2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 %************************************************************************
6 \section[FloatIn]{Floating Inwards pass}
8 %************************************************************************
10 The main purpose of @floatInwards@ is floating into branches of a
11 case, so that we don't allocate things, save them on the stack, and
12 then discover that they aren't needed in the chosen branch.
15 module FloatIn ( floatInwards ) where
17 #include "HsVersions.h"
19 import CmdLineOpts ( opt_D_verbose_core2core )
21 import CoreUtils ( exprIsValue, exprIsDupable )
22 import CoreLint ( beginPass, endPass )
23 import CoreFVs ( CoreExprWithFVs, freeVars, freeVarsOf )
24 import Id ( isOneShotLambda )
25 import Var ( Id, idType, isTyVar )
26 import Type ( isUnLiftedType )
28 import Util ( zipEqual, zipWithEqual )
32 Top-level interface function, @floatInwards@. Note that we do not
33 actually float any bindings downwards from the top-level.
36 floatInwards :: [CoreBind] -> IO [CoreBind]
40 beginPass "Float inwards";
41 let { binds' = map fi_top_bind binds };
42 endPass "Float inwards"
43 opt_D_verbose_core2core {- no specific flag for dumping float-in -}
48 fi_top_bind (NonRec binder rhs)
49 = NonRec binder (fiExpr [] (freeVars rhs))
50 fi_top_bind (Rec pairs)
51 = Rec [ (b, fiExpr [] (freeVars rhs)) | (b, rhs) <- pairs ]
54 %************************************************************************
56 \subsection{Mail from Andr\'e [edited]}
58 %************************************************************************
60 {\em Will wrote: What??? I thought the idea was to float as far
61 inwards as possible, no matter what. This is dropping all bindings
62 every time it sees a lambda of any kind. Help! }
64 You are assuming we DO DO full laziness AFTER floating inwards! We
65 have to [not float inside lambdas] if we don't.
67 If we indeed do full laziness after the floating inwards (we could
68 check the compilation flags for that) then I agree we could be more
69 aggressive and do float inwards past lambdas.
71 Actually we are not doing a proper full laziness (see below), which
72 was another reason for not floating inwards past a lambda.
74 This can easily be fixed. The problem is that we float lets outwards,
75 but there are a few expressions which are not let bound, like case
76 scrutinees and case alternatives. After floating inwards the
77 simplifier could decide to inline the let and the laziness would be
81 let a = expensive ==> \b -> case expensive of ...
82 in \ b -> case a of ...
87 to let bind the algebraic case scrutinees (done, I think) and
88 the case alternatives (except the ones with an
89 unboxed type)(not done, I think). This is best done in the
90 SetLevels.lhs module, which tags things with their level numbers.
92 do the full laziness pass (floating lets outwards).
94 simplify. The simplifier inlines the (trivial) lets that were
95 created but were not floated outwards.
98 With the fix I think Will's suggestion that we can gain even more from
99 strictness by floating inwards past lambdas makes sense.
101 We still gain even without going past lambdas, as things may be
102 strict in the (new) context of a branch (where it was floated to) or
105 let a = something case x of
106 in case x of alt1 -> case something of a -> a + a
107 alt1 -> a + a ==> alt2 -> b
110 let a = something let b = case something of a -> a + a
111 in let b = a + a ==> in (b,b)
114 Also, even if a is not found to be strict in the new context and is
115 still left as a let, if the branch is not taken (or b is not entered)
116 the closure for a is not built.
118 %************************************************************************
120 \subsection{Main floating-inwards code}
122 %************************************************************************
125 type FreeVarsSet = IdSet
127 type FloatingBinds = [(CoreBind, FreeVarsSet)]
128 -- In reverse dependency order (innermost bindiner first)
130 -- The FreeVarsSet is the free variables of the binding. In the case
131 -- of recursive bindings, the set doesn't include the bound
134 fiExpr :: FloatingBinds -- Binds we're trying to drop
135 -- as far "inwards" as possible
136 -> CoreExprWithFVs -- Input expr
137 -> CoreExpr -- Result
139 fiExpr to_drop (_, AnnVar v) = mkCoLets' to_drop (Var v)
141 fiExpr to_drop (_, AnnType ty) = ASSERT( null to_drop )
144 fiExpr to_drop (_, AnnLit lit) = Lit lit
147 Applications: we do float inside applications, mainly because we
148 need to get at all the arguments. The next simplifier run will
149 pull out any silly ones.
152 fiExpr to_drop (_,AnnApp fun arg)
153 = mkCoLets' drop_here (App (fiExpr fun_drop fun) (fiExpr arg_drop arg))
155 [drop_here, fun_drop, arg_drop] = sepBindsByDropPoint False [freeVarsOf fun, freeVarsOf arg] to_drop
158 We are careful about lambdas:
160 * We must be careful about floating inside inside a value lambda.
161 That risks losing laziness.
162 The float-out pass might rescue us, but then again it might not.
164 * We must be careful about type lambdas too. At one time we did, and
165 there is no risk of duplicating work thereby, but we do need to be
166 careful. In particular, here is a bad case (it happened in the
169 in let f = /\t -> \a -> ...
171 let f = /\t -> let v = ... in \a -> ...
172 This is bad as now f is an updatable closure (update PAP)
175 So we treat lambda in groups, using the following rule:
177 Float inside a group of lambdas only if
178 they are all either type lambdas or one-shot lambdas.
180 Otherwise drop all the bindings outside the group.
183 fiExpr to_drop (_, AnnLam b body)
184 = case collect [b] body of
186 | all is_ok bndrs -> mkLams bndrs (fiExpr to_drop real_body)
187 | otherwise -> mkCoLets' to_drop (mkLams bndrs (fiExpr [] real_body))
189 collect bs (_, AnnLam b body) = collect (b:bs) body
190 collect bs body = (reverse bs, body)
192 is_ok bndr = isTyVar bndr || isOneShotLambda bndr
195 We don't float lets inwards past an SCC.
196 ToDo: keep info on current cc, and when passing
197 one, if it is not the same, annotate all lets in binds with current
198 cc, change current cc to the new one and float binds into expr.
201 fiExpr to_drop (_, AnnNote note@(SCC cc) expr)
202 = -- Wimp out for now
203 mkCoLets' to_drop (Note note (fiExpr [] expr))
205 fiExpr to_drop (_, AnnNote InlineCall expr)
206 = -- Wimp out for InlineCall; keep it close
207 -- the the call it annotates
208 mkCoLets' to_drop (Note InlineCall (fiExpr [] expr))
210 fiExpr to_drop (_, AnnNote InlineMe expr)
211 = -- Ditto... don't float anything into an INLINE expression
212 mkCoLets' to_drop (Note InlineMe (fiExpr [] expr))
214 fiExpr to_drop (_, AnnNote note@(Coerce _ _) expr)
215 = -- Just float in past coercion
216 Note note (fiExpr to_drop expr)
218 fiExpr to_drop (_, AnnNote note@(TermUsg _) expr)
219 = -- Float in past term usage annotation
220 -- (for now; not sure if this is correct: KSW 1999-05)
221 Note note (fiExpr to_drop expr)
224 For @Lets@, the possible ``drop points'' for the \tr{to_drop}
225 bindings are: (a)~in the body, (b1)~in the RHS of a NonRec binding,
226 or~(b2), in each of the RHSs of the pairs of a @Rec@.
228 Note that we do {\em weird things} with this let's binding. Consider:
237 Look at the inner \tr{let}. As \tr{w} is used in both the bind and
238 body of the inner let, we could panic and leave \tr{w}'s binding where
239 it is. But \tr{v} is floatable further into the body of the inner let, and
240 {\em then} \tr{w} will also be only in the body of that inner let.
242 So: rather than drop \tr{w}'s binding here, we add it onto the list of
243 things to drop in the outer let's body, and let nature take its
247 fiExpr to_drop (_,AnnLet (AnnNonRec id rhs@(rhs_fvs, ann_rhs)) body)
248 = fiExpr new_to_drop body
250 body_fvs = freeVarsOf body
252 final_body_fvs | noFloatIntoRhs ann_rhs
253 || isUnLiftedType (idType id) = body_fvs `unionVarSet` rhs_fvs
254 | otherwise = body_fvs
255 -- See commments with letrec below
256 -- No point in floating in only to float straight out again
257 -- Ditto ok-for-speculation unlifted RHSs
259 [shared_binds, rhs_binds, body_binds] = sepBindsByDropPoint False [rhs_fvs, final_body_fvs] to_drop
261 new_to_drop = body_binds ++ -- the bindings used only in the body
262 [(NonRec id rhs', rhs_fvs')] ++ -- the new binding itself
263 shared_binds -- the bindings used both in rhs and body
265 -- Push rhs_binds into the right hand side of the binding
266 rhs' = fiExpr rhs_binds rhs
267 rhs_fvs' = rhs_fvs `unionVarSet` floatedBindsFVs rhs_binds
269 fiExpr to_drop (_,AnnLet (AnnRec bindings) body)
270 = fiExpr new_to_drop body
272 (binders, rhss) = unzip bindings
274 rhss_fvs = map freeVarsOf rhss
275 body_fvs = freeVarsOf body
277 -- Add to body_fvs the free vars of any RHS that has
278 -- a lambda at the top. This has the effect of making it seem
279 -- that such things are used in the body as well, and hence prevents
280 -- them getting floated in. The big idea is to avoid turning:
283 -- letrec f = \z. ...x#...f...
286 -- letrec f = let x# = y# +# 1# in \z. ...x#...f... in ...
288 -- Because now we can't float the let out again, because a letrec
289 -- can't have unboxed bindings.
291 final_body_fvs = foldr (unionVarSet . get_extras) body_fvs rhss
292 get_extras (rhs_fvs, rhs) | noFloatIntoRhs rhs = rhs_fvs
293 | otherwise = emptyVarSet
295 (shared_binds:body_binds:rhss_binds) = sepBindsByDropPoint False (final_body_fvs:rhss_fvs) to_drop
297 new_to_drop = -- the bindings used only in the body
299 -- the new binding itself
300 [(Rec (fi_bind rhss_binds bindings), rhs_fvs')] ++
301 -- the bindings used both in rhs and body or in more than one rhs
304 rhs_fvs' = unionVarSet (unionVarSets rhss_fvs)
305 (unionVarSets (map floatedBindsFVs rhss_binds))
307 -- Push rhs_binds into the right hand side of the binding
308 fi_bind :: [FloatingBinds] -- one per "drop pt" conjured w/ fvs_of_rhss
309 -> [(Id, CoreExprWithFVs)]
312 fi_bind to_drops pairs
313 = [ (binder, fiExpr to_drop rhs)
314 | ((binder, rhs), to_drop) <- zipEqual "fi_bind" pairs to_drops ]
317 For @Case@, the possible ``drop points'' for the \tr{to_drop}
318 bindings are: (a)~inside the scrutinee, (b)~inside one of the
319 alternatives/default [default FVs always {\em first}!].
322 fiExpr to_drop (_, AnnCase scrut case_bndr alts)
323 = mkCoLets' drop_here1 $
324 mkCoLets' drop_here2 $
325 Case (fiExpr scrut_drops scrut) case_bndr
326 (zipWith fi_alt alts_drops_s alts)
328 -- Float into the scrut and alts-considered-together just like App
329 [drop_here1, scrut_drops, alts_drops] = sepBindsByDropPoint False [scrut_fvs, all_alts_fvs] to_drop
331 -- Float into the alts with the is_case flag set
332 (drop_here2 : alts_drops_s) = sepBindsByDropPoint True alts_fvs alts_drops
334 scrut_fvs = freeVarsOf scrut
335 alts_fvs = map alt_fvs alts
336 all_alts_fvs = unionVarSets alts_fvs
337 alt_fvs (con, args, rhs) = foldl delVarSet (freeVarsOf rhs) (case_bndr:args)
338 -- Delete case_bndr and args from free vars of rhs
339 -- to get free vars of alt
341 fi_alt to_drop (con, args, rhs) = (con, args, fiExpr to_drop rhs)
343 noFloatIntoRhs (AnnNote InlineMe _) = True
344 noFloatIntoRhs (AnnLam b _) = not (isId b && isOneShotLambda b)
345 -- IMPORTANT: don't say 'True' for a RHS with a one-shot lambda at the top.
346 -- This makes a big difference for things like
347 -- f x# = let x = I# x#
348 -- in let j = \() -> ...x...
349 -- in if <condition> then normal-path else j ()
350 -- If x is used only in the error case join point, j, we must float the
351 -- boxing constructor into it, else we box it every time which is very bad
354 noFloatIntoRhs rhs = exprIsValue (deAnnotate' rhs) -- We'd just float rigt back out again...
358 %************************************************************************
360 \subsection{@sepBindsByDropPoint@}
362 %************************************************************************
364 This is the crucial function. The idea is: We have a wad of bindings
365 that we'd like to distribute inside a collection of {\em drop points};
366 insides the alternatives of a \tr{case} would be one example of some
367 drop points; the RHS and body of a non-recursive \tr{let} binding
368 would be another (2-element) collection.
370 So: We're given a list of sets-of-free-variables, one per drop point,
371 and a list of floating-inwards bindings. If a binding can go into
372 only one drop point (without suddenly making something out-of-scope),
373 in it goes. If a binding is used inside {\em multiple} drop points,
374 then it has to go in a you-must-drop-it-above-all-these-drop-points
377 We have to maintain the order on these drop-point-related lists.
381 :: Bool -- True <=> is case expression
382 -> [FreeVarsSet] -- One set of FVs per drop point
383 -> FloatingBinds -- Candidate floaters
384 -> [FloatingBinds] -- FIRST one is bindings which must not be floated
385 -- inside any drop point; the rest correspond
386 -- one-to-one with the input list of FV sets
388 -- Every input floater is returned somewhere in the result;
389 -- none are dropped, not even ones which don't seem to be
390 -- free in *any* of the drop-point fvs. Why? Because, for example,
391 -- a binding (let x = E in B) might have a specialised version of
392 -- x (say x') stored inside x, but x' isn't free in E or B.
394 type DropBox = (FreeVarsSet, FloatingBinds)
396 sepBindsByDropPoint is_case drop_pts []
397 = [] : [[] | p <- drop_pts] -- cut to the chase scene; it happens
399 sepBindsByDropPoint is_case drop_pts floaters
400 = go floaters (map (\fvs -> (fvs, [])) (emptyVarSet : drop_pts))
402 go :: FloatingBinds -> [DropBox] -> [FloatingBinds]
403 -- The *first* one in the argument list is the drop_here set
404 -- The FloatingBinds in the lists are in the reverse of
405 -- the normal FloatingBinds order; that is, they are the right way round!
407 go [] drop_boxes = map (reverse . snd) drop_boxes
409 go (bind_w_fvs@(bind, bind_fvs) : binds) drop_boxes@(here_box : fork_boxes)
412 -- "here" means the group of bindings dropped at the top of the fork
414 (used_here : used_in_flags) = [ any (`elemVarSet` fvs) (bindersOf bind)
415 | (fvs, drops) <- drop_boxes]
417 drop_here = used_here || not can_push
419 -- For case expressions we duplicate the binding if it is
420 -- reasonably small, and if it is not used in all the RHSs
421 -- This is good for situations like
426 -- E -> ...not mentioning x...
428 n_alts = length used_in_flags
429 n_used_alts = length [() | True <- used_in_flags]
431 can_push = n_used_alts == 1 -- Used in just one branch
432 || (is_case && -- We are looking at case alternatives
433 n_used_alts > 1 && -- It's used in more than one
434 n_used_alts < n_alts && -- ...but not all
435 bindIsDupable bind) -- and we can duplicate the binding
437 new_boxes | drop_here = (insert here_box : fork_boxes)
438 | otherwise = (here_box : new_fork_boxes)
440 new_fork_boxes = zipWithEqual "FloatIn.sepBinds" insert_maybe fork_boxes used_in_flags
442 insert :: DropBox -> DropBox
443 insert (fvs,drops) = (fvs `unionVarSet` bind_fvs, bind_w_fvs:drops)
445 insert_maybe box True = insert box
446 insert_maybe box False = box
449 floatedBindsFVs :: FloatingBinds -> FreeVarsSet
450 floatedBindsFVs binds = unionVarSets (map snd binds)
452 mkCoLets' :: FloatingBinds -> CoreExpr -> CoreExpr
453 mkCoLets' to_drop e = foldl (flip (Let . fst)) e to_drop
454 -- Remember to_drop is in *reverse* dependency order
456 bindIsDupable (Rec prs) = all (exprIsDupable . snd) prs
457 bindIsDupable (NonRec b r) = exprIsDupable r