%
-% (c) The AQUA Project, Glasgow University, 1993-1996
+% (c) The AQUA Project, Glasgow University, 1993-1998
%
\section[SimplUtils]{The simplifier utilities}
\begin{code}
-#include "HsVersions.h"
-
module SimplUtils (
+ simplBinder, simplBinders, simplIds,
+ transformRhs,
+ mkCase, findAlt, findDefault,
- floatExposesHNF,
-
- etaCoreExpr, mkRhsTyLam,
+ -- The continuation type
+ SimplCont(..), DupFlag(..), contIsDupable, contResultType,
+ pushArgs, discardCont, countValArgs, countArgs,
+ analyseCont, discardInline
- etaExpandCount,
-
- simplIdWantsToBeINLINEd,
-
- singleConstructorType, typeOkForCase
) where
-IMP_Ubiq(){-uitous-}
-#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ <= 201
-IMPORT_DELOOPER(SmplLoop) -- paranoia checking
-#endif
+#include "HsVersions.h"
import BinderInfo
-import CmdLineOpts ( opt_DoEtaReduction, SimplifierSwitch(..) )
+import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge )
import CoreSyn
-import CoreUnfold ( SimpleUnfolding, mkFormSummary, exprIsTrivial, FormSummary(..) )
-import Id ( idType, isBottomingId, addInlinePragma, addIdDemandInfo,
- idWantsToBeINLINEd, dataConArgTys, SYN_IE(Id),
- getIdArity, GenId{-instance Eq-}
+import PprCore ( {- instance Outputable Expr -} )
+import CoreUnfold ( isValueUnfolding )
+import CoreFVs ( exprFreeVars )
+import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, exprEtaExpandArity, bindNonRec )
+import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, lookupIdSubst )
+import Id ( Id, idType, isId, idName,
+ idOccInfo, idUnfolding,
+ mkId, idInfo
)
-import IdInfo ( ArityInfo(..), DemandInfo )
-import Maybes ( maybeToBool )
-import PrelVals ( augmentId, buildId )
-import PrimOp ( primOpIsCheap )
-import SimplEnv
+import IdInfo ( arityLowerBound, setOccInfo, vanillaIdInfo )
+import Maybes ( maybeToBool, catMaybes )
+import Name ( isLocalName, setNameUnique )
import SimplMonad
-import Type ( tyVarsOfType, mkForAllTys, mkTyVarTys, isPrimType, getTyVar_maybe,
- maybeAppDataTyConExpandingDicts, SYN_IE(Type)
+import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType, repType,
+ splitTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys
)
-import TyCon ( isDataTyCon )
-import TyVar ( elementOfTyVarSet,
- GenTyVar{-instance Eq-} )
-import Util ( isIn, panic, assertPanic )
+import TyCon ( tyConDataConsIfAvailable )
+import PprType ( {- instance Outputable Type -} )
+import DataCon ( dataConRepArity )
+import TysPrim ( statePrimTyCon )
+import Var ( setVarUnique )
+import VarSet
+import VarEnv ( SubstEnv, SubstResult(..) )
+import UniqSupply ( splitUniqSupply, uniqFromSupply )
+import Util ( zipWithEqual, mapAccumL )
+import Outputable
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{The continuation data type}
+%* *
+%************************************************************************
+
+\begin{code}
+data SimplCont -- Strict contexts
+ = Stop OutType -- Type of the result
+
+ | CoerceIt OutType -- The To-type, simplified
+ SimplCont
+
+ | InlinePlease -- This continuation makes a function very
+ SimplCont -- keen to inline itelf
+
+ | ApplyTo DupFlag
+ InExpr SubstEnv -- The argument, as yet unsimplified,
+ SimplCont -- and its subst-env
+
+ | Select DupFlag
+ InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
+ SimplCont
+
+ | ArgOf DupFlag -- An arbitrary strict context: the argument
+ -- of a strict function, or a primitive-arg fn
+ -- or a PrimOp
+ OutType -- The type of the expression being sought by the context
+ -- f (error "foo") ==> coerce t (error "foo")
+ -- when f is strict
+ -- We need to know the type t, to which to coerce.
+ (OutExpr -> SimplM OutExprStuff) -- What to do with the result
+
+instance Outputable SimplCont where
+ ppr (Stop _) = ptext SLIT("Stop")
+ ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
+ ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
+ ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
+ (nest 4 (ppr alts)) $$ ppr cont
+ ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
+ ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont
+
+data DupFlag = OkToDup | NoDup
+
+instance Outputable DupFlag where
+ ppr OkToDup = ptext SLIT("ok")
+ ppr NoDup = ptext SLIT("nodup")
+
+contIsDupable :: SimplCont -> Bool
+contIsDupable (Stop _) = True
+contIsDupable (ApplyTo OkToDup _ _ _) = True
+contIsDupable (ArgOf OkToDup _ _) = True
+contIsDupable (Select OkToDup _ _ _ _) = True
+contIsDupable (CoerceIt _ cont) = contIsDupable cont
+contIsDupable (InlinePlease cont) = contIsDupable cont
+contIsDupable other = False
+
+pushArgs :: SubstEnv -> [InExpr] -> SimplCont -> SimplCont
+pushArgs se [] cont = cont
+pushArgs se (arg:args) cont = ApplyTo NoDup arg se (pushArgs se args cont)
+
+discardCont :: SimplCont -- A continuation, expecting
+ -> SimplCont -- Replace the continuation with a suitable coerce
+discardCont (Stop to_ty) = Stop to_ty
+discardCont cont = CoerceIt to_ty (Stop to_ty)
+ where
+ to_ty = contResultType cont
+
+contResultType :: SimplCont -> OutType
+contResultType (Stop to_ty) = to_ty
+contResultType (ArgOf _ to_ty _) = to_ty
+contResultType (ApplyTo _ _ _ cont) = contResultType cont
+contResultType (CoerceIt _ cont) = contResultType cont
+contResultType (InlinePlease cont) = contResultType cont
+contResultType (Select _ _ _ _ cont) = contResultType cont
+
+countValArgs :: SimplCont -> Int
+countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
+countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
+countValArgs other = 0
+
+countArgs :: SimplCont -> Int
+countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
+countArgs other = 0
+\end{code}
+
+
+Comment about analyseCont
+~~~~~~~~~~~~~~~~~~~~~~~~~
+We want to avoid inlining an expression where there can't possibly be
+any gain, such as in an argument position. Hence, if the continuation
+is interesting (eg. a case scrutinee, application etc.) then we
+inline, otherwise we don't.
+
+Previously some_benefit used to return True only if the variable was
+applied to some value arguments. This didn't work:
+
+ let x = _coerce_ (T Int) Int (I# 3) in
+ case _coerce_ Int (T Int) x of
+ I# y -> ....
+
+we want to inline x, but can't see that it's a constructor in a case
+scrutinee position, and some_benefit is False.
+
+Another example:
+
+dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
+
+.... case dMonadST _@_ x0 of (a,b,c) -> ....
+
+we'd really like to inline dMonadST here, but we *don't* want to
+inline if the case expression is just
+ case x of y { DEFAULT -> ... }
+
+since we can just eliminate this case instead (x is in WHNF). Similar
+applies when x is bound to a lambda expression. Hence
+contIsInteresting looks for case expressions with just a single
+default case.
+
+\begin{code}
+analyseCont :: InScopeSet -> SimplCont
+ -> ([Bool], -- Arg-info flags; one for each value argument
+ Bool, -- Context of the result of the call is interesting
+ Bool) -- There was an InlinePlease
+
+analyseCont in_scope cont
+ = case cont of
+ -- The "lone-variable" case is important. I spent ages
+ -- messing about with unsatisfactory varaints, but this is nice.
+ -- The idea is that if a variable appear all alone
+ -- as an arg of lazy fn, or rhs Stop
+ -- as scrutinee of a case Select
+ -- as arg of a strict fn ArgOf
+ -- then we should not inline it (unless there is some other reason,
+ -- e.g. is is the sole occurrence).
+ -- Why not? At least in the case-scrutinee situation, turning
+ -- case x of y -> ...
+ -- into
+ -- let y = (a,b) in ...
+ -- is bad if the binding for x will remain.
+ --
+ -- Another example: I discovered that strings
+ -- were getting inlined straight back into applications of 'error'
+ -- because the latter is strict.
+ -- s = "foo"
+ -- f = \x -> ...(error s)...
+
+ -- Fundamentally such contexts should not ecourage inlining becuase
+ -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
+ -- so there's no gain.
+ --
+ -- However, even a type application isn't a lone variable. Consider
+ -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
+ -- We had better inline that sucker! The case won't see through it.
+
+ (Stop _) -> boring_result -- Don't inline a lone variable
+ (Select _ _ _ _ _) -> boring_result -- Ditto
+ (ArgOf _ _ _) -> boring_result -- Ditto
+ (ApplyTo _ (Type _) _ cont) -> analyse_ty_app cont
+ other -> analyse_app cont
+ where
+ boring_result = ([], False, False)
+
+ -- For now, I'm treating not treating a variable applied to types as
+ -- "lone". The motivating example was
+ -- f = /\a. \x. BIG
+ -- g = /\a. \y. h (f a)
+ -- There's no advantage in inlining f here, and perhaps
+ -- a significant disadvantage.
+ analyse_ty_app (Stop _) = boring_result
+ analyse_ty_app (ArgOf _ _ _) = boring_result
+ analyse_ty_app (Select _ _ _ _ _) = ([], True, False) -- See the $fMonadST example above
+ analyse_ty_app (ApplyTo _ (Type _) _ cont) = analyse_ty_app cont
+ analyse_ty_app cont = analyse_app cont
+
+ analyse_app (InlinePlease cont)
+ = case analyse_app cont of
+ (infos, icont, inline) -> (infos, icont, True)
+
+ analyse_app (ApplyTo _ arg subst cont)
+ | isValArg arg = case analyse_app cont of
+ (infos, icont, inline) -> (analyse_arg subst arg : infos, icont, inline)
+ | otherwise = analyse_app cont
+
+ analyse_app cont = ([], interesting_call_context cont, False)
+
+ -- An argument is interesting if it has *some* structure
+ -- We are here trying to avoid unfolding a function that
+ -- is applied only to variables that have no unfolding
+ -- (i.e. they are probably lambda bound): f x y z
+ -- There is little point in inlining f here.
+ analyse_arg :: SubstEnv -> InExpr -> Bool
+ analyse_arg subst (Var v) = case lookupIdSubst (mkSubst in_scope subst) v of
+ DoneId v' _ -> isValueUnfolding (idUnfolding v')
+ other -> False
+ analyse_arg subst (Type _) = False
+ analyse_arg subst (App fn (Type _)) = analyse_arg subst fn
+ analyse_arg subst (Note _ a) = analyse_arg subst a
+ analyse_arg subst other = True
+
+ interesting_call_context (Stop ty) = canUpdateInPlace ty
+ interesting_call_context (InlinePlease _) = True
+ interesting_call_context (Select _ _ _ _ _) = True
+ interesting_call_context (CoerceIt _ cont) = interesting_call_context cont
+ interesting_call_context (ApplyTo _ (Type _) _ cont) = interesting_call_context cont
+ interesting_call_context (ApplyTo _ _ _ _) = True
+ interesting_call_context (ArgOf _ _ _) = True
+ -- If this call is the arg of a strict function, the context
+ -- is a bit interesting. If we inline here, we may get useful
+ -- evaluation information to avoid repeated evals: e.g.
+ -- x + (y * z)
+ -- Here the contIsInteresting makes the '*' keener to inline,
+ -- which in turn exposes a constructor which makes the '+' inline.
+ -- Assuming that +,* aren't small enough to inline regardless.
+ --
+ -- It's also very important to inline in a strict context for things
+ -- like
+ -- foldr k z (f x)
+ -- Here, the context of (f x) is strict, and if f's unfolding is
+ -- a build it's *great* to inline it here. So we must ensure that
+ -- the context for (f x) is not totally uninteresting.
+
+
+discardInline :: SimplCont -> SimplCont
+discardInline (InlinePlease cont) = cont
+discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
+discardInline cont = cont
+
+-- 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.
+
+-- Note the repType: we want to look through newtypes for this purpose
+
+canUpdateInPlace ty = case splitTyConApp_maybe (repType ty) of {
+ Nothing -> False ;
+ Just (tycon, _) ->
+
+ case tyConDataConsIfAvailable tycon of
+ [dc] -> arity == 1 || arity == 2
+ where
+ arity = dataConRepArity dc
+ other -> False
+ }
\end{code}
-Floating
-~~~~~~~~
-The function @floatExposesHNF@ tells whether let/case floating will
-expose a head normal form. It is passed booleans indicating the
-desired strategy.
+
+%************************************************************************
+%* *
+\section{Dealing with a single binder}
+%* *
+%************************************************************************
+
+\begin{code}
+simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
+simplBinders bndrs thing_inside
+ = getSubst `thenSmpl` \ subst ->
+ let
+ (subst', bndrs') = substBndrs subst bndrs
+ in
+ seqBndrs bndrs' `seq`
+ setSubst subst' (thing_inside bndrs')
+
+simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
+simplBinder bndr thing_inside
+ = getSubst `thenSmpl` \ subst ->
+ let
+ (subst', bndr') = substBndr subst bndr
+ in
+ seqBndr bndr' `seq`
+ setSubst subst' (thing_inside bndr')
+
+
+-- Same semantics as simplBinders, but a little less
+-- plumbing and hence a little more efficient.
+-- Maybe not worth the candle?
+simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
+simplIds ids thing_inside
+ = getSubst `thenSmpl` \ subst ->
+ let
+ (subst', bndrs') = substIds subst ids
+ in
+ seqBndrs bndrs' `seq`
+ setSubst subst' (thing_inside bndrs')
+
+seqBndrs [] = ()
+seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
+
+seqBndr b | isTyVar b = b `seq` ()
+ | otherwise = seqType (idType b) `seq`
+ idInfo b `seq`
+ ()
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Transform a RHS}
+%* *
+%************************************************************************
+
+Try (a) eta expansion
+ (b) type-lambda swizzling
\begin{code}
-floatExposesHNF
- :: Bool -- Float let(rec)s out of rhs
- -> Bool -- Float cheap primops out of rhs
- -> Bool -- OK to duplicate code
- -> GenCoreExpr bdr Id tyvar uvar
- -> Bool
-
-floatExposesHNF float_lets float_primops ok_to_dup rhs
- = try rhs
+transformRhs :: InExpr -> SimplM InExpr
+transformRhs rhs
+ = tryEtaExpansion body `thenSmpl` \ body' ->
+ mkRhsTyLam tyvars body'
where
- try (Case (Prim _ _) (PrimAlts alts deflt) )
- | float_primops && (null alts || ok_to_dup)
- = or (try_deflt deflt : map try_alt alts)
-
- try (Let bind body) | float_lets = try body
-
- -- `build g'
- -- is like a HNF,
- -- because it *will* become one.
- -- likewise for `augment g h'
- --
- try (App (App (Var bld) _) _) | bld == buildId = True
- try (App (App (App (Var aug) _) _) _) | aug == augmentId = True
-
- try other = case mkFormSummary other of
- VarForm -> True
- ValueForm -> True
- other -> False
- {- but *not* necessarily "BottomForm"...
-
- We may want to float a let out of a let to expose WHNFs,
- but to do that to expose a "bottom" is a Bad Idea:
- let x = let y = ...
- in ...error ...y... -- manifestly bottom using y
- in ...
- =/=>
- let y = ...
- in let x = ...error ...y...
- in ...
-
- as y is only used in case of an error, we do not want
- to allocate it eagerly as that's a waste.
- -}
-
- try_alt (lit,rhs) = try rhs
-
- try_deflt NoDefault = False
- try_deflt (BindDefault _ rhs) = try rhs
+ (tyvars, body) = collectTyBinders rhs
\end{code}
-Local tyvar-lifting
-~~~~~~~~~~~~~~~~~~~
+%************************************************************************
+%* *
+\subsection{Local tyvar-lifting}
+%* *
+%************************************************************************
+
mkRhsTyLam tries this transformation, when the big lambda appears as
the RHS of a let(rec) binding:
into
letrec g' = /\a -> ... g' a ...
in
- let f = /\ a -> f a
+ let f = /\ a -> g' a
which is better. In effect, it means that big lambdas don't impede
let-floating.
where
G = F . Let {xi = xi' tvs}
-\begin{code}
-mkRhsTyLam [] body = returnSmpl body
+[May 1999] If we do this transformation *regardless* then we can
+end up with some pretty silly stuff. For example,
+
+ let
+ st = /\ s -> let { x1=r1 ; x2=r2 } in ...
+ in ..
+becomes
+ let y1 = /\s -> r1
+ y2 = /\s -> r2
+ st = /\s -> ...[y1 s/x1, y2 s/x2]
+ in ..
+
+Unless the "..." is a WHNF there is really no point in doing this.
+Indeed it can make things worse. Suppose x1 is used strictly,
+and is of the form
+
+ x1* = case f y of { (a,b) -> e }
-mkRhsTyLam tyvars body
+If we abstract this wrt the tyvar we then can't do the case inline
+as we would normally do.
+
+
+\begin{code}
+mkRhsTyLam tyvars body -- Only does something if there's a let
+ | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that
+ = returnSmpl (mkLams tyvars body)
+ | otherwise
= go (\x -> x) body
where
- tyvar_tys = mkTyVarTys tyvars
+ worth_it (Let _ e) = whnf_in_middle e
+ worth_it other = False
+ whnf_in_middle (Let _ e) = whnf_in_middle e
+ whnf_in_middle e = exprIsCheap e
+
+ main_tyvar_set = mkVarSet tyvars
go fn (Let bind@(NonRec var rhs) body) | exprIsTrivial rhs
= go (fn . Let bind) body
go fn (Let bind@(NonRec var rhs) body)
- = mk_poly var `thenSmpl` \ (var', rhs') ->
+ = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ body' ->
- returnSmpl (Let (NonRec var' (mkTyLam tyvars (fn rhs))) body')
+ returnSmpl (Let (NonRec var' (mkLams tyvars_here (fn rhs))) body')
+ where
+ tyvars_here = tyvars
+ -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
+ -- tyvars_here was an attempt to reduce the number of tyvars
+ -- wrt which the new binding is abstracted. But the naive
+ -- approach of abstract wrt the tyvars free in the Id's type
+ -- fails. Consider:
+ -- /\ a b -> let t :: (a,b) = (e1, e2)
+ -- x :: a = fst t
+ -- in ...
+ -- Here, b isn't free in x's type, but we must nevertheless
+ -- abstract wrt b as well, because t's type mentions b.
+ -- Since t is floated too, we'd end up with the bogus:
+ -- poly_t = /\ a b -> (e1, e2)
+ -- poly_x = /\ a -> fst (poly_t a *b*)
+ -- So for now we adopt the even more naive approach of
+ -- abstracting wrt *all* the tyvars. We'll see if that
+ -- gives rise to problems. SLPJ June 98
+
+ var_ty = idType var
go fn (Let (Rec prs) body)
- = mapAndUnzipSmpl mk_poly vars `thenSmpl` \ (vars', rhss') ->
+ = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
let
gn body = fn $ foldr Let body (zipWith mk_silly_bind vars rhss')
in
go gn body `thenSmpl` \ body' ->
- returnSmpl (Let (Rec (vars' `zip` [mkTyLam tyvars (gn rhs) | rhs <- rhss])) body')
+ returnSmpl (Let (Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])) body')
where
(vars,rhss) = unzip prs
+ tyvars_here = tyvars
+ -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
+ -- See notes with tyvars_here above
+
+ var_tys = map idType vars
+
+ go fn body = returnSmpl (mkLams tyvars (fn body))
+
+ mk_poly tyvars_here var
+ = getUniqueSmpl `thenSmpl` \ uniq ->
+ let
+ poly_name = setNameUnique (idName var) uniq -- Keep same name
+ poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
+
+ -- It's crucial to copy the occInfo of the original var, because
+ -- we're looking at occurrence-analysed but as yet unsimplified code!
+ -- In particular, we mustn't lose the loop breakers.
+ --
+ -- It's even right to retain single-occurrence or dead-var info:
+ -- Suppose we started with /\a -> let x = E in B
+ -- where x occurs once in E. Then we transform to:
+ -- let x' = /\a -> E in /\a -> let x* = x' a in B
+ -- where x* has an INLINE prag on it. Now, once x* is inlined,
+ -- the occurrences of x' will be just the occurrences originaly
+ -- pinned on x.
+ poly_info = vanillaIdInfo `setOccInfo` idOccInfo var
+
+ poly_id = mkId poly_name poly_ty poly_info
+ in
+ returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
- go fn body = returnSmpl (mkTyLam tyvars (fn body))
-
- mk_poly var
- = newId (mkForAllTys tyvars (idType var)) `thenSmpl` \ poly_id ->
- returnSmpl (poly_id, mkTyApp (Var poly_id) tyvar_tys)
-
- mk_silly_bind var rhs = NonRec (addInlinePragma var) rhs
- -- The addInlinePragma is really important! If we don't say
+ mk_silly_bind var rhs = NonRec var rhs
+ -- The Inline note is really important! If we don't say
-- INLINE on these silly little bindings then look what happens!
-- Suppose we start with:
--
-- * but then it gets inlined into the rhs of g*
-- * then the binding for g* is floated out of the /\b
-- * so we're back to square one
- -- The silly binding for g* must be INLINE, so that no inlining
- -- will happen in its RHS.
+ -- The silly binding for g* must be INLINEd, so that
+ -- we simply substitute for g* throughout.
\end{code}
-Eta reduction
-~~~~~~~~~~~~~
-@etaCoreExpr@ trys an eta reduction at the top level of a Core Expr.
-
-e.g. \ x y -> f x y ===> f
-
-It is used
- a) Before constructing an Unfolding, to
- try to make the unfolding smaller;
- b) In tidyCoreExpr, which is done just before converting to STG.
-
-But we only do this if it gets rid of a whole lambda, not part.
-The idea is that lambdas are often quite helpful: they indicate
-head normal forms, so we don't want to chuck them away lightly.
-But if they expose a simple variable then we definitely win. Even
-if they expose a type application we win. So we check for this special
-case.
-
-It does arise:
-
- f xs = [y | (y,_) <- xs]
-
-gives rise to a recursive function for the list comprehension, and
-f turns out to be just a single call to this recursive function.
-
-Doing eta on type lambdas is useful too:
-
- /\a -> <expr> a ===> <expr>
-
-where <expr> doesn't mention a.
-This is sometimes quite useful, because we can get the sequence:
- f ab d = let d1 = ...d... in
- letrec f' b x = ...d...(f' b)... in
- f' b
-specialise ==>
+%************************************************************************
+%* *
+\subsection{Eta expansion}
+%* *
+%************************************************************************
- f.Int b = letrec f' b x = ...dInt...(f' b)... in
- f' b
+ Try eta expansion for RHSs
-float ==>
+We go for:
+ \x1..xn -> N ==> \x1..xn y1..ym -> N y1..ym
+ AND
+ N E1..En ==> let z1=E1 .. zn=En in \y1..ym -> N z1..zn y1..ym
- f' b x = ...dInt...(f' b)...
- f.Int b = f' b
+where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere)
+wanting a suitable number of extra args.
-Now we really want to simplify to
+NB: the Ei may have unlifted type, but the simplifier (which is applied
+to the result) deals OK with this.
- f.Int = f'
-
-and then replace all the f's with f.Ints.
-
-N.B. We are careful not to partially eta-reduce a sequence of type
-applications since this breaks the specialiser:
-
- /\ a -> f Char# a =NO=> f Char#
+There is no point in looking for a combination of the two,
+because that would leave use with some lets sandwiched between lambdas;
+that's what the final test in the first equation is for.
\begin{code}
-etaCoreExpr :: CoreExpr -> CoreExpr
-
-
-etaCoreExpr expr@(Lam bndr body)
- | opt_DoEtaReduction
- = case etaCoreExpr body of
- App fun arg | eta_match bndr arg &&
- residual_ok fun
- -> fun -- Eta
- other -> expr -- Can't eliminate it, so do nothing at all
+tryEtaExpansion :: InExpr -> SimplM InExpr
+tryEtaExpansion rhs
+ | not opt_SimplDoLambdaEtaExpansion
+ || exprIsTrivial rhs -- Don't eta-expand a trival RHS
+ || null y_tys -- No useful expansion
+ || not (null x_bndrs || and trivial_args) -- Not (no x-binders or no z-binds)
+ = returnSmpl rhs
+
+ | otherwise -- Consider eta expansion
+ = newIds SLIT("y") y_tys $ ( \ y_bndrs ->
+ tick (EtaExpansion (head y_bndrs)) `thenSmpl_`
+ mapAndUnzipSmpl bind_z_arg (args `zip` trivial_args) `thenSmpl` (\ (maybe_z_binds, z_args) ->
+ returnSmpl (mkLams x_bndrs $
+ mkLets (catMaybes maybe_z_binds) $
+ mkLams y_bndrs $
+ mkApps (mkApps fun z_args) (map Var y_bndrs))))
where
- eta_match (ValBinder v) (VarArg v') = v == v'
- eta_match (TyBinder tv) (TyArg ty) = case getTyVar_maybe ty of
- Nothing -> False
- Just tv' -> tv == tv'
- eta_match bndr arg = False
-
- residual_ok :: CoreExpr -> Bool -- Checks for type application
- -- and function not one of the
- -- bound vars
-
- (VarArg v) `mentions` (ValBinder v') = v == v'
- (TyArg ty) `mentions` (TyBinder tv) = tv `elementOfTyVarSet` tyVarsOfType ty
- bndr `mentions` arg = False
-
- residual_ok (Var v)
- = not (VarArg v `mentions` bndr)
- residual_ok (App fun arg)
- | arg `mentions` bndr = False
- | otherwise = residual_ok fun
- residual_ok (Coerce coercion ty body)
- | TyArg ty `mentions` bndr = False
- | otherwise = residual_ok body
-
- residual_ok other = False -- Safe answer
- -- This last clause may seem conservative, but consider:
- -- primops, constructors, and literals, are impossible here
- -- let and case are unlikely (the argument would have been floated inside)
- -- SCCs we probably want to be conservative about (not sure, but it's safe to be)
+ (x_bndrs, body) = collectValBinders rhs
+ (fun, args) = collectArgs body
+ trivial_args = map exprIsTrivial args
+ fun_arity = exprEtaExpandArity fun
+
+ bind_z_arg (arg, trivial_arg)
+ | trivial_arg = returnSmpl (Nothing, arg)
+ | otherwise = newId SLIT("z") (exprType arg) $ \ z ->
+ returnSmpl (Just (NonRec z arg), Var z)
+
+ -- Note: I used to try to avoid the exprType call by using
+ -- the type of the binder. But this type doesn't necessarily
+ -- belong to the same substitution environment as this rhs;
+ -- and we are going to make extra term binders (y_bndrs) from the type
+ -- which will be processed with the rhs substitution environment.
+ -- This only went wrong in a mind bendingly complicated case.
+ (potential_extra_arg_tys, inner_ty) = splitFunTys (exprType body)
-etaCoreExpr expr = expr -- The common case
-\end{code}
+ y_tys :: [InType]
+ y_tys = take no_extras_wanted potential_extra_arg_tys
+ no_extras_wanted :: Int
+ no_extras_wanted = 0 `max`
+
+ -- We used to expand the arity to the previous arity fo the
+ -- function; but this is pretty dangerous. Consdier
+ -- f = \xy -> e
+ -- so that f has arity 2. Now float something into f's RHS:
+ -- f = let z = BIG in \xy -> e
+ -- The last thing we want to do now is to put some lambdas
+ -- outside, to get
+ -- f = \xy -> let z = BIG in e
+ --
+ -- (bndr_arity - no_of_xs) `max`
+
+ -- See if the body could obviously do with more args
+ (fun_arity - valArgCount args)
+
+-- This case is now deal with by exprEtaExpandArity
+ -- Finally, see if it's a state transformer, and xs is non-null
+ -- (so it's also a function not a thunk) in which
+ -- case we eta-expand on principle! This can waste work,
+ -- but usually doesn't.
+ -- I originally checked for a singleton type [ty] in this case
+ -- but then I found a situation in which I had
+ -- \ x -> let {..} in \ s -> f (...) s
+ -- AND f RETURNED A FUNCTION. That is, 's' wasn't the only
+ -- potential extra arg.
+-- case (x_bndrs, potential_extra_arg_tys) of
+-- (_:_, ty:_) -> case splitTyConApp_maybe ty of
+-- Just (tycon,_) | tycon == statePrimTyCon -> 1
+-- other -> 0
+-- other -> 0
+\end{code}
-Eta expansion
-~~~~~~~~~~~~~
-@etaExpandCount@ takes an expression, E, and returns an integer n,
-such that
-
- E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn)
-
-is a safe transformation. In particular, the transformation should
-not cause work to be duplicated, unless it is ``cheap'' (see
-@manifestlyCheap@ below).
-
-@etaExpandCount@ errs on the conservative side. It is always safe to
-return 0.
-An application of @error@ is special, because it can absorb as many
-arguments as you care to give it. For this special case we return
-100, to represent "infinity", which is a bit of a hack.
+%************************************************************************
+%* *
+\subsection{Case absorption and identity-case elimination}
+%* *
+%************************************************************************
\begin{code}
-etaExpandCount :: GenCoreExpr bdr Id tyvar uvar
- -> Int -- Number of extra args you can safely abstract
-
-etaExpandCount (Lam (ValBinder _) body)
- = 1 + etaExpandCount body
-
-etaExpandCount (Let bind body)
- | all manifestlyCheap (rhssOfBind bind)
- = etaExpandCount body
-
-etaExpandCount (Case scrut alts)
- | manifestlyCheap scrut
- = minimum [etaExpandCount rhs | rhs <- rhssOfAlts alts]
-
-etaExpandCount fun@(Var _) = eta_fun fun
-etaExpandCount (App fun arg)
- | notValArg arg = eta_fun fun
- | otherwise = case etaExpandCount fun of
- 0 -> 0
- n -> n-1 -- Knock off one
-
-etaExpandCount other = 0 -- Give up
- -- Lit, Con, Prim,
- -- non-val Lam,
- -- Scc (pessimistic; ToDo),
- -- Let with non-whnf rhs(s),
- -- Case with non-whnf scrutinee
-
------------------------------
-eta_fun :: GenCoreExpr bdr Id tv uv -- The function
- -> Int -- How many args it can safely be applied to
-
-eta_fun (App fun arg) | notValArg arg = eta_fun fun
-
-eta_fun expr@(Var v)
- | isBottomingId v -- Bottoming ids have "infinite arity"
- = 10000 -- Blargh. Infinite enough!
-
-eta_fun expr@(Var v) = idMinArity v
-
-eta_fun other = 0 -- Give up
+mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
\end{code}
-@manifestlyCheap@ looks at a Core expression and returns \tr{True} if
-it is obviously in weak head normal form, or is cheap to get to WHNF.
-By ``cheap'' we mean a computation we're willing to duplicate in order
-to bring a couple of lambdas together. The main examples of things
-which aren't WHNF but are ``cheap'' are:
+@mkCase@ tries the following transformation (if possible):
+
+case e of b { ==> case e of b {
+ p1 -> rhs1 p1 -> rhs1
+ ... ...
+ pm -> rhsm pm -> rhsm
+ _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
+ {or (prim) case b of b' { _ -> rhsn}}
+ pn -> rhsn ...
+ ... po -> rhso[b/b']
+ po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
+ _ -> rhsd
+}
+
+which merges two cases in one case when -- the default alternative of
+the outer case scrutises the same variable as the outer case This
+transformation is called Case Merging. It avoids that the same
+variable is scrutinised multiple times.
- * case e of
- pi -> ei
-
- where e, and all the ei are cheap; and
-
- * let x = e
- in b
+\begin{code}
+mkCase scrut outer_bndr outer_alts
+ | opt_SimplCaseMerge
+ && maybeToBool maybe_case_in_default
+
+ = tick (CaseMerge outer_bndr) `thenSmpl_`
+ returnSmpl (Case scrut outer_bndr new_alts)
+ -- Warning: don't call mkCase recursively!
+ -- Firstly, there's no point, because inner alts have already had
+ -- mkCase applied to them, so they won't have a case in their default
+ -- Secondly, if you do, you get an infinite loop, because the bindNonRec
+ -- in munge_rhs puts a case into the DEFAULT branch!
+ where
+ new_alts = outer_alts_without_deflt ++ munged_inner_alts
+ maybe_case_in_default = case findDefault outer_alts of
+ (outer_alts_without_default,
+ Just (Case (Var scrut_var) inner_bndr inner_alts))
+
+ | outer_bndr == scrut_var
+ -> Just (outer_alts_without_default, inner_bndr, inner_alts)
+ other -> Nothing
+
+ Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
+
+ -- Eliminate any inner alts which are shadowed by the outer ones
+ outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
+
+ munged_inner_alts = [ (con, args, munge_rhs rhs)
+ | (con, args, rhs) <- inner_alts,
+ not (con `elem` outer_cons) -- Eliminate shadowed inner alts
+ ]
+ munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
+\end{code}
- where e and b are cheap; and
+Now the identity-case transformation:
- * op x1 ... xn
+ case e of ===> e
+ True -> True;
+ False -> False
- where op is a cheap primitive operator
+and similar friends.
\begin{code}
-manifestlyCheap :: GenCoreExpr bndr Id tv uv -> Bool
-
-manifestlyCheap (Var _) = True
-manifestlyCheap (Lit _) = True
-manifestlyCheap (Con _ _) = True
-manifestlyCheap (SCC _ e) = manifestlyCheap e
-manifestlyCheap (Coerce _ _ e) = manifestlyCheap e
-manifestlyCheap (Lam x e) = if isValBinder x then True else manifestlyCheap e
-manifestlyCheap (Prim op _) = primOpIsCheap op
-
-manifestlyCheap (Let bind body)
- = manifestlyCheap body && all manifestlyCheap (rhssOfBind bind)
-
-manifestlyCheap (Case scrut alts)
- = manifestlyCheap scrut && all manifestlyCheap (rhssOfAlts alts)
-
-manifestlyCheap other_expr -- look for manifest partial application
- = case (collectArgs other_expr) of { (fun, _, _, vargs) ->
- case fun of
-
- Var f | isBottomingId f -> True -- Application of a function which
- -- always gives bottom; we treat this as
- -- a WHNF, because it certainly doesn't
- -- need to be shared!
-
- Var f -> let
- num_val_args = length vargs
- in
- num_val_args == 0 || -- Just a type application of
- -- a variable (f t1 t2 t3)
- -- counts as WHNF
- num_val_args < idMinArity f
-
- _ -> False
- }
+mkCase scrut case_bndr alts
+ | all identity_alt alts
+ = tick (CaseIdentity case_bndr) `thenSmpl_`
+ returnSmpl scrut
+ where
+ identity_alt (DEFAULT, [], Var v) = v == case_bndr
+ identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs
+ (mkConApp con (map Type arg_tys ++ map varToCoreExpr args))
+ identity_alt other = False
+ arg_tys = case splitTyConApp_maybe (idType case_bndr) of
+ Just (tycon, arg_tys) -> arg_tys
\end{code}
+The catch-all case
\begin{code}
-simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool
+mkCase other_scrut case_bndr other_alts
+ = returnSmpl (Case other_scrut case_bndr other_alts)
+\end{code}
-simplIdWantsToBeINLINEd id env
- = {- We used to arrange that in the final simplification pass we'd switch
- off all INLINE pragmas, so that we'd inline workers back into the
- body of their wrapper if the wrapper hadn't itself been inlined by then.
- This occurred especially for methods in dictionaries.
- We no longer do this:
- a) there's a good chance that the exported wrapper will get
- inlined in some importing scope, in which case we don't
- want to lose the w/w idea.
+\begin{code}
+findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
+findDefault [] = ([], Nothing)
+findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args )
+ ([], Just rhs)
+findDefault (alt : alts) = case findDefault alts of
+ (alts', deflt) -> (alt : alts', deflt)
+
+findAlt :: AltCon -> [CoreAlt] -> CoreAlt
+findAlt con alts
+ = go alts
+ where
+ go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
+ go (alt : alts) | matches alt = alt
+ | otherwise = go alts
- b) The occurrence analyser must agree about what has an
- INLINE pragma. Not hard, but delicate.
-
- c) if the worker gets inlined we have to tell the wrapepr
- that it's no longer a wrapper, else the interface file stuff
- asks for a worker that no longer exists.
-
- if switchIsSet env IgnoreINLINEPragma
- then False
- else
- -}
-
- idWantsToBeINLINEd id
-
-idMinArity id = case getIdArity id of
- UnknownArity -> 0
- ArityAtLeast n -> n
- ArityExactly n -> n
-
-singleConstructorType :: Type -> Bool
-singleConstructorType ty
- = case (maybeAppDataTyConExpandingDicts ty) of
- Just (tycon, ty_args, [con]) | isDataTyCon tycon -> True
- other -> False
-
-typeOkForCase :: Type -> Bool
-typeOkForCase ty
- = case (maybeAppDataTyConExpandingDicts ty) of
- Just (tycon, ty_args, []) -> False
- Just (tycon, ty_args, non_null_data_cons) | isDataTyCon tycon -> True
- other -> False
- -- Null data cons => type is abstract, which code gen can't
- -- currently handle. (ToDo: when return-in-heap is universal we
- -- don't need to worry about this.)
+ matches (DEFAULT, _, _) = True
+ matches (con1, _, _) = con == con1
\end{code}
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