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 ( dataConRepStrictness, dataConUnivTyVars )
22 import NewDemand ( isStrictDmd )
23 import PprCore ( pprParendExpr, pprCoreExpr )
24 import CoreUnfold ( mkUnfolding, callSiteInline )
26 import Rules ( lookupRule )
27 import BasicTypes ( isMarkedStrict )
28 import CostCentre ( currentCCS )
29 import TysPrim ( realWorldStatePrimTy )
30 import PrelInfo ( realWorldPrimId )
31 import BasicTypes ( TopLevelFlag(..), isTopLevel,
32 RecFlag(..), isNonRuleLoopBreaker )
33 import Maybes ( orElse )
39 The guts of the simplifier is in this module, but the driver loop for
40 the simplifier is in SimplCore.lhs.
43 -----------------------------------------
44 *** IMPORTANT NOTE ***
45 -----------------------------------------
46 The simplifier used to guarantee that the output had no shadowing, but
47 it does not do so any more. (Actually, it never did!) The reason is
48 documented with simplifyArgs.
51 -----------------------------------------
52 *** IMPORTANT NOTE ***
53 -----------------------------------------
54 Many parts of the simplifier return a bunch of "floats" as well as an
55 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
57 All "floats" are let-binds, not case-binds, but some non-rec lets may
58 be unlifted (with RHS ok-for-speculation).
62 -----------------------------------------
63 ORGANISATION OF FUNCTIONS
64 -----------------------------------------
66 - simplify all top-level binders
67 - for NonRec, call simplRecOrTopPair
68 - for Rec, call simplRecBind
71 ------------------------------
72 simplExpr (applied lambda) ==> simplNonRecBind
73 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
74 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
76 ------------------------------
77 simplRecBind [binders already simplfied]
78 - use simplRecOrTopPair on each pair in turn
80 simplRecOrTopPair [binder already simplified]
81 Used for: recursive bindings (top level and nested)
82 top-level non-recursive bindings
84 - check for PreInlineUnconditionally
88 Used for: non-top-level non-recursive bindings
89 beta reductions (which amount to the same thing)
90 Because it can deal with strict arts, it takes a
91 "thing-inside" and returns an expression
93 - check for PreInlineUnconditionally
94 - simplify binder, including its IdInfo
103 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
104 Used for: binding case-binder and constr args in a known-constructor case
105 - check for PreInLineUnconditionally
109 ------------------------------
110 simplLazyBind: [binder already simplified, RHS not]
111 Used for: recursive bindings (top level and nested)
112 top-level non-recursive bindings
113 non-top-level, but *lazy* non-recursive bindings
114 [must not be strict or unboxed]
115 Returns floats + an augmented environment, not an expression
116 - substituteIdInfo and add result to in-scope
117 [so that rules are available in rec rhs]
120 - float if exposes constructor or PAP
124 completeNonRecX: [binder and rhs both simplified]
125 - if the the thing needs case binding (unlifted and not ok-for-spec)
131 completeBind: [given a simplified RHS]
132 [used for both rec and non-rec bindings, top level and not]
133 - try PostInlineUnconditionally
134 - add unfolding [this is the only place we add an unfolding]
139 Right hand sides and arguments
140 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
141 In many ways we want to treat
142 (a) the right hand side of a let(rec), and
143 (b) a function argument
144 in the same way. But not always! In particular, we would
145 like to leave these arguments exactly as they are, so they
146 will match a RULE more easily.
151 It's harder to make the rule match if we ANF-ise the constructor,
152 or eta-expand the PAP:
154 f (let { a = g x; b = h x } in (a,b))
157 On the other hand if we see the let-defns
162 then we *do* want to ANF-ise and eta-expand, so that p and q
163 can be safely inlined.
165 Even floating lets out is a bit dubious. For let RHS's we float lets
166 out if that exposes a value, so that the value can be inlined more vigorously.
169 r = let x = e in (x,x)
171 Here, if we float the let out we'll expose a nice constructor. We did experiments
172 that showed this to be a generally good thing. But it was a bad thing to float
173 lets out unconditionally, because that meant they got allocated more often.
175 For function arguments, there's less reason to expose a constructor (it won't
176 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
177 So for the moment we don't float lets out of function arguments either.
182 For eta expansion, we want to catch things like
184 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
186 If the \x was on the RHS of a let, we'd eta expand to bring the two
187 lambdas together. And in general that's a good thing to do. Perhaps
188 we should eta expand wherever we find a (value) lambda? Then the eta
189 expansion at a let RHS can concentrate solely on the PAP case.
192 %************************************************************************
194 \subsection{Bindings}
196 %************************************************************************
199 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
201 simplTopBinds env binds
202 = do { -- Put all the top-level binders into scope at the start
203 -- so that if a transformation rule has unexpectedly brought
204 -- anything into scope, then we don't get a complaint about that.
205 -- It's rather as if the top-level binders were imported.
206 ; env <- simplRecBndrs env (bindersOfBinds binds)
207 ; dflags <- getDOptsSmpl
208 ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
209 dopt Opt_D_dump_rule_firings dflags
210 ; env' <- simpl_binds dump_flag env binds
211 ; freeTick SimplifierDone
212 ; return (getFloats env') }
214 -- We need to track the zapped top-level binders, because
215 -- they should have their fragile IdInfo zapped (notably occurrence info)
216 -- That's why we run down binds and bndrs' simultaneously.
218 -- The dump-flag emits a trace for each top-level binding, which
219 -- helps to locate the tracing for inlining and rule firing
220 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
221 simpl_binds dump env [] = return env
222 simpl_binds dump env (bind:binds) = do { env' <- trace dump bind $
224 ; simpl_binds dump env' binds }
226 trace True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
227 trace False bind = \x -> x
229 simpl_bind env (NonRec b r) = simplRecOrTopPair env TopLevel b r
230 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
234 %************************************************************************
236 \subsection{Lazy bindings}
238 %************************************************************************
240 simplRecBind is used for
241 * recursive bindings only
244 simplRecBind :: SimplEnv -> TopLevelFlag
247 simplRecBind env top_lvl pairs
248 = do { env' <- go (zapFloats env) pairs
249 ; return (env `addRecFloats` env') }
250 -- addFloats adds the floats from env',
251 -- *and* updates env with the in-scope set from env'
253 go env [] = return env
255 go env ((bndr, rhs) : pairs)
256 = do { env <- simplRecOrTopPair env top_lvl bndr rhs
260 simplOrTopPair is used for
261 * recursive bindings (whether top level or not)
262 * top-level non-recursive bindings
264 It assumes the binder has already been simplified, but not its IdInfo.
267 simplRecOrTopPair :: SimplEnv
269 -> InId -> InExpr -- Binder and rhs
270 -> SimplM SimplEnv -- Returns an env that includes the binding
272 simplRecOrTopPair env top_lvl bndr rhs
273 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
274 = do { tick (PreInlineUnconditionally bndr)
275 ; return (extendIdSubst env bndr (mkContEx env rhs)) }
278 = do { let bndr' = lookupRecBndr env bndr
279 (env', bndr'') = addLetIdInfo env bndr bndr'
280 ; simplLazyBind env' top_lvl Recursive bndr bndr'' rhs env' }
281 -- May not actually be recursive, but it doesn't matter
285 simplLazyBind is used for
286 * [simplRecOrTopPair] recursive bindings (whether top level or not)
287 * [simplRecOrTopPair] top-level non-recursive bindings
288 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
291 1. It assumes that the binder is *already* simplified,
292 and is in scope, and its IdInfo too, except unfolding
294 2. It assumes that the binder type is lifted.
296 3. It does not check for pre-inline-unconditionallly;
297 that should have been done already.
300 simplLazyBind :: SimplEnv
301 -> TopLevelFlag -> RecFlag
302 -> InId -> OutId -- Binder, both pre-and post simpl
303 -- The OutId has IdInfo, except arity, unfolding
304 -> InExpr -> SimplEnv -- The RHS and its environment
307 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
308 = do { let rhs_env = rhs_se `setInScope` env
309 rhs_cont = mkRhsStop (idType bndr1)
311 -- Simplify the RHS; note the mkRhsStop, which tells
312 -- the simplifier that this is the RHS of a let.
313 ; (rhs_env1, rhs1) <- simplExprF rhs_env rhs rhs_cont
315 -- If any of the floats can't be floated, give up now
316 -- (The canFloat predicate says True for empty floats.)
317 ; if (not (canFloat top_lvl is_rec False rhs_env1))
318 then completeBind env top_lvl bndr bndr1
319 (wrapFloats rhs_env1 rhs1)
321 -- ANF-ise a constructor or PAP rhs
322 { (rhs_env2, rhs2) <- prepareRhs rhs_env1 rhs1
323 ; (env', rhs3) <- chooseRhsFloats top_lvl is_rec False env rhs_env2 rhs2
324 ; completeBind env' top_lvl bndr bndr1 rhs3 } }
326 chooseRhsFloats :: TopLevelFlag -> RecFlag -> Bool
327 -> SimplEnv -- Env for the let
328 -> SimplEnv -- Env for the RHS, with RHS floats in it
329 -> OutExpr -- ..and the RHS itself
330 -> SimplM (SimplEnv, OutExpr) -- New env for let, and RHS
332 chooseRhsFloats top_lvl is_rec is_strict env rhs_env rhs
333 | not (isEmptyFloats rhs_env) -- Something to float
334 , canFloat top_lvl is_rec is_strict rhs_env -- ...that can float
335 , (isTopLevel top_lvl || exprIsCheap rhs) -- ...and we want to float
336 = do { tick LetFloatFromLet -- Float
337 ; return (addFloats env rhs_env, rhs) } -- Add the floats to the main env
338 | otherwise -- Don't float
339 = return (env, wrapFloats rhs_env rhs) -- Wrap the floats around the RHS
343 %************************************************************************
345 \subsection{simplNonRec}
347 %************************************************************************
349 A specialised variant of simplNonRec used when the RHS is already simplified,
350 notably in knownCon. It uses case-binding where necessary.
353 simplNonRecX :: SimplEnv
354 -> InId -- Old binder
355 -> OutExpr -- Simplified RHS
358 simplNonRecX env bndr new_rhs
359 = do { (env, bndr') <- simplBinder env bndr
360 ; completeNonRecX env NotTopLevel NonRecursive
361 (isStrictId bndr) bndr bndr' new_rhs }
363 completeNonRecX :: SimplEnv
364 -> TopLevelFlag -> RecFlag -> Bool
365 -> InId -- Old binder
366 -> OutId -- New binder
367 -> OutExpr -- Simplified RHS
370 completeNonRecX env top_lvl is_rec is_strict old_bndr new_bndr new_rhs
371 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
372 ; (env2, rhs2) <- chooseRhsFloats top_lvl is_rec is_strict env env1 rhs1
373 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
376 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
377 Doing so risks exponential behaviour, because new_rhs has been simplified once already
378 In the cases described by the folowing commment, postInlineUnconditionally will
379 catch many of the relevant cases.
380 -- This happens; for example, the case_bndr during case of
381 -- known constructor: case (a,b) of x { (p,q) -> ... }
382 -- Here x isn't mentioned in the RHS, so we don't want to
383 -- create the (dead) let-binding let x = (a,b) in ...
385 -- Similarly, single occurrences can be inlined vigourously
386 -- e.g. case (f x, g y) of (a,b) -> ....
387 -- If a,b occur once we can avoid constructing the let binding for them.
389 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
390 -- Consider case I# (quotInt# x y) of
391 -- I# v -> let w = J# v in ...
392 -- If we gaily inline (quotInt# x y) for v, we end up building an
394 -- let w = J# (quotInt# x y) in ...
395 -- because quotInt# can fail.
397 | preInlineUnconditionally env NotTopLevel bndr new_rhs
398 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
401 ----------------------------------
402 prepareRhs takes a putative RHS, checks whether it's a PAP or
403 constructor application and, if so, converts it to ANF, so that the
404 resulting thing can be inlined more easily. Thus
411 We also want to deal well cases like this
412 v = (f e1 `cast` co) e2
413 Here we want to make e1,e2 trivial and get
414 x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
415 That's what the 'go' loop in prepareRhs does
418 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
419 -- Adds new floats to the env iff that allows us to return a good RHS
420 prepareRhs env (Cast rhs co) -- Note [Float coercions]
421 = do { (env', rhs') <- makeTrivial env rhs
422 ; return (env', Cast rhs' co) }
425 = do { (is_val, env', rhs') <- go 0 env rhs
426 ; return (env', rhs') }
428 go n_val_args env (Cast rhs co)
429 = do { (is_val, env', rhs') <- go n_val_args env rhs
430 ; return (is_val, env', Cast rhs' co) }
431 go n_val_args env (App fun (Type ty))
432 = do { (is_val, env', rhs') <- go n_val_args env fun
433 ; return (is_val, env', App rhs' (Type ty)) }
434 go n_val_args env (App fun arg)
435 = do { (is_val, env', fun') <- go (n_val_args+1) env fun
437 True -> do { (env'', arg') <- makeTrivial env' arg
438 ; return (True, env'', App fun' arg') }
439 False -> return (False, env, App fun arg) }
440 go n_val_args env (Var fun)
441 = return (is_val, env, Var fun)
443 is_val = n_val_args > 0 -- There is at least one arg
444 -- ...and the fun a constructor or PAP
445 && (isDataConWorkId fun || n_val_args < idArity fun)
446 go n_val_args env other
447 = return (False, env, other)
450 Note [Float coercions]
451 ~~~~~~~~~~~~~~~~~~~~~~
452 When we find the binding
454 we'd like to transform it to
456 x = x `cast` co -- A trivial binding
457 There's a chance that e will be a constructor application or function, or something
458 like that, so moving the coerion to the usage site may well cancel the coersions
459 and lead to further optimisation. Example:
462 data instance T Int = T Int
464 foo :: Int -> Int -> Int
469 go n = case x of { T m -> go (n-m) }
470 -- This case should optimise
474 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
475 -- Binds the expression to a variable, if it's not trivial, returning the variable
479 | otherwise -- See Note [Take care] below
480 = do { var <- newId FSLIT("a") (exprType expr)
481 ; env <- completeNonRecX env NotTopLevel NonRecursive
483 ; return (env, substExpr env (Var var)) }
487 %************************************************************************
489 \subsection{Completing a lazy binding}
491 %************************************************************************
494 * deals only with Ids, not TyVars
495 * takes an already-simplified binder and RHS
496 * is used for both recursive and non-recursive bindings
497 * is used for both top-level and non-top-level bindings
499 It does the following:
500 - tries discarding a dead binding
501 - tries PostInlineUnconditionally
502 - add unfolding [this is the only place we add an unfolding]
505 It does *not* attempt to do let-to-case. Why? Because it is used for
506 - top-level bindings (when let-to-case is impossible)
507 - many situations where the "rhs" is known to be a WHNF
508 (so let-to-case is inappropriate).
510 Nor does it do the atomic-argument thing
513 completeBind :: SimplEnv
514 -> TopLevelFlag -- Flag stuck into unfolding
515 -> InId -- Old binder
516 -> OutId -> OutExpr -- New binder and RHS
518 -- completeBind may choose to do its work
519 -- * by extending the substitution (e.g. let x = y in ...)
520 -- * or by adding to the floats in the envt
522 completeBind env top_lvl old_bndr new_bndr new_rhs
523 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
524 -- Inline and discard the binding
525 = do { tick (PostInlineUnconditionally old_bndr)
526 ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
527 return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
528 -- Use the substitution to make quite, quite sure that the
529 -- substitution will happen, since we are going to discard the binding
534 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
537 -- Add the unfolding *only* for non-loop-breakers
538 -- Making loop breakers not have an unfolding at all
539 -- means that we can avoid tests in exprIsConApp, for example.
540 -- This is important: if exprIsConApp says 'yes' for a recursive
541 -- thing, then we can get into an infinite loop
544 -- If the unfolding is a value, the demand info may
545 -- go pear-shaped, so we nuke it. Example:
547 -- case x of (p,q) -> h p q x
548 -- Here x is certainly demanded. But after we've nuked
549 -- the case, we'll get just
550 -- let x = (a,b) in h a b x
551 -- and now x is not demanded (I'm assuming h is lazy)
552 -- This really happens. Similarly
553 -- let f = \x -> e in ...f..f...
554 -- After inlining f at some of its call sites the original binding may
555 -- (for example) be no longer strictly demanded.
556 -- The solution here is a bit ad hoc...
557 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
558 final_info | loop_breaker = new_bndr_info
559 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
560 | otherwise = info_w_unf
562 final_id = new_bndr `setIdInfo` final_info
564 -- These seqs forces the Id, and hence its IdInfo,
565 -- and hence any inner substitutions
567 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
568 return (addNonRec env final_id new_rhs)
570 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
571 loop_breaker = isNonRuleLoopBreaker occ_info
572 old_info = idInfo old_bndr
573 occ_info = occInfo old_info
578 %************************************************************************
580 \subsection[Simplify-simplExpr]{The main function: simplExpr}
582 %************************************************************************
584 The reason for this OutExprStuff stuff is that we want to float *after*
585 simplifying a RHS, not before. If we do so naively we get quadratic
586 behaviour as things float out.
588 To see why it's important to do it after, consider this (real) example:
602 a -- Can't inline a this round, cos it appears twice
606 Each of the ==> steps is a round of simplification. We'd save a
607 whole round if we float first. This can cascade. Consider
612 let f = let d1 = ..d.. in \y -> e
616 in \x -> ...(\y ->e)...
618 Only in this second round can the \y be applied, and it
619 might do the same again.
623 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
624 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
626 expr_ty' = substTy env (exprType expr)
627 -- The type in the Stop continuation, expr_ty', is usually not used
628 -- It's only needed when discarding continuations after finding
629 -- a function that returns bottom.
630 -- Hence the lazy substitution
633 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
634 -- Simplify an expression, given a continuation
635 simplExprC env expr cont
636 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
637 do { (env', expr') <- simplExprF (zapFloats env) expr cont
638 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
639 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
640 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
641 return (wrapFloats env' expr') }
643 --------------------------------------------------
644 simplExprF :: SimplEnv -> InExpr -> SimplCont
645 -> SimplM (SimplEnv, OutExpr)
647 simplExprF env e cont
648 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
649 simplExprF' env e cont
651 simplExprF' env (Var v) cont = simplVar env v cont
652 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
653 simplExprF' env (Note n expr) cont = simplNote env n expr cont
654 simplExprF' env (Cast body co) cont = simplCast env body co cont
655 simplExprF' env (App fun arg) cont = simplExprF env fun $
656 ApplyTo NoDup arg env cont
658 simplExprF' env expr@(Lam _ _) cont
659 = simplLam env (map zap bndrs) body cont
660 -- The main issue here is under-saturated lambdas
661 -- (\x1. \x2. e) arg1
662 -- Here x1 might have "occurs-once" occ-info, because occ-info
663 -- is computed assuming that a group of lambdas is applied
664 -- all at once. If there are too few args, we must zap the
667 n_args = countArgs cont
668 n_params = length bndrs
669 (bndrs, body) = collectBinders expr
670 zap | n_args >= n_params = \b -> b
671 | otherwise = \b -> if isTyVar b then b
673 -- NB: we count all the args incl type args
674 -- so we must count all the binders (incl type lambdas)
676 simplExprF' env (Type ty) cont
677 = ASSERT( contIsRhsOrArg cont )
678 do { ty' <- simplType env ty
679 ; rebuild env (Type ty') cont }
681 simplExprF' env (Case scrut bndr case_ty alts) cont
682 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
683 = -- Simplify the scrutinee with a Select continuation
684 simplExprF env scrut (Select NoDup bndr alts env cont)
687 = -- If case-of-case is off, simply simplify the case expression
688 -- in a vanilla Stop context, and rebuild the result around it
689 do { case_expr' <- simplExprC env scrut case_cont
690 ; rebuild env case_expr' cont }
692 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
693 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
695 simplExprF' env (Let (Rec pairs) body) cont
696 = do { env <- simplRecBndrs env (map fst pairs)
697 -- NB: bndrs' don't have unfoldings or rules
698 -- We add them as we go down
700 ; env <- simplRecBind env NotTopLevel pairs
701 ; simplExprF env body cont }
703 simplExprF' env (Let (NonRec bndr rhs) body) cont
704 = simplNonRecE env bndr (rhs, env) ([], body) cont
706 ---------------------------------
707 simplType :: SimplEnv -> InType -> SimplM OutType
708 -- Kept monadic just so we can do the seqType
710 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
711 seqType new_ty `seq` returnSmpl new_ty
713 new_ty = substTy env ty
717 %************************************************************************
719 \subsection{The main rebuilder}
721 %************************************************************************
724 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
725 -- At this point the substitution in the SimplEnv should be irrelevant
726 -- only the in-scope set and floats should matter
727 rebuild env expr cont
728 = -- pprTrace "rebuild" (ppr expr $$ ppr cont $$ ppr (seFloats env)) $
730 Stop {} -> return (env, expr)
731 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
732 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
733 StrictArg fun ty info cont -> rebuildCall env (fun `App` expr) (funResultTy ty) info cont
734 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
735 ; simplLam env' bs body cont }
736 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
737 ; rebuild env (App expr arg') cont }
741 %************************************************************************
745 %************************************************************************
748 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
749 -> SimplM (SimplEnv, OutExpr)
750 simplCast env body co cont
751 = do { co' <- simplType env co
752 ; simplExprF env body (addCoerce co' cont) }
754 addCoerce co cont = add_coerce co (coercionKind co) cont
756 add_coerce co (s1, k1) cont -- co :: ty~ty
757 | s1 `coreEqType` k1 = cont -- is a no-op
759 add_coerce co1 (s1, k2) (CoerceIt co2 cont)
760 | (l1, t1) <- coercionKind co2
761 -- coerce T1 S1 (coerce S1 K1 e)
764 -- coerce T1 K1 e, otherwise
766 -- For example, in the initial form of a worker
767 -- we may find (coerce T (coerce S (\x.e))) y
768 -- and we'd like it to simplify to e[y/x] in one round
770 , s1 `coreEqType` t1 = cont -- The coerces cancel out
771 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
773 add_coerce co (s1s2, t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
774 -- (f `cast` g) ty ---> (f ty) `cast` (g @ ty)
775 -- This implements the PushT rule from the paper
776 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
777 , not (isCoVar tyvar)
778 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
780 ty' = substTy arg_se arg_ty
782 -- ToDo: the PushC rule is not implemented at all
784 add_coerce co (s1s2, t1t2) (ApplyTo dup arg arg_se cont)
785 | not (isTypeArg arg) -- This implements the Push rule from the paper
786 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
787 -- co : s1s2 :=: t1t2
788 -- (coerce (T1->T2) (S1->S2) F) E
790 -- coerce T2 S2 (F (coerce S1 T1 E))
792 -- t1t2 must be a function type, T1->T2, because it's applied
793 -- to something but s1s2 might conceivably not be
795 -- When we build the ApplyTo we can't mix the out-types
796 -- with the InExpr in the argument, so we simply substitute
797 -- to make it all consistent. It's a bit messy.
798 -- But it isn't a common case.
800 -- Example of use: Trac #995
801 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
803 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
804 -- t2 :=: s2 with left and right on the curried form:
805 -- (->) t1 t2 :=: (->) s1 s2
806 [co1, co2] = decomposeCo 2 co
807 new_arg = mkCoerce (mkSymCoercion co1) arg'
808 arg' = substExpr arg_se arg
810 add_coerce co _ cont = CoerceIt co cont
814 %************************************************************************
818 %************************************************************************
821 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
822 -> SimplM (SimplEnv, OutExpr)
824 simplLam env [] body cont = simplExprF env body cont
826 -- Type-beta reduction
827 simplLam env (bndr:bndrs) body (ApplyTo _ (Type ty_arg) arg_se cont)
828 = ASSERT( isTyVar bndr )
829 do { tick (BetaReduction bndr)
830 ; ty_arg' <- simplType (arg_se `setInScope` env) ty_arg
831 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
833 -- Ordinary beta reduction
834 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
835 = do { tick (BetaReduction bndr)
836 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
838 -- Not enough args, so there are real lambdas left to put in the result
839 simplLam env bndrs body cont
840 = do { (env, bndrs') <- simplLamBndrs env bndrs
841 ; body' <- simplExpr env body
842 ; new_lam <- mkLam bndrs' body'
843 ; rebuild env new_lam cont }
846 simplNonRecE :: SimplEnv
847 -> InId -- The binder
848 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
849 -> ([InId], InExpr) -- Body of the let/lambda
852 -> SimplM (SimplEnv, OutExpr)
854 -- simplNonRecE is used for
855 -- * non-top-level non-recursive lets in expressions
858 -- It deals with strict bindings, via the StrictBind continuation,
859 -- which may abort the whole process
861 -- The "body" of the binding comes as a pair of ([InId],InExpr)
862 -- representing a lambda; so we recurse back to simplLam
863 -- Why? Because of the binder-occ-info-zapping done before
864 -- the call to simplLam in simplExprF (Lam ...)
866 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
867 | preInlineUnconditionally env NotTopLevel bndr rhs
868 = do { tick (PreInlineUnconditionally bndr)
869 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
872 = do { simplExprF (rhs_se `setFloats` env) rhs
873 (StrictBind bndr bndrs body env cont) }
876 = do { (env, bndr') <- simplBinder env bndr
877 ; env <- simplLazyBind env NotTopLevel NonRecursive bndr bndr' rhs rhs_se
878 ; simplLam env bndrs body cont }
882 %************************************************************************
886 %************************************************************************
889 -- Hack alert: we only distinguish subsumed cost centre stacks for the
890 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
891 simplNote env (SCC cc) e cont
892 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
893 ; rebuild env (mkSCC cc e') cont }
895 -- See notes with SimplMonad.inlineMode
896 simplNote env InlineMe e cont
897 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
898 = do { -- Don't inline inside an INLINE expression
899 e' <- simplExpr (setMode inlineMode env) e
900 ; rebuild env (mkInlineMe e') cont }
902 | otherwise -- Dissolve the InlineMe note if there's
903 -- an interesting context of any kind to combine with
904 -- (even a type application -- anything except Stop)
905 = simplExprF env e cont
907 simplNote env (CoreNote s) e cont
908 = simplExpr env e `thenSmpl` \ e' ->
909 rebuild env (Note (CoreNote s) e') cont
913 %************************************************************************
915 \subsection{Dealing with calls}
917 %************************************************************************
920 simplVar env var cont
921 = case substId env var of
922 DoneEx e -> simplExprF (zapSubstEnv env) e cont
923 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
924 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
925 -- Note [zapSubstEnv]
926 -- The template is already simplified, so don't re-substitute.
927 -- This is VITAL. Consider
929 -- let y = \z -> ...x... in
931 -- We'll clone the inner \x, adding x->x' in the id_subst
932 -- Then when we inline y, we must *not* replace x by x' in
933 -- the inlined copy!!
935 ---------------------------------------------------------
936 -- Dealing with a call site
938 completeCall env var cont
939 = do { dflags <- getDOptsSmpl
940 ; let (args,call_cont) = contArgs cont
941 -- The args are OutExprs, obtained by *lazily* substituting
942 -- in the args found in cont. These args are only examined
943 -- to limited depth (unless a rule fires). But we must do
944 -- the substitution; rule matching on un-simplified args would
947 ------------- First try rules ----------------
948 -- Do this before trying inlining. Some functions have
949 -- rules *and* are strict; in this case, we don't want to
950 -- inline the wrapper of the non-specialised thing; better
951 -- to call the specialised thing instead.
953 -- We used to use the black-listing mechanism to ensure that inlining of
954 -- the wrapper didn't occur for things that have specialisations till a
955 -- later phase, so but now we just try RULES first
957 -- You might think that we shouldn't apply rules for a loop breaker:
958 -- doing so might give rise to an infinite loop, because a RULE is
959 -- rather like an extra equation for the function:
960 -- RULE: f (g x) y = x+y
963 -- But it's too drastic to disable rules for loop breakers.
964 -- Even the foldr/build rule would be disabled, because foldr
965 -- is recursive, and hence a loop breaker:
966 -- foldr k z (build g) = g k z
967 -- So it's up to the programmer: rules can cause divergence
968 ; let in_scope = getInScope env
970 maybe_rule = case activeRule env of
971 Nothing -> Nothing -- No rules apply
972 Just act_fn -> lookupRule act_fn in_scope
974 ; case maybe_rule of {
975 Just (rule, rule_rhs) ->
976 tick (RuleFired (ru_name rule)) `thenSmpl_`
977 (if dopt Opt_D_dump_rule_firings dflags then
978 pprTrace "Rule fired" (vcat [
979 text "Rule:" <+> ftext (ru_name rule),
980 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
981 text "After: " <+> pprCoreExpr rule_rhs,
982 text "Cont: " <+> ppr call_cont])
985 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
986 -- The ruleArity says how many args the rule consumed
988 ; Nothing -> do -- No rules
990 ------------- Next try inlining ----------------
991 { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
992 n_val_args = length arg_infos
993 interesting_cont = interestingCallContext (notNull args)
996 active_inline = activeInline env var
997 maybe_inline = callSiteInline dflags active_inline
998 var arg_infos interesting_cont
999 ; case maybe_inline of {
1000 Just unfolding -- There is an inlining!
1001 -> do { tick (UnfoldingDone var)
1002 ; (if dopt Opt_D_dump_inlinings dflags then
1003 pprTrace "Inlining done" (vcat [
1004 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1005 text "Inlined fn: " <+> nest 2 (ppr unfolding),
1006 text "Cont: " <+> ppr call_cont])
1009 simplExprF env unfolding cont }
1011 ; Nothing -> -- No inlining!
1013 ------------- No inlining! ----------------
1014 -- Next, look for rules or specialisations that match
1016 rebuildCall env (Var var) (idType var)
1017 (mkArgInfo var n_val_args call_cont) cont
1020 rebuildCall :: SimplEnv
1021 -> OutExpr -> OutType -- Function and its type
1022 -> (Bool, [Bool]) -- See SimplUtils.mkArgInfo
1024 -> SimplM (SimplEnv, OutExpr)
1025 rebuildCall env fun fun_ty (has_rules, []) cont
1026 -- When we run out of strictness args, it means
1027 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
1028 -- Then we want to discard the entire strict continuation. E.g.
1029 -- * case (error "hello") of { ... }
1030 -- * (error "Hello") arg
1031 -- * f (error "Hello") where f is strict
1033 -- Then, especially in the first of these cases, we'd like to discard
1034 -- the continuation, leaving just the bottoming expression. But the
1035 -- type might not be right, so we may have to add a coerce.
1036 | not (contIsTrivial cont) -- Only do thia if there is a non-trivial
1037 = return (env, mk_coerce fun) -- contination to discard, else we do it
1038 where -- again and again!
1039 cont_ty = contResultType cont
1040 co = mkUnsafeCoercion fun_ty cont_ty
1041 mk_coerce expr | cont_ty `coreEqType` fun_ty = fun
1042 | otherwise = mkCoerce co fun
1044 rebuildCall env fun fun_ty info (ApplyTo _ (Type arg_ty) se cont)
1045 = do { ty' <- simplType (se `setInScope` env) arg_ty
1046 ; rebuildCall env (fun `App` Type ty') (applyTy fun_ty ty') info cont }
1048 rebuildCall env fun fun_ty (has_rules, str:strs) (ApplyTo _ arg arg_se cont)
1049 | str || isStrictType arg_ty -- Strict argument
1050 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1051 simplExprF (arg_se `setFloats` env) arg
1052 (StrictArg fun fun_ty (has_rules, strs) cont)
1055 | otherwise -- Lazy argument
1056 -- DO NOT float anything outside, hence simplExprC
1057 -- There is no benefit (unlike in a let-binding), and we'd
1058 -- have to be very careful about bogus strictness through
1059 -- floating a demanded let.
1060 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1061 (mkLazyArgStop arg_ty has_rules)
1062 ; rebuildCall env (fun `App` arg') res_ty (has_rules, strs) cont }
1064 (arg_ty, res_ty) = splitFunTy fun_ty
1066 rebuildCall env fun fun_ty info cont
1067 = rebuild env fun cont
1072 This part of the simplifier may break the no-shadowing invariant
1074 f (...(\a -> e)...) (case y of (a,b) -> e')
1075 where f is strict in its second arg
1076 If we simplify the innermost one first we get (...(\a -> e)...)
1077 Simplifying the second arg makes us float the case out, so we end up with
1078 case y of (a,b) -> f (...(\a -> e)...) e'
1079 So the output does not have the no-shadowing invariant. However, there is
1080 no danger of getting name-capture, because when the first arg was simplified
1081 we used an in-scope set that at least mentioned all the variables free in its
1082 static environment, and that is enough.
1084 We can't just do innermost first, or we'd end up with a dual problem:
1085 case x of (a,b) -> f e (...(\a -> e')...)
1087 I spent hours trying to recover the no-shadowing invariant, but I just could
1088 not think of an elegant way to do it. The simplifier is already knee-deep in
1089 continuations. We have to keep the right in-scope set around; AND we have
1090 to get the effect that finding (error "foo") in a strict arg position will
1091 discard the entire application and replace it with (error "foo"). Getting
1092 all this at once is TOO HARD!
1094 %************************************************************************
1096 Rebuilding a cse expression
1098 %************************************************************************
1100 Blob of helper functions for the "case-of-something-else" situation.
1103 ---------------------------------------------------------
1104 -- Eliminate the case if possible
1106 rebuildCase :: SimplEnv
1107 -> OutExpr -- Scrutinee
1108 -> InId -- Case binder
1109 -> [InAlt] -- Alternatives (inceasing order)
1111 -> SimplM (SimplEnv, OutExpr)
1113 --------------------------------------------------
1114 -- 1. Eliminate the case if there's a known constructor
1115 --------------------------------------------------
1117 rebuildCase env scrut case_bndr alts cont
1118 | Just (con,args) <- exprIsConApp_maybe scrut
1119 -- Works when the scrutinee is a variable with a known unfolding
1120 -- as well as when it's an explicit constructor application
1121 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1123 | Lit lit <- scrut -- No need for same treatment as constructors
1124 -- because literals are inlined more vigorously
1125 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1128 --------------------------------------------------
1129 -- 2. Eliminate the case if scrutinee is evaluated
1130 --------------------------------------------------
1132 rebuildCase env scrut case_bndr [(con,bndrs,rhs)] cont
1133 -- See if we can get rid of the case altogether
1134 -- See the extensive notes on case-elimination above
1135 -- mkCase made sure that if all the alternatives are equal,
1136 -- then there is now only one (DEFAULT) rhs
1137 | all isDeadBinder bndrs -- bndrs are [InId]
1139 -- Check that the scrutinee can be let-bound instead of case-bound
1140 , exprOkForSpeculation scrut
1141 -- OK not to evaluate it
1142 -- This includes things like (==# a# b#)::Bool
1143 -- so that we simplify
1144 -- case ==# a# b# of { True -> x; False -> x }
1147 -- This particular example shows up in default methods for
1148 -- comparision operations (e.g. in (>=) for Int.Int32)
1149 || exprIsHNF scrut -- It's already evaluated
1150 || var_demanded_later scrut -- It'll be demanded later
1152 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1153 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1154 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1155 -- its argument: case x of { y -> dataToTag# y }
1156 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1157 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1159 -- Also we don't want to discard 'seq's
1160 = do { tick (CaseElim case_bndr)
1161 ; env <- simplNonRecX env case_bndr scrut
1162 ; simplExprF env rhs cont }
1164 -- The case binder is going to be evaluated later,
1165 -- and the scrutinee is a simple variable
1166 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1167 && not (isTickBoxOp v)
1168 -- ugly hack; covering this case is what
1169 -- exprOkForSpeculation was intended for.
1170 var_demanded_later other = False
1173 --------------------------------------------------
1174 -- 3. Catch-all case
1175 --------------------------------------------------
1177 rebuildCase env scrut case_bndr alts cont
1178 = do { -- Prepare the continuation;
1179 -- The new subst_env is in place
1180 (env, dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1182 -- Simplify the alternatives
1183 ; (case_bndr', alts') <- simplAlts env scrut case_bndr alts dup_cont
1184 ; let res_ty' = contResultType dup_cont
1185 ; case_expr <- mkCase scrut case_bndr' res_ty' alts'
1187 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1188 -- The case binder *not* scope over the whole returned case-expression
1189 ; rebuild env case_expr nodup_cont }
1192 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1193 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1194 way, there's a chance that v will now only be used once, and hence
1197 Note [no-case-of-case]
1198 ~~~~~~~~~~~~~~~~~~~~~~
1199 There is a time we *don't* want to do that, namely when
1200 -fno-case-of-case is on. This happens in the first simplifier pass,
1201 and enhances full laziness. Here's the bad case:
1202 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1203 If we eliminate the inner case, we trap it inside the I# v -> arm,
1204 which might prevent some full laziness happening. I've seen this
1205 in action in spectral/cichelli/Prog.hs:
1206 [(m,n) | m <- [1..max], n <- [1..max]]
1207 Hence the check for NoCaseOfCase.
1209 Note [Suppressing the case binder-swap]
1210 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1211 There is another situation when it might make sense to suppress the
1212 case-expression binde-swap. If we have
1214 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1215 ...other cases .... }
1217 We'll perform the binder-swap for the outer case, giving
1219 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1220 ...other cases .... }
1222 But there is no point in doing it for the inner case, because w1 can't
1223 be inlined anyway. Furthermore, doing the case-swapping involves
1224 zapping w2's occurrence info (see paragraphs that follow), and that
1225 forces us to bind w2 when doing case merging. So we get
1227 case x of w1 { A -> let w2 = w1 in e1
1228 B -> let w2 = w1 in e2
1229 ...other cases .... }
1231 This is plain silly in the common case where w2 is dead.
1233 Even so, I can't see a good way to implement this idea. I tried
1234 not doing the binder-swap if the scrutinee was already evaluated
1235 but that failed big-time:
1239 case v of w { MkT x ->
1240 case x of x1 { I# y1 ->
1241 case x of x2 { I# y2 -> ...
1243 Notice that because MkT is strict, x is marked "evaluated". But to
1244 eliminate the last case, we must either make sure that x (as well as
1245 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1246 the binder-swap. So this whole note is a no-op.
1250 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1251 any occurrence info (eg IAmDead) in the case binder, because the
1252 case-binder now effectively occurs whenever v does. AND we have to do
1253 the same for the pattern-bound variables! Example:
1255 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1257 Here, b and p are dead. But when we move the argment inside the first
1258 case RHS, and eliminate the second case, we get
1260 case x of { (a,b) -> a b }
1262 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1265 Indeed, this can happen anytime the case binder isn't dead:
1266 case <any> of x { (a,b) ->
1267 case x of { (p,q) -> p } }
1268 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1269 The point is that we bring into the envt a binding
1271 after the outer case, and that makes (a,b) alive. At least we do unless
1272 the case binder is guaranteed dead.
1276 Consider case (v `cast` co) of x { I# ->
1277 ... (case (v `cast` co) of {...}) ...
1278 We'd like to eliminate the inner case. We can get this neatly by
1279 arranging that inside the outer case we add the unfolding
1280 v |-> x `cast` (sym co)
1281 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1284 Note [Case elimination]
1285 ~~~~~~~~~~~~~~~~~~~~~~~
1286 The case-elimination transformation discards redundant case expressions.
1287 Start with a simple situation:
1289 case x# of ===> e[x#/y#]
1292 (when x#, y# are of primitive type, of course). We can't (in general)
1293 do this for algebraic cases, because we might turn bottom into
1296 The code in SimplUtils.prepareAlts has the effect of generalise this
1297 idea to look for a case where we're scrutinising a variable, and we
1298 know that only the default case can match. For example:
1302 DEFAULT -> ...(case x of
1306 Here the inner case is first trimmed to have only one alternative, the
1307 DEFAULT, after which it's an instance of the previous case. This
1308 really only shows up in eliminating error-checking code.
1310 We also make sure that we deal with this very common case:
1315 Here we are using the case as a strict let; if x is used only once
1316 then we want to inline it. We have to be careful that this doesn't
1317 make the program terminate when it would have diverged before, so we
1319 - e is already evaluated (it may so if e is a variable)
1320 - x is used strictly, or
1322 Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
1324 case e of ===> case e of DEFAULT -> r
1328 Now again the case may be elminated by the CaseElim transformation.
1331 Further notes about case elimination
1332 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1333 Consider: test :: Integer -> IO ()
1336 Turns out that this compiles to:
1339 eta1 :: State# RealWorld ->
1340 case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
1342 (PrelNum.jtos eta ($w[] @ Char))
1344 of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
1346 Notice the strange '<' which has no effect at all. This is a funny one.
1347 It started like this:
1349 f x y = if x < 0 then jtos x
1350 else if y==0 then "" else jtos x
1352 At a particular call site we have (f v 1). So we inline to get
1354 if v < 0 then jtos x
1355 else if 1==0 then "" else jtos x
1357 Now simplify the 1==0 conditional:
1359 if v<0 then jtos v else jtos v
1361 Now common-up the two branches of the case:
1363 case (v<0) of DEFAULT -> jtos v
1365 Why don't we drop the case? Because it's strict in v. It's technically
1366 wrong to drop even unnecessary evaluations, and in practice they
1367 may be a result of 'seq' so we *definitely* don't want to drop those.
1368 I don't really know how to improve this situation.
1372 simplCaseBinder :: SimplEnv -> OutExpr -> InId -> SimplM (SimplEnv, OutId)
1373 simplCaseBinder env scrut case_bndr
1374 | switchIsOn (getSwitchChecker env) NoCaseOfCase
1375 -- See Note [no-case-of-case]
1376 = do { (env, case_bndr') <- simplBinder env case_bndr
1377 ; return (env, case_bndr') }
1379 simplCaseBinder env (Var v) case_bndr
1380 -- Failed try [see Note 2 above]
1381 -- not (isEvaldUnfolding (idUnfolding v))
1382 = do { (env, case_bndr') <- simplBinder env (zapOccInfo case_bndr)
1383 ; return (modifyInScope env v case_bndr', case_bndr') }
1384 -- We could extend the substitution instead, but it would be
1385 -- a hack because then the substitution wouldn't be idempotent
1386 -- any more (v is an OutId). And this does just as well.
1388 simplCaseBinder env (Cast (Var v) co) case_bndr -- Note [Case of cast]
1389 = do { (env, case_bndr') <- simplBinder env (zapOccInfo case_bndr)
1390 ; let rhs = Cast (Var case_bndr') (mkSymCoercion co)
1391 ; return (addBinderUnfolding env v rhs, case_bndr') }
1393 simplCaseBinder env other_scrut case_bndr
1394 = do { (env, case_bndr') <- simplBinder env case_bndr
1395 ; return (env, case_bndr') }
1397 zapOccInfo :: InId -> InId -- See Note [zapOccInfo]
1398 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1402 simplAlts does two things:
1404 1. Eliminate alternatives that cannot match, including the
1405 DEFAULT alternative.
1407 2. If the DEFAULT alternative can match only one possible constructor,
1408 then make that constructor explicit.
1410 case e of x { DEFAULT -> rhs }
1412 case e of x { (a,b) -> rhs }
1413 where the type is a single constructor type. This gives better code
1414 when rhs also scrutinises x or e.
1416 Here "cannot match" includes knowledge from GADTs
1418 It's a good idea do do this stuff before simplifying the alternatives, to
1419 avoid simplifying alternatives we know can't happen, and to come up with
1420 the list of constructors that are handled, to put into the IdInfo of the
1421 case binder, for use when simplifying the alternatives.
1423 Eliminating the default alternative in (1) isn't so obvious, but it can
1426 data Colour = Red | Green | Blue
1435 DEFAULT -> [ case y of ... ]
1437 If we inline h into f, the default case of the inlined h can't happen.
1438 If we don't notice this, we may end up filtering out *all* the cases
1439 of the inner case y, which give us nowhere to go!
1443 simplAlts :: SimplEnv
1445 -> InId -- Case binder
1446 -> [InAlt] -> SimplCont
1447 -> SimplM (OutId, [OutAlt]) -- Includes the continuation
1448 -- Like simplExpr, this just returns the simplified alternatives;
1449 -- it not return an environment
1451 simplAlts env scrut case_bndr alts cont'
1452 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1453 do { let alt_env = zapFloats env
1454 ; (alt_env, case_bndr') <- simplCaseBinder alt_env scrut case_bndr
1456 ; (imposs_deflt_cons, in_alts) <- prepareAlts scrut case_bndr' alts
1458 ; alts' <- mapM (simplAlt alt_env imposs_deflt_cons case_bndr' cont') in_alts
1459 ; return (case_bndr', alts') }
1461 ------------------------------------
1462 simplAlt :: SimplEnv
1463 -> [AltCon] -- These constructors can't be present when
1464 -- matching the DEFAULT alternative
1465 -> OutId -- The case binder
1470 simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1471 = ASSERT( null bndrs )
1472 do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
1473 -- Record the constructors that the case-binder *can't* be.
1474 ; rhs' <- simplExprC env' rhs cont'
1475 ; return (DEFAULT, [], rhs') }
1477 simplAlt env imposs_deflt_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1478 = ASSERT( null bndrs )
1479 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1480 ; rhs' <- simplExprC env' rhs cont'
1481 ; return (LitAlt lit, [], rhs') }
1483 simplAlt env imposs_deflt_cons case_bndr' cont' (DataAlt con, vs, rhs)
1484 = do { -- Deal with the pattern-bound variables
1485 (env, vs') <- simplBinders env (add_evals con vs)
1487 -- Mark the ones that are in ! positions in the
1488 -- data constructor as certainly-evaluated.
1489 ; let vs'' = add_evals con vs'
1491 -- Bind the case-binder to (con args)
1492 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1493 con_args = map Type inst_tys' ++ varsToCoreExprs vs''
1494 env' = addBinderUnfolding env case_bndr' (mkConApp con con_args)
1496 ; rhs' <- simplExprC env' rhs cont'
1497 ; return (DataAlt con, vs'', rhs') }
1499 -- add_evals records the evaluated-ness of the bound variables of
1500 -- a case pattern. This is *important*. Consider
1501 -- data T = T !Int !Int
1503 -- case x of { T a b -> T (a+1) b }
1505 -- We really must record that b is already evaluated so that we don't
1506 -- go and re-evaluate it when constructing the result.
1507 -- See Note [Data-con worker strictness] in MkId.lhs
1508 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1510 cat_evals dc vs strs
1514 go (v:vs) strs | isTyVar v = v : go vs strs
1515 go (v:vs) (str:strs)
1516 | isMarkedStrict str = evald_v : go vs strs
1517 | otherwise = zapped_v : go vs strs
1519 zapped_v = zap_occ_info v
1520 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1521 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1523 -- If the case binder is alive, then we add the unfolding
1525 -- to the envt; so vs are now very much alive
1526 -- Note [Aug06] I can't see why this actually matters
1527 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1528 | otherwise = zapOccInfo
1530 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1531 addBinderUnfolding env bndr rhs
1532 = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
1534 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1535 addBinderOtherCon env bndr cons
1536 = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
1540 %************************************************************************
1542 \subsection{Known constructor}
1544 %************************************************************************
1546 We are a bit careful with occurrence info. Here's an example
1548 (\x* -> case x of (a*, b) -> f a) (h v, e)
1550 where the * means "occurs once". This effectively becomes
1551 case (h v, e) of (a*, b) -> f a)
1553 let a* = h v; b = e in f a
1557 All this should happen in one sweep.
1560 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1561 -> InId -> [InAlt] -> SimplCont
1562 -> SimplM (SimplEnv, OutExpr)
1564 knownCon env scrut con args bndr alts cont
1565 = do { tick (KnownBranch bndr)
1566 ; knownAlt env scrut args bndr (findAlt con alts) cont }
1568 knownAlt env scrut args bndr (DEFAULT, bs, rhs) cont
1570 do { env <- simplNonRecX env bndr scrut
1571 -- This might give rise to a binding with non-atomic args
1572 -- like x = Node (f x) (g x)
1573 -- but simplNonRecX will atomic-ify it
1574 ; simplExprF env rhs cont }
1576 knownAlt env scrut args bndr (LitAlt lit, bs, rhs) cont
1578 do { env <- simplNonRecX env bndr scrut
1579 ; simplExprF env rhs cont }
1581 knownAlt env scrut args bndr (DataAlt dc, bs, rhs) cont
1582 = do { let dead_bndr = isDeadBinder bndr -- bndr is an InId
1583 n_drop_tys = length (dataConUnivTyVars dc)
1584 ; env <- bind_args env dead_bndr bs (drop n_drop_tys args)
1586 -- It's useful to bind bndr to scrut, rather than to a fresh
1587 -- binding x = Con arg1 .. argn
1588 -- because very often the scrut is a variable, so we avoid
1589 -- creating, and then subsequently eliminating, a let-binding
1590 -- BUT, if scrut is a not a variable, we must be careful
1591 -- about duplicating the arg redexes; in that case, make
1592 -- a new con-app from the args
1593 bndr_rhs = case scrut of
1596 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1597 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1598 -- args are aready OutExprs, but bs are InIds
1600 ; env <- simplNonRecX env bndr bndr_rhs
1601 ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env)) $
1602 simplExprF env rhs cont }
1605 bind_args env dead_bndr [] _ = return env
1607 bind_args env dead_bndr (b:bs) (Type ty : args)
1608 = ASSERT( isTyVar b )
1609 bind_args (extendTvSubst env b ty) dead_bndr bs args
1611 bind_args env dead_bndr (b:bs) (arg : args)
1613 do { let b' = if dead_bndr then b else zapOccInfo b
1614 -- Note that the binder might be "dead", because it doesn't occur
1615 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1616 -- Nevertheless we must keep it if the case-binder is alive, because it may
1617 -- be used in the con_app. See Note [zapOccInfo]
1618 ; env <- simplNonRecX env b' arg
1619 ; bind_args env dead_bndr bs args }
1621 bind_args _ _ _ _ = panic "bind_args"
1625 %************************************************************************
1627 \subsection{Duplicating continuations}
1629 %************************************************************************
1632 prepareCaseCont :: SimplEnv
1633 -> [InAlt] -> SimplCont
1634 -> SimplM (SimplEnv, SimplCont,SimplCont)
1635 -- Return a duplicatable continuation, a non-duplicable part
1636 -- plus some extra bindings (that scope over the entire
1639 -- No need to make it duplicatable if there's only one alternative
1640 prepareCaseCont env [alt] cont = return (env, cont, mkBoringStop (contResultType cont))
1641 prepareCaseCont env alts cont = mkDupableCont env cont
1645 mkDupableCont :: SimplEnv -> SimplCont
1646 -> SimplM (SimplEnv, SimplCont, SimplCont)
1648 mkDupableCont env cont
1649 | contIsDupable cont
1650 = returnSmpl (env, cont, mkBoringStop (contResultType cont))
1652 mkDupableCont env (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1654 mkDupableCont env (CoerceIt ty cont)
1655 = do { (env, dup, nodup) <- mkDupableCont env cont
1656 ; return (env, CoerceIt ty dup, nodup) }
1658 mkDupableCont env cont@(StrictBind bndr _ _ se _)
1659 = return (env, mkBoringStop (substTy se (idType bndr)), cont)
1660 -- See Note [Duplicating strict continuations]
1662 mkDupableCont env cont@(StrictArg _ fun_ty _ _)
1663 = return (env, mkBoringStop (funArgTy fun_ty), cont)
1664 -- See Note [Duplicating strict continuations]
1666 mkDupableCont env (ApplyTo _ arg se cont)
1667 = -- e.g. [...hole...] (...arg...)
1669 -- let a = ...arg...
1670 -- in [...hole...] a
1671 do { (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1672 ; arg <- simplExpr (se `setInScope` env) arg
1673 ; (env, arg) <- makeTrivial env arg
1674 ; let app_cont = ApplyTo OkToDup arg (zapSubstEnv env) dup_cont
1675 ; return (env, app_cont, nodup_cont) }
1677 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1678 -- See Note [Single-alternative case]
1679 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1680 -- | not (isDeadBinder case_bndr)
1681 | all isDeadBinder bs -- InIds
1682 = return (env, mkBoringStop scrut_ty, cont)
1684 scrut_ty = substTy se (idType case_bndr)
1686 mkDupableCont env (Select _ case_bndr alts se cont)
1687 = -- e.g. (case [...hole...] of { pi -> ei })
1689 -- let ji = \xij -> ei
1690 -- in case [...hole...] of { pi -> ji xij }
1691 do { tick (CaseOfCase case_bndr)
1692 ; (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1693 -- NB: call mkDupableCont here, *not* prepareCaseCont
1694 -- We must make a duplicable continuation, whereas prepareCaseCont
1695 -- doesn't when there is a single case branch
1697 ; let alt_env = se `setInScope` env
1698 ; (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1699 ; alts' <- mapM (simplAlt alt_env [] case_bndr' dup_cont) alts
1700 -- Safe to say that there are no handled-cons for the DEFAULT case
1701 -- NB: simplBinder does not zap deadness occ-info, so
1702 -- a dead case_bndr' will still advertise its deadness
1703 -- This is really important because in
1704 -- case e of b { (# p,q #) -> ... }
1705 -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
1706 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1707 -- In the new alts we build, we have the new case binder, so it must retain
1709 -- NB: we don't use alt_env further; it has the substEnv for
1710 -- the alternatives, and we don't want that
1712 ; (env, alts') <- mkDupableAlts env case_bndr' alts'
1713 ; return (env, -- Note [Duplicated env]
1714 Select OkToDup case_bndr' alts' (zapSubstEnv env)
1715 (mkBoringStop (contResultType dup_cont)),
1719 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1720 -> SimplM (SimplEnv, [InAlt])
1721 -- Absorbs the continuation into the new alternatives
1723 mkDupableAlts env case_bndr' alts
1726 go env [] = return (env, [])
1728 = do { (env, alt') <- mkDupableAlt env case_bndr' alt
1729 ; (env, alts') <- go env alts
1730 ; return (env, alt' : alts' ) }
1732 mkDupableAlt env case_bndr' (con, bndrs', rhs')
1733 | exprIsDupable rhs' -- Note [Small alternative rhs]
1734 = return (env, (con, bndrs', rhs'))
1736 = do { let rhs_ty' = exprType rhs'
1737 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1739 | isTyVar bndr = True -- Abstract over all type variables just in case
1740 | otherwise = not (isDeadBinder bndr)
1741 -- The deadness info on the new Ids is preserved by simplBinders
1743 ; (final_bndrs', final_args) -- Note [Join point abstraction]
1744 <- if (any isId used_bndrs')
1745 then return (used_bndrs', varsToCoreExprs used_bndrs')
1746 else do { rw_id <- newId FSLIT("w") realWorldStatePrimTy
1747 ; return ([rw_id], [Var realWorldPrimId]) }
1749 ; join_bndr <- newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty')
1750 -- Note [Funky mkPiTypes]
1752 ; let -- We make the lambdas into one-shot-lambdas. The
1753 -- join point is sure to be applied at most once, and doing so
1754 -- prevents the body of the join point being floated out by
1755 -- the full laziness pass
1756 really_final_bndrs = map one_shot final_bndrs'
1757 one_shot v | isId v = setOneShotLambda v
1759 join_rhs = mkLams really_final_bndrs rhs'
1760 join_call = mkApps (Var join_bndr) final_args
1762 ; return (addNonRec env join_bndr join_rhs, (con, bndrs', join_call)) }
1763 -- See Note [Duplicated env]
1766 Note [Duplicated env]
1767 ~~~~~~~~~~~~~~~~~~~~~
1768 Some of the alternatives are simplified, but have not been turned into a join point
1769 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
1770 bind the join point, because it might to do PostInlineUnconditionally, and
1771 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
1772 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
1773 at worst delays the join-point inlining.
1775 Note [Small alterantive rhs]
1776 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1777 It is worth checking for a small RHS because otherwise we
1778 get extra let bindings that may cause an extra iteration of the simplifier to
1779 inline back in place. Quite often the rhs is just a variable or constructor.
1780 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1781 iterations because the version with the let bindings looked big, and so wasn't
1782 inlined, but after the join points had been inlined it looked smaller, and so
1785 NB: we have to check the size of rhs', not rhs.
1786 Duplicating a small InAlt might invalidate occurrence information
1787 However, if it *is* dupable, we return the *un* simplified alternative,
1788 because otherwise we'd need to pair it up with an empty subst-env....
1789 but we only have one env shared between all the alts.
1790 (Remember we must zap the subst-env before re-simplifying something).
1791 Rather than do this we simply agree to re-simplify the original (small) thing later.
1793 Note [Funky mkPiTypes]
1794 ~~~~~~~~~~~~~~~~~~~~~~
1795 Notice the funky mkPiTypes. If the contructor has existentials
1796 it's possible that the join point will be abstracted over
1797 type varaibles as well as term variables.
1798 Example: Suppose we have
1799 data T = forall t. C [t]
1801 case (case e of ...) of
1803 We get the join point
1804 let j :: forall t. [t] -> ...
1805 j = /\t \xs::[t] -> rhs
1807 case (case e of ...) of
1808 C t xs::[t] -> j t xs
1810 Note [Join point abstaction]
1811 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1812 If we try to lift a primitive-typed something out
1813 for let-binding-purposes, we will *caseify* it (!),
1814 with potentially-disastrous strictness results. So
1815 instead we turn it into a function: \v -> e
1816 where v::State# RealWorld#. The value passed to this function
1817 is realworld#, which generates (almost) no code.
1819 There's a slight infelicity here: we pass the overall
1820 case_bndr to all the join points if it's used in *any* RHS,
1821 because we don't know its usage in each RHS separately
1823 We used to say "&& isUnLiftedType rhs_ty'" here, but now
1824 we make the join point into a function whenever used_bndrs'
1825 is empty. This makes the join-point more CPR friendly.
1826 Consider: let j = if .. then I# 3 else I# 4
1827 in case .. of { A -> j; B -> j; C -> ... }
1829 Now CPR doesn't w/w j because it's a thunk, so
1830 that means that the enclosing function can't w/w either,
1831 which is a lose. Here's the example that happened in practice:
1832 kgmod :: Int -> Int -> Int
1833 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1837 I have seen a case alternative like this:
1839 It's a bit silly to add the realWorld dummy arg in this case, making
1842 (the \v alone is enough to make CPR happy) but I think it's rare
1844 Note [Duplicating strict continuations]
1845 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1846 Do *not* duplicate StrictBind and StritArg continuations. We gain
1847 nothing by propagating them into the expressions, and we do lose a
1848 lot. Here's an example:
1849 && (case x of { T -> F; F -> T }) E
1850 Now, && is strict so we end up simplifying the case with
1851 an ArgOf continuation. If we let-bind it, we get
1853 let $j = \v -> && v E
1854 in simplExpr (case x of { T -> F; F -> T })
1856 And after simplifying more we get
1858 let $j = \v -> && v E
1859 in case x of { T -> $j F; F -> $j T }
1860 Which is a Very Bad Thing
1862 The desire not to duplicate is the entire reason that
1863 mkDupableCont returns a pair of continuations.
1865 The original plan had:
1866 e.g. (...strict-fn...) [...hole...]
1868 let $j = \a -> ...strict-fn...
1871 Note [Single-alternative cases]
1872 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1873 This case is just like the ArgOf case. Here's an example:
1877 case (case x of I# x' ->
1879 True -> I# (negate# x')
1880 False -> I# x') of y {
1882 Because the (case x) has only one alternative, we'll transform to
1884 case (case x' <# 0# of
1885 True -> I# (negate# x')
1886 False -> I# x') of y {
1888 But now we do *NOT* want to make a join point etc, giving
1890 let $j = \y -> MkT y
1892 True -> $j (I# (negate# x'))
1894 In this case the $j will inline again, but suppose there was a big
1895 strict computation enclosing the orginal call to MkT. Then, it won't
1896 "see" the MkT any more, because it's big and won't get duplicated.
1897 And, what is worse, nothing was gained by the case-of-case transform.
1899 When should use this case of mkDupableCont?
1900 However, matching on *any* single-alternative case is a *disaster*;
1901 e.g. case (case ....) of (a,b) -> (# a,b #)
1902 We must push the outer case into the inner one!
1905 * Match [(DEFAULT,_,_)], but in the common case of Int,
1906 the alternative-filling-in code turned the outer case into
1907 case (...) of y { I# _ -> MkT y }
1909 * Match on single alternative plus (not (isDeadBinder case_bndr))
1910 Rationale: pushing the case inwards won't eliminate the construction.
1911 But there's a risk of
1912 case (...) of y { (a,b) -> let z=(a,b) in ... }
1913 Now y looks dead, but it'll come alive again. Still, this
1914 seems like the best option at the moment.
1916 * Match on single alternative plus (all (isDeadBinder bndrs))
1917 Rationale: this is essentially seq.
1919 * Match when the rhs is *not* duplicable, and hence would lead to a
1920 join point. This catches the disaster-case above. We can test
1921 the *un-simplified* rhs, which is fine. It might get bigger or
1922 smaller after simplification; if it gets smaller, this case might
1923 fire next time round. NB also that we must test contIsDupable
1924 case_cont *btoo, because case_cont might be big!
1926 HOWEVER: I found that this version doesn't work well, because
1927 we can get let x = case (...) of { small } in ...case x...
1928 When x is inlined into its full context, we find that it was a bad
1929 idea to have pushed the outer case inside the (...) case.