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"
11 import DynFlags ( dopt, DynFlag(Opt_D_dump_inlinings),
15 import Type hiding ( substTy, extendTvSubst )
21 import TcGadt ( dataConCanMatch )
22 import DataCon ( dataConTyCon, dataConRepStrictness )
23 import TyCon ( tyConArity, isAlgTyCon, isNewTyCon, tyConDataCons_maybe )
25 import PprCore ( pprParendExpr, pprCoreExpr )
26 import CoreUnfold ( mkUnfolding, callSiteInline )
28 import Rules ( lookupRule )
29 import BasicTypes ( isMarkedStrict )
30 import CostCentre ( currentCCS )
31 import TysPrim ( realWorldStatePrimTy )
32 import PrelInfo ( realWorldPrimId )
33 import BasicTypes ( TopLevelFlag(..), isTopLevel,
34 RecFlag(..), isNonRuleLoopBreaker )
36 import Maybes ( orElse )
42 The guts of the simplifier is in this module, but the driver loop for
43 the simplifier is in SimplCore.lhs.
46 -----------------------------------------
47 *** IMPORTANT NOTE ***
48 -----------------------------------------
49 The simplifier used to guarantee that the output had no shadowing, but
50 it does not do so any more. (Actually, it never did!) The reason is
51 documented with simplifyArgs.
54 -----------------------------------------
55 *** IMPORTANT NOTE ***
56 -----------------------------------------
57 Many parts of the simplifier return a bunch of "floats" as well as an
58 expression. This is wrapped as a datatype SimplUtils.FloatsWith.
60 All "floats" are let-binds, not case-binds, but some non-rec lets may
61 be unlifted (with RHS ok-for-speculation).
65 -----------------------------------------
66 ORGANISATION OF FUNCTIONS
67 -----------------------------------------
69 - simplify all top-level binders
70 - for NonRec, call simplRecOrTopPair
71 - for Rec, call simplRecBind
74 ------------------------------
75 simplExpr (applied lambda) ==> simplNonRecBind
76 simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
77 simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
79 ------------------------------
80 simplRecBind [binders already simplfied]
81 - use simplRecOrTopPair on each pair in turn
83 simplRecOrTopPair [binder already simplified]
84 Used for: recursive bindings (top level and nested)
85 top-level non-recursive bindings
87 - check for PreInlineUnconditionally
91 Used for: non-top-level non-recursive bindings
92 beta reductions (which amount to the same thing)
93 Because it can deal with strict arts, it takes a
94 "thing-inside" and returns an expression
96 - check for PreInlineUnconditionally
97 - simplify binder, including its IdInfo
106 simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
107 Used for: binding case-binder and constr args in a known-constructor case
108 - check for PreInLineUnconditionally
112 ------------------------------
113 simplLazyBind: [binder already simplified, RHS not]
114 Used for: recursive bindings (top level and nested)
115 top-level non-recursive bindings
116 non-top-level, but *lazy* non-recursive bindings
117 [must not be strict or unboxed]
118 Returns floats + an augmented environment, not an expression
119 - substituteIdInfo and add result to in-scope
120 [so that rules are available in rec rhs]
123 - float if exposes constructor or PAP
127 completeNonRecX: [binder and rhs both simplified]
128 - if the the thing needs case binding (unlifted and not ok-for-spec)
134 completeBind: [given a simplified RHS]
135 [used for both rec and non-rec bindings, top level and not]
136 - try PostInlineUnconditionally
137 - add unfolding [this is the only place we add an unfolding]
142 Right hand sides and arguments
143 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
144 In many ways we want to treat
145 (a) the right hand side of a let(rec), and
146 (b) a function argument
147 in the same way. But not always! In particular, we would
148 like to leave these arguments exactly as they are, so they
149 will match a RULE more easily.
154 It's harder to make the rule match if we ANF-ise the constructor,
155 or eta-expand the PAP:
157 f (let { a = g x; b = h x } in (a,b))
160 On the other hand if we see the let-defns
165 then we *do* want to ANF-ise and eta-expand, so that p and q
166 can be safely inlined.
168 Even floating lets out is a bit dubious. For let RHS's we float lets
169 out if that exposes a value, so that the value can be inlined more vigorously.
172 r = let x = e in (x,x)
174 Here, if we float the let out we'll expose a nice constructor. We did experiments
175 that showed this to be a generally good thing. But it was a bad thing to float
176 lets out unconditionally, because that meant they got allocated more often.
178 For function arguments, there's less reason to expose a constructor (it won't
179 get inlined). Just possibly it might make a rule match, but I'm pretty skeptical.
180 So for the moment we don't float lets out of function arguments either.
185 For eta expansion, we want to catch things like
187 case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
189 If the \x was on the RHS of a let, we'd eta expand to bring the two
190 lambdas together. And in general that's a good thing to do. Perhaps
191 we should eta expand wherever we find a (value) lambda? Then the eta
192 expansion at a let RHS can concentrate solely on the PAP case.
195 %************************************************************************
197 \subsection{Bindings}
199 %************************************************************************
202 simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
204 simplTopBinds env binds
205 = do { -- Put all the top-level binders into scope at the start
206 -- so that if a transformation rule has unexpectedly brought
207 -- anything into scope, then we don't get a complaint about that.
208 -- It's rather as if the top-level binders were imported.
209 ; env <- simplRecBndrs env (bindersOfBinds binds)
210 ; dflags <- getDOptsSmpl
211 ; let dump_flag = dopt Opt_D_dump_inlinings dflags
212 ; env' <- simpl_binds dump_flag env binds
213 ; freeTick SimplifierDone
214 ; return (getFloats env') }
216 -- We need to track the zapped top-level binders, because
217 -- they should have their fragile IdInfo zapped (notably occurrence info)
218 -- That's why we run down binds and bndrs' simultaneously.
219 simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
220 simpl_binds dump env [] = return env
221 simpl_binds dump env (bind:binds) = do { env' <- trace dump bind $
223 ; simpl_binds dump env' binds }
225 trace True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
226 trace False bind = \x -> x
228 simpl_bind env (NonRec b r) = simplRecOrTopPair env TopLevel b r
229 simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
233 %************************************************************************
235 \subsection{Lazy bindings}
237 %************************************************************************
239 simplRecBind is used for
240 * recursive bindings only
243 simplRecBind :: SimplEnv -> TopLevelFlag
246 simplRecBind env top_lvl pairs
247 = do { env' <- go (zapFloats env) pairs
248 ; return (env `addRecFloats` env') }
249 -- addFloats adds the floats from env',
250 -- *and* updates env with the in-scope set from env'
252 go env [] = return env
254 go env ((bndr, rhs) : pairs)
255 = do { env <- simplRecOrTopPair env top_lvl bndr rhs
259 simplOrTopPair is used for
260 * recursive bindings (whether top level or not)
261 * top-level non-recursive bindings
263 It assumes the binder has already been simplified, but not its IdInfo.
266 simplRecOrTopPair :: SimplEnv
268 -> InId -> InExpr -- Binder and rhs
269 -> SimplM SimplEnv -- Returns an env that includes the binding
271 simplRecOrTopPair env top_lvl bndr rhs
272 | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
273 = do { tick (PreInlineUnconditionally bndr)
274 ; return (extendIdSubst env bndr (mkContEx env rhs)) }
277 = do { let bndr' = lookupRecBndr env bndr
278 (env', bndr'') = addLetIdInfo env bndr bndr'
279 ; simplLazyBind env' top_lvl Recursive bndr bndr'' rhs env' }
280 -- May not actually be recursive, but it doesn't matter
284 simplLazyBind is used for
285 * [simplRecOrTopPair] recursive bindings (whether top level or not)
286 * [simplRecOrTopPair] top-level non-recursive bindings
287 * [simplNonRecE] non-top-level *lazy* non-recursive bindings
290 1. It assumes that the binder is *already* simplified,
291 and is in scope, and its IdInfo too, except unfolding
293 2. It assumes that the binder type is lifted.
295 3. It does not check for pre-inline-unconditionallly;
296 that should have been done already.
299 simplLazyBind :: SimplEnv
300 -> TopLevelFlag -> RecFlag
301 -> InId -> OutId -- Binder, both pre-and post simpl
302 -- The OutId has IdInfo, except arity, unfolding
303 -> InExpr -> SimplEnv -- The RHS and its environment
306 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
307 = do { let rhs_env = rhs_se `setInScope` env
308 rhs_cont = mkRhsStop (idType bndr1)
310 -- Simplify the RHS; note the mkRhsStop, which tells
311 -- the simplifier that this is the RHS of a let.
312 ; (rhs_env1, rhs1) <- simplExprF rhs_env rhs rhs_cont
314 -- If any of the floats can't be floated, give up now
315 -- (The canFloat predicate says True for empty floats.)
316 ; if (not (canFloat top_lvl is_rec False rhs_env1))
317 then completeBind env top_lvl bndr bndr1
318 (wrapFloats rhs_env1 rhs1)
320 -- ANF-ise a constructor or PAP rhs
321 { (rhs_env2, rhs2) <- prepareRhs rhs_env1 rhs1
322 ; (env', rhs3) <- chooseRhsFloats top_lvl is_rec False env rhs_env2 rhs2
323 ; completeBind env' top_lvl bndr bndr1 rhs3 } }
325 chooseRhsFloats :: TopLevelFlag -> RecFlag -> Bool
326 -> SimplEnv -- Env for the let
327 -> SimplEnv -- Env for the RHS, with RHS floats in it
328 -> OutExpr -- ..and the RHS itself
329 -> SimplM (SimplEnv, OutExpr) -- New env for let, and RHS
331 chooseRhsFloats top_lvl is_rec is_strict env rhs_env rhs
332 | not (isEmptyFloats rhs_env) -- Something to float
333 , canFloat top_lvl is_rec is_strict rhs_env -- ...that can float
334 , (isTopLevel top_lvl || exprIsCheap rhs) -- ...and we want to float
335 = do { tick LetFloatFromLet -- Float
336 ; return (addFloats env rhs_env, rhs) } -- Add the floats to the main env
337 | otherwise -- Don't float
338 = return (env, wrapFloats rhs_env rhs) -- Wrap the floats around the RHS
342 %************************************************************************
344 \subsection{simplNonRec}
346 %************************************************************************
348 A specialised variant of simplNonRec used when the RHS is already simplified,
349 notably in knownCon. It uses case-binding where necessary.
352 simplNonRecX :: SimplEnv
353 -> InId -- Old binder
354 -> OutExpr -- Simplified RHS
357 simplNonRecX env bndr new_rhs
358 = do { (env, bndr') <- simplBinder env bndr
359 ; completeNonRecX env NotTopLevel NonRecursive
360 (isStrictBndr bndr) bndr bndr' new_rhs }
362 completeNonRecX :: SimplEnv
363 -> TopLevelFlag -> RecFlag -> Bool
364 -> InId -- Old binder
365 -> OutId -- New binder
366 -> OutExpr -- Simplified RHS
369 completeNonRecX env top_lvl is_rec is_strict old_bndr new_bndr new_rhs
370 = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
371 ; (env2, rhs2) <- chooseRhsFloats top_lvl is_rec is_strict env env1 rhs1
372 ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
375 {- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
376 Doing so risks exponential behaviour, because new_rhs has been simplified once already
377 In the cases described by the folowing commment, postInlineUnconditionally will
378 catch many of the relevant cases.
379 -- This happens; for example, the case_bndr during case of
380 -- known constructor: case (a,b) of x { (p,q) -> ... }
381 -- Here x isn't mentioned in the RHS, so we don't want to
382 -- create the (dead) let-binding let x = (a,b) in ...
384 -- Similarly, single occurrences can be inlined vigourously
385 -- e.g. case (f x, g y) of (a,b) -> ....
386 -- If a,b occur once we can avoid constructing the let binding for them.
388 Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
389 -- Consider case I# (quotInt# x y) of
390 -- I# v -> let w = J# v in ...
391 -- If we gaily inline (quotInt# x y) for v, we end up building an
393 -- let w = J# (quotInt# x y) in ...
394 -- because quotInt# can fail.
396 | preInlineUnconditionally env NotTopLevel bndr new_rhs
397 = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
400 prepareRhs takes a putative RHS, checks whether it's a PAP or
401 constructor application and, if so, converts it to ANF, so that the
402 resulting thing can be inlined more easily. Thus
410 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
411 -- Adds new floats to the env iff that allows us to return a good RHS
413 prepareRhs env (Cast rhs co) -- Note [Float coersions]
414 = do { (env', rhs') <- makeTrivial env rhs
415 ; return (env', Cast rhs' co) }
418 | (Var fun, args) <- collectArgs rhs -- It's an application
419 , let n_args = valArgCount args
420 , n_args > 0 -- ...but not a trivial one
421 , isDataConWorkId fun || n_args < idArity fun -- ...and it's a constructor or PAP
422 = go env (Var fun) args
424 go env fun [] = return (env, fun)
425 go env fun (arg : args) = do { (env', arg') <- makeTrivial env arg
426 ; go env' (App fun arg') args }
428 prepareRhs env rhs -- The default case
432 Note [Float coercions]
433 ~~~~~~~~~~~~~~~~~~~~~~
434 When we find the binding
436 we'd like to transform it to
438 x = x `cast` co -- A trivial binding
439 There's a chance that e will be a constructor application or function, or something
440 like that, so moving the coerion to the usage site may well cancel the coersions
441 and lead to further optimisation. Example:
444 data instance T Int = T Int
446 foo :: Int -> Int -> Int
451 go n = case x of { T m -> go (n-m) }
452 -- This case should optimise
456 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
457 -- Binds the expression to a variable, if it's not trivial, returning the variable
461 | otherwise -- See Note [Take care] below
462 = do { var <- newId FSLIT("a") (exprType expr)
463 ; env <- completeNonRecX env NotTopLevel NonRecursive
465 ; return (env, substExpr env (Var var)) }
469 %************************************************************************
471 \subsection{Completing a lazy binding}
473 %************************************************************************
476 * deals only with Ids, not TyVars
477 * takes an already-simplified binder and RHS
478 * is used for both recursive and non-recursive bindings
479 * is used for both top-level and non-top-level bindings
481 It does the following:
482 - tries discarding a dead binding
483 - tries PostInlineUnconditionally
484 - add unfolding [this is the only place we add an unfolding]
487 It does *not* attempt to do let-to-case. Why? Because it is used for
488 - top-level bindings (when let-to-case is impossible)
489 - many situations where the "rhs" is known to be a WHNF
490 (so let-to-case is inappropriate).
492 Nor does it do the atomic-argument thing
495 completeBind :: SimplEnv
496 -> TopLevelFlag -- Flag stuck into unfolding
497 -> InId -- Old binder
498 -> OutId -> OutExpr -- New binder and RHS
500 -- completeBind may choose to do its work
501 -- * by extending the substitution (e.g. let x = y in ...)
502 -- * or by adding to the floats in the envt
504 completeBind env top_lvl old_bndr new_bndr new_rhs
505 | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
506 -- Inline and discard the binding
507 = do { tick (PostInlineUnconditionally old_bndr)
508 ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
509 return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
510 -- Use the substitution to make quite, quite sure that the
511 -- substitution will happen, since we are going to discard the binding
516 new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
519 -- Add the unfolding *only* for non-loop-breakers
520 -- Making loop breakers not have an unfolding at all
521 -- means that we can avoid tests in exprIsConApp, for example.
522 -- This is important: if exprIsConApp says 'yes' for a recursive
523 -- thing, then we can get into an infinite loop
526 -- If the unfolding is a value, the demand info may
527 -- go pear-shaped, so we nuke it. Example:
529 -- case x of (p,q) -> h p q x
530 -- Here x is certainly demanded. But after we've nuked
531 -- the case, we'll get just
532 -- let x = (a,b) in h a b x
533 -- and now x is not demanded (I'm assuming h is lazy)
534 -- This really happens. Similarly
535 -- let f = \x -> e in ...f..f...
536 -- After inlining f at some of its call sites the original binding may
537 -- (for example) be no longer strictly demanded.
538 -- The solution here is a bit ad hoc...
539 info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
540 final_info | loop_breaker = new_bndr_info
541 | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
542 | otherwise = info_w_unf
544 final_id = new_bndr `setIdInfo` final_info
546 -- These seqs forces the Id, and hence its IdInfo,
547 -- and hence any inner substitutions
549 -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
550 return (addNonRec env final_id new_rhs)
552 unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
553 loop_breaker = isNonRuleLoopBreaker occ_info
554 old_info = idInfo old_bndr
555 occ_info = occInfo old_info
560 %************************************************************************
562 \subsection[Simplify-simplExpr]{The main function: simplExpr}
564 %************************************************************************
566 The reason for this OutExprStuff stuff is that we want to float *after*
567 simplifying a RHS, not before. If we do so naively we get quadratic
568 behaviour as things float out.
570 To see why it's important to do it after, consider this (real) example:
584 a -- Can't inline a this round, cos it appears twice
588 Each of the ==> steps is a round of simplification. We'd save a
589 whole round if we float first. This can cascade. Consider
594 let f = let d1 = ..d.. in \y -> e
598 in \x -> ...(\y ->e)...
600 Only in this second round can the \y be applied, and it
601 might do the same again.
605 simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
606 simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
608 expr_ty' = substTy env (exprType expr)
609 -- The type in the Stop continuation, expr_ty', is usually not used
610 -- It's only needed when discarding continuations after finding
611 -- a function that returns bottom.
612 -- Hence the lazy substitution
615 simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
616 -- Simplify an expression, given a continuation
617 simplExprC env expr cont
618 = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
619 do { (env', expr') <- simplExprF (zapFloats env) expr cont
620 ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
621 -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
622 -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
623 return (wrapFloats env' expr') }
625 --------------------------------------------------
626 simplExprF :: SimplEnv -> InExpr -> SimplCont
627 -> SimplM (SimplEnv, OutExpr)
629 simplExprF env e cont
630 = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
631 simplExprF' env e cont
633 simplExprF' env (Var v) cont = simplVar env v cont
634 simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
635 simplExprF' env (Note n expr) cont = simplNote env n expr cont
636 simplExprF' env (Cast body co) cont = simplCast env body co cont
637 simplExprF' env (App fun arg) cont = simplExprF env fun $
638 ApplyTo NoDup arg env cont
640 simplExprF' env expr@(Lam _ _) cont
641 = simplLam env (map zap bndrs) body cont
642 -- The main issue here is under-saturated lambdas
643 -- (\x1. \x2. e) arg1
644 -- Here x1 might have "occurs-once" occ-info, because occ-info
645 -- is computed assuming that a group of lambdas is applied
646 -- all at once. If there are too few args, we must zap the
649 n_args = countArgs cont
650 n_params = length bndrs
651 (bndrs, body) = collectBinders expr
652 zap | n_args >= n_params = \b -> b
653 | otherwise = \b -> if isTyVar b then b
655 -- NB: we count all the args incl type args
656 -- so we must count all the binders (incl type lambdas)
658 simplExprF' env (Type ty) cont
659 = ASSERT( contIsRhsOrArg cont )
660 do { ty' <- simplType env ty
661 ; rebuild env (Type ty') cont }
663 simplExprF' env (Case scrut bndr case_ty alts) cont
664 | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
665 = -- Simplify the scrutinee with a Select continuation
666 simplExprF env scrut (Select NoDup bndr alts env cont)
669 = -- If case-of-case is off, simply simplify the case expression
670 -- in a vanilla Stop context, and rebuild the result around it
671 do { case_expr' <- simplExprC env scrut case_cont
672 ; rebuild env case_expr' cont }
674 case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
675 case_ty' = substTy env case_ty -- c.f. defn of simplExpr
677 simplExprF' env (Let (Rec pairs) body) cont
678 = do { env <- simplRecBndrs env (map fst pairs)
679 -- NB: bndrs' don't have unfoldings or rules
680 -- We add them as we go down
682 ; env <- simplRecBind env NotTopLevel pairs
683 ; simplExprF env body cont }
685 simplExprF' env (Let (NonRec bndr rhs) body) cont
686 = simplNonRecE env bndr (rhs, env) ([], body) cont
688 ---------------------------------
689 simplType :: SimplEnv -> InType -> SimplM OutType
690 -- Kept monadic just so we can do the seqType
692 = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
693 seqType new_ty `seq` returnSmpl new_ty
695 new_ty = substTy env ty
699 %************************************************************************
701 \subsection{The main rebuilder}
703 %************************************************************************
706 rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
707 -- At this point the substitution in the SimplEnv should be irrelevant
708 -- only the in-scope set and floats should matter
709 rebuild env expr cont
710 = -- pprTrace "rebuild" (ppr expr $$ ppr cont $$ ppr (seFloats env)) $
712 Stop {} -> return (env, expr)
713 CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
714 Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
715 StrictArg fun ty info cont -> rebuildCall env (fun `App` expr) (funResultTy ty) info cont
716 StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
717 ; simplLam env' bs body cont }
718 ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
719 ; rebuild env (App expr arg') cont }
723 %************************************************************************
727 %************************************************************************
730 simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
731 -> SimplM (SimplEnv, OutExpr)
732 simplCast env body co cont
733 = do { co' <- simplType env co
734 ; simplExprF env body (addCoerce co' cont) }
736 addCoerce co cont = add_coerce co (coercionKind co) cont
738 add_coerce co (s1, k1) cont
739 | s1 `coreEqType` k1 = cont
740 add_coerce co1 (s1, k2) (CoerceIt co2 cont)
741 | (l1, t1) <- coercionKind co2
742 -- coerce T1 S1 (coerce S1 K1 e)
745 -- coerce T1 K1 e, otherwise
747 -- For example, in the initial form of a worker
748 -- we may find (coerce T (coerce S (\x.e))) y
749 -- and we'd like it to simplify to e[y/x] in one round
751 , s1 `coreEqType` t1 = cont -- The coerces cancel out
752 | otherwise = CoerceIt (mkTransCoercion co1 co2) cont
754 add_coerce co (s1s2, t1t2) (ApplyTo dup arg arg_se cont)
755 | not (isTypeArg arg) -- This whole case only works for value args
756 -- Could upgrade to have equiv thing for type apps too
757 , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
758 -- co : s1s2 :=: t1t2
759 -- (coerce (T1->T2) (S1->S2) F) E
761 -- coerce T2 S2 (F (coerce S1 T1 E))
763 -- t1t2 must be a function type, T1->T2, because it's applied
764 -- to something but s1s2 might conceivably not be
766 -- When we build the ApplyTo we can't mix the out-types
767 -- with the InExpr in the argument, so we simply substitute
768 -- to make it all consistent. It's a bit messy.
769 -- But it isn't a common case.
771 -- Example of use: Trac #995
772 = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
774 -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
775 -- t2 :=: s2 with left and right on the curried form:
776 -- (->) t1 t2 :=: (->) s1 s2
777 [co1, co2] = decomposeCo 2 co
778 new_arg = mkCoerce (mkSymCoercion co1) arg'
779 arg' = substExpr arg_se arg
781 add_coerce co _ cont = CoerceIt co cont
785 %************************************************************************
789 %************************************************************************
792 simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
793 -> SimplM (SimplEnv, OutExpr)
795 simplLam env [] body cont = simplExprF env body cont
797 -- Type-beta reduction
798 simplLam env (bndr:bndrs) body (ApplyTo _ (Type ty_arg) arg_se cont)
799 = ASSERT( isTyVar bndr )
800 do { tick (BetaReduction bndr)
801 ; ty_arg' <- simplType (arg_se `setInScope` env) ty_arg
802 ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
804 -- Ordinary beta reduction
805 simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
806 = do { tick (BetaReduction bndr)
807 ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
809 -- Not enough args, so there are real lambdas left to put in the result
810 simplLam env bndrs body cont
811 = do { (env, bndrs') <- simplLamBndrs env bndrs
812 ; body' <- simplExpr env body
813 ; new_lam <- mkLam bndrs' body'
814 ; rebuild env new_lam cont }
817 simplNonRecE :: SimplEnv
818 -> InId -- The binder
819 -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
820 -> ([InId], InExpr) -- Body of the let/lambda
823 -> SimplM (SimplEnv, OutExpr)
825 -- simplNonRecE is used for
826 -- * non-top-level non-recursive lets in expressions
829 -- It deals with strict bindings, via the StrictBind continuation,
830 -- which may abort the whole process
832 -- The "body" of the binding comes as a pair of ([InId],InExpr)
833 -- representing a lambda; so we recurse back to simplLam
834 -- Why? Because of the binder-occ-info-zapping done before
835 -- the call to simplLam in simplExprF (Lam ...)
837 simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
838 | preInlineUnconditionally env NotTopLevel bndr rhs
839 = do { tick (PreInlineUnconditionally bndr)
840 ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
843 = do { simplExprF (rhs_se `setFloats` env) rhs
844 (StrictBind bndr bndrs body env cont) }
847 = do { (env, bndr') <- simplBinder env bndr
848 ; env <- simplLazyBind env NotTopLevel NonRecursive bndr bndr' rhs rhs_se
849 ; simplLam env bndrs body cont }
853 %************************************************************************
857 %************************************************************************
860 -- Hack alert: we only distinguish subsumed cost centre stacks for the
861 -- purposes of inlining. All other CCCSs are mapped to currentCCS.
862 simplNote env (SCC cc) e cont
863 = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
864 ; rebuild env (mkSCC cc e') cont }
866 -- See notes with SimplMonad.inlineMode
867 simplNote env InlineMe e cont
868 | contIsRhsOrArg cont -- Totally boring continuation; see notes above
869 = do { -- Don't inline inside an INLINE expression
870 e' <- simplExpr (setMode inlineMode env) e
871 ; rebuild env (mkInlineMe e') cont }
873 | otherwise -- Dissolve the InlineMe note if there's
874 -- an interesting context of any kind to combine with
875 -- (even a type application -- anything except Stop)
876 = simplExprF env e cont
878 simplNote env (CoreNote s) e cont
879 = simplExpr env e `thenSmpl` \ e' ->
880 rebuild env (Note (CoreNote s) e') cont
884 %************************************************************************
886 \subsection{Dealing with calls}
888 %************************************************************************
891 simplVar env var cont
892 = case substId env var of
893 DoneEx e -> simplExprF (zapSubstEnv env) e cont
894 ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
895 DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
896 -- Note [zapSubstEnv]
897 -- The template is already simplified, so don't re-substitute.
898 -- This is VITAL. Consider
900 -- let y = \z -> ...x... in
902 -- We'll clone the inner \x, adding x->x' in the id_subst
903 -- Then when we inline y, we must *not* replace x by x' in
904 -- the inlined copy!!
906 ---------------------------------------------------------
907 -- Dealing with a call site
909 completeCall env var cont
910 = do { dflags <- getDOptsSmpl
911 ; let (args,call_cont) = contArgs cont
912 -- The args are OutExprs, obtained by *lazily* substituting
913 -- in the args found in cont. These args are only examined
914 -- to limited depth (unless a rule fires). But we must do
915 -- the substitution; rule matching on un-simplified args would
918 ------------- First try rules ----------------
919 -- Do this before trying inlining. Some functions have
920 -- rules *and* are strict; in this case, we don't want to
921 -- inline the wrapper of the non-specialised thing; better
922 -- to call the specialised thing instead.
924 -- We used to use the black-listing mechanism to ensure that inlining of
925 -- the wrapper didn't occur for things that have specialisations till a
926 -- later phase, so but now we just try RULES first
928 -- You might think that we shouldn't apply rules for a loop breaker:
929 -- doing so might give rise to an infinite loop, because a RULE is
930 -- rather like an extra equation for the function:
931 -- RULE: f (g x) y = x+y
934 -- But it's too drastic to disable rules for loop breakers.
935 -- Even the foldr/build rule would be disabled, because foldr
936 -- is recursive, and hence a loop breaker:
937 -- foldr k z (build g) = g k z
938 -- So it's up to the programmer: rules can cause divergence
939 ; let in_scope = getInScope env
941 maybe_rule = case activeRule env of
942 Nothing -> Nothing -- No rules apply
943 Just act_fn -> lookupRule act_fn in_scope
945 ; case maybe_rule of {
946 Just (rule, rule_rhs) ->
947 tick (RuleFired (ru_name rule)) `thenSmpl_`
948 (if dopt Opt_D_dump_inlinings dflags then
949 pprTrace "Rule fired" (vcat [
950 text "Rule:" <+> ftext (ru_name rule),
951 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
952 text "After: " <+> pprCoreExpr rule_rhs,
953 text "Cont: " <+> ppr call_cont])
956 simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
957 -- The ruleArity says how many args the rule consumed
959 ; Nothing -> do -- No rules
961 ------------- Next try inlining ----------------
962 { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
963 n_val_args = length arg_infos
964 interesting_cont = interestingCallContext (notNull args)
967 active_inline = activeInline env var
968 maybe_inline = callSiteInline dflags active_inline
969 var arg_infos interesting_cont
970 ; case maybe_inline of {
971 Just unfolding -- There is an inlining!
972 -> do { tick (UnfoldingDone var)
973 ; (if dopt Opt_D_dump_inlinings dflags then
974 pprTrace "Inlining done" (vcat [
975 text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
976 text "Inlined fn: " <+> nest 2 (ppr unfolding),
977 text "Cont: " <+> ppr call_cont])
980 simplExprF env unfolding cont }
982 ; Nothing -> -- No inlining!
984 ------------- No inlining! ----------------
985 -- Next, look for rules or specialisations that match
987 rebuildCall env (Var var) (idType var)
988 (mkArgInfo var n_val_args call_cont) cont
991 rebuildCall :: SimplEnv
992 -> OutExpr -> OutType -- Function and its type
993 -> (Bool, [Bool]) -- See SimplUtils.mkArgInfo
995 -> SimplM (SimplEnv, OutExpr)
996 rebuildCall env fun fun_ty (has_rules, []) cont
997 -- When we run out of strictness args, it means
998 -- that the call is definitely bottom; see SimplUtils.mkArgInfo
999 -- Then we want to discard the entire strict continuation. E.g.
1000 -- * case (error "hello") of { ... }
1001 -- * (error "Hello") arg
1002 -- * f (error "Hello") where f is strict
1004 -- Then, especially in the first of these cases, we'd like to discard
1005 -- the continuation, leaving just the bottoming expression. But the
1006 -- type might not be right, so we may have to add a coerce.
1007 | not (contIsTrivial cont) -- Only do thia if there is a non-trivial
1008 = return (env, mk_coerce fun) -- contination to discard, else we do it
1009 where -- again and again!
1010 cont_ty = contResultType cont
1011 co = mkUnsafeCoercion fun_ty cont_ty
1012 mk_coerce expr | cont_ty `coreEqType` fun_ty = fun
1013 | otherwise = mkCoerce co fun
1015 rebuildCall env fun fun_ty info (ApplyTo _ (Type arg_ty) se cont)
1016 = do { ty' <- simplType (se `setInScope` env) arg_ty
1017 ; rebuildCall env (fun `App` Type ty') (applyTy fun_ty ty') info cont }
1019 rebuildCall env fun fun_ty (has_rules, str:strs) (ApplyTo _ arg arg_se cont)
1020 | str || isStrictType arg_ty -- Strict argument
1021 = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
1022 simplExprF (arg_se `setFloats` env) arg
1023 (StrictArg fun fun_ty (has_rules, strs) cont)
1026 | otherwise -- Lazy argument
1027 -- DO NOT float anything outside, hence simplExprC
1028 -- There is no benefit (unlike in a let-binding), and we'd
1029 -- have to be very careful about bogus strictness through
1030 -- floating a demanded let.
1031 = do { arg' <- simplExprC (arg_se `setInScope` env) arg
1032 (mkLazyArgStop arg_ty has_rules)
1033 ; rebuildCall env (fun `App` arg') res_ty (has_rules, strs) cont }
1035 (arg_ty, res_ty) = splitFunTy fun_ty
1037 rebuildCall env fun fun_ty info cont
1038 = rebuild env fun cont
1043 This part of the simplifier may break the no-shadowing invariant
1045 f (...(\a -> e)...) (case y of (a,b) -> e')
1046 where f is strict in its second arg
1047 If we simplify the innermost one first we get (...(\a -> e)...)
1048 Simplifying the second arg makes us float the case out, so we end up with
1049 case y of (a,b) -> f (...(\a -> e)...) e'
1050 So the output does not have the no-shadowing invariant. However, there is
1051 no danger of getting name-capture, because when the first arg was simplified
1052 we used an in-scope set that at least mentioned all the variables free in its
1053 static environment, and that is enough.
1055 We can't just do innermost first, or we'd end up with a dual problem:
1056 case x of (a,b) -> f e (...(\a -> e')...)
1058 I spent hours trying to recover the no-shadowing invariant, but I just could
1059 not think of an elegant way to do it. The simplifier is already knee-deep in
1060 continuations. We have to keep the right in-scope set around; AND we have
1061 to get the effect that finding (error "foo") in a strict arg position will
1062 discard the entire application and replace it with (error "foo"). Getting
1063 all this at once is TOO HARD!
1065 %************************************************************************
1067 Rebuilding a cse expression
1069 %************************************************************************
1071 Blob of helper functions for the "case-of-something-else" situation.
1074 ---------------------------------------------------------
1075 -- Eliminate the case if possible
1077 rebuildCase :: SimplEnv
1078 -> OutExpr -- Scrutinee
1079 -> InId -- Case binder
1080 -> [InAlt] -- Alternatives (inceasing order)
1082 -> SimplM (SimplEnv, OutExpr)
1084 rebuildCase env scrut case_bndr alts cont
1085 | Just (con,args) <- exprIsConApp_maybe scrut
1086 -- Works when the scrutinee is a variable with a known unfolding
1087 -- as well as when it's an explicit constructor application
1088 = knownCon env scrut (DataAlt con) args case_bndr alts cont
1090 | Lit lit <- scrut -- No need for same treatment as constructors
1091 -- because literals are inlined more vigorously
1092 = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1095 = do { -- Prepare the continuation;
1096 -- The new subst_env is in place
1097 (env, dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1099 -- Simplify the alternatives
1100 ; (case_bndr', alts') <- simplAlts env scrut case_bndr alts dup_cont
1101 ; let res_ty' = contResultType dup_cont
1102 ; case_expr <- mkCase scrut case_bndr' res_ty' alts'
1104 -- Notice that rebuildDone returns the in-scope set from env, not alt_env
1105 -- The case binder *not* scope over the whole returned case-expression
1106 ; rebuild env case_expr nodup_cont }
1109 simplCaseBinder checks whether the scrutinee is a variable, v. If so,
1110 try to eliminate uses of v in the RHSs in favour of case_bndr; that
1111 way, there's a chance that v will now only be used once, and hence
1114 Note [no-case-of-case]
1115 ~~~~~~~~~~~~~~~~~~~~~~
1116 There is a time we *don't* want to do that, namely when
1117 -fno-case-of-case is on. This happens in the first simplifier pass,
1118 and enhances full laziness. Here's the bad case:
1119 f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
1120 If we eliminate the inner case, we trap it inside the I# v -> arm,
1121 which might prevent some full laziness happening. I've seen this
1122 in action in spectral/cichelli/Prog.hs:
1123 [(m,n) | m <- [1..max], n <- [1..max]]
1124 Hence the check for NoCaseOfCase.
1126 Note [Suppressing the case binder-swap]
1127 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1128 There is another situation when it might make sense to suppress the
1129 case-expression binde-swap. If we have
1131 case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
1132 ...other cases .... }
1134 We'll perform the binder-swap for the outer case, giving
1136 case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
1137 ...other cases .... }
1139 But there is no point in doing it for the inner case, because w1 can't
1140 be inlined anyway. Furthermore, doing the case-swapping involves
1141 zapping w2's occurrence info (see paragraphs that follow), and that
1142 forces us to bind w2 when doing case merging. So we get
1144 case x of w1 { A -> let w2 = w1 in e1
1145 B -> let w2 = w1 in e2
1146 ...other cases .... }
1148 This is plain silly in the common case where w2 is dead.
1150 Even so, I can't see a good way to implement this idea. I tried
1151 not doing the binder-swap if the scrutinee was already evaluated
1152 but that failed big-time:
1156 case v of w { MkT x ->
1157 case x of x1 { I# y1 ->
1158 case x of x2 { I# y2 -> ...
1160 Notice that because MkT is strict, x is marked "evaluated". But to
1161 eliminate the last case, we must either make sure that x (as well as
1162 x1) has unfolding MkT y1. THe straightforward thing to do is to do
1163 the binder-swap. So this whole note is a no-op.
1167 If we replace the scrutinee, v, by tbe case binder, then we have to nuke
1168 any occurrence info (eg IAmDead) in the case binder, because the
1169 case-binder now effectively occurs whenever v does. AND we have to do
1170 the same for the pattern-bound variables! Example:
1172 (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1174 Here, b and p are dead. But when we move the argment inside the first
1175 case RHS, and eliminate the second case, we get
1177 case x of { (a,b) -> a b }
1179 Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
1182 Indeed, this can happen anytime the case binder isn't dead:
1183 case <any> of x { (a,b) ->
1184 case x of { (p,q) -> p } }
1185 Here (a,b) both look dead, but come alive after the inner case is eliminated.
1186 The point is that we bring into the envt a binding
1188 after the outer case, and that makes (a,b) alive. At least we do unless
1189 the case binder is guaranteed dead.
1193 Consider case (v `cast` co) of x { I# ->
1194 ... (case (v `cast` co) of {...}) ...
1195 We'd like to eliminate the inner case. We can get this neatly by
1196 arranging that inside the outer case we add the unfolding
1197 v |-> x `cast` (sym co)
1198 to v. Then we should inline v at the inner case, cancel the casts, and away we go
1201 simplCaseBinder :: SimplEnv -> OutExpr -> InId -> SimplM (SimplEnv, OutId)
1202 simplCaseBinder env scrut case_bndr
1203 | switchIsOn (getSwitchChecker env) NoCaseOfCase
1204 -- See Note [no-case-of-case]
1205 = do { (env, case_bndr') <- simplBinder env case_bndr
1206 ; return (env, case_bndr') }
1208 simplCaseBinder env (Var v) case_bndr
1209 -- Failed try [see Note 2 above]
1210 -- not (isEvaldUnfolding (idUnfolding v))
1211 = do { (env, case_bndr') <- simplBinder env (zapOccInfo case_bndr)
1212 ; return (modifyInScope env v case_bndr', case_bndr') }
1213 -- We could extend the substitution instead, but it would be
1214 -- a hack because then the substitution wouldn't be idempotent
1215 -- any more (v is an OutId). And this does just as well.
1217 simplCaseBinder env (Cast (Var v) co) case_bndr -- Note [Case of cast]
1218 = do { (env, case_bndr') <- simplBinder env (zapOccInfo case_bndr)
1219 ; let rhs = Cast (Var case_bndr') (mkSymCoercion co)
1220 ; return (addBinderUnfolding env v rhs, case_bndr') }
1222 simplCaseBinder env other_scrut case_bndr
1223 = do { (env, case_bndr') <- simplBinder env case_bndr
1224 ; return (env, case_bndr') }
1226 zapOccInfo :: InId -> InId -- See Note [zapOccInfo]
1227 zapOccInfo b = b `setIdOccInfo` NoOccInfo
1231 simplAlts does two things:
1233 1. Eliminate alternatives that cannot match, including the
1234 DEFAULT alternative.
1236 2. If the DEFAULT alternative can match only one possible constructor,
1237 then make that constructor explicit.
1239 case e of x { DEFAULT -> rhs }
1241 case e of x { (a,b) -> rhs }
1242 where the type is a single constructor type. This gives better code
1243 when rhs also scrutinises x or e.
1245 Here "cannot match" includes knowledge from GADTs
1247 It's a good idea do do this stuff before simplifying the alternatives, to
1248 avoid simplifying alternatives we know can't happen, and to come up with
1249 the list of constructors that are handled, to put into the IdInfo of the
1250 case binder, for use when simplifying the alternatives.
1252 Eliminating the default alternative in (1) isn't so obvious, but it can
1255 data Colour = Red | Green | Blue
1264 DEFAULT -> [ case y of ... ]
1266 If we inline h into f, the default case of the inlined h can't happen.
1267 If we don't notice this, we may end up filtering out *all* the cases
1268 of the inner case y, which give us nowhere to go!
1272 simplAlts :: SimplEnv
1274 -> InId -- Case binder
1275 -> [InAlt] -> SimplCont
1276 -> SimplM (OutId, [OutAlt]) -- Includes the continuation
1277 -- Like simplExpr, this just returns the simplified alternatives;
1278 -- it not return an environment
1280 simplAlts env scrut case_bndr alts cont'
1281 = -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
1282 do { let alt_env = zapFloats env
1283 ; (alt_env, case_bndr') <- simplCaseBinder alt_env scrut case_bndr
1285 ; default_alts <- prepareDefault alt_env case_bndr' imposs_deflt_cons cont' maybe_deflt
1287 ; let inst_tys = tyConAppArgs (idType case_bndr')
1288 trimmed_alts = filter (is_possible inst_tys) alts_wo_default
1289 in_alts = mergeAlts default_alts trimmed_alts
1290 -- We need the mergeAlts in case the new default_alt
1291 -- has turned into a constructor alternative.
1293 ; alts' <- mapM (simplAlt alt_env imposs_cons case_bndr' cont') in_alts
1294 ; return (case_bndr', alts') }
1296 (alts_wo_default, maybe_deflt) = findDefault alts
1297 imposs_cons = case scrut of
1298 Var v -> otherCons (idUnfolding v)
1301 -- "imposs_deflt_cons" are handled either by the context,
1302 -- OR by a branch in this case expression. (Don't include DEFAULT!!)
1303 imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
1305 is_possible :: [Type] -> CoreAlt -> Bool
1306 is_possible tys (con, _, _) | con `elem` imposs_cons = False
1307 is_possible tys (DataAlt con, _, _) = dataConCanMatch tys con
1308 is_possible tys alt = True
1310 ------------------------------------
1311 prepareDefault :: SimplEnv
1312 -> OutId -- Case binder; need just for its type. Note that as an
1313 -- OutId, it has maximum information; this is important.
1314 -- Test simpl013 is an example
1315 -> [AltCon] -- These cons can't happen when matching the default
1318 -> SimplM [InAlt] -- One branch or none; still unsimplified
1319 -- We use a list because it's what mergeAlts expects
1321 prepareDefault env case_bndr' imposs_cons cont Nothing
1322 = return [] -- No default branch
1324 prepareDefault env case_bndr' imposs_cons cont (Just rhs)
1325 | -- This branch handles the case where we are
1326 -- scrutinisng an algebraic data type
1327 Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
1328 isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
1329 not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
1330 -- case x of { DEFAULT -> e }
1331 -- and we don't want to fill in a default for them!
1332 Just all_cons <- tyConDataCons_maybe tycon,
1333 not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
1334 -- which GHC allows, then the case expression will have at most a default
1335 -- alternative. We don't want to eliminate that alternative, because the
1336 -- invariant is that there's always one alternative. It's more convenient
1338 -- case x of { DEFAULT -> e }
1339 -- as it is, rather than transform it to
1340 -- error "case cant match"
1341 -- which would be quite legitmate. But it's a really obscure corner, and
1342 -- not worth wasting code on.
1344 let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
1345 is_possible con = not (con `elem` imposs_data_cons)
1346 && dataConCanMatch inst_tys con
1347 = case filter is_possible all_cons of
1348 [] -> return [] -- Eliminate the default alternative
1349 -- altogether if it can't match
1351 [con] -> -- It matches exactly one constructor, so fill it in
1352 do { tick (FillInCaseDefault case_bndr')
1353 ; us <- getUniquesSmpl
1354 ; let (ex_tvs, co_tvs, arg_ids) =
1355 dataConRepInstPat us con inst_tys
1356 ; return [(DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, rhs)] }
1358 two_or_more -> return [(DEFAULT, [], rhs)]
1361 = return [(DEFAULT, [], rhs)]
1363 ------------------------------------
1364 simplAlt :: SimplEnv
1365 -> [AltCon] -- These constructors can't be present when
1366 -- matching this alternative
1367 -> OutId -- The case binder
1372 -- Simplify an alternative, returning the type refinement for the
1373 -- alternative, if the alternative does any refinement at all
1375 simplAlt env handled_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
1376 = ASSERT( null bndrs )
1377 do { let env' = addBinderOtherCon env case_bndr' handled_cons
1378 -- Record the constructors that the case-binder *can't* be.
1379 ; rhs' <- simplExprC env' rhs cont'
1380 ; return (DEFAULT, [], rhs') }
1382 simplAlt env handled_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
1383 = ASSERT( null bndrs )
1384 do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
1385 ; rhs' <- simplExprC env' rhs cont'
1386 ; return (LitAlt lit, [], rhs') }
1388 simplAlt env handled_cons case_bndr' cont' (DataAlt con, vs, rhs)
1389 = do { -- Deal with the pattern-bound variables
1390 -- Mark the ones that are in ! positions in the data constructor
1391 -- as certainly-evaluated.
1392 -- NB: it happens that simplBinders does *not* erase the OtherCon
1393 -- form of unfolding, so it's ok to add this info before
1394 -- doing simplBinders
1395 (env, vs') <- simplBinders env (add_evals con vs)
1397 -- Bind the case-binder to (con args)
1398 ; let inst_tys' = tyConAppArgs (idType case_bndr')
1399 con_args = map Type inst_tys' ++ varsToCoreExprs vs'
1400 env' = addBinderUnfolding env case_bndr' (mkConApp con con_args)
1402 ; rhs' <- simplExprC env' rhs cont'
1403 ; return (DataAlt con, vs', rhs') }
1405 -- add_evals records the evaluated-ness of the bound variables of
1406 -- a case pattern. This is *important*. Consider
1407 -- data T = T !Int !Int
1409 -- case x of { T a b -> T (a+1) b }
1411 -- We really must record that b is already evaluated so that we don't
1412 -- go and re-evaluate it when constructing the result.
1413 -- See Note [Data-con worker strictness] in MkId.lhs
1414 add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
1416 cat_evals dc vs strs
1420 go (v:vs) strs | isTyVar v = v : go vs strs
1421 go (v:vs) (str:strs)
1422 | isMarkedStrict str = evald_v : go vs strs
1423 | otherwise = zapped_v : go vs strs
1425 zapped_v = zap_occ_info v
1426 evald_v = zapped_v `setIdUnfolding` evaldUnfolding
1427 go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
1429 -- If the case binder is alive, then we add the unfolding
1431 -- to the envt; so vs are now very much alive
1432 -- Note [Aug06] I can't see why this actually matters
1433 zap_occ_info | isDeadBinder case_bndr' = \id -> id
1434 | otherwise = zapOccInfo
1436 addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
1437 addBinderUnfolding env bndr rhs
1438 = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
1440 addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
1441 addBinderOtherCon env bndr cons
1442 = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
1446 %************************************************************************
1448 \subsection{Known constructor}
1450 %************************************************************************
1452 We are a bit careful with occurrence info. Here's an example
1454 (\x* -> case x of (a*, b) -> f a) (h v, e)
1456 where the * means "occurs once". This effectively becomes
1457 case (h v, e) of (a*, b) -> f a)
1459 let a* = h v; b = e in f a
1463 All this should happen in one sweep.
1466 knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
1467 -> InId -> [InAlt] -> SimplCont
1468 -> SimplM (SimplEnv, OutExpr)
1470 knownCon env scrut con args bndr alts cont
1471 = do { tick (KnownBranch bndr)
1472 ; knownAlt env scrut args bndr (findAlt con alts) cont }
1474 knownAlt env scrut args bndr (DEFAULT, bs, rhs) cont
1476 do { env <- simplNonRecX env bndr scrut
1477 -- This might give rise to a binding with non-atomic args
1478 -- like x = Node (f x) (g x)
1479 -- but simplNonRecX will atomic-ify it
1480 ; simplExprF env rhs cont }
1482 knownAlt env scrut args bndr (LitAlt lit, bs, rhs) cont
1484 do { env <- simplNonRecX env bndr scrut
1485 ; simplExprF env rhs cont }
1487 knownAlt env scrut args bndr (DataAlt dc, bs, rhs) cont
1488 = do { let dead_bndr = isDeadBinder bndr
1489 n_drop_tys = tyConArity (dataConTyCon dc)
1490 ; env <- bind_args env dead_bndr bs (drop n_drop_tys args)
1492 -- It's useful to bind bndr to scrut, rather than to a fresh
1493 -- binding x = Con arg1 .. argn
1494 -- because very often the scrut is a variable, so we avoid
1495 -- creating, and then subsequently eliminating, a let-binding
1496 -- BUT, if scrut is a not a variable, we must be careful
1497 -- about duplicating the arg redexes; in that case, make
1498 -- a new con-app from the args
1499 bndr_rhs = case scrut of
1502 con_app = mkConApp dc (take n_drop_tys args ++ con_args)
1503 con_args = [substExpr env (varToCoreExpr b) | b <- bs]
1504 -- args are aready OutExprs, but bs are InIds
1506 ; env <- simplNonRecX env bndr bndr_rhs
1507 ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env)) $
1508 simplExprF env rhs cont }
1511 bind_args env dead_bndr [] _ = return env
1513 bind_args env dead_bndr (b:bs) (Type ty : args)
1514 = ASSERT( isTyVar b )
1515 bind_args (extendTvSubst env b ty) dead_bndr bs args
1517 bind_args env dead_bndr (b:bs) (arg : args)
1519 do { let b' = if dead_bndr then b else zapOccInfo b
1520 -- Note that the binder might be "dead", because it doesn't occur
1521 -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
1522 -- Nevertheless we must keep it if the case-binder is alive, because it may
1523 -- be used in the con_app. See Note [zapOccInfo]
1524 ; env <- simplNonRecX env b' arg
1525 ; bind_args env dead_bndr bs args }
1527 bind_args _ _ _ _ = panic "bind_args"
1531 %************************************************************************
1533 \subsection{Duplicating continuations}
1535 %************************************************************************
1538 prepareCaseCont :: SimplEnv
1539 -> [InAlt] -> SimplCont
1540 -> SimplM (SimplEnv, SimplCont,SimplCont)
1541 -- Return a duplicatable continuation, a non-duplicable part
1542 -- plus some extra bindings (that scope over the entire
1545 -- No need to make it duplicatable if there's only one alternative
1546 prepareCaseCont env [alt] cont = return (env, cont, mkBoringStop (contResultType cont))
1547 prepareCaseCont env alts cont = mkDupableCont env cont
1551 mkDupableCont :: SimplEnv -> SimplCont
1552 -> SimplM (SimplEnv, SimplCont, SimplCont)
1554 mkDupableCont env cont
1555 | contIsDupable cont
1556 = returnSmpl (env, cont, mkBoringStop (contResultType cont))
1558 mkDupableCont env (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
1560 mkDupableCont env (CoerceIt ty cont)
1561 = do { (env, dup, nodup) <- mkDupableCont env cont
1562 ; return (env, CoerceIt ty dup, nodup) }
1564 mkDupableCont env cont@(StrictBind bndr _ _ se _)
1565 = return (env, mkBoringStop (substTy se (idType bndr)), cont)
1566 -- See Note [Duplicating strict continuations]
1568 mkDupableCont env cont@(StrictArg _ fun_ty _ _)
1569 = return (env, mkBoringStop (funArgTy fun_ty), cont)
1570 -- See Note [Duplicating strict continuations]
1572 mkDupableCont env (ApplyTo _ arg se cont)
1573 = -- e.g. [...hole...] (...arg...)
1575 -- let a = ...arg...
1576 -- in [...hole...] a
1577 do { (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1578 ; arg <- simplExpr (se `setInScope` env) arg
1579 ; (env, arg) <- makeTrivial env arg
1580 ; let app_cont = ApplyTo OkToDup arg (zapSubstEnv env) dup_cont
1581 ; return (env, app_cont, nodup_cont) }
1583 mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
1584 -- See Note [Single-alternative case]
1585 -- | not (exprIsDupable rhs && contIsDupable case_cont)
1586 -- | not (isDeadBinder case_bndr)
1587 | all isDeadBinder bs
1588 = return (env, mkBoringStop scrut_ty, cont)
1590 scrut_ty = substTy se (idType case_bndr)
1592 mkDupableCont env (Select _ case_bndr alts se cont)
1593 = -- e.g. (case [...hole...] of { pi -> ei })
1595 -- let ji = \xij -> ei
1596 -- in case [...hole...] of { pi -> ji xij }
1597 do { tick (CaseOfCase case_bndr)
1598 ; (env, dup_cont, nodup_cont) <- mkDupableCont env cont
1599 -- NB: call mkDupableCont here, *not* prepareCaseCont
1600 -- We must make a duplicable continuation, whereas prepareCaseCont
1601 -- doesn't when there is a single case branch
1603 ; let alt_env = se `setInScope` env
1604 ; (alt_env, case_bndr') <- simplBinder alt_env case_bndr
1605 ; alts' <- mapM (simplAlt alt_env [] case_bndr' dup_cont) alts
1606 -- Safe to say that there are no handled-cons for the DEFAULT case
1607 -- NB: simplBinder does not zap deadness occ-info, so
1608 -- a dead case_bndr' will still advertise its deadness
1609 -- This is really important because in
1610 -- case e of b { (# a,b #) -> ... }
1611 -- b is always dead, and indeed we are not allowed to bind b to (# a,b #),
1612 -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
1613 -- In the new alts we build, we have the new case binder, so it must retain
1615 -- NB: we don't use alt_env further; it has the substEnv for
1616 -- the alternatives, and we don't want that
1618 ; (env, alts') <- mkDupableAlts env case_bndr' alts'
1619 ; return (env, -- Note [Duplicated env]
1620 Select OkToDup case_bndr' alts' (zapSubstEnv env)
1621 (mkBoringStop (contResultType dup_cont)),
1625 mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
1626 -> SimplM (SimplEnv, [InAlt])
1627 -- Absorbs the continuation into the new alternatives
1629 mkDupableAlts env case_bndr' alts
1632 go env [] = return (env, [])
1634 = do { (env, alt') <- mkDupableAlt env case_bndr' alt
1635 ; (env, alts') <- go env alts
1636 ; return (env, alt' : alts' ) }
1638 mkDupableAlt env case_bndr' (con, bndrs', rhs')
1639 | exprIsDupable rhs' -- Note [Small alternative rhs]
1640 = return (env, (con, bndrs', rhs'))
1642 = do { let rhs_ty' = exprType rhs'
1643 used_bndrs' = filter abstract_over (case_bndr' : bndrs')
1645 | isTyVar bndr = True -- Abstract over all type variables just in case
1646 | otherwise = not (isDeadBinder bndr)
1647 -- The deadness info on the new Ids is preserved by simplBinders
1649 ; (final_bndrs', final_args) -- Note [Join point abstraction]
1650 <- if (any isId used_bndrs')
1651 then return (used_bndrs', varsToCoreExprs used_bndrs')
1652 else do { rw_id <- newId FSLIT("w") realWorldStatePrimTy
1653 ; return ([rw_id], [Var realWorldPrimId]) }
1655 ; join_bndr <- newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty')
1656 -- Note [Funky mkPiTypes]
1658 ; let -- We make the lambdas into one-shot-lambdas. The
1659 -- join point is sure to be applied at most once, and doing so
1660 -- prevents the body of the join point being floated out by
1661 -- the full laziness pass
1662 really_final_bndrs = map one_shot final_bndrs'
1663 one_shot v | isId v = setOneShotLambda v
1665 join_rhs = mkLams really_final_bndrs rhs'
1666 join_call = mkApps (Var join_bndr) final_args
1668 ; return (addNonRec env join_bndr join_rhs, (con, bndrs', join_call)) }
1669 -- See Note [Duplicated env]
1672 Note [Duplicated env]
1673 ~~~~~~~~~~~~~~~~~~~~~
1674 Some of the alternatives are simplified, but have not been turned into a join point
1675 So they *must* have an zapped subst-env. So we can't use completeNonRecX to
1676 bind the join point, because it might to do PostInlineUnconditionally, and
1677 we'd lose that when zapping the subst-env. We could have a per-alt subst-env,
1678 but zapping it (as we do in mkDupableCont, the Select case) is safe, and
1679 at worst delays the join-point inlining.
1681 Note [Small alterantive rhs]
1682 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1683 It is worth checking for a small RHS because otherwise we
1684 get extra let bindings that may cause an extra iteration of the simplifier to
1685 inline back in place. Quite often the rhs is just a variable or constructor.
1686 The Ord instance of Maybe in PrelMaybe.lhs, for example, took several extra
1687 iterations because the version with the let bindings looked big, and so wasn't
1688 inlined, but after the join points had been inlined it looked smaller, and so
1691 NB: we have to check the size of rhs', not rhs.
1692 Duplicating a small InAlt might invalidate occurrence information
1693 However, if it *is* dupable, we return the *un* simplified alternative,
1694 because otherwise we'd need to pair it up with an empty subst-env....
1695 but we only have one env shared between all the alts.
1696 (Remember we must zap the subst-env before re-simplifying something).
1697 Rather than do this we simply agree to re-simplify the original (small) thing later.
1699 Note [Funky mkPiTypes]
1700 ~~~~~~~~~~~~~~~~~~~~~~
1701 Notice the funky mkPiTypes. If the contructor has existentials
1702 it's possible that the join point will be abstracted over
1703 type varaibles as well as term variables.
1704 Example: Suppose we have
1705 data T = forall t. C [t]
1707 case (case e of ...) of
1709 We get the join point
1710 let j :: forall t. [t] -> ...
1711 j = /\t \xs::[t] -> rhs
1713 case (case e of ...) of
1714 C t xs::[t] -> j t xs
1716 Note [Join point abstaction]
1717 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1718 If we try to lift a primitive-typed something out
1719 for let-binding-purposes, we will *caseify* it (!),
1720 with potentially-disastrous strictness results. So
1721 instead we turn it into a function: \v -> e
1722 where v::State# RealWorld#. The value passed to this function
1723 is realworld#, which generates (almost) no code.
1725 There's a slight infelicity here: we pass the overall
1726 case_bndr to all the join points if it's used in *any* RHS,
1727 because we don't know its usage in each RHS separately
1729 We used to say "&& isUnLiftedType rhs_ty'" here, but now
1730 we make the join point into a function whenever used_bndrs'
1731 is empty. This makes the join-point more CPR friendly.
1732 Consider: let j = if .. then I# 3 else I# 4
1733 in case .. of { A -> j; B -> j; C -> ... }
1735 Now CPR doesn't w/w j because it's a thunk, so
1736 that means that the enclosing function can't w/w either,
1737 which is a lose. Here's the example that happened in practice:
1738 kgmod :: Int -> Int -> Int
1739 kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
1743 I have seen a case alternative like this:
1745 It's a bit silly to add the realWorld dummy arg in this case, making
1748 (the \v alone is enough to make CPR happy) but I think it's rare
1750 Note [Duplicating strict continuations]
1751 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1752 Do *not* duplicate StrictBind and StritArg continuations. We gain
1753 nothing by propagating them into the expressions, and we do lose a
1754 lot. Here's an example:
1755 && (case x of { T -> F; F -> T }) E
1756 Now, && is strict so we end up simplifying the case with
1757 an ArgOf continuation. If we let-bind it, we get
1759 let $j = \v -> && v E
1760 in simplExpr (case x of { T -> F; F -> T })
1762 And after simplifying more we get
1764 let $j = \v -> && v E
1765 in case x of { T -> $j F; F -> $j T }
1766 Which is a Very Bad Thing
1768 The desire not to duplicate is the entire reason that
1769 mkDupableCont returns a pair of continuations.
1771 The original plan had:
1772 e.g. (...strict-fn...) [...hole...]
1774 let $j = \a -> ...strict-fn...
1777 Note [Single-alternative cases]
1778 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1779 This case is just like the ArgOf case. Here's an example:
1783 case (case x of I# x' ->
1785 True -> I# (negate# x')
1786 False -> I# x') of y {
1788 Because the (case x) has only one alternative, we'll transform to
1790 case (case x' <# 0# of
1791 True -> I# (negate# x')
1792 False -> I# x') of y {
1794 But now we do *NOT* want to make a join point etc, giving
1796 let $j = \y -> MkT y
1798 True -> $j (I# (negate# x'))
1800 In this case the $j will inline again, but suppose there was a big
1801 strict computation enclosing the orginal call to MkT. Then, it won't
1802 "see" the MkT any more, because it's big and won't get duplicated.
1803 And, what is worse, nothing was gained by the case-of-case transform.
1805 When should use this case of mkDupableCont?
1806 However, matching on *any* single-alternative case is a *disaster*;
1807 e.g. case (case ....) of (a,b) -> (# a,b #)
1808 We must push the outer case into the inner one!
1811 * Match [(DEFAULT,_,_)], but in the common case of Int,
1812 the alternative-filling-in code turned the outer case into
1813 case (...) of y { I# _ -> MkT y }
1815 * Match on single alternative plus (not (isDeadBinder case_bndr))
1816 Rationale: pushing the case inwards won't eliminate the construction.
1817 But there's a risk of
1818 case (...) of y { (a,b) -> let z=(a,b) in ... }
1819 Now y looks dead, but it'll come alive again. Still, this
1820 seems like the best option at the moment.
1822 * Match on single alternative plus (all (isDeadBinder bndrs))
1823 Rationale: this is essentially seq.
1825 * Match when the rhs is *not* duplicable, and hence would lead to a
1826 join point. This catches the disaster-case above. We can test
1827 the *un-simplified* rhs, which is fine. It might get bigger or
1828 smaller after simplification; if it gets smaller, this case might
1829 fire next time round. NB also that we must test contIsDupable
1830 case_cont *btoo, because case_cont might be big!
1832 HOWEVER: I found that this version doesn't work well, because
1833 we can get let x = case (...) of { small } in ...case x...
1834 When x is inlined into its full context, we find that it was a bad
1835 idea to have pushed the outer case inside the (...) case.