2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[CoreUtils]{Utility functions on @Core@ syntax}
9 mkInlineMe, mkSCC, mkCoerce, mkCoerce2,
10 bindNonRec, needsCaseBinding,
11 mkIfThenElse, mkAltExpr, mkPiType, mkPiTypes,
13 -- Taking expressions apart
16 -- Properties of expressions
17 exprType, coreAltType,
18 exprIsDupable, exprIsTrivial, exprIsCheap,
19 exprIsValue,exprOkForSpeculation, exprIsBig,
20 exprIsConApp_maybe, exprIsBottom,
23 -- Arity and eta expansion
24 manifestArity, exprArity,
25 exprEtaExpandArity, etaExpand,
34 cheapEqExpr, eqExpr, applyTypeToArgs, applyTypeToArg
37 #include "HsVersions.h"
40 import GLAEXTS -- For `xori`
43 import PprCore ( pprCoreExpr )
44 import Var ( Var, isId, isTyVar )
46 import Name ( hashName, isDllName )
47 import Literal ( hashLiteral, literalType, litIsDupable,
48 litIsTrivial, isZeroLit, Literal( MachLabel ) )
49 import DataCon ( DataCon, dataConRepArity, dataConArgTys,
50 isVanillaDataCon, dataConTyCon )
51 import PrimOp ( PrimOp(..), primOpOkForSpeculation, primOpIsCheap )
52 import Id ( Id, idType, globalIdDetails, idNewStrictness,
53 mkWildId, idArity, idName, idUnfolding, idInfo,
54 isOneShotBndr, isStateHackType, isDataConWorkId_maybe, mkSysLocal,
55 isDataConWorkId, isBottomingId
57 import IdInfo ( GlobalIdDetails(..), megaSeqIdInfo )
58 import NewDemand ( appIsBottom )
59 import Type ( Type, mkFunTy, mkForAllTy, splitFunTy_maybe,
61 applyTys, isUnLiftedType, seqType, mkTyVarTy,
62 splitForAllTy_maybe, isForAllTy, splitRecNewType_maybe,
63 splitTyConApp_maybe, eqType, funResultTy, applyTy,
66 import TyCon ( tyConArity )
68 import TysWiredIn ( boolTy, trueDataCon, falseDataCon )
69 import CostCentre ( CostCentre )
70 import BasicTypes ( Arity )
71 import Unique ( Unique )
73 import TysPrim ( alphaTy ) -- Debugging only
74 import Util ( equalLength, lengthAtLeast )
78 %************************************************************************
80 \subsection{Find the type of a Core atom/expression}
82 %************************************************************************
85 exprType :: CoreExpr -> Type
87 exprType (Var var) = idType var
88 exprType (Lit lit) = literalType lit
89 exprType (Let _ body) = exprType body
91 exprType (Case _ _ ty alts) = ty
92 exprType (Note (Coerce ty _) e) = ty -- **! should take usage from e
93 exprType (Note other_note e) = exprType e
94 exprType (Lam binder expr) = mkPiType binder (exprType expr)
96 = case collectArgs e of
97 (fun, args) -> applyTypeToArgs e (exprType fun) args
99 exprType other = pprTrace "exprType" (pprCoreExpr other) alphaTy
101 coreAltType :: CoreAlt -> Type
102 coreAltType (_,_,rhs) = exprType rhs
105 @mkPiType@ makes a (->) type or a forall type, depending on whether
106 it is given a type variable or a term variable. We cleverly use the
107 lbvarinfo field to figure out the right annotation for the arrove in
108 case of a term variable.
111 mkPiType :: Var -> Type -> Type -- The more polymorphic version
112 mkPiTypes :: [Var] -> Type -> Type -- doesn't work...
114 mkPiTypes vs ty = foldr mkPiType ty vs
117 | isId v = mkFunTy (idType v) ty
118 | otherwise = mkForAllTy v ty
122 applyTypeToArg :: Type -> CoreExpr -> Type
123 applyTypeToArg fun_ty (Type arg_ty) = applyTy fun_ty arg_ty
124 applyTypeToArg fun_ty other_arg = funResultTy fun_ty
126 applyTypeToArgs :: CoreExpr -> Type -> [CoreExpr] -> Type
127 -- A more efficient version of applyTypeToArg
128 -- when we have several args
129 -- The first argument is just for debugging
130 applyTypeToArgs e op_ty [] = op_ty
132 applyTypeToArgs e op_ty (Type ty : args)
133 = -- Accumulate type arguments so we can instantiate all at once
136 go rev_tys (Type ty : args) = go (ty:rev_tys) args
137 go rev_tys rest_args = applyTypeToArgs e op_ty' rest_args
139 op_ty' = applyTys op_ty (reverse rev_tys)
141 applyTypeToArgs e op_ty (other_arg : args)
142 = case (splitFunTy_maybe op_ty) of
143 Just (_, res_ty) -> applyTypeToArgs e res_ty args
144 Nothing -> pprPanic "applyTypeToArgs" (pprCoreExpr e)
149 %************************************************************************
151 \subsection{Attaching notes}
153 %************************************************************************
155 mkNote removes redundant coercions, and SCCs where possible
159 mkNote :: Note -> CoreExpr -> CoreExpr
160 mkNote (Coerce to_ty from_ty) expr = mkCoerce2 to_ty from_ty expr
161 mkNote (SCC cc) expr = mkSCC cc expr
162 mkNote InlineMe expr = mkInlineMe expr
163 mkNote note expr = Note note expr
166 -- Slide InlineCall in around the function
167 -- No longer necessary I think (SLPJ Apr 99)
168 -- mkNote InlineCall (App f a) = App (mkNote InlineCall f) a
169 -- mkNote InlineCall (Var v) = Note InlineCall (Var v)
170 -- mkNote InlineCall expr = expr
173 Drop trivial InlineMe's. This is somewhat important, because if we have an unfolding
174 that looks like (Note InlineMe (Var v)), the InlineMe doesn't go away because it may
175 not be *applied* to anything.
177 We don't use exprIsTrivial here, though, because we sometimes generate worker/wrapper
180 f = inline_me (coerce t fw)
181 As usual, the inline_me prevents the worker from getting inlined back into the wrapper.
182 We want the split, so that the coerces can cancel at the call site.
184 However, we can get left with tiresome type applications. Notably, consider
185 f = /\ a -> let t = e in (t, w)
186 Then lifting the let out of the big lambda gives
188 f = /\ a -> let t = inline_me (t' a) in (t, w)
189 The inline_me is to stop the simplifier inlining t' right back
190 into t's RHS. In the next phase we'll substitute for t (since
191 its rhs is trivial) and *then* we could get rid of the inline_me.
192 But it hardly seems worth it, so I don't bother.
195 mkInlineMe (Var v) = Var v
196 mkInlineMe e = Note InlineMe e
202 mkCoerce :: Type -> CoreExpr -> CoreExpr
203 mkCoerce to_ty expr = mkCoerce2 to_ty (exprType expr) expr
205 mkCoerce2 :: Type -> Type -> CoreExpr -> CoreExpr
206 mkCoerce2 to_ty from_ty (Note (Coerce to_ty2 from_ty2) expr)
207 = ASSERT( from_ty `eqType` to_ty2 )
208 mkCoerce2 to_ty from_ty2 expr
210 mkCoerce2 to_ty from_ty expr
211 | to_ty `eqType` from_ty = expr
212 | otherwise = ASSERT( from_ty `eqType` exprType expr )
213 Note (Coerce to_ty from_ty) expr
217 mkSCC :: CostCentre -> Expr b -> Expr b
218 -- Note: Nested SCC's *are* preserved for the benefit of
219 -- cost centre stack profiling
220 mkSCC cc (Lit lit) = Lit lit
221 mkSCC cc (Lam x e) = Lam x (mkSCC cc e) -- Move _scc_ inside lambda
222 mkSCC cc (Note (SCC cc') e) = Note (SCC cc) (Note (SCC cc') e)
223 mkSCC cc (Note n e) = Note n (mkSCC cc e) -- Move _scc_ inside notes
224 mkSCC cc expr = Note (SCC cc) expr
228 %************************************************************************
230 \subsection{Other expression construction}
232 %************************************************************************
235 bindNonRec :: Id -> CoreExpr -> CoreExpr -> CoreExpr
236 -- (bindNonRec x r b) produces either
239 -- case r of x { _DEFAULT_ -> b }
241 -- depending on whether x is unlifted or not
242 -- It's used by the desugarer to avoid building bindings
243 -- that give Core Lint a heart attack. Actually the simplifier
244 -- deals with them perfectly well.
246 bindNonRec bndr rhs body
248 | needsCaseBinding (idType bndr) rhs = Case rhs bndr (exprType body) [(DEFAULT,[],body)]
249 | otherwise = Let (NonRec bndr rhs) body
251 needsCaseBinding ty rhs = isUnLiftedType ty && not (exprOkForSpeculation rhs)
252 -- Make a case expression instead of a let
253 -- These can arise either from the desugarer,
254 -- or from beta reductions: (\x.e) (x +# y)
258 mkAltExpr :: AltCon -> [CoreBndr] -> [Type] -> CoreExpr
259 -- This guy constructs the value that the scrutinee must have
260 -- when you are in one particular branch of a case
261 mkAltExpr (DataAlt con) args inst_tys
262 = mkConApp con (map Type inst_tys ++ map varToCoreExpr args)
263 mkAltExpr (LitAlt lit) [] []
266 mkIfThenElse :: CoreExpr -> CoreExpr -> CoreExpr -> CoreExpr
267 mkIfThenElse guard then_expr else_expr
269 -- Not going to be refining, so okay to take the type of the "then" clause
270 = Case guard (mkWildId boolTy) (exprType then_expr)
271 [ (DataAlt trueDataCon, [], then_expr),
272 (DataAlt falseDataCon, [], else_expr) ]
276 %************************************************************************
278 \subsection{Taking expressions apart}
280 %************************************************************************
282 The default alternative must be first, if it exists at all.
283 This makes it easy to find, though it makes matching marginally harder.
286 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
287 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null args ) (alts, Just rhs)
288 findDefault alts = (alts, Nothing)
290 findAlt :: AltCon -> [CoreAlt] -> CoreAlt
293 (deflt@(DEFAULT,_,_):alts) -> go alts deflt
294 other -> go alts panic_deflt
297 panic_deflt = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
300 go (alt@(con1,_,_) : alts) deflt | con == con1 = alt
301 | otherwise = ASSERT( not (con1 == DEFAULT) )
306 %************************************************************************
308 \subsection{Figuring out things about expressions}
310 %************************************************************************
312 @exprIsTrivial@ is true of expressions we are unconditionally happy to
313 duplicate; simple variables and constants, and type
314 applications. Note that primop Ids aren't considered
317 @exprIsBottom@ is true of expressions that are guaranteed to diverge
320 There used to be a gruesome test for (hasNoBinding v) in the
322 exprIsTrivial (Var v) | hasNoBinding v = idArity v == 0
323 The idea here is that a constructor worker, like $wJust, is
324 really short for (\x -> $wJust x), becuase $wJust has no binding.
325 So it should be treated like a lambda. Ditto unsaturated primops.
326 But now constructor workers are not "have-no-binding" Ids. And
327 completely un-applied primops and foreign-call Ids are sufficiently
328 rare that I plan to allow them to be duplicated and put up with
331 SCC notes. We do not treat (_scc_ "foo" x) as trivial, because
332 a) it really generates code, (and a heap object when it's
333 a function arg) to capture the cost centre
334 b) see the note [SCC-and-exprIsTrivial] in Simplify.simplLazyBind
337 exprIsTrivial (Var v) = True -- See notes above
338 exprIsTrivial (Type _) = True
339 exprIsTrivial (Lit lit) = litIsTrivial lit
340 exprIsTrivial (App e arg) = not (isRuntimeArg arg) && exprIsTrivial e
341 exprIsTrivial (Note (SCC _) e) = False -- See notes above
342 exprIsTrivial (Note _ e) = exprIsTrivial e
343 exprIsTrivial (Lam b body) = not (isRuntimeVar b) && exprIsTrivial body
344 exprIsTrivial other = False
348 @exprIsDupable@ is true of expressions that can be duplicated at a modest
349 cost in code size. This will only happen in different case
350 branches, so there's no issue about duplicating work.
352 That is, exprIsDupable returns True of (f x) even if
353 f is very very expensive to call.
355 Its only purpose is to avoid fruitless let-binding
356 and then inlining of case join points
360 exprIsDupable (Type _) = True
361 exprIsDupable (Var v) = True
362 exprIsDupable (Lit lit) = litIsDupable lit
363 exprIsDupable (Note InlineMe e) = True
364 exprIsDupable (Note _ e) = exprIsDupable e
368 go (Var v) n_args = True
369 go (App f a) n_args = n_args < dupAppSize
372 go other n_args = False
375 dupAppSize = 4 -- Size of application we are prepared to duplicate
378 @exprIsCheap@ looks at a Core expression and returns \tr{True} if
379 it is obviously in weak head normal form, or is cheap to get to WHNF.
380 [Note that that's not the same as exprIsDupable; an expression might be
381 big, and hence not dupable, but still cheap.]
383 By ``cheap'' we mean a computation we're willing to:
384 push inside a lambda, or
385 inline at more than one place
386 That might mean it gets evaluated more than once, instead of being
387 shared. The main examples of things which aren't WHNF but are
392 (where e, and all the ei are cheap)
395 (where e and b are cheap)
398 (where op is a cheap primitive operator)
401 (because we are happy to substitute it inside a lambda)
403 Notice that a variable is considered 'cheap': we can push it inside a lambda,
404 because sharing will make sure it is only evaluated once.
407 exprIsCheap :: CoreExpr -> Bool
408 exprIsCheap (Lit lit) = True
409 exprIsCheap (Type _) = True
410 exprIsCheap (Var _) = True
411 exprIsCheap (Note InlineMe e) = True
412 exprIsCheap (Note _ e) = exprIsCheap e
413 exprIsCheap (Lam x e) = isRuntimeVar x || exprIsCheap e
415 exprIsCheap (Case e _ _ alts) = exprIsCheap e &&
416 and [exprIsCheap rhs | (_,_,rhs) <- alts]
417 -- Experimentally, treat (case x of ...) as cheap
418 -- (and case __coerce x etc.)
419 -- This improves arities of overloaded functions where
420 -- there is only dictionary selection (no construction) involved
421 exprIsCheap (Let (NonRec x _) e)
422 | isUnLiftedType (idType x) = exprIsCheap e
424 -- strict lets always have cheap right hand sides, and
427 exprIsCheap other_expr
428 = go other_expr 0 True
430 go (Var f) n_args args_cheap
431 = (idAppIsCheap f n_args && args_cheap)
432 -- A constructor, cheap primop, or partial application
434 || idAppIsBottom f n_args
435 -- Application of a function which
436 -- always gives bottom; we treat this as cheap
437 -- because it certainly doesn't need to be shared!
439 go (App f a) n_args args_cheap
440 | not (isRuntimeArg a) = go f n_args args_cheap
441 | otherwise = go f (n_args + 1) (exprIsCheap a && args_cheap)
443 go other n_args args_cheap = False
445 idAppIsCheap :: Id -> Int -> Bool
446 idAppIsCheap id n_val_args
447 | n_val_args == 0 = True -- Just a type application of
448 -- a variable (f t1 t2 t3)
450 | otherwise = case globalIdDetails id of
451 DataConWorkId _ -> True
452 RecordSelId _ _ -> True -- I'm experimenting with making record selection
453 ClassOpId _ -> True -- look cheap, so we will substitute it inside a
454 -- lambda. Particularly for dictionary field selection
456 PrimOpId op -> primOpIsCheap op -- In principle we should worry about primops
457 -- that return a type variable, since the result
458 -- might be applied to something, but I'm not going
459 -- to bother to check the number of args
460 other -> n_val_args < idArity id
463 exprOkForSpeculation returns True of an expression that it is
465 * safe to evaluate even if normal order eval might not
466 evaluate the expression at all, or
468 * safe *not* to evaluate even if normal order would do so
472 the expression guarantees to terminate,
474 without raising an exception,
475 without causing a side effect (e.g. writing a mutable variable)
478 let x = case y# +# 1# of { r# -> I# r# }
481 case y# +# 1# of { r# ->
486 We can only do this if the (y+1) is ok for speculation: it has no
487 side effects, and can't diverge or raise an exception.
490 exprOkForSpeculation :: CoreExpr -> Bool
491 exprOkForSpeculation (Lit _) = True
492 exprOkForSpeculation (Type _) = True
493 exprOkForSpeculation (Var v) = isUnLiftedType (idType v)
494 exprOkForSpeculation (Note _ e) = exprOkForSpeculation e
495 exprOkForSpeculation other_expr
496 = case collectArgs other_expr of
497 (Var f, args) -> spec_ok (globalIdDetails f) args
501 spec_ok (DataConWorkId _) args
502 = True -- The strictness of the constructor has already
503 -- been expressed by its "wrapper", so we don't need
504 -- to take the arguments into account
506 spec_ok (PrimOpId op) args
507 | isDivOp op, -- Special case for dividing operations that fail
508 [arg1, Lit lit] <- args -- only if the divisor is zero
509 = not (isZeroLit lit) && exprOkForSpeculation arg1
510 -- Often there is a literal divisor, and this
511 -- can get rid of a thunk in an inner looop
514 = primOpOkForSpeculation op &&
515 all exprOkForSpeculation args
516 -- A bit conservative: we don't really need
517 -- to care about lazy arguments, but this is easy
519 spec_ok other args = False
521 isDivOp :: PrimOp -> Bool
522 -- True of dyadic operators that can fail
523 -- only if the second arg is zero
524 -- This function probably belongs in PrimOp, or even in
525 -- an automagically generated file.. but it's such a
526 -- special case I thought I'd leave it here for now.
527 isDivOp IntQuotOp = True
528 isDivOp IntRemOp = True
529 isDivOp WordQuotOp = True
530 isDivOp WordRemOp = True
531 isDivOp IntegerQuotRemOp = True
532 isDivOp IntegerDivModOp = True
533 isDivOp FloatDivOp = True
534 isDivOp DoubleDivOp = True
535 isDivOp other = False
540 exprIsBottom :: CoreExpr -> Bool -- True => definitely bottom
541 exprIsBottom e = go 0 e
543 -- n is the number of args
544 go n (Note _ e) = go n e
545 go n (Let _ e) = go n e
547 go n (Case e _ _ _) = go 0 e -- Just check the scrut
548 go n (App e _) = go (n+1) e
549 go n (Var v) = idAppIsBottom v n
551 go n (Lam _ _) = False
553 idAppIsBottom :: Id -> Int -> Bool
554 idAppIsBottom id n_val_args = appIsBottom (idNewStrictness id) n_val_args
557 @exprIsValue@ returns true for expressions that are certainly *already*
558 evaluated to *head* normal form. This is used to decide whether it's ok
561 case x of _ -> e ===> e
563 and to decide whether it's safe to discard a `seq`
565 So, it does *not* treat variables as evaluated, unless they say they are.
567 But it *does* treat partial applications and constructor applications
568 as values, even if their arguments are non-trivial, provided the argument
570 e.g. (:) (f x) (map f xs) is a value
571 map (...redex...) is a value
572 Because `seq` on such things completes immediately
574 For unlifted argument types, we have to be careful:
576 Suppose (f x) diverges; then C (f x) is not a value. True, but
577 this form is illegal (see the invariants in CoreSyn). Args of unboxed
578 type must be ok-for-speculation (or trivial).
581 exprIsValue :: CoreExpr -> Bool -- True => Value-lambda, constructor, PAP
582 exprIsValue (Var v) -- NB: There are no value args at this point
583 = isDataConWorkId v -- Catches nullary constructors,
584 -- so that [] and () are values, for example
585 || idArity v > 0 -- Catches (e.g.) primops that don't have unfoldings
586 || isEvaldUnfolding (idUnfolding v)
587 -- Check the thing's unfolding; it might be bound to a value
588 -- A worry: what if an Id's unfolding is just itself:
589 -- then we could get an infinite loop...
591 exprIsValue (Lit l) = True
592 exprIsValue (Type ty) = True -- Types are honorary Values;
593 -- we don't mind copying them
594 exprIsValue (Lam b e) = isRuntimeVar b || exprIsValue e
595 exprIsValue (Note _ e) = exprIsValue e
596 exprIsValue (App e (Type _)) = exprIsValue e
597 exprIsValue (App e a) = app_is_value e [a]
598 exprIsValue other = False
600 -- There is at least one value argument
601 app_is_value (Var fun) args
602 | isDataConWorkId fun -- Constructor apps are values
603 || idArity fun > valArgCount args -- Under-applied function
604 = check_args (idType fun) args
605 app_is_value (App f a) as = app_is_value f (a:as)
606 app_is_value other as = False
608 -- 'check_args' checks that unlifted-type args
609 -- are in fact guaranteed non-divergent
610 check_args fun_ty [] = True
611 check_args fun_ty (Type _ : args) = case splitForAllTy_maybe fun_ty of
612 Just (_, ty) -> check_args ty args
613 check_args fun_ty (arg : args)
614 | isUnLiftedType arg_ty = exprOkForSpeculation arg
615 | otherwise = check_args res_ty args
617 (arg_ty, res_ty) = splitFunTy fun_ty
621 exprIsConApp_maybe :: CoreExpr -> Maybe (DataCon, [CoreExpr])
622 exprIsConApp_maybe (Note (Coerce to_ty from_ty) expr)
623 = -- Maybe this is over the top, but here we try to turn
624 -- coerce (S,T) ( x, y )
626 -- ( coerce S x, coerce T y )
627 -- This happens in anger in PrelArrExts which has a coerce
628 -- case coerce memcpy a b of
630 -- where the memcpy is in the IO monad, but the call is in
632 case exprIsConApp_maybe expr of {
636 case splitTyConApp_maybe to_ty of {
638 Just (tc, tc_arg_tys) | tc /= dataConTyCon dc -> Nothing
639 | not (isVanillaDataCon dc) -> Nothing
641 -- Type constructor must match
642 -- We knock out existentials to keep matters simple(r)
644 arity = tyConArity tc
645 val_args = drop arity args
646 to_arg_tys = dataConArgTys dc tc_arg_tys
647 mk_coerce ty arg = mkCoerce ty arg
648 new_val_args = zipWith mk_coerce to_arg_tys val_args
650 ASSERT( all isTypeArg (take arity args) )
651 ASSERT( equalLength val_args to_arg_tys )
652 Just (dc, map Type tc_arg_tys ++ new_val_args)
655 exprIsConApp_maybe (Note _ expr)
656 = exprIsConApp_maybe expr
657 -- We ignore InlineMe notes in case we have
658 -- x = __inline_me__ (a,b)
659 -- All part of making sure that INLINE pragmas never hurt
660 -- Marcin tripped on this one when making dictionaries more inlinable
662 -- In fact, we ignore all notes. For example,
663 -- case _scc_ "foo" (C a b) of
665 -- should be optimised away, but it will be only if we look
666 -- through the SCC note.
668 exprIsConApp_maybe expr = analyse (collectArgs expr)
670 analyse (Var fun, args)
671 | Just con <- isDataConWorkId_maybe fun,
672 args `lengthAtLeast` dataConRepArity con
673 -- Might be > because the arity excludes type args
676 -- Look through unfoldings, but only cheap ones, because
677 -- we are effectively duplicating the unfolding
678 analyse (Var fun, [])
679 | let unf = idUnfolding fun,
681 = exprIsConApp_maybe (unfoldingTemplate unf)
683 analyse other = Nothing
688 %************************************************************************
690 \subsection{Eta reduction and expansion}
692 %************************************************************************
695 exprEtaExpandArity :: CoreExpr -> Arity
696 {- The Arity returned is the number of value args the
697 thing can be applied to without doing much work
699 exprEtaExpandArity is used when eta expanding
702 It returns 1 (or more) to:
703 case x of p -> \s -> ...
704 because for I/O ish things we really want to get that \s to the top.
705 We are prepared to evaluate x each time round the loop in order to get that
707 It's all a bit more subtle than it looks:
711 Consider one-shot lambdas
712 let x = expensive in \y z -> E
713 We want this to have arity 2 if the \y-abstraction is a 1-shot lambda
714 Hence the ArityType returned by arityType
716 2. The state-transformer hack
718 The one-shot lambda special cause is particularly important/useful for
719 IO state transformers, where we often get
720 let x = E in \ s -> ...
722 and the \s is a real-world state token abstraction. Such abstractions
723 are almost invariably 1-shot, so we want to pull the \s out, past the
724 let x=E, even if E is expensive. So we treat state-token lambdas as
725 one-shot even if they aren't really. The hack is in Id.isOneShotBndr.
727 3. Dealing with bottom
730 f = \x -> error "foo"
731 Here, arity 1 is fine. But if it is
735 then we want to get arity 2. Tecnically, this isn't quite right, because
737 should diverge, but it'll converge if we eta-expand f. Nevertheless, we
738 do so; it improves some programs significantly, and increasing convergence
739 isn't a bad thing. Hence the ABot/ATop in ArityType.
741 Actually, the situation is worse. Consider
745 Can we eta-expand here? At first the answer looks like "yes of course", but
748 This should diverge! But if we eta-expand, it won't. Again, we ignore this
749 "problem", because being scrupulous would lose an important transformation for
754 exprEtaExpandArity e = arityDepth (arityType e)
756 -- A limited sort of function type
757 data ArityType = AFun Bool ArityType -- True <=> one-shot
758 | ATop -- Know nothing
761 arityDepth :: ArityType -> Arity
762 arityDepth (AFun _ ty) = 1 + arityDepth ty
765 andArityType ABot at2 = at2
766 andArityType ATop at2 = ATop
767 andArityType (AFun t1 at1) (AFun t2 at2) = AFun (t1 && t2) (andArityType at1 at2)
768 andArityType at1 at2 = andArityType at2 at1
770 arityType :: CoreExpr -> ArityType
771 -- (go1 e) = [b1,..,bn]
772 -- means expression can be rewritten \x_b1 -> ... \x_bn -> body
773 -- where bi is True <=> the lambda is one-shot
775 arityType (Note n e) = arityType e
776 -- Not needed any more: etaExpand is cleverer
777 -- | ok_note n = arityType e
778 -- | otherwise = ATop
781 = mk (idArity v) (arg_tys (idType v))
783 mk :: Arity -> [Type] -> ArityType
784 -- The argument types are only to steer the "state hack"
785 -- Consider case x of
787 -- False -> \(s:RealWorld) -> e
788 -- where foo has arity 1. Then we want the state hack to
789 -- apply to foo too, so we can eta expand the case.
790 mk 0 tys | isBottomingId v = ABot
792 mk n (ty:tys) = AFun (isStateHackType ty) (mk (n-1) tys)
793 mk n [] = AFun False (mk (n-1) [])
795 arg_tys :: Type -> [Type] -- Ignore for-alls
797 | Just (_, ty') <- splitForAllTy_maybe ty = arg_tys ty'
798 | Just (arg,res) <- splitFunTy_maybe ty = arg : arg_tys res
801 -- Lambdas; increase arity
802 arityType (Lam x e) | isId x = AFun (isOneShotBndr x) (arityType e)
803 | otherwise = arityType e
805 -- Applications; decrease arity
806 arityType (App f (Type _)) = arityType f
807 arityType (App f a) = case arityType f of
808 AFun one_shot xs | exprIsCheap a -> xs
811 -- Case/Let; keep arity if either the expression is cheap
812 -- or it's a 1-shot lambda
813 -- The former is not really right for Haskell
814 -- f x = case x of { (a,b) -> \y. e }
816 -- f x y = case x of { (a,b) -> e }
817 -- The difference is observable using 'seq'
819 arityType (Case scrut _ _ alts) = case foldr1 andArityType [arityType rhs | (_,_,rhs) <- alts] of
820 xs@(AFun one_shot _) | one_shot -> xs
821 xs | exprIsCheap scrut -> xs
824 arityType (Let b e) = case arityType e of
825 xs@(AFun one_shot _) | one_shot -> xs
826 xs | all exprIsCheap (rhssOfBind b) -> xs
829 arityType other = ATop
831 {- NOT NEEDED ANY MORE: etaExpand is cleverer
832 ok_note InlineMe = False
834 -- Notice that we do not look through __inline_me__
835 -- This may seem surprising, but consider
836 -- f = _inline_me (\x -> e)
837 -- We DO NOT want to eta expand this to
838 -- f = \x -> (_inline_me (\x -> e)) x
839 -- because the _inline_me gets dropped now it is applied,
848 etaExpand :: Arity -- Result should have this number of value args
850 -> CoreExpr -> Type -- Expression and its type
852 -- (etaExpand n us e ty) returns an expression with
853 -- the same meaning as 'e', but with arity 'n'.
855 -- Given e' = etaExpand n us e ty
857 -- ty = exprType e = exprType e'
859 -- Note that SCCs are not treated specially. If we have
860 -- etaExpand 2 (\x -> scc "foo" e)
861 -- = (\xy -> (scc "foo" e) y)
862 -- So the costs of evaluating 'e' (not 'e y') are attributed to "foo"
864 etaExpand n us expr ty
865 | manifestArity expr >= n = expr -- The no-op case
866 | otherwise = eta_expand n us expr ty
869 -- manifestArity sees how many leading value lambdas there are
870 manifestArity :: CoreExpr -> Arity
871 manifestArity (Lam v e) | isId v = 1 + manifestArity e
872 | otherwise = manifestArity e
873 manifestArity (Note _ e) = manifestArity e
876 -- etaExpand deals with for-alls. For example:
878 -- where E :: forall a. a -> a
880 -- (/\b. \y::a -> E b y)
882 -- It deals with coerces too, though they are now rare
883 -- so perhaps the extra code isn't worth it
885 eta_expand n us expr ty
887 -- The ILX code generator requires eta expansion for type arguments
888 -- too, but alas the 'n' doesn't tell us how many of them there
889 -- may be. So we eagerly eta expand any big lambdas, and just
890 -- cross our fingers about possible loss of sharing in the ILX case.
891 -- The Right Thing is probably to make 'arity' include
892 -- type variables throughout the compiler. (ToDo.)
894 -- Saturated, so nothing to do
897 -- Short cut for the case where there already
898 -- is a lambda; no point in gratuitously adding more
899 eta_expand n us (Lam v body) ty
901 = Lam v (eta_expand n us body (applyTy ty (mkTyVarTy v)))
904 = Lam v (eta_expand (n-1) us body (funResultTy ty))
906 -- We used to have a special case that stepped inside Coerces here,
907 -- thus: eta_expand n us (Note note@(Coerce _ ty) e) _
908 -- = Note note (eta_expand n us e ty)
909 -- BUT this led to an infinite loop
910 -- Example: newtype T = MkT (Int -> Int)
911 -- eta_expand 1 (coerce (Int->Int) e)
912 -- --> coerce (Int->Int) (eta_expand 1 T e)
914 -- --> coerce (Int->Int) (coerce T
915 -- (\x::Int -> eta_expand 1 (coerce (Int->Int) e)))
916 -- by the splitNewType_maybe case below
919 eta_expand n us expr ty
920 = case splitForAllTy_maybe ty of {
921 Just (tv,ty') -> Lam tv (eta_expand n us (App expr (Type (mkTyVarTy tv))) ty')
925 case splitFunTy_maybe ty of {
926 Just (arg_ty, res_ty) -> Lam arg1 (eta_expand (n-1) us2 (App expr (Var arg1)) res_ty)
928 arg1 = mkSysLocal FSLIT("eta") uniq arg_ty
934 -- newtype T = MkT ([T] -> Int)
935 -- Consider eta-expanding this
938 -- coerce T (\x::[T] -> (coerce ([T]->Int) e) x)
939 -- Only try this for recursive newtypes; the non-recursive kind
940 -- are transparent anyway
942 case splitRecNewType_maybe ty of {
943 Just ty' -> mkCoerce2 ty ty' (eta_expand n us (mkCoerce2 ty' ty expr) ty') ;
944 Nothing -> pprTrace "Bad eta expand" (ppr n $$ ppr expr $$ ppr ty) expr
948 exprArity is a cheap-and-cheerful version of exprEtaExpandArity.
949 It tells how many things the expression can be applied to before doing
950 any work. It doesn't look inside cases, lets, etc. The idea is that
951 exprEtaExpandArity will do the hard work, leaving something that's easy
952 for exprArity to grapple with. In particular, Simplify uses exprArity to
953 compute the ArityInfo for the Id.
955 Originally I thought that it was enough just to look for top-level lambdas, but
956 it isn't. I've seen this
958 foo = PrelBase.timesInt
960 We want foo to get arity 2 even though the eta-expander will leave it
961 unchanged, in the expectation that it'll be inlined. But occasionally it
962 isn't, because foo is blacklisted (used in a rule).
964 Similarly, see the ok_note check in exprEtaExpandArity. So
965 f = __inline_me (\x -> e)
966 won't be eta-expanded.
968 And in any case it seems more robust to have exprArity be a bit more intelligent.
969 But note that (\x y z -> f x y z)
970 should have arity 3, regardless of f's arity.
973 exprArity :: CoreExpr -> Arity
976 go (Var v) = idArity v
977 go (Lam x e) | isId x = go e + 1
980 go (App e (Type t)) = go e
981 go (App f a) | exprIsCheap a = (go f - 1) `max` 0
982 -- NB: exprIsCheap a!
983 -- f (fac x) does not have arity 2,
984 -- even if f has arity 3!
985 -- NB: `max 0`! (\x y -> f x) has arity 2, even if f is
986 -- unknown, hence arity 0
990 %************************************************************************
992 \subsection{Equality}
994 %************************************************************************
996 @cheapEqExpr@ is a cheap equality test which bales out fast!
997 True => definitely equal
998 False => may or may not be equal
1001 cheapEqExpr :: Expr b -> Expr b -> Bool
1003 cheapEqExpr (Var v1) (Var v2) = v1==v2
1004 cheapEqExpr (Lit lit1) (Lit lit2) = lit1 == lit2
1005 cheapEqExpr (Type t1) (Type t2) = t1 `eqType` t2
1007 cheapEqExpr (App f1 a1) (App f2 a2)
1008 = f1 `cheapEqExpr` f2 && a1 `cheapEqExpr` a2
1010 cheapEqExpr _ _ = False
1012 exprIsBig :: Expr b -> Bool
1013 -- Returns True of expressions that are too big to be compared by cheapEqExpr
1014 exprIsBig (Lit _) = False
1015 exprIsBig (Var v) = False
1016 exprIsBig (Type t) = False
1017 exprIsBig (App f a) = exprIsBig f || exprIsBig a
1018 exprIsBig other = True
1023 eqExpr :: CoreExpr -> CoreExpr -> Bool
1024 -- Works ok at more general type, but only needed at CoreExpr
1025 -- Used in rule matching, so when we find a type we use
1026 -- eqTcType, which doesn't look through newtypes
1027 -- [And it doesn't risk falling into a black hole either.]
1029 = eq emptyVarEnv e1 e2
1031 -- The "env" maps variables in e1 to variables in ty2
1032 -- So when comparing lambdas etc,
1033 -- we in effect substitute v2 for v1 in e1 before continuing
1034 eq env (Var v1) (Var v2) = case lookupVarEnv env v1 of
1035 Just v1' -> v1' == v2
1038 eq env (Lit lit1) (Lit lit2) = lit1 == lit2
1039 eq env (App f1 a1) (App f2 a2) = eq env f1 f2 && eq env a1 a2
1040 eq env (Lam v1 e1) (Lam v2 e2) = eq (extendVarEnv env v1 v2) e1 e2
1041 eq env (Let (NonRec v1 r1) e1)
1042 (Let (NonRec v2 r2) e2) = eq env r1 r2 && eq (extendVarEnv env v1 v2) e1 e2
1043 eq env (Let (Rec ps1) e1)
1044 (Let (Rec ps2) e2) = equalLength ps1 ps2 &&
1045 and (zipWith eq_rhs ps1 ps2) &&
1048 env' = extendVarEnvList env [(v1,v2) | ((v1,_),(v2,_)) <- zip ps1 ps2]
1049 eq_rhs (_,r1) (_,r2) = eq env' r1 r2
1051 eq env (Case e1 v1 t1 a1)
1052 (Case e2 v2 t2 a2) = eq env e1 e2 &&
1054 equalLength a1 a2 &&
1055 and (zipWith (eq_alt env') a1 a2)
1057 env' = extendVarEnv env v1 v2
1059 eq env (Note n1 e1) (Note n2 e2) = eq_note env n1 n2 && eq env e1 e2
1060 eq env (Type t1) (Type t2) = t1 `eqType` t2
1061 eq env e1 e2 = False
1063 eq_list env [] [] = True
1064 eq_list env (e1:es1) (e2:es2) = eq env e1 e2 && eq_list env es1 es2
1065 eq_list env es1 es2 = False
1067 eq_alt env (c1,vs1,r1) (c2,vs2,r2) = c1==c2 &&
1068 eq (extendVarEnvList env (vs1 `zip` vs2)) r1 r2
1070 eq_note env (SCC cc1) (SCC cc2) = cc1 == cc2
1071 eq_note env (Coerce t1 f1) (Coerce t2 f2) = t1 `eqType` t2 && f1 `eqType` f2
1072 eq_note env InlineCall InlineCall = True
1073 eq_note env (CoreNote s1) (CoreNote s2) = s1 == s2
1074 eq_note env other1 other2 = False
1078 %************************************************************************
1080 \subsection{The size of an expression}
1082 %************************************************************************
1085 coreBindsSize :: [CoreBind] -> Int
1086 coreBindsSize bs = foldr ((+) . bindSize) 0 bs
1088 exprSize :: CoreExpr -> Int
1089 -- A measure of the size of the expressions
1090 -- It also forces the expression pretty drastically as a side effect
1091 exprSize (Var v) = v `seq` 1
1092 exprSize (Lit lit) = lit `seq` 1
1093 exprSize (App f a) = exprSize f + exprSize a
1094 exprSize (Lam b e) = varSize b + exprSize e
1095 exprSize (Let b e) = bindSize b + exprSize e
1097 exprSize (Case e b t as) = seqType t `seq` exprSize e + varSize b + 1 + foldr ((+) . altSize) 0 as
1098 exprSize (Note n e) = noteSize n + exprSize e
1099 exprSize (Type t) = seqType t `seq` 1
1101 noteSize (SCC cc) = cc `seq` 1
1102 noteSize (Coerce t1 t2) = seqType t1 `seq` seqType t2 `seq` 1
1103 noteSize InlineCall = 1
1104 noteSize InlineMe = 1
1105 noteSize (CoreNote s) = s `seq` 1 -- hdaume: core annotations
1107 varSize :: Var -> Int
1108 varSize b | isTyVar b = 1
1109 | otherwise = seqType (idType b) `seq`
1110 megaSeqIdInfo (idInfo b) `seq`
1113 varsSize = foldr ((+) . varSize) 0
1115 bindSize (NonRec b e) = varSize b + exprSize e
1116 bindSize (Rec prs) = foldr ((+) . pairSize) 0 prs
1118 pairSize (b,e) = varSize b + exprSize e
1120 altSize (c,bs,e) = c `seq` varsSize bs + exprSize e
1124 %************************************************************************
1126 \subsection{Hashing}
1128 %************************************************************************
1131 hashExpr :: CoreExpr -> Int
1132 hashExpr e | hash < 0 = 77 -- Just in case we hit -maxInt
1135 hash = abs (hash_expr e) -- Negative numbers kill UniqFM
1137 hash_expr (Note _ e) = hash_expr e
1138 hash_expr (Let (NonRec b r) e) = hashId b
1139 hash_expr (Let (Rec ((b,r):_)) e) = hashId b
1141 hash_expr (Case _ b _ _) = hashId b
1142 hash_expr (App f e) = hash_expr f * fast_hash_expr e
1143 hash_expr (Var v) = hashId v
1144 hash_expr (Lit lit) = hashLiteral lit
1145 hash_expr (Lam b _) = hashId b
1146 hash_expr (Type t) = trace "hash_expr: type" 1 -- Shouldn't happen
1148 fast_hash_expr (Var v) = hashId v
1149 fast_hash_expr (Lit lit) = hashLiteral lit
1150 fast_hash_expr (App f (Type _)) = fast_hash_expr f
1151 fast_hash_expr (App f a) = fast_hash_expr a
1152 fast_hash_expr (Lam b _) = hashId b
1153 fast_hash_expr other = 1
1156 hashId id = hashName (idName id)
1159 %************************************************************************
1161 \subsection{Determining non-updatable right-hand-sides}
1163 %************************************************************************
1165 Top-level constructor applications can usually be allocated
1166 statically, but they can't if the constructor, or any of the
1167 arguments, come from another DLL (because we can't refer to static
1168 labels in other DLLs).
1170 If this happens we simply make the RHS into an updatable thunk,
1171 and 'exectute' it rather than allocating it statically.
1174 rhsIsStatic :: CoreExpr -> Bool
1175 -- This function is called only on *top-level* right-hand sides
1176 -- Returns True if the RHS can be allocated statically, with
1177 -- no thunks involved at all.
1179 -- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
1180 -- refers to, CAFs; and (ii) in CoreToStg to decide whether to put an
1181 -- update flag on it.
1183 -- The basic idea is that rhsIsStatic returns True only if the RHS is
1184 -- (a) a value lambda
1185 -- (b) a saturated constructor application with static args
1187 -- BUT watch out for
1188 -- (i) Any cross-DLL references kill static-ness completely
1189 -- because they must be 'executed' not statically allocated
1191 -- (ii) We treat partial applications as redexes, because in fact we
1192 -- make a thunk for them that runs and builds a PAP
1193 -- at run-time. The only appliations that are treated as
1194 -- static are *saturated* applications of constructors.
1196 -- We used to try to be clever with nested structures like this:
1197 -- ys = (:) w ((:) w [])
1198 -- on the grounds that CorePrep will flatten ANF-ise it later.
1199 -- But supporting this special case made the function much more
1200 -- complicated, because the special case only applies if there are no
1201 -- enclosing type lambdas:
1202 -- ys = /\ a -> Foo (Baz ([] a))
1203 -- Here the nested (Baz []) won't float out to top level in CorePrep.
1205 -- But in fact, even without -O, nested structures at top level are
1206 -- flattened by the simplifier, so we don't need to be super-clever here.
1210 -- f = \x::Int. x+7 TRUE
1211 -- p = (True,False) TRUE
1213 -- d = (fst p, False) FALSE because there's a redex inside
1214 -- (this particular one doesn't happen but...)
1216 -- h = D# (1.0## /## 2.0##) FALSE (redex again)
1217 -- n = /\a. Nil a TRUE
1219 -- t = /\a. (:) (case w a of ...) (Nil a) FALSE (redex)
1222 -- This is a bit like CoreUtils.exprIsValue, with the following differences:
1223 -- a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
1225 -- b) (C x xs), where C is a contructors is updatable if the application is
1228 -- c) don't look through unfolding of f in (f x).
1230 -- When opt_RuntimeTypes is on, we keep type lambdas and treat
1231 -- them as making the RHS re-entrant (non-updatable).
1233 rhsIsStatic rhs = is_static False rhs
1235 is_static :: Bool -- True <=> in a constructor argument; must be atomic
1238 is_static False (Lam b e) = isRuntimeVar b || is_static False e
1240 is_static in_arg (Note (SCC _) e) = False
1241 is_static in_arg (Note _ e) = is_static in_arg e
1243 is_static in_arg (Lit lit)
1245 MachLabel _ _ -> False
1247 -- A MachLabel (foreign import "&foo") in an argument
1248 -- prevents a constructor application from being static. The
1249 -- reason is that it might give rise to unresolvable symbols
1250 -- in the object file: under Linux, references to "weak"
1251 -- symbols from the data segment give rise to "unresolvable
1252 -- relocation" errors at link time This might be due to a bug
1253 -- in the linker, but we'll work around it here anyway.
1256 is_static in_arg other_expr = go other_expr 0
1258 go (Var f) n_val_args
1259 | not (isDllName (idName f))
1260 = saturated_data_con f n_val_args
1261 || (in_arg && n_val_args == 0)
1262 -- A naked un-applied variable is *not* deemed a static RHS
1264 -- Reason: better to update so that the indirection gets shorted
1265 -- out, and the true value will be seen
1266 -- NB: if you change this, you'll break the invariant that THUNK_STATICs
1267 -- are always updatable. If you do so, make sure that non-updatable
1268 -- ones have enough space for their static link field!
1270 go (App f a) n_val_args
1271 | isTypeArg a = go f n_val_args
1272 | not in_arg && is_static True a = go f (n_val_args + 1)
1273 -- The (not in_arg) checks that we aren't in a constructor argument;
1274 -- if we are, we don't allow (value) applications of any sort
1276 -- NB. In case you wonder, args are sometimes not atomic. eg.
1277 -- x = D# (1.0## /## 2.0##)
1278 -- can't float because /## can fail.
1280 go (Note (SCC _) f) n_val_args = False
1281 go (Note _ f) n_val_args = go f n_val_args
1283 go other n_val_args = False
1285 saturated_data_con f n_val_args
1286 = case isDataConWorkId_maybe f of
1287 Just dc -> n_val_args == dataConRepArity dc