2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Pattern-matching bindings (HsBinds and MonoBinds)
8 Handles @HsBinds@; those at the top level require different handling,
9 in that the @Rec@/@NonRec@/etc structure is thrown away (whereas at
10 lower levels it is preserved with @let@/@letrec@s).
13 module DsBinds ( dsTopLHsBinds, dsLHsBinds, decomposeRuleLhs,
18 #include "HsVersions.h"
20 import {-# SOURCE #-} DsExpr( dsLExpr, dsExpr )
21 import {-# SOURCE #-} Match( matchWrapper )
28 import HsSyn -- lots of things
29 import CoreSyn -- lots of things
35 import TysPrim ( anyTypeOfKind )
40 import Var ( Var, TyVar, tyVarKind )
48 import BasicTypes hiding ( TopLevel )
50 import StaticFlags ( opt_DsMultiTyVar )
51 import Util ( mapSnd, mapAndUnzip, lengthExceeds )
57 %************************************************************************
59 \subsection[dsMonoBinds]{Desugaring a @MonoBinds@}
61 %************************************************************************
64 dsTopLHsBinds :: AutoScc -> LHsBinds Id -> DsM [(Id,CoreExpr)]
65 dsTopLHsBinds auto_scc binds = ds_lhs_binds auto_scc binds
67 dsLHsBinds :: LHsBinds Id -> DsM [(Id,CoreExpr)]
68 dsLHsBinds binds = ds_lhs_binds NoSccs binds
71 ------------------------
72 ds_lhs_binds :: AutoScc -> LHsBinds Id -> DsM [(Id,CoreExpr)]
73 -- scc annotation policy (see below)
74 ds_lhs_binds auto_scc binds = foldM (dsLHsBind auto_scc) [] (bagToList binds)
77 -> [(Id,CoreExpr)] -- Put this on the end (avoid quadratic append)
79 -> DsM [(Id,CoreExpr)] -- Result
80 dsLHsBind auto_scc rest (L loc bind)
81 = putSrcSpanDs loc $ dsHsBind auto_scc rest bind
84 -> [(Id,CoreExpr)] -- Put this on the end (avoid quadratic append)
86 -> DsM [(Id,CoreExpr)] -- Result
88 dsHsBind _ rest (VarBind var expr) = do
89 core_expr <- dsLExpr expr
91 -- Dictionary bindings are always VarMonoBinds, so
92 -- we only need do this here
93 core_expr' <- addDictScc var core_expr
94 return ((var, core_expr') : rest)
96 dsHsBind _ rest (FunBind { fun_id = L _ fun, fun_matches = matches,
97 fun_co_fn = co_fn, fun_tick = tick, fun_infix = inf }) = do
98 (args, body) <- matchWrapper (FunRhs (idName fun) inf) matches
99 body' <- mkOptTickBox tick body
100 rhs <- dsCoercion co_fn (return (mkLams args body'))
101 return ((fun,rhs) : rest)
103 dsHsBind _ rest (PatBind { pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) = do
104 body_expr <- dsGuarded grhss ty
105 sel_binds <- mkSelectorBinds pat body_expr
106 return (sel_binds ++ rest)
108 {- Note [Rules and inlining]
109 ~~~~~~~~~~~~~~~~~~~~~~~~~
110 Common special case: no type or dictionary abstraction
111 This is a bit less trivial than you might suppose
112 The naive way woudl be to desguar to something like
113 f_lcl = ...f_lcl... -- The "binds" from AbsBinds
114 M.f = f_lcl -- Generated from "exports"
115 But we don't want that, because if M.f isn't exported,
116 it'll be inlined unconditionally at every call site (its rhs is
117 trivial). That would be ok unless it has RULES, which would
118 thereby be completely lost. Bad, bad, bad.
120 Instead we want to generate
123 Now all is cool. The RULES are attached to M.f (by SimplCore),
124 and f_lcl is rapidly inlined away.
126 This does not happen in the same way to polymorphic binds,
127 because they desugar to
128 M.f = /\a. let f_lcl = ...f_lcl... in f_lcl
129 Although I'm a bit worried about whether full laziness might
130 float the f_lcl binding out and then inline M.f at its call site -}
132 dsHsBind auto_scc rest (AbsBinds [] [] exports binds)
133 = do { core_prs <- ds_lhs_binds NoSccs binds
134 ; let env = mkABEnv exports
135 do_one (lcl_id, rhs) | Just (_, gbl_id, _, prags) <- lookupVarEnv env lcl_id
136 = addInlinePrags prags gbl_id $
137 addAutoScc auto_scc gbl_id rhs
138 | otherwise = (lcl_id, rhs)
139 locals' = [(lcl_id, Var gbl_id) | (_, gbl_id, lcl_id, _) <- exports]
140 -- Note [Rules and inlining]
141 ; return (map do_one core_prs ++ locals' ++ rest) }
142 -- No Rec needed here (contrast the other AbsBinds cases)
143 -- because we can rely on the enclosing dsBind to wrap in Rec
146 {- Note [Abstracting over tyvars only]
147 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
148 When abstracting over type variable only (not dictionaries), we don't really need to
149 built a tuple and select from it, as we do in the general case. Instead we can take
151 AbsBinds [a,b] [ ([a,b], fg, fl, _),
159 fg = /\ab. let B in e1
160 gg = /\b. let a = () in let B in S(e2)
161 h = /\ab. let B in e3
163 where B is the *non-recursive* binding
166 h = h a b -- See (b); note shadowing!
168 Notice (a) g has a different number of type variables to f, so we must
169 use the mkArbitraryType thing to fill in the gaps.
170 We use a type-let to do that.
172 (b) The local variable h isn't in the exports, and rather than
173 clone a fresh copy we simply replace h by (h a b), where
174 the two h's have different types! Shadowing happens here,
175 which looks confusing but works fine.
177 (c) The result is *still* quadratic-sized if there are a lot of
178 small bindings. So if there are more than some small
179 number (10), we filter the binding set B by the free
180 variables of the particular RHS. Tiresome.
182 Why got to this trouble? It's a common case, and it removes the
183 quadratic-sized tuple desugaring. Less clutter, hopefullly faster
184 compilation, especially in a case where there are a *lot* of
189 dsHsBind auto_scc rest (AbsBinds tyvars [] exports binds)
190 | opt_DsMultiTyVar -- This (static) debug flag just lets us
191 -- switch on and off this optimisation to
192 -- see if it has any impact; it is on by default
193 = -- Note [Abstracting over tyvars only]
194 do { core_prs <- ds_lhs_binds NoSccs binds
196 ; let arby_env = mkArbitraryTypeEnv tyvars exports
197 (lg_binds, core_prs') = mapAndUnzip do_one core_prs
198 bndrs = mkVarSet (map fst core_prs)
200 add_lets | core_prs `lengthExceeds` 10 = add_some
201 | otherwise = mkLets lg_binds
202 add_some rhs = mkLets [ NonRec b r | NonRec b r <- lg_binds
203 , b `elemVarSet` fvs] rhs
205 fvs = exprSomeFreeVars (`elemVarSet` bndrs) rhs
207 env = mkABEnv exports
210 | Just (id_tvs, gbl_id, _, prags) <- lookupVarEnv env lcl_id
211 = (NonRec lcl_id (mkTyApps (Var gbl_id) (mkTyVarTys id_tvs)),
212 addInlinePrags prags gbl_id $
213 addAutoScc auto_scc gbl_id $
215 mkLets [ NonRec tv (Type (lookupVarEnv_NF arby_env tv))
216 | tv <- tyvars, not (tv `elem` id_tvs)] $
219 = (NonRec lcl_id (mkTyApps (Var non_exp_gbl_id) (mkTyVarTys tyvars)),
220 (non_exp_gbl_id, mkLams tyvars (add_lets rhs)))
222 non_exp_gbl_id = setIdType lcl_id (mkForAllTys tyvars (idType lcl_id))
224 ; return (core_prs' ++ rest) }
226 -- Another common case: one exported variable
227 -- Non-recursive bindings come through this way
228 dsHsBind auto_scc rest
229 (AbsBinds all_tyvars dicts [(tyvars, global, local, prags)] binds)
230 = ASSERT( all (`elem` tyvars) all_tyvars ) do
231 core_prs <- ds_lhs_binds NoSccs binds
233 -- Always treat the binds as recursive, because the typechecker
234 -- makes rather mixed-up dictionary bindings
235 core_bind = Rec core_prs
237 mb_specs <- mapM (dsSpec all_tyvars dicts tyvars global local core_bind) prags
239 (spec_binds, rules) = unzip (catMaybes mb_specs)
240 global' = addIdSpecialisations global rules
241 rhs' = mkLams tyvars $ mkLams dicts $ Let core_bind (Var local)
242 bind = addInlinePrags prags global' $ addAutoScc auto_scc global' rhs'
244 return (bind : spec_binds ++ rest)
246 dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds)
247 = do { core_prs <- ds_lhs_binds NoSccs binds
248 ; let env = mkABEnv exports
249 do_one (lcl_id,rhs) | Just (_, gbl_id, _, prags) <- lookupVarEnv env lcl_id
250 = addInlinePrags prags lcl_id $
251 addAutoScc auto_scc gbl_id rhs
252 | otherwise = (lcl_id,rhs)
254 -- Rec because of mixed-up dictionary bindings
255 core_bind = Rec (map do_one core_prs)
257 tup_expr = mkBigCoreVarTup locals
258 tup_ty = exprType tup_expr
259 poly_tup_expr = mkLams all_tyvars $ mkLams dicts $
260 Let core_bind tup_expr
261 locals = [local | (_, _, local, _) <- exports]
262 local_tys = map idType locals
264 ; poly_tup_id <- newSysLocalDs (exprType poly_tup_expr)
266 ; let mk_bind ((tyvars, global, local, prags), n) -- locals!!n == local
267 = -- Need to make fresh locals to bind in the selector,
268 -- because some of the tyvars will be bound to 'Any'
269 do { let ty_args = map mk_ty_arg all_tyvars
270 substitute = substTyWith all_tyvars ty_args
271 ; locals' <- newSysLocalsDs (map substitute local_tys)
272 ; tup_id <- newSysLocalDs (substitute tup_ty)
273 ; mb_specs <- mapM (dsSpec all_tyvars dicts tyvars global
276 ; let (spec_binds, rules) = unzip (catMaybes mb_specs)
277 global' = addIdSpecialisations global rules
278 rhs = mkLams tyvars $ mkLams dicts $
279 mkTupleSelector locals' (locals' !! n) tup_id $
280 mkVarApps (mkTyApps (Var poly_tup_id) ty_args)
282 ; return ((global', rhs) : spec_binds) }
285 | all_tyvar `elem` tyvars = mkTyVarTy all_tyvar
286 | otherwise = dsMkArbitraryType all_tyvar
288 ; export_binds_s <- mapM mk_bind (exports `zip` [0..])
289 -- don't scc (auto-)annotate the tuple itself.
291 ; return ((poly_tup_id, poly_tup_expr) :
292 (concat export_binds_s ++ rest)) }
294 mkABEnv :: [([TyVar], Id, Id, [LPrag])] -> VarEnv ([TyVar], Id, Id, [LPrag])
295 -- Takes the exports of a AbsBinds, and returns a mapping
296 -- lcl_id -> (tyvars, gbl_id, lcl_id, prags)
297 mkABEnv exports = mkVarEnv [ (lcl_id, export) | export@(_, _, lcl_id, _) <- exports]
300 dsSpec :: [TyVar] -> [DictId] -> [TyVar]
301 -> Id -> Id -- Global, local
303 -> DsM (Maybe ((Id,CoreExpr), -- Binding for specialised Id
304 CoreRule)) -- Rule for the Global Id
307 -- f :: (Eq a, Ix b) => a -> b -> b
308 -- {-# SPECIALISE f :: Ix b => Int -> b -> b #-}
310 -- AbsBinds [ab] [d1,d2] [([ab], f, f_mono, prags)] binds
312 -- SpecPrag (/\b.\(d:Ix b). f Int b dInt d)
313 -- (forall b. Ix b => Int -> b -> b)
315 -- Rule: forall b,(d:Ix b). f Int b dInt d = f_spec b d
317 -- Spec bind: f_spec = Let f = /\ab \(d1:Eq a)(d2:Ix b). let binds in f_mono
318 -- /\b.\(d:Ix b). in f Int b dInt d
319 -- The idea is that f occurs just once, so it'll be
320 -- inlined and specialised
322 -- Given SpecPrag (/\as.\ds. f es) t, we have
323 -- the defn f_spec as ds = let-nonrec f = /\fas\fds. let f_mono = <f-rhs> in f_mono
325 -- and the RULE forall as, ds. f es = f_spec as ds
327 -- It is *possible* that 'es' does not mention all of the dictionaries 'ds'
328 -- (a bit silly, because then the
329 dsSpec _ _ _ _ _ _ (L _ (InlinePrag {}))
332 dsSpec all_tvs dicts tvs poly_id mono_id mono_bind
333 (L loc (SpecPrag spec_expr spec_ty inl))
335 do { let poly_name = idName poly_id
336 ; spec_name <- newLocalName poly_name
337 ; ds_spec_expr <- dsExpr spec_expr
338 ; case (decomposeRuleLhs ds_spec_expr) of {
339 Nothing -> do { warnDs decomp_msg; return Nothing } ;
341 Just (bndrs, _fn, args) ->
343 -- Check for dead binders: Note [Unused spec binders]
344 case filter isDeadBinder bndrs of {
345 bs | not (null bs) -> do { warnDs (dead_msg bs); return Nothing }
348 { let f_body = fix_up (Let mono_bind (Var mono_id))
350 local_poly = setIdNotExported poly_id
351 -- Very important to make the 'f' non-exported,
352 -- else it won't be inlined!
353 spec_id = mkLocalId spec_name spec_ty
354 spec_rhs = Let (NonRec local_poly poly_f_body) ds_spec_expr
355 poly_f_body = mkLams (tvs ++ dicts) f_body
357 extra_dict_bndrs = [localiseId d -- See Note [Constant rule dicts]
358 | d <- varSetElems (exprFreeVars ds_spec_expr)
360 -- Note [Const rule dicts]
362 rule = mkLocalRule (mkFastString ("SPEC " ++ showSDoc (ppr poly_name)))
363 AlwaysActive poly_name
364 (extra_dict_bndrs ++ bndrs) args
365 (mkVarApps (Var spec_id) bndrs)
366 ; return (Just (addInlineInfo inl spec_id spec_rhs, rule))
369 -- Bind to Any any of all_ptvs that aren't
370 -- relevant for this particular function
371 fix_up body | null void_tvs = body
372 | otherwise = mkTyApps (mkLams void_tvs body) $
373 map dsMkArbitraryType void_tvs
375 void_tvs = all_tvs \\ tvs
377 dead_msg bs = vcat [ sep [ptext (sLit "Useless constraint") <> plural bs
378 <+> ptext (sLit "in specialied type:"),
379 nest 2 (pprTheta (map get_pred bs))]
380 , ptext (sLit "SPECIALISE pragma ignored")]
381 get_pred b = ASSERT( isId b ) expectJust "dsSpec" (tcSplitPredTy_maybe (idType b))
383 decomp_msg = hang (ptext (sLit "Specialisation too complicated to desugar; ignored"))
387 mkArbitraryTypeEnv :: [TyVar] -> [([TyVar], a, b, c)] -> TyVarEnv Type
388 -- If any of the tyvars is missing from any of the lists in
389 -- the second arg, return a binding in the result
390 mkArbitraryTypeEnv tyvars exports
391 = go emptyVarEnv exports
394 go env ((ltvs, _, _, _) : exports)
397 env' = foldl extend env [tv | tv <- tyvars
398 , not (tv `elem` ltvs)
399 , not (tv `elemVarEnv` env)]
401 extend env tv = extendVarEnv env tv (dsMkArbitraryType tv)
403 dsMkArbitraryType :: TcTyVar -> Type
404 dsMkArbitraryType tv = anyTypeOfKind (tyVarKind tv)
407 Note [Unused spec binders]
408 ~~~~~~~~~~~~~~~~~~~~~~~~~~
411 {-# SPECIALISE f :: Eq a => a -> a #-}
412 It's true that this *is* a more specialised type, but the rule
413 we get is something like this:
416 Note that the rule is bogus, becuase it mentions a 'd' that is
417 not bound on the LHS! But it's a silly specialisation anyway, becuase
418 the constraint is unused. We could bind 'd' to (error "unused")
419 but it seems better to reject the program because it's almost certainly
420 a mistake. That's what the isDeadBinder call detects.
422 Note [Const rule dicts]
423 ~~~~~~~~~~~~~~~~~~~~~~~
424 When the LHS of a specialisation rule, (/\as\ds. f es) has a free dict,
425 which is presumably in scope at the function definition site, we can quantify
426 over it too. *Any* dict with that type will do.
428 So for example when you have
431 {-# SPECIALISE f :: Int -> Int #-}
433 Then we get the SpecPrag
434 SpecPrag (f Int dInt) Int
436 And from that we want the rule
438 RULE forall dInt. f Int dInt = f_spec
439 f_spec = let f = <rhs> in f Int dInt
441 But be careful! That dInt might be GHC.Base.$fOrdInt, which is an External
442 Name, and you can't bind them in a lambda or forall without getting things
443 confused. Hence the use of 'localiseId' to make it Internal.
446 %************************************************************************
448 \subsection{Adding inline pragmas}
450 %************************************************************************
453 decomposeRuleLhs :: CoreExpr -> Maybe ([Var], Id, [CoreExpr])
454 -- Take apart the LHS of a RULE. It's suuposed to look like
455 -- /\a. f a Int dOrdInt
456 -- or /\a.\d:Ord a. let { dl::Ord [a] = dOrdList a d } in f [a] dl
457 -- That is, the RULE binders are lambda-bound
458 -- Returns Nothing if the LHS isn't of the expected shape
460 = case (decomp emptyVarEnv body) of
462 Just (fn, args) -> Just (bndrs, fn, args)
464 occ_lhs = occurAnalyseExpr lhs
465 -- The occurrence-analysis does two things
466 -- (a) identifies unused binders: Note [Unused spec binders]
467 -- (b) sorts dict bindings into NonRecs
468 -- so they can be inlined by 'decomp'
469 (bndrs, body) = collectBinders occ_lhs
471 -- Substitute dicts in the LHS args, so that there
472 -- aren't any lets getting in the way
473 -- Note that we substitute the function too; we might have this as
474 -- a LHS: let f71 = M.f Int in f71
475 decomp env (Let (NonRec dict rhs) body)
476 = decomp (extendVarEnv env dict (simpleSubst env rhs)) body
478 decomp env (Case scrut bndr ty [(DEFAULT, _, body)])
479 | isDeadBinder bndr -- Note [Matching seqId]
480 = Just (seqId, [Type (idType bndr), Type ty,
481 simpleSubst env scrut, simpleSubst env body])
484 = case collectArgs (simpleSubst env body) of
485 (Var fn, args) -> Just (fn, args)
488 simpleSubst :: IdEnv CoreExpr -> CoreExpr -> CoreExpr
489 -- Similar to CoreSubst.substExpr, except that
490 -- (a) Takes no account of capture; at this point there is no shadowing
491 -- (b) Can have a GlobalId (imported) in its domain
492 -- (c) Ids only; no types are substituted
493 -- (d) Does not insist (as does CoreSubst.lookupIdSubst) that the
494 -- in-scope set mentions all LocalIds mentioned in the argument of the subst
496 -- (b) and (d) are the reasons we can't use CoreSubst
498 -- (I had a note that (b) is "no longer relevant", and indeed it doesn't
499 -- look relevant here. Perhaps there was another caller of simpleSubst.)
501 simpleSubst subst expr
504 go (Var v) = lookupVarEnv subst v `orElse` Var v
505 go (Cast e co) = Cast (go e) co
506 go (Type ty) = Type ty
507 go (Lit lit) = Lit lit
508 go (App fun arg) = App (go fun) (go arg)
509 go (Note note e) = Note note (go e)
510 go (Lam bndr body) = Lam bndr (go body)
511 go (Let (NonRec bndr rhs) body) = Let (NonRec bndr (go rhs)) (go body)
512 go (Let (Rec pairs) body) = Let (Rec (mapSnd go pairs)) (go body)
513 go (Case scrut bndr ty alts) = Case (go scrut) bndr ty
514 [(c,bs,go r) | (c,bs,r) <- alts]
516 addInlinePrags :: [LPrag] -> Id -> CoreExpr -> (Id,CoreExpr)
517 addInlinePrags prags bndr rhs
518 = case [inl | L _ (InlinePrag inl) <- prags] of
520 (inl:_) -> addInlineInfo inl bndr rhs
522 addInlineInfo :: InlineSpec -> Id -> CoreExpr -> (Id,CoreExpr)
523 addInlineInfo (Inline prag is_inline) bndr rhs
524 = (attach_pragma bndr prag, wrap_inline is_inline rhs)
526 attach_pragma bndr prag
527 | isDefaultInlinePragma prag = bndr
528 | otherwise = bndr `setInlinePragma` prag
530 wrap_inline True body = mkInlineMe body
531 wrap_inline False body = body
536 The desugarer turns (seq e r) into (case e of _ -> r), via a special-case hack
537 and this code turns it back into an application of seq!
538 See Note [Rules for seq] in MkId for the details.
541 %************************************************************************
543 \subsection[addAutoScc]{Adding automatic sccs}
545 %************************************************************************
548 data AutoScc = NoSccs
549 | AddSccs Module (Id -> Bool)
550 -- The (Id->Bool) says which Ids to add SCCs to
552 addAutoScc :: AutoScc
555 -> CoreExpr -- Scc'd Rhs
557 addAutoScc NoSccs _ rhs
559 addAutoScc (AddSccs mod add_scc) id rhs
560 | add_scc id = mkSCC (mkAutoCC id mod NotCafCC) rhs
564 If profiling and dealing with a dict binding,
565 wrap the dict in @_scc_ DICT <dict>@:
568 addDictScc :: Id -> CoreExpr -> DsM CoreExpr
569 addDictScc _ rhs = return rhs
571 {- DISABLED for now (need to somehow make up a name for the scc) -- SDM
572 | not ( opt_SccProfilingOn && opt_AutoSccsOnDicts)
573 || not (isDictId var)
574 = return rhs -- That's easy: do nothing
577 = do (mod, grp) <- getModuleAndGroupDs
578 -- ToDo: do -dicts-all flag (mark dict things with individual CCs)
579 return (Note (SCC (mkAllDictsCC mod grp False)) rhs)
584 %************************************************************************
588 %************************************************************************
592 dsCoercion :: HsWrapper -> DsM CoreExpr -> DsM CoreExpr
593 dsCoercion WpHole thing_inside = thing_inside
594 dsCoercion (WpCompose c1 c2) thing_inside = dsCoercion c1 (dsCoercion c2 thing_inside)
595 dsCoercion (WpCast co) thing_inside = do { expr <- thing_inside
596 ; return (Cast expr co) }
597 dsCoercion (WpLam id) thing_inside = do { expr <- thing_inside
598 ; return (Lam id expr) }
599 dsCoercion (WpTyLam tv) thing_inside = do { expr <- thing_inside
600 ; return (Lam tv expr) }
601 dsCoercion (WpApp v) thing_inside
602 | isTyVar v = do { expr <- thing_inside
603 {- Probably a coercion var -} ; return (App expr (Type (mkTyVarTy v))) }
604 | otherwise = do { expr <- thing_inside
605 {- An Id -} ; return (App expr (Var v)) }
606 dsCoercion (WpTyApp ty) thing_inside = do { expr <- thing_inside
607 ; return (App expr (Type ty)) }
608 dsCoercion WpInline thing_inside = do { expr <- thing_inside
609 ; return (mkInlineMe expr) }
610 dsCoercion (WpLet bs) thing_inside = do { prs <- dsLHsBinds bs
611 ; expr <- thing_inside
612 ; return (Let (Rec prs) expr) }