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.
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module FloatOut ( floatOutwards ) where
18 #include "HsVersions.h"
23 import DynFlags ( DynFlags, DynFlag(..), FloatOutSwitches(..) )
24 import ErrUtils ( dumpIfSet_dyn )
25 import CostCentre ( dupifyCC, CostCentre )
26 import Id ( Id, idType )
27 import Type ( isUnLiftedType )
28 import CoreLint ( showPass, endPass )
29 import SetLevels ( Level(..), LevelledExpr, LevelledBind,
30 setLevels, ltMajLvl, ltLvl, isTopLvl )
31 import UniqSupply ( UniqSupply )
32 import List ( partition )
43 To float out sub-expressions that can thereby get outside
44 a non-one-shot value lambda, and hence may be shared.
47 To achieve this we may need to do two thing:
49 a) Let-bind the sub-expression:
51 f (g x) ==> let lvl = f (g x) in lvl
53 Now we can float the binding for 'lvl'.
55 b) More than that, we may need to abstract wrt a type variable
57 \x -> ... /\a -> let v = ...a... in ....
59 Here the binding for v mentions 'a' but not 'x'. So we
60 abstract wrt 'a', to give this binding for 'v':
65 Now the binding for vp can float out unimpeded.
66 I can't remember why this case seemed important enough to
67 deal with, but I certainly found cases where important floats
68 didn't happen if we did not abstract wrt tyvars.
70 With this in mind we can also achieve another goal: lambda lifting.
71 We can make an arbitrary (function) binding float to top level by
72 abstracting wrt *all* local variables, not just type variables, leaving
73 a binding that can be floated right to top level. Whether or not this
74 happens is controlled by a flag.
80 At the moment we never float a binding out to between two adjacent
84 \x y -> let t = x+x in ...
86 \x -> let t = x+x in \y -> ...
88 Reason: this is less efficient in the case where the original lambda
89 is never partially applied.
91 But there's a case I've seen where this might not be true. Consider:
97 elem' x (y:ys) = x==y || elem' x ys
99 It turns out that this generates a subexpression of the form
101 \deq x ys -> let eq = eqFromEqDict deq in ...
103 vwhich might usefully be separated to
105 \deq -> let eq = eqFromEqDict deq in \xy -> ...
107 Well, maybe. We don't do this at the moment.
110 type FloatBind = (Level, CoreBind) -- INVARIANT: a FloatBind is always lifted
111 type FloatBinds = [FloatBind]
114 %************************************************************************
116 \subsection[floatOutwards]{@floatOutwards@: let-floating interface function}
118 %************************************************************************
121 floatOutwards :: FloatOutSwitches
124 -> [CoreBind] -> IO [CoreBind]
126 floatOutwards float_sws dflags us pgm
128 showPass dflags float_msg ;
130 let { annotated_w_levels = setLevels float_sws pgm us ;
131 (fss, binds_s') = unzip (map floatTopBind annotated_w_levels)
134 dumpIfSet_dyn dflags Opt_D_verbose_core2core "Levels added:"
135 (vcat (map ppr annotated_w_levels));
137 let { (tlets, ntlets, lams) = get_stats (sum_stats fss) };
139 dumpIfSet_dyn dflags Opt_D_dump_simpl_stats "FloatOut stats:"
140 (hcat [ int tlets, ptext SLIT(" Lets floated to top level; "),
141 int ntlets, ptext SLIT(" Lets floated elsewhere; from "),
142 int lams, ptext SLIT(" Lambda groups")]);
144 endPass dflags float_msg Opt_D_verbose_core2core (concat binds_s')
145 {- no specific flag for dumping float-out -}
148 float_msg = showSDoc (text "Float out" <+> parens (sws float_sws))
149 sws (FloatOutSw lam const) = pp_not lam <+> text "lambdas" <> comma <+>
150 pp_not const <+> text "constants"
152 pp_not False = text "not"
155 = case (floatBind bind) of { (fs, floats) ->
156 (fs, floatsToBinds floats)
160 %************************************************************************
162 \subsection[FloatOut-Bind]{Floating in a binding (the business end)}
164 %************************************************************************
168 floatBind :: LevelledBind -> (FloatStats, FloatBinds)
170 floatBind (NonRec (TB name level) rhs)
171 = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
172 (fs, rhs_floats ++ [(level, NonRec name rhs')]) }
174 floatBind bind@(Rec pairs)
175 = case (unzip3 (map do_pair pairs)) of { (fss, rhss_floats, new_pairs) ->
176 let rhs_floats = concat rhss_floats in
178 if not (isTopLvl bind_dest_lvl) then
179 -- Find which bindings float out at least one lambda beyond this one
180 -- These ones can't mention the binders, because they couldn't
181 -- be escaping a major level if so.
182 -- The ones that are not going further can join the letrec;
183 -- they may not be mutually recursive but the occurrence analyser will
185 case (partitionByMajorLevel bind_dest_lvl rhs_floats) of { (floats', heres) ->
186 (sum_stats fss, floats' ++ [(bind_dest_lvl, Rec (floatsToBindPairs heres ++ new_pairs))]) }
188 -- In a recursive binding, *destined for* the top level
189 -- (only), the rhs floats may contain references to the
190 -- bound things. For example
191 -- f = ...(let v = ...f... in b) ...
192 -- might get floated to
195 -- and hence we must (pessimistically) make all the floats recursive
196 -- with the top binding. Later dependency analysis will unravel it.
198 -- This can only happen for bindings destined for the top level,
199 -- because only then will partitionByMajorLevel allow through a binding
200 -- that only differs in its minor level
201 (sum_stats fss, [(bind_dest_lvl, Rec (new_pairs ++ floatsToBindPairs rhs_floats))])
204 bind_dest_lvl = getBindLevel bind
206 do_pair (TB name level, rhs)
207 = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
208 (fs, rhs_floats, (name, rhs'))
212 %************************************************************************
214 \subsection[FloatOut-Expr]{Floating in expressions}
216 %************************************************************************
219 floatExpr, floatRhs, floatCaseAlt
222 -> (FloatStats, FloatBinds, CoreExpr)
224 floatCaseAlt lvl arg -- Used rec rhss, and case-alternative rhss
225 = case (floatExpr lvl arg) of { (fsa, floats, arg') ->
226 case (partitionByMajorLevel lvl floats) of { (floats', heres) ->
227 -- Dump bindings that aren't going to escape from a lambda;
228 -- in particular, we must dump the ones that are bound by
229 -- the rec or case alternative
230 (fsa, floats', install heres arg') }}
232 floatRhs lvl arg -- Used for nested non-rec rhss, and fn args
233 -- See Note [Floating out of RHS]
234 = case (floatExpr lvl arg) of { (fsa, floats, arg') ->
235 if exprIsCheap arg' then
238 case (partitionByMajorLevel lvl floats) of { (floats', heres) ->
239 (fsa, floats', install heres arg') }}
241 -- Note [Floating out of RHSs]
242 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~
243 -- Dump bindings that aren't going to escape from a lambda
244 -- This isn't a scoping issue (the binder isn't in scope in the RHS
245 -- of a non-rec binding)
246 -- Rather, it is to avoid floating the x binding out of
247 -- f (let x = e in b)
248 -- unnecessarily. But we first test for values or trival rhss,
249 -- because (in particular) we don't want to insert new bindings between
250 -- the "=" and the "\". E.g.
251 -- f = \x -> let <bind> in <body>
253 -- f = let <bind> in \x -> <body>
254 -- (a) The simplifier will immediately float it further out, so we may
255 -- as well do so right now; in general, keeping rhss as manifest
257 -- (b) If a float-in pass follows immediately, it might add yet more
258 -- bindings just after the '='. And some of them might (correctly)
259 -- be strict even though the 'let f' is lazy, because f, being a value,
260 -- gets its demand-info zapped by the simplifier.
262 -- We use exprIsCheap because that is also what's used by the simplifier
263 -- to decide whether to float a let out of a let
265 floatExpr _ (Var v) = (zeroStats, [], Var v)
266 floatExpr _ (Type ty) = (zeroStats, [], Type ty)
267 floatExpr _ (Lit lit) = (zeroStats, [], Lit lit)
269 floatExpr lvl (App e a)
270 = case (floatExpr lvl e) of { (fse, floats_e, e') ->
271 case (floatRhs lvl a) of { (fsa, floats_a, a') ->
272 (fse `add_stats` fsa, floats_e ++ floats_a, App e' a') }}
274 floatExpr lvl lam@(Lam _ _)
276 (bndrs_w_lvls, body) = collectBinders lam
277 bndrs = [b | TB b _ <- bndrs_w_lvls]
278 lvls = [l | TB b l <- bndrs_w_lvls]
280 -- For the all-tyvar case we are prepared to pull
281 -- the lets out, to implement the float-out-of-big-lambda
282 -- transform; but otherwise we only float bindings that are
283 -- going to escape a value lambda.
284 -- In particular, for one-shot lambdas we don't float things
285 -- out; we get no saving by so doing.
286 partition_fn | all isTyVar bndrs = partitionByLevel
287 | otherwise = partitionByMajorLevel
289 case (floatExpr (last lvls) body) of { (fs, floats, body') ->
291 -- Dump any bindings which absolutely cannot go any further
292 case (partition_fn (head lvls) floats) of { (floats', heres) ->
294 (add_to_stats fs floats', floats', mkLams bndrs (install heres body'))
297 floatExpr lvl (Note note@(SCC cc) expr)
298 = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
300 -- Annotate bindings floated outwards past an scc expression
301 -- with the cc. We mark that cc as "duplicated", though.
303 annotated_defns = annotate (dupifyCC cc) floating_defns
305 (fs, annotated_defns, Note note expr') }
307 annotate :: CostCentre -> FloatBinds -> FloatBinds
309 annotate dupd_cc defn_groups
310 = [ (level, ann_bind floater) | (level, floater) <- defn_groups ]
312 ann_bind (NonRec binder rhs)
313 = NonRec binder (mkSCC dupd_cc rhs)
316 = Rec [(binder, mkSCC dupd_cc rhs) | (binder, rhs) <- pairs]
318 floatExpr lvl (Note InlineMe expr) -- Other than SCCs
319 = case floatExpr InlineCtxt expr of { (fs, floating_defns, expr') ->
320 -- There can be some floating_defns, arising from
321 -- ordinary lets that were there all the time. It seems
322 -- more efficient to test once here than to avoid putting
323 -- them into floating_defns (which would mean testing for
324 -- inlineCtxt at every let)
325 (fs, [], Note InlineMe (install floating_defns expr')) }
326 -- See Note [FloatOut inside INLINE] in SetLevels
327 -- I'm guessing that floating_dens should be empty
329 floatExpr lvl (Note note expr) -- Other than SCCs
330 = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
331 (fs, floating_defns, Note note expr') }
333 floatExpr lvl (Cast expr co)
334 = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
335 (fs, floating_defns, Cast expr' co) }
337 floatExpr lvl (Let (NonRec (TB bndr bndr_lvl) rhs) body)
338 | isUnLiftedType (idType bndr) -- Treat unlifted lets just like a case
339 -- I.e. floatExpr for rhs, floatCaseAlt for body
340 = case floatExpr lvl rhs of { (fs, rhs_floats, rhs') ->
341 case floatCaseAlt bndr_lvl body of { (fs, body_floats, body') ->
342 (fs, rhs_floats ++ body_floats, Let (NonRec bndr rhs') body') }}
344 floatExpr lvl (Let bind body)
345 = case (floatBind bind) of { (fsb, bind_floats) ->
346 case (floatExpr lvl body) of { (fse, body_floats, body') ->
348 bind_floats ++ body_floats,
351 floatExpr lvl (Case scrut (TB case_bndr case_lvl) ty alts)
352 = case floatExpr lvl scrut of { (fse, fde, scrut') ->
353 case floatList float_alt alts of { (fsa, fda, alts') ->
354 (add_stats fse fsa, fda ++ fde, Case scrut' case_bndr ty alts')
357 -- Use floatCaseAlt for the alternatives, so that we
358 -- don't gratuitiously float bindings out of the RHSs
359 float_alt (con, bs, rhs)
360 = case (floatCaseAlt case_lvl rhs) of { (fs, rhs_floats, rhs') ->
361 (fs, rhs_floats, (con, [b | TB b _ <- bs], rhs')) }
364 floatList :: (a -> (FloatStats, FloatBinds, b)) -> [a] -> (FloatStats, FloatBinds, [b])
365 floatList f [] = (zeroStats, [], [])
366 floatList f (a:as) = case f a of { (fs_a, binds_a, b) ->
367 case floatList f as of { (fs_as, binds_as, bs) ->
368 (fs_a `add_stats` fs_as, binds_a ++ binds_as, b:bs) }}
371 %************************************************************************
373 \subsection{Utility bits for floating stats}
375 %************************************************************************
377 I didn't implement this with unboxed numbers. I don't want to be too
378 strict in this stuff, as it is rarely turned on. (WDP 95/09)
382 = FlS Int -- Number of top-floats * lambda groups they've been past
383 Int -- Number of non-top-floats * lambda groups they've been past
384 Int -- Number of lambda (groups) seen
386 get_stats (FlS a b c) = (a, b, c)
388 zeroStats = FlS 0 0 0
390 sum_stats xs = foldr add_stats zeroStats xs
392 add_stats (FlS a1 b1 c1) (FlS a2 b2 c2)
393 = FlS (a1 + a2) (b1 + b2) (c1 + c2)
395 add_to_stats (FlS a b c) floats
396 = FlS (a + length top_floats) (b + length other_floats) (c + 1)
398 (top_floats, other_floats) = partition to_very_top floats
400 to_very_top (my_lvl, _) = isTopLvl my_lvl
404 %************************************************************************
406 \subsection{Utility bits for floating}
408 %************************************************************************
411 getBindLevel (NonRec (TB _ lvl) _) = lvl
412 getBindLevel (Rec (((TB _ lvl), _) : _)) = lvl
416 partitionByMajorLevel, partitionByLevel
417 :: Level -- Partitioning level
419 -> FloatBinds -- Defns to be divided into 2 piles...
421 -> (FloatBinds, -- Defns with level strictly < partition level,
422 FloatBinds) -- The rest
425 partitionByMajorLevel ctxt_lvl defns
426 = partition float_further defns
428 -- Float it if we escape a value lambda, or if we get to the top level
429 float_further (my_lvl, bind) = my_lvl `ltMajLvl` ctxt_lvl || isTopLvl my_lvl
430 -- The isTopLvl part says that if we can get to the top level, say "yes" anyway
436 -- which is as it should be
438 partitionByLevel ctxt_lvl defns
439 = partition float_further defns
441 float_further (my_lvl, _) = my_lvl `ltLvl` ctxt_lvl
445 floatsToBinds :: FloatBinds -> [CoreBind]
446 floatsToBinds floats = map snd floats
448 floatsToBindPairs :: FloatBinds -> [(Id,CoreExpr)]
450 floatsToBindPairs floats = concat (map mk_pairs floats)
452 mk_pairs (_, Rec pairs) = pairs
453 mk_pairs (_, NonRec binder rhs) = [(binder,rhs)]
455 install :: FloatBinds -> CoreExpr -> CoreExpr
457 install defn_groups expr
458 = foldr install_group expr defn_groups
460 install_group (_, defns) body = Let defns body