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}
7 module WwLib ( mkWwBodies, mkWWstr, mkWorkerArgs ) where
9 #include "HsVersions.h"
12 import CoreUtils ( exprType )
13 import Id ( Id, idType, mkSysLocal, idNewDemandInfo, setIdNewDemandInfo,
14 isOneShotLambda, setOneShotLambda, setIdUnfolding,
17 import IdInfo ( vanillaIdInfo )
18 import DataCon ( splitProductType_maybe, splitProductType )
19 import NewDemand ( Demand(..), DmdResult(..), Demands(..) )
20 import MkId ( realWorldPrimId, voidArgId, eRROR_CSTRING_ID )
21 import TysWiredIn ( tupleCon )
22 import Type ( Type, isUnLiftedType, mkFunTys,
23 splitForAllTys, splitFunTys, splitNewType_maybe, isAlgType
25 import Literal ( Literal(MachStr) )
26 import BasicTypes ( Boxity(..) )
27 import Var ( Var, isId )
28 import UniqSupply ( returnUs, thenUs, getUniquesUs, UniqSM )
29 import Util ( zipWithEqual )
31 import List ( zipWith4 )
35 %************************************************************************
37 \subsection[mkWrapperAndWorker]{@mkWrapperAndWorker@}
39 %************************************************************************
41 Here's an example. The original function is:
44 g :: forall a . Int -> [a] -> a
52 From this, we want to produce:
54 -- wrapper (an unfolding)
55 g :: forall a . Int -> [a] -> a
60 -- call the worker; don't forget the type args!
63 $wg :: forall a . Int# -> [a] -> a
65 $wg = /\ a -> \ x# ys ->
69 case x of -- note: body of g moved intact
74 Something we have to be careful about: Here's an example:
77 -- "f" strictness: U(P)U(P)
78 f (I# a) (I# b) = a +# b
80 g = f -- "g" strictness same as "f"
83 \tr{f} will get a worker all nice and friendly-like; that's good.
84 {\em But we don't want a worker for \tr{g}}, even though it has the
85 same strictness as \tr{f}. Doing so could break laziness, at best.
87 Consequently, we insist that the number of strictness-info items is
88 exactly the same as the number of lambda-bound arguments. (This is
89 probably slightly paranoid, but OK in practice.) If it isn't the
90 same, we ``revise'' the strictness info, so that we won't propagate
91 the unusable strictness-info into the interfaces.
94 %************************************************************************
96 \subsection{The worker wrapper core}
98 %************************************************************************
100 @mkWwBodies@ is called when doing the worker/wrapper split inside a module.
103 mkWwBodies :: Type -- Type of original function
104 -> [Demand] -- Strictness of original function
105 -> DmdResult -- Info about function result
106 -> [Bool] -- One-shot-ness of the function
107 -> UniqSM ([Demand], -- Demands for worker (value) args
108 Id -> CoreExpr, -- Wrapper body, lacking only the worker Id
109 CoreExpr -> CoreExpr) -- Worker body, lacking the original function rhs
111 -- wrap_fn_args E = \x y -> E
112 -- work_fn_args E = E x y
114 -- wrap_fn_str E = case x of { (a,b) ->
115 -- case a of { (a1,a2) ->
117 -- work_fn_str E = \a2 a2 b y ->
118 -- let a = (a1,a2) in
122 mkWwBodies fun_ty demands res_info one_shots
123 = mkWWargs fun_ty demands one_shots' `thenUs` \ (wrap_args, wrap_fn_args, work_fn_args, res_ty) ->
124 mkWWcpr res_ty res_info `thenUs` \ (wrap_fn_cpr, work_fn_cpr, cpr_res_ty) ->
125 mkWWstr wrap_args `thenUs` \ (work_args, wrap_fn_str, work_fn_str) ->
127 (work_lam_args, work_call_args) = mkWorkerArgs work_args cpr_res_ty
129 returnUs ([idNewDemandInfo v | v <- work_args, isId v],
130 Note InlineMe . wrap_fn_args . wrap_fn_cpr . wrap_fn_str . applyToVars work_call_args . Var,
131 mkLams work_lam_args . work_fn_str . work_fn_cpr . work_fn_args)
132 -- We use an INLINE unconditionally, even if the wrapper turns out to be
133 -- something trivial like
135 -- f = __inline__ (coerce T fw)
136 -- The point is to propagate the coerce to f's call sites, so even though
137 -- f's RHS is now trivial (size 1) we still want the __inline__ to prevent
138 -- fw from being inlined into f's RHS
140 one_shots' = one_shots ++ repeat False
144 %************************************************************************
146 \subsection{Making wrapper args}
148 %************************************************************************
150 During worker-wrapper stuff we may end up with an unlifted thing
151 which we want to let-bind without losing laziness. So we
152 add a void argument. E.g.
154 f = /\a -> \x y z -> E::Int# -- E does not mentione x,y,z
156 fw = /\ a -> \void -> E
157 f = /\ a -> \x y z -> fw realworld
159 We use the state-token type which generates no code.
162 mkWorkerArgs :: [Var]
163 -> Type -- Type of body
164 -> ([Var], -- Lambda bound args
165 [Var]) -- Args at call site
166 mkWorkerArgs args res_ty
167 | any isId args || not (isUnLiftedType res_ty)
170 = (args ++ [voidArgId], args ++ [realWorldPrimId])
174 %************************************************************************
176 \subsection{Coercion stuff}
178 %************************************************************************
181 We really want to "look through" coerces.
182 Reason: I've seen this situation:
184 let f = coerce T (\s -> E)
190 If only we w/w'd f, we'd get
191 let f = coerce T (\s -> fw s)
195 Now we'll inline f to get
203 Now we'll see that fw has arity 1, and will arity expand
204 the \x to get what we want.
207 -- mkWWargs is driven off the function type and arity.
208 -- It chomps bites off foralls, arrows, newtypes
209 -- and keeps repeating that until it's satisfied the supplied arity
213 -> [Bool] -- True for a one-shot arg; ** may be infinite **
214 -> UniqSM ([Var], -- Wrapper args
215 CoreExpr -> CoreExpr, -- Wrapper fn
216 CoreExpr -> CoreExpr, -- Worker fn
217 Type) -- Type of wrapper body
219 mkWWargs fun_ty demands one_shots
220 | Just rep_ty <- splitNewType_maybe fun_ty
221 -- The newtype case is for when the function has
222 -- a recursive newtype after the arrow (rare)
223 -- We check for arity >= 0 to avoid looping in the case
224 -- of a function whose type is, in effect, infinite
225 -- [Arity is driven by looking at the term, not just the type.]
227 -- It's also important when we have a function returning (say) a pair
228 -- wrapped in a recursive newtype, at least if CPR analysis can look
229 -- through such newtypes, which it probably can since they are
231 = mkWWargs rep_ty demands one_shots `thenUs` \ (wrap_args, wrap_fn_args, work_fn_args, res_ty) ->
233 Note (Coerce fun_ty rep_ty) . wrap_fn_args,
234 work_fn_args . Note (Coerce rep_ty fun_ty),
238 = getUniquesUs `thenUs` \ wrap_uniqs ->
240 (tyvars, tau) = splitForAllTys fun_ty
241 (arg_tys, body_ty) = splitFunTys tau
243 n_demands = length demands
244 n_arg_tys = length arg_tys
245 n_args = n_demands `min` n_arg_tys
247 new_fun_ty = mkFunTys (drop n_demands arg_tys) body_ty
248 new_demands = drop n_arg_tys demands
249 new_one_shots = drop n_args one_shots
251 val_args = zipWith4 mk_wrap_arg wrap_uniqs arg_tys demands one_shots
252 wrap_args = tyvars ++ val_args
254 {- ASSERT( not (null tyvars) || not (null arg_tys) ) -}
255 if (null tyvars) && (null arg_tys) then
256 pprTrace "mkWWargs" (ppr fun_ty $$ ppr demands)
257 returnUs ([], id, id, fun_ty)
262 new_one_shots `thenUs` \ (more_wrap_args, wrap_fn_args, work_fn_args, res_ty) ->
264 returnUs (wrap_args ++ more_wrap_args,
265 mkLams wrap_args . wrap_fn_args,
266 work_fn_args . applyToVars wrap_args,
270 = returnUs ([], id, id, fun_ty)
273 applyToVars :: [Var] -> CoreExpr -> CoreExpr
274 applyToVars vars fn = mkVarApps fn vars
276 mk_wrap_arg uniq ty dmd one_shot
277 = set_one_shot one_shot (setIdNewDemandInfo (mkSysLocal SLIT("w") uniq ty) dmd)
279 set_one_shot True id = setOneShotLambda id
280 set_one_shot False id = id
284 %************************************************************************
286 \subsection{Strictness stuff}
288 %************************************************************************
291 mkWWstr :: [Var] -- Wrapper args; have their demand info on them
292 -- *Includes type variables*
293 -> UniqSM ([Var], -- Worker args
294 CoreExpr -> CoreExpr, -- Wrapper body, lacking the worker call
295 -- and without its lambdas
296 -- This fn adds the unboxing
298 CoreExpr -> CoreExpr) -- Worker body, lacking the original body of the function,
299 -- and lacking its lambdas.
300 -- This fn does the reboxing
302 ----------------------
305 ----------------------
307 = returnUs ([], nop_fn, nop_fn)
310 = mkWWstr_one arg `thenUs` \ (args1, wrap_fn1, work_fn1) ->
311 mkWWstr args `thenUs` \ (args2, wrap_fn2, work_fn2) ->
312 returnUs (args1 ++ args2, wrap_fn1 . wrap_fn2, work_fn1 . work_fn2)
315 ----------------------
316 -- mkWWstr_one wrap_arg = (work_args, wrap_fn, work_fn)
317 -- * wrap_fn assumes wrap_arg is in scope,
318 -- brings into scope work_args (via cases)
319 -- * work_fn assumes work_args are in scope, a
320 -- brings into scope wrap_arg (via lets)
324 = returnUs ([arg], nop_fn, nop_fn)
327 = case idNewDemandInfo arg of
329 -- Absent case. We don't deal with absence for unlifted types,
330 -- though, because it's not so easy to manufacture a placeholder
331 -- We'll see if this turns out to be a problem
332 Abs | not (isUnLiftedType (idType arg)) ->
333 returnUs ([], nop_fn, mk_absent_let arg)
337 | Just (arg_tycon, tycon_arg_tys, data_con, inst_con_arg_tys)
338 <- splitProductType_maybe (idType arg)
339 -> getUniquesUs `thenUs` \ uniqs ->
341 unpk_args = zipWith mk_ww_local uniqs inst_con_arg_tys
342 unpk_args_w_ds = zipWithEqual "mkWWstr" set_worker_arg_info unpk_args cs
343 unbox_fn = mk_unpk_case arg unpk_args data_con arg_tycon
344 rebox_fn = Let (NonRec arg con_app)
345 con_app = mkConApp data_con (map Type tycon_arg_tys ++ map Var unpk_args)
347 mkWWstr unpk_args_w_ds `thenUs` \ (worker_args, wrap_fn, work_fn) ->
348 returnUs (worker_args, unbox_fn . wrap_fn, work_fn . rebox_fn)
349 -- Don't pass the arg, rebox instead
351 -- `seq` demand; evaluate in wrapper in the hope
352 -- of dropping seqs in the worker
355 arg_w_unf = arg `setIdUnfolding` mkOtherCon []
356 -- Tell the worker arg that it's sure to be evaluated
357 -- so that internal seqs can be dropped
359 returnUs ([arg_w_unf], mk_seq_case arg, nop_fn)
360 -- Pass the arg, anyway, even if it is in theory discarded
363 -- x gets a (Seq Drop []) demand, but if we fail to pass it to the worker
364 -- we ABSOLUTELY MUST record that x is evaluated in the wrapper.
366 -- f x y = x `seq` fw y
367 -- fw y = let x{Evald} = error "oops" in (x `seq` y)
368 -- If we don't pin on the "Evald" flag, the seq doesn't disappear, and
369 -- we end up evaluating the absent thunk.
370 -- But the Evald flag is pretty weird, and I worry that it might disappear
371 -- during simplification, so for now I've just nuked this whole case
374 other_demand -> returnUs ([arg], nop_fn, nop_fn)
377 -- If the wrapper argument is a one-shot lambda, then
378 -- so should (all) the corresponding worker arguments be
379 -- This bites when we do w/w on a case join point
380 set_worker_arg_info worker_arg demand = set_one_shot (setIdNewDemandInfo worker_arg demand)
382 set_one_shot | isOneShotLambda arg = setOneShotLambda
383 | otherwise = \x -> x
387 %************************************************************************
389 \subsection{CPR stuff}
391 %************************************************************************
394 @mkWWcpr@ takes the worker/wrapper pair produced from the strictness
395 info and adds in the CPR transformation. The worker returns an
396 unboxed tuple containing non-CPR components. The wrapper takes this
397 tuple and re-produces the correct structured output.
399 The non-CPR results appear ordered in the unboxed tuple as if by a
400 left-to-right traversal of the result structure.
404 mkWWcpr :: Type -- function body type
405 -> DmdResult -- CPR analysis results
406 -> UniqSM (CoreExpr -> CoreExpr, -- New wrapper
407 CoreExpr -> CoreExpr, -- New worker
408 Type) -- Type of worker's body
410 mkWWcpr body_ty RetCPR
411 | not (isAlgType body_ty)
412 = WARN( True, text "mkWWcpr: non-algebraic body type" <+> ppr body_ty )
413 returnUs (id, id, body_ty)
415 | n_con_args == 1 && isUnLiftedType con_arg_ty1
416 -- Special case when there is a single result of unlifted type
417 = getUniquesUs `thenUs` \ (work_uniq : arg_uniq : _) ->
419 work_wild = mk_ww_local work_uniq body_ty
420 arg = mk_ww_local arg_uniq con_arg_ty1
422 returnUs (\ wkr_call -> Case wkr_call arg [(DEFAULT, [], mkConApp data_con (map Type tycon_arg_tys ++ [Var arg]))],
423 \ body -> workerCase body work_wild [(DataAlt data_con, [arg], Var arg)],
426 | otherwise -- The general case
427 = getUniquesUs `thenUs` \ uniqs ->
429 (wrap_wild : work_wild : args) = zipWith mk_ww_local uniqs (ubx_tup_ty : body_ty : con_arg_tys)
430 arg_vars = map Var args
431 ubx_tup_con = tupleCon Unboxed n_con_args
432 ubx_tup_ty = exprType ubx_tup_app
433 ubx_tup_app = mkConApp ubx_tup_con (map Type con_arg_tys ++ arg_vars)
434 con_app = mkConApp data_con (map Type tycon_arg_tys ++ arg_vars)
436 returnUs (\ wkr_call -> Case wkr_call wrap_wild [(DataAlt ubx_tup_con, args, con_app)],
437 \ body -> workerCase body work_wild [(DataAlt data_con, args, ubx_tup_app)],
440 (_, tycon_arg_tys, data_con, con_arg_tys) = splitProductType "mkWWcpr" body_ty
441 n_con_args = length con_arg_tys
442 con_arg_ty1 = head con_arg_tys
444 mkWWcpr body_ty other -- No CPR info
445 = returnUs (id, id, body_ty)
447 -- If the original function looked like
448 -- f = \ x -> _scc_ "foo" E
450 -- then we want the CPR'd worker to look like
451 -- \ x -> _scc_ "foo" (case E of I# x -> x)
452 -- and definitely not
453 -- \ x -> case (_scc_ "foo" E) of I# x -> x)
455 -- This transform doesn't move work or allocation
456 -- from one cost centre to another
458 workerCase (Note (SCC cc) e) arg alts = Note (SCC cc) (Case e arg alts)
459 workerCase e arg alts = Case e arg alts
463 %************************************************************************
465 \subsection{Utilities}
467 %************************************************************************
471 mk_absent_let arg body
472 | not (isUnLiftedType arg_ty)
473 = Let (NonRec arg abs_rhs) body
475 = panic "WwLib: haven't done mk_absent_let for primitives yet"
478 -- abs_rhs = mkTyApps (Var aBSENT_ERROR_ID) [arg_ty]
479 abs_rhs = mkApps (Var eRROR_CSTRING_ID) [Type arg_ty, Lit (MachStr (_PK_ msg))]
480 msg = "Oops! Entered absent arg " ++ showSDocDebug (ppr arg <+> ppr (idType arg))
482 mk_unpk_case arg unpk_args boxing_con boxing_tycon body
485 (sanitiseCaseBndr arg)
486 [(DataAlt boxing_con, unpk_args, body)]
488 mk_seq_case arg body = Case (Var arg) (sanitiseCaseBndr arg) [(DEFAULT, [], body)]
490 sanitiseCaseBndr :: Id -> Id
491 -- The argument we are scrutinising has the right type to be
492 -- a case binder, so it's convenient to re-use it for that purpose.
493 -- But we *must* throw away all its IdInfo. In particular, the argument
494 -- will have demand info on it, and that demand info may be incorrect for
495 -- the case binder. e.g. case ww_arg of ww_arg { I# x -> ... }
496 -- Quite likely ww_arg isn't used in '...'. The case may get discarded
497 -- if the case binder says "I'm demanded". This happened in a situation
498 -- like (x+y) `seq` ....
499 sanitiseCaseBndr id = id `setIdInfo` vanillaIdInfo
501 mk_ww_local uniq ty = mkSysLocal SLIT("ww") uniq ty