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 CoreLint ( beginPass, endPass )
22 import Const ( isDataCon )
23 import CoreFVs ( CoreExprWithFVs, freeVars, freeVarsOf )
24 import Id ( isOneShotLambda )
25 import Var ( Id, idType, isTyVar )
26 import Type ( isUnLiftedType )
28 import Util ( zipEqual )
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 (_, AnnCon c args)
145 = mkCoLets' drop_here (Con c args')
147 (drop_here : arg_drops) = sepBindsByDropPoint (map freeVarsOf args) to_drop
148 args' = zipWith fiExpr arg_drops args
151 Applications: we do float inside applications, mainly because we
152 need to get at all the arguments. The next simplifier run will
153 pull out any silly ones.
156 fiExpr to_drop (_,AnnApp fun arg)
157 = mkCoLets' drop_here (App (fiExpr fun_drop fun) (fiExpr arg_drop arg))
159 [drop_here, fun_drop, arg_drop] = sepBindsByDropPoint [freeVarsOf fun, freeVarsOf arg] to_drop
162 We are careful about lambdas:
164 * We must be careful about floating inside inside a value lambda.
165 That risks losing laziness.
166 The float-out pass might rescue us, but then again it might not.
168 * We must be careful about type lambdas too. At one time we did, and
169 there is no risk of duplicating work thereby, but we do need to be
170 careful. In particular, here is a bad case (it happened in the
173 in let f = /\t -> \a -> ...
175 let f = /\t -> let v = ... in \a -> ...
176 This is bad as now f is an updatable closure (update PAP)
179 So we treat lambda in groups, using the following rule:
181 Float inside a group of lambdas only if
182 they are all either type lambdas or one-shot lambdas.
184 Otherwise drop all the bindings outside the group.
187 fiExpr to_drop (_, AnnLam b body)
188 = case collect [b] body of
190 | all is_ok bndrs -> mkLams bndrs (fiExpr to_drop real_body)
191 | otherwise -> mkCoLets' to_drop (mkLams bndrs (fiExpr [] real_body))
193 collect bs (_, AnnLam b body) = collect (b:bs) body
194 collect bs body = (reverse bs, body)
196 is_ok bndr = isTyVar bndr || isOneShotLambda bndr
199 We don't float lets inwards past an SCC.
200 ToDo: keep info on current cc, and when passing
201 one, if it is not the same, annotate all lets in binds with current
202 cc, change current cc to the new one and float binds into expr.
205 fiExpr to_drop (_, AnnNote note@(SCC cc) expr)
206 = -- Wimp out for now
207 mkCoLets' to_drop (Note note (fiExpr [] expr))
209 fiExpr to_drop (_, AnnNote InlineCall expr)
210 = -- Wimp out for InlineCall; keep it close
211 -- the the call it annotates
212 mkCoLets' to_drop (Note InlineCall (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@(Coerce _ _) expr)
219 = -- Just float in past coercion
220 Note note (fiExpr to_drop expr)
222 fiExpr to_drop (_, AnnNote note@(TermUsg _) expr)
223 = -- Float in past term usage annotation
224 -- (for now; not sure if this is correct: KSW 1999-05)
225 Note note (fiExpr to_drop expr)
228 For @Lets@, the possible ``drop points'' for the \tr{to_drop}
229 bindings are: (a)~in the body, (b1)~in the RHS of a NonRec binding,
230 or~(b2), in each of the RHSs of the pairs of a @Rec@.
232 Note that we do {\em weird things} with this let's binding. Consider:
241 Look at the inner \tr{let}. As \tr{w} is used in both the bind and
242 body of the inner let, we could panic and leave \tr{w}'s binding where
243 it is. But \tr{v} is floatable further into the body of the inner let, and
244 {\em then} \tr{w} will also be only in the body of that inner let.
246 So: rather than drop \tr{w}'s binding here, we add it onto the list of
247 things to drop in the outer let's body, and let nature take its
251 fiExpr to_drop (_,AnnLet (AnnNonRec id rhs@(rhs_fvs, ann_rhs)) body)
252 = fiExpr new_to_drop body
254 body_fvs = freeVarsOf body
256 final_body_fvs | noFloatIntoRhs ann_rhs
257 || isUnLiftedType (idType id) = body_fvs `unionVarSet` rhs_fvs
258 | otherwise = body_fvs
259 -- See commments with letrec below
260 -- No point in floating in only to float straight out again
261 -- Ditto ok-for-speculation unlifted RHSs
263 [shared_binds, rhs_binds, body_binds] = sepBindsByDropPoint [rhs_fvs, final_body_fvs] to_drop
265 new_to_drop = body_binds ++ -- the bindings used only in the body
266 [(NonRec id rhs', rhs_fvs')] ++ -- the new binding itself
267 shared_binds -- the bindings used both in rhs and body
269 -- Push rhs_binds into the right hand side of the binding
270 rhs' = fiExpr rhs_binds rhs
271 rhs_fvs' = rhs_fvs `unionVarSet` floatedBindsFVs rhs_binds
273 fiExpr to_drop (_,AnnLet (AnnRec bindings) body)
274 = fiExpr new_to_drop body
276 (binders, rhss) = unzip bindings
278 rhss_fvs = map freeVarsOf rhss
279 body_fvs = freeVarsOf body
281 -- Add to body_fvs the free vars of any RHS that has
282 -- a lambda at the top. This has the effect of making it seem
283 -- that such things are used in the body as well, and hence prevents
284 -- them getting floated in. The big idea is to avoid turning:
287 -- letrec f = \z. ...x#...f...
290 -- letrec f = let x# = y# +# 1# in \z. ...x#...f... in ...
292 -- Because now we can't float the let out again, because a letrec
293 -- can't have unboxed bindings.
295 final_body_fvs = foldr (unionVarSet . get_extras) body_fvs rhss
296 get_extras (rhs_fvs, rhs) | noFloatIntoRhs rhs = rhs_fvs
297 | otherwise = emptyVarSet
299 (shared_binds:body_binds:rhss_binds) = sepBindsByDropPoint (final_body_fvs:rhss_fvs) to_drop
301 new_to_drop = -- the bindings used only in the body
303 -- the new binding itself
304 [(Rec (fi_bind rhss_binds bindings), rhs_fvs')] ++
305 -- the bindings used both in rhs and body or in more than one rhs
308 rhs_fvs' = unionVarSet (unionVarSets rhss_fvs)
309 (unionVarSets (map floatedBindsFVs rhss_binds))
311 -- Push rhs_binds into the right hand side of the binding
312 fi_bind :: [FloatingBinds] -- one per "drop pt" conjured w/ fvs_of_rhss
313 -> [(Id, CoreExprWithFVs)]
316 fi_bind to_drops pairs
317 = [ (binder, fiExpr to_drop rhs)
318 | ((binder, rhs), to_drop) <- zipEqual "fi_bind" pairs to_drops ]
321 For @Case@, the possible ``drop points'' for the \tr{to_drop}
322 bindings are: (a)~inside the scrutinee, (b)~inside one of the
323 alternatives/default [default FVs always {\em first}!].
326 fiExpr to_drop (_, AnnCase scrut case_bndr alts)
327 = mkCoLets' drop_here (Case (fiExpr scrut_drops scrut) case_bndr
328 (zipWith fi_alt alts_drops alts))
330 (drop_here : scrut_drops : alts_drops) = sepBindsByDropPoint (scrut_fvs : alts_fvs) to_drop
331 scrut_fvs = freeVarsOf scrut
332 alts_fvs = map alt_fvs alts
333 alt_fvs (con, args, rhs) = foldl delVarSet (freeVarsOf rhs) (case_bndr:args)
334 -- Delete case_bndr and args from free vars of rhs
335 -- to get free vars of alt
337 fi_alt to_drop (con, args, rhs) = (con, args, fiExpr to_drop rhs)
339 noFloatIntoRhs (AnnNote InlineMe _) = True
340 noFloatIntoRhs (AnnLam _ _) = True
341 noFloatIntoRhs (AnnCon con _) = isDataCon con
342 noFloatIntoRhs other = False
346 %************************************************************************
348 \subsection{@sepBindsByDropPoint@}
350 %************************************************************************
352 This is the crucial function. The idea is: We have a wad of bindings
353 that we'd like to distribute inside a collection of {\em drop points};
354 insides the alternatives of a \tr{case} would be one example of some
355 drop points; the RHS and body of a non-recursive \tr{let} binding
356 would be another (2-element) collection.
358 So: We're given a list of sets-of-free-variables, one per drop point,
359 and a list of floating-inwards bindings. If a binding can go into
360 only one drop point (without suddenly making something out-of-scope),
361 in it goes. If a binding is used inside {\em multiple} drop points,
362 then it has to go in a you-must-drop-it-above-all-these-drop-points
365 We have to maintain the order on these drop-point-related lists.
369 :: [FreeVarsSet] -- One set of FVs per drop point
370 -> FloatingBinds -- Candidate floaters
371 -> [FloatingBinds] -- FIRST one is bindings which must not be floated
372 -- inside any drop point; the rest correspond
373 -- one-to-one with the input list of FV sets
375 -- Every input floater is returned somewhere in the result;
376 -- none are dropped, not even ones which don't seem to be
377 -- free in *any* of the drop-point fvs. Why? Because, for example,
378 -- a binding (let x = E in B) might have a specialised version of
379 -- x (say x') stored inside x, but x' isn't free in E or B.
381 sepBindsByDropPoint drop_pts []
382 = [] : [[] | p <- drop_pts] -- cut to the chase scene; it happens
384 sepBindsByDropPoint drop_pts floaters
385 = go floaters (map (\fvs -> (fvs, [])) (emptyVarSet : drop_pts))
387 go :: FloatingBinds -> [(FreeVarsSet, FloatingBinds)] -> [FloatingBinds]
388 -- The *first* one in the argument list is the drop_here set
389 -- The FloatingBinds in the lists are in the reverse of
390 -- the normal FloatingBinds order; that is, they are the right way round!
392 go [] drop_boxes = map (reverse . snd) drop_boxes
394 go (bind_w_fvs@(bind, bind_fvs) : binds) drop_boxes
395 = go binds (insert drop_boxes (drop_here : used_in_flags))
396 -- insert puts the find in box whose True flag comes first
398 (used_here : used_in_flags) = [ any (`elemVarSet` fvs) (bindersOf bind)
399 | (fvs, drops) <- drop_boxes]
401 drop_here = used_here || not (exactlyOneTrue used_in_flags)
403 insert ((fvs,drops) : drop_boxes) (True : _)
404 = ((fvs `unionVarSet` bind_fvs, bind_w_fvs:drops) : drop_boxes)
405 insert (drop_box : drop_boxes) (False : others)
406 = drop_box : insert drop_boxes others
407 insert _ _ = panic "sepBindsByDropPoint" -- Should never happen
409 exactlyOneTrue :: [Bool] -> Bool
410 exactlyOneTrue flags = case [() | True <- flags] of
414 floatedBindsFVs :: FloatingBinds -> FreeVarsSet
415 floatedBindsFVs binds = unionVarSets (map snd binds)
417 mkCoLets' :: FloatingBinds -> CoreExpr -> CoreExpr
418 mkCoLets' to_drop e = foldl (flip (Let . fst)) e to_drop
419 -- Remember to_drop is in *reverse* dependency order