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 ( certainlyWillInline )
13 import CoreLint ( showPass, endPass )
14 import CoreUtils ( exprType )
15 import MkId ( mkWorkerId )
16 import Id ( Id, idType, idStrictness, idArity, isOneShotLambda,
17 setIdStrictness, idInlinePragma,
18 setIdWorkerInfo, idCprInfo, setInlinePragma )
19 import Type ( Type, isNewType, splitForAllTys, splitFunTys )
20 import IdInfo ( mkStrictnessInfo, noStrictnessInfo, StrictnessInfo(..),
21 CprInfo(..), InlinePragInfo(..), isNeverInlinePrag,
24 import Demand ( Demand )
25 import UniqSupply ( UniqSupply, initUs_, returnUs, thenUs, mapUs, getUniqueUs, UniqSM )
31 We take Core bindings whose binders have:
35 \item Strictness attached (by the front-end of the strictness
38 \item Constructed Product Result information attached by the CPR
43 and we return some ``plain'' bindings which have been
44 worker/wrapper-ified, meaning:
48 \item Functions have been split into workers and wrappers where
49 appropriate. If a function has both strictness and CPR properties
50 then only one worker/wrapper doing both transformations is produced;
52 \item Binders' @IdInfos@ have been updated to reflect the existence of
53 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
54 info for exported values).
59 wwTopBinds :: DynFlags
64 wwTopBinds dflags us binds
66 showPass dflags "Worker Wrapper binds";
68 -- Create worker/wrappers, and mark binders with their
69 -- "strictness info" [which encodes their worker/wrapper-ness]
70 let { binds' = workersAndWrappers us binds };
72 endPass dflags "Worker Wrapper binds"
73 Opt_D_dump_worker_wrapper binds'
79 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
81 workersAndWrappers us top_binds
83 mapUs wwBind top_binds `thenUs` \ top_binds' ->
84 returnUs (concat top_binds')
87 %************************************************************************
89 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
91 %************************************************************************
93 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
94 turn. Non-recursive case first, then recursive...
98 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
99 -- the caller will convert to Expr/Binding,
102 wwBind (NonRec binder rhs)
103 = wwExpr rhs `thenUs` \ new_rhs ->
104 tryWW True {- non-recursive -} binder new_rhs `thenUs` \ new_pairs ->
105 returnUs [NonRec b e | (b,e) <- new_pairs]
106 -- Generated bindings must be non-recursive
107 -- because the original binding was.
109 ------------------------------
112 = mapUs do_one pairs `thenUs` \ new_pairs ->
113 returnUs [Rec (concat new_pairs)]
115 do_one (binder, rhs) = wwExpr rhs `thenUs` \ new_rhs ->
116 tryWW False {- recursive -} binder new_rhs
119 @wwExpr@ basically just walks the tree, looking for appropriate
120 annotations that can be used. Remember it is @wwBind@ that does the
121 matching by looking for strict arguments of the correct type.
122 @wwExpr@ is a version that just returns the ``Plain'' Tree.
125 wwExpr :: CoreExpr -> UniqSM CoreExpr
127 wwExpr e@(Type _) = returnUs e
128 wwExpr e@(Var _) = returnUs e
129 wwExpr e@(Lit _) = returnUs e
131 wwExpr (Lam binder expr)
132 = wwExpr expr `thenUs` \ new_expr ->
133 returnUs (Lam binder new_expr)
136 = wwExpr f `thenUs` \ new_f ->
137 wwExpr a `thenUs` \ new_a ->
138 returnUs (App new_f new_a)
140 wwExpr (Note note expr)
141 = wwExpr expr `thenUs` \ new_expr ->
142 returnUs (Note note new_expr)
144 wwExpr (Let bind expr)
145 = wwBind bind `thenUs` \ intermediate_bind ->
146 wwExpr expr `thenUs` \ new_expr ->
147 returnUs (mkLets intermediate_bind new_expr)
149 wwExpr (Case expr binder alts)
150 = wwExpr expr `thenUs` \ new_expr ->
151 mapUs ww_alt alts `thenUs` \ new_alts ->
152 returnUs (Case new_expr binder new_alts)
154 ww_alt (con, binders, rhs)
155 = wwExpr rhs `thenUs` \ new_rhs ->
156 returnUs (con, binders, new_rhs)
159 %************************************************************************
161 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
163 %************************************************************************
165 @tryWW@ just accumulates arguments, converts strictness info from the
166 front-end into the proper form, then calls @mkWwBodies@ to do
169 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
170 already been w-w'd! (You can end up with several liked-named Ids
171 bouncing around at the same time---absolute mischief.) So the
172 criterion we use is: if an Id already has an unfolding (for whatever
173 reason), then we don't w-w it.
175 The only reason this is monadised is for the unique supply.
178 tryWW :: Bool -- True <=> a non-recursive binding
179 -> Id -- The fn binder
180 -> CoreExpr -- The bound rhs; its innards
182 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
183 -- if one, then no worker (only
184 -- the orig "wrapper" lives on);
185 -- if two, then a worker and a
187 tryWW non_rec fn_id rhs
188 | isNeverInlinePrag inline_prag || arity == 0
189 = -- Don't split things that will never be inlined
190 returnUs [ (fn_id, rhs) ]
192 | non_rec && not do_coerce_ww && certainlyWillInline fn_id
193 -- No point in worker/wrappering a function that is going to be
194 -- INLINEd wholesale anyway. If the strictness analyser is run
195 -- twice, this test also prevents wrappers (which are INLINEd)
196 -- from being re-done.
198 -- The do_coerce_ww test is so that
199 -- a function with a coerce should w/w to get rid
200 -- of the coerces, which can significantly improve its arity.
201 -- Example: f [] = return [] :: IO [Int]
202 -- f (x:xs) = return (x:xs)
203 -- If we aren't careful we end up with
204 -- f = \ x -> case x of {
205 -- x:xs -> __coerce (IO [Int]) (\ s -> (# s, x:xs #)
209 -- OUT OF DATE NOTE, kept for info:
210 -- It's out of date because now wrappers look very cheap
211 -- even when they are inlined.
212 -- In this case we add an INLINE pragma to the RHS. Why?
215 -- g = \yz -> ... -- And g is strict
216 -- Then f is small, so we don't w/w it. But g is big, and we do, so
217 -- g's wrapper will get inlined in f's RHS, which makes f look big now.
218 -- So f doesn't get inlined, but it is strict and we have failed to w/w it.
219 = returnUs [ (fn_id, rhs) ]
221 | not (do_strict_ww || do_cpr_ww || do_coerce_ww)
222 = returnUs [ (fn_id, rhs) ]
224 | otherwise -- Do w/w split
225 = mkWwBodies fun_ty arity wrap_dmds result_bot one_shots cpr_info `thenUs` \ (work_demands, wrap_fn, work_fn) ->
226 getUniqueUs `thenUs` \ work_uniq ->
228 work_rhs = work_fn rhs
229 proto_work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
230 `setInlinePragma` inline_prag
232 work_id | has_strictness = proto_work_id `setIdStrictness` mkStrictnessInfo (work_demands, result_bot)
233 | otherwise = proto_work_id
235 wrap_rhs = wrap_fn work_id
236 wrap_id = fn_id `setIdStrictness` wrapper_strictness
237 `setIdWorkerInfo` HasWorker work_id arity
238 `setInlinePragma` NoInlinePragInfo -- Put it on the worker instead
239 -- Add info to the wrapper:
240 -- (a) we want to set its arity
241 -- (b) we want to pin on its revised strictness info
242 -- (c) we pin on its worker id
244 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
245 -- Worker first, because wrapper mentions it
246 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
248 fun_ty = idType fn_id
249 arity = idArity fn_id -- The arity is set by the simplifier using exprEtaExpandArity
250 -- So it may be more than the number of top-level-visible lambdas
252 inline_prag = idInlinePragma fn_id
254 strictness_info = idStrictness fn_id
255 has_strictness = case strictness_info of
256 StrictnessInfo _ _ -> True
257 NoStrictnessInfo -> False
258 (arg_demands, result_bot) = case strictness_info of
259 StrictnessInfo d r -> (d, r)
260 NoStrictnessInfo -> ([], False)
262 wrap_dmds = setUnpackStrategy arg_demands
263 do_strict_ww = WARN( has_strictness && not result_bot && arity < length arg_demands && worthSplitting wrap_dmds result_bot,
264 text "Insufficient arity" <+> ppr fn_id <+> ppr arity <+> ppr arg_demands )
265 (result_bot || arity >= length arg_demands) -- Only if there's enough visible arity
266 && -- (else strictness info isn't valid)
268 worthSplitting wrap_dmds result_bot -- And it's useful
269 -- worthSplitting returns False for an empty list of demands,
270 -- and hence do_strict_ww is False if arity is zero
271 -- Also it's false if there is no strictness (arg_demands is [])
273 wrapper_strictness | has_strictness = mkStrictnessInfo (wrap_dmds, result_bot)
274 | otherwise = noStrictnessInfo
276 -------------------------------------------------------------
277 cpr_info = idCprInfo fn_id
278 do_cpr_ww = arity > 0 &&
283 -------------------------------------------------------------
284 do_coerce_ww = check_for_coerce arity fun_ty
285 -- We are willing to do a w/w even if the arity is zero.
291 -------------------------------------------------------------
292 one_shots = get_one_shots rhs
294 -- See if there's a Coerce before we run out of arity;
295 -- if so, it's worth trying a w/w split. Reason: we find
296 -- functions like f = coerce (\s -> e)
297 -- and g = \x -> coerce (\s -> e)
298 -- and they may have no useful strictness or cpr info, but if we
299 -- do the w/w thing we get rid of the coerces.
301 check_for_coerce arity ty
302 = length arg_tys <= arity && isNewType res_ty
303 -- Don't look further than arity args,
304 -- but if there are arity or fewer, see if there's
305 -- a newtype in the corner
307 (_, tau) = splitForAllTys ty
308 (arg_tys, res_ty) = splitFunTys tau
310 -- If the original function has one-shot arguments, it is important to
311 -- make the wrapper and worker have corresponding one-shot arguments too.
312 -- Otherwise we spuriously float stuff out of case-expression join points,
313 -- which is very annoying.
314 get_one_shots (Lam b e)
315 | isId b = isOneShotLambda b : get_one_shots e
316 | otherwise = get_one_shots e
317 get_one_shots (Note _ e) = get_one_shots e
318 get_one_shots other = noOneShotInfo
323 %************************************************************************
325 \subsection{The worker wrapper core}
327 %************************************************************************
329 @mkWrapper@ is called when importing a function. We have the type of
330 the function and the name of its worker, and we want to make its body (the wrapper).
333 mkWrapper :: Type -- Wrapper type
335 -> [Demand] -- Wrapper strictness info
336 -> Bool -- Function returns bottom
337 -> CprInfo -- Wrapper cpr info
338 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
340 mkWrapper fun_ty arity demands res_bot cpr_info
341 = mkWwBodies fun_ty arity demands res_bot noOneShotInfo cpr_info `thenUs` \ (_, wrap_fn, _) ->
344 noOneShotInfo = repeat False