2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1996
4 \section[SaAbsInt]{Abstract interpreter for strictness analysis}
16 #include "HsVersions.h"
18 import CmdLineOpts ( opt_AllStrict, opt_NumbersStrict )
20 import CoreUnfold ( Unfolding(..), FormSummary )
21 import CoreUtils ( unTagBinders )
22 import Id ( idType, getIdStrictness, getIdUnfolding,
23 dataConTyCon, dataConArgTys, Id
25 import IdInfo ( StrictnessInfo(..) )
26 import Demand ( Demand(..), wwPrim, wwStrict, wwEnum, wwUnpackData, wwUnpackNew )
27 import MagicUFs ( MagicUnfoldingFun )
28 import Maybes ( maybeToBool )
29 import PrimOp ( PrimOp(..) )
31 import TyCon ( isProductTyCon, isEnumerationTyCon, isNewTyCon,
34 import BasicTypes ( NewOrData(..) )
35 import Type ( splitAlgTyConApp_maybe,
36 isUnpointedType, Type )
37 import TysWiredIn ( intTyCon, integerTyCon, doubleTyCon,
38 floatTyCon, wordTyCon, addrTyCon
40 import Util ( isIn, isn'tIn, nOfThem, zipWithEqual, trace )
43 returnsRealWorld x = False -- ToDo: panic "SaAbsInt.returnsRealWorld (ToDo)"
46 %************************************************************************
48 \subsection[AbsVal-ops]{Operations on @AbsVals@}
50 %************************************************************************
52 Least upper bound, greatest lower bound.
55 lub, glb :: AbsVal -> AbsVal -> AbsVal
57 lub val1 val2 | isBot val1 = val2 -- The isBot test includes the case where
58 lub val1 val2 | isBot val2 = val1 -- one of the val's is a function which
59 -- always returns bottom, such as \y.x,
60 -- when x is bound to bottom.
62 lub (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "lub" lub xs ys)
64 lub _ _ = AbsTop -- Crude, but conservative
65 -- The crudity only shows up if there
66 -- are functions involved
68 -- Slightly funny glb; for absence analysis only;
69 -- AbsBot is the safe answer.
71 -- Using anyBot rather than just testing for AbsBot is important.
76 -- g = \x y z -> case x of
80 -- Now, the abstract value of the branches of the case will be an
81 -- AbsFun, but when testing for z's absence we want to spot that it's
82 -- an AbsFun which can't possibly return AbsBot. So when glb'ing we
83 -- mustn't be too keen to bale out and return AbsBot; the anyBot test
84 -- spots that (f x) can't possibly return AbsBot.
86 -- We have also tripped over the following interesting case:
91 -- Now, suppose f is bound to AbsTop. Does this expression mention z?
92 -- Obviously not. But the case will take the glb of AbsTop (for f) and
93 -- an AbsFun (for \y->1). We should not bale out and give AbsBot, because
94 -- that would say that it *does* mention z (or anything else for that matter).
95 -- Nor can we always return AbsTop, because the AbsFun might be something
96 -- like (\y->z), which obviously does mention z. The point is that we're
97 -- glbing two functions, and AbsTop is not actually the top of the function
98 -- lattice. It is more like (\xyz -> x|y|z); that is, AbsTop returns
99 -- poison iff any of its arguments do.
101 -- Deal with functions specially, because AbsTop isn't the
102 -- top of their domain.
105 | is_fun v1 || is_fun v2
106 = if not (anyBot v1) && not (anyBot v2)
112 is_fun (AbsFun _ _ _) = True
113 is_fun (AbsApproxFun _ _) = True -- Not used, but the glb works ok
116 -- The non-functional cases are quite straightforward
118 glb (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "glb" glb xs ys)
123 glb _ _ = AbsBot -- Be pessimistic
129 -> AbsVal -- Value of scrutinee
130 -> [AbsVal] -- Value of branches (at least one)
133 -- For strictness analysis, see if the scrutinee is bottom; if so
134 -- return bottom; otherwise, the lub of the branches.
136 combineCaseValues StrAnal AbsBot branches = AbsBot
137 combineCaseValues StrAnal other_scrutinee branches
138 -- Scrutinee can only be AbsBot, AbsProd or AbsTop
139 = ASSERT(ok_scrutinee)
143 = case other_scrutinee of {
144 AbsTop -> True; -- i.e., cool
145 AbsProd _ -> True; -- ditto
146 _ -> False -- party over
149 -- For absence analysis, check if the scrutinee is all poison (isBot)
150 -- If so, return poison (AbsBot); otherwise, any nested poison will come
151 -- out from looking at the branches, so just glb together the branches
152 -- to get the worst one.
154 combineCaseValues AbsAnal AbsBot branches = AbsBot
155 combineCaseValues AbsAnal other_scrutinee branches
156 -- Scrutinee can only be AbsBot, AbsProd or AbsTop
157 = ASSERT(ok_scrutinee)
159 result = foldr1 glb branches
161 tracer = if at_least_one_AbsFun && at_least_one_AbsTop
163 pprTrace "combineCase:" (ppr branches)
172 = case other_scrutinee of {
173 AbsTop -> True; -- i.e., cool
174 AbsProd _ -> True; -- ditto
175 _ -> False -- party over
178 at_least_one_AbsFun = foldr ((||) . is_AbsFun) False branches
179 at_least_one_AbsTop = foldr ((||) . is_AbsTop) False branches
180 no_AbsBots = foldr ((&&) . is_not_AbsBot) True branches
182 is_AbsFun x = case x of { AbsFun _ _ _ -> True; _ -> False }
183 is_AbsTop x = case x of { AbsTop -> True; _ -> False }
184 is_not_AbsBot x = case x of { AbsBot -> False; _ -> True }
187 @isBot@ returns True if its argument is (a representation of) bottom. The
188 ``representation'' part is because we need to detect the bottom {\em function}
189 too. To detect the bottom function, bind its args to top, and see if it
192 Used only in strictness analysis:
194 isBot :: AbsVal -> Bool
197 isBot (AbsFun arg body env) = isBot (absEval StrAnal body env)
198 -- Don't bother to extend the envt because
199 -- unbound variables default to AbsTop anyway
203 Used only in absence analysis:
205 anyBot :: AbsVal -> Bool
207 anyBot AbsBot = True -- poisoned!
208 anyBot AbsTop = False
209 anyBot (AbsProd vals) = any anyBot vals
210 anyBot (AbsFun arg body env) = anyBot (absEval AbsAnal body env)
211 anyBot (AbsApproxFun _ _) = False
213 -- AbsApproxFun can only arise in absence analysis from the Demand
214 -- info of an imported value; whatever it is we're looking for is
215 -- certainly not present over in the imported value.
218 @widen@ takes an @AbsVal@, $val$, and returns and @AbsVal@ which is
219 approximated by $val$. Furthermore, the result has no @AbsFun@s in
220 it, so it can be compared for equality by @sameVal@.
223 widen :: AnalysisKind -> AbsVal -> AbsVal
225 widen StrAnal (AbsFun arg body env)
226 = AbsApproxFun (findDemandStrOnly env body arg)
227 (widen StrAnal abs_body)
229 abs_body = absEval StrAnal body env
232 This stuff is now instead handled neatly by the fact that AbsApproxFun
233 contains an AbsVal inside it. SLPJ Jan 97
235 | isBot abs_body = AbsBot
236 -- It's worth checking for a function which is unconditionally
239 -- f x y = let g y = case x of ...
240 -- in (g ..) + (g ..)
242 -- Here, when we are considering strictness of f in x, we'll
243 -- evaluate the body of f with x bound to bottom. The current
244 -- strategy is to bind g to its *widened* value; without the isBot
245 -- (...) test above, we'd bind g to an AbsApproxFun, and deliver
246 -- Top, not Bot as the value of f's rhs. The test spots the
247 -- unconditional bottom-ness of g when x is bottom. (Another
248 -- alternative here would be to bind g to its exact abstract
249 -- value, but that entails lots of potential re-computation, at
250 -- every application of g.)
253 widen StrAnal (AbsProd vals) = AbsProd (map (widen StrAnal) vals)
254 widen StrAnal other_val = other_val
257 widen AbsAnal (AbsFun arg body env)
258 | anyBot abs_body = AbsBot
259 -- In the absence-analysis case it's *essential* to check
260 -- that the function has no poison in its body. If it does,
261 -- anywhere, then the whole function is poisonous.
264 = AbsApproxFun (findDemandAbsOnly env body arg)
265 (widen AbsAnal abs_body)
267 abs_body = absEval AbsAnal body env
269 widen AbsAnal (AbsProd vals) = AbsProd (map (widen AbsAnal) vals)
271 -- It's desirable to do a good job of widening for product
275 -- in ...(case p of (x,y) -> x)...
277 -- Now, is y absent in this expression? Currently the
278 -- analyser widens p before looking at p's scope, to avoid
279 -- lots of recomputation in the case where p is a function.
280 -- So if widening doesn't have a case for products, we'll
281 -- widen p to AbsBot (since when searching for absence in y we
282 -- bind y to poison ie AbsBot), and now we are lost.
284 widen AbsAnal other_val = other_val
286 -- WAS: if anyBot val then AbsBot else AbsTop
287 -- Nowadays widen is doing a better job on functions for absence analysis.
290 @crudeAbsWiden@ is used just for absence analysis, and always
291 returns AbsTop or AbsBot, so it widens to a two-point domain
294 crudeAbsWiden :: AbsVal -> AbsVal
295 crudeAbsWiden val = if anyBot val then AbsBot else AbsTop
298 @sameVal@ compares two abstract values for equality. It can't deal with
299 @AbsFun@, but that should have been removed earlier in the day by @widen@.
302 sameVal :: AbsVal -> AbsVal -> Bool -- Can't handle AbsFun!
305 sameVal (AbsFun _ _ _) _ = panic "sameVal: AbsFun: arg1"
306 sameVal _ (AbsFun _ _ _) = panic "sameVal: AbsFun: arg2"
309 sameVal AbsBot AbsBot = True
310 sameVal AbsBot other = False -- widen has reduced AbsFun bots to AbsBot
312 sameVal AbsTop AbsTop = True
313 sameVal AbsTop other = False -- Right?
315 sameVal (AbsProd vals1) (AbsProd vals2) = and (zipWithEqual "sameVal" sameVal vals1 vals2)
316 sameVal (AbsProd _) AbsTop = False
317 sameVal (AbsProd _) AbsBot = False
319 sameVal (AbsApproxFun str1 v1) (AbsApproxFun str2 v2) = str1 == str2 && sameVal v1 v2
320 sameVal (AbsApproxFun _ _) AbsTop = False
321 sameVal (AbsApproxFun _ _) AbsBot = False
323 sameVal val1 val2 = panic "sameVal: type mismatch or AbsFun encountered"
327 @evalStrictness@ compares a @Demand@ with an abstract value, returning
328 @True@ iff the abstract value is {\em less defined} than the demand.
329 (@True@ is the exciting answer; @False@ is always safe.)
332 evalStrictness :: Demand
334 -> Bool -- True iff the value is sure
335 -- to be less defined than the Demand
337 evalStrictness (WwLazy _) _ = False
338 evalStrictness WwStrict val = isBot val
339 evalStrictness WwEnum val = isBot val
341 evalStrictness (WwUnpack NewType _ (demand:_)) val
342 = evalStrictness demand val
344 evalStrictness (WwUnpack DataType _ demand_info) val
348 AbsProd vals -> or (zipWithEqual "evalStrictness" evalStrictness demand_info vals)
349 _ -> trace "evalStrictness?" False
351 evalStrictness WwPrim val
355 other -> -- A primitive value should be defined, never bottom;
356 -- hence this paranoia check
357 pprPanic "evalStrictness: WwPrim:" (ppr other)
360 For absence analysis, we're interested in whether "poison" in the
361 argument (ie a bottom therein) can propagate to the result of the
362 function call; that is, whether the specified demand can {\em
363 possibly} hit poison.
366 evalAbsence (WwLazy True) _ = False -- Can't possibly hit poison
367 -- with Absent demand
369 evalAbsence (WwUnpack NewType _ (demand:_)) val
370 = evalAbsence demand val
372 evalAbsence (WwUnpack DataType _ demand_info) val
374 AbsTop -> False -- No poison in here
375 AbsBot -> True -- Pure poison
376 AbsProd vals -> or (zipWithEqual "evalAbsence" evalAbsence demand_info vals)
377 _ -> panic "evalAbsence: other"
379 evalAbsence other val = anyBot val
380 -- The demand is conservative; even "Lazy" *might* evaluate the
381 -- argument arbitrarily so we have to look everywhere for poison
384 %************************************************************************
386 \subsection[absEval]{Evaluate an expression in the abstract domain}
388 %************************************************************************
391 -- The isBottomingId stuf is now dealt with via the Id's strictness info
392 -- absId anal var env | isBottomingId var
394 -- StrAnal -> AbsBot -- See discussion below
395 -- AbsAnal -> AbsTop -- Just want to see if there's any poison in
401 case (lookupAbsValEnv env var, getIdStrictness var, getIdUnfolding var) of
403 (Just abs_val, _, _) ->
404 abs_val -- Bound in the environment
406 (Nothing, NoStrictnessInfo, CoreUnfolding _ _ unfolding) ->
407 -- We have an unfolding for the expr
408 -- Assume the unfolding has no free variables since it
409 -- came from inside the Id
410 absEval anal (unTagBinders unfolding) env
411 -- Notice here that we only look in the unfolding if we don't
412 -- have strictness info (an unusual situation).
413 -- We could have chosen to look in the unfolding if it exists,
414 -- and only try the strictness info if it doesn't, and that would
415 -- give more accurate results, at the cost of re-abstract-interpreting
416 -- the unfolding every time.
417 -- We found only one place where the look-at-unfolding-first
418 -- method gave better results, which is in the definition of
419 -- showInt in the Prelude. In its defintion, fromIntegral is
420 -- not inlined (it's big) but ab-interp-ing its unfolding gave
421 -- a better result than looking at its strictness only.
422 -- showInt :: Integral a => a -> [Char] -> [Char]
423 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
424 -- "U(U(U(U(SA)AAAAAAAAL)AA)AAAAASAAASA)" {...} _N_ _N_ #-}
426 -- showInt :: Integral a => a -> [Char] -> [Char]
427 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
428 -- "U(U(U(U(SL)LLLLLLLLL)LL)LLLLLSLLLLL)" _N_ _N_ #-}
431 (Nothing, strictness_info, _) ->
432 -- Includes MagicUnfolding, NoUnfolding
433 -- Try the strictness info
434 absValFromStrictness anal strictness_info
436 -- pprTrace "absId:" (hcat [ppr var, ptext SLIT("=:"), pp_anal anal, text SLIT(":="),ppr result]) $
439 pp_anal StrAnal = ptext SLIT("STR")
440 pp_anal AbsAnal = ptext SLIT("ABS")
442 absEvalAtom anal (VarArg v) env = absId anal v env
443 absEvalAtom anal (LitArg _) env = AbsTop
447 absEval :: AnalysisKind -> CoreExpr -> AbsValEnv -> AbsVal
449 absEval anal (Var var) env = absId anal var env
451 absEval anal (Lit _) env = AbsTop
452 -- What if an unboxed literal? That's OK: it terminates, so its
453 -- abstract value is AbsTop.
455 -- For absence analysis, a literal certainly isn't the "poison" variable
458 Discussion about \tr{error} (following/quoting Lennart): Any expression
459 \tr{error e} is regarded as bottom (with HBC, with the
460 \tr{-ffail-strict} flag, on with \tr{-O}).
462 Regarding it as bottom gives much better strictness properties for
466 f (x:xs) y = f xs (x+y)
468 f [] _ = error "no match"
470 f (x:xs) y = f xs (x+y)
472 is strict in \tr{y}, which you really want. But, it may lead to
473 transformations that turn a call to \tr{error} into non-termination.
474 (The odds of this happening aren't good.)
477 Things are a little different for absence analysis, because we want
478 to make sure that any poison (?????)
481 absEval StrAnal (Prim SeqOp [TyArg _, e]) env
483 if isBot (absEvalAtom StrAnal e env) then AbsBot else AbsTop
484 -- This is a special case to ensure that seq# is strict in its argument.
485 -- The comments below (for most normal PrimOps) do not apply.
487 absEval StrAnal (Prim op es) env = AbsTop
488 -- The arguments are all of unboxed type, so they will already
489 -- have been eval'd. If the boxed version was bottom, we'll
490 -- already have returned bottom.
492 -- Actually, I believe we are saying that either (1) the
493 -- primOp uses unboxed args and they've been eval'ed, so
494 -- there's no need to force strictness here, _or_ the primOp
495 -- uses boxed args and we don't know whether or not it's
496 -- strict, so we assume laziness. (JSM)
498 absEval AbsAnal (Prim op as) env
499 = if any anyBot [absEvalAtom AbsAnal a env | a <- as, isValArg a]
502 -- For absence analysis, we want to see if the poison shows up...
504 absEval anal (Con con as) env
505 | isProductTyCon (dataConTyCon con)
506 = --pprTrace "absEval.Con" (cat[ text "con: ", (ppr con), text "args: ", interppSP as]) $
507 AbsProd [absEvalAtom anal a env | a <- as, isValArg a]
509 | otherwise -- Not single-constructor
511 StrAnal -> -- Strictness case: it's easy: it certainly terminates
513 AbsAnal -> -- In the absence case we need to be more
514 -- careful: look to see if there's any
515 -- poison in the components
516 if any anyBot [absEvalAtom AbsAnal a env | a <- as, isValArg a]
522 absEval anal (Lam (ValBinder binder) body) env
523 = AbsFun binder body env
524 absEval anal (Lam other_binder expr) env
525 = absEval anal expr env
526 absEval anal (App f a) env | isValArg a
527 = absApply anal (absEval anal f env) (absEvalAtom anal a env)
528 absEval anal (App expr _) env
529 = absEval anal expr env
532 For primitive cases, just GLB the branches, then LUB with the expr part.
535 absEval anal (Case expr (PrimAlts alts deflt)) env
537 expr_val = absEval anal expr env
538 abs_alts = [ absEval anal rhs env | (_, rhs) <- alts ]
539 -- PrimAlts don't bind anything, so no need
540 -- to extend the environment
542 abs_deflt = absEvalDefault anal expr_val deflt env
544 combineCaseValues anal expr_val
545 (abs_deflt ++ abs_alts)
547 absEval anal (Case expr (AlgAlts alts deflt)) env
549 expr_val = absEval anal expr env
550 abs_alts = [ absEvalAlgAlt anal expr_val alt env | alt <- alts ]
551 abs_deflt = absEvalDefault anal expr_val deflt env
555 combineCaseValues anal expr_val
556 (abs_deflt ++ abs_alts)
561 _ -> pprTrace "absCase:ABS:" (($$) (hsep [ppr expr, ppr result, ppr expr_val, ppr abs_deflt, ppr abs_alts]) (ppr (keysFM env `zip` eltsFM env)))
567 For @Lets@ we widen the value we get. This is nothing to
568 do with fixpointing. The reason is so that we don't get an explosion
569 in the amount of computation. For example, consider:
581 If we bind @f@ and @g@ to their exact abstract value, then we'll
582 ``execute'' one call to @f@ and {\em two} calls to @g@. This can blow
583 up exponentially. Widening cuts it off by making a fixed
584 approximation to @f@ and @g@, so that the bodies of @f@ and @g@ are
585 not evaluated again at all when they are called.
587 Of course, this can lose useful joint strictness, which is sad. An
588 alternative approach would be to try with a certain amount of ``fuel''
589 and be prepared to bale out.
592 absEval anal (Let (NonRec binder e1) e2) env
594 new_env = addOneToAbsValEnv env binder (widen anal (absEval anal e1 env))
596 -- The binder of a NonRec should *not* be of unboxed type,
597 -- hence no need to strictly evaluate the Rhs.
598 absEval anal e2 new_env
600 absEval anal (Let (Rec pairs) body) env
602 (binders,rhss) = unzip pairs
603 rhs_vals = cheapFixpoint anal binders rhss env -- Returns widened values
604 new_env = growAbsValEnvList env (binders `zip` rhs_vals)
606 absEval anal body new_env
608 absEval anal (Note note expr) env = absEval anal expr env
612 absEvalAlgAlt :: AnalysisKind -> AbsVal -> (Id,[Id],CoreExpr) -> AbsValEnv -> AbsVal
614 absEvalAlgAlt anal (AbsProd arg_vals) (con, args, rhs) env
615 = -- The scrutinee is a product value, so it must be of a single-constr
616 -- type; so the constructor in this alternative must be the right one
617 -- so we can go ahead and bind the constructor args to the components
618 -- of the product value.
619 ASSERT(length arg_vals == length args)
621 new_env = growAbsValEnvList env (args `zip` arg_vals)
623 absEval anal rhs new_env
625 absEvalAlgAlt anal other_scrutinee (con, args, rhs) env
626 = -- Scrutinised value is Top or Bot (it can't be a function!)
627 -- So just evaluate the rhs with all constr args bound to Top.
628 -- (If the scrutinee is Top we'll never evaluated this function
631 absEval anal rhs rhs_env
633 rhs_env = growAbsValEnvList env (args `zip` repeat AbsTop)
634 -- We must extend the environment, because
635 -- there might be shadowing
638 = case other_scrutinee of {
639 AbsTop -> True; -- i.e., OK
640 AbsBot -> True; -- ditto
641 _ -> False -- party over
645 absEvalDefault :: AnalysisKind
646 -> AbsVal -- Value of scrutinee
649 -> [AbsVal] -- Empty or singleton
651 absEvalDefault anal scrut_val NoDefault env = []
652 absEvalDefault anal scrut_val (BindDefault binder expr) env
653 = [absEval anal expr (addOneToAbsValEnv env binder scrut_val)]
656 %************************************************************************
658 \subsection[absApply]{Apply an abstract function to an abstract argument}
660 %************************************************************************
665 absApply :: AnalysisKind -> AbsVal -> AbsVal -> AbsVal
667 absApply anal AbsBot arg = AbsBot
668 -- AbsBot represents the abstract bottom *function* too
670 absApply StrAnal AbsTop arg = AbsTop
671 absApply AbsAnal AbsTop arg = if anyBot arg
674 -- To be conservative, we have to assume that a function about
675 -- which we know nothing (AbsTop) might look at some part of
679 An @AbsFun@ with only one more argument needed---bind it and eval the
680 result. A @Lam@ with two or more args: return another @AbsFun@ with
681 an augmented environment.
684 absApply anal (AbsFun binder body env) arg
685 = absEval anal body (addOneToAbsValEnv env binder arg)
689 absApply StrAnal (AbsApproxFun demand val) arg
690 = if evalStrictness demand arg
694 absApply AbsAnal (AbsApproxFun demand val) arg
695 = if evalAbsence demand arg
700 absApply anal f@(AbsProd _) arg = pprPanic ("absApply: Duff function: AbsProd." ++ show anal) ((ppr f) <+> (ppr arg))
707 %************************************************************************
709 \subsection[findStrictness]{Determine some binders' strictness}
711 %************************************************************************
713 @findStrictness@ applies the function \tr{\ ids -> expr} to
714 \tr{[bot,top,top,...]}, \tr{[top,bot,top,top,...]}, etc., (i.e., once
715 with @AbsBot@ in each argument position), and evaluates the resulting
716 abstract value; it returns a vector of @Demand@s saying whether the
717 result of doing this is guaranteed to be bottom. This tells the
718 strictness of the function in each of the arguments.
720 If an argument is of unboxed type, then we declare that function to be
721 strict in that argument.
723 We don't really have to make up all those lists of mostly-@AbsTops@;
724 unbound variables in an @AbsValEnv@ are implicitly mapped to that.
726 See notes on @addStrictnessInfoToId@.
729 findStrictness :: [Type] -- Types of args in which strictness is wanted
730 -> AbsVal -- Abstract strictness value of function
731 -> AbsVal -- Abstract absence value of function
732 -> [Demand] -- Resulting strictness annotation
734 findStrictness [] str_val abs_val = []
736 findStrictness (ty:tys) str_val abs_val
738 demand = findRecDemand str_fn abs_fn ty
739 str_fn val = absApply StrAnal str_val val
740 abs_fn val = absApply AbsAnal abs_val val
742 demands = findStrictness tys
743 (absApply StrAnal str_val AbsTop)
744 (absApply AbsAnal abs_val AbsTop)
751 findDemandStrOnly str_env expr binder -- Only strictness environment available
752 = findRecDemand str_fn abs_fn (idType binder)
754 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
755 abs_fn val = AbsBot -- Always says poison; so it looks as if
756 -- nothing is absent; safe
758 findDemandAbsOnly abs_env expr binder -- Only absence environment available
759 = findRecDemand str_fn abs_fn (idType binder)
761 str_fn val = AbsBot -- Always says non-termination;
762 -- that'll make findRecDemand peer into the
763 -- structure of the value.
764 abs_fn val = absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)
767 findDemand str_env abs_env expr binder
768 = findRecDemand str_fn abs_fn (idType binder)
770 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
771 abs_fn val = absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)
774 @findRecDemand@ is where we finally convert strictness/absence info
775 into ``Demands'' which we can pin on Ids (etc.).
777 NOTE: What do we do if something is {\em both} strict and absent?
778 Should \tr{f x y z = error "foo"} says that \tr{f}'s arguments are all
779 strict (because of bottoming effect of \tr{error}) or all absent
780 (because they're not used)?
782 Well, for practical reasons, we prefer absence over strictness. In
783 particular, it makes the ``default defaults'' for class methods (the
784 ones that say \tr{defm.foo dict = error "I don't exist"}) come out
785 nicely [saying ``the dict isn't used''], rather than saying it is
786 strict in every component of the dictionary [massive gratuitious
787 casing to take the dict apart].
789 But you could have examples where going for strictness would be better
790 than absence. Consider:
792 let x = something big
797 If \tr{x} is marked absent in \tr{f}, but not strict, and \tr{g} is
798 lazy, then the thunk for \tr{x} will be built. If \tr{f} was strict,
799 then we'd let-to-case it:
801 case something big of
807 findRecDemand :: (AbsVal -> AbsVal) -- The strictness function
808 -> (AbsVal -> AbsVal) -- The absence function
809 -> Type -- The type of the argument
812 findRecDemand str_fn abs_fn ty
813 = if isUnpointedType ty then -- It's a primitive type!
816 else if not (anyBot (abs_fn AbsBot)) then -- It's absent
817 -- We prefer absence over strictness: see NOTE above.
820 else if not (opt_AllStrict ||
821 (opt_NumbersStrict && is_numeric_type ty) ||
822 (isBot (str_fn AbsBot))) then
823 WwLazy False -- It's not strict and we're not pretending
825 else -- It's strict (or we're pretending it is)!
827 case (splitAlgTyConApp_maybe ty) of
831 Just (tycon,tycon_arg_tys,[data_con]) | isProductTyCon tycon ->
832 -- Non-recursive, single constructor case
834 cmpnt_tys = dataConArgTys data_con tycon_arg_tys
835 prod_len = length cmpnt_tys
838 if isNewTyCon tycon then -- A newtype!
839 ASSERT( null (tail cmpnt_tys) )
841 demand = findRecDemand str_fn abs_fn (head cmpnt_tys)
843 case demand of -- No point in unpacking unless there is more to see inside
844 WwUnpack _ _ _ -> wwUnpackNew demand
852 str_fn (mkMainlyTopProd prod_len i cmpnt_val)
855 abs_fn (mkMainlyTopProd prod_len i cmpnt_val)
858 | (cmpnt_ty, i) <- cmpnt_tys `zip` [1..] ]
860 if null compt_strict_infos then
861 if isEnumerationTyCon tycon then wwEnum else wwStrict
863 wwUnpackData compt_strict_infos
866 -- Multi-constr data types, *or* an abstract data
867 -- types, *or* things we don't have a way of conveying
868 -- the info over module boundaries (class ops,
869 -- superdict sels, dfns).
870 if isEnumerationTyCon tycon then
876 = case (splitAlgTyConApp_maybe ty) of -- NB: duplicates stuff done above
880 [intTyCon, integerTyCon,
881 doubleTyCon, floatTyCon,
882 wordTyCon, addrTyCon]
884 _{-something else-} -> False
886 is_elem = isIn "is_numeric_type"
888 -- mkMainlyTopProd: make an AbsProd that is all AbsTops ("n"-1 of
889 -- them) except for a given value in the "i"th position.
891 mkMainlyTopProd :: Int -> Int -> AbsVal -> AbsVal
893 mkMainlyTopProd n i val
895 befores = nOfThem (i-1) AbsTop
896 afters = nOfThem (n-i) AbsTop
898 AbsProd (befores ++ (val : afters))
901 %************************************************************************
903 \subsection[fixpoint]{Fixpointer for the strictness analyser}
905 %************************************************************************
907 The @fixpoint@ functions take a list of \tr{(binder, expr)} pairs, an
908 environment, and returns the abstract value of each binder.
910 The @cheapFixpoint@ function makes a conservative approximation,
911 by binding each of the variables to Top in their own right hand sides.
912 That allows us to make rapid progress, at the cost of a less-than-wonderful
916 cheapFixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
918 cheapFixpoint AbsAnal [id] [rhs] env
919 = [crudeAbsWiden (absEval AbsAnal rhs new_env)]
921 new_env = addOneToAbsValEnv env id AbsTop -- Unsafe starting point!
922 -- In the just-one-binding case, we guarantee to
923 -- find a fixed point in just one iteration,
924 -- because we are using only a two-point domain.
925 -- This improves matters in cases like:
927 -- f x y = letrec g = ...g...
930 -- Here, y isn't used at all, but if g is bound to
931 -- AbsBot we simply get AbsBot as the next
934 cheapFixpoint anal ids rhss env
935 = [widen anal (absEval anal rhs new_env) | rhs <- rhss]
936 -- We do just one iteration, starting from a safe
937 -- approximation. This won't do a good job in situations
939 -- \x -> letrec f = ...g...
943 -- Here, f will end up bound to Top after one iteration,
944 -- and hence we won't spot the strictness in x.
945 -- (A second iteration would solve this. ToDo: try the effect of
946 -- really searching for a fixed point.)
948 new_env = growAbsValEnvList env [(id,safe_val) | id <- ids]
951 = case anal of -- The safe starting point
957 mkLookupFun :: (key -> key -> Bool) -- Equality predicate
958 -> (key -> key -> Bool) -- Less-than predicate
959 -> [(key,val)] -- The assoc list
961 -> Maybe val -- The corresponding value
963 mkLookupFun eq lt alist s
964 = case [a | (s',a) <- alist, s' `eq` s] of
970 fixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
972 fixpoint anal [] _ env = []
974 fixpoint anal ids rhss env
975 = fix_loop initial_vals
978 = case anal of -- The (unsafe) starting point
979 StrAnal -> if (returnsRealWorld (idType id))
980 then AbsTop -- this is a massively horrible hack (SLPJ 95/05)
984 initial_vals = [ initial_val id | id <- ids ]
986 fix_loop :: [AbsVal] -> [AbsVal]
988 fix_loop current_widened_vals
990 new_env = growAbsValEnvList env (ids `zip` current_widened_vals)
991 new_vals = [ absEval anal rhs new_env | rhs <- rhss ]
992 new_widened_vals = map (widen anal) new_vals
994 if (and (zipWith sameVal current_widened_vals new_widened_vals)) then
997 -- NB: I was too chicken to make that a zipWithEqual,
998 -- lest I jump into a black hole. WDP 96/02
1000 -- Return the widened values. We might get a slightly
1001 -- better value by returning new_vals (which we used to
1002 -- do, see below), but alas that means that whenever the
1003 -- function is called we have to re-execute it, which is
1008 -- Return the un-widened values which may be a bit better
1009 -- than the widened ones, and are guaranteed safe, since
1010 -- they are one iteration beyond current_widened_vals,
1011 -- which itself is a fixed point.
1013 fix_loop new_widened_vals
1016 For absence analysis, we make do with a very very simple approach:
1017 look for convergence in a two-point domain.
1019 We used to use just one iteration, starting with the variables bound
1020 to @AbsBot@, which is safe.
1022 Prior to that, we used one iteration starting from @AbsTop@ (which
1023 isn't safe). Why isn't @AbsTop@ safe? Consider:
1031 Here, if p is @AbsBot@, then we'd better {\em not} end up with a ``fixed
1032 point'' of @d@ being @(AbsTop, AbsTop)@! An @AbsBot@ initial value is
1033 safe because it gives poison more often than really necessary, and
1034 thus may miss some absence, but will never claim absence when it ain't
1037 Anyway, one iteration starting with everything bound to @AbsBot@ give
1042 Here, f would always end up bound to @AbsBot@, which ain't very
1043 clever, because then it would introduce poison whenever it was
1044 applied. Much better to start with f bound to @AbsTop@, and widen it
1045 to @AbsBot@ if any poison shows up. In effect we look for convergence
1046 in the two-point @AbsTop@/@AbsBot@ domain.
1048 What we miss (compared with the cleverer strictness analysis) is
1049 spotting that in this case
1051 f = \ x y -> ...y...(f x y')...
1053 \tr{x} is actually absent, since it is only passed round the loop, never
1054 used. But who cares about missing that?
1056 NB: despite only having a two-point domain, we may still have many
1057 iterations, because there are several variables involved at once.