Commit 32bb9e87 by simonpj@microsoft.com

### Yet another go at CoreArity

Amazingly, there were still Wrong Things in the arity analysis,
exposed by my fiddling with eta expansion.

and tidied it all up.  I hope it's better this time.
 ... ... @@ -376,51 +376,54 @@ Note [ArityType] ~~~~~~~~~~~~~~~~ ArityType is the result of a compositional analysis on expressions, from which we can decide the real arity of the expression (extracted with function getArity). with function exprEtaExpandArity). Here is what the fields mean. If e has ArityType (AT as r), where n = length as, then Here is what the fields mean. If an arbitrary expression 'f' has ArityType 'at', then * If r is ABot then (e x1..xn) definitely diverges Partial applications may or may not diverge * If at = ABot n, then (f x1..xn) definitely diverges. Partial applications to fewer than n args may *or may not* diverge. * If r is ACheap then (e x1..x(n-1)) is cheap, including any nested sub-expressions inside e (say e is (f e1 e2) then e1,e2 are cheap too) We allow ourselves to eta-expand bottoming functions, even if doing so may lose some seq sharing, let x = in \y. error (g x y) ==> \y. let x = in error (g x y) * e, (e x1), ... (e x1 ... x(n-1)) are definitely really functions, or bottom, not casts from a data type So eta expansion is dynamically ok; see Note [State hack and bottoming functions], the part about catch# * If at = ATop as, and n=length as, then expanding 'f' to (\x1..xn. f x1 .. xn) loses no sharing, assuming the calls of f respect the one-shot-ness of of its definition. NB 'f' is an arbitary expression, eg (f = g e1 e2). This 'f' can have ArityType as ATop, with length as > 0, only if e1 e2 are themselves. * In both cases, f, (f x1), ... (f x1 ... f(n-1)) are definitely really functions, or bottom, but *not* casts from a data type, in at least one case branch. (If it's a function in one case branch but an unsafe cast from a data type in another, the program is bogus.) So eta expansion is dynamically ok; see Note [State hack and bottoming functions], the part about catch# Example: f = \x\y. let v = in \s(one-shot) \t(one-shot). blah 'f' has ArityType [ManyShot,ManyShot,OneShot,OneShot] The one-shot-ness means we can, in effect, push that 'let' inside the \st. We regard ABot as stronger than ACheap; ie if ABot holds we don't bother about ACheap Suppose f = \xy. x+y Then f :: AT [False,False] ACheap f v :: AT [False] ACheap f :: AT [False] ATop Note the ArityRes flag tells whether the whole expression is cheap. Note also that having a non-empty 'as' doesn't mean it has that arity; see (f ) which does not have arity 1! The key function getArity extracts the arity (which in turn guides eta-expansion) from ArityType. * If the term is cheap or diverges we can certainly eta expand it e.g. (f x) where x has arity 2 * If its a function whose first arg is one-shot (probably via the state hack) we can eta expand it e.g. (getChar ) Then f :: AT [False,False] ATop f v :: AT [False] ATop f :: AT [] ATop -------------------- Main arity code ---------------------------- \begin{code} -- See Note [ArityType] data ArityType = AT [OneShot] ArityRes data ArityType = ATop [OneShot] | ABot Arity -- There is always an explicit lambda -- to justify the [OneShot] -- to justify the [OneShot], or the Arity type OneShot = Bool -- False <=> Know nothing -- True <=> Can definitely float inside this lambda ... ... @@ -428,10 +431,8 @@ type OneShot = Bool -- False <=> Know nothing -- is marked one-shot, or because it's a state lambda -- and we have the state hack on data ArityRes = ATop | ACheap | ABot vanillaArityType :: ArityType vanillaArityType = AT [] ATop -- Totally uninformative vanillaArityType = ATop [] -- Totally uninformative -- ^ The Arity returned is the number of value args the [_$_] -- expression can be applied to without doing much work ... ... @@ -440,52 +441,89 @@ exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity -- e ==> \xy -> e x y exprEtaExpandArity dflags e = case (arityType dicts_cheap e) of AT (a:as) res | want_eta a res -> 1 + length as _ -> 0 ATop (os:oss) | os || has_lam e -> 1 + length oss -- Note [Eta expanding thunks] | otherwise -> 0 ATop [] -> 0 ABot n -> n where want_eta one_shot ATop = one_shot want_eta _ _ = True dicts_cheap = dopt Opt_DictsCheap dflags has_lam (Note _ e) = has_lam e has_lam (Lam b e) = isId b || has_lam e has_lam _ = False getBotArity :: ArityType -> Maybe Arity -- Arity of a divergent function getBotArity (AT as ABot) = Just (length as) getBotArity _ = Nothing getBotArity (ABot n) = Just n getBotArity _ = Nothing \end{code} Note [Eta expanding thunks] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we see f = case y of p -> \x -> blah should we eta-expand it? Well, if 'x' is a one-shot state token then 'yes' because 'f' will only be applied once. But otherwise we (conservatively) say no. My main reason is to avoid expanding PAPSs f = g d ==> f = \x. g d x because that might in turn make g inline (if it has an inline pragma), which we might not want. After all, INLINE pragmas say "inline only when saturate" so we don't want to be too gung-ho about saturating! \begin{code} arityLam :: Id -> ArityType -> ArityType arityLam id (AT as r) = AT (isOneShotBndr id : as) r arityLam id (ATop as) = ATop (isOneShotBndr id : as) arityLam _ (ABot n) = ABot (n+1) floatIn :: Bool -> ArityType -> ArityType -- We have something like (let x = E in b), -- where b has the given arity type. floatIn c (AT as r) = AT as (extendArityRes r c) floatIn _ (ABot n) = ABot n floatIn True (ATop as) = ATop as floatIn False (ATop as) = ATop (takeWhile id as) -- If E is not cheap, keep arity only for one-shots arityApp :: ArityType -> CoreExpr -> ArityType -- Processing (fun arg) where at is the ArityType of fun, arityApp (AT [] r) arg = AT [] (extendArityRes r (exprIsCheap arg)) arityApp (AT (_:as) r) arg = AT as (extendArityRes r (exprIsCheap arg)) extendArityRes :: ArityRes -> Bool -> ArityRes extendArityRes ABot _ = ABot extendArityRes ACheap True = ACheap extendArityRes _ _ = ATop -- Knock off an argument and behave like 'let' arityApp (ABot 0) _ = ABot 0 arityApp (ABot n) _ = ABot (n-1) arityApp (ATop []) _ = ATop [] arityApp (ATop (_:as)) arg = floatIn (exprIsCheap arg) (ATop as) andArityType :: ArityType -> ArityType -> ArityType -- Used for branches of a 'case' andArityType (AT as1 r1) (AT as2 r2) = AT (go_as as1 as2) (go_r r1 r2) where go_r ABot ABot = ABot go_r ABot ACheap = ACheap go_r ACheap ABot = ACheap go_r ACheap ACheap = ACheap go_r _ _ = ATop go_as (os1:as1) (os2:as2) = (os1 || os2) : go_as as1 as2 go_as [] as2 = as2 go_as as1 [] = as1 andArityType (ABot n1) (ABot n2) = ABot (n1 min n2) andArityType (ATop as) (ABot _) = ATop as andArityType (ABot _) (ATop bs) = ATop bs andArityType (ATop as) (ATop bs) = ATop (as combine bs) where -- See Note [Combining case branches] combine (a:as) (b:bs) = (a && b) : combine as bs combine [] bs = take_one_shots bs combine as [] = take_one_shots as take_one_shots [] = [] take_one_shots (one_shot : as) | one_shot = True : take_one_shots as | otherwise = [] \end{code} Note [Combining case branches] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider go = \x. let z = go e0 go2 = \x. case x of True -> z False -> \s(one-shot). e1 in go2 x We *really* want to eta-expand go and go2. When combining the barnches of the case we have ATop [] andAT ATop [True] and we want to get ATop [True]. But if the inner lambda wasn't one-shot we don't want to do this. (We need a proper arity analysis to justify that.) \begin{code} --------------------------- ... ... @@ -493,16 +531,13 @@ arityType :: Bool -> CoreExpr -> ArityType arityType _ (Var v) | Just strict_sig <- idStrictness_maybe v , (ds, res) <- splitStrictSig strict_sig = mk_arity (length ds) res , let arity = length ds = if isBotRes res then ABot arity else ATop (take arity one_shots) | otherwise = mk_arity (idArity v) TopRes = ATop (take (idArity v) one_shots) where mk_arity id_arity res | isBotRes res = AT (take id_arity one_shots) ABot | id_arity>0 = AT (take id_arity one_shots) ACheap | otherwise = AT [] ATop one_shots :: [Bool] -- One-shot-ness derived from the type one_shots = typeArity (idType v) -- Lambdas; increase arity ... ... @@ -645,7 +680,7 @@ etaExpand n orig_expr -- Note [Eta expansion and SCCs] go 0 expr = expr go n (Lam v body) | isTyCoVar v = Lam v (go n body) | otherwise = Lam v (go (n-1) body) | otherwise = Lam v (go (n-1) body) go n (Cast expr co) = Cast (go n expr) co go n expr = -- pprTrace "ee" (vcat [ppr orig_expr, ppr expr, ppr etas])$ etaInfoAbs etas (etaInfoApp subst' expr etas) ... ...