\begin{code}
-- | Arit and eta expansion
module CoreArity (
- manifestArity, exprArity,
- exprEtaExpandArity, etaExpand
+ manifestArity, exprArity, exprBotStrictness_maybe,
+ exprEtaExpandArity, etaExpand
) where
#include "HsVersions.h"
import CoreSyn
import CoreFVs
import CoreUtils
+import NewDemand
+import TyCon ( isRecursiveTyCon )
import qualified CoreSubst
import CoreSubst ( Subst, substBndr, substBndrs, substExpr
, mkEmptySubst, isEmptySubst )
import Var
import VarEnv
-#if mingw32_TARGET_OS
-import Packages
-#endif
import Id
import Type
+import TcType ( isDictLikeTy )
import Coercion
import BasicTypes
import Unique
import Outputable
import DynFlags
+import StaticFlags ( opt_NoStateHack )
import FastString
-import Maybes
-
-import GHC.Exts -- For `xori`
\end{code}
%************************************************************************
%************************************************************************
%* *
-\subsection{Eta reduction and expansion}
+ Eta expansion
%* *
%************************************************************************
-exprEtaExpandArity is used when eta expanding
- e ==> \xy -> e x y
-
-It returns 1 (or more) to:
- case x of p -> \s -> ...
-because for I/O ish things we really want to get that \s to the top.
-We are prepared to evaluate x each time round the loop in order to get that
+\begin{code}
+-- ^ The Arity returned is the number of value args the
+-- expression can be applied to without doing much work
+exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity
+-- exprEtaExpandArity is used when eta expanding
+-- e ==> \xy -> e x y
+exprEtaExpandArity dflags e
+ = applyStateHack e (arityType dicts_cheap e)
+ where
+ dicts_cheap = dopt Opt_DictsCheap dflags
+
+exprBotStrictness_maybe :: CoreExpr -> Maybe (Arity, StrictSig)
+-- A cheap and cheerful function that identifies bottoming functions
+-- and gives them a suitable strictness signatures. It's used during
+-- float-out
+exprBotStrictness_maybe e
+ = case arityType False e of
+ AT _ ATop -> Nothing
+ AT a ABot -> Just (a, mkStrictSig (mkTopDmdType (replicate a topDmd) BotRes))
+\end{code}
+
+Note [Definition of arity]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+The "arity" of an expression 'e' is n if
+ applying 'e' to *fewer* than n *value* arguments
+ converges rapidly
-It's all a bit more subtle than it looks:
+Or, to put it another way
-1. One-shot lambdas
+ there is no work lost in duplicating the partial
+ application (e x1 .. x(n-1))
-Consider one-shot lambdas
- let x = expensive in \y z -> E
-We want this to have arity 2 if the \y-abstraction is a 1-shot lambda
-Hence the ArityType returned by arityType
+In the divegent case, no work is lost by duplicating because if the thing
+is evaluated once, that's the end of the program.
-2. The state-transformer hack
+Or, to put it another way, in any context C
-The one-shot lambda special cause is particularly important/useful for
-IO state transformers, where we often get
- let x = E in \ s -> ...
+ C[ (\x1 .. xn. e x1 .. xn) ]
+ is as efficient as
+ C[ e ]
-and the \s is a real-world state token abstraction. Such abstractions
-are almost invariably 1-shot, so we want to pull the \s out, past the
-let x=E, even if E is expensive. So we treat state-token lambdas as
-one-shot even if they aren't really. The hack is in Id.isOneShotBndr.
-3. Dealing with bottom
+It's all a bit more subtle than it looks:
-Consider also
- f = \x -> error "foo"
-Here, arity 1 is fine. But if it is
- f = \x -> case x of
- True -> error "foo"
- False -> \y -> x+y
-then we want to get arity 2. Tecnically, this isn't quite right, because
- (f True) `seq` 1
-should diverge, but it'll converge if we eta-expand f. Nevertheless, we
-do so; it improves some programs significantly, and increasing convergence
-isn't a bad thing. Hence the ABot/ATop in ArityType.
+Note [Arity of case expressions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We treat the arity of
+ case x of p -> \s -> ...
+as 1 (or more) because for I/O ish things we really want to get that
+\s to the top. We are prepared to evaluate x each time round the loop
+in order to get that.
-Actually, the situation is worse. Consider
+This isn't really right in the presence of seq. Consider
f = \x -> case x of
True -> \y -> x+y
False -> \y -> x-y
many programs.
-4. Newtypes
+1. Note [One-shot lambdas]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider one-shot lambdas
+ let x = expensive in \y z -> E
+We want this to have arity 1 if the \y-abstraction is a 1-shot lambda.
+3. Note [Dealing with bottom]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ f = \x -> error "foo"
+Here, arity 1 is fine. But if it is
+ f = \x -> case x of
+ True -> error "foo"
+ False -> \y -> x+y
+then we want to get arity 2. Technically, this isn't quite right, because
+ (f True) `seq` 1
+should diverge, but it'll converge if we eta-expand f. Nevertheless, we
+do so; it improves some programs significantly, and increasing convergence
+isn't a bad thing. Hence the ABot/ATop in ArityType.
+
+
+4. Note [Newtype arity]
+~~~~~~~~~~~~~~~~~~~~~~~~
Non-recursive newtypes are transparent, and should not get in the way.
We do (currently) eta-expand recursive newtypes too. So if we have, say
When we eta-expand e to arity 1: eta_expand 1 e T
we want to get: coerce T (\x::[T] -> (coerce ([T]->Int) e) x)
-HOWEVER, note that if you use coerce bogusly you can ge
- coerce Int negate
-And since negate has arity 2, you might try to eta expand. But you can't
-decopose Int to a function type. Hence the final case in eta_expand.
-
+ HOWEVER, note that if you use coerce bogusly you can ge
+ coerce Int negate
+ And since negate has arity 2, you might try to eta expand. But you can't
+ decopose Int to a function type. Hence the final case in eta_expand.
+
+Note [The state-transformer hack]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we have
+ f = e
+where e has arity n. Then, if we know from the context that f has
+a usage type like
+ t1 -> ... -> tn -1-> t(n+1) -1-> ... -1-> tm -> ...
+then we can expand the arity to m. This usage type says that
+any application (x e1 .. en) will be applied to uniquely to (m-n) more args
+Consider f = \x. let y = <expensive>
+ in case x of
+ True -> foo
+ False -> \(s:RealWorld) -> e
+where foo has arity 1. Then we want the state hack to
+apply to foo too, so we can eta expand the case.
+
+Then we expect that if f is applied to one arg, it'll be applied to two
+(that's the hack -- we don't really know, and sometimes it's false)
+See also Id.isOneShotBndr.
\begin{code}
--- ^ The Arity returned is the number of value args the
--- expression can be applied to without doing much work
-exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity
-exprEtaExpandArity dflags e = arityDepth (arityType dflags e)
+applyStateHack :: CoreExpr -> ArityType -> Arity
+applyStateHack e (AT orig_arity is_bot)
+ | opt_NoStateHack = orig_arity
+ | ABot <- is_bot = orig_arity -- Note [State hack and bottoming functions]
+ | otherwise = go orig_ty orig_arity
+ where -- Note [The state-transformer hack]
+ orig_ty = exprType e
+ go :: Type -> Arity -> Arity
+ go ty arity -- This case analysis should match that in eta_expand
+ | Just (_, ty') <- splitForAllTy_maybe ty = go ty' arity
+
+ | Just (tc,tys) <- splitTyConApp_maybe ty
+ , Just (ty', _) <- instNewTyCon_maybe tc tys
+ , not (isRecursiveTyCon tc) = go ty' arity
+ -- Important to look through non-recursive newtypes, so that, eg
+ -- (f x) where f has arity 2, f :: Int -> IO ()
+ -- Here we want to get arity 1 for the result!
+
+ | Just (arg,res) <- splitFunTy_maybe ty
+ , arity > 0 || isStateHackType arg = 1 + go res (arity-1)
+{-
+ = if arity > 0 then 1 + go res (arity-1)
+ else if isStateHackType arg then
+ pprTrace "applystatehack" (vcat [ppr orig_arity, ppr orig_ty,
+ ppr ty, ppr res, ppr e]) $
+ 1 + go res (arity-1)
+ else WARN( arity > 0, ppr arity ) 0
+-}
+ | otherwise = WARN( arity > 0, ppr arity ) 0
+\end{code}
--- A limited sort of function type
-data ArityType = AFun Bool ArityType -- True <=> one-shot
- | ATop -- Know nothing
- | ABot -- Diverges
+Note [State hack and bottoming functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's a terrible idea to use the state hack on a bottoming function.
+Here's what happens (Trac #2861):
+
+ f :: String -> IO T
+ f = \p. error "..."
+
+Eta-expand, using the state hack:
+
+ f = \p. (\s. ((error "...") |> g1) s) |> g2
+ g1 :: IO T ~ (S -> (S,T))
+ g2 :: (S -> (S,T)) ~ IO T
-arityDepth :: ArityType -> Arity
-arityDepth (AFun _ ty) = 1 + arityDepth ty
-arityDepth _ = 0
+Extrude the g2
-andArityType :: ArityType -> ArityType -> ArityType
-andArityType ABot at2 = at2
-andArityType ATop _ = ATop
-andArityType (AFun t1 at1) (AFun t2 at2) = AFun (t1 && t2) (andArityType at1 at2)
-andArityType at1 at2 = andArityType at2 at1
+ f' = \p. \s. ((error "...") |> g1) s
+ f = f' |> (String -> g2)
-arityType :: DynFlags -> CoreExpr -> ArityType
- -- (go1 e) = [b1,..,bn]
- -- means expression can be rewritten \x_b1 -> ... \x_bn -> body
- -- where bi is True <=> the lambda is one-shot
+Discard args for bottomming function
-arityType dflags (Note _ e) = arityType dflags e
--- Not needed any more: etaExpand is cleverer
--- removed: | ok_note n = arityType dflags e
--- removed: | otherwise = ATop
+ f' = \p. \s. ((error "...") |> g1 |> g3
+ g3 :: (S -> (S,T)) ~ (S,T)
-arityType dflags (Cast e _) = arityType dflags e
+Extrude g1.g3
+ f'' = \p. \s. (error "...")
+ f' = f'' |> (String -> S -> g1.g3)
+
+And now we can repeat the whole loop. Aargh! The bug is in applying the
+state hack to a function which then swallows the argument.
+
+
+-------------------- Main arity code ----------------------------
+\begin{code}
+-- If e has ArityType (AT n r), then the term 'e'
+-- * Must be applied to at least n *value* args
+-- before doing any significant work
+-- * It will not diverge before being applied to n
+-- value arguments
+-- * If 'r' is ABot, then it guarantees to diverge if
+-- applied to n arguments (or more)
+
+data ArityType = AT Arity ArityRes
+data ArityRes = ATop -- Know nothing
+ | ABot -- Diverges
+
+vanillaArityType :: ArityType
+vanillaArityType = AT 0 ATop -- Totally uninformative
+
+incArity :: ArityType -> ArityType
+incArity (AT a r) = AT (a+1) r
+
+decArity :: ArityType -> ArityType
+decArity (AT 0 r) = AT 0 r
+decArity (AT a r) = AT (a-1) r
+
+andArityType :: ArityType -> ArityType -> ArityType -- Used for branches of a 'case'
+andArityType (AT a1 ATop) (AT a2 ATop) = AT (a1 `min` a2) ATop
+andArityType (AT _ ABot) (AT a2 ATop) = AT a2 ATop
+andArityType (AT a1 ATop) (AT _ ABot) = AT a1 ATop
+andArityType (AT a1 ABot) (AT a2 ABot) = AT (a1 `max` a2) ABot
+
+trimArity :: Bool -> ArityType -> ArityType
+-- We have something like (let x = E in b), where b has the given
+-- arity type. Then
+-- * If E is cheap we can push it inside as far as we like
+-- * If b eventually diverges, we allow ourselves to push inside
+-- arbitrarily, even though that is not quite right
+trimArity _cheap (AT a ABot) = AT a ABot
+trimArity True (AT a ATop) = AT a ATop
+trimArity False (AT _ ATop) = AT 0 ATop -- Bale out
+
+---------------------------
+arityType :: Bool -> CoreExpr -> ArityType
arityType _ (Var v)
- = mk (idArity v) (arg_tys (idType v))
- where
- mk :: Arity -> [Type] -> ArityType
- -- The argument types are only to steer the "state hack"
- -- Consider case x of
- -- True -> foo
- -- False -> \(s:RealWorld) -> e
- -- where foo has arity 1. Then we want the state hack to
- -- apply to foo too, so we can eta expand the case.
- mk 0 tys | isBottomingId v = ABot
- | (ty:_) <- tys, isStateHackType ty = AFun True ATop
- | otherwise = ATop
- mk n (ty:tys) = AFun (isStateHackType ty) (mk (n-1) tys)
- mk n [] = AFun False (mk (n-1) [])
-
- arg_tys :: Type -> [Type] -- Ignore for-alls
- arg_tys ty
- | Just (_, ty') <- splitForAllTy_maybe ty = arg_tys ty'
- | Just (arg,res) <- splitFunTy_maybe ty = arg : arg_tys res
- | otherwise = []
+ | Just strict_sig <- idNewStrictness_maybe v
+ , (ds, res) <- splitStrictSig strict_sig
+ , isBotRes res
+ = AT (length ds) ABot -- Function diverges
+ | otherwise
+ = AT (idArity v) ATop
-- Lambdas; increase arity
-arityType dflags (Lam x e)
- | isId x = AFun (isOneShotBndr x) (arityType dflags e)
- | otherwise = arityType dflags e
+arityType dicts_cheap (Lam x e)
+ | isId x = incArity (arityType dicts_cheap e)
+ | otherwise = arityType dicts_cheap e
-- Applications; decrease arity
-arityType dflags (App f (Type _)) = arityType dflags f
-arityType dflags (App f a)
- = case arityType dflags f of
- ABot -> ABot -- If function diverges, ignore argument
- ATop -> ATop -- No no info about function
- AFun _ xs
- | exprIsCheap a -> xs
- | otherwise -> ATop
-
+arityType dicts_cheap (App fun (Type _))
+ = arityType dicts_cheap fun
+arityType dicts_cheap (App fun arg )
+ = trimArity (exprIsCheap arg) (decArity (arityType dicts_cheap fun))
+
-- Case/Let; keep arity if either the expression is cheap
-- or it's a 1-shot lambda
-- The former is not really right for Haskell
-- ===>
-- f x y = case x of { (a,b) -> e }
-- The difference is observable using 'seq'
-arityType dflags (Case scrut _ _ alts)
- = case foldr1 andArityType [arityType dflags rhs | (_,_,rhs) <- alts] of
- xs | exprIsCheap scrut -> xs
- AFun one_shot _ | one_shot -> AFun True ATop
- _ -> ATop
-
-arityType dflags (Let b e)
- = case arityType dflags e of
- xs | cheap_bind b -> xs
- AFun one_shot _ | one_shot -> AFun True ATop
- _ -> ATop
+arityType dicts_cheap (Case scrut _ _ alts)
+ = trimArity (exprIsCheap scrut)
+ (foldr1 andArityType [arityType dicts_cheap rhs | (_,_,rhs) <- alts])
+
+arityType dicts_cheap (Let b e)
+ = trimArity (cheap_bind b) (arityType dicts_cheap e)
where
cheap_bind (NonRec b e) = is_cheap (b,e)
cheap_bind (Rec prs) = all is_cheap prs
- is_cheap (b,e) = (dopt Opt_DictsCheap dflags && isDictId b)
+ is_cheap (b,e) = (dicts_cheap && isDictLikeTy (idType b))
|| exprIsCheap e
-- If the experimental -fdicts-cheap flag is on, we eta-expand through
-- dictionary bindings. This improves arities. Thereby, it also
-- means that full laziness is less prone to floating out the
- -- application of a function to its dictionary arguments, which
+ -- application of a function to its dictionary arguments, which
-- can thereby lose opportunities for fusion. Example:
-- foo :: Ord a => a -> ...
-- foo = /\a \(d:Ord a). let d' = ...d... in \(x:a). ....
--
-- The (foo DInt) is floated out, and makes ineffective a RULE
-- foo (bar x) = ...
- --
+ --
-- One could go further and make exprIsCheap reply True to any
-- dictionary-typed expression, but that's more work.
+ --
+ -- See Note [Dictionary-like types] in TcType.lhs for why we use
+ -- isDictLikeTy here rather than isDictTy
-arityType _ _ = ATop
+arityType dicts_cheap (Note _ e) = arityType dicts_cheap e
+arityType dicts_cheap (Cast e _) = arityType dicts_cheap e
+arityType _ _ = vanillaArityType
\end{code}
-
-
+
+
%************************************************************************
%* *
The main eta-expander
a subsequent clean-up phase of the Simplifier to de-crapify the result,
means you can't really use it in CorePrep, which is painful.
+Note [Eta expansion and SCCs]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note that SCCs are not treated specially by etaExpand. If we have
+ etaExpand 2 (\x -> scc "foo" e)
+ = (\xy -> (scc "foo" e) y)
+So the costs of evaluating 'e' (not 'e y') are attributed to "foo"
+
\begin{code}
-- | @etaExpand n us e ty@ returns an expression with
-- the same meaning as @e@, but with arity @n@.
etaExpand :: Arity -- ^ Result should have this number of value args
-> CoreExpr -- ^ Expression to expand
-> CoreExpr
--- Note that SCCs are not treated specially. If we have
--- etaExpand 2 (\x -> scc "foo" e)
--- = (\xy -> (scc "foo" e) y)
--- So the costs of evaluating 'e' (not 'e y') are attributed to "foo"
-
-- etaExpand deals with for-alls. For example:
-- etaExpand 1 E
-- where E :: forall a. a -> a
= go n orig_expr
where
-- Strip off existing lambdas
+ -- Note [Eta expansion and SCCs]
go 0 expr = expr
go n (Lam v body) | isTyVar v = Lam v (go n body)
| otherwise = Lam v (go (n-1) body)
- go n (Note InlineMe expr) = Note InlineMe (go n expr)
- -- Note [Eta expansion and SCCs]
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)
projects / ghc-hetmet.git / blobdiff