2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[CoreRules]{Transformation rules}
8 RuleBase, prepareRuleBase, lookupRule,
10 ProtoCoreRule(..), pprProtoCoreRule,
14 #include "HsVersions.h"
16 import CoreSyn -- All of it
17 import OccurAnal ( occurAnalyseExpr, tagBinders, UsageDetails )
18 import BinderInfo ( markMany )
19 import CoreFVs ( exprFreeVars, idRuleVars, ruleSomeLhsFreeVars )
20 import CoreUnfold ( Unfolding(..) )
21 import CoreUtils ( whnfOrBottom, eqExpr )
22 import PprCore ( pprCoreRule )
23 import Subst ( Subst, InScopeSet, substBndr, lookupSubst, extendSubst,
24 mkSubst, substEnv, setSubstEnv, emptySubst, isInScope,
25 unBindSubst, bindSubstList, unBindSubstList,
27 import Id ( Id, getIdUnfolding,
28 getIdSpecialisation, setIdSpecialisation,
29 setIdNoDiscard, maybeModifyIdInfo, modifyIdInfo
31 import IdInfo ( zapLamIdInfo, setSpecInfo, specInfo )
32 import Name ( Name, isLocallyDefined )
33 import Var ( isTyVar, isId )
36 import Type ( mkTyVarTy, getTyVar_maybe )
37 import qualified Unify ( match )
38 import CmdLineOpts ( opt_D_dump_simpl, opt_D_verbose_core2core )
41 import ErrUtils ( dumpIfSet )
43 import Maybes ( maybeToBool )
44 import List ( partition )
45 import Util ( sortLt )
49 %************************************************************************
51 \subsection[specialisation-IdInfo]{Specialisation info about an @Id@}
53 %************************************************************************
55 A @CoreRule@ holds details of one rule for an @Id@, which
56 includes its specialisations.
58 For example, if a rule for @f@ contains the mapping:
60 forall a b d. [Type (List a), Type b, Var d] ===> f' a b
62 then when we find an application of f to matching types, we simply replace
63 it by the matching RHS:
65 f (List Int) Bool dict ===> f' Int Bool
67 All the stuff about how many dictionaries to discard, and what types
68 to apply the specialised function to, are handled by the fact that the
69 Rule contains a template for the result of the specialisation.
71 There is one more exciting case, which is dealt with in exactly the same
72 way. If the specialised value is unboxed then it is lifted at its
73 definition site and unlifted at its uses. For example:
75 pi :: forall a. Num a => a
77 might have a specialisation
79 [Int#] ===> (case pi' of Lift pi# -> pi#)
81 where pi' :: Lift Int# is the specialised version of pi.
84 %************************************************************************
88 %************************************************************************
91 matchRules :: InScopeSet -> [CoreRule] -> [CoreExpr] -> Maybe (FAST_STRING, CoreExpr, [CoreExpr])
92 -- See comments on matchRule
93 matchRules in_scope [] args = Nothing
94 matchRules in_scope (rule:rules) args
95 = case matchRule in_scope rule args of
96 Just result -> Just result
97 Nothing -> matchRules in_scope rules args
100 matchRule :: InScopeSet -> CoreRule -> [CoreExpr] -> Maybe (FAST_STRING, CoreExpr, [CoreExpr])
102 -- If (matchRule rule args) returns Just (name,rhs,args')
103 -- then (f args) matches the rule, and the corresponding
104 -- rewritten RHS is (rhs args').
106 -- The bndrs and rhs is occurrence-analysed
111 -- forall f g x. map f (map g x) ==> map (f . g) x
113 -- CoreRule "map/map"
114 -- [f,g,x] -- tpl_vars
115 -- [f,map g x] -- tpl_args
116 -- map (f.g) x) -- rhs
118 -- Then the call: matchRule the_rule [e1,map e2 e3]
119 -- = Just ("map/map", \f,g,x -> rhs, [e1,e2,e3])
121 -- Any 'surplus' arguments in the input are simply put on the end
125 -- A1. No top-level variable is bound in the target
126 -- A2. No template variable is bound in the target
127 -- A3. No lambda bound template variable is free in any subexpression of the target
129 -- To see why A1 is necessary, consider matching
130 -- \x->f against \f->f
131 -- When we meet the lambdas we substitute [f/x] in the template (a no-op),
132 -- and then erroneously succeed in matching f against f.
134 -- To see why A2 is needed consider matching
135 -- forall a. \b->b against \a->3
136 -- When we meet the lambdas we substitute [a/b] in the template, and then
137 -- erroneously succeed in matching what looks like the template variable 'a' against 3.
139 -- A3 is needed to validate the rule that says
142 -- (\x->E) matches (\x->F x)
145 matchRule in_scope (Rule rn tpl_vars tpl_args rhs) args
146 = go tpl_args args emptySubst
147 -- We used to use the in_scope set, but I don't think that's necessary
148 -- After all, the result is going to be simplified again with that in_scope set
150 tpl_var_set = mkVarSet tpl_vars
152 -----------------------
154 go (tpl_arg:tpl_args) (arg:args) subst = match tpl_arg arg tpl_var_set (go tpl_args args) subst
156 -- Two easy ways to terminate
157 go [] [] subst = Just (rn, mkLams tpl_vars rhs, mk_result_args subst tpl_vars)
158 go [] args subst = Just (rn, mkLams tpl_vars rhs, mk_result_args subst tpl_vars ++ args)
160 -- One tiresome way to terminate: check for excess unmatched
161 -- template arguments
163 = case eta_complete tpl_args (mkVarSet leftovers) of
164 Just leftovers' -> Just (rn, mkLams done (mkLams leftovers' rhs),
165 mk_result_args subst done)
166 Nothing -> Nothing -- Failure
168 (done, leftovers) = partition (\v -> maybeToBool (lookupSubstEnv subst_env v))
169 (map zapOccInfo tpl_vars)
171 subst_env = substEnv subst
173 -----------------------
174 eta_complete [] vars = ASSERT( isEmptyVarSet vars )
176 eta_complete (Type ty:tpl_args) vars
177 = case getTyVar_maybe ty of
178 Just tv | tv `elemVarSet` vars
179 -> case eta_complete tpl_args (vars `delVarSet` tv) of
180 Just vars' -> Just (tv:vars')
184 eta_complete (Var v:tpl_args) vars
185 | v `elemVarSet` vars
186 = case eta_complete tpl_args (vars `delVarSet` v) of
187 Just vars' -> Just (v:vars')
190 eta_complete other vars = Nothing
192 -----------------------
193 mk_result_args subst vs = map go vs
195 senv = substEnv subst
196 go v = case lookupSubstEnv senv v of
197 Just (DoneEx ex) -> ex
198 Just (DoneTy ty) -> Type ty
199 -- Substitution should bind them all!
201 zapOccInfo bndr | isTyVar bndr = bndr
202 | otherwise = maybeModifyIdInfo zapLamIdInfo bndr
206 type Matcher result = IdOrTyVarSet -- Template variables
207 -> (Subst -> Maybe result) -- Continuation if success
208 -> Subst -> Maybe result -- Substitution so far -> result
209 -- The *SubstEnv* in these Substs apply to the TEMPLATE only
211 -- The *InScopeSet* in these Substs gives variables bound so far in the
212 -- target term. So when matching forall a. (\x. a x) against (\y. y y)
213 -- while processing the body of the lambdas, the in-scope set will be {y}.
214 -- That lets us do the occurs-check when matching 'a' against 'y'
216 match :: CoreExpr -- Template
217 -> CoreExpr -- Target
222 match (Var v1) e2 tpl_vars kont subst
223 = case lookupSubst subst v1 of
224 Nothing | v1 `elemVarSet` tpl_vars -- v1 is a template variable
225 -> if (any (`isInScope` subst) (varSetElems (exprFreeVars e2))) then
226 match_fail -- Occurs check failure
227 -- e.g. match forall a. (\x-> a x) against (\y. y y)
229 kont (extendSubst subst v1 (DoneEx e2))
232 | eqExpr (Var v1) e2 -> kont subst
233 -- v1 is not a template variable, so it must be a global constant
235 Just (DoneEx e2') | eqExpr e2' e2 -> kont subst
239 match (Con c1 es1) (Con c2 es2) tpl_vars kont subst
241 = matches es1 es2 tpl_vars kont subst
243 match (App f1 a1) (App f2 a2) tpl_vars kont subst
244 = match f1 f2 tpl_vars (match a1 a2 tpl_vars kont) subst
246 match (Lam x1 e1) (Lam x2 e2) tpl_vars kont subst
247 = bind [x1] [x2] (match e1 e2) tpl_vars kont subst
249 -- This rule does eta expansion
250 -- (\x.M) ~ N iff M ~ N x
252 match (Lam x1 e1) e2 tpl_vars kont subst
253 = bind [x1] [x1] (match e1 (App e2 (mkVarArg x1))) tpl_vars kont subst
255 -- Eta expansion the other way
256 -- M ~ (\y.N) iff \y.M y ~ \y.N
258 -- Remembering that by (A), y can't be free in M, we get this
259 match e1 (Lam x2 e2) tpl_vars kont subst
260 = bind [x2] [x2] (match (App e1 (mkVarArg x2)) e2) tpl_vars kont subst
262 match (Case e1 x1 alts1) (Case e2 x2 alts2) tpl_vars kont subst
263 = match e1 e2 tpl_vars case_kont subst
265 case_kont subst = bind [x1] [x2] (match_alts alts1 (sortLt lt_alt alts2))
268 match (Type ty1) (Type ty2) tpl_vars kont subst
269 = match_ty ty1 ty2 tpl_vars kont subst
271 match (Note (Coerce to1 from1) e1) (Note (Coerce to2 from2) e2)
273 = (match_ty to1 to2 tpl_vars $
274 match_ty from1 from2 tpl_vars $
275 match e1 e2 tpl_vars kont) subst
278 {- I don't buy this let-rule any more
279 The let rule fails on matching
280 forall f,x,xs. f (x:xs)
282 f (let y = e in (y:[]))
283 because we just get x->y, which is bogus.
285 -- This is an interesting rule: we simply ignore lets in the
286 -- term being matched against! The unfolding inside it is (by assumption)
287 -- already inside any occurrences of the bound variables, so we'll expand
288 -- them when we encounter them. Meanwhile, we can't get false matches because
289 -- (also by assumption) the term being matched has no shadowing.
290 match e1 (Let bind e2) tpl_vars kont subst
291 = match e1 e2 tpl_vars kont subst
294 -- Here is another important rule: if the term being matched is a
295 -- variable, we expand it so long as its unfolding is a WHNF
296 -- (Its occurrence information is not necessarily up to date,
297 -- so we don't use it.)
298 match e1 (Var v2) tpl_vars kont subst
299 = case getIdUnfolding v2 of
300 CoreUnfolding form guidance unfolding
302 -> match e1 unfolding tpl_vars kont subst
306 -- We can't cope with lets in the template
308 match e1 e2 tpl_vars kont subst = match_fail
311 ------------------------------------------
312 match_alts [] [] tpl_vars kont subst
314 match_alts ((c1,vs1,r1):alts1) ((c2,vs2,r2):alts2) tpl_vars kont subst
316 = bind vs1 vs2 (match r1 r2) tpl_vars
317 (match_alts alts1 alts2 tpl_vars kont)
319 match_alts alts1 alts2 tpl_vars kont subst = match_fail
321 lt_alt (con1, _, _) (con2, _, _) = con1 < con2
323 ----------------------------------------
324 bind :: [CoreBndr] -- Template binders
325 -> [CoreBndr] -- Target binders
328 -- This makes uses of assumption (A) above. For example,
330 -- Template: (\x.y) (y is free)
331 -- Target : (\y.y) (y is bound)
332 -- We rename x to y in the template... but then erroneously
333 -- match y against y. But this can't happen because of (A)
334 bind vs1 vs2 matcher tpl_vars kont subst
335 = ASSERT( all not_in_subst vs1)
336 matcher tpl_vars kont' subst'
338 kont' subst'' = kont (unBindSubstList subst'' vs1 vs2)
339 subst' = bindSubstList subst vs1 vs2
341 -- The unBindSubst relies on no shadowing in the template
342 not_in_subst v = not (maybeToBool (lookupSubst subst v))
344 ----------------------------------------
345 match_ty ty1 ty2 tpl_vars kont subst
346 = case Unify.match ty1 ty2 tpl_vars Just (substEnv subst) of
347 Nothing -> match_fail
348 Just senv' -> kont (setSubstEnv subst senv')
350 ----------------------------------------
351 matches [] [] tpl_vars kont subst
353 matches (e:es) (e':es') tpl_vars kont subst
354 = match e e' tpl_vars (matches es es' tpl_vars kont) subst
355 matches es es' tpl_vars kont subst
358 ----------------------------------------
359 mkVarArg :: CoreBndr -> CoreArg
360 mkVarArg v | isId v = Var v
361 | otherwise = Type (mkTyVarTy v)
364 %************************************************************************
366 \subsection{Adding a new rule}
368 %************************************************************************
371 addRule :: Id -> CoreRules -> CoreRule -> CoreRules
373 -- Insert the new rule just before a rule that is *less specific*
374 -- than the new one; or at the end if there isn't such a one.
375 -- In this way we make sure that when looking up, the first match
376 -- is the most specific.
378 -- We make no check for rules that unify without one dominating
379 -- the other. Arguably this would be a bug.
381 addRule id (Rules rules rhs_fvs) (Rule str tpl_vars tpl_args rhs)
382 = Rules (insert rules) (rhs_fvs `unionVarSet` new_rhs_fvs)
384 new_rule = Rule str tpl_vars' tpl_args rhs'
385 -- Add occ info to tpl_vars, rhs
387 (rhs_uds, rhs') = occurAnalyseExpr isLocallyDefined rhs
388 (rhs_uds1, tpl_vars') = tagBinders rhs_uds tpl_vars
390 insert [] = [new_rule]
391 insert (rule:rules) | new_is_more_specific rule = (new_rule:rule:rules)
392 | otherwise = rule : insert rules
394 new_is_more_specific rule = maybeToBool (matchRule tpl_var_set rule tpl_args)
396 tpl_var_set = mkVarSet tpl_vars'
397 -- Actually we should probably include the free vars of tpl_args,
398 -- but I can't be bothered
400 new_rhs_fvs = (exprFreeVars rhs' `minusVarSet` tpl_var_set) `delVarSet` id
402 -- Don't include the Id in its own rhs free-var set.
403 -- Otherwise the occurrence analyser makes bindings recursive
404 -- that shoudn't be. E.g.
405 -- RULE: f (f x y) z ==> f x (f y z)
407 addIdSpecialisations :: Id -> [([CoreBndr], [CoreExpr], CoreExpr)] -> Id
408 addIdSpecialisations id spec_stuff
409 = setIdSpecialisation id new_rules
411 rule_name = _PK_ ("SPEC " ++ showSDoc (ppr id))
412 new_rules = foldr add (getIdSpecialisation id) spec_stuff
413 add (vars, args, rhs) rules = addRule id rules (Rule rule_name vars args rhs)
417 %************************************************************************
419 \subsection{Preparing the rule base
421 %************************************************************************
426 Bool -- True <=> this rule was defined in this module,
427 Id -- What Id is it for
428 CoreRule -- The rule itself
431 pprProtoCoreRule (ProtoCoreRule _ fn rule) = pprCoreRule (Just fn) rule
433 lookupRule :: InScopeSet -> Id -> [CoreExpr] -> Maybe (FAST_STRING, CoreExpr, [CoreExpr])
434 lookupRule in_scope fn args
435 = case getIdSpecialisation fn of
436 Rules rules _ -> matchRules in_scope rules args
438 orphanRule :: ProtoCoreRule -> Bool
439 -- An "orphan rule" is one that is defined in this
440 -- module, but of ran *imported* function. We need
441 -- to track these separately when generating the interface file
442 orphanRule (ProtoCoreRule local fn _)
443 = local && not (isLocallyDefined fn)
447 %************************************************************************
449 \subsection{Getting the rules ready}
451 %************************************************************************
454 type RuleBase = (IdSet, -- Imported Ids that have rules attached
455 IdSet) -- Ids (whether local or imported) mentioned on
456 -- LHS of some rule; these should be black listed
458 -- The rule Ids and LHS Ids are black-listed; that is, they aren't inlined
459 -- so that the opportunity to apply the rule isn't lost too soon
461 prepareRuleBase :: [CoreBind] -> [ProtoCoreRule] -> ([CoreBind], RuleBase)
462 prepareRuleBase binds rules
463 = (map zap_bind binds, (imported_rule_ids, rule_lhs_fvs))
465 (rule_ids, rule_lhs_fvs) = foldr add_rule (emptyVarSet, emptyVarSet) rules
466 imported_rule_ids = filterVarSet (not . isLocallyDefined) rule_ids
468 -- rule_fvs is the set of all variables mentioned in rules
469 rule_fvs = foldVarSet (unionVarSet . idRuleVars) rule_lhs_fvs rule_ids
471 -- Attach the rules for each locally-defined Id to that Id.
472 -- - This makes the rules easier to look up
473 -- - It means that transformation rules and specialisations for
474 -- locally defined Ids are handled uniformly
475 -- - It keeps alive things that are referred to only from a rule
476 -- (the occurrence analyser knows about rules attached to Ids)
477 -- - It makes sure that, when we apply a rule, the free vars
478 -- of the RHS are more likely to be in scope
480 -- The LHS and RHS Ids are marked 'no-discard'.
481 -- This means that the binding won't be discarded EVEN if the binding
482 -- ends up being trivial (v = w) -- the simplifier would usually just
483 -- substitute w for v throughout, but we don't apply the substitution to
484 -- the rules (maybe we should?), so this substitution would make the rule
486 zap_bind (NonRec b r) = NonRec (zap_bndr b) r
487 zap_bind (Rec prs) = Rec [(zap_bndr b, r) | (b,r) <- prs]
489 zap_bndr bndr = case lookupVarSet rule_ids bndr of
490 Just bndr' -> setIdNoDiscard bndr'
491 Nothing | bndr `elemVarSet` rule_fvs -> setIdNoDiscard bndr
494 add_rule (ProtoCoreRule _ id rule)
495 (rule_id_set, rule_fvs)
496 = (rule_id_set `extendVarSet` new_id,
497 rule_fvs `unionVarSet` extendVarSet lhs_fvs id)
499 new_id = case lookupVarSet rule_id_set id of
500 Just id' -> addRuleToId id' rule
501 Nothing -> addRuleToId id rule
502 lhs_fvs = ruleSomeLhsFreeVars isId rule
503 -- Find *all* the free Ids of the LHS, not just
504 -- locally defined ones!!
506 addRuleToId id rule = setIdSpecialisation id (addRule id (getIdSpecialisation id) rule)