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 )
46 We take Core bindings whose binders have:
50 \item Strictness attached (by the front-end of the strictness
53 \item Constructed Product Result information attached by the CPR
58 and we return some ``plain'' bindings which have been
59 worker/wrapper-ified, meaning:
63 \item Functions have been split into workers and wrappers where
64 appropriate. If a function has both strictness and CPR properties
65 then only one worker/wrapper doing both transformations is produced;
67 \item Binders' @IdInfos@ have been updated to reflect the existence of
68 these workers/wrappers (this is where we get STRICTNESS and CPR pragma
69 info for exported values).
74 wwTopBinds :: DynFlags
79 wwTopBinds dflags us binds
81 showPass dflags "Worker Wrapper binds";
83 -- Create worker/wrappers, and mark binders with their
84 -- "strictness info" [which encodes their worker/wrapper-ness]
85 let { binds' = workersAndWrappers us binds };
87 endPass dflags "Worker Wrapper binds"
88 Opt_D_dump_worker_wrapper binds'
94 workersAndWrappers :: UniqSupply -> [CoreBind] -> [CoreBind]
96 workersAndWrappers us top_binds
98 top_binds' <- mapM wwBind top_binds
99 return (concat top_binds')
102 %************************************************************************
104 \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@}
106 %************************************************************************
108 @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in
109 turn. Non-recursive case first, then recursive...
113 -> UniqSM [CoreBind] -- returns a WwBinding intermediate form;
114 -- the caller will convert to Expr/Binding,
117 wwBind (NonRec binder rhs) = do
118 new_rhs <- wwExpr rhs
119 new_pairs <- tryWW NonRecursive binder new_rhs
120 return [NonRec b e | (b,e) <- new_pairs]
121 -- Generated bindings must be non-recursive
122 -- because the original binding was.
125 = return . Rec <$> concatMapM do_one pairs
127 do_one (binder, rhs) = do new_rhs <- wwExpr rhs
128 tryWW Recursive binder new_rhs
131 @wwExpr@ basically just walks the tree, looking for appropriate
132 annotations that can be used. Remember it is @wwBind@ that does the
133 matching by looking for strict arguments of the correct type.
134 @wwExpr@ is a version that just returns the ``Plain'' Tree.
137 wwExpr :: CoreExpr -> UniqSM CoreExpr
139 wwExpr e@(Type _) = return e
140 wwExpr e@(Lit _) = return e
141 wwExpr e@(Note InlineMe expr) = return e
142 -- Don't w/w inside InlineMe's
145 | v `hasKey` lazyIdKey = return lazyIdUnfolding
146 | otherwise = return e
147 -- HACK alert: Inline 'lazy' after strictness analysis
148 -- (but not inside InlineMe's)
150 wwExpr (Lam binder expr)
151 = Lam binder <$> wwExpr expr
154 = App <$> wwExpr f <*> wwExpr a
156 wwExpr (Note note expr)
157 = Note note <$> wwExpr expr
159 wwExpr (Cast expr co) = do
160 new_expr <- wwExpr expr
161 return (Cast new_expr co)
163 wwExpr (Let bind expr)
164 = mkLets <$> wwBind bind <*> wwExpr expr
166 wwExpr (Case expr binder ty alts) = do
167 new_expr <- wwExpr expr
168 new_alts <- mapM ww_alt alts
169 return (Case new_expr binder ty new_alts)
171 ww_alt (con, binders, rhs) = do
172 new_rhs <- wwExpr rhs
173 return (con, binders, new_rhs)
176 %************************************************************************
178 \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair}
180 %************************************************************************
182 @tryWW@ just accumulates arguments, converts strictness info from the
183 front-end into the proper form, then calls @mkWwBodies@ to do
186 We have to BE CAREFUL that we don't worker-wrapperize an Id that has
187 already been w-w'd! (You can end up with several liked-named Ids
188 bouncing around at the same time---absolute mischief.) So the
189 criterion we use is: if an Id already has an unfolding (for whatever
190 reason), then we don't w-w it.
192 The only reason this is monadised is for the unique supply.
194 Note [Don't w/w inline things]
195 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
196 It's very important to refrain from w/w-ing an INLINE function
197 If we do so by mistake we transform
198 f = __inline (\x -> E)
200 f = __inline (\x -> case x of (a,b) -> fw E)
201 fw = \ab -> (__inline (\x -> E)) (a,b)
202 and the original __inline now vanishes, so E is no longer
203 inside its __inline wrapper. Death! Disaster!
205 Furthermore, if the programmer has marked something as INLINE,
206 we may lose by w/w'ing it.
208 If the strictness analyser is run twice, this test also prevents
209 wrappers (which are INLINEd) from being re-done.
211 Notice that we refrain from w/w'ing an INLINE function even if it is
212 in a recursive group. It might not be the loop breaker. (We could
213 test for loop-breaker-hood, but I'm not sure that ever matters.)
217 -> Id -- The fn binder
218 -> CoreExpr -- The bound rhs; its innards
220 -> UniqSM [(Id, CoreExpr)] -- either *one* or *two* pairs;
221 -- if one, then no worker (only
222 -- the orig "wrapper" lives on);
223 -- if two, then a worker and a
225 tryWW is_rec fn_id rhs
226 | -- isNonRec is_rec && -- Now omitted: see Note [Don't w/w inline things]
227 certainlyWillInline unfolding
229 || isNeverActive inline_prag
230 -- No point in worker/wrappering if the thing is never inlined!
231 -- Because the no-inline prag will prevent the wrapper ever
232 -- being inlined at a call site.
233 = return [ (new_fn_id, rhs) ]
235 | is_thunk && worthSplittingThunk maybe_fn_dmd res_info
236 = ASSERT2( isNonRec is_rec, ppr new_fn_id ) -- The thunk must be non-recursive
237 splitThunk new_fn_id rhs
239 | is_fun && worthSplittingFun wrap_dmds res_info
240 = splitFun new_fn_id fn_info wrap_dmds res_info inline_prag rhs
243 = return [ (new_fn_id, rhs) ]
246 fn_info = idInfo fn_id
247 maybe_fn_dmd = newDemandInfo fn_info
248 unfolding = unfoldingInfo fn_info
249 inline_prag = inlinePragInfo fn_info
251 -- In practice it always will have a strictness
252 -- signature, even if it's a uninformative one
253 strict_sig = newStrictnessInfo fn_info `orElse` topSig
254 StrictSig (DmdType env wrap_dmds res_info) = strict_sig
256 -- new_fn_id has the DmdEnv zapped.
257 -- (a) it is never used again
258 -- (b) it wastes space
259 -- (c) it becomes incorrect as things are cloned, because
260 -- we don't push the substitution into it
261 new_fn_id | isEmptyVarEnv env = fn_id
262 | otherwise = fn_id `setIdNewStrictness`
263 StrictSig (mkTopDmdType wrap_dmds res_info)
265 is_fun = notNull wrap_dmds
266 is_thunk = not is_fun && not (exprIsHNF rhs)
268 ---------------------
269 splitFun fn_id fn_info wrap_dmds res_info inline_prag rhs
270 = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) )
272 -- The arity should match the signature
273 (work_demands, wrap_fn, work_fn) <- mkWwBodies fun_ty wrap_dmds res_info one_shots
274 ; work_uniq <- getUniqueM
276 work_rhs = work_fn rhs
277 work_id = mkWorkerId work_uniq fn_id (exprType work_rhs)
278 `setInlinePragma` inline_prag
279 -- Any inline pragma (which sets when inlining is active)
280 -- on the original function is duplicated on the worker and wrapper
281 -- It *matters* that the pragma stays on the wrapper
282 -- It seems sensible to have it on the worker too, although we
283 -- can't think of a compelling reason. (In ptic, INLINE things are
285 `setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
286 -- Even though we may not be at top level,
287 -- it's ok to give it an empty DmdEnv
288 `setIdArity` (exprArity work_rhs)
289 -- Set the arity so that the Core Lint check that the
290 -- arity is consistent with the demand type goes through
292 wrap_rhs = wrap_fn work_id
293 wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
295 ; return ([(work_id, work_rhs), (wrap_id, wrap_rhs)]) })
296 -- Worker first, because wrapper mentions it
297 -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it
299 fun_ty = idType fn_id
301 arity = arityInfo fn_info -- The arity is set by the simplifier using exprEtaExpandArity
302 -- So it may be more than the number of top-level-visible lambdas
304 work_res_info | isBotRes res_info = BotRes -- Cpr stuff done by wrapper
307 one_shots = get_one_shots rhs
309 -- If the original function has one-shot arguments, it is important to
310 -- make the wrapper and worker have corresponding one-shot arguments too.
311 -- Otherwise we spuriously float stuff out of case-expression join points,
312 -- which is very annoying.
313 get_one_shots (Lam b e)
314 | isIdVar b = isOneShotLambda b : get_one_shots e
315 | otherwise = get_one_shots e
316 get_one_shots (Note _ e) = get_one_shots e
317 get_one_shots other = noOneShotInfo
322 Suppose x is used strictly (never mind whether it has the CPR
329 splitThunk transforms like this:
332 x* = case x-rhs of { I# a -> I# a }
335 Now simplifier will transform to
338 I# a -> let x* = I# a
341 which is what we want. Now suppose x-rhs is itself a case:
343 x-rhs = case e of { T -> I# a; F -> I# b }
345 The join point will abstract over a, rather than over (which is
346 what would have happened before) which is fine.
348 Notice that x certainly has the CPR property now!
350 In fact, splitThunk uses the function argument w/w splitting
351 function, so that if x's demand is deeper (say U(U(L,L),L))
352 then the splitting will go deeper too.
355 -- splitThunk converts the *non-recursive* binding
360 -- I# y -> let x = I# y in x }
361 -- See comments above. Is it not beautifully short?
363 splitThunk fn_id rhs = do
364 (_, wrap_fn, work_fn) <- mkWWstr [fn_id]
365 return [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
369 %************************************************************************
371 \subsection{Functions over Demands}
373 %************************************************************************
376 worthSplittingFun :: [Demand] -> DmdResult -> Bool
377 -- True <=> the wrapper would not be an identity function
378 worthSplittingFun ds res
379 = any worth_it ds || returnsCPR res
380 -- worthSplitting returns False for an empty list of demands,
381 -- and hence do_strict_ww is False if arity is zero and there is no CPR
382 -- See Note [Worker-wrapper for bottoming functions]
384 worth_it Abs = True -- Absent arg
385 worth_it (Eval (Prod ds)) = True -- Product arg to evaluate
386 worth_it other = False
388 worthSplittingThunk :: Maybe Demand -- Demand on the thunk
389 -> DmdResult -- CPR info for the thunk
391 worthSplittingThunk maybe_dmd res
392 = worth_it maybe_dmd || returnsCPR res
394 -- Split if the thing is unpacked
395 worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
396 worth_it other = False
399 Note [Worker-wrapper for bottoming functions]
400 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
401 We used not to split if the result is bottom.
402 [Justification: there's no efficiency to be gained.]
404 But it's sometimes bad not to make a wrapper. Consider
405 fw = \x# -> let x = I# x# in case e of
409 The re-boxing code won't go away unless error_fn gets a wrapper too.
410 [We don't do reboxing now, but in general it's better to pass an
411 unboxed thing to f, and have it reboxed in the error cases....]
414 %************************************************************************
416 \subsection{The worker wrapper core}
418 %************************************************************************
420 @mkWrapper@ is called when importing a function. We have the type of
421 the function and the name of its worker, and we want to make its body (the wrapper).
424 mkWrapper :: Type -- Wrapper type
425 -> StrictSig -- Wrapper strictness info
426 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
428 mkWrapper fun_ty (StrictSig (DmdType _ demands res_info)) = do
429 (_, wrap_fn, _) <- mkWwBodies fun_ty demands res_info noOneShotInfo
432 noOneShotInfo = repeat False