2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[WorkWrap]{Worker/wrapper-generating back-end of strictness analyser}
8 -- The above warning supression flag is a temporary kludge.
9 -- While working on this module you are encouraged to remove it and fix
10 -- any warnings in the module. See
11 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
14 module WorkWrap ( wwTopBinds, mkWrapper ) where
16 #include "HsVersions.h"
19 import CoreUnfold ( certainlyWillInline )
20 import CoreLint ( showPass, endPass )
21 import CoreUtils ( exprType, exprIsHNF, exprArity )
22 import Id ( Id, idType, isOneShotLambda,
23 setIdNewStrictness, mkWorkerId,
24 setIdWorkerInfo, setInlinePragma,
26 import MkId ( lazyIdKey, lazyIdUnfolding )
28 import IdInfo ( WorkerInfo(..), arityInfo,
29 newDemandInfo, newStrictnessInfo, unfoldingInfo, inlinePragInfo
31 import NewDemand ( Demand(..), StrictSig(..), DmdType(..), DmdResult(..),
32 Demands(..), mkTopDmdType, isBotRes, returnsCPR, topSig, isAbsent
34 import UniqSupply ( UniqSupply, initUs_, returnUs, thenUs, mapUs, getUniqueUs, UniqSM )
35 import Unique ( hasKey )
36 import BasicTypes ( RecFlag(..), isNonRec, isNeverActive )
37 import VarEnv ( isEmptyVarEnv )
38 import Maybes ( orElse )
41 import Util ( lengthIs, notNull )
45 We take Core bindings whose binders have:
49 \item Strictness attached (by the front-end of the strictness
52 \item Constructed Product Result information attached by the CPR
57 and we return some ``plain'' bindings which have been
58 worker/wrapper-ified, meaning:
62 \item Functions have been split into workers and wrappers where
63 appropriate. If a function has both strictness and CPR properties
64 then only one worker/wrapper doing both transformations is produced;
66 \item Binders' @IdInfos@ have been updated to reflect the existence of
67 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
68 info for exported values).
73 wwTopBinds :: DynFlags
78 wwTopBinds dflags us binds
80 showPass dflags "Worker Wrapper binds";
82 -- Create worker/wrappers, and mark binders with their
83 -- "strictness info" [which encodes their worker/wrapper-ness]
84 let { binds' = workersAndWrappers us binds };
86 endPass dflags "Worker Wrapper binds"
87 Opt_D_dump_worker_wrapper binds'
93 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
95 workersAndWrappers us top_binds
97 mapUs wwBind top_binds `thenUs` \ top_binds' ->
98 returnUs (concat top_binds')
101 %************************************************************************
103 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
105 %************************************************************************
107 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
108 turn. Non-recursive case first, then recursive...
112 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
113 -- the caller will convert to Expr/Binding,
116 wwBind (NonRec binder rhs)
117 = wwExpr rhs `thenUs` \ new_rhs ->
118 tryWW NonRecursive binder new_rhs `thenUs` \ new_pairs ->
119 returnUs [NonRec b e | (b,e) <- new_pairs]
120 -- Generated bindings must be non-recursive
121 -- because the original binding was.
124 = mapUs do_one pairs `thenUs` \ new_pairs ->
125 returnUs [Rec (concat new_pairs)]
127 do_one (binder, rhs) = wwExpr rhs `thenUs` \ new_rhs ->
128 tryWW Recursive binder new_rhs
131 @wwExpr@ basically just walks the tree, looking for appropriate
132 annotations that can be used. Remember it is @wwBind@ that does the
133 matching by looking for strict arguments of the correct type.
134 @wwExpr@ is a version that just returns the ``Plain'' Tree.
137 wwExpr :: CoreExpr -> UniqSM CoreExpr
139 wwExpr e@(Type _) = returnUs e
140 wwExpr e@(Lit _) = returnUs e
141 wwExpr e@(Note InlineMe expr) = returnUs e
142 -- Don't w/w inside InlineMe's
145 | v `hasKey` lazyIdKey = returnUs lazyIdUnfolding
146 | otherwise = returnUs e
147 -- HACK alert: Inline 'lazy' after strictness analysis
148 -- (but not inside InlineMe's)
150 wwExpr (Lam binder expr)
151 = wwExpr expr `thenUs` \ new_expr ->
152 returnUs (Lam binder new_expr)
155 = wwExpr f `thenUs` \ new_f ->
156 wwExpr a `thenUs` \ new_a ->
157 returnUs (App new_f new_a)
159 wwExpr (Note note expr)
160 = wwExpr expr `thenUs` \ new_expr ->
161 returnUs (Note note new_expr)
163 wwExpr (Cast expr co)
164 = wwExpr expr `thenUs` \ new_expr ->
165 returnUs (Cast new_expr co)
167 wwExpr (Let bind expr)
168 = wwBind bind `thenUs` \ intermediate_bind ->
169 wwExpr expr `thenUs` \ new_expr ->
170 returnUs (mkLets intermediate_bind new_expr)
172 wwExpr (Case expr binder ty alts)
173 = wwExpr expr `thenUs` \ new_expr ->
174 mapUs ww_alt alts `thenUs` \ new_alts ->
175 returnUs (Case new_expr binder ty new_alts)
177 ww_alt (con, binders, rhs)
178 = wwExpr rhs `thenUs` \ new_rhs ->
179 returnUs (con, binders, new_rhs)
182 %************************************************************************
184 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
186 %************************************************************************
188 @tryWW@ just accumulates arguments, converts strictness info from the
189 front-end into the proper form, then calls @mkWwBodies@ to do
192 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
193 already been w-w'd! (You can end up with several liked-named Ids
194 bouncing around at the same time---absolute mischief.) So the
195 criterion we use is: if an Id already has an unfolding (for whatever
196 reason), then we don't w-w it.
198 The only reason this is monadised is for the unique supply.
202 -> Id -- The fn binder
203 -> CoreExpr -- The bound rhs; its innards
205 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
206 -- if one, then no worker (only
207 -- the orig "wrapper" lives on);
208 -- if two, then a worker and a
210 tryWW is_rec fn_id rhs
211 | isNonRec is_rec && certainlyWillInline unfolding
212 -- No point in worker/wrappering a function that is going to be
213 -- INLINEd wholesale anyway. If the strictness analyser is run
214 -- twice, this test also prevents wrappers (which are INLINEd)
215 -- from being re-done.
217 -- It's very important to refrain from w/w-ing an INLINE function
218 -- If we do so by mistake we transform
219 -- f = __inline (\x -> E)
221 -- f = __inline (\x -> case x of (a,b) -> fw E)
222 -- fw = \ab -> (__inline (\x -> E)) (a,b)
223 -- and the original __inline now vanishes, so E is no longer
224 -- inside its __inline wrapper. Death! Disaster!
226 || isNeverActive inline_prag
227 -- No point in worker/wrappering if the thing is never inlined!
228 -- Because the no-inline prag will prevent the wrapper ever
229 -- being inlined at a call site.
230 = returnUs [ (new_fn_id, rhs) ]
232 | is_thunk && worthSplittingThunk maybe_fn_dmd res_info
233 = ASSERT2( isNonRec is_rec, ppr new_fn_id ) -- The thunk must be non-recursive
234 splitThunk new_fn_id rhs
236 | is_fun && worthSplittingFun wrap_dmds res_info
237 = splitFun new_fn_id fn_info wrap_dmds res_info inline_prag rhs
240 = returnUs [ (new_fn_id, rhs) ]
243 fn_info = idInfo fn_id
244 maybe_fn_dmd = newDemandInfo fn_info
245 unfolding = unfoldingInfo fn_info
246 inline_prag = inlinePragInfo fn_info
248 -- In practice it always will have a strictness
249 -- signature, even if it's a uninformative one
250 strict_sig = newStrictnessInfo fn_info `orElse` topSig
251 StrictSig (DmdType env wrap_dmds res_info) = strict_sig
253 -- new_fn_id has the DmdEnv zapped.
254 -- (a) it is never used again
255 -- (b) it wastes space
256 -- (c) it becomes incorrect as things are cloned, because
257 -- we don't push the substitution into it
258 new_fn_id | isEmptyVarEnv env = fn_id
259 | otherwise = fn_id `setIdNewStrictness`
260 StrictSig (mkTopDmdType wrap_dmds res_info)
262 is_fun = notNull wrap_dmds
263 is_thunk = not is_fun && not (exprIsHNF rhs)
265 ---------------------
266 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
267 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
268 -- The arity should match the signature
269 mkWwBodies fun_ty wrap_dmds res_info one_shots `thenUs` \ (work_demands, wrap_fn, work_fn) ->
270 getUniqueUs `thenUs` \ work_uniq ->
272 work_rhs = work_fn rhs
273 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
274 `setInlinePragma` inline_prag
275 -- Any inline pragma (which sets when inlining is active)
276 -- on the original function is duplicated on the worker and wrapper
277 -- It *matters* that the pragma stays on the wrapper
278 -- It seems sensible to have it on the worker too, although we
279 -- can't think of a compelling reason. (In ptic, INLINE things are
281 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
282 -- Even though we may not be at top level,
283 -- it's ok to give it an empty DmdEnv
284 `setIdArity` (exprArity work_rhs)
285 -- Set the arity so that the Core Lint check that the
286 -- arity is consistent with the demand type goes through
288 wrap_rhs = wrap_fn work_id
289 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
292 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
293 -- Worker first, because wrapper mentions it
294 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
296 fun_ty = idType fn_id
298 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
299 -- So it may be more than the number of top-level-visible lambdas
301 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
304 one_shots = get_one_shots rhs
306 -- If the original function has one-shot arguments, it is important to
307 -- make the wrapper and worker have corresponding one-shot arguments too.
308 -- Otherwise we spuriously float stuff out of case-expression join points,
309 -- which is very annoying.
310 get_one_shots (Lam b e)
311 | isId b = isOneShotLambda b : get_one_shots e
312 | otherwise = get_one_shots e
313 get_one_shots (Note _ e) = get_one_shots e
314 get_one_shots other = noOneShotInfo
319 Suppose x is used strictly (never mind whether it has the CPR
326 splitThunk transforms like this:
329 x* = case x-rhs of { I# a -> I# a }
332 Now simplifier will transform to
335 I# a -> let x* = I# a
338 which is what we want. Now suppose x-rhs is itself a case:
340 x-rhs = case e of { T -> I# a; F -> I# b }
342 The join point will abstract over a, rather than over (which is
343 what would have happened before) which is fine.
345 Notice that x certainly has the CPR property now!
347 In fact, splitThunk uses the function argument w/w splitting
348 function, so that if x's demand is deeper (say U(U(L,L),L))
349 then the splitting will go deeper too.
352 -- splitThunk converts the *non-recursive* binding
357 -- I# y -> let x = I# y in x }
358 -- See comments above. Is it not beautifully short?
361 = mkWWstr [fn_id] `thenUs` \ (_, wrap_fn, work_fn) ->
362 returnUs [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
366 %************************************************************************
368 \subsection{Functions over Demands}
370 %************************************************************************
373 worthSplittingFun :: [Demand] -> DmdResult -> Bool
374 -- True <=> the wrapper would not be an identity function
375 worthSplittingFun ds res
376 = any worth_it ds || returnsCPR res
377 -- worthSplitting returns False for an empty list of demands,
378 -- and hence do_strict_ww is False if arity is zero and there is no CPR
380 -- We used not to split if the result is bottom.
381 -- [Justification: there's no efficiency to be gained.]
382 -- But it's sometimes bad not to make a wrapper. Consider
383 -- fw = \x# -> let x = I# x# in case e of
386 -- p3 -> the real stuff
387 -- The re-boxing code won't go away unless error_fn gets a wrapper too.
388 -- [We don't do reboxing now, but in general it's better to pass
389 -- an unboxed thing to f, and have it reboxed in the error cases....]
391 worth_it Abs = True -- Absent arg
392 worth_it (Eval (Prod ds)) = True -- Product arg to evaluate
393 worth_it other = False
395 worthSplittingThunk :: Maybe Demand -- Demand on the thunk
396 -> DmdResult -- CPR info for the thunk
398 worthSplittingThunk maybe_dmd res
399 = worth_it maybe_dmd || returnsCPR res
401 -- Split if the thing is unpacked
402 worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
403 worth_it other = False
408 %************************************************************************
410 \subsection{The worker wrapper core}
412 %************************************************************************
414 @mkWrapper@ is called when importing a function. We have the type of
415 the function and the name of its worker, and we want to make its body (the wrapper).
418 mkWrapper :: Type -- Wrapper type
419 -> StrictSig -- Wrapper strictness info
420 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
422 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info))
423 = mkWwBodies fun_ty demands res_info noOneShotInfo `thenUs` \ (_, wrap_fn, _) ->
426 noOneShotInfo = repeat False