import VarSet
import BasicTypes
import Util
+import MonadUtils
import Outputable
+
import List( nub )
\end{code}
Bool -- True <=> There is something interesting about
-- the context, and hence the inliner
-- should be a bit keener (see interestingCallContext)
- -- Two cases:
- -- (a) This is the RHS of a thunk whose type suggests
- -- that update-in-place would be possible
- -- (b) This is an argument of a function that has RULES
+ -- Specifically:
+ -- This is an argument of a function that has RULES
-- Inlining the call might allow the rule to fire
| CoerceIt -- C `cast` co
mkBoringStop ty = Stop ty AnArg False
mkLazyArgStop :: OutType -> Bool -> SimplCont
-mkLazyArgStop ty has_rules = Stop ty AnArg (canUpdateInPlace ty || has_rules)
+mkLazyArgStop ty has_rules = Stop ty AnArg has_rules
mkRhsStop :: OutType -> SimplCont
-mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty)
+mkRhsStop ty = Stop ty AnRhs False
-------------------
contIsRhsOrArg (Stop {}) = True
go (StrictBind {}) = False -- ??
go (CoerceIt _ c) = go c
go (Stop _ _ interesting) = interesting
-
--------------------
-canUpdateInPlace :: Type -> Bool
--- Consider let x = <wurble> in ...
--- If <wurble> returns an explicit constructor, we might be able
--- to do update in place. So we treat even a thunk RHS context
--- as interesting if update in place is possible. We approximate
--- this by seeing if the type has a single constructor with a
--- small arity. But arity zero isn't good -- we share the single copy
--- for that case, so no point in sharing.
-
-canUpdateInPlace ty
- | not opt_UF_UpdateInPlace = False
- | otherwise
- = case splitTyConApp_maybe ty of
- Nothing -> False
- Just (tycon, _) -> case tyConDataCons_maybe tycon of
- Just [dc] -> arity == 1 || arity == 2
- where
- arity = dataConRepArity dc
- other -> False
\end{code}
; return (mkLams bndrs body') }
| otherwise
- = returnSmpl (mkLams bndrs body)
+ = return (mkLams bndrs body)
\end{code}
Note [Casts and lambdas]
-- if this is indeed a right-hand side; otherwise
-- we end up floating the thing out, only for float-in
-- to float it right back in again!
- = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
- returnSmpl (floats, mkLams bndrs body')
+ = do (floats, body') <- tryRhsTyLam env bndrs body
+ return (floats, mkLams bndrs body')
-}
%************************************************************************
%* *
-\subsection{Eta expansion and reduction}
+ Eta reduction
%* *
%************************************************************************
-We try for eta reduction here, but *only* if we get all the
-way to an exprIsTrivial expression.
-We don't want to remove extra lambdas unless we are going
-to avoid allocating this thing altogether
+Note [Eta reduction conditions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We try for eta reduction here, but *only* if we get all the way to an
+trivial expression. We don't want to remove extra lambdas unless we
+are going to avoid allocating this thing altogether.
+
+There are some particularly delicate points here:
+
+* Eta reduction is not valid in general:
+ \x. bot /= bot
+ This matters, partly for old-fashioned correctness reasons but,
+ worse, getting it wrong can yield a seg fault. Consider
+ f = \x.f x
+ h y = case (case y of { True -> f `seq` True; False -> False }) of
+ True -> ...; False -> ...
+
+ If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
+ says f=bottom, and replaces the (f `seq` True) with just
+ (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
+ *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
+ the definition again, so that it does not termninate after all.
+ Result: seg-fault because the boolean case actually gets a function value.
+ See Trac #1947.
+
+ So it's important to to the right thing.
+
+* We need to be careful if we just look at f's arity. Currently (Dec07),
+ f's arity is visible in its own RHS (see Note [Arity robustness] in
+ SimplEnv) so we must *not* trust the arity when checking that 'f' is
+ a value. Instead, look at the unfolding.
+
+ However for GlobalIds we can look at the arity; and for primops we
+ must, since they have no unfolding.
+
+* Regardless of whether 'f' is a vlaue, we always want to
+ reduce (/\a -> f a) to f
+ This came up in a RULE: foldr (build (/\a -> g a))
+ did not match foldr (build (/\b -> ...something complex...))
+ The type checker can insert these eta-expanded versions,
+ with both type and dictionary lambdas; hence the slightly
+ ad-hoc isDictId
+
+These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
+Alas.
\begin{code}
tryEtaReduce :: [OutBndr] -> OutExpr -> Maybe OutExpr
tryEtaReduce bndrs body
- -- We don't use CoreUtils.etaReduce, because we can be more
- -- efficient here:
- -- (a) we already have the binders
- -- (b) we can do the triviality test before computing the free vars
= go (reverse bndrs) body
where
go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
go [] fun | ok_fun fun = Just fun -- Success!
go _ _ = Nothing -- Failure!
- ok_fun fun = exprIsTrivial fun
- && not (any (`elemVarSet` (exprFreeVars fun)) bndrs)
- && (exprIsHNF fun || all ok_lam bndrs)
+ -- Note [Eta reduction conditions]
+ ok_fun (App fun (Type ty))
+ | not (any (`elemVarSet` tyVarsOfType ty) bndrs)
+ = ok_fun fun
+ ok_fun (Var fun_id)
+ = not (fun_id `elem` bndrs)
+ && (ok_fun_id fun_id || all ok_lam bndrs)
+ ok_fun _fun = False
+
+ ok_fun_id fun
+ | isLocalId fun = isEvaldUnfolding (idUnfolding fun)
+ | isDataConWorkId fun = True
+ | isGlobalId fun = idArity fun > 0
+
ok_lam v = isTyVar v || isDictId v
- -- The exprIsHNF is because eta reduction is not
- -- valid in general: \x. bot /= bot
- -- So we need to be sure that the "fun" is a value.
- --
- -- However, we always want to reduce (/\a -> f a) to f
- -- This came up in a RULE: foldr (build (/\a -> g a))
- -- did not match foldr (build (/\b -> ...something complex...))
- -- The type checker can insert these eta-expanded versions,
- -- with both type and dictionary lambdas; hence the slightly
- -- ad-hoc isDictTy
ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
\end{code}
- Try eta expansion for RHSs
+%************************************************************************
+%* *
+ Eta expansion
+%* *
+%************************************************************************
+
We go for:
f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
* N is a NORMAL FORM (i.e. no redexes anywhere)
wanting a suitable number of extra args.
+The biggest reason for doing this is for cases like
+
+ f = \x -> case x of
+ True -> \y -> e1
+ False -> \y -> e2
+
+Here we want to get the lambdas together. A good exmaple is the nofib
+program fibheaps, which gets 25% more allocation if you don't do this
+eta-expansion.
+
We may have to sandwich some coerces between the lambdas
to make the types work. exprEtaExpandArity looks through coerces
when computing arity; and etaExpand adds the coerces as necessary when
\begin{code}
tryEtaExpansion :: DynFlags -> OutExpr -> SimplM OutExpr
-- There is at least one runtime binder in the binders
-tryEtaExpansion dflags body
- = getUniquesSmpl `thenSmpl` \ us ->
- returnSmpl (etaExpand fun_arity us body (exprType body))
+tryEtaExpansion dflags body = do
+ us <- getUniquesM
+ return (etaExpand fun_arity us body (exprType body))
where
fun_arity = exprEtaExpandArity dflags body
\end{code}
abstractFloats :: [OutTyVar] -> SimplEnv -> OutExpr -> SimplM ([OutBind], OutExpr)
abstractFloats main_tvs body_env body
= ASSERT( notNull body_floats )
- do { (subst, float_binds) <- mapAccumLSmpl abstract empty_subst body_floats
+ do { (subst, float_binds) <- mapAccumLM abstract empty_subst body_floats
; return (float_binds, CoreSubst.substExpr subst body) }
where
main_tv_set = mkVarSet main_tvs
-- gives rise to problems. SLPJ June 98
abstract subst (Rec prs)
- = do { (poly_ids, poly_apps) <- mapAndUnzipSmpl (mk_poly tvs_here) ids
+ = do { (poly_ids, poly_apps) <- mapAndUnzipM (mk_poly tvs_here) ids
; let subst' = CoreSubst.extendSubstList subst (ids `zip` poly_apps)
poly_rhss = [mkLams tvs_here (CoreSubst.substExpr subst' rhs) | rhs <- rhss]
; return (subst', Rec (poly_ids `zip` poly_rhss)) }
tvs_here = main_tvs
mk_poly tvs_here var
- = do { uniq <- getUniqueSmpl
+ = do { uniq <- getUniqueM
; let poly_name = setNameUnique (idName var) uniq -- Keep same name
poly_ty = mkForAllTys tvs_here (idType var) -- But new type of course
poly_id = mkLocalId poly_name poly_ty
[con] -> -- It matches exactly one constructor, so fill it in
do { tick (FillInCaseDefault case_bndr)
- ; us <- getUniquesSmpl
+ ; us <- getUniquesM
; let (ex_tvs, co_tvs, arg_ids) =
dataConRepInstPat us con inst_tys
; return [(DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, deflt_rhs)] }
mkCase scrut case_bndr ty alts -- Identity case
| all identity_alt alts
- = tick (CaseIdentity case_bndr) `thenSmpl_`
- returnSmpl (re_cast scrut)
+ = do tick (CaseIdentity case_bndr)
+ return (re_cast scrut)
where
identity_alt (con, args, rhs) = check_eq con args (de_cast rhs)
--------------------------------------------------
-- Catch-all
--------------------------------------------------
-mkCase scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts)
+mkCase scrut bndr ty alts = return (Case scrut bndr ty alts)
\end{code}