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 )
20 import DataCon ( dataConTyCon, dataConRepStrictness, dataConUnivTyVars )
21 import TyCon ( tyConArity )
23 import NewDemand ( isStrictDmd )
24 import PprCore ( pprParendExpr, pprCoreExpr )
25 import CoreUnfold ( mkUnfolding, callSiteInline )
27 import Rules ( lookupRule )
28 import BasicTypes ( isMarkedStrict )
29 import CostCentre ( currentCCS )
30 import TysPrim ( realWorldStatePrimTy )
31 import PrelInfo ( realWorldPrimId )
32 import BasicTypes ( TopLevelFlag(..), isTopLevel,
33 RecFlag(..), isNonRuleLoopBreaker )
34 import Maybes ( orElse )
40 The guts of the simplifier is in this module, but the driver loop for
41 the simplifier is in SimplCore.lhs.
44 -----------------------------------------
45 *** IMPORTANT NOTE ***
46 -----------------------------------------
47 The simplifier used to guarantee that the output had no shadowing, but
48 it does not do so any more. (Actually, it never did!) The reason is
49 documented with simplifyArgs.
52 -----------------------------------------
53 *** IMPORTANT NOTE ***
54 -----------------------------------------
55 Many parts of the simplifier return a bunch of "floats" as well as an
56 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
58 All "floats" are let-binds, not case-binds, but some non-rec lets may
59 be unlifted (with RHS ok-for-speculation).
63 -----------------------------------------
64 ORGANISATION OF FUNCTIONS
65 -----------------------------------------
67 - simplify all top-level binders
68 - for NonRec, call simplRecOrTopPair
69 - for Rec, call simplRecBind
72 ------------------------------
73 simplExpr (applied lambda) ==> simplNonRecBind
74 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
75 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
77 ------------------------------
78 simplRecBind [binders already simplfied]
79 - use simplRecOrTopPair on each pair in turn
81 simplRecOrTopPair [binder already simplified]
82 Used for: recursive bindings (top level and nested)
83 top-level non-recursive bindings
85 - check for PreInlineUnconditionally
89 Used for: non-top-level non-recursive bindings
90 beta reductions (which amount to the same thing)
91 Because it can deal with strict arts, it takes a
92 "thing-inside" and returns an expression
94 - check for PreInlineUnconditionally
95 - simplify binder, including its IdInfo
104 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
105 Used for: binding case-binder and constr args in a known-constructor case
106 - check for PreInLineUnconditionally
110 ------------------------------
111 simplLazyBind: [binder already simplified, RHS not]
112 Used for: recursive bindings (top level and nested)
113 top-level non-recursive bindings
114 non-top-level, but *lazy* non-recursive bindings
115 [must not be strict or unboxed]
116 Returns floats + an augmented environment, not an expression
117 - substituteIdInfo and add result to in-scope
118 [so that rules are available in rec rhs]
121 - float if exposes constructor or PAP
125 completeNonRecX: [binder and rhs both simplified]
126 - if the the thing needs case binding (unlifted and not ok-for-spec)
132 completeBind: [given a simplified RHS]
133 [used for both rec and non-rec bindings, top level and not]
134 - try PostInlineUnconditionally
135 - add unfolding [this is the only place we add an unfolding]
140 Right hand sides and arguments
141 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
142 In many ways we want to treat
143 (a) the right hand side of a let(rec), and
144 (b) a function argument
145 in the same way. But not always! In particular, we would
146 like to leave these arguments exactly as they are, so they
147 will match a RULE more easily.
152 It's harder to make the rule match if we ANF-ise the constructor,
153 or eta-expand the PAP:
155 f (let { a = g x; b = h x } in (a,b))
158 On the other hand if we see the let-defns
163 then we *do* want to ANF-ise and eta-expand, so that p and q
164 can be safely inlined.
166 Even floating lets out is a bit dubious. For let RHS's we float lets
167 out if that exposes a value, so that the value can be inlined more vigorously.
170 r = let x = e in (x,x)
172 Here, if we float the let out we'll expose a nice constructor. We did experiments
173 that showed this to be a generally good thing. But it was a bad thing to float
174 lets out unconditionally, because that meant they got allocated more often.
176 For function arguments, there's less reason to expose a constructor (it won't
177 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
178 So for the moment we don't float lets out of function arguments either.
183 For eta expansion, we want to catch things like
185 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
187 If the \x was on the RHS of a let, we'd eta expand to bring the two
188 lambdas together. And in general that's a good thing to do. Perhaps
189 we should eta expand wherever we find a (value) lambda? Then the eta
190 expansion at a let RHS can concentrate solely on the PAP case.
193 %************************************************************************
195 \subsection{Bindings}
197 %************************************************************************
200 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
202 simplTopBinds env binds
203 = do { -- Put all the top-level binders into scope at the start
204 -- so that if a transformation rule has unexpectedly brought
205 -- anything into scope, then we don't get a complaint about that.
206 -- It's rather as if the top-level binders were imported.
207 ; env <- simplRecBndrs env (bindersOfBinds binds)
208 ; dflags <- getDOptsSmpl
209 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
210 dopt Opt_D_dump_rule_firings dflags
211 ; env' <- simpl_binds dump_flag env binds
212 ; freeTick SimplifierDone
213 ; return (getFloats env') }
215 -- We need to track the zapped top-level binders, because
216 -- they should have their fragile IdInfo zapped (notably occurrence info)
217 -- That's why we run down binds and bndrs' simultaneously.
219 -- The dump-flag emits a trace for each top-level binding, which
220 -- helps to locate the tracing for inlining and rule firing
221 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
222 simpl_binds dump env [] = return env
223 simpl_binds dump env (bind:binds) = do { env' <- trace dump bind $
225 ; simpl_binds dump env' binds }
227 trace True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
228 trace False bind = \x -> x
230 simpl_bind env (NonRec b r) = simplRecOrTopPair env TopLevel b r
231 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
235 %************************************************************************
237 \subsection{Lazy bindings}
239 %************************************************************************
241 simplRecBind is used for
242 * recursive bindings only
245 simplRecBind :: SimplEnv -> TopLevelFlag
248 simplRecBind env top_lvl pairs
249 = do { env' <- go (zapFloats env) pairs
250 ; return (env `addRecFloats` env') }
251 -- addFloats adds the floats from env',
252 -- *and* updates env with the in-scope set from env'
254 go env [] = return env
256 go env ((bndr, rhs) : pairs)
257 = do { env <- simplRecOrTopPair env top_lvl bndr rhs
261 simplOrTopPair is used for
262 * recursive bindings (whether top level or not)
263 * top-level non-recursive bindings
265 It assumes the binder has already been simplified, but not its IdInfo.
268 simplRecOrTopPair :: SimplEnv
270 -> InId -> InExpr -- Binder and rhs
271 -> SimplM SimplEnv -- Returns an env that includes the binding
273 simplRecOrTopPair env top_lvl bndr rhs
274 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
275 = do { tick (PreInlineUnconditionally bndr)
276 ; return (extendIdSubst env bndr (mkContEx env rhs)) }
279 = do { let bndr' = lookupRecBndr env bndr
280 (env', bndr'') = addLetIdInfo env bndr bndr'
281 ; simplLazyBind env' top_lvl Recursive bndr bndr'' rhs env' }
282 -- May not actually be recursive, but it doesn't matter
286 simplLazyBind is used for
287 * [simplRecOrTopPair] recursive bindings (whether top level or not)
288 * [simplRecOrTopPair] top-level non-recursive bindings
289 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
292 1. It assumes that the binder is *already* simplified,
293 and is in scope, and its IdInfo too, except unfolding
295 2. It assumes that the binder type is lifted.
297 3. It does not check for pre-inline-unconditionallly;
298 that should have been done already.
301 simplLazyBind :: SimplEnv
302 -> TopLevelFlag -> RecFlag
303 -> InId -> OutId -- Binder, both pre-and post simpl
304 -- The OutId has IdInfo, except arity, unfolding
305 -> InExpr -> SimplEnv -- The RHS and its environment
308 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
309 = do { let rhs_env = rhs_se `setInScope` env
310 rhs_cont = mkRhsStop (idType bndr1)
312 -- Simplify the RHS; note the mkRhsStop, which tells
313 -- the simplifier that this is the RHS of a let.
314 ; (rhs_env1, rhs1) <- simplExprF rhs_env rhs rhs_cont
316 -- If any of the floats can't be floated, give up now
317 -- (The canFloat predicate says True for empty floats.)
318 ; if (not (canFloat top_lvl is_rec False rhs_env1))
319 then completeBind env top_lvl bndr bndr1
320 (wrapFloats rhs_env1 rhs1)
322 -- ANF-ise a constructor or PAP rhs
323 { (rhs_env2, rhs2) <- prepareRhs rhs_env1 rhs1
324 ; (env', rhs3) <- chooseRhsFloats top_lvl is_rec False env rhs_env2 rhs2
325 ; completeBind env' top_lvl bndr bndr1 rhs3 } }
327 chooseRhsFloats :: TopLevelFlag -> RecFlag -> Bool
328 -> SimplEnv -- Env for the let
329 -> SimplEnv -- Env for the RHS, with RHS floats in it
330 -> OutExpr -- ..and the RHS itself
331 -> SimplM (SimplEnv, OutExpr) -- New env for let, and RHS
333 chooseRhsFloats top_lvl is_rec is_strict env rhs_env rhs
334 | not (isEmptyFloats rhs_env) -- Something to float
335 , canFloat top_lvl is_rec is_strict rhs_env -- ...that can float
336 , (isTopLevel top_lvl || exprIsCheap rhs) -- ...and we want to float
337 = do { tick LetFloatFromLet -- Float
338 ; return (addFloats env rhs_env, rhs) } -- Add the floats to the main env
339 | otherwise -- Don't float
340 = return (env, wrapFloats rhs_env rhs) -- Wrap the floats around the RHS
344 %************************************************************************
346 \subsection{simplNonRec}
348 %************************************************************************
350 A specialised variant of simplNonRec used when the RHS is already simplified,
351 notably in knownCon. It uses case-binding where necessary.
354 simplNonRecX :: SimplEnv
355 -> InId -- Old binder
356 -> OutExpr -- Simplified RHS
359 simplNonRecX env bndr new_rhs
360 = do { (env, bndr') <- simplBinder env bndr
361 ; completeNonRecX env NotTopLevel NonRecursive
362 (isStrictId bndr) bndr bndr' new_rhs }
364 completeNonRecX :: SimplEnv
365 -> TopLevelFlag -> RecFlag -> Bool
366 -> InId -- Old binder
367 -> OutId -- New binder
368 -> OutExpr -- Simplified RHS
371 completeNonRecX env top_lvl is_rec is_strict old_bndr new_bndr new_rhs
372 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
373 ; (env2, rhs2) <- chooseRhsFloats top_lvl is_rec is_strict env env1 rhs1
374 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
377 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
378 Doing so risks exponential behaviour, because new_rhs has been simplified once already
379 In the cases described by the folowing commment, postInlineUnconditionally will
380 catch many of the relevant cases.
381 -- This happens; for example, the case_bndr during case of
382 -- known constructor: case (a,b) of x { (p,q) -> ... }
383 -- Here x isn't mentioned in the RHS, so we don't want to
384 -- create the (dead) let-binding let x = (a,b) in ...
386 -- Similarly, single occurrences can be inlined vigourously
387 -- e.g. case (f x, g y) of (a,b) -> ....
388 -- If a,b occur once we can avoid constructing the let binding for them.
390 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
391 -- Consider case I# (quotInt# x y) of
392 -- I# v -> let w = J# v in ...
393 -- If we gaily inline (quotInt# x y) for v, we end up building an
395 -- let w = J# (quotInt# x y) in ...
396 -- because quotInt# can fail.
398 | preInlineUnconditionally env NotTopLevel bndr new_rhs
399 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
402 ----------------------------------
403 prepareRhs takes a putative RHS, checks whether it's a PAP or
404 constructor application and, if so, converts it to ANF, so that the
405 resulting thing can be inlined more easily. Thus
412 We also want to deal well cases like this
413 v = (f e1 `cast` co) e2
414 Here we want to make e1,e2 trivial and get
415 x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
416 That's what the 'go' loop in prepareRhs does
419 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
420 -- Adds new floats to the env iff that allows us to return a good RHS
421 prepareRhs env (Cast rhs co) -- Note [Float coercions]
422 = do { (env', rhs') <- makeTrivial env rhs
423 ; return (env', Cast rhs' co) }
426 = do { (is_val, env', rhs') <- go 0 env rhs
427 ; return (env', rhs') }
429 go n_val_args env (Cast rhs co)
430 = do { (is_val, env', rhs') <- go n_val_args env rhs
431 ; return (is_val, env', Cast rhs' co) }
432 go n_val_args env (App fun (Type ty))
433 = do { (is_val, env', rhs') <- go n_val_args env fun
434 ; return (is_val, env', App rhs' (Type ty)) }
435 go n_val_args env (App fun arg)
436 = do { (is_val, env', fun') <- go (n_val_args+1) env fun
438 True -> do { (env'', arg') <- makeTrivial env' arg
439 ; return (True, env'', App fun' arg') }
440 False -> return (False, env, App fun arg) }
441 go n_val_args env (Var fun)
442 = return (is_val, env, Var fun)
444 is_val = n_val_args > 0 -- There is at least one arg
445 -- ...and the fun a constructor or PAP
446 && (isDataConWorkId fun || n_val_args < idArity fun)
447 go n_val_args env other
448 = return (False, env, other)
451 Note [Float coercions]
452 ~~~~~~~~~~~~~~~~~~~~~~
453 When we find the binding
455 we'd like to transform it to
457 x = x `cast` co -- A trivial binding
458 There's a chance that e will be a constructor application or function, or something
459 like that, so moving the coerion to the usage site may well cancel the coersions
460 and lead to further optimisation. Example:
463 data instance T Int = T Int
465 foo :: Int -> Int -> Int
470 go n = case x of { T m -> go (n-m) }
471 -- This case should optimise
475 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
476 -- Binds the expression to a variable, if it's not trivial, returning the variable
480 | otherwise -- See Note [Take care] below
481 = do { var <- newId FSLIT("a") (exprType expr)
482 ; env <- completeNonRecX env NotTopLevel NonRecursive
484 ; return (env, substExpr env (Var var)) }
488 %************************************************************************
490 \subsection{Completing a lazy binding}
492 %************************************************************************
495 * deals only with Ids, not TyVars
496 * takes an already-simplified binder and RHS
497 * is used for both recursive and non-recursive bindings
498 * is used for both top-level and non-top-level bindings
500 It does the following:
501 - tries discarding a dead binding
502 - tries PostInlineUnconditionally
503 - add unfolding [this is the only place we add an unfolding]
506 It does *not* attempt to do let-to-case. Why? Because it is used for
507 - top-level bindings (when let-to-case is impossible)
508 - many situations where the "rhs" is known to be a WHNF
509 (so let-to-case is inappropriate).
511 Nor does it do the atomic-argument thing
514 completeBind :: SimplEnv
515 -> TopLevelFlag -- Flag stuck into unfolding
516 -> InId -- Old binder
517 -> OutId -> OutExpr -- New binder and RHS
519 -- completeBind may choose to do its work
520 -- * by extending the substitution (e.g. let x = y in ...)
521 -- * or by adding to the floats in the envt
523 completeBind env top_lvl old_bndr new_bndr new_rhs
524 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
525 -- Inline and discard the binding
526 = do { tick (PostInlineUnconditionally old_bndr)
527 ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
528 return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
529 -- Use the substitution to make quite, quite sure that the
530 -- substitution will happen, since we are going to discard the binding
535 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
538 -- Add the unfolding *only* for non-loop-breakers
539 -- Making loop breakers not have an unfolding at all
540 -- means that we can avoid tests in exprIsConApp, for example.
541 -- This is important: if exprIsConApp says 'yes' for a recursive
542 -- thing, then we can get into an infinite loop
545 -- If the unfolding is a value, the demand info may
546 -- go pear-shaped, so we nuke it. Example:
548 -- case x of (p,q) -> h p q x
549 -- Here x is certainly demanded. But after we've nuked
550 -- the case, we'll get just
551 -- let x = (a,b) in h a b x
552 -- and now x is not demanded (I'm assuming h is lazy)
553 -- This really happens. Similarly
554 -- let f = \x -> e in ...f..f...
555 -- After inlining f at some of its call sites the original binding may
556 -- (for example) be no longer strictly demanded.
557 -- The solution here is a bit ad hoc...
558 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
559 final_info | loop_breaker = new_bndr_info
560 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
561 | otherwise = info_w_unf
563 final_id = new_bndr `setIdInfo` final_info
565 -- These seqs forces the Id, and hence its IdInfo,
566 -- and hence any inner substitutions
568 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
569 return (addNonRec env final_id new_rhs)
571 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
572 loop_breaker = isNonRuleLoopBreaker occ_info
573 old_info = idInfo old_bndr
574 occ_info = occInfo old_info
579 %************************************************************************
581 \subsection[Simplify-simplExpr]{The main function: simplExpr}
583 %************************************************************************
585 The reason for this OutExprStuff stuff is that we want to float *after*
586 simplifying a RHS, not before. If we do so naively we get quadratic
587 behaviour as things float out.
589 To see why it's important to do it after, consider this (real) example:
603 a -- Can't inline a this round, cos it appears twice
607 Each of the ==> steps is a round of simplification. We'd save a
608 whole round if we float first. This can cascade. Consider
613 let f = let d1 = ..d.. in \y -> e
617 in \x -> ...(\y ->e)...
619 Only in this second round can the \y be applied, and it
620 might do the same again.
624 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
625 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
627 expr_ty' = substTy env (exprType expr)
628 -- The type in the Stop continuation, expr_ty', is usually not used
629 -- It's only needed when discarding continuations after finding
630 -- a function that returns bottom.
631 -- Hence the lazy substitution
634 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
635 -- Simplify an expression, given a continuation
636 simplExprC env expr cont
637 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
638 do { (env', expr') <- simplExprF (zapFloats env) expr cont
639 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
640 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
641 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
642 return (wrapFloats env' expr') }
644 --------------------------------------------------
645 simplExprF :: SimplEnv -> InExpr -> SimplCont
646 -> SimplM (SimplEnv, OutExpr)
648 simplExprF env e cont
649 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
650 simplExprF' env e cont
652 simplExprF' env (Var v) cont = simplVar env v cont
653 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
654 simplExprF' env (Note n expr) cont = simplNote env n expr cont
655 simplExprF' env (Cast body co) cont = simplCast env body co cont
656 simplExprF' env (App fun arg) cont = simplExprF env fun $
657 ApplyTo NoDup arg env cont
659 simplExprF' env expr@(Lam _ _) cont
660 = simplLam env (map zap bndrs) body cont
661 -- The main issue here is under-saturated lambdas
662 -- (\x1. \x2. e) arg1
663 -- Here x1 might have "occurs-once" occ-info, because occ-info
664 -- is computed assuming that a group of lambdas is applied
665 -- all at once. If there are too few args, we must zap the
668 n_args = countArgs cont
669 n_params = length bndrs
670 (bndrs, body) = collectBinders expr
671 zap | n_args >= n_params = \b -> b
672 | otherwise = \b -> if isTyVar b then b
674 -- NB: we count all the args incl type args
675 -- so we must count all the binders (incl type lambdas)
677 simplExprF' env (Type ty) cont
678 = ASSERT( contIsRhsOrArg cont )
679 do { ty' <- simplType env ty
680 ; rebuild env (Type ty') cont }
682 simplExprF' env (Case scrut bndr case_ty alts) cont
683 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
684 = -- Simplify the scrutinee with a Select continuation
685 simplExprF env scrut (Select NoDup bndr alts env cont)
688 = -- If case-of-case is off, simply simplify the case expression
689 -- in a vanilla Stop context, and rebuild the result around it
690 do { case_expr' <- simplExprC env scrut case_cont
691 ; rebuild env case_expr' cont }
693 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
694 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
696 simplExprF' env (Let (Rec pairs) body) cont
697 = do { env <- simplRecBndrs env (map fst pairs)
698 -- NB: bndrs' don't have unfoldings or rules
699 -- We add them as we go down
701 ; env <- simplRecBind env NotTopLevel pairs
702 ; simplExprF env body cont }
704 simplExprF' env (Let (NonRec bndr rhs) body) cont
705 = simplNonRecE env bndr (rhs, env) ([], body) cont
707 ---------------------------------
708 simplType :: SimplEnv -> InType -> SimplM OutType
709 -- Kept monadic just so we can do the seqType
711 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
712 seqType new_ty `seq` returnSmpl new_ty
714 new_ty = substTy env ty
718 %************************************************************************
720 \subsection{The main rebuilder}
722 %************************************************************************
725 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
726 -- At this point the substitution in the SimplEnv should be irrelevant
727 -- only the in-scope set and floats should matter
728 rebuild env expr cont
729 = -- pprTrace "rebuild" (ppr expr $$ ppr cont $$ ppr (seFloats env)) $
731 Stop {} -> return (env, expr)
732 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
733 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
734 StrictArg fun ty info cont -> rebuildCall env (fun `App` expr) (funResultTy ty) info cont
735 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
736 ; simplLam env' bs body cont }
737 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
738 ; rebuild env (App expr arg') cont }
742 %************************************************************************
746 %************************************************************************
749 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
750 -> SimplM (SimplEnv, OutExpr)
751 simplCast env body co cont
752 = do { co' <- simplType env co
753 ; simplExprF env body (addCoerce co' cont) }
755 addCoerce co cont = add_coerce co (coercionKind co) cont
757 add_coerce co (s1, k1) cont -- co :: ty~ty
758 | s1 `coreEqType` k1 = cont -- is a no-op
760 add_coerce co1 (s1, k2) (CoerceIt co2 cont)
761 | (l1, t1) <- coercionKind co2
762 -- coerce T1 S1 (coerce S1 K1 e)
765 -- coerce T1 K1 e, otherwise
767 -- For example, in the initial form of a worker
768 -- we may find (coerce T (coerce S (\x.e))) y
769 -- and we'd like it to simplify to e[y/x] in one round
771 , s1 `coreEqType` t1 = cont -- The coerces cancel out
772 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
774 add_coerce co (s1s2, t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
775 -- (f `cast` g) ty ---> (f ty) `cast` (g @ ty)
776 -- This implements the PushT rule from the paper
777 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
778 , not (isCoVar tyvar)
779 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
781 ty' = substTy arg_se arg_ty
783 -- ToDo: the PushC rule is not implemented at all
785 add_coerce co (s1s2, t1t2) (ApplyTo dup arg arg_se cont)
786 | not (isTypeArg arg) -- This implements the Push rule from the paper
787 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
788 -- co : s1s2 :=: t1t2
789 -- (coerce (T1->T2) (S1->S2) F) E
791 -- coerce T2 S2 (F (coerce S1 T1 E))
793 -- t1t2 must be a function type, T1->T2, because it's applied
794 -- to something but s1s2 might conceivably not be
796 -- When we build the ApplyTo we can't mix the out-types
797 -- with the InExpr in the argument, so we simply substitute
798 -- to make it all consistent. It's a bit messy.
799 -- But it isn't a common case.
801 -- Example of use: Trac #995
802 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
804 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
805 -- t2 :=: s2 with left and right on the curried form:
806 -- (->) t1 t2 :=: (->) s1 s2
807 [co1, co2] = decomposeCo 2 co
808 new_arg = mkCoerce (mkSymCoercion co1) arg'
809 arg' = substExpr arg_se arg
811 add_coerce co _ cont = CoerceIt co cont
815 %************************************************************************
819 %************************************************************************
822 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
823 -> SimplM (SimplEnv, OutExpr)
825 simplLam env [] body cont = simplExprF env body cont
827 -- Type-beta reduction
828 simplLam env (bndr:bndrs) body (ApplyTo _ (Type ty_arg) arg_se cont)
829 = ASSERT( isTyVar bndr )
830 do { tick (BetaReduction bndr)
831 ; ty_arg' <- simplType (arg_se `setInScope` env) ty_arg
832 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
834 -- Ordinary beta reduction
835 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
836 = do { tick (BetaReduction bndr)
837 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
839 -- Not enough args, so there are real lambdas left to put in the result
840 simplLam env bndrs body cont
841 = do { (env, bndrs') <- simplLamBndrs env bndrs
842 ; body' <- simplExpr env body
843 ; new_lam <- mkLam bndrs' body'
844 ; rebuild env new_lam cont }
847 simplNonRecE :: SimplEnv
848 -> InId -- The binder
849 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
850 -> ([InId], InExpr) -- Body of the let/lambda
853 -> SimplM (SimplEnv, OutExpr)
855 -- simplNonRecE is used for
856 -- * non-top-level non-recursive lets in expressions
859 -- It deals with strict bindings, via the StrictBind continuation,
860 -- which may abort the whole process
862 -- The "body" of the binding comes as a pair of ([InId],InExpr)
863 -- representing a lambda; so we recurse back to simplLam
864 -- Why? Because of the binder-occ-info-zapping done before
865 -- the call to simplLam in simplExprF (Lam ...)
867 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
868 | preInlineUnconditionally env NotTopLevel bndr rhs
869 = do { tick (PreInlineUnconditionally bndr)
870 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
873 = do { simplExprF (rhs_se `setFloats` env) rhs
874 (StrictBind bndr bndrs body env cont) }
877 = do { (env, bndr') <- simplBinder env bndr
878 ; env <- simplLazyBind env NotTopLevel NonRecursive bndr bndr' rhs rhs_se
879 ; simplLam env bndrs body cont }
883 %************************************************************************
887 %************************************************************************
890 -- Hack alert: we only distinguish subsumed cost centre stacks for the
891 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
892 simplNote env (SCC cc) e cont
893 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
894 ; rebuild env (mkSCC cc e') cont }
896 -- See notes with SimplMonad.inlineMode
897 simplNote env InlineMe e cont
898 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
899 = do { -- Don't inline inside an INLINE expression
900 e' <- simplExpr (setMode inlineMode env) e
901 ; rebuild env (mkInlineMe e') cont }
903 | otherwise -- Dissolve the InlineMe note if there's
904 -- an interesting context of any kind to combine with
905 -- (even a type application -- anything except Stop)
906 = simplExprF env e cont
908 simplNote env (CoreNote s) e cont
909 = simplExpr env e `thenSmpl` \ e' ->
910 rebuild env (Note (CoreNote s) e') cont
914 %************************************************************************
916 \subsection{Dealing with calls}
918 %************************************************************************
921 simplVar env var cont
922 = case substId env var of
923 DoneEx e -> simplExprF (zapSubstEnv env) e cont
924 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
925 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
926 -- Note [zapSubstEnv]
927 -- The template is already simplified, so don't re-substitute.
928 -- This is VITAL. Consider
930 -- let y = \z -> ...x... in
932 -- We'll clone the inner \x, adding x->x' in the id_subst
933 -- Then when we inline y, we must *not* replace x by x' in
934 -- the inlined copy!!
936 ---------------------------------------------------------
937 -- Dealing with a call site
939 completeCall env var cont
940 = do { dflags <- getDOptsSmpl
941 ; let (args,call_cont) = contArgs cont
942 -- The args are OutExprs, obtained by *lazily* substituting
943 -- in the args found in cont. These args are only examined
944 -- to limited depth (unless a rule fires). But we must do
945 -- the substitution; rule matching on un-simplified args would
948 ------------- First try rules ----------------
949 -- Do this before trying inlining. Some functions have
950 -- rules *and* are strict; in this case, we don't want to
951 -- inline the wrapper of the non-specialised thing; better
952 -- to call the specialised thing instead.
954 -- We used to use the black-listing mechanism to ensure that inlining of
955 -- the wrapper didn't occur for things that have specialisations till a
956 -- later phase, so but now we just try RULES first
958 -- You might think that we shouldn't apply rules for a loop breaker:
959 -- doing so might give rise to an infinite loop, because a RULE is
960 -- rather like an extra equation for the function:
961 -- RULE: f (g x) y = x+y
964 -- But it's too drastic to disable rules for loop breakers.
965 -- Even the foldr/build rule would be disabled, because foldr
966 -- is recursive, and hence a loop breaker:
967 -- foldr k z (build g) = g k z
968 -- So it's up to the programmer: rules can cause divergence
969 ; let in_scope = getInScope env
971 maybe_rule = case activeRule env of
972 Nothing -> Nothing -- No rules apply
973 Just act_fn -> lookupRule act_fn in_scope
975 ; case maybe_rule of {
976 Just (rule, rule_rhs) ->
977 tick (RuleFired (ru_name rule)) `thenSmpl_`
978 (if dopt Opt_D_dump_rule_firings dflags then
979 pprTrace "Rule fired" (vcat [
980 text "Rule:" <+> ftext (ru_name rule),
981 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
982 text "After: " <+> pprCoreExpr rule_rhs,
983 text "Cont: " <+> ppr call_cont])
986 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
987 -- The ruleArity says how many args the rule consumed
989 ; Nothing -> do -- No rules
991 ------------- Next try inlining ----------------
992 { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
993 n_val_args = length arg_infos
994 interesting_cont = interestingCallContext (notNull args)
997 active_inline = activeInline env var
998 maybe_inline = callSiteInline dflags active_inline
999 var arg_infos interesting_cont
1000 ; case maybe_inline of {
1001 Just unfolding -- There is an inlining!
1002 -> do { tick (UnfoldingDone var)
1003 ; (if dopt Opt_D_dump_inlinings dflags then
1004 pprTrace "Inlining done" (vcat [
1005 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1006 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1007 text "Cont: " <+> ppr call_cont])
1010 simplExprF env unfolding cont }
1012 ; Nothing -> -- No inlining!
1014 ------------- No inlining! ----------------
1015 -- Next, look for rules or specialisations that match
1017 rebuildCall env (Var var) (idType var)
1018 (mkArgInfo var n_val_args call_cont) cont
1021 rebuildCall :: SimplEnv
1022 -> OutExpr -> OutType -- Function and its type
1023 -> (Bool, [Bool]) -- See SimplUtils.mkArgInfo
1025 -> SimplM (SimplEnv, OutExpr)
1026 rebuildCall env fun fun_ty (has_rules, []) cont
1027 -- When we run out of strictness args, it means
1028 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1029 -- Then we want to discard the entire strict continuation. E.g.
1030 -- * case (error "hello") of { ... }
1031 -- * (error "Hello") arg
1032 -- * f (error "Hello") where f is strict
1034 -- Then, especially in the first of these cases, we'd like to discard
1035 -- the continuation, leaving just the bottoming expression. But the
1036 -- type might not be right, so we may have to add a coerce.
1037 | not (contIsTrivial cont) -- Only do thia if there is a non-trivial
1038 = return (env, mk_coerce fun) -- contination to discard, else we do it
1039 where -- again and again!
1040 cont_ty = contResultType cont
1041 co = mkUnsafeCoercion fun_ty cont_ty
1042 mk_coerce expr | cont_ty `coreEqType` fun_ty = fun
1043 | otherwise = mkCoerce co fun
1045 rebuildCall env fun fun_ty info (ApplyTo _ (Type arg_ty) se cont)
1046 = do { ty' <- simplType (se `setInScope` env) arg_ty
1047 ; rebuildCall env (fun `App` Type ty') (applyTy fun_ty ty') info cont }
1049 rebuildCall env fun fun_ty (has_rules, str:strs) (ApplyTo _ arg arg_se cont)
1050 | str || isStrictType arg_ty -- Strict argument
1051 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1052 simplExprF (arg_se `setFloats` env) arg
1053 (StrictArg fun fun_ty (has_rules, strs) cont)
1056 | otherwise -- Lazy argument
1057 -- DO NOT float anything outside, hence simplExprC
1058 -- There is no benefit (unlike in a let-binding), and we'd
1059 -- have to be very careful about bogus strictness through
1060 -- floating a demanded let.
1061 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1062 (mkLazyArgStop arg_ty has_rules)
1063 ; rebuildCall env (fun `App` arg') res_ty (has_rules, strs) cont }
1065 (arg_ty, res_ty) = splitFunTy fun_ty
1067 rebuildCall env fun fun_ty info cont
1068 = rebuild env fun cont
1073 This part of the simplifier may break the no-shadowing invariant
1075 f (...(\a -> e)...) (case y of (a,b) -> e')
1076 where f is strict in its second arg
1077 If we simplify the innermost one first we get (...(\a -> e)...)
1078 Simplifying the second arg makes us float the case out, so we end up with
1079 case y of (a,b) -> f (...(\a -> e)...) e'
1080 So the output does not have the no-shadowing invariant. However, there is
1081 no danger of getting name-capture, because when the first arg was simplified
1082 we used an in-scope set that at least mentioned all the variables free in its
1083 static environment, and that is enough.
1085 We can't just do innermost first, or we'd end up with a dual problem:
1086 case x of (a,b) -> f e (...(\a -> e')...)
1088 I spent hours trying to recover the no-shadowing invariant, but I just could
1089 not think of an elegant way to do it. The simplifier is already knee-deep in
1090 continuations. We have to keep the right in-scope set around; AND we have
1091 to get the effect that finding (error "foo") in a strict arg position will
1092 discard the entire application and replace it with (error "foo"). Getting
1093 all this at once is TOO HARD!
1095 %************************************************************************
1097 Rebuilding a cse expression
1099 %************************************************************************
1101 Blob of helper functions for the "case-of-something-else" situation.
1104 ---------------------------------------------------------
1105 -- Eliminate the case if possible
1107 rebuildCase :: SimplEnv
1108 -> OutExpr -- Scrutinee
1109 -> InId -- Case binder
1110 -> [InAlt] -- Alternatives (inceasing order)
1112 -> SimplM (SimplEnv, OutExpr)
1114 --------------------------------------------------
1115 -- 1. Eliminate the case if there's a known constructor
1116 --------------------------------------------------
1118 rebuildCase env scrut case_bndr alts cont
1119 | Just (con,args) <- exprIsConApp_maybe scrut
1120 -- Works when the scrutinee is a variable with a known unfolding
1121 -- as well as when it's an explicit constructor application
1122 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1124 | Lit lit <- scrut -- No need for same treatment as constructors
1125 -- because literals are inlined more vigorously
1126 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1129 --------------------------------------------------
1130 -- 2. Eliminate the case if scrutinee is evaluated
1131 --------------------------------------------------
1133 rebuildCase env scrut case_bndr [(con,bndrs,rhs)] cont
1134 -- See if we can get rid of the case altogether
1135 -- See the extensive notes on case-elimination above
1136 -- mkCase made sure that if all the alternatives are equal,
1137 -- then there is now only one (DEFAULT) rhs
1138 | all isDeadBinder bndrs -- bndrs are [InId]
1140 -- Check that the scrutinee can be let-bound instead of case-bound
1141 , exprOkForSpeculation scrut
1142 -- OK not to evaluate it
1143 -- This includes things like (==# a# b#)::Bool
1144 -- so that we simplify
1145 -- case ==# a# b# of { True -> x; False -> x }
1148 -- This particular example shows up in default methods for
1149 -- comparision operations (e.g. in (>=) for Int.Int32)
1150 || exprIsHNF scrut -- It's already evaluated
1151 || var_demanded_later scrut -- It'll be demanded later
1153 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1154 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1155 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1156 -- its argument: case x of { y -> dataToTag# y }
1157 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1158 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1160 -- Also we don't want to discard 'seq's
1161 = do { tick (CaseElim case_bndr)
1162 ; env <- simplNonRecX env case_bndr scrut
1163 ; simplExprF env rhs cont }
1165 -- The case binder is going to be evaluated later,
1166 -- and the scrutinee is a simple variable
1167 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1168 var_demanded_later other = False
1171 --------------------------------------------------
1172 -- 3. Catch-all case
1173 --------------------------------------------------
1175 rebuildCase env scrut case_bndr alts cont
1176 = do { -- Prepare the continuation;
1177 -- The new subst_env is in place
1178 (env, dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1180 -- Simplify the alternatives
1181 ; (case_bndr', alts') <- simplAlts env scrut case_bndr alts dup_cont
1182 ; let res_ty' = contResultType dup_cont
1183 ; case_expr <- mkCase scrut case_bndr' res_ty' alts'
1185 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1186 -- The case binder *not* scope over the whole returned case-expression
1187 ; rebuild env case_expr nodup_cont }
1190 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1191 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1192 way, there's a chance that v will now only be used once, and hence
1195 Note [no-case-of-case]
1196 ~~~~~~~~~~~~~~~~~~~~~~
1197 There is a time we *don't* want to do that, namely when
1198 -fno-case-of-case is on. This happens in the first simplifier pass,
1199 and enhances full laziness. Here's the bad case:
1200 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1201 If we eliminate the inner case, we trap it inside the I# v -> arm,
1202 which might prevent some full laziness happening. I've seen this
1203 in action in spectral/cichelli/Prog.hs:
1204 [(m,n) | m <- [1..max], n <- [1..max]]
1205 Hence the check for NoCaseOfCase.
1207 Note [Suppressing the case binder-swap]
1208 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1209 There is another situation when it might make sense to suppress the
1210 case-expression binde-swap. If we have
1212 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1213 ...other cases .... }
1215 We'll perform the binder-swap for the outer case, giving
1217 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1218 ...other cases .... }
1220 But there is no point in doing it for the inner case, because w1 can't
1221 be inlined anyway. Furthermore, doing the case-swapping involves
1222 zapping w2's occurrence info (see paragraphs that follow), and that
1223 forces us to bind w2 when doing case merging. So we get
1225 case x of w1 { A -> let w2 = w1 in e1
1226 B -> let w2 = w1 in e2
1227 ...other cases .... }
1229 This is plain silly in the common case where w2 is dead.
1231 Even so, I can't see a good way to implement this idea. I tried
1232 not doing the binder-swap if the scrutinee was already evaluated
1233 but that failed big-time:
1237 case v of w { MkT x ->
1238 case x of x1 { I# y1 ->
1239 case x of x2 { I# y2 -> ...
1241 Notice that because MkT is strict, x is marked "evaluated". But to
1242 eliminate the last case, we must either make sure that x (as well as
1243 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1244 the binder-swap. So this whole note is a no-op.
1248 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1249 any occurrence info (eg IAmDead) in the case binder, because the
1250 case-binder now effectively occurs whenever v does. AND we have to do
1251 the same for the pattern-bound variables! Example:
1253 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1255 Here, b and p are dead. But when we move the argment inside the first
1256 case RHS, and eliminate the second case, we get
1258 case x of { (a,b) -> a b }
1260 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1263 Indeed, this can happen anytime the case binder isn't dead:
1264 case <any> of x { (a,b) ->
1265 case x of { (p,q) -> p } }
1266 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1267 The point is that we bring into the envt a binding
1269 after the outer case, and that makes (a,b) alive. At least we do unless
1270 the case binder is guaranteed dead.
1274 Consider case (v `cast` co) of x { I# ->
1275 ... (case (v `cast` co) of {...}) ...
1276 We'd like to eliminate the inner case. We can get this neatly by
1277 arranging that inside the outer case we add the unfolding
1278 v |-> x `cast` (sym co)
1279 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1282 Note [Case elimination]
1283 ~~~~~~~~~~~~~~~~~~~~~~~
1284 The case-elimination transformation discards redundant case expressions.
1285 Start with a simple situation:
1287 case x# of ===> e[x#/y#]
1290 (when x#, y# are of primitive type, of course). We can't (in general)
1291 do this for algebraic cases, because we might turn bottom into
1294 The code in SimplUtils.prepareAlts has the effect of generalise this
1295 idea to look for a case where we're scrutinising a variable, and we
1296 know that only the default case can match. For example:
1300 DEFAULT -> ...(case x of
1304 Here the inner case is first trimmed to have only one alternative, the
1305 DEFAULT, after which it's an instance of the previous case. This
1306 really only shows up in eliminating error-checking code.
1308 We also make sure that we deal with this very common case:
1313 Here we are using the case as a strict let; if x is used only once
1314 then we want to inline it. We have to be careful that this doesn't
1315 make the program terminate when it would have diverged before, so we
1317 - e is already evaluated (it may so if e is a variable)
1318 - x is used strictly, or
1320 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1322 case e of ===> case e of DEFAULT -> r
1326 Now again the case may be elminated by the CaseElim transformation.
1329 Further notes about case elimination
1330 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1331 Consider: test :: Integer -> IO ()
1334 Turns out that this compiles to:
1337 eta1 :: State# RealWorld ->
1338 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1340 (PrelNum.jtos eta ($w[] @ Char))
1342 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1344 Notice the strange '<' which has no effect at all. This is a funny one.
1345 It started like this:
1347 f x y = if x < 0 then jtos x
1348 else if y==0 then "" else jtos x
1350 At a particular call site we have (f v 1). So we inline to get
1352 if v < 0 then jtos x
1353 else if 1==0 then "" else jtos x
1355 Now simplify the 1==0 conditional:
1357 if v<0 then jtos v else jtos v
1359 Now common-up the two branches of the case:
1361 case (v<0) of DEFAULT -> jtos v
1363 Why don't we drop the case? Because it's strict in v. It's technically
1364 wrong to drop even unnecessary evaluations, and in practice they
1365 may be a result of 'seq' so we *definitely* don't want to drop those.
1366 I don't really know how to improve this situation.
1370 simplCaseBinder :: SimplEnv -> OutExpr -> InId -> SimplM (SimplEnv, OutId)
1371 simplCaseBinder env scrut case_bndr
1372 | switchIsOn (getSwitchChecker env) NoCaseOfCase
1373 -- See Note [no-case-of-case]
1374 = do { (env, case_bndr') <- simplBinder env case_bndr
1375 ; return (env, case_bndr') }
1377 simplCaseBinder env (Var v) case_bndr
1378 -- Failed try [see Note 2 above]
1379 -- not (isEvaldUnfolding (idUnfolding v))
1380 = do { (env, case_bndr') <- simplBinder env (zapOccInfo case_bndr)
1381 ; return (modifyInScope env v case_bndr', case_bndr') }
1382 -- We could extend the substitution instead, but it would be
1383 -- a hack because then the substitution wouldn't be idempotent
1384 -- any more (v is an OutId). And this does just as well.
1386 simplCaseBinder env (Cast (Var v) co) case_bndr -- Note [Case of cast]
1387 = do { (env, case_bndr') <- simplBinder env (zapOccInfo case_bndr)
1388 ; let rhs = Cast (Var case_bndr') (mkSymCoercion co)
1389 ; return (addBinderUnfolding env v rhs, case_bndr') }
1391 simplCaseBinder env other_scrut case_bndr
1392 = do { (env, case_bndr') <- simplBinder env case_bndr
1393 ; return (env, case_bndr') }
1395 zapOccInfo :: InId -> InId -- See Note [zapOccInfo]
1396 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1400 simplAlts does two things:
1402 1. Eliminate alternatives that cannot match, including the
1403 DEFAULT alternative.
1405 2. If the DEFAULT alternative can match only one possible constructor,
1406 then make that constructor explicit.
1408 case e of x { DEFAULT -> rhs }
1410 case e of x { (a,b) -> rhs }
1411 where the type is a single constructor type. This gives better code
1412 when rhs also scrutinises x or e.
1414 Here "cannot match" includes knowledge from GADTs
1416 It's a good idea do do this stuff before simplifying the alternatives, to
1417 avoid simplifying alternatives we know can't happen, and to come up with
1418 the list of constructors that are handled, to put into the IdInfo of the
1419 case binder, for use when simplifying the alternatives.
1421 Eliminating the default alternative in (1) isn't so obvious, but it can
1424 data Colour = Red | Green | Blue
1433 DEFAULT -> [ case y of ... ]
1435 If we inline h into f, the default case of the inlined h can't happen.
1436 If we don't notice this, we may end up filtering out *all* the cases
1437 of the inner case y, which give us nowhere to go!
1441 simplAlts :: SimplEnv
1443 -> InId -- Case binder
1444 -> [InAlt] -> SimplCont
1445 -> SimplM (OutId, [OutAlt]) -- Includes the continuation
1446 -- Like simplExpr, this just returns the simplified alternatives;
1447 -- it not return an environment
1449 simplAlts env scrut case_bndr alts cont'
1450 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1451 do { let alt_env = zapFloats env
1452 ; (alt_env, case_bndr') <- simplCaseBinder alt_env scrut case_bndr
1454 ; (imposs_deflt_cons, in_alts) <- prepareAlts scrut case_bndr' alts
1456 ; alts' <- mapM (simplAlt alt_env imposs_deflt_cons case_bndr' cont') in_alts
1457 ; return (case_bndr', alts') }
1459 ------------------------------------
1460 simplAlt :: SimplEnv
1461 -> [AltCon] -- These constructors can't be present when
1462 -- matching the DEFAULT alternative
1463 -> OutId -- The case binder
1468 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1469 = ASSERT( null bndrs )
1470 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1471 -- Record the constructors that the case-binder *can't* be.
1472 ; rhs' <- simplExprC env' rhs cont'
1473 ; return (DEFAULT, [], rhs') }
1475 simplAlt env imposs_deflt_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1476 = ASSERT( null bndrs )
1477 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1478 ; rhs' <- simplExprC env' rhs cont'
1479 ; return (LitAlt lit, [], rhs') }
1481 simplAlt env imposs_deflt_cons case_bndr' cont' (DataAlt con, vs, rhs)
1482 = do { -- Deal with the pattern-bound variables
1483 (env, vs') <- simplBinders env (add_evals con vs)
1485 -- Mark the ones that are in ! positions in the
1486 -- data constructor as certainly-evaluated.
1487 ; let vs'' = add_evals con vs'
1489 -- Bind the case-binder to (con args)
1490 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1491 con_args = map Type inst_tys' ++ varsToCoreExprs vs''
1492 env' = addBinderUnfolding env case_bndr' (mkConApp con con_args)
1494 ; rhs' <- simplExprC env' rhs cont'
1495 ; return (DataAlt con, vs'', rhs') }
1497 -- add_evals records the evaluated-ness of the bound variables of
1498 -- a case pattern. This is *important*. Consider
1499 -- data T = T !Int !Int
1501 -- case x of { T a b -> T (a+1) b }
1503 -- We really must record that b is already evaluated so that we don't
1504 -- go and re-evaluate it when constructing the result.
1505 -- See Note [Data-con worker strictness] in MkId.lhs
1506 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1508 cat_evals dc vs strs
1512 go (v:vs) strs | isTyVar v = v : go vs strs
1513 go (v:vs) (str:strs)
1514 | isMarkedStrict str = evald_v : go vs strs
1515 | otherwise = zapped_v : go vs strs
1517 zapped_v = zap_occ_info v
1518 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1519 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1521 -- If the case binder is alive, then we add the unfolding
1523 -- to the envt; so vs are now very much alive
1524 -- Note [Aug06] I can't see why this actually matters
1525 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1526 | otherwise = zapOccInfo
1528 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1529 addBinderUnfolding env bndr rhs
1530 = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
1532 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1533 addBinderOtherCon env bndr cons
1534 = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
1538 %************************************************************************
1540 \subsection{Known constructor}
1542 %************************************************************************
1544 We are a bit careful with occurrence info. Here's an example
1546 (\x* -> case x of (a*, b) -> f a) (h v, e)
1548 where the * means "occurs once". This effectively becomes
1549 case (h v, e) of (a*, b) -> f a)
1551 let a* = h v; b = e in f a
1555 All this should happen in one sweep.
1558 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1559 -> InId -> [InAlt] -> SimplCont
1560 -> SimplM (SimplEnv, OutExpr)
1562 knownCon env scrut con args bndr alts cont
1563 = do { tick (KnownBranch bndr)
1564 ; knownAlt env scrut args bndr (findAlt con alts) cont }
1566 knownAlt env scrut args bndr (DEFAULT, bs, rhs) cont
1568 do { env <- simplNonRecX env bndr scrut
1569 -- This might give rise to a binding with non-atomic args
1570 -- like x = Node (f x) (g x)
1571 -- but simplNonRecX will atomic-ify it
1572 ; simplExprF env rhs cont }
1574 knownAlt env scrut args bndr (LitAlt lit, bs, rhs) cont
1576 do { env <- simplNonRecX env bndr scrut
1577 ; simplExprF env rhs cont }
1579 knownAlt env scrut args bndr (DataAlt dc, bs, rhs) cont
1580 = do { let dead_bndr = isDeadBinder bndr -- bndr is an InId
1581 n_drop_tys = length (dataConUnivTyVars dc)
1582 ; env <- bind_args env dead_bndr bs (drop n_drop_tys args)
1584 -- It's useful to bind bndr to scrut, rather than to a fresh
1585 -- binding x = Con arg1 .. argn
1586 -- because very often the scrut is a variable, so we avoid
1587 -- creating, and then subsequently eliminating, a let-binding
1588 -- BUT, if scrut is a not a variable, we must be careful
1589 -- about duplicating the arg redexes; in that case, make
1590 -- a new con-app from the args
1591 bndr_rhs = case scrut of
1594 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1595 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1596 -- args are aready OutExprs, but bs are InIds
1598 ; env <- simplNonRecX env bndr bndr_rhs
1599 ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env)) $
1600 simplExprF env rhs cont }
1603 bind_args env dead_bndr [] _ = return env
1605 bind_args env dead_bndr (b:bs) (Type ty : args)
1606 = ASSERT( isTyVar b )
1607 bind_args (extendTvSubst env b ty) dead_bndr bs args
1609 bind_args env dead_bndr (b:bs) (arg : args)
1611 do { let b' = if dead_bndr then b else zapOccInfo b
1612 -- Note that the binder might be "dead", because it doesn't occur
1613 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1614 -- Nevertheless we must keep it if the case-binder is alive, because it may
1615 -- be used in the con_app. See Note [zapOccInfo]
1616 ; env <- simplNonRecX env b' arg
1617 ; bind_args env dead_bndr bs args }
1619 bind_args _ _ _ _ = panic "bind_args"
1623 %************************************************************************
1625 \subsection{Duplicating continuations}
1627 %************************************************************************
1630 prepareCaseCont :: SimplEnv
1631 -> [InAlt] -> SimplCont
1632 -> SimplM (SimplEnv, SimplCont,SimplCont)
1633 -- Return a duplicatable continuation, a non-duplicable part
1634 -- plus some extra bindings (that scope over the entire
1637 -- No need to make it duplicatable if there's only one alternative
1638 prepareCaseCont env [alt] cont = return (env, cont, mkBoringStop (contResultType cont))
1639 prepareCaseCont env alts cont = mkDupableCont env cont
1643 mkDupableCont :: SimplEnv -> SimplCont
1644 -> SimplM (SimplEnv, SimplCont, SimplCont)
1646 mkDupableCont env cont
1647 | contIsDupable cont
1648 = returnSmpl (env, cont, mkBoringStop (contResultType cont))
1650 mkDupableCont env (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1652 mkDupableCont env (CoerceIt ty cont)
1653 = do { (env, dup, nodup) <- mkDupableCont env cont
1654 ; return (env, CoerceIt ty dup, nodup) }
1656 mkDupableCont env cont@(StrictBind bndr _ _ se _)
1657 = return (env, mkBoringStop (substTy se (idType bndr)), cont)
1658 -- See Note [Duplicating strict continuations]
1660 mkDupableCont env cont@(StrictArg _ fun_ty _ _)
1661 = return (env, mkBoringStop (funArgTy fun_ty), cont)
1662 -- See Note [Duplicating strict continuations]
1664 mkDupableCont env (ApplyTo _ arg se cont)
1665 = -- e.g. [...hole...] (...arg...)
1667 -- let a = ...arg...
1668 -- in [...hole...] a
1669 do { (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1670 ; arg <- simplExpr (se `setInScope` env) arg
1671 ; (env, arg) <- makeTrivial env arg
1672 ; let app_cont = ApplyTo OkToDup arg (zapSubstEnv env) dup_cont
1673 ; return (env, app_cont, nodup_cont) }
1675 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1676 -- See Note [Single-alternative case]
1677 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1678 -- | not (isDeadBinder case_bndr)
1679 | all isDeadBinder bs -- InIds
1680 = return (env, mkBoringStop scrut_ty, cont)
1682 scrut_ty = substTy se (idType case_bndr)
1684 mkDupableCont env (Select _ case_bndr alts se cont)
1685 = -- e.g. (case [...hole...] of { pi -> ei })
1687 -- let ji = \xij -> ei
1688 -- in case [...hole...] of { pi -> ji xij }
1689 do { tick (CaseOfCase case_bndr)
1690 ; (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1691 -- NB: call mkDupableCont here, *not* prepareCaseCont
1692 -- We must make a duplicable continuation, whereas prepareCaseCont
1693 -- doesn't when there is a single case branch
1695 ; let alt_env = se `setInScope` env
1696 ; (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1697 ; alts' <- mapM (simplAlt alt_env [] case_bndr' dup_cont) alts
1698 -- Safe to say that there are no handled-cons for the DEFAULT case
1699 -- NB: simplBinder does not zap deadness occ-info, so
1700 -- a dead case_bndr' will still advertise its deadness
1701 -- This is really important because in
1702 -- case e of b { (# p,q #) -> ... }
1703 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1704 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1705 -- In the new alts we build, we have the new case binder, so it must retain
1707 -- NB: we don't use alt_env further; it has the substEnv for
1708 -- the alternatives, and we don't want that
1710 ; (env, alts') <- mkDupableAlts env case_bndr' alts'
1711 ; return (env, -- Note [Duplicated env]
1712 Select OkToDup case_bndr' alts' (zapSubstEnv env)
1713 (mkBoringStop (contResultType dup_cont)),
1717 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1718 -> SimplM (SimplEnv, [InAlt])
1719 -- Absorbs the continuation into the new alternatives
1721 mkDupableAlts env case_bndr' alts
1724 go env [] = return (env, [])
1726 = do { (env, alt') <- mkDupableAlt env case_bndr' alt
1727 ; (env, alts') <- go env alts
1728 ; return (env, alt' : alts' ) }
1730 mkDupableAlt env case_bndr' (con, bndrs', rhs')
1731 | exprIsDupable rhs' -- Note [Small alternative rhs]
1732 = return (env, (con, bndrs', rhs'))
1734 = do { let rhs_ty' = exprType rhs'
1735 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1737 | isTyVar bndr = True -- Abstract over all type variables just in case
1738 | otherwise = not (isDeadBinder bndr)
1739 -- The deadness info on the new Ids is preserved by simplBinders
1741 ; (final_bndrs', final_args) -- Note [Join point abstraction]
1742 <- if (any isId used_bndrs')
1743 then return (used_bndrs', varsToCoreExprs used_bndrs')
1744 else do { rw_id <- newId FSLIT("w") realWorldStatePrimTy
1745 ; return ([rw_id], [Var realWorldPrimId]) }
1747 ; join_bndr <- newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty')
1748 -- Note [Funky mkPiTypes]
1750 ; let -- We make the lambdas into one-shot-lambdas. The
1751 -- join point is sure to be applied at most once, and doing so
1752 -- prevents the body of the join point being floated out by
1753 -- the full laziness pass
1754 really_final_bndrs = map one_shot final_bndrs'
1755 one_shot v | isId v = setOneShotLambda v
1757 join_rhs = mkLams really_final_bndrs rhs'
1758 join_call = mkApps (Var join_bndr) final_args
1760 ; return (addNonRec env join_bndr join_rhs, (con, bndrs', join_call)) }
1761 -- See Note [Duplicated env]
1764 Note [Duplicated env]
1765 ~~~~~~~~~~~~~~~~~~~~~
1766 Some of the alternatives are simplified, but have not been turned into a join point
1767 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
1768 bind the join point, because it might to do PostInlineUnconditionally, and
1769 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
1770 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
1771 at worst delays the join-point inlining.
1773 Note [Small alterantive rhs]
1774 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1775 It is worth checking for a small RHS because otherwise we
1776 get extra let bindings that may cause an extra iteration of the simplifier to
1777 inline back in place. Quite often the rhs is just a variable or constructor.
1778 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1779 iterations because the version with the let bindings looked big, and so wasn't
1780 inlined, but after the join points had been inlined it looked smaller, and so
1783 NB: we have to check the size of rhs', not rhs.
1784 Duplicating a small InAlt might invalidate occurrence information
1785 However, if it *is* dupable, we return the *un* simplified alternative,
1786 because otherwise we'd need to pair it up with an empty subst-env....
1787 but we only have one env shared between all the alts.
1788 (Remember we must zap the subst-env before re-simplifying something).
1789 Rather than do this we simply agree to re-simplify the original (small) thing later.
1791 Note [Funky mkPiTypes]
1792 ~~~~~~~~~~~~~~~~~~~~~~
1793 Notice the funky mkPiTypes. If the contructor has existentials
1794 it's possible that the join point will be abstracted over
1795 type varaibles as well as term variables.
1796 Example: Suppose we have
1797 data T = forall t. C [t]
1799 case (case e of ...) of
1801 We get the join point
1802 let j :: forall t. [t] -> ...
1803 j = /\t \xs::[t] -> rhs
1805 case (case e of ...) of
1806 C t xs::[t] -> j t xs
1808 Note [Join point abstaction]
1809 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1810 If we try to lift a primitive-typed something out
1811 for let-binding-purposes, we will *caseify* it (!),
1812 with potentially-disastrous strictness results. So
1813 instead we turn it into a function: \v -> e
1814 where v::State# RealWorld#. The value passed to this function
1815 is realworld#, which generates (almost) no code.
1817 There's a slight infelicity here: we pass the overall
1818 case_bndr to all the join points if it's used in *any* RHS,
1819 because we don't know its usage in each RHS separately
1821 We used to say "&& isUnLiftedType rhs_ty'" here, but now
1822 we make the join point into a function whenever used_bndrs'
1823 is empty. This makes the join-point more CPR friendly.
1824 Consider: let j = if .. then I# 3 else I# 4
1825 in case .. of { A -> j; B -> j; C -> ... }
1827 Now CPR doesn't w/w j because it's a thunk, so
1828 that means that the enclosing function can't w/w either,
1829 which is a lose. Here's the example that happened in practice:
1830 kgmod :: Int -> Int -> Int
1831 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1835 I have seen a case alternative like this:
1837 It's a bit silly to add the realWorld dummy arg in this case, making
1840 (the \v alone is enough to make CPR happy) but I think it's rare
1842 Note [Duplicating strict continuations]
1843 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1844 Do *not* duplicate StrictBind and StritArg continuations. We gain
1845 nothing by propagating them into the expressions, and we do lose a
1846 lot. Here's an example:
1847 && (case x of { T -> F; F -> T }) E
1848 Now, && is strict so we end up simplifying the case with
1849 an ArgOf continuation. If we let-bind it, we get
1851 let $j = \v -> && v E
1852 in simplExpr (case x of { T -> F; F -> T })
1854 And after simplifying more we get
1856 let $j = \v -> && v E
1857 in case x of { T -> $j F; F -> $j T }
1858 Which is a Very Bad Thing
1860 The desire not to duplicate is the entire reason that
1861 mkDupableCont returns a pair of continuations.
1863 The original plan had:
1864 e.g. (...strict-fn...) [...hole...]
1866 let $j = \a -> ...strict-fn...
1869 Note [Single-alternative cases]
1870 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1871 This case is just like the ArgOf case. Here's an example:
1875 case (case x of I# x' ->
1877 True -> I# (negate# x')
1878 False -> I# x') of y {
1880 Because the (case x) has only one alternative, we'll transform to
1882 case (case x' <# 0# of
1883 True -> I# (negate# x')
1884 False -> I# x') of y {
1886 But now we do *NOT* want to make a join point etc, giving
1888 let $j = \y -> MkT y
1890 True -> $j (I# (negate# x'))
1892 In this case the $j will inline again, but suppose there was a big
1893 strict computation enclosing the orginal call to MkT. Then, it won't
1894 "see" the MkT any more, because it's big and won't get duplicated.
1895 And, what is worse, nothing was gained by the case-of-case transform.
1897 When should use this case of mkDupableCont?
1898 However, matching on *any* single-alternative case is a *disaster*;
1899 e.g. case (case ....) of (a,b) -> (# a,b #)
1900 We must push the outer case into the inner one!
1903 * Match [(DEFAULT,_,_)], but in the common case of Int,
1904 the alternative-filling-in code turned the outer case into
1905 case (...) of y { I# _ -> MkT y }
1907 * Match on single alternative plus (not (isDeadBinder case_bndr))
1908 Rationale: pushing the case inwards won't eliminate the construction.
1909 But there's a risk of
1910 case (...) of y { (a,b) -> let z=(a,b) in ... }
1911 Now y looks dead, but it'll come alive again. Still, this
1912 seems like the best option at the moment.
1914 * Match on single alternative plus (all (isDeadBinder bndrs))
1915 Rationale: this is essentially seq.
1917 * Match when the rhs is *not* duplicable, and hence would lead to a
1918 join point. This catches the disaster-case above. We can test
1919 the *un-simplified* rhs, which is fine. It might get bigger or
1920 smaller after simplification; if it gets smaller, this case might
1921 fire next time round. NB also that we must test contIsDupable
1922 case_cont *btoo, because case_cont might be big!
1924 HOWEVER: I found that this version doesn't work well, because
1925 we can get let x = case (...) of { small } in ...case x...
1926 When x is inlined into its full context, we find that it was a bad
1927 idea to have pushed the outer case inside the (...) case.