import CoreFVs
import CoreMonad ( endPass, CoreToDo(..) )
import CoreSyn
+import CoreSubst
import Type
import Coercion
import TyCon
import Outputable
import MonadUtils
import FastString
+import Data.List ( mapAccumL )
import Control.Monad
\end{code}
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]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cpePair top_lvl is_rec is_strict_or_unlifted env bndr rhs
= do { (floats1, rhs1) <- cpeRhsE env rhs
- ; (floats2, rhs2)
+ -- 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 (floats1, cpeEtaExpand arity rhs1)
+ 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 rhs1
- ; return (addFloat floats1 float, cpeEtaExpand arity (Var v)) })
-
- ; (floats3, rhs')
- <- if want_float floats2 rhs2
- then return (floats2, rhs2)
- else -- Non-empty floats will wrap rhs1
- -- But: rhs1 might have lambdas, and we can't
- -- put them inside a wrapBinds
- do { body2 <- rhsToBodyNF rhs2
- ; return (emptyFloats, wrapBinds floats2 body2) }
+ ; let float = mkFloat False False v rhs2
+ ; return (addFloat floats2 float, cpeEtaExpand arity (Var v)) })
-- Record if the binder is evaluated
; let bndr' | exprIsHNF rhs' = bndr `setIdUnfolding` evaldUnfolding
; 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]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
; 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)
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}
==> 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) &&
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}
\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
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)
get b _ = pprPanic "corePrepPgm" (ppr b)
-------------------------------------------
-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
projects / ghc-hetmet.git / blobdiff