2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[SpecConstr]{Specialise over constructors}
11 #include "HsVersions.h"
14 import CoreLint ( showPass, endPass )
15 import CoreUtils ( exprType, eqExpr, mkPiTypes )
16 import CoreFVs ( exprsFreeVars )
17 import CoreTidy ( pprTidyIdRules )
18 import WwLib ( mkWorkerArgs )
19 import DataCon ( dataConRepArity )
20 import Type ( tyConAppArgs )
21 import Id ( Id, idName, idType, idSpecialisation,
23 mkUserLocal, mkSysLocal )
27 import Name ( nameOccName, nameSrcLoc )
28 import Rules ( addIdSpecialisations )
29 import OccName ( mkSpecOcc )
30 import ErrUtils ( dumpIfSet_dyn )
31 import CmdLineOpts ( DynFlags, DynFlag(..) )
32 import BasicTypes ( Activation(..) )
35 import Maybes ( orElse )
36 import Util ( mapAccumL, lengthAtLeast, notNull )
37 import List ( nubBy, partition )
42 -----------------------------------------------------
44 -----------------------------------------------------
49 drop n (x:xs) = drop (n-1) xs
51 After the first time round, we could pass n unboxed. This happens in
52 numerical code too. Here's what it looks like in Core:
54 drop n xs = case xs of
59 _ -> drop (I# (n# -# 1#)) xs
61 Notice that the recursive call has an explicit constructor as argument.
62 Noticing this, we can make a specialised version of drop
64 RULE: drop (I# n#) xs ==> drop' n# xs
66 drop' n# xs = let n = I# n# in ...orig RHS...
68 Now the simplifier will apply the specialisation in the rhs of drop', giving
70 drop' n# xs = case xs of
74 _ -> drop (n# -# 1#) xs
78 We'd also like to catch cases where a parameter is carried along unchanged,
79 but evaluated each time round the loop:
81 f i n = if i>0 || i>n then i else f (i*2) n
83 Here f isn't strict in n, but we'd like to avoid evaluating it each iteration.
84 In Core, by the time we've w/wd (f is strict in i) we get
86 f i# n = case i# ># 0 of
88 True -> case n of n' { I# n# ->
91 True -> f (i# *# 2#) n'
93 At the call to f, we see that the argument, n is know to be (I# n#),
94 and n is evaluated elsewhere in the body of f, so we can play the same
95 trick as above. However we don't want to do that if the boxed version
96 of n is needed (else we'd avoid the eval but pay more for re-boxing n).
97 So in this case we want that the *only* uses of n are in case statements.
102 * A self-recursive function. Ignore mutual recursion for now,
103 because it's less common, and the code is simpler for self-recursion.
107 a) At a recursive call, one or more parameters is an explicit
108 constructor application
110 That same parameter is scrutinised by a case somewhere in
111 the RHS of the function
115 b) At a recursive call, one or more parameters has an unfolding
116 that is an explicit constructor application
118 That same parameter is scrutinised by a case somewhere in
119 the RHS of the function
121 Those are the only uses of the parameter
124 There's a bit of a complication with type arguments. If the call
127 f p = ...f ((:) [a] x xs)...
129 then our specialised function look like
131 f_spec x xs = let p = (:) [a] x xs in ....as before....
133 This only makes sense if either
134 a) the type variable 'a' is in scope at the top of f, or
135 b) the type variable 'a' is an argument to f (and hence fs)
137 Actually, (a) may hold for value arguments too, in which case
138 we may not want to pass them. Supose 'x' is in scope at f's
139 defn, but xs is not. Then we'd like
141 f_spec xs = let p = (:) [a] x xs in ....as before....
143 Similarly (b) may hold too. If x is already an argument at the
144 call, no need to pass it again.
146 Finally, if 'a' is not in scope at the call site, we could abstract
147 it as we do the term variables:
149 f_spec a x xs = let p = (:) [a] x xs in ...as before...
151 So the grand plan is:
153 * abstract the call site to a constructor-only pattern
154 e.g. C x (D (f p) (g q)) ==> C s1 (D s2 s3)
156 * Find the free variables of the abstracted pattern
158 * Pass these variables, less any that are in scope at
162 NOTICE that we only abstract over variables that are not in scope,
163 so we're in no danger of shadowing variables used in "higher up"
167 %************************************************************************
169 \subsection{Top level wrapper stuff}
171 %************************************************************************
174 specConstrProgram :: DynFlags -> UniqSupply -> [CoreBind] -> IO [CoreBind]
175 specConstrProgram dflags us binds
177 showPass dflags "SpecConstr"
179 let (binds', _) = initUs us (go emptyScEnv binds)
181 endPass dflags "SpecConstr" Opt_D_dump_spec binds'
183 dumpIfSet_dyn dflags Opt_D_dump_rules "Top-level specialisations"
184 (vcat (map pprTidyIdRules (concat (map bindersOf binds'))))
188 go env [] = returnUs []
189 go env (bind:binds) = scBind env bind `thenUs` \ (env', _, bind') ->
190 go env' binds `thenUs` \ binds' ->
191 returnUs (bind' : binds')
195 %************************************************************************
197 \subsection{Environment: goes downwards}
199 %************************************************************************
202 data ScEnv = SCE { scope :: VarEnv HowBound,
203 -- Binds all non-top-level variables in scope
208 type ConstrEnv = IdEnv (AltCon, [CoreArg])
209 -- Variables known to be bound to a constructor
210 -- in a particular case alternative
212 emptyScEnv = SCE { scope = emptyVarEnv, cons = emptyVarEnv }
214 data HowBound = RecFun -- These are the recursive functions for which
215 -- we seek interesting call patterns
217 | RecArg -- These are those functions' arguments; we are
218 -- interested to see if those arguments are scrutinised
220 | Other -- We track all others so we know what's in scope
221 -- This is used in spec_one to check what needs to be
222 -- passed as a parameter and what is in scope at the
223 -- function definition site
225 instance Outputable HowBound where
226 ppr RecFun = text "RecFun"
227 ppr RecArg = text "RecArg"
228 ppr Other = text "Other"
230 lookupScopeEnv env v = lookupVarEnv (scope env) v
232 extendBndrs env bndrs = env { scope = extendVarEnvList (scope env) [(b,Other) | b <- bndrs] }
233 extendBndr env bndr = env { scope = extendVarEnv (scope env) bndr Other }
238 -- we want to bind b, and perhaps scrut too, to (C x y)
239 extendCaseBndrs :: ScEnv -> Id -> CoreExpr -> AltCon -> [Var] -> ScEnv
240 extendCaseBndrs env case_bndr scrut DEFAULT alt_bndrs
241 = extendBndrs env (case_bndr : alt_bndrs)
243 extendCaseBndrs env case_bndr scrut con alt_bndrs
245 Var v -> -- Bind the scrutinee in the ConstrEnv if it's a variable
246 -- Also forget if the scrutinee is a RecArg, because we're
247 -- now in the branch of a case, and we don't want to
248 -- record a non-scrutinee use of v if we have
249 -- case v of { (a,b) -> ...(f v)... }
250 SCE { scope = extendVarEnv (scope env1) v Other,
251 cons = extendVarEnv (cons env1) v (con,args) }
255 env1 = SCE { scope = extendVarEnvList (scope env) [(b,Other) | b <- case_bndr : alt_bndrs],
256 cons = extendVarEnv (cons env) case_bndr (con,args) }
258 args = map Type (tyConAppArgs (idType case_bndr)) ++
259 map varToCoreExpr alt_bndrs
261 -- When we encounter a recursive function binding
263 -- we want to extend the scope env with bindings
264 -- that record that f is a RecFn and x,y are RecArgs
265 extendRecBndr env fn bndrs
266 = env { scope = scope env `extendVarEnvList`
267 ((fn,RecFun): [(bndr,RecArg) | bndr <- bndrs]) }
271 %************************************************************************
273 \subsection{Usage information: flows upwards}
275 %************************************************************************
280 calls :: !(IdEnv ([Call])), -- Calls
281 -- The functions are a subset of the
282 -- RecFuns in the ScEnv
284 occs :: !(IdEnv ArgOcc) -- Information on argument occurrences
285 } -- The variables are a subset of the
286 -- RecArg in the ScEnv
288 type Call = (ConstrEnv, [CoreArg])
289 -- The arguments of the call, together with the
290 -- env giving the constructor bindings at the call site
292 nullUsage = SCU { calls = emptyVarEnv, occs = emptyVarEnv }
294 combineUsage u1 u2 = SCU { calls = plusVarEnv_C (++) (calls u1) (calls u2),
295 occs = plusVarEnv_C combineOcc (occs u1) (occs u2) }
297 combineUsages [] = nullUsage
298 combineUsages us = foldr1 combineUsage us
300 data ArgOcc = CaseScrut
304 instance Outputable ArgOcc where
305 ppr CaseScrut = ptext SLIT("case-scrut")
306 ppr OtherOcc = ptext SLIT("other-occ")
307 ppr Both = ptext SLIT("case-scrut and other")
309 combineOcc CaseScrut CaseScrut = CaseScrut
310 combineOcc OtherOcc OtherOcc = OtherOcc
311 combineOcc _ _ = Both
315 %************************************************************************
317 \subsection{The main recursive function}
319 %************************************************************************
321 The main recursive function gathers up usage information, and
322 creates specialised versions of functions.
325 scExpr :: ScEnv -> CoreExpr -> UniqSM (ScUsage, CoreExpr)
326 -- The unique supply is needed when we invent
327 -- a new name for the specialised function and its args
329 scExpr env e@(Type t) = returnUs (nullUsage, e)
330 scExpr env e@(Lit l) = returnUs (nullUsage, e)
331 scExpr env e@(Var v) = returnUs (varUsage env v OtherOcc, e)
332 scExpr env (Note n e) = scExpr env e `thenUs` \ (usg,e') ->
333 returnUs (usg, Note n e')
334 scExpr env (Lam b e) = scExpr (extendBndr env b) e `thenUs` \ (usg,e') ->
335 returnUs (usg, Lam b e')
337 scExpr env (Case scrut b alts)
338 = sc_scrut scrut `thenUs` \ (scrut_usg, scrut') ->
339 mapAndUnzipUs sc_alt alts `thenUs` \ (alts_usgs, alts') ->
340 returnUs (combineUsages alts_usgs `combineUsage` scrut_usg,
343 sc_scrut e@(Var v) = returnUs (varUsage env v CaseScrut, e)
344 sc_scrut e = scExpr env e
346 sc_alt (con,bs,rhs) = scExpr env1 rhs `thenUs` \ (usg,rhs') ->
347 returnUs (usg, (con,bs,rhs'))
349 env1 = extendCaseBndrs env b scrut con bs
351 scExpr env (Let bind body)
352 = scBind env bind `thenUs` \ (env', bind_usg, bind') ->
353 scExpr env' body `thenUs` \ (body_usg, body') ->
354 returnUs (bind_usg `combineUsage` body_usg, Let bind' body')
356 scExpr env e@(App _ _)
358 (fn, args) = collectArgs e
360 mapAndUnzipUs (scExpr env) args `thenUs` \ (usgs, args') ->
362 arg_usg = combineUsages usgs
363 fn_usg | Var f <- fn,
364 Just RecFun <- lookupScopeEnv env f
365 = SCU { calls = unitVarEnv f [(cons env, args)],
370 returnUs (arg_usg `combineUsage` fn_usg, mkApps fn args')
371 -- Don't bother to look inside fn;
372 -- it's almost always a variable
374 ----------------------
375 scBind :: ScEnv -> CoreBind -> UniqSM (ScEnv, ScUsage, CoreBind)
376 scBind env (Rec [(fn,rhs)])
378 = scExpr env_fn_body body `thenUs` \ (usg, body') ->
380 SCU { calls = calls, occs = occs } = usg
382 specialise env fn bndrs body usg `thenUs` \ (rules, spec_prs) ->
383 returnUs (extendBndr env fn, -- For the body of the letrec, just
384 -- extend the env with Other to record
385 -- that it's in scope; no funny RecFun business
386 SCU { calls = calls `delVarEnv` fn, occs = occs `delVarEnvList` val_bndrs},
387 Rec ((fn `addIdSpecialisations` rules, mkLams bndrs body') : spec_prs))
389 (bndrs,body) = collectBinders rhs
390 val_bndrs = filter isId bndrs
391 env_fn_body = extendRecBndr env fn bndrs
394 = mapAndUnzipUs do_one prs `thenUs` \ (usgs, prs') ->
395 returnUs (extendBndrs env (map fst prs), combineUsages usgs, Rec prs')
397 do_one (bndr,rhs) = scExpr env rhs `thenUs` \ (usg, rhs') ->
398 returnUs (usg, (bndr,rhs'))
400 scBind env (NonRec bndr rhs)
401 = scExpr env rhs `thenUs` \ (usg, rhs') ->
402 returnUs (extendBndr env bndr, usg, NonRec bndr rhs')
404 ----------------------
406 | Just RecArg <- lookupScopeEnv env v = SCU { calls = emptyVarEnv,
407 occs = unitVarEnv v use }
408 | otherwise = nullUsage
412 %************************************************************************
414 \subsection{The specialiser}
416 %************************************************************************
421 -> [CoreBndr] -> CoreExpr -- Its RHS
422 -> ScUsage -- Info on usage
423 -> UniqSM ([CoreRule], -- Rules
424 [(Id,CoreExpr)]) -- Bindings
426 specialise env fn bndrs body (SCU {calls=calls, occs=occs})
427 = getUs `thenUs` \ us ->
429 all_calls = lookupVarEnv calls fn `orElse` []
431 good_calls :: [[CoreArg]]
433 | (con_env, call_args) <- all_calls,
434 call_args `lengthAtLeast` n_bndrs, -- App is saturated
435 let call = (bndrs `zip` call_args),
436 any (good_arg con_env occs) call, -- At least one arg is a constr app
437 let (_, pats) = argsToPats con_env us call_args
440 mapAndUnzipUs (spec_one env fn (mkLams bndrs body))
441 (nubBy same_call good_calls `zip` [1..])
443 n_bndrs = length bndrs
444 same_call as1 as2 = and (zipWith eqExpr as1 as2)
446 ---------------------
447 good_arg :: ConstrEnv -> IdEnv ArgOcc -> (CoreBndr, CoreArg) -> Bool
448 good_arg con_env arg_occs (bndr, arg)
449 = case is_con_app_maybe con_env arg of
450 Just _ -> bndr_usg_ok arg_occs bndr arg
453 bndr_usg_ok :: IdEnv ArgOcc -> Var -> CoreArg -> Bool
454 bndr_usg_ok arg_occs bndr arg
455 = case lookupVarEnv arg_occs bndr of
456 Just CaseScrut -> True -- Used only by case scrutiny
457 Just Both -> case arg of -- Used by case and elsewhere
458 App _ _ -> True -- so the arg should be an explicit con app
460 other -> False -- Not used, or used wonkily
463 ---------------------
466 -> CoreExpr -- Rhs of the original function
468 -> UniqSM (CoreRule, (Id,CoreExpr)) -- Rule and binding
470 -- spec_one creates a specialised copy of the function, together
471 -- with a rule for using it. I'm very proud of how short this
472 -- function is, considering what it does :-).
478 f = /\b \y::[(a,b)] -> ....f (b,c) ((:) (a,(b,c)) (x,v) (h w))...
479 [c::*, v::(b,c) are presumably bound by the (...) part]
481 f_spec = /\ b c \ v::(b,c) hw::[(a,(b,c))] ->
482 (...entire RHS of f...) (b,c) ((:) (a,(b,c)) (x,v) hw)
484 RULE: forall b::* c::*, -- Note, *not* forall a, x
488 f (b,c) ((:) (a,(b,c)) (x,v) hw) = f_spec b c v hw
491 spec_one env fn rhs (pats, rule_number)
492 = getUniqueUs `thenUs` \ spec_uniq ->
495 fn_loc = nameSrcLoc fn_name
496 spec_occ = mkSpecOcc (nameOccName fn_name)
497 pat_fvs = varSetElems (exprsFreeVars pats)
498 vars_to_bind = filter not_avail pat_fvs
499 not_avail v = not (v `elemVarEnv` scope env)
500 -- Put the type variables first; the type of a term
501 -- variable may mention a type variable
502 (tvs, ids) = partition isTyVar vars_to_bind
504 spec_body = mkApps rhs pats
505 body_ty = exprType spec_body
507 (spec_lam_args, spec_call_args) = mkWorkerArgs bndrs body_ty
508 -- Usual w/w hack to avoid generating
509 -- a spec_rhs of unlifted type and no args
511 rule_name = _PK_ ("SC:" ++ showSDoc (ppr fn <> int rule_number))
512 spec_rhs = mkLams spec_lam_args spec_body
513 spec_id = mkUserLocal spec_occ spec_uniq (mkPiTypes spec_lam_args body_ty) fn_loc
514 rule = Rule rule_name specConstrActivation
515 bndrs pats (mkVarApps (Var spec_id) spec_call_args)
517 returnUs (rule, (spec_id, spec_rhs))
519 -- In which phase should the specialise-constructor rules be active?
520 -- Originally I made them always-active, but Manuel found that
521 -- this defeated some clever user-written rules. So Plan B
522 -- is to make them active only in Phase 0; after all, currently,
523 -- the specConstr transformation is only run after the simplifier
524 -- has reached Phase 0. In general one would want it to be
525 -- flag-controllable, but for now I'm leaving it baked in
527 specConstrActivation :: Activation
528 specConstrActivation = ActiveAfter 0 -- Baked in; see comments above
531 %************************************************************************
533 \subsection{Argument analysis}
535 %************************************************************************
537 This code deals with analysing call-site arguments to see whether
538 they are constructor applications.
541 -- argToPat takes an actual argument, and returns an abstracted
542 -- version, consisting of just the "constructor skeleton" of the
543 -- argument, with non-constructor sub-expression replaced by new
544 -- placeholder variables. For example:
545 -- C a (D (f x) (g y)) ==> C p1 (D p2 p3)
547 argToPat :: ConstrEnv -> UniqSupply -> CoreArg -> (UniqSupply, CoreExpr)
548 argToPat env us (Type ty)
552 | Just (dc,args) <- is_con_app_maybe env arg
554 (us',args') = argsToPats env us args
556 (us', mk_con_app dc args')
558 argToPat env us (Var v) -- Don't uniqify existing vars,
559 = (us, Var v) -- so that we can spot when we pass them twice
562 = (us1, Var (mkSysLocal FSLIT("sc") (uniqFromSupply us2) (exprType arg)))
564 (us1,us2) = splitUniqSupply us
566 argsToPats :: ConstrEnv -> UniqSupply -> [CoreArg] -> (UniqSupply, [CoreExpr])
567 argsToPats env us args = mapAccumL (argToPat env) us args
572 is_con_app_maybe :: ConstrEnv -> CoreExpr -> Maybe (AltCon, [CoreExpr])
573 is_con_app_maybe env (Var v)
575 -- You might think we could look in the idUnfolding here
576 -- but that doesn't take account of which branch of a
577 -- case we are in, which is the whole point
579 is_con_app_maybe env (Lit lit)
580 = Just (LitAlt lit, [])
582 is_con_app_maybe env expr
583 = case collectArgs expr of
584 (Var fun, args) | Just con <- isDataConId_maybe fun,
585 args `lengthAtLeast` dataConRepArity con
586 -- Might be > because the arity excludes type args
587 -> Just (DataAlt con,args)
591 mk_con_app :: AltCon -> [CoreArg] -> CoreExpr
592 mk_con_app (LitAlt lit) [] = Lit lit
593 mk_con_app (DataAlt con) args = mkConApp con args