%
-% (c) The AQUA Project, Glasgow University, 1993-1995
+% (c) The AQUA Project, Glasgow University, 1993-1998
%
\section[SimplUtils]{The simplifier utilities}
\begin{code}
-#include "HsVersions.h"
-
module SimplUtils (
-
- floatExposesHNF,
-
- mkCoTyLamTryingEta, mkCoLamTryingEta,
-
- etaExpandCount,
-
- mkIdentityAlts,
-
- simplIdWantsToBeINLINEd,
-
- type_ok_for_let_to_case
+ simplBinder, simplBinders, simplIds,
+ mkRhsTyLam,
+ etaCoreExpr,
+ etaExpandCount,
+ mkCase, findAlt, findDefault
) where
-IMPORT_Trace -- ToDo: rm (debugging)
-import Pretty
-
-import TaggedCore
-import PlainCore
-import SimplEnv
-import SimplMonad
+#include "HsVersions.h"
import BinderInfo
-
-import AbsPrel ( primOpIsCheap, realWorldStateTy,
- buildId, augmentId
- IF_ATTACK_PRAGMAS(COMMA realWorldTy)
- IF_ATTACK_PRAGMAS(COMMA tagOf_PrimOp)
- IF_ATTACK_PRAGMAS(COMMA pprPrimOp)
+import CmdLineOpts ( opt_DoEtaReduction, switchIsOn, SimplifierSwitch(..) )
+import CoreSyn
+import CoreUtils ( exprIsCheap, exprIsTrivial, exprFreeVars, cheapEqExpr,
+ FormSummary(..),
+ substId, substIds
)
-import AbsUniType ( extractTyVarsFromTy, getTyVarMaybe, isPrimType,
- splitTypeWithDictsAsArgs, getUniDataTyCon_maybe,
- applyTy, isFunType, TyVar, TyVarTemplate
- IF_ATTACK_PRAGMAS(COMMA cmpTyVar COMMA cmpClass)
+import Id ( Id, idType, isBottomingId, getIdArity, isId, idName,
+ getInlinePragma, setInlinePragma,
+ getIdDemandInfo
)
-import Id ( getInstantiatedDataConSig, isDataCon, getIdUniType,
- getIdArity, isBottomingId, idWantsToBeINLINEd,
- DataCon(..), Id
+import IdInfo ( arityLowerBound, InlinePragInfo(..) )
+import Demand ( isStrict )
+import Maybes ( maybeToBool )
+import Const ( Con(..) )
+import Name ( isLocalName )
+import SimplMonad
+import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, mkTyVarTys,
+ splitTyConApp_maybe, mkTyVarTy, substTyVar
)
-import IdInfo
-import CmdLineOpts ( SimplifierSwitch(..) )
-import Maybes ( maybeToBool, Maybe(..) )
-import Outputable -- isExported ...
-import Util
-\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.
-
-\begin{code}
-floatExposesHNF
- :: Bool -- Float let(rec)s out of rhs
- -> Bool -- Float cheap primops out of rhs
- -> Bool -- OK to duplicate code
- -> CoreExpr bdr Id
- -> Bool
-
-floatExposesHNF float_lets float_primops ok_to_dup rhs
- = try rhs
- where
- try (CoCase (CoPrim _ _ _) (CoPrimAlts alts deflt) )
- | float_primops && (null alts || ok_to_dup)
- = or (try_deflt deflt : map try_alt alts)
-
- try (CoLet bind body) | float_lets = try body
-
- -- `build g'
- -- is like a HNF,
- -- because it *will* become one.
- -- likewise for `augment g h'
- --
- try (CoApp (CoTyApp (CoVar bld) _) _) | bld == buildId = True
- try (CoApp (CoApp (CoTyApp (CoVar bld) _) _) _) | bld == augmentId = True
-
- try other = manifestlyWHNF other
- {- but *not* necessarily "manifestlyBottom other"...
-
- 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 CoNoDefault = False
- try_deflt (CoBindDefault _ rhs) = try rhs
+import Var ( setVarUnique )
+import VarSet
+import UniqSupply ( splitUniqSupply, uniqFromSupply )
+import Util ( zipWithEqual, mapAccumL )
+import Outputable
\end{code}
-Eta reduction on ordinary lambdas
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-We have a go at doing
+%************************************************************************
+%* *
+\section{Dealing with a single binder}
+%* *
+%************************************************************************
- \ x y -> f x y ===> f
-
-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.
+When we hit a binder we may need to
+ (a) apply the the type envt (if non-empty) to its type
+ (b) apply the type envt and id envt to its SpecEnv (if it has one)
+ (c) give it a new unique to avoid name clashes
\begin{code}
-mkCoLamTryingEta :: [Id] -- Args to the lambda
- -> PlainCoreExpr -- Lambda body
- -> PlainCoreExpr
-
-mkCoLamTryingEta [] body = body
+simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
+simplBinders bndrs thing_inside
+ = getSwitchChecker `thenSmpl` \ sw_chkr ->
+ getSimplBinderStuff `thenSmpl` \ stuff ->
+ let
+ must_clone = switchIsOn sw_chkr SimplPleaseClone
+ (stuff', bndrs') = mapAccumL (subst_binder must_clone) stuff bndrs
+ in
+ setSimplBinderStuff stuff' $
+ thing_inside bndrs'
+
+simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
+simplBinder bndr thing_inside
+ = getSwitchChecker `thenSmpl` \ sw_chkr ->
+ getSimplBinderStuff `thenSmpl` \ stuff ->
+ let
+ must_clone = switchIsOn sw_chkr SimplPleaseClone
+ (stuff', bndr') = subst_binder must_clone stuff bndr
+ in
+ setSimplBinderStuff stuff' $
+ 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
+ = getSwitchChecker `thenSmpl` \ sw_chkr ->
+ getSimplBinderStuff `thenSmpl` \ (ty_subst, id_subst, in_scope, us) ->
+ let
+ must_clone = switchIsOn sw_chkr SimplPleaseClone
+ (id_subst', in_scope', us', ids') = substIds (simpl_clone_fn must_clone)
+ ty_subst id_subst in_scope us ids
+ in
+ setSimplBinderStuff (ty_subst, id_subst', in_scope', us') $
+ thing_inside ids'
+
+subst_binder must_clone (ty_subst, id_subst, in_scope, us) bndr
+ | isTyVar bndr
+ = case substTyVar ty_subst in_scope bndr of
+ (ty_subst', in_scope', bndr') -> ((ty_subst', id_subst, in_scope', us), bndr')
-mkCoLamTryingEta orig_ids body
- = reduce_it (reverse orig_ids) body
- where
- bale_out = mkCoLam orig_ids body
-
- reduce_it [] residual
- | residual_ok residual = residual
- | otherwise = bale_out
-
- reduce_it (id:ids) (CoApp fun (CoVarAtom arg))
- | id == arg
- && getIdUniType id /= realWorldStateTy
- -- *never* eta-reduce away a PrimIO state token! (WDP 94/11)
- = reduce_it ids fun
-
- reduce_it ids other = bale_out
-
- is_elem = isIn "mkCoLamTryingEta"
-
- -----------
- residual_ok :: PlainCoreExpr -> Bool -- Checks for type application
- -- and function not one of the
- -- bound vars
- residual_ok (CoTyApp fun ty) = residual_ok fun
- residual_ok (CoVar v) = not (v `is_elem` orig_ids) -- Fun mustn't be one of
- -- the bound ids
- residual_ok other = False
+ | otherwise
+ = case substId (simpl_clone_fn must_clone) ty_subst id_subst in_scope us bndr of
+ (id_subst', in_scope', us', bndr')
+ -> ((ty_subst, id_subst', in_scope', us'), bndr')
+
+simpl_clone_fn must_clone in_scope us id
+ | (must_clone && isLocalName (idName id))
+ || id `elemVarSet` in_scope
+ = case splitUniqSupply us of
+ (us1, us2) -> Just (us1, setVarUnique id (uniqFromSupply us2))
+
+ | otherwise
+ = Nothing
\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).
+%************************************************************************
+%* *
+\subsection{Local tyvar-lifting}
+%* *
+%************************************************************************
-@etaExpandCount@ errs on the conservative side. It is always safe to return 0.
+mkRhsTyLam tries this transformation, when the big lambda appears as
+the RHS of a let(rec) binding:
-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.
+ /\abc -> let(rec) x = e in b
+ ==>
+ let(rec) x' = /\abc -> let x = x' a b c in e
+ in
+ /\abc -> let x = x' a b c in b
-\begin{code}
-etaExpandCount :: CoreExpr bdr Id
- -> Int -- Number of extra args you can safely abstract
+This is good because it can turn things like:
-etaExpandCount (CoLam ids body)
- = length ids + etaExpandCount body
+ let f = /\a -> letrec g = ... g ... in g
+into
+ letrec g' = /\a -> ... g' a ...
+ in
+ let f = /\ a -> g' a
-etaExpandCount (CoLet bind body)
- | all manifestlyCheap (rhssOfBind bind)
- = etaExpandCount body
-
-etaExpandCount (CoCase scrut alts)
- | manifestlyCheap scrut
- = minimum [etaExpandCount rhs | rhs <- rhssOfAlts alts]
+which is better. In effect, it means that big lambdas don't impede
+let-floating.
-etaExpandCount (CoApp fun _) = case etaExpandCount fun of
- 0 -> 0
- n -> n-1 -- Knock off one
+This optimisation is CRUCIAL in eliminating the junk introduced by
+desugaring mutually recursive definitions. Don't eliminate it lightly!
-etaExpandCount fun@(CoTyApp _ _) = eta_fun fun
-etaExpandCount fun@(CoVar _) = eta_fun fun
+So far as the implemtation is concerned:
-etaExpandCount other = 0 -- Give up
- -- CoLit, CoCon, CoPrim,
- -- CoTyLam,
- -- CoScc (pessimistic; ToDo),
- -- CoLet with non-whnf rhs(s),
- -- CoCase with non-whnf scrutinee
+ Invariant: go F e = /\tvs -> F e
+
+ Equalities:
+ go F (Let x=e in b)
+ = Let x' = /\tvs -> F e
+ in
+ go G b
+ where
+ G = F . Let x = x' tvs
+
+ go F (Letrec xi=ei in b)
+ = Letrec {xi' = /\tvs -> G ei}
+ in
+ go G b
+ where
+ G = F . Let {xi = xi' tvs}
-eta_fun :: CoreExpr bdr Id -- The function
- -> Int -- How many args it can safely be applied to
+\begin{code}
+mkRhsTyLam (Lam b e)
+ | isTyVar b = case collectTyBinders e of
+ (bs,body) -> mkRhsTyLam_help (b:bs) body
-eta_fun (CoTyApp fun ty) = eta_fun fun
+mkRhsTyLam other_expr -- No-op if not a type lambda
+ = returnSmpl other_expr
-eta_fun expr@(CoVar v)
- | isBottomingId v -- Bottoming ids have "infinite arity"
- = 10000 -- Blargh. Infinite enough!
-eta_fun expr@(CoVar v)
- | maybeToBool arity_maybe -- We know the arity
- = arity
+mkRhsTyLam_help tyvars body
+ = go (\x -> x) body
where
- arity_maybe = arityMaybe (getIdArity v)
- arity = case arity_maybe of { Just arity -> arity }
-
-eta_fun other = 0 -- Give up
+ 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 tyvars_here var `thenSmpl` \ (var', rhs') ->
+ go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ 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 a'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 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` [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
+ = newId (mkForAllTys tyvars_here (idType var)) $ \ poly_id ->
+ let
+ -- It's crucial to copy the inline-prag 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.
+ --
+ -- *However* we don't want to retain a single-occurrence or dead-var info
+ -- because we're adding a load of "silly bindings" of the form
+ -- var _U_ = poly_var t1 t2
+ -- with a must-inline pragma on the silly binding to prevent the
+ -- poly-var from being inlined right back in. Since poly_var now
+ -- occurs inside an INLINE binding, it should be given a ManyOcc,
+ -- else it may get inlined unconditionally
+ poly_inline_prag = case getInlinePragma var of
+ ICanSafelyBeINLINEd _ _ -> NoInlinePragInfo
+ IAmDead -> NoInlinePragInfo
+ var_inline_prag -> var_inline_prag
+
+ poly_id' = setInlinePragma poly_id poly_inline_prag
+ in
+ returnSmpl (poly_id', mkTyApps (Var poly_id') (mkTyVarTys tyvars_here))
+
+ mk_silly_bind var rhs = NonRec (setInlinePragma var IWantToBeINLINEd) rhs
+ -- The addInlinePragma is really important! If we don't say
+ -- INLINE on these silly little bindings then look what happens!
+ -- Suppose we start with:
+ --
+ -- x = let g = /\a -> \x -> f x x
+ -- in
+ -- /\ b -> let g* = g b in E
+ --
+ -- Then: * the binding for g gets floated out
+ -- * 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.
+ -- PS: Jun 98: actually this isn't important any more;
+ -- inlineUnconditionally will catch the type applicn
+ -- and inline it unconditionally, without ever trying
+ -- to simplify the RHS
\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:
- * case e of
- pi -> ei
+%************************************************************************
+%* *
+\subsection{Eta reduction}
+%* *
+%************************************************************************
- where e, and all the ei are cheap; and
+@etaCoreExpr@ trys an eta reduction at the top level of a Core Expr.
- * let x = e
- in b
+e.g. \ x y -> f x y ===> f
- where e and b are cheap; and
+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.
- * op x1 ... xn
+But we only do this if
+ i) 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.
- where op is a cheap primitive operator
+ ii) It exposes a simple variable or a type application; in short
+ it exposes a "trivial" expression. (exprIsTrivial)
\begin{code}
-manifestlyCheap :: CoreExpr bndr Id -> Bool
-
-manifestlyCheap (CoVar _) = True
-manifestlyCheap (CoLit _) = True
-manifestlyCheap (CoCon _ _ _) = True
-manifestlyCheap (CoLam _ _) = True
-manifestlyCheap (CoTyLam _ e) = manifestlyCheap e
-manifestlyCheap (CoSCC _ e) = manifestlyCheap e
+etaCoreExpr :: CoreExpr -> CoreExpr
+ -- ToDo: we should really check that we don't turn a non-bottom
+ -- lambda into a bottom variable. Sigh
-manifestlyCheap (CoPrim op _ _) = primOpIsCheap op
+etaCoreExpr expr@(Lam bndr body)
+ | opt_DoEtaReduction
+ = check (reverse binders) body
+ where
+ (binders, body) = collectBinders expr
-manifestlyCheap (CoLet bind body)
- = manifestlyCheap body && all manifestlyCheap (rhssOfBind bind)
+ check [] body
+ | exprIsTrivial body && not (any (`elemVarSet` body_fvs) binders)
+ = body -- Success!
+ where
+ body_fvs = exprFreeVars body
-manifestlyCheap (CoCase scrut alts)
- = manifestlyCheap scrut && all manifestlyCheap (rhssOfAlts alts)
+ check (b : bs) (App fun arg)
+ | (varToCoreExpr b `cheapEqExpr` arg)
+ && not (is_strict_binder b)
+ = check bs fun
-manifestlyCheap other_expr -- look for manifest partial application
- = case (collectArgs other_expr) of { (fun, args) ->
- case fun of
+ check _ _ = expr -- Bale out
- CoVar 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!
+ -- We don't want to eta-abstract (\x -> f x) if x carries a "strict"
+ -- demand info. That demand info conveys useful information to the
+ -- call site, via the let-to-case transform, so we don't want to discard it.
+ is_strict_binder b = isId b && isStrict (getIdDemandInfo b)
+
+etaCoreExpr expr = expr -- The common case
+\end{code}
+
- CoVar f -> let
- num_val_args = length [ a | (ValArg a) <- args ]
- in
- num_val_args == 0 || -- Just a type application of
- -- a variable (f t1 t2 t3)
- -- counts as WHNF
- case (arityMaybe (getIdArity f)) of
- Nothing -> False
- Just arity -> num_val_args < arity
+%************************************************************************
+%* *
+\subsection{Eta expansion}
+%* *
+%************************************************************************
- _ -> False
- }
+@etaExpandCount@ takes an expression, E, and returns an integer n,
+such that
+ E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn)
--- ToDo: Move to CoreFuns
+is a safe transformation. In particular, the transformation should
+not cause work to be duplicated, unless it is ``cheap'' (see
+@manifestlyCheap@ below).
-rhssOfBind :: CoreBinding bndr bdee -> [CoreExpr bndr bdee]
+@etaExpandCount@ errs on the conservative side. It is always safe to
+return 0.
-rhssOfBind (CoNonRec _ rhs) = [rhs]
-rhssOfBind (CoRec pairs) = [rhs | (_,rhs) <- pairs]
+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.
-rhssOfAlts :: CoreCaseAlternatives bndr bdee -> [CoreExpr bndr bdee]
+\begin{code}
+etaExpandCount :: CoreExpr
+ -> Int -- Number of extra args you can safely abstract
-rhssOfAlts (CoAlgAlts alts deflt) = rhssOfDeflt deflt ++
- [rhs | (_,_,rhs) <- alts]
-rhssOfAlts (CoPrimAlts alts deflt) = rhssOfDeflt deflt ++
- [rhs | (_,rhs) <- alts]
-rhssOfDeflt CoNoDefault = []
-rhssOfDeflt (CoBindDefault _ rhs) = [rhs]
-\end{code}
+etaExpandCount (Lam b body)
+ | isId b
+ = 1 + etaExpandCount body
-Eta reduction on type lambdas
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-We have a go at doing
+etaExpandCount (Let bind body)
+ | all exprIsCheap (rhssOfBind bind)
+ = etaExpandCount body
- /\a -> <expr> a ===> <expr>
+etaExpandCount (Case scrut _ alts)
+ | exprIsCheap scrut
+ = minimum [etaExpandCount rhs | (_,_,rhs) <- alts]
-where <expr> doesn't mention a.
-This is sometimes quite useful, because we can get the sequence:
+etaExpandCount fun@(Var _) = eta_fun fun
- f ab d = let d1 = ...d... in
- letrec f' b x = ...d...(f' b)... in
- f' b
-specialise ==>
+etaExpandCount (App fun (Type ty))
+ = eta_fun fun
+etaExpandCount (App fun arg)
+ | exprIsCheap arg = case etaExpandCount fun of
+ 0 -> 0
+ n -> n-1 -- Knock off one
- f.Int b = letrec f' b x = ...dInt...(f' b)... in
- f' b
+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
-float ==>
+-----------------------------
+eta_fun :: CoreExpr -- The function
+ -> Int -- How many args it can safely be applied to
- f' b x = ...dInt...(f' b)...
- f.Int b = f' b
+eta_fun (App fun (Type ty)) = eta_fun fun
-Now we really want to simplify to
+eta_fun (Var v)
+ | isBottomingId v -- Bottoming ids have "infinite arity"
+ = 10000 -- Blargh. Infinite enough!
- f.Int = f'
+eta_fun (Var v) = arityLowerBound (getIdArity v)
-and then replace all the f's with f.Ints.
+eta_fun other = 0 -- Give up
+\end{code}
-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#
+%************************************************************************
+%* *
+\subsection{Case absorption and identity-case elimination}
+%* *
+%************************************************************************
\begin{code}
-mkCoTyLamTryingEta :: [TyVar] -> PlainCoreExpr -> PlainCoreExpr
-
-mkCoTyLamTryingEta tyvars tylam_body
- = if
- tyvars == tyvar_args && -- Same args in same order
- check_fun fun -- Function left is ok
- then
- -- Eta reduction worked
- fun
- else
- -- The vastly common case
- mkCoTyLam tyvars tylam_body
- where
- (tyvar_args, fun) = strip_tyvar_args [] tylam_body
-
- strip_tyvar_args args_so_far tyapp@(CoTyApp fun ty)
- = case getTyVarMaybe ty of
- Just tyvar_arg -> strip_tyvar_args (tyvar_arg:args_so_far) fun
- Nothing -> (args_so_far, tyapp)
-
- strip_tyvar_args args_so_far fun
- = (args_so_far, fun)
-
- check_fun (CoVar f) = True -- Claim: tyvars not mentioned by type of f
- check_fun other = False
-
-{- OLD:
-mkCoTyLamTryingEta :: TyVar -> PlainCoreExpr -> PlainCoreExpr
+mkCase :: SwitchChecker -> OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
+\end{code}
-mkCoTyLamTryingEta tyvar body
- = case body of
- CoTyApp fun ty ->
- case getTyVarMaybe ty of
- Just tyvar' | tyvar == tyvar' &&
- ok fun -> fun
- -- Ha! So it's /\ a -> fun a, and fun is "ok"
+@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.
- other -> CoTyLam tyvar body
- other -> CoTyLam tyvar body
+\begin{code}
+mkCase sw_chkr scrut outer_bndr outer_alts
+ | switchIsOn sw_chkr SimplCaseMerge
+ && maybeToBool maybe_case_in_default
+
+ = tick CaseMerge `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
- is_elem = isIn "mkCoTyLamTryingEta"
-
- ok :: PlainCoreExpr -> Bool -- Returns True iff the expression doesn't
- -- mention tyvar
-
- ok (CoVar v) = True -- Claim: tyvar not mentioned by type of v
- ok (CoApp fun arg) = ok fun -- Claim: tyvar not mentioned by type of arg
- ok (CoTyApp fun ty) = not (tyvar `is_elem` extractTyVarsFromTy ty) &&
- ok fun
- ok other = False
--}
+ 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}
-Let to case
-~~~~~~~~~~~
-
-Given a type generate the case alternatives
-
- C a b -> C a b
+Now the identity-case transformation:
-if there's one constructor, or
-
- x -> x
-
-if there's many, or if it's a primitive type.
+ case e of ===> e
+ True -> True;
+ False -> False
+and similar friends.
\begin{code}
-mkIdentityAlts
- :: UniType -- type of RHS
- -> SmplM InAlts -- result
-
-mkIdentityAlts rhs_ty
- | isPrimType rhs_ty
- = newId rhs_ty `thenSmpl` \ binder ->
- returnSmpl (CoPrimAlts [] (CoBindDefault (binder, bad_occ_info) (CoVar binder)))
-
- | otherwise
- = case getUniDataTyCon_maybe rhs_ty of
- Just (tycon, ty_args, [data_con]) -> -- algebraic type suitable for unpacking
- let
- (_,inst_con_arg_tys,_) = getInstantiatedDataConSig data_con ty_args
- in
- newIds inst_con_arg_tys `thenSmpl` \ new_bindees ->
- let
- new_binders = [ (b, bad_occ_info) | b <- new_bindees ]
- in
- returnSmpl (
- CoAlgAlts
- [(data_con, new_binders, CoCon data_con ty_args (map CoVarAtom new_bindees))]
- CoNoDefault
- )
-
- _ -> -- Multi-constructor or abstract algebraic type
- newId rhs_ty `thenSmpl` \ binder ->
- returnSmpl (CoAlgAlts [] (CoBindDefault (binder,bad_occ_info) (CoVar binder)))
+mkCase sw_chkr scrut case_bndr alts
+ | all identity_alt alts
+ = tick CaseIdentity `thenSmpl_`
+ returnSmpl scrut
where
- bad_occ_info = ManyOcc 0 -- Non-committal!
+ identity_alt (DEFAULT, [], Var v) = v == case_bndr
+ identity_alt (con, args, Con con' args') = con == con' &&
+ and (zipWithEqual "mkCase"
+ cheapEqExpr
+ (map Type arg_tys ++ map varToCoreExpr args)
+ 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 sw_chkr other_scrut case_bndr other_alts
+ = returnSmpl (Case other_scrut case_bndr other_alts)
+\end{code}
-simplIdWantsToBeINLINEd id env
- = if switchIsSet env IgnoreINLINEPragma
- then False
- else idWantsToBeINLINEd id
-type_ok_for_let_to_case :: UniType -> Bool
+\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 :: Con -> [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
-type_ok_for_let_to_case ty
- = case getUniDataTyCon_maybe ty of
- Nothing -> False
- Just (tycon, ty_args, []) -> False
- Just (tycon, ty_args, non_null_data_cons) -> True
- -- Null data cons => type is abstract
+ matches (DEFAULT, _, _) = True
+ matches (con1, _, _) = con == con1
\end{code}
projects / ghc-hetmet.git / blobdiff