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
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 top_binds' <- mapM wwBind top_binds
98 return (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) = do
117 new_rhs <- wwExpr rhs
118 new_pairs <- tryWW NonRecursive binder new_rhs
119 return [NonRec b e | (b,e) <- new_pairs]
120 -- Generated bindings must be non-recursive
121 -- because the original binding was.
124 = return . Rec <$> concatMapM do_one pairs
126 do_one (binder, rhs) = do new_rhs <- wwExpr rhs
127 tryWW Recursive binder new_rhs
130 @wwExpr@ basically just walks the tree, looking for appropriate
131 annotations that can be used. Remember it is @wwBind@ that does the
132 matching by looking for strict arguments of the correct type.
133 @wwExpr@ is a version that just returns the ``Plain'' Tree.
136 wwExpr :: CoreExpr -> UniqSM CoreExpr
138 wwExpr e@(Type _) = return e
139 wwExpr e@(Lit _) = return e
140 wwExpr e@(Note InlineMe expr) = return e
141 -- Don't w/w inside InlineMe's
144 | v `hasKey` lazyIdKey = return lazyIdUnfolding
145 | otherwise = return e
146 -- HACK alert: Inline 'lazy' after strictness analysis
147 -- (but not inside InlineMe's)
149 wwExpr (Lam binder expr)
150 = Lam binder <$> wwExpr expr
153 = App <$> wwExpr f <*> wwExpr a
155 wwExpr (Note note expr)
156 = Note note <$> wwExpr expr
158 wwExpr (Cast expr co) = do
159 new_expr <- wwExpr expr
160 return (Cast new_expr co)
162 wwExpr (Let bind expr)
163 = mkLets <$> wwBind bind <*> wwExpr expr
165 wwExpr (Case expr binder ty alts) = do
166 new_expr <- wwExpr expr
167 new_alts <- mapM ww_alt alts
168 return (Case new_expr binder ty new_alts)
170 ww_alt (con, binders, rhs) = do
171 new_rhs <- wwExpr rhs
172 return (con, binders, new_rhs)
175 %************************************************************************
177 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
179 %************************************************************************
181 @tryWW@ just accumulates arguments, converts strictness info from the
182 front-end into the proper form, then calls @mkWwBodies@ to do
185 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
186 already been w-w'd! (You can end up with several liked-named Ids
187 bouncing around at the same time---absolute mischief.) So the
188 criterion we use is: if an Id already has an unfolding (for whatever
189 reason), then we don't w-w it.
191 The only reason this is monadised is for the unique supply.
193 Note [Don't w/w inline things]
194 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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!
204 Furthermore, if the programmer has marked something as INLINE,
205 we may lose by w/w'ing it.
207 If the strictness analyser is run twice, this test also prevents
208 wrappers (which are INLINEd) from being re-done.
210 Notice that we refrain from w/w'ing an INLINE function even if it is
211 in a recursive group. It might not be the loop breaker. (We could
212 test for loop-breaker-hood, but I'm not sure that ever matters.)
216 -> Id -- The fn binder
217 -> CoreExpr -- The bound rhs; its innards
219 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
220 -- if one, then no worker (only
221 -- the orig "wrapper" lives on);
222 -- if two, then a worker and a
224 tryWW is_rec fn_id rhs
225 | -- isNonRec is_rec && -- Now omitted: see Note [Don't w/w inline things]
226 certainlyWillInline unfolding
228 || isNeverActive inline_prag
229 -- No point in worker/wrappering if the thing is never inlined!
230 -- Because the no-inline prag will prevent the wrapper ever
231 -- being inlined at a call site.
232 = return [ (new_fn_id, rhs) ]
234 | is_thunk && worthSplittingThunk maybe_fn_dmd res_info
235 = ASSERT2( isNonRec is_rec, ppr new_fn_id ) -- The thunk must be non-recursive
236 splitThunk new_fn_id rhs
238 | is_fun && worthSplittingFun wrap_dmds res_info
239 = splitFun new_fn_id fn_info wrap_dmds res_info inline_prag rhs
242 = return [ (new_fn_id, rhs) ]
245 fn_info = idInfo fn_id
246 maybe_fn_dmd = newDemandInfo fn_info
247 unfolding = unfoldingInfo fn_info
248 inline_prag = inlinePragInfo fn_info
250 -- In practice it always will have a strictness
251 -- signature, even if it's a uninformative one
252 strict_sig = newStrictnessInfo fn_info `orElse` topSig
253 StrictSig (DmdType env wrap_dmds res_info) = strict_sig
255 -- new_fn_id has the DmdEnv zapped.
256 -- (a) it is never used again
257 -- (b) it wastes space
258 -- (c) it becomes incorrect as things are cloned, because
259 -- we don't push the substitution into it
260 new_fn_id | isEmptyVarEnv env = fn_id
261 | otherwise = fn_id `setIdNewStrictness`
262 StrictSig (mkTopDmdType wrap_dmds res_info)
264 is_fun = notNull wrap_dmds
265 is_thunk = not is_fun && not (exprIsHNF rhs)
267 ---------------------
268 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
269 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) ) do
270 -- The arity should match the signature
271 (work_demands, wrap_fn, work_fn) <- mkWwBodies fun_ty wrap_dmds res_info one_shots
272 work_uniq <- getUniqueM
274 work_rhs = work_fn rhs
275 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
276 `setInlinePragma` inline_prag
277 -- Any inline pragma (which sets when inlining is active)
278 -- on the original function is duplicated on the worker and wrapper
279 -- It *matters* that the pragma stays on the wrapper
280 -- It seems sensible to have it on the worker too, although we
281 -- can't think of a compelling reason. (In ptic, INLINE things are
283 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
284 -- Even though we may not be at top level,
285 -- it's ok to give it an empty DmdEnv
286 `setIdArity` (exprArity work_rhs)
287 -- Set the arity so that the Core Lint check that the
288 -- arity is consistent with the demand type goes through
290 wrap_rhs = wrap_fn work_id
291 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
293 return ([(work_id, work_rhs), (wrap_id, wrap_rhs)])
294 -- Worker first, because wrapper mentions it
295 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
297 fun_ty = idType fn_id
299 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
300 -- So it may be more than the number of top-level-visible lambdas
302 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
305 one_shots = get_one_shots rhs
307 -- If the original function has one-shot arguments, it is important to
308 -- make the wrapper and worker have corresponding one-shot arguments too.
309 -- Otherwise we spuriously float stuff out of case-expression join points,
310 -- which is very annoying.
311 get_one_shots (Lam b e)
312 | isId b = isOneShotLambda b : get_one_shots e
313 | otherwise = get_one_shots e
314 get_one_shots (Note _ e) = get_one_shots e
315 get_one_shots other = noOneShotInfo
320 Suppose x is used strictly (never mind whether it has the CPR
327 splitThunk transforms like this:
330 x* = case x-rhs of { I# a -> I# a }
333 Now simplifier will transform to
336 I# a -> let x* = I# a
339 which is what we want. Now suppose x-rhs is itself a case:
341 x-rhs = case e of { T -> I# a; F -> I# b }
343 The join point will abstract over a, rather than over (which is
344 what would have happened before) which is fine.
346 Notice that x certainly has the CPR property now!
348 In fact, splitThunk uses the function argument w/w splitting
349 function, so that if x's demand is deeper (say U(U(L,L),L))
350 then the splitting will go deeper too.
353 -- splitThunk converts the *non-recursive* binding
358 -- I# y -> let x = I# y in x }
359 -- See comments above. Is it not beautifully short?
361 splitThunk fn_id rhs = do
362 (_, wrap_fn, work_fn) <- mkWWstr [fn_id]
363 return [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
367 %************************************************************************
369 \subsection{Functions over Demands}
371 %************************************************************************
374 worthSplittingFun :: [Demand] -> DmdResult -> Bool
375 -- True <=> the wrapper would not be an identity function
376 worthSplittingFun ds res
377 = any worth_it ds || returnsCPR res
378 -- worthSplitting returns False for an empty list of demands,
379 -- and hence do_strict_ww is False if arity is zero and there is no CPR
380 -- See Note [Worker-wrapper for bottoming functions]
382 worth_it Abs = True -- Absent arg
383 worth_it (Eval (Prod ds)) = True -- Product arg to evaluate
384 worth_it other = False
386 worthSplittingThunk :: Maybe Demand -- Demand on the thunk
387 -> DmdResult -- CPR info for the thunk
389 worthSplittingThunk maybe_dmd res
390 = worth_it maybe_dmd || returnsCPR res
392 -- Split if the thing is unpacked
393 worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
394 worth_it other = False
397 Note [Worker-wrapper for bottoming functions]
398 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
399 We used not to split if the result is bottom.
400 [Justification: there's no efficiency to be gained.]
402 But it's sometimes bad not to make a wrapper. Consider
403 fw = \x# -> let x = I# x# in case e of
407 The re-boxing code won't go away unless error_fn gets a wrapper too.
408 [We don't do reboxing now, but in general it's better to pass an
409 unboxed thing to f, and have it reboxed in the error cases....]
412 %************************************************************************
414 \subsection{The worker wrapper core}
416 %************************************************************************
418 @mkWrapper@ is called when importing a function. We have the type of
419 the function and the name of its worker, and we want to make its body (the wrapper).
422 mkWrapper :: Type -- Wrapper type
423 -> StrictSig -- Wrapper strictness info
424 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
426 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info)) = do
427 (_, wrap_fn, _) <- mkWwBodies fun_ty demands res_info noOneShotInfo
430 noOneShotInfo = repeat False