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
690 | DataToTagOp <- op -- See Note [dataToTag speculation]
694 = primOpOkForSpeculation op &&
695 all exprOkForSpeculation args
696 -- A bit conservative: we don't really need
697 -- to care about lazy arguments, but this is easy
699 spec_ok (DFunId _ new_type) _ = not new_type
700 -- DFuns terminate, unless the dict is implemented with a newtype
701 -- in which case they may not
705 -- | True of dyadic operators that can fail only if the second arg is zero!
706 isDivOp :: PrimOp -> Bool
707 -- This function probably belongs in PrimOp, or even in
708 -- an automagically generated file.. but it's such a
709 -- special case I thought I'd leave it here for now.
710 isDivOp IntQuotOp = True
711 isDivOp IntRemOp = True
712 isDivOp WordQuotOp = True
713 isDivOp WordRemOp = True
714 isDivOp FloatDivOp = True
715 isDivOp DoubleDivOp = True
719 Note [exprOkForSpeculation: case expressions]
720 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
722 It's always sound for exprOkForSpeculation to return False, and we
723 don't want it to take too long, so it bales out on complicated-looking
724 terms. Notably lets, which can be stacked very deeply; and in any
725 case the argument of exprOkForSpeculation is usually in a strict context,
726 so any lets will have been floated away.
728 However, we keep going on case-expressions. An example like this one
729 showed up in DPH code:
732 foo n = (if n < 5 then 1 else 2) `seq` foo (n-1)
734 If exprOkForSpeculation doesn't look through case expressions, you get this:
736 \ (ww :: GHC.Prim.Int#) ->
738 __DEFAULT -> case (case <# ds 5 of _ {
739 GHC.Types.False -> lvl1;
740 GHC.Types.True -> lvl})
742 T.$wfoo (GHC.Prim.-# ds_XkE 1) };
746 The inner case is redundant, and should be nuked.
748 Note [dataToTag speculation]
749 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
751 f x = let v::Int# = dataToTag# x
753 We say "yes", even though 'x' may not be evaluated. Reasons
755 * dataToTag#'s strictness means that its argument often will be
756 evaluated, but FloatOut makes that temporarily untrue
757 case x of y -> let v = dataToTag# y in ...
759 case x of y -> let v = dataToTag# x in ...
760 Note that we look at 'x' instead of 'y' (this is to improve
761 floating in FloatOut). So Lint complains.
763 Moreover, it really *might* improve floating to let the
766 * CorePrep makes sure dataToTag#'s argument is evaluated, just
767 before code gen. Until then, it's not guaranteed
770 %************************************************************************
772 exprIsHNF, exprIsConLike
774 %************************************************************************
777 -- Note [exprIsHNF] See also Note [exprIsCheap and exprIsHNF]
779 -- | exprIsHNF returns true for expressions that are certainly /already/
780 -- evaluated to /head/ normal form. This is used to decide whether it's ok
783 -- > case x of _ -> e
789 -- and to decide whether it's safe to discard a 'seq'.
791 -- So, it does /not/ treat variables as evaluated, unless they say they are.
792 -- However, it /does/ treat partial applications and constructor applications
793 -- as values, even if their arguments are non-trivial, provided the argument
794 -- type is lifted. For example, both of these are values:
796 -- > (:) (f x) (map f xs)
797 -- > map (...redex...)
799 -- because 'seq' on such things completes immediately.
801 -- For unlifted argument types, we have to be careful:
805 -- Suppose @f x@ diverges; then @C (f x)@ is not a value. However this can't
806 -- happen: see "CoreSyn#let_app_invariant". This invariant states that arguments of
807 -- unboxed type must be ok-for-speculation (or trivial).
808 exprIsHNF :: CoreExpr -> Bool -- True => Value-lambda, constructor, PAP
809 exprIsHNF = exprIsHNFlike isDataConWorkId isEvaldUnfolding
813 -- | Similar to 'exprIsHNF' but includes CONLIKE functions as well as
814 -- data constructors. Conlike arguments are considered interesting by the
816 exprIsConLike :: CoreExpr -> Bool -- True => lambda, conlike, PAP
817 exprIsConLike = exprIsHNFlike isConLikeId isConLikeUnfolding
819 -- | Returns true for values or value-like expressions. These are lambdas,
820 -- constructors / CONLIKE functions (as determined by the function argument)
823 exprIsHNFlike :: (Var -> Bool) -> (Unfolding -> Bool) -> CoreExpr -> Bool
824 exprIsHNFlike is_con is_con_unf = is_hnf_like
826 is_hnf_like (Var v) -- NB: There are no value args at this point
827 = is_con v -- Catches nullary constructors,
828 -- so that [] and () are values, for example
829 || idArity v > 0 -- Catches (e.g.) primops that don't have unfoldings
830 || is_con_unf (idUnfolding v)
831 -- Check the thing's unfolding; it might be bound to a value
832 -- We don't look through loop breakers here, which is a bit conservative
833 -- but otherwise I worry that if an Id's unfolding is just itself,
834 -- we could get an infinite loop
836 is_hnf_like (Lit _) = True
837 is_hnf_like (Type _) = True -- Types are honorary Values;
838 -- we don't mind copying them
839 is_hnf_like (Lam b e) = isRuntimeVar b || is_hnf_like e
840 is_hnf_like (Note _ e) = is_hnf_like e
841 is_hnf_like (Cast e _) = is_hnf_like e
842 is_hnf_like (App e (Type _)) = is_hnf_like e
843 is_hnf_like (App e a) = app_is_value e [a]
844 is_hnf_like (Let _ e) = is_hnf_like e -- Lazy let(rec)s don't affect us
845 is_hnf_like _ = False
847 -- There is at least one value argument
848 app_is_value :: CoreExpr -> [CoreArg] -> Bool
849 app_is_value (Var fun) args
850 = idArity fun > valArgCount args -- Under-applied function
851 || is_con fun -- or constructor-like
852 app_is_value (Note _ f) as = app_is_value f as
853 app_is_value (Cast f _) as = app_is_value f as
854 app_is_value (App f a) as = app_is_value f (a:as)
855 app_is_value _ _ = False
859 %************************************************************************
861 Instantiating data constructors
863 %************************************************************************
865 These InstPat functions go here to avoid circularity between DataCon and Id
868 dataConRepInstPat, dataConOrigInstPat :: [Unique] -> DataCon -> [Type] -> ([TyVar], [CoVar], [Id])
869 dataConRepFSInstPat :: [FastString] -> [Unique] -> DataCon -> [Type] -> ([TyVar], [CoVar], [Id])
871 dataConRepInstPat = dataConInstPat dataConRepArgTys (repeat ((fsLit "ipv")))
872 dataConRepFSInstPat = dataConInstPat dataConRepArgTys
873 dataConOrigInstPat = dataConInstPat dc_arg_tys (repeat ((fsLit "ipv")))
875 dc_arg_tys dc = map mkPredTy (dataConEqTheta dc) ++ map mkPredTy (dataConDictTheta dc) ++ dataConOrigArgTys dc
876 -- Remember to include the existential dictionaries
878 dataConInstPat :: (DataCon -> [Type]) -- function used to find arg tys
879 -> [FastString] -- A long enough list of FSs to use for names
880 -> [Unique] -- An equally long list of uniques, at least one for each binder
882 -> [Type] -- Types to instantiate the universally quantified tyvars
883 -> ([TyVar], [CoVar], [Id]) -- Return instantiated variables
884 -- dataConInstPat arg_fun fss us con inst_tys returns a triple
885 -- (ex_tvs, co_tvs, arg_ids),
887 -- ex_tvs are intended to be used as binders for existential type args
889 -- co_tvs are intended to be used as binders for coercion args and the kinds
890 -- of these vars have been instantiated by the inst_tys and the ex_tys
891 -- The co_tvs include both GADT equalities (dcEqSpec) and
892 -- programmer-specified equalities (dcEqTheta)
894 -- arg_ids are indended to be used as binders for value arguments,
895 -- and their types have been instantiated with inst_tys and ex_tys
896 -- The arg_ids include both dicts (dcDictTheta) and
897 -- programmer-specified arguments (after rep-ing) (deRepArgTys)
900 -- The following constructor T1
903 -- T1 :: forall b. Int -> b -> T(a,b)
906 -- has representation type
907 -- forall a. forall a1. forall b. (a ~ (a1,b)) =>
910 -- dataConInstPat fss us T1 (a1',b') will return
912 -- ([a1'', b''], [c :: (a1', b')~(a1'', b'')], [x :: Int, y :: b''])
914 -- where the double-primed variables are created with the FastStrings and
915 -- Uniques given as fss and us
916 dataConInstPat arg_fun fss uniqs con inst_tys
917 = (ex_bndrs, co_bndrs, arg_ids)
919 univ_tvs = dataConUnivTyVars con
920 ex_tvs = dataConExTyVars con
921 arg_tys = arg_fun con
922 eq_spec = dataConEqSpec con
923 eq_theta = dataConEqTheta con
924 eq_preds = eqSpecPreds eq_spec ++ eq_theta
927 n_co = length eq_preds
929 -- split the Uniques and FastStrings
930 (ex_uniqs, uniqs') = splitAt n_ex uniqs
931 (co_uniqs, id_uniqs) = splitAt n_co uniqs'
933 (ex_fss, fss') = splitAt n_ex fss
934 (co_fss, id_fss) = splitAt n_co fss'
936 -- Make existential type variables
937 ex_bndrs = zipWith3 mk_ex_var ex_uniqs ex_fss ex_tvs
938 mk_ex_var uniq fs var = mkTyVar new_name kind
940 new_name = mkSysTvName uniq fs
943 -- Make the instantiating substitution
944 subst = zipOpenTvSubst (univ_tvs ++ ex_tvs) (inst_tys ++ map mkTyVarTy ex_bndrs)
946 -- Make new coercion vars, instantiating kind
947 co_bndrs = zipWith3 mk_co_var co_uniqs co_fss eq_preds
948 mk_co_var uniq fs eq_pred = mkCoVar new_name co_kind
950 new_name = mkSysTvName uniq fs
951 co_kind = substTy subst (mkPredTy eq_pred)
953 -- make value vars, instantiating types
954 mk_id_var uniq fs ty = mkUserLocal (mkVarOccFS fs) uniq (substTy subst ty) noSrcSpan
955 arg_ids = zipWith3 mk_id_var id_uniqs id_fss arg_tys
959 %************************************************************************
963 %************************************************************************
966 -- | A cheap equality test which bales out fast!
967 -- If it returns @True@ the arguments are definitely equal,
968 -- otherwise, they may or may not be equal.
970 -- See also 'exprIsBig'
971 cheapEqExpr :: Expr b -> Expr b -> Bool
973 cheapEqExpr (Var v1) (Var v2) = v1==v2
974 cheapEqExpr (Lit lit1) (Lit lit2) = lit1 == lit2
975 cheapEqExpr (Type t1) (Type t2) = t1 `coreEqType` t2
977 cheapEqExpr (App f1 a1) (App f2 a2)
978 = f1 `cheapEqExpr` f2 && a1 `cheapEqExpr` a2
980 cheapEqExpr (Cast e1 t1) (Cast e2 t2)
981 = e1 `cheapEqExpr` e2 && t1 `coreEqCoercion` t2
983 cheapEqExpr _ _ = False
987 exprIsBig :: Expr b -> Bool
988 -- ^ Returns @True@ of expressions that are too big to be compared by 'cheapEqExpr'
989 exprIsBig (Lit _) = False
990 exprIsBig (Var _) = False
991 exprIsBig (Type _) = False
992 exprIsBig (Lam _ e) = exprIsBig e
993 exprIsBig (App f a) = exprIsBig f || exprIsBig a
994 exprIsBig (Cast e _) = exprIsBig e -- Hopefully coercions are not too big!
999 eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
1000 -- Compares for equality, modulo alpha
1001 eqExpr in_scope e1 e2
1002 = eqExprX id_unf (mkRnEnv2 in_scope) e1 e2
1004 id_unf _ = noUnfolding -- Don't expand
1008 eqExprX :: IdUnfoldingFun -> RnEnv2 -> CoreExpr -> CoreExpr -> Bool
1009 -- ^ Compares expressions for equality, modulo alpha.
1010 -- Does /not/ look through newtypes or predicate types
1011 -- Used in rule matching, and also CSE
1013 eqExprX id_unfolding_fun env e1 e2
1016 go env (Var v1) (Var v2)
1017 | rnOccL env v1 == rnOccR env v2
1020 -- The next two rules expand non-local variables
1021 -- C.f. Note [Expanding variables] in Rules.lhs
1022 -- and Note [Do not expand locally-bound variables] in Rules.lhs
1024 | not (locallyBoundL env v1)
1025 , Just e1' <- expandUnfolding_maybe (id_unfolding_fun (lookupRnInScope env v1))
1026 = go (nukeRnEnvL env) e1' e2
1029 | not (locallyBoundR env v2)
1030 , Just e2' <- expandUnfolding_maybe (id_unfolding_fun (lookupRnInScope env v2))
1031 = go (nukeRnEnvR env) e1 e2'
1033 go _ (Lit lit1) (Lit lit2) = lit1 == lit2
1034 go env (Type t1) (Type t2) = tcEqTypeX env t1 t2
1035 go env (Cast e1 co1) (Cast e2 co2) = tcEqTypeX env co1 co2 && go env e1 e2
1036 go env (App f1 a1) (App f2 a2) = go env f1 f2 && go env a1 a2
1037 go env (Note n1 e1) (Note n2 e2) = go_note n1 n2 && go env e1 e2
1039 go env (Lam b1 e1) (Lam b2 e2)
1040 = tcEqTypeX env (varType b1) (varType b2) -- False for Id/TyVar combination
1041 && go (rnBndr2 env b1 b2) e1 e2
1043 go env (Let (NonRec v1 r1) e1) (Let (NonRec v2 r2) e2)
1044 = go env r1 r2 -- No need to check binder types, since RHSs match
1045 && go (rnBndr2 env v1 v2) e1 e2
1047 go env (Let (Rec ps1) e1) (Let (Rec ps2) e2)
1048 = all2 (go env') rs1 rs2 && go env' e1 e2
1050 (bs1,rs1) = unzip ps1
1051 (bs2,rs2) = unzip ps2
1052 env' = rnBndrs2 env bs1 bs2
1054 go env (Case e1 b1 _ a1) (Case e2 b2 _ a2)
1056 && tcEqTypeX env (idType b1) (idType b2)
1057 && all2 (go_alt (rnBndr2 env b1 b2)) a1 a2
1062 go_alt env (c1, bs1, e1) (c2, bs2, e2)
1063 = c1 == c2 && go (rnBndrs2 env bs1 bs2) e1 e2
1066 go_note (SCC cc1) (SCC cc2) = cc1 == cc2
1067 go_note (CoreNote s1) (CoreNote s2) = s1 == s2
1074 locallyBoundL, locallyBoundR :: RnEnv2 -> Var -> Bool
1075 locallyBoundL rn_env v = inRnEnvL rn_env v
1076 locallyBoundR rn_env v = inRnEnvR rn_env v
1080 %************************************************************************
1082 \subsection{The size of an expression}
1084 %************************************************************************
1087 coreBindsSize :: [CoreBind] -> Int
1088 coreBindsSize bs = foldr ((+) . bindSize) 0 bs
1090 exprSize :: CoreExpr -> Int
1091 -- ^ A measure of the size of the expressions, strictly greater than 0
1092 -- It also forces the expression pretty drastically as a side effect
1093 exprSize (Var v) = v `seq` 1
1094 exprSize (Lit lit) = lit `seq` 1
1095 exprSize (App f a) = exprSize f + exprSize a
1096 exprSize (Lam b e) = varSize b + exprSize e
1097 exprSize (Let b e) = bindSize b + exprSize e
1098 exprSize (Case e b t as) = seqType t `seq` exprSize e + varSize b + 1 + foldr ((+) . altSize) 0 as
1099 exprSize (Cast e co) = (seqType co `seq` 1) + exprSize e
1100 exprSize (Note n e) = noteSize n + exprSize e
1101 exprSize (Type t) = seqType t `seq` 1
1103 noteSize :: Note -> Int
1104 noteSize (SCC cc) = cc `seq` 1
1105 noteSize (CoreNote s) = s `seq` 1 -- hdaume: core annotations
1107 varSize :: Var -> Int
1108 varSize b | isTyCoVar b = 1
1109 | otherwise = seqType (idType b) `seq`
1110 megaSeqIdInfo (idInfo b) `seq`
1113 varsSize :: [Var] -> Int
1114 varsSize = sum . map varSize
1116 bindSize :: CoreBind -> Int
1117 bindSize (NonRec b e) = varSize b + exprSize e
1118 bindSize (Rec prs) = foldr ((+) . pairSize) 0 prs
1120 pairSize :: (Var, CoreExpr) -> Int
1121 pairSize (b,e) = varSize b + exprSize e
1123 altSize :: CoreAlt -> Int
1124 altSize (c,bs,e) = c `seq` varsSize bs + exprSize e
1128 %************************************************************************
1130 \subsection{Hashing}
1132 %************************************************************************
1135 hashExpr :: CoreExpr -> Int
1136 -- ^ Two expressions that hash to the same @Int@ may be equal (but may not be)
1137 -- Two expressions that hash to the different Ints are definitely unequal.
1139 -- The emphasis is on a crude, fast hash, rather than on high precision.
1141 -- But unequal here means \"not identical\"; two alpha-equivalent
1142 -- expressions may hash to the different Ints.
1144 -- We must be careful that @\\x.x@ and @\\y.y@ map to the same hash code,
1145 -- (at least if we want the above invariant to be true).
1147 hashExpr e = fromIntegral (hash_expr (1,emptyVarEnv) e .&. 0x7fffffff)
1148 -- UniqFM doesn't like negative Ints
1150 type HashEnv = (Int, VarEnv Int) -- Hash code for bound variables
1152 hash_expr :: HashEnv -> CoreExpr -> Word32
1153 -- Word32, because we're expecting overflows here, and overflowing
1154 -- signed types just isn't cool. In C it's even undefined.
1155 hash_expr env (Note _ e) = hash_expr env e
1156 hash_expr env (Cast e _) = hash_expr env e
1157 hash_expr env (Var v) = hashVar env v
1158 hash_expr _ (Lit lit) = fromIntegral (hashLiteral lit)
1159 hash_expr env (App f e) = hash_expr env f * fast_hash_expr env e
1160 hash_expr env (Let (NonRec b r) e) = hash_expr (extend_env env b) e * fast_hash_expr env r
1161 hash_expr env (Let (Rec ((b,_):_)) e) = hash_expr (extend_env env b) e
1162 hash_expr env (Case e _ _ _) = hash_expr env e
1163 hash_expr env (Lam b e) = hash_expr (extend_env env b) e
1164 hash_expr _ (Type _) = WARN(True, text "hash_expr: type") 1
1165 -- Shouldn't happen. Better to use WARN than trace, because trace
1166 -- prevents the CPR optimisation kicking in for hash_expr.
1168 fast_hash_expr :: HashEnv -> CoreExpr -> Word32
1169 fast_hash_expr env (Var v) = hashVar env v
1170 fast_hash_expr env (Type t) = fast_hash_type env t
1171 fast_hash_expr _ (Lit lit) = fromIntegral (hashLiteral lit)
1172 fast_hash_expr env (Cast e _) = fast_hash_expr env e
1173 fast_hash_expr env (Note _ e) = fast_hash_expr env e
1174 fast_hash_expr env (App _ a) = fast_hash_expr env a -- A bit idiosyncratic ('a' not 'f')!
1175 fast_hash_expr _ _ = 1
1177 fast_hash_type :: HashEnv -> Type -> Word32
1178 fast_hash_type env ty
1179 | Just tv <- getTyVar_maybe ty = hashVar env tv
1180 | Just (tc,tys) <- splitTyConApp_maybe ty = let hash_tc = fromIntegral (hashName (tyConName tc))
1181 in foldr (\t n -> fast_hash_type env t + n) hash_tc tys
1184 extend_env :: HashEnv -> Var -> (Int, VarEnv Int)
1185 extend_env (n,env) b = (n+1, extendVarEnv env b n)
1187 hashVar :: HashEnv -> Var -> Word32
1189 = fromIntegral (lookupVarEnv env v `orElse` hashName (idName v))
1193 %************************************************************************
1197 %************************************************************************
1199 Note [Eta reduction conditions]
1200 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1201 We try for eta reduction here, but *only* if we get all the way to an
1202 trivial expression. We don't want to remove extra lambdas unless we
1203 are going to avoid allocating this thing altogether.
1205 There are some particularly delicate points here:
1207 * Eta reduction is not valid in general:
1209 This matters, partly for old-fashioned correctness reasons but,
1210 worse, getting it wrong can yield a seg fault. Consider
1212 h y = case (case y of { True -> f `seq` True; False -> False }) of
1213 True -> ...; False -> ...
1215 If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
1216 says f=bottom, and replaces the (f `seq` True) with just
1217 (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
1218 *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
1219 the definition again, so that it does not termninate after all.
1220 Result: seg-fault because the boolean case actually gets a function value.
1223 So it's important to to the right thing.
1225 * Note [Arity care]: we need to be careful if we just look at f's
1226 arity. Currently (Dec07), f's arity is visible in its own RHS (see
1227 Note [Arity robustness] in SimplEnv) so we must *not* trust the
1228 arity when checking that 'f' is a value. Otherwise we will
1233 Which might change a terminiating program (think (f `seq` e)) to a
1234 non-terminating one. So we check for being a loop breaker first.
1236 However for GlobalIds we can look at the arity; and for primops we
1237 must, since they have no unfolding.
1239 * Regardless of whether 'f' is a value, we always want to
1240 reduce (/\a -> f a) to f
1241 This came up in a RULE: foldr (build (/\a -> g a))
1242 did not match foldr (build (/\b -> ...something complex...))
1243 The type checker can insert these eta-expanded versions,
1244 with both type and dictionary lambdas; hence the slightly
1247 * Never *reduce* arity. For example
1249 Then if h has arity 1 we don't want to eta-reduce because then
1250 f's arity would decrease, and that is bad
1252 These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
1255 Note [Eta reduction with casted arguments]
1256 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1258 (\(x:t3). f (x |> g)) :: t3 -> t2
1262 This should be eta-reduced to
1266 So we need to accumulate a coercion, pushing it inward (past
1267 variable arguments only) thus:
1268 f (x |> co_arg) |> co --> (f |> (sym co_arg -> co)) x
1269 f (x:t) |> co --> (f |> (t -> co)) x
1270 f @ a |> co --> (f |> (forall a.co)) @ a
1271 f @ (g:t1~t2) |> co --> (f |> (t1~t2 => co)) @ (g:t1~t2)
1272 These are the equations for ok_arg.
1274 It's true that we could also hope to eta reduce these:
1277 But the simplifier pushes those casts outwards, so we don't
1278 need to address that here.
1281 tryEtaReduce :: [Var] -> CoreExpr -> Maybe CoreExpr
1282 tryEtaReduce bndrs body
1283 = go (reverse bndrs) body (IdCo (exprType body))
1285 incoming_arity = count isId bndrs
1287 go :: [Var] -- Binders, innermost first, types [a3,a2,a1]
1288 -> CoreExpr -- Of type tr
1289 -> CoercionI -- Of type tr ~ ts
1290 -> Maybe CoreExpr -- Of type a1 -> a2 -> a3 -> ts
1291 -- See Note [Eta reduction with casted arguments]
1292 -- for why we have an accumulating coercion
1294 | ok_fun fun = Just (mkCoerceI co fun)
1296 go (b : bs) (App fun arg) co
1297 | Just co' <- ok_arg b arg co
1300 go _ _ _ = Nothing -- Failure!
1303 -- Note [Eta reduction conditions]
1304 ok_fun (App fun (Type ty))
1305 | not (any (`elemVarSet` tyVarsOfType ty) bndrs)
1308 = not (fun_id `elem` bndrs)
1309 && (ok_fun_id fun_id || all ok_lam bndrs)
1313 ok_fun_id fun = fun_arity fun >= incoming_arity
1316 fun_arity fun -- See Note [Arity care]
1317 | isLocalId fun && isLoopBreaker (idOccInfo fun) = 0
1318 | otherwise = idArity fun
1321 ok_lam v = isTyCoVar v || isDictId v
1324 ok_arg :: Var -- Of type bndr_t
1325 -> CoreExpr -- Of type arg_t
1326 -> CoercionI -- Of kind (t1~t2)
1327 -> Maybe CoercionI -- Of type (arg_t -> t1 ~ bndr_t -> t2)
1328 -- (and similarly for tyvars, coercion args)
1329 -- See Note [Eta reduction with casted arguments]
1330 ok_arg bndr (Type ty) co
1331 | Just tv <- getTyVar_maybe ty
1332 , bndr == tv = Just (mkForAllTyCoI tv co)
1333 ok_arg bndr (Var v) co
1334 | bndr == v = Just (mkFunTyCoI (IdCo (idType bndr)) co)
1335 ok_arg bndr (Cast (Var v) co_arg) co
1336 | bndr == v = Just (mkFunTyCoI (ACo (mkSymCoercion co_arg)) co)
1337 -- The simplifier combines multiple casts into one,
1338 -- so we can have a simple-minded pattern match here
1339 ok_arg _ _ _ = Nothing
1343 %************************************************************************
1345 \subsection{Determining non-updatable right-hand-sides}
1347 %************************************************************************
1349 Top-level constructor applications can usually be allocated
1350 statically, but they can't if the constructor, or any of the
1351 arguments, come from another DLL (because we can't refer to static
1352 labels in other DLLs).
1354 If this happens we simply make the RHS into an updatable thunk,
1355 and 'execute' it rather than allocating it statically.
1358 -- | This function is called only on *top-level* right-hand sides.
1359 -- Returns @True@ if the RHS can be allocated statically in the output,
1360 -- with no thunks involved at all.
1361 rhsIsStatic :: (Name -> Bool) -> CoreExpr -> Bool
1362 -- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
1363 -- refers to, CAFs; (ii) in CoreToStg to decide whether to put an
1364 -- update flag on it and (iii) in DsExpr to decide how to expand
1367 -- The basic idea is that rhsIsStatic returns True only if the RHS is
1368 -- (a) a value lambda
1369 -- (b) a saturated constructor application with static args
1371 -- BUT watch out for
1372 -- (i) Any cross-DLL references kill static-ness completely
1373 -- because they must be 'executed' not statically allocated
1374 -- ("DLL" here really only refers to Windows DLLs, on other platforms,
1375 -- this is not necessary)
1377 -- (ii) We treat partial applications as redexes, because in fact we
1378 -- make a thunk for them that runs and builds a PAP
1379 -- at run-time. The only appliations that are treated as
1380 -- static are *saturated* applications of constructors.
1382 -- We used to try to be clever with nested structures like this:
1383 -- ys = (:) w ((:) w [])
1384 -- on the grounds that CorePrep will flatten ANF-ise it later.
1385 -- But supporting this special case made the function much more
1386 -- complicated, because the special case only applies if there are no
1387 -- enclosing type lambdas:
1388 -- ys = /\ a -> Foo (Baz ([] a))
1389 -- Here the nested (Baz []) won't float out to top level in CorePrep.
1391 -- But in fact, even without -O, nested structures at top level are
1392 -- flattened by the simplifier, so we don't need to be super-clever here.
1396 -- f = \x::Int. x+7 TRUE
1397 -- p = (True,False) TRUE
1399 -- d = (fst p, False) FALSE because there's a redex inside
1400 -- (this particular one doesn't happen but...)
1402 -- h = D# (1.0## /## 2.0##) FALSE (redex again)
1403 -- n = /\a. Nil a TRUE
1405 -- t = /\a. (:) (case w a of ...) (Nil a) FALSE (redex)
1408 -- This is a bit like CoreUtils.exprIsHNF, with the following differences:
1409 -- a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
1411 -- b) (C x xs), where C is a contructor is updatable if the application is
1414 -- c) don't look through unfolding of f in (f x).
1416 rhsIsStatic _is_dynamic_name rhs = is_static False rhs
1418 is_static :: Bool -- True <=> in a constructor argument; must be atomic
1421 is_static False (Lam b e) = isRuntimeVar b || is_static False e
1422 is_static in_arg (Note n e) = notSccNote n && is_static in_arg e
1423 is_static in_arg (Cast e _) = is_static in_arg e
1425 is_static _ (Lit lit)
1427 MachLabel _ _ _ -> False
1429 -- A MachLabel (foreign import "&foo") in an argument
1430 -- prevents a constructor application from being static. The
1431 -- reason is that it might give rise to unresolvable symbols
1432 -- in the object file: under Linux, references to "weak"
1433 -- symbols from the data segment give rise to "unresolvable
1434 -- relocation" errors at link time This might be due to a bug
1435 -- in the linker, but we'll work around it here anyway.
1438 is_static in_arg other_expr = go other_expr 0
1440 go (Var f) n_val_args
1441 #if mingw32_TARGET_OS
1442 | not (_is_dynamic_name (idName f))
1444 = saturated_data_con f n_val_args
1445 || (in_arg && n_val_args == 0)
1446 -- A naked un-applied variable is *not* deemed a static RHS
1448 -- Reason: better to update so that the indirection gets shorted
1449 -- out, and the true value will be seen
1450 -- NB: if you change this, you'll break the invariant that THUNK_STATICs
1451 -- are always updatable. If you do so, make sure that non-updatable
1452 -- ones have enough space for their static link field!
1454 go (App f a) n_val_args
1455 | isTypeArg a = go f n_val_args
1456 | not in_arg && is_static True a = go f (n_val_args + 1)
1457 -- The (not in_arg) checks that we aren't in a constructor argument;
1458 -- if we are, we don't allow (value) applications of any sort
1460 -- NB. In case you wonder, args are sometimes not atomic. eg.
1461 -- x = D# (1.0## /## 2.0##)
1462 -- can't float because /## can fail.
1464 go (Note n f) n_val_args = notSccNote n && go f n_val_args
1465 go (Cast e _) n_val_args = go e n_val_args
1468 saturated_data_con f n_val_args
1469 = case isDataConWorkId_maybe f of
1470 Just dc -> n_val_args == dataConRepArity dc