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(..),
24 Demands(..), mkTopDmdType, isBotRes, returnsCPR, topSig, isAbsent
26 import UniqSupply ( UniqSupply, initUs_, returnUs, thenUs, mapUs, getUniqueUs, UniqSM )
27 import BasicTypes ( RecFlag(..), isNonRec, Activation(..) )
28 import VarEnv ( isEmptyVarEnv )
29 import Maybes ( orElse )
32 import Util ( lengthIs )
36 We take Core bindings whose binders have:
40 \item Strictness attached (by the front-end of the strictness
43 \item Constructed Product Result information attached by the CPR
48 and we return some ``plain'' bindings which have been
49 worker/wrapper-ified, meaning:
53 \item Functions have been split into workers and wrappers where
54 appropriate. If a function has both strictness and CPR properties
55 then only one worker/wrapper doing both transformations is produced;
57 \item Binders' @IdInfos@ have been updated to reflect the existence of
58 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
59 info for exported values).
64 wwTopBinds :: DynFlags
69 wwTopBinds dflags us binds
71 showPass dflags "Worker Wrapper binds";
73 -- Create worker/wrappers, and mark binders with their
74 -- "strictness info" [which encodes their worker/wrapper-ness]
75 let { binds' = workersAndWrappers us binds };
77 endPass dflags "Worker Wrapper binds"
78 Opt_D_dump_worker_wrapper binds'
84 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
86 workersAndWrappers us top_binds
88 mapUs wwBind top_binds `thenUs` \ top_binds' ->
89 returnUs (concat top_binds')
92 %************************************************************************
94 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
96 %************************************************************************
98 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
99 turn. Non-recursive case first, then recursive...
103 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
104 -- the caller will convert to Expr/Binding,
107 wwBind (NonRec binder rhs)
108 = wwExpr rhs `thenUs` \ new_rhs ->
109 tryWW NonRecursive binder new_rhs `thenUs` \ new_pairs ->
110 returnUs [NonRec b e | (b,e) <- new_pairs]
111 -- Generated bindings must be non-recursive
112 -- because the original binding was.
115 = mapUs do_one pairs `thenUs` \ new_pairs ->
116 returnUs [Rec (concat new_pairs)]
118 do_one (binder, rhs) = wwExpr rhs `thenUs` \ new_rhs ->
119 tryWW Recursive binder new_rhs
122 @wwExpr@ basically just walks the tree, looking for appropriate
123 annotations that can be used. Remember it is @wwBind@ that does the
124 matching by looking for strict arguments of the correct type.
125 @wwExpr@ is a version that just returns the ``Plain'' Tree.
128 wwExpr :: CoreExpr -> UniqSM CoreExpr
130 wwExpr e@(Type _) = returnUs e
131 wwExpr e@(Var _) = returnUs e
132 wwExpr e@(Lit _) = returnUs e
134 wwExpr (Lam binder expr)
135 = wwExpr expr `thenUs` \ new_expr ->
136 returnUs (Lam binder new_expr)
139 = wwExpr f `thenUs` \ new_f ->
140 wwExpr a `thenUs` \ new_a ->
141 returnUs (App new_f new_a)
143 wwExpr (Note note expr)
144 = wwExpr expr `thenUs` \ new_expr ->
145 returnUs (Note note new_expr)
147 wwExpr (Let bind expr)
148 = wwBind bind `thenUs` \ intermediate_bind ->
149 wwExpr expr `thenUs` \ new_expr ->
150 returnUs (mkLets intermediate_bind new_expr)
152 wwExpr (Case expr binder alts)
153 = wwExpr expr `thenUs` \ new_expr ->
154 mapUs ww_alt alts `thenUs` \ new_alts ->
155 returnUs (Case new_expr binder new_alts)
157 ww_alt (con, binders, rhs)
158 = wwExpr rhs `thenUs` \ new_rhs ->
159 returnUs (con, binders, new_rhs)
162 %************************************************************************
164 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
166 %************************************************************************
168 @tryWW@ just accumulates arguments, converts strictness info from the
169 front-end into the proper form, then calls @mkWwBodies@ to do
172 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
173 already been w-w'd! (You can end up with several liked-named Ids
174 bouncing around at the same time---absolute mischief.) So the
175 criterion we use is: if an Id already has an unfolding (for whatever
176 reason), then we don't w-w it.
178 The only reason this is monadised is for the unique supply.
182 -> Id -- The fn binder
183 -> CoreExpr -- The bound rhs; its innards
185 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
186 -- if one, then no worker (only
187 -- the orig "wrapper" lives on);
188 -- if two, then a worker and a
190 tryWW is_rec fn_id rhs
191 | isNonRec is_rec && certainlyWillInline unfolding
192 -- No point in worker/wrappering a function that is going to be
193 -- INLINEd wholesale anyway. If the strictness analyser is run
194 -- twice, this test also prevents wrappers (which are INLINEd)
195 -- from being re-done.
197 -- It's very important to refrain from w/w-ing an INLINE function
198 -- If we do so by mistake we transform
199 -- f = __inline (\x -> E)
201 -- f = __inline (\x -> case x of (a,b) -> fw E)
202 -- fw = \ab -> (__inline (\x -> E)) (a,b)
203 -- and the original __inline now vanishes, so E is no longer
204 -- inside its __inline wrapper. Death! Disaster!
205 = returnUs [ (fn_id', rhs) ]
207 | is_thunk && worthSplittingThunk fn_dmd res_info
208 = ASSERT( isNonRec is_rec ) -- The thunk must be non-recursive
209 splitThunk fn_id' rhs
211 | is_fun && worthSplittingFun wrap_dmds res_info
212 = splitFun fn_id' fn_info wrap_dmds res_info inline_prag rhs
215 = returnUs [ (fn_id', rhs) ]
218 fn_info = idInfo fn_id
219 fn_dmd = newDemandInfo fn_info
220 unfolding = unfoldingInfo fn_info
221 inline_prag = inlinePragInfo fn_info
222 maybe_sig = newStrictnessInfo fn_info
224 -- In practice it always will have a strictness
225 -- signature, even if it's a uninformative one
226 strict_sig = newStrictnessInfo fn_info `orElse` topSig
227 StrictSig (DmdType env wrap_dmds res_info) = strict_sig
229 -- fn_id' has the DmdEnv zapped.
230 -- (a) it is never used again
231 -- (b) it wastes space
232 -- (c) it becomes incorrect as things are cloned, because
233 -- we don't push the substitution into it
234 fn_id' | isEmptyVarEnv env = fn_id
235 | otherwise = fn_id `setIdNewStrictness`
236 StrictSig (mkTopDmdType wrap_dmds res_info)
238 is_fun = not (null wrap_dmds)
239 is_thunk = not is_fun && not (exprIsValue rhs)
241 ---------------------
242 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
243 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
244 -- The arity should match the signature
245 mkWwBodies fun_ty wrap_dmds res_info one_shots `thenUs` \ (work_demands, wrap_fn, work_fn) ->
246 getUniqueUs `thenUs` \ work_uniq ->
248 work_rhs = work_fn rhs
249 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
250 `setInlinePragma` inline_prag
251 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
252 -- Even though we may not be at top level,
253 -- it's ok to give it an empty DmdEnv
255 wrap_rhs = wrap_fn work_id
256 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
257 `setInlinePragma` AlwaysActive -- Zap any inline pragma;
258 -- Put it on the worker instead
260 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
261 -- Worker first, because wrapper mentions it
262 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
264 fun_ty = idType fn_id
266 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
267 -- So it may be more than the number of top-level-visible lambdas
269 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
272 one_shots = get_one_shots rhs
274 -- If the original function has one-shot arguments, it is important to
275 -- make the wrapper and worker have corresponding one-shot arguments too.
276 -- Otherwise we spuriously float stuff out of case-expression join points,
277 -- which is very annoying.
278 get_one_shots (Lam b e)
279 | isId b = isOneShotLambda b : get_one_shots e
280 | otherwise = get_one_shots e
281 get_one_shots (Note _ e) = get_one_shots e
282 get_one_shots other = noOneShotInfo
287 Suppose x is used strictly (never mind whether it has the CPR
294 splitThunk transforms like this:
297 x* = case x-rhs of { I# a -> I# a }
300 Now simplifier will transform to
303 I# a -> let x* = I# b
306 which is what we want. Now suppose x-rhs is itself a case:
308 x-rhs = case e of { T -> I# a; F -> I# b }
310 The join point will abstract over a, rather than over (which is
311 what would have happened before) which is fine.
313 Notice that x certainly has the CPR property now!
315 In fact, splitThunk uses the function argument w/w splitting
316 function, so that if x's demand is deeper (say U(U(L,L),L))
317 then the splitting will go deeper too.
320 -- splitThunk converts the *non-recursive* binding
325 -- I# y -> let x = I# y in x }
326 -- See comments above. Is it not beautifully short?
329 = mkWWstr [fn_id] `thenUs` \ (_, wrap_fn, work_fn) ->
330 returnUs [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
334 %************************************************************************
336 \subsection{Functions over Demands}
338 %************************************************************************
341 worthSplittingFun :: [Demand] -> DmdResult -> Bool
342 -- True <=> the wrapper would not be an identity function
343 worthSplittingFun ds res
344 = any worth_it ds || returnsCPR res
345 -- worthSplitting returns False for an empty list of demands,
346 -- and hence do_strict_ww is False if arity is zero and there is no CPR
348 -- We used not to split if the result is bottom.
349 -- [Justification: there's no efficiency to be gained.]
350 -- But it's sometimes bad not to make a wrapper. Consider
351 -- fw = \x# -> let x = I# x# in case e of
354 -- p3 -> the real stuff
355 -- The re-boxing code won't go away unless error_fn gets a wrapper too.
356 -- [We don't do reboxing now, but in general it's better to pass
357 -- an unboxed thing to f, and have it reboxed in the error cases....]
359 worth_it Abs = True -- Absent arg
360 worth_it (Eval (Prod ds)) = True -- Product arg to evaluate
361 worth_it other = False
363 worthSplittingThunk :: Demand -- Demand on the thunk
364 -> DmdResult -- CPR info for the thunk
366 worthSplittingThunk dmd res
367 = worth_it dmd || returnsCPR res
369 -- Split if the thing is unpacked
370 worth_it (Eval (Prod ds)) = not (all isAbsent ds)
371 worth_it other = False
376 %************************************************************************
378 \subsection{The worker wrapper core}
380 %************************************************************************
382 @mkWrapper@ is called when importing a function. We have the type of
383 the function and the name of its worker, and we want to make its body (the wrapper).
386 mkWrapper :: Type -- Wrapper type
387 -> StrictSig -- Wrapper strictness info
388 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
390 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info))
391 = mkWwBodies fun_ty demands res_info noOneShotInfo `thenUs` \ (_, wrap_fn, _) ->
394 noOneShotInfo = repeat False