2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[WorkWrap]{Worker/wrapper-generating back-end of strictness analyser}
7 module WorkWrap ( wwTopBinds, mkWrapper ) where
9 #include "HsVersions.h"
12 import CoreUnfold ( Unfolding, certainlyWillInline )
13 import CmdLineOpts ( opt_UF_CreationThreshold , opt_D_verbose_core2core,
14 opt_D_dump_worker_wrapper
16 import CoreLint ( beginPass, endPass )
17 import CoreUtils ( exprType, exprEtaExpandArity )
18 import MkId ( mkWorkerId )
19 import Id ( Id, idType, idStrictness, idArity, isOneShotLambda,
20 setIdStrictness, idInlinePragma,
21 setIdWorkerInfo, idCprInfo, setInlinePragma )
22 import Type ( Type, isNewType, splitForAllTys, splitFunTys )
23 import IdInfo ( mkStrictnessInfo, noStrictnessInfo, StrictnessInfo(..),
24 CprInfo(..), exactArity, InlinePragInfo(..), isNeverInlinePrag,
27 import Demand ( Demand, wwLazy )
28 import UniqSupply ( UniqSupply, initUs_, returnUs, thenUs, mapUs, getUniqueUs, UniqSM )
33 We take Core bindings whose binders have:
37 \item Strictness attached (by the front-end of the strictness
40 \item Constructed Product Result information attached by the CPR
45 and we return some ``plain'' bindings which have been
46 worker/wrapper-ified, meaning:
50 \item Functions have been split into workers and wrappers where
51 appropriate. If a function has both strictness and CPR properties
52 then only one worker/wrapper doing both transformations is produced;
54 \item Binders' @IdInfos@ have been updated to reflect the existence of
55 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
56 info for exported values).
61 wwTopBinds :: UniqSupply
67 beginPass "Worker Wrapper binds";
69 -- Create worker/wrappers, and mark binders with their
70 -- "strictness info" [which encodes their worker/wrapper-ness]
71 let { binds' = workersAndWrappers us binds };
73 endPass "Worker Wrapper binds" (opt_D_dump_worker_wrapper ||
74 opt_D_verbose_core2core) binds'
80 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
82 workersAndWrappers us top_binds
84 mapUs wwBind top_binds `thenUs` \ top_binds' ->
85 returnUs (concat top_binds')
88 %************************************************************************
90 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
92 %************************************************************************
94 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
95 turn. Non-recursive case first, then recursive...
99 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
100 -- the caller will convert to Expr/Binding,
103 wwBind (NonRec binder rhs)
104 = wwExpr rhs `thenUs` \ new_rhs ->
105 tryWW True {- non-recursive -} binder new_rhs `thenUs` \ new_pairs ->
106 returnUs [NonRec b e | (b,e) <- new_pairs]
107 -- Generated bindings must be non-recursive
108 -- because the original binding was.
110 ------------------------------
113 = mapUs do_one pairs `thenUs` \ new_pairs ->
114 returnUs [Rec (concat new_pairs)]
116 do_one (binder, rhs) = wwExpr rhs `thenUs` \ new_rhs ->
117 tryWW False {- recursive -} binder new_rhs
120 @wwExpr@ basically just walks the tree, looking for appropriate
121 annotations that can be used. Remember it is @wwBind@ that does the
122 matching by looking for strict arguments of the correct type.
123 @wwExpr@ is a version that just returns the ``Plain'' Tree.
126 wwExpr :: CoreExpr -> UniqSM CoreExpr
128 wwExpr e@(Type _) = returnUs e
129 wwExpr e@(Var _) = returnUs e
130 wwExpr e@(Lit _) = returnUs e
132 wwExpr (Lam binder expr)
133 = wwExpr expr `thenUs` \ new_expr ->
134 returnUs (Lam binder new_expr)
137 = wwExpr f `thenUs` \ new_f ->
138 wwExpr a `thenUs` \ new_a ->
139 returnUs (App new_f new_a)
141 wwExpr (Note note expr)
142 = wwExpr expr `thenUs` \ new_expr ->
143 returnUs (Note note new_expr)
145 wwExpr (Let bind expr)
146 = wwBind bind `thenUs` \ intermediate_bind ->
147 wwExpr expr `thenUs` \ new_expr ->
148 returnUs (mkLets intermediate_bind new_expr)
150 wwExpr (Case expr binder alts)
151 = wwExpr expr `thenUs` \ new_expr ->
152 mapUs ww_alt alts `thenUs` \ new_alts ->
153 returnUs (Case new_expr binder new_alts)
155 ww_alt (con, binders, rhs)
156 = wwExpr rhs `thenUs` \ new_rhs ->
157 returnUs (con, binders, new_rhs)
160 %************************************************************************
162 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
164 %************************************************************************
166 @tryWW@ just accumulates arguments, converts strictness info from the
167 front-end into the proper form, then calls @mkWwBodies@ to do
170 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
171 already been w-w'd! (You can end up with several liked-named Ids
172 bouncing around at the same time---absolute mischief.) So the
173 criterion we use is: if an Id already has an unfolding (for whatever
174 reason), then we don't w-w it.
176 The only reason this is monadised is for the unique supply.
179 tryWW :: Bool -- True <=> a non-recursive binding
180 -> Id -- The fn binder
181 -> CoreExpr -- The bound rhs; its innards
183 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
184 -- if one, then no worker (only
185 -- the orig "wrapper" lives on);
186 -- if two, then a worker and a
188 tryWW non_rec fn_id rhs
189 | isNeverInlinePrag inline_prag || arity == 0
190 = -- Don't split things that will never be inlined
191 returnUs [ (fn_id, rhs) ]
193 | non_rec && not do_coerce_ww && certainlyWillInline fn_id
194 -- No point in worker/wrappering a function that is going to be
195 -- INLINEd wholesale anyway. If the strictness analyser is run
196 -- twice, this test also prevents wrappers (which are INLINEd)
197 -- from being re-done.
199 -- The do_coerce_ww test is so that
200 -- a function with a coerce should w/w to get rid
201 -- of the coerces, which can significantly improve its arity.
202 -- Example: f [] = return [] :: IO [Int]
203 -- f (x:xs) = return (x:xs)
204 -- If we aren't careful we end up with
205 -- f = \ x -> case x of {
206 -- x:xs -> __coerce (IO [Int]) (\ s -> (# s, x:xs #)
210 -- OUT OF DATE NOTE, kept for info:
211 -- It's out of date because now wrappers look very cheap
212 -- even when they are inlined.
213 -- In this case we add an INLINE pragma to the RHS. Why?
216 -- g = \yz -> ... -- And g is strict
217 -- Then f is small, so we don't w/w it. But g is big, and we do, so
218 -- g's wrapper will get inlined in f's RHS, which makes f look big now.
219 -- So f doesn't get inlined, but it is strict and we have failed to w/w it.
220 = returnUs [ (fn_id, rhs) ]
222 | not (do_strict_ww || do_cpr_ww || do_coerce_ww)
223 = returnUs [ (fn_id, rhs) ]
225 | otherwise -- Do w/w split
226 = mkWwBodies fun_ty arity wrap_dmds result_bot one_shots cpr_info `thenUs` \ (work_demands, wrap_fn, work_fn) ->
227 getUniqueUs `thenUs` \ work_uniq ->
229 work_rhs = work_fn rhs
230 proto_work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
231 `setInlinePragma` inline_prag
233 work_id | has_strictness = proto_work_id `setIdStrictness` mkStrictnessInfo (work_demands, result_bot)
234 | otherwise = proto_work_id
236 wrap_rhs = wrap_fn work_id
237 wrap_id = fn_id `setIdStrictness` wrapper_strictness
238 `setIdWorkerInfo` HasWorker work_id arity
239 `setInlinePragma` NoInlinePragInfo -- Put it on the worker instead
240 -- Add info to the wrapper:
241 -- (a) we want to set its arity
242 -- (b) we want to pin on its revised strictness info
243 -- (c) we pin on its worker id
245 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
246 -- Worker first, because wrapper mentions it
247 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
249 fun_ty = idType fn_id
250 arity = idArity fn_id -- The arity is set by the simplifier using exprEtaExpandArity
251 -- So it may be more than the number of top-level-visible lambdas
253 inline_prag = idInlinePragma fn_id
255 strictness_info = idStrictness fn_id
256 has_strictness = case strictness_info of
257 StrictnessInfo _ _ -> True
258 NoStrictnessInfo -> False
259 (arg_demands, result_bot) = case strictness_info of
260 StrictnessInfo d r -> (d, r)
261 NoStrictnessInfo -> ([], False)
263 wrap_dmds = setUnpackStrategy arg_demands
264 do_strict_ww = WARN( has_strictness && not result_bot && arity < length arg_demands && worthSplitting wrap_dmds result_bot,
265 text "Insufficient arity" <+> ppr fn_id <+> ppr arity <+> ppr arg_demands )
266 (result_bot || arity >= length arg_demands) -- Only if there's enough visible arity
267 && -- (else strictness info isn't valid)
269 worthSplitting wrap_dmds result_bot -- And it's useful
270 -- worthSplitting returns False for an empty list of demands,
271 -- and hence do_strict_ww is False if arity is zero
272 -- Also it's false if there is no strictness (arg_demands is [])
274 wrapper_strictness | has_strictness = mkStrictnessInfo (wrap_dmds, result_bot)
275 | otherwise = noStrictnessInfo
277 -------------------------------------------------------------
278 cpr_info = idCprInfo fn_id
279 do_cpr_ww = arity > 0 &&
284 -------------------------------------------------------------
285 do_coerce_ww = check_for_coerce arity fun_ty
286 -- We are willing to do a w/w even if the arity is zero.
292 -------------------------------------------------------------
293 one_shots = get_one_shots rhs
295 -- See if there's a Coerce before we run out of arity;
296 -- if so, it's worth trying a w/w split. Reason: we find
297 -- functions like f = coerce (\s -> e)
298 -- and g = \x -> coerce (\s -> e)
299 -- and they may have no useful strictness or cpr info, but if we
300 -- do the w/w thing we get rid of the coerces.
302 check_for_coerce arity ty
303 = length arg_tys <= arity && isNewType res_ty
304 -- Don't look further than arity args,
305 -- but if there are arity or fewer, see if there's
306 -- a newtype in the corner
308 (_, tau) = splitForAllTys ty
309 (arg_tys, res_ty) = splitFunTys tau
311 -- If the original function has one-shot arguments, it is important to
312 -- make the wrapper and worker have corresponding one-shot arguments too.
313 -- Otherwise we spuriously float stuff out of case-expression join points,
314 -- which is very annoying.
315 get_one_shots (Lam b e)
316 | isId b = isOneShotLambda b : get_one_shots e
317 | otherwise = get_one_shots e
318 get_one_shots (Note _ e) = get_one_shots e
319 get_one_shots other = noOneShotInfo
324 %************************************************************************
326 \subsection{The worker wrapper core}
328 %************************************************************************
330 @mkWrapper@ is called when importing a function. We have the type of
331 the function and the name of its worker, and we want to make its body (the wrapper).
334 mkWrapper :: Type -- Wrapper type
336 -> [Demand] -- Wrapper strictness info
337 -> Bool -- Function returns bottom
338 -> CprInfo -- Wrapper cpr info
339 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
341 mkWrapper fun_ty arity demands res_bot cpr_info
342 = mkWwBodies fun_ty arity demands res_bot noOneShotInfo cpr_info `thenUs` \ (_, wrap_fn, _) ->
345 noOneShotInfo = repeat False