2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
10 module DmdAnal ( dmdAnalPgm, dmdAnalTopRhs,
11 both {- needed by WwLib -}
14 #include "HsVersions.h"
16 import CmdLineOpts ( DynFlags, DynFlag(..), opt_MaxWorkerArgs )
17 import NewDemand -- All of it
20 import CoreUtils ( exprIsValue, exprArity )
21 import DataCon ( dataConTyCon )
22 import TyCon ( isProductTyCon, isRecursiveTyCon )
23 import Id ( Id, idType, idInlinePragma,
24 isDataConId, isGlobalId, idArity,
26 idDemandInfo, idStrictness, idCprInfo, idName,
28 idNewStrictness, idNewStrictness_maybe,
29 setIdNewStrictness, idNewDemandInfo,
30 idNewDemandInfo_maybe,
34 import IdInfo ( newStrictnessFromOld, newDemand )
38 import UniqFM ( plusUFM_C, addToUFM_Directly, lookupUFM_Directly,
39 keysUFM, minusUFM, ufmToList, filterUFM )
40 import Type ( isUnLiftedType )
41 import CoreLint ( showPass, endPass )
42 import Util ( mapAndUnzip, mapAccumL, mapAccumR, lengthIs )
43 import BasicTypes ( Arity, TopLevelFlag(..), isTopLevel, isNeverActive,
45 import Maybes ( orElse, expectJust )
51 * set a noinline pragma on bottoming Ids
53 * Consider f x = x+1 `fatbar` error (show x)
54 We'd like to unbox x, even if that means reboxing it in the error case.
57 instance Outputable TopLevelFlag where
61 %************************************************************************
63 \subsection{Top level stuff}
65 %************************************************************************
68 dmdAnalPgm :: DynFlags -> [CoreBind] -> IO [CoreBind]
69 dmdAnalPgm dflags binds
71 showPass dflags "Demand analysis" ;
72 let { binds_plus_dmds = do_prog binds } ;
74 endPass dflags "Demand analysis"
75 Opt_D_dump_stranal binds_plus_dmds ;
77 -- Only if OLD_STRICTNESS is on, because only then is the old
78 -- strictness analyser run
79 let { dmd_changes = get_changes binds_plus_dmds } ;
80 printDump (text "Changes in demands" $$ dmd_changes) ;
82 return binds_plus_dmds
85 do_prog :: [CoreBind] -> [CoreBind]
86 do_prog binds = snd $ mapAccumL dmdAnalTopBind emptySigEnv binds
88 dmdAnalTopBind :: SigEnv
91 dmdAnalTopBind sigs (NonRec id rhs)
93 ( _, _, (_, rhs1)) = dmdAnalRhs TopLevel NonRecursive sigs (id, rhs)
94 (sigs2, _, (id2, rhs2)) = dmdAnalRhs TopLevel NonRecursive sigs (id, rhs1)
95 -- Do two passes to improve CPR information
96 -- See comments with ignore_cpr_info in mk_sig_ty
97 -- and with extendSigsWithLam
99 (sigs2, NonRec id2 rhs2)
101 dmdAnalTopBind sigs (Rec pairs)
103 (sigs', _, pairs') = dmdFix TopLevel sigs pairs
104 -- We get two iterations automatically
105 -- c.f. the NonRec case above
111 dmdAnalTopRhs :: CoreExpr -> (StrictSig, CoreExpr)
112 -- Analyse the RHS and return
113 -- a) appropriate strictness info
114 -- b) the unfolding (decorated with stricntess info)
118 arity = exprArity rhs
119 (rhs_ty, rhs') = dmdAnal emptySigEnv (vanillaCall arity) rhs
120 sig = mkTopSigTy rhs rhs_ty
123 %************************************************************************
125 \subsection{The analyser itself}
127 %************************************************************************
130 dmdAnal :: SigEnv -> Demand -> CoreExpr -> (DmdType, CoreExpr)
132 dmdAnal sigs Abs e = (topDmdType, e)
135 | not (isStrictDmd dmd)
137 (res_ty, e') = dmdAnal sigs evalDmd e
139 (deferType res_ty, e')
140 -- It's important not to analyse e with a lazy demand because
141 -- a) When we encounter case s of (a,b) ->
142 -- we demand s with U(d1d2)... but if the overall demand is lazy
143 -- that is wrong, and we'd need to reduce the demand on s,
144 -- which is inconvenient
145 -- b) More important, consider
146 -- f (let x = R in x+x), where f is lazy
147 -- We still want to mark x as demanded, because it will be when we
148 -- enter the let. If we analyse f's arg with a Lazy demand, we'll
149 -- just mark x as Lazy
150 -- c) The application rule wouldn't be right either
151 -- Evaluating (f x) in a L demand does *not* cause
152 -- evaluation of f in a C(L) demand!
155 dmdAnal sigs dmd (Lit lit)
156 = (topDmdType, Lit lit)
158 dmdAnal sigs dmd (Var var)
159 = (dmdTransform sigs var dmd, Var var)
161 dmdAnal sigs dmd (Note n e)
162 = (dmd_ty, Note n e')
164 (dmd_ty, e') = dmdAnal sigs dmd' e
166 Coerce _ _ -> evalDmd -- This coerce usually arises from a recursive
167 other -> dmd -- newtype, and we don't want to look inside them
168 -- for exactly the same reason that we don't look
169 -- inside recursive products -- we might not reach
170 -- a fixpoint. So revert to a vanilla Eval demand
172 dmdAnal sigs dmd (App fun (Type ty))
173 = (fun_ty, App fun' (Type ty))
175 (fun_ty, fun') = dmdAnal sigs dmd fun
177 -- Lots of the other code is there to make this
178 -- beautiful, compositional, application rule :-)
179 dmdAnal sigs dmd e@(App fun arg) -- Non-type arguments
180 = let -- [Type arg handled above]
181 (fun_ty, fun') = dmdAnal sigs (Call dmd) fun
182 (arg_ty, arg') = dmdAnal sigs arg_dmd arg
183 (arg_dmd, res_ty) = splitDmdTy fun_ty
185 (res_ty `bothType` arg_ty, App fun' arg')
187 dmdAnal sigs dmd (Lam var body)
190 (body_ty, body') = dmdAnal sigs dmd body
192 (body_ty, Lam var body')
194 | Call body_dmd <- dmd -- A call demand: good!
196 sigs' = extendSigsWithLam sigs var
197 (body_ty, body') = dmdAnal sigs' body_dmd body
198 (lam_ty, var') = annotateLamIdBndr body_ty var
200 (lam_ty, Lam var' body')
202 | otherwise -- Not enough demand on the lambda; but do the body
203 = let -- anyway to annotate it and gather free var info
204 (body_ty, body') = dmdAnal sigs evalDmd body
205 (lam_ty, var') = annotateLamIdBndr body_ty var
207 (deferType lam_ty, Lam var' body')
209 dmdAnal sigs dmd (Case scrut case_bndr [alt@(DataAlt dc,bndrs,rhs)])
210 | let tycon = dataConTyCon dc,
211 isProductTyCon tycon,
212 not (isRecursiveTyCon tycon)
214 sigs_alt = extendSigEnv NotTopLevel sigs case_bndr case_bndr_sig
215 (alt_ty, alt') = dmdAnalAlt sigs_alt dmd alt
216 (alt_ty1, case_bndr') = annotateBndr alt_ty case_bndr
217 (_, bndrs', _) = alt'
218 case_bndr_sig = cprSig
219 -- Inside the alternative, the case binder has the CPR property.
220 -- Meaning that a case on it will successfully cancel.
222 -- f True x = case x of y { I# x' -> if x' ==# 3 then y else I# 8 }
225 -- We want f to have the CPR property:
226 -- f b x = case fw b x of { r -> I# r }
227 -- fw True x = case x of y { I# x' -> if x' ==# 3 then x' else 8 }
230 -- Figure out whether the demand on the case binder is used, and use
231 -- that to set the scrut_dmd. This is utterly essential.
232 -- Consider f x = case x of y { (a,b) -> k y a }
233 -- If we just take scrut_demand = U(L,A), then we won't pass x to the
234 -- worker, so the worker will rebuild
235 -- x = (a, absent-error)
236 -- and that'll crash.
237 -- So at one stage I had:
238 -- dead_case_bndr = isAbsentDmd (idNewDemandInfo case_bndr')
239 -- keepity | dead_case_bndr = Drop
240 -- | otherwise = Keep
243 -- case x of y { (a,b) -> h y + a }
244 -- where h : U(LL) -> T
245 -- The above code would compute a Keep for x, since y is not Abs, which is silly
246 -- The insight is, of course, that a demand on y is a demand on the
247 -- scrutinee, so we need to `both` it with the scrut demand
249 scrut_dmd = Eval (Prod [idNewDemandInfo b | b <- bndrs', isId b])
251 idNewDemandInfo case_bndr'
253 (scrut_ty, scrut') = dmdAnal sigs scrut_dmd scrut
255 (alt_ty1 `bothType` scrut_ty, Case scrut' case_bndr' [alt'])
257 dmdAnal sigs dmd (Case scrut case_bndr alts)
259 (alt_tys, alts') = mapAndUnzip (dmdAnalAlt sigs dmd) alts
260 (scrut_ty, scrut') = dmdAnal sigs evalDmd scrut
261 (alt_ty, case_bndr') = annotateBndr (foldr1 lubType alt_tys) case_bndr
263 -- pprTrace "dmdAnal:Case" (ppr alts $$ ppr alt_tys)
264 (alt_ty `bothType` scrut_ty, Case scrut' case_bndr' alts')
266 dmdAnal sigs dmd (Let (NonRec id rhs) body)
268 (sigs', lazy_fv, (id1, rhs')) = dmdAnalRhs NotTopLevel NonRecursive sigs (id, rhs)
269 (body_ty, body') = dmdAnal sigs' dmd body
270 (body_ty1, id2) = annotateBndr body_ty id1
271 body_ty2 = addLazyFVs body_ty1 lazy_fv
273 -- If the actual demand is better than the vanilla call
274 -- demand, you might think that we might do better to re-analyse
275 -- the RHS with the stronger demand.
276 -- But (a) That seldom happens, because it means that *every* path in
277 -- the body of the let has to use that stronger demand
278 -- (b) It often happens temporarily in when fixpointing, because
279 -- the recursive function at first seems to place a massive demand.
280 -- But we don't want to go to extra work when the function will
281 -- probably iterate to something less demanding.
282 -- In practice, all the times the actual demand on id2 is more than
283 -- the vanilla call demand seem to be due to (b). So we don't
284 -- bother to re-analyse the RHS.
285 (body_ty2, Let (NonRec id2 rhs') body')
287 dmdAnal sigs dmd (Let (Rec pairs) body)
289 bndrs = map fst pairs
290 (sigs', lazy_fv, pairs') = dmdFix NotTopLevel sigs pairs
291 (body_ty, body') = dmdAnal sigs' dmd body
292 body_ty1 = addLazyFVs body_ty lazy_fv
294 sigs' `seq` body_ty `seq`
296 (body_ty2, _) = annotateBndrs body_ty1 bndrs
297 -- Don't bother to add demand info to recursive
298 -- binders as annotateBndr does;
299 -- being recursive, we can't treat them strictly.
300 -- But we do need to remove the binders from the result demand env
302 (body_ty2, Let (Rec pairs') body')
305 dmdAnalAlt sigs dmd (con,bndrs,rhs)
307 (rhs_ty, rhs') = dmdAnal sigs dmd rhs
308 (alt_ty, bndrs') = annotateBndrs rhs_ty bndrs
310 (alt_ty, (con, bndrs', rhs'))
313 %************************************************************************
315 \subsection{Bindings}
317 %************************************************************************
320 dmdFix :: TopLevelFlag
321 -> SigEnv -- Does not include bindings for this binding
324 [(Id,CoreExpr)]) -- Binders annotated with stricness info
326 dmdFix top_lvl sigs orig_pairs
327 = loop 1 initial_sigs orig_pairs
329 bndrs = map fst orig_pairs
330 initial_sigs = extendSigEnvList sigs [(id, (initialSig id, top_lvl)) | id <- bndrs]
333 -> SigEnv -- Already contains the current sigs
335 -> (SigEnv, DmdEnv, [(Id,CoreExpr)])
338 = (sigs', lazy_fv, pairs')
339 -- Note: use pairs', not pairs. pairs' is the result of
340 -- processing the RHSs with sigs (= sigs'), whereas pairs
341 -- is the result of processing the RHSs with the *previous*
342 -- iteration of sigs.
344 | n >= 10 = pprTrace "dmdFix loop" (ppr n <+> (vcat
345 [ text "Sigs:" <+> ppr [(id,lookup sigs id, lookup sigs' id) | (id,_) <- pairs],
346 text "env:" <+> ppr (ufmToList sigs),
347 text "binds:" <+> pprCoreBinding (Rec pairs)]))
348 (emptySigEnv, lazy_fv, orig_pairs) -- Safe output
349 -- The lazy_fv part is really important! orig_pairs has no strictness
350 -- info, including nothing about free vars. But if we have
351 -- letrec f = ....y..... in ...f...
352 -- where 'y' is free in f, we must record that y is mentioned,
353 -- otherwise y will get recorded as absent altogether
355 | otherwise = loop (n+1) sigs' pairs'
357 found_fixpoint = all (same_sig sigs sigs') bndrs
358 -- Use the new signature to do the next pair
359 -- The occurrence analyser has arranged them in a good order
360 -- so this can significantly reduce the number of iterations needed
361 ((sigs',lazy_fv), pairs') = mapAccumL (my_downRhs top_lvl) (sigs, emptyDmdEnv) pairs
363 my_downRhs top_lvl (sigs,lazy_fv) (id,rhs)
364 = -- pprTrace "downRhs {" (ppr id <+> (ppr old_sig))
366 -- pprTrace "downRhsEnd" (ppr id <+> ppr new_sig <+> char '}' )
367 ((sigs', lazy_fv'), pair')
370 (sigs', lazy_fv1, pair') = dmdAnalRhs top_lvl Recursive sigs (id,rhs)
371 lazy_fv' = plusUFM_C both lazy_fv lazy_fv1
372 -- old_sig = lookup sigs id
373 -- new_sig = lookup sigs' id
375 same_sig sigs sigs' var = lookup sigs var == lookup sigs' var
376 lookup sigs var = case lookupVarEnv sigs var of
379 -- Get an initial strictness signature from the Id
380 -- itself. That way we make use of earlier iterations
381 -- of the fixpoint algorithm. (Cunning plan.)
382 -- Note that the cunning plan extends to the DmdEnv too,
383 -- since it is part of the strictness signature
384 initialSig id = idNewStrictness_maybe id `orElse` botSig
386 dmdAnalRhs :: TopLevelFlag -> RecFlag
387 -> SigEnv -> (Id, CoreExpr)
388 -> (SigEnv, DmdEnv, (Id, CoreExpr))
389 -- Process the RHS of the binding, add the strictness signature
390 -- to the Id, and augment the environment with the signature as well.
392 dmdAnalRhs top_lvl rec_flag sigs (id, rhs)
393 = (sigs', lazy_fv, (id', rhs'))
395 arity = idArity id -- The idArity should be up to date
396 -- The simplifier was run just beforehand
397 (rhs_dmd_ty, rhs') = dmdAnal sigs (vanillaCall arity) rhs
398 (lazy_fv, sig_ty) = WARN( arity /= dmdTypeDepth rhs_dmd_ty, ppr id )
399 mkSigTy top_lvl rec_flag id rhs rhs_dmd_ty
400 id' = id `setIdNewStrictness` sig_ty
401 sigs' = extendSigEnv top_lvl sigs id sig_ty
404 %************************************************************************
406 \subsection{Strictness signatures and types}
408 %************************************************************************
411 mkTopSigTy :: CoreExpr -> DmdType -> StrictSig
412 -- Take a DmdType and turn it into a StrictSig
413 -- NB: not used for never-inline things; hence False
414 mkTopSigTy rhs dmd_ty = snd (mk_sig_ty False False rhs dmd_ty)
416 mkSigTy :: TopLevelFlag -> RecFlag -> Id -> CoreExpr -> DmdType -> (DmdEnv, StrictSig)
417 mkSigTy top_lvl rec_flag id rhs dmd_ty
418 = mk_sig_ty never_inline thunk_cpr_ok rhs dmd_ty
420 never_inline = isNeverActive (idInlinePragma id)
421 maybe_id_dmd = idNewDemandInfo_maybe id
422 -- Is Nothing the first time round
425 | isTopLevel top_lvl = False -- Top level things don't get
426 -- their demandInfo set at all
427 | isRec rec_flag = False -- Ditto recursive things
428 | Just dmd <- maybe_id_dmd = isStrictDmd dmd
429 | otherwise = True -- Optimistic, first time round
433 The thunk_cpr_ok stuff [CPR-AND-STRICTNESS]
434 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
435 If the rhs is a thunk, we usually forget the CPR info, because
436 it is presumably shared (else it would have been inlined, and
437 so we'd lose sharing if w/w'd it into a function.
439 However, if the strictness analyser has figured out (in a previous
440 iteration) that it's strict, then we DON'T need to forget the CPR info.
441 Instead we can retain the CPR info and do the thunk-splitting transform
442 (see WorkWrap.splitThunk).
444 This made a big difference to PrelBase.modInt, which had something like
445 modInt = \ x -> let r = ... -> I# v in
446 ...body strict in r...
447 r's RHS isn't a value yet; but modInt returns r in various branches, so
448 if r doesn't have the CPR property then neither does modInt
449 Another case I found in practice (in Complex.magnitude), looks like this:
450 let k = if ... then I# a else I# b
451 in ... body strict in k ....
452 (For this example, it doesn't matter whether k is returned as part of
453 the overall result; but it does matter that k's RHS has the CPR property.)
454 Left to itself, the simplifier will make a join point thus:
455 let $j k = ...body strict in k...
456 if ... then $j (I# a) else $j (I# b)
457 With thunk-splitting, we get instead
458 let $j x = let k = I#x in ...body strict in k...
459 in if ... then $j a else $j b
460 This is much better; there's a good chance the I# won't get allocated.
462 The difficulty with this is that we need the strictness type to
463 look at the body... but we now need the body to calculate the demand
464 on the variable, so we can decide whether its strictness type should
465 have a CPR in it or not. Simple solution:
466 a) use strictness info from the previous iteration
467 b) make sure we do at least 2 iterations, by doing a second
468 round for top-level non-recs. Top level recs will get at
469 least 2 iterations except for totally-bottom functions
470 which aren't very interesting anyway.
472 NB: strictly_demanded is never true of a top-level Id, or of a recursive Id.
474 The Nothing case in thunk_cpr_ok [CPR-AND-STRICTNESS]
475 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
476 Demand info now has a 'Nothing' state, just like strictness info.
477 The analysis works from 'dangerous' towards a 'safe' state; so we
478 start with botSig for 'Nothing' strictness infos, and we start with
479 "yes, it's demanded" for 'Nothing' in the demand info. The
480 fixpoint iteration will sort it all out.
482 We can't start with 'not-demanded' because then consider
486 if ... then t else I# y else f x'
488 In the first iteration we'd have no demand info for x, so assume
489 not-demanded; then we'd get TopRes for f's CPR info. Next iteration
490 we'd see that t was demanded, and so give it the CPR property, but
491 by now f has TopRes, so it will stay TopRes.
493 Instead, with the Nothing setting the first time round, we say
494 'yes t is demanded' the first time.
496 However, this does mean that for non-recursive bindings we must
497 iterate twice to be sure of not getting over-optimistic CPR info,
498 in the case where t turns out to be not-demanded. This is handled
503 mk_sig_ty never_inline thunk_cpr_ok rhs (DmdType fv dmds res)
504 | never_inline && not (isBotRes res)
506 -- Don't strictness-analyse NOINLINE things. Why not? Because
507 -- the NOINLINE says "don't expose any of the inner workings at the call
508 -- site" and the strictness is certainly an inner working.
510 -- More concretely, the demand analyser discovers the following strictness
511 -- for unsafePerformIO: C(U(AV))
513 -- unsafePerformIO (\s -> let r = f x in
514 -- case writeIORef v r s of (# s1, _ #) ->
516 -- The strictness analyser will find that the binding for r is strict,
517 -- (becuase of uPIO's strictness sig), and so it'll evaluate it before
518 -- doing the writeIORef. This actually makes tests/lib/should_run/memo002
521 -- Solution: don't expose the strictness of unsafePerformIO.
523 -- But we do want to expose the strictness of error functions,
524 -- which are also often marked NOINLINE
525 -- {-# NOINLINE foo #-}
526 -- foo x = error ("wubble buggle" ++ x)
527 -- So (hack, hack) we only drop the strictness for non-bottom things
528 -- This is all very unsatisfactory.
529 = (deferEnv fv, topSig)
532 = (lazy_fv, mkStrictSig dmd_ty)
534 dmd_ty = DmdType strict_fv final_dmds res'
536 lazy_fv = filterUFM (not . isStrictDmd) fv
537 strict_fv = filterUFM isStrictDmd fv
538 -- We put the strict FVs in the DmdType of the Id, so
539 -- that at its call sites we unleash demands on its strict fvs.
540 -- An example is 'roll' in imaginary/wheel-sieve2
541 -- Something like this:
543 -- go y = if ... then roll (x-1) else x+1
546 -- We want to see that roll is strict in x, which is because
547 -- go is called. So we put the DmdEnv for x in go's DmdType.
550 -- f :: Int -> Int -> Int
551 -- f x y = let t = x+1
552 -- h z = if z==0 then t else
553 -- if z==1 then x+1 else
557 -- Calling h does indeed evaluate x, but we can only see
558 -- that if we unleash a demand on x at the call site for t.
560 -- Incidentally, here's a place where lambda-lifting h would
561 -- lose the cigar --- we couldn't see the joint strictness in t/x
564 -- We don't want to put *all* the fv's from the RHS into the
565 -- DmdType, because that makes fixpointing very slow --- the
566 -- DmdType gets full of lazy demands that are slow to converge.
568 final_dmds = setUnpackStrategy dmds
569 -- Set the unpacking strategy
572 RetCPR | ignore_cpr_info -> TopRes
574 ignore_cpr_info = not (exprIsValue rhs || thunk_cpr_ok)
577 The unpack strategy determines whether we'll *really* unpack the argument,
578 or whether we'll just remember its strictness. If unpacking would give
579 rise to a *lot* of worker args, we may decide not to unpack after all.
582 setUnpackStrategy :: [Demand] -> [Demand]
584 = snd (go (opt_MaxWorkerArgs - nonAbsentArgs ds) ds)
586 go :: Int -- Max number of args available for sub-components of [Demand]
588 -> (Int, [Demand]) -- Args remaining after subcomponents of [Demand] are unpacked
590 go n (Eval (Prod cs) : ds)
591 | n' >= 0 = Eval (Prod cs') `cons` go n'' ds
592 | otherwise = Box (Eval (Prod cs)) `cons` go n ds
595 n' = n + 1 - non_abs_args
596 -- Add one to the budget 'cos we drop the top-level arg
597 non_abs_args = nonAbsentArgs cs
598 -- Delete # of non-absent args to which we'll now be committed
600 go n (d:ds) = d `cons` go n ds
603 cons d (n,ds) = (n, d:ds)
605 nonAbsentArgs :: [Demand] -> Int
607 nonAbsentArgs (Abs : ds) = nonAbsentArgs ds
608 nonAbsentArgs (d : ds) = 1 + nonAbsentArgs ds
612 %************************************************************************
614 \subsection{Strictness signatures and types}
616 %************************************************************************
619 splitDmdTy :: DmdType -> (Demand, DmdType)
620 -- Split off one function argument
621 -- We already have a suitable demand on all
622 -- free vars, so no need to add more!
623 splitDmdTy (DmdType fv (dmd:dmds) res_ty) = (dmd, DmdType fv dmds res_ty)
624 splitDmdTy ty@(DmdType fv [] res_ty) = (resTypeArgDmd res_ty, ty)
628 unitVarDmd var dmd = DmdType (unitVarEnv var dmd) [] TopRes
630 addVarDmd top_lvl dmd_ty@(DmdType fv ds res) var dmd
631 | isTopLevel top_lvl = dmd_ty -- Don't record top level things
632 | otherwise = DmdType (extendVarEnv fv var dmd) ds res
634 addLazyFVs (DmdType fv ds res) lazy_fvs
635 = DmdType both_fv1 ds res
637 both_fv = (plusUFM_C both fv lazy_fvs)
638 both_fv1 = modifyEnv (isBotRes res) (`both` Bot) lazy_fvs fv both_fv
639 -- This modifyEnv is vital. Consider
640 -- let f = \x -> (x,y)
642 -- Here, y is treated as a lazy-fv of f, but we must `both` that L
643 -- demand with the bottom coming up from 'error'
645 -- I got a loop in the fixpointer without this, due to an interaction
646 -- with the lazy_fv filtering in mkSigTy. Roughly, it was
648 -- = letrec g y = x `fatbar`
649 -- letrec h z = z + ...g...
652 -- In the initial iteration for f, f=Bot
653 -- Suppose h is found to be strict in z, but the occurrence of g in its RHS
654 -- is lazy. Now consider the fixpoint iteration for g, esp the demands it
655 -- places on its free variables. Suppose it places none. Then the
656 -- x `fatbar` ...call to h...
657 -- will give a x->V demand for x. That turns into a L demand for x,
658 -- which floats out of the defn for h. Without the modifyEnv, that
659 -- L demand doesn't get both'd with the Bot coming up from the inner
660 -- call to f. So we just get an L demand for x for g.
662 -- A better way to say this is that the lazy-fv filtering should give the
663 -- same answer as putting the lazy fv demands in the function's type.
665 annotateBndr :: DmdType -> Var -> (DmdType, Var)
666 -- The returned env has the var deleted
667 -- The returned var is annotated with demand info
668 -- No effect on the argument demands
669 annotateBndr dmd_ty@(DmdType fv ds res) var
670 | isTyVar var = (dmd_ty, var)
671 | otherwise = (DmdType fv' ds res, setIdNewDemandInfo var dmd)
673 (fv', dmd) = removeFV fv var res
675 annotateBndrs = mapAccumR annotateBndr
677 annotateLamIdBndr dmd_ty@(DmdType fv ds res) id
678 -- For lambdas we add the demand to the argument demands
679 -- Only called for Ids
681 (DmdType fv' (hacked_dmd:ds) res, setIdNewDemandInfo id hacked_dmd)
683 (fv', dmd) = removeFV fv id res
684 hacked_dmd = argDemand dmd
685 -- This call to argDemand is vital, because otherwise we label
686 -- a lambda binder with demand 'B'. But in terms of calling
687 -- conventions that's Abs, because we don't pass it. But
688 -- when we do a w/w split we get
689 -- fw x = (\x y:B -> ...) x (error "oops")
690 -- And then the simplifier things the 'B' is a strict demand
691 -- and evaluates the (error "oops"). Sigh
693 removeFV fv id res = (fv', zapUnlifted id dmd)
695 fv' = fv `delVarEnv` id
696 dmd = lookupVarEnv fv id `orElse` deflt
697 deflt | isBotRes res = Bot
700 -- For unlifted-type variables, we are only
701 -- interested in Bot/Abs/Box Abs
702 zapUnlifted is Bot = Bot
703 zapUnlifted id Abs = Abs
704 zapUnlifted id dmd | isUnLiftedType (idType id) = lazyDmd
708 %************************************************************************
710 \subsection{Strictness signatures}
712 %************************************************************************
715 type SigEnv = VarEnv (StrictSig, TopLevelFlag)
716 -- We use the SigEnv to tell us whether to
717 -- record info about a variable in the DmdEnv
718 -- We do so if it's a LocalId, but not top-level
720 -- The DmdEnv gives the demand on the free vars of the function
721 -- when it is given enough args to satisfy the strictness signature
723 emptySigEnv = emptyVarEnv
725 extendSigEnv :: TopLevelFlag -> SigEnv -> Id -> StrictSig -> SigEnv
726 extendSigEnv top_lvl env var sig = extendVarEnv env var (sig, top_lvl)
728 extendSigEnvList = extendVarEnvList
730 extendSigsWithLam :: SigEnv -> Id -> SigEnv
731 -- Extend the SigEnv when we meet a lambda binder
732 -- If the binder is marked demanded with a product demand, then give it a CPR
733 -- signature, because in the likely event that this is a lambda on a fn defn
734 -- [we only use this when the lambda is being consumed with a call demand],
735 -- it'll be w/w'd and so it will be CPR-ish.
737 -- NOTE: see notes [CPR-AND-STRICTNESS]
739 -- Also note that we only want to do this for something that
740 -- definitely has product type, else we may get over-optimistic
741 -- CPR results (e.g. from \x -> x!).
743 extendSigsWithLam sigs id
744 = case idNewDemandInfo_maybe id of
745 Nothing -> extendVarEnv sigs id (cprSig, NotTopLevel)
746 Just (Eval (Prod ds)) -> extendVarEnv sigs id (cprSig, NotTopLevel)
750 dmdTransform :: SigEnv -- The strictness environment
751 -> Id -- The function
752 -> Demand -- The demand on the function
753 -> DmdType -- The demand type of the function in this context
754 -- Returned DmdEnv includes the demand on
755 -- this function plus demand on its free variables
757 dmdTransform sigs var dmd
759 ------ DATA CONSTRUCTOR
760 | isDataConId var -- Data constructor
762 StrictSig dmd_ty = idNewStrictness var -- It must have a strictness sig
763 DmdType _ _ con_res = dmd_ty
766 if arity == call_depth then -- Saturated, so unleash the demand
768 -- Important! If we Keep the constructor application, then
769 -- we need the demands the constructor places (always lazy)
770 -- If not, we don't need to. For example:
771 -- f p@(x,y) = (p,y) -- S(AL)
773 -- It's vital that we don't calculate Absent for a!
774 dmd_ds = case res_dmd of
775 Box (Eval ds) -> mapDmds box ds
779 -- ds can be empty, when we are just seq'ing the thing
780 -- If so we must make up a suitable bunch of demands
781 arg_ds = case dmd_ds of
782 Poly d -> replicate arity d
783 Prod ds -> ASSERT( ds `lengthIs` arity ) ds
786 mkDmdType emptyDmdEnv arg_ds con_res
787 -- Must remember whether it's a product, hence con_res, not TopRes
791 ------ IMPORTED FUNCTION
792 | isGlobalId var, -- Imported function
793 let StrictSig dmd_ty = idNewStrictness var
794 = if dmdTypeDepth dmd_ty <= call_depth then -- Saturated, so unleash the demand
799 ------ LOCAL LET/REC BOUND THING
800 | Just (StrictSig dmd_ty, top_lvl) <- lookupVarEnv sigs var
802 fn_ty | dmdTypeDepth dmd_ty <= call_depth = dmd_ty
803 | otherwise = deferType dmd_ty
804 -- NB: it's important to use deferType, and not just return topDmdType
805 -- Consider let { f x y = p + x } in f 1
806 -- The application isn't saturated, but we must nevertheless propagate
807 -- a lazy demand for p!
809 addVarDmd top_lvl fn_ty var dmd
811 ------ LOCAL NON-LET/REC BOUND THING
812 | otherwise -- Default case
816 (call_depth, res_dmd) = splitCallDmd dmd
820 %************************************************************************
824 %************************************************************************
827 splitCallDmd :: Demand -> (Int, Demand)
828 splitCallDmd (Call d) = case splitCallDmd d of
830 splitCallDmd d = (0, d)
832 vanillaCall :: Arity -> Demand
833 vanillaCall 0 = evalDmd
834 vanillaCall n = Call (vanillaCall (n-1))
836 deferType :: DmdType -> DmdType
837 deferType (DmdType fv _ _) = DmdType (deferEnv fv) [] TopRes
838 -- Notice that we throw away info about both arguments and results
839 -- For example, f = let ... in \x -> x
840 -- We don't want to get a stricness type V->T for f.
843 deferEnv :: DmdEnv -> DmdEnv
844 deferEnv fv = mapVarEnv defer fv
848 argDemand :: Demand -> Demand
849 -- The 'Defer' demands are just Lazy at function boundaries
850 -- Ugly! Ask John how to improve it.
851 argDemand Top = lazyDmd
852 argDemand (Defer d) = lazyDmd
853 argDemand (Eval ds) = Eval (mapDmds argDemand ds)
854 argDemand (Box Bot) = evalDmd
855 argDemand (Box d) = box (argDemand d)
856 argDemand Bot = Abs -- Don't pass args that are consumed (only) by bottom
861 betterStrictness :: StrictSig -> StrictSig -> Bool
862 betterStrictness (StrictSig t1) (StrictSig t2) = betterDmdType t1 t2
864 betterDmdType t1 t2 = (t1 `lubType` t2) == t2
866 betterDemand :: Demand -> Demand -> Bool
867 -- If d1 `better` d2, and d2 `better` d2, then d1==d2
868 betterDemand d1 d2 = (d1 `lub` d2) == d2
872 -------------------------
873 -- Consider (if x then y else []) with demand V
874 -- Then the first branch gives {y->V} and the second
875 -- *implicitly* has {y->A}. So we must put {y->(V `lub` A)}
876 -- in the result env.
877 lubType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
878 = DmdType lub_fv2 (lub_ds ds1 ds2) (r1 `lubRes` r2)
880 lub_fv = plusUFM_C lub fv1 fv2
881 lub_fv1 = modifyEnv (not (isBotRes r1)) absLub fv2 fv1 lub_fv
882 lub_fv2 = modifyEnv (not (isBotRes r2)) absLub fv1 fv2 lub_fv1
883 -- lub is the identity for Bot
885 -- Extend the shorter argument list to match the longer
886 lub_ds (d1:ds1) (d2:ds2) = lub d1 d2 : lub_ds ds1 ds2
888 lub_ds ds1 [] = map (`lub` resTypeArgDmd r2) ds1
889 lub_ds [] ds2 = map (resTypeArgDmd r1 `lub`) ds2
891 -----------------------------------
892 -- (t1 `bothType` t2) takes the argument/result info from t1,
893 -- using t2 just for its free-var info
894 -- NB: Don't forget about r2! It might be BotRes, which is
895 -- a bottom demand on all the in-scope variables.
896 -- Peter: can this be done more neatly?
897 bothType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
898 = DmdType both_fv2 ds1 (r1 `bothRes` r2)
900 both_fv = plusUFM_C both fv1 fv2
901 both_fv1 = modifyEnv (isBotRes r1) (`both` Bot) fv2 fv1 both_fv
902 both_fv2 = modifyEnv (isBotRes r2) (`both` Bot) fv1 fv2 both_fv1
903 -- both is the identity for Abs
910 lubRes RetCPR RetCPR = RetCPR
911 lubRes r1 r2 = TopRes
913 -- If either diverges, the whole thing does
914 -- Otherwise take CPR info from the first
915 bothRes r1 BotRes = BotRes
920 modifyEnv :: Bool -- No-op if False
921 -> (Demand -> Demand) -- The zapper
922 -> DmdEnv -> DmdEnv -- Env1 and Env2
923 -> DmdEnv -> DmdEnv -- Transform this env
924 -- Zap anything in Env1 but not in Env2
925 -- Assume: dom(env) includes dom(Env1) and dom(Env2)
927 modifyEnv need_to_modify zapper env1 env2 env
928 | need_to_modify = foldr zap env (keysUFM (env1 `minusUFM` env2))
931 zap uniq env = addToUFM_Directly env uniq (zapper current_val)
933 current_val = expectJust "modifyEnv" (lookupUFM_Directly env uniq)
937 %************************************************************************
939 \subsection{LUB and BOTH}
941 %************************************************************************
944 lub :: Demand -> Demand -> Demand
947 lub Abs d2 = absLub d2
949 lub (Defer ds1) d2 = defer (Eval ds1 `lub` d2)
951 lub (Call d1) (Call d2) = Call (d1 `lub` d2)
952 lub d1@(Call _) (Box d2) = d1 `lub` d2 -- Just strip the box
953 lub d1@(Call _) d2@(Eval _) = d2 -- Presumably seq or vanilla eval
954 lub d1@(Call _) d2 = d2 `lub` d1 -- Bot, Abs, Top
956 -- For the Eval case, we use these approximation rules
957 -- Box Bot <= Eval (Box Bot ...)
958 -- Box Top <= Defer (Box Bot ...)
959 -- Box (Eval ds) <= Eval (map Box ds)
960 lub (Eval ds1) (Eval ds2) = Eval (ds1 `lubs` ds2)
961 lub (Eval ds1) (Box Bot) = Eval (mapDmds (`lub` Box Bot) ds1)
962 lub (Eval ds1) (Box (Eval ds2)) = Eval (ds1 `lubs` mapDmds box ds2)
963 lub (Eval ds1) (Box Abs) = deferEval (mapDmds (`lub` Box Bot) ds1)
964 lub d1@(Eval _) d2 = d2 `lub` d1 -- Bot,Abs,Top,Call,Defer
966 lub (Box d1) (Box d2) = box (d1 `lub` d2)
967 lub d1@(Box _) d2 = d2 `lub` d1
969 lubs = zipWithDmds lub
971 ---------------------
972 -- box is the smart constructor for Box
973 -- It computes <B,bot> & d
974 -- INVARIANT: (Box d) => d = Bot, Abs, Eval
975 -- Seems to be no point in allowing (Box (Call d))
976 box (Call d) = Call d -- The odd man out. Why?
978 box (Defer _) = lazyDmd
979 box Top = lazyDmd -- Box Abs and Box Top
980 box Abs = lazyDmd -- are the same <B,L>
981 box d = Box d -- Bot, Eval
984 defer :: Demand -> Demand
986 -- defer is the smart constructor for Defer
987 -- The idea is that (Defer ds) = <U(ds), L>
989 -- It specifies what happens at a lazy function argument
990 -- or a lambda; the L* operator
991 -- Set the strictness part to L, but leave
992 -- the boxity side unaffected
993 -- It also ensures that Defer (Eval [LLLL]) = L
998 defer (Call _) = lazyDmd -- Approximation here?
999 defer (Box _) = lazyDmd
1000 defer (Defer ds) = Defer ds
1001 defer (Eval ds) = deferEval ds
1003 -- deferEval ds = defer (Eval ds)
1004 deferEval ds | allTop ds = Top
1005 | otherwise = Defer ds
1007 ---------------------
1008 absLub :: Demand -> Demand
1009 -- Computes (Abs `lub` d)
1010 -- For the Bot case consider
1011 -- f x y = if ... then x else error x
1012 -- Then for y we get Abs `lub` Bot, and we really
1017 absLub (Call _) = Top
1018 absLub (Box _) = Top
1019 absLub (Eval ds) = Defer (absLubs ds) -- Or (Defer ds)?
1020 absLub (Defer ds) = Defer (absLubs ds) -- Or (Defer ds)?
1022 absLubs = mapDmds absLub
1025 both :: Demand -> Demand -> Demand
1031 both Bot (Eval ds) = Eval (mapDmds (`both` Bot) ds)
1034 -- From 'error' itself we get demand Bot on x
1035 -- From the arg demand on x we get
1036 -- x :-> evalDmd = Box (Eval (Poly Abs))
1037 -- So we get Bot `both` Box (Eval (Poly Abs))
1038 -- = Seq Keep (Poly Bot)
1041 -- f x = if ... then error (fst x) else fst x
1042 -- Then we get (Eval (Box Bot, Bot) `lub` Eval (SA))
1044 -- which is what we want.
1047 both Top Bot = errDmd
1050 both Top (Box d) = Box d
1051 both Top (Call d) = Call d
1052 both Top (Eval ds) = Eval (mapDmds (`both` Top) ds)
1053 both Top (Defer ds) -- = defer (Top `both` Eval ds)
1054 -- = defer (Eval (mapDmds (`both` Top) ds))
1055 = deferEval (mapDmds (`both` Top) ds)
1058 both (Box d1) (Box d2) = box (d1 `both` d2)
1059 both (Box d1) d2@(Call _) = box (d1 `both` d2)
1060 both (Box d1) d2@(Eval _) = box (d1 `both` d2)
1061 both (Box d1) (Defer d2) = Box d1
1062 both d1@(Box _) d2 = d2 `both` d1
1064 both (Call d1) (Call d2) = Call (d1 `both` d2)
1065 both (Call d1) (Eval ds2) = Call d1 -- Could do better for (Poly Bot)?
1066 both (Call d1) (Defer ds2) = Call d1 -- Ditto
1067 both d1@(Call _) d2 = d1 `both` d1
1069 both (Eval ds1) (Eval ds2) = Eval (ds1 `boths` ds2)
1070 both (Eval ds1) (Defer ds2) = Eval (ds1 `boths` mapDmds defer ds2)
1071 both d1@(Eval ds1) d2 = d2 `both` d1
1073 both (Defer ds1) (Defer ds2) = deferEval (ds1 `boths` ds2)
1074 both d1@(Defer ds1) d2 = d2 `both` d1
1076 boths = zipWithDmds both
1081 %************************************************************************
1083 \subsection{Miscellaneous
1085 %************************************************************************
1089 #ifdef OLD_STRICTNESS
1090 get_changes binds = vcat (map get_changes_bind binds)
1092 get_changes_bind (Rec pairs) = vcat (map get_changes_pr pairs)
1093 get_changes_bind (NonRec id rhs) = get_changes_pr (id,rhs)
1095 get_changes_pr (id,rhs)
1096 = get_changes_var id $$ get_changes_expr rhs
1099 | isId var = get_changes_str var $$ get_changes_dmd var
1102 get_changes_expr (Type t) = empty
1103 get_changes_expr (Var v) = empty
1104 get_changes_expr (Lit l) = empty
1105 get_changes_expr (Note n e) = get_changes_expr e
1106 get_changes_expr (App e1 e2) = get_changes_expr e1 $$ get_changes_expr e2
1107 get_changes_expr (Lam b e) = {- get_changes_var b $$ -} get_changes_expr e
1108 get_changes_expr (Let b e) = get_changes_bind b $$ get_changes_expr e
1109 get_changes_expr (Case e b a) = get_changes_expr e $$ {- get_changes_var b $$ -} vcat (map get_changes_alt a)
1111 get_changes_alt (con,bs,rhs) = {- vcat (map get_changes_var bs) $$ -} get_changes_expr rhs
1114 | new_better && old_better = empty
1115 | new_better = message "BETTER"
1116 | old_better = message "WORSE"
1117 | otherwise = message "INCOMPARABLE"
1119 message word = text word <+> text "strictness for" <+> ppr id <+> info
1120 info = (text "Old" <+> ppr old) $$ (text "New" <+> ppr new)
1121 new = squashSig (idNewStrictness id) -- Don't report spurious diffs that the old
1122 -- strictness analyser can't track
1123 old = newStrictnessFromOld (idName id) (idArity id) (idStrictness id) (idCprInfo id)
1124 old_better = old `betterStrictness` new
1125 new_better = new `betterStrictness` old
1128 | isUnLiftedType (idType id) = empty -- Not useful
1129 | new_better && old_better = empty
1130 | new_better = message "BETTER"
1131 | old_better = message "WORSE"
1132 | otherwise = message "INCOMPARABLE"
1134 message word = text word <+> text "demand for" <+> ppr id <+> info
1135 info = (text "Old" <+> ppr old) $$ (text "New" <+> ppr new)
1136 new = squashDmd (argDemand (idNewDemandInfo id)) -- To avoid spurious improvements
1138 old = newDemand (idDemandInfo id)
1139 new_better = new `betterDemand` old
1140 old_better = old `betterDemand` new
1143 squashSig (StrictSig (DmdType fv ds res))
1144 = StrictSig (DmdType emptyDmdEnv (map squashDmd ds) res)
1146 -- squash just gets rid of call demands
1147 -- which the old analyser doesn't track
1148 squashDmd (Call d) = evalDmd
1149 squashDmd (Box d) = Box (squashDmd d)
1150 squashDmd (Eval ds) = Eval (mapDmds squashDmd ds)
1151 squashDmd (Defer ds) = Defer (mapDmds squashDmd ds)