2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section[SimplUtils]{The simplifier utilities}
8 simplBinder, simplBinders, simplRecIds, simplLetId, simplLamBinders,
12 -- The continuation type
13 SimplCont(..), DupFlag(..), LetRhsFlag(..),
14 contIsDupable, contResultType,
15 countValArgs, countArgs, pushContArgs,
16 mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg,
17 getContArgs, interestingCallContext, interestingArg, isStrictType
21 #include "HsVersions.h"
23 import CmdLineOpts ( SimplifierSwitch(..),
24 opt_SimplDoLambdaEtaExpansion, opt_SimplDoEtaReduction,
25 opt_SimplCaseMerge, opt_UF_UpdateInPlace
28 import CoreUtils ( cheapEqExpr, exprType,
29 etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce,
30 findDefault, exprOkForSpeculation, exprIsValue
32 import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr )
33 import Id ( Id, idType, idInfo,
34 mkSysLocal, hasNoBinding, isDeadBinder, idNewDemandInfo,
35 idUnfolding, idNewStrictness
37 import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
39 import Type ( Type, seqType,
40 splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
41 splitRepFunTys, isStrictType
43 import OccName ( UserFS )
44 import TyCon ( tyConDataConsIfAvailable, isDataTyCon )
45 import DataCon ( dataConRepArity, dataConSig, dataConArgTys )
46 import Var ( mkSysTyVar, tyVarKind )
47 import Util ( lengthExceeds, mapAccumL )
52 %************************************************************************
54 \subsection{The continuation data type}
56 %************************************************************************
59 data SimplCont -- Strict contexts
60 = Stop OutType -- Type of the result
62 Bool -- True <=> This is the RHS of a thunk whose type suggests
63 -- that update-in-place would be possible
64 -- (This makes the inliner a little keener.)
66 | CoerceIt OutType -- The To-type, simplified
69 | InlinePlease -- This continuation makes a function very
70 SimplCont -- keen to inline itelf
73 InExpr SimplEnv -- The argument, as yet unsimplified,
74 SimplCont -- and its environment
77 InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
80 | ArgOf DupFlag -- An arbitrary strict context: the argument
81 -- of a strict function, or a primitive-arg fn
84 OutType -- cont_ty: the type of the expression being sought by the context
85 -- f (error "foo") ==> coerce t (error "foo")
87 -- We need to know the type t, to which to coerce.
88 (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
89 -- The result expression in the OutExprStuff has type cont_ty
91 data LetRhsFlag = AnArg -- It's just an argument not a let RHS
92 | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
94 instance Outputable LetRhsFlag where
95 ppr AnArg = ptext SLIT("arg")
96 ppr AnRhs = ptext SLIT("rhs")
98 instance Outputable SimplCont where
99 ppr (Stop _ is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs)
100 ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
101 ppr (ArgOf dup _ _ _) = ptext SLIT("ArgOf...") <+> ppr dup
102 ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
103 (nest 4 (ppr alts)) $$ ppr cont
104 ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
105 ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
107 data DupFlag = OkToDup | NoDup
109 instance Outputable DupFlag where
110 ppr OkToDup = ptext SLIT("ok")
111 ppr NoDup = ptext SLIT("nodup")
115 mkBoringStop :: OutType -> SimplCont
116 mkBoringStop ty = Stop ty AnArg (canUpdateInPlace ty)
118 mkStop :: OutType -> LetRhsFlag -> SimplCont
119 mkStop ty is_rhs = Stop ty is_rhs (canUpdateInPlace ty)
121 contIsRhs :: SimplCont -> Bool
122 contIsRhs (Stop _ AnRhs _) = True
123 contIsRhs (ArgOf _ AnRhs _ _) = True
124 contIsRhs other = False
126 contIsRhsOrArg (Stop _ _ _) = True
127 contIsRhsOrArg (ArgOf _ _ _ _) = True
128 contIsRhsOrArg other = False
131 contIsDupable :: SimplCont -> Bool
132 contIsDupable (Stop _ _ _) = True
133 contIsDupable (ApplyTo OkToDup _ _ _) = True
134 contIsDupable (ArgOf OkToDup _ _ _) = True
135 contIsDupable (Select OkToDup _ _ _ _) = True
136 contIsDupable (CoerceIt _ cont) = contIsDupable cont
137 contIsDupable (InlinePlease cont) = contIsDupable cont
138 contIsDupable other = False
141 discardableCont :: SimplCont -> Bool
142 discardableCont (Stop _ _ _) = False
143 discardableCont (CoerceIt _ cont) = discardableCont cont
144 discardableCont (InlinePlease cont) = discardableCont cont
145 discardableCont other = True
147 discardCont :: SimplCont -- A continuation, expecting
148 -> SimplCont -- Replace the continuation with a suitable coerce
149 discardCont cont = case cont of
150 Stop to_ty is_rhs _ -> cont
151 other -> CoerceIt to_ty (mkBoringStop to_ty)
153 to_ty = contResultType cont
156 contResultType :: SimplCont -> OutType
157 contResultType (Stop to_ty _ _) = to_ty
158 contResultType (ArgOf _ _ to_ty _) = to_ty
159 contResultType (ApplyTo _ _ _ cont) = contResultType cont
160 contResultType (CoerceIt _ cont) = contResultType cont
161 contResultType (InlinePlease cont) = contResultType cont
162 contResultType (Select _ _ _ _ cont) = contResultType cont
165 countValArgs :: SimplCont -> Int
166 countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
167 countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
168 countValArgs other = 0
170 countArgs :: SimplCont -> Int
171 countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
175 pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont
176 -- Pushes args with the specified environment
177 pushContArgs env [] cont = cont
178 pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont)
183 getContArgs :: SwitchChecker
184 -> OutId -> SimplCont
185 -> ([(InExpr, SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
186 SimplCont, -- Remaining continuation
187 Bool) -- Whether we came across an InlineCall
188 -- getContArgs id k = (args, k', inl)
189 -- args are the leading ApplyTo items in k
190 -- (i.e. outermost comes first)
191 -- augmented with demand info from the functionn
192 getContArgs chkr fun orig_cont
194 -- Ignore strictness info if the no-case-of-case
195 -- flag is on. Strictness changes evaluation order
196 -- and that can change full laziness
197 stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
198 | otherwise = computed_stricts
200 go [] stricts False orig_cont
202 ----------------------------
205 go acc ss inl (ApplyTo _ arg@(Type _) se cont)
206 = go ((arg,se,False) : acc) ss inl cont
207 -- NB: don't bother to instantiate the function type
210 go acc (s:ss) inl (ApplyTo _ arg se cont)
211 = go ((arg,se,s) : acc) ss inl cont
213 -- An Inline continuation
214 go acc ss inl (InlinePlease cont)
215 = go acc ss True cont
217 -- We're run out of arguments, or else we've run out of demands
218 -- The latter only happens if the result is guaranteed bottom
219 -- This is the case for
220 -- * case (error "hello") of { ... }
221 -- * (error "Hello") arg
222 -- * f (error "Hello") where f is strict
225 | null ss && discardableCont cont = (reverse acc, discardCont cont, inl)
226 | otherwise = (reverse acc, cont, inl)
228 ----------------------------
229 vanilla_stricts, computed_stricts :: [Bool]
230 vanilla_stricts = repeat False
231 computed_stricts = zipWith (||) fun_stricts arg_stricts
233 ----------------------------
234 (val_arg_tys, _) = splitRepFunTys (idType fun)
235 arg_stricts = map isStrictType val_arg_tys ++ repeat False
236 -- These argument types are used as a cheap and cheerful way to find
237 -- unboxed arguments, which must be strict. But it's an InType
238 -- and so there might be a type variable where we expect a function
239 -- type (the substitution hasn't happened yet). And we don't bother
240 -- doing the type applications for a polymorphic function.
241 -- Hence the split*Rep*FunTys
243 ----------------------------
244 -- If fun_stricts is finite, it means the function returns bottom
245 -- after that number of value args have been consumed
246 -- Otherwise it's infinite, extended with False
248 = case splitStrictSig (idNewStrictness fun) of
249 (demands, result_info)
250 | not (demands `lengthExceeds` countValArgs orig_cont)
251 -> -- Enough args, use the strictness given.
252 -- For bottoming functions we used to pretend that the arg
253 -- is lazy, so that we don't treat the arg as an
254 -- interesting context. This avoids substituting
255 -- top-level bindings for (say) strings into
256 -- calls to error. But now we are more careful about
257 -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
258 if isBotRes result_info then
259 map isStrictDmd demands -- Finite => result is bottom
261 map isStrictDmd demands ++ vanilla_stricts
263 other -> vanilla_stricts -- Not enough args, or no strictness
266 interestingArg :: OutExpr -> Bool
267 -- An argument is interesting if it has *some* structure
268 -- We are here trying to avoid unfolding a function that
269 -- is applied only to variables that have no unfolding
270 -- (i.e. they are probably lambda bound): f x y z
271 -- There is little point in inlining f here.
272 interestingArg (Var v) = hasSomeUnfolding (idUnfolding v)
273 -- Was: isValueUnfolding (idUnfolding v')
274 -- But that seems over-pessimistic
275 interestingArg (Type _) = False
276 interestingArg (App fn (Type _)) = interestingArg fn
277 interestingArg (Note _ a) = interestingArg a
278 interestingArg other = True
279 -- Consider let x = 3 in f x
280 -- The substitution will contain (x -> ContEx 3), and we want to
281 -- to say that x is an interesting argument.
282 -- But consider also (\x. f x y) y
283 -- The substitution will contain (x -> ContEx y), and we want to say
284 -- that x is not interesting (assuming y has no unfolding)
287 Comment about interestingCallContext
288 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
289 We want to avoid inlining an expression where there can't possibly be
290 any gain, such as in an argument position. Hence, if the continuation
291 is interesting (eg. a case scrutinee, application etc.) then we
292 inline, otherwise we don't.
294 Previously some_benefit used to return True only if the variable was
295 applied to some value arguments. This didn't work:
297 let x = _coerce_ (T Int) Int (I# 3) in
298 case _coerce_ Int (T Int) x of
301 we want to inline x, but can't see that it's a constructor in a case
302 scrutinee position, and some_benefit is False.
306 dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
308 .... case dMonadST _@_ x0 of (a,b,c) -> ....
310 we'd really like to inline dMonadST here, but we *don't* want to
311 inline if the case expression is just
313 case x of y { DEFAULT -> ... }
315 since we can just eliminate this case instead (x is in WHNF). Similar
316 applies when x is bound to a lambda expression. Hence
317 contIsInteresting looks for case expressions with just a single
321 interestingCallContext :: Bool -- False <=> no args at all
322 -> Bool -- False <=> no value args
324 -- The "lone-variable" case is important. I spent ages
325 -- messing about with unsatisfactory varaints, but this is nice.
326 -- The idea is that if a variable appear all alone
327 -- as an arg of lazy fn, or rhs Stop
328 -- as scrutinee of a case Select
329 -- as arg of a strict fn ArgOf
330 -- then we should not inline it (unless there is some other reason,
331 -- e.g. is is the sole occurrence). We achieve this by making
332 -- interestingCallContext return False for a lone variable.
334 -- Why? At least in the case-scrutinee situation, turning
335 -- let x = (a,b) in case x of y -> ...
337 -- let x = (a,b) in case (a,b) of y -> ...
339 -- let x = (a,b) in let y = (a,b) in ...
340 -- is bad if the binding for x will remain.
342 -- Another example: I discovered that strings
343 -- were getting inlined straight back into applications of 'error'
344 -- because the latter is strict.
346 -- f = \x -> ...(error s)...
348 -- Fundamentally such contexts should not ecourage inlining because
349 -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
350 -- so there's no gain.
352 -- However, even a type application or coercion isn't a lone variable.
354 -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
355 -- We had better inline that sucker! The case won't see through it.
357 -- For now, I'm treating treating a variable applied to types
358 -- in a *lazy* context "lone". The motivating example was
360 -- g = /\a. \y. h (f a)
361 -- There's no advantage in inlining f here, and perhaps
362 -- a significant disadvantage. Hence some_val_args in the Stop case
364 interestingCallContext some_args some_val_args cont
367 interesting (InlinePlease _) = True
368 interesting (Select _ _ _ _ _) = some_args
369 interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
370 -- Perhaps True is a bit over-keen, but I've
371 -- seen (coerce f) x, where f has an INLINE prag,
372 -- So we have to give some motivaiton for inlining it
373 interesting (ArgOf _ _ _ _) = some_val_args
374 interesting (Stop ty _ upd_in_place) = some_val_args && upd_in_place
375 interesting (CoerceIt _ cont) = interesting cont
376 -- If this call is the arg of a strict function, the context
377 -- is a bit interesting. If we inline here, we may get useful
378 -- evaluation information to avoid repeated evals: e.g.
380 -- Here the contIsInteresting makes the '*' keener to inline,
381 -- which in turn exposes a constructor which makes the '+' inline.
382 -- Assuming that +,* aren't small enough to inline regardless.
384 -- It's also very important to inline in a strict context for things
387 -- Here, the context of (f x) is strict, and if f's unfolding is
388 -- a build it's *great* to inline it here. So we must ensure that
389 -- the context for (f x) is not totally uninteresting.
393 canUpdateInPlace :: Type -> Bool
394 -- Consider let x = <wurble> in ...
395 -- If <wurble> returns an explicit constructor, we might be able
396 -- to do update in place. So we treat even a thunk RHS context
397 -- as interesting if update in place is possible. We approximate
398 -- this by seeing if the type has a single constructor with a
399 -- small arity. But arity zero isn't good -- we share the single copy
400 -- for that case, so no point in sharing.
403 | not opt_UF_UpdateInPlace = False
405 = case splitTyConApp_maybe ty of
407 Just (tycon, _) -> case tyConDataConsIfAvailable tycon of
408 [dc] -> arity == 1 || arity == 2
410 arity = dataConRepArity dc
416 %************************************************************************
418 \section{Dealing with a single binder}
420 %************************************************************************
422 These functions are in the monad only so that they can be made strict via seq.
425 simplBinders :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
426 simplBinders env bndrs
428 (subst', bndrs') = Subst.simplBndrs (getSubst env) bndrs
430 seqBndrs bndrs' `seq`
431 returnSmpl (setSubst env subst', bndrs')
433 simplBinder :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder)
436 (subst', bndr') = Subst.simplBndr (getSubst env) bndr
439 returnSmpl (setSubst env subst', bndr')
442 simplLamBinders :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
443 simplLamBinders env bndrs
445 (subst', bndrs') = mapAccumL Subst.simplLamBndr (getSubst env) bndrs
447 seqBndrs bndrs' `seq`
448 returnSmpl (setSubst env subst', bndrs')
450 simplRecIds :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
453 (subst', ids') = mapAccumL Subst.simplLetId (getSubst env) ids
456 returnSmpl (setSubst env subst', ids')
458 simplLetId :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder)
461 (subst', id') = Subst.simplLetId (getSubst env) id
464 returnSmpl (setSubst env subst', id')
467 seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
469 seqBndr b | isTyVar b = b `seq` ()
470 | otherwise = seqType (idType b) `seq`
477 newId :: UserFS -> Type -> SimplM Id
478 newId fs ty = getUniqueSmpl `thenSmpl` \ uniq ->
479 returnSmpl (mkSysLocal fs uniq ty)
483 %************************************************************************
485 \subsection{Rebuilding a lambda}
487 %************************************************************************
490 mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
494 a) eta reduction, if that gives a trivial expression
495 b) eta expansion [only if there are some value lambdas]
496 c) floating lets out through big lambdas
497 [only if all tyvar lambdas, and only if this lambda
501 mkLam env bndrs body cont
502 | opt_SimplDoEtaReduction,
503 Just etad_lam <- tryEtaReduce bndrs body
504 = tick (EtaReduction (head bndrs)) `thenSmpl_`
505 returnSmpl (emptyFloats env, etad_lam)
507 | opt_SimplDoLambdaEtaExpansion,
508 any isRuntimeVar bndrs
509 = tryEtaExpansion body `thenSmpl` \ body' ->
510 returnSmpl (emptyFloats env, mkLams bndrs body')
512 {- Sept 01: I'm experimenting with getting the
513 full laziness pass to float out past big lambdsa
514 | all isTyVar bndrs, -- Only for big lambdas
515 contIsRhs cont -- Only try the rhs type-lambda floating
516 -- if this is indeed a right-hand side; otherwise
517 -- we end up floating the thing out, only for float-in
518 -- to float it right back in again!
519 = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
520 returnSmpl (floats, mkLams bndrs body')
524 = returnSmpl (emptyFloats env, mkLams bndrs body)
528 %************************************************************************
530 \subsection{Eta expansion and reduction}
532 %************************************************************************
534 We try for eta reduction here, but *only* if we get all the
535 way to an exprIsTrivial expression.
536 We don't want to remove extra lambdas unless we are going
537 to avoid allocating this thing altogether
540 tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr
541 tryEtaReduce bndrs body
542 -- We don't use CoreUtils.etaReduce, because we can be more
544 -- (a) we already have the binders
545 -- (b) we can do the triviality test before computing the free vars
546 -- [in fact I take the simple path and look for just a variable]
547 = go (reverse bndrs) body
549 go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
550 go [] (Var fun) | ok_fun fun = Just (Var fun) -- Success!
551 go _ _ = Nothing -- Failure!
553 ok_fun fun = not (fun `elem` bndrs) && not (hasNoBinding fun)
554 ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
558 Try eta expansion for RHSs
561 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
564 where (in both cases)
566 * The xi can include type variables
568 * The yi are all value variables
570 * N is a NORMAL FORM (i.e. no redexes anywhere)
571 wanting a suitable number of extra args.
573 We may have to sandwich some coerces between the lambdas
574 to make the types work. exprEtaExpandArity looks through coerces
575 when computing arity; and etaExpand adds the coerces as necessary when
576 actually computing the expansion.
579 tryEtaExpansion :: OutExpr -> SimplM OutExpr
580 -- There is at least one runtime binder in the binders
582 | arity_is_manifest -- Some lambdas but not enough
586 = getUniquesSmpl `thenSmpl` \ us ->
587 returnSmpl (etaExpand fun_arity us body (exprType body))
589 (fun_arity, arity_is_manifest) = exprEtaExpandArity body
593 %************************************************************************
595 \subsection{Floating lets out of big lambdas}
597 %************************************************************************
599 tryRhsTyLam tries this transformation, when the big lambda appears as
600 the RHS of a let(rec) binding:
602 /\abc -> let(rec) x = e in b
604 let(rec) x' = /\abc -> let x = x' a b c in e
606 /\abc -> let x = x' a b c in b
608 This is good because it can turn things like:
610 let f = /\a -> letrec g = ... g ... in g
612 letrec g' = /\a -> ... g' a ...
616 which is better. In effect, it means that big lambdas don't impede
619 This optimisation is CRUCIAL in eliminating the junk introduced by
620 desugaring mutually recursive definitions. Don't eliminate it lightly!
622 So far as the implementation is concerned:
624 Invariant: go F e = /\tvs -> F e
628 = Let x' = /\tvs -> F e
632 G = F . Let x = x' tvs
634 go F (Letrec xi=ei in b)
635 = Letrec {xi' = /\tvs -> G ei}
639 G = F . Let {xi = xi' tvs}
641 [May 1999] If we do this transformation *regardless* then we can
642 end up with some pretty silly stuff. For example,
645 st = /\ s -> let { x1=r1 ; x2=r2 } in ...
650 st = /\s -> ...[y1 s/x1, y2 s/x2]
653 Unless the "..." is a WHNF there is really no point in doing this.
654 Indeed it can make things worse. Suppose x1 is used strictly,
657 x1* = case f y of { (a,b) -> e }
659 If we abstract this wrt the tyvar we then can't do the case inline
660 as we would normally do.
664 {- Trying to do this in full laziness
666 tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
667 -- Call ensures that all the binders are type variables
669 tryRhsTyLam env tyvars body -- Only does something if there's a let
670 | not (all isTyVar tyvars)
671 || not (worth_it body) -- inside a type lambda,
672 = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
675 = go env (\x -> x) body
678 worth_it e@(Let _ _) = whnf_in_middle e
681 whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
682 whnf_in_middle (Let _ e) = whnf_in_middle e
683 whnf_in_middle e = exprIsCheap e
685 main_tyvar_set = mkVarSet tyvars
687 go env fn (Let bind@(NonRec var rhs) body)
689 = go env (fn . Let bind) body
691 go env fn (Let (NonRec var rhs) body)
692 = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
693 addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
694 go env (fn . Let (mk_silly_bind var rhs')) body
698 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
699 -- Abstract only over the type variables free in the rhs
700 -- wrt which the new binding is abstracted. But the naive
701 -- approach of abstract wrt the tyvars free in the Id's type
703 -- /\ a b -> let t :: (a,b) = (e1, e2)
706 -- Here, b isn't free in x's type, but we must nevertheless
707 -- abstract wrt b as well, because t's type mentions b.
708 -- Since t is floated too, we'd end up with the bogus:
709 -- poly_t = /\ a b -> (e1, e2)
710 -- poly_x = /\ a -> fst (poly_t a *b*)
711 -- So for now we adopt the even more naive approach of
712 -- abstracting wrt *all* the tyvars. We'll see if that
713 -- gives rise to problems. SLPJ June 98
715 go env fn (Let (Rec prs) body)
716 = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
718 gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
719 pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
721 addAuxiliaryBind env (Rec pairs) $ \ env ->
724 (vars,rhss) = unzip prs
725 tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
726 -- See notes with tyvars_here above
728 go env fn body = returnSmpl (emptyFloats env, fn body)
730 mk_poly tyvars_here var
731 = getUniqueSmpl `thenSmpl` \ uniq ->
733 poly_name = setNameUnique (idName var) uniq -- Keep same name
734 poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
735 poly_id = mkLocalId poly_name poly_ty
737 -- In the olden days, it was crucial to copy the occInfo of the original var,
738 -- because we were looking at occurrence-analysed but as yet unsimplified code!
739 -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
740 -- at already simplified code, so it doesn't matter
742 -- It's even right to retain single-occurrence or dead-var info:
743 -- Suppose we started with /\a -> let x = E in B
744 -- where x occurs once in B. Then we transform to:
745 -- let x' = /\a -> E in /\a -> let x* = x' a in B
746 -- where x* has an INLINE prag on it. Now, once x* is inlined,
747 -- the occurrences of x' will be just the occurrences originally
750 returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
752 mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
753 -- Suppose we start with:
755 -- x = /\ a -> let g = G in E
757 -- Then we'll float to get
759 -- x = let poly_g = /\ a -> G
760 -- in /\ a -> let g = poly_g a in E
762 -- But now the occurrence analyser will see just one occurrence
763 -- of poly_g, not inside a lambda, so the simplifier will
764 -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
765 -- (I used to think that the "don't inline lone occurrences" stuff
766 -- would stop this happening, but since it's the *only* occurrence,
767 -- PreInlineUnconditionally kicks in first!)
769 -- Solution: put an INLINE note on g's RHS, so that poly_g seems
770 -- to appear many times. (NB: mkInlineMe eliminates
771 -- such notes on trivial RHSs, so do it manually.)
776 %************************************************************************
778 \subsection{Case absorption and identity-case elimination}
780 %************************************************************************
782 mkCase puts a case expression back together, trying various transformations first.
785 mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
787 mkCase scrut case_bndr alts
788 = mkAlts scrut case_bndr alts `thenSmpl` \ better_alts ->
789 mkCase1 scrut case_bndr better_alts
793 mkAlts tries these things:
795 1. If several alternatives are identical, merge them into
796 a single DEFAULT alternative. I've occasionally seen this
797 making a big difference:
799 case e of =====> case e of
800 C _ -> f x D v -> ....v....
801 D v -> ....v.... DEFAULT -> f x
804 The point is that we merge common RHSs, at least for the DEFAULT case.
805 [One could do something more elaborate but I've never seen it needed.]
806 To avoid an expensive test, we just merge branches equal to the *first*
807 alternative; this picks up the common cases
808 a) all branches equal
809 b) some branches equal to the DEFAULT (which occurs first)
811 2. If the DEFAULT alternative can match only one possible constructor,
812 then make that constructor explicit.
814 case e of x { DEFAULT -> rhs }
816 case e of x { (a,b) -> rhs }
817 where the type is a single constructor type. This gives better code
818 when rhs also scrutinises x or e.
821 case e of b { ==> case e of b {
822 p1 -> rhs1 p1 -> rhs1
824 pm -> rhsm pm -> rhsm
825 _ -> case b of b' { pn -> let b'=b in rhsn
827 ... po -> let b'=b in rhso
828 po -> rhso _ -> let b'=b in rhsd
832 which merges two cases in one case when -- the default alternative of
833 the outer case scrutises the same variable as the outer case This
834 transformation is called Case Merging. It avoids that the same
835 variable is scrutinised multiple times.
838 The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
844 where @is@ was something like
846 p `is` n = p /= (-1) && p == n
848 This gave rise to a horrible sequence of cases
855 and similarly in cascade for all the join points!
860 --------------------------------------------------
861 -- 1. Merge identical branches
862 --------------------------------------------------
863 mkAlts scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
864 | all isDeadBinder bndrs1, -- Remember the default
865 length filtered_alts < length con_alts -- alternative comes first
866 = tick (AltMerge case_bndr) `thenSmpl_`
867 returnSmpl better_alts
869 filtered_alts = filter keep con_alts
870 keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
871 better_alts = (DEFAULT, [], rhs1) : filtered_alts
874 --------------------------------------------------
875 -- 2. Fill in missing constructor
876 --------------------------------------------------
878 mkAlts scrut case_bndr alts
879 | Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr),
880 isDataTyCon tycon, -- It's a data type
881 (alts_no_deflt, Just rhs) <- findDefault alts,
882 -- There is a DEFAULT case
883 [missing_con] <- filter is_missing (tyConDataConsIfAvailable tycon)
884 -- There is just one missing constructor!
885 = tick (FillInCaseDefault case_bndr) `thenSmpl_`
886 getUniquesSmpl `thenSmpl` \ tv_uniqs ->
887 getUniquesSmpl `thenSmpl` \ id_uniqs ->
889 (_,_,ex_tyvars,_,_,_) = dataConSig missing_con
890 ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
891 mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
892 arg_ids = zipWith (mkSysLocal SLIT("a")) id_uniqs arg_tys
893 arg_tys = dataConArgTys missing_con (inst_tys ++ mkTyVarTys ex_tyvars')
894 better_alts = (DataAlt missing_con, ex_tyvars' ++ arg_ids, rhs) : alts_no_deflt
896 returnSmpl better_alts
898 impossible_cons = otherCons (idUnfolding case_bndr)
899 handled_data_cons = [data_con | DataAlt data_con <- impossible_cons] ++
900 [data_con | (DataAlt data_con, _, _) <- alts]
901 is_missing con = not (con `elem` handled_data_cons)
903 --------------------------------------------------
904 -- 3. Merge nested cases
905 --------------------------------------------------
907 mkAlts scrut outer_bndr outer_alts
908 | opt_SimplCaseMerge,
909 (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
910 Just (Case (Var scrut_var) inner_bndr inner_alts) <- maybe_outer_deflt,
911 scruting_same_var scrut_var
913 = let -- Eliminate any inner alts which are shadowed by the outer ones
914 outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
916 munged_inner_alts = [ (con, args, munge_rhs rhs)
917 | (con, args, rhs) <- inner_alts,
918 not (con `elem` outer_cons) -- Eliminate shadowed inner alts
920 munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
922 (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
924 new_alts = add_default maybe_inner_default
925 (outer_alts_without_deflt ++ inner_con_alts)
927 tick (CaseMerge outer_bndr) `thenSmpl_`
929 -- Warning: don't call mkAlts recursively!
930 -- Firstly, there's no point, because inner alts have already had
931 -- mkCase applied to them, so they won't have a case in their default
932 -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
933 -- in munge_rhs may put a case into the DEFAULT branch!
935 -- We are scrutinising the same variable if it's
936 -- the outer case-binder, or if the outer case scrutinises a variable
937 -- (and it's the same). Testing both allows us not to replace the
938 -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
939 scruting_same_var = case scrut of
940 Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
941 other -> \ v -> v == outer_bndr
943 add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
944 add_default Nothing alts = alts
947 --------------------------------------------------
949 --------------------------------------------------
951 mkAlts scrut case_bndr other_alts = returnSmpl other_alts
956 =================================================================================
958 mkCase1 tries these things
960 1. Eliminate the case altogether if possible
971 Start with a simple situation:
973 case x# of ===> e[x#/y#]
976 (when x#, y# are of primitive type, of course). We can't (in general)
977 do this for algebraic cases, because we might turn bottom into
980 Actually, we generalise this idea to look for a case where we're
981 scrutinising a variable, and we know that only the default case can
986 other -> ...(case x of
990 Here the inner case can be eliminated. This really only shows up in
991 eliminating error-checking code.
993 We also make sure that we deal with this very common case:
998 Here we are using the case as a strict let; if x is used only once
999 then we want to inline it. We have to be careful that this doesn't
1000 make the program terminate when it would have diverged before, so we
1002 - x is used strictly, or
1003 - e is already evaluated (it may so if e is a variable)
1005 Lastly, we generalise the transformation to handle this:
1011 We only do this for very cheaply compared r's (constructors, literals
1012 and variables). If pedantic bottoms is on, we only do it when the
1013 scrutinee is a PrimOp which can't fail.
1015 We do it *here*, looking at un-simplified alternatives, because we
1016 have to check that r doesn't mention the variables bound by the
1017 pattern in each alternative, so the binder-info is rather useful.
1019 So the case-elimination algorithm is:
1021 1. Eliminate alternatives which can't match
1023 2. Check whether all the remaining alternatives
1024 (a) do not mention in their rhs any of the variables bound in their pattern
1025 and (b) have equal rhss
1027 3. Check we can safely ditch the case:
1028 * PedanticBottoms is off,
1029 or * the scrutinee is an already-evaluated variable
1030 or * the scrutinee is a primop which is ok for speculation
1031 -- ie we want to preserve divide-by-zero errors, and
1032 -- calls to error itself!
1034 or * [Prim cases] the scrutinee is a primitive variable
1036 or * [Alg cases] the scrutinee is a variable and
1037 either * the rhs is the same variable
1038 (eg case x of C a b -> x ===> x)
1039 or * there is only one alternative, the default alternative,
1040 and the binder is used strictly in its scope.
1041 [NB this is helped by the "use default binder where
1042 possible" transformation; see below.]
1045 If so, then we can replace the case with one of the rhss.
1049 --------------------------------------------------
1050 -- 1. Eliminate the case altogether if poss
1051 --------------------------------------------------
1053 mkCase1 scrut case_bndr [(con,bndrs,rhs)]
1054 -- See if we can get rid of the case altogether
1055 -- See the extensive notes on case-elimination above
1056 -- mkCase made sure that if all the alternatives are equal,
1057 -- then there is now only one (DEFAULT) rhs
1058 | all isDeadBinder bndrs,
1060 -- Check that the scrutinee can be let-bound instead of case-bound
1061 exprOkForSpeculation scrut
1062 -- OK not to evaluate it
1063 -- This includes things like (==# a# b#)::Bool
1064 -- so that we simplify
1065 -- case ==# a# b# of { True -> x; False -> x }
1068 -- This particular example shows up in default methods for
1069 -- comparision operations (e.g. in (>=) for Int.Int32)
1070 || exprIsValue scrut -- It's already evaluated
1071 || var_demanded_later scrut -- It'll be demanded later
1073 -- || not opt_SimplPedanticBottoms) -- Or we don't care!
1074 -- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
1075 -- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
1076 -- its argument: case x of { y -> dataToTag# y }
1077 -- Here we must *not* discard the case, because dataToTag# just fetches the tag from
1078 -- the info pointer. So we'll be pedantic all the time, and see if that gives any
1080 = tick (CaseElim case_bndr) `thenSmpl_`
1081 returnSmpl (bindCaseBndr case_bndr scrut rhs)
1084 -- The case binder is going to be evaluated later,
1085 -- and the scrutinee is a simple variable
1086 var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1087 var_demanded_later other = False
1090 --------------------------------------------------
1092 --------------------------------------------------
1094 mkCase1 scrut case_bndr alts -- Identity case
1095 | all identity_alt alts
1096 = tick (CaseIdentity case_bndr) `thenSmpl_`
1097 returnSmpl (re_note scrut)
1099 identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
1101 identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
1102 identity_rhs (LitAlt lit) _ = Lit lit
1103 identity_rhs DEFAULT _ = Var case_bndr
1105 arg_tys = map Type (tyConAppArgs (idType case_bndr))
1108 -- case coerce T e of x { _ -> coerce T' x }
1109 -- And we definitely want to eliminate this case!
1110 -- So we throw away notes from the RHS, and reconstruct
1111 -- (at least an approximation) at the other end
1112 de_note (Note _ e) = de_note e
1115 -- re_note wraps a coerce if it might be necessary
1116 re_note scrut = case head alts of
1117 (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
1121 --------------------------------------------------
1123 --------------------------------------------------
1124 mkCase1 scrut bndr alts = returnSmpl (Case scrut bndr alts)
1128 When adding auxiliary bindings for the case binder, it's worth checking if
1129 its dead, because it often is, and occasionally these mkCase transformations
1130 cascade rather nicely.
1133 bindCaseBndr bndr rhs body
1134 | isDeadBinder bndr = body
1135 | otherwise = bindNonRec bndr rhs body