2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[Simplify]{The main module of the simplifier}
8 -- The above warning supression flag is a temporary kludge.
9 -- While working on this module you are encouraged to remove it and fix
10 -- any warnings in the module. See
11 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
14 module Simplify ( simplTopBinds, simplExpr ) where
16 #include "HsVersions.h"
20 import Type hiding ( substTy, extendTvSubst )
27 import FamInstEnv ( topNormaliseType )
28 import DataCon ( dataConRepStrictness, dataConUnivTyVars )
30 import NewDemand ( isStrictDmd )
31 import PprCore ( pprParendExpr, pprCoreExpr )
32 import CoreUnfold ( mkUnfolding, callSiteInline )
34 import Rules ( lookupRule )
35 import BasicTypes ( isMarkedStrict )
36 import CostCentre ( currentCCS )
37 import TysPrim ( realWorldStatePrimTy )
38 import PrelInfo ( realWorldPrimId )
39 import BasicTypes ( TopLevelFlag(..), isTopLevel,
40 RecFlag(..), isNonRuleLoopBreaker )
41 import Maybes ( orElse )
42 import Data.List ( mapAccumL )
48 The guts of the simplifier is in this module, but the driver loop for
49 the simplifier is in SimplCore.lhs.
52 -----------------------------------------
53 *** IMPORTANT NOTE ***
54 -----------------------------------------
55 The simplifier used to guarantee that the output had no shadowing, but
56 it does not do so any more. (Actually, it never did!) The reason is
57 documented with simplifyArgs.
60 -----------------------------------------
61 *** IMPORTANT NOTE ***
62 -----------------------------------------
63 Many parts of the simplifier return a bunch of "floats" as well as an
64 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
66 All "floats" are let-binds, not case-binds, but some non-rec lets may
67 be unlifted (with RHS ok-for-speculation).
71 -----------------------------------------
72 ORGANISATION OF FUNCTIONS
73 -----------------------------------------
75 - simplify all top-level binders
76 - for NonRec, call simplRecOrTopPair
77 - for Rec, call simplRecBind
80 ------------------------------
81 simplExpr (applied lambda) ==> simplNonRecBind
82 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
83 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
85 ------------------------------
86 simplRecBind [binders already simplfied]
87 - use simplRecOrTopPair on each pair in turn
89 simplRecOrTopPair [binder already simplified]
90 Used for: recursive bindings (top level and nested)
91 top-level non-recursive bindings
93 - check for PreInlineUnconditionally
97 Used for: non-top-level non-recursive bindings
98 beta reductions (which amount to the same thing)
99 Because it can deal with strict arts, it takes a
100 "thing-inside" and returns an expression
102 - check for PreInlineUnconditionally
103 - simplify binder, including its IdInfo
112 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
113 Used for: binding case-binder and constr args in a known-constructor case
114 - check for PreInLineUnconditionally
118 ------------------------------
119 simplLazyBind: [binder already simplified, RHS not]
120 Used for: recursive bindings (top level and nested)
121 top-level non-recursive bindings
122 non-top-level, but *lazy* non-recursive bindings
123 [must not be strict or unboxed]
124 Returns floats + an augmented environment, not an expression
125 - substituteIdInfo and add result to in-scope
126 [so that rules are available in rec rhs]
129 - float if exposes constructor or PAP
133 completeNonRecX: [binder and rhs both simplified]
134 - if the the thing needs case binding (unlifted and not ok-for-spec)
140 completeBind: [given a simplified RHS]
141 [used for both rec and non-rec bindings, top level and not]
142 - try PostInlineUnconditionally
143 - add unfolding [this is the only place we add an unfolding]
148 Right hand sides and arguments
149 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
150 In many ways we want to treat
151 (a) the right hand side of a let(rec), and
152 (b) a function argument
153 in the same way. But not always! In particular, we would
154 like to leave these arguments exactly as they are, so they
155 will match a RULE more easily.
160 It's harder to make the rule match if we ANF-ise the constructor,
161 or eta-expand the PAP:
163 f (let { a = g x; b = h x } in (a,b))
166 On the other hand if we see the let-defns
171 then we *do* want to ANF-ise and eta-expand, so that p and q
172 can be safely inlined.
174 Even floating lets out is a bit dubious. For let RHS's we float lets
175 out if that exposes a value, so that the value can be inlined more vigorously.
178 r = let x = e in (x,x)
180 Here, if we float the let out we'll expose a nice constructor. We did experiments
181 that showed this to be a generally good thing. But it was a bad thing to float
182 lets out unconditionally, because that meant they got allocated more often.
184 For function arguments, there's less reason to expose a constructor (it won't
185 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
186 So for the moment we don't float lets out of function arguments either.
191 For eta expansion, we want to catch things like
193 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
195 If the \x was on the RHS of a let, we'd eta expand to bring the two
196 lambdas together. And in general that's a good thing to do. Perhaps
197 we should eta expand wherever we find a (value) lambda? Then the eta
198 expansion at a let RHS can concentrate solely on the PAP case.
201 %************************************************************************
203 \subsection{Bindings}
205 %************************************************************************
208 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
210 simplTopBinds env binds
211 = do { -- Put all the top-level binders into scope at the start
212 -- so that if a transformation rule has unexpectedly brought
213 -- anything into scope, then we don't get a complaint about that.
214 -- It's rather as if the top-level binders were imported.
215 ; env <- simplRecBndrs env (bindersOfBinds binds)
216 ; dflags <- getDOptsSmpl
217 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
218 dopt Opt_D_dump_rule_firings dflags
219 ; env' <- simpl_binds dump_flag env binds
220 ; freeTick SimplifierDone
221 ; return (getFloats env') }
223 -- We need to track the zapped top-level binders, because
224 -- they should have their fragile IdInfo zapped (notably occurrence info)
225 -- That's why we run down binds and bndrs' simultaneously.
227 -- The dump-flag emits a trace for each top-level binding, which
228 -- helps to locate the tracing for inlining and rule firing
229 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
230 simpl_binds dump env [] = return env
231 simpl_binds dump env (bind:binds) = do { env' <- trace dump bind $
233 ; simpl_binds dump env' binds }
235 trace True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
236 trace False bind = \x -> x
238 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
239 simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel b b' r
241 (env', b') = addBndrRules env b (lookupRecBndr env b)
245 %************************************************************************
247 \subsection{Lazy bindings}
249 %************************************************************************
251 simplRecBind is used for
252 * recursive bindings only
255 simplRecBind :: SimplEnv -> TopLevelFlag
258 simplRecBind env top_lvl pairs
259 = do { let (env_with_info, triples) = mapAccumL add_rules env pairs
260 ; env' <- go (zapFloats env_with_info) triples
261 ; return (env `addRecFloats` env') }
262 -- addFloats adds the floats from env',
263 -- *and* updates env with the in-scope set from env'
265 add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
266 -- Add the (substituted) rules to the binder
267 add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
269 (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
271 go env [] = return env
273 go env ((old_bndr, new_bndr, rhs) : pairs)
274 = do { env <- simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
278 simplOrTopPair is used for
279 * recursive bindings (whether top level or not)
280 * top-level non-recursive bindings
282 It assumes the binder has already been simplified, but not its IdInfo.
285 simplRecOrTopPair :: SimplEnv
287 -> InId -> OutBndr -> InExpr -- Binder and rhs
288 -> SimplM SimplEnv -- Returns an env that includes the binding
290 simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
291 | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
292 = do { tick (PreInlineUnconditionally old_bndr)
293 ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
296 = simplLazyBind env top_lvl Recursive old_bndr new_bndr rhs env
297 -- May not actually be recursive, but it doesn't matter
301 simplLazyBind is used for
302 * [simplRecOrTopPair] recursive bindings (whether top level or not)
303 * [simplRecOrTopPair] top-level non-recursive bindings
304 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
307 1. It assumes that the binder is *already* simplified,
308 and is in scope, and its IdInfo too, except unfolding
310 2. It assumes that the binder type is lifted.
312 3. It does not check for pre-inline-unconditionallly;
313 that should have been done already.
316 simplLazyBind :: SimplEnv
317 -> TopLevelFlag -> RecFlag
318 -> InId -> OutId -- Binder, both pre-and post simpl
319 -- The OutId has IdInfo, except arity, unfolding
320 -> InExpr -> SimplEnv -- The RHS and its environment
323 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
324 = do { let rhs_env = rhs_se `setInScope` env
325 (tvs, body) = collectTyBinders rhs
326 ; (body_env, tvs') <- simplBinders rhs_env tvs
327 -- See Note [Floating and type abstraction]
330 -- Simplify the RHS; note the mkRhsStop, which tells
331 -- the simplifier that this is the RHS of a let.
332 ; let rhs_cont = mkRhsStop (applyTys (idType bndr1) (mkTyVarTys tvs'))
333 ; (body_env1, body1) <- simplExprF body_env body rhs_cont
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 ; return (extendFloats env poly_binds, rhs') }
354 ; completeBind env' top_lvl bndr bndr1 rhs' }
357 A specialised variant of simplNonRec used when the RHS is already simplified,
358 notably in knownCon. It uses case-binding where necessary.
361 simplNonRecX :: SimplEnv
362 -> InId -- Old binder
363 -> OutExpr -- Simplified RHS
366 simplNonRecX env bndr new_rhs
367 = do { (env, bndr') <- simplBinder env bndr
368 ; completeNonRecX env NotTopLevel NonRecursive
369 (isStrictId bndr) bndr bndr' new_rhs }
371 completeNonRecX :: SimplEnv
372 -> TopLevelFlag -> RecFlag -> Bool
373 -> InId -- Old binder
374 -> OutId -- New binder
375 -> OutExpr -- Simplified RHS
378 completeNonRecX env top_lvl is_rec is_strict old_bndr new_bndr new_rhs
379 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
381 if doFloatFromRhs top_lvl is_rec 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, env', rhs') <- go 0 env rhs
440 ; return (env', rhs') }
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)
460 go n_val_args env other
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 NotTopLevel NonRecursive
514 ; return (env, substExpr env (Var var)) }
518 %************************************************************************
520 \subsection{Completing a lazy binding}
522 %************************************************************************
525 * deals only with Ids, not TyVars
526 * takes an already-simplified binder and RHS
527 * is used for both recursive and non-recursive bindings
528 * is used for both top-level and non-top-level bindings
530 It does the following:
531 - tries discarding a dead binding
532 - tries PostInlineUnconditionally
533 - add unfolding [this is the only place we add an unfolding]
536 It does *not* attempt to do let-to-case. Why? Because it is used for
537 - top-level bindings (when let-to-case is impossible)
538 - many situations where the "rhs" is known to be a WHNF
539 (so let-to-case is inappropriate).
541 Nor does it do the atomic-argument thing
544 completeBind :: SimplEnv
545 -> TopLevelFlag -- Flag stuck into unfolding
546 -> InId -- Old binder
547 -> OutId -> OutExpr -- New binder and RHS
549 -- completeBind may choose to do its work
550 -- * by extending the substitution (e.g. let x = y in ...)
551 -- * or by adding to the floats in the envt
553 completeBind env top_lvl old_bndr new_bndr new_rhs
554 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
555 -- Inline and discard the binding
556 = do { tick (PostInlineUnconditionally old_bndr)
557 ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
558 return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
559 -- Use the substitution to make quite, quite sure that the
560 -- substitution will happen, since we are going to discard the binding
565 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
568 -- Add the unfolding *only* for non-loop-breakers
569 -- Making loop breakers not have an unfolding at all
570 -- means that we can avoid tests in exprIsConApp, for example.
571 -- This is important: if exprIsConApp says 'yes' for a recursive
572 -- thing, then we can get into an infinite loop
575 -- If the unfolding is a value, the demand info may
576 -- go pear-shaped, so we nuke it. Example:
578 -- case x of (p,q) -> h p q x
579 -- Here x is certainly demanded. But after we've nuked
580 -- the case, we'll get just
581 -- let x = (a,b) in h a b x
582 -- and now x is not demanded (I'm assuming h is lazy)
583 -- This really happens. Similarly
584 -- let f = \x -> e in ...f..f...
585 -- After inlining f at some of its call sites the original binding may
586 -- (for example) be no longer strictly demanded.
587 -- The solution here is a bit ad hoc...
588 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
589 `setWorkerInfo` worker_info
591 final_info | loop_breaker = new_bndr_info
592 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
593 | otherwise = info_w_unf
595 final_id = new_bndr `setIdInfo` final_info
597 -- These seqs forces the Id, and hence its IdInfo,
598 -- and hence any inner substitutions
600 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
601 return (addNonRec env final_id new_rhs)
603 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
604 worker_info = substWorker env (workerInfo old_info)
605 loop_breaker = isNonRuleLoopBreaker occ_info
606 old_info = idInfo old_bndr
607 occ_info = occInfo old_info
612 %************************************************************************
614 \subsection[Simplify-simplExpr]{The main function: simplExpr}
616 %************************************************************************
618 The reason for this OutExprStuff stuff is that we want to float *after*
619 simplifying a RHS, not before. If we do so naively we get quadratic
620 behaviour as things float out.
622 To see why it's important to do it after, consider this (real) example:
636 a -- Can't inline a this round, cos it appears twice
640 Each of the ==> steps is a round of simplification. We'd save a
641 whole round if we float first. This can cascade. Consider
646 let f = let d1 = ..d.. in \y -> e
650 in \x -> ...(\y ->e)...
652 Only in this second round can the \y be applied, and it
653 might do the same again.
657 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
658 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
660 expr_ty' = substTy env (exprType expr)
661 -- The type in the Stop continuation, expr_ty', is usually not used
662 -- It's only needed when discarding continuations after finding
663 -- a function that returns bottom.
664 -- Hence the lazy substitution
667 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
668 -- Simplify an expression, given a continuation
669 simplExprC env expr cont
670 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
671 do { (env', expr') <- simplExprF (zapFloats env) expr cont
672 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
673 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
674 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
675 return (wrapFloats env' expr') }
677 --------------------------------------------------
678 simplExprF :: SimplEnv -> InExpr -> SimplCont
679 -> SimplM (SimplEnv, OutExpr)
681 simplExprF env e cont
682 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
683 simplExprF' env e cont
685 simplExprF' env (Var v) cont = simplVar env v cont
686 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
687 simplExprF' env (Note n expr) cont = simplNote env n expr cont
688 simplExprF' env (Cast body co) cont = simplCast env body co cont
689 simplExprF' env (App fun arg) cont = simplExprF env fun $
690 ApplyTo NoDup arg env cont
692 simplExprF' env expr@(Lam _ _) cont
693 = simplLam env (map zap bndrs) body cont
694 -- The main issue here is under-saturated lambdas
695 -- (\x1. \x2. e) arg1
696 -- Here x1 might have "occurs-once" occ-info, because occ-info
697 -- is computed assuming that a group of lambdas is applied
698 -- all at once. If there are too few args, we must zap the
701 n_args = countArgs cont
702 n_params = length bndrs
703 (bndrs, body) = collectBinders expr
704 zap | n_args >= n_params = \b -> b
705 | otherwise = \b -> if isTyVar b then b
707 -- NB: we count all the args incl type args
708 -- so we must count all the binders (incl type lambdas)
710 simplExprF' env (Type ty) cont
711 = ASSERT( contIsRhsOrArg cont )
712 do { ty' <- simplType env ty
713 ; rebuild env (Type ty') cont }
715 simplExprF' env (Case scrut bndr case_ty alts) cont
716 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
717 = -- Simplify the scrutinee with a Select continuation
718 simplExprF env scrut (Select NoDup bndr alts env cont)
721 = -- If case-of-case is off, simply simplify the case expression
722 -- in a vanilla Stop context, and rebuild the result around it
723 do { case_expr' <- simplExprC env scrut case_cont
724 ; rebuild env case_expr' cont }
726 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
727 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
729 simplExprF' env (Let (Rec pairs) body) cont
730 = do { env <- simplRecBndrs env (map fst pairs)
731 -- NB: bndrs' don't have unfoldings or rules
732 -- We add them as we go down
734 ; env <- simplRecBind env NotTopLevel pairs
735 ; simplExprF env body cont }
737 simplExprF' env (Let (NonRec bndr rhs) body) cont
738 = simplNonRecE env bndr (rhs, env) ([], body) cont
740 ---------------------------------
741 simplType :: SimplEnv -> InType -> SimplM OutType
742 -- Kept monadic just so we can do the seqType
744 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
745 seqType new_ty `seq` returnSmpl new_ty
747 new_ty = substTy env ty
751 %************************************************************************
753 \subsection{The main rebuilder}
755 %************************************************************************
758 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
759 -- At this point the substitution in the SimplEnv should be irrelevant
760 -- only the in-scope set and floats should matter
761 rebuild env expr cont
762 = -- pprTrace "rebuild" (ppr expr $$ ppr cont $$ ppr (seFloats env)) $
764 Stop {} -> return (env, expr)
765 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
766 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
767 StrictArg fun ty info cont -> rebuildCall env (fun `App` expr) (funResultTy ty) info cont
768 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
769 ; simplLam env' bs body cont }
770 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
771 ; rebuild env (App expr arg') cont }
775 %************************************************************************
779 %************************************************************************
782 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
783 -> SimplM (SimplEnv, OutExpr)
784 simplCast env body co cont
785 = do { co' <- simplType env co
786 ; simplExprF env body (addCoerce co' cont) }
788 addCoerce co cont = add_coerce co (coercionKind co) cont
790 add_coerce co (s1, k1) cont -- co :: ty~ty
791 | s1 `coreEqType` k1 = cont -- is a no-op
793 add_coerce co1 (s1, k2) (CoerceIt co2 cont)
794 | (l1, t1) <- coercionKind co2
795 -- coerce T1 S1 (coerce S1 K1 e)
798 -- coerce T1 K1 e, otherwise
800 -- For example, in the initial form of a worker
801 -- we may find (coerce T (coerce S (\x.e))) y
802 -- and we'd like it to simplify to e[y/x] in one round
804 , s1 `coreEqType` t1 = cont -- The coerces cancel out
805 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
807 add_coerce co (s1s2, t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
808 -- (f `cast` g) ty ---> (f ty) `cast` (g @ ty)
809 -- This implements the PushT rule from the paper
810 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
811 , not (isCoVar tyvar)
812 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
814 ty' = substTy arg_se arg_ty
816 -- ToDo: the PushC rule is not implemented at all
818 add_coerce co (s1s2, t1t2) (ApplyTo dup arg arg_se cont)
819 | not (isTypeArg arg) -- This implements the Push rule from the paper
820 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
821 -- co : s1s2 :=: t1t2
822 -- (coerce (T1->T2) (S1->S2) F) E
824 -- coerce T2 S2 (F (coerce S1 T1 E))
826 -- t1t2 must be a function type, T1->T2, because it's applied
827 -- to something but s1s2 might conceivably not be
829 -- When we build the ApplyTo we can't mix the out-types
830 -- with the InExpr in the argument, so we simply substitute
831 -- to make it all consistent. It's a bit messy.
832 -- But it isn't a common case.
834 -- Example of use: Trac #995
835 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
837 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
838 -- t2 :=: s2 with left and right on the curried form:
839 -- (->) t1 t2 :=: (->) s1 s2
840 [co1, co2] = decomposeCo 2 co
841 new_arg = mkCoerce (mkSymCoercion co1) arg'
842 arg' = substExpr arg_se arg
844 add_coerce co _ cont = CoerceIt co cont
848 %************************************************************************
852 %************************************************************************
855 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
856 -> SimplM (SimplEnv, OutExpr)
858 simplLam env [] body cont = simplExprF env body cont
860 -- Type-beta reduction
861 simplLam env (bndr:bndrs) body (ApplyTo _ (Type ty_arg) arg_se cont)
862 = ASSERT( isTyVar bndr )
863 do { tick (BetaReduction bndr)
864 ; ty_arg' <- simplType (arg_se `setInScope` env) ty_arg
865 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
867 -- Ordinary beta reduction
868 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
869 = do { tick (BetaReduction bndr)
870 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
872 -- Not enough args, so there are real lambdas left to put in the result
873 simplLam env bndrs body cont
874 = do { (env, bndrs') <- simplLamBndrs env bndrs
875 ; body' <- simplExpr env body
876 ; new_lam <- mkLam bndrs' body'
877 ; rebuild env new_lam cont }
880 simplNonRecE :: SimplEnv
881 -> InId -- The binder
882 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
883 -> ([InId], InExpr) -- Body of the let/lambda
886 -> SimplM (SimplEnv, OutExpr)
888 -- simplNonRecE is used for
889 -- * non-top-level non-recursive lets in expressions
892 -- It deals with strict bindings, via the StrictBind continuation,
893 -- which may abort the whole process
895 -- The "body" of the binding comes as a pair of ([InId],InExpr)
896 -- representing a lambda; so we recurse back to simplLam
897 -- Why? Because of the binder-occ-info-zapping done before
898 -- the call to simplLam in simplExprF (Lam ...)
900 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
901 | preInlineUnconditionally env NotTopLevel bndr rhs
902 = do { tick (PreInlineUnconditionally bndr)
903 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
906 = do { simplExprF (rhs_se `setFloats` env) rhs
907 (StrictBind bndr bndrs body env cont) }
910 = do { (env1, bndr1) <- simplNonRecBndr env bndr
911 ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
912 ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
913 ; simplLam env3 bndrs body cont }
917 %************************************************************************
921 %************************************************************************
924 -- Hack alert: we only distinguish subsumed cost centre stacks for the
925 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
926 simplNote env (SCC cc) e cont
927 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
928 ; rebuild env (mkSCC cc e') cont }
930 -- See notes with SimplMonad.inlineMode
931 simplNote env InlineMe e cont
932 | Just (inside, outside) <- splitInlineCont cont -- Boring boring continuation; see notes above
933 = do { -- Don't inline inside an INLINE expression
934 e' <- simplExprC (setMode inlineMode env) e inside
935 ; rebuild env (mkInlineMe e') outside }
937 | otherwise -- Dissolve the InlineMe note if there's
938 -- an interesting context of any kind to combine with
939 -- (even a type application -- anything except Stop)
940 = simplExprF env e cont
942 simplNote env (CoreNote s) e cont
943 = simplExpr env e `thenSmpl` \ e' ->
944 rebuild env (Note (CoreNote s) e') cont
948 %************************************************************************
950 \subsection{Dealing with calls}
952 %************************************************************************
955 simplVar env var cont
956 = case substId env var of
957 DoneEx e -> simplExprF (zapSubstEnv env) e cont
958 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
959 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
960 -- Note [zapSubstEnv]
961 -- The template is already simplified, so don't re-substitute.
962 -- This is VITAL. Consider
964 -- let y = \z -> ...x... in
966 -- We'll clone the inner \x, adding x->x' in the id_subst
967 -- Then when we inline y, we must *not* replace x by x' in
968 -- the inlined copy!!
970 ---------------------------------------------------------
971 -- Dealing with a call site
973 completeCall env var cont
974 = do { dflags <- getDOptsSmpl
975 ; let (args,call_cont) = contArgs cont
976 -- The args are OutExprs, obtained by *lazily* substituting
977 -- in the args found in cont. These args are only examined
978 -- to limited depth (unless a rule fires). But we must do
979 -- the substitution; rule matching on un-simplified args would
982 ------------- First try rules ----------------
983 -- Do this before trying inlining. Some functions have
984 -- rules *and* are strict; in this case, we don't want to
985 -- inline the wrapper of the non-specialised thing; better
986 -- to call the specialised thing instead.
988 -- We used to use the black-listing mechanism to ensure that inlining of
989 -- the wrapper didn't occur for things that have specialisations till a
990 -- later phase, so but now we just try RULES first
992 -- Note [Rules for recursive functions]
993 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
994 -- You might think that we shouldn't apply rules for a loop breaker:
995 -- doing so might give rise to an infinite loop, because a RULE is
996 -- rather like an extra equation for the function:
997 -- RULE: f (g x) y = x+y
1000 -- But it's too drastic to disable rules for loop breakers.
1001 -- Even the foldr/build rule would be disabled, because foldr
1002 -- is recursive, and hence a loop breaker:
1003 -- foldr k z (build g) = g k z
1004 -- So it's up to the programmer: rules can cause divergence
1006 ; let in_scope = getInScope env
1007 maybe_rule = case activeRule dflags env of
1008 Nothing -> Nothing -- No rules apply
1009 Just act_fn -> lookupRule act_fn in_scope
1011 ; case maybe_rule of {
1012 Just (rule, rule_rhs) ->
1013 tick (RuleFired (ru_name rule)) `thenSmpl_`
1014 (if dopt Opt_D_dump_rule_firings dflags then
1015 pprTrace "Rule fired" (vcat [
1016 text "Rule:" <+> ftext (ru_name rule),
1017 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1018 text "After: " <+> pprCoreExpr rule_rhs,
1019 text "Cont: " <+> ppr call_cont])
1022 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
1023 -- The ruleArity says how many args the rule consumed
1025 ; Nothing -> do -- No rules
1027 ------------- Next try inlining ----------------
1028 { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
1029 n_val_args = length arg_infos
1030 interesting_cont = interestingCallContext (notNull args)
1033 active_inline = activeInline env var
1034 maybe_inline = callSiteInline dflags active_inline
1035 var arg_infos interesting_cont
1036 ; case maybe_inline of {
1037 Just unfolding -- There is an inlining!
1038 -> do { tick (UnfoldingDone var)
1039 ; (if dopt Opt_D_dump_inlinings dflags then
1040 pprTrace ("Inlining done" ++ showSDoc (ppr var)) (vcat [
1041 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1042 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1043 text "Cont: " <+> ppr call_cont])
1046 simplExprF env unfolding cont }
1048 ; Nothing -> -- No inlining!
1050 ------------- No inlining! ----------------
1051 -- Next, look for rules or specialisations that match
1053 rebuildCall env (Var var) (idType var)
1054 (mkArgInfo var n_val_args call_cont) cont
1057 rebuildCall :: SimplEnv
1058 -> OutExpr -> OutType -- Function and its type
1059 -> (Bool, [Bool]) -- See SimplUtils.mkArgInfo
1061 -> SimplM (SimplEnv, OutExpr)
1062 rebuildCall env fun fun_ty (has_rules, []) cont
1063 -- When we run out of strictness args, it means
1064 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1065 -- Then we want to discard the entire strict continuation. E.g.
1066 -- * case (error "hello") of { ... }
1067 -- * (error "Hello") arg
1068 -- * f (error "Hello") where f is strict
1070 -- Then, especially in the first of these cases, we'd like to discard
1071 -- the continuation, leaving just the bottoming expression. But the
1072 -- type might not be right, so we may have to add a coerce.
1073 | not (contIsTrivial cont) -- Only do this if there is a non-trivial
1074 = return (env, mk_coerce fun) -- contination to discard, else we do it
1075 where -- again and again!
1076 cont_ty = contResultType cont
1077 co = mkUnsafeCoercion fun_ty cont_ty
1078 mk_coerce expr | cont_ty `coreEqType` fun_ty = fun
1079 | otherwise = mkCoerce co fun
1081 rebuildCall env fun fun_ty info (ApplyTo _ (Type arg_ty) se cont)
1082 = do { ty' <- simplType (se `setInScope` env) arg_ty
1083 ; rebuildCall env (fun `App` Type ty') (applyTy fun_ty ty') info cont }
1085 rebuildCall env fun fun_ty (has_rules, str:strs) (ApplyTo _ arg arg_se cont)
1086 | str || isStrictType arg_ty -- Strict argument
1087 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1088 simplExprF (arg_se `setFloats` env) arg
1089 (StrictArg fun fun_ty (has_rules, strs) cont)
1092 | otherwise -- Lazy argument
1093 -- DO NOT float anything outside, hence simplExprC
1094 -- There is no benefit (unlike in a let-binding), and we'd
1095 -- have to be very careful about bogus strictness through
1096 -- floating a demanded let.
1097 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1098 (mkLazyArgStop arg_ty has_rules)
1099 ; rebuildCall env (fun `App` arg') res_ty (has_rules, strs) cont }
1101 (arg_ty, res_ty) = splitFunTy fun_ty
1103 rebuildCall env fun fun_ty info cont
1104 = rebuild env fun cont
1109 This part of the simplifier may break the no-shadowing invariant
1111 f (...(\a -> e)...) (case y of (a,b) -> e')
1112 where f is strict in its second arg
1113 If we simplify the innermost one first we get (...(\a -> e)...)
1114 Simplifying the second arg makes us float the case out, so we end up with
1115 case y of (a,b) -> f (...(\a -> e)...) e'
1116 So the output does not have the no-shadowing invariant. However, there is
1117 no danger of getting name-capture, because when the first arg was simplified
1118 we used an in-scope set that at least mentioned all the variables free in its
1119 static environment, and that is enough.
1121 We can't just do innermost first, or we'd end up with a dual problem:
1122 case x of (a,b) -> f e (...(\a -> e')...)
1124 I spent hours trying to recover the no-shadowing invariant, but I just could
1125 not think of an elegant way to do it. The simplifier is already knee-deep in
1126 continuations. We have to keep the right in-scope set around; AND we have
1127 to get the effect that finding (error "foo") in a strict arg position will
1128 discard the entire application and replace it with (error "foo"). Getting
1129 all this at once is TOO HARD!
1131 %************************************************************************
1133 Rebuilding a cse expression
1135 %************************************************************************
1137 Blob of helper functions for the "case-of-something-else" situation.
1140 ---------------------------------------------------------
1141 -- Eliminate the case if possible
1143 rebuildCase :: SimplEnv
1144 -> OutExpr -- Scrutinee
1145 -> InId -- Case binder
1146 -> [InAlt] -- Alternatives (inceasing order)
1148 -> SimplM (SimplEnv, OutExpr)
1150 --------------------------------------------------
1151 -- 1. Eliminate the case if there's a known constructor
1152 --------------------------------------------------
1154 rebuildCase env scrut case_bndr alts cont
1155 | Just (con,args) <- exprIsConApp_maybe scrut
1156 -- Works when the scrutinee is a variable with a known unfolding
1157 -- as well as when it's an explicit constructor application
1158 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1160 | Lit lit <- scrut -- No need for same treatment as constructors
1161 -- because literals are inlined more vigorously
1162 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1165 --------------------------------------------------
1166 -- 2. Eliminate the case if scrutinee is evaluated
1167 --------------------------------------------------
1169 rebuildCase env scrut case_bndr [(con,bndrs,rhs)] cont
1170 -- See if we can get rid of the case altogether
1171 -- See the extensive notes on case-elimination above
1172 -- mkCase made sure that if all the alternatives are equal,
1173 -- then there is now only one (DEFAULT) rhs
1174 | all isDeadBinder bndrs -- bndrs are [InId]
1176 -- Check that the scrutinee can be let-bound instead of case-bound
1177 , exprOkForSpeculation scrut
1178 -- OK not to evaluate it
1179 -- This includes things like (==# a# b#)::Bool
1180 -- so that we simplify
1181 -- case ==# a# b# of { True -> x; False -> x }
1184 -- This particular example shows up in default methods for
1185 -- comparision operations (e.g. in (>=) for Int.Int32)
1186 || exprIsHNF scrut -- It's already evaluated
1187 || var_demanded_later scrut -- It'll be demanded later
1189 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1190 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1191 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1192 -- its argument: case x of { y -> dataToTag# y }
1193 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1194 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1196 -- Also we don't want to discard 'seq's
1197 = do { tick (CaseElim case_bndr)
1198 ; env <- simplNonRecX env case_bndr scrut
1199 ; simplExprF env rhs cont }
1201 -- The case binder is going to be evaluated later,
1202 -- and the scrutinee is a simple variable
1203 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1204 && not (isTickBoxOp v)
1205 -- ugly hack; covering this case is what
1206 -- exprOkForSpeculation was intended for.
1207 var_demanded_later other = False
1210 --------------------------------------------------
1211 -- 3. Catch-all case
1212 --------------------------------------------------
1214 rebuildCase env scrut case_bndr alts cont
1215 = do { -- Prepare the continuation;
1216 -- The new subst_env is in place
1217 (env, dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1219 -- Simplify the alternatives
1220 ; (scrut', case_bndr', alts') <- simplAlts env scrut case_bndr alts dup_cont
1221 ; let res_ty' = contResultType dup_cont
1222 ; case_expr <- mkCase scrut' case_bndr' res_ty' alts'
1224 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1225 -- The case binder *not* scope over the whole returned case-expression
1226 ; rebuild env case_expr nodup_cont }
1229 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1230 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1231 way, there's a chance that v will now only be used once, and hence
1234 Note [no-case-of-case]
1235 ~~~~~~~~~~~~~~~~~~~~~~
1236 There is a time we *don't* want to do that, namely when
1237 -fno-case-of-case is on. This happens in the first simplifier pass,
1238 and enhances full laziness. Here's the bad case:
1239 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1240 If we eliminate the inner case, we trap it inside the I# v -> arm,
1241 which might prevent some full laziness happening. I've seen this
1242 in action in spectral/cichelli/Prog.hs:
1243 [(m,n) | m <- [1..max], n <- [1..max]]
1244 Hence the check for NoCaseOfCase.
1246 Note [Suppressing the case binder-swap]
1247 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1248 There is another situation when it might make sense to suppress the
1249 case-expression binde-swap. If we have
1251 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1252 ...other cases .... }
1254 We'll perform the binder-swap for the outer case, giving
1256 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1257 ...other cases .... }
1259 But there is no point in doing it for the inner case, because w1 can't
1260 be inlined anyway. Furthermore, doing the case-swapping involves
1261 zapping w2's occurrence info (see paragraphs that follow), and that
1262 forces us to bind w2 when doing case merging. So we get
1264 case x of w1 { A -> let w2 = w1 in e1
1265 B -> let w2 = w1 in e2
1266 ...other cases .... }
1268 This is plain silly in the common case where w2 is dead.
1270 Even so, I can't see a good way to implement this idea. I tried
1271 not doing the binder-swap if the scrutinee was already evaluated
1272 but that failed big-time:
1276 case v of w { MkT x ->
1277 case x of x1 { I# y1 ->
1278 case x of x2 { I# y2 -> ...
1280 Notice that because MkT is strict, x is marked "evaluated". But to
1281 eliminate the last case, we must either make sure that x (as well as
1282 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1283 the binder-swap. So this whole note is a no-op.
1287 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1288 any occurrence info (eg IAmDead) in the case binder, because the
1289 case-binder now effectively occurs whenever v does. AND we have to do
1290 the same for the pattern-bound variables! Example:
1292 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1294 Here, b and p are dead. But when we move the argment inside the first
1295 case RHS, and eliminate the second case, we get
1297 case x of { (a,b) -> a b }
1299 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1302 Indeed, this can happen anytime the case binder isn't dead:
1303 case <any> of x { (a,b) ->
1304 case x of { (p,q) -> p } }
1305 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1306 The point is that we bring into the envt a binding
1308 after the outer case, and that makes (a,b) alive. At least we do unless
1309 the case binder is guaranteed dead.
1313 Consider case (v `cast` co) of x { I# ->
1314 ... (case (v `cast` co) of {...}) ...
1315 We'd like to eliminate the inner case. We can get this neatly by
1316 arranging that inside the outer case we add the unfolding
1317 v |-> x `cast` (sym co)
1318 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1320 Note [Improving seq]
1323 type family F :: * -> *
1324 type instance F Int = Int
1326 ... case e of x { DEFAULT -> rhs } ...
1328 where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
1330 case e `cast` co of x'::Int
1331 I# x# -> let x = x' `cast` sym co
1334 so that 'rhs' can take advantage of hte form of x'. Notice that Note
1335 [Case of cast] may then apply to the result.
1337 This showed up in Roman's experiments. Example:
1338 foo :: F Int -> Int -> Int
1339 foo t n = t `seq` bar n
1342 bar n = bar (n - case t of TI i -> i)
1343 Here we'd like to avoid repeated evaluating t inside the loop, by
1344 taking advantage of the `seq`.
1346 At one point I did transformation in LiberateCase, but it's more robust here.
1347 (Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
1348 LiberateCase gets to see it.)
1350 Note [Case elimination]
1351 ~~~~~~~~~~~~~~~~~~~~~~~
1352 The case-elimination transformation discards redundant case expressions.
1353 Start with a simple situation:
1355 case x# of ===> e[x#/y#]
1358 (when x#, y# are of primitive type, of course). We can't (in general)
1359 do this for algebraic cases, because we might turn bottom into
1362 The code in SimplUtils.prepareAlts has the effect of generalise this
1363 idea to look for a case where we're scrutinising a variable, and we
1364 know that only the default case can match. For example:
1368 DEFAULT -> ...(case x of
1372 Here the inner case is first trimmed to have only one alternative, the
1373 DEFAULT, after which it's an instance of the previous case. This
1374 really only shows up in eliminating error-checking code.
1376 We also make sure that we deal with this very common case:
1381 Here we are using the case as a strict let; if x is used only once
1382 then we want to inline it. We have to be careful that this doesn't
1383 make the program terminate when it would have diverged before, so we
1385 - e is already evaluated (it may so if e is a variable)
1386 - x is used strictly, or
1388 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1390 case e of ===> case e of DEFAULT -> r
1394 Now again the case may be elminated by the CaseElim transformation.
1397 Further notes about case elimination
1398 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1399 Consider: test :: Integer -> IO ()
1402 Turns out that this compiles to:
1405 eta1 :: State# RealWorld ->
1406 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1408 (PrelNum.jtos eta ($w[] @ Char))
1410 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1412 Notice the strange '<' which has no effect at all. This is a funny one.
1413 It started like this:
1415 f x y = if x < 0 then jtos x
1416 else if y==0 then "" else jtos x
1418 At a particular call site we have (f v 1). So we inline to get
1420 if v < 0 then jtos x
1421 else if 1==0 then "" else jtos x
1423 Now simplify the 1==0 conditional:
1425 if v<0 then jtos v else jtos v
1427 Now common-up the two branches of the case:
1429 case (v<0) of DEFAULT -> jtos v
1431 Why don't we drop the case? Because it's strict in v. It's technically
1432 wrong to drop even unnecessary evaluations, and in practice they
1433 may be a result of 'seq' so we *definitely* don't want to drop those.
1434 I don't really know how to improve this situation.
1438 simplCaseBinder :: SimplEnv -> OutExpr -> OutId -> [InAlt]
1439 -> SimplM (SimplEnv, OutExpr, OutId)
1440 simplCaseBinder env scrut case_bndr alts
1441 = do { (env1, case_bndr1) <- simplBinder env case_bndr
1443 ; fam_envs <- getFamEnvs
1444 ; (env2, scrut2, case_bndr2) <- improve_seq fam_envs env1 scrut
1445 case_bndr case_bndr1 alts
1446 -- Note [Improving seq]
1448 ; let (env3, case_bndr3) = improve_case_bndr env2 scrut2 case_bndr2
1449 -- Note [Case of cast]
1451 ; return (env3, scrut2, case_bndr3) }
1454 improve_seq fam_envs env1 scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
1455 | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
1456 = do { case_bndr2 <- newId FSLIT("nt") ty2
1457 ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
1458 env2 = extendIdSubst env1 case_bndr rhs
1459 ; return (env2, scrut `Cast` co, case_bndr2) }
1461 improve_seq fam_envs env1 scrut case_bndr case_bndr1 alts
1462 = return (env1, scrut, case_bndr1)
1465 improve_case_bndr env scrut case_bndr
1466 | switchIsOn (getSwitchChecker env) NoCaseOfCase
1467 -- See Note [no-case-of-case]
1470 | otherwise -- Failed try; see Note [Suppressing the case binder-swap]
1471 -- not (isEvaldUnfolding (idUnfolding v))
1473 Var v -> (modifyInScope env1 v case_bndr', case_bndr')
1474 -- Note about using modifyInScope for v here
1475 -- We could extend the substitution instead, but it would be
1476 -- a hack because then the substitution wouldn't be idempotent
1477 -- any more (v is an OutId). And this does just as well.
1479 Cast (Var v) co -> (addBinderUnfolding env1 v rhs, case_bndr')
1481 rhs = Cast (Var case_bndr') (mkSymCoercion co)
1483 other -> (env, case_bndr)
1485 case_bndr' = zapOccInfo case_bndr
1486 env1 = modifyInScope env case_bndr case_bndr'
1489 zapOccInfo :: InId -> InId -- See Note [zapOccInfo]
1490 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1494 simplAlts does two things:
1496 1. Eliminate alternatives that cannot match, including the
1497 DEFAULT alternative.
1499 2. If the DEFAULT alternative can match only one possible constructor,
1500 then make that constructor explicit.
1502 case e of x { DEFAULT -> rhs }
1504 case e of x { (a,b) -> rhs }
1505 where the type is a single constructor type. This gives better code
1506 when rhs also scrutinises x or e.
1508 Here "cannot match" includes knowledge from GADTs
1510 It's a good idea do do this stuff before simplifying the alternatives, to
1511 avoid simplifying alternatives we know can't happen, and to come up with
1512 the list of constructors that are handled, to put into the IdInfo of the
1513 case binder, for use when simplifying the alternatives.
1515 Eliminating the default alternative in (1) isn't so obvious, but it can
1518 data Colour = Red | Green | Blue
1527 DEFAULT -> [ case y of ... ]
1529 If we inline h into f, the default case of the inlined h can't happen.
1530 If we don't notice this, we may end up filtering out *all* the cases
1531 of the inner case y, which give us nowhere to go!
1535 simplAlts :: SimplEnv
1537 -> InId -- Case binder
1538 -> [InAlt] -> SimplCont
1539 -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
1540 -- Like simplExpr, this just returns the simplified alternatives;
1541 -- it not return an environment
1543 simplAlts env scrut case_bndr alts cont'
1544 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1545 do { let alt_env = zapFloats env
1546 ; (alt_env, scrut', case_bndr') <- simplCaseBinder alt_env scrut case_bndr alts
1548 ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env scrut case_bndr' alts
1550 ; alts' <- mapM (simplAlt alt_env imposs_deflt_cons case_bndr' cont') in_alts
1551 ; return (scrut', case_bndr', alts') }
1553 ------------------------------------
1554 simplAlt :: SimplEnv
1555 -> [AltCon] -- These constructors can't be present when
1556 -- matching the DEFAULT alternative
1557 -> OutId -- The case binder
1562 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1563 = ASSERT( null bndrs )
1564 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1565 -- Record the constructors that the case-binder *can't* be.
1566 ; rhs' <- simplExprC env' rhs cont'
1567 ; return (DEFAULT, [], rhs') }
1569 simplAlt env imposs_deflt_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1570 = ASSERT( null bndrs )
1571 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1572 ; rhs' <- simplExprC env' rhs cont'
1573 ; return (LitAlt lit, [], rhs') }
1575 simplAlt env imposs_deflt_cons case_bndr' cont' (DataAlt con, vs, rhs)
1576 = do { -- Deal with the pattern-bound variables
1577 (env, vs') <- simplBinders env (add_evals con vs)
1579 -- Mark the ones that are in ! positions in the
1580 -- data constructor as certainly-evaluated.
1581 ; let vs'' = add_evals con vs'
1583 -- Bind the case-binder to (con args)
1584 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1585 con_args = map Type inst_tys' ++ varsToCoreExprs vs''
1586 env' = addBinderUnfolding env case_bndr' (mkConApp con con_args)
1588 ; rhs' <- simplExprC env' rhs cont'
1589 ; return (DataAlt con, vs'', rhs') }
1591 -- add_evals records the evaluated-ness of the bound variables of
1592 -- a case pattern. This is *important*. Consider
1593 -- data T = T !Int !Int
1595 -- case x of { T a b -> T (a+1) b }
1597 -- We really must record that b is already evaluated so that we don't
1598 -- go and re-evaluate it when constructing the result.
1599 -- See Note [Data-con worker strictness] in MkId.lhs
1600 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1602 cat_evals dc vs strs
1606 go (v:vs) strs | isTyVar v = v : go vs strs
1607 go (v:vs) (str:strs)
1608 | isMarkedStrict str = evald_v : go vs strs
1609 | otherwise = zapped_v : go vs strs
1611 zapped_v = zap_occ_info v
1612 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1613 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1615 -- If the case binder is alive, then we add the unfolding
1617 -- to the envt; so vs are now very much alive
1618 -- Note [Aug06] I can't see why this actually matters
1619 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1620 | otherwise = zapOccInfo
1622 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1623 addBinderUnfolding env bndr rhs
1624 = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
1626 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1627 addBinderOtherCon env bndr cons
1628 = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
1632 %************************************************************************
1634 \subsection{Known constructor}
1636 %************************************************************************
1638 We are a bit careful with occurrence info. Here's an example
1640 (\x* -> case x of (a*, b) -> f a) (h v, e)
1642 where the * means "occurs once". This effectively becomes
1643 case (h v, e) of (a*, b) -> f a)
1645 let a* = h v; b = e in f a
1649 All this should happen in one sweep.
1652 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1653 -> InId -> [InAlt] -> SimplCont
1654 -> SimplM (SimplEnv, OutExpr)
1656 knownCon env scrut con args bndr alts cont
1657 = do { tick (KnownBranch bndr)
1658 ; knownAlt env scrut args bndr (findAlt con alts) cont }
1660 knownAlt env scrut args bndr (DEFAULT, bs, rhs) cont
1662 do { env <- simplNonRecX env bndr scrut
1663 -- This might give rise to a binding with non-atomic args
1664 -- like x = Node (f x) (g x)
1665 -- but simplNonRecX will atomic-ify it
1666 ; simplExprF env rhs cont }
1668 knownAlt env scrut args bndr (LitAlt lit, bs, rhs) cont
1670 do { env <- simplNonRecX env bndr scrut
1671 ; simplExprF env rhs cont }
1673 knownAlt env scrut args bndr (DataAlt dc, bs, rhs) cont
1674 = do { let dead_bndr = isDeadBinder bndr -- bndr is an InId
1675 n_drop_tys = length (dataConUnivTyVars dc)
1676 ; env <- bind_args env dead_bndr bs (drop n_drop_tys args)
1678 -- It's useful to bind bndr to scrut, rather than to a fresh
1679 -- binding x = Con arg1 .. argn
1680 -- because very often the scrut is a variable, so we avoid
1681 -- creating, and then subsequently eliminating, a let-binding
1682 -- BUT, if scrut is a not a variable, we must be careful
1683 -- about duplicating the arg redexes; in that case, make
1684 -- a new con-app from the args
1685 bndr_rhs = case scrut of
1688 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1689 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1690 -- args are aready OutExprs, but bs are InIds
1692 ; env <- simplNonRecX env bndr bndr_rhs
1693 ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env)) $
1694 simplExprF env rhs cont }
1697 bind_args env dead_bndr [] _ = return env
1699 bind_args env dead_bndr (b:bs) (Type ty : args)
1700 = ASSERT( isTyVar b )
1701 bind_args (extendTvSubst env b ty) dead_bndr bs args
1703 bind_args env dead_bndr (b:bs) (arg : args)
1705 do { let b' = if dead_bndr then b else zapOccInfo b
1706 -- Note that the binder might be "dead", because it doesn't occur
1707 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1708 -- Nevertheless we must keep it if the case-binder is alive, because it may
1709 -- be used in the con_app. See Note [zapOccInfo]
1710 ; env <- simplNonRecX env b' arg
1711 ; bind_args env dead_bndr bs args }
1714 pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr args $$
1715 text "scrut:" <+> ppr scrut
1719 %************************************************************************
1721 \subsection{Duplicating continuations}
1723 %************************************************************************
1726 prepareCaseCont :: SimplEnv
1727 -> [InAlt] -> SimplCont
1728 -> SimplM (SimplEnv, SimplCont,SimplCont)
1729 -- Return a duplicatable continuation, a non-duplicable part
1730 -- plus some extra bindings (that scope over the entire
1733 -- No need to make it duplicatable if there's only one alternative
1734 prepareCaseCont env [alt] cont = return (env, cont, mkBoringStop (contResultType cont))
1735 prepareCaseCont env alts cont = mkDupableCont env cont
1739 mkDupableCont :: SimplEnv -> SimplCont
1740 -> SimplM (SimplEnv, SimplCont, SimplCont)
1742 mkDupableCont env cont
1743 | contIsDupable cont
1744 = returnSmpl (env, cont, mkBoringStop (contResultType cont))
1746 mkDupableCont env (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1748 mkDupableCont env (CoerceIt ty cont)
1749 = do { (env, dup, nodup) <- mkDupableCont env cont
1750 ; return (env, CoerceIt ty dup, nodup) }
1752 mkDupableCont env cont@(StrictBind bndr _ _ se _)
1753 = return (env, mkBoringStop (substTy se (idType bndr)), cont)
1754 -- See Note [Duplicating strict continuations]
1756 mkDupableCont env cont@(StrictArg _ fun_ty _ _)
1757 = return (env, mkBoringStop (funArgTy fun_ty), cont)
1758 -- See Note [Duplicating strict continuations]
1760 mkDupableCont env (ApplyTo _ arg se cont)
1761 = -- e.g. [...hole...] (...arg...)
1763 -- let a = ...arg...
1764 -- in [...hole...] a
1765 do { (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1766 ; arg <- simplExpr (se `setInScope` env) arg
1767 ; (env, arg) <- makeTrivial env arg
1768 ; let app_cont = ApplyTo OkToDup arg (zapSubstEnv env) dup_cont
1769 ; return (env, app_cont, nodup_cont) }
1771 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1772 -- See Note [Single-alternative case]
1773 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1774 -- | not (isDeadBinder case_bndr)
1775 | all isDeadBinder bs -- InIds
1776 = return (env, mkBoringStop scrut_ty, cont)
1778 scrut_ty = substTy se (idType case_bndr)
1780 mkDupableCont env (Select _ case_bndr alts se cont)
1781 = -- e.g. (case [...hole...] of { pi -> ei })
1783 -- let ji = \xij -> ei
1784 -- in case [...hole...] of { pi -> ji xij }
1785 do { tick (CaseOfCase case_bndr)
1786 ; (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1787 -- NB: call mkDupableCont here, *not* prepareCaseCont
1788 -- We must make a duplicable continuation, whereas prepareCaseCont
1789 -- doesn't when there is a single case branch
1791 ; let alt_env = se `setInScope` env
1792 ; (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1793 ; alts' <- mapM (simplAlt alt_env [] case_bndr' dup_cont) alts
1794 -- Safe to say that there are no handled-cons for the DEFAULT case
1795 -- NB: simplBinder does not zap deadness occ-info, so
1796 -- a dead case_bndr' will still advertise its deadness
1797 -- This is really important because in
1798 -- case e of b { (# p,q #) -> ... }
1799 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1800 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1801 -- In the new alts we build, we have the new case binder, so it must retain
1803 -- NB: we don't use alt_env further; it has the substEnv for
1804 -- the alternatives, and we don't want that
1806 ; (env, alts') <- mkDupableAlts env case_bndr' alts'
1807 ; return (env, -- Note [Duplicated env]
1808 Select OkToDup case_bndr' alts' (zapSubstEnv env)
1809 (mkBoringStop (contResultType dup_cont)),
1813 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1814 -> SimplM (SimplEnv, [InAlt])
1815 -- Absorbs the continuation into the new alternatives
1817 mkDupableAlts env case_bndr' alts
1820 go env [] = return (env, [])
1822 = do { (env, alt') <- mkDupableAlt env case_bndr' alt
1823 ; (env, alts') <- go env alts
1824 ; return (env, alt' : alts' ) }
1826 mkDupableAlt env case_bndr' (con, bndrs', rhs')
1827 | exprIsDupable rhs' -- Note [Small alternative rhs]
1828 = return (env, (con, bndrs', rhs'))
1830 = do { let rhs_ty' = exprType rhs'
1831 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1833 | isTyVar bndr = True -- Abstract over all type variables just in case
1834 | otherwise = not (isDeadBinder bndr)
1835 -- The deadness info on the new Ids is preserved by simplBinders
1837 ; (final_bndrs', final_args) -- Note [Join point abstraction]
1838 <- if (any isId used_bndrs')
1839 then return (used_bndrs', varsToCoreExprs used_bndrs')
1840 else do { rw_id <- newId FSLIT("w") realWorldStatePrimTy
1841 ; return ([rw_id], [Var realWorldPrimId]) }
1843 ; join_bndr <- newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty')
1844 -- Note [Funky mkPiTypes]
1846 ; let -- We make the lambdas into one-shot-lambdas. The
1847 -- join point is sure to be applied at most once, and doing so
1848 -- prevents the body of the join point being floated out by
1849 -- the full laziness pass
1850 really_final_bndrs = map one_shot final_bndrs'
1851 one_shot v | isId v = setOneShotLambda v
1853 join_rhs = mkLams really_final_bndrs rhs'
1854 join_call = mkApps (Var join_bndr) final_args
1856 ; return (addNonRec env join_bndr join_rhs, (con, bndrs', join_call)) }
1857 -- See Note [Duplicated env]
1860 Note [Duplicated env]
1861 ~~~~~~~~~~~~~~~~~~~~~
1862 Some of the alternatives are simplified, but have not been turned into a join point
1863 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
1864 bind the join point, because it might to do PostInlineUnconditionally, and
1865 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
1866 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
1867 at worst delays the join-point inlining.
1869 Note [Small alterantive rhs]
1870 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1871 It is worth checking for a small RHS because otherwise we
1872 get extra let bindings that may cause an extra iteration of the simplifier to
1873 inline back in place. Quite often the rhs is just a variable or constructor.
1874 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1875 iterations because the version with the let bindings looked big, and so wasn't
1876 inlined, but after the join points had been inlined it looked smaller, and so
1879 NB: we have to check the size of rhs', not rhs.
1880 Duplicating a small InAlt might invalidate occurrence information
1881 However, if it *is* dupable, we return the *un* simplified alternative,
1882 because otherwise we'd need to pair it up with an empty subst-env....
1883 but we only have one env shared between all the alts.
1884 (Remember we must zap the subst-env before re-simplifying something).
1885 Rather than do this we simply agree to re-simplify the original (small) thing later.
1887 Note [Funky mkPiTypes]
1888 ~~~~~~~~~~~~~~~~~~~~~~
1889 Notice the funky mkPiTypes. If the contructor has existentials
1890 it's possible that the join point will be abstracted over
1891 type varaibles as well as term variables.
1892 Example: Suppose we have
1893 data T = forall t. C [t]
1895 case (case e of ...) of
1897 We get the join point
1898 let j :: forall t. [t] -> ...
1899 j = /\t \xs::[t] -> rhs
1901 case (case e of ...) of
1902 C t xs::[t] -> j t xs
1904 Note [Join point abstaction]
1905 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1906 If we try to lift a primitive-typed something out
1907 for let-binding-purposes, we will *caseify* it (!),
1908 with potentially-disastrous strictness results. So
1909 instead we turn it into a function: \v -> e
1910 where v::State# RealWorld#. The value passed to this function
1911 is realworld#, which generates (almost) no code.
1913 There's a slight infelicity here: we pass the overall
1914 case_bndr to all the join points if it's used in *any* RHS,
1915 because we don't know its usage in each RHS separately
1917 We used to say "&& isUnLiftedType rhs_ty'" here, but now
1918 we make the join point into a function whenever used_bndrs'
1919 is empty. This makes the join-point more CPR friendly.
1920 Consider: let j = if .. then I# 3 else I# 4
1921 in case .. of { A -> j; B -> j; C -> ... }
1923 Now CPR doesn't w/w j because it's a thunk, so
1924 that means that the enclosing function can't w/w either,
1925 which is a lose. Here's the example that happened in practice:
1926 kgmod :: Int -> Int -> Int
1927 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1931 I have seen a case alternative like this:
1933 It's a bit silly to add the realWorld dummy arg in this case, making
1936 (the \v alone is enough to make CPR happy) but I think it's rare
1938 Note [Duplicating strict continuations]
1939 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1940 Do *not* duplicate StrictBind and StritArg continuations. We gain
1941 nothing by propagating them into the expressions, and we do lose a
1942 lot. Here's an example:
1943 && (case x of { T -> F; F -> T }) E
1944 Now, && is strict so we end up simplifying the case with
1945 an ArgOf continuation. If we let-bind it, we get
1947 let $j = \v -> && v E
1948 in simplExpr (case x of { T -> F; F -> T })
1950 And after simplifying more we get
1952 let $j = \v -> && v E
1953 in case x of { T -> $j F; F -> $j T }
1954 Which is a Very Bad Thing
1956 The desire not to duplicate is the entire reason that
1957 mkDupableCont returns a pair of continuations.
1959 The original plan had:
1960 e.g. (...strict-fn...) [...hole...]
1962 let $j = \a -> ...strict-fn...
1965 Note [Single-alternative cases]
1966 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1967 This case is just like the ArgOf case. Here's an example:
1971 case (case x of I# x' ->
1973 True -> I# (negate# x')
1974 False -> I# x') of y {
1976 Because the (case x) has only one alternative, we'll transform to
1978 case (case x' <# 0# of
1979 True -> I# (negate# x')
1980 False -> I# x') of y {
1982 But now we do *NOT* want to make a join point etc, giving
1984 let $j = \y -> MkT y
1986 True -> $j (I# (negate# x'))
1988 In this case the $j will inline again, but suppose there was a big
1989 strict computation enclosing the orginal call to MkT. Then, it won't
1990 "see" the MkT any more, because it's big and won't get duplicated.
1991 And, what is worse, nothing was gained by the case-of-case transform.
1993 When should use this case of mkDupableCont?
1994 However, matching on *any* single-alternative case is a *disaster*;
1995 e.g. case (case ....) of (a,b) -> (# a,b #)
1996 We must push the outer case into the inner one!
1999 * Match [(DEFAULT,_,_)], but in the common case of Int,
2000 the alternative-filling-in code turned the outer case into
2001 case (...) of y { I# _ -> MkT y }
2003 * Match on single alternative plus (not (isDeadBinder case_bndr))
2004 Rationale: pushing the case inwards won't eliminate the construction.
2005 But there's a risk of
2006 case (...) of y { (a,b) -> let z=(a,b) in ... }
2007 Now y looks dead, but it'll come alive again. Still, this
2008 seems like the best option at the moment.
2010 * Match on single alternative plus (all (isDeadBinder bndrs))
2011 Rationale: this is essentially seq.
2013 * Match when the rhs is *not* duplicable, and hence would lead to a
2014 join point. This catches the disaster-case above. We can test
2015 the *un-simplified* rhs, which is fine. It might get bigger or
2016 smaller after simplification; if it gets smaller, this case might
2017 fire next time round. NB also that we must test contIsDupable
2018 case_cont *btoo, because case_cont might be big!
2020 HOWEVER: I found that this version doesn't work well, because
2021 we can get let x = case (...) of { small } in ...case x...
2022 When x is inlined into its full context, we find that it was a bad
2023 idea to have pushed the outer case inside the (...) case.