2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[FloatOut]{Float bindings outwards (towards the top level)}
6 ``Long-distance'' floating of bindings towards the top level.
9 module FloatOut ( floatOutwards ) where
14 import DynFlags ( DynFlags, DynFlag(..), FloatOutSwitches(..) )
15 import ErrUtils ( dumpIfSet_dyn )
16 import CostCentre ( dupifyCC, CostCentre )
17 import Id ( Id, idType )
18 import Type ( isUnLiftedType )
19 import SetLevels ( Level(..), LevelledExpr, LevelledBind,
20 setLevels, ltMajLvl, ltLvl, isTopLvl )
21 import UniqSupply ( UniqSupply )
33 To float out sub-expressions that can thereby get outside
34 a non-one-shot value lambda, and hence may be shared.
37 To achieve this we may need to do two thing:
39 a) Let-bind the sub-expression:
41 f (g x) ==> let lvl = f (g x) in lvl
43 Now we can float the binding for 'lvl'.
45 b) More than that, we may need to abstract wrt a type variable
47 \x -> ... /\a -> let v = ...a... in ....
49 Here the binding for v mentions 'a' but not 'x'. So we
50 abstract wrt 'a', to give this binding for 'v':
55 Now the binding for vp can float out unimpeded.
56 I can't remember why this case seemed important enough to
57 deal with, but I certainly found cases where important floats
58 didn't happen if we did not abstract wrt tyvars.
60 With this in mind we can also achieve another goal: lambda lifting.
61 We can make an arbitrary (function) binding float to top level by
62 abstracting wrt *all* local variables, not just type variables, leaving
63 a binding that can be floated right to top level. Whether or not this
64 happens is controlled by a flag.
70 At the moment we never float a binding out to between two adjacent
74 \x y -> let t = x+x in ...
76 \x -> let t = x+x in \y -> ...
78 Reason: this is less efficient in the case where the original lambda
79 is never partially applied.
81 But there's a case I've seen where this might not be true. Consider:
87 elem' x (y:ys) = x==y || elem' x ys
89 It turns out that this generates a subexpression of the form
91 \deq x ys -> let eq = eqFromEqDict deq in ...
93 vwhich might usefully be separated to
95 \deq -> let eq = eqFromEqDict deq in \xy -> ...
97 Well, maybe. We don't do this at the moment.
100 type FloatBind = (Level, CoreBind) -- INVARIANT: a FloatBind is always lifted
101 type FloatBinds = [FloatBind]
104 %************************************************************************
106 \subsection[floatOutwards]{@floatOutwards@: let-floating interface function}
108 %************************************************************************
111 floatOutwards :: FloatOutSwitches
114 -> [CoreBind] -> IO [CoreBind]
116 floatOutwards float_sws dflags us pgm
118 let { annotated_w_levels = setLevels float_sws pgm us ;
119 (fss, binds_s') = unzip (map floatTopBind annotated_w_levels)
122 dumpIfSet_dyn dflags Opt_D_verbose_core2core "Levels added:"
123 (vcat (map ppr annotated_w_levels));
125 let { (tlets, ntlets, lams) = get_stats (sum_stats fss) };
127 dumpIfSet_dyn dflags Opt_D_dump_simpl_stats "FloatOut stats:"
128 (hcat [ int tlets, ptext (sLit " Lets floated to top level; "),
129 int ntlets, ptext (sLit " Lets floated elsewhere; from "),
130 int lams, ptext (sLit " Lambda groups")]);
132 return (concat binds_s')
135 floatTopBind :: LevelledBind -> (FloatStats, [CoreBind])
137 = case (floatBind bind) of { (fs, floats) ->
138 (fs, floatsToBinds floats)
142 %************************************************************************
144 \subsection[FloatOut-Bind]{Floating in a binding (the business end)}
146 %************************************************************************
150 floatBind :: LevelledBind -> (FloatStats, FloatBinds)
152 floatBind (NonRec (TB name level) rhs)
153 = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
154 (fs, rhs_floats ++ [(level, NonRec name rhs')]) }
156 floatBind bind@(Rec pairs)
157 = case (unzip3 (map do_pair pairs)) of { (fss, rhss_floats, new_pairs) ->
158 let rhs_floats = concat rhss_floats in
160 if not (isTopLvl bind_dest_lvl) then
161 -- Find which bindings float out at least one lambda beyond this one
162 -- These ones can't mention the binders, because they couldn't
163 -- be escaping a major level if so.
164 -- The ones that are not going further can join the letrec;
165 -- they may not be mutually recursive but the occurrence analyser will
167 case (partitionByMajorLevel bind_dest_lvl rhs_floats) of { (floats', heres) ->
168 (sum_stats fss, floats' ++ [(bind_dest_lvl, Rec (floatsToBindPairs heres ++ new_pairs))]) }
170 -- In a recursive binding, *destined for* the top level
171 -- (only), the rhs floats may contain references to the
172 -- bound things. For example
173 -- f = ...(let v = ...f... in b) ...
174 -- might get floated to
177 -- and hence we must (pessimistically) make all the floats recursive
178 -- with the top binding. Later dependency analysis will unravel it.
180 -- This can only happen for bindings destined for the top level,
181 -- because only then will partitionByMajorLevel allow through a binding
182 -- that only differs in its minor level
183 (sum_stats fss, [(bind_dest_lvl, Rec (new_pairs ++ floatsToBindPairs rhs_floats))])
186 bind_dest_lvl = getBindLevel bind
188 do_pair (TB name level, rhs)
189 = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
190 (fs, rhs_floats, (name, rhs'))
194 %************************************************************************
196 \subsection[FloatOut-Expr]{Floating in expressions}
198 %************************************************************************
201 floatExpr, floatRhs, floatCaseAlt
204 -> (FloatStats, FloatBinds, CoreExpr)
206 floatCaseAlt lvl arg -- Used rec rhss, and case-alternative rhss
207 = case (floatExpr lvl arg) of { (fsa, floats, arg') ->
208 case (partitionByMajorLevel lvl floats) of { (floats', heres) ->
209 -- Dump bindings that aren't going to escape from a lambda;
210 -- in particular, we must dump the ones that are bound by
211 -- the rec or case alternative
212 (fsa, floats', install heres arg') }}
214 floatRhs lvl arg -- Used for nested non-rec rhss, and fn args
215 -- See Note [Floating out of RHS]
216 = case (floatExpr lvl arg) of { (fsa, floats, arg') ->
217 if exprIsCheap arg' then
220 case (partitionByMajorLevel lvl floats) of { (floats', heres) ->
221 (fsa, floats', install heres arg') }}
223 -- Note [Floating out of RHSs]
224 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~
225 -- Dump bindings that aren't going to escape from a lambda
226 -- This isn't a scoping issue (the binder isn't in scope in the RHS
227 -- of a non-rec binding)
228 -- Rather, it is to avoid floating the x binding out of
229 -- f (let x = e in b)
230 -- unnecessarily. But we first test for values or trival rhss,
231 -- because (in particular) we don't want to insert new bindings between
232 -- the "=" and the "\". E.g.
233 -- f = \x -> let <bind> in <body>
235 -- f = let <bind> in \x -> <body>
236 -- (a) The simplifier will immediately float it further out, so we may
237 -- as well do so right now; in general, keeping rhss as manifest
239 -- (b) If a float-in pass follows immediately, it might add yet more
240 -- bindings just after the '='. And some of them might (correctly)
241 -- be strict even though the 'let f' is lazy, because f, being a value,
242 -- gets its demand-info zapped by the simplifier.
244 -- We use exprIsCheap because that is also what's used by the simplifier
245 -- to decide whether to float a let out of a let
247 floatExpr _ (Var v) = (zeroStats, [], Var v)
248 floatExpr _ (Type ty) = (zeroStats, [], Type ty)
249 floatExpr _ (Lit lit) = (zeroStats, [], Lit lit)
251 floatExpr lvl (App e a)
252 = case (floatExpr lvl e) of { (fse, floats_e, e') ->
253 case (floatRhs lvl a) of { (fsa, floats_a, a') ->
254 (fse `add_stats` fsa, floats_e ++ floats_a, App e' a') }}
256 floatExpr _ lam@(Lam _ _)
258 (bndrs_w_lvls, body) = collectBinders lam
259 bndrs = [b | TB b _ <- bndrs_w_lvls]
260 lvls = [l | TB _ l <- bndrs_w_lvls]
262 -- For the all-tyvar case we are prepared to pull
263 -- the lets out, to implement the float-out-of-big-lambda
264 -- transform; but otherwise we only float bindings that are
265 -- going to escape a value lambda.
266 -- In particular, for one-shot lambdas we don't float things
267 -- out; we get no saving by so doing.
268 partition_fn | all isTyVar bndrs = partitionByLevel
269 | otherwise = partitionByMajorLevel
271 case (floatExpr (last lvls) body) of { (fs, floats, body') ->
273 -- Dump any bindings which absolutely cannot go any further
274 case (partition_fn (head lvls) floats) of { (floats', heres) ->
276 (add_to_stats fs floats', floats', mkLams bndrs (install heres body'))
279 floatExpr lvl (Note note@(SCC cc) expr)
280 = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
282 -- Annotate bindings floated outwards past an scc expression
283 -- with the cc. We mark that cc as "duplicated", though.
285 annotated_defns = annotate (dupifyCC cc) floating_defns
287 (fs, annotated_defns, Note note expr') }
289 annotate :: CostCentre -> FloatBinds -> FloatBinds
291 annotate dupd_cc defn_groups
292 = [ (level, ann_bind floater) | (level, floater) <- defn_groups ]
294 ann_bind (NonRec binder rhs)
295 = NonRec binder (mkSCC dupd_cc rhs)
298 = Rec [(binder, mkSCC dupd_cc rhs) | (binder, rhs) <- pairs]
300 floatExpr lvl (Note note expr) -- Other than SCCs
301 = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
302 (fs, floating_defns, Note note expr') }
304 floatExpr lvl (Cast expr co)
305 = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
306 (fs, floating_defns, Cast expr' co) }
308 floatExpr lvl (Let (NonRec (TB bndr bndr_lvl) rhs) body)
309 | isUnLiftedType (idType bndr) -- Treat unlifted lets just like a case
310 -- I.e. floatExpr for rhs, floatCaseAlt for body
311 = case floatExpr lvl rhs of { (_, rhs_floats, rhs') ->
312 case floatCaseAlt bndr_lvl body of { (fs, body_floats, body') ->
313 (fs, rhs_floats ++ body_floats, Let (NonRec bndr rhs') body') }}
315 floatExpr lvl (Let bind body)
316 = case (floatBind bind) of { (fsb, bind_floats) ->
317 case (floatExpr lvl body) of { (fse, body_floats, body') ->
319 bind_floats ++ body_floats,
322 floatExpr lvl (Case scrut (TB case_bndr case_lvl) ty alts)
323 = case floatExpr lvl scrut of { (fse, fde, scrut') ->
324 case floatList float_alt alts of { (fsa, fda, alts') ->
325 (add_stats fse fsa, fda ++ fde, Case scrut' case_bndr ty alts')
328 -- Use floatCaseAlt for the alternatives, so that we
329 -- don't gratuitiously float bindings out of the RHSs
330 float_alt (con, bs, rhs)
331 = case (floatCaseAlt case_lvl rhs) of { (fs, rhs_floats, rhs') ->
332 (fs, rhs_floats, (con, [b | TB b _ <- bs], rhs')) }
335 floatList :: (a -> (FloatStats, FloatBinds, b)) -> [a] -> (FloatStats, FloatBinds, [b])
336 floatList _ [] = (zeroStats, [], [])
337 floatList f (a:as) = case f a of { (fs_a, binds_a, b) ->
338 case floatList f as of { (fs_as, binds_as, bs) ->
339 (fs_a `add_stats` fs_as, binds_a ++ binds_as, b:bs) }}
342 %************************************************************************
344 \subsection{Utility bits for floating stats}
346 %************************************************************************
348 I didn't implement this with unboxed numbers. I don't want to be too
349 strict in this stuff, as it is rarely turned on. (WDP 95/09)
353 = FlS Int -- Number of top-floats * lambda groups they've been past
354 Int -- Number of non-top-floats * lambda groups they've been past
355 Int -- Number of lambda (groups) seen
357 get_stats :: FloatStats -> (Int, Int, Int)
358 get_stats (FlS a b c) = (a, b, c)
360 zeroStats :: FloatStats
361 zeroStats = FlS 0 0 0
363 sum_stats :: [FloatStats] -> FloatStats
364 sum_stats xs = foldr add_stats zeroStats xs
366 add_stats :: FloatStats -> FloatStats -> FloatStats
367 add_stats (FlS a1 b1 c1) (FlS a2 b2 c2)
368 = FlS (a1 + a2) (b1 + b2) (c1 + c2)
370 add_to_stats :: FloatStats -> [(Level, Bind CoreBndr)] -> FloatStats
371 add_to_stats (FlS a b c) floats
372 = FlS (a + length top_floats) (b + length other_floats) (c + 1)
374 (top_floats, other_floats) = partition to_very_top floats
376 to_very_top (my_lvl, _) = isTopLvl my_lvl
380 %************************************************************************
382 \subsection{Utility bits for floating}
384 %************************************************************************
387 getBindLevel :: Bind (TaggedBndr Level) -> Level
388 getBindLevel (NonRec (TB _ lvl) _) = lvl
389 getBindLevel (Rec (((TB _ lvl), _) : _)) = lvl
390 getBindLevel (Rec []) = panic "getBindLevel Rec []"
394 partitionByMajorLevel, partitionByLevel
395 :: Level -- Partitioning level
397 -> FloatBinds -- Defns to be divided into 2 piles...
399 -> (FloatBinds, -- Defns with level strictly < partition level,
400 FloatBinds) -- The rest
403 partitionByMajorLevel ctxt_lvl defns
404 = partition float_further defns
406 -- Float it if we escape a value lambda, or if we get to the top level
407 float_further (my_lvl, _) = my_lvl `ltMajLvl` ctxt_lvl || isTopLvl my_lvl
408 -- The isTopLvl part says that if we can get to the top level, say "yes" anyway
414 -- which is as it should be
416 partitionByLevel ctxt_lvl defns
417 = partition float_further defns
419 float_further (my_lvl, _) = my_lvl `ltLvl` ctxt_lvl
423 floatsToBinds :: FloatBinds -> [CoreBind]
424 floatsToBinds floats = map snd floats
426 floatsToBindPairs :: FloatBinds -> [(Id,CoreExpr)]
428 floatsToBindPairs floats = concat (map mk_pairs floats)
430 mk_pairs (_, Rec pairs) = pairs
431 mk_pairs (_, NonRec binder rhs) = [(binder,rhs)]
433 install :: FloatBinds -> CoreExpr -> CoreExpr
435 install defn_groups expr
436 = foldr install_group expr defn_groups
438 install_group (_, defns) body = Let defns body