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 ( DynFlags, DynFlag(..) )
21 import CoreUtils ( exprIsValue, exprIsDupable )
22 import CoreLint ( showPass, 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 :: DynFlags -> [CoreBind] -> IO [CoreBind]
38 floatInwards dflags binds
40 showPass dflags "Float inwards";
41 let { binds' = map fi_top_bind binds };
42 endPass dflags "Float inwards" Opt_D_verbose_core2core binds'
43 {- no specific flag for dumping float-in -}
47 fi_top_bind (NonRec binder rhs)
48 = NonRec binder (fiExpr [] (freeVars rhs))
49 fi_top_bind (Rec pairs)
50 = Rec [ (b, fiExpr [] (freeVars rhs)) | (b, rhs) <- pairs ]
53 %************************************************************************
55 \subsection{Mail from Andr\'e [edited]}
57 %************************************************************************
59 {\em Will wrote: What??? I thought the idea was to float as far
60 inwards as possible, no matter what. This is dropping all bindings
61 every time it sees a lambda of any kind. Help! }
63 You are assuming we DO DO full laziness AFTER floating inwards! We
64 have to [not float inside lambdas] if we don't.
66 If we indeed do full laziness after the floating inwards (we could
67 check the compilation flags for that) then I agree we could be more
68 aggressive and do float inwards past lambdas.
70 Actually we are not doing a proper full laziness (see below), which
71 was another reason for not floating inwards past a lambda.
73 This can easily be fixed. The problem is that we float lets outwards,
74 but there are a few expressions which are not let bound, like case
75 scrutinees and case alternatives. After floating inwards the
76 simplifier could decide to inline the let and the laziness would be
80 let a = expensive ==> \b -> case expensive of ...
81 in \ b -> case a of ...
86 to let bind the algebraic case scrutinees (done, I think) and
87 the case alternatives (except the ones with an
88 unboxed type)(not done, I think). This is best done in the
89 SetLevels.lhs module, which tags things with their level numbers.
91 do the full laziness pass (floating lets outwards).
93 simplify. The simplifier inlines the (trivial) lets that were
94 created but were not floated outwards.
97 With the fix I think Will's suggestion that we can gain even more from
98 strictness by floating inwards past lambdas makes sense.
100 We still gain even without going past lambdas, as things may be
101 strict in the (new) context of a branch (where it was floated to) or
104 let a = something case x of
105 in case x of alt1 -> case something of a -> a + a
106 alt1 -> a + a ==> alt2 -> b
109 let a = something let b = case something of a -> a + a
110 in let b = a + a ==> in (b,b)
113 Also, even if a is not found to be strict in the new context and is
114 still left as a let, if the branch is not taken (or b is not entered)
115 the closure for a is not built.
117 %************************************************************************
119 \subsection{Main floating-inwards code}
121 %************************************************************************
124 type FreeVarsSet = IdSet
126 type FloatingBinds = [(CoreBind, FreeVarsSet)]
127 -- In reverse dependency order (innermost bindiner first)
129 -- The FreeVarsSet is the free variables of the binding. In the case
130 -- of recursive bindings, the set doesn't include the bound
133 fiExpr :: FloatingBinds -- Binds we're trying to drop
134 -- as far "inwards" as possible
135 -> CoreExprWithFVs -- Input expr
136 -> CoreExpr -- Result
138 fiExpr to_drop (_, AnnVar v) = mkCoLets' to_drop (Var v)
140 fiExpr to_drop (_, AnnType ty) = ASSERT( null to_drop )
143 fiExpr to_drop (_, AnnLit lit) = Lit lit
146 Applications: we do float inside applications, mainly because we
147 need to get at all the arguments. The next simplifier run will
148 pull out any silly ones.
151 fiExpr to_drop (_,AnnApp fun arg)
152 = mkCoLets' drop_here (App (fiExpr fun_drop fun) (fiExpr arg_drop arg))
154 [drop_here, fun_drop, arg_drop] = sepBindsByDropPoint False [freeVarsOf fun, freeVarsOf arg] to_drop
157 We are careful about lambdas:
159 * We must be careful about floating inside inside a value lambda.
160 That risks losing laziness.
161 The float-out pass might rescue us, but then again it might not.
163 * We must be careful about type lambdas too. At one time we did, and
164 there is no risk of duplicating work thereby, but we do need to be
165 careful. In particular, here is a bad case (it happened in the
168 in let f = /\t -> \a -> ...
170 let f = /\t -> let v = ... in \a -> ...
171 This is bad as now f is an updatable closure (update PAP)
174 So we treat lambda in groups, using the following rule:
176 Float inside a group of lambdas only if
177 they are all either type lambdas or one-shot lambdas.
179 Otherwise drop all the bindings outside the group.
182 fiExpr to_drop (_, AnnLam b body)
183 = case collect [b] body of
185 -- | all is_ok bndrs -> mkLams bndrs (fiExpr to_drop real_body)
186 -- [July 01: I'm experiment with getting the full laziness
187 -- pass to floats bindings out past big lambdas (instead of the simplifier)
188 -- so I don't want the float-in pass to just push them right back in.
189 -- I'm going to try just dumping all bindings outside lambdas.]
190 | otherwise -> mkCoLets' to_drop (mkLams bndrs (fiExpr [] real_body))
192 collect bs (_, AnnLam b body) = collect (b:bs) body
193 collect bs body = (reverse bs, body)
195 -- is_ok bndr = isTyVar bndr || isOneShotLambda bndr
198 We don't float lets inwards past an SCC.
199 ToDo: keep info on current cc, and when passing
200 one, if it is not the same, annotate all lets in binds with current
201 cc, change current cc to the new one and float binds into expr.
204 fiExpr to_drop (_, AnnNote note@(SCC cc) expr)
205 = -- Wimp out for now
206 mkCoLets' to_drop (Note note (fiExpr [] expr))
208 fiExpr to_drop (_, AnnNote InlineCall expr)
209 = -- Wimp out for InlineCall; keep it close
210 -- the the call it annotates
211 mkCoLets' to_drop (Note InlineCall (fiExpr [] expr))
213 fiExpr to_drop (_, AnnNote InlineMe expr)
214 = -- Ditto... don't float anything into an INLINE expression
215 mkCoLets' to_drop (Note InlineMe (fiExpr [] expr))
217 fiExpr to_drop (_, AnnNote note@(Coerce _ _) expr)
218 = -- Just float in past coercion
219 Note note (fiExpr to_drop expr)
222 For @Lets@, the possible ``drop points'' for the \tr{to_drop}
223 bindings are: (a)~in the body, (b1)~in the RHS of a NonRec binding,
224 or~(b2), in each of the RHSs of the pairs of a @Rec@.
226 Note that we do {\em weird things} with this let's binding. Consider:
235 Look at the inner \tr{let}. As \tr{w} is used in both the bind and
236 body of the inner let, we could panic and leave \tr{w}'s binding where
237 it is. But \tr{v} is floatable further into the body of the inner let, and
238 {\em then} \tr{w} will also be only in the body of that inner let.
240 So: rather than drop \tr{w}'s binding here, we add it onto the list of
241 things to drop in the outer let's body, and let nature take its
245 fiExpr to_drop (_,AnnLet (AnnNonRec id rhs@(rhs_fvs, ann_rhs)) body)
246 = fiExpr new_to_drop body
248 body_fvs = freeVarsOf body
250 final_body_fvs | noFloatIntoRhs ann_rhs
251 || isUnLiftedType (idType id) = body_fvs `unionVarSet` rhs_fvs
252 | otherwise = body_fvs
253 -- See commments with letrec below
254 -- No point in floating in only to float straight out again
255 -- Ditto ok-for-speculation unlifted RHSs
257 [shared_binds, rhs_binds, body_binds] = sepBindsByDropPoint False [rhs_fvs, final_body_fvs] to_drop
259 new_to_drop = body_binds ++ -- the bindings used only in the body
260 [(NonRec id rhs', rhs_fvs')] ++ -- the new binding itself
261 shared_binds -- the bindings used both in rhs and body
263 -- Push rhs_binds into the right hand side of the binding
264 rhs' = fiExpr rhs_binds rhs
265 rhs_fvs' = rhs_fvs `unionVarSet` floatedBindsFVs rhs_binds
267 fiExpr to_drop (_,AnnLet (AnnRec bindings) body)
268 = fiExpr new_to_drop body
270 rhss = map snd bindings
272 rhss_fvs = map freeVarsOf rhss
273 body_fvs = freeVarsOf body
275 -- Add to body_fvs the free vars of any RHS that has
276 -- a lambda at the top. This has the effect of making it seem
277 -- that such things are used in the body as well, and hence prevents
278 -- them getting floated in. The big idea is to avoid turning:
281 -- letrec f = \z. ...x#...f...
284 -- letrec f = let x# = y# +# 1# in \z. ...x#...f... in ...
286 -- Because now we can't float the let out again, because a letrec
287 -- can't have unboxed bindings.
289 final_body_fvs = foldr (unionVarSet . get_extras) body_fvs rhss
290 get_extras (rhs_fvs, rhs) | noFloatIntoRhs rhs = rhs_fvs
291 | otherwise = emptyVarSet
293 (shared_binds:body_binds:rhss_binds) = sepBindsByDropPoint False (final_body_fvs:rhss_fvs) to_drop
295 new_to_drop = -- the bindings used only in the body
297 -- the new binding itself
298 [(Rec (fi_bind rhss_binds bindings), rhs_fvs')] ++
299 -- the bindings used both in rhs and body or in more than one rhs
302 rhs_fvs' = unionVarSet (unionVarSets rhss_fvs)
303 (unionVarSets (map floatedBindsFVs rhss_binds))
305 -- Push rhs_binds into the right hand side of the binding
306 fi_bind :: [FloatingBinds] -- one per "drop pt" conjured w/ fvs_of_rhss
307 -> [(Id, CoreExprWithFVs)]
310 fi_bind to_drops pairs
311 = [ (binder, fiExpr to_drop rhs)
312 | ((binder, rhs), to_drop) <- zipEqual "fi_bind" pairs to_drops ]
315 For @Case@, the possible ``drop points'' for the \tr{to_drop}
316 bindings are: (a)~inside the scrutinee, (b)~inside one of the
317 alternatives/default [default FVs always {\em first}!].
320 fiExpr to_drop (_, AnnCase scrut case_bndr alts)
321 = mkCoLets' drop_here1 $
322 mkCoLets' drop_here2 $
323 Case (fiExpr scrut_drops scrut) case_bndr
324 (zipWith fi_alt alts_drops_s alts)
326 -- Float into the scrut and alts-considered-together just like App
327 [drop_here1, scrut_drops, alts_drops] = sepBindsByDropPoint False [scrut_fvs, all_alts_fvs] to_drop
329 -- Float into the alts with the is_case flag set
330 (drop_here2 : alts_drops_s) = sepBindsByDropPoint True alts_fvs alts_drops
332 scrut_fvs = freeVarsOf scrut
333 alts_fvs = map alt_fvs alts
334 all_alts_fvs = unionVarSets alts_fvs
335 alt_fvs (con, args, rhs) = foldl delVarSet (freeVarsOf rhs) (case_bndr:args)
336 -- Delete case_bndr and args from free vars of rhs
337 -- to get free vars of alt
339 fi_alt to_drop (con, args, rhs) = (con, args, fiExpr to_drop rhs)
341 noFloatIntoRhs (AnnNote InlineMe _) = True
342 noFloatIntoRhs (AnnLam b _) = not (isId b && isOneShotLambda b)
343 -- IMPORTANT: don't say 'True' for a RHS with a one-shot lambda at the top.
344 -- This makes a big difference for things like
345 -- f x# = let x = I# x#
346 -- in let j = \() -> ...x...
347 -- in if <condition> then normal-path else j ()
348 -- If x is used only in the error case join point, j, we must float the
349 -- boxing constructor into it, else we box it every time which is very bad
352 noFloatIntoRhs rhs = exprIsValue (deAnnotate' rhs) -- We'd just float rigt back out again...
356 %************************************************************************
358 \subsection{@sepBindsByDropPoint@}
360 %************************************************************************
362 This is the crucial function. The idea is: We have a wad of bindings
363 that we'd like to distribute inside a collection of {\em drop points};
364 insides the alternatives of a \tr{case} would be one example of some
365 drop points; the RHS and body of a non-recursive \tr{let} binding
366 would be another (2-element) collection.
368 So: We're given a list of sets-of-free-variables, one per drop point,
369 and a list of floating-inwards bindings. If a binding can go into
370 only one drop point (without suddenly making something out-of-scope),
371 in it goes. If a binding is used inside {\em multiple} drop points,
372 then it has to go in a you-must-drop-it-above-all-these-drop-points
375 We have to maintain the order on these drop-point-related lists.
379 :: Bool -- True <=> is case expression
380 -> [FreeVarsSet] -- One set of FVs per drop point
381 -> FloatingBinds -- Candidate floaters
382 -> [FloatingBinds] -- FIRST one is bindings which must not be floated
383 -- inside any drop point; the rest correspond
384 -- one-to-one with the input list of FV sets
386 -- Every input floater is returned somewhere in the result;
387 -- none are dropped, not even ones which don't seem to be
388 -- free in *any* of the drop-point fvs. Why? Because, for example,
389 -- a binding (let x = E in B) might have a specialised version of
390 -- x (say x') stored inside x, but x' isn't free in E or B.
392 type DropBox = (FreeVarsSet, FloatingBinds)
394 sepBindsByDropPoint is_case drop_pts []
395 = [] : [[] | p <- drop_pts] -- cut to the chase scene; it happens
397 sepBindsByDropPoint is_case drop_pts floaters
398 = go floaters (map (\fvs -> (fvs, [])) (emptyVarSet : drop_pts))
400 go :: FloatingBinds -> [DropBox] -> [FloatingBinds]
401 -- The *first* one in the argument list is the drop_here set
402 -- The FloatingBinds in the lists are in the reverse of
403 -- the normal FloatingBinds order; that is, they are the right way round!
405 go [] drop_boxes = map (reverse . snd) drop_boxes
407 go (bind_w_fvs@(bind, bind_fvs) : binds) drop_boxes@(here_box : fork_boxes)
410 -- "here" means the group of bindings dropped at the top of the fork
412 (used_here : used_in_flags) = [ any (`elemVarSet` fvs) (bindersOf bind)
413 | (fvs, drops) <- drop_boxes]
415 drop_here = used_here || not can_push
417 -- For case expressions we duplicate the binding if it is
418 -- reasonably small, and if it is not used in all the RHSs
419 -- This is good for situations like
424 -- E -> ...not mentioning x...
426 n_alts = length used_in_flags
427 n_used_alts = length [() | True <- used_in_flags]
429 can_push = n_used_alts == 1 -- Used in just one branch
430 || (is_case && -- We are looking at case alternatives
431 n_used_alts > 1 && -- It's used in more than one
432 n_used_alts < n_alts && -- ...but not all
433 bindIsDupable bind) -- and we can duplicate the binding
435 new_boxes | drop_here = (insert here_box : fork_boxes)
436 | otherwise = (here_box : new_fork_boxes)
438 new_fork_boxes = zipWithEqual "FloatIn.sepBinds" insert_maybe fork_boxes used_in_flags
440 insert :: DropBox -> DropBox
441 insert (fvs,drops) = (fvs `unionVarSet` bind_fvs, bind_w_fvs:drops)
443 insert_maybe box True = insert box
444 insert_maybe box False = box
447 floatedBindsFVs :: FloatingBinds -> FreeVarsSet
448 floatedBindsFVs binds = unionVarSets (map snd binds)
450 mkCoLets' :: FloatingBinds -> CoreExpr -> CoreExpr
451 mkCoLets' to_drop e = foldl (flip (Let . fst)) e to_drop
452 -- Remember to_drop is in *reverse* dependency order
454 bindIsDupable (Rec prs) = all (exprIsDupable . snd) prs
455 bindIsDupable (NonRec b r) = exprIsDupable r