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.
200 Note [Don't w/w inline things]
201 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
202 It's very important to refrain from w/w-ing an INLINE function
203 If we do so by mistake we transform
204 f = __inline (\x -> E)
206 f = __inline (\x -> case x of (a,b) -> fw E)
207 fw = \ab -> (__inline (\x -> E)) (a,b)
208 and the original __inline now vanishes, so E is no longer
209 inside its __inline wrapper. Death! Disaster!
211 Furthermore, if the programmer has marked something as INLINE,
212 we may lose by w/w'ing it.
214 If the strictness analyser is run twice, this test also prevents
215 wrappers (which are INLINEd) from being re-done.
217 Notice that we refrain from w/w'ing an INLINE function even if it is
218 in a recursive group. It might not be the loop breaker. (We could
219 test for loop-breaker-hood, but I'm not sure that ever matters.)
223 -> Id -- The fn binder
224 -> CoreExpr -- The bound rhs; its innards
226 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
227 -- if one, then no worker (only
228 -- the orig "wrapper" lives on);
229 -- if two, then a worker and a
231 tryWW is_rec fn_id rhs
232 | -- isNonRec is_rec && -- Now omitted: see Note [Don't w/w inline things]
233 certainlyWillInline unfolding
235 || isNeverActive inline_prag
236 -- No point in worker/wrappering if the thing is never inlined!
237 -- Because the no-inline prag will prevent the wrapper ever
238 -- being inlined at a call site.
239 = returnUs [ (new_fn_id, rhs) ]
241 | is_thunk && worthSplittingThunk maybe_fn_dmd res_info
242 = ASSERT2( isNonRec is_rec, ppr new_fn_id ) -- The thunk must be non-recursive
243 splitThunk new_fn_id rhs
245 | is_fun && worthSplittingFun wrap_dmds res_info
246 = splitFun new_fn_id fn_info wrap_dmds res_info inline_prag rhs
249 = returnUs [ (new_fn_id, rhs) ]
252 fn_info = idInfo fn_id
253 maybe_fn_dmd = newDemandInfo fn_info
254 unfolding = unfoldingInfo fn_info
255 inline_prag = inlinePragInfo fn_info
257 -- In practice it always will have a strictness
258 -- signature, even if it's a uninformative one
259 strict_sig = newStrictnessInfo fn_info `orElse` topSig
260 StrictSig (DmdType env wrap_dmds res_info) = strict_sig
262 -- new_fn_id has the DmdEnv zapped.
263 -- (a) it is never used again
264 -- (b) it wastes space
265 -- (c) it becomes incorrect as things are cloned, because
266 -- we don't push the substitution into it
267 new_fn_id | isEmptyVarEnv env = fn_id
268 | otherwise = fn_id `setIdNewStrictness`
269 StrictSig (mkTopDmdType wrap_dmds res_info)
271 is_fun = notNull wrap_dmds
272 is_thunk = not is_fun && not (exprIsHNF rhs)
274 ---------------------
275 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
276 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
277 -- The arity should match the signature
278 mkWwBodies fun_ty wrap_dmds res_info one_shots `thenUs` \ (work_demands, wrap_fn, work_fn) ->
279 getUniqueUs `thenUs` \ work_uniq ->
281 work_rhs = work_fn rhs
282 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
283 `setInlinePragma` inline_prag
284 -- Any inline pragma (which sets when inlining is active)
285 -- on the original function is duplicated on the worker and wrapper
286 -- It *matters* that the pragma stays on the wrapper
287 -- It seems sensible to have it on the worker too, although we
288 -- can't think of a compelling reason. (In ptic, INLINE things are
290 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
291 -- Even though we may not be at top level,
292 -- it's ok to give it an empty DmdEnv
293 `setIdArity` (exprArity work_rhs)
294 -- Set the arity so that the Core Lint check that the
295 -- arity is consistent with the demand type goes through
297 wrap_rhs = wrap_fn work_id
298 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
301 returnUs ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
302 -- Worker first, because wrapper mentions it
303 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
305 fun_ty = idType fn_id
307 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
308 -- So it may be more than the number of top-level-visible lambdas
310 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
313 one_shots = get_one_shots rhs
315 -- If the original function has one-shot arguments, it is important to
316 -- make the wrapper and worker have corresponding one-shot arguments too.
317 -- Otherwise we spuriously float stuff out of case-expression join points,
318 -- which is very annoying.
319 get_one_shots (Lam b e)
320 | isId b = isOneShotLambda b : get_one_shots e
321 | otherwise = get_one_shots e
322 get_one_shots (Note _ e) = get_one_shots e
323 get_one_shots other = noOneShotInfo
328 Suppose x is used strictly (never mind whether it has the CPR
335 splitThunk transforms like this:
338 x* = case x-rhs of { I# a -> I# a }
341 Now simplifier will transform to
344 I# a -> let x* = I# a
347 which is what we want. Now suppose x-rhs is itself a case:
349 x-rhs = case e of { T -> I# a; F -> I# b }
351 The join point will abstract over a, rather than over (which is
352 what would have happened before) which is fine.
354 Notice that x certainly has the CPR property now!
356 In fact, splitThunk uses the function argument w/w splitting
357 function, so that if x's demand is deeper (say U(U(L,L),L))
358 then the splitting will go deeper too.
361 -- splitThunk converts the *non-recursive* binding
366 -- I# y -> let x = I# y in x }
367 -- See comments above. Is it not beautifully short?
370 = mkWWstr [fn_id] `thenUs` \ (_, wrap_fn, work_fn) ->
371 returnUs [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
375 %************************************************************************
377 \subsection{Functions over Demands}
379 %************************************************************************
382 worthSplittingFun :: [Demand] -> DmdResult -> Bool
383 -- True <=> the wrapper would not be an identity function
384 worthSplittingFun ds res
385 = any worth_it ds || returnsCPR res
386 -- worthSplitting returns False for an empty list of demands,
387 -- and hence do_strict_ww is False if arity is zero and there is no CPR
388 -- See Note [Worker-wrapper for bottoming functions]
390 worth_it Abs = True -- Absent arg
391 worth_it (Eval (Prod ds)) = True -- Product arg to evaluate
392 worth_it other = False
394 worthSplittingThunk :: Maybe Demand -- Demand on the thunk
395 -> DmdResult -- CPR info for the thunk
397 worthSplittingThunk maybe_dmd res
398 = worth_it maybe_dmd || returnsCPR res
400 -- Split if the thing is unpacked
401 worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
402 worth_it other = False
405 Note [Worker-wrapper for bottoming functions]
406 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
407 We used not to split if the result is bottom.
408 [Justification: there's no efficiency to be gained.]
410 But it's sometimes bad not to make a wrapper. Consider
411 fw = \x# -> let x = I# x# in case e of
415 The re-boxing code won't go away unless error_fn gets a wrapper too.
416 [We don't do reboxing now, but in general it's better to pass an
417 unboxed thing to f, and have it reboxed in the error cases....]
420 %************************************************************************
422 \subsection{The worker wrapper core}
424 %************************************************************************
426 @mkWrapper@ is called when importing a function. We have the type of
427 the function and the name of its worker, and we want to make its body (the wrapper).
430 mkWrapper :: Type -- Wrapper type
431 -> StrictSig -- Wrapper strictness info
432 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
434 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info))
435 = mkWwBodies fun_ty demands res_info noOneShotInfo `thenUs` \ (_, wrap_fn, _) ->
438 noOneShotInfo = repeat False