2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[SaAbsInt]{Abstract interpreter for strictness analysis}
9 findDemand, findDemandAlts,
16 #include "HsVersions.h"
18 import CmdLineOpts ( opt_AllStrict, opt_NumbersStrict )
20 import CoreUnfold ( Unfolding, maybeUnfoldingTemplate )
21 import Id ( Id, idType, idArity, idStrictness, idUnfolding, isDataConId_maybe )
22 import DataCon ( dataConTyCon, splitProductType_maybe, dataConRepArgTys )
23 import IdInfo ( StrictnessInfo(..) )
24 import Demand ( Demand(..), wwPrim, wwStrict, wwEnum, wwUnpackData, wwLazy,
27 import TyCon ( isProductTyCon, isRecursiveTyCon, isEnumerationTyCon, isNewTyCon )
28 import BasicTypes ( Arity, NewOrData(..) )
29 import Type ( splitAlgTyConApp_maybe,
30 isUnLiftedType, Type )
31 import TyCon ( tyConUnique )
32 import PrelInfo ( numericTyKeys )
33 import Util ( isIn, nOfThem, zipWithEqual )
37 %************************************************************************
39 \subsection[AbsVal-ops]{Operations on @AbsVals@}
41 %************************************************************************
43 Least upper bound, greatest lower bound.
46 lub, glb :: AbsVal -> AbsVal -> AbsVal
48 lub AbsBot val2 = val2
49 lub val1 AbsBot = val1
51 lub (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "lub" lub xs ys)
53 lub _ _ = AbsTop -- Crude, but conservative
54 -- The crudity only shows up if there
55 -- are functions involved
57 -- Slightly funny glb; for absence analysis only;
58 -- AbsBot is the safe answer.
60 -- Using anyBot rather than just testing for AbsBot is important.
65 -- g = \x y z -> case x of
69 -- Now, the abstract value of the branches of the case will be an
70 -- AbsFun, but when testing for z's absence we want to spot that it's
71 -- an AbsFun which can't possibly return AbsBot. So when glb'ing we
72 -- mustn't be too keen to bale out and return AbsBot; the anyBot test
73 -- spots that (f x) can't possibly return AbsBot.
75 -- We have also tripped over the following interesting case:
80 -- Now, suppose f is bound to AbsTop. Does this expression mention z?
81 -- Obviously not. But the case will take the glb of AbsTop (for f) and
82 -- an AbsFun (for \y->1). We should not bale out and give AbsBot, because
83 -- that would say that it *does* mention z (or anything else for that matter).
84 -- Nor can we always return AbsTop, because the AbsFun might be something
85 -- like (\y->z), which obviously does mention z. The point is that we're
86 -- glbing two functions, and AbsTop is not actually the top of the function
87 -- lattice. It is more like (\xyz -> x|y|z); that is, AbsTop returns
88 -- poison iff any of its arguments do.
90 -- Deal with functions specially, because AbsTop isn't the
91 -- top of their domain.
94 | is_fun v1 || is_fun v2
95 = if not (anyBot v1) && not (anyBot v2)
101 is_fun (AbsFun _ _) = True
102 is_fun (AbsApproxFun _ _) = True -- Not used, but the glb works ok
105 -- The non-functional cases are quite straightforward
107 glb (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "glb" glb xs ys)
112 glb _ _ = AbsBot -- Be pessimistic
115 @isBot@ returns True if its argument is (a representation of) bottom. The
116 ``representation'' part is because we need to detect the bottom {\em function}
117 too. To detect the bottom function, bind its args to top, and see if it
120 Used only in strictness analysis:
122 isBot :: AbsVal -> Bool
125 isBot other = False -- Functions aren't bottom any more
128 Used only in absence analysis:
131 anyBot :: AbsVal -> Bool
133 anyBot AbsBot = True -- poisoned!
134 anyBot AbsTop = False
135 anyBot (AbsProd vals) = any anyBot vals
136 anyBot (AbsFun bndr_ty abs_fn) = anyBot (abs_fn AbsTop)
137 anyBot (AbsApproxFun _ val) = anyBot val
140 @widen@ takes an @AbsVal@, $val$, and returns and @AbsVal@ which is
141 approximated by $val$. Furthermore, the result has no @AbsFun@s in
142 it, so it can be compared for equality by @sameVal@.
145 widen :: AnalysisKind -> AbsVal -> AbsVal
147 -- Widening is complicated by the fact that funtions are lifted
148 widen StrAnal the_fn@(AbsFun bndr_ty _)
149 = case widened_body of
150 AbsApproxFun ds val -> AbsApproxFun (d : ds) val
152 d = findRecDemand str_fn abs_fn bndr_ty
153 str_fn val = isBot (foldl (absApply StrAnal) the_fn
154 (val : [AbsTop | d <- ds]))
156 other -> AbsApproxFun [d] widened_body
158 d = findRecDemand str_fn abs_fn bndr_ty
159 str_fn val = isBot (absApply StrAnal the_fn val)
161 widened_body = widen StrAnal (absApply StrAnal the_fn AbsTop)
162 abs_fn val = False -- Always says poison; so it looks as if
163 -- nothing is absent; safe
166 This stuff is now instead handled neatly by the fact that AbsApproxFun
167 contains an AbsVal inside it. SLPJ Jan 97
169 | isBot abs_body = AbsBot
170 -- It's worth checking for a function which is unconditionally
173 -- f x y = let g y = case x of ...
174 -- in (g ..) + (g ..)
176 -- Here, when we are considering strictness of f in x, we'll
177 -- evaluate the body of f with x bound to bottom. The current
178 -- strategy is to bind g to its *widened* value; without the isBot
179 -- (...) test above, we'd bind g to an AbsApproxFun, and deliver
180 -- Top, not Bot as the value of f's rhs. The test spots the
181 -- unconditional bottom-ness of g when x is bottom. (Another
182 -- alternative here would be to bind g to its exact abstract
183 -- value, but that entails lots of potential re-computation, at
184 -- every application of g.)
187 widen StrAnal (AbsProd vals) = AbsProd (map (widen StrAnal) vals)
188 widen StrAnal other_val = other_val
191 widen AbsAnal the_fn@(AbsFun bndr_ty _)
192 | anyBot widened_body = AbsBot
193 -- In the absence-analysis case it's *essential* to check
194 -- that the function has no poison in its body. If it does,
195 -- anywhere, then the whole function is poisonous.
198 = case widened_body of
199 AbsApproxFun ds val -> AbsApproxFun (d : ds) val
201 d = findRecDemand str_fn abs_fn bndr_ty
202 abs_fn val = not (anyBot (foldl (absApply AbsAnal) the_fn
203 (val : [AbsTop | d <- ds])))
205 other -> AbsApproxFun [d] widened_body
207 d = findRecDemand str_fn abs_fn bndr_ty
208 abs_fn val = not (anyBot (absApply AbsAnal the_fn val))
210 widened_body = widen AbsAnal (absApply AbsAnal the_fn AbsTop)
211 str_fn val = True -- Always says non-termination;
212 -- that'll make findRecDemand peer into the
213 -- structure of the value.
215 widen AbsAnal (AbsProd vals) = AbsProd (map (widen AbsAnal) vals)
217 -- It's desirable to do a good job of widening for product
221 -- in ...(case p of (x,y) -> x)...
223 -- Now, is y absent in this expression? Currently the
224 -- analyser widens p before looking at p's scope, to avoid
225 -- lots of recomputation in the case where p is a function.
226 -- So if widening doesn't have a case for products, we'll
227 -- widen p to AbsBot (since when searching for absence in y we
228 -- bind y to poison ie AbsBot), and now we are lost.
230 widen AbsAnal other_val = other_val
232 -- WAS: if anyBot val then AbsBot else AbsTop
233 -- Nowadays widen is doing a better job on functions for absence analysis.
236 @crudeAbsWiden@ is used just for absence analysis, and always
237 returns AbsTop or AbsBot, so it widens to a two-point domain
240 crudeAbsWiden :: AbsVal -> AbsVal
241 crudeAbsWiden val = if anyBot val then AbsBot else AbsTop
244 @sameVal@ compares two abstract values for equality. It can't deal with
245 @AbsFun@, but that should have been removed earlier in the day by @widen@.
248 sameVal :: AbsVal -> AbsVal -> Bool -- Can't handle AbsFun!
251 sameVal (AbsFun _ _) _ = panic "sameVal: AbsFun: arg1"
252 sameVal _ (AbsFun _ _) = panic "sameVal: AbsFun: arg2"
255 sameVal AbsBot AbsBot = True
256 sameVal AbsBot other = False -- widen has reduced AbsFun bots to AbsBot
258 sameVal AbsTop AbsTop = True
259 sameVal AbsTop other = False -- Right?
261 sameVal (AbsProd vals1) (AbsProd vals2) = and (zipWithEqual "sameVal" sameVal vals1 vals2)
262 sameVal (AbsProd _) AbsTop = False
263 sameVal (AbsProd _) AbsBot = False
265 sameVal (AbsApproxFun str1 v1) (AbsApproxFun str2 v2) = str1 == str2 && sameVal v1 v2
266 sameVal (AbsApproxFun _ _) AbsTop = False
267 sameVal (AbsApproxFun _ _) AbsBot = False
269 sameVal val1 val2 = panic "sameVal: type mismatch or AbsFun encountered"
273 @evalStrictness@ compares a @Demand@ with an abstract value, returning
274 @True@ iff the abstract value is {\em less defined} than the demand.
275 (@True@ is the exciting answer; @False@ is always safe.)
278 evalStrictness :: Demand
280 -> Bool -- True iff the value is sure
281 -- to be less defined than the Demand
283 evalStrictness (WwLazy _) _ = False
284 evalStrictness WwStrict val = isBot val
285 evalStrictness WwEnum val = isBot val
287 evalStrictness (WwUnpack NewType _ (demand:_)) val
288 = evalStrictness demand val
290 evalStrictness (WwUnpack DataType _ demand_info) val
294 AbsProd vals -> or (zipWithEqual "evalStrictness" evalStrictness demand_info vals)
295 _ -> pprTrace "evalStrictness?" empty False
297 evalStrictness WwPrim val
300 AbsBot -> True -- Can happen: consider f (g x), where g is a
301 -- recursive function returning an Int# that diverges
303 other -> pprPanic "evalStrictness: WwPrim:" (ppr other)
306 For absence analysis, we're interested in whether "poison" in the
307 argument (ie a bottom therein) can propagate to the result of the
308 function call; that is, whether the specified demand can {\em
309 possibly} hit poison.
312 evalAbsence (WwLazy True) _ = False -- Can't possibly hit poison
313 -- with Absent demand
315 evalAbsence (WwUnpack NewType _ (demand:_)) val
316 = evalAbsence demand val
318 evalAbsence (WwUnpack DataType _ demand_info) val
320 AbsTop -> False -- No poison in here
321 AbsBot -> True -- Pure poison
322 AbsProd vals -> or (zipWithEqual "evalAbsence" evalAbsence demand_info vals)
323 _ -> panic "evalAbsence: other"
325 evalAbsence other val = anyBot val
326 -- The demand is conservative; even "Lazy" *might* evaluate the
327 -- argument arbitrarily so we have to look everywhere for poison
330 %************************************************************************
332 \subsection[absEval]{Evaluate an expression in the abstract domain}
334 %************************************************************************
337 -- The isBottomingId stuf is now dealt with via the Id's strictness info
338 -- absId anal var env | isBottomingId var
340 -- StrAnal -> AbsBot -- See discussion below
341 -- AbsAnal -> AbsTop -- Just want to see if there's any poison in
345 = case (lookupAbsValEnv env var,
346 isDataConId_maybe var,
348 maybeUnfoldingTemplate (idUnfolding var)) of
350 (Just abs_val, _, _, _) ->
351 abs_val -- Bound in the environment
353 (_, Just data_con, _, _) | isProductTyCon tycon &&
354 not (isRecursiveTyCon tycon)
355 -> -- A product. We get infinite loops if we don't
356 -- check for recursive products!
357 -- The strictness info on the constructor
358 -- isn't expressive enough to contain its abstract value
359 productAbsVal (dataConRepArgTys data_con) []
361 tycon = dataConTyCon data_con
363 (_, _, NoStrictnessInfo, Just unfolding) ->
364 -- We have an unfolding for the expr
365 -- Assume the unfolding has no free variables since it
366 -- came from inside the Id
367 absEval anal unfolding env
368 -- Notice here that we only look in the unfolding if we don't
369 -- have strictness info (an unusual situation).
370 -- We could have chosen to look in the unfolding if it exists,
371 -- and only try the strictness info if it doesn't, and that would
372 -- give more accurate results, at the cost of re-abstract-interpreting
373 -- the unfolding every time.
374 -- We found only one place where the look-at-unfolding-first
375 -- method gave better results, which is in the definition of
376 -- showInt in the Prelude. In its defintion, fromIntegral is
377 -- not inlined (it's big) but ab-interp-ing its unfolding gave
378 -- a better result than looking at its strictness only.
379 -- showInt :: Integral a => a -> [Char] -> [Char]
380 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
381 -- "U(U(U(U(SA)AAAAAAAAL)AA)AAAAASAAASA)" {...} _N_ _N_ #-}
383 -- showInt :: Integral a => a -> [Char] -> [Char]
384 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
385 -- "U(U(U(U(SL)LLLLLLLLL)LL)LLLLLSLLLLL)" _N_ _N_ #-}
388 (_, _, strictness_info, _) ->
389 -- Includes NoUnfolding
390 -- Try the strictness info
391 absValFromStrictness anal strictness_info
393 productAbsVal [] rev_abs_args = AbsProd (reverse rev_abs_args)
394 productAbsVal (arg_ty : arg_tys) rev_abs_args = AbsFun arg_ty (\ abs_arg -> productAbsVal arg_tys (abs_arg : rev_abs_args))
398 absEval :: AnalysisKind -> CoreExpr -> AbsValEnv -> AbsVal
400 absEval anal (Type ty) env = AbsTop
401 absEval anal (Var var) env = absId anal var env
404 Discussion about error (following/quoting Lennart): Any expression
405 'error e' is regarded as bottom (with HBC, with the -ffail-strict
408 Regarding it as bottom gives much better strictness properties for
412 f (x:xs) y = f xs (x+y)
414 f [] _ = error "no match"
416 f (x:xs) y = f xs (x+y)
418 is strict in y, which you really want. But, it may lead to
419 transformations that turn a call to \tr{error} into non-termination.
420 (The odds of this happening aren't good.)
422 Things are a little different for absence analysis, because we want
423 to make sure that any poison (?????)
426 absEval anal (Lit _) env = AbsTop
427 -- Literals terminate (strictness) and are not poison (absence)
431 absEval anal (Lam bndr body) env
432 | isTyVar bndr = absEval anal body env -- Type lambda
433 | otherwise = AbsFun (idType bndr) abs_fn -- Value lambda
435 abs_fn arg = absEval anal body (addOneToAbsValEnv env bndr arg)
437 absEval anal (App expr (Type ty)) env
438 = absEval anal expr env -- Type appplication
439 absEval anal (App f val_arg) env
440 = absApply anal (absEval anal f env) -- Value applicationn
441 (absEval anal val_arg env)
445 absEval anal expr@(Case scrut case_bndr alts) env
447 scrut_val = absEval anal scrut env
448 alts_env = addOneToAbsValEnv env case_bndr scrut_val
450 case (scrut_val, alts) of
451 (AbsBot, _) -> AbsBot
453 (AbsProd arg_vals, [(con, bndrs, rhs)])
455 -- The scrutinee is a product value, so it must be of a single-constr
456 -- type; so the constructor in this alternative must be the right one
457 -- so we can go ahead and bind the constructor args to the components
458 -- of the product value.
459 ASSERT(length arg_vals == length val_bndrs)
460 absEval anal rhs rhs_env
462 val_bndrs = filter isId bndrs
463 rhs_env = growAbsValEnvList alts_env (val_bndrs `zip` arg_vals)
465 other -> absEvalAlts anal alts alts_env
468 For @Lets@ we widen the value we get. This is nothing to
469 do with fixpointing. The reason is so that we don't get an explosion
470 in the amount of computation. For example, consider:
482 If we bind @f@ and @g@ to their exact abstract value, then we'll
483 ``execute'' one call to @f@ and {\em two} calls to @g@. This can blow
484 up exponentially. Widening cuts it off by making a fixed
485 approximation to @f@ and @g@, so that the bodies of @f@ and @g@ are
486 not evaluated again at all when they are called.
488 Of course, this can lose useful joint strictness, which is sad. An
489 alternative approach would be to try with a certain amount of ``fuel''
490 and be prepared to bale out.
493 absEval anal (Let (NonRec binder e1) e2) env
495 new_env = addOneToAbsValEnv env binder (widen anal (absEval anal e1 env))
497 -- The binder of a NonRec should *not* be of unboxed type,
498 -- hence no need to strictly evaluate the Rhs.
499 absEval anal e2 new_env
501 absEval anal (Let (Rec pairs) body) env
503 (binders,rhss) = unzip pairs
504 rhs_vals = cheapFixpoint anal binders rhss env -- Returns widened values
505 new_env = growAbsValEnvList env (binders `zip` rhs_vals)
507 absEval anal body new_env
509 absEval anal (Note note expr) env = absEval anal expr env
513 absEvalAlts :: AnalysisKind -> [CoreAlt] -> AbsValEnv -> AbsVal
514 absEvalAlts anal alts env
515 = combine anal (map go alts)
517 combine StrAnal = foldr1 lub -- Diverge only if all diverge
518 combine AbsAnal = foldr1 glb -- Find any poison
521 = absEval anal rhs rhs_env
523 rhs_env = growAbsValEnvList env (filter isId bndrs `zip` repeat AbsTop)
526 %************************************************************************
528 \subsection[absApply]{Apply an abstract function to an abstract argument}
530 %************************************************************************
535 absApply :: AnalysisKind -> AbsVal -> AbsVal -> AbsVal
537 absApply anal AbsBot arg = AbsBot
538 -- AbsBot represents the abstract bottom *function* too
540 absApply StrAnal AbsTop arg = AbsTop
541 absApply AbsAnal AbsTop arg = if anyBot arg
544 -- To be conservative, we have to assume that a function about
545 -- which we know nothing (AbsTop) might look at some part of
549 An @AbsFun@ with only one more argument needed---bind it and eval the
550 result. A @Lam@ with two or more args: return another @AbsFun@ with
551 an augmented environment.
554 absApply anal (AbsFun bndr_ty abs_fn) arg = abs_fn arg
558 absApply StrAnal (AbsApproxFun (d:ds) val) arg
561 other -> AbsApproxFun ds val' -- Result is non-bot if there are still args
563 val' | evalStrictness d arg = AbsBot
566 absApply AbsAnal (AbsApproxFun (d:ds) val) arg
567 = if evalAbsence d arg
568 then AbsBot -- Poison in arg means poison in the application
571 other -> AbsApproxFun ds val
574 absApply anal f@(AbsProd _) arg
575 = pprPanic ("absApply: Duff function: AbsProd." ++ show anal) ((ppr f) <+> (ppr arg))
582 %************************************************************************
584 \subsection[findStrictness]{Determine some binders' strictness}
586 %************************************************************************
590 -> AbsVal -- Abstract strictness value of function
591 -> AbsVal -- Abstract absence value of function
592 -> StrictnessInfo -- Resulting strictness annotation
594 findStrictness id (AbsApproxFun str_ds str_res) (AbsApproxFun abs_ds _)
595 -- You might think there's really no point in describing detailed
596 -- strictness for a divergent function;
597 -- If it's fully applied we get bottom regardless of the
598 -- argument. If it's not fully applied we don't get bottom.
599 -- Finally, we don't want to regard the args of a divergent function
600 -- as 'interesting' for inlining purposes (see Simplify.prepareArgs)
602 -- HOWEVER, if we make diverging functions appear lazy, they
603 -- don't get wrappers, and then we get dreadful reboxing.
604 -- See notes with WwLib.worthSplitting
605 = StrictnessInfo (combineDemands id str_ds abs_ds) (isBot str_res)
607 findStrictness id str_val abs_val = NoStrictnessInfo
609 -- The list of absence demands passed to combineDemands
610 -- can be shorter than the list of absence demands
612 -- lookup = \ dEq -> letrec {
613 -- lookup = \ key ds -> ...lookup...
616 -- Here the strictness value takes three args, but the absence value
617 -- takes only one, for reasons I don't quite understand (see cheapFixpoint)
619 combineDemands id orig_str_ds orig_abs_ds
620 = go orig_str_ds orig_abs_ds
622 go str_ds abs_ds = zipWith mk_dmd str_ds (abs_ds ++ repeat wwLazy)
624 mk_dmd str_dmd (WwLazy True) = WARN( case str_dmd of { WwLazy _ -> False; other -> True },
625 ppr id <+> ppr orig_str_ds <+> ppr orig_abs_ds )
626 WwLazy True -- Best of all
627 mk_dmd (WwUnpack nd u str_ds)
628 (WwUnpack _ _ abs_ds) = WwUnpack nd u (go str_ds abs_ds)
630 mk_dmd str_dmd abs_dmd = str_dmd
635 findDemand dmd str_env abs_env expr binder
636 = findRecDemand str_fn abs_fn (idType binder)
638 str_fn val = evalStrictness dmd (absEval StrAnal expr (addOneToAbsValEnv str_env binder val))
639 abs_fn val = not (evalAbsence dmd (absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)))
641 findDemandAlts dmd str_env abs_env alts binder
642 = findRecDemand str_fn abs_fn (idType binder)
644 str_fn val = evalStrictness dmd (absEvalAlts StrAnal alts (addOneToAbsValEnv str_env binder val))
645 abs_fn val = not (evalAbsence dmd (absEvalAlts AbsAnal alts (addOneToAbsValEnv abs_env binder val)))
648 @findRecDemand@ is where we finally convert strictness/absence info
649 into ``Demands'' which we can pin on Ids (etc.).
651 NOTE: What do we do if something is {\em both} strict and absent?
652 Should \tr{f x y z = error "foo"} says that \tr{f}'s arguments are all
653 strict (because of bottoming effect of \tr{error}) or all absent
654 (because they're not used)?
656 Well, for practical reasons, we prefer absence over strictness. In
657 particular, it makes the ``default defaults'' for class methods (the
658 ones that say \tr{defm.foo dict = error "I don't exist"}) come out
659 nicely [saying ``the dict isn't used''], rather than saying it is
660 strict in every component of the dictionary [massive gratuitious
661 casing to take the dict apart].
663 But you could have examples where going for strictness would be better
664 than absence. Consider:
666 let x = something big
671 If \tr{x} is marked absent in \tr{f}, but not strict, and \tr{g} is
672 lazy, then the thunk for \tr{x} will be built. If \tr{f} was strict,
673 then we'd let-to-case it:
675 case something big of
681 findRecDemand :: (AbsVal -> Bool) -- True => function applied to this value yields Bot
682 -> (AbsVal -> Bool) -- True => function applied to this value yields no poison
683 -> Type -- The type of the argument
686 findRecDemand str_fn abs_fn ty
687 = if isUnLiftedType ty then -- It's a primitive type!
690 else if abs_fn AbsBot then -- It's absent
691 -- We prefer absence over strictness: see NOTE above.
694 else if not (opt_AllStrict ||
695 (opt_NumbersStrict && is_numeric_type ty) ||
697 WwLazy False -- It's not strict and we're not pretending
699 else -- It's strict (or we're pretending it is)!
701 case splitProductType_maybe ty of
703 Nothing -> wwStrict -- Could have a test for wwEnum, but
704 -- we don't exploit it yet, so don't bother
706 Just (tycon,_,data_con,cmpnt_tys) -- Single constructor case
707 | isNewTyCon tycon -- A newtype!
708 -> ASSERT( null (tail cmpnt_tys) )
710 demand = findRecDemand str_fn abs_fn (head cmpnt_tys)
714 | null compt_strict_infos -- A nullary data type
715 || isRecursiveTyCon tycon -- Recursive data type; don't unpack
718 | otherwise -- Some other data type
719 -> wwUnpackData compt_strict_infos
722 prod_len = length cmpnt_tys
726 str_fn (mkMainlyTopProd prod_len i cmpnt_val)
729 abs_fn (mkMainlyTopProd prod_len i cmpnt_val)
732 | (cmpnt_ty, i) <- cmpnt_tys `zip` [1..] ]
736 = case (splitAlgTyConApp_maybe ty) of -- NB: duplicates stuff done above
739 | tyConUnique tycon `is_elem` numericTyKeys
741 _{-something else-} -> False
743 is_elem = isIn "is_numeric_type"
745 -- mkMainlyTopProd: make an AbsProd that is all AbsTops ("n"-1 of
746 -- them) except for a given value in the "i"th position.
748 mkMainlyTopProd :: Int -> Int -> AbsVal -> AbsVal
750 mkMainlyTopProd n i val
752 befores = nOfThem (i-1) AbsTop
753 afters = nOfThem (n-i) AbsTop
755 AbsProd (befores ++ (val : afters))
758 %************************************************************************
760 \subsection[fixpoint]{Fixpointer for the strictness analyser}
762 %************************************************************************
764 The @fixpoint@ functions take a list of \tr{(binder, expr)} pairs, an
765 environment, and returns the abstract value of each binder.
767 The @cheapFixpoint@ function makes a conservative approximation,
768 by binding each of the variables to Top in their own right hand sides.
769 That allows us to make rapid progress, at the cost of a less-than-wonderful
773 cheapFixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
775 cheapFixpoint AbsAnal [id] [rhs] env
776 = [crudeAbsWiden (absEval AbsAnal rhs new_env)]
778 new_env = addOneToAbsValEnv env id AbsTop -- Unsafe starting point!
779 -- In the just-one-binding case, we guarantee to
780 -- find a fixed point in just one iteration,
781 -- because we are using only a two-point domain.
782 -- This improves matters in cases like:
784 -- f x y = letrec g = ...g...
787 -- Here, y isn't used at all, but if g is bound to
788 -- AbsBot we simply get AbsBot as the next
791 cheapFixpoint anal ids rhss env
792 = [widen anal (absEval anal rhs new_env) | rhs <- rhss]
793 -- We do just one iteration, starting from a safe
794 -- approximation. This won't do a good job in situations
796 -- \x -> letrec f = ...g...
800 -- Here, f will end up bound to Top after one iteration,
801 -- and hence we won't spot the strictness in x.
802 -- (A second iteration would solve this. ToDo: try the effect of
803 -- really searching for a fixed point.)
805 new_env = growAbsValEnvList env [(id,safe_val) | id <- ids]
808 = case anal of -- The safe starting point
814 mkLookupFun :: (key -> key -> Bool) -- Equality predicate
815 -> (key -> key -> Bool) -- Less-than predicate
816 -> [(key,val)] -- The assoc list
818 -> Maybe val -- The corresponding value
820 mkLookupFun eq lt alist s
821 = case [a | (s',a) <- alist, s' `eq` s] of
827 fixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
829 fixpoint anal [] _ env = []
831 fixpoint anal ids rhss env
832 = fix_loop initial_vals
835 = case anal of -- The (unsafe) starting point
838 -- At one stage for StrAnal we said:
839 -- if (returnsRealWorld (idType id))
840 -- then AbsTop -- this is a massively horrible hack (SLPJ 95/05)
841 -- but no one has the foggiest idea what this hack did,
842 -- and returnsRealWorld was a stub that always returned False
843 -- So this comment is all that is left of the hack!
845 initial_vals = [ initial_val id | id <- ids ]
847 fix_loop :: [AbsVal] -> [AbsVal]
849 fix_loop current_widened_vals
851 new_env = growAbsValEnvList env (ids `zip` current_widened_vals)
852 new_vals = [ absEval anal rhs new_env | rhs <- rhss ]
853 new_widened_vals = map (widen anal) new_vals
855 if (and (zipWith sameVal current_widened_vals new_widened_vals)) then
858 -- NB: I was too chicken to make that a zipWithEqual,
859 -- lest I jump into a black hole. WDP 96/02
861 -- Return the widened values. We might get a slightly
862 -- better value by returning new_vals (which we used to
863 -- do, see below), but alas that means that whenever the
864 -- function is called we have to re-execute it, which is
869 -- Return the un-widened values which may be a bit better
870 -- than the widened ones, and are guaranteed safe, since
871 -- they are one iteration beyond current_widened_vals,
872 -- which itself is a fixed point.
874 fix_loop new_widened_vals
877 For absence analysis, we make do with a very very simple approach:
878 look for convergence in a two-point domain.
880 We used to use just one iteration, starting with the variables bound
881 to @AbsBot@, which is safe.
883 Prior to that, we used one iteration starting from @AbsTop@ (which
884 isn't safe). Why isn't @AbsTop@ safe? Consider:
892 Here, if p is @AbsBot@, then we'd better {\em not} end up with a ``fixed
893 point'' of @d@ being @(AbsTop, AbsTop)@! An @AbsBot@ initial value is
894 safe because it gives poison more often than really necessary, and
895 thus may miss some absence, but will never claim absence when it ain't
898 Anyway, one iteration starting with everything bound to @AbsBot@ give
903 Here, f would always end up bound to @AbsBot@, which ain't very
904 clever, because then it would introduce poison whenever it was
905 applied. Much better to start with f bound to @AbsTop@, and widen it
906 to @AbsBot@ if any poison shows up. In effect we look for convergence
907 in the two-point @AbsTop@/@AbsBot@ domain.
909 What we miss (compared with the cleverer strictness analysis) is
910 spotting that in this case
912 f = \ x y -> ...y...(f x y')...
914 \tr{x} is actually absent, since it is only passed round the loop, never
915 used. But who cares about missing that?
917 NB: despite only having a two-point domain, we may still have many
918 iterations, because there are several variables involved at once.