2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
4 \section[WwLib]{A library for the ``worker/wrapper'' back-end to the strictness analyser}
9 worthSplitting, setUnpackStrategy
12 #include "HsVersions.h"
15 import CoreUtils ( exprType )
16 import Id ( Id, idType, mkSysLocal, idDemandInfo, setIdDemandInfo,
17 isOneShotLambda, setOneShotLambda,
20 import IdInfo ( CprInfo(..), noCprInfo, vanillaIdInfo )
21 import DataCon ( DataCon, splitProductType )
22 import Demand ( Demand(..), wwLazy, wwPrim )
23 import PrelInfo ( realWorldPrimId, aBSENT_ERROR_ID )
24 import TysPrim ( realWorldStatePrimTy )
25 import TysWiredIn ( tupleCon )
26 import Type ( isUnLiftedType,
27 splitForAllTys, splitFunTys, isAlgType,
32 import BasicTypes ( NewOrData(..), Arity, Boxity(..) )
33 import Var ( TyVar, Var, isId )
34 import UniqSupply ( returnUs, thenUs, getUniqueUs, getUniquesUs,
36 import Util ( zipWithEqual, zipEqual, lengthExceeds )
38 import List ( zipWith4 )
42 %************************************************************************
44 \subsection[mkWrapperAndWorker]{@mkWrapperAndWorker@}
46 %************************************************************************
48 ************ WARNING ******************
49 these comments are rather out of date
50 *****************************************
52 @mkWrapperAndWorker@ is given:
55 The {\em original function} \tr{f}, of the form:
57 f = /\ tyvars -> \ args -> body
59 The original-binder \tr{f}, the \tr{tyvars}, \tr{args}, and \tr{body}
62 We use the Id \tr{f} mostly to get its type.
65 Strictness information about \tr{f}, in the form of a list of
72 @mkWrapperAndWorker@ produces (A BIT OUT-OF-DATE...):
75 Maybe @Nothing@: no worker/wrappering going on in this case. This can
76 happen (a)~if the strictness info says that there is nothing
77 interesting to do or (b)~if *any* of the argument types corresponding
78 to ``active'' arg postitions is abstract or will be to the outside
79 world (i.e., {\em this} module can see the constructors, but nobody
80 else will be able to). An ``active'' arg position is one which the
81 wrapper has to unpack. An importing module can't do this unpacking,
82 so it simply has to give up and call the wrapper only.
85 Maybe \tr{Just (wrapper_Id, wrapper_body, worker_Id, worker_body)}.
87 The @wrapper_Id@ is just the one that was passed in, with its
88 strictness IdInfo updated.
91 The \tr{body} of the original function may not be given (i.e., it's
92 BOTTOM), in which case you'd jolly well better not tug on the
95 Here's an example. The original function is:
97 g :: forall a . Int -> [a] -> a
105 From this, we want to produce:
107 -- wrapper (an unfolding)
108 g :: forall a . Int -> [a] -> a
110 g = /\ a -> \ x ys ->
112 I# x# -> g.wrk a x# ys
113 -- call the worker; don't forget the type args!
116 g.wrk :: forall a . Int# -> [a] -> a
118 g.wrk = /\ a -> \ x# ys ->
122 case x of -- note: body of g moved intact
127 Something we have to be careful about: Here's an example:
129 -- "f" strictness: U(P)U(P)
130 f (I# a) (I# b) = a +# b
132 g = f -- "g" strictness same as "f"
134 \tr{f} will get a worker all nice and friendly-like; that's good.
135 {\em But we don't want a worker for \tr{g}}, even though it has the
136 same strictness as \tr{f}. Doing so could break laziness, at best.
138 Consequently, we insist that the number of strictness-info items is
139 exactly the same as the number of lambda-bound arguments. (This is
140 probably slightly paranoid, but OK in practice.) If it isn't the
141 same, we ``revise'' the strictness info, so that we won't propagate
142 the unusable strictness-info into the interfaces.
145 %************************************************************************
147 \subsection{Functions over Demands}
149 %************************************************************************
152 mAX_WORKER_ARGS :: Int -- ToDo: set via flag
155 setUnpackStrategy :: [Demand] -> [Demand]
157 = snd (go (mAX_WORKER_ARGS - nonAbsentArgs ds) ds)
159 go :: Int -- Max number of args available for sub-components of [Demand]
161 -> (Int, [Demand]) -- Args remaining after subcomponents of [Demand] are unpacked
163 go n (WwUnpack nd _ cs : ds) | n' >= 0
164 = WwUnpack nd True cs' `cons` go n'' ds
166 = WwUnpack nd False cs `cons` go n ds
168 n' = n + 1 - nonAbsentArgs cs
169 -- Add one because we don't pass the top-level arg any more
170 -- Delete # of non-absent args to which we'll now be committed
173 go n (d:ds) = d `cons` go n ds
176 cons d (n,ds) = (n, d:ds)
178 nonAbsentArgs :: [Demand] -> Int
180 nonAbsentArgs (WwLazy True : ds) = nonAbsentArgs ds
181 nonAbsentArgs (d : ds) = 1 + nonAbsentArgs ds
183 worthSplitting :: [Demand]
184 -> Bool -- Result is bottom
185 -> Bool -- True <=> the wrapper would not be an identity function
186 worthSplitting ds result_bot = any worth_it ds
187 -- We used not to split if the result is bottom.
188 -- [Justification: there's no efficiency to be gained.]
189 -- But it's sometimes bad not to make a wrapper. Consider
190 -- fw = \x# -> let x = I# x# in case e of
193 -- p3 -> the real stuff
194 -- The re-boxing code won't go away unless error_fn gets a wrapper too.
197 worth_it (WwLazy True) = True -- Absent arg
198 worth_it (WwUnpack _ True _) = True -- Arg to unpack
199 worth_it WwStrict = False -- Don't w/w just because of strictness
200 worth_it other = False
202 allAbsent :: [Demand] -> Bool
203 allAbsent ds = all absent ds
205 absent (WwLazy is_absent) = is_absent
206 absent (WwUnpack _ True cs) = allAbsent cs
211 %************************************************************************
213 \subsection{The worker wrapper core}
215 %************************************************************************
217 @mkWwBodies@ is called when doing the worker/wrapper split inside a module.
220 mkWwBodies :: Type -- Type of original function
221 -> Arity -- Arity of original function
222 -> [Demand] -- Strictness of original function
223 -> Bool -- True <=> function returns bottom
224 -> [Bool] -- One-shot-ness of the function
225 -> CprInfo -- Result of CPR analysis
226 -> UniqSM ([Demand], -- Demands for worker (value) args
227 Id -> CoreExpr, -- Wrapper body, lacking only the worker Id
228 CoreExpr -> CoreExpr) -- Worker body, lacking the original function rhs
230 mkWwBodies fun_ty arity demands res_bot one_shots cpr_info
231 = mkWWargs fun_ty arity demands' res_bot one_shots' `thenUs` \ (wrap_args, wrap_fn_args, work_fn_args, res_ty) ->
232 mkWWstr wrap_args `thenUs` \ (work_dmds, wrap_fn_str, work_fn_str) ->
233 mkWWcpr res_ty cpr_info `thenUs` \ (wrap_fn_cpr, work_fn_cpr, cpr_res_ty) ->
234 mkWWfixup cpr_res_ty work_dmds `thenUs` \ (final_work_dmds, wrap_fn_fixup, work_fn_fixup) ->
236 returnUs (final_work_dmds,
237 Note InlineMe . wrap_fn_args . wrap_fn_cpr . wrap_fn_str . wrap_fn_fixup . Var,
238 work_fn_fixup . work_fn_str . work_fn_cpr . work_fn_args)
239 -- We use an INLINE unconditionally, even if the wrapper turns out to be
240 -- something trivial like
242 -- f = __inline__ (coerce T fw)
243 -- The point is to propagate the coerce to f's call sites, so even though
244 -- f's RHS is now trivial (size 1) we still want the __inline__ to prevent
245 -- fw from being inlined into f's RHS
247 demands' = demands ++ repeat wwLazy
248 one_shots' = one_shots ++ repeat False
252 %************************************************************************
254 \subsection{Coercion stuff}
256 %************************************************************************
259 We really want to "look through" coerces.
260 Reason: I've seen this situation:
262 let f = coerce T (\s -> E)
268 If only we w/w'd f, we'd get
269 let f = coerce T (\s -> fw s)
273 Now we'll inline f to get
281 Now we'll see that fw has arity 1, and will arity expand
282 the \x to get what we want.
285 -- mkWWargs is driven off the function type and arity.
286 -- It chomps bites off foralls, arrows, newtypes
287 -- and keeps repeating that until it's satisfied the supplied arity
289 mkWWargs :: Type -> Arity
290 -> [Demand] -> Bool -> [Bool] -- Both these will in due course be derived
291 -- from the type. The [Bool] is True for a one-shot arg.
292 -- ** Both are infinite, extended with neutral values if necy **
293 -> UniqSM ([Var], -- Wrapper args
294 CoreExpr -> CoreExpr, -- Wrapper fn
295 CoreExpr -> CoreExpr, -- Worker fn
296 Type) -- Type of wrapper body
298 mkWWargs fun_ty arity demands res_bot one_shots
299 | (res_bot || arity > 0) && (not (null tyvars) || n_arg_tys > 0)
300 -- If the function returns bottom, we feel free to
301 -- build lots of wrapper args:
302 -- \x. let v=E in \y. bottom
303 -- = \xy. let v=E in bottom
304 = getUniquesUs n_args `thenUs` \ wrap_uniqs ->
306 val_args = zipWith4 mk_wrap_arg wrap_uniqs arg_tys demands one_shots
307 wrap_args = tyvars ++ val_args
311 (drop n_args demands)
313 (drop n_args one_shots) `thenUs` \ (more_wrap_args, wrap_fn_args, work_fn_args, res_ty) ->
315 returnUs (wrap_args ++ more_wrap_args,
316 mkLams wrap_args . wrap_fn_args,
317 work_fn_args . applyToVars wrap_args,
320 (tyvars, tau) = splitForAllTys fun_ty
321 (arg_tys, body_ty) = splitFunTys tau
322 n_arg_tys = length arg_tys
323 n_args | res_bot = n_arg_tys
324 | otherwise = arity `min` n_arg_tys
325 new_fun_ty | n_args == n_arg_tys = body_ty
326 | otherwise = mkFunTys (drop n_args arg_tys) body_ty
328 mkWWargs fun_ty arity demands res_bot one_shots
329 = case splitNewType_maybe fun_ty of
330 Nothing -> returnUs ([], id, id, fun_ty)
331 Just rep_ty -> mkWWargs rep_ty arity demands res_bot one_shots `thenUs` \ (wrap_args, wrap_fn_args, work_fn_args, res_ty) ->
333 Note (Coerce fun_ty rep_ty) . wrap_fn_args,
334 work_fn_args . Note (Coerce rep_ty fun_ty),
338 applyToVars :: [Var] -> CoreExpr -> CoreExpr
339 applyToVars vars fn = mkVarApps fn vars
341 mk_wrap_arg uniq ty dmd one_shot
342 = set_one_shot one_shot (setIdDemandInfo (mkSysLocal SLIT("w") uniq ty) dmd)
344 set_one_shot True id = setOneShotLambda id
345 set_one_shot False id = id
349 %************************************************************************
351 \subsection{Fixup stuff}
353 %************************************************************************
356 mkWWfixup res_ty work_dmds
357 | null work_dmds && isUnLiftedType res_ty
358 -- Horrid special case. If the worker would have no arguments, and the
359 -- function returns a primitive type value, that would make the worker into
360 -- an unboxed value. We box it by passing a dummy void argument, thus:
362 -- f = /\abc. \xyz. fw abc void
363 -- fw = /\abc. \v. body
365 -- We use the state-token type which generates no code
366 = getUniqueUs `thenUs` \ void_arg_uniq ->
368 void_arg = mk_ww_local void_arg_uniq realWorldStatePrimTy
371 \ call_to_worker -> App call_to_worker (Var realWorldPrimId),
372 \ worker_body -> Lam void_arg worker_body)
375 = returnUs (work_dmds, id, id)
379 %************************************************************************
381 \subsection{Strictness stuff}
383 %************************************************************************
386 mkWWstr :: [Var] -- Wrapper args; have their demand info on them
387 -- *Includes type variables*
388 -> UniqSM ([Demand], -- Demand on worker (value) args
389 CoreExpr -> CoreExpr, -- Wrapper body, lacking the worker call
390 -- and without its lambdas
391 -- This fn adds the unboxing, and makes the
392 -- call passing the unboxed things
394 CoreExpr -> CoreExpr) -- Worker body, lacking the original body of the function,
395 -- but *with* lambdas
398 = mk_ww_str wrap_args `thenUs` \ (work_args, wrap_fn, work_fn) ->
399 returnUs ( [idDemandInfo v | v <- work_args, isId v],
400 \ wrapper_body -> wrap_fn (mkVarApps wrapper_body work_args),
401 \ worker_body -> mkLams work_args (work_fn worker_body))
406 \ wrapper_body -> wrapper_body,
407 \ worker_body -> worker_body)
412 = mk_ww_str ds `thenUs` \ (worker_args, wrap_fn, work_fn) ->
413 returnUs (arg : worker_args, wrap_fn, work_fn)
416 = case idDemandInfo arg of
420 mk_ww_str ds `thenUs` \ (worker_args, wrap_fn, work_fn) ->
421 returnUs (worker_args, wrap_fn, mk_absent_let arg . work_fn)
424 WwUnpack new_or_data True cs ->
425 getUniquesUs (length inst_con_arg_tys) `thenUs` \ uniqs ->
427 unpk_args = zipWith mk_ww_local uniqs inst_con_arg_tys
428 unpk_args_w_ds = zipWithEqual "mk_ww_str" set_worker_arg_info unpk_args cs
430 mk_ww_str (unpk_args_w_ds ++ ds) `thenUs` \ (worker_args, wrap_fn, work_fn) ->
431 returnUs (worker_args,
432 mk_unpk_case new_or_data arg unpk_args data_con arg_tycon . wrap_fn,
433 work_fn . mk_pk_let new_or_data arg data_con tycon_arg_tys unpk_args)
435 (arg_tycon, tycon_arg_tys, data_con, inst_con_arg_tys) = splitProductType "mk_ww_str" (idType arg)
439 mk_ww_str ds `thenUs` \ (worker_args, wrap_fn, work_fn) ->
440 returnUs (arg : worker_args, wrap_fn, work_fn)
442 -- If the wrapper argument is a one-shot lambda, then
443 -- so should (all) the corresponding worker arguments be
444 -- This bites when we do w/w on a case join point
445 set_worker_arg_info worker_arg demand = set_one_shot (setIdDemandInfo worker_arg demand)
447 set_one_shot | isOneShotLambda arg = setOneShotLambda
448 | otherwise = \x -> x
452 %************************************************************************
454 \subsection{CPR stuff}
456 %************************************************************************
459 @mkWWcpr@ takes the worker/wrapper pair produced from the strictness
460 info and adds in the CPR transformation. The worker returns an
461 unboxed tuple containing non-CPR components. The wrapper takes this
462 tuple and re-produces the correct structured output.
464 The non-CPR results appear ordered in the unboxed tuple as if by a
465 left-to-right traversal of the result structure.
469 mkWWcpr :: Type -- function body type
470 -> CprInfo -- CPR analysis results
471 -> UniqSM (CoreExpr -> CoreExpr, -- New wrapper
472 CoreExpr -> CoreExpr, -- New worker
473 Type) -- Type of worker's body
475 mkWWcpr body_ty NoCPRInfo
476 = returnUs (id, id, body_ty) -- Must be just the strictness transf.
478 mkWWcpr body_ty ReturnsCPR
479 | not (isAlgType body_ty)
480 = WARN( True, text "mkWWcpr: non-algebraic body type" <+> ppr body_ty )
481 returnUs (id, id, body_ty)
483 | n_con_args == 1 && isUnLiftedType con_arg_ty1
484 -- Special case when there is a single result of unlifted type
485 = getUniquesUs 2 `thenUs` \ [work_uniq, arg_uniq] ->
487 work_wild = mk_ww_local work_uniq body_ty
488 arg = mk_ww_local arg_uniq con_arg_ty1
490 returnUs (\ wkr_call -> Case wkr_call arg [(DEFAULT, [], mkConApp data_con (map Type tycon_arg_tys ++ [Var arg]))],
491 \ body -> Case body work_wild [(DataAlt data_con, [arg], Var arg)],
494 | otherwise -- The general case
495 = getUniquesUs (n_con_args + 2) `thenUs` \ uniqs ->
497 (wrap_wild : work_wild : args) = zipWith mk_ww_local uniqs (ubx_tup_ty : body_ty : con_arg_tys)
498 arg_vars = map Var args
499 ubx_tup_con = tupleCon Unboxed n_con_args
500 ubx_tup_ty = exprType ubx_tup_app
501 ubx_tup_app = mkConApp ubx_tup_con (map Type con_arg_tys ++ arg_vars)
502 con_app = mkConApp data_con (map Type tycon_arg_tys ++ arg_vars)
504 returnUs (\ wkr_call -> Case wkr_call wrap_wild [(DataAlt ubx_tup_con, args, con_app)],
505 \ body -> Case body work_wild [(DataAlt data_con, args, ubx_tup_app)],
508 (tycon, tycon_arg_tys, data_con, con_arg_tys) = splitProductType "mkWWcpr" body_ty
509 n_con_args = length con_arg_tys
510 con_arg_ty1 = head con_arg_tys
514 %************************************************************************
516 \subsection{Utilities}
518 %************************************************************************
522 mk_absent_let arg body
523 | not (isUnLiftedType arg_ty)
524 = Let (NonRec arg (mkTyApps (Var aBSENT_ERROR_ID) [arg_ty])) body
526 = panic "WwLib: haven't done mk_absent_let for primitives yet"
530 mk_unpk_case NewType arg unpk_args boxing_con boxing_tycon body
531 -- A newtype! Use a coercion not a case
532 = ASSERT( null other_args )
533 Case (Note (Coerce (idType unpk_arg) (idType arg)) (Var arg))
534 (sanitiseCaseBndr unpk_arg)
537 (unpk_arg:other_args) = unpk_args
539 mk_unpk_case DataType arg unpk_args boxing_con boxing_tycon body
542 (sanitiseCaseBndr arg)
543 [(DataAlt boxing_con, unpk_args, body)]
545 sanitiseCaseBndr :: Id -> Id
546 -- The argument we are scrutinising has the right type to be
547 -- a case binder, so it's convenient to re-use it for that purpose.
548 -- But we *must* throw away all its IdInfo. In particular, the argument
549 -- will have demand info on it, and that demand info may be incorrect for
550 -- the case binder. e.g. case ww_arg of ww_arg { I# x -> ... }
551 -- Quite likely ww_arg isn't used in '...'. The case may get discarded
552 -- if the case binder says "I'm demanded". This happened in a situation
553 -- like (x+y) `seq` ....
554 sanitiseCaseBndr id = id `setIdInfo` vanillaIdInfo
556 mk_pk_let NewType arg boxing_con con_tys unpk_args body
557 = ASSERT( null other_args )
558 Let (NonRec arg (Note (Coerce (idType arg) (idType unpk_arg)) (Var unpk_arg))) body
560 (unpk_arg:other_args) = unpk_args
562 mk_pk_let DataType arg boxing_con con_tys unpk_args body
563 = Let (NonRec arg (mkConApp boxing_con con_args)) body
565 con_args = map Type con_tys ++ map Var unpk_args
568 mk_ww_local uniq ty = mkSysLocal SLIT("ww") uniq ty