{- (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 \section[CoreRules]{Rewrite rules} -} {-# LANGUAGE CPP #-} -- | Functions for collecting together and applying rewrite rules to a module. -- The 'CoreRule' datatype itself is declared elsewhere. module GHC.Core.Rules ( -- ** Constructing emptyRuleBase, mkRuleBase, extendRuleBaseList, unionRuleBase, pprRuleBase, -- ** Checking rule applications ruleCheckProgram, -- ** Manipulating 'RuleInfo' rules extendRuleInfo, addRuleInfo, addIdSpecialisations, -- * Misc. CoreRule helpers rulesOfBinds, getRules, pprRulesForUser, lookupRule, mkRule, roughTopNames, initRuleOpts ) where #include "HsVersions.h" import GHC.Prelude import GHC.Core -- All of it import GHC.Unit.Types ( primUnitId, bignumUnitId ) import GHC.Unit.Module ( Module ) import GHC.Unit.Module.Env import GHC.Core.Subst import GHC.Core.SimpleOpt ( exprIsLambda_maybe ) import GHC.Core.FVs ( exprFreeVars, exprsFreeVars, bindFreeVars , rulesFreeVarsDSet, exprsOrphNames, exprFreeVarsList ) import GHC.Core.Utils ( exprType, eqExpr, mkTick, mkTicks , stripTicksTopT, stripTicksTopE , isJoinBind, mkCastMCo ) import GHC.Core.Ppr ( pprRules ) import GHC.Core.Type as Type ( Type, TCvSubst, extendTvSubst, extendCvSubst , mkEmptyTCvSubst, getTyVar_maybe, substTy ) import GHC.Tc.Utils.TcType ( tcSplitTyConApp_maybe ) import GHC.Builtin.Types ( anyTypeOfKind ) import GHC.Core.Coercion as Coercion import GHC.Core.Tidy ( tidyRules ) import GHC.Types.Id import GHC.Types.Id.Info ( RuleInfo( RuleInfo ) ) import GHC.Types.Var import GHC.Types.Var.Env import GHC.Types.Var.Set import GHC.Types.Name ( Name, NamedThing(..), nameIsLocalOrFrom ) import GHC.Types.Name.Set import GHC.Types.Name.Env import GHC.Types.Unique.FM import GHC.Types.Tickish import GHC.Core.Unify as Unify ( ruleMatchTyKiX ) import GHC.Types.Basic import GHC.Driver.Session ( DynFlags, gopt, targetPlatform, homeUnitId_ ) import GHC.Driver.Ppr import GHC.Driver.Flags import GHC.Utils.Outputable import GHC.Utils.Panic import GHC.Data.FastString import GHC.Data.Maybe import GHC.Data.Bag import GHC.Utils.Misc as Utils import Data.List (sortBy, mapAccumL, isPrefixOf) import Data.Function ( on ) import Control.Monad ( guard ) {- Note [Overall plumbing for rules] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * After the desugarer: - The ModGuts initially contains mg_rules :: [CoreRule] of locally-declared rules for imported Ids. - Locally-declared rules for locally-declared Ids are attached to the IdInfo for that Id. See Note [Attach rules to local ids] in GHC.HsToCore.Binds * GHC.Iface.Tidy strips off all the rules from local Ids and adds them to mg_rules, so that the ModGuts has *all* the locally-declared rules. * The HomePackageTable contains a ModDetails for each home package module. Each contains md_rules :: [CoreRule] of rules declared in that module. The HomePackageTable grows as ghc --make does its up-sweep. In batch mode (ghc -c), the HPT is empty; all imported modules are treated by the "external" route, discussed next, regardless of which package they come from. * The ExternalPackageState has a single eps_rule_base :: RuleBase for Ids in other packages. This RuleBase simply grow monotonically, as ghc --make compiles one module after another. During simplification, interface files may get demand-loaded, as the simplifier explores the unfoldings for Ids it has in its hand. (Via an unsafePerformIO; the EPS is really a cache.) That in turn may make the EPS rule-base grow. In contrast, the HPT never grows in this way. * The result of all this is that during Core-to-Core optimisation there are four sources of rules: (a) Rules in the IdInfo of the Id they are a rule for. These are easy: fast to look up, and if you apply a substitution then it'll be applied to the IdInfo as a matter of course. (b) Rules declared in this module for imported Ids, kept in the ModGuts. If you do a substitution, you'd better apply the substitution to these. There are seldom many of these. (c) Rules declared in the HomePackageTable. These never change. (d) Rules in the ExternalPackageTable. These can grow in response to lazy demand-loading of interfaces. * At the moment (c) is carried in a reader-monad way by the GHC.Core.Opt.Monad. The HomePackageTable doesn't have a single RuleBase because technically we should only be able to "see" rules "below" this module; so we generate a RuleBase for (c) by combing rules from all the modules "below" us. That's why we can't just select the home-package RuleBase from HscEnv. [NB: we are inconsistent here. We should do the same for external packages, but we don't. Same for type-class instances.] * So in the outer simplifier loop, we combine (b-d) into a single RuleBase, reading (b) from the ModGuts, (c) from the GHC.Core.Opt.Monad, and (d) from its mutable variable [Of course this means that we won't see new EPS rules that come in during a single simplifier iteration, but that probably does not matter.] ************************************************************************ * * \subsection[specialisation-IdInfo]{Specialisation info about an @Id@} * * ************************************************************************ A @CoreRule@ holds details of one rule for an @Id@, which includes its specialisations. For example, if a rule for @f@ contains the mapping: \begin{verbatim} forall a b d. [Type (List a), Type b, Var d] ===> f' a b \end{verbatim} then when we find an application of f to matching types, we simply replace it by the matching RHS: \begin{verbatim} f (List Int) Bool dict ===> f' Int Bool \end{verbatim} All the stuff about how many dictionaries to discard, and what types to apply the specialised function to, are handled by the fact that the Rule contains a template for the result of the specialisation. There is one more exciting case, which is dealt with in exactly the same way. If the specialised value is unboxed then it is lifted at its definition site and unlifted at its uses. For example: pi :: forall a. Num a => a might have a specialisation [Int#] ===> (case pi' of Lift pi# -> pi#) where pi' :: Lift Int# is the specialised version of pi. -} mkRule :: Module -> Bool -> Bool -> RuleName -> Activation -> Name -> [CoreBndr] -> [CoreExpr] -> CoreExpr -> CoreRule -- ^ Used to make 'CoreRule' for an 'Id' defined in the module being -- compiled. See also 'GHC.Core.CoreRule' mkRule this_mod is_auto is_local name act fn bndrs args rhs = Rule { ru_name = name, ru_fn = fn, ru_act = act, ru_bndrs = bndrs, ru_args = args, ru_rhs = rhs, ru_rough = roughTopNames args, ru_origin = this_mod, ru_orphan = orph, ru_auto = is_auto, ru_local = is_local } where -- Compute orphanhood. See Note [Orphans] in GHC.Core.InstEnv -- A rule is an orphan only if none of the variables -- mentioned on its left-hand side are locally defined lhs_names = extendNameSet (exprsOrphNames args) fn -- Since rules get eventually attached to one of the free names -- from the definition when compiling the ABI hash, we should make -- it deterministic. This chooses the one with minimal OccName -- as opposed to uniq value. local_lhs_names = filterNameSet (nameIsLocalOrFrom this_mod) lhs_names orph = chooseOrphanAnchor local_lhs_names -------------- roughTopNames :: [CoreExpr] -> [Maybe Name] -- ^ Find the \"top\" free names of several expressions. -- Such names are either: -- -- 1. The function finally being applied to in an application chain -- (if that name is a GlobalId: see "GHC.Types.Var#globalvslocal"), or -- -- 2. The 'TyCon' if the expression is a 'Type' -- -- This is used for the fast-match-check for rules; -- if the top names don't match, the rest can't roughTopNames args = map roughTopName args roughTopName :: CoreExpr -> Maybe Name roughTopName (Type ty) = case tcSplitTyConApp_maybe ty of Just (tc,_) -> Just (getName tc) Nothing -> Nothing roughTopName (Coercion _) = Nothing roughTopName (App f _) = roughTopName f roughTopName (Var f) | isGlobalId f -- Note [Care with roughTopName] , isDataConWorkId f || idArity f > 0 = Just (idName f) roughTopName (Tick t e) | tickishFloatable t = roughTopName e roughTopName _ = Nothing ruleCantMatch :: [Maybe Name] -> [Maybe Name] -> Bool -- ^ @ruleCantMatch tpl actual@ returns True only if @actual@ -- definitely can't match @tpl@ by instantiating @tpl@. -- It's only a one-way match; unlike instance matching we -- don't consider unification. -- -- Notice that [_$_] -- @ruleCantMatch [Nothing] [Just n2] = False@ -- Reason: a template variable can be instantiated by a constant -- Also: -- @ruleCantMatch [Just n1] [Nothing] = False@ -- Reason: a local variable @v@ in the actuals might [_$_] ruleCantMatch (Just n1 : ts) (Just n2 : as) = n1 /= n2 || ruleCantMatch ts as ruleCantMatch (_ : ts) (_ : as) = ruleCantMatch ts as ruleCantMatch _ _ = False {- Note [Care with roughTopName] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this module M where { x = a:b } module N where { ...f x... RULE f (p:q) = ... } You'd expect the rule to match, because the matcher can look through the unfolding of 'x'. So we must avoid roughTopName returning 'M.x' for the call (f x), or else it'll say "can't match" and we won't even try!! However, suppose we have RULE g (M.h x) = ... foo = ...(g (M.k v)).... where k is a *function* exported by M. We never really match functions (lambdas) except by name, so in this case it seems like a good idea to treat 'M.k' as a roughTopName of the call. -} pprRulesForUser :: [CoreRule] -> SDoc -- (a) tidy the rules -- (b) sort them into order based on the rule name -- (c) suppress uniques (unless -dppr-debug is on) -- This combination makes the output stable so we can use in testing -- It's here rather than in GHC.Core.Ppr because it calls tidyRules pprRulesForUser rules = withPprStyle defaultUserStyle $ pprRules $ sortBy (lexicalCompareFS `on` ruleName) $ tidyRules emptyTidyEnv rules {- ************************************************************************ * * RuleInfo: the rules in an IdInfo * * ************************************************************************ -} extendRuleInfo :: RuleInfo -> [CoreRule] -> RuleInfo extendRuleInfo (RuleInfo rs1 fvs1) rs2 = RuleInfo (rs2 ++ rs1) (rulesFreeVarsDSet rs2 `unionDVarSet` fvs1) addRuleInfo :: RuleInfo -> RuleInfo -> RuleInfo addRuleInfo (RuleInfo rs1 fvs1) (RuleInfo rs2 fvs2) = RuleInfo (rs1 ++ rs2) (fvs1 `unionDVarSet` fvs2) addIdSpecialisations :: Id -> [CoreRule] -> Id addIdSpecialisations id rules | null rules = id | otherwise = setIdSpecialisation id $ extendRuleInfo (idSpecialisation id) rules -- | Gather all the rules for locally bound identifiers from the supplied bindings rulesOfBinds :: [CoreBind] -> [CoreRule] rulesOfBinds binds = concatMap (concatMap idCoreRules . bindersOf) binds getRules :: RuleEnv -> Id -> [CoreRule] -- See Note [Where rules are found] getRules (RuleEnv { re_base = rule_base, re_visible_orphs = orphs }) fn = idCoreRules fn ++ filter (ruleIsVisible orphs) imp_rules where imp_rules = lookupNameEnv rule_base (idName fn) `orElse` [] ruleIsVisible :: ModuleSet -> CoreRule -> Bool ruleIsVisible _ BuiltinRule{} = True ruleIsVisible vis_orphs Rule { ru_orphan = orph, ru_origin = origin } = notOrphan orph || origin `elemModuleSet` vis_orphs {- Note [Where rules are found] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The rules for an Id come from two places: (a) the ones it is born with, stored inside the Id itself (idCoreRules fn), (b) rules added in other modules, stored in the global RuleBase (imp_rules) It's tempting to think that - LocalIds have only (a) - non-LocalIds have only (b) but that isn't quite right: - PrimOps and ClassOps are born with a bunch of rules inside the Id, even when they are imported - The rules in GHC.Core.Opt.ConstantFold.builtinRules should be active even in the module defining the Id (when it's a LocalId), but the rules are kept in the global RuleBase ************************************************************************ * * RuleBase * * ************************************************************************ -} -- RuleBase itself is defined in GHC.Core, along with CoreRule emptyRuleBase :: RuleBase emptyRuleBase = emptyNameEnv mkRuleBase :: [CoreRule] -> RuleBase mkRuleBase rules = extendRuleBaseList emptyRuleBase rules extendRuleBaseList :: RuleBase -> [CoreRule] -> RuleBase extendRuleBaseList rule_base new_guys = foldl' extendRuleBase rule_base new_guys unionRuleBase :: RuleBase -> RuleBase -> RuleBase unionRuleBase rb1 rb2 = plusNameEnv_C (++) rb1 rb2 extendRuleBase :: RuleBase -> CoreRule -> RuleBase extendRuleBase rule_base rule = extendNameEnv_Acc (:) Utils.singleton rule_base (ruleIdName rule) rule pprRuleBase :: RuleBase -> SDoc pprRuleBase rules = pprUFM rules $ \rss -> vcat [ pprRules (tidyRules emptyTidyEnv rs) | rs <- rss ] {- ************************************************************************ * * Matching * * ************************************************************************ -} -- | The main rule matching function. Attempts to apply all (active) -- supplied rules to this instance of an application in a given -- context, returning the rule applied and the resulting expression if -- successful. lookupRule :: RuleOpts -> InScopeEnv -> (Activation -> Bool) -- When rule is active -> Id -> [CoreExpr] -> [CoreRule] -> Maybe (CoreRule, CoreExpr) -- See Note [Extra args in the target] -- See comments on matchRule lookupRule opts rule_env@(in_scope,_) is_active fn args rules = -- pprTrace "matchRules" (ppr fn <+> ppr args $$ ppr rules ) $ case go [] rules of [] -> Nothing (m:ms) -> Just (findBest in_scope (fn,args') m ms) where rough_args = map roughTopName args -- Strip ticks from arguments, see note [Tick annotations in RULE -- matching]. We only collect ticks if a rule actually matches - -- this matters for performance tests. args' = map (stripTicksTopE tickishFloatable) args ticks = concatMap (stripTicksTopT tickishFloatable) args go :: [(CoreRule,CoreExpr)] -> [CoreRule] -> [(CoreRule,CoreExpr)] go ms [] = ms go ms (r:rs) | Just e <- matchRule opts rule_env is_active fn args' rough_args r = go ((r,mkTicks ticks e):ms) rs | otherwise = -- pprTrace "match failed" (ppr r $$ ppr args $$ -- ppr [ (arg_id, unfoldingTemplate unf) -- | Var arg_id <- args -- , let unf = idUnfolding arg_id -- , isCheapUnfolding unf] ) go ms rs findBest :: InScopeSet -> (Id, [CoreExpr]) -> (CoreRule,CoreExpr) -> [(CoreRule,CoreExpr)] -> (CoreRule,CoreExpr) -- All these pairs matched the expression -- Return the pair the most specific rule -- The (fn,args) is just for overlap reporting findBest _ _ (rule,ans) [] = (rule,ans) findBest in_scope target (rule1,ans1) ((rule2,ans2):prs) | isMoreSpecific in_scope rule1 rule2 = findBest in_scope target (rule1,ans1) prs | isMoreSpecific in_scope rule2 rule1 = findBest in_scope target (rule2,ans2) prs | debugIsOn = let pp_rule rule = ifPprDebug (ppr rule) (doubleQuotes (ftext (ruleName rule))) in pprTrace "Rules.findBest: rule overlap (Rule 1 wins)" (vcat [ whenPprDebug $ text "Expression to match:" <+> ppr fn <+> sep (map ppr args) , text "Rule 1:" <+> pp_rule rule1 , text "Rule 2:" <+> pp_rule rule2]) $ findBest in_scope target (rule1,ans1) prs | otherwise = findBest in_scope target (rule1,ans1) prs where (fn,args) = target isMoreSpecific :: InScopeSet -> CoreRule -> CoreRule -> Bool -- This tests if one rule is more specific than another -- We take the view that a BuiltinRule is less specific than -- anything else, because we want user-define rules to "win" -- In particular, class ops have a built-in rule, but we -- any user-specific rules to win -- eg (#4397) -- truncate :: (RealFrac a, Integral b) => a -> b -- {-# RULES "truncate/Double->Int" truncate = double2Int #-} -- double2Int :: Double -> Int -- We want the specific RULE to beat the built-in class-op rule isMoreSpecific _ (BuiltinRule {}) _ = False isMoreSpecific _ (Rule {}) (BuiltinRule {}) = True isMoreSpecific in_scope (Rule { ru_bndrs = bndrs1, ru_args = args1 }) (Rule { ru_bndrs = bndrs2, ru_args = args2 , ru_name = rule_name2, ru_rhs = rhs2 }) = isJust (matchN (full_in_scope, id_unfolding_fun) rule_name2 bndrs2 args2 args1 rhs2) where id_unfolding_fun _ = NoUnfolding -- Don't expand in templates full_in_scope = in_scope `extendInScopeSetList` bndrs1 noBlackList :: Activation -> Bool noBlackList _ = False -- Nothing is black listed {- Note [Extra args in the target] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we find a matching rule, we return (Just (rule, rhs)), /but/ the rule firing has only consumed as many of the input args as the ruleArity says. The unused arguments are handled by the code in GHC.Core.Opt.Simplify.tryRules, using the arity of the returned rule. E.g. Rule "foo": forall a b. f p1 p2 = rhs Target: f e1 e2 e3 Then lookupRule returns Just (Rule "foo", rhs), where Rule "foo" has ruleArity 2. The real rewrite is f e1 e2 e3 ==> rhs e3 You might think it'd be cleaner for lookupRule to deal with the leftover arguments, by applying 'rhs' to them, but the main call in the Simplifier works better as it is. Reason: the 'args' passed to lookupRule are the result of a lazy substitution Historical note: At one stage I tried to match even if there are more args in the /template/ than the target. I now think this is probably a bad idea. Should the template (map f xs) match (map g)? I think not. For a start, in general eta expansion wastes work. SLPJ July 99 -} ------------------------------------ matchRule :: RuleOpts -> InScopeEnv -> (Activation -> Bool) -> Id -> [CoreExpr] -> [Maybe Name] -> CoreRule -> Maybe CoreExpr -- If (matchRule rule args) returns Just (name,rhs) -- then (f args) matches the rule, and the corresponding -- rewritten RHS is rhs -- -- The returned expression is occurrence-analysed -- -- Example -- -- The rule -- forall f g x. map f (map g x) ==> map (f . g) x -- is stored -- CoreRule "map/map" -- [f,g,x] -- tpl_vars -- [f,map g x] -- tpl_args -- map (f.g) x) -- rhs -- -- Then the call: matchRule the_rule [e1,map e2 e3] -- = Just ("map/map", (\f,g,x -> rhs) e1 e2 e3) -- -- Any 'surplus' arguments in the input are simply put on the end -- of the output. matchRule opts rule_env _is_active fn args _rough_args (BuiltinRule { ru_try = match_fn }) -- Built-in rules can't be switched off, it seems = case match_fn opts rule_env fn args of Nothing -> Nothing Just expr -> Just expr matchRule _ rule_env is_active _ args rough_args (Rule { ru_name = rule_name, ru_act = act, ru_rough = tpl_tops , ru_bndrs = tpl_vars, ru_args = tpl_args, ru_rhs = rhs }) | not (is_active act) = Nothing | ruleCantMatch tpl_tops rough_args = Nothing | otherwise = matchN rule_env rule_name tpl_vars tpl_args args rhs -- | Initialize RuleOpts from DynFlags initRuleOpts :: DynFlags -> RuleOpts initRuleOpts dflags = RuleOpts { roPlatform = targetPlatform dflags , roNumConstantFolding = gopt Opt_NumConstantFolding dflags , roExcessRationalPrecision = gopt Opt_ExcessPrecision dflags -- disable bignum rules in ghc-prim and ghc-bignum itself , roBignumRules = homeUnitId_ dflags /= primUnitId && homeUnitId_ dflags /= bignumUnitId } --------------------------------------- matchN :: InScopeEnv -> RuleName -> [Var] -> [CoreExpr] -> [CoreExpr] -> CoreExpr -- ^ Target; can have more elements than the template -> Maybe CoreExpr -- For a given match template and context, find bindings to wrap around -- the entire result and what should be substituted for each template variable. -- -- Fail if there are too few actual arguments from the target to match the template -- -- See Note [Extra args in the target] -- If there are too /many/ actual arguments, we simply ignore the -- trailing ones, returning the result of applying the rule to a prefix -- of the actual arguments. matchN (in_scope, id_unf) rule_name tmpl_vars tmpl_es target_es rhs = do { rule_subst <- match_exprs init_menv emptyRuleSubst tmpl_es target_es ; let (_, matched_es) = mapAccumL (lookup_tmpl rule_subst) (mkEmptyTCvSubst in_scope) $ tmpl_vars `zip` tmpl_vars1 bind_wrapper = rs_binds rule_subst -- Floated bindings; see Note [Matching lets] ; return (bind_wrapper $ mkLams tmpl_vars rhs `mkApps` matched_es) } where (init_rn_env, tmpl_vars1) = mapAccumL rnBndrL (mkRnEnv2 in_scope) tmpl_vars -- See Note [Cloning the template binders] init_menv = RV { rv_tmpls = mkVarSet tmpl_vars1 , rv_lcl = init_rn_env , rv_fltR = mkEmptySubst (rnInScopeSet init_rn_env) , rv_unf = id_unf } lookup_tmpl :: RuleSubst -> TCvSubst -> (InVar,OutVar) -> (TCvSubst, CoreExpr) -- Need to return a RuleSubst solely for the benefit of mk_fake_ty lookup_tmpl (RS { rs_tv_subst = tv_subst, rs_id_subst = id_subst }) tcv_subst (tmpl_var, tmpl_var1) | isId tmpl_var1 = case lookupVarEnv id_subst tmpl_var1 of Just e | Coercion co <- e -> (Type.extendCvSubst tcv_subst tmpl_var1 co, Coercion co) | otherwise -> (tcv_subst, e) Nothing | Just refl_co <- isReflCoVar_maybe tmpl_var1 , let co = Coercion.substCo tcv_subst refl_co -> -- See Note [Unbound RULE binders] (Type.extendCvSubst tcv_subst tmpl_var1 co, Coercion co) | otherwise -> unbound tmpl_var | otherwise = (Type.extendTvSubst tcv_subst tmpl_var1 ty', Type ty') where ty' = case lookupVarEnv tv_subst tmpl_var1 of Just ty -> ty Nothing -> fake_ty -- See Note [Unbound RULE binders] fake_ty = anyTypeOfKind (Type.substTy tcv_subst (tyVarKind tmpl_var1)) -- This substitution is the sole reason we accumulate -- TCvSubst in lookup_tmpl unbound tmpl_var = pprPanic "Template variable unbound in rewrite rule" $ vcat [ text "Variable:" <+> ppr tmpl_var <+> dcolon <+> ppr (varType tmpl_var) , text "Rule" <+> pprRuleName rule_name , text "Rule bndrs:" <+> ppr tmpl_vars , text "LHS args:" <+> ppr tmpl_es , text "Actual args:" <+> ppr target_es ] {- Note [Unbound RULE binders] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It can be the case that the binder in a rule is not actually bound on the LHS: * Type variables. Type synonyms with phantom args can give rise to unbound template type variables. Consider this (#10689, simplCore/should_compile/T10689): type Foo a b = b f :: Eq a => a -> Bool f x = x==x {-# RULES "foo" forall (x :: Foo a Char). f x = True #-} finkle = f 'c' The rule looks like forall (a::*) (d::Eq Char) (x :: Foo a Char). f (Foo a Char) d x = True Matching the rule won't bind 'a', and legitimately so. We fudge by pretending that 'a' is bound to (Any :: *). * Coercion variables. On the LHS of a RULE for a local binder we might have RULE forall (c :: a~b). f (x |> c) = e Now, if that binding is inlined, so that a=b=Int, we'd get RULE forall (c :: Int~Int). f (x |> c) = e and now when we simplify the LHS (Simplify.simplRule) we optCoercion (look at the CoVarCo case) will turn that 'c' into Refl: RULE forall (c :: Int~Int). f (x |> ) = e and then perhaps drop it altogether. Now 'c' is unbound. It's tricky to be sure this never happens, so instead I say it's OK to have an unbound coercion binder in a RULE provided its type is (c :: t~t). Then, when the RULE fires we can substitute for c. This actually happened (in a RULE for a local function) in #13410, and also in test T10602. Note [Cloning the template binders] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following match (example 1): Template: forall x. f x Target: f (x+1) This should succeed, because the template variable 'x' has nothing to do with the 'x' in the target. Likewise this one (example 2): Template: forall x. f (\x.x) Target: f (\y.y) We achieve this simply by using rnBndrL to clone the template binders if they are already in scope. ------ Historical note ------- At one point I tried simply adding the template binders to the in-scope set /without/ cloning them, but that failed in a horribly obscure way in #14777. Problem was that during matching we look up target-term variables in the in-scope set (see Note [Lookup in-scope]). If a target-term variable happens to name-clash with a template variable, that lookup will find the template variable, which is /utterly/ bogus. In #14777, this transformed a term variable into a type variable, and then crashed when we wanted its idInfo. ------ End of historical note ------- ************************************************************************ * * The main matcher * * ********************************************************************* -} data RuleMatchEnv = RV { rv_lcl :: RnEnv2 -- Renamings for *local bindings* -- (lambda/case) , rv_tmpls :: VarSet -- Template variables -- (after applying envL of rv_lcl) , rv_fltR :: Subst -- Renamings for floated let-bindings -- (domain disjoint from envR of rv_lcl) -- See Note [Matching lets] -- N.B. The InScopeSet of rv_fltR is always ignored; -- see (4) in Note [Matching lets]. , rv_unf :: IdUnfoldingFun } {- Note [rv_lcl in RuleMatchEnv] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider matching Template: \x->f Target: \f->f where 'f' is free in the template. When we meet the lambdas we must remember to rename f :-> f' in the target, as well as x :-> f in the template. The rv_lcl::RnEnv2 does that. Similarly, consider matching Template: {a} \b->b Target: \a->3 We must rename the \a. Otherwise when we meet the lambdas we might substitute [b :-> a] in the template, and then erroneously succeed in matching what looks like the template variable 'a' against 3. So we must add the template vars to the in-scope set before starting; see `init_menv` in `matchN`. -} rvInScopeEnv :: RuleMatchEnv -> InScopeEnv rvInScopeEnv renv = (rnInScopeSet (rv_lcl renv), rv_unf renv) -- * The domain of the TvSubstEnv and IdSubstEnv are the template -- variables passed into the match. -- -- * The BindWrapper in a RuleSubst are the bindings floated out -- from nested matches; see the Let case of match, below -- data RuleSubst = RS { rs_tv_subst :: TvSubstEnv -- Range is the , rs_id_subst :: IdSubstEnv -- template variables , rs_binds :: BindWrapper -- Floated bindings , rs_bndrs :: [Var] -- Variables bound by floated lets } type BindWrapper = CoreExpr -> CoreExpr -- See Notes [Matching lets] and [Matching cases] -- we represent the floated bindings as a core-to-core function emptyRuleSubst :: RuleSubst emptyRuleSubst = RS { rs_tv_subst = emptyVarEnv, rs_id_subst = emptyVarEnv , rs_binds = \e -> e, rs_bndrs = [] } ---------------------- match_exprs :: RuleMatchEnv -> RuleSubst -> [CoreExpr] -- Templates -> [CoreExpr] -- Targets -> Maybe RuleSubst -- If the targets are longer than templates, succeed, simply ignoring -- the leftover targets. This matters in the call in matchN. -- -- Precondition: corresponding elements of es1 and es2 have the same -- type, assumuing earlier elements match -- Example: f :: forall v. v -> blah -- match_exprs [Type a, y::a] [Type Int, 3] -- Then, after matching Type a against Type Int, -- the type of (y::a) matches that of (3::Int) match_exprs _ subst [] _ = Just subst match_exprs renv subst (e1:es1) (e2:es2) = do { subst' <- match renv subst e1 e2 MRefl ; match_exprs renv subst' es1 es2 } match_exprs _ _ _ _ = Nothing -- I now think this is probably a bad idea. -- Should the template (map f xs) match (map g)? I think not. -- For a start, in general eta expansion wastes work. -- SLPJ July 99 {- Note [Casts in the target] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As far as possible we don't want casts in the target to get in the way of matching. E.g. * (let bind in e) |> co * (case e of alts) |> co * (\ a b. f a b) |> co In the first two cases we want to float the cast inwards so we can match on the let/case. This is not important in practice because the Simplifier does this anyway. But the third case /is/ important: we don't want the cast to get in the way of eta-reduction. See Note [Cancel reflexive casts] for a real life example. The most convenient thing is to make 'match' take an MCoercion argument, thus: * The main matching function match env subst template target mco matches template ~ (target |> mco) * Invariant: typeof( subst(template) ) = typeof( target |> mco ) Note that for applications (e1 e2) ~ (d1 d2) |> co where 'co' is non-reflexive, we simply fail. You might wonder about (e1 e2) ~ ((d1 |> co1) d2) |> co2 but the Simplifer pushes the casts in an application to to the right, if it can, so this doesn't really arise. Note [Coercion arguments] ~~~~~~~~~~~~~~~~~~~~~~~~~ What if we have (f co) in the template, where the 'co' is a coercion argument to f? Right now we have nothing in place to ensure that a coercion /argument/ in the template is a variable. We really should, perhaps by abstracting over that variable. C.f. the treatment of dictionaries in GHC.HsToCore.Binds.decompseRuleLhs. For now, though, we simply behave badly, by failing in match_co. We really should never rely on matching the structure of a coercion (which is just a proof). Note [Casts in the template] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the definition f x = e, and SpecConstr on call pattern f ((e1,e2) |> co) We'll make a RULE RULE forall a,b,g. f ((a,b)|> g) = $sf a b g $sf a b g = e[ ((a,b)|> g) / x ] So here is the invariant: In the template, in a cast (e |> co), the cast `co` is always a /variable/. Matching should bind that variable to an actual coercion, so that we can use it in $sf. So a Cast on the LHS (the template) calls match_co, which succeeds when the template cast is a variable -- which it always is. That is why match_co has so few cases. See also * Note [Coercion arguments] * Note [Matching coercion variables] in GHC.Core.Unify. * Note [Cast swizzling on rule LHSs] in GHC.Core.Opt.Simplify.Utils: sm_cast_swizzle is switched off in the template of a RULE -} ---------------------- match :: RuleMatchEnv -> RuleSubst -- Substitution applies to template only -> CoreExpr -- Template -> CoreExpr -- Target -> MCoercion -> Maybe RuleSubst -- Postcondition (TypeInv): if matching succeeds, then -- typeof( subst(template) ) = typeof( target |> mco ) -- But this is /not/ a pre-condition! The types of template and target -- may differ, see the (App e1 e2) case -- -- Invariant (CoInv): if mco :: ty ~ ty, then it is MRefl, not MCo co -- See Note [Cancel reflexive casts] -- -- See the notes with Unify.match, which matches types -- Everything is very similar for terms ------------------------ Ticks --------------------- -- We look through certain ticks. See Note [Tick annotations in RULE matching] match renv subst e1 (Tick t e2) mco | tickishFloatable t = match renv subst' e1 e2 mco | otherwise = Nothing where subst' = subst { rs_binds = rs_binds subst . mkTick t } match renv subst e@(Tick t e1) e2 mco | tickishFloatable t -- Ignore floatable ticks in rule template. = match renv subst e1 e2 mco | otherwise = pprPanic "Tick in rule" (ppr e) ------------------------ Types --------------------- match renv subst (Type ty1) (Type ty2) _mco = match_ty renv subst ty1 ty2 ------------------------ Coercions --------------------- -- See Note [Coercion argument] for why this isn't really right match renv subst (Coercion co1) (Coercion co2) MRefl = match_co renv subst co1 co2 -- The MCo case corresponds to matching co ~ (co2 |> co3) -- and I have no idea what to do there -- or even if it can occur -- Failing seems the simplest thing to do; it's certainly safe. ------------------------ Casts --------------------- -- See Note [Casts in the template] -- Note [Casts in the target] -- Note [Cancel reflexive casts] match renv subst e1 (Cast e2 co2) mco = match renv subst e1 e2 (checkReflexiveMCo (mkTransMCoR co2 mco)) -- checkReflexiveMCo: cancel casts if possible -- This is important: see Note [Cancel reflexive casts] match renv subst (Cast e1 co1) e2 mco = -- See Note [Casts in the template] do { let co2 = case mco of MRefl -> mkRepReflCo (exprType e2) MCo co2 -> co2 ; subst1 <- match_co renv subst co1 co2 -- If match_co succeeds, then (exprType e1) = (exprType e2) -- Hence the MRefl in the next line ; match renv subst1 e1 e2 MRefl } ------------------------ Literals --------------------- match _ subst (Lit lit1) (Lit lit2) mco | lit1 == lit2 = ASSERT2(isReflMCo mco, ppr mco) Just subst ------------------------ Variables --------------------- -- The Var case follows closely what happens in GHC.Core.Unify.match match renv subst (Var v1) e2 mco = match_var renv subst v1 (mkCastMCo e2 mco) match renv subst e1 (Var v2) mco -- Note [Expanding variables] | not (inRnEnvR rn_env v2) -- Note [Do not expand locally-bound variables] , Just e2' <- expandUnfolding_maybe (rv_unf renv v2') = match (renv { rv_lcl = nukeRnEnvR rn_env }) subst e1 e2' mco where v2' = lookupRnInScope rn_env v2 rn_env = rv_lcl renv -- Notice that we look up v2 in the in-scope set -- See Note [Lookup in-scope] -- No need to apply any renaming first (hence no rnOccR) -- because of the not-inRnEnvR ------------------------ Applications --------------------- -- Note the match on MRefl! We fail if there is a cast in the target -- (e1 e2) ~ (d1 d2) |> co -- See Note [Cancel reflexive casts]: in the Cast equations for 'match' -- we agressively ensure that if MCo is reflective, it really is MRefl. match renv subst (App f1 a1) (App f2 a2) MRefl = do { subst' <- match renv subst f1 f2 MRefl ; match renv subst' a1 a2 MRefl } ------------------------ Float lets --------------------- match renv subst e1 (Let bind e2) mco | -- pprTrace "match:Let" (vcat [ppr bind, ppr $ okToFloat (rv_lcl renv) (bindFreeVars bind)]) $ not (isJoinBind bind) -- can't float join point out of argument position , okToFloat (rv_lcl renv) (bindFreeVars bind) -- See Note [Matching lets] = match (renv { rv_fltR = flt_subst' , rv_lcl = rv_lcl renv `extendRnInScopeSetList` new_bndrs }) -- We are floating the let-binding out, as if it had enclosed -- the entire target from Day 1. So we must add its binders to -- the in-scope set (#20200) (subst { rs_binds = rs_binds subst . Let bind' , rs_bndrs = new_bndrs ++ rs_bndrs subst }) e1 e2 mco | otherwise = Nothing where in_scope = rnInScopeSet (rv_lcl renv) `extendInScopeSetList` rs_bndrs subst -- in_scope: see (4) in Note [Matching lets] flt_subst = rv_fltR renv `setInScope` in_scope (flt_subst', bind') = substBind flt_subst bind new_bndrs = bindersOf bind' ------------------------ Lambdas --------------------- match renv subst (Lam x1 e1) e2 mco | Just (x2, e2', ts) <- exprIsLambda_maybe (rvInScopeEnv renv) (mkCastMCo e2 mco) -- See Note [Lambdas in the template] = let renv' = rnMatchBndr2 renv x1 x2 subst' = subst { rs_binds = rs_binds subst . flip (foldr mkTick) ts } in match renv' subst' e1 e2' MRefl match renv subst e1 e2@(Lam {}) mco | Just (renv', e2') <- eta_reduce renv e2 -- See Note [Eta reduction in the target] = match renv' subst e1 e2' mco {- Note [Lambdas in the template] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we match Template: (\x. blah_template) Target: (\y. blah_target) then we want to match inside the lambdas, using rv_lcl to match up x and y. But what about this? Template (\x. (blah1 |> cv)) Target (\y. blah2) |> co This happens quite readily, because the Simplifier generally moves casts outside lambdas: see Note [Casts and lambdas] in GHC.Core.Opt.Simplify.Utils. So, tiresomely, we want to push `co` back inside, which is what `exprIsLambda_maybe` does. But we've stripped off that cast, so now we need to put it back, hence mkCastMCo. Unlike the target, where we attempt eta-reduction, we do not attempt to eta-reduce the template, and may therefore fail on Template: \x. f True x Target f True It's not especially easy to deal with eta reducing the template, and never happens, because no one write eta-expanded left-hand-sides. -} ------------------------ Case expression --------------------- {- Disabled: see Note [Matching cases] below match renv (tv_subst, id_subst, binds) e1 (Case scrut case_bndr ty [(con, alt_bndrs, rhs)]) | exprOkForSpeculation scrut -- See Note [Matching cases] , okToFloat rn_env bndrs (exprFreeVars scrut) = match (renv { me_env = rn_env' }) (tv_subst, id_subst, binds . case_wrap) e1 rhs where rn_env = me_env renv rn_env' = extendRnInScopeList rn_env bndrs bndrs = case_bndr : alt_bndrs case_wrap rhs' = Case scrut case_bndr ty [(con, alt_bndrs, rhs')] -} match renv subst (Case e1 x1 ty1 alts1) (Case e2 x2 ty2 alts2) mco = do { subst1 <- match_ty renv subst ty1 ty2 ; subst2 <- match renv subst1 e1 e2 MRefl ; let renv' = rnMatchBndr2 renv x1 x2 ; match_alts renv' subst2 alts1 alts2 mco -- Alts are both sorted } -- Everything else fails match _ _ _e1 _e2 _mco = -- pprTrace "Failing at" ((text "e1:" <+> ppr _e1) $$ (text "e2:" <+> ppr _e2)) $ Nothing ------------- eta_reduce :: RuleMatchEnv -> CoreExpr -> Maybe (RuleMatchEnv, CoreExpr) -- See Note [Eta reduction in the target] eta_reduce renv e@(Lam {}) = go renv id [] e where go :: RuleMatchEnv -> BindWrapper -> [Var] -> CoreExpr -> Maybe (RuleMatchEnv, CoreExpr) go renv bw vs (Let b e) = go renv (bw . Let b) vs e go renv bw vs (Lam v e) = go renv' bw (v':vs) e where (rn_env', v') = rnBndrR (rv_lcl renv) v renv' = renv { rv_lcl = rn_env' } go renv bw (v:vs) (App f arg) | Var a <- arg, v == rnOccR (rv_lcl renv) a = go renv bw vs f | Type ty <- arg, Just tv <- getTyVar_maybe ty , v == rnOccR (rv_lcl renv) tv = go renv bw vs f go renv bw [] e = Just (renv, bw e) go _ _ (_:_) _ = Nothing eta_reduce _ _ = Nothing {- Note [Eta reduction in the target] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we are faced with this (#19790) Template {x} f x Target (\a b c. let blah in f x a b c) You might wonder why we have an eta-expanded target (see first subtle point below), but regardless of how it came about, we'd like eta-expansion not to impede matching. So eta_reduce does on-the-fly eta-reduction of the target expression. Given (\a b c. let blah in e a b c), it returns (let blah in e). Subtle points: * Consider a target: \x. f x In the main eta-reducer we do not eta-reduce this, because doing so might reduce the arity of the expression (from 1 to zero, because of ). But for rule-matching we /do/ want to match template (f a) against target (\x. f x), with a := This is a compelling reason for not relying on the Simplifier's eta-reducer. * The Lam case of eta_reduce renames as it goes. Consider (\x. \x. f x x). We should not eta-reduce this. As we go we rename the first x to x1, and the second to x2; then both argument x's are x2. * eta_reduce does /not/ need to check that the bindings 'blah' and expression 'e' don't mention a b c; but it /does/ extend the rv_lcl RnEnv2 (see rn_bndr in eta_reduce). * If 'blah' mentions the binders, the let-float rule won't fire; and * if 'e' mentions the binders we we'll also fail to match e.g. because of the exprFreeVars test in match_tmpl_var. Example: Template: {x} f a -- Some top-level 'a' Target: (\a b. f a a b) -- The \a shadows top level 'a' Then eta_reduce will /succeed/, with (rnEnvR = [a :-> a'], f a) The returned RnEnv will map [a :-> a'], where a' is fresh. (There is no need to rename 'b' because (in this example) it is not in scope. So it's as if we'd returned (f a') from eta_reduce; the renaming applied to the target is simply deferred. Note [Cancel reflexive casts] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is an example (from #19790) which we want to catch (f x) ~ (\a b. (f x |> co) a b) |> sym co where f :: Int -> Stream co :: Stream ~ T1 -> T2 -> T3 when we eta-reduce (\a b. blah a b) to 'blah', we'll get (f x) ~ (f x) |> co |> sym co and we really want to spot that the co/sym-co cancels out. Hence * We keep an invariant that the MCoercion is always MRefl if the MCoercion is reflextve * We maintain this invariant via the call to checkReflexiveMCo in the Cast case of 'match'. -} ------------- match_co :: RuleMatchEnv -> RuleSubst -> Coercion -> Coercion -> Maybe RuleSubst -- We only match if the template is a coercion variable or Refl: -- see Note [Casts in the template] -- Like 'match' it is /not/ guaranteed that -- coercionKind template = coercionKind target -- But if match_co succeeds, it /is/ guaranteed that -- coercionKind (subst template) = coercionKind target match_co renv subst co1 co2 | Just cv <- getCoVar_maybe co1 = match_var renv subst cv (Coercion co2) | Just (ty1, r1) <- isReflCo_maybe co1 = do { (ty2, r2) <- isReflCo_maybe co2 ; guard (r1 == r2) ; match_ty renv subst ty1 ty2 } | debugIsOn = pprTrace "match_co: needs more cases" (ppr co1 $$ ppr co2) Nothing -- Currently just deals with CoVarCo and Refl | otherwise = Nothing ------------- rnMatchBndr2 :: RuleMatchEnv -> Var -> Var -> RuleMatchEnv rnMatchBndr2 renv x1 x2 = renv { rv_lcl = rnBndr2 (rv_lcl renv) x1 x2 , rv_fltR = delBndr (rv_fltR renv) x2 } ------------------------------------------ match_alts :: RuleMatchEnv -> RuleSubst -> [CoreAlt] -- Template -> [CoreAlt] -> MCoercion -- Target -> Maybe RuleSubst match_alts _ subst [] [] _ = return subst match_alts renv subst (Alt c1 vs1 r1:alts1) (Alt c2 vs2 r2:alts2) mco | c1 == c2 = do { subst1 <- match renv' subst r1 r2 mco ; match_alts renv subst1 alts1 alts2 mco } where renv' = foldl' mb renv (vs1 `zip` vs2) mb renv (v1,v2) = rnMatchBndr2 renv v1 v2 match_alts _ _ _ _ _ = Nothing ------------------------------------------ okToFloat :: RnEnv2 -> VarSet -> Bool okToFloat rn_env bind_fvs = allVarSet not_captured bind_fvs where not_captured fv = not (inRnEnvR rn_env fv) ------------------------------------------ match_var :: RuleMatchEnv -> RuleSubst -> Var -- Template -> CoreExpr -- Target -> Maybe RuleSubst match_var renv@(RV { rv_tmpls = tmpls, rv_lcl = rn_env, rv_fltR = flt_env }) subst v1 e2 | v1' `elemVarSet` tmpls = match_tmpl_var renv subst v1' e2 | otherwise -- v1' is not a template variable; check for an exact match with e2 = case e2 of -- Remember, envR of rn_env is disjoint from rv_fltR Var v2 | Just v2' <- rnOccR_maybe rn_env v2 -> -- v2 was bound by a nested lambda or case if v1' == v2' then Just subst else Nothing -- v2 is not bound nestedly; it is free -- in the whole expression being matched -- So it will be in the InScopeSet for flt_env (#20200) | Var v2' <- lookupIdSubst flt_env v2 , v1' == v2' -> Just subst | otherwise -> Nothing _ -> Nothing where v1' = rnOccL rn_env v1 -- If the template is -- forall x. f x (\x -> x) = ... -- Then the x inside the lambda isn't the -- template x, so we must rename first! ------------------------------------------ match_tmpl_var :: RuleMatchEnv -> RuleSubst -> Var -- Template -> CoreExpr -- Target -> Maybe RuleSubst match_tmpl_var renv@(RV { rv_lcl = rn_env, rv_fltR = flt_env }) subst@(RS { rs_id_subst = id_subst, rs_bndrs = let_bndrs }) v1' e2 | any (inRnEnvR rn_env) (exprFreeVarsList e2) = Nothing -- Skolem-escape failure -- e.g. match forall a. (\x-> a x) against (\y. y y) | Just e1' <- lookupVarEnv id_subst v1' = if eqExpr (rnInScopeSet rn_env) e1' e2' then Just subst else Nothing | otherwise -- See Note [Matching variable types] = do { subst' <- match_ty renv subst (idType v1') (exprType e2) ; return (subst' { rs_id_subst = id_subst' }) } where -- e2' is the result of applying flt_env to e2 e2' | null let_bndrs = e2 | otherwise = substExpr flt_env e2 id_subst' = extendVarEnv (rs_id_subst subst) v1' e2' -- No further renaming to do on e2', -- because no free var of e2' is in the rnEnvR of the envt ------------------------------------------ match_ty :: RuleMatchEnv -> RuleSubst -> Type -- Template -> Type -- Target -> Maybe RuleSubst -- Matching Core types: use the matcher in GHC.Tc.Utils.TcType. -- Notice that we treat newtypes as opaque. For example, suppose -- we have a specialised version of a function at a newtype, say -- newtype T = MkT Int -- We only want to replace (f T) with f', not (f Int). match_ty renv subst ty1 ty2 = do { tv_subst' <- Unify.ruleMatchTyKiX (rv_tmpls renv) (rv_lcl renv) tv_subst ty1 ty2 ; return (subst { rs_tv_subst = tv_subst' }) } where tv_subst = rs_tv_subst subst {- Note [Matching variable types] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When matching x ~ e, where 'x' is a template variable, we must check that x's type matches e's type, to establish (TypeInv). For example forall (c::Char->Int) (x::Char). f (c x) = "RULE FIRED" We must not match on, say (f (pred (3::Int))). It's actually quite difficult to come up with an example that shows you need type matching, esp since matching is left-to-right, so type args get matched first. But it's possible (e.g. simplrun008) and this is the Right Thing to do. An alternative would be to make (TypeInf) into a /pre-condition/. It is threatened only by the App rule. So when matching an application (e1 e2) ~ (d1 d2) would be to collect args of the application chain, match the types of the head, then match arg-by-arg. However that alternative seems a bit more complicated. And by matching types at variables we do one match_ty for each template variable, rather than one for each application chain. Usually there are fewer template variables, although for simple rules it could be the other way around. Note [Expanding variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is another Very Important rule: if the term being matched is a variable, we expand it so long as its unfolding is "expandable". (Its occurrence information is not necessarily up to date, so we don't use it.) By "expandable" we mean a WHNF or a "constructor-like" application. This is the key reason for "constructor-like" Ids. If we have {-# NOINLINE [1] CONLIKE g #-} {-# RULE f (g x) = h x #-} then in the term let v = g 3 in ....(f v).... we want to make the rule fire, to replace (f v) with (h 3). Note [Do not expand locally-bound variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Do *not* expand locally-bound variables, else there's a worry that the unfolding might mention variables that are themselves renamed. Example case x of y { (p,q) -> ...y... } Don't expand 'y' to (p,q) because p,q might themselves have been renamed. Essentially we only expand unfoldings that are "outside" the entire match. Hence, (a) the guard (not (isLocallyBoundR v2)) (b) when we expand we nuke the renaming envt (nukeRnEnvR). Note [Tick annotations in RULE matching] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We used to unconditionally look through ticks in both template and expression being matched. This is actually illegal for counting or cost-centre-scoped ticks, because we have no place to put them without changing entry counts and/or costs. So now we just fail the match in these cases. On the other hand, where we are allowed to insert new cost into the tick scope, we can float them upwards to the rule application site. Moreover, we may encounter ticks in the template of a rule. There are a few ways in which these may be introduced (e.g. #18162, #17619). Such ticks are ignored by the matcher. See Note [Simplifying rules] in GHC.Core.Opt.Simplify.Utils for details. cf Note [Tick annotations in call patterns] in GHC.Core.Opt.SpecConstr Note [Matching lets] ~~~~~~~~~~~~~~~~~~~~ Matching a let-expression. Consider RULE forall x. f (g x) = and target expression f (let { w=R } in g E)) Then we'd like the rule to match, to generate let { w=R } in (\x. ) E In effect, we want to float the let-binding outward, to enable the match to happen. This is the WHOLE REASON for accumulating bindings in the RuleSubst We can only do this if the free variables of R are not bound by the part of the target expression outside the let binding; e.g. f (\v. let w = v+1 in g E) Here we obviously cannot float the let-binding for w. Hence the use of okToFloat. There are a couple of tricky points: (a) What if floating the binding captures a variable that is free in the entire expression? f (let v = x+1 in v) v --> NOT! let v = x+1 in f (x+1) v (b) What if the let shadows a local binding? f (\v -> (v, let v = x+1 in (v,v)) --> NOT! let v = x+1 in f (\v -> (v, (v,v))) (c) What if two non-nested let bindings bind the same variable? f (let v = e1 in b1) (let v = e2 in b2) --> NOT! let v = e1 in let v = e2 in (f b2 b2) See testsuite test `T4814`. Our cunning plan is this: (1) Along with the growing substitution for template variables we maintain a growing set of floated let-bindings (rs_binds) plus the set of variables thus bound (rs_bndrs). (2) The RnEnv2 in the MatchEnv binds only the local binders in the term (lambdas, case), not the floated let-bndrs. (3) When we encounter a `let` in the term to be matched, in the Let case of `match`, we use `okToFloat` to check that it does not mention any locally bound (lambda, case) variables. If so we fail. (4) In the Let case of `match`, we use GHC.Core.Subst.substBind to freshen the binding (which, remember (3), mentions no locally bound variables), in a lexically-scoped way (via rv_fltR in MatchEnv). The subtle point is that we want an in-scope set for this substitution that includes /two/ sets: * The in-scope variables at this point, so that we avoid using those local names for the floated binding; points (a) and (b) above. * All "earlier" floated bindings, so that we avoid using the same name for two different floated bindings; point (c) above. Because we have to compute the in-scope set here, the in-scope set stored in `rv_fltR` is always ignored; we leave it only because it's convenient to have `rv_fltR :: Subst` (with an always-ignored `InScopeSet`) rather than storing three separate substitutions. (5) We apply that freshening substitution, in a lexically-scoped way to the term, although lazily; this is the rv_fltR field. See #4814, which is an issue resulting from getting this wrong. Note [Matching cases] ~~~~~~~~~~~~~~~~~~~~~ {- NOTE: This idea is currently disabled. It really only works if the primops involved are OkForSpeculation, and, since they have side effects readIntOfAddr and touch are not. Maybe we'll get back to this later . -} Consider f (case readIntOffAddr# p# i# realWorld# of { (# s#, n# #) -> case touch# fp s# of { _ -> I# n# } } ) This happened in a tight loop generated by stream fusion that Roman encountered. We'd like to treat this just like the let case, because the primops concerned are ok-for-speculation. That is, we'd like to behave as if it had been case readIntOffAddr# p# i# realWorld# of { (# s#, n# #) -> case touch# fp s# of { _ -> f (I# n# } } ) Note [Lookup in-scope] ~~~~~~~~~~~~~~~~~~~~~~ Consider this example foo :: Int -> Maybe Int -> Int foo 0 (Just n) = n foo m (Just n) = foo (m-n) (Just n) SpecConstr sees this fragment: case w_smT of wild_Xf [Just A] { Data.Maybe.Nothing -> lvl_smf; Data.Maybe.Just n_acT [Just S(L)] -> case n_acT of wild1_ams [Just A] { GHC.Base.I# y_amr [Just L] -> $wfoo_smW (GHC.Prim.-# ds_Xmb y_amr) wild_Xf }}; and correctly generates the rule RULES: "SC:$wfoo1" [0] __forall {y_amr [Just L] :: GHC.Prim.Int# sc_snn :: GHC.Prim.Int#} $wfoo_smW sc_snn (Data.Maybe.Just @ GHC.Base.Int (GHC.Base.I# y_amr)) = $s$wfoo_sno y_amr sc_snn ;] BUT we must ensure that this rule matches in the original function! Note that the call to $wfoo is $wfoo_smW (GHC.Prim.-# ds_Xmb y_amr) wild_Xf During matching we expand wild_Xf to (Just n_acT). But then we must also expand n_acT to (I# y_amr). And we can only do that if we look up n_acT in the in-scope set, because in wild_Xf's unfolding it won't have an unfolding at all. That is why the 'lookupRnInScope' call in the (Var v2) case of 'match' is so important. ************************************************************************ * * Rule-check the program * * ************************************************************************ We want to know what sites have rules that could have fired but didn't. This pass runs over the tree (without changing it) and reports such. -} -- | Report partial matches for rules beginning with the specified -- string for the purposes of error reporting ruleCheckProgram :: RuleOpts -- ^ Rule options -> CompilerPhase -- ^ Rule activation test -> String -- ^ Rule pattern -> (Id -> [CoreRule]) -- ^ Rules for an Id -> CoreProgram -- ^ Bindings to check in -> SDoc -- ^ Resulting check message ruleCheckProgram ropts phase rule_pat rules binds | isEmptyBag results = text "Rule check results: no rule application sites" | otherwise = vcat [text "Rule check results:", line, vcat [ p $$ line | p <- bagToList results ] ] where env = RuleCheckEnv { rc_is_active = isActive phase , rc_id_unf = idUnfolding -- Not quite right -- Should use activeUnfolding , rc_pattern = rule_pat , rc_rules = rules , rc_ropts = ropts } results = unionManyBags (map (ruleCheckBind env) binds) line = text (replicate 20 '-') data RuleCheckEnv = RuleCheckEnv { rc_is_active :: Activation -> Bool, rc_id_unf :: IdUnfoldingFun, rc_pattern :: String, rc_rules :: Id -> [CoreRule], rc_ropts :: RuleOpts } ruleCheckBind :: RuleCheckEnv -> CoreBind -> Bag SDoc -- The Bag returned has one SDoc for each call site found ruleCheckBind env (NonRec _ r) = ruleCheck env r ruleCheckBind env (Rec prs) = unionManyBags [ruleCheck env r | (_,r) <- prs] ruleCheck :: RuleCheckEnv -> CoreExpr -> Bag SDoc ruleCheck _ (Var _) = emptyBag ruleCheck _ (Lit _) = emptyBag ruleCheck _ (Type _) = emptyBag ruleCheck _ (Coercion _) = emptyBag ruleCheck env (App f a) = ruleCheckApp env (App f a) [] ruleCheck env (Tick _ e) = ruleCheck env e ruleCheck env (Cast e _) = ruleCheck env e ruleCheck env (Let bd e) = ruleCheckBind env bd `unionBags` ruleCheck env e ruleCheck env (Lam _ e) = ruleCheck env e ruleCheck env (Case e _ _ as) = ruleCheck env e `unionBags` unionManyBags [ruleCheck env r | Alt _ _ r <- as] ruleCheckApp :: RuleCheckEnv -> Expr CoreBndr -> [Arg CoreBndr] -> Bag SDoc ruleCheckApp env (App f a) as = ruleCheck env a `unionBags` ruleCheckApp env f (a:as) ruleCheckApp env (Var f) as = ruleCheckFun env f as ruleCheckApp env other _ = ruleCheck env other ruleCheckFun :: RuleCheckEnv -> Id -> [CoreExpr] -> Bag SDoc -- Produce a report for all rules matching the predicate -- saying why it doesn't match the specified application ruleCheckFun env fn args | null name_match_rules = emptyBag | otherwise = unitBag (ruleAppCheck_help env fn args name_match_rules) where name_match_rules = filter match (rc_rules env fn) match rule = (rc_pattern env) `isPrefixOf` unpackFS (ruleName rule) ruleAppCheck_help :: RuleCheckEnv -> Id -> [CoreExpr] -> [CoreRule] -> SDoc ruleAppCheck_help env fn args rules = -- The rules match the pattern, so we want to print something vcat [text "Expression:" <+> ppr (mkApps (Var fn) args), vcat (map check_rule rules)] where n_args = length args i_args = args `zip` [1::Int ..] rough_args = map roughTopName args check_rule rule = rule_herald rule <> colon <+> rule_info (rc_ropts env) rule rule_herald (BuiltinRule { ru_name = name }) = text "Builtin rule" <+> doubleQuotes (ftext name) rule_herald (Rule { ru_name = name }) = text "Rule" <+> doubleQuotes (ftext name) rule_info opts rule | Just _ <- matchRule opts (emptyInScopeSet, rc_id_unf env) noBlackList fn args rough_args rule = text "matches (which is very peculiar!)" rule_info _ (BuiltinRule {}) = text "does not match" rule_info _ (Rule { ru_act = act, ru_bndrs = rule_bndrs, ru_args = rule_args}) | not (rc_is_active env act) = text "active only in later phase" | n_args < n_rule_args = text "too few arguments" | n_mismatches == n_rule_args = text "no arguments match" | n_mismatches == 0 = text "all arguments match (considered individually), but rule as a whole does not" | otherwise = text "arguments" <+> ppr mismatches <+> text "do not match (1-indexing)" where n_rule_args = length rule_args n_mismatches = length mismatches mismatches = [i | (rule_arg, (arg,i)) <- rule_args `zip` i_args, not (isJust (match_fn rule_arg arg))] lhs_fvs = exprsFreeVars rule_args -- Includes template tyvars match_fn rule_arg arg = match renv emptyRuleSubst rule_arg arg MRefl where in_scope = mkInScopeSet (lhs_fvs `unionVarSet` exprFreeVars arg) renv = RV { rv_lcl = mkRnEnv2 in_scope , rv_tmpls = mkVarSet rule_bndrs , rv_fltR = mkEmptySubst in_scope , rv_unf = rc_id_unf env }