2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1996
4 \section[SaAbsInt]{Abstract interpreter for strictness analysis}
7 #include "HsVersions.h"
21 import CoreUnfold ( Unfolding(..), UfExpr, RdrName, SimpleUnfolding(..), FormSummary )
22 import CoreUtils ( unTagBinders )
23 import Id ( idType, getIdStrictness, getIdUnfolding,
24 dataConTyCon, dataConArgTys
26 import IdInfo ( StrictnessInfo(..),
27 wwPrim, wwStrict, wwEnum, wwUnpack
29 import Demand ( Demand(..) )
30 import MagicUFs ( MagicUnfoldingFun )
31 import Maybes ( maybeToBool )
32 import Outputable ( Outputable(..){-instance * []-} )
33 import PprStyle ( PprStyle(..) )
34 import Pretty ( ppStr )
35 import PrimOp ( PrimOp(..) )
37 import TyCon ( maybeTyConSingleCon, isEnumerationTyCon,
40 import Type ( maybeAppDataTyConExpandingDicts, isPrimType )
41 import TysWiredIn ( intTyCon, integerTyCon, doubleTyCon,
42 floatTyCon, wordTyCon, addrTyCon
44 import Util ( isIn, isn'tIn, nOfThem, zipWithEqual,
45 pprTrace, panic, pprPanic, assertPanic
48 returnsRealWorld x = False -- ToDo: panic "SaAbsInt.returnsRealWorld (ToDo)"
51 %************************************************************************
53 \subsection[AbsVal-ops]{Operations on @AbsVals@}
55 %************************************************************************
57 Least upper bound, greatest lower bound.
60 lub, glb :: AbsVal -> AbsVal -> AbsVal
62 lub val1 val2 | isBot val1 = val2 -- The isBot test includes the case where
63 lub val1 val2 | isBot val2 = val1 -- one of the val's is a function which
64 -- always returns bottom, such as \y.x,
65 -- when x is bound to bottom.
67 lub (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "lub" lub xs ys)
69 lub _ _ = AbsTop -- Crude, but conservative
70 -- The crudity only shows up if there
71 -- are functions involved
73 -- Slightly funny glb; for absence analysis only;
74 -- AbsBot is the safe answer.
76 -- Using anyBot rather than just testing for AbsBot is important.
81 -- g = \x y z -> case x of
85 -- Now, the abstract value of the branches of the case will be an
86 -- AbsFun, but when testing for z's absence we want to spot that it's
87 -- an AbsFun which can't possibly return AbsBot. So when glb'ing we
88 -- mustn't be too keen to bale out and return AbsBot; the anyBot test
89 -- spots that (f x) can't possibly return AbsBot.
91 -- We have also tripped over the following interesting case:
96 -- Now, suppose f is bound to AbsTop. Does this expression mention z?
97 -- Obviously not. But the case will take the glb of AbsTop (for f) and
98 -- an AbsFun (for \y->1). We should not bale out and give AbsBot, because
99 -- that would say that it *does* mention z (or anything else for that matter).
100 -- Nor can we always return AbsTop, because the AbsFun might be something
101 -- like (\y->z), which obviously does mention z. The point is that we're
102 -- glbing two functions, and AbsTop is not actually the top of the function
103 -- lattice. It is more like (\xyz -> x|y|z); that is, AbsTop returns
104 -- poison iff any of its arguments do.
106 -- Deal with functions specially, because AbsTop isn't the
107 -- top of their domain.
110 | is_fun v1 || is_fun v2
111 = if not (anyBot v1) && not (anyBot v2)
117 is_fun (AbsFun _ _ _) = True
118 is_fun (AbsApproxFun _) = True -- Not used, but the glb works ok
121 -- The non-functional cases are quite straightforward
123 glb (AbsProd xs) (AbsProd ys) = AbsProd (zipWithEqual "glb" glb xs ys)
128 glb _ _ = AbsBot -- Be pessimistic
134 -> AbsVal -- Value of scrutinee
135 -> [AbsVal] -- Value of branches (at least one)
138 -- For strictness analysis, see if the scrutinee is bottom; if so
139 -- return bottom; otherwise, the lub of the branches.
141 combineCaseValues StrAnal AbsBot branches = AbsBot
142 combineCaseValues StrAnal other_scrutinee branches
143 -- Scrutinee can only be AbsBot, AbsProd or AbsTop
144 = ASSERT(ok_scrutinee)
148 = case other_scrutinee of {
149 AbsTop -> True; -- i.e., cool
150 AbsProd _ -> True; -- ditto
151 _ -> False -- party over
154 -- For absence analysis, check if the scrutinee is all poison (isBot)
155 -- If so, return poison (AbsBot); otherwise, any nested poison will come
156 -- out from looking at the branches, so just glb together the branches
157 -- to get the worst one.
159 combineCaseValues AbsAnal AbsBot branches = AbsBot
160 combineCaseValues AbsAnal other_scrutinee branches
161 -- Scrutinee can only be AbsBot, AbsProd or AbsTop
162 = ASSERT(ok_scrutinee)
164 result = foldr1 glb branches
166 tracer = if at_least_one_AbsFun && at_least_one_AbsTop
168 pprTrace "combineCase:" (ppr PprDebug branches)
177 = case other_scrutinee of {
178 AbsTop -> True; -- i.e., cool
179 AbsProd _ -> True; -- ditto
180 _ -> False -- party over
183 at_least_one_AbsFun = foldr ((||) . is_AbsFun) False branches
184 at_least_one_AbsTop = foldr ((||) . is_AbsTop) False branches
185 no_AbsBots = foldr ((&&) . is_not_AbsBot) True branches
187 is_AbsFun x = case x of { AbsFun _ _ _ -> True; _ -> False }
188 is_AbsTop x = case x of { AbsTop -> True; _ -> False }
189 is_not_AbsBot x = case x of { AbsBot -> False; _ -> True }
192 @isBot@ returns True if its argument is (a representation of) bottom. The
193 ``representation'' part is because we need to detect the bottom {\em function}
194 too. To detect the bottom function, bind its args to top, and see if it
197 Used only in strictness analysis:
199 isBot :: AbsVal -> Bool
202 isBot (AbsFun args body env) = isBot (absEval StrAnal body env)
203 -- Don't bother to extend the envt because
204 -- unbound variables default to AbsTop anyway
208 Used only in absence analysis:
210 anyBot :: AbsVal -> Bool
212 anyBot AbsBot = True -- poisoned!
213 anyBot AbsTop = False
214 anyBot (AbsProd vals) = any anyBot vals
215 anyBot (AbsFun args body env) = anyBot (absEval AbsAnal body env)
216 anyBot (AbsApproxFun demands) = False
218 -- AbsApproxFun can only arise in absence analysis from the Demand
219 -- info of an imported value; whatever it is we're looking for is
220 -- certainly not present over in the imported value.
223 @widen@ takes an @AbsVal@, $val$, and returns and @AbsVal@ which is
224 approximated by $val$. Furthermore, the result has no @AbsFun@s in
225 it, so it can be compared for equality by @sameVal@.
228 widen :: AnalysisKind -> AbsVal -> AbsVal
230 widen StrAnal (AbsFun args body env)
231 | isBot (absEval StrAnal body env) = AbsBot
233 = ASSERT (not (null args))
234 AbsApproxFun (map (findDemandStrOnly env body) args)
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.)
252 widen StrAnal (AbsProd vals) = AbsProd (map (widen StrAnal) vals)
253 widen StrAnal other_val = other_val
256 widen AbsAnal (AbsFun args body env)
257 | anyBot (absEval AbsAnal body env) = AbsBot
258 -- In the absence-analysis case it's *essential* to check
259 -- that the function has no poison in its body. If it does,
260 -- anywhere, then the whole function is poisonous.
263 = ASSERT (not (null args))
264 AbsApproxFun (map (findDemandAbsOnly env body) args)
266 widen AbsAnal (AbsProd vals) = AbsProd (map (widen AbsAnal) vals)
268 -- It's desirable to do a good job of widening for product
272 -- in ...(case p of (x,y) -> x)...
274 -- Now, is y absent in this expression? Currently the
275 -- analyser widens p before looking at p's scope, to avoid
276 -- lots of recomputation in the case where p is a function.
277 -- So if widening doesn't have a case for products, we'll
278 -- widen p to AbsBot (since when searching for absence in y we
279 -- bind y to poison ie AbsBot), and now we are lost.
281 widen AbsAnal other_val = other_val
283 -- WAS: if anyBot val then AbsBot else AbsTop
284 -- Nowadays widen is doing a better job on functions for absence analysis.
287 @crudeAbsWiden@ is used just for absence analysis, and always
288 returns AbsTop or AbsBot, so it widens to a two-point domain
291 crudeAbsWiden :: AbsVal -> AbsVal
292 crudeAbsWiden val = if anyBot val then AbsBot else AbsTop
295 @sameVal@ compares two abstract values for equality. It can't deal with
296 @AbsFun@, but that should have been removed earlier in the day by @widen@.
299 sameVal :: AbsVal -> AbsVal -> Bool -- Can't handle AbsFun!
302 sameVal (AbsFun _ _ _) _ = panic "sameVal: AbsFun: arg1"
303 sameVal _ (AbsFun _ _ _) = panic "sameVal: AbsFun: arg2"
306 sameVal AbsBot AbsBot = True
307 sameVal AbsBot other = False -- widen has reduced AbsFun bots to AbsBot
309 sameVal AbsTop AbsTop = True
310 sameVal AbsTop other = False -- Right?
312 sameVal (AbsProd vals1) (AbsProd vals2) = and (zipWithEqual "sameVal" sameVal vals1 vals2)
313 sameVal (AbsProd _) AbsTop = False
314 sameVal (AbsProd _) AbsBot = False
316 sameVal (AbsApproxFun str1) (AbsApproxFun str2) = str1 == str2
317 sameVal (AbsApproxFun _) AbsTop = False
318 sameVal (AbsApproxFun _) AbsBot = False
320 sameVal val1 val2 = panic "sameVal: type mismatch or AbsFun encountered"
324 @evalStrictness@ compares a @Demand@ with an abstract value, returning
325 @True@ iff the abstract value is {\em less defined} than the demand.
326 (@True@ is the exciting answer; @False@ is always safe.)
329 evalStrictness :: Demand
331 -> Bool -- True iff the value is sure
332 -- to be less defined than the Demand
334 evalStrictness (WwLazy _) _ = False
335 evalStrictness WwStrict val = isBot val
336 evalStrictness WwEnum val = isBot val
338 evalStrictness (WwUnpack demand_info) val
342 AbsProd vals -> or (zipWithEqual "evalStrictness" evalStrictness demand_info vals)
343 _ -> trace "evalStrictness?" False
345 evalStrictness WwPrim val
349 other -> -- A primitive value should be defined, never bottom;
350 -- hence this paranoia check
351 pprPanic "evalStrictness: WwPrim:" (ppr PprDebug other)
354 For absence analysis, we're interested in whether "poison" in the
355 argument (ie a bottom therein) can propagate to the result of the
356 function call; that is, whether the specified demand can {\em
357 possibly} hit poison.
360 evalAbsence (WwLazy True) _ = False -- Can't possibly hit poison
361 -- with Absent demand
363 evalAbsence (WwUnpack demand_info) val
365 AbsTop -> False -- No poison in here
366 AbsBot -> True -- Pure poison
367 AbsProd vals -> or (zipWithEqual "evalAbsence" evalAbsence demand_info vals)
368 _ -> panic "evalAbsence: other"
370 evalAbsence other val = anyBot val
371 -- The demand is conservative; even "Lazy" *might* evaluate the
372 -- argument arbitrarily so we have to look everywhere for poison
375 %************************************************************************
377 \subsection[absEval]{Evaluate an expression in the abstract domain}
379 %************************************************************************
382 -- The isBottomingId stuf is now dealt with via the Id's strictness info
383 -- absId anal var env | isBottomingId var
385 -- StrAnal -> AbsBot -- See discussion below
386 -- AbsAnal -> AbsTop -- Just want to see if there's any poison in
392 case (lookupAbsValEnv env var, getIdStrictness var, getIdUnfolding var) of
394 (Just abs_val, _, _) ->
395 abs_val -- Bound in the environment
397 (Nothing, noStrictnessInfo, CoreUnfolding (SimpleUnfolding _ _ unfolding)) ->
398 -- We have an unfolding for the expr
399 -- Assume the unfolding has no free variables since it
400 -- came from inside the Id
401 absEval anal (unTagBinders unfolding) env
402 -- Notice here that we only look in the unfolding if we don't
403 -- have strictness info (an unusual situation).
404 -- We could have chosen to look in the unfolding if it exists,
405 -- and only try the strictness info if it doesn't, and that would
406 -- give more accurate results, at the cost of re-abstract-interpreting
407 -- the unfolding every time.
408 -- We found only one place where the look-at-unfolding-first
409 -- method gave better results, which is in the definition of
410 -- showInt in the Prelude. In its defintion, fromIntegral is
411 -- not inlined (it's big) but ab-interp-ing its unfolding gave
412 -- a better result than looking at its strictness only.
413 -- showInt :: Integral a => a -> [Char] -> [Char]
414 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
415 -- "U(U(U(U(SA)AAAAAAAAL)AA)AAAAASAAASA)" {...} _N_ _N_ #-}
417 -- showInt :: Integral a => a -> [Char] -> [Char]
418 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
419 -- "U(U(U(U(SL)LLLLLLLLL)LL)LLLLLSLLLLL)" _N_ _N_ #-}
422 (Nothing, strictness_info, _) ->
423 -- Includes MagicUnfolding, NoUnfolding
424 -- Try the strictness info
425 absValFromStrictness anal strictness_info
427 -- pprTrace "absId:" (ppBesides [ppr PprDebug var, ppStr "=:", pp_anal anal, ppStr ":=",ppr PprDebug result]) $
430 pp_anal StrAnal = ppStr "STR"
431 pp_anal AbsAnal = ppStr "ABS"
433 absEvalAtom anal (VarArg v) env = absId anal v env
434 absEvalAtom anal (LitArg _) env = AbsTop
438 absEval :: AnalysisKind -> CoreExpr -> AbsValEnv -> AbsVal
440 absEval anal (Var var) env = absId anal var env
442 absEval anal (Lit _) env = AbsTop
443 -- What if an unboxed literal? That's OK: it terminates, so its
444 -- abstract value is AbsTop.
446 -- For absence analysis, a literal certainly isn't the "poison" variable
449 Discussion about \tr{error} (following/quoting Lennart): Any expression
450 \tr{error e} is regarded as bottom (with HBC, with the
451 \tr{-ffail-strict} flag, on with \tr{-O}).
453 Regarding it as bottom gives much better strictness properties for
457 f (x:xs) y = f xs (x+y)
459 f [] _ = error "no match"
461 f (x:xs) y = f xs (x+y)
463 is strict in \tr{y}, which you really want. But, it may lead to
464 transformations that turn a call to \tr{error} into non-termination.
465 (The odds of this happening aren't good.)
468 Things are a little different for absence analysis, because we want
469 to make sure that any poison (?????)
472 absEval StrAnal (Prim SeqOp [TyArg _, e]) env
474 if isBot (absEvalAtom StrAnal e env) then AbsBot else AbsTop
475 -- This is a special case to ensure that seq# is strict in its argument.
476 -- The comments below (for most normal PrimOps) do not apply.
478 absEval StrAnal (Prim op es) env = AbsTop
479 -- The arguments are all of unboxed type, so they will already
480 -- have been eval'd. If the boxed version was bottom, we'll
481 -- already have returned bottom.
483 -- Actually, I believe we are saying that either (1) the
484 -- primOp uses unboxed args and they've been eval'ed, so
485 -- there's no need to force strictness here, _or_ the primOp
486 -- uses boxed args and we don't know whether or not it's
487 -- strict, so we assume laziness. (JSM)
489 absEval AbsAnal (Prim op as) env
490 = if any anyBot [absEvalAtom AbsAnal a env | a <- as, isValArg a]
493 -- For absence analysis, we want to see if the poison shows up...
495 absEval anal (Con con as) env
497 = AbsProd [absEvalAtom anal a env | a <- as, isValArg a]
499 | otherwise -- Not single-constructor
501 StrAnal -> -- Strictness case: it's easy: it certainly terminates
503 AbsAnal -> -- In the absence case we need to be more
504 -- careful: look to see if there's any
505 -- poison in the components
506 if any anyBot [absEvalAtom AbsAnal a env | a <- as, isValArg a]
510 has_single_con = maybeToBool (maybeTyConSingleCon (dataConTyCon con))
514 absEval anal (Lam (ValBinder binder) body) env
515 = AbsFun [binder] body env
516 absEval anal (Lam other_binder expr) env
517 = absEval anal expr env
518 absEval anal (App f a) env | isValArg a
519 = absApply anal (absEval anal f env) (absEvalAtom anal a env)
520 absEval anal (App expr _) env
521 = absEval anal expr env
524 For primitive cases, just GLB the branches, then LUB with the expr part.
527 absEval anal (Case expr (PrimAlts alts deflt)) env
529 expr_val = absEval anal expr env
530 abs_alts = [ absEval anal rhs env | (_, rhs) <- alts ]
531 -- Don't bother to extend envt, because unbound vars
532 -- default to the conservative AbsTop
534 abs_deflt = absEvalDefault anal expr_val deflt env
536 combineCaseValues anal expr_val
537 (abs_deflt ++ abs_alts)
539 absEval anal (Case expr (AlgAlts alts deflt)) env
541 expr_val = absEval anal expr env
542 abs_alts = [ absEvalAlgAlt anal expr_val alt env | alt <- alts ]
543 abs_deflt = absEvalDefault anal expr_val deflt env
547 combineCaseValues anal expr_val
548 (abs_deflt ++ abs_alts)
553 _ -> pprTrace "absCase:ABS:" (ppAbove (ppCat [ppr PprDebug expr, ppr PprDebug result, ppr PprDebug expr_val, ppr PprDebug abs_deflt, ppr PprDebug abs_alts]) (ppr PprDebug (keysFM env `zip` eltsFM env)))
559 For @Lets@ we widen the value we get. This is nothing to
560 do with fixpointing. The reason is so that we don't get an explosion
561 in the amount of computation. For example, consider:
573 If we bind @f@ and @g@ to their exact abstract value, then we'll
574 ``execute'' one call to @f@ and {\em two} calls to @g@. This can blow
575 up exponentially. Widening cuts it off by making a fixed
576 approximation to @f@ and @g@, so that the bodies of @f@ and @g@ are
577 not evaluated again at all when they are called.
579 Of course, this can lose useful joint strictness, which is sad. An
580 alternative approach would be to try with a certain amount of ``fuel''
581 and be prepared to bale out.
584 absEval anal (Let (NonRec binder e1) e2) env
586 new_env = addOneToAbsValEnv env binder (widen anal (absEval anal e1 env))
588 -- The binder of a NonRec should *not* be of unboxed type,
589 -- hence no need to strictly evaluate the Rhs.
590 absEval anal e2 new_env
592 absEval anal (Let (Rec pairs) body) env
594 (binders,rhss) = unzip pairs
595 rhs_vals = cheapFixpoint anal binders rhss env -- Returns widened values
596 new_env = growAbsValEnvList env (binders `zip` rhs_vals)
598 absEval anal body new_env
600 absEval anal (SCC cc expr) env = absEval anal expr env
601 absEval anal (Coerce c ty expr) env = absEval anal expr env
605 absEvalAlgAlt :: AnalysisKind -> AbsVal -> (Id,[Id],CoreExpr) -> AbsValEnv -> AbsVal
607 absEvalAlgAlt anal (AbsProd arg_vals) (con, args, rhs) env
608 = -- The scrutinee is a product value, so it must be of a single-constr
609 -- type; so the constructor in this alternative must be the right one
610 -- so we can go ahead and bind the constructor args to the components
611 -- of the product value.
612 ASSERT(length arg_vals == length args)
614 new_env = growAbsValEnvList env (args `zip` arg_vals)
616 absEval anal rhs new_env
618 absEvalAlgAlt anal other_scrutinee (con, args, rhs) env
619 = -- Scrutinised value is Top or Bot (it can't be a function!)
620 -- So just evaluate the rhs with all constr args bound to Top.
621 -- (If the scrutinee is Top we'll never evaluated this function
627 = case other_scrutinee of {
628 AbsTop -> True; -- i.e., OK
629 AbsBot -> True; -- ditto
630 _ -> False -- party over
634 absEvalDefault :: AnalysisKind
635 -> AbsVal -- Value of scrutinee
638 -> [AbsVal] -- Empty or singleton
640 absEvalDefault anal scrut_val NoDefault env = []
641 absEvalDefault anal scrut_val (BindDefault binder expr) env
642 = [absEval anal expr (addOneToAbsValEnv env binder scrut_val)]
645 %************************************************************************
647 \subsection[absApply]{Apply an abstract function to an abstract argument}
649 %************************************************************************
654 absApply :: AnalysisKind -> AbsVal -> AbsVal -> AbsVal
656 absApply anal AbsBot arg = AbsBot
657 -- AbsBot represents the abstract bottom *function* too
659 absApply StrAnal AbsTop arg = AbsTop
660 absApply AbsAnal AbsTop arg = if anyBot arg
663 -- To be conservative, we have to assume that a function about
664 -- which we know nothing (AbsTop) might look at some part of
668 An @AbsFun@ with only one more argument needed---bind it and eval the
669 result. A @Lam@ with two or more args: return another @AbsFun@ with
670 an augmented environment.
673 absApply anal (AbsFun [binder] body env) arg
674 = absEval anal body (addOneToAbsValEnv env binder arg)
676 absApply anal (AbsFun (binder:bs) body env) arg
677 = AbsFun bs body (addOneToAbsValEnv env binder arg)
681 absApply StrAnal (AbsApproxFun (arg1_demand:ds)) arg
682 = if evalStrictness arg1_demand arg
686 other -> AbsApproxFun ds
688 absApply AbsAnal (AbsApproxFun (arg1_demand:ds)) arg
689 = if evalAbsence arg1_demand arg
693 other -> AbsApproxFun ds
696 absApply anal (AbsApproxFun []) arg = panic ("absApply: Duff function: AbsApproxFun." ++ show anal)
697 absApply anal (AbsFun [] _ _) arg = panic ("absApply: Duff function: AbsFun." ++ show anal)
698 absApply anal (AbsProd _) arg = panic ("absApply: Duff function: AbsProd." ++ show anal)
705 %************************************************************************
707 \subsection[findStrictness]{Determine some binders' strictness}
709 %************************************************************************
711 @findStrictness@ applies the function \tr{\ ids -> expr} to
712 \tr{[bot,top,top,...]}, \tr{[top,bot,top,top,...]}, etc., (i.e., once
713 with @AbsBot@ in each argument position), and evaluates the resulting
714 abstract value; it returns a vector of @Demand@s saying whether the
715 result of doing this is guaranteed to be bottom. This tells the
716 strictness of the function in each of the arguments.
718 If an argument is of unboxed type, then we declare that function to be
719 strict in that argument.
721 We don't really have to make up all those lists of mostly-@AbsTops@;
722 unbound variables in an @AbsValEnv@ are implicitly mapped to that.
724 See notes on @addStrictnessInfoToId@.
727 findStrictness :: StrAnalFlags
728 -> [Type] -- Types of args in which strictness is wanted
729 -> AbsVal -- Abstract strictness value of function
730 -> AbsVal -- Abstract absence value of function
731 -> [Demand] -- Resulting strictness annotation
733 findStrictness strflags [] str_val abs_val = []
735 findStrictness strflags (ty:tys) str_val abs_val
737 demand = findRecDemand strflags [] str_fn abs_fn ty
738 str_fn val = absApply StrAnal str_val val
739 abs_fn val = absApply AbsAnal abs_val val
741 demands = findStrictness strflags tys
742 (absApply StrAnal str_val AbsTop)
743 (absApply AbsAnal abs_val AbsTop)
750 findDemandStrOnly str_env expr binder -- Only strictness environment available
751 = findRecDemand strflags [] str_fn abs_fn (idType binder)
753 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
754 abs_fn val = AbsBot -- Always says poison; so it looks as if
755 -- nothing is absent; safe
756 strflags = getStrAnalFlags str_env
758 findDemandAbsOnly abs_env expr binder -- Only absence environment available
759 = findRecDemand strflags [] 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)
765 strflags = getStrAnalFlags abs_env
768 findDemand str_env abs_env expr binder
769 = findRecDemand strflags [] str_fn abs_fn (idType binder)
771 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
772 abs_fn val = absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)
773 strflags = getStrAnalFlags str_env
776 @findRecDemand@ is where we finally convert strictness/absence info
777 into ``Demands'' which we can pin on Ids (etc.).
779 NOTE: What do we do if something is {\em both} strict and absent?
780 Should \tr{f x y z = error "foo"} says that \tr{f}'s arguments are all
781 strict (because of bottoming effect of \tr{error}) or all absent
782 (because they're not used)?
784 Well, for practical reasons, we prefer absence over strictness. In
785 particular, it makes the ``default defaults'' for class methods (the
786 ones that say \tr{defm.foo dict = error "I don't exist"}) come out
787 nicely [saying ``the dict isn't used''], rather than saying it is
788 strict in every component of the dictionary [massive gratuitious
789 casing to take the dict apart].
791 But you could have examples where going for strictness would be better
792 than absence. Consider:
794 let x = something big
799 If \tr{x} is marked absent in \tr{f}, but not strict, and \tr{g} is
800 lazy, then the thunk for \tr{x} will be built. If \tr{f} was strict,
801 then we'd let-to-case it:
803 case something big of
809 findRecDemand :: StrAnalFlags
810 -> [TyCon] -- TyCons already seen; used to avoid
811 -- zooming into recursive types
812 -> (AbsVal -> AbsVal) -- The strictness function
813 -> (AbsVal -> AbsVal) -- The absence function
814 -> Type -- The type of the argument
817 findRecDemand strflags seen str_fn abs_fn ty
818 = if isPrimType ty then -- It's a primitive type!
821 else if not (anyBot (abs_fn AbsBot)) then -- It's absent
822 -- We prefer absence over strictness: see NOTE above.
825 else if not (all_strict ||
826 (num_strict && is_numeric_type ty) ||
827 (isBot (str_fn AbsBot))) then
828 WwLazy False -- It's not strict and we're not pretending
830 else -- It's strict (or we're pretending it is)!
832 case (maybeAppDataTyConExpandingDicts ty) of
836 Just (tycon,tycon_arg_tys,[data_con]) | tycon `not_elem` seen ->
837 -- Single constructor case, tycon not already seen higher up
839 cmpnt_tys = dataConArgTys data_con tycon_arg_tys
840 prod_len = length cmpnt_tys
843 = [ findRecDemand strflags (tycon:seen)
845 str_fn (mkMainlyTopProd prod_len i cmpnt_val)
848 abs_fn (mkMainlyTopProd prod_len i cmpnt_val)
851 | (cmpnt_ty, i) <- cmpnt_tys `zip` [1..] ]
853 if null compt_strict_infos then
854 if isEnumerationTyCon tycon then wwEnum else wwStrict
856 wwUnpack compt_strict_infos
858 not_elem = isn'tIn "findRecDemand"
861 -- Multi-constr data types, *or* an abstract data
862 -- types, *or* things we don't have a way of conveying
863 -- the info over module boundaries (class ops,
864 -- superdict sels, dfns).
865 if isEnumerationTyCon tycon then
870 (all_strict, num_strict) = strflags
873 = case (maybeAppDataTyConExpandingDicts ty) of -- NB: duplicates stuff done above
877 [intTyCon, integerTyCon,
878 doubleTyCon, floatTyCon,
879 wordTyCon, addrTyCon]
881 _{-something else-} -> False
883 is_elem = isIn "is_numeric_type"
885 -- mkMainlyTopProd: make an AbsProd that is all AbsTops ("n"-1 of
886 -- them) except for a given value in the "i"th position.
888 mkMainlyTopProd :: Int -> Int -> AbsVal -> AbsVal
890 mkMainlyTopProd n i val
892 befores = nOfThem (i-1) AbsTop
893 afters = nOfThem (n-i) AbsTop
895 AbsProd (befores ++ (val : afters))
898 %************************************************************************
900 \subsection[fixpoint]{Fixpointer for the strictness analyser}
902 %************************************************************************
904 The @fixpoint@ functions take a list of \tr{(binder, expr)} pairs, an
905 environment, and returns the abstract value of each binder.
907 The @cheapFixpoint@ function makes a conservative approximation,
908 by binding each of the variables to Top in their own right hand sides.
909 That allows us to make rapid progress, at the cost of a less-than-wonderful
913 cheapFixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
915 cheapFixpoint AbsAnal [id] [rhs] env
916 = [crudeAbsWiden (absEval AbsAnal rhs new_env)]
918 new_env = addOneToAbsValEnv env id AbsTop -- Unsafe starting point!
919 -- In the just-one-binding case, we guarantee to
920 -- find a fixed point in just one iteration,
921 -- because we are using only a two-point domain.
922 -- This improves matters in cases like:
924 -- f x y = letrec g = ...g...
927 -- Here, y isn't used at all, but if g is bound to
928 -- AbsBot we simply get AbsBot as the next
931 cheapFixpoint anal ids rhss env
932 = [widen anal (absEval anal rhs new_env) | rhs <- rhss]
933 -- We do just one iteration, starting from a safe
934 -- approximation. This won't do a good job in situations
936 -- \x -> letrec f = ...g...
940 -- Here, f will end up bound to Top after one iteration,
941 -- and hence we won't spot the strictness in x.
942 -- (A second iteration would solve this. ToDo: try the effect of
943 -- really searching for a fixed point.)
945 new_env = growAbsValEnvList env [(id,safe_val) | id <- ids]
948 = case anal of -- The safe starting point
954 mkLookupFun :: (key -> key -> Bool) -- Equality predicate
955 -> (key -> key -> Bool) -- Less-than predicate
956 -> [(key,val)] -- The assoc list
958 -> Maybe val -- The corresponding value
960 mkLookupFun eq lt alist s
961 = case [a | (s',a) <- alist, s' `eq` s] of
967 fixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
969 fixpoint anal [] _ env = []
971 fixpoint anal ids rhss env
972 = fix_loop initial_vals
975 = case anal of -- The (unsafe) starting point
976 StrAnal -> if (returnsRealWorld (idType id))
977 then AbsTop -- this is a massively horrible hack (SLPJ 95/05)
981 initial_vals = [ initial_val id | id <- ids ]
983 fix_loop :: [AbsVal] -> [AbsVal]
985 fix_loop current_widened_vals
987 new_env = growAbsValEnvList env (ids `zip` current_widened_vals)
988 new_vals = [ absEval anal rhs new_env | rhs <- rhss ]
989 new_widened_vals = map (widen anal) new_vals
991 if (and (zipWith sameVal current_widened_vals new_widened_vals)) then
994 -- NB: I was too chicken to make that a zipWithEqual,
995 -- lest I jump into a black hole. WDP 96/02
997 -- Return the widened values. We might get a slightly
998 -- better value by returning new_vals (which we used to
999 -- do, see below), but alas that means that whenever the
1000 -- function is called we have to re-execute it, which is
1005 -- Return the un-widened values which may be a bit better
1006 -- than the widened ones, and are guaranteed safe, since
1007 -- they are one iteration beyond current_widened_vals,
1008 -- which itself is a fixed point.
1010 fix_loop new_widened_vals
1013 For absence analysis, we make do with a very very simple approach:
1014 look for convergence in a two-point domain.
1016 We used to use just one iteration, starting with the variables bound
1017 to @AbsBot@, which is safe.
1019 Prior to that, we used one iteration starting from @AbsTop@ (which
1020 isn't safe). Why isn't @AbsTop@ safe? Consider:
1028 Here, if p is @AbsBot@, then we'd better {\em not} end up with a ``fixed
1029 point'' of @d@ being @(AbsTop, AbsTop)@! An @AbsBot@ initial value is
1030 safe because it gives poison more often than really necessary, and
1031 thus may miss some absence, but will never claim absence when it ain't
1034 Anyway, one iteration starting with everything bound to @AbsBot@ give
1039 Here, f would always end up bound to @AbsBot@, which ain't very
1040 clever, because then it would introduce poison whenever it was
1041 applied. Much better to start with f bound to @AbsTop@, and widen it
1042 to @AbsBot@ if any poison shows up. In effect we look for convergence
1043 in the two-point @AbsTop@/@AbsBot@ domain.
1045 What we miss (compared with the cleverer strictness analysis) is
1046 spotting that in this case
1048 f = \ x y -> ...y...(f x y')...
1050 \tr{x} is actually absent, since it is only passed round the loop, never
1051 used. But who cares about missing that?
1053 NB: despite only having a two-point domain, we may still have many
1054 iterations, because there are several variables involved at once.