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,
29 exprIsCheap, exprIsExpandable, exprIsCheap', CheapAppFun,
30 exprIsHNF, exprOkForSpeculation, exprIsBig, exprIsConLike,
31 rhsIsStatic, isCheapApp, isExpandableApp,
33 -- * Expression and bindings size
34 coreBindsSize, exprSize,
40 cheapEqExpr, eqExpr, eqExprX,
45 -- * Manipulating data constructors and types
46 applyTypeToArgs, applyTypeToArg,
47 dataConOrigInstPat, dataConRepInstPat, dataConRepFSInstPat
50 #include "HsVersions.h"
64 import TcType ( isPredTy )
72 import PrelNames( absentErrorIdKey )
81 %************************************************************************
83 \subsection{Find the type of a Core atom/expression}
85 %************************************************************************
88 exprType :: CoreExpr -> Type
89 -- ^ Recover the type of a well-typed Core expression. Fails when
90 -- applied to the actual 'CoreSyn.Type' expression as it cannot
91 -- really be said to have a type
92 exprType (Var var) = idType var
93 exprType (Lit lit) = literalType lit
94 exprType (Let _ body) = exprType body
95 exprType (Case _ _ ty _) = ty
96 exprType (Cast _ co) = snd (coercionKind co)
97 exprType (Note _ e) = exprType e
98 exprType (Lam binder expr) = mkPiType binder (exprType expr)
100 = case collectArgs e of
101 (fun, args) -> applyTypeToArgs e (exprType fun) args
103 exprType other = pprTrace "exprType" (pprCoreExpr other) alphaTy
105 coreAltType :: CoreAlt -> Type
106 -- ^ Returns the type of the alternatives right hand side
107 coreAltType (_,bs,rhs)
108 | any bad_binder bs = expandTypeSynonyms ty
109 | otherwise = ty -- Note [Existential variables and silly type synonyms]
112 free_tvs = tyVarsOfType ty
113 bad_binder b = isTyCoVar b && b `elemVarSet` free_tvs
115 coreAltsType :: [CoreAlt] -> Type
116 -- ^ Returns the type of the first alternative, which should be the same as for all alternatives
117 coreAltsType (alt:_) = coreAltType alt
118 coreAltsType [] = panic "corAltsType"
121 Note [Existential variables and silly type synonyms]
122 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
124 data T = forall a. T (Funny a)
129 Now, the type of 'x' is (Funny a), where 'a' is existentially quantified.
130 That means that 'exprType' and 'coreAltsType' may give a result that *appears*
131 to mention an out-of-scope type variable. See Trac #3409 for a more real-world
134 Various possibilities suggest themselves:
136 - Ignore the problem, and make Lint not complain about such variables
138 - Expand all type synonyms (or at least all those that discard arguments)
139 This is tricky, because at least for top-level things we want to
140 retain the type the user originally specified.
142 - Expand synonyms on the fly, when the problem arises. That is what
143 we are doing here. It's not too expensive, I think.
146 mkPiType :: EvVar -> Type -> Type
147 -- ^ Makes a @(->)@ type or a forall type, depending
148 -- on whether it is given a type variable or a term variable.
149 mkPiTypes :: [EvVar] -> Type -> Type
150 -- ^ 'mkPiType' for multiple type or value arguments
153 | isId v = mkFunTy (idType v) ty
154 | otherwise = mkForAllTy v ty
156 mkPiTypes vs ty = foldr mkPiType ty vs
160 applyTypeToArg :: Type -> CoreExpr -> Type
161 -- ^ Determines the type resulting from applying an expression to a function with the given type
162 applyTypeToArg fun_ty (Type arg_ty) = applyTy fun_ty arg_ty
163 applyTypeToArg fun_ty _ = funResultTy fun_ty
165 applyTypeToArgs :: CoreExpr -> Type -> [CoreExpr] -> Type
166 -- ^ A more efficient version of 'applyTypeToArg' when we have several arguments.
167 -- The first argument is just for debugging, and gives some context
168 applyTypeToArgs _ op_ty [] = op_ty
170 applyTypeToArgs e op_ty (Type ty : args)
171 = -- Accumulate type arguments so we can instantiate all at once
174 go rev_tys (Type ty : args) = go (ty:rev_tys) args
175 go rev_tys rest_args = applyTypeToArgs e op_ty' rest_args
177 op_ty' = applyTysD msg op_ty (reverse rev_tys)
178 msg = ptext (sLit "applyTypeToArgs") <+>
181 applyTypeToArgs e op_ty (_ : args)
182 = case (splitFunTy_maybe op_ty) of
183 Just (_, res_ty) -> applyTypeToArgs e res_ty args
184 Nothing -> pprPanic "applyTypeToArgs" (panic_msg e op_ty)
186 panic_msg :: CoreExpr -> Type -> SDoc
187 panic_msg e op_ty = pprCoreExpr e $$ ppr op_ty
190 %************************************************************************
192 \subsection{Attaching notes}
194 %************************************************************************
197 -- | Wrap the given expression in the coercion, dropping identity coercions and coalescing nested coercions
198 mkCoerceI :: CoercionI -> CoreExpr -> CoreExpr
199 mkCoerceI (IdCo _) e = e
200 mkCoerceI (ACo co) e = mkCoerce co e
202 -- | Wrap the given expression in the coercion safely, coalescing nested coercions
203 mkCoerce :: Coercion -> CoreExpr -> CoreExpr
204 mkCoerce co (Cast expr co2)
205 = ASSERT(let { (from_ty, _to_ty) = coercionKind co;
206 (_from_ty2, to_ty2) = coercionKind co2} in
207 from_ty `coreEqType` to_ty2 )
208 mkCoerce (mkTransCoercion co2 co) expr
211 = let (from_ty, _to_ty) = coercionKind co in
212 -- if to_ty `coreEqType` from_ty
215 WARN(not (from_ty `coreEqType` exprType expr), text "Trying to coerce" <+> text "(" <> ppr expr $$ text "::" <+> ppr (exprType expr) <> text ")" $$ ppr co $$ pprEqPred (coercionKind co))
220 -- | Wraps the given expression in the cost centre unless
221 -- in a way that maximises their utility to the user
222 mkSCC :: CostCentre -> Expr b -> Expr b
223 -- Note: Nested SCC's *are* preserved for the benefit of
224 -- cost centre stack profiling
225 mkSCC _ (Lit lit) = Lit lit
226 mkSCC cc (Lam x e) = Lam x (mkSCC cc e) -- Move _scc_ inside lambda
227 mkSCC cc (Note (SCC cc') e) = Note (SCC cc) (Note (SCC cc') e)
228 mkSCC cc (Note n e) = Note n (mkSCC cc e) -- Move _scc_ inside notes
229 mkSCC cc (Cast e co) = Cast (mkSCC cc e) co -- Move _scc_ inside cast
230 mkSCC cc expr = Note (SCC cc) expr
234 %************************************************************************
236 \subsection{Other expression construction}
238 %************************************************************************
241 bindNonRec :: Id -> CoreExpr -> CoreExpr -> CoreExpr
242 -- ^ @bindNonRec x r b@ produces either:
248 -- > case r of x { _DEFAULT_ -> b }
250 -- depending on whether we have to use a @case@ or @let@
251 -- binding for the expression (see 'needsCaseBinding').
252 -- It's used by the desugarer to avoid building bindings
253 -- that give Core Lint a heart attack, although actually
254 -- the simplifier deals with them perfectly well. See
255 -- also 'MkCore.mkCoreLet'
256 bindNonRec bndr rhs body
257 | needsCaseBinding (idType bndr) rhs = Case rhs bndr (exprType body) [(DEFAULT, [], body)]
258 | otherwise = Let (NonRec bndr rhs) body
260 -- | Tests whether we have to use a @case@ rather than @let@ binding for this expression
261 -- as per the invariants of 'CoreExpr': see "CoreSyn#let_app_invariant"
262 needsCaseBinding :: Type -> CoreExpr -> Bool
263 needsCaseBinding ty rhs = isUnLiftedType ty && not (exprOkForSpeculation rhs)
264 -- Make a case expression instead of a let
265 -- These can arise either from the desugarer,
266 -- or from beta reductions: (\x.e) (x +# y)
270 mkAltExpr :: AltCon -- ^ Case alternative constructor
271 -> [CoreBndr] -- ^ Things bound by the pattern match
272 -> [Type] -- ^ The type arguments to the case alternative
274 -- ^ This guy constructs the value that the scrutinee must have
275 -- given that you are in one particular branch of a case
276 mkAltExpr (DataAlt con) args inst_tys
277 = mkConApp con (map Type inst_tys ++ varsToCoreExprs args)
278 mkAltExpr (LitAlt lit) [] []
280 mkAltExpr (LitAlt _) _ _ = panic "mkAltExpr LitAlt"
281 mkAltExpr DEFAULT _ _ = panic "mkAltExpr DEFAULT"
285 %************************************************************************
287 \subsection{Taking expressions apart}
289 %************************************************************************
291 The default alternative must be first, if it exists at all.
292 This makes it easy to find, though it makes matching marginally harder.
295 -- | Extract the default case alternative
296 findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
297 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null args ) (alts, Just rhs)
298 findDefault alts = (alts, Nothing)
300 isDefaultAlt :: CoreAlt -> Bool
301 isDefaultAlt (DEFAULT, _, _) = True
302 isDefaultAlt _ = False
305 -- | Find the case alternative corresponding to a particular
306 -- constructor: panics if no such constructor exists
307 findAlt :: AltCon -> [CoreAlt] -> Maybe CoreAlt
308 -- A "Nothing" result *is* legitmiate
309 -- See Note [Unreachable code]
312 (deflt@(DEFAULT,_,_):alts) -> go alts (Just deflt)
316 go (alt@(con1,_,_) : alts) deflt
317 = case con `cmpAltCon` con1 of
318 LT -> deflt -- Missed it already; the alts are in increasing order
320 GT -> ASSERT( not (con1 == DEFAULT) ) go alts deflt
322 ---------------------------------
323 mergeAlts :: [CoreAlt] -> [CoreAlt] -> [CoreAlt]
324 -- ^ Merge alternatives preserving order; alternatives in
325 -- the first argument shadow ones in the second
326 mergeAlts [] as2 = as2
327 mergeAlts as1 [] = as1
328 mergeAlts (a1:as1) (a2:as2)
329 = case a1 `cmpAlt` a2 of
330 LT -> a1 : mergeAlts as1 (a2:as2)
331 EQ -> a1 : mergeAlts as1 as2 -- Discard a2
332 GT -> a2 : mergeAlts (a1:as1) as2
335 ---------------------------------
336 trimConArgs :: AltCon -> [CoreArg] -> [CoreArg]
339 -- > case (C a b x y) of
342 -- We want to drop the leading type argument of the scrutinee
343 -- leaving the arguments to match agains the pattern
345 trimConArgs DEFAULT args = ASSERT( null args ) []
346 trimConArgs (LitAlt _) args = ASSERT( null args ) []
347 trimConArgs (DataAlt dc) args = dropList (dataConUnivTyVars dc) args
350 Note [Unreachable code]
351 ~~~~~~~~~~~~~~~~~~~~~~~
352 It is possible (although unusual) for GHC to find a case expression
353 that cannot match. For example:
355 data Col = Red | Green | Blue
359 _ -> ...(case x of { Green -> e1; Blue -> e2 })...
361 Suppose that for some silly reason, x isn't substituted in the case
362 expression. (Perhaps there's a NOINLINE on it, or profiling SCC stuff
363 gets in the way; cf Trac #3118.) Then the full-lazines pass might produce
367 lvl = case x of { Green -> e1; Blue -> e2 })
372 Now if x gets inlined, we won't be able to find a matching alternative
373 for 'Red'. That's because 'lvl' is unreachable. So rather than crashing
374 we generate (error "Inaccessible alternative").
376 Similar things can happen (augmented by GADTs) when the Simplifier
377 filters down the matching alternatives in Simplify.rebuildCase.
380 %************************************************************************
384 %************************************************************************
388 @exprIsTrivial@ is true of expressions we are unconditionally happy to
389 duplicate; simple variables and constants, and type
390 applications. Note that primop Ids aren't considered
393 Note [Variable are trivial]
394 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
395 There used to be a gruesome test for (hasNoBinding v) in the
397 exprIsTrivial (Var v) | hasNoBinding v = idArity v == 0
398 The idea here is that a constructor worker, like \$wJust, is
399 really short for (\x -> \$wJust x), becuase \$wJust has no binding.
400 So it should be treated like a lambda. Ditto unsaturated primops.
401 But now constructor workers are not "have-no-binding" Ids. And
402 completely un-applied primops and foreign-call Ids are sufficiently
403 rare that I plan to allow them to be duplicated and put up with
406 Note [SCCs are trivial]
407 ~~~~~~~~~~~~~~~~~~~~~~~
408 We used not to treat (_scc_ "foo" x) as trivial, because it really
409 generates code, (and a heap object when it's a function arg) to
410 capture the cost centre. However, the profiling system discounts the
411 allocation costs for such "boxing thunks" whereas the extra costs of
412 *not* inlining otherwise-trivial bindings can be high, and are hard to
416 exprIsTrivial :: CoreExpr -> Bool
417 exprIsTrivial (Var _) = True -- See Note [Variables are trivial]
418 exprIsTrivial (Type _) = True
419 exprIsTrivial (Lit lit) = litIsTrivial lit
420 exprIsTrivial (App e arg) = not (isRuntimeArg arg) && exprIsTrivial e
421 exprIsTrivial (Note _ e) = exprIsTrivial e -- See Note [SCCs are trivial]
422 exprIsTrivial (Cast e _) = exprIsTrivial e
423 exprIsTrivial (Lam b body) = not (isRuntimeVar b) && exprIsTrivial body
424 exprIsTrivial _ = False
428 %************************************************************************
432 %************************************************************************
436 @exprIsDupable@ is true of expressions that can be duplicated at a modest
437 cost in code size. This will only happen in different case
438 branches, so there's no issue about duplicating work.
440 That is, exprIsDupable returns True of (f x) even if
441 f is very very expensive to call.
443 Its only purpose is to avoid fruitless let-binding
444 and then inlining of case join points
448 exprIsDupable :: CoreExpr -> Bool
449 exprIsDupable (Type _) = True
450 exprIsDupable (Var _) = True
451 exprIsDupable (Lit lit) = litIsDupable lit
452 exprIsDupable (Note _ e) = exprIsDupable e
453 exprIsDupable (Cast e _) = exprIsDupable e
458 go (App f a) n_args = n_args < dupAppSize
464 dupAppSize = 4 -- Size of application we are prepared to duplicate
467 %************************************************************************
469 exprIsCheap, exprIsExpandable
471 %************************************************************************
473 Note [exprIsCheap] See also Note [Interaction of exprIsCheap and lone variables]
474 ~~~~~~~~~~~~~~~~~~ in CoreUnfold.lhs
475 @exprIsCheap@ looks at a Core expression and returns \tr{True} if
476 it is obviously in weak head normal form, or is cheap to get to WHNF.
477 [Note that that's not the same as exprIsDupable; an expression might be
478 big, and hence not dupable, but still cheap.]
480 By ``cheap'' we mean a computation we're willing to:
481 push inside a lambda, or
482 inline at more than one place
483 That might mean it gets evaluated more than once, instead of being
484 shared. The main examples of things which aren't WHNF but are
489 (where e, and all the ei are cheap)
492 (where e and b are cheap)
495 (where op is a cheap primitive operator)
498 (because we are happy to substitute it inside a lambda)
500 Notice that a variable is considered 'cheap': we can push it inside a lambda,
501 because sharing will make sure it is only evaluated once.
503 Note [exprIsCheap and exprIsHNF]
504 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
505 Note that exprIsHNF does not imply exprIsCheap. Eg
506 let x = fac 20 in Just x
507 This responds True to exprIsHNF (you can discard a seq), but
508 False to exprIsCheap.
511 exprIsCheap :: CoreExpr -> Bool
512 exprIsCheap = exprIsCheap' isCheapApp
514 exprIsExpandable :: CoreExpr -> Bool
515 exprIsExpandable = exprIsCheap' isExpandableApp -- See Note [CONLIKE pragma] in BasicTypes
517 type CheapAppFun = Id -> Int -> Bool
518 exprIsCheap' :: CheapAppFun -> CoreExpr -> Bool
519 exprIsCheap' _ (Lit _) = True
520 exprIsCheap' _ (Type _) = True
521 exprIsCheap' _ (Var _) = True
522 exprIsCheap' good_app (Note _ e) = exprIsCheap' good_app e
523 exprIsCheap' good_app (Cast e _) = exprIsCheap' good_app e
524 exprIsCheap' good_app (Lam x e) = isRuntimeVar x
525 || exprIsCheap' good_app e
527 exprIsCheap' good_app (Case e _ _ alts) = exprIsCheap' good_app e &&
528 and [exprIsCheap' good_app rhs | (_,_,rhs) <- alts]
529 -- Experimentally, treat (case x of ...) as cheap
530 -- (and case __coerce x etc.)
531 -- This improves arities of overloaded functions where
532 -- there is only dictionary selection (no construction) involved
534 exprIsCheap' good_app (Let (NonRec x _) e)
535 | isUnLiftedType (idType x) = exprIsCheap' good_app e
537 -- Strict lets always have cheap right hand sides,
538 -- and do no allocation, so just look at the body
539 -- Non-strict lets do allocation so we don't treat them as cheap
542 exprIsCheap' good_app other_expr -- Applications and variables
545 -- Accumulate value arguments, then decide
546 go (App f a) val_args | isRuntimeArg a = go f (a:val_args)
547 | otherwise = go f val_args
549 go (Var _) [] = True -- Just a type application of a variable
550 -- (f t1 t2 t3) counts as WHNF
552 = case idDetails f of
553 RecSelId {} -> go_sel args
554 ClassOpId {} -> go_sel args
555 PrimOpId op -> go_primop op args
556 _ | good_app f (length args) -> go_pap args
557 | isBottomingId f -> True
559 -- Application of a function which
560 -- always gives bottom; we treat this as cheap
561 -- because it certainly doesn't need to be shared!
566 go_pap args = all exprIsTrivial args
567 -- For constructor applications and primops, check that all
568 -- the args are trivial. We don't want to treat as cheap, say,
570 -- We'll put up with one constructor application, but not dozens
573 go_primop op args = primOpIsCheap op && all (exprIsCheap' good_app) args
574 -- In principle we should worry about primops
575 -- that return a type variable, since the result
576 -- might be applied to something, but I'm not going
577 -- to bother to check the number of args
580 go_sel [arg] = exprIsCheap' good_app arg -- I'm experimenting with making record selection
581 go_sel _ = False -- look cheap, so we will substitute it inside a
582 -- lambda. Particularly for dictionary field selection.
583 -- BUT: Take care with (sel d x)! The (sel d) might be cheap, but
584 -- there's no guarantee that (sel d x) will be too. Hence (n_val_args == 1)
586 isCheapApp :: CheapAppFun
587 isCheapApp fn n_val_args
589 || n_val_args < idArity fn
591 isExpandableApp :: CheapAppFun
592 isExpandableApp fn n_val_args
594 || n_val_args < idArity fn
595 || go n_val_args (idType fn)
597 -- See if all the arguments are PredTys (implicit params or classes)
598 -- If so we'll regard it as expandable; see Note [Expandable overloadings]
601 | Just (_, ty) <- splitForAllTy_maybe ty = go n_val_args ty
602 | Just (arg, ty) <- splitFunTy_maybe ty
603 , isPredTy arg = go (n_val_args-1) ty
607 Note [Expandable overloadings]
608 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
609 Suppose the user wrote this
610 {-# RULE forall x. foo (negate x) = h x #-}
611 f x = ....(foo (negate x))....
612 He'd expect the rule to fire. But since negate is overloaded, we might
614 f = \d -> let n = negate d in \x -> ...foo (n x)...
615 So we treat the application of a function (negate in this case) to a
616 *dictionary* as expandable. In effect, every function is CONLIKE when
617 it's applied only to dictionaries.
620 %************************************************************************
624 %************************************************************************
627 -- | 'exprOkForSpeculation' returns True of an expression that is:
629 -- * Safe to evaluate even if normal order eval might not
630 -- evaluate the expression at all, or
632 -- * Safe /not/ to evaluate even if normal order would do so
634 -- Precisely, it returns @True@ iff:
636 -- * The expression guarantees to terminate,
638 -- * without raising an exception,
639 -- * without causing a side effect (e.g. writing a mutable variable)
641 -- Note that if @exprIsHNF e@, then @exprOkForSpecuation e@.
642 -- As an example of the considerations in this test, consider:
644 -- > let x = case y# +# 1# of { r# -> I# r# }
647 -- being translated to:
649 -- > case y# +# 1# of { r# ->
654 -- We can only do this if the @y + 1@ is ok for speculation: it has no
655 -- side effects, and can't diverge or raise an exception.
656 exprOkForSpeculation :: CoreExpr -> Bool
657 exprOkForSpeculation (Lit _) = True
658 exprOkForSpeculation (Type _) = True
659 -- Tick boxes are *not* suitable for speculation
660 exprOkForSpeculation (Var v) = isUnLiftedType (idType v)
661 && not (isTickBoxOp v)
662 exprOkForSpeculation (Note _ e) = exprOkForSpeculation e
663 exprOkForSpeculation (Cast e _) = exprOkForSpeculation e
665 exprOkForSpeculation (Case e _ _ alts)
666 = exprOkForSpeculation e -- Note [exprOkForSpeculation: case expressions]
667 && all (\(_,_,rhs) -> exprOkForSpeculation rhs) alts
669 exprOkForSpeculation other_expr
670 = case collectArgs other_expr of
671 (Var f, args) | f `hasKey` absentErrorIdKey -- Note [Absent error Id]
672 -> all exprOkForSpeculation args -- in WwLib
674 -> spec_ok (idDetails f) args
678 spec_ok (DataConWorkId _) _
679 = True -- The strictness of the constructor has already
680 -- been expressed by its "wrapper", so we don't need
681 -- to take the arguments into account
683 spec_ok (PrimOpId op) args
684 | isDivOp op, -- Special case for dividing operations that fail
685 [arg1, Lit lit] <- args -- only if the divisor is zero
686 = not (isZeroLit lit) && exprOkForSpeculation arg1
687 -- Often there is a literal divisor, and this
688 -- can get rid of a thunk in an inner looop
691 = primOpOkForSpeculation op &&
692 all exprOkForSpeculation args
693 -- A bit conservative: we don't really need
694 -- to care about lazy arguments, but this is easy
696 spec_ok (DFunId _ new_type) _ = not new_type
697 -- DFuns terminate, unless the dict is implemented with a newtype
698 -- in which case they may not
702 -- | True of dyadic operators that can fail only if the second arg is zero!
703 isDivOp :: PrimOp -> Bool
704 -- This function probably belongs in PrimOp, or even in
705 -- an automagically generated file.. but it's such a
706 -- special case I thought I'd leave it here for now.
707 isDivOp IntQuotOp = True
708 isDivOp IntRemOp = True
709 isDivOp WordQuotOp = True
710 isDivOp WordRemOp = True
711 isDivOp FloatDivOp = True
712 isDivOp DoubleDivOp = True
716 Note [exprOkForSpeculation: case expressions]
717 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
719 It's always sound for exprOkForSpeculation to return False, and we
720 don't want it to take too long, so it bales out on complicated-looking
721 terms. Notably lets, which can be stacked very deeply; and in any
722 case the argument of exprOkForSpeculation is usually in a strict context,
723 so any lets will have been floated away.
725 However, we keep going on case-expressions. An example like this one
726 showed up in DPH code:
729 foo n = (if n < 5 then 1 else 2) `seq` foo (n-1)
731 If exprOkForSpeculation doesn't look through case expressions, you get this:
733 \ (ww :: GHC.Prim.Int#) ->
735 __DEFAULT -> case (case <# ds 5 of _ {
736 GHC.Types.False -> lvl1;
737 GHC.Types.True -> lvl})
739 T.$wfoo (GHC.Prim.-# ds_XkE 1) };
743 The inner case is redundant, and should be nuked.
746 %************************************************************************
748 exprIsHNF, exprIsConLike
750 %************************************************************************
753 -- Note [exprIsHNF] See also Note [exprIsCheap and exprIsHNF]
755 -- | exprIsHNF returns true for expressions that are certainly /already/
756 -- evaluated to /head/ normal form. This is used to decide whether it's ok
759 -- > case x of _ -> e
765 -- and to decide whether it's safe to discard a 'seq'.
767 -- So, it does /not/ treat variables as evaluated, unless they say they are.
768 -- However, it /does/ treat partial applications and constructor applications
769 -- as values, even if their arguments are non-trivial, provided the argument
770 -- type is lifted. For example, both of these are values:
772 -- > (:) (f x) (map f xs)
773 -- > map (...redex...)
775 -- because 'seq' on such things completes immediately.
777 -- For unlifted argument types, we have to be careful:
781 -- Suppose @f x@ diverges; then @C (f x)@ is not a value. However this can't
782 -- happen: see "CoreSyn#let_app_invariant". This invariant states that arguments of
783 -- unboxed type must be ok-for-speculation (or trivial).
784 exprIsHNF :: CoreExpr -> Bool -- True => Value-lambda, constructor, PAP
785 exprIsHNF = exprIsHNFlike isDataConWorkId isEvaldUnfolding
789 -- | Similar to 'exprIsHNF' but includes CONLIKE functions as well as
790 -- data constructors. Conlike arguments are considered interesting by the
792 exprIsConLike :: CoreExpr -> Bool -- True => lambda, conlike, PAP
793 exprIsConLike = exprIsHNFlike isConLikeId isConLikeUnfolding
795 -- | Returns true for values or value-like expressions. These are lambdas,
796 -- constructors / CONLIKE functions (as determined by the function argument)
799 exprIsHNFlike :: (Var -> Bool) -> (Unfolding -> Bool) -> CoreExpr -> Bool
800 exprIsHNFlike is_con is_con_unf = is_hnf_like
802 is_hnf_like (Var v) -- NB: There are no value args at this point
803 = is_con v -- Catches nullary constructors,
804 -- so that [] and () are values, for example
805 || idArity v > 0 -- Catches (e.g.) primops that don't have unfoldings
806 || is_con_unf (idUnfolding v)
807 -- Check the thing's unfolding; it might be bound to a value
808 -- We don't look through loop breakers here, which is a bit conservative
809 -- but otherwise I worry that if an Id's unfolding is just itself,
810 -- we could get an infinite loop
812 is_hnf_like (Lit _) = True
813 is_hnf_like (Type _) = True -- Types are honorary Values;
814 -- we don't mind copying them
815 is_hnf_like (Lam b e) = isRuntimeVar b || is_hnf_like e
816 is_hnf_like (Note _ e) = is_hnf_like e
817 is_hnf_like (Cast e _) = is_hnf_like e
818 is_hnf_like (App e (Type _)) = is_hnf_like e
819 is_hnf_like (App e a) = app_is_value e [a]
820 is_hnf_like (Let _ e) = is_hnf_like e -- Lazy let(rec)s don't affect us
821 is_hnf_like _ = False
823 -- There is at least one value argument
824 app_is_value :: CoreExpr -> [CoreArg] -> Bool
825 app_is_value (Var fun) args
826 = idArity fun > valArgCount args -- Under-applied function
827 || is_con fun -- or constructor-like
828 app_is_value (Note _ f) as = app_is_value f as
829 app_is_value (Cast f _) as = app_is_value f as
830 app_is_value (App f a) as = app_is_value f (a:as)
831 app_is_value _ _ = False
835 %************************************************************************
837 Instantiating data constructors
839 %************************************************************************
841 These InstPat functions go here to avoid circularity between DataCon and Id
844 dataConRepInstPat, dataConOrigInstPat :: [Unique] -> DataCon -> [Type] -> ([TyVar], [CoVar], [Id])
845 dataConRepFSInstPat :: [FastString] -> [Unique] -> DataCon -> [Type] -> ([TyVar], [CoVar], [Id])
847 dataConRepInstPat = dataConInstPat dataConRepArgTys (repeat ((fsLit "ipv")))
848 dataConRepFSInstPat = dataConInstPat dataConRepArgTys
849 dataConOrigInstPat = dataConInstPat dc_arg_tys (repeat ((fsLit "ipv")))
851 dc_arg_tys dc = map mkPredTy (dataConEqTheta dc) ++ map mkPredTy (dataConDictTheta dc) ++ dataConOrigArgTys dc
852 -- Remember to include the existential dictionaries
854 dataConInstPat :: (DataCon -> [Type]) -- function used to find arg tys
855 -> [FastString] -- A long enough list of FSs to use for names
856 -> [Unique] -- An equally long list of uniques, at least one for each binder
858 -> [Type] -- Types to instantiate the universally quantified tyvars
859 -> ([TyVar], [CoVar], [Id]) -- Return instantiated variables
860 -- dataConInstPat arg_fun fss us con inst_tys returns a triple
861 -- (ex_tvs, co_tvs, arg_ids),
863 -- ex_tvs are intended to be used as binders for existential type args
865 -- co_tvs are intended to be used as binders for coercion args and the kinds
866 -- of these vars have been instantiated by the inst_tys and the ex_tys
867 -- The co_tvs include both GADT equalities (dcEqSpec) and
868 -- programmer-specified equalities (dcEqTheta)
870 -- arg_ids are indended to be used as binders for value arguments,
871 -- and their types have been instantiated with inst_tys and ex_tys
872 -- The arg_ids include both dicts (dcDictTheta) and
873 -- programmer-specified arguments (after rep-ing) (deRepArgTys)
876 -- The following constructor T1
879 -- T1 :: forall b. Int -> b -> T(a,b)
882 -- has representation type
883 -- forall a. forall a1. forall b. (a ~ (a1,b)) =>
886 -- dataConInstPat fss us T1 (a1',b') will return
888 -- ([a1'', b''], [c :: (a1', b')~(a1'', b'')], [x :: Int, y :: b''])
890 -- where the double-primed variables are created with the FastStrings and
891 -- Uniques given as fss and us
892 dataConInstPat arg_fun fss uniqs con inst_tys
893 = (ex_bndrs, co_bndrs, arg_ids)
895 univ_tvs = dataConUnivTyVars con
896 ex_tvs = dataConExTyVars con
897 arg_tys = arg_fun con
898 eq_spec = dataConEqSpec con
899 eq_theta = dataConEqTheta con
900 eq_preds = eqSpecPreds eq_spec ++ eq_theta
903 n_co = length eq_preds
905 -- split the Uniques and FastStrings
906 (ex_uniqs, uniqs') = splitAt n_ex uniqs
907 (co_uniqs, id_uniqs) = splitAt n_co uniqs'
909 (ex_fss, fss') = splitAt n_ex fss
910 (co_fss, id_fss) = splitAt n_co fss'
912 -- Make existential type variables
913 ex_bndrs = zipWith3 mk_ex_var ex_uniqs ex_fss ex_tvs
914 mk_ex_var uniq fs var = mkTyVar new_name kind
916 new_name = mkSysTvName uniq fs
919 -- Make the instantiating substitution
920 subst = zipOpenTvSubst (univ_tvs ++ ex_tvs) (inst_tys ++ map mkTyVarTy ex_bndrs)
922 -- Make new coercion vars, instantiating kind
923 co_bndrs = zipWith3 mk_co_var co_uniqs co_fss eq_preds
924 mk_co_var uniq fs eq_pred = mkCoVar new_name co_kind
926 new_name = mkSysTvName uniq fs
927 co_kind = substTy subst (mkPredTy eq_pred)
929 -- make value vars, instantiating types
930 mk_id_var uniq fs ty = mkUserLocal (mkVarOccFS fs) uniq (substTy subst ty) noSrcSpan
931 arg_ids = zipWith3 mk_id_var id_uniqs id_fss arg_tys
935 %************************************************************************
939 %************************************************************************
942 -- | A cheap equality test which bales out fast!
943 -- If it returns @True@ the arguments are definitely equal,
944 -- otherwise, they may or may not be equal.
946 -- See also 'exprIsBig'
947 cheapEqExpr :: Expr b -> Expr b -> Bool
949 cheapEqExpr (Var v1) (Var v2) = v1==v2
950 cheapEqExpr (Lit lit1) (Lit lit2) = lit1 == lit2
951 cheapEqExpr (Type t1) (Type t2) = t1 `coreEqType` t2
953 cheapEqExpr (App f1 a1) (App f2 a2)
954 = f1 `cheapEqExpr` f2 && a1 `cheapEqExpr` a2
956 cheapEqExpr (Cast e1 t1) (Cast e2 t2)
957 = e1 `cheapEqExpr` e2 && t1 `coreEqCoercion` t2
959 cheapEqExpr _ _ = False
963 exprIsBig :: Expr b -> Bool
964 -- ^ Returns @True@ of expressions that are too big to be compared by 'cheapEqExpr'
965 exprIsBig (Lit _) = False
966 exprIsBig (Var _) = False
967 exprIsBig (Type _) = False
968 exprIsBig (Lam _ e) = exprIsBig e
969 exprIsBig (App f a) = exprIsBig f || exprIsBig a
970 exprIsBig (Cast e _) = exprIsBig e -- Hopefully coercions are not too big!
975 eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
976 -- Compares for equality, modulo alpha
977 eqExpr in_scope e1 e2
978 = eqExprX id_unf (mkRnEnv2 in_scope) e1 e2
980 id_unf _ = noUnfolding -- Don't expand
984 eqExprX :: IdUnfoldingFun -> RnEnv2 -> CoreExpr -> CoreExpr -> Bool
985 -- ^ Compares expressions for equality, modulo alpha.
986 -- Does /not/ look through newtypes or predicate types
987 -- Used in rule matching, and also CSE
989 eqExprX id_unfolding_fun env e1 e2
992 go env (Var v1) (Var v2)
993 | rnOccL env v1 == rnOccR env v2
996 -- The next two rules expand non-local variables
997 -- C.f. Note [Expanding variables] in Rules.lhs
998 -- and Note [Do not expand locally-bound variables] in Rules.lhs
1000 | not (locallyBoundL env v1)
1001 , Just e1' <- expandUnfolding_maybe (id_unfolding_fun (lookupRnInScope env v1))
1002 = go (nukeRnEnvL env) e1' e2
1005 | not (locallyBoundR env v2)
1006 , Just e2' <- expandUnfolding_maybe (id_unfolding_fun (lookupRnInScope env v2))
1007 = go (nukeRnEnvR env) e1 e2'
1009 go _ (Lit lit1) (Lit lit2) = lit1 == lit2
1010 go env (Type t1) (Type t2) = tcEqTypeX env t1 t2
1011 go env (Cast e1 co1) (Cast e2 co2) = tcEqTypeX env co1 co2 && go env e1 e2
1012 go env (App f1 a1) (App f2 a2) = go env f1 f2 && go env a1 a2
1013 go env (Note n1 e1) (Note n2 e2) = go_note n1 n2 && go env e1 e2
1015 go env (Lam b1 e1) (Lam b2 e2)
1016 = tcEqTypeX env (varType b1) (varType b2) -- False for Id/TyVar combination
1017 && go (rnBndr2 env b1 b2) e1 e2
1019 go env (Let (NonRec v1 r1) e1) (Let (NonRec v2 r2) e2)
1020 = go env r1 r2 -- No need to check binder types, since RHSs match
1021 && go (rnBndr2 env v1 v2) e1 e2
1023 go env (Let (Rec ps1) e1) (Let (Rec ps2) e2)
1024 = all2 (go env') rs1 rs2 && go env' e1 e2
1026 (bs1,rs1) = unzip ps1
1027 (bs2,rs2) = unzip ps2
1028 env' = rnBndrs2 env bs1 bs2
1030 go env (Case e1 b1 _ a1) (Case e2 b2 _ a2)
1032 && tcEqTypeX env (idType b1) (idType b2)
1033 && all2 (go_alt (rnBndr2 env b1 b2)) a1 a2
1038 go_alt env (c1, bs1, e1) (c2, bs2, e2)
1039 = c1 == c2 && go (rnBndrs2 env bs1 bs2) e1 e2
1042 go_note (SCC cc1) (SCC cc2) = cc1 == cc2
1043 go_note (CoreNote s1) (CoreNote s2) = s1 == s2
1050 locallyBoundL, locallyBoundR :: RnEnv2 -> Var -> Bool
1051 locallyBoundL rn_env v = inRnEnvL rn_env v
1052 locallyBoundR rn_env v = inRnEnvR rn_env v
1056 %************************************************************************
1058 \subsection{The size of an expression}
1060 %************************************************************************
1063 coreBindsSize :: [CoreBind] -> Int
1064 coreBindsSize bs = foldr ((+) . bindSize) 0 bs
1066 exprSize :: CoreExpr -> Int
1067 -- ^ A measure of the size of the expressions, strictly greater than 0
1068 -- It also forces the expression pretty drastically as a side effect
1069 exprSize (Var v) = v `seq` 1
1070 exprSize (Lit lit) = lit `seq` 1
1071 exprSize (App f a) = exprSize f + exprSize a
1072 exprSize (Lam b e) = varSize b + exprSize e
1073 exprSize (Let b e) = bindSize b + exprSize e
1074 exprSize (Case e b t as) = seqType t `seq` exprSize e + varSize b + 1 + foldr ((+) . altSize) 0 as
1075 exprSize (Cast e co) = (seqType co `seq` 1) + exprSize e
1076 exprSize (Note n e) = noteSize n + exprSize e
1077 exprSize (Type t) = seqType t `seq` 1
1079 noteSize :: Note -> Int
1080 noteSize (SCC cc) = cc `seq` 1
1081 noteSize (CoreNote s) = s `seq` 1 -- hdaume: core annotations
1083 varSize :: Var -> Int
1084 varSize b | isTyCoVar b = 1
1085 | otherwise = seqType (idType b) `seq`
1086 megaSeqIdInfo (idInfo b) `seq`
1089 varsSize :: [Var] -> Int
1090 varsSize = sum . map varSize
1092 bindSize :: CoreBind -> Int
1093 bindSize (NonRec b e) = varSize b + exprSize e
1094 bindSize (Rec prs) = foldr ((+) . pairSize) 0 prs
1096 pairSize :: (Var, CoreExpr) -> Int
1097 pairSize (b,e) = varSize b + exprSize e
1099 altSize :: CoreAlt -> Int
1100 altSize (c,bs,e) = c `seq` varsSize bs + exprSize e
1104 %************************************************************************
1106 \subsection{Hashing}
1108 %************************************************************************
1111 hashExpr :: CoreExpr -> Int
1112 -- ^ Two expressions that hash to the same @Int@ may be equal (but may not be)
1113 -- Two expressions that hash to the different Ints are definitely unequal.
1115 -- The emphasis is on a crude, fast hash, rather than on high precision.
1117 -- But unequal here means \"not identical\"; two alpha-equivalent
1118 -- expressions may hash to the different Ints.
1120 -- We must be careful that @\\x.x@ and @\\y.y@ map to the same hash code,
1121 -- (at least if we want the above invariant to be true).
1123 hashExpr e = fromIntegral (hash_expr (1,emptyVarEnv) e .&. 0x7fffffff)
1124 -- UniqFM doesn't like negative Ints
1126 type HashEnv = (Int, VarEnv Int) -- Hash code for bound variables
1128 hash_expr :: HashEnv -> CoreExpr -> Word32
1129 -- Word32, because we're expecting overflows here, and overflowing
1130 -- signed types just isn't cool. In C it's even undefined.
1131 hash_expr env (Note _ e) = hash_expr env e
1132 hash_expr env (Cast e _) = hash_expr env e
1133 hash_expr env (Var v) = hashVar env v
1134 hash_expr _ (Lit lit) = fromIntegral (hashLiteral lit)
1135 hash_expr env (App f e) = hash_expr env f * fast_hash_expr env e
1136 hash_expr env (Let (NonRec b r) e) = hash_expr (extend_env env b) e * fast_hash_expr env r
1137 hash_expr env (Let (Rec ((b,_):_)) e) = hash_expr (extend_env env b) e
1138 hash_expr env (Case e _ _ _) = hash_expr env e
1139 hash_expr env (Lam b e) = hash_expr (extend_env env b) e
1140 hash_expr _ (Type _) = WARN(True, text "hash_expr: type") 1
1141 -- Shouldn't happen. Better to use WARN than trace, because trace
1142 -- prevents the CPR optimisation kicking in for hash_expr.
1144 fast_hash_expr :: HashEnv -> CoreExpr -> Word32
1145 fast_hash_expr env (Var v) = hashVar env v
1146 fast_hash_expr env (Type t) = fast_hash_type env t
1147 fast_hash_expr _ (Lit lit) = fromIntegral (hashLiteral lit)
1148 fast_hash_expr env (Cast e _) = fast_hash_expr env e
1149 fast_hash_expr env (Note _ e) = fast_hash_expr env e
1150 fast_hash_expr env (App _ a) = fast_hash_expr env a -- A bit idiosyncratic ('a' not 'f')!
1151 fast_hash_expr _ _ = 1
1153 fast_hash_type :: HashEnv -> Type -> Word32
1154 fast_hash_type env ty
1155 | Just tv <- getTyVar_maybe ty = hashVar env tv
1156 | Just (tc,tys) <- splitTyConApp_maybe ty = let hash_tc = fromIntegral (hashName (tyConName tc))
1157 in foldr (\t n -> fast_hash_type env t + n) hash_tc tys
1160 extend_env :: HashEnv -> Var -> (Int, VarEnv Int)
1161 extend_env (n,env) b = (n+1, extendVarEnv env b n)
1163 hashVar :: HashEnv -> Var -> Word32
1165 = fromIntegral (lookupVarEnv env v `orElse` hashName (idName v))
1169 %************************************************************************
1173 %************************************************************************
1175 Note [Eta reduction conditions]
1176 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1177 We try for eta reduction here, but *only* if we get all the way to an
1178 trivial expression. We don't want to remove extra lambdas unless we
1179 are going to avoid allocating this thing altogether.
1181 There are some particularly delicate points here:
1183 * Eta reduction is not valid in general:
1185 This matters, partly for old-fashioned correctness reasons but,
1186 worse, getting it wrong can yield a seg fault. Consider
1188 h y = case (case y of { True -> f `seq` True; False -> False }) of
1189 True -> ...; False -> ...
1191 If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
1192 says f=bottom, and replaces the (f `seq` True) with just
1193 (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
1194 *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
1195 the definition again, so that it does not termninate after all.
1196 Result: seg-fault because the boolean case actually gets a function value.
1199 So it's important to to the right thing.
1201 * Note [Arity care]: we need to be careful if we just look at f's
1202 arity. Currently (Dec07), f's arity is visible in its own RHS (see
1203 Note [Arity robustness] in SimplEnv) so we must *not* trust the
1204 arity when checking that 'f' is a value. Otherwise we will
1209 Which might change a terminiating program (think (f `seq` e)) to a
1210 non-terminating one. So we check for being a loop breaker first.
1212 However for GlobalIds we can look at the arity; and for primops we
1213 must, since they have no unfolding.
1215 * Regardless of whether 'f' is a value, we always want to
1216 reduce (/\a -> f a) to f
1217 This came up in a RULE: foldr (build (/\a -> g a))
1218 did not match foldr (build (/\b -> ...something complex...))
1219 The type checker can insert these eta-expanded versions,
1220 with both type and dictionary lambdas; hence the slightly
1223 * Never *reduce* arity. For example
1225 Then if h has arity 1 we don't want to eta-reduce because then
1226 f's arity would decrease, and that is bad
1228 These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
1231 Note [Eta reduction with casted arguments]
1232 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1234 (\(x:t3). f (x |> g)) :: t3 -> t2
1238 This should be eta-reduced to
1242 So we need to accumulate a coercion, pushing it inward (past
1243 variable arguments only) thus:
1244 f (x |> co_arg) |> co --> (f |> (sym co_arg -> co)) x
1245 f (x:t) |> co --> (f |> (t -> co)) x
1246 f @ a |> co --> (f |> (forall a.co)) @ a
1247 f @ (g:t1~t2) |> co --> (f |> (t1~t2 => co)) @ (g:t1~t2)
1248 These are the equations for ok_arg.
1250 It's true that we could also hope to eta reduce these:
1253 But the simplifier pushes those casts outwards, so we don't
1254 need to address that here.
1257 tryEtaReduce :: [Var] -> CoreExpr -> Maybe CoreExpr
1258 tryEtaReduce bndrs body
1259 = go (reverse bndrs) body (IdCo (exprType body))
1261 incoming_arity = count isId bndrs
1263 go :: [Var] -- Binders, innermost first, types [a3,a2,a1]
1264 -> CoreExpr -- Of type tr
1265 -> CoercionI -- Of type tr ~ ts
1266 -> Maybe CoreExpr -- Of type a1 -> a2 -> a3 -> ts
1267 -- See Note [Eta reduction with casted arguments]
1268 -- for why we have an accumulating coercion
1270 | ok_fun fun = Just (mkCoerceI co fun)
1272 go (b : bs) (App fun arg) co
1273 | Just co' <- ok_arg b arg co
1276 go _ _ _ = Nothing -- Failure!
1279 -- Note [Eta reduction conditions]
1280 ok_fun (App fun (Type ty))
1281 | not (any (`elemVarSet` tyVarsOfType ty) bndrs)
1284 = not (fun_id `elem` bndrs)
1285 && (ok_fun_id fun_id || all ok_lam bndrs)
1289 ok_fun_id fun = fun_arity fun >= incoming_arity
1292 fun_arity fun -- See Note [Arity care]
1293 | isLocalId fun && isLoopBreaker (idOccInfo fun) = 0
1294 | otherwise = idArity fun
1297 ok_lam v = isTyCoVar v || isDictId v
1300 ok_arg :: Var -- Of type bndr_t
1301 -> CoreExpr -- Of type arg_t
1302 -> CoercionI -- Of kind (t1~t2)
1303 -> Maybe CoercionI -- Of type (arg_t -> t1 ~ bndr_t -> t2)
1304 -- (and similarly for tyvars, coercion args)
1305 -- See Note [Eta reduction with casted arguments]
1306 ok_arg bndr (Type ty) co
1307 | Just tv <- getTyVar_maybe ty
1308 , bndr == tv = Just (mkForAllTyCoI tv co)
1309 ok_arg bndr (Var v) co
1310 | bndr == v = Just (mkFunTyCoI (IdCo (idType bndr)) co)
1311 ok_arg bndr (Cast (Var v) co_arg) co
1312 | bndr == v = Just (mkFunTyCoI (ACo (mkSymCoercion co_arg)) co)
1313 -- The simplifier combines multiple casts into one,
1314 -- so we can have a simple-minded pattern match here
1315 ok_arg _ _ _ = Nothing
1319 %************************************************************************
1321 \subsection{Determining non-updatable right-hand-sides}
1323 %************************************************************************
1325 Top-level constructor applications can usually be allocated
1326 statically, but they can't if the constructor, or any of the
1327 arguments, come from another DLL (because we can't refer to static
1328 labels in other DLLs).
1330 If this happens we simply make the RHS into an updatable thunk,
1331 and 'execute' it rather than allocating it statically.
1334 -- | This function is called only on *top-level* right-hand sides.
1335 -- Returns @True@ if the RHS can be allocated statically in the output,
1336 -- with no thunks involved at all.
1337 rhsIsStatic :: (Name -> Bool) -> CoreExpr -> Bool
1338 -- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
1339 -- refers to, CAFs; (ii) in CoreToStg to decide whether to put an
1340 -- update flag on it and (iii) in DsExpr to decide how to expand
1343 -- The basic idea is that rhsIsStatic returns True only if the RHS is
1344 -- (a) a value lambda
1345 -- (b) a saturated constructor application with static args
1347 -- BUT watch out for
1348 -- (i) Any cross-DLL references kill static-ness completely
1349 -- because they must be 'executed' not statically allocated
1350 -- ("DLL" here really only refers to Windows DLLs, on other platforms,
1351 -- this is not necessary)
1353 -- (ii) We treat partial applications as redexes, because in fact we
1354 -- make a thunk for them that runs and builds a PAP
1355 -- at run-time. The only appliations that are treated as
1356 -- static are *saturated* applications of constructors.
1358 -- We used to try to be clever with nested structures like this:
1359 -- ys = (:) w ((:) w [])
1360 -- on the grounds that CorePrep will flatten ANF-ise it later.
1361 -- But supporting this special case made the function much more
1362 -- complicated, because the special case only applies if there are no
1363 -- enclosing type lambdas:
1364 -- ys = /\ a -> Foo (Baz ([] a))
1365 -- Here the nested (Baz []) won't float out to top level in CorePrep.
1367 -- But in fact, even without -O, nested structures at top level are
1368 -- flattened by the simplifier, so we don't need to be super-clever here.
1372 -- f = \x::Int. x+7 TRUE
1373 -- p = (True,False) TRUE
1375 -- d = (fst p, False) FALSE because there's a redex inside
1376 -- (this particular one doesn't happen but...)
1378 -- h = D# (1.0## /## 2.0##) FALSE (redex again)
1379 -- n = /\a. Nil a TRUE
1381 -- t = /\a. (:) (case w a of ...) (Nil a) FALSE (redex)
1384 -- This is a bit like CoreUtils.exprIsHNF, with the following differences:
1385 -- a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
1387 -- b) (C x xs), where C is a contructor is updatable if the application is
1390 -- c) don't look through unfolding of f in (f x).
1392 rhsIsStatic _is_dynamic_name rhs = is_static False rhs
1394 is_static :: Bool -- True <=> in a constructor argument; must be atomic
1397 is_static False (Lam b e) = isRuntimeVar b || is_static False e
1398 is_static in_arg (Note n e) = notSccNote n && is_static in_arg e
1399 is_static in_arg (Cast e _) = is_static in_arg e
1401 is_static _ (Lit lit)
1403 MachLabel _ _ _ -> False
1405 -- A MachLabel (foreign import "&foo") in an argument
1406 -- prevents a constructor application from being static. The
1407 -- reason is that it might give rise to unresolvable symbols
1408 -- in the object file: under Linux, references to "weak"
1409 -- symbols from the data segment give rise to "unresolvable
1410 -- relocation" errors at link time This might be due to a bug
1411 -- in the linker, but we'll work around it here anyway.
1414 is_static in_arg other_expr = go other_expr 0
1416 go (Var f) n_val_args
1417 #if mingw32_TARGET_OS
1418 | not (_is_dynamic_name (idName f))
1420 = saturated_data_con f n_val_args
1421 || (in_arg && n_val_args == 0)
1422 -- A naked un-applied variable is *not* deemed a static RHS
1424 -- Reason: better to update so that the indirection gets shorted
1425 -- out, and the true value will be seen
1426 -- NB: if you change this, you'll break the invariant that THUNK_STATICs
1427 -- are always updatable. If you do so, make sure that non-updatable
1428 -- ones have enough space for their static link field!
1430 go (App f a) n_val_args
1431 | isTypeArg a = go f n_val_args
1432 | not in_arg && is_static True a = go f (n_val_args + 1)
1433 -- The (not in_arg) checks that we aren't in a constructor argument;
1434 -- if we are, we don't allow (value) applications of any sort
1436 -- NB. In case you wonder, args are sometimes not atomic. eg.
1437 -- x = D# (1.0## /## 2.0##)
1438 -- can't float because /## can fail.
1440 go (Note n f) n_val_args = notSccNote n && go f n_val_args
1441 go (Cast e _) n_val_args = go e n_val_args
1444 saturated_data_con f n_val_args
1445 = case isDataConWorkId_maybe f of
1446 Just dc -> n_val_args == dataConRepArity dc