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 PrimOp ( primOpStrictness )
22 import Id ( Id, idType, getIdStrictness, getIdUnfolding )
23 import Const ( Con(..) )
24 import DataCon ( dataConTyCon, splitProductType_maybe )
25 import IdInfo ( StrictnessInfo(..) )
26 import Demand ( Demand(..), wwPrim, wwStrict, wwEnum, wwUnpackData,
29 import TyCon ( isProductTyCon, isEnumerationTyCon, isNewTyCon )
30 import BasicTypes ( NewOrData(..) )
31 import Type ( splitAlgTyConApp_maybe,
32 isUnLiftedType, Type )
33 import TyCon ( tyConUnique )
34 import PrelInfo ( numericTyKeys )
35 import Util ( isIn, nOfThem, zipWithEqual )
39 %************************************************************************
41 \subsection[AbsVal-ops]{Operations on @AbsVals@}
43 %************************************************************************
45 Least upper bound, greatest lower bound.
48 lub, glb :: AbsVal -> AbsVal -> AbsVal
50 lub val1 val2 | isBot val1 = val2 -- The isBot test includes the case where
51 lub val1 val2 | isBot val2 = val1 -- one of the val's is a function which
52 -- always returns bottom, such as \y.x,
53 -- when x is bound to bottom.
55 lub (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "lub" lub xs ys)
57 lub _ _ = AbsTop -- Crude, but conservative
58 -- The crudity only shows up if there
59 -- are functions involved
61 -- Slightly funny glb; for absence analysis only;
62 -- AbsBot is the safe answer.
64 -- Using anyBot rather than just testing for AbsBot is important.
69 -- g = \x y z -> case x of
73 -- Now, the abstract value of the branches of the case will be an
74 -- AbsFun, but when testing for z's absence we want to spot that it's
75 -- an AbsFun which can't possibly return AbsBot. So when glb'ing we
76 -- mustn't be too keen to bale out and return AbsBot; the anyBot test
77 -- spots that (f x) can't possibly return AbsBot.
79 -- We have also tripped over the following interesting case:
84 -- Now, suppose f is bound to AbsTop. Does this expression mention z?
85 -- Obviously not. But the case will take the glb of AbsTop (for f) and
86 -- an AbsFun (for \y->1). We should not bale out and give AbsBot, because
87 -- that would say that it *does* mention z (or anything else for that matter).
88 -- Nor can we always return AbsTop, because the AbsFun might be something
89 -- like (\y->z), which obviously does mention z. The point is that we're
90 -- glbing two functions, and AbsTop is not actually the top of the function
91 -- lattice. It is more like (\xyz -> x|y|z); that is, AbsTop returns
92 -- poison iff any of its arguments do.
94 -- Deal with functions specially, because AbsTop isn't the
95 -- top of their domain.
98 | is_fun v1 || is_fun v2
99 = if not (anyBot v1) && not (anyBot v2)
105 is_fun (AbsFun _ _ _) = True
106 is_fun (AbsApproxFun _ _) = True -- Not used, but the glb works ok
109 -- The non-functional cases are quite straightforward
111 glb (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "glb" glb xs ys)
116 glb _ _ = AbsBot -- Be pessimistic
119 @isBot@ returns True if its argument is (a representation of) bottom. The
120 ``representation'' part is because we need to detect the bottom {\em function}
121 too. To detect the bottom function, bind its args to top, and see if it
124 Used only in strictness analysis:
126 isBot :: AbsVal -> Bool
129 isBot other = False -- Functions aren't bottom any more
133 Used only in absence analysis:
135 anyBot :: AbsVal -> Bool
137 anyBot AbsBot = True -- poisoned!
138 anyBot AbsTop = False
139 anyBot (AbsProd vals) = any anyBot vals
140 anyBot (AbsFun bndr body env) = anyBot (absEval AbsAnal body (addOneToAbsValEnv env bndr AbsTop))
141 anyBot (AbsApproxFun _ val) = anyBot val
144 @widen@ takes an @AbsVal@, $val$, and returns and @AbsVal@ which is
145 approximated by $val$. Furthermore, the result has no @AbsFun@s in
146 it, so it can be compared for equality by @sameVal@.
149 widen :: AnalysisKind -> AbsVal -> AbsVal
151 -- Widening is complicated by the fact that funtions are lifted
152 widen StrAnal the_fn@(AbsFun bndr body env)
153 = case widened_body of
154 AbsApproxFun ds val -> AbsApproxFun (d : ds) val
156 d = findRecDemand str_fn abs_fn bndr_ty
157 str_fn val = foldl (absApply StrAnal) the_fn
158 (val : [AbsTop | d <- ds])
160 other -> AbsApproxFun [d] widened_body
162 d = findRecDemand str_fn abs_fn bndr_ty
163 str_fn val = absApply StrAnal the_fn val
165 bndr_ty = idType bndr
166 widened_body = widen StrAnal (absApply StrAnal the_fn AbsTop)
167 abs_fn val = AbsBot -- Always says poison; so it looks as if
168 -- nothing is absent; safe
171 This stuff is now instead handled neatly by the fact that AbsApproxFun
172 contains an AbsVal inside it. SLPJ Jan 97
174 | isBot abs_body = AbsBot
175 -- It's worth checking for a function which is unconditionally
178 -- f x y = let g y = case x of ...
179 -- in (g ..) + (g ..)
181 -- Here, when we are considering strictness of f in x, we'll
182 -- evaluate the body of f with x bound to bottom. The current
183 -- strategy is to bind g to its *widened* value; without the isBot
184 -- (...) test above, we'd bind g to an AbsApproxFun, and deliver
185 -- Top, not Bot as the value of f's rhs. The test spots the
186 -- unconditional bottom-ness of g when x is bottom. (Another
187 -- alternative here would be to bind g to its exact abstract
188 -- value, but that entails lots of potential re-computation, at
189 -- every application of g.)
192 widen StrAnal (AbsProd vals) = AbsProd (map (widen StrAnal) vals)
193 widen StrAnal other_val = other_val
196 widen AbsAnal the_fn@(AbsFun bndr body env)
197 | anyBot widened_body = AbsBot
198 -- In the absence-analysis case it's *essential* to check
199 -- that the function has no poison in its body. If it does,
200 -- anywhere, then the whole function is poisonous.
203 = case widened_body of
204 AbsApproxFun ds val -> AbsApproxFun (d : ds) val
206 d = findRecDemand str_fn abs_fn bndr_ty
207 abs_fn val = foldl (absApply AbsAnal) the_fn
208 (val : [AbsTop | d <- ds])
210 other -> AbsApproxFun [d] widened_body
212 d = findRecDemand str_fn abs_fn bndr_ty
213 abs_fn val = absApply AbsAnal the_fn val
215 bndr_ty = idType bndr
216 widened_body = widen AbsAnal (absApply AbsAnal the_fn AbsTop)
217 str_fn val = AbsBot -- Always says non-termination;
218 -- that'll make findRecDemand peer into the
219 -- structure of the value.
221 widen AbsAnal (AbsProd vals) = AbsProd (map (widen AbsAnal) vals)
223 -- It's desirable to do a good job of widening for product
227 -- in ...(case p of (x,y) -> x)...
229 -- Now, is y absent in this expression? Currently the
230 -- analyser widens p before looking at p's scope, to avoid
231 -- lots of recomputation in the case where p is a function.
232 -- So if widening doesn't have a case for products, we'll
233 -- widen p to AbsBot (since when searching for absence in y we
234 -- bind y to poison ie AbsBot), and now we are lost.
236 widen AbsAnal other_val = other_val
238 -- WAS: if anyBot val then AbsBot else AbsTop
239 -- Nowadays widen is doing a better job on functions for absence analysis.
242 @crudeAbsWiden@ is used just for absence analysis, and always
243 returns AbsTop or AbsBot, so it widens to a two-point domain
246 crudeAbsWiden :: AbsVal -> AbsVal
247 crudeAbsWiden val = if anyBot val then AbsBot else AbsTop
250 @sameVal@ compares two abstract values for equality. It can't deal with
251 @AbsFun@, but that should have been removed earlier in the day by @widen@.
254 sameVal :: AbsVal -> AbsVal -> Bool -- Can't handle AbsFun!
257 sameVal (AbsFun _ _ _) _ = panic "sameVal: AbsFun: arg1"
258 sameVal _ (AbsFun _ _ _) = panic "sameVal: AbsFun: arg2"
261 sameVal AbsBot AbsBot = True
262 sameVal AbsBot other = False -- widen has reduced AbsFun bots to AbsBot
264 sameVal AbsTop AbsTop = True
265 sameVal AbsTop other = False -- Right?
267 sameVal (AbsProd vals1) (AbsProd vals2) = and (zipWithEqual "sameVal" sameVal vals1 vals2)
268 sameVal (AbsProd _) AbsTop = False
269 sameVal (AbsProd _) AbsBot = False
271 sameVal (AbsApproxFun str1 v1) (AbsApproxFun str2 v2) = str1 == str2 && sameVal v1 v2
272 sameVal (AbsApproxFun _ _) AbsTop = False
273 sameVal (AbsApproxFun _ _) AbsBot = False
275 sameVal val1 val2 = panic "sameVal: type mismatch or AbsFun encountered"
279 @evalStrictness@ compares a @Demand@ with an abstract value, returning
280 @True@ iff the abstract value is {\em less defined} than the demand.
281 (@True@ is the exciting answer; @False@ is always safe.)
284 evalStrictness :: Demand
286 -> Bool -- True iff the value is sure
287 -- to be less defined than the Demand
289 evalStrictness (WwLazy _) _ = False
290 evalStrictness WwStrict val = isBot val
291 evalStrictness WwEnum val = isBot val
293 evalStrictness (WwUnpack NewType _ (demand:_)) val
294 = evalStrictness demand val
296 evalStrictness (WwUnpack DataType _ demand_info) val
300 AbsProd vals -> or (zipWithEqual "evalStrictness" evalStrictness demand_info vals)
301 _ -> pprTrace "evalStrictness?" empty False
303 evalStrictness WwPrim val
306 AbsBot -> True -- Can happen: consider f (g x), where g is a
307 -- recursive function returning an Int# that diverges
309 other -> pprPanic "evalStrictness: WwPrim:" (ppr other)
312 For absence analysis, we're interested in whether "poison" in the
313 argument (ie a bottom therein) can propagate to the result of the
314 function call; that is, whether the specified demand can {\em
315 possibly} hit poison.
318 evalAbsence (WwLazy True) _ = False -- Can't possibly hit poison
319 -- with Absent demand
321 evalAbsence (WwUnpack NewType _ (demand:_)) val
322 = evalAbsence demand val
324 evalAbsence (WwUnpack DataType _ demand_info) val
326 AbsTop -> False -- No poison in here
327 AbsBot -> True -- Pure poison
328 AbsProd vals -> or (zipWithEqual "evalAbsence" evalAbsence demand_info vals)
329 _ -> panic "evalAbsence: other"
331 evalAbsence other val = anyBot val
332 -- The demand is conservative; even "Lazy" *might* evaluate the
333 -- argument arbitrarily so we have to look everywhere for poison
336 %************************************************************************
338 \subsection[absEval]{Evaluate an expression in the abstract domain}
340 %************************************************************************
343 -- The isBottomingId stuf is now dealt with via the Id's strictness info
344 -- absId anal var env | isBottomingId var
346 -- StrAnal -> AbsBot -- See discussion below
347 -- AbsAnal -> AbsTop -- Just want to see if there's any poison in
351 = case (lookupAbsValEnv env var, getIdStrictness var, maybeUnfoldingTemplate (getIdUnfolding var)) of
353 (Just abs_val, _, _) ->
354 abs_val -- Bound in the environment
356 (Nothing, NoStrictnessInfo, Just unfolding) ->
357 -- We have an unfolding for the expr
358 -- Assume the unfolding has no free variables since it
359 -- came from inside the Id
360 absEval anal unfolding env
361 -- Notice here that we only look in the unfolding if we don't
362 -- have strictness info (an unusual situation).
363 -- We could have chosen to look in the unfolding if it exists,
364 -- and only try the strictness info if it doesn't, and that would
365 -- give more accurate results, at the cost of re-abstract-interpreting
366 -- the unfolding every time.
367 -- We found only one place where the look-at-unfolding-first
368 -- method gave better results, which is in the definition of
369 -- showInt in the Prelude. In its defintion, fromIntegral is
370 -- not inlined (it's big) but ab-interp-ing its unfolding gave
371 -- a better result than looking at its strictness only.
372 -- showInt :: Integral a => a -> [Char] -> [Char]
373 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
374 -- "U(U(U(U(SA)AAAAAAAAL)AA)AAAAASAAASA)" {...} _N_ _N_ #-}
376 -- showInt :: Integral a => a -> [Char] -> [Char]
377 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
378 -- "U(U(U(U(SL)LLLLLLLLL)LL)LLLLLSLLLLL)" _N_ _N_ #-}
381 (Nothing, strictness_info, _) ->
382 -- Includes NoUnfolding
383 -- Try the strictness info
384 absValFromStrictness anal strictness_info
388 absEval :: AnalysisKind -> CoreExpr -> AbsValEnv -> AbsVal
390 absEval anal (Type ty) env = AbsTop
391 absEval anal (Var var) env = absId anal var env
394 Discussion about error (following/quoting Lennart): Any expression
395 'error e' is regarded as bottom (with HBC, with the -ffail-strict
398 Regarding it as bottom gives much better strictness properties for
402 f (x:xs) y = f xs (x+y)
404 f [] _ = error "no match"
406 f (x:xs) y = f xs (x+y)
408 is strict in y, which you really want. But, it may lead to
409 transformations that turn a call to \tr{error} into non-termination.
410 (The odds of this happening aren't good.)
412 Things are a little different for absence analysis, because we want
413 to make sure that any poison (?????)
416 absEval anal (Con (Literal _) args) env
417 = -- Literals terminate (strictness) and are not poison (absence)
420 absEval anal (Con (PrimOp op) args) env
421 = -- Not all PrimOps evaluate all their arguments
422 if or (zipWith (check_arg anal)
423 [absEval anal arg env | arg <- args, isValArg arg]
427 StrAnal | result_bot -> AbsBot
430 (arg_demands, result_bot) = primOpStrictness op
431 check_arg StrAnal arg dmd = evalStrictness dmd arg
432 check_arg AbsAnal arg dmd = evalAbsence dmd arg
434 absEval anal (Con (DataCon con) args) env
435 | isProductTyCon (dataConTyCon con)
436 = -- Products; filter out type arguments
437 AbsProd [absEval anal a env | a <- args, isValArg a]
439 | otherwise -- Not single-constructor
441 StrAnal -> -- Strictness case: it's easy: it certainly terminates
443 AbsAnal -> -- In the absence case we need to be more
444 -- careful: look to see if there's any
445 -- poison in the components
446 if any anyBot [absEval AbsAnal arg env | arg <- args]
452 absEval anal (Lam bndr body) env
453 | isTyVar bndr = absEval anal body env -- Type lambda
454 | otherwise = AbsFun bndr body env -- Value lambda
456 absEval anal (App expr (Type ty)) env
457 = absEval anal expr env -- Type appplication
458 absEval anal (App f val_arg) env
459 = absApply anal (absEval anal f env) -- Value applicationn
460 (absEval anal val_arg env)
464 absEval anal expr@(Case scrut case_bndr alts) env
466 scrut_val = absEval anal scrut env
467 alts_env = addOneToAbsValEnv env case_bndr scrut_val
469 case (scrut_val, alts) of
470 (AbsBot, _) -> AbsBot
472 (AbsProd arg_vals, [(con, bndrs, rhs)])
474 -- The scrutinee is a product value, so it must be of a single-constr
475 -- type; so the constructor in this alternative must be the right one
476 -- so we can go ahead and bind the constructor args to the components
477 -- of the product value.
478 ASSERT(length arg_vals == length val_bndrs)
479 absEval anal rhs rhs_env
481 val_bndrs = filter isId bndrs
482 rhs_env = growAbsValEnvList alts_env (val_bndrs `zip` arg_vals)
484 other -> absEvalAlts anal alts alts_env
487 For @Lets@ we widen the value we get. This is nothing to
488 do with fixpointing. The reason is so that we don't get an explosion
489 in the amount of computation. For example, consider:
501 If we bind @f@ and @g@ to their exact abstract value, then we'll
502 ``execute'' one call to @f@ and {\em two} calls to @g@. This can blow
503 up exponentially. Widening cuts it off by making a fixed
504 approximation to @f@ and @g@, so that the bodies of @f@ and @g@ are
505 not evaluated again at all when they are called.
507 Of course, this can lose useful joint strictness, which is sad. An
508 alternative approach would be to try with a certain amount of ``fuel''
509 and be prepared to bale out.
512 absEval anal (Let (NonRec binder e1) e2) env
514 new_env = addOneToAbsValEnv env binder (widen anal (absEval anal e1 env))
516 -- The binder of a NonRec should *not* be of unboxed type,
517 -- hence no need to strictly evaluate the Rhs.
518 absEval anal e2 new_env
520 absEval anal (Let (Rec pairs) body) env
522 (binders,rhss) = unzip pairs
523 rhs_vals = cheapFixpoint anal binders rhss env -- Returns widened values
524 new_env = growAbsValEnvList env (binders `zip` rhs_vals)
526 absEval anal body new_env
528 absEval anal (Note note expr) env = absEval anal expr env
532 absEvalAlts :: AnalysisKind -> [CoreAlt] -> AbsValEnv -> AbsVal
533 absEvalAlts anal alts env
534 = combine anal (map go alts)
536 combine StrAnal = foldr1 lub -- Diverge only if all diverge
537 combine AbsAnal = foldr1 glb -- Find any poison
540 = absEval anal rhs rhs_env
542 rhs_env = growAbsValEnvList env (filter isId bndrs `zip` repeat AbsTop)
545 %************************************************************************
547 \subsection[absApply]{Apply an abstract function to an abstract argument}
549 %************************************************************************
554 absApply :: AnalysisKind -> AbsVal -> AbsVal -> AbsVal
556 absApply anal AbsBot arg = AbsBot
557 -- AbsBot represents the abstract bottom *function* too
559 absApply StrAnal AbsTop arg = AbsTop
560 absApply AbsAnal AbsTop arg = if anyBot arg
563 -- To be conservative, we have to assume that a function about
564 -- which we know nothing (AbsTop) might look at some part of
568 An @AbsFun@ with only one more argument needed---bind it and eval the
569 result. A @Lam@ with two or more args: return another @AbsFun@ with
570 an augmented environment.
573 absApply anal (AbsFun binder body env) arg
574 = absEval anal body (addOneToAbsValEnv env binder arg)
578 absApply StrAnal (AbsApproxFun (d:ds) val) arg
581 other -> AbsApproxFun ds val' -- Result is non-bot if there are still args
583 val' | evalStrictness d arg = AbsBot
586 absApply AbsAnal (AbsApproxFun (d:ds) val) arg
587 = if evalAbsence d arg
588 then AbsBot -- Poison in arg means poison in the application
591 other -> AbsApproxFun ds val
594 absApply anal f@(AbsProd _) arg = pprPanic ("absApply: Duff function: AbsProd." ++ show anal) ((ppr f) <+> (ppr arg))
601 %************************************************************************
603 \subsection[findStrictness]{Determine some binders' strictness}
605 %************************************************************************
607 @findStrictness@ applies the function \tr{\ ids -> expr} to
608 \tr{[bot,top,top,...]}, \tr{[top,bot,top,top,...]}, etc., (i.e., once
609 with @AbsBot@ in each argument position), and evaluates the resulting
610 abstract value; it returns a vector of @Demand@s saying whether the
611 result of doing this is guaranteed to be bottom. This tells the
612 strictness of the function in each of the arguments.
614 If an argument is of unboxed type, then we declare that function to be
615 strict in that argument.
617 We don't really have to make up all those lists of mostly-@AbsTops@;
618 unbound variables in an @AbsValEnv@ are implicitly mapped to that.
620 See notes on @addStrictnessInfoToId@.
623 findStrictness :: [Type] -- Types of args in which strictness is wanted
624 -> AbsVal -- Abstract strictness value of function
625 -> AbsVal -- Abstract absence value of function
626 -> ([Demand], Bool) -- Resulting strictness annotation
628 findStrictness tys str_val abs_val
629 = (map find_str tys_w_index, isBot (foldl (absApply StrAnal) str_val all_tops))
631 tys_w_index = tys `zip` [(1::Int) ..]
633 find_str (ty,n) = findRecDemand str_fn abs_fn ty
635 str_fn val = foldl (absApply StrAnal) str_val
636 (map (mk_arg val n) tys_w_index)
638 abs_fn val = foldl (absApply AbsAnal) abs_val
639 (map (mk_arg val n) tys_w_index)
641 mk_arg val n (_,m) | m==n = val
644 all_tops = [AbsTop | _ <- tys]
649 findDemand str_env abs_env expr binder
650 = findRecDemand str_fn abs_fn (idType binder)
652 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
653 abs_fn val = absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)
655 findDemandAlts str_env abs_env alts binder
656 = findRecDemand str_fn abs_fn (idType binder)
658 str_fn val = absEvalAlts StrAnal alts (addOneToAbsValEnv str_env binder val)
659 abs_fn val = absEvalAlts AbsAnal alts (addOneToAbsValEnv abs_env binder val)
662 @findRecDemand@ is where we finally convert strictness/absence info
663 into ``Demands'' which we can pin on Ids (etc.).
665 NOTE: What do we do if something is {\em both} strict and absent?
666 Should \tr{f x y z = error "foo"} says that \tr{f}'s arguments are all
667 strict (because of bottoming effect of \tr{error}) or all absent
668 (because they're not used)?
670 Well, for practical reasons, we prefer absence over strictness. In
671 particular, it makes the ``default defaults'' for class methods (the
672 ones that say \tr{defm.foo dict = error "I don't exist"}) come out
673 nicely [saying ``the dict isn't used''], rather than saying it is
674 strict in every component of the dictionary [massive gratuitious
675 casing to take the dict apart].
677 But you could have examples where going for strictness would be better
678 than absence. Consider:
680 let x = something big
685 If \tr{x} is marked absent in \tr{f}, but not strict, and \tr{g} is
686 lazy, then the thunk for \tr{x} will be built. If \tr{f} was strict,
687 then we'd let-to-case it:
689 case something big of
695 findRecDemand :: (AbsVal -> AbsVal) -- The strictness function
696 -> (AbsVal -> AbsVal) -- The absence function
697 -> Type -- The type of the argument
700 findRecDemand str_fn abs_fn ty
701 = if isUnLiftedType ty then -- It's a primitive type!
704 else if not (anyBot (abs_fn AbsBot)) then -- It's absent
705 -- We prefer absence over strictness: see NOTE above.
708 else if not (opt_AllStrict ||
709 (opt_NumbersStrict && is_numeric_type ty) ||
710 (isBot (str_fn AbsBot))) then
711 WwLazy False -- It's not strict and we're not pretending
713 else -- It's strict (or we're pretending it is)!
715 case splitProductType_maybe ty of
717 Nothing -> wwStrict -- Could have a test for wwEnum, but
718 -- we don't exploit it yet, so don't bother
720 Just (tycon,_,data_con,cmpnt_tys) -- Non-recursive, single constructor case
721 | isNewTyCon tycon -- A newtype!
722 -> ASSERT( null (tail cmpnt_tys) )
724 demand = findRecDemand str_fn abs_fn (head cmpnt_tys)
728 | null compt_strict_infos -- A nullary data type
731 | otherwise -- Some other data type
732 -> wwUnpackData compt_strict_infos
735 prod_len = length cmpnt_tys
739 str_fn (mkMainlyTopProd prod_len i cmpnt_val)
742 abs_fn (mkMainlyTopProd prod_len i cmpnt_val)
745 | (cmpnt_ty, i) <- cmpnt_tys `zip` [1..] ]
749 = case (splitAlgTyConApp_maybe ty) of -- NB: duplicates stuff done above
752 | tyConUnique tycon `is_elem` numericTyKeys
754 _{-something else-} -> False
756 is_elem = isIn "is_numeric_type"
758 -- mkMainlyTopProd: make an AbsProd that is all AbsTops ("n"-1 of
759 -- them) except for a given value in the "i"th position.
761 mkMainlyTopProd :: Int -> Int -> AbsVal -> AbsVal
763 mkMainlyTopProd n i val
765 befores = nOfThem (i-1) AbsTop
766 afters = nOfThem (n-i) AbsTop
768 AbsProd (befores ++ (val : afters))
771 %************************************************************************
773 \subsection[fixpoint]{Fixpointer for the strictness analyser}
775 %************************************************************************
777 The @fixpoint@ functions take a list of \tr{(binder, expr)} pairs, an
778 environment, and returns the abstract value of each binder.
780 The @cheapFixpoint@ function makes a conservative approximation,
781 by binding each of the variables to Top in their own right hand sides.
782 That allows us to make rapid progress, at the cost of a less-than-wonderful
786 cheapFixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
788 cheapFixpoint AbsAnal [id] [rhs] env
789 = [crudeAbsWiden (absEval AbsAnal rhs new_env)]
791 new_env = addOneToAbsValEnv env id AbsTop -- Unsafe starting point!
792 -- In the just-one-binding case, we guarantee to
793 -- find a fixed point in just one iteration,
794 -- because we are using only a two-point domain.
795 -- This improves matters in cases like:
797 -- f x y = letrec g = ...g...
800 -- Here, y isn't used at all, but if g is bound to
801 -- AbsBot we simply get AbsBot as the next
804 cheapFixpoint anal ids rhss env
805 = [widen anal (absEval anal rhs new_env) | rhs <- rhss]
806 -- We do just one iteration, starting from a safe
807 -- approximation. This won't do a good job in situations
809 -- \x -> letrec f = ...g...
813 -- Here, f will end up bound to Top after one iteration,
814 -- and hence we won't spot the strictness in x.
815 -- (A second iteration would solve this. ToDo: try the effect of
816 -- really searching for a fixed point.)
818 new_env = growAbsValEnvList env [(id,safe_val) | id <- ids]
821 = case anal of -- The safe starting point
827 mkLookupFun :: (key -> key -> Bool) -- Equality predicate
828 -> (key -> key -> Bool) -- Less-than predicate
829 -> [(key,val)] -- The assoc list
831 -> Maybe val -- The corresponding value
833 mkLookupFun eq lt alist s
834 = case [a | (s',a) <- alist, s' `eq` s] of
840 fixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
842 fixpoint anal [] _ env = []
844 fixpoint anal ids rhss env
845 = fix_loop initial_vals
848 = case anal of -- The (unsafe) starting point
851 -- At one stage for StrAnal we said:
852 -- if (returnsRealWorld (idType id))
853 -- then AbsTop -- this is a massively horrible hack (SLPJ 95/05)
854 -- but no one has the foggiest idea what this hack did,
855 -- and returnsRealWorld was a stub that always returned False
856 -- So this comment is all that is left of the hack!
858 initial_vals = [ initial_val id | id <- ids ]
860 fix_loop :: [AbsVal] -> [AbsVal]
862 fix_loop current_widened_vals
864 new_env = growAbsValEnvList env (ids `zip` current_widened_vals)
865 new_vals = [ absEval anal rhs new_env | rhs <- rhss ]
866 new_widened_vals = map (widen anal) new_vals
868 if (and (zipWith sameVal current_widened_vals new_widened_vals)) then
871 -- NB: I was too chicken to make that a zipWithEqual,
872 -- lest I jump into a black hole. WDP 96/02
874 -- Return the widened values. We might get a slightly
875 -- better value by returning new_vals (which we used to
876 -- do, see below), but alas that means that whenever the
877 -- function is called we have to re-execute it, which is
882 -- Return the un-widened values which may be a bit better
883 -- than the widened ones, and are guaranteed safe, since
884 -- they are one iteration beyond current_widened_vals,
885 -- which itself is a fixed point.
887 fix_loop new_widened_vals
890 For absence analysis, we make do with a very very simple approach:
891 look for convergence in a two-point domain.
893 We used to use just one iteration, starting with the variables bound
894 to @AbsBot@, which is safe.
896 Prior to that, we used one iteration starting from @AbsTop@ (which
897 isn't safe). Why isn't @AbsTop@ safe? Consider:
905 Here, if p is @AbsBot@, then we'd better {\em not} end up with a ``fixed
906 point'' of @d@ being @(AbsTop, AbsTop)@! An @AbsBot@ initial value is
907 safe because it gives poison more often than really necessary, and
908 thus may miss some absence, but will never claim absence when it ain't
911 Anyway, one iteration starting with everything bound to @AbsBot@ give
916 Here, f would always end up bound to @AbsBot@, which ain't very
917 clever, because then it would introduce poison whenever it was
918 applied. Much better to start with f bound to @AbsTop@, and widen it
919 to @AbsBot@ if any poison shows up. In effect we look for convergence
920 in the two-point @AbsTop@/@AbsBot@ domain.
922 What we miss (compared with the cleverer strictness analysis) is
923 spotting that in this case
925 f = \ x y -> ...y...(f x y')...
927 \tr{x} is actually absent, since it is only passed round the loop, never
928 used. But who cares about missing that?
930 NB: despite only having a two-point domain, we may still have many
931 iterations, because there are several variables involved at once.