the alternatives are simplified, but have not been turned into a join point
1922 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
1923 bind the join point, because it might to do PostInlineUnconditionally, and
1924 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
1925 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
1926 at worst delays the join-point inlining.
1928 Note [Small alterantive rhs]
1929 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1930 It is worth checking for a small RHS because otherwise we
1931 get extra let bindings that may cause an extra iteration of the simplifier to
1932 inline back in place. Quite often the rhs is just a variable or constructor.
1933 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1934 iterations because the version with the let bindings looked big, and so wasn't
1935 inlined, but after the join points had been inlined it looked smaller, and so
1938 NB: we have to check the size of rhs', not rhs.
1939 Duplicating a small InAlt might invalidate occurrence information
1940 However, if it *is* dupable, we return the *un* simplified alternative,
1941 because otherwise we'd need to pair it up with an empty subst-env....
1942 but we only have one env shared between all the alts.
1943 (Remember we must zap the subst-env before re-simplifying something).
1944 Rather than do this we simply agree to re-simplify the original (small) thing later.
1946 Note [Funky mkPiTypes]
1947 ~~~~~~~~~~~~~~~~~~~~~~
1948 Notice the funky mkPiTypes. If the contructor has existentials
1949 it's possible that the join point will be abstracted over
1950 type varaibles as well as term variables.
1951 Example: Suppose we have
1952 data T = forall t. C [t]
1954 case (case e of ...) of
1956 We get the join point
1957 let j :: forall t. [t] -> ...
1958 j = /\t \xs::[t] -> rhs
1960 case (case e of ...) of
1961 C t xs::[t] -> j t xs
1963 Note [Join point abstaction]
1964 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1965 If we try to lift a primitive-typed something out
1966 for let-binding-purposes, we will *caseify* it (!),
1967 with potentially-disastrous strictness results. So
1968 instead we turn it into a function: \v -> e
1969 where v::State# RealWorld#. The value passed to this function
1970 is realworld#, which generates (almost) no code.
1972 There's a slight infelicity here: we pass the overall
1973 case_bndr to all the join points if it's used in *any* RHS,
1974 because we don't know its usage in each RHS separately
1976 We used to say "&& isUnLiftedType rhs_ty'" here, but now
1977 we make the join point into a function whenever used_bndrs'
1978 is empty. This makes the join-point more CPR friendly.
1979 Consider: let j = if .. then I# 3 else I# 4
1980 in case .. of { A -> j; B -> j; C -> ... }
1982 Now CPR doesn't w/w j because it's a thunk, so
1983 that means that the enclosing function can't w/w either,
1984 which is a lose. Here's the example that happened in practice:
1985 kgmod :: Int -> Int -> Int
1986 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1990 I have seen a case alternative like this:
1992 It's a bit silly to add the realWorld dummy arg in this case, making
1995 (the \v alone is enough to make CPR happy) but I think it's rare
1997 Note [Duplicating strict continuations]
1998 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1999 Do *not* duplicate StrictBind and StritArg continuations. We gain
2000 nothing by propagating them into the expressions, and we do lose a
2001 lot. Here's an example:
2002 && (case x of { T -> F; F -> T }) E
2003 Now, && is strict so we end up simplifying the case with
2004 an ArgOf continuation. If we let-bind it, we get
2006 let $j = \v -> && v E
2007 in simplExpr (case x of { T -> F; F -> T })
2009 And after simplifying more we get
2011 let $j = \v -> && v E
2012 in case x of { T -> $j F; F -> $j T }
2013 Which is a Very Bad Thing
2015 The desire not to duplicate is the entire reason that
2016 mkDupableCont returns a pair of continuations.
2018 The original plan had:
2019 e.g. (...strict-fn...) [...hole...]
2021 let $j = \a -> ...strict-fn...
2024 Note [Single-alternative cases]
2025 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2026 This case is just like the ArgOf case. Here's an example:
2030 case (case x of I# x' ->
2032 True -> I# (negate# x')
2033 False -> I# x') of y {
2035 Because the (case x) has only one alternative, we'll transform to
2037 case (case x' <# 0# of
2038 True -> I# (negate# x')
2039 False -> I# x') of y {
2041 But now we do *NOT* want to make a join point etc, giving
2043 let $j = \y -> MkT y
2045 True -> $j (I# (negate# x'))
2047 In this case the $j will inline again, but suppose there was a big
2048 strict computation enclosing the orginal call to MkT. Then, it won't
2049 "see" the MkT any more, because it's big and won't get duplicated.
2050 And, what is worse, nothing was gained by the case-of-case transform.
2052 When should use this case of mkDupableCont?
2053 However, matching on *any* single-alternative case is a *disaster*;
2054 e.g. case (case ....) of (a,b) -> (# a,b #)
2055 We must push the outer case into the inner one!
2058 * Match [(DEFAULT,_,_)], but in the common case of Int,
2059 the alternative-filling-in code turned the outer case into
2060 case (...) of y { I# _ -> MkT y }
2062 * Match on single alternative plus (not (isDeadBinder case_bndr))
2063 Rationale: pushing the case inwards won't eliminate the construction.
2064 But there's a risk of
2065 case (...) of y { (a,b) -> let z=(a,b) in ... }
2066 Now y looks dead, but it'll come alive again. Still, this
2067 seems like the best option at the moment.
2069 * Match on single alternative plus (all (isDeadBinder bndrs))
2070 Rationale: this is essentially seq.
2072 * Match when the rhs is *not* duplicable, and hence would lead to a
2073 join point. This catches the disaster-case above. We can test
2074 the *un-simplified* rhs, which is fine. It might get bigger or
2075 smaller after simplification; if it gets smaller, this case might
2076 fire next time round. NB also that we must test contIsDupable
2077 case_cont *btoo, because case_cont might be big!
2079 HOWEVER: I found that this version doesn't work well, because
2080 we can get let x = case (...) of { small } in ...case x...
2081 When x is inlined into its full context, we find that it was a bad
2082 idea to have pushed the outer case inside the (...) case.
2084 Note [Single-alternative-unlifted]
2085 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2086 Here's another single-alternative where we really want to do case-of-case:
2094 case y_s6X of tpl_s7m {
2095 M1.Mk1 ipv_s70 -> ipv_s70;
2096 M1.Mk2 ipv_s72 -> ipv_s72;
2102 case x_s74 of tpl_s7n {
2103 M1.Mk1 ipv_s77 -> ipv_s77;
2104 M1.Mk2 ipv_s79 -> ipv_s79;
2108 { __DEFAULT -> ==# [wild1_s7b wild_s7c];
2112 So the outer case is doing *nothing at all*, other than serving as a
2113 join-point. In this case we really want to do case-of-case and decide
2114 whether to use a real join point or just duplicate the continuation.
2116 Hence: check whether the case binder's type is unlifted, because then
2117 the outer case is *not* a seq.