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, exprIsValue )
15 import Id ( Id, idType, isOneShotLambda,
16 setIdNewStrictness, mkWorkerId,
17 setIdWorkerInfo, setInlinePragma,
20 import IdInfo ( WorkerInfo(..), arityInfo,
21 newDemandInfo, newStrictnessInfo, unfoldingInfo, inlinePragInfo
23 import NewDemand ( Demand(..), StrictSig(..), DmdType(..), DmdResult(..), Keepity(..),
24 mkTopDmdType, isBotRes, returnsCPR, topSig
26 import UniqSupply ( UniqSupply, initUs_, returnUs, thenUs, mapUs, getUniqueUs, UniqSM )
27 import BasicTypes ( RecFlag(..), isNonRec, Activation(..) )
28 import Maybes ( orElse )
31 import Util ( lengthIs )
35 We take Core bindings whose binders have:
39 \item Strictness attached (by the front-end of the strictness
42 \item Constructed Product Result information attached by the CPR
47 and we return some ``plain'' bindings which have been
48 worker/wrapper-ified, meaning:
52 \item Functions have been split into workers and wrappers where
53 appropriate. If a function has both strictness and CPR properties
54 then only one worker/wrapper doing both transformations is produced;
56 \item Binders' @IdInfos@ have been updated to reflect the existence of
57 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
58 info for exported values).
63 wwTopBinds :: DynFlags
68 wwTopBinds dflags us binds
70 showPass dflags "Worker Wrapper binds";
72 -- Create worker/wrappers, and mark binders with their
73 -- "strictness info" [which encodes their worker/wrapper-ness]
74 let { binds' = workersAndWrappers us binds };
76 endPass dflags "Worker Wrapper binds"
77 Opt_D_dump_worker_wrapper binds'
83 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
85 workersAndWrappers us top_binds
87 mapUs wwBind top_binds `thenUs` \ top_binds' ->
88 returnUs (concat top_binds')
91 %************************************************************************
93 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
95 %************************************************************************
97 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
98 turn. Non-recursive case first, then recursive...
102 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
103 -- the caller will convert to Expr/Binding,
106 wwBind (NonRec binder rhs)
107 = wwExpr rhs `thenUs` \ new_rhs ->
108 tryWW NonRecursive binder new_rhs `thenUs` \ new_pairs ->
109 returnUs [NonRec b e | (b,e) <- new_pairs]
110 -- Generated bindings must be non-recursive
111 -- because the original binding was.
114 = mapUs do_one pairs `thenUs` \ new_pairs ->
115 returnUs [Rec (concat new_pairs)]
117 do_one (binder, rhs) = wwExpr rhs `thenUs` \ new_rhs ->
118 tryWW Recursive binder new_rhs
121 @wwExpr@ basically just walks the tree, looking for appropriate
122 annotations that can be used. Remember it is @wwBind@ that does the
123 matching by looking for strict arguments of the correct type.
124 @wwExpr@ is a version that just returns the ``Plain'' Tree.
127 wwExpr :: CoreExpr -> UniqSM CoreExpr
129 wwExpr e@(Type _) = returnUs e
130 wwExpr e@(Var _) = returnUs e
131 wwExpr e@(Lit _) = returnUs e
133 wwExpr (Lam binder expr)
134 = wwExpr expr `thenUs` \ new_expr ->
135 returnUs (Lam binder new_expr)
138 = wwExpr f `thenUs` \ new_f ->
139 wwExpr a `thenUs` \ new_a ->
140 returnUs (App new_f new_a)
142 wwExpr (Note note expr)
143 = wwExpr expr `thenUs` \ new_expr ->
144 returnUs (Note note new_expr)
146 wwExpr (Let bind expr)
147 = wwBind bind `thenUs` \ intermediate_bind ->
148 wwExpr expr `thenUs` \ new_expr ->
149 returnUs (mkLets intermediate_bind new_expr)
151 wwExpr (Case expr binder alts)
152 = wwExpr expr `thenUs` \ new_expr ->
153 mapUs ww_alt alts `thenUs` \ new_alts ->
154 returnUs (Case new_expr binder new_alts)
156 ww_alt (con, binders, rhs)
157 = wwExpr rhs `thenUs` \ new_rhs ->
158 returnUs (con, binders, new_rhs)
161 %************************************************************************
163 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
165 %************************************************************************
167 @tryWW@ just accumulates arguments, converts strictness info from the
168 front-end into the proper form, then calls @mkWwBodies@ to do
171 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
172 already been w-w'd! (You can end up with several liked-named Ids
173 bouncing around at the same time---absolute mischief.) So the
174 criterion we use is: if an Id already has an unfolding (for whatever
175 reason), then we don't w-w it.
177 The only reason this is monadised is for the unique supply.
181 -> Id -- The fn binder
182 -> CoreExpr -- The bound rhs; its innards
184 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
185 -- if one, then no worker (only
186 -- the orig "wrapper" lives on);
187 -- if two, then a worker and a
189 tryWW is_rec fn_id rhs
190 | isNonRec is_rec && certainlyWillInline unfolding
191 -- No point in worker/wrappering a function that is going to be
192 -- INLINEd wholesale anyway. If the strictness analyser is run
193 -- twice, this test also prevents wrappers (which are INLINEd)
194 -- from being re-done.
196 -- It's very important to refrain from w/w-ing an INLINE function
197 -- If we do so by mistake we transform
198 -- f = __inline (\x -> E)
200 -- f = __inline (\x -> case x of (a,b) -> fw E)
201 -- fw = \ab -> (__inline (\x -> E)) (a,b)
202 -- and the original __inline now vanishes, so E is no longer
203 -- inside its __inline wrapper. Death! Disaster!
204 = returnUs [ (fn_id, rhs) ]
206 | is_thunk && worthSplittingThunk fn_dmd res_info
207 = ASSERT( isNonRec is_rec ) -- The thunk must be non-recursive
210 | is_fun && worthSplittingFun wrap_dmds res_info
211 = splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
214 = returnUs [ (fn_id, rhs) ]
217 fn_info = idInfo fn_id
218 fn_dmd = newDemandInfo fn_info
219 unfolding = unfoldingInfo fn_info
220 inline_prag = inlinePragInfo fn_info
221 strict_sig = newStrictnessInfo fn_info `orElse` topSig
223 StrictSig (DmdType _ wrap_dmds res_info) = strict_sig
225 is_fun = not (null wrap_dmds)
226 is_thunk = not is_fun && not (exprIsValue rhs)
228 ---------------------
229 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
230 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
231 -- The arity should match the signature
232 mkWwBodies fun_ty wrap_dmds res_info one_shots `thenUs` \ (work_demands, wrap_fn, work_fn) ->
233 getUniqueUs `thenUs` \ work_uniq ->
235 work_rhs = work_fn rhs
236 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
237 `setInlinePragma` inline_prag
238 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
239 -- Even though we may not be at top level,
240 -- it's ok to give it an empty DmdEnv
242 wrap_rhs = wrap_fn work_id
243 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
244 `setInlinePragma` AlwaysActive -- Zap any inline pragma;
245 -- Put it on the worker instead
247 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
248 -- Worker first, because wrapper mentions it
249 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
251 fun_ty = idType fn_id
253 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
254 -- So it may be more than the number of top-level-visible lambdas
256 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
259 one_shots = get_one_shots rhs
261 -- If the original function has one-shot arguments, it is important to
262 -- make the wrapper and worker have corresponding one-shot arguments too.
263 -- Otherwise we spuriously float stuff out of case-expression join points,
264 -- which is very annoying.
265 get_one_shots (Lam b e)
266 | isId b = isOneShotLambda b : get_one_shots e
267 | otherwise = get_one_shots e
268 get_one_shots (Note _ e) = get_one_shots e
269 get_one_shots other = noOneShotInfo
274 Suppose x is used strictly (never mind whether it has the CPR
281 splitThunk transforms like this:
284 x* = case x-rhs of { I# a -> I# a }
287 Now simplifier will transform to
290 I# a -> let x* = I# b
293 which is what we want. Now suppose x-rhs is itself a case:
295 x-rhs = case e of { T -> I# a; F -> I# b }
297 The join point will abstract over a, rather than over (which is
298 what would have happened before) which is fine.
300 Notice that x certainly has the CPR property now!
302 In fact, splitThunk uses the function argument w/w splitting
303 function, so that if x's demand is deeper (say U(U(L,L),L))
304 then the splitting will go deeper too.
307 -- splitThunk converts the *non-recursive* binding
312 -- I# y -> let x = I# y in x }
313 -- See comments above. Is it not beautifully short?
316 = mkWWstr [fn_id] `thenUs` \ (_, wrap_fn, work_fn) ->
317 returnUs [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
321 %************************************************************************
323 \subsection{Functions over Demands}
325 %************************************************************************
328 worthSplittingFun :: [Demand] -> DmdResult -> Bool
329 -- True <=> the wrapper would not be an identity function
330 worthSplittingFun ds res
331 = any worth_it ds || returnsCPR res
332 -- worthSplitting returns False for an empty list of demands,
333 -- and hence do_strict_ww is False if arity is zero and there is no CPR
335 -- We used not to split if the result is bottom.
336 -- [Justification: there's no efficiency to be gained.]
337 -- But it's sometimes bad not to make a wrapper. Consider
338 -- fw = \x# -> let x = I# x# in case e of
341 -- p3 -> the real stuff
342 -- The re-boxing code won't go away unless error_fn gets a wrapper too.
343 -- [We don't do reboxing now, but in general it's better to pass
344 -- an unboxed thing to f, and have it reboxed in the error cases....]
346 worth_it Abs = True -- Absent arg
347 worth_it (Seq _ ds) = True -- Arg to evaluate
348 worth_it other = False
350 worthSplittingThunk :: Demand -- Demand on the thunk
351 -> DmdResult -- CPR info for the thunk
353 worthSplittingThunk dmd res
354 = worth_it dmd || returnsCPR res
356 -- Split if the thing is unpacked
357 worth_it (Seq Defer ds) = False
358 worth_it (Seq _ ds) = any not_abs ds
359 worth_it other = False
367 %************************************************************************
369 \subsection{The worker wrapper core}
371 %************************************************************************
373 @mkWrapper@ is called when importing a function. We have the type of
374 the function and the name of its worker, and we want to make its body (the wrapper).
377 mkWrapper :: Type -- Wrapper type
378 -> StrictSig -- Wrapper strictness info
379 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
381 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info))
382 = mkWwBodies fun_ty demands res_info noOneShotInfo `thenUs` \ (_, wrap_fn, _) ->
385 noOneShotInfo = repeat False