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 DynFlags ( DynFlags, DynFlag(..) )
21 import CoreUtils ( exprIsHNF, exprIsDupable )
22 import CoreLint ( showPass, endPass )
23 import CoreFVs ( CoreExprWithFVs, freeVars, freeVarsOf, idRuleVars )
24 import Id ( isOneShotBndr, idType )
26 import Type ( isUnLiftedType )
28 import Util ( zipEqual, zipWithEqual, count )
33 Top-level interface function, @floatInwards@. Note that we do not
34 actually float any bindings downwards from the top-level.
37 floatInwards :: DynFlags -> [CoreBind] -> IO [CoreBind]
39 floatInwards dflags binds
41 showPass dflags "Float inwards";
42 let { binds' = map fi_top_bind binds };
43 endPass dflags "Float inwards" Opt_D_verbose_core2core binds'
44 {- 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 binder 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 )
143 fiExpr to_drop (_, AnnCast expr co)
144 = Cast (fiExpr to_drop expr) co -- Just float in past coercion
146 fiExpr _ (_, AnnLit lit) = Lit lit
149 Applications: we do float inside applications, mainly because we
150 need to get at all the arguments. The next simplifier run will
151 pull out any silly ones.
154 fiExpr to_drop (_,AnnApp fun arg)
155 = mkCoLets' drop_here (App (fiExpr fun_drop fun) (fiExpr arg_drop arg))
157 [drop_here, fun_drop, arg_drop] = sepBindsByDropPoint False [freeVarsOf fun, freeVarsOf arg] to_drop
160 We are careful about lambdas:
162 * We must be careful about floating inside inside a value lambda.
163 That risks losing laziness.
164 The float-out pass might rescue us, but then again it might not.
166 * We must be careful about type lambdas too. At one time we did, and
167 there is no risk of duplicating work thereby, but we do need to be
168 careful. In particular, here is a bad case (it happened in the
171 in let f = /\t -> \a -> ...
173 let f = /\t -> let v = ... in \a -> ...
174 This is bad as now f is an updatable closure (update PAP)
177 So we treat lambda in groups, using the following rule:
179 Float inside a group of lambdas only if
180 they are all either type lambdas or one-shot lambdas.
182 Otherwise drop all the bindings outside the group.
185 -- Hack alert! We only float in through one-shot lambdas,
186 -- not (as you might guess) through big lambdas.
187 -- Reason: we float *out* past big lambdas (see the test in the Lam
188 -- case of FloatOut.floatExpr) and we don't want to float straight
191 -- It *is* important to float into one-shot lambdas, however;
192 -- see the remarks with noFloatIntoRhs.
193 fiExpr to_drop lam@(_, AnnLam _ _)
194 | all is_one_shot bndrs -- Float in
195 = mkLams bndrs (fiExpr to_drop body)
197 | otherwise -- Dump it all here
198 = mkCoLets' to_drop (mkLams bndrs (fiExpr [] body))
201 (bndrs, body) = collectAnnBndrs lam
204 We don't float lets inwards past an SCC.
205 ToDo: keep info on current cc, and when passing
206 one, if it is not the same, annotate all lets in binds with current
207 cc, change current cc to the new one and float binds into expr.
210 fiExpr to_drop (_, AnnNote note@(SCC _) expr)
211 = -- Wimp out for now
212 mkCoLets' to_drop (Note note (fiExpr [] expr))
214 fiExpr to_drop (_, AnnNote InlineMe expr)
215 = -- Ditto... don't float anything into an INLINE expression
216 mkCoLets' to_drop (Note InlineMe (fiExpr [] expr))
218 fiExpr to_drop (_, AnnNote note@(CoreNote _) expr)
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
244 Note [extra_fvs (1): avoid floating into RHS]
245 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
246 Consdider let x=\y....t... in body. We do not necessarily want to float
247 a binding for t into the RHS, because it'll immediately be floated out
248 again. (It won't go inside the lambda else we risk losing work.)
249 In letrec, we need to be more careful still. We don't want to transform
252 letrec f = \z. ...x#...f...
255 letrec f = let x# = y# +# 1# in \z. ...x#...f... in ...
256 because now we can't float the let out again, because a letrec
257 can't have unboxed bindings.
259 So we make "extra_fvs" which is the rhs_fvs of such bindings, and
260 arrange to dump bindings that bind extra_fvs before the entire let.
262 Note [extra_fvs (s): free variables of rules]
263 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
264 Consider let x{rule mentioning y} = rhs in body
265 Here y is not free in rhs or body; but we still want to dump bindings
266 that bind y outside the let. So we augment extra_fvs with the
271 fiExpr to_drop (_,AnnLet (AnnNonRec id rhs@(rhs_fvs, ann_rhs)) body)
272 = fiExpr new_to_drop body
274 body_fvs = freeVarsOf body
276 rule_fvs = idRuleVars id -- See Note [extra_fvs (2): free variables of rules]
277 extra_fvs | noFloatIntoRhs ann_rhs
278 || isUnLiftedType (idType id) = rule_fvs `unionVarSet` rhs_fvs
279 | otherwise = rule_fvs
280 -- See Note [extra_fvs (2): avoid floating into RHS]
281 -- No point in floating in only to float straight out again
282 -- Ditto ok-for-speculation unlifted RHSs
284 [shared_binds, extra_binds, rhs_binds, body_binds]
285 = sepBindsByDropPoint False [extra_fvs, rhs_fvs, body_fvs] to_drop
287 new_to_drop = body_binds ++ -- the bindings used only in the body
288 [(NonRec id rhs', rhs_fvs')] ++ -- the new binding itself
289 extra_binds ++ -- bindings from extra_fvs
290 shared_binds -- the bindings used both in rhs and body
292 -- Push rhs_binds into the right hand side of the binding
293 rhs' = fiExpr rhs_binds rhs
294 rhs_fvs' = rhs_fvs `unionVarSet` floatedBindsFVs rhs_binds
296 fiExpr to_drop (_,AnnLet (AnnRec bindings) body)
297 = fiExpr new_to_drop body
299 rhss = map snd bindings
301 rhss_fvs = map freeVarsOf rhss
302 body_fvs = freeVarsOf body
304 -- See Note [extra_fvs (1,2)]
305 extra_fvs = foldr (unionVarSet . get_extras) emptyVarSet bindings
306 get_extras (id, (rhs_fvs, rhs))
307 | noFloatIntoRhs rhs = idRuleVars id `unionVarSet` rhs_fvs
308 | otherwise = idRuleVars id
310 (shared_binds:extra_binds:body_binds:rhss_binds)
311 = sepBindsByDropPoint False (extra_fvs:body_fvs:rhss_fvs) to_drop
313 new_to_drop = body_binds ++ -- the bindings used only in the body
314 [(Rec (fi_bind rhss_binds bindings), rhs_fvs')] ++
315 -- The new binding itself
316 extra_binds ++ -- Note [extra_fvs (1,2)]
317 shared_binds -- Used in more than one place
319 rhs_fvs' = unionVarSet (unionVarSets rhss_fvs)
320 (unionVarSets (map floatedBindsFVs rhss_binds))
322 -- Push rhs_binds into the right hand side of the binding
323 fi_bind :: [FloatingBinds] -- one per "drop pt" conjured w/ fvs_of_rhss
324 -> [(Id, CoreExprWithFVs)]
327 fi_bind to_drops pairs
328 = [ (binder, fiExpr to_drop rhs)
329 | ((binder, rhs), to_drop) <- zipEqual "fi_bind" pairs to_drops ]
332 For @Case@, the possible ``drop points'' for the \tr{to_drop}
333 bindings are: (a)~inside the scrutinee, (b)~inside one of the
334 alternatives/default [default FVs always {\em first}!].
337 fiExpr to_drop (_, AnnCase scrut case_bndr ty alts)
338 = mkCoLets' drop_here1 $
339 mkCoLets' drop_here2 $
340 Case (fiExpr scrut_drops scrut) case_bndr ty
341 (zipWith fi_alt alts_drops_s alts)
343 -- Float into the scrut and alts-considered-together just like App
344 [drop_here1, scrut_drops, alts_drops] = sepBindsByDropPoint False [scrut_fvs, all_alts_fvs] to_drop
346 -- Float into the alts with the is_case flag set
347 (drop_here2 : alts_drops_s) = sepBindsByDropPoint True alts_fvs alts_drops
349 scrut_fvs = freeVarsOf scrut
350 alts_fvs = map alt_fvs alts
351 all_alts_fvs = unionVarSets alts_fvs
352 alt_fvs (_con, args, rhs) = foldl delVarSet (freeVarsOf rhs) (case_bndr:args)
353 -- Delete case_bndr and args from free vars of rhs
354 -- to get free vars of alt
356 fi_alt to_drop (con, args, rhs) = (con, args, fiExpr to_drop rhs)
358 noFloatIntoRhs :: AnnExpr' Var (UniqFM Var) -> Bool
359 noFloatIntoRhs (AnnNote InlineMe _) = True
360 noFloatIntoRhs (AnnLam b _) = not (is_one_shot b)
361 -- IMPORTANT: don't say 'True' for a RHS with a one-shot lambda at the top.
362 -- This makes a big difference for things like
363 -- f x# = let x = I# x#
364 -- in let j = \() -> ...x...
365 -- in if <condition> then normal-path else j ()
366 -- If x is used only in the error case join point, j, we must float the
367 -- boxing constructor into it, else we box it every time which is very bad
370 noFloatIntoRhs rhs = exprIsHNF (deAnnotate' rhs) -- We'd just float right back out again...
372 is_one_shot :: Var -> Bool
373 is_one_shot b = isIdVar b && isOneShotBndr b
377 %************************************************************************
379 \subsection{@sepBindsByDropPoint@}
381 %************************************************************************
383 This is the crucial function. The idea is: We have a wad of bindings
384 that we'd like to distribute inside a collection of {\em drop points};
385 insides the alternatives of a \tr{case} would be one example of some
386 drop points; the RHS and body of a non-recursive \tr{let} binding
387 would be another (2-element) collection.
389 So: We're given a list of sets-of-free-variables, one per drop point,
390 and a list of floating-inwards bindings. If a binding can go into
391 only one drop point (without suddenly making something out-of-scope),
392 in it goes. If a binding is used inside {\em multiple} drop points,
393 then it has to go in a you-must-drop-it-above-all-these-drop-points
396 We have to maintain the order on these drop-point-related lists.
400 :: Bool -- True <=> is case expression
401 -> [FreeVarsSet] -- One set of FVs per drop point
402 -> FloatingBinds -- Candidate floaters
403 -> [FloatingBinds] -- FIRST one is bindings which must not be floated
404 -- inside any drop point; the rest correspond
405 -- one-to-one with the input list of FV sets
407 -- Every input floater is returned somewhere in the result;
408 -- none are dropped, not even ones which don't seem to be
409 -- free in *any* of the drop-point fvs. Why? Because, for example,
410 -- a binding (let x = E in B) might have a specialised version of
411 -- x (say x') stored inside x, but x' isn't free in E or B.
413 type DropBox = (FreeVarsSet, FloatingBinds)
415 sepBindsByDropPoint _is_case drop_pts []
416 = [] : [[] | _ <- drop_pts] -- cut to the chase scene; it happens
418 sepBindsByDropPoint is_case drop_pts floaters
419 = go floaters (map (\fvs -> (fvs, [])) (emptyVarSet : drop_pts))
421 go :: FloatingBinds -> [DropBox] -> [FloatingBinds]
422 -- The *first* one in the argument list is the drop_here set
423 -- The FloatingBinds in the lists are in the reverse of
424 -- the normal FloatingBinds order; that is, they are the right way round!
426 go [] drop_boxes = map (reverse . snd) drop_boxes
428 go (bind_w_fvs@(bind, bind_fvs) : binds) drop_boxes@(here_box : fork_boxes)
431 -- "here" means the group of bindings dropped at the top of the fork
433 (used_here : used_in_flags) = [ any (`elemVarSet` fvs) (bindersOf bind)
434 | (fvs, _) <- drop_boxes]
436 drop_here = used_here || not can_push
438 -- For case expressions we duplicate the binding if it is
439 -- reasonably small, and if it is not used in all the RHSs
440 -- This is good for situations like
445 -- E -> ...not mentioning x...
447 n_alts = length used_in_flags
448 n_used_alts = count id used_in_flags -- returns number of Trues in list.
450 can_push = n_used_alts == 1 -- Used in just one branch
451 || (is_case && -- We are looking at case alternatives
452 n_used_alts > 1 && -- It's used in more than one
453 n_used_alts < n_alts && -- ...but not all
454 bindIsDupable bind) -- and we can duplicate the binding
456 new_boxes | drop_here = (insert here_box : fork_boxes)
457 | otherwise = (here_box : new_fork_boxes)
459 new_fork_boxes = zipWithEqual "FloatIn.sepBinds" insert_maybe fork_boxes used_in_flags
461 insert :: DropBox -> DropBox
462 insert (fvs,drops) = (fvs `unionVarSet` bind_fvs, bind_w_fvs:drops)
464 insert_maybe box True = insert box
465 insert_maybe box False = box
467 go _ _ = panic "sepBindsByDropPoint/go"
470 floatedBindsFVs :: FloatingBinds -> FreeVarsSet
471 floatedBindsFVs binds = unionVarSets (map snd binds)
473 mkCoLets' :: FloatingBinds -> CoreExpr -> CoreExpr
474 mkCoLets' to_drop e = foldl (flip (Let . fst)) e to_drop
475 -- Remember to_drop is in *reverse* dependency order
477 bindIsDupable :: Bind CoreBndr -> Bool
478 bindIsDupable (Rec prs) = all (exprIsDupable . snd) prs
479 bindIsDupable (NonRec _ r) = exprIsDupable r