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 )
34 We take Core bindings whose binders have:
38 \item Strictness attached (by the front-end of the strictness
41 \item Constructed Product Result information attached by the CPR
46 and we return some ``plain'' bindings which have been
47 worker/wrapper-ified, meaning:
51 \item Functions have been split into workers and wrappers where
52 appropriate. If a function has both strictness and CPR properties
53 then only one worker/wrapper doing both transformations is produced;
55 \item Binders' @IdInfos@ have been updated to reflect the existence of
56 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
57 info for exported values).
62 wwTopBinds :: DynFlags
67 wwTopBinds dflags us binds
69 showPass dflags "Worker Wrapper binds";
71 -- Create worker/wrappers, and mark binders with their
72 -- "strictness info" [which encodes their worker/wrapper-ness]
73 let { binds' = workersAndWrappers us binds };
75 endPass dflags "Worker Wrapper binds"
76 Opt_D_dump_worker_wrapper binds'
82 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
84 workersAndWrappers us top_binds
86 mapUs wwBind top_binds `thenUs` \ top_binds' ->
87 returnUs (concat top_binds')
90 %************************************************************************
92 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
94 %************************************************************************
96 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
97 turn. Non-recursive case first, then recursive...
101 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
102 -- the caller will convert to Expr/Binding,
105 wwBind (NonRec binder rhs)
106 = wwExpr rhs `thenUs` \ new_rhs ->
107 tryWW NonRecursive binder new_rhs `thenUs` \ new_pairs ->
108 returnUs [NonRec b e | (b,e) <- new_pairs]
109 -- Generated bindings must be non-recursive
110 -- because the original binding was.
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 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.
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 is_rec fn_id rhs
189 | isNonRec is_rec && certainlyWillInline unfolding
190 -- No point in worker/wrappering a function that is going to be
191 -- INLINEd wholesale anyway. If the strictness analyser is run
192 -- twice, this test also prevents wrappers (which are INLINEd)
193 -- from being re-done.
195 -- It's very important to refrain from w/w-ing an INLINE function
196 -- If we do so by mistake we transform
197 -- f = __inline (\x -> E)
199 -- f = __inline (\x -> case x of (a,b) -> fw E)
200 -- fw = \ab -> (__inline (\x -> E)) (a,b)
201 -- and the original __inline now vanishes, so E is no longer
202 -- inside its __inline wrapper. Death! Disaster!
203 = returnUs [ (fn_id, rhs) ]
205 | is_thunk && worthSplittingThunk fn_dmd res_info
206 = ASSERT( isNonRec is_rec ) -- The thunk must be non-recursive
209 | is_fun && worthSplittingFun wrap_dmds res_info
210 = splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
213 = returnUs [ (fn_id, rhs) ]
216 fn_info = idInfo fn_id
217 fn_dmd = newDemandInfo fn_info
218 unfolding = unfoldingInfo fn_info
219 inline_prag = inlinePragInfo fn_info
220 strict_sig = newStrictnessInfo fn_info `orElse` topSig
222 StrictSig (DmdType _ wrap_dmds res_info) = strict_sig
224 is_fun = not (null wrap_dmds)
225 is_thunk = not is_fun && not (exprIsValue rhs)
227 ---------------------
228 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
229 = WARN( arity /= length wrap_dmds, ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
230 -- The arity should match the signature
231 mkWwBodies fun_ty wrap_dmds res_info one_shots `thenUs` \ (work_demands, wrap_fn, work_fn) ->
232 getUniqueUs `thenUs` \ work_uniq ->
234 work_rhs = work_fn rhs
235 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
236 `setInlinePragma` inline_prag
237 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
238 -- Even though we may not be at top level,
239 -- it's ok to give it an empty DmdEnv
241 wrap_rhs = wrap_fn work_id
242 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
243 `setInlinePragma` AlwaysActive -- Zap any inline pragma;
244 -- Put it on the worker instead
246 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
247 -- Worker first, because wrapper mentions it
248 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
250 fun_ty = idType fn_id
252 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
253 -- So it may be more than the number of top-level-visible lambdas
255 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
258 one_shots = get_one_shots rhs
260 -- If the original function has one-shot arguments, it is important to
261 -- make the wrapper and worker have corresponding one-shot arguments too.
262 -- Otherwise we spuriously float stuff out of case-expression join points,
263 -- which is very annoying.
264 get_one_shots (Lam b e)
265 | isId b = isOneShotLambda b : get_one_shots e
266 | otherwise = get_one_shots e
267 get_one_shots (Note _ e) = get_one_shots e
268 get_one_shots other = noOneShotInfo
273 Suppose x is used strictly (never mind whether it has the CPR
280 splitThunk transforms like this:
283 x* = case x-rhs of { I# a -> I# a }
286 Now simplifier will transform to
289 I# a -> let x* = I# b
292 which is what we want. Now suppose x-rhs is itself a case:
294 x-rhs = case e of { T -> I# a; F -> I# b }
296 The join point will abstract over a, rather than over (which is
297 what would have happened before) which is fine.
299 Notice that x certainly has the CPR property now!
301 In fact, splitThunk uses the function argument w/w splitting
302 function, so that if x's demand is deeper (say U(U(L,L),L))
303 then the splitting will go deeper too.
306 -- splitThunk converts the *non-recursive* binding
311 -- I# y -> let x = I# y in x }
312 -- See comments above. Is it not beautifully short?
315 = mkWWstr [fn_id] `thenUs` \ (_, wrap_fn, work_fn) ->
316 returnUs [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
320 %************************************************************************
322 \subsection{Functions over Demands}
324 %************************************************************************
327 worthSplittingFun :: [Demand] -> DmdResult -> Bool
328 -- True <=> the wrapper would not be an identity function
329 worthSplittingFun ds res
330 = any worth_it ds || returnsCPR res
331 -- worthSplitting returns False for an empty list of demands,
332 -- and hence do_strict_ww is False if arity is zero and there is no CPR
334 -- We used not to split if the result is bottom.
335 -- [Justification: there's no efficiency to be gained.]
336 -- But it's sometimes bad not to make a wrapper. Consider
337 -- fw = \x# -> let x = I# x# in case e of
340 -- p3 -> the real stuff
341 -- The re-boxing code won't go away unless error_fn gets a wrapper too.
342 -- [We don't do reboxing now, but in general it's better to pass
343 -- an unboxed thing to f, and have it reboxed in the error cases....]
345 worth_it Abs = True -- Absent arg
346 worth_it (Seq _ ds) = True -- Arg to evaluate
347 worth_it other = False
349 worthSplittingThunk :: Demand -- Demand on the thunk
350 -> DmdResult -- CPR info for the thunk
352 worthSplittingThunk dmd res
353 = worth_it dmd || returnsCPR res
355 -- Split if the thing is unpacked
356 worth_it (Seq Defer ds) = False
357 worth_it (Seq _ ds) = any not_abs ds
358 worth_it other = False
366 %************************************************************************
368 \subsection{The worker wrapper core}
370 %************************************************************************
372 @mkWrapper@ is called when importing a function. We have the type of
373 the function and the name of its worker, and we want to make its body (the wrapper).
376 mkWrapper :: Type -- Wrapper type
377 -> StrictSig -- Wrapper strictness info
378 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
380 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info))
381 = mkWwBodies fun_ty demands res_info noOneShotInfo `thenUs` \ (_, wrap_fn, _) ->
384 noOneShotInfo = repeat False