2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1996
4 \section[SaAbsInt]{Abstract interpreter for strictness analysis}
7 #include "HsVersions.h"
20 import CmdLineOpts ( opt_AllStrict, opt_NumbersStrict )
22 import CoreUnfold ( Unfolding(..), UfExpr, RdrName, SimpleUnfolding(..), FormSummary )
23 import CoreUtils ( unTagBinders )
24 import Id ( idType, getIdStrictness, getIdUnfolding,
25 dataConTyCon, dataConArgTys, SYN_IE(Id)
27 import IdInfo ( StrictnessInfo(..) )
28 import Demand ( Demand(..), wwPrim, wwStrict, wwEnum, wwUnpackData, wwUnpackNew )
29 import MagicUFs ( MagicUnfoldingFun )
30 import Maybes ( maybeToBool )
32 import Pretty --TEMP:( Doc, ptext )
33 import PrimOp ( PrimOp(..) )
35 import TyCon ( maybeTyConSingleCon, isEnumerationTyCon, isNewTyCon,
38 import BasicTypes ( NewOrData(..) )
39 import Type ( maybeAppDataTyConExpandingDicts,
40 isPrimType, SYN_IE(Type) )
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 arg 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 arg body env) = anyBot (absEval AbsAnal body env)
216 anyBot (AbsApproxFun _ _) = 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 arg body env)
231 = AbsApproxFun (findDemandStrOnly env body arg)
232 (widen StrAnal abs_body)
234 abs_body = absEval StrAnal body env
237 This stuff is now instead handled neatly by the fact that AbsApproxFun
238 contains an AbsVal inside it. SLPJ Jan 97
240 | isBot abs_body = AbsBot
241 -- It's worth checking for a function which is unconditionally
244 -- f x y = let g y = case x of ...
245 -- in (g ..) + (g ..)
247 -- Here, when we are considering strictness of f in x, we'll
248 -- evaluate the body of f with x bound to bottom. The current
249 -- strategy is to bind g to its *widened* value; without the isBot
250 -- (...) test above, we'd bind g to an AbsApproxFun, and deliver
251 -- Top, not Bot as the value of f's rhs. The test spots the
252 -- unconditional bottom-ness of g when x is bottom. (Another
253 -- alternative here would be to bind g to its exact abstract
254 -- value, but that entails lots of potential re-computation, at
255 -- every application of g.)
258 widen StrAnal (AbsProd vals) = AbsProd (map (widen StrAnal) vals)
259 widen StrAnal other_val = other_val
262 widen AbsAnal (AbsFun arg body env)
263 | anyBot abs_body = AbsBot
264 -- In the absence-analysis case it's *essential* to check
265 -- that the function has no poison in its body. If it does,
266 -- anywhere, then the whole function is poisonous.
269 = AbsApproxFun (findDemandAbsOnly env body arg)
270 (widen AbsAnal abs_body)
272 abs_body = absEval AbsAnal body env
274 widen AbsAnal (AbsProd vals) = AbsProd (map (widen AbsAnal) vals)
276 -- It's desirable to do a good job of widening for product
280 -- in ...(case p of (x,y) -> x)...
282 -- Now, is y absent in this expression? Currently the
283 -- analyser widens p before looking at p's scope, to avoid
284 -- lots of recomputation in the case where p is a function.
285 -- So if widening doesn't have a case for products, we'll
286 -- widen p to AbsBot (since when searching for absence in y we
287 -- bind y to poison ie AbsBot), and now we are lost.
289 widen AbsAnal other_val = other_val
291 -- WAS: if anyBot val then AbsBot else AbsTop
292 -- Nowadays widen is doing a better job on functions for absence analysis.
295 @crudeAbsWiden@ is used just for absence analysis, and always
296 returns AbsTop or AbsBot, so it widens to a two-point domain
299 crudeAbsWiden :: AbsVal -> AbsVal
300 crudeAbsWiden val = if anyBot val then AbsBot else AbsTop
303 @sameVal@ compares two abstract values for equality. It can't deal with
304 @AbsFun@, but that should have been removed earlier in the day by @widen@.
307 sameVal :: AbsVal -> AbsVal -> Bool -- Can't handle AbsFun!
310 sameVal (AbsFun _ _ _) _ = panic "sameVal: AbsFun: arg1"
311 sameVal _ (AbsFun _ _ _) = panic "sameVal: AbsFun: arg2"
314 sameVal AbsBot AbsBot = True
315 sameVal AbsBot other = False -- widen has reduced AbsFun bots to AbsBot
317 sameVal AbsTop AbsTop = True
318 sameVal AbsTop other = False -- Right?
320 sameVal (AbsProd vals1) (AbsProd vals2) = and (zipWithEqual "sameVal" sameVal vals1 vals2)
321 sameVal (AbsProd _) AbsTop = False
322 sameVal (AbsProd _) AbsBot = False
324 sameVal (AbsApproxFun str1 v1) (AbsApproxFun str2 v2) = str1 == str2 && sameVal v1 v1
325 sameVal (AbsApproxFun _ _) AbsTop = False
326 sameVal (AbsApproxFun _ _) AbsBot = False
328 sameVal val1 val2 = panic "sameVal: type mismatch or AbsFun encountered"
332 @evalStrictness@ compares a @Demand@ with an abstract value, returning
333 @True@ iff the abstract value is {\em less defined} than the demand.
334 (@True@ is the exciting answer; @False@ is always safe.)
337 evalStrictness :: Demand
339 -> Bool -- True iff the value is sure
340 -- to be less defined than the Demand
342 evalStrictness (WwLazy _) _ = False
343 evalStrictness WwStrict val = isBot val
344 evalStrictness WwEnum val = isBot val
346 evalStrictness (WwUnpack NewType _ (demand:_)) val
347 = evalStrictness demand val
349 evalStrictness (WwUnpack DataType _ demand_info) val
353 AbsProd vals -> or (zipWithEqual "evalStrictness" evalStrictness demand_info vals)
354 _ -> trace "evalStrictness?" False
356 evalStrictness WwPrim val
360 other -> -- A primitive value should be defined, never bottom;
361 -- hence this paranoia check
362 pprPanic "evalStrictness: WwPrim:" (ppr PprDebug other)
365 For absence analysis, we're interested in whether "poison" in the
366 argument (ie a bottom therein) can propagate to the result of the
367 function call; that is, whether the specified demand can {\em
368 possibly} hit poison.
371 evalAbsence (WwLazy True) _ = False -- Can't possibly hit poison
372 -- with Absent demand
374 evalAbsence (WwUnpack NewType _ (demand:_)) val
375 = evalAbsence demand val
377 evalAbsence (WwUnpack DataType _ demand_info) val
379 AbsTop -> False -- No poison in here
380 AbsBot -> True -- Pure poison
381 AbsProd vals -> or (zipWithEqual "evalAbsence" evalAbsence demand_info vals)
382 _ -> panic "evalAbsence: other"
384 evalAbsence other val = anyBot val
385 -- The demand is conservative; even "Lazy" *might* evaluate the
386 -- argument arbitrarily so we have to look everywhere for poison
389 %************************************************************************
391 \subsection[absEval]{Evaluate an expression in the abstract domain}
393 %************************************************************************
396 -- The isBottomingId stuf is now dealt with via the Id's strictness info
397 -- absId anal var env | isBottomingId var
399 -- StrAnal -> AbsBot -- See discussion below
400 -- AbsAnal -> AbsTop -- Just want to see if there's any poison in
406 case (lookupAbsValEnv env var, getIdStrictness var, getIdUnfolding var) of
408 (Just abs_val, _, _) ->
409 abs_val -- Bound in the environment
411 (Nothing, NoStrictnessInfo, CoreUnfolding (SimpleUnfolding _ _ unfolding)) ->
412 -- We have an unfolding for the expr
413 -- Assume the unfolding has no free variables since it
414 -- came from inside the Id
415 absEval anal (unTagBinders unfolding) env
416 -- Notice here that we only look in the unfolding if we don't
417 -- have strictness info (an unusual situation).
418 -- We could have chosen to look in the unfolding if it exists,
419 -- and only try the strictness info if it doesn't, and that would
420 -- give more accurate results, at the cost of re-abstract-interpreting
421 -- the unfolding every time.
422 -- We found only one place where the look-at-unfolding-first
423 -- method gave better results, which is in the definition of
424 -- showInt in the Prelude. In its defintion, fromIntegral is
425 -- not inlined (it's big) but ab-interp-ing its unfolding gave
426 -- a better result than looking at its strictness only.
427 -- showInt :: Integral a => a -> [Char] -> [Char]
428 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
429 -- "U(U(U(U(SA)AAAAAAAAL)AA)AAAAASAAASA)" {...} _N_ _N_ #-}
431 -- showInt :: Integral a => a -> [Char] -> [Char]
432 -- ! {-# GHC_PRAGMA _A_ 1 _U_ 122 _S_
433 -- "U(U(U(U(SL)LLLLLLLLL)LL)LLLLLSLLLLL)" _N_ _N_ #-}
436 (Nothing, strictness_info, _) ->
437 -- Includes MagicUnfolding, NoUnfolding
438 -- Try the strictness info
439 absValFromStrictness anal strictness_info
441 -- pprTrace "absId:" (hcat [ppr PprDebug var, ptext SLIT("=:"), pp_anal anal, text SLIT(":="),ppr PprDebug result]) $
444 pp_anal StrAnal = ptext SLIT("STR")
445 pp_anal AbsAnal = ptext SLIT("ABS")
447 absEvalAtom anal (VarArg v) env = absId anal v env
448 absEvalAtom anal (LitArg _) env = AbsTop
452 absEval :: AnalysisKind -> CoreExpr -> AbsValEnv -> AbsVal
454 absEval anal (Var var) env = absId anal var env
456 absEval anal (Lit _) env = AbsTop
457 -- What if an unboxed literal? That's OK: it terminates, so its
458 -- abstract value is AbsTop.
460 -- For absence analysis, a literal certainly isn't the "poison" variable
463 Discussion about \tr{error} (following/quoting Lennart): Any expression
464 \tr{error e} is regarded as bottom (with HBC, with the
465 \tr{-ffail-strict} flag, on with \tr{-O}).
467 Regarding it as bottom gives much better strictness properties for
471 f (x:xs) y = f xs (x+y)
473 f [] _ = error "no match"
475 f (x:xs) y = f xs (x+y)
477 is strict in \tr{y}, which you really want. But, it may lead to
478 transformations that turn a call to \tr{error} into non-termination.
479 (The odds of this happening aren't good.)
482 Things are a little different for absence analysis, because we want
483 to make sure that any poison (?????)
486 absEval StrAnal (Prim SeqOp [TyArg _, e]) env
488 if isBot (absEvalAtom StrAnal e env) then AbsBot else AbsTop
489 -- This is a special case to ensure that seq# is strict in its argument.
490 -- The comments below (for most normal PrimOps) do not apply.
492 absEval StrAnal (Prim op es) env = AbsTop
493 -- The arguments are all of unboxed type, so they will already
494 -- have been eval'd. If the boxed version was bottom, we'll
495 -- already have returned bottom.
497 -- Actually, I believe we are saying that either (1) the
498 -- primOp uses unboxed args and they've been eval'ed, so
499 -- there's no need to force strictness here, _or_ the primOp
500 -- uses boxed args and we don't know whether or not it's
501 -- strict, so we assume laziness. (JSM)
503 absEval AbsAnal (Prim op as) env
504 = if any anyBot [absEvalAtom AbsAnal a env | a <- as, isValArg a]
507 -- For absence analysis, we want to see if the poison shows up...
509 absEval anal (Con con as) env
511 = --pprTrace "absEval.Con" (cat[ text "con: ", (ppr PprDebug con), text "args: ", interppSP PprDebug as]) $
512 AbsProd [absEvalAtom anal a env | a <- as, isValArg a]
514 | otherwise -- Not single-constructor
516 StrAnal -> -- Strictness case: it's easy: it certainly terminates
518 AbsAnal -> -- In the absence case we need to be more
519 -- careful: look to see if there's any
520 -- poison in the components
521 if any anyBot [absEvalAtom AbsAnal a env | a <- as, isValArg a]
525 has_single_con = maybeToBool (maybeTyConSingleCon (dataConTyCon con))
529 absEval anal (Lam (ValBinder binder) body) env
530 = AbsFun binder body env
531 absEval anal (Lam other_binder expr) env
532 = absEval anal expr env
533 absEval anal (App f a) env | isValArg a
534 = absApply anal (absEval anal f env) (absEvalAtom anal a env)
535 absEval anal (App expr _) env
536 = absEval anal expr env
539 For primitive cases, just GLB the branches, then LUB with the expr part.
542 absEval anal (Case expr (PrimAlts alts deflt)) env
544 expr_val = absEval anal expr env
545 abs_alts = [ absEval anal rhs env | (_, rhs) <- alts ]
546 -- Don't bother to extend envt, because unbound vars
547 -- default to the conservative AbsTop
549 abs_deflt = absEvalDefault anal expr_val deflt env
551 combineCaseValues anal expr_val
552 (abs_deflt ++ abs_alts)
554 absEval anal (Case expr (AlgAlts alts deflt)) env
556 expr_val = absEval anal expr env
557 abs_alts = [ absEvalAlgAlt anal expr_val alt env | alt <- alts ]
558 abs_deflt = absEvalDefault anal expr_val deflt env
562 combineCaseValues anal expr_val
563 (abs_deflt ++ abs_alts)
568 _ -> pprTrace "absCase:ABS:" (($$) (hsep [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)))
574 For @Lets@ we widen the value we get. This is nothing to
575 do with fixpointing. The reason is so that we don't get an explosion
576 in the amount of computation. For example, consider:
588 If we bind @f@ and @g@ to their exact abstract value, then we'll
589 ``execute'' one call to @f@ and {\em two} calls to @g@. This can blow
590 up exponentially. Widening cuts it off by making a fixed
591 approximation to @f@ and @g@, so that the bodies of @f@ and @g@ are
592 not evaluated again at all when they are called.
594 Of course, this can lose useful joint strictness, which is sad. An
595 alternative approach would be to try with a certain amount of ``fuel''
596 and be prepared to bale out.
599 absEval anal (Let (NonRec binder e1) e2) env
601 new_env = addOneToAbsValEnv env binder (widen anal (absEval anal e1 env))
603 -- The binder of a NonRec should *not* be of unboxed type,
604 -- hence no need to strictly evaluate the Rhs.
605 absEval anal e2 new_env
607 absEval anal (Let (Rec pairs) body) env
609 (binders,rhss) = unzip pairs
610 rhs_vals = cheapFixpoint anal binders rhss env -- Returns widened values
611 new_env = growAbsValEnvList env (binders `zip` rhs_vals)
613 absEval anal body new_env
615 absEval anal (SCC cc expr) env = absEval anal expr env
616 absEval anal (Coerce c ty expr) env = absEval anal expr env
620 absEvalAlgAlt :: AnalysisKind -> AbsVal -> (Id,[Id],CoreExpr) -> AbsValEnv -> AbsVal
622 absEvalAlgAlt anal (AbsProd arg_vals) (con, args, rhs) env
623 = -- The scrutinee is a product value, so it must be of a single-constr
624 -- type; so the constructor in this alternative must be the right one
625 -- so we can go ahead and bind the constructor args to the components
626 -- of the product value.
627 ASSERT(length arg_vals == length args)
629 new_env = growAbsValEnvList env (args `zip` arg_vals)
631 absEval anal rhs new_env
633 absEvalAlgAlt anal other_scrutinee (con, args, rhs) env
634 = -- Scrutinised value is Top or Bot (it can't be a function!)
635 -- So just evaluate the rhs with all constr args bound to Top.
636 -- (If the scrutinee is Top we'll never evaluated this function
642 = case other_scrutinee of {
643 AbsTop -> True; -- i.e., OK
644 AbsBot -> True; -- ditto
645 _ -> False -- party over
649 absEvalDefault :: AnalysisKind
650 -> AbsVal -- Value of scrutinee
653 -> [AbsVal] -- Empty or singleton
655 absEvalDefault anal scrut_val NoDefault env = []
656 absEvalDefault anal scrut_val (BindDefault binder expr) env
657 = [absEval anal expr (addOneToAbsValEnv env binder scrut_val)]
660 %************************************************************************
662 \subsection[absApply]{Apply an abstract function to an abstract argument}
664 %************************************************************************
669 absApply :: AnalysisKind -> AbsVal -> AbsVal -> AbsVal
671 absApply anal AbsBot arg = AbsBot
672 -- AbsBot represents the abstract bottom *function* too
674 absApply StrAnal AbsTop arg = AbsTop
675 absApply AbsAnal AbsTop arg = if anyBot arg
678 -- To be conservative, we have to assume that a function about
679 -- which we know nothing (AbsTop) might look at some part of
683 An @AbsFun@ with only one more argument needed---bind it and eval the
684 result. A @Lam@ with two or more args: return another @AbsFun@ with
685 an augmented environment.
688 absApply anal (AbsFun binder body env) arg
689 = absEval anal body (addOneToAbsValEnv env binder arg)
693 absApply StrAnal (AbsApproxFun demand val) arg
694 = if evalStrictness demand arg
698 absApply AbsAnal (AbsApproxFun demand val) arg
699 = if evalAbsence demand arg
704 absApply anal f@(AbsProd _) arg = pprPanic ("absApply: Duff function: AbsProd." ++ show anal) ((ppr PprDebug f) <+> (ppr PprDebug arg))
711 %************************************************************************
713 \subsection[findStrictness]{Determine some binders' strictness}
715 %************************************************************************
717 @findStrictness@ applies the function \tr{\ ids -> expr} to
718 \tr{[bot,top,top,...]}, \tr{[top,bot,top,top,...]}, etc., (i.e., once
719 with @AbsBot@ in each argument position), and evaluates the resulting
720 abstract value; it returns a vector of @Demand@s saying whether the
721 result of doing this is guaranteed to be bottom. This tells the
722 strictness of the function in each of the arguments.
724 If an argument is of unboxed type, then we declare that function to be
725 strict in that argument.
727 We don't really have to make up all those lists of mostly-@AbsTops@;
728 unbound variables in an @AbsValEnv@ are implicitly mapped to that.
730 See notes on @addStrictnessInfoToId@.
733 findStrictness :: [Type] -- Types of args in which strictness is wanted
734 -> AbsVal -- Abstract strictness value of function
735 -> AbsVal -- Abstract absence value of function
736 -> [Demand] -- Resulting strictness annotation
738 findStrictness [] str_val abs_val = []
740 findStrictness (ty:tys) str_val abs_val
742 demand = findRecDemand [] str_fn abs_fn ty
743 str_fn val = absApply StrAnal str_val val
744 abs_fn val = absApply AbsAnal abs_val val
746 demands = findStrictness tys
747 (absApply StrAnal str_val AbsTop)
748 (absApply AbsAnal abs_val AbsTop)
755 findDemandStrOnly str_env expr binder -- Only strictness environment available
756 = findRecDemand [] str_fn abs_fn (idType binder)
758 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
759 abs_fn val = AbsBot -- Always says poison; so it looks as if
760 -- nothing is absent; safe
762 findDemandAbsOnly abs_env expr binder -- Only absence environment available
763 = findRecDemand [] str_fn abs_fn (idType binder)
765 str_fn val = AbsBot -- Always says non-termination;
766 -- that'll make findRecDemand peer into the
767 -- structure of the value.
768 abs_fn val = absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)
771 findDemand str_env abs_env expr binder
772 = findRecDemand [] str_fn abs_fn (idType binder)
774 str_fn val = absEval StrAnal expr (addOneToAbsValEnv str_env binder val)
775 abs_fn val = absEval AbsAnal expr (addOneToAbsValEnv abs_env binder val)
778 @findRecDemand@ is where we finally convert strictness/absence info
779 into ``Demands'' which we can pin on Ids (etc.).
781 NOTE: What do we do if something is {\em both} strict and absent?
782 Should \tr{f x y z = error "foo"} says that \tr{f}'s arguments are all
783 strict (because of bottoming effect of \tr{error}) or all absent
784 (because they're not used)?
786 Well, for practical reasons, we prefer absence over strictness. In
787 particular, it makes the ``default defaults'' for class methods (the
788 ones that say \tr{defm.foo dict = error "I don't exist"}) come out
789 nicely [saying ``the dict isn't used''], rather than saying it is
790 strict in every component of the dictionary [massive gratuitious
791 casing to take the dict apart].
793 But you could have examples where going for strictness would be better
794 than absence. Consider:
796 let x = something big
801 If \tr{x} is marked absent in \tr{f}, but not strict, and \tr{g} is
802 lazy, then the thunk for \tr{x} will be built. If \tr{f} was strict,
803 then we'd let-to-case it:
805 case something big of
811 findRecDemand :: [TyCon] -- TyCons already seen; used to avoid
812 -- zooming into recursive types
813 -> (AbsVal -> AbsVal) -- The strictness function
814 -> (AbsVal -> AbsVal) -- The absence function
815 -> Type -- The type of the argument
818 findRecDemand seen str_fn abs_fn ty
819 = if isPrimType ty then -- It's a primitive type!
822 else if not (anyBot (abs_fn AbsBot)) then -- It's absent
823 -- We prefer absence over strictness: see NOTE above.
826 else if not (opt_AllStrict ||
827 (opt_NumbersStrict && is_numeric_type ty) ||
828 (isBot (str_fn AbsBot))) then
829 WwLazy False -- It's not strict and we're not pretending
831 else -- It's strict (or we're pretending it is)!
833 case (maybeAppDataTyConExpandingDicts ty) of
837 Just (tycon,tycon_arg_tys,[data_con]) | tycon `not_elem` seen ->
838 -- Single constructor case, tycon not already seen higher up
841 cmpnt_tys = dataConArgTys data_con tycon_arg_tys
842 prod_len = length cmpnt_tys
845 if isNewTyCon tycon then -- A newtype!
846 ASSERT( null (tail cmpnt_tys) )
848 demand = findRecDemand (tycon:seen) str_fn abs_fn (head cmpnt_tys)
850 case demand of -- No point in unpacking unless there is more to see inside
851 WwUnpack _ _ _ -> wwUnpackNew demand
857 = [ findRecDemand (tycon:seen)
859 str_fn (mkMainlyTopProd prod_len i cmpnt_val)
862 abs_fn (mkMainlyTopProd prod_len i cmpnt_val)
865 | (cmpnt_ty, i) <- cmpnt_tys `zip` [1..] ]
867 if null compt_strict_infos then
868 if isEnumerationTyCon tycon then wwEnum else wwStrict
870 wwUnpackData compt_strict_infos
872 not_elem = isn'tIn "findRecDemand"
875 -- Multi-constr data types, *or* an abstract data
876 -- types, *or* things we don't have a way of conveying
877 -- the info over module boundaries (class ops,
878 -- superdict sels, dfns).
879 if isEnumerationTyCon tycon then
885 = case (maybeAppDataTyConExpandingDicts ty) of -- NB: duplicates stuff done above
889 [intTyCon, integerTyCon,
890 doubleTyCon, floatTyCon,
891 wordTyCon, addrTyCon]
893 _{-something else-} -> False
895 is_elem = isIn "is_numeric_type"
897 -- mkMainlyTopProd: make an AbsProd that is all AbsTops ("n"-1 of
898 -- them) except for a given value in the "i"th position.
900 mkMainlyTopProd :: Int -> Int -> AbsVal -> AbsVal
902 mkMainlyTopProd n i val
904 befores = nOfThem (i-1) AbsTop
905 afters = nOfThem (n-i) AbsTop
907 AbsProd (befores ++ (val : afters))
910 %************************************************************************
912 \subsection[fixpoint]{Fixpointer for the strictness analyser}
914 %************************************************************************
916 The @fixpoint@ functions take a list of \tr{(binder, expr)} pairs, an
917 environment, and returns the abstract value of each binder.
919 The @cheapFixpoint@ function makes a conservative approximation,
920 by binding each of the variables to Top in their own right hand sides.
921 That allows us to make rapid progress, at the cost of a less-than-wonderful
925 cheapFixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
927 cheapFixpoint AbsAnal [id] [rhs] env
928 = [crudeAbsWiden (absEval AbsAnal rhs new_env)]
930 new_env = addOneToAbsValEnv env id AbsTop -- Unsafe starting point!
931 -- In the just-one-binding case, we guarantee to
932 -- find a fixed point in just one iteration,
933 -- because we are using only a two-point domain.
934 -- This improves matters in cases like:
936 -- f x y = letrec g = ...g...
939 -- Here, y isn't used at all, but if g is bound to
940 -- AbsBot we simply get AbsBot as the next
943 cheapFixpoint anal ids rhss env
944 = [widen anal (absEval anal rhs new_env) | rhs <- rhss]
945 -- We do just one iteration, starting from a safe
946 -- approximation. This won't do a good job in situations
948 -- \x -> letrec f = ...g...
952 -- Here, f will end up bound to Top after one iteration,
953 -- and hence we won't spot the strictness in x.
954 -- (A second iteration would solve this. ToDo: try the effect of
955 -- really searching for a fixed point.)
957 new_env = growAbsValEnvList env [(id,safe_val) | id <- ids]
960 = case anal of -- The safe starting point
966 mkLookupFun :: (key -> key -> Bool) -- Equality predicate
967 -> (key -> key -> Bool) -- Less-than predicate
968 -> [(key,val)] -- The assoc list
970 -> Maybe val -- The corresponding value
972 mkLookupFun eq lt alist s
973 = case [a | (s',a) <- alist, s' `eq` s] of
979 fixpoint :: AnalysisKind -> [Id] -> [CoreExpr] -> AbsValEnv -> [AbsVal]
981 fixpoint anal [] _ env = []
983 fixpoint anal ids rhss env
984 = fix_loop initial_vals
987 = case anal of -- The (unsafe) starting point
988 StrAnal -> if (returnsRealWorld (idType id))
989 then AbsTop -- this is a massively horrible hack (SLPJ 95/05)
993 initial_vals = [ initial_val id | id <- ids ]
995 fix_loop :: [AbsVal] -> [AbsVal]
997 fix_loop current_widened_vals
999 new_env = growAbsValEnvList env (ids `zip` current_widened_vals)
1000 new_vals = [ absEval anal rhs new_env | rhs <- rhss ]
1001 new_widened_vals = map (widen anal) new_vals
1003 if (and (zipWith sameVal current_widened_vals new_widened_vals)) then
1004 current_widened_vals
1006 -- NB: I was too chicken to make that a zipWithEqual,
1007 -- lest I jump into a black hole. WDP 96/02
1009 -- Return the widened values. We might get a slightly
1010 -- better value by returning new_vals (which we used to
1011 -- do, see below), but alas that means that whenever the
1012 -- function is called we have to re-execute it, which is
1017 -- Return the un-widened values which may be a bit better
1018 -- than the widened ones, and are guaranteed safe, since
1019 -- they are one iteration beyond current_widened_vals,
1020 -- which itself is a fixed point.
1022 fix_loop new_widened_vals
1025 For absence analysis, we make do with a very very simple approach:
1026 look for convergence in a two-point domain.
1028 We used to use just one iteration, starting with the variables bound
1029 to @AbsBot@, which is safe.
1031 Prior to that, we used one iteration starting from @AbsTop@ (which
1032 isn't safe). Why isn't @AbsTop@ safe? Consider:
1040 Here, if p is @AbsBot@, then we'd better {\em not} end up with a ``fixed
1041 point'' of @d@ being @(AbsTop, AbsTop)@! An @AbsBot@ initial value is
1042 safe because it gives poison more often than really necessary, and
1043 thus may miss some absence, but will never claim absence when it ain't
1046 Anyway, one iteration starting with everything bound to @AbsBot@ give
1051 Here, f would always end up bound to @AbsBot@, which ain't very
1052 clever, because then it would introduce poison whenever it was
1053 applied. Much better to start with f bound to @AbsTop@, and widen it
1054 to @AbsBot@ if any poison shows up. In effect we look for convergence
1055 in the two-point @AbsTop@/@AbsBot@ domain.
1057 What we miss (compared with the cleverer strictness analysis) is
1058 spotting that in this case
1060 f = \ x y -> ...y...(f x y')...
1062 \tr{x} is actually absent, since it is only passed round the loop, never
1063 used. But who cares about missing that?
1065 NB: despite only having a two-point domain, we may still have many
1066 iterations, because there are several variables involved at once.