2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Utility functions on @Core@ syntax
9 {-# OPTIONS -fno-warn-incomplete-patterns #-}
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 -- | Commonly useful utilites for manipulating the Core language
18 -- * Constructing expressions
19 mkSCC, mkCoerce, mkCoerceI,
20 bindNonRec, needsCaseBinding,
21 mkAltExpr, mkPiType, mkPiTypes,
23 -- * Taking expressions apart
24 findDefault, findAlt, isDefaultAlt, mergeAlts, trimConArgs,
26 -- * Properties of expressions
27 exprType, coreAltType, coreAltsType,
28 exprIsDupable, exprIsTrivial, exprIsCheap, exprIsExpandable,
29 exprIsHNF, exprOkForSpeculation, exprIsBig, exprIsConLike,
30 rhsIsStatic, isCheapApp, isExpandableApp,
32 -- * Expression and bindings size
33 coreBindsSize, exprSize,
39 cheapEqExpr, eqExpr, eqExprX,
44 -- * Manipulating data constructors and types
45 applyTypeToArgs, applyTypeToArg,
46 dataConOrigInstPat, dataConRepInstPat, dataConRepFSInstPat
49 #include "HsVersions.h"
63 import TcType ( isPredTy )
71 import PrelNames( absentErrorIdKey )
80 %************************************************************************
82 \subsection{Find the type of a Core atom/expression}
84 %************************************************************************
87 exprType :: CoreExpr -> Type
88 -- ^ Recover the type of a well-typed Core expression. Fails when
89 -- applied to the actual 'CoreSyn.Type' expression as it cannot
90 -- really be said to have a type
91 exprType (Var var) = idType var
92 exprType (Lit lit) = literalType lit
93 exprType (Let _ body) = exprType body
94 exprType (Case _ _ ty _) = ty
95 exprType (Cast _ co) = snd (coercionKind co)
96 exprType (Note _ e) = exprType e
97 exprType (Lam binder expr) = mkPiType binder (exprType expr)
99 = case collectArgs e of
100 (fun, args) -> applyTypeToArgs e (exprType fun) args
102 exprType other = pprTrace "exprType" (pprCoreExpr other) alphaTy
104 coreAltType :: CoreAlt -> Type
105 -- ^ Returns the type of the alternatives right hand side
106 coreAltType (_,bs,rhs)
107 | any bad_binder bs = expandTypeSynonyms ty
108 | otherwise = ty -- Note [Existential variables and silly type synonyms]
111 free_tvs = tyVarsOfType ty
112 bad_binder b = isTyCoVar b && b `elemVarSet` free_tvs
114 coreAltsType :: [CoreAlt] -> Type
115 -- ^ Returns the type of the first alternative, which should be the same as for all alternatives
116 coreAltsType (alt:_) = coreAltType alt
117 coreAltsType [] = panic "corAltsType"
120 Note [Existential variables and silly type synonyms]
121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
123 data T = forall a. T (Funny a)
128 Now, the type of 'x' is (Funny a), where 'a' is existentially quantified.
129 That means that 'exprType' and 'coreAltsType' may give a result that *appears*
130 to mention an out-of-scope type variable. See Trac #3409 for a more real-world
133 Various possibilities suggest themselves:
135 - Ignore the problem, and make Lint not complain about such variables
137 - Expand all type synonyms (or at least all those that discard arguments)
138 This is tricky, because at least for top-level things we want to
139 retain the type the user originally specified.
141 - Expand synonyms on the fly, when the problem arises. That is what
142 we are doing here. It's not too expensive, I think.
145 mkPiType :: EvVar -> Type -> Type
146 -- ^ Makes a @(->)@ type or a forall type, depending
147 -- on whether it is given a type variable or a term variable.
148 mkPiTypes :: [EvVar] -> Type -> Type
149 -- ^ 'mkPiType' for multiple type or value arguments
152 | isId v = mkFunTy (idType v) ty
153 | otherwise = mkForAllTy v ty
155 mkPiTypes vs ty = foldr mkPiType ty vs
159 applyTypeToArg :: Type -> CoreExpr -> Type
160 -- ^ Determines the type resulting from applying an expression to a function with the given type
161 applyTypeToArg fun_ty (Type arg_ty) = applyTy fun_ty arg_ty
162 applyTypeToArg fun_ty _ = funResultTy fun_ty
164 applyTypeToArgs :: CoreExpr -> Type -> [CoreExpr] -> Type
165 -- ^ A more efficient version of 'applyTypeToArg' when we have several arguments.
166 -- The first argument is just for debugging, and gives some context
167 applyTypeToArgs _ op_ty [] = op_ty
169 applyTypeToArgs e op_ty (Type ty : args)
170 = -- Accumulate type arguments so we can instantiate all at once
173 go rev_tys (Type ty : args) = go (ty:rev_tys) args
174 go rev_tys rest_args = applyTypeToArgs e op_ty' rest_args
176 op_ty' = applyTysD msg op_ty (reverse rev_tys)
177 msg = ptext (sLit "applyTypeToArgs") <+>
180 applyTypeToArgs e op_ty (_ : args)
181 = case (splitFunTy_maybe op_ty) of
182 Just (_, res_ty) -> applyTypeToArgs e res_ty args
183 Nothing -> pprPanic "applyTypeToArgs" (panic_msg e op_ty)
185 panic_msg :: CoreExpr -> Type -> SDoc
186 panic_msg e op_ty = pprCoreExpr e $$ ppr op_ty
189 %************************************************************************
191 \subsection{Attaching notes}
193 %************************************************************************
196 -- | Wrap the given expression in the coercion, dropping identity coercions and coalescing nested coercions
197 mkCoerceI :: CoercionI -> CoreExpr -> CoreExpr
198 mkCoerceI (IdCo _) e = e
199 mkCoerceI (ACo co) e = mkCoerce co e
201 -- | Wrap the given expression in the coercion safely, coalescing nested coercions
202 mkCoerce :: Coercion -> CoreExpr -> CoreExpr
203 mkCoerce co (Cast expr co2)
204 = ASSERT(let { (from_ty, _to_ty) = coercionKind co;
205 (_from_ty2, to_ty2) = coercionKind co2} in
206 from_ty `coreEqType` to_ty2 )
207 mkCoerce (mkTransCoercion co2 co) expr
210 = let (from_ty, _to_ty) = coercionKind co in
211 -- if to_ty `coreEqType` from_ty
214 WARN(not (from_ty `coreEqType` exprType expr), text "Trying to coerce" <+> text "(" <> ppr expr $$ text "::" <+> ppr (exprType expr) <> text ")" $$ ppr co $$ pprEqPred (coercionKind co))
219 -- | Wraps the given expression in the cost centre unless
220 -- in a way that maximises their utility to the user
221 mkSCC :: CostCentre -> Expr b -> Expr b
222 -- Note: Nested SCC's *are* preserved for the benefit of
223 -- cost centre stack profiling
224 mkSCC _ (Lit lit) = Lit lit
225 mkSCC cc (Lam x e) = Lam x (mkSCC cc e) -- Move _scc_ inside lambda
226 mkSCC cc (Note (SCC cc') e) = Note (SCC cc) (Note (SCC cc') e)
227 mkSCC cc (Note n e) = Note n (mkSCC cc e) -- Move _scc_ inside notes
228 mkSCC cc (Cast e co) = Cast (mkSCC cc e) co -- Move _scc_ inside cast
229 mkSCC cc expr = Note (SCC cc) expr
233 %************************************************************************
235 \subsection{Other expression construction}
237 %************************************************************************
240 bindNonRec :: Id -> CoreExpr -> CoreExpr -> CoreExpr
241 -- ^ @bindNonRec x r b@ produces either:
247 -- > case r of x { _DEFAULT_ -> b }
249 -- depending on whether we have to use a @case@ or @let@
250 -- binding for the expression (see 'needsCaseBinding').
251 -- It's used by the desugarer to avoid building bindings
252 -- that give Core Lint a heart attack, although actually
253 -- the simplifier deals with them perfectly well. See
254 -- also 'MkCore.mkCoreLet'
255 bindNonRec bndr rhs body
256 | needsCaseBinding (idType bndr) rhs = Case rhs bndr (exprType body) [(DEFAULT, [], body)]
257 | otherwise = Let (NonRec bndr rhs) body
259 -- | Tests whether we have to use a @case@ rather than @let@ binding for this expression
260 -- as per the invariants of 'CoreExpr': see "CoreSyn#let_app_invariant"
261 needsCaseBinding :: Type -> CoreExpr -> Bool
262 needsCaseBinding ty rhs = isUnLiftedType ty && not (exprOkForSpeculation rhs)
263 -- Make a case expression instead of a let
264 -- These can arise either from the desugarer,
265 -- or from beta reductions: (\x.e) (x +# y)
269 mkAltExpr :: AltCon -- ^ Case alternative constructor
270 -> [CoreBndr] -- ^ Things bound by the pattern match
271 -> [Type] -- ^ The type arguments to the case alternative
273 -- ^ This guy constructs the value that the scrutinee must have
274 -- given that you are in one particular branch of a case
275 mkAltExpr (DataAlt con) args inst_tys
276 = mkConApp con (map Type inst_tys ++ varsToCoreExprs args)
277 mkAltExpr (LitAlt lit) [] []
279 mkAltExpr (LitAlt _) _ _ = panic "mkAltExpr LitAlt"
280 mkAltExpr DEFAULT _ _ = panic "mkAltExpr DEFAULT"
284 %************************************************************************
286 \subsection{Taking expressions apart}
288 %************************************************************************
290 The default alternative must be first, if it exists at all.
291 This makes it easy to find, though it makes matching marginally harder.
294 -- | Extract the default case alternative
295 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
296 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null args ) (alts, Just rhs)
297 findDefault alts = (alts, Nothing)
299 isDefaultAlt :: CoreAlt -> Bool
300 isDefaultAlt (DEFAULT, _, _) = True
301 isDefaultAlt _ = False
304 -- | Find the case alternative corresponding to a particular
305 -- constructor: panics if no such constructor exists
306 findAlt :: AltCon -> [CoreAlt] -> Maybe CoreAlt
307 -- A "Nothing" result *is* legitmiate
308 -- See Note [Unreachable code]
311 (deflt@(DEFAULT,_,_):alts) -> go alts (Just deflt)
315 go (alt@(con1,_,_) : alts) deflt
316 = case con `cmpAltCon` con1 of
317 LT -> deflt -- Missed it already; the alts are in increasing order
319 GT -> ASSERT( not (con1 == DEFAULT) ) go alts deflt
321 ---------------------------------
322 mergeAlts :: [CoreAlt] -> [CoreAlt] -> [CoreAlt]
323 -- ^ Merge alternatives preserving order; alternatives in
324 -- the first argument shadow ones in the second
325 mergeAlts [] as2 = as2
326 mergeAlts as1 [] = as1
327 mergeAlts (a1:as1) (a2:as2)
328 = case a1 `cmpAlt` a2 of
329 LT -> a1 : mergeAlts as1 (a2:as2)
330 EQ -> a1 : mergeAlts as1 as2 -- Discard a2
331 GT -> a2 : mergeAlts (a1:as1) as2
334 ---------------------------------
335 trimConArgs :: AltCon -> [CoreArg] -> [CoreArg]
338 -- > case (C a b x y) of
341 -- We want to drop the leading type argument of the scrutinee
342 -- leaving the arguments to match agains the pattern
344 trimConArgs DEFAULT args = ASSERT( null args ) []
345 trimConArgs (LitAlt _) args = ASSERT( null args ) []
346 trimConArgs (DataAlt dc) args = dropList (dataConUnivTyVars dc) args
349 Note [Unreachable code]
350 ~~~~~~~~~~~~~~~~~~~~~~~
351 It is possible (although unusual) for GHC to find a case expression
352 that cannot match. For example:
354 data Col = Red | Green | Blue
358 _ -> ...(case x of { Green -> e1; Blue -> e2 })...
360 Suppose that for some silly reason, x isn't substituted in the case
361 expression. (Perhaps there's a NOINLINE on it, or profiling SCC stuff
362 gets in the way; cf Trac #3118.) Then the full-lazines pass might produce
366 lvl = case x of { Green -> e1; Blue -> e2 })
371 Now if x gets inlined, we won't be able to find a matching alternative
372 for 'Red'. That's because 'lvl' is unreachable. So rather than crashing
373 we generate (error "Inaccessible alternative").
375 Similar things can happen (augmented by GADTs) when the Simplifier
376 filters down the matching alternatives in Simplify.rebuildCase.
379 %************************************************************************
383 %************************************************************************
387 @exprIsTrivial@ is true of expressions we are unconditionally happy to
388 duplicate; simple variables and constants, and type
389 applications. Note that primop Ids aren't considered
392 Note [Variable are trivial]
393 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
394 There used to be a gruesome test for (hasNoBinding v) in the
396 exprIsTrivial (Var v) | hasNoBinding v = idArity v == 0
397 The idea here is that a constructor worker, like \$wJust, is
398 really short for (\x -> \$wJust x), becuase \$wJust has no binding.
399 So it should be treated like a lambda. Ditto unsaturated primops.
400 But now constructor workers are not "have-no-binding" Ids. And
401 completely un-applied primops and foreign-call Ids are sufficiently
402 rare that I plan to allow them to be duplicated and put up with
405 Note [SCCs are trivial]
406 ~~~~~~~~~~~~~~~~~~~~~~~
407 We used not to treat (_scc_ "foo" x) as trivial, because it really
408 generates code, (and a heap object when it's a function arg) to
409 capture the cost centre. However, the profiling system discounts the
410 allocation costs for such "boxing thunks" whereas the extra costs of
411 *not* inlining otherwise-trivial bindings can be high, and are hard to
415 exprIsTrivial :: CoreExpr -> Bool
416 exprIsTrivial (Var _) = True -- See Note [Variables are trivial]
417 exprIsTrivial (Type _) = True
418 exprIsTrivial (Lit lit) = litIsTrivial lit
419 exprIsTrivial (App e arg) = not (isRuntimeArg arg) && exprIsTrivial e
420 exprIsTrivial (Note _ e) = exprIsTrivial e -- See Note [SCCs are trivial]
421 exprIsTrivial (Cast e _) = exprIsTrivial e
422 exprIsTrivial (Lam b body) = not (isRuntimeVar b) && exprIsTrivial body
423 exprIsTrivial _ = False
427 %************************************************************************
431 %************************************************************************
435 @exprIsDupable@ is true of expressions that can be duplicated at a modest
436 cost in code size. This will only happen in different case
437 branches, so there's no issue about duplicating work.
439 That is, exprIsDupable returns True of (f x) even if
440 f is very very expensive to call.
442 Its only purpose is to avoid fruitless let-binding
443 and then inlining of case join points
447 exprIsDupable :: CoreExpr -> Bool
448 exprIsDupable (Type _) = True
449 exprIsDupable (Var _) = True
450 exprIsDupable (Lit lit) = litIsDupable lit
451 exprIsDupable (Note _ e) = exprIsDupable e
452 exprIsDupable (Cast e _) = exprIsDupable e
457 go (App f a) n_args = n_args < dupAppSize
463 dupAppSize = 4 -- Size of application we are prepared to duplicate
466 %************************************************************************
468 exprIsCheap, exprIsExpandable
470 %************************************************************************
472 Note [exprIsCheap] See also Note [Interaction of exprIsCheap and lone variables]
473 ~~~~~~~~~~~~~~~~~~ in CoreUnfold.lhs
474 @exprIsCheap@ looks at a Core expression and returns \tr{True} if
475 it is obviously in weak head normal form, or is cheap to get to WHNF.
476 [Note that that's not the same as exprIsDupable; an expression might be
477 big, and hence not dupable, but still cheap.]
479 By ``cheap'' we mean a computation we're willing to:
480 push inside a lambda, or
481 inline at more than one place
482 That might mean it gets evaluated more than once, instead of being
483 shared. The main examples of things which aren't WHNF but are
488 (where e, and all the ei are cheap)
491 (where e and b are cheap)
494 (where op is a cheap primitive operator)
497 (because we are happy to substitute it inside a lambda)
499 Notice that a variable is considered 'cheap': we can push it inside a lambda,
500 because sharing will make sure it is only evaluated once.
502 Note [exprIsCheap and exprIsHNF]
503 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
504 Note that exprIsHNF does not imply exprIsCheap. Eg
505 let x = fac 20 in Just x
506 This responds True to exprIsHNF (you can discard a seq), but
507 False to exprIsCheap.
510 exprIsCheap :: CoreExpr -> Bool
511 exprIsCheap = exprIsCheap' isCheapApp
513 exprIsExpandable :: CoreExpr -> Bool
514 exprIsExpandable = exprIsCheap' isExpandableApp -- See Note [CONLIKE pragma] in BasicTypes
517 exprIsCheap' :: (Id -> Int -> Bool) -> CoreExpr -> Bool
518 exprIsCheap' _ (Lit _) = True
519 exprIsCheap' _ (Type _) = True
520 exprIsCheap' _ (Var _) = True
521 exprIsCheap' good_app (Note _ e) = exprIsCheap' good_app e
522 exprIsCheap' good_app (Cast e _) = exprIsCheap' good_app e
523 exprIsCheap' good_app (Lam x e) = isRuntimeVar x
524 || exprIsCheap' good_app e
526 exprIsCheap' good_app (Case e _ _ alts) = exprIsCheap' good_app e &&
527 and [exprIsCheap' good_app rhs | (_,_,rhs) <- alts]
528 -- Experimentally, treat (case x of ...) as cheap
529 -- (and case __coerce x etc.)
530 -- This improves arities of overloaded functions where
531 -- there is only dictionary selection (no construction) involved
533 exprIsCheap' good_app (Let (NonRec x _) e)
534 | isUnLiftedType (idType x) = exprIsCheap' good_app e
536 -- Strict lets always have cheap right hand sides,
537 -- and do no allocation, so just look at the body
538 -- Non-strict lets do allocation so we don't treat them as cheap
541 exprIsCheap' good_app other_expr -- Applications and variables
544 -- Accumulate value arguments, then decide
545 go (App f a) val_args | isRuntimeArg a = go f (a:val_args)
546 | otherwise = go f val_args
548 go (Var _) [] = True -- Just a type application of a variable
549 -- (f t1 t2 t3) counts as WHNF
551 = case idDetails f of
552 RecSelId {} -> go_sel args
553 ClassOpId {} -> go_sel args
554 PrimOpId op -> go_primop op args
555 _ | good_app f (length args) -> go_pap args
556 | isBottomingId f -> True
558 -- Application of a function which
559 -- always gives bottom; we treat this as cheap
560 -- because it certainly doesn't need to be shared!
565 go_pap args = all exprIsTrivial args
566 -- For constructor applications and primops, check that all
567 -- the args are trivial. We don't want to treat as cheap, say,
569 -- We'll put up with one constructor application, but not dozens
572 go_primop op args = primOpIsCheap op && all (exprIsCheap' good_app) args
573 -- In principle we should worry about primops
574 -- that return a type variable, since the result
575 -- might be applied to something, but I'm not going
576 -- to bother to check the number of args
579 go_sel [arg] = exprIsCheap' good_app arg -- I'm experimenting with making record selection
580 go_sel _ = False -- look cheap, so we will substitute it inside a
581 -- lambda. Particularly for dictionary field selection.
582 -- BUT: Take care with (sel d x)! The (sel d) might be cheap, but
583 -- there's no guarantee that (sel d x) will be too. Hence (n_val_args == 1)
585 isCheapApp :: Id -> Int -> Bool
586 isCheapApp fn n_val_args
588 || n_val_args < idArity fn
590 isExpandableApp :: Id -> Int -> Bool
591 isExpandableApp fn n_val_args
593 || n_val_args < idArity fn
594 || go n_val_args (idType fn)
596 -- See if all the arguments are PredTys (implicit params or classes)
597 -- If so we'll regard it as expandable; see Note [Expandable overloadings]
600 | Just (_, ty) <- splitForAllTy_maybe ty = go n_val_args ty
601 | Just (arg, ty) <- splitFunTy_maybe ty
602 , isPredTy arg = go (n_val_args-1) ty
606 Note [Expandable overloadings]
607 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
608 Suppose the user wrote this
609 {-# RULE forall x. foo (negate x) = h x #-}
610 f x = ....(foo (negate x))....
611 He'd expect the rule to fire. But since negate is overloaded, we might
613 f = \d -> let n = negate d in \x -> ...foo (n x)...
614 So we treat the application of a function (negate in this case) to a
615 *dictionary* as expandable. In effect, every function is CONLIKE when
616 it's applied only to dictionaries.
619 %************************************************************************
623 %************************************************************************
626 -- | 'exprOkForSpeculation' returns True of an expression that is:
628 -- * Safe to evaluate even if normal order eval might not
629 -- evaluate the expression at all, or
631 -- * Safe /not/ to evaluate even if normal order would do so
633 -- Precisely, it returns @True@ iff:
635 -- * The expression guarantees to terminate,
637 -- * without raising an exception,
638 -- * without causing a side effect (e.g. writing a mutable variable)
640 -- Note that if @exprIsHNF e@, then @exprOkForSpecuation e@.
641 -- As an example of the considerations in this test, consider:
643 -- > let x = case y# +# 1# of { r# -> I# r# }
646 -- being translated to:
648 -- > case y# +# 1# of { r# ->
653 -- We can only do this if the @y + 1@ is ok for speculation: it has no
654 -- side effects, and can't diverge or raise an exception.
655 exprOkForSpeculation :: CoreExpr -> Bool
656 exprOkForSpeculation (Lit _) = True
657 exprOkForSpeculation (Type _) = True
658 -- Tick boxes are *not* suitable for speculation
659 exprOkForSpeculation (Var v) = isUnLiftedType (idType v)
660 && not (isTickBoxOp v)
661 exprOkForSpeculation (Note _ e) = exprOkForSpeculation e
662 exprOkForSpeculation (Cast e _) = exprOkForSpeculation e
664 exprOkForSpeculation (Case e _ _ alts)
665 = exprOkForSpeculation e -- Note [exprOkForSpeculation: case expressions]
666 && all (\(_,_,rhs) -> exprOkForSpeculation rhs) alts
668 exprOkForSpeculation other_expr
669 = case collectArgs other_expr of
670 (Var f, args) | f `hasKey` absentErrorIdKey -- Note [Absent error Id]
671 -> all exprOkForSpeculation args -- in WwLib
673 -> spec_ok (idDetails f) args
677 spec_ok (DataConWorkId _) _
678 = True -- The strictness of the constructor has already
679 -- been expressed by its "wrapper", so we don't need
680 -- to take the arguments into account
682 spec_ok (PrimOpId op) args
683 | isDivOp op, -- Special case for dividing operations that fail
684 [arg1, Lit lit] <- args -- only if the divisor is zero
685 = not (isZeroLit lit) && exprOkForSpeculation arg1
686 -- Often there is a literal divisor, and this
687 -- can get rid of a thunk in an inner looop
690 = primOpOkForSpeculation op &&
691 all exprOkForSpeculation args
692 -- A bit conservative: we don't really need
693 -- to care about lazy arguments, but this is easy
695 spec_ok (DFunId new_type) _ = not new_type
696 -- DFuns terminate, unless the dict is implemented with a newtype
697 -- in which case they may not
701 -- | True of dyadic operators that can fail only if the second arg is zero!
702 isDivOp :: PrimOp -> Bool
703 -- This function probably belongs in PrimOp, or even in
704 -- an automagically generated file.. but it's such a
705 -- special case I thought I'd leave it here for now.
706 isDivOp IntQuotOp = True
707 isDivOp IntRemOp = True
708 isDivOp WordQuotOp = True
709 isDivOp WordRemOp = True
710 isDivOp FloatDivOp = True
711 isDivOp DoubleDivOp = True
715 Note [exprOkForSpeculation: case expressions]
716 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
718 It's always sound for exprOkForSpeculation to return False, and we
719 don't want it to take too long, so it bales out on complicated-looking
720 terms. Notably lets, which can be stacked very deeply; and in any
721 case the argument of exprOkForSpeculation is usually in a strict context,
722 so any lets will have been floated away.
724 However, we keep going on case-expressions. An example like this one
725 showed up in DPH code:
728 foo n = (if n < 5 then 1 else 2) `seq` foo (n-1)
730 If exprOkForSpeculation doesn't look through case expressions, you get this:
732 \ (ww :: GHC.Prim.Int#) ->
734 __DEFAULT -> case (case <# ds 5 of _ {
735 GHC.Types.False -> lvl1;
736 GHC.Types.True -> lvl})
738 T.$wfoo (GHC.Prim.-# ds_XkE 1) };
742 The inner case is redundant, and should be nuked.
745 %************************************************************************
747 exprIsHNF, exprIsConLike
749 %************************************************************************
752 -- Note [exprIsHNF] See also Note [exprIsCheap and exprIsHNF]
754 -- | exprIsHNF returns true for expressions that are certainly /already/
755 -- evaluated to /head/ normal form. This is used to decide whether it's ok
758 -- > case x of _ -> e
764 -- and to decide whether it's safe to discard a 'seq'.
766 -- So, it does /not/ treat variables as evaluated, unless they say they are.
767 -- However, it /does/ treat partial applications and constructor applications
768 -- as values, even if their arguments are non-trivial, provided the argument
769 -- type is lifted. For example, both of these are values:
771 -- > (:) (f x) (map f xs)
772 -- > map (...redex...)
774 -- because 'seq' on such things completes immediately.
776 -- For unlifted argument types, we have to be careful:
780 -- Suppose @f x@ diverges; then @C (f x)@ is not a value. However this can't
781 -- happen: see "CoreSyn#let_app_invariant". This invariant states that arguments of
782 -- unboxed type must be ok-for-speculation (or trivial).
783 exprIsHNF :: CoreExpr -> Bool -- True => Value-lambda, constructor, PAP
784 exprIsHNF = exprIsHNFlike isDataConWorkId isEvaldUnfolding
788 -- | Similar to 'exprIsHNF' but includes CONLIKE functions as well as
789 -- data constructors. Conlike arguments are considered interesting by the
791 exprIsConLike :: CoreExpr -> Bool -- True => lambda, conlike, PAP
792 exprIsConLike = exprIsHNFlike isConLikeId isConLikeUnfolding
794 -- | Returns true for values or value-like expressions. These are lambdas,
795 -- constructors / CONLIKE functions (as determined by the function argument)
798 exprIsHNFlike :: (Var -> Bool) -> (Unfolding -> Bool) -> CoreExpr -> Bool
799 exprIsHNFlike is_con is_con_unf = is_hnf_like
801 is_hnf_like (Var v) -- NB: There are no value args at this point
802 = is_con v -- Catches nullary constructors,
803 -- so that [] and () are values, for example
804 || idArity v > 0 -- Catches (e.g.) primops that don't have unfoldings
805 || is_con_unf (idUnfolding v)
806 -- Check the thing's unfolding; it might be bound to a value
807 -- We don't look through loop breakers here, which is a bit conservative
808 -- but otherwise I worry that if an Id's unfolding is just itself,
809 -- we could get an infinite loop
811 is_hnf_like (Lit _) = True
812 is_hnf_like (Type _) = True -- Types are honorary Values;
813 -- we don't mind copying them
814 is_hnf_like (Lam b e) = isRuntimeVar b || is_hnf_like e
815 is_hnf_like (Note _ e) = is_hnf_like e
816 is_hnf_like (Cast e _) = is_hnf_like e
817 is_hnf_like (App e (Type _)) = is_hnf_like e
818 is_hnf_like (App e a) = app_is_value e [a]
819 is_hnf_like (Let _ e) = is_hnf_like e -- Lazy let(rec)s don't affect us
820 is_hnf_like _ = False
822 -- There is at least one value argument
823 app_is_value :: CoreExpr -> [CoreArg] -> Bool
824 app_is_value (Var fun) args
825 = idArity fun > valArgCount args -- Under-applied function
826 || is_con fun -- or constructor-like
827 app_is_value (Note _ f) as = app_is_value f as
828 app_is_value (Cast f _) as = app_is_value f as
829 app_is_value (App f a) as = app_is_value f (a:as)
830 app_is_value _ _ = False
834 %************************************************************************
836 Instantiating data constructors
838 %************************************************************************
840 These InstPat functions go here to avoid circularity between DataCon and Id
843 dataConRepInstPat, dataConOrigInstPat :: [Unique] -> DataCon -> [Type] -> ([TyVar], [CoVar], [Id])
844 dataConRepFSInstPat :: [FastString] -> [Unique] -> DataCon -> [Type] -> ([TyVar], [CoVar], [Id])
846 dataConRepInstPat = dataConInstPat dataConRepArgTys (repeat ((fsLit "ipv")))
847 dataConRepFSInstPat = dataConInstPat dataConRepArgTys
848 dataConOrigInstPat = dataConInstPat dc_arg_tys (repeat ((fsLit "ipv")))
850 dc_arg_tys dc = map mkPredTy (dataConEqTheta dc) ++ map mkPredTy (dataConDictTheta dc) ++ dataConOrigArgTys dc
851 -- Remember to include the existential dictionaries
853 dataConInstPat :: (DataCon -> [Type]) -- function used to find arg tys
854 -> [FastString] -- A long enough list of FSs to use for names
855 -> [Unique] -- An equally long list of uniques, at least one for each binder
857 -> [Type] -- Types to instantiate the universally quantified tyvars
858 -> ([TyVar], [CoVar], [Id]) -- Return instantiated variables
859 -- dataConInstPat arg_fun fss us con inst_tys returns a triple
860 -- (ex_tvs, co_tvs, arg_ids),
862 -- ex_tvs are intended to be used as binders for existential type args
864 -- co_tvs are intended to be used as binders for coercion args and the kinds
865 -- of these vars have been instantiated by the inst_tys and the ex_tys
866 -- The co_tvs include both GADT equalities (dcEqSpec) and
867 -- programmer-specified equalities (dcEqTheta)
869 -- arg_ids are indended to be used as binders for value arguments,
870 -- and their types have been instantiated with inst_tys and ex_tys
871 -- The arg_ids include both dicts (dcDictTheta) and
872 -- programmer-specified arguments (after rep-ing) (deRepArgTys)
875 -- The following constructor T1
878 -- T1 :: forall b. Int -> b -> T(a,b)
881 -- has representation type
882 -- forall a. forall a1. forall b. (a ~ (a1,b)) =>
885 -- dataConInstPat fss us T1 (a1',b') will return
887 -- ([a1'', b''], [c :: (a1', b')~(a1'', b'')], [x :: Int, y :: b''])
889 -- where the double-primed variables are created with the FastStrings and
890 -- Uniques given as fss and us
891 dataConInstPat arg_fun fss uniqs con inst_tys
892 = (ex_bndrs, co_bndrs, arg_ids)
894 univ_tvs = dataConUnivTyVars con
895 ex_tvs = dataConExTyVars con
896 arg_tys = arg_fun con
897 eq_spec = dataConEqSpec con
898 eq_theta = dataConEqTheta con
899 eq_preds = eqSpecPreds eq_spec ++ eq_theta
902 n_co = length eq_preds
904 -- split the Uniques and FastStrings
905 (ex_uniqs, uniqs') = splitAt n_ex uniqs
906 (co_uniqs, id_uniqs) = splitAt n_co uniqs'
908 (ex_fss, fss') = splitAt n_ex fss
909 (co_fss, id_fss) = splitAt n_co fss'
911 -- Make existential type variables
912 ex_bndrs = zipWith3 mk_ex_var ex_uniqs ex_fss ex_tvs
913 mk_ex_var uniq fs var = mkTyVar new_name kind
915 new_name = mkSysTvName uniq fs
918 -- Make the instantiating substitution
919 subst = zipOpenTvSubst (univ_tvs ++ ex_tvs) (inst_tys ++ map mkTyVarTy ex_bndrs)
921 -- Make new coercion vars, instantiating kind
922 co_bndrs = zipWith3 mk_co_var co_uniqs co_fss eq_preds
923 mk_co_var uniq fs eq_pred = mkCoVar new_name co_kind
925 new_name = mkSysTvName uniq fs
926 co_kind = substTy subst (mkPredTy eq_pred)
928 -- make value vars, instantiating types
929 mk_id_var uniq fs ty = mkUserLocal (mkVarOccFS fs) uniq (substTy subst ty) noSrcSpan
930 arg_ids = zipWith3 mk_id_var id_uniqs id_fss arg_tys
934 %************************************************************************
938 %************************************************************************
941 -- | A cheap equality test which bales out fast!
942 -- If it returns @True@ the arguments are definitely equal,
943 -- otherwise, they may or may not be equal.
945 -- See also 'exprIsBig'
946 cheapEqExpr :: Expr b -> Expr b -> Bool
948 cheapEqExpr (Var v1) (Var v2) = v1==v2
949 cheapEqExpr (Lit lit1) (Lit lit2) = lit1 == lit2
950 cheapEqExpr (Type t1) (Type t2) = t1 `coreEqType` t2
952 cheapEqExpr (App f1 a1) (App f2 a2)
953 = f1 `cheapEqExpr` f2 && a1 `cheapEqExpr` a2
955 cheapEqExpr (Cast e1 t1) (Cast e2 t2)
956 = e1 `cheapEqExpr` e2 && t1 `coreEqCoercion` t2
958 cheapEqExpr _ _ = False
962 exprIsBig :: Expr b -> Bool
963 -- ^ Returns @True@ of expressions that are too big to be compared by 'cheapEqExpr'
964 exprIsBig (Lit _) = False
965 exprIsBig (Var _) = False
966 exprIsBig (Type _) = False
967 exprIsBig (Lam _ e) = exprIsBig e
968 exprIsBig (App f a) = exprIsBig f || exprIsBig a
969 exprIsBig (Cast e _) = exprIsBig e -- Hopefully coercions are not too big!
974 eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
975 -- Compares for equality, modulo alpha
976 eqExpr in_scope e1 e2
977 = eqExprX id_unf (mkRnEnv2 in_scope) e1 e2
979 id_unf _ = noUnfolding -- Don't expand
983 eqExprX :: IdUnfoldingFun -> RnEnv2 -> CoreExpr -> CoreExpr -> Bool
984 -- ^ Compares expressions for equality, modulo alpha.
985 -- Does /not/ look through newtypes or predicate types
986 -- Used in rule matching, and also CSE
988 eqExprX id_unfolding_fun env e1 e2
991 go env (Var v1) (Var v2)
992 | rnOccL env v1 == rnOccR env v2
995 -- The next two rules expand non-local variables
996 -- C.f. Note [Expanding variables] in Rules.lhs
997 -- and Note [Do not expand locally-bound variables] in Rules.lhs
999 | not (locallyBoundL env v1)
1000 , Just e1' <- expandUnfolding_maybe (id_unfolding_fun (lookupRnInScope env v1))
1001 = go (nukeRnEnvL env) e1' e2
1004 | not (locallyBoundR env v2)
1005 , Just e2' <- expandUnfolding_maybe (id_unfolding_fun (lookupRnInScope env v2))
1006 = go (nukeRnEnvR env) e1 e2'
1008 go _ (Lit lit1) (Lit lit2) = lit1 == lit2
1009 go env (Type t1) (Type t2) = tcEqTypeX env t1 t2
1010 go env (Cast e1 co1) (Cast e2 co2) = tcEqTypeX env co1 co2 && go env e1 e2
1011 go env (App f1 a1) (App f2 a2) = go env f1 f2 && go env a1 a2
1012 go env (Note n1 e1) (Note n2 e2) = go_note n1 n2 && go env e1 e2
1014 go env (Lam b1 e1) (Lam b2 e2)
1015 = tcEqTypeX env (varType b1) (varType b2) -- False for Id/TyVar combination
1016 && go (rnBndr2 env b1 b2) e1 e2
1018 go env (Let (NonRec v1 r1) e1) (Let (NonRec v2 r2) e2)
1019 = go env r1 r2 -- No need to check binder types, since RHSs match
1020 && go (rnBndr2 env v1 v2) e1 e2
1022 go env (Let (Rec ps1) e1) (Let (Rec ps2) e2)
1023 = all2 (go env') rs1 rs2 && go env' e1 e2
1025 (bs1,rs1) = unzip ps1
1026 (bs2,rs2) = unzip ps2
1027 env' = rnBndrs2 env bs1 bs2
1029 go env (Case e1 b1 _ a1) (Case e2 b2 _ a2)
1031 && tcEqTypeX env (idType b1) (idType b2)
1032 && all2 (go_alt (rnBndr2 env b1 b2)) a1 a2
1037 go_alt env (c1, bs1, e1) (c2, bs2, e2)
1038 = c1 == c2 && go (rnBndrs2 env bs1 bs2) e1 e2
1041 go_note (SCC cc1) (SCC cc2) = cc1 == cc2
1042 go_note (CoreNote s1) (CoreNote s2) = s1 == s2
1049 locallyBoundL, locallyBoundR :: RnEnv2 -> Var -> Bool
1050 locallyBoundL rn_env v = inRnEnvL rn_env v
1051 locallyBoundR rn_env v = inRnEnvR rn_env v
1055 %************************************************************************
1057 \subsection{The size of an expression}
1059 %************************************************************************
1062 coreBindsSize :: [CoreBind] -> Int
1063 coreBindsSize bs = foldr ((+) . bindSize) 0 bs
1065 exprSize :: CoreExpr -> Int
1066 -- ^ A measure of the size of the expressions, strictly greater than 0
1067 -- It also forces the expression pretty drastically as a side effect
1068 exprSize (Var v) = v `seq` 1
1069 exprSize (Lit lit) = lit `seq` 1
1070 exprSize (App f a) = exprSize f + exprSize a
1071 exprSize (Lam b e) = varSize b + exprSize e
1072 exprSize (Let b e) = bindSize b + exprSize e
1073 exprSize (Case e b t as) = seqType t `seq` exprSize e + varSize b + 1 + foldr ((+) . altSize) 0 as
1074 exprSize (Cast e co) = (seqType co `seq` 1) + exprSize e
1075 exprSize (Note n e) = noteSize n + exprSize e
1076 exprSize (Type t) = seqType t `seq` 1
1078 noteSize :: Note -> Int
1079 noteSize (SCC cc) = cc `seq` 1
1080 noteSize (CoreNote s) = s `seq` 1 -- hdaume: core annotations
1082 varSize :: Var -> Int
1083 varSize b | isTyCoVar b = 1
1084 | otherwise = seqType (idType b) `seq`
1085 megaSeqIdInfo (idInfo b) `seq`
1088 varsSize :: [Var] -> Int
1089 varsSize = sum . map varSize
1091 bindSize :: CoreBind -> Int
1092 bindSize (NonRec b e) = varSize b + exprSize e
1093 bindSize (Rec prs) = foldr ((+) . pairSize) 0 prs
1095 pairSize :: (Var, CoreExpr) -> Int
1096 pairSize (b,e) = varSize b + exprSize e
1098 altSize :: CoreAlt -> Int
1099 altSize (c,bs,e) = c `seq` varsSize bs + exprSize e
1103 %************************************************************************
1105 \subsection{Hashing}
1107 %************************************************************************
1110 hashExpr :: CoreExpr -> Int
1111 -- ^ Two expressions that hash to the same @Int@ may be equal (but may not be)
1112 -- Two expressions that hash to the different Ints are definitely unequal.
1114 -- The emphasis is on a crude, fast hash, rather than on high precision.
1116 -- But unequal here means \"not identical\"; two alpha-equivalent
1117 -- expressions may hash to the different Ints.
1119 -- We must be careful that @\\x.x@ and @\\y.y@ map to the same hash code,
1120 -- (at least if we want the above invariant to be true).
1122 hashExpr e = fromIntegral (hash_expr (1,emptyVarEnv) e .&. 0x7fffffff)
1123 -- UniqFM doesn't like negative Ints
1125 type HashEnv = (Int, VarEnv Int) -- Hash code for bound variables
1127 hash_expr :: HashEnv -> CoreExpr -> Word32
1128 -- Word32, because we're expecting overflows here, and overflowing
1129 -- signed types just isn't cool. In C it's even undefined.
1130 hash_expr env (Note _ e) = hash_expr env e
1131 hash_expr env (Cast e _) = hash_expr env e
1132 hash_expr env (Var v) = hashVar env v
1133 hash_expr _ (Lit lit) = fromIntegral (hashLiteral lit)
1134 hash_expr env (App f e) = hash_expr env f * fast_hash_expr env e
1135 hash_expr env (Let (NonRec b r) e) = hash_expr (extend_env env b) e * fast_hash_expr env r
1136 hash_expr env (Let (Rec ((b,_):_)) e) = hash_expr (extend_env env b) e
1137 hash_expr env (Case e _ _ _) = hash_expr env e
1138 hash_expr env (Lam b e) = hash_expr (extend_env env b) e
1139 hash_expr _ (Type _) = WARN(True, text "hash_expr: type") 1
1140 -- Shouldn't happen. Better to use WARN than trace, because trace
1141 -- prevents the CPR optimisation kicking in for hash_expr.
1143 fast_hash_expr :: HashEnv -> CoreExpr -> Word32
1144 fast_hash_expr env (Var v) = hashVar env v
1145 fast_hash_expr env (Type t) = fast_hash_type env t
1146 fast_hash_expr _ (Lit lit) = fromIntegral (hashLiteral lit)
1147 fast_hash_expr env (Cast e _) = fast_hash_expr env e
1148 fast_hash_expr env (Note _ e) = fast_hash_expr env e
1149 fast_hash_expr env (App _ a) = fast_hash_expr env a -- A bit idiosyncratic ('a' not 'f')!
1150 fast_hash_expr _ _ = 1
1152 fast_hash_type :: HashEnv -> Type -> Word32
1153 fast_hash_type env ty
1154 | Just tv <- getTyVar_maybe ty = hashVar env tv
1155 | Just (tc,tys) <- splitTyConApp_maybe ty = let hash_tc = fromIntegral (hashName (tyConName tc))
1156 in foldr (\t n -> fast_hash_type env t + n) hash_tc tys
1159 extend_env :: HashEnv -> Var -> (Int, VarEnv Int)
1160 extend_env (n,env) b = (n+1, extendVarEnv env b n)
1162 hashVar :: HashEnv -> Var -> Word32
1164 = fromIntegral (lookupVarEnv env v `orElse` hashName (idName v))
1168 %************************************************************************
1172 %************************************************************************
1174 Note [Eta reduction conditions]
1175 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1176 We try for eta reduction here, but *only* if we get all the way to an
1177 trivial expression. We don't want to remove extra lambdas unless we
1178 are going to avoid allocating this thing altogether.
1180 There are some particularly delicate points here:
1182 * Eta reduction is not valid in general:
1184 This matters, partly for old-fashioned correctness reasons but,
1185 worse, getting it wrong can yield a seg fault. Consider
1187 h y = case (case y of { True -> f `seq` True; False -> False }) of
1188 True -> ...; False -> ...
1190 If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
1191 says f=bottom, and replaces the (f `seq` True) with just
1192 (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
1193 *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
1194 the definition again, so that it does not termninate after all.
1195 Result: seg-fault because the boolean case actually gets a function value.
1198 So it's important to to the right thing.
1200 * Note [Arity care]: we need to be careful if we just look at f's
1201 arity. Currently (Dec07), f's arity is visible in its own RHS (see
1202 Note [Arity robustness] in SimplEnv) so we must *not* trust the
1203 arity when checking that 'f' is a value. Otherwise we will
1208 Which might change a terminiating program (think (f `seq` e)) to a
1209 non-terminating one. So we check for being a loop breaker first.
1211 However for GlobalIds we can look at the arity; and for primops we
1212 must, since they have no unfolding.
1214 * Regardless of whether 'f' is a value, we always want to
1215 reduce (/\a -> f a) to f
1216 This came up in a RULE: foldr (build (/\a -> g a))
1217 did not match foldr (build (/\b -> ...something complex...))
1218 The type checker can insert these eta-expanded versions,
1219 with both type and dictionary lambdas; hence the slightly
1222 * Never *reduce* arity. For example
1224 Then if h has arity 1 we don't want to eta-reduce because then
1225 f's arity would decrease, and that is bad
1227 These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
1230 Note [Eta reduction with casted arguments]
1231 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1233 (\(x:t3). f (x |> g)) :: t3 -> t2
1237 This should be eta-reduced to
1241 So we need to accumulate a coercion, pushing it inward (past
1242 variable arguments only) thus:
1243 f (x |> co_arg) |> co --> (f |> (sym co_arg -> co)) x
1244 f (x:t) |> co --> (f |> (t -> co)) x
1245 f @ a |> co --> (f |> (forall a.co)) @ a
1246 f @ (g:t1~t2) |> co --> (f |> (t1~t2 => co)) @ (g:t1~t2)
1247 These are the equations for ok_arg.
1249 It's true that we could also hope to eta reduce these:
1252 But the simplifier pushes those casts outwards, so we don't
1253 need to address that here.
1256 tryEtaReduce :: [Var] -> CoreExpr -> Maybe CoreExpr
1257 tryEtaReduce bndrs body
1258 = go (reverse bndrs) body (IdCo (exprType body))
1260 incoming_arity = count isId bndrs
1262 go :: [Var] -- Binders, innermost first, types [a3,a2,a1]
1263 -> CoreExpr -- Of type tr
1264 -> CoercionI -- Of type tr ~ ts
1265 -> Maybe CoreExpr -- Of type a1 -> a2 -> a3 -> ts
1266 -- See Note [Eta reduction with casted arguments]
1267 -- for why we have an accumulating coercion
1269 | ok_fun fun = Just (mkCoerceI co fun)
1271 go (b : bs) (App fun arg) co
1272 | Just co' <- ok_arg b arg co
1275 go _ _ _ = Nothing -- Failure!
1278 -- Note [Eta reduction conditions]
1279 ok_fun (App fun (Type ty))
1280 | not (any (`elemVarSet` tyVarsOfType ty) bndrs)
1283 = not (fun_id `elem` bndrs)
1284 && (ok_fun_id fun_id || all ok_lam bndrs)
1288 ok_fun_id fun = fun_arity fun >= incoming_arity
1291 fun_arity fun -- See Note [Arity care]
1292 | isLocalId fun && isLoopBreaker (idOccInfo fun) = 0
1293 | otherwise = idArity fun
1296 ok_lam v = isTyCoVar v || isDictId v
1299 ok_arg :: Var -- Of type bndr_t
1300 -> CoreExpr -- Of type arg_t
1301 -> CoercionI -- Of kind (t1~t2)
1302 -> Maybe CoercionI -- Of type (arg_t -> t1 ~ bndr_t -> t2)
1303 -- (and similarly for tyvars, coercion args)
1304 -- See Note [Eta reduction with casted arguments]
1305 ok_arg bndr (Type ty) co
1306 | Just tv <- getTyVar_maybe ty
1307 , bndr == tv = Just (mkForAllTyCoI tv co)
1308 ok_arg bndr (Var v) co
1309 | bndr == v = Just (mkFunTyCoI (IdCo (idType bndr)) co)
1310 ok_arg bndr (Cast (Var v) co_arg) co
1311 | bndr == v = Just (mkFunTyCoI (ACo (mkSymCoercion co_arg)) co)
1312 -- The simplifier combines multiple casts into one,
1313 -- so we can have a simple-minded pattern match here
1314 ok_arg _ _ _ = Nothing
1318 %************************************************************************
1320 \subsection{Determining non-updatable right-hand-sides}
1322 %************************************************************************
1324 Top-level constructor applications can usually be allocated
1325 statically, but they can't if the constructor, or any of the
1326 arguments, come from another DLL (because we can't refer to static
1327 labels in other DLLs).
1329 If this happens we simply make the RHS into an updatable thunk,
1330 and 'execute' it rather than allocating it statically.
1333 -- | This function is called only on *top-level* right-hand sides.
1334 -- Returns @True@ if the RHS can be allocated statically in the output,
1335 -- with no thunks involved at all.
1336 rhsIsStatic :: (Name -> Bool) -> CoreExpr -> Bool
1337 -- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
1338 -- refers to, CAFs; (ii) in CoreToStg to decide whether to put an
1339 -- update flag on it and (iii) in DsExpr to decide how to expand
1342 -- The basic idea is that rhsIsStatic returns True only if the RHS is
1343 -- (a) a value lambda
1344 -- (b) a saturated constructor application with static args
1346 -- BUT watch out for
1347 -- (i) Any cross-DLL references kill static-ness completely
1348 -- because they must be 'executed' not statically allocated
1349 -- ("DLL" here really only refers to Windows DLLs, on other platforms,
1350 -- this is not necessary)
1352 -- (ii) We treat partial applications as redexes, because in fact we
1353 -- make a thunk for them that runs and builds a PAP
1354 -- at run-time. The only appliations that are treated as
1355 -- static are *saturated* applications of constructors.
1357 -- We used to try to be clever with nested structures like this:
1358 -- ys = (:) w ((:) w [])
1359 -- on the grounds that CorePrep will flatten ANF-ise it later.
1360 -- But supporting this special case made the function much more
1361 -- complicated, because the special case only applies if there are no
1362 -- enclosing type lambdas:
1363 -- ys = /\ a -> Foo (Baz ([] a))
1364 -- Here the nested (Baz []) won't float out to top level in CorePrep.
1366 -- But in fact, even without -O, nested structures at top level are
1367 -- flattened by the simplifier, so we don't need to be super-clever here.
1371 -- f = \x::Int. x+7 TRUE
1372 -- p = (True,False) TRUE
1374 -- d = (fst p, False) FALSE because there's a redex inside
1375 -- (this particular one doesn't happen but...)
1377 -- h = D# (1.0## /## 2.0##) FALSE (redex again)
1378 -- n = /\a. Nil a TRUE
1380 -- t = /\a. (:) (case w a of ...) (Nil a) FALSE (redex)
1383 -- This is a bit like CoreUtils.exprIsHNF, with the following differences:
1384 -- a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
1386 -- b) (C x xs), where C is a contructor is updatable if the application is
1389 -- c) don't look through unfolding of f in (f x).
1391 rhsIsStatic _is_dynamic_name rhs = is_static False rhs
1393 is_static :: Bool -- True <=> in a constructor argument; must be atomic
1396 is_static False (Lam b e) = isRuntimeVar b || is_static False e
1397 is_static in_arg (Note n e) = notSccNote n && is_static in_arg e
1398 is_static in_arg (Cast e _) = is_static in_arg e
1400 is_static _ (Lit lit)
1402 MachLabel _ _ _ -> False
1404 -- A MachLabel (foreign import "&foo") in an argument
1405 -- prevents a constructor application from being static. The
1406 -- reason is that it might give rise to unresolvable symbols
1407 -- in the object file: under Linux, references to "weak"
1408 -- symbols from the data segment give rise to "unresolvable
1409 -- relocation" errors at link time This might be due to a bug
1410 -- in the linker, but we'll work around it here anyway.
1413 is_static in_arg other_expr = go other_expr 0
1415 go (Var f) n_val_args
1416 #if mingw32_TARGET_OS
1417 | not (_is_dynamic_name (idName f))
1419 = saturated_data_con f n_val_args
1420 || (in_arg && n_val_args == 0)
1421 -- A naked un-applied variable is *not* deemed a static RHS
1423 -- Reason: better to update so that the indirection gets shorted
1424 -- out, and the true value will be seen
1425 -- NB: if you change this, you'll break the invariant that THUNK_STATICs
1426 -- are always updatable. If you do so, make sure that non-updatable
1427 -- ones have enough space for their static link field!
1429 go (App f a) n_val_args
1430 | isTypeArg a = go f n_val_args
1431 | not in_arg && is_static True a = go f (n_val_args + 1)
1432 -- The (not in_arg) checks that we aren't in a constructor argument;
1433 -- if we are, we don't allow (value) applications of any sort
1435 -- NB. In case you wonder, args are sometimes not atomic. eg.
1436 -- x = D# (1.0## /## 2.0##)
1437 -- can't float because /## can fail.
1439 go (Note n f) n_val_args = notSccNote n && go f n_val_args
1440 go (Cast e _) n_val_args = go e n_val_args
1443 saturated_data_con f n_val_args
1444 = case isDataConWorkId_maybe f of
1445 Just dc -> n_val_args == dataConRepArity dc