From: simonpj@microsoft.com Date: Fri, 8 Jan 2010 08:45:47 +0000 (+0000) Subject: Respect SPECIALISE pragmas even for apparently-non-overloaded things X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=99e1bcf5cf97b3b69df99ec7b2bfb9b1076d3516 Respect SPECIALISE pragmas even for apparently-non-overloaded things This is an implementation matter really (the key word is "apparently"!). See Note [Specialising in no-dict case] in DsBinds. It showed up when compiling GHC.Float. --- diff --git a/compiler/deSugar/DsBinds.lhs b/compiler/deSugar/DsBinds.lhs index 9e29c96..31cb75d 100644 --- a/compiler/deSugar/DsBinds.lhs +++ b/compiler/deSugar/DsBinds.lhs @@ -55,7 +55,6 @@ import Util ( count, lengthExceeds ) import MonadUtils import Control.Monad -import Data.List \end{code} %************************************************************************ @@ -115,90 +114,28 @@ dsHsBind _ rest ; sel_binds <- mkSelectorBinds pat body_expr ; return (sel_binds ++ rest) } -{- Note [Rules and inlining] - ~~~~~~~~~~~~~~~~~~~~~~~~~ - Common special case: no type or dictionary abstraction - This is a bit less trivial than you might suppose - The naive way woudl be to desguar to something like - f_lcl = ...f_lcl... -- The "binds" from AbsBinds - M.f = f_lcl -- Generated from "exports" - But we don't want that, because if M.f isn't exported, - it'll be inlined unconditionally at every call site (its rhs is - trivial). That would be ok unless it has RULES, which would - thereby be completely lost. Bad, bad, bad. - - Instead we want to generate - M.f = ...f_lcl... - f_lcl = M.f - Now all is cool. The RULES are attached to M.f (by SimplCore), - and f_lcl is rapidly inlined away. - - This does not happen in the same way to polymorphic binds, - because they desugar to - M.f = /\a. let f_lcl = ...f_lcl... in f_lcl - Although I'm a bit worried about whether full laziness might - float the f_lcl binding out and then inline M.f at its call site -} - dsHsBind auto_scc rest (AbsBinds [] [] exports binds) = do { core_prs <- ds_lhs_binds NoSccs binds ; let env = mkABEnv exports do_one (lcl_id, rhs) | Just (_, gbl_id, _, spec_prags) <- lookupVarEnv env lcl_id - = WARN( hasSpecPrags spec_prags, pprTcSpecPrags gbl_id spec_prags ) -- Not overloaded - makeCorePair gbl_id False 0 (addAutoScc auto_scc gbl_id rhs) + = do { let rhs' = addAutoScc auto_scc gbl_id rhs + ; (spec_binds, rules) <- dsSpecs gbl_id (Let (Rec core_prs) rhs') spec_prags + -- See Note [Specialising in no-dict case] + ; let gbl_id' = addIdSpecialisations gbl_id rules + main_bind = makeCorePair gbl_id' False 0 rhs' + ; return (main_bind : spec_binds) } - | otherwise = (lcl_id, rhs) + | otherwise = return [(lcl_id, rhs)] locals' = [(lcl_id, Var gbl_id) | (_, gbl_id, lcl_id, _) <- exports] -- Note [Rules and inlining] - ; return (map do_one core_prs ++ locals' ++ rest) } + ; export_binds <- mapM do_one core_prs + ; return (concat export_binds ++ locals' ++ rest) } -- No Rec needed here (contrast the other AbsBinds cases) -- because we can rely on the enclosing dsBind to wrap in Rec -{- Note [Abstracting over tyvars only] - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - When abstracting over type variable only (not dictionaries), we don't really need to - built a tuple and select from it, as we do in the general case. Instead we can take - - AbsBinds [a,b] [ ([a,b], fg, fl, _), - ([b], gg, gl, _) ] - { fl = e1 - gl = e2 - h = e3 } - - and desugar it to - - fg = /\ab. let B in e1 - gg = /\b. let a = () in let B in S(e2) - h = /\ab. let B in e3 - - where B is the *non-recursive* binding - fl = fg a b - gl = gg b - h = h a b -- See (b); note shadowing! - - Notice (a) g has a different number of type variables to f, so we must - use the mkArbitraryType thing to fill in the gaps. - We use a type-let to do that. - - (b) The local variable h isn't in the exports, and rather than - clone a fresh copy we simply replace h by (h a b), where - the two h's have different types! Shadowing happens here, - which looks confusing but works fine. - - (c) The result is *still* quadratic-sized if there are a lot of - small bindings. So if there are more than some small - number (10), we filter the binding set B by the free - variables of the particular RHS. Tiresome. - - Why got to this trouble? It's a common case, and it removes the - quadratic-sized tuple desugaring. Less clutter, hopefullly faster - compilation, especially in a case where there are a *lot* of - bindings. --} - - dsHsBind auto_scc rest (AbsBinds tyvars [] exports binds) | opt_DsMultiTyVar -- This (static) debug flag just lets us -- switch on and off this optimisation to @@ -225,21 +162,22 @@ dsHsBind auto_scc rest (AbsBinds tyvars [] exports binds) do_one lg_binds (lcl_id, rhs) | Just (id_tvs, gbl_id, _, spec_prags) <- lookupVarEnv env lcl_id - = WARN( hasSpecPrags spec_prags, pprTcSpecPrags gbl_id spec_prags ) -- Not overloaded - (let rhs' = addAutoScc auto_scc gbl_id $ - mkLams id_tvs $ - mkLets [ NonRec tv (Type (lookupVarEnv_NF arby_env tv)) - | tv <- tyvars, not (tv `elem` id_tvs)] $ - add_lets lg_binds rhs - in return (mk_lg_bind lcl_id gbl_id id_tvs, - makeCorePair gbl_id False 0 rhs')) + = do { let rhs' = addAutoScc auto_scc gbl_id $ + mkLams id_tvs $ + mkLets [ NonRec tv (Type (lookupVarEnv_NF arby_env tv)) + | tv <- tyvars, not (tv `elem` id_tvs)] $ + add_lets lg_binds rhs + ; (spec_binds, rules) <- dsSpecs gbl_id rhs' spec_prags + ; let gbl_id' = addIdSpecialisations gbl_id rules + main_bind = makeCorePair gbl_id' False 0 rhs' + ; return (mk_lg_bind lcl_id gbl_id' id_tvs, main_bind : spec_binds) } | otherwise = do { non_exp_gbl_id <- newUniqueId lcl_id (mkForAllTys tyvars (idType lcl_id)) ; return (mk_lg_bind lcl_id non_exp_gbl_id tyvars, - (non_exp_gbl_id, mkLams tyvars (add_lets lg_binds rhs))) } + [(non_exp_gbl_id, mkLams tyvars (add_lets lg_binds rhs))]) } ; (_, core_prs') <- fixDs (\ ~(lg_binds, _) -> mapAndUnzipM (do_one lg_binds) core_prs) - ; return (core_prs' ++ rest) } + ; return (concat core_prs' ++ rest) } -- Another common case: one exported variable -- Non-recursive bindings come through this way @@ -253,13 +191,12 @@ dsHsBind auto_scc rest ; let -- Always treat the binds as recursive, because the -- typechecker makes rather mixed-up dictionary bindings core_bind = Rec core_prs + rhs = addAutoScc auto_scc global $ + mkLams tyvars $ mkLams dicts $ Let core_bind (Var local) - ; (spec_binds, rules) <- dsSpecs all_tyvars dicts tyvars global - local core_bind prags + ; (spec_binds, rules) <- dsSpecs global rhs prags ; let global' = addIdSpecialisations global rules - rhs = addAutoScc auto_scc global $ - mkLams tyvars $ mkLams dicts $ Let core_bind (Var local) main_bind = makeCorePair global' (isDefaultMethod prags) (dictArity dicts) rhs @@ -275,30 +212,30 @@ dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds) -- Rec because of mixed-up dictionary bindings core_bind = Rec (map do_one core_prs) - tup_expr = mkBigCoreVarTup locals - tup_ty = exprType tup_expr - poly_tup_expr = mkLams all_tyvars $ mkLams dicts $ - Let core_bind tup_expr - locals = [local | (_, _, local, _) <- exports] - local_tys = map idType locals + tup_expr = mkBigCoreVarTup locals + tup_ty = exprType tup_expr + poly_tup_rhs = mkLams all_tyvars $ mkLams dicts $ + Let core_bind tup_expr + locals = [local | (_, _, local, _) <- exports] + local_tys = map idType locals - ; poly_tup_id <- newSysLocalDs (exprType poly_tup_expr) + ; poly_tup_id <- newSysLocalDs (exprType poly_tup_rhs) - ; let mk_bind ((tyvars, global, local, spec_prags), n) -- locals!!n == local + ; let mk_bind ((tyvars, global, _, spec_prags), n) -- locals!!n == local = -- Need to make fresh locals to bind in the selector, -- because some of the tyvars will be bound to 'Any' do { let ty_args = map mk_ty_arg all_tyvars substitute = substTyWith all_tyvars ty_args ; locals' <- newSysLocalsDs (map substitute local_tys) ; tup_id <- newSysLocalDs (substitute tup_ty) - ; (spec_binds, rules) <- dsSpecs all_tyvars dicts tyvars global local - core_bind - spec_prags - ; let global' = addIdSpecialisations global rules - rhs = mkLams tyvars $ mkLams dicts $ + ; let rhs = mkLams tyvars $ mkLams dicts $ mkTupleSelector locals' (locals' !! n) tup_id $ mkVarApps (mkTyApps (Var poly_tup_id) ty_args) dicts + ; (spec_binds, rules) <- dsSpecs global + (Let (NonRec poly_tup_id poly_tup_rhs) rhs) + spec_prags + ; let global' = addIdSpecialisations global rules ; return ((global', rhs) : spec_binds) } where mk_ty_arg all_tyvar @@ -308,7 +245,7 @@ dsHsBind auto_scc rest (AbsBinds all_tyvars dicts exports binds) ; export_binds_s <- mapM mk_bind (exports `zip` [0..]) -- Don't scc (auto-)annotate the tuple itself. - ; return ((poly_tup_id, poly_tup_expr) : + ; return ((poly_tup_id, poly_tup_rhs) : (concat export_binds_s ++ rest)) } ------------------------ @@ -348,6 +285,89 @@ mkABEnv :: [([TyVar], Id, Id, TcSpecPrags)] -> AbsBindEnv mkABEnv exports = mkVarEnv [ (lcl_id, export) | export@(_, _, lcl_id, _) <- exports] \end{code} +Note [Rules and inlining] +~~~~~~~~~~~~~~~~~~~~~~~~~ +Common special case: no type or dictionary abstraction +This is a bit less trivial than you might suppose +The naive way woudl be to desguar to something like + f_lcl = ...f_lcl... -- The "binds" from AbsBinds + M.f = f_lcl -- Generated from "exports" +But we don't want that, because if M.f isn't exported, +it'll be inlined unconditionally at every call site (its rhs is +trivial). That would be ok unless it has RULES, which would +thereby be completely lost. Bad, bad, bad. + +Instead we want to generate + M.f = ...f_lcl... + f_lcl = M.f +Now all is cool. The RULES are attached to M.f (by SimplCore), +and f_lcl is rapidly inlined away. + +This does not happen in the same way to polymorphic binds, +because they desugar to + M.f = /\a. let f_lcl = ...f_lcl... in f_lcl +Although I'm a bit worried about whether full laziness might +float the f_lcl binding out and then inline M.f at its call site -} + +Note [Specialising in no-dict case] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Even if there are no tyvars or dicts, we may have specialisation pragmas. +Class methods can generate + AbsBinds [] [] [( ... spec-prag] + { AbsBinds [tvs] [dicts] ...blah } +So the overloading is in the nested AbsBinds. A good example is in GHC.Float: + + class (Real a, Fractional a) => RealFrac a where + round :: (Integral b) => a -> b + + instance RealFrac Float where + {-# SPECIALIZE round :: Float -> Int #-} + +The top-level AbsBinds for $cround has no tyvars or dicts (because the +instance does not). But the method is locally overloaded! + +Note [Abstracting over tyvars only] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When abstracting over type variable only (not dictionaries), we don't really need to +built a tuple and select from it, as we do in the general case. Instead we can take + + AbsBinds [a,b] [ ([a,b], fg, fl, _), + ([b], gg, gl, _) ] + { fl = e1 + gl = e2 + h = e3 } + +and desugar it to + + fg = /\ab. let B in e1 + gg = /\b. let a = () in let B in S(e2) + h = /\ab. let B in e3 + +where B is the *non-recursive* binding + fl = fg a b + gl = gg b + h = h a b -- See (b); note shadowing! + +Notice (a) g has a different number of type variables to f, so we must + use the mkArbitraryType thing to fill in the gaps. + We use a type-let to do that. + + (b) The local variable h isn't in the exports, and rather than + clone a fresh copy we simply replace h by (h a b), where + the two h's have different types! Shadowing happens here, + which looks confusing but works fine. + + (c) The result is *still* quadratic-sized if there are a lot of + small bindings. So if there are more than some small + number (10), we filter the binding set B by the free + variables of the particular RHS. Tiresome. + +Why got to this trouble? It's a common case, and it removes the +quadratic-sized tuple desugaring. Less clutter, hopefullly faster +compilation, especially in a case where there are a *lot* of +bindings. + + Note [Eta-expanding INLINE things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider @@ -387,6 +407,7 @@ Note [Implementing SPECIALISE pragmas] Example: f :: (Eq a, Ix b) => a -> b -> Bool {-# SPECIALISE f :: (Ix p, Ix q) => Int -> (p,q) -> Bool #-} + f = From this the typechecker generates @@ -396,8 +417,8 @@ From this the typechecker generates -> forall p q. (Ix p, Ix q) => XXX[ Int/a, (p,q)/b ]) Note that wrap_fn can transform *any* function with the right type prefix - forall ab. (Eq a, Ix b) => -regardless of . It's sort of polymorphic in . This is + forall ab. (Eq a, Ix b) => XXX +regardless of XXX. It's sort of polymorphic in XXX. This is useful: we use the same wrapper to transform each of the class ops, as well as the dict. @@ -406,7 +427,7 @@ From these we generate: Rule: forall p, q, (dp:Ix p), (dq:Ix q). f Int (p,q) dInt ($dfInPair dp dq) = f_spec p q dp dq - Spec bind: f_spec = wrap_fn (/\ab \d1 d2. Let binds in f_mono) + Spec bind: f_spec = wrap_fn Note that @@ -414,18 +435,18 @@ Note that $dfIxPair dp dq), and that is essential because the dp, dq are needed on the RHS. - * The RHS of f_spec has a *copy* of 'binds', so that it can fully - specialise it. + * The RHS of f_spec, has a *copy* of 'binds', so that it + can fully specialise it. \begin{code} ------------------------ -dsSpecs :: [TyVar] -> [DictId] -> [TyVar] - -> Id -> Id -- Global, local - -> CoreBind -> TcSpecPrags +dsSpecs :: Id -- The polymorphic Id + -> CoreExpr -- Its rhs + -> TcSpecPrags -> DsM ( [(Id,CoreExpr)] -- Binding for specialised Ids , [CoreRule] ) -- Rules for the Global Ids -- See Note [Implementing SPECIALISE pragmas] -dsSpecs all_tvs dicts tvs poly_id mono_id mono_bind prags +dsSpecs poly_id poly_rhs prags = case prags of IsDefaultMethod -> return ([], []) SpecPrags sps -> do { pairs <- mapMaybeM spec_one sps @@ -452,8 +473,7 @@ dsSpecs all_tvs dicts tvs poly_id mono_id mono_bind prags { (spec_unf, unf_pairs) <- specUnfolding wrap_fn (realIdUnfolding poly_id) - ; let f_body = fix_up (Let mono_bind (Var mono_id)) - spec_ty = exprType ds_spec_expr + ; let spec_ty = exprType ds_spec_expr spec_id = mkLocalId spec_name spec_ty `setInlinePragma` inl_prag `setIdUnfolding` spec_unf @@ -472,20 +492,12 @@ dsSpecs all_tvs dicts tvs poly_id mono_id mono_bind prags (extra_dict_bndrs ++ bndrs) args (mkVarApps (Var spec_id) bndrs) - spec_rhs = wrap_fn (mkLams (tvs ++ dicts) f_body) + spec_rhs = wrap_fn poly_rhs spec_pair = makeCorePair spec_id False (dictArity bndrs) spec_rhs ; return (Just (spec_pair : unf_pairs, rule)) } } } } - -- Bind to Any any of all_ptvs that aren't - -- relevant for this particular function - fix_up body | null void_tvs = body - | otherwise = mkTyApps (mkLams void_tvs body) $ - map dsMkArbitraryType void_tvs - - void_tvs = all_tvs \\ tvs - dead_msg bs = vcat [ sep [ptext (sLit "Useless constraint") <> plural bs <+> ptext (sLit "in specialied type:"), nest 2 (pprTheta (map get_pred bs))]