2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
11 -- The above warning supression flag is a temporary kludge.
12 -- While working on this module you are encouraged to remove it and fix
13 -- any warnings in the module. See
14 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 module DmdAnal ( dmdAnalPgm, dmdAnalTopRhs,
18 both {- needed by WwLib -}
21 #include "HsVersions.h"
23 import DynFlags ( DynFlags, DynFlag(..) )
24 import StaticFlags ( opt_MaxWorkerArgs )
25 import NewDemand -- All of it
28 import CoreUtils ( exprIsHNF, exprIsTrivial, exprArity )
29 import DataCon ( dataConTyCon )
30 import TyCon ( isProductTyCon, isRecursiveTyCon )
31 import Id ( Id, idType, idInlinePragma,
32 isDataConWorkId, isGlobalId, idArity,
34 idDemandInfo, idStrictness, idCprInfo, idName,
36 idNewStrictness, idNewStrictness_maybe,
37 setIdNewStrictness, idNewDemandInfo,
38 idNewDemandInfo_maybe,
42 import IdInfo ( newStrictnessFromOld, newDemand )
46 import TysWiredIn ( unboxedPairDataCon )
47 import TysPrim ( realWorldStatePrimTy )
48 import UniqFM ( plusUFM_C, addToUFM_Directly, lookupUFM_Directly,
49 keysUFM, minusUFM, ufmToList, filterUFM )
50 import Type ( isUnLiftedType, coreEqType, splitTyConApp_maybe )
51 import Coercion ( coercionKind )
52 import CoreLint ( showPass, endPass )
53 import Util ( mapAndUnzip, lengthIs )
54 import BasicTypes ( Arity, TopLevelFlag(..), isTopLevel, isNeverActive,
56 import Maybes ( orElse, expectJust )
64 * set a noinline pragma on bottoming Ids
66 * Consider f x = x+1 `fatbar` error (show x)
67 We'd like to unbox x, even if that means reboxing it in the error case.
70 %************************************************************************
72 \subsection{Top level stuff}
74 %************************************************************************
77 dmdAnalPgm :: DynFlags -> [CoreBind] -> IO [CoreBind]
78 dmdAnalPgm dflags binds
80 showPass dflags "Demand analysis" ;
81 let { binds_plus_dmds = do_prog binds } ;
83 endPass dflags "Demand analysis"
84 Opt_D_dump_stranal binds_plus_dmds ;
86 -- Only if OLD_STRICTNESS is on, because only then is the old
87 -- strictness analyser run
88 let { dmd_changes = get_changes binds_plus_dmds } ;
89 printDump (text "Changes in demands" $$ dmd_changes) ;
91 return binds_plus_dmds
94 do_prog :: [CoreBind] -> [CoreBind]
95 do_prog binds = snd $ mapAccumL dmdAnalTopBind emptySigEnv binds
97 dmdAnalTopBind :: SigEnv
100 dmdAnalTopBind sigs (NonRec id rhs)
102 ( _, _, (_, rhs1)) = dmdAnalRhs TopLevel NonRecursive sigs (id, rhs)
103 (sigs2, _, (id2, rhs2)) = dmdAnalRhs TopLevel NonRecursive sigs (id, rhs1)
104 -- Do two passes to improve CPR information
105 -- See comments with ignore_cpr_info in mk_sig_ty
106 -- and with extendSigsWithLam
108 (sigs2, NonRec id2 rhs2)
110 dmdAnalTopBind sigs (Rec pairs)
112 (sigs', _, pairs') = dmdFix TopLevel sigs pairs
113 -- We get two iterations automatically
114 -- c.f. the NonRec case above
120 dmdAnalTopRhs :: CoreExpr -> (StrictSig, CoreExpr)
121 -- Analyse the RHS and return
122 -- a) appropriate strictness info
123 -- b) the unfolding (decorated with stricntess info)
127 call_dmd = vanillaCall (exprArity rhs)
128 (_, rhs1) = dmdAnal emptySigEnv call_dmd rhs
129 (rhs_ty, rhs2) = dmdAnal emptySigEnv call_dmd rhs1
130 sig = mkTopSigTy rhs rhs_ty
131 -- Do two passes; see notes with extendSigsWithLam
132 -- Otherwise we get bogus CPR info for constructors like
133 -- newtype T a = MkT a
134 -- The constructor looks like (\x::T a -> x), modulo the coerce
135 -- extendSigsWithLam will optimistically give x a CPR tag the
136 -- first time, which is wrong in the end.
139 %************************************************************************
141 \subsection{The analyser itself}
143 %************************************************************************
146 dmdAnal :: SigEnv -> Demand -> CoreExpr -> (DmdType, CoreExpr)
148 dmdAnal sigs Abs e = (topDmdType, e)
151 | not (isStrictDmd dmd)
153 (res_ty, e') = dmdAnal sigs evalDmd e
155 (deferType res_ty, e')
156 -- It's important not to analyse e with a lazy demand because
157 -- a) When we encounter case s of (a,b) ->
158 -- we demand s with U(d1d2)... but if the overall demand is lazy
159 -- that is wrong, and we'd need to reduce the demand on s,
160 -- which is inconvenient
161 -- b) More important, consider
162 -- f (let x = R in x+x), where f is lazy
163 -- We still want to mark x as demanded, because it will be when we
164 -- enter the let. If we analyse f's arg with a Lazy demand, we'll
165 -- just mark x as Lazy
166 -- c) The application rule wouldn't be right either
167 -- Evaluating (f x) in a L demand does *not* cause
168 -- evaluation of f in a C(L) demand!
171 dmdAnal sigs dmd (Lit lit)
172 = (topDmdType, Lit lit)
174 dmdAnal sigs dmd (Var var)
175 = (dmdTransform sigs var dmd, Var var)
177 dmdAnal sigs dmd (Cast e co)
178 = (dmd_ty, Cast e' co)
180 (dmd_ty, e') = dmdAnal sigs dmd' e
181 to_co = snd (coercionKind co)
183 | Just (tc, args) <- splitTyConApp_maybe to_co
184 , isRecursiveTyCon tc = evalDmd
186 -- This coerce usually arises from a recursive
187 -- newtype, and we don't want to look inside them
188 -- for exactly the same reason that we don't look
189 -- inside recursive products -- we might not reach
190 -- a fixpoint. So revert to a vanilla Eval demand
192 dmdAnal sigs dmd (Note n e)
193 = (dmd_ty, Note n e')
195 (dmd_ty, e') = dmdAnal sigs dmd e
197 dmdAnal sigs dmd (App fun (Type ty))
198 = (fun_ty, App fun' (Type ty))
200 (fun_ty, fun') = dmdAnal sigs dmd fun
202 -- Lots of the other code is there to make this
203 -- beautiful, compositional, application rule :-)
204 dmdAnal sigs dmd e@(App fun arg) -- Non-type arguments
205 = let -- [Type arg handled above]
206 (fun_ty, fun') = dmdAnal sigs (Call dmd) fun
207 (arg_ty, arg') = dmdAnal sigs arg_dmd arg
208 (arg_dmd, res_ty) = splitDmdTy fun_ty
210 (res_ty `bothType` arg_ty, App fun' arg')
212 dmdAnal sigs dmd (Lam var body)
215 (body_ty, body') = dmdAnal sigs dmd body
217 (body_ty, Lam var body')
219 | Call body_dmd <- dmd -- A call demand: good!
221 sigs' = extendSigsWithLam sigs var
222 (body_ty, body') = dmdAnal sigs' body_dmd body
223 (lam_ty, var') = annotateLamIdBndr body_ty var
225 (lam_ty, Lam var' body')
227 | otherwise -- Not enough demand on the lambda; but do the body
228 = let -- anyway to annotate it and gather free var info
229 (body_ty, body') = dmdAnal sigs evalDmd body
230 (lam_ty, var') = annotateLamIdBndr body_ty var
232 (deferType lam_ty, Lam var' body')
234 dmdAnal sigs dmd (Case scrut case_bndr ty [alt@(DataAlt dc,bndrs,rhs)])
235 | let tycon = dataConTyCon dc,
236 isProductTyCon tycon,
237 not (isRecursiveTyCon tycon)
239 sigs_alt = extendSigEnv NotTopLevel sigs case_bndr case_bndr_sig
240 (alt_ty, alt') = dmdAnalAlt sigs_alt dmd alt
241 (alt_ty1, case_bndr') = annotateBndr alt_ty case_bndr
242 (_, bndrs', _) = alt'
243 case_bndr_sig = cprSig
244 -- Inside the alternative, the case binder has the CPR property.
245 -- Meaning that a case on it will successfully cancel.
247 -- f True x = case x of y { I# x' -> if x' ==# 3 then y else I# 8 }
250 -- We want f to have the CPR property:
251 -- f b x = case fw b x of { r -> I# r }
252 -- fw True x = case x of y { I# x' -> if x' ==# 3 then x' else 8 }
255 -- Figure out whether the demand on the case binder is used, and use
256 -- that to set the scrut_dmd. This is utterly essential.
257 -- Consider f x = case x of y { (a,b) -> k y a }
258 -- If we just take scrut_demand = U(L,A), then we won't pass x to the
259 -- worker, so the worker will rebuild
260 -- x = (a, absent-error)
261 -- and that'll crash.
262 -- So at one stage I had:
263 -- dead_case_bndr = isAbsentDmd (idNewDemandInfo case_bndr')
264 -- keepity | dead_case_bndr = Drop
265 -- | otherwise = Keep
268 -- case x of y { (a,b) -> h y + a }
269 -- where h : U(LL) -> T
270 -- The above code would compute a Keep for x, since y is not Abs, which is silly
271 -- The insight is, of course, that a demand on y is a demand on the
272 -- scrutinee, so we need to `both` it with the scrut demand
274 alt_dmd = Eval (Prod [idNewDemandInfo b | b <- bndrs', isId b])
275 scrut_dmd = alt_dmd `both`
276 idNewDemandInfo case_bndr'
278 (scrut_ty, scrut') = dmdAnal sigs scrut_dmd scrut
280 (alt_ty1 `bothType` scrut_ty, Case scrut' case_bndr' ty [alt'])
282 dmdAnal sigs dmd (Case scrut case_bndr ty alts)
284 (alt_tys, alts') = mapAndUnzip (dmdAnalAlt sigs dmd) alts
285 (scrut_ty, scrut') = dmdAnal sigs evalDmd scrut
286 (alt_ty, case_bndr') = annotateBndr (foldr1 lubType alt_tys) case_bndr
288 -- pprTrace "dmdAnal:Case" (ppr alts $$ ppr alt_tys)
289 (alt_ty `bothType` scrut_ty, Case scrut' case_bndr' ty alts')
291 dmdAnal sigs dmd (Let (NonRec id rhs) body)
293 (sigs', lazy_fv, (id1, rhs')) = dmdAnalRhs NotTopLevel NonRecursive sigs (id, rhs)
294 (body_ty, body') = dmdAnal sigs' dmd body
295 (body_ty1, id2) = annotateBndr body_ty id1
296 body_ty2 = addLazyFVs body_ty1 lazy_fv
298 -- If the actual demand is better than the vanilla call
299 -- demand, you might think that we might do better to re-analyse
300 -- the RHS with the stronger demand.
301 -- But (a) That seldom happens, because it means that *every* path in
302 -- the body of the let has to use that stronger demand
303 -- (b) It often happens temporarily in when fixpointing, because
304 -- the recursive function at first seems to place a massive demand.
305 -- But we don't want to go to extra work when the function will
306 -- probably iterate to something less demanding.
307 -- In practice, all the times the actual demand on id2 is more than
308 -- the vanilla call demand seem to be due to (b). So we don't
309 -- bother to re-analyse the RHS.
310 (body_ty2, Let (NonRec id2 rhs') body')
312 dmdAnal sigs dmd (Let (Rec pairs) body)
314 bndrs = map fst pairs
315 (sigs', lazy_fv, pairs') = dmdFix NotTopLevel sigs pairs
316 (body_ty, body') = dmdAnal sigs' dmd body
317 body_ty1 = addLazyFVs body_ty lazy_fv
319 sigs' `seq` body_ty `seq`
321 (body_ty2, _) = annotateBndrs body_ty1 bndrs
322 -- Don't bother to add demand info to recursive
323 -- binders as annotateBndr does;
324 -- being recursive, we can't treat them strictly.
325 -- But we do need to remove the binders from the result demand env
327 (body_ty2, Let (Rec pairs') body')
330 dmdAnalAlt sigs dmd (con,bndrs,rhs)
332 (rhs_ty, rhs') = dmdAnal sigs dmd rhs
333 (alt_ty, bndrs') = annotateBndrs rhs_ty bndrs
334 final_alt_ty | io_hack_reqd = alt_ty `lubType` topDmdType
337 -- There's a hack here for I/O operations. Consider
338 -- case foo x s of { (# s, r #) -> y }
339 -- Is this strict in 'y'. Normally yes, but what if 'foo' is an I/O
340 -- operation that simply terminates the program (not in an erroneous way)?
341 -- In that case we should not evaluate y before the call to 'foo'.
342 -- Hackish solution: spot the IO-like situation and add a virtual branch,
346 -- other -> return ()
347 -- So the 'y' isn't necessarily going to be evaluated
349 -- A more complete example where this shows up is:
350 -- do { let len = <expensive> ;
351 -- ; when (...) (exitWith ExitSuccess)
354 io_hack_reqd = con == DataAlt unboxedPairDataCon &&
355 idType (head bndrs) `coreEqType` realWorldStatePrimTy
357 (final_alt_ty, (con, bndrs', rhs'))
360 %************************************************************************
362 \subsection{Bindings}
364 %************************************************************************
367 dmdFix :: TopLevelFlag
368 -> SigEnv -- Does not include bindings for this binding
371 [(Id,CoreExpr)]) -- Binders annotated with stricness info
373 dmdFix top_lvl sigs orig_pairs
374 = loop 1 initial_sigs orig_pairs
376 bndrs = map fst orig_pairs
377 initial_sigs = extendSigEnvList sigs [(id, (initialSig id, top_lvl)) | id <- bndrs]
380 -> SigEnv -- Already contains the current sigs
382 -> (SigEnv, DmdEnv, [(Id,CoreExpr)])
385 = (sigs', lazy_fv, pairs')
386 -- Note: use pairs', not pairs. pairs' is the result of
387 -- processing the RHSs with sigs (= sigs'), whereas pairs
388 -- is the result of processing the RHSs with the *previous*
389 -- iteration of sigs.
391 | n >= 10 = pprTrace "dmdFix loop" (ppr n <+> (vcat
392 [ text "Sigs:" <+> ppr [(id,lookup sigs id, lookup sigs' id) | (id,_) <- pairs],
393 text "env:" <+> ppr (ufmToList sigs),
394 text "binds:" <+> pprCoreBinding (Rec pairs)]))
395 (emptySigEnv, lazy_fv, orig_pairs) -- Safe output
396 -- The lazy_fv part is really important! orig_pairs has no strictness
397 -- info, including nothing about free vars. But if we have
398 -- letrec f = ....y..... in ...f...
399 -- where 'y' is free in f, we must record that y is mentioned,
400 -- otherwise y will get recorded as absent altogether
402 | otherwise = loop (n+1) sigs' pairs'
404 found_fixpoint = all (same_sig sigs sigs') bndrs
405 -- Use the new signature to do the next pair
406 -- The occurrence analyser has arranged them in a good order
407 -- so this can significantly reduce the number of iterations needed
408 ((sigs',lazy_fv), pairs') = mapAccumL (my_downRhs top_lvl) (sigs, emptyDmdEnv) pairs
410 my_downRhs top_lvl (sigs,lazy_fv) (id,rhs)
411 = -- pprTrace "downRhs {" (ppr id <+> (ppr old_sig))
413 -- pprTrace "downRhsEnd" (ppr id <+> ppr new_sig <+> char '}' )
414 ((sigs', lazy_fv'), pair')
417 (sigs', lazy_fv1, pair') = dmdAnalRhs top_lvl Recursive sigs (id,rhs)
418 lazy_fv' = plusUFM_C both lazy_fv lazy_fv1
419 -- old_sig = lookup sigs id
420 -- new_sig = lookup sigs' id
422 same_sig sigs sigs' var = lookup sigs var == lookup sigs' var
423 lookup sigs var = case lookupVarEnv sigs var of
426 -- Get an initial strictness signature from the Id
427 -- itself. That way we make use of earlier iterations
428 -- of the fixpoint algorithm. (Cunning plan.)
429 -- Note that the cunning plan extends to the DmdEnv too,
430 -- since it is part of the strictness signature
431 initialSig id = idNewStrictness_maybe id `orElse` botSig
433 dmdAnalRhs :: TopLevelFlag -> RecFlag
434 -> SigEnv -> (Id, CoreExpr)
435 -> (SigEnv, DmdEnv, (Id, CoreExpr))
436 -- Process the RHS of the binding, add the strictness signature
437 -- to the Id, and augment the environment with the signature as well.
439 dmdAnalRhs top_lvl rec_flag sigs (id, rhs)
440 = (sigs', lazy_fv, (id', rhs'))
442 arity = idArity id -- The idArity should be up to date
443 -- The simplifier was run just beforehand
444 (rhs_dmd_ty, rhs') = dmdAnal sigs (vanillaCall arity) rhs
445 (lazy_fv, sig_ty) = WARN( arity /= dmdTypeDepth rhs_dmd_ty && not (exprIsTrivial rhs), ppr id )
446 -- The RHS can be eta-reduced to just a variable,
447 -- in which case we should not complain.
448 mkSigTy top_lvl rec_flag id rhs rhs_dmd_ty
449 id' = id `setIdNewStrictness` sig_ty
450 sigs' = extendSigEnv top_lvl sigs id sig_ty
453 %************************************************************************
455 \subsection{Strictness signatures and types}
457 %************************************************************************
460 mkTopSigTy :: CoreExpr -> DmdType -> StrictSig
461 -- Take a DmdType and turn it into a StrictSig
462 -- NB: not used for never-inline things; hence False
463 mkTopSigTy rhs dmd_ty = snd (mk_sig_ty False False rhs dmd_ty)
465 mkSigTy :: TopLevelFlag -> RecFlag -> Id -> CoreExpr -> DmdType -> (DmdEnv, StrictSig)
466 mkSigTy top_lvl rec_flag id rhs dmd_ty
467 = mk_sig_ty never_inline thunk_cpr_ok rhs dmd_ty
469 never_inline = isNeverActive (idInlinePragma id)
470 maybe_id_dmd = idNewDemandInfo_maybe id
471 -- Is Nothing the first time round
474 | isTopLevel top_lvl = False -- Top level things don't get
475 -- their demandInfo set at all
476 | isRec rec_flag = False -- Ditto recursive things
477 | Just dmd <- maybe_id_dmd = isStrictDmd dmd
478 | otherwise = True -- Optimistic, first time round
482 The thunk_cpr_ok stuff [CPR-AND-STRICTNESS]
483 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
484 If the rhs is a thunk, we usually forget the CPR info, because
485 it is presumably shared (else it would have been inlined, and
486 so we'd lose sharing if w/w'd it into a function). E.g.
488 let r = case expensive of
492 If we marked r as having the CPR property, then we'd w/w into
494 let $wr = \() -> case expensive of
500 But now r is a thunk, which won't be inlined, so we are no further ahead.
503 f x = let r = case expensive of (a,b) -> (b,a)
504 in if foo r then r else (x,x)
506 Does f have the CPR property? Well, no.
508 However, if the strictness analyser has figured out (in a previous
509 iteration) that it's strict, then we DON'T need to forget the CPR info.
510 Instead we can retain the CPR info and do the thunk-splitting transform
511 (see WorkWrap.splitThunk).
513 This made a big difference to PrelBase.modInt, which had something like
514 modInt = \ x -> let r = ... -> I# v in
515 ...body strict in r...
516 r's RHS isn't a value yet; but modInt returns r in various branches, so
517 if r doesn't have the CPR property then neither does modInt
518 Another case I found in practice (in Complex.magnitude), looks like this:
519 let k = if ... then I# a else I# b
520 in ... body strict in k ....
521 (For this example, it doesn't matter whether k is returned as part of
522 the overall result; but it does matter that k's RHS has the CPR property.)
523 Left to itself, the simplifier will make a join point thus:
524 let $j k = ...body strict in k...
525 if ... then $j (I# a) else $j (I# b)
526 With thunk-splitting, we get instead
527 let $j x = let k = I#x in ...body strict in k...
528 in if ... then $j a else $j b
529 This is much better; there's a good chance the I# won't get allocated.
531 The difficulty with this is that we need the strictness type to
532 look at the body... but we now need the body to calculate the demand
533 on the variable, so we can decide whether its strictness type should
534 have a CPR in it or not. Simple solution:
535 a) use strictness info from the previous iteration
536 b) make sure we do at least 2 iterations, by doing a second
537 round for top-level non-recs. Top level recs will get at
538 least 2 iterations except for totally-bottom functions
539 which aren't very interesting anyway.
541 NB: strictly_demanded is never true of a top-level Id, or of a recursive Id.
543 The Nothing case in thunk_cpr_ok [CPR-AND-STRICTNESS]
544 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
545 Demand info now has a 'Nothing' state, just like strictness info.
546 The analysis works from 'dangerous' towards a 'safe' state; so we
547 start with botSig for 'Nothing' strictness infos, and we start with
548 "yes, it's demanded" for 'Nothing' in the demand info. The
549 fixpoint iteration will sort it all out.
551 We can't start with 'not-demanded' because then consider
555 if ... then t else I# y else f x'
557 In the first iteration we'd have no demand info for x, so assume
558 not-demanded; then we'd get TopRes for f's CPR info. Next iteration
559 we'd see that t was demanded, and so give it the CPR property, but by
560 now f has TopRes, so it will stay TopRes. Instead, with the Nothing
561 setting the first time round, we say 'yes t is demanded' the first
564 However, this does mean that for non-recursive bindings we must
565 iterate twice to be sure of not getting over-optimistic CPR info,
566 in the case where t turns out to be not-demanded. This is handled
571 mk_sig_ty never_inline thunk_cpr_ok rhs (DmdType fv dmds res)
572 = (lazy_fv, mkStrictSig dmd_ty)
574 dmd_ty = DmdType strict_fv final_dmds res'
576 lazy_fv = filterUFM (not . isStrictDmd) fv
577 strict_fv = filterUFM isStrictDmd fv
578 -- We put the strict FVs in the DmdType of the Id, so
579 -- that at its call sites we unleash demands on its strict fvs.
580 -- An example is 'roll' in imaginary/wheel-sieve2
581 -- Something like this:
583 -- go y = if ... then roll (x-1) else x+1
586 -- We want to see that roll is strict in x, which is because
587 -- go is called. So we put the DmdEnv for x in go's DmdType.
590 -- f :: Int -> Int -> Int
591 -- f x y = let t = x+1
592 -- h z = if z==0 then t else
593 -- if z==1 then x+1 else
597 -- Calling h does indeed evaluate x, but we can only see
598 -- that if we unleash a demand on x at the call site for t.
600 -- Incidentally, here's a place where lambda-lifting h would
601 -- lose the cigar --- we couldn't see the joint strictness in t/x
604 -- We don't want to put *all* the fv's from the RHS into the
605 -- DmdType, because that makes fixpointing very slow --- the
606 -- DmdType gets full of lazy demands that are slow to converge.
608 final_dmds = setUnpackStrategy dmds
609 -- Set the unpacking strategy
612 RetCPR | ignore_cpr_info -> TopRes
614 ignore_cpr_info = not (exprIsHNF rhs || thunk_cpr_ok)
617 The unpack strategy determines whether we'll *really* unpack the argument,
618 or whether we'll just remember its strictness. If unpacking would give
619 rise to a *lot* of worker args, we may decide not to unpack after all.
622 setUnpackStrategy :: [Demand] -> [Demand]
624 = snd (go (opt_MaxWorkerArgs - nonAbsentArgs ds) ds)
626 go :: Int -- Max number of args available for sub-components of [Demand]
628 -> (Int, [Demand]) -- Args remaining after subcomponents of [Demand] are unpacked
630 go n (Eval (Prod cs) : ds)
631 | n' >= 0 = Eval (Prod cs') `cons` go n'' ds
632 | otherwise = Box (Eval (Prod cs)) `cons` go n ds
635 n' = n + 1 - non_abs_args
636 -- Add one to the budget 'cos we drop the top-level arg
637 non_abs_args = nonAbsentArgs cs
638 -- Delete # of non-absent args to which we'll now be committed
640 go n (d:ds) = d `cons` go n ds
643 cons d (n,ds) = (n, d:ds)
645 nonAbsentArgs :: [Demand] -> Int
647 nonAbsentArgs (Abs : ds) = nonAbsentArgs ds
648 nonAbsentArgs (d : ds) = 1 + nonAbsentArgs ds
652 %************************************************************************
654 \subsection{Strictness signatures and types}
656 %************************************************************************
659 unitVarDmd var dmd = DmdType (unitVarEnv var dmd) [] TopRes
661 addVarDmd top_lvl dmd_ty@(DmdType fv ds res) var dmd
662 | isTopLevel top_lvl = dmd_ty -- Don't record top level things
663 | otherwise = DmdType (extendVarEnv fv var dmd) ds res
665 addLazyFVs (DmdType fv ds res) lazy_fvs
666 = DmdType both_fv1 ds res
668 both_fv = (plusUFM_C both fv lazy_fvs)
669 both_fv1 = modifyEnv (isBotRes res) (`both` Bot) lazy_fvs fv both_fv
670 -- This modifyEnv is vital. Consider
671 -- let f = \x -> (x,y)
673 -- Here, y is treated as a lazy-fv of f, but we must `both` that L
674 -- demand with the bottom coming up from 'error'
676 -- I got a loop in the fixpointer without this, due to an interaction
677 -- with the lazy_fv filtering in mkSigTy. Roughly, it was
679 -- = letrec g y = x `fatbar`
680 -- letrec h z = z + ...g...
683 -- In the initial iteration for f, f=Bot
684 -- Suppose h is found to be strict in z, but the occurrence of g in its RHS
685 -- is lazy. Now consider the fixpoint iteration for g, esp the demands it
686 -- places on its free variables. Suppose it places none. Then the
687 -- x `fatbar` ...call to h...
688 -- will give a x->V demand for x. That turns into a L demand for x,
689 -- which floats out of the defn for h. Without the modifyEnv, that
690 -- L demand doesn't get both'd with the Bot coming up from the inner
691 -- call to f. So we just get an L demand for x for g.
693 -- A better way to say this is that the lazy-fv filtering should give the
694 -- same answer as putting the lazy fv demands in the function's type.
696 annotateBndr :: DmdType -> Var -> (DmdType, Var)
697 -- The returned env has the var deleted
698 -- The returned var is annotated with demand info
699 -- No effect on the argument demands
700 annotateBndr dmd_ty@(DmdType fv ds res) var
701 | isTyVar var = (dmd_ty, var)
702 | otherwise = (DmdType fv' ds res, setIdNewDemandInfo var dmd)
704 (fv', dmd) = removeFV fv var res
706 annotateBndrs = mapAccumR annotateBndr
708 annotateLamIdBndr dmd_ty@(DmdType fv ds res) id
709 -- For lambdas we add the demand to the argument demands
710 -- Only called for Ids
712 (DmdType fv' (hacked_dmd:ds) res, setIdNewDemandInfo id hacked_dmd)
714 (fv', dmd) = removeFV fv id res
715 hacked_dmd = argDemand dmd
716 -- This call to argDemand is vital, because otherwise we label
717 -- a lambda binder with demand 'B'. But in terms of calling
718 -- conventions that's Abs, because we don't pass it. But
719 -- when we do a w/w split we get
720 -- fw x = (\x y:B -> ...) x (error "oops")
721 -- And then the simplifier things the 'B' is a strict demand
722 -- and evaluates the (error "oops"). Sigh
724 removeFV fv id res = (fv', zapUnlifted id dmd)
726 fv' = fv `delVarEnv` id
727 dmd = lookupVarEnv fv id `orElse` deflt
728 deflt | isBotRes res = Bot
731 -- For unlifted-type variables, we are only
732 -- interested in Bot/Abs/Box Abs
733 zapUnlifted is Bot = Bot
734 zapUnlifted id Abs = Abs
735 zapUnlifted id dmd | isUnLiftedType (idType id) = lazyDmd
739 %************************************************************************
741 \subsection{Strictness signatures}
743 %************************************************************************
746 type SigEnv = VarEnv (StrictSig, TopLevelFlag)
747 -- We use the SigEnv to tell us whether to
748 -- record info about a variable in the DmdEnv
749 -- We do so if it's a LocalId, but not top-level
751 -- The DmdEnv gives the demand on the free vars of the function
752 -- when it is given enough args to satisfy the strictness signature
754 emptySigEnv = emptyVarEnv
756 extendSigEnv :: TopLevelFlag -> SigEnv -> Id -> StrictSig -> SigEnv
757 extendSigEnv top_lvl env var sig = extendVarEnv env var (sig, top_lvl)
759 extendSigEnvList = extendVarEnvList
761 extendSigsWithLam :: SigEnv -> Id -> SigEnv
762 -- Extend the SigEnv when we meet a lambda binder
763 -- If the binder is marked demanded with a product demand, then give it a CPR
764 -- signature, because in the likely event that this is a lambda on a fn defn
765 -- [we only use this when the lambda is being consumed with a call demand],
766 -- it'll be w/w'd and so it will be CPR-ish. E.g.
767 -- f = \x::(Int,Int). if ...strict in x... then
771 -- We want f to have the CPR property because x does, by the time f has been w/w'd
773 -- Also note that we only want to do this for something that
774 -- definitely has product type, else we may get over-optimistic
775 -- CPR results (e.g. from \x -> x!).
777 extendSigsWithLam sigs id
778 = case idNewDemandInfo_maybe id of
779 Nothing -> extendVarEnv sigs id (cprSig, NotTopLevel)
780 -- Optimistic in the Nothing case;
781 -- See notes [CPR-AND-STRICTNESS]
782 Just (Eval (Prod ds)) -> extendVarEnv sigs id (cprSig, NotTopLevel)
786 dmdTransform :: SigEnv -- The strictness environment
787 -> Id -- The function
788 -> Demand -- The demand on the function
789 -> DmdType -- The demand type of the function in this context
790 -- Returned DmdEnv includes the demand on
791 -- this function plus demand on its free variables
793 dmdTransform sigs var dmd
795 ------ DATA CONSTRUCTOR
796 | isDataConWorkId var -- Data constructor
798 StrictSig dmd_ty = idNewStrictness var -- It must have a strictness sig
799 DmdType _ _ con_res = dmd_ty
802 if arity == call_depth then -- Saturated, so unleash the demand
804 -- Important! If we Keep the constructor application, then
805 -- we need the demands the constructor places (always lazy)
806 -- If not, we don't need to. For example:
807 -- f p@(x,y) = (p,y) -- S(AL)
809 -- It's vital that we don't calculate Absent for a!
810 dmd_ds = case res_dmd of
811 Box (Eval ds) -> mapDmds box ds
815 -- ds can be empty, when we are just seq'ing the thing
816 -- If so we must make up a suitable bunch of demands
817 arg_ds = case dmd_ds of
818 Poly d -> replicate arity d
819 Prod ds -> ASSERT( ds `lengthIs` arity ) ds
822 mkDmdType emptyDmdEnv arg_ds con_res
823 -- Must remember whether it's a product, hence con_res, not TopRes
827 ------ IMPORTED FUNCTION
828 | isGlobalId var, -- Imported function
829 let StrictSig dmd_ty = idNewStrictness var
830 = if dmdTypeDepth dmd_ty <= call_depth then -- Saturated, so unleash the demand
835 ------ LOCAL LET/REC BOUND THING
836 | Just (StrictSig dmd_ty, top_lvl) <- lookupVarEnv sigs var
838 fn_ty | dmdTypeDepth dmd_ty <= call_depth = dmd_ty
839 | otherwise = deferType dmd_ty
840 -- NB: it's important to use deferType, and not just return topDmdType
841 -- Consider let { f x y = p + x } in f 1
842 -- The application isn't saturated, but we must nevertheless propagate
843 -- a lazy demand for p!
845 addVarDmd top_lvl fn_ty var dmd
847 ------ LOCAL NON-LET/REC BOUND THING
848 | otherwise -- Default case
852 (call_depth, res_dmd) = splitCallDmd dmd
856 %************************************************************************
860 %************************************************************************
863 splitDmdTy :: DmdType -> (Demand, DmdType)
864 -- Split off one function argument
865 -- We already have a suitable demand on all
866 -- free vars, so no need to add more!
867 splitDmdTy (DmdType fv (dmd:dmds) res_ty) = (dmd, DmdType fv dmds res_ty)
868 splitDmdTy ty@(DmdType fv [] res_ty) = (resTypeArgDmd res_ty, ty)
870 splitCallDmd :: Demand -> (Int, Demand)
871 splitCallDmd (Call d) = case splitCallDmd d of
873 splitCallDmd d = (0, d)
875 vanillaCall :: Arity -> Demand
876 vanillaCall 0 = evalDmd
877 vanillaCall n = Call (vanillaCall (n-1))
879 deferType :: DmdType -> DmdType
880 deferType (DmdType fv _ _) = DmdType (deferEnv fv) [] TopRes
881 -- Notice that we throw away info about both arguments and results
882 -- For example, f = let ... in \x -> x
883 -- We don't want to get a stricness type V->T for f.
885 deferEnv :: DmdEnv -> DmdEnv
886 deferEnv fv = mapVarEnv defer fv
890 argDemand :: Demand -> Demand
891 -- The 'Defer' demands are just Lazy at function boundaries
892 -- Ugly! Ask John how to improve it.
893 argDemand Top = lazyDmd
894 argDemand (Defer d) = lazyDmd
895 argDemand (Eval ds) = Eval (mapDmds argDemand ds)
896 argDemand (Box Bot) = evalDmd
897 argDemand (Box d) = box (argDemand d)
898 argDemand Bot = Abs -- Don't pass args that are consumed (only) by bottom
903 -------------------------
904 -- Consider (if x then y else []) with demand V
905 -- Then the first branch gives {y->V} and the second
906 -- *implicitly* has {y->A}. So we must put {y->(V `lub` A)}
907 -- in the result env.
908 lubType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
909 = DmdType lub_fv2 (lub_ds ds1 ds2) (r1 `lubRes` r2)
911 lub_fv = plusUFM_C lub fv1 fv2
912 lub_fv1 = modifyEnv (not (isBotRes r1)) absLub fv2 fv1 lub_fv
913 lub_fv2 = modifyEnv (not (isBotRes r2)) absLub fv1 fv2 lub_fv1
914 -- lub is the identity for Bot
916 -- Extend the shorter argument list to match the longer
917 lub_ds (d1:ds1) (d2:ds2) = lub d1 d2 : lub_ds ds1 ds2
919 lub_ds ds1 [] = map (`lub` resTypeArgDmd r2) ds1
920 lub_ds [] ds2 = map (resTypeArgDmd r1 `lub`) ds2
922 -----------------------------------
923 -- (t1 `bothType` t2) takes the argument/result info from t1,
924 -- using t2 just for its free-var info
925 -- NB: Don't forget about r2! It might be BotRes, which is
926 -- a bottom demand on all the in-scope variables.
927 -- Peter: can this be done more neatly?
928 bothType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
929 = DmdType both_fv2 ds1 (r1 `bothRes` r2)
931 both_fv = plusUFM_C both fv1 fv2
932 both_fv1 = modifyEnv (isBotRes r1) (`both` Bot) fv2 fv1 both_fv
933 both_fv2 = modifyEnv (isBotRes r2) (`both` Bot) fv1 fv2 both_fv1
934 -- both is the identity for Abs
941 lubRes RetCPR RetCPR = RetCPR
942 lubRes r1 r2 = TopRes
944 -- If either diverges, the whole thing does
945 -- Otherwise take CPR info from the first
946 bothRes r1 BotRes = BotRes
951 modifyEnv :: Bool -- No-op if False
952 -> (Demand -> Demand) -- The zapper
953 -> DmdEnv -> DmdEnv -- Env1 and Env2
954 -> DmdEnv -> DmdEnv -- Transform this env
955 -- Zap anything in Env1 but not in Env2
956 -- Assume: dom(env) includes dom(Env1) and dom(Env2)
958 modifyEnv need_to_modify zapper env1 env2 env
959 | need_to_modify = foldr zap env (keysUFM (env1 `minusUFM` env2))
962 zap uniq env = addToUFM_Directly env uniq (zapper current_val)
964 current_val = expectJust "modifyEnv" (lookupUFM_Directly env uniq)
968 %************************************************************************
970 \subsection{LUB and BOTH}
972 %************************************************************************
975 lub :: Demand -> Demand -> Demand
978 lub Abs d2 = absLub d2
980 lub (Defer ds1) d2 = defer (Eval ds1 `lub` d2)
982 lub (Call d1) (Call d2) = Call (d1 `lub` d2)
983 lub d1@(Call _) (Box d2) = d1 `lub` d2 -- Just strip the box
984 lub d1@(Call _) d2@(Eval _) = d2 -- Presumably seq or vanilla eval
985 lub d1@(Call _) d2 = d2 `lub` d1 -- Bot, Abs, Top
987 -- For the Eval case, we use these approximation rules
988 -- Box Bot <= Eval (Box Bot ...)
989 -- Box Top <= Defer (Box Bot ...)
990 -- Box (Eval ds) <= Eval (map Box ds)
991 lub (Eval ds1) (Eval ds2) = Eval (ds1 `lubs` ds2)
992 lub (Eval ds1) (Box Bot) = Eval (mapDmds (`lub` Box Bot) ds1)
993 lub (Eval ds1) (Box (Eval ds2)) = Eval (ds1 `lubs` mapDmds box ds2)
994 lub (Eval ds1) (Box Abs) = deferEval (mapDmds (`lub` Box Bot) ds1)
995 lub d1@(Eval _) d2 = d2 `lub` d1 -- Bot,Abs,Top,Call,Defer
997 lub (Box d1) (Box d2) = box (d1 `lub` d2)
998 lub d1@(Box _) d2 = d2 `lub` d1
1000 lubs ds1 ds2 = zipWithDmds lub ds1 ds2
1002 ---------------------
1003 -- box is the smart constructor for Box
1004 -- It computes <B,bot> & d
1005 -- INVARIANT: (Box d) => d = Bot, Abs, Eval
1006 -- Seems to be no point in allowing (Box (Call d))
1007 box (Call d) = Call d -- The odd man out. Why?
1009 box (Defer _) = lazyDmd
1010 box Top = lazyDmd -- Box Abs and Box Top
1011 box Abs = lazyDmd -- are the same <B,L>
1012 box d = Box d -- Bot, Eval
1015 defer :: Demand -> Demand
1017 -- defer is the smart constructor for Defer
1018 -- The idea is that (Defer ds) = <U(ds), L>
1020 -- It specifies what happens at a lazy function argument
1021 -- or a lambda; the L* operator
1022 -- Set the strictness part to L, but leave
1023 -- the boxity side unaffected
1024 -- It also ensures that Defer (Eval [LLLL]) = L
1029 defer (Call _) = lazyDmd -- Approximation here?
1030 defer (Box _) = lazyDmd
1031 defer (Defer ds) = Defer ds
1032 defer (Eval ds) = deferEval ds
1034 -- deferEval ds = defer (Eval ds)
1035 deferEval ds | allTop ds = Top
1036 | otherwise = Defer ds
1038 ---------------------
1039 absLub :: Demand -> Demand
1040 -- Computes (Abs `lub` d)
1041 -- For the Bot case consider
1042 -- f x y = if ... then x else error x
1043 -- Then for y we get Abs `lub` Bot, and we really
1048 absLub (Call _) = Top
1049 absLub (Box _) = Top
1050 absLub (Eval ds) = Defer (absLubs ds) -- Or (Defer ds)?
1051 absLub (Defer ds) = Defer (absLubs ds) -- Or (Defer ds)?
1053 absLubs = mapDmds absLub
1056 both :: Demand -> Demand -> Demand
1062 both Bot (Eval ds) = Eval (mapDmds (`both` Bot) ds)
1065 -- From 'error' itself we get demand Bot on x
1066 -- From the arg demand on x we get
1067 -- x :-> evalDmd = Box (Eval (Poly Abs))
1068 -- So we get Bot `both` Box (Eval (Poly Abs))
1069 -- = Seq Keep (Poly Bot)
1072 -- f x = if ... then error (fst x) else fst x
1073 -- Then we get (Eval (Box Bot, Bot) `lub` Eval (SA))
1075 -- which is what we want.
1078 both Top Bot = errDmd
1081 both Top (Box d) = Box d
1082 both Top (Call d) = Call d
1083 both Top (Eval ds) = Eval (mapDmds (`both` Top) ds)
1084 both Top (Defer ds) -- = defer (Top `both` Eval ds)
1085 -- = defer (Eval (mapDmds (`both` Top) ds))
1086 = deferEval (mapDmds (`both` Top) ds)
1089 both (Box d1) (Box d2) = box (d1 `both` d2)
1090 both (Box d1) d2@(Call _) = box (d1 `both` d2)
1091 both (Box d1) d2@(Eval _) = box (d1 `both` d2)
1092 both (Box d1) (Defer d2) = Box d1
1093 both d1@(Box _) d2 = d2 `both` d1
1095 both (Call d1) (Call d2) = Call (d1 `both` d2)
1096 both (Call d1) (Eval ds2) = Call d1 -- Could do better for (Poly Bot)?
1097 both (Call d1) (Defer ds2) = Call d1 -- Ditto
1098 both d1@(Call _) d2 = d1 `both` d1
1100 both (Eval ds1) (Eval ds2) = Eval (ds1 `boths` ds2)
1101 both (Eval ds1) (Defer ds2) = Eval (ds1 `boths` mapDmds defer ds2)
1102 both d1@(Eval ds1) d2 = d2 `both` d1
1104 both (Defer ds1) (Defer ds2) = deferEval (ds1 `boths` ds2)
1105 both d1@(Defer ds1) d2 = d2 `both` d1
1107 boths ds1 ds2 = zipWithDmds both ds1 ds2
1112 %************************************************************************
1114 \subsection{Miscellaneous
1116 %************************************************************************
1120 #ifdef OLD_STRICTNESS
1121 get_changes binds = vcat (map get_changes_bind binds)
1123 get_changes_bind (Rec pairs) = vcat (map get_changes_pr pairs)
1124 get_changes_bind (NonRec id rhs) = get_changes_pr (id,rhs)
1126 get_changes_pr (id,rhs)
1127 = get_changes_var id $$ get_changes_expr rhs
1130 | isId var = get_changes_str var $$ get_changes_dmd var
1133 get_changes_expr (Type t) = empty
1134 get_changes_expr (Var v) = empty
1135 get_changes_expr (Lit l) = empty
1136 get_changes_expr (Note n e) = get_changes_expr e
1137 get_changes_expr (App e1 e2) = get_changes_expr e1 $$ get_changes_expr e2
1138 get_changes_expr (Lam b e) = {- get_changes_var b $$ -} get_changes_expr e
1139 get_changes_expr (Let b e) = get_changes_bind b $$ get_changes_expr e
1140 get_changes_expr (Case e b a) = get_changes_expr e $$ {- get_changes_var b $$ -} vcat (map get_changes_alt a)
1142 get_changes_alt (con,bs,rhs) = {- vcat (map get_changes_var bs) $$ -} get_changes_expr rhs
1145 | new_better && old_better = empty
1146 | new_better = message "BETTER"
1147 | old_better = message "WORSE"
1148 | otherwise = message "INCOMPARABLE"
1150 message word = text word <+> text "strictness for" <+> ppr id <+> info
1151 info = (text "Old" <+> ppr old) $$ (text "New" <+> ppr new)
1152 new = squashSig (idNewStrictness id) -- Don't report spurious diffs that the old
1153 -- strictness analyser can't track
1154 old = newStrictnessFromOld (idName id) (idArity id) (idStrictness id) (idCprInfo id)
1155 old_better = old `betterStrictness` new
1156 new_better = new `betterStrictness` old
1159 | isUnLiftedType (idType id) = empty -- Not useful
1160 | new_better && old_better = empty
1161 | new_better = message "BETTER"
1162 | old_better = message "WORSE"
1163 | otherwise = message "INCOMPARABLE"
1165 message word = text word <+> text "demand for" <+> ppr id <+> info
1166 info = (text "Old" <+> ppr old) $$ (text "New" <+> ppr new)
1167 new = squashDmd (argDemand (idNewDemandInfo id)) -- To avoid spurious improvements
1169 old = newDemand (idDemandInfo id)
1170 new_better = new `betterDemand` old
1171 old_better = old `betterDemand` new
1173 betterStrictness :: StrictSig -> StrictSig -> Bool
1174 betterStrictness (StrictSig t1) (StrictSig t2) = betterDmdType t1 t2
1176 betterDmdType t1 t2 = (t1 `lubType` t2) == t2
1178 betterDemand :: Demand -> Demand -> Bool
1179 -- If d1 `better` d2, and d2 `better` d2, then d1==d2
1180 betterDemand d1 d2 = (d1 `lub` d2) == d2
1182 squashSig (StrictSig (DmdType fv ds res))
1183 = StrictSig (DmdType emptyDmdEnv (map squashDmd ds) res)
1185 -- squash just gets rid of call demands
1186 -- which the old analyser doesn't track
1187 squashDmd (Call d) = evalDmd
1188 squashDmd (Box d) = Box (squashDmd d)
1189 squashDmd (Eval ds) = Eval (mapDmds squashDmd ds)
1190 squashDmd (Defer ds) = Defer (mapDmds squashDmd ds)