2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1996
4 \section[WwLib]{A library for the ``worker/wrapper'' back-end to the strictness analyser}
10 worthSplitting, setUnpackStrategy,
14 #include "HsVersions.h"
17 import Id ( GenId, idType, mkSysLocal, dataConArgTys, isDataCon, isNewCon, Id )
18 import IdInfo ( Demand(..) )
19 import PrelVals ( aBSENT_ERROR_ID, voidId )
20 import TysPrim ( voidTy )
21 import SrcLoc ( noSrcLoc )
22 import Type ( isUnpointedType, mkTyVarTys, mkFunTys,
23 splitForAllTys, splitFunTys,
24 splitAlgTyConApp_maybe,
27 import TyCon ( isNewTyCon, isDataTyCon )
28 import BasicTypes ( NewOrData(..) )
29 import TyVar ( TyVar )
30 import PprType ( GenType, GenTyVar )
31 import UniqSupply ( returnUs, thenUs, getUniques, getUnique, UniqSM )
32 import Util ( zipEqual )
36 %************************************************************************
38 \subsection[datatype-WwLib]{@WwBinding@: a datatype for worker/wrapper-ing}
40 %************************************************************************
42 In the worker/wrapper stuff, we want to carry around @CoreBindings@ in
43 an ``intermediate form'' that can later be turned into a \tr{let} or
44 \tr{case} (depending on strictness info).
49 | WwCase (CoreExpr -> CoreExpr)
50 -- the "case" will be a "strict let" of the form:
55 -- (instead of "let <blah> = rhs in body")
57 -- The expr you pass to the function is "body" (the
58 -- expression that goes "in the corner").
61 %************************************************************************
63 \subsection[mkWrapperAndWorker]{@mkWrapperAndWorker@}
65 %************************************************************************
67 ************ WARNING ******************
68 these comments are rather out of date
69 *****************************************
71 @mkWrapperAndWorker@ is given:
74 The {\em original function} \tr{f}, of the form:
76 f = /\ tyvars -> \ args -> body
78 The original-binder \tr{f}, the \tr{tyvars}, \tr{args}, and \tr{body}
81 We use the Id \tr{f} mostly to get its type.
84 Strictness information about \tr{f}, in the form of a list of
91 @mkWrapperAndWorker@ produces (A BIT OUT-OF-DATE...):
94 Maybe @Nothing@: no worker/wrappering going on in this case. This can
95 happen (a)~if the strictness info says that there is nothing
96 interesting to do or (b)~if *any* of the argument types corresponding
97 to ``active'' arg postitions is abstract or will be to the outside
98 world (i.e., {\em this} module can see the constructors, but nobody
99 else will be able to). An ``active'' arg position is one which the
100 wrapper has to unpack. An importing module can't do this unpacking,
101 so it simply has to give up and call the wrapper only.
104 Maybe \tr{Just (wrapper_Id, wrapper_body, worker_Id, worker_body)}.
106 The @wrapper_Id@ is just the one that was passed in, with its
107 strictness IdInfo updated.
110 The \tr{body} of the original function may not be given (i.e., it's
111 BOTTOM), in which case you'd jolly well better not tug on the
114 Here's an example. The original function is:
116 g :: forall a . Int -> [a] -> a
118 g = /\ a -> \ x ys ->
124 From this, we want to produce:
126 -- wrapper (an unfolding)
127 g :: forall a . Int -> [a] -> a
129 g = /\ a -> \ x ys ->
131 I# x# -> g.wrk a x# ys
132 -- call the worker; don't forget the type args!
135 g.wrk :: forall a . Int# -> [a] -> a
137 g.wrk = /\ a -> \ x# ys ->
141 case x of -- note: body of g moved intact
146 Something we have to be careful about: Here's an example:
148 -- "f" strictness: U(P)U(P)
149 f (I# a) (I# b) = a +# b
151 g = f -- "g" strictness same as "f"
153 \tr{f} will get a worker all nice and friendly-like; that's good.
154 {\em But we don't want a worker for \tr{g}}, even though it has the
155 same strictness as \tr{f}. Doing so could break laziness, at best.
157 Consequently, we insist that the number of strictness-info items is
158 exactly the same as the number of lambda-bound arguments. (This is
159 probably slightly paranoid, but OK in practice.) If it isn't the
160 same, we ``revise'' the strictness info, so that we won't propagate
161 the unusable strictness-info into the interfaces.
164 %************************************************************************
166 \subsection{Functions over Demands}
168 %************************************************************************
171 mAX_WORKER_ARGS :: Int -- ToDo: set via flag
174 setUnpackStrategy :: [Demand] -> [Demand]
176 = snd (go (mAX_WORKER_ARGS - nonAbsentArgs ds) ds)
178 go :: Int -- Max number of args available for sub-components of [Demand]
180 -> (Int, [Demand]) -- Args remaining after subcomponents of [Demand] are unpacked
182 go n (WwUnpack nd _ cs : ds) | n' >= 0
183 = WwUnpack nd True cs' `cons` go n'' ds
185 = WwUnpack nd False cs `cons` go n ds
187 n' = n + 1 - nonAbsentArgs cs
188 -- Add one because we don't pass the top-level arg any more
189 -- Delete # of non-absent args to which we'll now be committed
192 go n (d:ds) = d `cons` go n ds
195 cons d (n,ds) = (n, d:ds)
197 nonAbsentArgs :: [Demand] -> Int
199 nonAbsentArgs (WwLazy True : ds) = nonAbsentArgs ds
200 nonAbsentArgs (d : ds) = 1 + nonAbsentArgs ds
202 worthSplitting :: [Demand] -> Bool -- True <=> the wrapper would not be an identity function
203 worthSplitting [] = False
204 worthSplitting (WwLazy True : ds) = True -- Absent arg
205 worthSplitting (WwUnpack _ True _ : ds) = True -- Arg to unpack
206 worthSplitting (d : ds) = worthSplitting ds
208 allAbsent :: [Demand] -> Bool
209 allAbsent (WwLazy True : ds) = allAbsent ds
210 allAbsent (WwUnpack _ True cs : ds) = allAbsent cs && allAbsent ds
211 allAbsent (d : ds) = False
216 %************************************************************************
218 \subsection{The worker wrapper core}
220 %************************************************************************
222 @mkWrapper@ is called when importing a function. We have the type of
223 the function and the name of its worker, and we want to make its body (the wrapper).
226 mkWrapper :: Type -- Wrapper type
227 -> [Demand] -- Wrapper strictness info
228 -> UniqSM (Id -> CoreExpr) -- Wrapper body, missing worker Id
230 mkWrapper fun_ty demands
232 n_wrap_args = length demands
234 getUniques n_wrap_args `thenUs` \ wrap_uniqs ->
236 (tyvars, tau_ty) = splitForAllTys fun_ty
237 (arg_tys, body_ty) = splitFunTys tau_ty
238 -- The "expanding dicts" part here is important, even for the splitForAll
239 -- The imported thing might be a dictionary, such as Functor Foo
240 -- But Functor Foo = forall a b. (a->b) -> Foo a -> Foo b
241 -- and as such might have some strictness info attached.
242 -- Then we need to have enough args to zip to the strictness info
244 wrap_args = zipWith mk_ww_local wrap_uniqs arg_tys
245 leftover_arg_tys = drop n_wrap_args arg_tys
246 final_body_ty = mkFunTys leftover_arg_tys body_ty
248 mkWwBodies tyvars wrap_args final_body_ty demands `thenUs` \ (wrap_fn, _, _) ->
252 @mkWwBodies@ is called when doing the worker/wrapper split inside a module.
255 mkWwBodies :: [TyVar] -> [Id] -> Type -- Original fn args and body type
256 -> [Demand] -- Strictness info for original fn; corresp 1-1 with args
257 -> UniqSM (Id -> CoreExpr, -- Wrapper body, lacking only the worker Id
258 CoreExpr -> CoreExpr, -- Worker body, lacking the original function body
259 [Demand]) -- Strictness info for worker
261 mkWwBodies tyvars args body_ty demands
262 | allAbsent demands &&
263 isUnpointedType body_ty
264 = -- Horrid special case. If the worker would have no arguments, and the
265 -- function returns a primitive type value, that would make the worker into
266 -- an unboxed value. We box it by passing a dummy void argument, thus:
268 -- f = /\abc. \xyz. fw abc void
269 -- fw = /\abc. \v. body
271 getUnique `thenUs` \ void_arg_uniq ->
273 void_arg = mk_ww_local void_arg_uniq voidTy
275 returnUs (\ work_id -> mkLam tyvars args (App (mkTyApp (Var work_id) (mkTyVarTys tyvars)) (VarArg voidId)),
276 \ body -> mkLam tyvars [void_arg] body,
279 mkWwBodies tyvars args body_ty demands
282 args_w_demands = zipEqual "mkWwBodies" args demands
284 mkWW args_w_demands `thenUs` \ (wrap_fn, work_args_w_demands, work_fn) ->
286 (work_args, work_demands) = unzip work_args_w_demands
288 returnUs (\ work_id -> mkLam tyvars args (wrap_fn (mkTyApp (Var work_id) (mkTyVarTys tyvars))),
289 \ body -> mkLam tyvars work_args (work_fn body),
295 mkWW :: [(Id,Demand)]
296 -> UniqSM (CoreExpr -> CoreExpr, -- Wrapper body, lacking the inner call to the worker
297 -- and without its lambdas
298 [(Id,Demand)], -- Worker args and their demand infos
299 CoreExpr -> CoreExpr) -- Worker body, lacking the original body of the function
304 = returnUs (\ wrapper_body -> wrapper_body,
306 \ worker_body -> worker_body)
310 mkWW ((arg,WwLazy True) : ds)
311 = mkWW ds `thenUs` \ (wrap_fn, worker_args, work_fn) ->
312 returnUs (\ wrapper_body -> wrap_fn wrapper_body,
314 \ worker_body -> mk_absent_let arg (work_fn worker_body))
318 mkWW ((arg,WwUnpack new_or_data True cs) : ds)
319 = getUniques (length inst_con_arg_tys) `thenUs` \ uniqs ->
321 unpk_args = zipWith mk_ww_local uniqs inst_con_arg_tys
322 unpk_args_w_ds = zipEqual "mkWW" unpk_args cs
324 mkWW (unpk_args_w_ds ++ ds) `thenUs` \ (wrap_fn, worker_args, work_fn) ->
325 returnUs (\ wrapper_body -> mk_unpk_case new_or_data arg unpk_args data_con arg_tycon (wrap_fn wrapper_body),
327 \ worker_body -> work_fn (mk_pk_let new_or_data arg data_con tycon_arg_tys unpk_args worker_body))
329 inst_con_arg_tys = dataConArgTys data_con tycon_arg_tys
330 (arg_tycon, tycon_arg_tys, data_con)
331 = case (splitAlgTyConApp_maybe (idType arg)) of
333 Just (arg_tycon, tycon_arg_tys, [data_con]) ->
334 -- The main event: a single-constructor data type
335 (arg_tycon, tycon_arg_tys, data_con)
337 Just (_, _, data_cons) -> pprPanic "mk_ww_arg_processing: not one constr (interface files not consistent/up to date ?)" ((ppr arg) <+> (ppr (idType arg)))
338 Nothing -> panic "mk_ww_arg_processing: not datatype"
342 mkWW ((arg,other_demand) : ds)
343 = mkWW ds `thenUs` \ (wrap_fn, worker_args, work_fn) ->
344 returnUs (\ wrapper_body -> wrap_fn (App wrapper_body (VarArg arg)),
345 (arg,other_demand) : worker_args,
350 %************************************************************************
352 \subsection{Utilities}
354 %************************************************************************
358 mk_absent_let arg body
359 | not (isUnpointedType arg_ty)
360 = Let (NonRec arg (mkTyApp (Var aBSENT_ERROR_ID) [arg_ty])) body
362 = panic "WwLib: haven't done mk_absent_let for primitives yet"
366 mk_unpk_case NewType arg unpk_args boxing_con boxing_tycon body
367 -- A newtype! Use a coercion not a case
368 = ASSERT( null other_args && isNewTyCon boxing_tycon )
369 Let (NonRec unpk_arg (Coerce (CoerceOut boxing_con) (idType unpk_arg) (Var arg)))
372 (unpk_arg:other_args) = unpk_args
374 mk_unpk_case DataType arg unpk_args boxing_con boxing_tycon body
376 = ASSERT( isDataTyCon boxing_tycon )
378 (AlgAlts [(boxing_con, unpk_args, body)]
382 mk_pk_let NewType arg boxing_con con_tys unpk_args body
383 = ASSERT( null other_args && isNewCon boxing_con )
384 Let (NonRec arg (Coerce (CoerceIn boxing_con) (idType arg) (Var unpk_arg))) body
386 (unpk_arg:other_args) = unpk_args
388 mk_pk_let DataType arg boxing_con con_tys unpk_args body
389 = ASSERT( isDataCon boxing_con )
390 Let (NonRec arg (Con boxing_con con_args)) body
392 con_args = map TyArg con_tys ++ map VarArg unpk_args
396 = mkSysLocal SLIT("ww") uniq ty noSrcLoc