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 FreeVars ( CoreExprWithFVs, freeVars, freeVarsOf )
25 import Util ( zipEqual )
29 Top-level interface function, @floatInwards@. Note that we do not
30 actually float any bindings downwards from the top-level.
33 floatInwards :: [CoreBind] -> IO [CoreBind]
37 beginPass "Float inwards";
38 let { binds' = map fi_top_bind binds };
39 endPass "Float inwards"
40 opt_D_verbose_core2core {- no specific flag for dumping float-in -}
45 fi_top_bind (NonRec binder rhs)
46 = NonRec binder (fiExpr [] (freeVars rhs))
47 fi_top_bind (Rec pairs)
48 = Rec [ (b, fiExpr [] (freeVars rhs)) | (b, rhs) <- pairs ]
51 %************************************************************************
53 \subsection{Mail from Andr\'e [edited]}
55 %************************************************************************
57 {\em Will wrote: What??? I thought the idea was to float as far
58 inwards as possible, no matter what. This is dropping all bindings
59 every time it sees a lambda of any kind. Help! }
61 You are assuming we DO DO full laziness AFTER floating inwards! We
62 have to [not float inside lambdas] if we don't.
64 If we indeed do full laziness after the floating inwards (we could
65 check the compilation flags for that) then I agree we could be more
66 aggressive and do float inwards past lambdas.
68 Actually we are not doing a proper full laziness (see below), which
69 was another reason for not floating inwards past a lambda.
71 This can easily be fixed. The problem is that we float lets outwards,
72 but there are a few expressions which are not let bound, like case
73 scrutinees and case alternatives. After floating inwards the
74 simplifier could decide to inline the let and the laziness would be
78 let a = expensive ==> \b -> case expensive of ...
79 in \ b -> case a of ...
84 to let bind the algebraic case scrutinees (done, I think) and
85 the case alternatives (except the ones with an
86 unboxed type)(not done, I think). This is best done in the
87 SetLevels.lhs module, which tags things with their level numbers.
89 do the full laziness pass (floating lets outwards).
91 simplify. The simplifier inlines the (trivial) lets that were
92 created but were not floated outwards.
95 With the fix I think Will's suggestion that we can gain even more from
96 strictness by floating inwards past lambdas makes sense.
98 We still gain even without going past lambdas, as things may be
99 strict in the (new) context of a branch (where it was floated to) or
102 let a = something case x of
103 in case x of alt1 -> case something of a -> a + a
104 alt1 -> a + a ==> alt2 -> b
107 let a = something let b = case something of a -> a + a
108 in let b = a + a ==> in (b,b)
111 Also, even if a is not found to be strict in the new context and is
112 still left as a let, if the branch is not taken (or b is not entered)
113 the closure for a is not built.
115 %************************************************************************
117 \subsection{Main floating-inwards code}
119 %************************************************************************
122 type FreeVarsSet = IdSet
124 type FloatingBinds = [(CoreBind, FreeVarsSet)]
125 -- In reverse dependency order (innermost bindiner first)
127 -- The FreeVarsSet is the free variables of the binding. In the case
128 -- of recursive bindings, the set doesn't include the bound
131 fiExpr :: FloatingBinds -- Binds we're trying to drop
132 -- as far "inwards" as possible
133 -> CoreExprWithFVs -- Input expr
134 -> CoreExpr -- Result
136 fiExpr to_drop (_, AnnVar v) = mkCoLets' to_drop (Var v)
138 fiExpr to_drop (_, AnnType ty) = ASSERT( null to_drop )
141 fiExpr to_drop (_, AnnCon c args)
142 = mkCoLets' drop_here (Con c args')
144 (drop_here : arg_drops) = sepBindsByDropPoint (map freeVarsOf args) to_drop
145 args' = zipWith fiExpr arg_drops args
148 Applications: we do float inside applications, mainly because we
149 need to get at all the arguments. The next simplifier run will
150 pull out any silly ones.
153 fiExpr to_drop (_,AnnApp fun arg)
154 = mkCoLets' drop_here (App (fiExpr fun_drop fun) (fiExpr arg_drop arg))
156 [drop_here, fun_drop, arg_drop] = sepBindsByDropPoint [freeVarsOf fun, freeVarsOf arg] to_drop
159 We are careful about lambdas:
161 * We never float inside a value lambda. That risks losing laziness.
162 The float-out pass might rescue us, but then again it might not.
164 * We don't float inside type lambdas either. 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 the simple thing is never to float inside big lambda either.
176 Maybe we'll find cases when that loses something important; if
177 so we can modify the decision.
180 fiExpr to_drop (_, AnnLam b body)
181 = mkCoLets' to_drop (Lam b (fiExpr [] body))
184 We don't float lets inwards past an SCC.
185 ToDo: keep info on current cc, and when passing
186 one, if it is not the same, annotate all lets in binds with current
187 cc, change current cc to the new one and float binds into expr.
190 fiExpr to_drop (_, AnnNote note@(SCC cc) expr)
191 = -- Wimp out for now
192 mkCoLets' to_drop (Note note (fiExpr [] expr))
194 fiExpr to_drop (_, AnnNote InlineCall expr)
195 = -- Wimp out for InlineCall; keep it close
196 -- the the call it annotates
197 mkCoLets' to_drop (Note InlineCall (fiExpr [] expr))
199 fiExpr to_drop (_, AnnNote note@(Coerce _ _) expr)
200 = -- Just float in past coercion
201 Note note (fiExpr to_drop expr)
204 For @Lets@, the possible ``drop points'' for the \tr{to_drop}
205 bindings are: (a)~in the body, (b1)~in the RHS of a NonRec binding,
206 or~(b2), in each of the RHSs of the pairs of a @Rec@.
208 Note that we do {\em weird things} with this let's binding. Consider:
217 Look at the inner \tr{let}. As \tr{w} is used in both the bind and
218 body of the inner let, we could panic and leave \tr{w}'s binding where
219 it is. But \tr{v} is floatable into the body of the inner let, and
220 {\em then} \tr{w} will also be only in the body of that inner let.
222 So: rather than drop \tr{w}'s binding here, we add it onto the list of
223 things to drop in the outer let's body, and let nature take its
227 fiExpr to_drop (_,AnnLet (AnnNonRec id rhs) body)
228 = fiExpr new_to_drop body
230 rhs_fvs = freeVarsOf rhs
231 body_fvs = freeVarsOf body
233 [shared_binds, rhs_binds, body_binds] = sepBindsByDropPoint [rhs_fvs, body_fvs] to_drop
235 new_to_drop = body_binds ++ -- the bindings used only in the body
236 [(NonRec id rhs', rhs_fvs')] ++ -- the new binding itself
237 shared_binds -- the bindings used both in rhs and body
239 -- Push rhs_binds into the right hand side of the binding
240 rhs' = fiExpr rhs_binds rhs
241 rhs_fvs' = rhs_fvs `unionVarSet` floatedBindsFVs rhs_binds
243 fiExpr to_drop (_,AnnLet (AnnRec bindings) body)
244 = fiExpr new_to_drop body
246 (binders, rhss) = unzip bindings
248 rhss_fvs = map freeVarsOf rhss
249 body_fvs = freeVarsOf body
251 (shared_binds:body_binds:rhss_binds) = sepBindsByDropPoint (body_fvs:rhss_fvs) to_drop
253 new_to_drop = -- the bindings used only in the body
255 -- the new binding itself
256 [(Rec (fi_bind rhss_binds bindings), rhs_fvs')] ++
257 -- the bindings used both in rhs and body or in more than one rhs
260 rhs_fvs' = unionVarSet (unionVarSets rhss_fvs)
261 (unionVarSets (map floatedBindsFVs rhss_binds))
263 -- Push rhs_binds into the right hand side of the binding
264 fi_bind :: [FloatingBinds] -- one per "drop pt" conjured w/ fvs_of_rhss
265 -> [(Id, CoreExprWithFVs)]
268 fi_bind to_drops pairs
269 = [ (binder, fiExpr to_drop rhs)
270 | ((binder, rhs), to_drop) <- zipEqual "fi_bind" pairs to_drops ]
273 For @Case@, the possible ``drop points'' for the \tr{to_drop}
274 bindings are: (a)~inside the scrutinee, (b)~inside one of the
275 alternatives/default [default FVs always {\em first}!].
278 fiExpr to_drop (_, AnnCase scrut case_bndr alts)
279 = mkCoLets' drop_here (Case (fiExpr scrut_drops scrut) case_bndr
280 (zipWith fi_alt alts_drops alts))
282 (drop_here : scrut_drops : alts_drops) = sepBindsByDropPoint (scrut_fvs : alts_fvs) to_drop
283 scrut_fvs = freeVarsOf scrut
284 alts_fvs = map alt_fvs alts
285 alt_fvs (con, args, rhs) = foldl delVarSet (freeVarsOf rhs) (case_bndr:args)
286 -- Delete case_bndr and args from free vars of rhs
287 -- to get free vars of alt
289 fi_alt to_drop (con, args, rhs) = (con, args, fiExpr to_drop rhs)
293 %************************************************************************
295 \subsection{@sepBindsByDropPoint@}
297 %************************************************************************
299 This is the crucial function. The idea is: We have a wad of bindings
300 that we'd like to distribute inside a collection of {\em drop points};
301 insides the alternatives of a \tr{case} would be one example of some
302 drop points; the RHS and body of a non-recursive \tr{let} binding
303 would be another (2-element) collection.
305 So: We're given a list of sets-of-free-variables, one per drop point,
306 and a list of floating-inwards bindings. If a binding can go into
307 only one drop point (without suddenly making something out-of-scope),
308 in it goes. If a binding is used inside {\em multiple} drop points,
309 then it has to go in a you-must-drop-it-above-all-these-drop-points
312 We have to maintain the order on these drop-point-related lists.
316 :: [FreeVarsSet] -- One set of FVs per drop point
317 -> FloatingBinds -- Candidate floaters
318 -> [FloatingBinds] -- FIRST one is bindings which must not be floated
319 -- inside any drop point; the rest correspond
320 -- one-to-one with the input list of FV sets
322 -- Every input floater is returned somewhere in the result;
323 -- none are dropped, not even ones which don't seem to be
324 -- free in *any* of the drop-point fvs. Why? Because, for example,
325 -- a binding (let x = E in B) might have a specialised version of
326 -- x (say x') stored inside x, but x' isn't free in E or B.
328 sepBindsByDropPoint drop_pts []
329 = [] : [[] | p <- drop_pts] -- cut to the chase scene; it happens
331 sepBindsByDropPoint drop_pts floaters
332 = go floaters (map (\fvs -> (fvs, [])) (emptyVarSet : drop_pts))
334 go :: FloatingBinds -> [(FreeVarsSet, FloatingBinds)] -> [FloatingBinds]
335 -- The *first* one in the argument list is the drop_here set
336 -- The FloatingBinds in the lists are in the reverse of
337 -- the normal FloatingBinds order; that is, they are the right way round!
339 go [] drop_boxes = map (reverse . snd) drop_boxes
341 go (bind_w_fvs@(bind, bind_fvs) : binds) drop_boxes
342 = go binds (insert drop_boxes (drop_here : used_in_flags))
343 -- insert puts the find in box whose True flag comes first
345 (used_here : used_in_flags) = [ any (`elemVarSet` fvs) (bindersOf bind)
346 | (fvs, drops) <- drop_boxes]
348 drop_here = used_here || not (exactlyOneTrue used_in_flags)
350 insert ((fvs,drops) : drop_boxes) (True : _)
351 = ((fvs `unionVarSet` bind_fvs, bind_w_fvs:drops) : drop_boxes)
352 insert (drop_box : drop_boxes) (False : others)
353 = drop_box : insert drop_boxes others
354 insert _ _ = panic "sepBindsByDropPoint" -- Should never happen
356 exactlyOneTrue :: [Bool] -> Bool
357 exactlyOneTrue flags = case [() | True <- flags] of
361 floatedBindsFVs :: FloatingBinds -> FreeVarsSet
362 floatedBindsFVs binds = unionVarSets (map snd binds)
364 mkCoLets' :: FloatingBinds -> CoreExpr -> CoreExpr
365 mkCoLets' to_drop e = foldl (flip (Let . fst)) e to_drop
366 -- Remember to_drop is in *reverse* dependency order