+ ImpRuleUsage
+%* *
+%************************************************************************
+
+\begin{code}
+type ImpRuleUsage = NameEnv UsageDetails
+ -- Maps an *imported* Id f to the UsageDetails for *local* Ids
+ -- used on the RHS for a *local* rule for f.
+\end{code}
+
+Note [ImpRuleUsage]
+~~~~~~~~~~~~~~~~
+Consider this, where A.g is an imported Id
+
+ f x = A.g x
+ {-# RULE "foo" forall x. A.g x = f x #-}
+
+Obviously there's a loop, but the danger is that the occurrence analyser
+will say that 'f' is not a loop breaker. Then the simplifier will
+optimise 'f' to
+ f x = f x
+and then gaily inline 'f'. Result infinite loop. More realistically,
+these kind of rules are generated when specialising imported INLINABLE Ids.
+
+Solution: treat an occurrence of A.g as an occurrence of all the local Ids
+that occur on the RULE's RHS. This mapping from imported Id to local Ids
+is held in occ_rule_fvs.
+
+\begin{code}
+findImpRuleUsage :: Maybe (Activation -> Bool) -> [CoreRule] -> ImpRuleUsage
+-- Find the *local* Ids that can be reached transitively,
+-- via local rules, from each *imported* Id.
+-- Sigh: this function seems more complicated than it is really worth
+findImpRuleUsage Nothing _ = emptyNameEnv
+findImpRuleUsage (Just is_active) rules
+ = mkNameEnv [ (f, mapUFM (\_ -> NoOccInfo) ls)
+ | f <- rule_names
+ , let ls = find_lcl_deps f
+ , not (isEmptyVarSet ls) ]
+ where
+ rule_names = map ru_fn rules
+ rule_name_set = mkNameSet rule_names
+
+ imp_deps :: NameEnv VarSet
+ -- (f,g) means imported Id 'g' appears in RHS of
+ -- rule for imported Id 'f', *or* does so transitively
+ imp_deps = foldr add_imp emptyNameEnv rules
+ add_imp rule acc
+ | is_active (ruleActivation rule)
+ = extendNameEnv_C unionVarSet acc (ru_fn rule)
+ (exprSomeFreeVars keep_imp (ru_rhs rule))
+ | otherwise = acc
+ keep_imp v = isId v && (idName v `elemNameSet` rule_name_set)
+ full_imp_deps = transClosureFV (ufmToList imp_deps)
+
+ lcl_deps :: NameEnv VarSet
+ -- (f, l) means localId 'l' appears immediately
+ -- in the RHS of a rule for imported Id 'f'
+ -- Remember, many rules might have the same ru_fn
+ -- so we do need to fold
+ lcl_deps = foldr add_lcl emptyNameEnv rules
+ add_lcl rule acc = extendNameEnv_C unionVarSet acc (ru_fn rule)
+ (exprFreeIds (ru_rhs rule))
+
+ find_lcl_deps :: Name -> VarSet
+ find_lcl_deps f
+ = foldVarSet (unionVarSet . lookup_lcl . idName) (lookup_lcl f)
+ (lookupNameEnv full_imp_deps f `orElse` emptyVarSet)
+ lookup_lcl :: Name -> VarSet
+ lookup_lcl g = lookupNameEnv lcl_deps g `orElse` emptyVarSet
+
+-------------
+transClosureFV :: Uniquable a => [(a, VarSet)] -> UniqFM VarSet
+-- If (f,g), (g,h) are in the input, then (f,h) is in the output
+transClosureFV fv_list
+ | no_change = env
+ | otherwise = transClosureFV new_fv_list
+ where
+ env = listToUFM fv_list
+ (no_change, new_fv_list) = mapAccumL bump True fv_list
+ bump no_change (b,fvs)
+ | no_change_here = (no_change, (b,fvs))
+ | otherwise = (False, (b,new_fvs))
+ where
+ (new_fvs, no_change_here) = extendFvs env fvs
+
+-------------
+extendFvs :: UniqFM VarSet -> VarSet -> (VarSet, Bool)
+-- (extendFVs env s) returns
+-- (s `union` env(s), env(s) `subset` s)
+extendFvs env s
+ = foldVarSet add (s, True) s
+ where
+ add v (vs, no_change_so_far)
+ = case lookupUFM env v of
+ Just fvs | not (fvs `subVarSet` s)
+ -> (vs `unionVarSet` fvs, False)
+ _ -> (vs, no_change_so_far)
+\end{code}
+
+
+%************************************************************************
+%* *
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