Coercion Quantification
[ghc.git] / compiler / coreSyn / CoreUtils.hs
1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4
5
6 Utility functions on @Core@ syntax
7 -}
8
9 {-# LANGUAGE CPP #-}
10
11 -- | Commonly useful utilites for manipulating the Core language
12 module CoreUtils (
13 -- * Constructing expressions
14 mkCast,
15 mkTick, mkTicks, mkTickNoHNF, tickHNFArgs,
16 bindNonRec, needsCaseBinding,
17 mkAltExpr,
18
19 -- * Taking expressions apart
20 findDefault, addDefault, findAlt, isDefaultAlt,
21 mergeAlts, trimConArgs,
22 filterAlts, combineIdenticalAlts, refineDefaultAlt,
23
24 -- * Properties of expressions
25 exprType, coreAltType, coreAltsType, isExprLevPoly,
26 exprIsDupable, exprIsTrivial, getIdFromTrivialExpr, exprIsBottom,
27 getIdFromTrivialExpr_maybe,
28 exprIsCheap, exprIsExpandable, exprIsCheapX, CheapAppFun,
29 exprIsHNF, exprOkForSpeculation, exprOkForSideEffects, exprIsWorkFree,
30 exprIsBig, exprIsConLike,
31 rhsIsStatic, isCheapApp, isExpandableApp,
32 exprIsTickedString, exprIsTickedString_maybe,
33 exprIsTopLevelBindable,
34 altsAreExhaustive,
35
36 -- * Equality
37 cheapEqExpr, cheapEqExpr', eqExpr,
38 diffExpr, diffBinds,
39
40 -- * Eta reduction
41 tryEtaReduce,
42
43 -- * Manipulating data constructors and types
44 exprToType, exprToCoercion_maybe,
45 applyTypeToArgs, applyTypeToArg,
46 dataConRepInstPat, dataConRepFSInstPat,
47 isEmptyTy,
48
49 -- * Working with ticks
50 stripTicksTop, stripTicksTopE, stripTicksTopT,
51 stripTicksE, stripTicksT,
52
53 -- * StaticPtr
54 collectMakeStaticArgs,
55
56 -- * Join points
57 isJoinBind
58 ) where
59
60 #include "HsVersions.h"
61
62 import GhcPrelude
63
64 import CoreSyn
65 import PrelNames ( makeStaticName )
66 import PprCore
67 import CoreFVs( exprFreeVars )
68 import Var
69 import SrcLoc
70 import VarEnv
71 import VarSet
72 import Name
73 import Literal
74 import DataCon
75 import PrimOp
76 import Id
77 import IdInfo
78 import PrelNames( absentErrorIdKey )
79 import Type
80 import TyCoRep( TyCoBinder(..), TyBinder )
81 import Coercion
82 import TyCon
83 import Unique
84 import Outputable
85 import TysPrim
86 import DynFlags
87 import FastString
88 import Maybes
89 import ListSetOps ( minusList )
90 import BasicTypes ( Arity, isConLike )
91 import Platform
92 import Util
93 import Pair
94 import Data.ByteString ( ByteString )
95 import Data.Function ( on )
96 import Data.List
97 import Data.Ord ( comparing )
98 import OrdList
99 import qualified Data.Set as Set
100 import UniqSet
101
102 {-
103 ************************************************************************
104 * *
105 \subsection{Find the type of a Core atom/expression}
106 * *
107 ************************************************************************
108 -}
109
110 exprType :: CoreExpr -> Type
111 -- ^ Recover the type of a well-typed Core expression. Fails when
112 -- applied to the actual 'CoreSyn.Type' expression as it cannot
113 -- really be said to have a type
114 exprType (Var var) = idType var
115 exprType (Lit lit) = literalType lit
116 exprType (Coercion co) = coercionType co
117 exprType (Let bind body)
118 | NonRec tv rhs <- bind -- See Note [Type bindings]
119 , Type ty <- rhs = substTyWithUnchecked [tv] [ty] (exprType body)
120 | otherwise = exprType body
121 exprType (Case _ _ ty _) = ty
122 exprType (Cast _ co) = pSnd (coercionKind co)
123 exprType (Tick _ e) = exprType e
124 exprType (Lam binder expr) = mkLamType binder (exprType expr)
125 exprType e@(App _ _)
126 = case collectArgs e of
127 (fun, args) -> applyTypeToArgs e (exprType fun) args
128
129 exprType other = pprTrace "exprType" (pprCoreExpr other) alphaTy
130
131 coreAltType :: CoreAlt -> Type
132 -- ^ Returns the type of the alternatives right hand side
133 coreAltType alt@(_,bs,rhs)
134 = case occCheckExpand bs rhs_ty of
135 -- Note [Existential variables and silly type synonyms]
136 Just ty -> ty
137 Nothing -> pprPanic "coreAltType" (pprCoreAlt alt $$ ppr rhs_ty)
138 where
139 rhs_ty = exprType rhs
140
141 coreAltsType :: [CoreAlt] -> Type
142 -- ^ Returns the type of the first alternative, which should be the same as for all alternatives
143 coreAltsType (alt:_) = coreAltType alt
144 coreAltsType [] = panic "corAltsType"
145
146 -- | Is this expression levity polymorphic? This should be the
147 -- same as saying (isKindLevPoly . typeKind . exprType) but
148 -- much faster.
149 isExprLevPoly :: CoreExpr -> Bool
150 isExprLevPoly = go
151 where
152 go (Var _) = False -- no levity-polymorphic binders
153 go (Lit _) = False -- no levity-polymorphic literals
154 go e@(App f _) | not (go_app f) = False
155 | otherwise = check_type e
156 go (Lam _ _) = False
157 go (Let _ e) = go e
158 go e@(Case {}) = check_type e -- checking type is fast
159 go e@(Cast {}) = check_type e
160 go (Tick _ e) = go e
161 go e@(Type {}) = pprPanic "isExprLevPoly ty" (ppr e)
162 go (Coercion {}) = False -- this case can happen in SetLevels
163
164 check_type = isTypeLevPoly . exprType -- slow approach
165
166 -- if the function is a variable (common case), check its
167 -- levityInfo. This might mean we don't need to look up and compute
168 -- on the type. Spec of these functions: return False if there is
169 -- no possibility, ever, of this expression becoming levity polymorphic,
170 -- no matter what it's applied to; return True otherwise.
171 -- returning True is always safe. See also Note [Levity info] in
172 -- IdInfo
173 go_app (Var id) = not (isNeverLevPolyId id)
174 go_app (Lit _) = False
175 go_app (App f _) = go_app f
176 go_app (Lam _ e) = go_app e
177 go_app (Let _ e) = go_app e
178 go_app (Case _ _ ty _) = resultIsLevPoly ty
179 go_app (Cast _ co) = resultIsLevPoly (pSnd $ coercionKind co)
180 go_app (Tick _ e) = go_app e
181 go_app e@(Type {}) = pprPanic "isExprLevPoly app ty" (ppr e)
182 go_app e@(Coercion {}) = pprPanic "isExprLevPoly app co" (ppr e)
183
184
185 {-
186 Note [Type bindings]
187 ~~~~~~~~~~~~~~~~~~~~
188 Core does allow type bindings, although such bindings are
189 not much used, except in the output of the desugarer.
190 Example:
191 let a = Int in (\x:a. x)
192 Given this, exprType must be careful to substitute 'a' in the
193 result type (Trac #8522).
194
195 Note [Existential variables and silly type synonyms]
196 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
197 Consider
198 data T = forall a. T (Funny a)
199 type Funny a = Bool
200 f :: T -> Bool
201 f (T x) = x
202
203 Now, the type of 'x' is (Funny a), where 'a' is existentially quantified.
204 That means that 'exprType' and 'coreAltsType' may give a result that *appears*
205 to mention an out-of-scope type variable. See Trac #3409 for a more real-world
206 example.
207
208 Various possibilities suggest themselves:
209
210 - Ignore the problem, and make Lint not complain about such variables
211
212 - Expand all type synonyms (or at least all those that discard arguments)
213 This is tricky, because at least for top-level things we want to
214 retain the type the user originally specified.
215
216 - Expand synonyms on the fly, when the problem arises. That is what
217 we are doing here. It's not too expensive, I think.
218
219 Note that there might be existentially quantified coercion variables, too.
220 -}
221
222 -- Not defined with applyTypeToArg because you can't print from CoreSyn.
223 applyTypeToArgs :: CoreExpr -> Type -> [CoreExpr] -> Type
224 -- ^ A more efficient version of 'applyTypeToArg' when we have several arguments.
225 -- The first argument is just for debugging, and gives some context
226 applyTypeToArgs e op_ty args
227 = go op_ty args
228 where
229 go op_ty [] = op_ty
230 go op_ty (Type ty : args) = go_ty_args op_ty [ty] args
231 go op_ty (Coercion co : args) = go_ty_args op_ty [mkCoercionTy co] args
232 go op_ty (_ : args) | Just (_, res_ty) <- splitFunTy_maybe op_ty
233 = go res_ty args
234 go _ _ = pprPanic "applyTypeToArgs" panic_msg
235
236 -- go_ty_args: accumulate type arguments so we can
237 -- instantiate all at once with piResultTys
238 go_ty_args op_ty rev_tys (Type ty : args)
239 = go_ty_args op_ty (ty:rev_tys) args
240 go_ty_args op_ty rev_tys (Coercion co : args)
241 = go_ty_args op_ty (mkCoercionTy co : rev_tys) args
242 go_ty_args op_ty rev_tys args
243 = go (piResultTys op_ty (reverse rev_tys)) args
244
245 panic_msg = vcat [ text "Expression:" <+> pprCoreExpr e
246 , text "Type:" <+> ppr op_ty
247 , text "Args:" <+> ppr args ]
248
249
250 {-
251 ************************************************************************
252 * *
253 \subsection{Attaching notes}
254 * *
255 ************************************************************************
256 -}
257
258 -- | Wrap the given expression in the coercion safely, dropping
259 -- identity coercions and coalescing nested coercions
260 mkCast :: CoreExpr -> CoercionR -> CoreExpr
261 mkCast e co
262 | ASSERT2( coercionRole co == Representational
263 , text "coercion" <+> ppr co <+> ptext (sLit "passed to mkCast")
264 <+> ppr e <+> text "has wrong role" <+> ppr (coercionRole co) )
265 isReflCo co
266 = e
267
268 mkCast (Coercion e_co) co
269 | isCoercionType (pSnd (coercionKind co))
270 -- The guard here checks that g has a (~#) on both sides,
271 -- otherwise decomposeCo fails. Can in principle happen
272 -- with unsafeCoerce
273 = Coercion (mkCoCast e_co co)
274
275 mkCast (Cast expr co2) co
276 = WARN(let { Pair from_ty _to_ty = coercionKind co;
277 Pair _from_ty2 to_ty2 = coercionKind co2} in
278 not (from_ty `eqType` to_ty2),
279 vcat ([ text "expr:" <+> ppr expr
280 , text "co2:" <+> ppr co2
281 , text "co:" <+> ppr co ]) )
282 mkCast expr (mkTransCo co2 co)
283
284 mkCast (Tick t expr) co
285 = Tick t (mkCast expr co)
286
287 mkCast expr co
288 = let Pair from_ty _to_ty = coercionKind co in
289 WARN( not (from_ty `eqType` exprType expr),
290 text "Trying to coerce" <+> text "(" <> ppr expr
291 $$ text "::" <+> ppr (exprType expr) <> text ")"
292 $$ ppr co $$ ppr (coercionType co) )
293 (Cast expr co)
294
295 -- | Wraps the given expression in the source annotation, dropping the
296 -- annotation if possible.
297 mkTick :: Tickish Id -> CoreExpr -> CoreExpr
298 mkTick t orig_expr = mkTick' id id orig_expr
299 where
300 -- Some ticks (cost-centres) can be split in two, with the
301 -- non-counting part having laxer placement properties.
302 canSplit = tickishCanSplit t && tickishPlace (mkNoCount t) /= tickishPlace t
303
304 mkTick' :: (CoreExpr -> CoreExpr) -- ^ apply after adding tick (float through)
305 -> (CoreExpr -> CoreExpr) -- ^ apply before adding tick (float with)
306 -> CoreExpr -- ^ current expression
307 -> CoreExpr
308 mkTick' top rest expr = case expr of
309
310 -- Cost centre ticks should never be reordered relative to each
311 -- other. Therefore we can stop whenever two collide.
312 Tick t2 e
313 | ProfNote{} <- t2, ProfNote{} <- t -> top $ Tick t $ rest expr
314
315 -- Otherwise we assume that ticks of different placements float
316 -- through each other.
317 | tickishPlace t2 /= tickishPlace t -> mkTick' (top . Tick t2) rest e
318
319 -- For annotations this is where we make sure to not introduce
320 -- redundant ticks.
321 | tickishContains t t2 -> mkTick' top rest e
322 | tickishContains t2 t -> orig_expr
323 | otherwise -> mkTick' top (rest . Tick t2) e
324
325 -- Ticks don't care about types, so we just float all ticks
326 -- through them. Note that it's not enough to check for these
327 -- cases top-level. While mkTick will never produce Core with type
328 -- expressions below ticks, such constructs can be the result of
329 -- unfoldings. We therefore make an effort to put everything into
330 -- the right place no matter what we start with.
331 Cast e co -> mkTick' (top . flip Cast co) rest e
332 Coercion co -> Coercion co
333
334 Lam x e
335 -- Always float through type lambdas. Even for non-type lambdas,
336 -- floating is allowed for all but the most strict placement rule.
337 | not (isRuntimeVar x) || tickishPlace t /= PlaceRuntime
338 -> mkTick' (top . Lam x) rest e
339
340 -- If it is both counting and scoped, we split the tick into its
341 -- two components, often allowing us to keep the counting tick on
342 -- the outside of the lambda and push the scoped tick inside.
343 -- The point of this is that the counting tick can probably be
344 -- floated, and the lambda may then be in a position to be
345 -- beta-reduced.
346 | canSplit
347 -> top $ Tick (mkNoScope t) $ rest $ Lam x $ mkTick (mkNoCount t) e
348
349 App f arg
350 -- Always float through type applications.
351 | not (isRuntimeArg arg)
352 -> mkTick' (top . flip App arg) rest f
353
354 -- We can also float through constructor applications, placement
355 -- permitting. Again we can split.
356 | isSaturatedConApp expr && (tickishPlace t==PlaceCostCentre || canSplit)
357 -> if tickishPlace t == PlaceCostCentre
358 then top $ rest $ tickHNFArgs t expr
359 else top $ Tick (mkNoScope t) $ rest $ tickHNFArgs (mkNoCount t) expr
360
361 Var x
362 | notFunction && tickishPlace t == PlaceCostCentre
363 -> orig_expr
364 | notFunction && canSplit
365 -> top $ Tick (mkNoScope t) $ rest expr
366 where
367 -- SCCs can be eliminated on variables provided the variable
368 -- is not a function. In these cases the SCC makes no difference:
369 -- the cost of evaluating the variable will be attributed to its
370 -- definition site. When the variable refers to a function, however,
371 -- an SCC annotation on the variable affects the cost-centre stack
372 -- when the function is called, so we must retain those.
373 notFunction = not (isFunTy (idType x))
374
375 Lit{}
376 | tickishPlace t == PlaceCostCentre
377 -> orig_expr
378
379 -- Catch-all: Annotate where we stand
380 _any -> top $ Tick t $ rest expr
381
382 mkTicks :: [Tickish Id] -> CoreExpr -> CoreExpr
383 mkTicks ticks expr = foldr mkTick expr ticks
384
385 isSaturatedConApp :: CoreExpr -> Bool
386 isSaturatedConApp e = go e []
387 where go (App f a) as = go f (a:as)
388 go (Var fun) args
389 = isConLikeId fun && idArity fun == valArgCount args
390 go (Cast f _) as = go f as
391 go _ _ = False
392
393 mkTickNoHNF :: Tickish Id -> CoreExpr -> CoreExpr
394 mkTickNoHNF t e
395 | exprIsHNF e = tickHNFArgs t e
396 | otherwise = mkTick t e
397
398 -- push a tick into the arguments of a HNF (call or constructor app)
399 tickHNFArgs :: Tickish Id -> CoreExpr -> CoreExpr
400 tickHNFArgs t e = push t e
401 where
402 push t (App f (Type u)) = App (push t f) (Type u)
403 push t (App f arg) = App (push t f) (mkTick t arg)
404 push _t e = e
405
406 -- | Strip ticks satisfying a predicate from top of an expression
407 stripTicksTop :: (Tickish Id -> Bool) -> Expr b -> ([Tickish Id], Expr b)
408 stripTicksTop p = go []
409 where go ts (Tick t e) | p t = go (t:ts) e
410 go ts other = (reverse ts, other)
411
412 -- | Strip ticks satisfying a predicate from top of an expression,
413 -- returning the remaining expression
414 stripTicksTopE :: (Tickish Id -> Bool) -> Expr b -> Expr b
415 stripTicksTopE p = go
416 where go (Tick t e) | p t = go e
417 go other = other
418
419 -- | Strip ticks satisfying a predicate from top of an expression,
420 -- returning the ticks
421 stripTicksTopT :: (Tickish Id -> Bool) -> Expr b -> [Tickish Id]
422 stripTicksTopT p = go []
423 where go ts (Tick t e) | p t = go (t:ts) e
424 go ts _ = ts
425
426 -- | Completely strip ticks satisfying a predicate from an
427 -- expression. Note this is O(n) in the size of the expression!
428 stripTicksE :: (Tickish Id -> Bool) -> Expr b -> Expr b
429 stripTicksE p expr = go expr
430 where go (App e a) = App (go e) (go a)
431 go (Lam b e) = Lam b (go e)
432 go (Let b e) = Let (go_bs b) (go e)
433 go (Case e b t as) = Case (go e) b t (map go_a as)
434 go (Cast e c) = Cast (go e) c
435 go (Tick t e)
436 | p t = go e
437 | otherwise = Tick t (go e)
438 go other = other
439 go_bs (NonRec b e) = NonRec b (go e)
440 go_bs (Rec bs) = Rec (map go_b bs)
441 go_b (b, e) = (b, go e)
442 go_a (c,bs,e) = (c,bs, go e)
443
444 stripTicksT :: (Tickish Id -> Bool) -> Expr b -> [Tickish Id]
445 stripTicksT p expr = fromOL $ go expr
446 where go (App e a) = go e `appOL` go a
447 go (Lam _ e) = go e
448 go (Let b e) = go_bs b `appOL` go e
449 go (Case e _ _ as) = go e `appOL` concatOL (map go_a as)
450 go (Cast e _) = go e
451 go (Tick t e)
452 | p t = t `consOL` go e
453 | otherwise = go e
454 go _ = nilOL
455 go_bs (NonRec _ e) = go e
456 go_bs (Rec bs) = concatOL (map go_b bs)
457 go_b (_, e) = go e
458 go_a (_, _, e) = go e
459
460 {-
461 ************************************************************************
462 * *
463 \subsection{Other expression construction}
464 * *
465 ************************************************************************
466 -}
467
468 bindNonRec :: Id -> CoreExpr -> CoreExpr -> CoreExpr
469 -- ^ @bindNonRec x r b@ produces either:
470 --
471 -- > let x = r in b
472 --
473 -- or:
474 --
475 -- > case r of x { _DEFAULT_ -> b }
476 --
477 -- depending on whether we have to use a @case@ or @let@
478 -- binding for the expression (see 'needsCaseBinding').
479 -- It's used by the desugarer to avoid building bindings
480 -- that give Core Lint a heart attack, although actually
481 -- the simplifier deals with them perfectly well. See
482 -- also 'MkCore.mkCoreLet'
483 bindNonRec bndr rhs body
484 | isTyVar bndr = let_bind
485 | isCoVar bndr = if isCoArg rhs then let_bind
486 {- See Note [Binding coercions] -} else case_bind
487 | isJoinId bndr = let_bind
488 | needsCaseBinding (idType bndr) rhs = case_bind
489 | otherwise = let_bind
490 where
491 case_bind = Case rhs bndr (exprType body) [(DEFAULT, [], body)]
492 let_bind = Let (NonRec bndr rhs) body
493
494 -- | Tests whether we have to use a @case@ rather than @let@ binding for this expression
495 -- as per the invariants of 'CoreExpr': see "CoreSyn#let_app_invariant"
496 needsCaseBinding :: Type -> CoreExpr -> Bool
497 needsCaseBinding ty rhs = isUnliftedType ty && not (exprOkForSpeculation rhs)
498 -- Make a case expression instead of a let
499 -- These can arise either from the desugarer,
500 -- or from beta reductions: (\x.e) (x +# y)
501
502 mkAltExpr :: AltCon -- ^ Case alternative constructor
503 -> [CoreBndr] -- ^ Things bound by the pattern match
504 -> [Type] -- ^ The type arguments to the case alternative
505 -> CoreExpr
506 -- ^ This guy constructs the value that the scrutinee must have
507 -- given that you are in one particular branch of a case
508 mkAltExpr (DataAlt con) args inst_tys
509 = mkConApp con (map Type inst_tys ++ varsToCoreExprs args)
510 mkAltExpr (LitAlt lit) [] []
511 = Lit lit
512 mkAltExpr (LitAlt _) _ _ = panic "mkAltExpr LitAlt"
513 mkAltExpr DEFAULT _ _ = panic "mkAltExpr DEFAULT"
514
515 {- Note [Binding coercions]
516 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
517 Consider binding a CoVar, c = e. Then, we must atisfy
518 Note [CoreSyn type and coercion invariant] in CoreSyn,
519 which allows only (Coercion co) on the RHS.
520
521 ************************************************************************
522 * *
523 Operations oer case alternatives
524 * *
525 ************************************************************************
526
527 The default alternative must be first, if it exists at all.
528 This makes it easy to find, though it makes matching marginally harder.
529 -}
530
531 -- | Extract the default case alternative
532 findDefault :: [(AltCon, [a], b)] -> ([(AltCon, [a], b)], Maybe b)
533 findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null args ) (alts, Just rhs)
534 findDefault alts = (alts, Nothing)
535
536 addDefault :: [(AltCon, [a], b)] -> Maybe b -> [(AltCon, [a], b)]
537 addDefault alts Nothing = alts
538 addDefault alts (Just rhs) = (DEFAULT, [], rhs) : alts
539
540 isDefaultAlt :: (AltCon, a, b) -> Bool
541 isDefaultAlt (DEFAULT, _, _) = True
542 isDefaultAlt _ = False
543
544 -- | Find the case alternative corresponding to a particular
545 -- constructor: panics if no such constructor exists
546 findAlt :: AltCon -> [(AltCon, a, b)] -> Maybe (AltCon, a, b)
547 -- A "Nothing" result *is* legitimate
548 -- See Note [Unreachable code]
549 findAlt con alts
550 = case alts of
551 (deflt@(DEFAULT,_,_):alts) -> go alts (Just deflt)
552 _ -> go alts Nothing
553 where
554 go [] deflt = deflt
555 go (alt@(con1,_,_) : alts) deflt
556 = case con `cmpAltCon` con1 of
557 LT -> deflt -- Missed it already; the alts are in increasing order
558 EQ -> Just alt
559 GT -> ASSERT( not (con1 == DEFAULT) ) go alts deflt
560
561 {- Note [Unreachable code]
562 ~~~~~~~~~~~~~~~~~~~~~~~~~~
563 It is possible (although unusual) for GHC to find a case expression
564 that cannot match. For example:
565
566 data Col = Red | Green | Blue
567 x = Red
568 f v = case x of
569 Red -> ...
570 _ -> ...(case x of { Green -> e1; Blue -> e2 })...
571
572 Suppose that for some silly reason, x isn't substituted in the case
573 expression. (Perhaps there's a NOINLINE on it, or profiling SCC stuff
574 gets in the way; cf Trac #3118.) Then the full-lazines pass might produce
575 this
576
577 x = Red
578 lvl = case x of { Green -> e1; Blue -> e2 })
579 f v = case x of
580 Red -> ...
581 _ -> ...lvl...
582
583 Now if x gets inlined, we won't be able to find a matching alternative
584 for 'Red'. That's because 'lvl' is unreachable. So rather than crashing
585 we generate (error "Inaccessible alternative").
586
587 Similar things can happen (augmented by GADTs) when the Simplifier
588 filters down the matching alternatives in Simplify.rebuildCase.
589 -}
590
591 ---------------------------------
592 mergeAlts :: [(AltCon, a, b)] -> [(AltCon, a, b)] -> [(AltCon, a, b)]
593 -- ^ Merge alternatives preserving order; alternatives in
594 -- the first argument shadow ones in the second
595 mergeAlts [] as2 = as2
596 mergeAlts as1 [] = as1
597 mergeAlts (a1:as1) (a2:as2)
598 = case a1 `cmpAlt` a2 of
599 LT -> a1 : mergeAlts as1 (a2:as2)
600 EQ -> a1 : mergeAlts as1 as2 -- Discard a2
601 GT -> a2 : mergeAlts (a1:as1) as2
602
603
604 ---------------------------------
605 trimConArgs :: AltCon -> [CoreArg] -> [CoreArg]
606 -- ^ Given:
607 --
608 -- > case (C a b x y) of
609 -- > C b x y -> ...
610 --
611 -- We want to drop the leading type argument of the scrutinee
612 -- leaving the arguments to match against the pattern
613
614 trimConArgs DEFAULT args = ASSERT( null args ) []
615 trimConArgs (LitAlt _) args = ASSERT( null args ) []
616 trimConArgs (DataAlt dc) args = dropList (dataConUnivTyVars dc) args
617
618 filterAlts :: TyCon -- ^ Type constructor of scrutinee's type (used to prune possibilities)
619 -> [Type] -- ^ And its type arguments
620 -> [AltCon] -- ^ 'imposs_cons': constructors known to be impossible due to the form of the scrutinee
621 -> [(AltCon, [Var], a)] -- ^ Alternatives
622 -> ([AltCon], [(AltCon, [Var], a)])
623 -- Returns:
624 -- 1. Constructors that will never be encountered by the
625 -- *default* case (if any). A superset of imposs_cons
626 -- 2. The new alternatives, trimmed by
627 -- a) remove imposs_cons
628 -- b) remove constructors which can't match because of GADTs
629 --
630 -- NB: the final list of alternatives may be empty:
631 -- This is a tricky corner case. If the data type has no constructors,
632 -- which GHC allows, or if the imposs_cons covers all constructors (after taking
633 -- account of GADTs), then no alternatives can match.
634 --
635 -- If callers need to preserve the invariant that there is always at least one branch
636 -- in a "case" statement then they will need to manually add a dummy case branch that just
637 -- calls "error" or similar.
638 filterAlts _tycon inst_tys imposs_cons alts
639 = (imposs_deflt_cons, addDefault trimmed_alts maybe_deflt)
640 where
641 (alts_wo_default, maybe_deflt) = findDefault alts
642 alt_cons = [con | (con,_,_) <- alts_wo_default]
643
644 trimmed_alts = filterOut (impossible_alt inst_tys) alts_wo_default
645
646 imposs_cons_set = Set.fromList imposs_cons
647 imposs_deflt_cons =
648 imposs_cons ++ filterOut (`Set.member` imposs_cons_set) alt_cons
649 -- "imposs_deflt_cons" are handled
650 -- EITHER by the context,
651 -- OR by a non-DEFAULT branch in this case expression.
652
653 impossible_alt :: [Type] -> (AltCon, a, b) -> Bool
654 impossible_alt _ (con, _, _) | con `Set.member` imposs_cons_set = True
655 impossible_alt inst_tys (DataAlt con, _, _) = dataConCannotMatch inst_tys con
656 impossible_alt _ _ = False
657
658 -- | Refine the default alternative to a 'DataAlt', if there is a unique way to do so.
659 -- See Note [Refine Default Alts]
660 refineDefaultAlt :: [Unique] -- ^ Uniques for constructing new binders
661 -> TyCon -- ^ Type constructor of scrutinee's type
662 -> [Type] -- ^ Type arguments of scrutinee's type
663 -> [AltCon] -- ^ Constructors that cannot match the DEFAULT (if any)
664 -> [CoreAlt]
665 -> (Bool, [CoreAlt]) -- ^ 'True', if a default alt was replaced with a 'DataAlt'
666 refineDefaultAlt us tycon tys imposs_deflt_cons all_alts
667 | (DEFAULT,_,rhs) : rest_alts <- all_alts
668 , isAlgTyCon tycon -- It's a data type, tuple, or unboxed tuples.
669 , not (isNewTyCon tycon) -- We can have a newtype, if we are just doing an eval:
670 -- case x of { DEFAULT -> e }
671 -- and we don't want to fill in a default for them!
672 , Just all_cons <- tyConDataCons_maybe tycon
673 , let imposs_data_cons = mkUniqSet [con | DataAlt con <- imposs_deflt_cons]
674 -- We now know it's a data type, so we can use
675 -- UniqSet rather than Set (more efficient)
676 impossible con = con `elementOfUniqSet` imposs_data_cons
677 || dataConCannotMatch tys con
678 = case filterOut impossible all_cons of
679 -- Eliminate the default alternative
680 -- altogether if it can't match:
681 [] -> (False, rest_alts)
682
683 -- It matches exactly one constructor, so fill it in:
684 [con] -> (True, mergeAlts rest_alts [(DataAlt con, ex_tvs ++ arg_ids, rhs)])
685 -- We need the mergeAlts to keep the alternatives in the right order
686 where
687 (ex_tvs, arg_ids) = dataConRepInstPat us con tys
688
689 -- It matches more than one, so do nothing
690 _ -> (False, all_alts)
691
692 | debugIsOn, isAlgTyCon tycon, null (tyConDataCons tycon)
693 , not (isFamilyTyCon tycon || isAbstractTyCon tycon)
694 -- Check for no data constructors
695 -- This can legitimately happen for abstract types and type families,
696 -- so don't report that
697 = (False, all_alts)
698
699 | otherwise -- The common case
700 = (False, all_alts)
701
702 {- Note [Refine Default Alts]
703
704 refineDefaultAlt replaces the DEFAULT alt with a constructor if there is one
705 possible value it could be.
706
707 The simplest example being
708
709 foo :: () -> ()
710 foo x = case x of !_ -> ()
711
712 rewrites to
713
714 foo :: () -> ()
715 foo x = case x of () -> ()
716
717 There are two reasons in general why this is desirable.
718
719 1. We can simplify inner expressions
720
721 In this example we can eliminate the inner case by refining the outer case.
722 If we don't refine it, we are left with both case expressions.
723
724 ```
725 {-# LANGUAGE BangPatterns #-}
726 module Test where
727
728 mid x = x
729 {-# NOINLINE mid #-}
730
731 data Foo = Foo1 ()
732
733 test :: Foo -> ()
734 test x =
735 case x of
736 !_ -> mid (case x of
737 Foo1 x1 -> x1)
738
739 ```
740
741 refineDefaultAlt fills in the DEFAULT here with `Foo ip1` and then x
742 becomes bound to `Foo ip1` so is inlined into the other case which
743 causes the KnownBranch optimisation to kick in.
744
745
746 2. combineIdenticalAlts does a better job
747
748 Simon Jakobi also points out that that combineIdenticalAlts will do a better job
749 if we refine the DEFAULT first.
750
751 ```
752 data D = C0 | C1 | C2
753
754 case e of
755 DEFAULT -> e0
756 C0 -> e1
757 C1 -> e1
758 ```
759
760 When we apply combineIdenticalAlts to this expression, it can't
761 combine the alts for C0 and C1, as we already have a default case.
762
763 If we apply refineDefaultAlt first, we get
764
765 ```
766 case e of
767 C0 -> e1
768 C1 -> e1
769 C2 -> e0
770 ```
771
772 and combineIdenticalAlts can turn that into
773
774 ```
775 case e of
776 DEFAULT -> e1
777 C2 -> e0
778 ```
779
780 It isn't obvious that refineDefaultAlt does this but if you look at its one
781 call site in SimplUtils then the `imposs_deflt_cons` argument is populated with
782 constructors which are matched elsewhere.
783
784 -}
785
786
787
788
789 {- Note [Combine identical alternatives]
790 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
791 If several alternatives are identical, merge them into a single
792 DEFAULT alternative. I've occasionally seen this making a big
793 difference:
794
795 case e of =====> case e of
796 C _ -> f x D v -> ....v....
797 D v -> ....v.... DEFAULT -> f x
798 DEFAULT -> f x
799
800 The point is that we merge common RHSs, at least for the DEFAULT case.
801 [One could do something more elaborate but I've never seen it needed.]
802 To avoid an expensive test, we just merge branches equal to the *first*
803 alternative; this picks up the common cases
804 a) all branches equal
805 b) some branches equal to the DEFAULT (which occurs first)
806
807 The case where Combine Identical Alternatives transformation showed up
808 was like this (base/Foreign/C/Err/Error.hs):
809
810 x | p `is` 1 -> e1
811 | p `is` 2 -> e2
812 ...etc...
813
814 where @is@ was something like
815
816 p `is` n = p /= (-1) && p == n
817
818 This gave rise to a horrible sequence of cases
819
820 case p of
821 (-1) -> $j p
822 1 -> e1
823 DEFAULT -> $j p
824
825 and similarly in cascade for all the join points!
826
827 NB: it's important that all this is done in [InAlt], *before* we work
828 on the alternatives themselves, because Simplify.simplAlt may zap the
829 occurrence info on the binders in the alternatives, which in turn
830 defeats combineIdenticalAlts (see Trac #7360).
831
832 Note [Care with impossible-constructors when combining alternatives]
833 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
834 Suppose we have (Trac #10538)
835 data T = A | B | C | D
836
837 case x::T of (Imposs-default-cons {A,B})
838 DEFAULT -> e1
839 A -> e2
840 B -> e1
841
842 When calling combineIdentialAlts, we'll have computed that the
843 "impossible constructors" for the DEFAULT alt is {A,B}, since if x is
844 A or B we'll take the other alternatives. But suppose we combine B
845 into the DEFAULT, to get
846
847 case x::T of (Imposs-default-cons {A})
848 DEFAULT -> e1
849 A -> e2
850
851 Then we must be careful to trim the impossible constructors to just {A},
852 else we risk compiling 'e1' wrong!
853
854 Not only that, but we take care when there is no DEFAULT beforehand,
855 because we are introducing one. Consider
856
857 case x of (Imposs-default-cons {A,B,C})
858 A -> e1
859 B -> e2
860 C -> e1
861
862 Then when combining the A and C alternatives we get
863
864 case x of (Imposs-default-cons {B})
865 DEFAULT -> e1
866 B -> e2
867
868 Note that we have a new DEFAULT branch that we didn't have before. So
869 we need delete from the "impossible-default-constructors" all the
870 known-con alternatives that we have eliminated. (In Trac #11172 we
871 missed the first one.)
872
873 -}
874
875 combineIdenticalAlts :: [AltCon] -- Constructors that cannot match DEFAULT
876 -> [CoreAlt]
877 -> (Bool, -- True <=> something happened
878 [AltCon], -- New constructors that cannot match DEFAULT
879 [CoreAlt]) -- New alternatives
880 -- See Note [Combine identical alternatives]
881 -- True <=> we did some combining, result is a single DEFAULT alternative
882 combineIdenticalAlts imposs_deflt_cons ((con1,bndrs1,rhs1) : rest_alts)
883 | all isDeadBinder bndrs1 -- Remember the default
884 , not (null elim_rest) -- alternative comes first
885 = (True, imposs_deflt_cons', deflt_alt : filtered_rest)
886 where
887 (elim_rest, filtered_rest) = partition identical_to_alt1 rest_alts
888 deflt_alt = (DEFAULT, [], mkTicks (concat tickss) rhs1)
889
890 -- See Note [Care with impossible-constructors when combining alternatives]
891 imposs_deflt_cons' = imposs_deflt_cons `minusList` elim_cons
892 elim_cons = elim_con1 ++ map fstOf3 elim_rest
893 elim_con1 = case con1 of -- Don't forget con1!
894 DEFAULT -> [] -- See Note [
895 _ -> [con1]
896
897 cheapEqTicked e1 e2 = cheapEqExpr' tickishFloatable e1 e2
898 identical_to_alt1 (_con,bndrs,rhs)
899 = all isDeadBinder bndrs && rhs `cheapEqTicked` rhs1
900 tickss = map (stripTicksT tickishFloatable . thdOf3) elim_rest
901
902 combineIdenticalAlts imposs_cons alts
903 = (False, imposs_cons, alts)
904
905 {- *********************************************************************
906 * *
907 exprIsTrivial
908 * *
909 ************************************************************************
910
911 Note [exprIsTrivial]
912 ~~~~~~~~~~~~~~~~~~~~
913 @exprIsTrivial@ is true of expressions we are unconditionally happy to
914 duplicate; simple variables and constants, and type
915 applications. Note that primop Ids aren't considered
916 trivial unless
917
918 Note [Variables are trivial]
919 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
920 There used to be a gruesome test for (hasNoBinding v) in the
921 Var case:
922 exprIsTrivial (Var v) | hasNoBinding v = idArity v == 0
923 The idea here is that a constructor worker, like \$wJust, is
924 really short for (\x -> \$wJust x), because \$wJust has no binding.
925 So it should be treated like a lambda. Ditto unsaturated primops.
926 But now constructor workers are not "have-no-binding" Ids. And
927 completely un-applied primops and foreign-call Ids are sufficiently
928 rare that I plan to allow them to be duplicated and put up with
929 saturating them.
930
931 Note [Tick trivial]
932 ~~~~~~~~~~~~~~~~~~~
933 Ticks are only trivial if they are pure annotations. If we treat
934 "tick<n> x" as trivial, it will be inlined inside lambdas and the
935 entry count will be skewed, for example. Furthermore "scc<n> x" will
936 turn into just "x" in mkTick.
937
938 Note [Empty case is trivial]
939 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
940 The expression (case (x::Int) Bool of {}) is just a type-changing
941 case used when we are sure that 'x' will not return. See
942 Note [Empty case alternatives] in CoreSyn.
943
944 If the scrutinee is trivial, then so is the whole expression; and the
945 CoreToSTG pass in fact drops the case expression leaving only the
946 scrutinee.
947
948 Having more trivial expressions is good. Moreover, if we don't treat
949 it as trivial we may land up with let-bindings like
950 let v = case x of {} in ...
951 and after CoreToSTG that gives
952 let v = x in ...
953 and that confuses the code generator (Trac #11155). So best to kill
954 it off at source.
955 -}
956
957 exprIsTrivial :: CoreExpr -> Bool
958 exprIsTrivial (Var _) = True -- See Note [Variables are trivial]
959 exprIsTrivial (Type _) = True
960 exprIsTrivial (Coercion _) = True
961 exprIsTrivial (Lit lit) = litIsTrivial lit
962 exprIsTrivial (App e arg) = not (isRuntimeArg arg) && exprIsTrivial e
963 exprIsTrivial (Lam b e) = not (isRuntimeVar b) && exprIsTrivial e
964 exprIsTrivial (Tick t e) = not (tickishIsCode t) && exprIsTrivial e
965 -- See Note [Tick trivial]
966 exprIsTrivial (Cast e _) = exprIsTrivial e
967 exprIsTrivial (Case e _ _ []) = exprIsTrivial e -- See Note [Empty case is trivial]
968 exprIsTrivial _ = False
969
970 {-
971 Note [getIdFromTrivialExpr]
972 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
973 When substituting in a breakpoint we need to strip away the type cruft
974 from a trivial expression and get back to the Id. The invariant is
975 that the expression we're substituting was originally trivial
976 according to exprIsTrivial, AND the expression is not a literal.
977 See Note [substTickish] for how breakpoint substitution preserves
978 this extra invariant.
979
980 We also need this functionality in CorePrep to extract out Id of a
981 function which we are saturating. However, in this case we don't know
982 if the variable actually refers to a literal; thus we use
983 'getIdFromTrivialExpr_maybe' to handle this case. See test
984 T12076lit for an example where this matters.
985 -}
986
987 getIdFromTrivialExpr :: CoreExpr -> Id
988 getIdFromTrivialExpr e
989 = fromMaybe (pprPanic "getIdFromTrivialExpr" (ppr e))
990 (getIdFromTrivialExpr_maybe e)
991
992 getIdFromTrivialExpr_maybe :: CoreExpr -> Maybe Id
993 -- See Note [getIdFromTrivialExpr]
994 getIdFromTrivialExpr_maybe e = go e
995 where go (Var v) = Just v
996 go (App f t) | not (isRuntimeArg t) = go f
997 go (Tick t e) | not (tickishIsCode t) = go e
998 go (Cast e _) = go e
999 go (Lam b e) | not (isRuntimeVar b) = go e
1000 go _ = Nothing
1001
1002 {-
1003 exprIsBottom is a very cheap and cheerful function; it may return
1004 False for bottoming expressions, but it never costs much to ask. See
1005 also CoreArity.exprBotStrictness_maybe, but that's a bit more
1006 expensive.
1007 -}
1008
1009 exprIsBottom :: CoreExpr -> Bool
1010 -- See Note [Bottoming expressions]
1011 exprIsBottom e
1012 | isEmptyTy (exprType e)
1013 = True
1014 | otherwise
1015 = go 0 e
1016 where
1017 go n (Var v) = isBottomingId v && n >= idArity v
1018 go n (App e a) | isTypeArg a = go n e
1019 | otherwise = go (n+1) e
1020 go n (Tick _ e) = go n e
1021 go n (Cast e _) = go n e
1022 go n (Let _ e) = go n e
1023 go n (Lam v e) | isTyVar v = go n e
1024 go _ (Case _ _ _ alts) = null alts
1025 -- See Note [Empty case alternatives] in CoreSyn
1026 go _ _ = False
1027
1028 {- Note [Bottoming expressions]
1029 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1030 A bottoming expression is guaranteed to diverge, or raise an
1031 exception. We can test for it in two different ways, and exprIsBottom
1032 checks for both of these situations:
1033
1034 * Visibly-bottom computations. For example
1035 (error Int "Hello")
1036 is visibly bottom. The strictness analyser also finds out if
1037 a function diverges or raises an exception, and puts that info
1038 in its strictness signature.
1039
1040 * Empty types. If a type is empty, its only inhabitant is bottom.
1041 For example:
1042 data T
1043 f :: T -> Bool
1044 f = \(x:t). case x of Bool {}
1045 Since T has no data constructors, the case alternatives are of course
1046 empty. However note that 'x' is not bound to a visibly-bottom value;
1047 it's the *type* that tells us it's going to diverge.
1048
1049 A GADT may also be empty even though it has constructors:
1050 data T a where
1051 T1 :: a -> T Bool
1052 T2 :: T Int
1053 ...(case (x::T Char) of {})...
1054 Here (T Char) is uninhabited. A more realistic case is (Int ~ Bool),
1055 which is likewise uninhabited.
1056
1057
1058 ************************************************************************
1059 * *
1060 exprIsDupable
1061 * *
1062 ************************************************************************
1063
1064 Note [exprIsDupable]
1065 ~~~~~~~~~~~~~~~~~~~~
1066 @exprIsDupable@ is true of expressions that can be duplicated at a modest
1067 cost in code size. This will only happen in different case
1068 branches, so there's no issue about duplicating work.
1069
1070 That is, exprIsDupable returns True of (f x) even if
1071 f is very very expensive to call.
1072
1073 Its only purpose is to avoid fruitless let-binding
1074 and then inlining of case join points
1075 -}
1076
1077 exprIsDupable :: DynFlags -> CoreExpr -> Bool
1078 exprIsDupable dflags e
1079 = isJust (go dupAppSize e)
1080 where
1081 go :: Int -> CoreExpr -> Maybe Int
1082 go n (Type {}) = Just n
1083 go n (Coercion {}) = Just n
1084 go n (Var {}) = decrement n
1085 go n (Tick _ e) = go n e
1086 go n (Cast e _) = go n e
1087 go n (App f a) | Just n' <- go n a = go n' f
1088 go n (Lit lit) | litIsDupable dflags lit = decrement n
1089 go _ _ = Nothing
1090
1091 decrement :: Int -> Maybe Int
1092 decrement 0 = Nothing
1093 decrement n = Just (n-1)
1094
1095 dupAppSize :: Int
1096 dupAppSize = 8 -- Size of term we are prepared to duplicate
1097 -- This is *just* big enough to make test MethSharing
1098 -- inline enough join points. Really it should be
1099 -- smaller, and could be if we fixed Trac #4960.
1100
1101 {-
1102 ************************************************************************
1103 * *
1104 exprIsCheap, exprIsExpandable
1105 * *
1106 ************************************************************************
1107
1108 Note [exprIsWorkFree]
1109 ~~~~~~~~~~~~~~~~~~~~~
1110 exprIsWorkFree is used when deciding whether to inline something; we
1111 don't inline it if doing so might duplicate work, by peeling off a
1112 complete copy of the expression. Here we do not want even to
1113 duplicate a primop (Trac #5623):
1114 eg let x = a #+ b in x +# x
1115 we do not want to inline/duplicate x
1116
1117 Previously we were a bit more liberal, which led to the primop-duplicating
1118 problem. However, being more conservative did lead to a big regression in
1119 one nofib benchmark, wheel-sieve1. The situation looks like this:
1120
1121 let noFactor_sZ3 :: GHC.Types.Int -> GHC.Types.Bool
1122 noFactor_sZ3 = case s_adJ of _ { GHC.Types.I# x_aRs ->
1123 case GHC.Prim.<=# x_aRs 2 of _ {
1124 GHC.Types.False -> notDivBy ps_adM qs_adN;
1125 GHC.Types.True -> lvl_r2Eb }}
1126 go = \x. ...(noFactor (I# y))....(go x')...
1127
1128 The function 'noFactor' is heap-allocated and then called. Turns out
1129 that 'notDivBy' is strict in its THIRD arg, but that is invisible to
1130 the caller of noFactor, which therefore cannot do w/w and
1131 heap-allocates noFactor's argument. At the moment (May 12) we are just
1132 going to put up with this, because the previous more aggressive inlining
1133 (which treated 'noFactor' as work-free) was duplicating primops, which
1134 in turn was making inner loops of array calculations runs slow (#5623)
1135
1136 Note [Case expressions are work-free]
1137 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1138 Are case-expressions work-free? Consider
1139 let v = case x of (p,q) -> p
1140 go = \y -> ...case v of ...
1141 Should we inline 'v' at its use site inside the loop? At the moment
1142 we do. I experimented with saying that case are *not* work-free, but
1143 that increased allocation slightly. It's a fairly small effect, and at
1144 the moment we go for the slightly more aggressive version which treats
1145 (case x of ....) as work-free if the alternatives are.
1146
1147 Moreover it improves arities of overloaded functions where
1148 there is only dictionary selection (no construction) involved
1149
1150 Note [exprIsCheap] See also Note [Interaction of exprIsCheap and lone variables]
1151 ~~~~~~~~~~~~~~~~~~ in CoreUnfold.hs
1152 @exprIsCheap@ looks at a Core expression and returns \tr{True} if
1153 it is obviously in weak head normal form, or is cheap to get to WHNF.
1154 [Note that that's not the same as exprIsDupable; an expression might be
1155 big, and hence not dupable, but still cheap.]
1156
1157 By ``cheap'' we mean a computation we're willing to:
1158 push inside a lambda, or
1159 inline at more than one place
1160 That might mean it gets evaluated more than once, instead of being
1161 shared. The main examples of things which aren't WHNF but are
1162 ``cheap'' are:
1163
1164 * case e of
1165 pi -> ei
1166 (where e, and all the ei are cheap)
1167
1168 * let x = e in b
1169 (where e and b are cheap)
1170
1171 * op x1 ... xn
1172 (where op is a cheap primitive operator)
1173
1174 * error "foo"
1175 (because we are happy to substitute it inside a lambda)
1176
1177 Notice that a variable is considered 'cheap': we can push it inside a lambda,
1178 because sharing will make sure it is only evaluated once.
1179
1180 Note [exprIsCheap and exprIsHNF]
1181 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1182 Note that exprIsHNF does not imply exprIsCheap. Eg
1183 let x = fac 20 in Just x
1184 This responds True to exprIsHNF (you can discard a seq), but
1185 False to exprIsCheap.
1186
1187 Note [Arguments and let-bindings exprIsCheapX]
1188 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1189 What predicate should we apply to the argument of an application, or the
1190 RHS of a let-binding?
1191
1192 We used to say "exprIsTrivial arg" due to concerns about duplicating
1193 nested constructor applications, but see #4978. So now we just recursively
1194 use exprIsCheapX.
1195
1196 We definitely want to treat let and app the same. The principle here is
1197 that
1198 let x = blah in f x
1199 should behave equivalently to
1200 f blah
1201
1202 This in turn means that the 'letrec g' does not prevent eta expansion
1203 in this (which it previously was):
1204 f = \x. let v = case x of
1205 True -> letrec g = \w. blah
1206 in g
1207 False -> \x. x
1208 in \w. v True
1209 -}
1210
1211 --------------------
1212 exprIsWorkFree :: CoreExpr -> Bool -- See Note [exprIsWorkFree]
1213 exprIsWorkFree = exprIsCheapX isWorkFreeApp
1214
1215 exprIsCheap :: CoreExpr -> Bool
1216 exprIsCheap = exprIsCheapX isCheapApp
1217
1218 exprIsCheapX :: CheapAppFun -> CoreExpr -> Bool
1219 exprIsCheapX ok_app e
1220 = ok e
1221 where
1222 ok e = go 0 e
1223
1224 -- n is the number of value arguments
1225 go n (Var v) = ok_app v n
1226 go _ (Lit {}) = True
1227 go _ (Type {}) = True
1228 go _ (Coercion {}) = True
1229 go n (Cast e _) = go n e
1230 go n (Case scrut _ _ alts) = ok scrut &&
1231 and [ go n rhs | (_,_,rhs) <- alts ]
1232 go n (Tick t e) | tickishCounts t = False
1233 | otherwise = go n e
1234 go n (Lam x e) | isRuntimeVar x = n==0 || go (n-1) e
1235 | otherwise = go n e
1236 go n (App f e) | isRuntimeArg e = go (n+1) f && ok e
1237 | otherwise = go n f
1238 go n (Let (NonRec _ r) e) = go n e && ok r
1239 go n (Let (Rec prs) e) = go n e && all (ok . snd) prs
1240
1241 -- Case: see Note [Case expressions are work-free]
1242 -- App, Let: see Note [Arguments and let-bindings exprIsCheapX]
1243
1244
1245 {- Note [exprIsExpandable]
1246 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1247 An expression is "expandable" if we are willing to duplicate it, if doing
1248 so might make a RULE or case-of-constructor fire. Consider
1249 let x = (a,b)
1250 y = build g
1251 in ....(case x of (p,q) -> rhs)....(foldr k z y)....
1252
1253 We don't inline 'x' or 'y' (see Note [Lone variables] in CoreUnfold),
1254 but we do want
1255
1256 * the case-expression to simplify
1257 (via exprIsConApp_maybe, exprIsLiteral_maybe)
1258
1259 * the foldr/build RULE to fire
1260 (by expanding the unfolding during rule matching)
1261
1262 So we classify the unfolding of a let-binding as "expandable" (via the
1263 uf_expandable field) if we want to do this kind of on-the-fly
1264 expansion. Specifically:
1265
1266 * True of constructor applications (K a b)
1267
1268 * True of applications of a "CONLIKE" Id; see Note [CONLIKE pragma] in BasicTypes.
1269 (NB: exprIsCheap might not be true of this)
1270
1271 * False of case-expressions. If we have
1272 let x = case ... in ...(case x of ...)...
1273 we won't simplify. We have to inline x. See Trac #14688.
1274
1275 * False of let-expressions (same reason); and in any case we
1276 float lets out of an RHS if doing so will reveal an expandable
1277 application (see SimplEnv.doFloatFromRhs).
1278
1279 * Take care: exprIsExpandable should /not/ be true of primops. I
1280 found this in test T5623a:
1281 let q = /\a. Ptr a (a +# b)
1282 in case q @ Float of Ptr v -> ...q...
1283
1284 q's inlining should not be expandable, else exprIsConApp_maybe will
1285 say that (q @ Float) expands to (Ptr a (a +# b)), and that will
1286 duplicate the (a +# b) primop, which we should not do lightly.
1287 (It's quite hard to trigger this bug, but T13155 does so for GHC 8.0.)
1288 -}
1289
1290 -------------------------------------
1291 exprIsExpandable :: CoreExpr -> Bool
1292 -- See Note [exprIsExpandable]
1293 exprIsExpandable e
1294 = ok e
1295 where
1296 ok e = go 0 e
1297
1298 -- n is the number of value arguments
1299 go n (Var v) = isExpandableApp v n
1300 go _ (Lit {}) = True
1301 go _ (Type {}) = True
1302 go _ (Coercion {}) = True
1303 go n (Cast e _) = go n e
1304 go n (Tick t e) | tickishCounts t = False
1305 | otherwise = go n e
1306 go n (Lam x e) | isRuntimeVar x = n==0 || go (n-1) e
1307 | otherwise = go n e
1308 go n (App f e) | isRuntimeArg e = go (n+1) f && ok e
1309 | otherwise = go n f
1310 go _ (Case {}) = False
1311 go _ (Let {}) = False
1312
1313
1314 -------------------------------------
1315 type CheapAppFun = Id -> Arity -> Bool
1316 -- Is an application of this function to n *value* args
1317 -- always cheap, assuming the arguments are cheap?
1318 -- True mainly of data constructors, partial applications;
1319 -- but with minor variations:
1320 -- isWorkFreeApp
1321 -- isCheapApp
1322 -- isExpandableApp
1323
1324 isWorkFreeApp :: CheapAppFun
1325 isWorkFreeApp fn n_val_args
1326 | n_val_args == 0 -- No value args
1327 = True
1328 | n_val_args < idArity fn -- Partial application
1329 = True
1330 | otherwise
1331 = case idDetails fn of
1332 DataConWorkId {} -> True
1333 _ -> False
1334
1335 isCheapApp :: CheapAppFun
1336 isCheapApp fn n_val_args
1337 | isWorkFreeApp fn n_val_args = True
1338 | isBottomingId fn = True -- See Note [isCheapApp: bottoming functions]
1339 | otherwise
1340 = case idDetails fn of
1341 DataConWorkId {} -> True -- Actually handled by isWorkFreeApp
1342 RecSelId {} -> n_val_args == 1 -- See Note [Record selection]
1343 ClassOpId {} -> n_val_args == 1
1344 PrimOpId op -> primOpIsCheap op
1345 _ -> False
1346 -- In principle we should worry about primops
1347 -- that return a type variable, since the result
1348 -- might be applied to something, but I'm not going
1349 -- to bother to check the number of args
1350
1351 isExpandableApp :: CheapAppFun
1352 isExpandableApp fn n_val_args
1353 | isWorkFreeApp fn n_val_args = True
1354 | otherwise
1355 = case idDetails fn of
1356 DataConWorkId {} -> True -- Actually handled by isWorkFreeApp
1357 RecSelId {} -> n_val_args == 1 -- See Note [Record selection]
1358 ClassOpId {} -> n_val_args == 1
1359 PrimOpId {} -> False
1360 _ | isBottomingId fn -> False
1361 -- See Note [isExpandableApp: bottoming functions]
1362 | isConLike (idRuleMatchInfo fn) -> True
1363 | all_args_are_preds -> True
1364 | otherwise -> False
1365
1366 where
1367 -- See if all the arguments are PredTys (implicit params or classes)
1368 -- If so we'll regard it as expandable; see Note [Expandable overloadings]
1369 all_args_are_preds = all_pred_args n_val_args (idType fn)
1370
1371 all_pred_args n_val_args ty
1372 | n_val_args == 0
1373 = True
1374
1375 | Just (bndr, ty) <- splitPiTy_maybe ty
1376 = caseBinder bndr
1377 (\_tv -> all_pred_args n_val_args ty)
1378 (\bndr_ty -> isPredTy bndr_ty && all_pred_args (n_val_args-1) ty)
1379
1380 | otherwise
1381 = False
1382
1383 {- Note [isCheapApp: bottoming functions]
1384 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1385 I'm not sure why we have a special case for bottoming
1386 functions in isCheapApp. Maybe we don't need it.
1387
1388 Note [isExpandableApp: bottoming functions]
1389 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1390 It's important that isExpandableApp does not respond True to bottoming
1391 functions. Recall undefined :: HasCallStack => a
1392 Suppose isExpandableApp responded True to (undefined d), and we had:
1393
1394 x = undefined <dict-expr>
1395
1396 Then Simplify.prepareRhs would ANF the RHS:
1397
1398 d = <dict-expr>
1399 x = undefined d
1400
1401 This is already bad: we gain nothing from having x bound to (undefined
1402 var), unlike the case for data constructors. Worse, we get the
1403 simplifier loop described in OccurAnal Note [Cascading inlines].
1404 Suppose x occurs just once; OccurAnal.occAnalNonRecRhs decides x will
1405 certainly_inline; so we end up inlining d right back into x; but in
1406 the end x doesn't inline because it is bottom (preInlineUnconditionally);
1407 so the process repeats.. We could elaborate the certainly_inline logic
1408 some more, but it's better just to treat bottoming bindings as
1409 non-expandable, because ANFing them is a bad idea in the first place.
1410
1411 Note [Record selection]
1412 ~~~~~~~~~~~~~~~~~~~~~~~~~~
1413 I'm experimenting with making record selection
1414 look cheap, so we will substitute it inside a
1415 lambda. Particularly for dictionary field selection.
1416
1417 BUT: Take care with (sel d x)! The (sel d) might be cheap, but
1418 there's no guarantee that (sel d x) will be too. Hence (n_val_args == 1)
1419
1420 Note [Expandable overloadings]
1421 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1422 Suppose the user wrote this
1423 {-# RULE forall x. foo (negate x) = h x #-}
1424 f x = ....(foo (negate x))....
1425 He'd expect the rule to fire. But since negate is overloaded, we might
1426 get this:
1427 f = \d -> let n = negate d in \x -> ...foo (n x)...
1428 So we treat the application of a function (negate in this case) to a
1429 *dictionary* as expandable. In effect, every function is CONLIKE when
1430 it's applied only to dictionaries.
1431
1432
1433 ************************************************************************
1434 * *
1435 exprOkForSpeculation
1436 * *
1437 ************************************************************************
1438 -}
1439
1440 -----------------------------
1441 -- | 'exprOkForSpeculation' returns True of an expression that is:
1442 --
1443 -- * Safe to evaluate even if normal order eval might not
1444 -- evaluate the expression at all, or
1445 --
1446 -- * Safe /not/ to evaluate even if normal order would do so
1447 --
1448 -- It is usually called on arguments of unlifted type, but not always
1449 -- In particular, Simplify.rebuildCase calls it on lifted types
1450 -- when a 'case' is a plain 'seq'. See the example in
1451 -- Note [exprOkForSpeculation: case expressions] below
1452 --
1453 -- Precisely, it returns @True@ iff:
1454 -- a) The expression guarantees to terminate,
1455 -- b) soon,
1456 -- c) without causing a write side effect (e.g. writing a mutable variable)
1457 -- d) without throwing a Haskell exception
1458 -- e) without risking an unchecked runtime exception (array out of bounds,
1459 -- divide by zero)
1460 --
1461 -- For @exprOkForSideEffects@ the list is the same, but omitting (e).
1462 --
1463 -- Note that
1464 -- exprIsHNF implies exprOkForSpeculation
1465 -- exprOkForSpeculation implies exprOkForSideEffects
1466 --
1467 -- See Note [PrimOp can_fail and has_side_effects] in PrimOp
1468 -- and Note [Transformations affected by can_fail and has_side_effects]
1469 --
1470 -- As an example of the considerations in this test, consider:
1471 --
1472 -- > let x = case y# +# 1# of { r# -> I# r# }
1473 -- > in E
1474 --
1475 -- being translated to:
1476 --
1477 -- > case y# +# 1# of { r# ->
1478 -- > let x = I# r#
1479 -- > in E
1480 -- > }
1481 --
1482 -- We can only do this if the @y + 1@ is ok for speculation: it has no
1483 -- side effects, and can't diverge or raise an exception.
1484
1485 exprOkForSpeculation, exprOkForSideEffects :: CoreExpr -> Bool
1486 exprOkForSpeculation = expr_ok primOpOkForSpeculation
1487 exprOkForSideEffects = expr_ok primOpOkForSideEffects
1488
1489 expr_ok :: (PrimOp -> Bool) -> CoreExpr -> Bool
1490 expr_ok _ (Lit _) = True
1491 expr_ok _ (Type _) = True
1492 expr_ok _ (Coercion _) = True
1493
1494 expr_ok primop_ok (Var v) = app_ok primop_ok v []
1495 expr_ok primop_ok (Cast e _) = expr_ok primop_ok e
1496 expr_ok primop_ok (Lam b e)
1497 | isTyVar b = expr_ok primop_ok e
1498 | otherwise = True
1499
1500
1501 -- Tick annotations that *tick* cannot be speculated, because these
1502 -- are meant to identify whether or not (and how often) the particular
1503 -- source expression was evaluated at runtime.
1504 expr_ok primop_ok (Tick tickish e)
1505 | tickishCounts tickish = False
1506 | otherwise = expr_ok primop_ok e
1507
1508 expr_ok _ (Let {}) = False
1509 -- Lets can be stacked deeply, so just give up.
1510 -- In any case, the argument of exprOkForSpeculation is
1511 -- usually in a strict context, so any lets will have been
1512 -- floated away.
1513
1514 expr_ok primop_ok (Case scrut bndr _ alts)
1515 = -- See Note [exprOkForSpeculation: case expressions]
1516 expr_ok primop_ok scrut
1517 && isUnliftedType (idType bndr)
1518 && all (\(_,_,rhs) -> expr_ok primop_ok rhs) alts
1519 && altsAreExhaustive alts
1520
1521 expr_ok primop_ok other_expr
1522 = case collectArgs other_expr of
1523 (expr, args) | Var f <- stripTicksTopE (not . tickishCounts) expr
1524 -> app_ok primop_ok f args
1525 _ -> False
1526
1527 -----------------------------
1528 app_ok :: (PrimOp -> Bool) -> Id -> [CoreExpr] -> Bool
1529 app_ok primop_ok fun args
1530 = case idDetails fun of
1531 DFunId new_type -> not new_type
1532 -- DFuns terminate, unless the dict is implemented
1533 -- with a newtype in which case they may not
1534
1535 DataConWorkId {} -> True
1536 -- The strictness of the constructor has already
1537 -- been expressed by its "wrapper", so we don't need
1538 -- to take the arguments into account
1539
1540 PrimOpId op
1541 | isDivOp op
1542 , [arg1, Lit lit] <- args
1543 -> not (isZeroLit lit) && expr_ok primop_ok arg1
1544 -- Special case for dividing operations that fail
1545 -- In general they are NOT ok-for-speculation
1546 -- (which primop_ok will catch), but they ARE OK
1547 -- if the divisor is definitely non-zero.
1548 -- Often there is a literal divisor, and this
1549 -- can get rid of a thunk in an inner loop
1550
1551 | SeqOp <- op -- See Note [seq# and expr_ok]
1552 -> all (expr_ok primop_ok) args
1553
1554 | otherwise
1555 -> primop_ok op -- Check the primop itself
1556 && and (zipWith arg_ok arg_tys args) -- Check the arguments
1557
1558 _other -> isUnliftedType (idType fun) -- c.f. the Var case of exprIsHNF
1559 || idArity fun > n_val_args -- Partial apps
1560 || (n_val_args == 0 &&
1561 isEvaldUnfolding (idUnfolding fun)) -- Let-bound values
1562 where
1563 n_val_args = valArgCount args
1564 where
1565 (arg_tys, _) = splitPiTys (idType fun)
1566
1567 arg_ok :: TyBinder -> CoreExpr -> Bool
1568 arg_ok (Named _) _ = True -- A type argument
1569 arg_ok (Anon ty) arg -- A term argument
1570 | isUnliftedType ty = expr_ok primop_ok arg
1571 | otherwise = True -- See Note [Primops with lifted arguments]
1572
1573 -----------------------------
1574 altsAreExhaustive :: [Alt b] -> Bool
1575 -- True <=> the case alternatives are definiely exhaustive
1576 -- False <=> they may or may not be
1577 altsAreExhaustive []
1578 = False -- Should not happen
1579 altsAreExhaustive ((con1,_,_) : alts)
1580 = case con1 of
1581 DEFAULT -> True
1582 LitAlt {} -> False
1583 DataAlt c -> alts `lengthIs` (tyConFamilySize (dataConTyCon c) - 1)
1584 -- It is possible to have an exhaustive case that does not
1585 -- enumerate all constructors, notably in a GADT match, but
1586 -- we behave conservatively here -- I don't think it's important
1587 -- enough to deserve special treatment
1588
1589 -- | True of dyadic operators that can fail only if the second arg is zero!
1590 isDivOp :: PrimOp -> Bool
1591 -- This function probably belongs in PrimOp, or even in
1592 -- an automagically generated file.. but it's such a
1593 -- special case I thought I'd leave it here for now.
1594 isDivOp IntQuotOp = True
1595 isDivOp IntRemOp = True
1596 isDivOp WordQuotOp = True
1597 isDivOp WordRemOp = True
1598 isDivOp FloatDivOp = True
1599 isDivOp DoubleDivOp = True
1600 isDivOp _ = False
1601
1602 {- Note [exprOkForSpeculation: case expressions]
1603 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1604 exprOkForSpeculation accepts very special case expressions.
1605 Reason: (a ==# b) is ok-for-speculation, but the litEq rules
1606 in PrelRules convert it (a ==# 3#) to
1607 case a of { DEAFULT -> 0#; 3# -> 1# }
1608 for excellent reasons described in
1609 PrelRules Note [The litEq rule: converting equality to case].
1610 So, annoyingly, we want that case expression to be
1611 ok-for-speculation too. Bother.
1612
1613 But we restrict it sharply:
1614
1615 * We restrict it to unlifted scrutinees. Consider this:
1616 case x of y {
1617 DEFAULT -> ... (let v::Int# = case y of { True -> e1
1618 ; False -> e2 }
1619 in ...) ...
1620
1621 Does the RHS of v satisfy the let/app invariant? Previously we said
1622 yes, on the grounds that y is evaluated. But the binder-swap done
1623 by SetLevels would transform the inner alternative to
1624 DEFAULT -> ... (let v::Int# = case x of { ... }
1625 in ...) ....
1626 which does /not/ satisfy the let/app invariant, because x is
1627 not evaluated. See Note [Binder-swap during float-out]
1628 in SetLevels. To avoid this awkwardness it seems simpler
1629 to stick to unlifted scrutinees where the issue does not
1630 arise.
1631
1632 * We restrict it to exhaustive alternatives. A non-exhaustive
1633 case manifestly isn't ok-for-speculation. Consider
1634 case e of x { DEAFULT ->
1635 ...(case x of y
1636 A -> ...
1637 _ -> ...(case (case x of { B -> p; C -> p }) of
1638 I# r -> blah)...
1639 If SetLevesls considers the inner nested case as ok-for-speculation
1640 it can do case-floating (see Note [Floating cases] in SetLevels).
1641 So we'd float to:
1642 case e of x { DEAFULT ->
1643 case (case x of { B -> p; C -> p }) of I# r ->
1644 ...(case x of y
1645 A -> ...
1646 _ -> ...blah...)...
1647 which is utterly bogus (seg fault); see Trac #5453.
1648
1649 Similarly, this is a valid program (albeit a slightly dodgy one)
1650 let v = case x of { B -> ...; C -> ... }
1651 in case x of
1652 A -> ...
1653 _ -> ...v...v....
1654 Should v be considered ok-for-speculation? Its scrutinee may be
1655 evaluated, but the alternatives are incomplete so we should not
1656 evaluate it strictly.
1657
1658 Now, all this is for lifted types, but it'd be the same for any
1659 finite unlifted type. We don't have many of them, but we might
1660 add unlifted algebraic types in due course.
1661
1662 ----- Historical note: Trac #3717: --------
1663 foo :: Int -> Int
1664 foo 0 = 0
1665 foo n = (if n < 5 then 1 else 2) `seq` foo (n-1)
1666
1667 In earlier GHCs, we got this:
1668 T.$wfoo =
1669 \ (ww :: GHC.Prim.Int#) ->
1670 case ww of ds {
1671 __DEFAULT -> case (case <# ds 5 of _ {
1672 GHC.Types.False -> lvl1;
1673 GHC.Types.True -> lvl})
1674 of _ { __DEFAULT ->
1675 T.$wfoo (GHC.Prim.-# ds_XkE 1) };
1676 0 -> 0 }
1677
1678 Before join-points etc we could only get rid of two cases (which are
1679 redundant) by recognising that th e(case <# ds 5 of { ... }) is
1680 ok-for-speculation, even though it has /lifted/ type. But now join
1681 points do the job nicely.
1682 ------- End of historical note ------------
1683
1684
1685 Note [Primops with lifted arguments]
1686 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1687 Is this ok-for-speculation (see Trac #13027)?
1688 reallyUnsafePtrEq# a b
1689 Well, yes. The primop accepts lifted arguments and does not
1690 evaluate them. Indeed, in general primops are, well, primitive
1691 and do not perform evaluation.
1692
1693 There is one primop, dataToTag#, which does /require/ a lifted
1694 argument to be evaluated. To ensure this, CorePrep adds an
1695 eval if it can't see the argument is definitely evaluated
1696 (see [dataToTag magic] in CorePrep).
1697
1698 We make no attempt to guarantee that dataToTag#'s argument is
1699 evaluated here. Main reason: it's very fragile to test for the
1700 evaluatedness of a lifted argument. Consider
1701 case x of y -> let v = dataToTag# y in ...
1702
1703 where x/y have type Int, say. 'y' looks evaluated (by the enclosing
1704 case) so all is well. Now the FloatOut pass does a binder-swap (for
1705 very good reasons), changing to
1706 case x of y -> let v = dataToTag# x in ...
1707
1708 See also Note [dataToTag#] in primops.txt.pp.
1709
1710 Bottom line:
1711 * in exprOkForSpeculation we simply ignore all lifted arguments.
1712 * except see Note [seq# and expr_ok] for an exception
1713
1714
1715 Note [seq# and expr_ok]
1716 ~~~~~~~~~~~~~~~~~~~~~~~
1717 Recall that
1718 seq# :: forall a s . a -> State# s -> (# State# s, a #)
1719 must always evaluate its first argument. So it's really a
1720 counter-example to Note [Primops with lifted arguments]. In
1721 the case of seq# we must check the argument to seq#. Remember
1722 item (d) of the specification of exprOkForSpeculation:
1723
1724 -- Precisely, it returns @True@ iff:
1725 -- a) The expression guarantees to terminate,
1726 ...
1727 -- d) without throwing a Haskell exception
1728
1729 The lack of this special case caused Trac #5129 to go bad again.
1730 See comment:24 and following
1731
1732
1733 ************************************************************************
1734 * *
1735 exprIsHNF, exprIsConLike
1736 * *
1737 ************************************************************************
1738 -}
1739
1740 -- Note [exprIsHNF] See also Note [exprIsCheap and exprIsHNF]
1741 -- ~~~~~~~~~~~~~~~~
1742 -- | exprIsHNF returns true for expressions that are certainly /already/
1743 -- evaluated to /head/ normal form. This is used to decide whether it's ok
1744 -- to change:
1745 --
1746 -- > case x of _ -> e
1747 --
1748 -- into:
1749 --
1750 -- > e
1751 --
1752 -- and to decide whether it's safe to discard a 'seq'.
1753 --
1754 -- So, it does /not/ treat variables as evaluated, unless they say they are.
1755 -- However, it /does/ treat partial applications and constructor applications
1756 -- as values, even if their arguments are non-trivial, provided the argument
1757 -- type is lifted. For example, both of these are values:
1758 --
1759 -- > (:) (f x) (map f xs)
1760 -- > map (...redex...)
1761 --
1762 -- because 'seq' on such things completes immediately.
1763 --
1764 -- For unlifted argument types, we have to be careful:
1765 --
1766 -- > C (f x :: Int#)
1767 --
1768 -- Suppose @f x@ diverges; then @C (f x)@ is not a value. However this can't
1769 -- happen: see "CoreSyn#let_app_invariant". This invariant states that arguments of
1770 -- unboxed type must be ok-for-speculation (or trivial).
1771 exprIsHNF :: CoreExpr -> Bool -- True => Value-lambda, constructor, PAP
1772 exprIsHNF = exprIsHNFlike isDataConWorkId isEvaldUnfolding
1773
1774 -- | Similar to 'exprIsHNF' but includes CONLIKE functions as well as
1775 -- data constructors. Conlike arguments are considered interesting by the
1776 -- inliner.
1777 exprIsConLike :: CoreExpr -> Bool -- True => lambda, conlike, PAP
1778 exprIsConLike = exprIsHNFlike isConLikeId isConLikeUnfolding
1779
1780 -- | Returns true for values or value-like expressions. These are lambdas,
1781 -- constructors / CONLIKE functions (as determined by the function argument)
1782 -- or PAPs.
1783 --
1784 exprIsHNFlike :: (Var -> Bool) -> (Unfolding -> Bool) -> CoreExpr -> Bool
1785 exprIsHNFlike is_con is_con_unf = is_hnf_like
1786 where
1787 is_hnf_like (Var v) -- NB: There are no value args at this point
1788 = id_app_is_value v 0 -- Catches nullary constructors,
1789 -- so that [] and () are values, for example
1790 -- and (e.g.) primops that don't have unfoldings
1791 || is_con_unf (idUnfolding v)
1792 -- Check the thing's unfolding; it might be bound to a value
1793 -- We don't look through loop breakers here, which is a bit conservative
1794 -- but otherwise I worry that if an Id's unfolding is just itself,
1795 -- we could get an infinite loop
1796
1797 is_hnf_like (Lit _) = True
1798 is_hnf_like (Type _) = True -- Types are honorary Values;
1799 -- we don't mind copying them
1800 is_hnf_like (Coercion _) = True -- Same for coercions
1801 is_hnf_like (Lam b e) = isRuntimeVar b || is_hnf_like e
1802 is_hnf_like (Tick tickish e) = not (tickishCounts tickish)
1803 && is_hnf_like e
1804 -- See Note [exprIsHNF Tick]
1805 is_hnf_like (Cast e _) = is_hnf_like e
1806 is_hnf_like (App e a)
1807 | isValArg a = app_is_value e 1
1808 | otherwise = is_hnf_like e
1809 is_hnf_like (Let _ e) = is_hnf_like e -- Lazy let(rec)s don't affect us
1810 is_hnf_like _ = False
1811
1812 -- There is at least one value argument
1813 -- 'n' is number of value args to which the expression is applied
1814 app_is_value :: CoreExpr -> Int -> Bool
1815 app_is_value (Var f) nva = id_app_is_value f nva
1816 app_is_value (Tick _ f) nva = app_is_value f nva
1817 app_is_value (Cast f _) nva = app_is_value f nva
1818 app_is_value (App f a) nva
1819 | isValArg a = app_is_value f (nva + 1)
1820 | otherwise = app_is_value f nva
1821 app_is_value _ _ = False
1822
1823 id_app_is_value id n_val_args
1824 = is_con id
1825 || idArity id > n_val_args
1826 || id `hasKey` absentErrorIdKey -- See Note [aBSENT_ERROR_ID] in MkCore
1827 -- absentError behaves like an honorary data constructor
1828
1829
1830 {-
1831 Note [exprIsHNF Tick]
1832
1833 We can discard source annotations on HNFs as long as they aren't
1834 tick-like:
1835
1836 scc c (\x . e) => \x . e
1837 scc c (C x1..xn) => C x1..xn
1838
1839 So we regard these as HNFs. Tick annotations that tick are not
1840 regarded as HNF if the expression they surround is HNF, because the
1841 tick is there to tell us that the expression was evaluated, so we
1842 don't want to discard a seq on it.
1843 -}
1844
1845 -- | Can we bind this 'CoreExpr' at the top level?
1846 exprIsTopLevelBindable :: CoreExpr -> Type -> Bool
1847 -- See Note [CoreSyn top-level string literals]
1848 -- Precondition: exprType expr = ty
1849 -- Top-level literal strings can't even be wrapped in ticks
1850 -- see Note [CoreSyn top-level string literals] in CoreSyn
1851 exprIsTopLevelBindable expr ty
1852 = not (isUnliftedType ty)
1853 || exprIsTickedString expr
1854
1855 -- | Check if the expression is zero or more Ticks wrapped around a literal
1856 -- string.
1857 exprIsTickedString :: CoreExpr -> Bool
1858 exprIsTickedString = isJust . exprIsTickedString_maybe
1859
1860 -- | Extract a literal string from an expression that is zero or more Ticks
1861 -- wrapped around a literal string. Returns Nothing if the expression has a
1862 -- different shape.
1863 -- Used to "look through" Ticks in places that need to handle literal strings.
1864 exprIsTickedString_maybe :: CoreExpr -> Maybe ByteString
1865 exprIsTickedString_maybe (Lit (MachStr bs)) = Just bs
1866 exprIsTickedString_maybe (Tick t e)
1867 -- we don't tick literals with CostCentre ticks, compare to mkTick
1868 | tickishPlace t == PlaceCostCentre = Nothing
1869 | otherwise = exprIsTickedString_maybe e
1870 exprIsTickedString_maybe _ = Nothing
1871
1872 {-
1873 ************************************************************************
1874 * *
1875 Instantiating data constructors
1876 * *
1877 ************************************************************************
1878
1879 These InstPat functions go here to avoid circularity between DataCon and Id
1880 -}
1881
1882 dataConRepInstPat :: [Unique] -> DataCon -> [Type] -> ([TyCoVar], [Id])
1883 dataConRepFSInstPat :: [FastString] -> [Unique] -> DataCon -> [Type] -> ([TyCoVar], [Id])
1884
1885 dataConRepInstPat = dataConInstPat (repeat ((fsLit "ipv")))
1886 dataConRepFSInstPat = dataConInstPat
1887
1888 dataConInstPat :: [FastString] -- A long enough list of FSs to use for names
1889 -> [Unique] -- An equally long list of uniques, at least one for each binder
1890 -> DataCon
1891 -> [Type] -- Types to instantiate the universally quantified tyvars
1892 -> ([TyCoVar], [Id]) -- Return instantiated variables
1893 -- dataConInstPat arg_fun fss us con inst_tys returns a tuple
1894 -- (ex_tvs, arg_ids),
1895 --
1896 -- ex_tvs are intended to be used as binders for existential type args
1897 --
1898 -- arg_ids are indended to be used as binders for value arguments,
1899 -- and their types have been instantiated with inst_tys and ex_tys
1900 -- The arg_ids include both evidence and
1901 -- programmer-specified arguments (both after rep-ing)
1902 --
1903 -- Example.
1904 -- The following constructor T1
1905 --
1906 -- data T a where
1907 -- T1 :: forall b. Int -> b -> T(a,b)
1908 -- ...
1909 --
1910 -- has representation type
1911 -- forall a. forall a1. forall b. (a ~ (a1,b)) =>
1912 -- Int -> b -> T a
1913 --
1914 -- dataConInstPat fss us T1 (a1',b') will return
1915 --
1916 -- ([a1'', b''], [c :: (a1', b')~(a1'', b''), x :: Int, y :: b''])
1917 --
1918 -- where the double-primed variables are created with the FastStrings and
1919 -- Uniques given as fss and us
1920 dataConInstPat fss uniqs con inst_tys
1921 = ASSERT( univ_tvs `equalLength` inst_tys )
1922 (ex_bndrs, arg_ids)
1923 where
1924 univ_tvs = dataConUnivTyVars con
1925 ex_tvs = dataConExTyCoVars con
1926 arg_tys = dataConRepArgTys con
1927 arg_strs = dataConRepStrictness con -- 1-1 with arg_tys
1928 n_ex = length ex_tvs
1929
1930 -- split the Uniques and FastStrings
1931 (ex_uniqs, id_uniqs) = splitAt n_ex uniqs
1932 (ex_fss, id_fss) = splitAt n_ex fss
1933
1934 -- Make the instantiating substitution for universals
1935 univ_subst = zipTvSubst univ_tvs inst_tys
1936
1937 -- Make existential type variables, applying and extending the substitution
1938 (full_subst, ex_bndrs) = mapAccumL mk_ex_var univ_subst
1939 (zip3 ex_tvs ex_fss ex_uniqs)
1940
1941 mk_ex_var :: TCvSubst -> (TyCoVar, FastString, Unique) -> (TCvSubst, TyCoVar)
1942 mk_ex_var subst (tv, fs, uniq) = (Type.extendTCvSubstWithClone subst tv
1943 new_tv
1944 , new_tv)
1945 where
1946 new_tv | isTyVar tv
1947 = mkTyVar (mkSysTvName uniq fs) kind
1948 | otherwise
1949 = mkCoVar (mkSystemVarName uniq fs) kind
1950 kind = Type.substTyUnchecked subst (varType tv)
1951
1952 -- Make value vars, instantiating types
1953 arg_ids = zipWith4 mk_id_var id_uniqs id_fss arg_tys arg_strs
1954 mk_id_var uniq fs ty str
1955 = setCaseBndrEvald str $ -- See Note [Mark evaluated arguments]
1956 mkLocalIdOrCoVar name (Type.substTy full_subst ty)
1957 where
1958 name = mkInternalName uniq (mkVarOccFS fs) noSrcSpan
1959
1960 {-
1961 Note [Mark evaluated arguments]
1962 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1963 When pattern matching on a constructor with strict fields, the binder
1964 can have an 'evaldUnfolding'. Moreover, it *should* have one, so that
1965 when loading an interface file unfolding like:
1966 data T = MkT !Int
1967 f x = case x of { MkT y -> let v::Int# = case y of I# n -> n+1
1968 in ... }
1969 we don't want Lint to complain. The 'y' is evaluated, so the
1970 case in the RHS of the binding for 'v' is fine. But only if we
1971 *know* that 'y' is evaluated.
1972
1973 c.f. add_evals in Simplify.simplAlt
1974
1975 ************************************************************************
1976 * *
1977 Equality
1978 * *
1979 ************************************************************************
1980 -}
1981
1982 -- | A cheap equality test which bales out fast!
1983 -- If it returns @True@ the arguments are definitely equal,
1984 -- otherwise, they may or may not be equal.
1985 --
1986 -- See also 'exprIsBig'
1987 cheapEqExpr :: Expr b -> Expr b -> Bool
1988 cheapEqExpr = cheapEqExpr' (const False)
1989
1990 -- | Cheap expression equality test, can ignore ticks by type.
1991 cheapEqExpr' :: (Tickish Id -> Bool) -> Expr b -> Expr b -> Bool
1992 cheapEqExpr' ignoreTick = go_s
1993 where go_s = go `on` stripTicksTopE ignoreTick
1994 go (Var v1) (Var v2) = v1 == v2
1995 go (Lit lit1) (Lit lit2) = lit1 == lit2
1996 go (Type t1) (Type t2) = t1 `eqType` t2
1997 go (Coercion c1) (Coercion c2) = c1 `eqCoercion` c2
1998
1999 go (App f1 a1) (App f2 a2)
2000 = f1 `go_s` f2 && a1 `go_s` a2
2001
2002 go (Cast e1 t1) (Cast e2 t2)
2003 = e1 `go_s` e2 && t1 `eqCoercion` t2
2004
2005 go (Tick t1 e1) (Tick t2 e2)
2006 = t1 == t2 && e1 `go_s` e2
2007
2008 go _ _ = False
2009 {-# INLINE go #-}
2010 {-# INLINE cheapEqExpr' #-}
2011
2012 exprIsBig :: Expr b -> Bool
2013 -- ^ Returns @True@ of expressions that are too big to be compared by 'cheapEqExpr'
2014 exprIsBig (Lit _) = False
2015 exprIsBig (Var _) = False
2016 exprIsBig (Type _) = False
2017 exprIsBig (Coercion _) = False
2018 exprIsBig (Lam _ e) = exprIsBig e
2019 exprIsBig (App f a) = exprIsBig f || exprIsBig a
2020 exprIsBig (Cast e _) = exprIsBig e -- Hopefully coercions are not too big!
2021 exprIsBig (Tick _ e) = exprIsBig e
2022 exprIsBig _ = True
2023
2024 eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
2025 -- Compares for equality, modulo alpha
2026 eqExpr in_scope e1 e2
2027 = go (mkRnEnv2 in_scope) e1 e2
2028 where
2029 go env (Var v1) (Var v2)
2030 | rnOccL env v1 == rnOccR env v2
2031 = True
2032
2033 go _ (Lit lit1) (Lit lit2) = lit1 == lit2
2034 go env (Type t1) (Type t2) = eqTypeX env t1 t2
2035 go env (Coercion co1) (Coercion co2) = eqCoercionX env co1 co2
2036 go env (Cast e1 co1) (Cast e2 co2) = eqCoercionX env co1 co2 && go env e1 e2
2037 go env (App f1 a1) (App f2 a2) = go env f1 f2 && go env a1 a2
2038 go env (Tick n1 e1) (Tick n2 e2) = eqTickish env n1 n2 && go env e1 e2
2039
2040 go env (Lam b1 e1) (Lam b2 e2)
2041 = eqTypeX env (varType b1) (varType b2) -- False for Id/TyVar combination
2042 && go (rnBndr2 env b1 b2) e1 e2
2043
2044 go env (Let (NonRec v1 r1) e1) (Let (NonRec v2 r2) e2)
2045 = go env r1 r2 -- No need to check binder types, since RHSs match
2046 && go (rnBndr2 env v1 v2) e1 e2
2047
2048 go env (Let (Rec ps1) e1) (Let (Rec ps2) e2)
2049 = equalLength ps1 ps2
2050 && all2 (go env') rs1 rs2 && go env' e1 e2
2051 where
2052 (bs1,rs1) = unzip ps1
2053 (bs2,rs2) = unzip ps2
2054 env' = rnBndrs2 env bs1 bs2
2055
2056 go env (Case e1 b1 t1 a1) (Case e2 b2 t2 a2)
2057 | null a1 -- See Note [Empty case alternatives] in TrieMap
2058 = null a2 && go env e1 e2 && eqTypeX env t1 t2
2059 | otherwise
2060 = go env e1 e2 && all2 (go_alt (rnBndr2 env b1 b2)) a1 a2
2061
2062 go _ _ _ = False
2063
2064 -----------
2065 go_alt env (c1, bs1, e1) (c2, bs2, e2)
2066 = c1 == c2 && go (rnBndrs2 env bs1 bs2) e1 e2
2067
2068 eqTickish :: RnEnv2 -> Tickish Id -> Tickish Id -> Bool
2069 eqTickish env (Breakpoint lid lids) (Breakpoint rid rids)
2070 = lid == rid && map (rnOccL env) lids == map (rnOccR env) rids
2071 eqTickish _ l r = l == r
2072
2073 -- | Finds differences between core expressions, modulo alpha and
2074 -- renaming. Setting @top@ means that the @IdInfo@ of bindings will be
2075 -- checked for differences as well.
2076 diffExpr :: Bool -> RnEnv2 -> CoreExpr -> CoreExpr -> [SDoc]
2077 diffExpr _ env (Var v1) (Var v2) | rnOccL env v1 == rnOccR env v2 = []
2078 diffExpr _ _ (Lit lit1) (Lit lit2) | lit1 == lit2 = []
2079 diffExpr _ env (Type t1) (Type t2) | eqTypeX env t1 t2 = []
2080 diffExpr _ env (Coercion co1) (Coercion co2)
2081 | eqCoercionX env co1 co2 = []
2082 diffExpr top env (Cast e1 co1) (Cast e2 co2)
2083 | eqCoercionX env co1 co2 = diffExpr top env e1 e2
2084 diffExpr top env (Tick n1 e1) e2
2085 | not (tickishIsCode n1) = diffExpr top env e1 e2
2086 diffExpr top env e1 (Tick n2 e2)
2087 | not (tickishIsCode n2) = diffExpr top env e1 e2
2088 diffExpr top env (Tick n1 e1) (Tick n2 e2)
2089 | eqTickish env n1 n2 = diffExpr top env e1 e2
2090 -- The error message of failed pattern matches will contain
2091 -- generated names, which are allowed to differ.
2092 diffExpr _ _ (App (App (Var absent) _) _)
2093 (App (App (Var absent2) _) _)
2094 | isBottomingId absent && isBottomingId absent2 = []
2095 diffExpr top env (App f1 a1) (App f2 a2)
2096 = diffExpr top env f1 f2 ++ diffExpr top env a1 a2
2097 diffExpr top env (Lam b1 e1) (Lam b2 e2)
2098 | eqTypeX env (varType b1) (varType b2) -- False for Id/TyVar combination
2099 = diffExpr top (rnBndr2 env b1 b2) e1 e2
2100 diffExpr top env (Let bs1 e1) (Let bs2 e2)
2101 = let (ds, env') = diffBinds top env (flattenBinds [bs1]) (flattenBinds [bs2])
2102 in ds ++ diffExpr top env' e1 e2
2103 diffExpr top env (Case e1 b1 t1 a1) (Case e2 b2 t2 a2)
2104 | equalLength a1 a2 && not (null a1) || eqTypeX env t1 t2
2105 -- See Note [Empty case alternatives] in TrieMap
2106 = diffExpr top env e1 e2 ++ concat (zipWith diffAlt a1 a2)
2107 where env' = rnBndr2 env b1 b2
2108 diffAlt (c1, bs1, e1) (c2, bs2, e2)
2109 | c1 /= c2 = [text "alt-cons " <> ppr c1 <> text " /= " <> ppr c2]
2110 | otherwise = diffExpr top (rnBndrs2 env' bs1 bs2) e1 e2
2111 diffExpr _ _ e1 e2
2112 = [fsep [ppr e1, text "/=", ppr e2]]
2113
2114 -- | Finds differences between core bindings, see @diffExpr@.
2115 --
2116 -- The main problem here is that while we expect the binds to have the
2117 -- same order in both lists, this is not guaranteed. To do this
2118 -- properly we'd either have to do some sort of unification or check
2119 -- all possible mappings, which would be seriously expensive. So
2120 -- instead we simply match single bindings as far as we can. This
2121 -- leaves us just with mutually recursive and/or mismatching bindings,
2122 -- which we then speculatively match by ordering them. It's by no means
2123 -- perfect, but gets the job done well enough.
2124 diffBinds :: Bool -> RnEnv2 -> [(Var, CoreExpr)] -> [(Var, CoreExpr)]
2125 -> ([SDoc], RnEnv2)
2126 diffBinds top env binds1 = go (length binds1) env binds1
2127 where go _ env [] []
2128 = ([], env)
2129 go fuel env binds1 binds2
2130 -- No binds left to compare? Bail out early.
2131 | null binds1 || null binds2
2132 = (warn env binds1 binds2, env)
2133 -- Iterated over all binds without finding a match? Then
2134 -- try speculatively matching binders by order.
2135 | fuel == 0
2136 = if not $ env `inRnEnvL` fst (head binds1)
2137 then let env' = uncurry (rnBndrs2 env) $ unzip $
2138 zip (sort $ map fst binds1) (sort $ map fst binds2)
2139 in go (length binds1) env' binds1 binds2
2140 -- If we have already tried that, give up
2141 else (warn env binds1 binds2, env)
2142 go fuel env ((bndr1,expr1):binds1) binds2
2143 | let matchExpr (bndr,expr) =
2144 (not top || null (diffIdInfo env bndr bndr1)) &&
2145 null (diffExpr top (rnBndr2 env bndr1 bndr) expr1 expr)
2146 , (binds2l, (bndr2,_):binds2r) <- break matchExpr binds2
2147 = go (length binds1) (rnBndr2 env bndr1 bndr2)
2148 binds1 (binds2l ++ binds2r)
2149 | otherwise -- No match, so push back (FIXME O(n^2))
2150 = go (fuel-1) env (binds1++[(bndr1,expr1)]) binds2
2151 go _ _ _ _ = panic "diffBinds: impossible" -- GHC isn't smart enough
2152
2153 -- We have tried everything, but couldn't find a good match. So
2154 -- now we just return the comparison results when we pair up
2155 -- the binds in a pseudo-random order.
2156 warn env binds1 binds2 =
2157 concatMap (uncurry (diffBind env)) (zip binds1' binds2') ++
2158 unmatched "unmatched left-hand:" (drop l binds1') ++
2159 unmatched "unmatched right-hand:" (drop l binds2')
2160 where binds1' = sortBy (comparing fst) binds1
2161 binds2' = sortBy (comparing fst) binds2
2162 l = min (length binds1') (length binds2')
2163 unmatched _ [] = []
2164 unmatched txt bs = [text txt $$ ppr (Rec bs)]
2165 diffBind env (bndr1,expr1) (bndr2,expr2)
2166 | ds@(_:_) <- diffExpr top env expr1 expr2
2167 = locBind "in binding" bndr1 bndr2 ds
2168 | otherwise
2169 = diffIdInfo env bndr1 bndr2
2170
2171 -- | Find differences in @IdInfo@. We will especially check whether
2172 -- the unfoldings match, if present (see @diffUnfold@).
2173 diffIdInfo :: RnEnv2 -> Var -> Var -> [SDoc]
2174 diffIdInfo env bndr1 bndr2
2175 | arityInfo info1 == arityInfo info2
2176 && cafInfo info1 == cafInfo info2
2177 && oneShotInfo info1 == oneShotInfo info2
2178 && inlinePragInfo info1 == inlinePragInfo info2
2179 && occInfo info1 == occInfo info2
2180 && demandInfo info1 == demandInfo info2
2181 && callArityInfo info1 == callArityInfo info2
2182 && levityInfo info1 == levityInfo info2
2183 = locBind "in unfolding of" bndr1 bndr2 $
2184 diffUnfold env (unfoldingInfo info1) (unfoldingInfo info2)
2185 | otherwise
2186 = locBind "in Id info of" bndr1 bndr2
2187 [fsep [pprBndr LetBind bndr1, text "/=", pprBndr LetBind bndr2]]
2188 where info1 = idInfo bndr1; info2 = idInfo bndr2
2189
2190 -- | Find differences in unfoldings. Note that we will not check for
2191 -- differences of @IdInfo@ in unfoldings, as this is generally
2192 -- redundant, and can lead to an exponential blow-up in complexity.
2193 diffUnfold :: RnEnv2 -> Unfolding -> Unfolding -> [SDoc]
2194 diffUnfold _ NoUnfolding NoUnfolding = []
2195 diffUnfold _ BootUnfolding BootUnfolding = []
2196 diffUnfold _ (OtherCon cs1) (OtherCon cs2) | cs1 == cs2 = []
2197 diffUnfold env (DFunUnfolding bs1 c1 a1)
2198 (DFunUnfolding bs2 c2 a2)
2199 | c1 == c2 && equalLength bs1 bs2
2200 = concatMap (uncurry (diffExpr False env')) (zip a1 a2)
2201 where env' = rnBndrs2 env bs1 bs2
2202 diffUnfold env (CoreUnfolding t1 _ _ v1 cl1 wf1 x1 g1)
2203 (CoreUnfolding t2 _ _ v2 cl2 wf2 x2 g2)
2204 | v1 == v2 && cl1 == cl2
2205 && wf1 == wf2 && x1 == x2 && g1 == g2
2206 = diffExpr False env t1 t2
2207 diffUnfold _ uf1 uf2
2208 = [fsep [ppr uf1, text "/=", ppr uf2]]
2209
2210 -- | Add location information to diff messages
2211 locBind :: String -> Var -> Var -> [SDoc] -> [SDoc]
2212 locBind loc b1 b2 diffs = map addLoc diffs
2213 where addLoc d = d $$ nest 2 (parens (text loc <+> bindLoc))
2214 bindLoc | b1 == b2 = ppr b1
2215 | otherwise = ppr b1 <> char '/' <> ppr b2
2216
2217 {-
2218 ************************************************************************
2219 * *
2220 Eta reduction
2221 * *
2222 ************************************************************************
2223
2224 Note [Eta reduction conditions]
2225 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2226 We try for eta reduction here, but *only* if we get all the way to an
2227 trivial expression. We don't want to remove extra lambdas unless we
2228 are going to avoid allocating this thing altogether.
2229
2230 There are some particularly delicate points here:
2231
2232 * We want to eta-reduce if doing so leaves a trivial expression,
2233 *including* a cast. For example
2234 \x. f |> co --> f |> co
2235 (provided co doesn't mention x)
2236
2237 * Eta reduction is not valid in general:
2238 \x. bot /= bot
2239 This matters, partly for old-fashioned correctness reasons but,
2240 worse, getting it wrong can yield a seg fault. Consider
2241 f = \x.f x
2242 h y = case (case y of { True -> f `seq` True; False -> False }) of
2243 True -> ...; False -> ...
2244
2245 If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
2246 says f=bottom, and replaces the (f `seq` True) with just
2247 (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
2248 *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
2249 the definition again, so that it does not termninate after all.
2250 Result: seg-fault because the boolean case actually gets a function value.
2251 See Trac #1947.
2252
2253 So it's important to do the right thing.
2254
2255 * Note [Arity care]: we need to be careful if we just look at f's
2256 arity. Currently (Dec07), f's arity is visible in its own RHS (see
2257 Note [Arity robustness] in SimplEnv) so we must *not* trust the
2258 arity when checking that 'f' is a value. Otherwise we will
2259 eta-reduce
2260 f = \x. f x
2261 to
2262 f = f
2263 Which might change a terminating program (think (f `seq` e)) to a
2264 non-terminating one. So we check for being a loop breaker first.
2265
2266 However for GlobalIds we can look at the arity; and for primops we
2267 must, since they have no unfolding.
2268
2269 * Regardless of whether 'f' is a value, we always want to
2270 reduce (/\a -> f a) to f
2271 This came up in a RULE: foldr (build (/\a -> g a))
2272 did not match foldr (build (/\b -> ...something complex...))
2273 The type checker can insert these eta-expanded versions,
2274 with both type and dictionary lambdas; hence the slightly
2275 ad-hoc isDictId
2276
2277 * Never *reduce* arity. For example
2278 f = \xy. g x y
2279 Then if h has arity 1 we don't want to eta-reduce because then
2280 f's arity would decrease, and that is bad
2281
2282 These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
2283 Alas.
2284
2285 Note [Eta reduction with casted arguments]
2286 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2287 Consider
2288 (\(x:t3). f (x |> g)) :: t3 -> t2
2289 where
2290 f :: t1 -> t2
2291 g :: t3 ~ t1
2292 This should be eta-reduced to
2293
2294 f |> (sym g -> t2)
2295
2296 So we need to accumulate a coercion, pushing it inward (past
2297 variable arguments only) thus:
2298 f (x |> co_arg) |> co --> (f |> (sym co_arg -> co)) x
2299 f (x:t) |> co --> (f |> (t -> co)) x
2300 f @ a |> co --> (f |> (forall a.co)) @ a
2301 f @ (g:t1~t2) |> co --> (f |> (t1~t2 => co)) @ (g:t1~t2)
2302 These are the equations for ok_arg.
2303
2304 It's true that we could also hope to eta reduce these:
2305 (\xy. (f x |> g) y)
2306 (\xy. (f x y) |> g)
2307 But the simplifier pushes those casts outwards, so we don't
2308 need to address that here.
2309 -}
2310
2311 tryEtaReduce :: [Var] -> CoreExpr -> Maybe CoreExpr
2312 tryEtaReduce bndrs body
2313 = go (reverse bndrs) body (mkRepReflCo (exprType body))
2314 where
2315 incoming_arity = count isId bndrs
2316
2317 go :: [Var] -- Binders, innermost first, types [a3,a2,a1]
2318 -> CoreExpr -- Of type tr
2319 -> Coercion -- Of type tr ~ ts
2320 -> Maybe CoreExpr -- Of type a1 -> a2 -> a3 -> ts
2321 -- See Note [Eta reduction with casted arguments]
2322 -- for why we have an accumulating coercion
2323 go [] fun co
2324 | ok_fun fun
2325 , let used_vars = exprFreeVars fun `unionVarSet` tyCoVarsOfCo co
2326 , not (any (`elemVarSet` used_vars) bndrs)
2327 = Just (mkCast fun co) -- Check for any of the binders free in the result
2328 -- including the accumulated coercion
2329
2330 go bs (Tick t e) co
2331 | tickishFloatable t
2332 = fmap (Tick t) $ go bs e co
2333 -- Float app ticks: \x -> Tick t (e x) ==> Tick t e
2334
2335 go (b : bs) (App fun arg) co
2336 | Just (co', ticks) <- ok_arg b arg co
2337 = fmap (flip (foldr mkTick) ticks) $ go bs fun co'
2338 -- Float arg ticks: \x -> e (Tick t x) ==> Tick t e
2339
2340 go _ _ _ = Nothing -- Failure!
2341
2342 ---------------
2343 -- Note [Eta reduction conditions]
2344 ok_fun (App fun (Type {})) = ok_fun fun
2345 ok_fun (Cast fun _) = ok_fun fun
2346 ok_fun (Tick _ expr) = ok_fun expr
2347 ok_fun (Var fun_id) = ok_fun_id fun_id || all ok_lam bndrs
2348 ok_fun _fun = False
2349
2350 ---------------
2351 ok_fun_id fun = fun_arity fun >= incoming_arity
2352
2353 ---------------
2354 fun_arity fun -- See Note [Arity care]
2355 | isLocalId fun
2356 , isStrongLoopBreaker (idOccInfo fun) = 0
2357 | arity > 0 = arity
2358 | isEvaldUnfolding (idUnfolding fun) = 1
2359 -- See Note [Eta reduction of an eval'd function]
2360 | otherwise = 0
2361 where
2362 arity = idArity fun
2363
2364 ---------------
2365 ok_lam v = isTyVar v || isEvVar v
2366
2367 ---------------
2368 ok_arg :: Var -- Of type bndr_t
2369 -> CoreExpr -- Of type arg_t
2370 -> Coercion -- Of kind (t1~t2)
2371 -> Maybe (Coercion -- Of type (arg_t -> t1 ~ bndr_t -> t2)
2372 -- (and similarly for tyvars, coercion args)
2373 , [Tickish Var])
2374 -- See Note [Eta reduction with casted arguments]
2375 ok_arg bndr (Type ty) co
2376 | Just tv <- getTyVar_maybe ty
2377 , bndr == tv = Just (mkHomoForAllCos [tv] co, [])
2378 ok_arg bndr (Var v) co
2379 | bndr == v = let reflCo = mkRepReflCo (idType bndr)
2380 in Just (mkFunCo Representational reflCo co, [])
2381 ok_arg bndr (Cast e co_arg) co
2382 | (ticks, Var v) <- stripTicksTop tickishFloatable e
2383 , bndr == v
2384 = Just (mkFunCo Representational (mkSymCo co_arg) co, ticks)
2385 -- The simplifier combines multiple casts into one,
2386 -- so we can have a simple-minded pattern match here
2387 ok_arg bndr (Tick t arg) co
2388 | tickishFloatable t, Just (co', ticks) <- ok_arg bndr arg co
2389 = Just (co', t:ticks)
2390
2391 ok_arg _ _ _ = Nothing
2392
2393 {-
2394 Note [Eta reduction of an eval'd function]
2395 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2396 In Haskell it is not true that f = \x. f x
2397 because f might be bottom, and 'seq' can distinguish them.
2398
2399 But it *is* true that f = f `seq` \x. f x
2400 and we'd like to simplify the latter to the former. This amounts
2401 to the rule that
2402 * when there is just *one* value argument,
2403 * f is not bottom
2404 we can eta-reduce \x. f x ===> f
2405
2406 This turned up in Trac #7542.
2407
2408
2409 ************************************************************************
2410 * *
2411 \subsection{Determining non-updatable right-hand-sides}
2412 * *
2413 ************************************************************************
2414
2415 Top-level constructor applications can usually be allocated
2416 statically, but they can't if the constructor, or any of the
2417 arguments, come from another DLL (because we can't refer to static
2418 labels in other DLLs).
2419
2420 If this happens we simply make the RHS into an updatable thunk,
2421 and 'execute' it rather than allocating it statically.
2422 -}
2423
2424 -- | This function is called only on *top-level* right-hand sides.
2425 -- Returns @True@ if the RHS can be allocated statically in the output,
2426 -- with no thunks involved at all.
2427 rhsIsStatic
2428 :: Platform
2429 -> (Name -> Bool) -- Which names are dynamic
2430 -> (LitNumType -> Integer -> Maybe CoreExpr)
2431 -- Desugaring for some literals (disgusting)
2432 -- C.f. Note [Disgusting computation of CafRefs] in TidyPgm
2433 -> CoreExpr -> Bool
2434 -- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
2435 -- refers to, CAFs; (ii) in CoreToStg to decide whether to put an
2436 -- update flag on it and (iii) in DsExpr to decide how to expand
2437 -- list literals
2438 --
2439 -- The basic idea is that rhsIsStatic returns True only if the RHS is
2440 -- (a) a value lambda
2441 -- (b) a saturated constructor application with static args
2442 --
2443 -- BUT watch out for
2444 -- (i) Any cross-DLL references kill static-ness completely
2445 -- because they must be 'executed' not statically allocated
2446 -- ("DLL" here really only refers to Windows DLLs, on other platforms,
2447 -- this is not necessary)
2448 --
2449 -- (ii) We treat partial applications as redexes, because in fact we
2450 -- make a thunk for them that runs and builds a PAP
2451 -- at run-time. The only applications that are treated as
2452 -- static are *saturated* applications of constructors.
2453
2454 -- We used to try to be clever with nested structures like this:
2455 -- ys = (:) w ((:) w [])
2456 -- on the grounds that CorePrep will flatten ANF-ise it later.
2457 -- But supporting this special case made the function much more
2458 -- complicated, because the special case only applies if there are no
2459 -- enclosing type lambdas:
2460 -- ys = /\ a -> Foo (Baz ([] a))
2461 -- Here the nested (Baz []) won't float out to top level in CorePrep.
2462 --
2463 -- But in fact, even without -O, nested structures at top level are
2464 -- flattened by the simplifier, so we don't need to be super-clever here.
2465 --
2466 -- Examples
2467 --
2468 -- f = \x::Int. x+7 TRUE
2469 -- p = (True,False) TRUE
2470 --
2471 -- d = (fst p, False) FALSE because there's a redex inside
2472 -- (this particular one doesn't happen but...)
2473 --
2474 -- h = D# (1.0## /## 2.0##) FALSE (redex again)
2475 -- n = /\a. Nil a TRUE
2476 --
2477 -- t = /\a. (:) (case w a of ...) (Nil a) FALSE (redex)
2478 --
2479 --
2480 -- This is a bit like CoreUtils.exprIsHNF, with the following differences:
2481 -- a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
2482 --
2483 -- b) (C x xs), where C is a constructor is updatable if the application is
2484 -- dynamic
2485 --
2486 -- c) don't look through unfolding of f in (f x).
2487
2488 rhsIsStatic platform is_dynamic_name cvt_literal rhs = is_static False rhs
2489 where
2490 is_static :: Bool -- True <=> in a constructor argument; must be atomic
2491 -> CoreExpr -> Bool
2492
2493 is_static False (Lam b e) = isRuntimeVar b || is_static False e
2494 is_static in_arg (Tick n e) = not (tickishIsCode n)
2495 && is_static in_arg e
2496 is_static in_arg (Cast e _) = is_static in_arg e
2497 is_static _ (Coercion {}) = True -- Behaves just like a literal
2498 is_static in_arg (Lit (LitNumber nt i _)) = case cvt_literal nt i of
2499 Just e -> is_static in_arg e
2500 Nothing -> True
2501 is_static _ (Lit (MachLabel {})) = False
2502 is_static _ (Lit _) = True
2503 -- A MachLabel (foreign import "&foo") in an argument
2504 -- prevents a constructor application from being static. The
2505 -- reason is that it might give rise to unresolvable symbols
2506 -- in the object file: under Linux, references to "weak"
2507 -- symbols from the data segment give rise to "unresolvable
2508 -- relocation" errors at link time This might be due to a bug
2509 -- in the linker, but we'll work around it here anyway.
2510 -- SDM 24/2/2004
2511
2512 is_static in_arg other_expr = go other_expr 0
2513 where
2514 go (Var f) n_val_args
2515 | (platformOS platform /= OSMinGW32) ||
2516 not (is_dynamic_name (idName f))
2517 = saturated_data_con f n_val_args
2518 || (in_arg && n_val_args == 0)
2519 -- A naked un-applied variable is *not* deemed a static RHS
2520 -- E.g. f = g
2521 -- Reason: better to update so that the indirection gets shorted
2522 -- out, and the true value will be seen
2523 -- NB: if you change this, you'll break the invariant that THUNK_STATICs
2524 -- are always updatable. If you do so, make sure that non-updatable
2525 -- ones have enough space for their static link field!
2526
2527 go (App f a) n_val_args
2528 | isTypeArg a = go f n_val_args
2529 | not in_arg && is_static True a = go f (n_val_args + 1)
2530 -- The (not in_arg) checks that we aren't in a constructor argument;
2531 -- if we are, we don't allow (value) applications of any sort
2532 --
2533 -- NB. In case you wonder, args are sometimes not atomic. eg.
2534 -- x = D# (1.0## /## 2.0##)
2535 -- can't float because /## can fail.
2536
2537 go (Tick n f) n_val_args = not (tickishIsCode n) && go f n_val_args
2538 go (Cast e _) n_val_args = go e n_val_args
2539 go _ _ = False
2540
2541 saturated_data_con f n_val_args
2542 = case isDataConWorkId_maybe f of
2543 Just dc -> n_val_args == dataConRepArity dc
2544 Nothing -> False
2545
2546 {-
2547 ************************************************************************
2548 * *
2549 \subsection{Type utilities}
2550 * *
2551 ************************************************************************
2552 -}
2553
2554 -- | True if the type has no non-bottom elements, e.g. when it is an empty
2555 -- datatype, or a GADT with non-satisfiable type parameters, e.g. Int :~: Bool.
2556 -- See Note [Bottoming expressions]
2557 --
2558 -- See Note [No alternatives lint check] for another use of this function.
2559 isEmptyTy :: Type -> Bool
2560 isEmptyTy ty
2561 -- Data types where, given the particular type parameters, no data
2562 -- constructor matches, are empty.
2563 -- This includes data types with no constructors, e.g. Data.Void.Void.
2564 | Just (tc, inst_tys) <- splitTyConApp_maybe ty
2565 , Just dcs <- tyConDataCons_maybe tc
2566 , all (dataConCannotMatch inst_tys) dcs
2567 = True
2568 | otherwise
2569 = False
2570
2571 {-
2572 *****************************************************
2573 *
2574 * StaticPtr
2575 *
2576 *****************************************************
2577 -}
2578
2579 -- | @collectMakeStaticArgs (makeStatic t srcLoc e)@ yields
2580 -- @Just (makeStatic, t, srcLoc, e)@.
2581 --
2582 -- Returns @Nothing@ for every other expression.
2583 collectMakeStaticArgs
2584 :: CoreExpr -> Maybe (CoreExpr, Type, CoreExpr, CoreExpr)
2585 collectMakeStaticArgs e
2586 | (fun@(Var b), [Type t, loc, arg], _) <- collectArgsTicks (const True) e
2587 , idName b == makeStaticName = Just (fun, t, loc, arg)
2588 collectMakeStaticArgs _ = Nothing
2589
2590 {-
2591 ************************************************************************
2592 * *
2593 \subsection{Join points}
2594 * *
2595 ************************************************************************
2596 -}
2597
2598 -- | Does this binding bind a join point (or a recursive group of join points)?
2599 isJoinBind :: CoreBind -> Bool
2600 isJoinBind (NonRec b _) = isJoinId b
2601 isJoinBind (Rec ((b, _) : _)) = isJoinId b
2602 isJoinBind _ = False