2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[StrictAnal]{``Simple'' Mycroft-style strictness analyser}
6 The original version(s) of all strictness-analyser code (except the
7 Semantique analyser) was written by Andy Gill.
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 #ifndef OLD_STRICTNESS
18 module StrictAnal ( ) where
22 module StrictAnal ( saBinds ) where
24 #include "HsVersions.h"
26 import DynFlags ( DynFlags, DynFlag(..) )
28 import Id ( setIdStrictness, setInlinePragma,
29 idDemandInfo, setIdDemandInfo, isBottomingId,
32 import CoreLint ( showPass, endPass )
33 import ErrUtils ( dumpIfSet_dyn )
36 import Demand ( Demand, wwStrict, isStrict, isLazy )
37 import Util ( zipWith3Equal, stretchZipWith, compareLength )
38 import BasicTypes ( Activation( NeverActive ) )
44 %************************************************************************
46 \subsection[Thoughts]{Random thoughts}
48 %************************************************************************
50 A note about worker-wrappering. If we have
53 f = let v = <expensive>
56 and we deduce that f is strict, it is nevertheless NOT safe to worker-wapper to
58 f = \x -> case x of Int x# -> fw x#
59 fw = \x# -> let x = Int x#
64 because this obviously loses laziness, since now <expensive>
65 is done each time. Alas.
67 WATCH OUT! This can mean that something is unboxed only to be
68 boxed again. For example
72 Here g is strict, and *will* split into worker-wrapper. A call to
73 g, with the wrapper inlined will then be
75 case arg of Int a# -> gw a#
77 Now g calls f, which has no wrapper, so it has to box it.
79 gw = \a# -> f (Int a#)
84 %************************************************************************
86 \subsection[iface-StrictAnal]{Interface to the outside world}
88 %************************************************************************
90 @saBinds@ decorates bindings with strictness info. A later
91 worker-wrapper pass can use this info to create wrappers and
95 saBinds :: DynFlags -> [CoreBind] -> IO [CoreBind]
98 showPass dflags "Strictness analysis";
100 -- Mark each binder with its strictness
101 #ifndef OMIT_STRANAL_STATS
102 let { (binds_w_strictness, sa_stats) = runState $ (saTopBinds binds) nullSaStats };
103 dumpIfSet_dyn dflags Opt_D_dump_simpl_stats "Strictness analysis statistics"
106 let { binds_w_strictness = unSaM $ saTopBindsBinds binds };
109 endPass dflags "Strictness analysis" Opt_D_dump_stranal
114 %************************************************************************
116 \subsection[saBinds]{Strictness analysis of bindings}
118 %************************************************************************
120 [Some of the documentation about types, etc., in \tr{SaLib} may be
121 helpful for understanding this module.]
123 @saTopBinds@ tags each binder in the program with its @Demand@.
124 That tells how each binder is {\em used}; if @Strict@, then the binder
125 is sure to be evaluated to HNF; if @NonStrict@ it may or may not be;
126 if @Absent@, then it certainly is not used. [DATED; ToDo: update]
128 (The above info is actually recorded for posterity in each binder's
129 IdInfo, notably its @DemandInfo@.)
131 We proceed by analysing the bindings top-to-bottom, building up an
132 environment which maps @Id@s to their abstract values (i.e., an
133 @AbsValEnv@ maps an @Id@ to its @AbsVal@).
136 saTopBinds :: [CoreBind] -> SaM [CoreBind] -- not exported
140 starting_abs_env = nullAbsValEnv
142 do_it starting_abs_env starting_abs_env binds
144 do_it _ _ [] = return []
145 do_it senv aenv (b:bs) = do
146 (senv2, aenv2, new_b) <- saTopBind senv aenv b
147 new_bs <- do_it senv2 aenv2 bs
148 return (new_b : new_bs)
151 @saTopBind@ is only used for the top level. We don't add any demand
152 info to these ids because we can't work it out. In any case, it
153 doesn't do us any good to know whether top-level binders are sure to
154 be used; we can't turn top-level @let@s into @case@s.
157 saTopBind :: StrictEnv -> AbsenceEnv
159 -> SaM (StrictEnv, AbsenceEnv, CoreBind)
161 saTopBind str_env abs_env (NonRec binder rhs) = do
162 new_rhs <- saExpr minDemand str_env abs_env rhs
164 str_rhs = absEval StrAnal rhs str_env
165 abs_rhs = absEval AbsAnal rhs abs_env
167 widened_str_rhs = widen StrAnal str_rhs
168 widened_abs_rhs = widen AbsAnal abs_rhs
169 -- The widening above is done for efficiency reasons.
170 -- See notes on Let case in SaAbsInt.lhs
173 = addStrictnessInfoToTopId
174 widened_str_rhs widened_abs_rhs
177 -- Augment environments with a mapping of the
178 -- binder to its abstract values, computed by absEval
179 new_str_env = addOneToAbsValEnv str_env binder widened_str_rhs
180 new_abs_env = addOneToAbsValEnv abs_env binder widened_abs_rhs
182 return (new_str_env, new_abs_env, NonRec new_binder new_rhs)
184 saTopBind str_env abs_env (Rec pairs)
186 (binders,rhss) = unzip pairs
187 str_rhss = fixpoint StrAnal binders rhss str_env
188 abs_rhss = fixpoint AbsAnal binders rhss abs_env
189 -- fixpoint returns widened values
190 new_str_env = growAbsValEnvList str_env (binders `zip` str_rhss)
191 new_abs_env = growAbsValEnvList abs_env (binders `zip` abs_rhss)
192 new_binders = zipWith3Equal "saTopBind" addStrictnessInfoToTopId
193 str_rhss abs_rhss binders
195 new_rhss <- mapM (saExpr minDemand new_str_env new_abs_env) rhss
197 new_pairs = new_binders `zip` new_rhss
199 return (new_str_env, new_abs_env, Rec new_pairs)
202 -- Top level divergent bindings are marked NOINLINE
203 -- This avoids fruitless inlining of top level error functions
204 addStrictnessInfoToTopId str_val abs_val bndr
205 = if isBottomingId new_id then
206 new_id `setInlinePragma` NeverActive
210 new_id = addStrictnessInfoToId str_val abs_val bndr
213 %************************************************************************
215 \subsection[saExpr]{Strictness analysis of an expression}
217 %************************************************************************
219 @saExpr@ computes the strictness of an expression within a given
223 saExpr :: Demand -> StrictEnv -> AbsenceEnv -> CoreExpr -> SaM CoreExpr
224 -- The demand is the least demand we expect on the
225 -- expression. WwStrict is the least, because we're only
226 -- interested in the expression at all if it's being evaluated,
227 -- but the demand may be more. E.g.
229 -- where f has strictness u(LL), will evaluate E with demand u(LL)
232 minDemands = repeat minDemand
234 -- When we find an application, do the arguments
235 -- with demands gotten from the function
236 saApp str_env abs_env (fun, args) = do
237 args' <- sequence sa_args
238 fun' <- saExpr minDemand str_env abs_env fun
239 return (mkApps fun' args')
241 arg_dmds = case fun of
242 Var var -> case lookupAbsValEnv str_env var of
243 Just (AbsApproxFun ds _)
244 | compareLength ds args /= LT
245 -- 'ds' is at least as long as 'args'.
250 sa_args = stretchZipWith isTypeArg (error "saApp:dmd")
252 -- The arg_dmds are for value args only, we need to skip
253 -- over the type args when pairing up with the demands
254 -- Hence the stretchZipWith
256 sa_arg arg dmd = saExpr dmd' str_env abs_env arg
258 -- Bring arg demand up to minDemand
259 dmd' | isLazy dmd = minDemand
262 saExpr _ _ _ e@(Var _) = return e
263 saExpr _ _ _ e@(Lit _) = return e
264 saExpr _ _ _ e@(Type _) = return e
266 saExpr dmd str_env abs_env (Lam bndr body)
267 = do -- Don't bother to set the demand-info on a lambda binder
268 -- We do that only for let(rec)-bound functions
269 new_body <- saExpr minDemand str_env abs_env body
270 return (Lam bndr new_body)
272 saExpr dmd str_env abs_env e@(App fun arg)
273 = saApp str_env abs_env (collectArgs e)
275 saExpr dmd str_env abs_env (Note note expr) = do
276 new_expr <- saExpr dmd str_env abs_env expr
277 return (Note note new_expr)
279 saExpr dmd str_env abs_env (Case expr case_bndr alts) = do
280 new_expr <- saExpr minDemand str_env abs_env expr
281 new_alts <- mapM sa_alt alts
283 new_case_bndr = addDemandInfoToCaseBndr dmd str_env abs_env alts case_bndr
284 return (Case new_expr new_case_bndr new_alts)
286 sa_alt (con, binders, rhs) = do
287 new_rhs <- saExpr dmd str_env abs_env rhs
289 new_binders = map add_demand_info binders
290 add_demand_info bndr | isTyVar bndr = bndr
291 | otherwise = addDemandInfoToId dmd str_env abs_env rhs bndr
293 tickCases new_binders -- stats
294 return (con, new_binders, new_rhs)
296 saExpr dmd str_env abs_env (Let (NonRec binder rhs) body) = do
297 -- Analyse the RHS in the environment at hand
299 -- Find the demand on the RHS
300 rhs_dmd = findDemand dmd str_env abs_env body binder
302 -- Bind this binder to the abstract value of the RHS; analyse
303 -- the body of the `let' in the extended environment.
304 str_rhs_val = absEval StrAnal rhs str_env
305 abs_rhs_val = absEval AbsAnal rhs abs_env
307 widened_str_rhs = widen StrAnal str_rhs_val
308 widened_abs_rhs = widen AbsAnal abs_rhs_val
309 -- The widening above is done for efficiency reasons.
310 -- See notes on Let case in SaAbsInt.lhs
312 new_str_env = addOneToAbsValEnv str_env binder widened_str_rhs
313 new_abs_env = addOneToAbsValEnv abs_env binder widened_abs_rhs
315 -- Now determine the strictness of this binder; use that info
316 -- to record DemandInfo/StrictnessInfo in the binder.
317 new_binder = addStrictnessInfoToId
318 widened_str_rhs widened_abs_rhs
319 (binder `setIdDemandInfo` rhs_dmd)
321 tickLet new_binder -- stats
322 new_rhs <- saExpr rhs_dmd str_env abs_env rhs
323 new_body <- saExpr dmd new_str_env new_abs_env body
324 return (Let (NonRec new_binder new_rhs) new_body)
326 saExpr dmd str_env abs_env (Let (Rec pairs) body) = do
328 (binders,rhss) = unzip pairs
329 str_vals = fixpoint StrAnal binders rhss str_env
330 abs_vals = fixpoint AbsAnal binders rhss abs_env
331 -- fixpoint returns widened values
332 new_str_env = growAbsValEnvList str_env (binders `zip` str_vals)
333 new_abs_env = growAbsValEnvList abs_env (binders `zip` abs_vals)
335 new_body <- saExpr dmd new_str_env new_abs_env body
336 new_rhss <- mapM (saExpr minDemand new_str_env new_abs_env) rhss
338 -- DON'T add demand info in a Rec!
339 -- a) it's useless: we can't do let-to-case
340 -- b) it's incorrect. Consider
341 -- letrec x = ...y...
344 -- When we ask whether y is demanded we'll bind y to bottom and
345 -- evaluate the body of the letrec. But that will result in our
346 -- deciding that y is absent, which is plain wrong!
347 -- It's much easier simply not to do this.
349 improved_binders = zipWith3Equal "saExpr" addStrictnessInfoToId
350 str_vals abs_vals binders
352 new_pairs = improved_binders `zip` new_rhss
354 return (Let (Rec new_pairs) new_body)
358 %************************************************************************
360 \subsection[computeInfos]{Add computed info to binders}
362 %************************************************************************
364 Important note (Sept 93). @addStrictnessInfoToId@ is used only for
365 let(rec) bound variables, and is use to attach the strictness (not
366 demand) info to the binder. We are careful to restrict this
367 strictness info to the lambda-bound arguments which are actually
368 visible, at the top level, lest we accidentally lose laziness by
369 eagerly looking for an "extra" argument. So we "dig for lambdas" in a
370 rather syntactic way.
372 A better idea might be to have some kind of arity analysis to
373 tell how many args could safely be grabbed.
376 addStrictnessInfoToId
377 :: AbsVal -- Abstract strictness value
378 -> AbsVal -- Ditto absence
380 -> Id -- Augmented with strictness
382 addStrictnessInfoToId str_val abs_val binder
383 = binder `setIdStrictness` findStrictness binder str_val abs_val
387 addDemandInfoToId :: Demand -> StrictEnv -> AbsenceEnv
388 -> CoreExpr -- The scope of the id
390 -> Id -- Id augmented with Demand info
392 addDemandInfoToId dmd str_env abs_env expr binder
393 = binder `setIdDemandInfo` (findDemand dmd str_env abs_env expr binder)
395 addDemandInfoToCaseBndr dmd str_env abs_env alts binder
396 = binder `setIdDemandInfo` (findDemandAlts dmd str_env abs_env alts binder)
399 %************************************************************************
401 \subsection{Monad used herein for stats}
403 %************************************************************************
407 = SaStats FastInt FastInt -- total/marked-demanded lambda-bound
408 FastInt FastInt -- total/marked-demanded case-bound
409 FastInt FastInt -- total/marked-demanded let-bound
410 -- (excl. top-level; excl. letrecs)
412 nullSaStats = SaStats
413 (_ILIT(0)) (_ILIT(0))
414 (_ILIT(0)) (_ILIT(0))
415 (_ILIT(0)) (_ILIT(0))
417 tickLambda :: Id -> SaM ()
418 tickCases :: [CoreBndr] -> SaM ()
419 tickLet :: Id -> SaM ()
421 #ifndef OMIT_STRANAL_STATS
422 type SaM a = State SaStats a
424 tickLambda var = modify $ \(SaStats tlam dlam tc dc tlet dlet)
425 -> case (tick_demanded var (0,0)) of { (totB, demandedB) ->
426 let tot = iUnbox totB ; demanded = iUnbox demandedB
427 in SaStats (tlam +# tot) (dlam +# demanded) tc dc tlet dlet)
429 tickCases vars = modify $ \(SaStats tlam dlam tc dc tlet dlet)
430 = case (foldr tick_demanded (0,0) vars) of { (totB, demandedB) ->
431 let tot = iUnbox totB ; demanded = iUnbox demandedB
432 in SaStats tlam dlam (tc +# tot) (dc +# demanded) tlet dlet)
434 tickLet var = modify $ \(SaStats tlam dlam tc dc tlet dlet)
435 = case (tick_demanded var (0,0)) of { (totB, demandedB) ->
436 let tot = iUnbox totB ; demanded = iUnbox demandedB
437 in SaStats tlam dlam tc dc (tlet +# tot) (dlet +# demanded))
439 tick_demanded var (tot, demanded)
440 | isTyVar var = (tot, demanded)
443 if (isStrict (idDemandInfo var))
447 pp_stats (SaStats tlam dlam tc dc tlet dlet)
448 = hcat [ptext SLIT("Lambda vars: "), int (iBox dlam), char '/', int (iBox tlam),
449 ptext SLIT("; Case vars: "), int (iBox dc), char '/', int (iBox tc),
450 ptext SLIT("; Let vars: "), int (iBox dlet), char '/', int (iBox tlet)
453 #else /* OMIT_STRANAL_STATS */
455 newtype SaM a = SaM { unSaM :: a }
457 instance Monad SaM where
461 tickLambda var = panic "OMIT_STRANAL_STATS: tickLambda"
462 tickCases vars = panic "OMIT_STRANAL_STATS: tickCases"
463 tickLet var = panic "OMIT_STRANAL_STATS: tickLet"
465 #endif /* OMIT_STRANAL_STATS */
467 #endif /* OLD_STRICTNESS */