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 ( isCheapUnfolding, unfoldingTemplate )
21 import CoreUtils ( eqExpr )
22 import PprCore ( pprCoreRule )
23 import Subst ( Subst, InScopeSet, substBndr, lookupSubst, extendSubst,
24 mkSubst, substEnv, setSubstEnv, emptySubst, isInScope,
25 unBindSubst, bindSubstList, unBindSubstList, substInScope
27 import Id ( Id, getIdUnfolding, zapLamIdInfo,
28 getIdSpecialisation, setIdSpecialisation,
29 setIdNoDiscard, maybeModifyIdInfo, modifyIdInfo
31 import IdInfo ( 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@(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
162 go tpl_args [] subst = Nothing -- Failure
165 {- The code below tries to match even if there are more
166 template args than real args.
168 I now think this is probably a bad idea.
169 Should the template (map f xs) match (map g)? I think not.
170 For a start, in general eta expansion wastes work.
173 = case eta_complete tpl_args (mkVarSet leftovers) of
174 Just leftovers' -> Just (rn, mkLams done (mkLams leftovers' rhs),
175 mk_result_args subst done)
176 Nothing -> Nothing -- Failure
178 (done, leftovers) = partition (\v -> maybeToBool (lookupSubstEnv subst_env v))
179 (map zapOccInfo tpl_vars)
181 subst_env = substEnv subst
183 -----------------------
184 eta_complete [] vars = ASSERT( isEmptyVarSet vars )
186 eta_complete (Type ty:tpl_args) vars
187 = case getTyVar_maybe ty of
188 Just tv | tv `elemVarSet` vars
189 -> case eta_complete tpl_args (vars `delVarSet` tv) of
190 Just vars' -> Just (tv:vars')
194 eta_complete (Var v:tpl_args) vars
195 | v `elemVarSet` vars
196 = case eta_complete tpl_args (vars `delVarSet` v) of
197 Just vars' -> Just (v:vars')
200 eta_complete other vars = Nothing
203 -----------------------
204 mk_result_args subst vs = map go vs
206 senv = substEnv subst
207 go v = case lookupSubstEnv senv v of
208 Just (DoneEx ex) -> ex
209 Just (DoneTy ty) -> Type ty
210 -- Substitution should bind them all!
213 zapOccInfo bndr | isTyVar bndr = bndr
214 | otherwise = zapLamIdInfo bndr
218 type Matcher result = IdOrTyVarSet -- Template variables
219 -> (Subst -> Maybe result) -- Continuation if success
220 -> Subst -> Maybe result -- Substitution so far -> result
221 -- The *SubstEnv* in these Substs apply to the TEMPLATE only
223 -- The *InScopeSet* in these Substs gives variables bound so far in the
224 -- target term. So when matching forall a. (\x. a x) against (\y. y y)
225 -- while processing the body of the lambdas, the in-scope set will be {y}.
226 -- That lets us do the occurs-check when matching 'a' against 'y'
228 match :: CoreExpr -- Template
229 -> CoreExpr -- Target
234 match (Var v1) e2 tpl_vars kont subst
235 = case lookupSubst subst v1 of
236 Nothing | v1 `elemVarSet` tpl_vars -- v1 is a template variable
237 -> if (any (`isInScope` subst) (varSetElems (exprFreeVars e2))) then
238 match_fail -- Occurs check failure
239 -- e.g. match forall a. (\x-> a x) against (\y. y y)
241 kont (extendSubst subst v1 (DoneEx e2))
244 | eqExpr (Var v1) e2 -> kont subst
245 -- v1 is not a template variable, so it must be a global constant
247 Just (DoneEx e2') | eqExpr e2' e2 -> kont subst
251 match (Con c1 es1) (Con c2 es2) tpl_vars kont subst
253 = matches es1 es2 tpl_vars kont subst
255 match (App f1 a1) (App f2 a2) tpl_vars kont subst
256 = match f1 f2 tpl_vars (match a1 a2 tpl_vars kont) subst
258 match (Lam x1 e1) (Lam x2 e2) tpl_vars kont subst
259 = bind [x1] [x2] (match e1 e2) tpl_vars kont subst
261 -- This rule does eta expansion
262 -- (\x.M) ~ N iff M ~ N x
264 match (Lam x1 e1) e2 tpl_vars kont subst
265 = bind [x1] [x1] (match e1 (App e2 (mkVarArg x1))) tpl_vars kont subst
267 -- Eta expansion the other way
268 -- M ~ (\y.N) iff \y.M y ~ \y.N
270 -- Remembering that by (A), y can't be free in M, we get this
271 match e1 (Lam x2 e2) tpl_vars kont subst
272 = bind [new_id] [x2] (match (App e1 (mkVarArg new_id)) e2) tpl_vars kont subst
274 new_id = uniqAway (substInScope subst) x2
275 -- This uniqAway is actually needed. Here's the example:
276 -- rule: foldr (mapFB (:) f) [] = mapList
277 -- target: foldr (\x. mapFB k f x) []
279 -- k = \x. mapFB ... x
280 -- The first \x is ok, but when we inline k, hoping it might
281 -- match (:) we find a second \x.
283 match (Case e1 x1 alts1) (Case e2 x2 alts2) tpl_vars kont subst
284 = match e1 e2 tpl_vars case_kont subst
286 case_kont subst = bind [x1] [x2] (match_alts alts1 (sortLt lt_alt alts2))
289 match (Type ty1) (Type ty2) tpl_vars kont subst
290 = match_ty ty1 ty2 tpl_vars kont subst
292 match (Note (Coerce to1 from1) e1) (Note (Coerce to2 from2) e2)
294 = (match_ty to1 to2 tpl_vars $
295 match_ty from1 from2 tpl_vars $
296 match e1 e2 tpl_vars kont) subst
299 {- I don't buy this let-rule any more
300 The let rule fails on matching
301 forall f,x,xs. f (x:xs)
303 f (let y = e in (y:[]))
304 because we just get x->y, which is bogus.
306 -- This is an interesting rule: we simply ignore lets in the
307 -- term being matched against! The unfolding inside it is (by assumption)
308 -- already inside any occurrences of the bound variables, so we'll expand
309 -- them when we encounter them. Meanwhile, we can't get false matches because
310 -- (also by assumption) the term being matched has no shadowing.
311 match e1 (Let bind e2) tpl_vars kont subst
312 = match e1 e2 tpl_vars kont subst
315 -- Here is another important rule: if the term being matched is a
316 -- variable, we expand it so long as its unfolding is a WHNF
317 -- (Its occurrence information is not necessarily up to date,
318 -- so we don't use it.)
319 match e1 (Var v2) tpl_vars kont subst
320 | isCheapUnfolding unfolding
321 = match e1 (unfoldingTemplate unfolding) tpl_vars kont subst
323 unfolding = getIdUnfolding v2
326 -- We can't cope with lets in the template
328 match e1 e2 tpl_vars kont subst = match_fail
331 ------------------------------------------
332 match_alts [] [] tpl_vars kont subst
334 match_alts ((c1,vs1,r1):alts1) ((c2,vs2,r2):alts2) tpl_vars kont subst
336 = bind vs1 vs2 (match r1 r2) tpl_vars
337 (match_alts alts1 alts2 tpl_vars kont)
339 match_alts alts1 alts2 tpl_vars kont subst = match_fail
341 lt_alt (con1, _, _) (con2, _, _) = con1 < con2
343 ----------------------------------------
344 bind :: [CoreBndr] -- Template binders
345 -> [CoreBndr] -- Target binders
348 -- This makes uses of assumption (A) above. For example,
350 -- Template: (\x.y) (y is free)
351 -- Target : (\y.y) (y is bound)
352 -- We rename x to y in the template... but then erroneously
353 -- match y against y. But this can't happen because of (A)
354 bind vs1 vs2 matcher tpl_vars kont subst
355 = WARN( not (all not_in_subst vs1), bug_msg )
356 matcher tpl_vars kont' subst'
358 kont' subst'' = kont (unBindSubstList subst'' vs1 vs2)
359 subst' = bindSubstList subst vs1 vs2
361 -- The unBindSubst relies on no shadowing in the template
362 not_in_subst v = not (maybeToBool (lookupSubst subst v))
363 bug_msg = sep [ppr vs1, ppr vs2]
365 ----------------------------------------
366 match_ty ty1 ty2 tpl_vars kont subst
367 = case Unify.match ty1 ty2 tpl_vars Just (substEnv subst) of
368 Nothing -> match_fail
369 Just senv' -> kont (setSubstEnv subst senv')
371 ----------------------------------------
372 matches [] [] tpl_vars kont subst
374 matches (e:es) (e':es') tpl_vars kont subst
375 = match e e' tpl_vars (matches es es' tpl_vars kont) subst
376 matches es es' tpl_vars kont subst
379 ----------------------------------------
380 mkVarArg :: CoreBndr -> CoreArg
381 mkVarArg v | isId v = Var v
382 | otherwise = Type (mkTyVarTy v)
385 %************************************************************************
387 \subsection{Adding a new rule}
389 %************************************************************************
392 addRule :: Id -> CoreRules -> CoreRule -> CoreRules
394 -- Insert the new rule just before a rule that is *less specific*
395 -- than the new one; or at the end if there isn't such a one.
396 -- In this way we make sure that when looking up, the first match
397 -- is the most specific.
399 -- We make no check for rules that unify without one dominating
400 -- the other. Arguably this would be a bug.
402 addRule id (Rules rules rhs_fvs) (Rule str tpl_vars tpl_args rhs)
403 = Rules (insert rules) (rhs_fvs `unionVarSet` new_rhs_fvs)
405 new_rule = Rule str tpl_vars' tpl_args rhs'
406 -- Add occ info to tpl_vars, rhs
408 (rhs_uds, rhs') = occurAnalyseExpr isLocallyDefined rhs
409 (rhs_uds1, tpl_vars') = tagBinders rhs_uds tpl_vars
411 insert [] = [new_rule]
412 insert (rule:rules) | new_is_more_specific rule = (new_rule:rule:rules)
413 | otherwise = rule : insert rules
415 new_is_more_specific rule = maybeToBool (matchRule tpl_var_set rule tpl_args)
417 tpl_var_set = mkVarSet tpl_vars'
418 -- Actually we should probably include the free vars of tpl_args,
419 -- but I can't be bothered
421 new_rhs_fvs = (exprFreeVars rhs' `minusVarSet` tpl_var_set) `delVarSet` id
423 -- Don't include the Id in its own rhs free-var set.
424 -- Otherwise the occurrence analyser makes bindings recursive
425 -- that shoudn't be. E.g.
426 -- RULE: f (f x y) z ==> f x (f y z)
428 addIdSpecialisations :: Id -> [([CoreBndr], [CoreExpr], CoreExpr)] -> Id
429 addIdSpecialisations id spec_stuff
430 = setIdSpecialisation id new_rules
432 rule_name = _PK_ ("SPEC " ++ showSDoc (ppr id))
433 new_rules = foldr add (getIdSpecialisation id) spec_stuff
434 add (vars, args, rhs) rules = addRule id rules (Rule rule_name vars args rhs)
438 %************************************************************************
440 \subsection{Preparing the rule base
442 %************************************************************************
447 Bool -- True <=> this rule was defined in this module,
448 Id -- What Id is it for
449 CoreRule -- The rule itself
452 pprProtoCoreRule (ProtoCoreRule _ fn rule) = pprCoreRule (Just fn) rule
454 lookupRule :: InScopeSet -> Id -> [CoreExpr] -> Maybe (FAST_STRING, CoreExpr, [CoreExpr])
455 lookupRule in_scope fn args
456 = case getIdSpecialisation fn of
457 Rules rules _ -> matchRules in_scope rules args
459 orphanRule :: ProtoCoreRule -> Bool
460 -- An "orphan rule" is one that is defined in this
461 -- module, but of ran *imported* function. We need
462 -- to track these separately when generating the interface file
463 orphanRule (ProtoCoreRule local fn _)
464 = local && not (isLocallyDefined fn)
468 %************************************************************************
470 \subsection{Getting the rules ready}
472 %************************************************************************
475 type RuleBase = (IdSet, -- Imported Ids that have rules attached
476 IdSet) -- Ids (whether local or imported) mentioned on
477 -- LHS of some rule; these should be black listed
479 -- The rule Ids and LHS Ids are black-listed; that is, they aren't inlined
480 -- so that the opportunity to apply the rule isn't lost too soon
482 prepareRuleBase :: [CoreBind] -> [ProtoCoreRule] -> ([CoreBind], RuleBase)
483 prepareRuleBase binds rules
484 = (map zap_bind binds, (imported_rule_ids, rule_lhs_fvs))
486 (rule_ids, rule_lhs_fvs) = foldr add_rule (emptyVarSet, emptyVarSet) rules
487 imported_rule_ids = filterVarSet (not . isLocallyDefined) rule_ids
489 -- rule_fvs is the set of all variables mentioned in rules
490 rule_fvs = foldVarSet (unionVarSet . idRuleVars) rule_lhs_fvs rule_ids
492 -- Attach the rules for each locally-defined Id to that Id.
493 -- - This makes the rules easier to look up
494 -- - It means that transformation rules and specialisations for
495 -- locally defined Ids are handled uniformly
496 -- - It keeps alive things that are referred to only from a rule
497 -- (the occurrence analyser knows about rules attached to Ids)
498 -- - It makes sure that, when we apply a rule, the free vars
499 -- of the RHS are more likely to be in scope
501 -- The LHS and RHS Ids are marked 'no-discard'.
502 -- This means that the binding won't be discarded EVEN if the binding
503 -- ends up being trivial (v = w) -- the simplifier would usually just
504 -- substitute w for v throughout, but we don't apply the substitution to
505 -- the rules (maybe we should?), so this substitution would make the rule
507 zap_bind (NonRec b r) = NonRec (zap_bndr b) r
508 zap_bind (Rec prs) = Rec [(zap_bndr b, r) | (b,r) <- prs]
510 zap_bndr bndr = case lookupVarSet rule_ids bndr of
511 Just bndr' -> setIdNoDiscard bndr'
512 Nothing | bndr `elemVarSet` rule_fvs -> setIdNoDiscard bndr
515 add_rule (ProtoCoreRule _ id rule)
516 (rule_id_set, rule_fvs)
517 = (rule_id_set `extendVarSet` new_id,
518 rule_fvs `unionVarSet` extendVarSet lhs_fvs id)
520 new_id = case lookupVarSet rule_id_set id of
521 Just id' -> addRuleToId id' rule
522 Nothing -> addRuleToId id rule
523 lhs_fvs = ruleSomeLhsFreeVars isId rule
524 -- Find *all* the free Ids of the LHS, not just
525 -- locally defined ones!!
527 addRuleToId id rule = setIdSpecialisation id (addRule id (getIdSpecialisation id) rule)