X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FcoreSyn%2FCorePrep.lhs;h=42379b4c0141763505586553ab6ff1cfff5ce369;hb=3dceaeff2bc7358afefad08bb1e24dbed6b2c611;hp=89ec98f6f27738b3dbeb5ebc57047e85f75c88c3;hpb=0abcc75505992b925ca1b6fed6c97cb105b6fe96;p=ghc-hetmet.git diff --git a/compiler/coreSyn/CorePrep.lhs b/compiler/coreSyn/CorePrep.lhs index 89ec98f..42379b4 100644 --- a/compiler/coreSyn/CorePrep.lhs +++ b/compiler/coreSyn/CorePrep.lhs @@ -15,12 +15,14 @@ import PrelNames ( lazyIdKey, hasKey ) import CoreUtils import CoreArity import CoreFVs -import CoreLint +import CoreMonad ( endPass, CoreToDo(..) ) import CoreSyn +import CoreSubst +import OccurAnal ( occurAnalyseExpr ) import Type import Coercion import TyCon -import NewDemand +import Demand import Var import VarSet import VarEnv @@ -38,6 +40,7 @@ import Util import Outputable import MonadUtils import FastString +import Data.List ( mapAccumL ) import Control.Monad \end{code} @@ -86,7 +89,6 @@ The goal of this pass is to prepare for code generation. 8. Inject bindings for the "implicit" Ids: * Constructor wrappers * Constructor workers - * Record selectors We want curried definitions for all of these in case they aren't inlined by some caller. @@ -148,7 +150,7 @@ corePrepPgm dflags binds data_tycons = do floats2 <- corePrepTopBinds implicit_binds return (deFloatTop (floats1 `appendFloats` floats2)) - endPass dflags "CorePrep" Opt_D_dump_prep binds_out + endPass dflags CorePrep binds_out [] return binds_out corePrepExpr :: DynFlags -> CoreExpr -> IO CoreExpr @@ -196,24 +198,38 @@ And then x will actually end up case-bound Note [CafInfo and floating] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ -What happens to the CafInfo on the floated bindings? By default, all -the CafInfos will be set to MayHaveCafRefs, which is safe. - -This might be pessimistic, because the floated binding might not refer -to any CAFs and the GC will end up doing more traversal than is -necessary, but it's still better than not floating the bindings at -all, because then the GC would have to traverse the structure in the -heap instead. Given this, we decided not to try to get the CafInfo on -the floated bindings correct, because it looks difficult. - -But that means we can't float anything out of a NoCafRefs binding. -Consider f = g (h x) -If f is NoCafRefs, we don't want to convert to - sat = h x - f = g sat -where sat conservatively says HasCafRefs, because now f's info -is wrong. I don't think this is common, so we simply switch off -floating in this case. +What happens when we try to float bindings to the top level? At this +point all the CafInfo is supposed to be correct, and we must make certain +that is true of the new top-level bindings. There are two cases +to consider + +a) The top-level binding is marked asCafRefs. In that case we are + basically fine. The floated bindings had better all be lazy lets, + so they can float to top level, but they'll all have HasCafRefs + (the default) which is safe. + +b) The top-level binding is marked NoCafRefs. This really happens + Example. CoreTidy produces + $fApplicativeSTM [NoCafRefs] = D:Alternative retry# ...blah... + Now CorePrep has to eta-expand to + $fApplicativeSTM = let sat = \xy. retry x y + in D:Alternative sat ...blah... + So what we *want* is + sat [NoCafRefs] = \xy. retry x y + $fApplicativeSTM [NoCafRefs] = D:Alternative sat ...blah... + + So, gruesomely, we must set the NoCafRefs flag on the sat bindings, + *and* substutite the modified 'sat' into the old RHS. + + It should be the case that 'sat' is itself [NoCafRefs] (a value, no + cafs) else the original top-level binding would not itself have been + marked [NoCafRefs]. The DEBUG check in CoreToStg for + consistentCafInfo will find this. + +This is all very gruesome and horrible. It would be better to figure +out CafInfo later, after CorePrep. We'll do that in due course. +Meanwhile this horrible hack works. + Note [Data constructor workers] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -233,6 +249,61 @@ always fully applied, and the bindings are just there to support partial applications. But it's easier to let them through. +Note [Dead code in CorePrep] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Imagine that we got an input program like this: + + f :: Show b => Int -> (Int, b -> Maybe Int -> Int) + f x = (g True (Just x) + g () (Just x), g) + where + g :: Show a => a -> Maybe Int -> Int + g _ Nothing = x + g y (Just z) = if z > 100 then g y (Just (z + length (show y))) else g y unknown + +After specialisation and SpecConstr, we would get something like this: + + f :: Show b => Int -> (Int, b -> Maybe Int -> Int) + f x = (g$Bool_True_Just x + g$Unit_Unit_Just x, g) + where + {-# RULES g $dBool = g$Bool + g $dUnit = g$Unit #-} + g = ... + {-# RULES forall x. g$Bool True (Just x) = g$Bool_True_Just x #-} + g$Bool = ... + {-# RULES forall x. g$Unit () (Just x) = g$Unit_Unit_Just x #-} + g$Unit = ... + g$Bool_True_Just = ... + g$Unit_Unit_Just = ... + +Note that the g$Bool and g$Unit functions are actually dead code: they are only kept +alive by the occurrence analyser because they are referred to by the rules of g, +which is being kept alive by the fact that it is used (unspecialised) in the returned pair. + +However, at the CorePrep stage there is no way that the rules for g will ever fire, +and it really seems like a shame to produce an output program that goes to the trouble +of allocating a closure for the unreachable g$Bool and g$Unit functions. + +The way we fix this is to: + * In cloneBndr, drop all unfoldings/rules + * In deFloatTop, run the occurrence analyser on each top-level RHS to drop + the dead local bindings + +The reason we don't just OccAnal the whole output of CorePrep is that the tidier +ensures that all top-level binders are GlobalIds, so they don't show up in the free +variables any longer. So if you run the occurrence analyser on the output of CoreTidy +(or later) you e.g. turn this program: + + Rec { + f = ... f ... + } + +Into this one: + + f = ... f ... + +(Since f is not considered to be free in its own RHS.) + + %************************************************************************ %* * The main code @@ -245,7 +316,7 @@ cpeBind :: TopLevelFlag -> UniqSM (CorePrepEnv, Floats) cpeBind top_lvl env (NonRec bndr rhs) = do { (_, bndr1) <- cloneBndr env bndr - ; let is_strict = isStrictDmd (idNewDemandInfo bndr) + ; let is_strict = isStrictDmd (idDemandInfo bndr) is_unlifted = isUnLiftedType (idType bndr) ; (floats, bndr2, rhs2) <- cpePair top_lvl NonRecursive (is_strict || is_unlifted) @@ -263,7 +334,7 @@ cpeBind top_lvl env (Rec pairs) ; stuff <- zipWithM (cpePair top_lvl Recursive False env') bndrs1 rhss ; let (floats_s, bndrs2, rhss2) = unzip3 stuff - all_pairs = foldrOL add_float (bndrs1 `zip` rhss2) + all_pairs = foldrOL add_float (bndrs2 `zip` rhss2) (concatFloats floats_s) ; return (extendCorePrepEnvList env (bndrs `zip` bndrs2), unitFloat (FloatLet (Rec all_pairs))) } @@ -277,42 +348,69 @@ cpeBind top_lvl env (Rec pairs) --------------- cpePair :: TopLevelFlag -> RecFlag -> RhsDemand -> CorePrepEnv -> Id -> CoreExpr - -> UniqSM (Floats, Id, CoreExpr) + -> UniqSM (Floats, Id, CpeRhs) -- Used for all bindings cpePair top_lvl is_rec is_strict_or_unlifted env bndr rhs = do { (floats1, rhs1) <- cpeRhsE env rhs - ; let (rhs1_bndrs, _) = collectBinders rhs1 - ; (floats2, rhs2) - <- if want_float floats1 rhs1 - then return (floats1, rhs1) - else -- Non-empty floats will wrap rhs1 - -- But: rhs1 might have lambdas, and we can't - -- put them inside a wrapBinds - if valBndrCount rhs1_bndrs <= arity - then -- Lambdas in rhs1 will be nuked by eta expansion - return (emptyFloats, wrapBinds floats1 rhs1) - - else do { body1 <- rhsToBodyNF rhs1 - ; return (emptyFloats, wrapBinds floats1 body1) } - - ; (floats3, rhs') -- Note [Silly extra arguments] - <- if manifestArity rhs2 <= arity + + -- See if we are allowed to float this stuff out of the RHS + ; (floats2, rhs2) <- float_from_rhs floats1 rhs1 + + -- Make the arity match up + ; (floats3, rhs') + <- if manifestArity rhs1 <= arity then return (floats2, cpeEtaExpand arity rhs2) else WARN(True, text "CorePrep: silly extra arguments:" <+> ppr bndr) + -- Note [Silly extra arguments] (do { v <- newVar (idType bndr) ; let float = mkFloat False False v rhs2 ; return (addFloat floats2 float, cpeEtaExpand arity (Var v)) }) - -- Record if the binder is evaluated + -- Record if the binder is evaluated + -- and otherwise trim off the unfolding altogether + -- It's not used by the code generator; getting rid of it reduces + -- heap usage and, since we may be changing uniques, we'd have + -- to substitute to keep it right ; let bndr' | exprIsHNF rhs' = bndr `setIdUnfolding` evaldUnfolding - | otherwise = bndr + | otherwise = bndr `setIdUnfolding` noUnfolding ; return (floats3, bndr', rhs') } where arity = idArity bndr -- We must match this arity - want_float floats rhs - | isTopLevel top_lvl = wantFloatTop bndr floats - | otherwise = wantFloatNested is_rec is_strict_or_unlifted floats rhs + + --------------------- + float_from_rhs floats rhs + | isEmptyFloats floats = return (emptyFloats, rhs) + | isTopLevel top_lvl = float_top floats rhs + | otherwise = float_nested floats rhs + + --------------------- + float_nested floats rhs + | wantFloatNested is_rec is_strict_or_unlifted floats rhs + = return (floats, rhs) + | otherwise = dont_float floats rhs + + --------------------- + float_top floats rhs -- Urhgh! See Note [CafInfo and floating] + | mayHaveCafRefs (idCafInfo bndr) + , allLazyTop floats + = return (floats, rhs) + + -- So the top-level binding is marked NoCafRefs + | Just (floats', rhs') <- canFloatFromNoCaf floats rhs + = return (floats', rhs') + + | otherwise + = dont_float floats rhs + + --------------------- + dont_float floats rhs + -- Non-empty floats, but do not want to float from rhs + -- So wrap the rhs in the floats + -- But: rhs1 might have lambdas, and we can't + -- put them inside a wrapBinds + = do { body <- rhsToBodyNF rhs + ; return (emptyFloats, wrapBinds floats body) } {- Note [Silly extra arguments] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -428,9 +526,9 @@ rhsToBody (Cast e co) ; return (floats, Cast e' co) } rhsToBody expr@(Lam {}) - | Just no_lam_result <- tryEtaReduce bndrs body + | Just no_lam_result <- tryEtaReducePrep bndrs body = return (emptyFloats, no_lam_result) - | all isTyVar bndrs -- Type lambdas are ok + | all isTyCoVar bndrs -- Type lambdas are ok = return (emptyFloats, expr) | otherwise -- Some value lambdas = do { fn <- newVar (exprType expr) @@ -498,7 +596,7 @@ cpeApp env expr ; let v2 = lookupCorePrepEnv env v1 ; return (Var v2, (Var v2, depth), idType v2, emptyFloats, stricts) } where - stricts = case idNewStrictness v of + stricts = case idStrictness v of StrictSig (DmdType _ demands _) | listLengthCmp demands depth /= GT -> demands -- length demands <= depth @@ -535,10 +633,7 @@ cpeApp env expr cpeArg :: CorePrepEnv -> RhsDemand -> CoreArg -> Type -> UniqSM (Floats, CpeTriv) cpeArg env is_strict arg arg_ty - | cpe_ExprIsTrivial arg -- Do not eta expand etc a trivial argument - = cpeBody env arg -- Must still do substitution though - | otherwise - = do { (floats1, arg1) <- cpeRhsE env arg -- arg1 can be a lambda + = do { (floats1, arg1) <- cpeRhsE env arg -- arg1 can be a lambda ; (floats2, arg2) <- if want_float floats1 arg1 then return (floats1, arg1) else do { body1 <- rhsToBodyNF arg1 @@ -546,10 +641,13 @@ cpeArg env is_strict arg arg_ty -- Else case: arg1 might have lambdas, and we can't -- put them inside a wrapBinds - ; v <- newVar arg_ty + ; if cpe_ExprIsTrivial arg2 -- Do not eta expand a trivial argument + then return (floats2, arg2) + else do + { v <- newVar arg_ty ; let arg3 = cpeEtaExpand (exprArity arg2) arg2 arg_float = mkFloat is_strict is_unlifted v arg3 - ; return (addFloat floats2 arg_float, Var v) } + ; return (addFloat floats2 arg_float, Var v) } } where is_unlifted = isUnLiftedType arg_ty want_float = wantFloatNested NonRecursive (is_strict || is_unlifted) @@ -595,9 +693,7 @@ maybeSaturate fn expr n_args ------------- saturateDataToTag :: CpeApp -> UniqSM CpeApp --- Horrid: ensure that the arg of data2TagOp is evaluated --- (data2tag x) --> (case x of y -> data2tag y) --- (yuk yuk) take into account the lambdas we've now introduced +-- See Note [dataToTag magic] saturateDataToTag sat_expr = do { let (eta_bndrs, eta_body) = collectBinders sat_expr ; eta_body' <- eval_data2tag_arg eta_body @@ -621,7 +717,14 @@ saturateDataToTag sat_expr = pprPanic "eval_data2tag" (ppr other) \end{code} +Note [dataToTag magic] +~~~~~~~~~~~~~~~~~~~~~~ +Horrid: we must ensure that the arg of data2TagOp is evaluated + (data2tag x) --> (case x of y -> data2tag y) +(yuk yuk) take into account the lambdas we've now introduced +How might it not be evaluated? Well, we might have floated it out +of the scope of a `seq`, or dropped the `seq` altogether. %************************************************************************ @@ -641,7 +744,6 @@ ignoreNote :: Note -> Bool -- want to get this: -- unzip = /\ab \xs. (__inline_me__ ...) a b xs ignoreNote (CoreNote _) = True -ignoreNote InlineMe = True ignoreNote _other = False @@ -651,10 +753,9 @@ cpe_ExprIsTrivial (Var _) = True cpe_ExprIsTrivial (Type _) = True cpe_ExprIsTrivial (Lit _) = True cpe_ExprIsTrivial (App e arg) = isTypeArg arg && cpe_ExprIsTrivial e -cpe_ExprIsTrivial (Note (SCC _) _) = False -cpe_ExprIsTrivial (Note _ e) = cpe_ExprIsTrivial e +cpe_ExprIsTrivial (Note n e) = notSccNote n && cpe_ExprIsTrivial e cpe_ExprIsTrivial (Cast e _) = cpe_ExprIsTrivial e -cpe_ExprIsTrivial (Lam b body) | isTyVar b = cpe_ExprIsTrivial body +cpe_ExprIsTrivial (Lam b body) | isTyCoVar b = cpe_ExprIsTrivial body cpe_ExprIsTrivial _ = False \end{code} @@ -699,7 +800,7 @@ Instead CoreArity.etaExpand gives f = /\a -> \y -> let s = h 3 in g s y \begin{code} -cpeEtaExpand :: Arity -> CoreExpr -> CoreExpr +cpeEtaExpand :: Arity -> CpeRhs -> CpeRhs cpeEtaExpand arity expr | arity == 0 = expr | otherwise = etaExpand arity expr @@ -717,8 +818,8 @@ get to a partial application: ==> case x of { p -> map f } \begin{code} -tryEtaReduce :: [CoreBndr] -> CoreExpr -> Maybe CoreExpr -tryEtaReduce bndrs expr@(App _ _) +tryEtaReducePrep :: [CoreBndr] -> CoreExpr -> Maybe CoreExpr +tryEtaReducePrep bndrs expr@(App _ _) | ok_to_eta_reduce f && n_remaining >= 0 && and (zipWith ok bndrs last_args) && @@ -738,15 +839,15 @@ tryEtaReduce bndrs expr@(App _ _) ok_to_eta_reduce (Var f) = not (hasNoBinding f) ok_to_eta_reduce _ = False --safe. ToDo: generalise -tryEtaReduce bndrs (Let bind@(NonRec _ r) body) +tryEtaReducePrep bndrs (Let bind@(NonRec _ r) body) | not (any (`elemVarSet` fvs) bndrs) - = case tryEtaReduce bndrs body of + = case tryEtaReducePrep bndrs body of Just e -> Just (Let bind e) Nothing -> Nothing where fvs = exprFreeVars r -tryEtaReduce _ _ = Nothing +tryEtaReducePrep _ _ = Nothing \end{code} @@ -766,18 +867,37 @@ type RhsDemand = Bool -- True => used strictly; hence not top-level, non-recurs \begin{code} data FloatingBind - = FloatLet CoreBind -- Rhs of bindings are CpeRhss - | FloatCase Id CpeBody Bool -- The bool indicates "ok-for-speculation" + = FloatLet CoreBind -- Rhs of bindings are CpeRhss + -- They are always of lifted type; + -- unlifted ones are done with FloatCase + + | FloatCase + Id CpeBody + Bool -- The bool indicates "ok-for-speculation" data Floats = Floats OkToSpec (OrdList FloatingBind) +instance Outputable FloatingBind where + ppr (FloatLet b) = ppr b + ppr (FloatCase b r ok) = brackets (ppr ok) <+> ppr b <+> equals <+> ppr r + +instance Outputable Floats where + ppr (Floats flag fs) = ptext (sLit "Floats") <> brackets (ppr flag) <+> + braces (vcat (map ppr (fromOL fs))) + +instance Outputable OkToSpec where + ppr OkToSpec = ptext (sLit "OkToSpec") + ppr IfUnboxedOk = ptext (sLit "IfUnboxedOk") + ppr NotOkToSpec = ptext (sLit "NotOkToSpec") + -- Can we float these binds out of the rhs of a let? We cache this decision -- to avoid having to recompute it in a non-linear way when there are -- deeply nested lets. data OkToSpec - = NotOkToSpec -- definitely not - | OkToSpec -- yes - | IfUnboxedOk -- only if floating an unboxed binding is ok + = OkToSpec -- Lazy bindings of lifted type + | IfUnboxedOk -- A mixture of lazy lifted bindings and n + -- ok-to-speculate unlifted bindings + | NotOkToSpec -- Some not-ok-to-speculate unlifted bindings mkFloat :: Bool -> Bool -> Id -> CpeRhs -> FloatingBind mkFloat is_strict is_unlifted bndr rhs @@ -795,7 +915,7 @@ emptyFloats = Floats OkToSpec nilOL isEmptyFloats :: Floats -> Bool isEmptyFloats (Floats _ bs) = isNilOL bs -wrapBinds :: Floats -> CoreExpr -> CoreExpr +wrapBinds :: Floats -> CpeBody -> CpeBody wrapBinds (Floats _ binds) body = foldrOL mk_bind body binds where @@ -832,24 +952,65 @@ combine IfUnboxedOk _ = IfUnboxedOk combine _ IfUnboxedOk = IfUnboxedOk combine _ _ = OkToSpec -instance Outputable FloatingBind where - ppr (FloatLet bind) = text "FloatLet" <+> ppr bind - ppr (FloatCase b rhs spec) = text "FloatCase" <+> ppr b <+> ppr spec <+> equals <+> ppr rhs - deFloatTop :: Floats -> [CoreBind] -- For top level only; we don't expect any FloatCases deFloatTop (Floats _ floats) = foldrOL get [] floats where - get (FloatLet b) bs = b:bs + get (FloatLet b) bs = occurAnalyseRHSs b : bs get b _ = pprPanic "corePrepPgm" (ppr b) + + -- See Note [Dead code in CorePrep] + occurAnalyseRHSs (NonRec x e) = NonRec x (occurAnalyseExpr e) + occurAnalyseRHSs (Rec xes) = Rec [(x, occurAnalyseExpr e) | (x, e) <- xes] ------------------------------------------- -wantFloatTop :: Id -> Floats -> Bool +canFloatFromNoCaf :: Floats -> CpeRhs -> Maybe (Floats, CpeRhs) -- Note [CafInfo and floating] -wantFloatTop bndr floats = isEmptyFloats floats - || (mayHaveCafRefs (idCafInfo bndr) - && allLazyTop floats) +canFloatFromNoCaf (Floats ok_to_spec fs) rhs + | OkToSpec <- ok_to_spec -- Worth trying + , Just (subst, fs') <- go (emptySubst, nilOL) (fromOL fs) + = Just (Floats OkToSpec fs', subst_expr subst rhs) + | otherwise + = Nothing + where + subst_expr = substExpr (text "CorePrep") + + go :: (Subst, OrdList FloatingBind) -> [FloatingBind] + -> Maybe (Subst, OrdList FloatingBind) + + go (subst, fbs_out) [] = Just (subst, fbs_out) + + go (subst, fbs_out) (FloatLet (NonRec b r) : fbs_in) + | rhs_ok r + = go (subst', fbs_out `snocOL` new_fb) fbs_in + where + (subst', b') = set_nocaf_bndr subst b + new_fb = FloatLet (NonRec b' (subst_expr subst r)) + + go (subst, fbs_out) (FloatLet (Rec prs) : fbs_in) + | all rhs_ok rs + = go (subst', fbs_out `snocOL` new_fb) fbs_in + where + (bs,rs) = unzip prs + (subst', bs') = mapAccumL set_nocaf_bndr subst bs + rs' = map (subst_expr subst') rs + new_fb = FloatLet (Rec (bs' `zip` rs')) + + go _ _ = Nothing -- Encountered a caffy binding + + ------------ + set_nocaf_bndr subst bndr + = (extendIdSubst subst bndr (Var bndr'), bndr') + where + bndr' = bndr `setIdCafInfo` NoCafRefs + + ------------ + rhs_ok :: CoreExpr -> Bool + -- We can only float to top level from a NoCaf thing if + -- the new binding is static. However it can't mention + -- any non-static things or it would *already* be Caffy + rhs_ok = rhsIsStatic (\_ -> False) wantFloatNested :: RecFlag -> Bool -> Floats -> CpeRhs -> Bool wantFloatNested is_rec strict_or_unlifted floats rhs @@ -911,7 +1072,13 @@ cloneBndr :: CorePrepEnv -> Var -> UniqSM (CorePrepEnv, Var) cloneBndr env bndr | isLocalId bndr = do bndr' <- setVarUnique bndr <$> getUniqueM - return (extendCorePrepEnv env bndr bndr', bndr') + + -- We are going to OccAnal soon, so drop (now-useless) rules/unfoldings + -- so that we can drop more stuff as dead code. + -- See also Note [Dead code in CorePrep] + let bndr'' = bndr' `setIdUnfolding` noUnfolding + `setIdSpecialisation` emptySpecInfo + return (extendCorePrepEnv env bndr bndr'', bndr'') | otherwise -- Top level things, which we don't want -- to clone, have become GlobalIds by now