2 % (c) The AQUA Project, Glasgow University, 1994-1998
4 \section[LambdaLift]{A STG-code lambda lifter}
7 module LambdaLift ( liftProgram ) where
9 #include "HsVersions.h"
13 import Bag ( Bag, emptyBag, unionBags, unitBag, snocBag, bagToList )
14 import Id ( mkUserId, idType, setIdArity, Id )
17 import IdInfo ( exactArity )
18 import Module ( Module )
19 import Name ( mkTopName )
20 import Type ( splitForAllTys, mkForAllTys, mkFunTys, Type )
21 import UniqSupply ( uniqFromSupply, splitUniqSupply, UniqSupply )
22 import Util ( zipEqual )
23 import Panic ( panic, assertPanic )
26 This is the lambda lifter. It turns lambda abstractions into
27 supercombinators on a selective basis:
29 * Let-no-escaped bindings are never lifted. That's one major reason
30 why the lambda lifter is done in STG.
32 * Non-recursive bindings whose RHS is a lambda abstractions are lifted,
33 provided all the occurrences of the bound variable is in a function
34 postition. In this example, f will be lifted:
42 $f p q r x = e -- Supercombinator
44 ..($f p q r a1)...($f p q r a2)...
46 NOTE that the original binding is eliminated.
48 But in this case, f won't be lifted:
55 Why? Because we have to heap-allocate a closure for f thus:
57 $f p q r x = e -- Supercombinator
62 ..(g f)...($f p q r a2)..
64 so it might as well be the original lambda abstraction.
66 We also do not lift if the function has an occurrence with no arguments, e.g.
72 as this form is more efficient than if we create a partial application
74 $f p q r x = e -- Supercombinator
78 * Recursive bindings *all* of whose RHSs are lambda abstractions are
80 - all the occurrences of all the binders are in a function position
81 - there aren't ``too many'' free variables.
83 Same reasoning as before for the function-position stuff. The ``too many
84 free variable'' part comes from considering the (potentially many)
85 recursive calls, which may now have lots of free vars.
89 * 2 might be already ``too many'' variables to abstract.
90 The problem is that the increase in the number of free variables
91 of closures refering to the lifted function (which is always # of
92 abstracted args - 1) may increase heap allocation a lot.
93 Expeiments are being done to check this...
95 * We do not lambda lift if the function has at least one occurrence
96 without any arguments. This caused lots of problems. Ex:
97 h = \ x -> ... let y = ...
98 in let let f = \x -> ...y...
102 h = \ x -> ... let y = ...
105 now f y is a partial application, so it will be updated, and this
109 --- NOT RELEVANT FOR STG ----
110 * All ``lone'' lambda abstractions are lifted. Notably this means lambda
112 - in a case alternative: case e of True -> (\x->b)
113 - in the body of a let: let x=e in (\y->b)
114 -----------------------------
116 %************************************************************************
118 \subsection[Lift-expressions]{The main function: liftExpr}
120 %************************************************************************
123 liftProgram :: Module -> UniqSupply -> [StgBinding] -> [StgBinding]
124 liftProgram mod us prog = concat (runLM mod Nothing us (mapLM liftTopBind prog))
127 liftTopBind :: StgBinding -> LiftM [StgBinding]
128 liftTopBind (StgNonRec id rhs)
129 = dontLiftRhs rhs `thenLM` \ (rhs', rhs_info) ->
130 returnLM (getScBinds rhs_info ++ [StgNonRec id rhs'])
132 liftTopBind (StgRec pairs)
133 = mapAndUnzipLM dontLiftRhs rhss `thenLM` \ (rhss', rhs_infos) ->
134 returnLM ([co_rec_ify (StgRec (ids `zip` rhss') :
135 getScBinds (unionLiftInfos rhs_infos))
138 (ids, rhss) = unzip pairs
144 -> LiftM (StgExpr, LiftInfo)
147 liftExpr expr@(StgCon con args _) = returnLM (expr, emptyLiftInfo)
149 liftExpr expr@(StgApp v args)
150 = lookUp v `thenLM` \ ~(sc, sc_args) -> -- NB the ~. We don't want to
151 -- poke these bindings too early!
152 returnLM (StgApp sc (map StgVarArg sc_args ++ args),
154 -- The lvs field is probably wrong, but we reconstruct it
155 -- anyway following lambda lifting
157 liftExpr (StgCase scrut lv1 lv2 bndr srt alts)
158 = liftExpr scrut `thenLM` \ (scrut', scrut_info) ->
159 lift_alts alts `thenLM` \ (alts', alts_info) ->
160 returnLM (StgCase scrut' lv1 lv2 bndr srt alts', scrut_info `unionLiftInfo` alts_info)
162 lift_alts (StgAlgAlts ty alg_alts deflt)
163 = mapAndUnzipLM lift_alg_alt alg_alts `thenLM` \ (alg_alts', alt_infos) ->
164 lift_deflt deflt `thenLM` \ (deflt', deflt_info) ->
165 returnLM (StgAlgAlts ty alg_alts' deflt', foldr unionLiftInfo deflt_info alt_infos)
167 lift_alts (StgPrimAlts ty prim_alts deflt)
168 = mapAndUnzipLM lift_prim_alt prim_alts `thenLM` \ (prim_alts', alt_infos) ->
169 lift_deflt deflt `thenLM` \ (deflt', deflt_info) ->
170 returnLM (StgPrimAlts ty prim_alts' deflt', foldr unionLiftInfo deflt_info alt_infos)
172 lift_alg_alt (con, args, use_mask, rhs)
173 = liftExpr rhs `thenLM` \ (rhs', rhs_info) ->
174 returnLM ((con, args, use_mask, rhs'), rhs_info)
176 lift_prim_alt (lit, rhs)
177 = liftExpr rhs `thenLM` \ (rhs', rhs_info) ->
178 returnLM ((lit, rhs'), rhs_info)
180 lift_deflt StgNoDefault = returnLM (StgNoDefault, emptyLiftInfo)
181 lift_deflt (StgBindDefault rhs)
182 = liftExpr rhs `thenLM` \ (rhs', rhs_info) ->
183 returnLM (StgBindDefault rhs', rhs_info)
186 Now the interesting cases. Let no escape isn't lifted. We turn it
187 back into a let, to play safe, because we have to redo that pass after
191 liftExpr (StgLetNoEscape _ _ (StgNonRec binder rhs) body)
192 = dontLiftRhs rhs `thenLM` \ (rhs', rhs_info) ->
193 liftExpr body `thenLM` \ (body', body_info) ->
194 returnLM (StgLet (StgNonRec binder rhs') body',
195 rhs_info `unionLiftInfo` body_info)
197 liftExpr (StgLetNoEscape _ _ (StgRec pairs) body)
198 = liftExpr body `thenLM` \ (body', body_info) ->
199 mapAndUnzipLM dontLiftRhs rhss `thenLM` \ (rhss', rhs_infos) ->
200 returnLM (StgLet (StgRec (zipEqual "liftExpr" binders rhss')) body',
201 foldr unionLiftInfo body_info rhs_infos)
203 (binders,rhss) = unzip pairs
207 liftExpr (StgLet (StgNonRec binder rhs) body)
208 | not (isLiftable rhs)
209 = dontLiftRhs rhs `thenLM` \ (rhs', rhs_info) ->
210 liftExpr body `thenLM` \ (body', body_info) ->
211 returnLM (StgLet (StgNonRec binder rhs') body',
212 rhs_info `unionLiftInfo` body_info)
214 | otherwise -- It's a lambda
215 = -- Do the body of the let
216 fixLM (\ ~(sc_inline, _, _) ->
217 addScInlines [binder] [sc_inline] (
219 ) `thenLM` \ (body', body_info) ->
222 dontLiftRhs rhs `thenLM` \ (rhs', rhs_info) ->
224 -- All occurrences in function position, so lambda lift
225 getFinalFreeVars (rhsFreeVars rhs) `thenLM` \ final_free_vars ->
227 mkScPieces final_free_vars (binder,rhs') `thenLM` \ (sc_inline, sc_bind) ->
231 nonRecScBind rhs_info sc_bind `unionLiftInfo` body_info)
233 ) `thenLM` \ (_, expr', final_info) ->
235 returnLM (expr', final_info)
237 liftExpr (StgLet (StgRec pairs) body)
239 | not (all isLiftableRec rhss)
240 = liftExpr body `thenLM` \ (body', body_info) ->
241 mapAndUnzipLM dontLiftRhs rhss `thenLM` \ (rhss', rhs_infos) ->
242 returnLM (StgLet (StgRec (zipEqual "liftExpr2" binders rhss')) body',
243 foldr unionLiftInfo body_info rhs_infos)
245 | otherwise -- All rhss are liftable
246 = -- Do the body of the let
247 fixLM (\ ~(sc_inlines, _, _) ->
248 addScInlines binders sc_inlines (
250 liftExpr body `thenLM` \ (body', body_info) ->
251 mapAndUnzipLM dontLiftRhs rhss `thenLM` \ (rhss', rhs_infos) ->
253 -- Find the free vars of all the rhss,
254 -- excluding the binders themselves.
255 rhs_free_vars = unionVarSets (map rhsFreeVars rhss)
259 rhs_info = unionLiftInfos rhs_infos
261 getFinalFreeVars rhs_free_vars `thenLM` \ final_free_vars ->
263 mapAndUnzipLM (mkScPieces final_free_vars) (binders `zip` rhss')
264 `thenLM` \ (sc_inlines, sc_pairs) ->
265 returnLM (sc_inlines,
267 recScBind rhs_info sc_pairs `unionLiftInfo` body_info)
269 )) `thenLM` \ (_, expr', final_info) ->
271 returnLM (expr', final_info)
273 (binders,rhss) = unzip pairs
277 liftExpr (StgSCC cc expr)
278 = liftExpr expr `thenLM` \ (expr2, expr_info) ->
279 returnLM (StgSCC cc expr2, expr_info)
282 A binding is liftable if it's a *function* (args not null) and never
283 occurs in an argument position.
286 isLiftable :: StgRhs -> Bool
288 isLiftable (StgRhsClosure _ (StgBinderInfo arg_occ _ _ _ unapplied_occ) _ fvs _ args _)
290 -- Experimental evidence suggests we should lift only if we will be
291 -- abstracting up to 4 fvs.
293 = if not (null args || -- Not a function
294 unapplied_occ || -- Has an occ with no args at all
295 arg_occ || -- Occurs in arg position
296 length fvs > 4 -- Too many free variables
298 then {-trace ("LL: " ++ show (length fvs))-} True
300 isLiftable other_rhs = False
302 isLiftableRec :: StgRhs -> Bool
304 -- this is just the same as for non-rec, except we only lift to
305 -- abstract up to 1 argument this avoids undoing Static Argument
306 -- Transformation work
308 {- Andre's longer comment about isLiftableRec: 1996/01:
310 A rec binding is "liftable" (according to our heuristics) if:
312 * all occurrences have arguments,
313 * does not occur in an argument position and
314 * has up to *2* free variables (including the rec binding variable
317 The point is: my experiments show that SAT is more important than LL.
318 Therefore if we still want to do LL, for *recursive* functions, we do
319 not want LL to undo what SAT did. We do this by avoiding LL recursive
320 functions that have more than 2 fvs, since if this recursive function
321 was created by SAT (we don't know!), it would have at least 3 fvs: one
322 for the rec binding itself and 2 more for the static arguments (note:
323 this matches with the choice of performing SAT to have at least 2
324 static arguments, if we change things there we should change things
328 isLiftableRec (StgRhsClosure _ (StgBinderInfo arg_occ _ _ _ unapplied_occ) _ fvs _ args _)
329 = if not (null args || -- Not a function
330 unapplied_occ || -- Has an occ with no args at all
331 arg_occ || -- Occurs in arg position
332 length fvs > 2 -- Too many free variables
334 then {-trace ("LLRec: " ++ show (length fvs))-} True
336 isLiftableRec other_rhs = False
338 rhsFreeVars :: StgRhs -> IdSet
339 rhsFreeVars (StgRhsClosure _ _ _ fvs _ _ _) = mkVarSet fvs
340 rhsFreeVars other = panic "rhsFreeVars"
343 dontLiftRhs is like liftExpr, except that it does not lift a top-level
344 lambda abstraction. It is used for the right-hand sides of
345 definitions where we've decided *not* to lift: for example, top-level
346 ones or mutually-recursive ones where not all are lambdas.
349 dontLiftRhs :: StgRhs -> LiftM (StgRhs, LiftInfo)
351 dontLiftRhs rhs@(StgRhsCon cc v args) = returnLM (rhs, emptyLiftInfo)
353 dontLiftRhs (StgRhsClosure cc bi srt fvs upd args body)
354 = liftExpr body `thenLM` \ (body', body_info) ->
355 returnLM (StgRhsClosure cc bi srt fvs upd args body', body_info)
359 mkScPieces :: IdSet -- Extra args for the supercombinator
360 -> (Id, StgRhs) -- The processed RHS and original Id
361 -> LiftM ((Id,[Id]), -- Replace abstraction with this;
362 -- the set is its free vars
363 (Id,StgRhs)) -- Binding for supercombinator
365 mkScPieces extra_arg_set (id, StgRhsClosure cc bi srt _ upd args body)
366 = ASSERT( n_args > 0 )
367 -- Construct the rhs of the supercombinator, and its Id
368 newSupercombinator sc_ty arity `thenLM` \ sc_id ->
369 returnLM ((sc_id, extra_args), (sc_id, sc_rhs))
372 extra_args = varSetElems extra_arg_set
373 arity = n_args + length extra_args
375 -- Construct the supercombinator type
376 type_of_original_id = idType id
377 extra_arg_tys = map idType extra_args
378 (tyvars, rest) = splitForAllTys type_of_original_id
379 sc_ty = mkForAllTys tyvars (mkFunTys extra_arg_tys rest)
381 sc_rhs = StgRhsClosure cc bi srt [] upd (extra_args ++ args) body
385 %************************************************************************
387 \subsection[Lift-monad]{The LiftM monad}
389 %************************************************************************
391 The monad is used only to distribute global stuff, and the unique supply.
394 type LiftM a = Module
397 -> (IdEnv -- Domain = candidates for lifting
398 (Id, -- The supercombinator
399 [Id]) -- Args to apply it to
404 type LiftFlags = Maybe Int -- No of fvs reqd to float recursive
405 -- binding; Nothing == infinity
408 runLM :: Module -> LiftFlags -> UniqSupply -> LiftM a -> a
409 runLM mod flags us m = m mod flags us emptyVarEnv
411 thenLM :: LiftM a -> (a -> LiftM b) -> LiftM b
412 thenLM m k mod ci us idenv
413 = k (m mod ci us1 idenv) mod ci us2 idenv
415 (us1, us2) = splitUniqSupply us
417 returnLM :: a -> LiftM a
418 returnLM a mod ci us idenv = a
420 fixLM :: (a -> LiftM a) -> LiftM a
421 fixLM k mod ci us idenv = r
423 r = k r mod ci us idenv
425 mapLM :: (a -> LiftM b) -> [a] -> LiftM [b]
426 mapLM f [] = returnLM []
427 mapLM f (a:as) = f a `thenLM` \ r ->
428 mapLM f as `thenLM` \ rs ->
431 mapAndUnzipLM :: (a -> LiftM (b,c)) -> [a] -> LiftM ([b],[c])
432 mapAndUnzipLM f [] = returnLM ([],[])
433 mapAndUnzipLM f (a:as) = f a `thenLM` \ (b,c) ->
434 mapAndUnzipLM f as `thenLM` \ (bs,cs) ->
435 returnLM (b:bs, c:cs)
439 newSupercombinator :: Type
443 newSupercombinator ty arity mod ci us idenv
444 = mkUserId (mkTopName uniq mod SLIT("_ll")) ty
445 `setIdArity` exactArity arity
446 -- ToDo: rm the setIdArity? Just let subsequent stg-saturation pass do it?
448 uniq = uniqFromSupply us
450 lookUp :: Id -> LiftM (Id,[Id])
451 lookUp v mod ci us idenv
452 = case (lookupVarEnv idenv v) of
453 Just result -> result
456 addScInlines :: [Id] -> [(Id,[Id])] -> LiftM a -> LiftM a
457 addScInlines ids values m mod ci us idenv
460 idenv' = extendVarEnvList idenv (ids `zip_lazy` values)
462 -- zip_lazy zips two things together but matches lazily on the
463 -- second argument. This is important, because the ids are know here,
464 -- but the things they are bound to are decided only later
466 zip_lazy (x:xs) ~(y:ys) = (x,y) : zip_lazy xs ys
469 -- The free vars reported by the free-var analyser will include
470 -- some ids, f, which are to be replaced by ($f a b c), where $f
471 -- is the supercombinator. Hence instead of f being a free var,
476 -- f a = ...y1..y2.....
483 -- Here the free vars of g are {f,z}; but f will be lambda-lifted
484 -- with free vars {y1,y2}, so the "real~ free vars of g are {y1,y2,z}.
486 getFinalFreeVars :: IdSet -> LiftM IdSet
488 getFinalFreeVars free_vars mod ci us idenv
489 = unionVarSets (map munge_it (varSetElems free_vars))
491 munge_it :: Id -> IdSet -- Takes a free var and maps it to the "real"
493 munge_it id = case (lookupVarEnv idenv id) of
494 Just (_, args) -> mkVarSet args
495 Nothing -> unitVarSet id
499 %************************************************************************
501 \subsection[Lift-info]{The LiftInfo type}
503 %************************************************************************
506 type LiftInfo = Bag StgBinding -- Float to top
508 emptyLiftInfo = emptyBag
510 unionLiftInfo :: LiftInfo -> LiftInfo -> LiftInfo
511 unionLiftInfo binds1 binds2 = binds1 `unionBags` binds2
513 unionLiftInfos :: [LiftInfo] -> LiftInfo
514 unionLiftInfos infos = foldr unionLiftInfo emptyLiftInfo infos
516 mkScInfo :: StgBinding -> LiftInfo
517 mkScInfo bind = unitBag bind
519 nonRecScBind :: LiftInfo -- From body of supercombinator
520 -> (Id, StgRhs) -- Supercombinator and its rhs
522 nonRecScBind binds (sc_id,sc_rhs) = binds `snocBag` (StgNonRec sc_id sc_rhs)
525 -- In the recursive case, all the SCs from the RHSs of the recursive group
526 -- are dealing with might potentially mention the new, recursive SCs.
527 -- So we flatten the whole lot into a single recursive group.
529 recScBind :: LiftInfo -- From body of supercombinator
530 -> [(Id,StgRhs)] -- Supercombinator rhs
533 recScBind binds pairs = unitBag (co_rec_ify (StgRec pairs : bagToList binds))
535 co_rec_ify :: [StgBinding] -> StgBinding
536 co_rec_ify binds = StgRec (concat (map f binds))
538 f (StgNonRec id rhs) = [(id,rhs)]
539 f (StgRec pairs) = pairs
542 getScBinds :: LiftInfo -> [StgBinding]
543 getScBinds binds = bagToList binds
545 looksLikeSATRhs [(f,StgRhsClosure _ _ _ _ _ ls _)] (StgApp f' args)
546 = (f == f') && (length args == length ls)
547 looksLikeSATRhs _ _ = False