%
-% (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 (
-
- floatExposesHNF,
-
- mkTyLamTryingEta, mkValLamTryingEta,
-
- etaExpandCount,
-
- mkIdentityAlts,
-
- simplIdWantsToBeINLINEd,
-
- type_ok_for_let_to_case
+ simplBinder, simplBinders, simplIds,
+ transformRhs,
+ etaCoreExpr,
+ mkCase, findAlt, findDefault,
+ mkCoerce
) where
-IMP_Ubiq(){-uitous-}
-IMPORT_DELOOPER(SmplLoop) -- paranoia checking
+#include "HsVersions.h"
import BinderInfo
-import CmdLineOpts ( SimplifierSwitch(..) )
+import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge )
import CoreSyn
-import CoreUtils ( manifestlyWHNF )
-import Id ( idType, isBottomingId, idWantsToBeINLINEd, dataConArgTys,
- getIdArity, GenId{-instance Eq-}
+import CoreFVs ( exprFreeVars )
+import CoreUtils ( exprIsCheap, exprIsTrivial, cheapEqExpr, coreExprType,
+ exprIsWHNF, FormSummary(..)
)
-import IdInfo ( arityMaybe )
-import Maybes ( maybeToBool )
-import PrelVals ( augmentId, buildId )
-import PrimOp ( primOpIsCheap )
-import SimplEnv
+import Subst ( substBndrs, substBndr, substIds )
+import Id ( Id, idType, getIdArity, isId, idName,
+ getInlinePragma, setInlinePragma,
+ getIdDemandInfo, mkId
+ )
+import IdInfo ( arityLowerBound, InlinePragInfo(..), setInlinePragInfo, vanillaIdInfo )
+import Maybes ( maybeToBool, catMaybes )
+import Const ( Con(..) )
+import Name ( isLocalName, setNameUnique )
import SimplMonad
-import Type ( eqTy, isPrimType, maybeAppDataTyConExpandingDicts, getTyVar_maybe )
-import TysWiredIn ( realWorldStateTy )
-import TyVar ( GenTyVar{-instance Eq-} )
-import Util ( isIn, panic )
-
+import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys,
+ splitTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys
+ )
+import TysPrim ( statePrimTyCon )
+import Var ( setVarUnique )
+import VarSet
+import UniqSupply ( splitUniqSupply, uniqFromSupply )
+import Util ( zipWithEqual, mapAccumL )
+import Outputable
\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}
-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
- 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 = 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 NoDefault = False
- try_deflt (BindDefault _ rhs) = try rhs
+simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
+simplBinders bndrs thing_inside
+ = getSubst `thenSmpl` \ subst ->
+ let
+ (subst', bndrs') = substBndrs subst bndrs
+ in
+ 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
+ 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
+ setSubst subst' $
+ thing_inside bndrs'
\end{code}
-Eta reduction on ordinary lambdas
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-We have a go at doing
-
- \ 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.
+%************************************************************************
+%* *
+\subsection{Transform a RHS}
+%* *
+%************************************************************************
-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.
+Try (a) eta expansion
+ (b) type-lambda swizzling
\begin{code}
-mkValLamTryingEta :: [Id] -- Args to the lambda
- -> CoreExpr -- Lambda body
- -> CoreExpr
-
-mkValLamTryingEta [] body = body
-
-mkValLamTryingEta orig_ids body
- = reduce_it (reverse orig_ids) body
+transformRhs :: InExpr -> SimplM InExpr
+transformRhs rhs
+ = tryEtaExpansion body `thenSmpl` \ body' ->
+ mkRhsTyLam tyvars body'
where
- bale_out = mkValLam orig_ids body
-
- reduce_it [] residual
- | residual_ok residual = residual
- | otherwise = bale_out
-
- reduce_it (id:ids) (App fun (VarArg arg))
- | id == arg
- && not (idType id `eqTy` 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 "mkValLamTryingEta"
-
- -----------
- residual_ok :: CoreExpr -> Bool -- Checks for type application
- -- and function not one of the
- -- bound vars
-
- residual_ok (Var v) = not (v `is_elem` orig_ids)
- -- Fun mustn't be one of the bound ids
- residual_ok (App fun arg)
- | notValArg arg = residual_ok fun
- residual_ok other = False
+ (tyvars, body) = collectTyBinders rhs
\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.
+%************************************************************************
+%* *
+\subsection{Local tyvar-lifting}
+%* *
+%************************************************************************
+
+mkRhsTyLam tries this transformation, when the big lambda appears as
+the RHS of a let(rec) binding:
+
+ /\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
+
+This is good because it can turn things like:
+
+ let f = /\a -> letrec g = ... g ... in g
+into
+ letrec g' = /\a -> ... g' a ...
+ in
+ let f = /\ a -> g' a
+
+which is better. In effect, it means that big lambdas don't impede
+let-floating.
+
+This optimisation is CRUCIAL in eliminating the junk introduced by
+desugaring mutually recursive definitions. Don't eliminate it lightly!
+
+So far as the implemtation is concerned:
+
+ 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}
+
+[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 }
+
+If we abstract this wrt the tyvar we then can't do the case inline
+as we would normally do.
-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.
\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)
- | maybeToBool arity_maybe -- We know the arity
- = arity
+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
- arity_maybe = arityMaybe (getIdArity v)
- arity = case arity_maybe of { Just arity -> arity }
-
-eta_fun other = 0 -- Give up
+ 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 = exprIsWHNF 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 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 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 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
+ = 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 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.
+ --
+ -- 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 `setInlinePragInfo` getInlinePragma var
+
+ poly_id = mkId poly_name poly_ty poly_info
+ in
+ returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
+
+ mk_silly_bind var rhs = NonRec (setInlinePragma var IMustBeINLINEd) 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 IMustBeINLINEs, so that
+ -- we simply substitute for g* throughout.
\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 expansion}
+%* *
+%************************************************************************
- where e, and all the ei are cheap; and
+ Try eta expansion for RHSs
- * let x = e
- in b
+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
- where e and b are cheap; and
+where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere)
+wanting a suitable number of extra args.
- * op x1 ... xn
+NB: the Ei may have unlifted type, but the simplifier (which is applied
+to the result) deals OK with this).
- where op is a cheap primitive operator
+There is no point in looking for a combination of the two,
+because that would leave use with some lets sandwiched between lambdas;
+but it's awkward to detect that case, so we don't bother.
\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
- case (arityMaybe (getIdArity f)) of
- Nothing -> False
- Just arity -> num_val_args < arity
-
- _ -> False
- }
+tryEtaExpansion :: InExpr -> SimplM InExpr
+tryEtaExpansion rhs
+ | not opt_SimplDoLambdaEtaExpansion
+ || exprIsTrivial rhs -- Don't eta-expand a trival RHS
+ || null y_tys -- No useful expansion
+ = returnSmpl rhs
+
+ | otherwise -- Consider eta expansion
+ = newIds y_tys ( \ y_bndrs ->
+ tick (EtaExpansion (head y_bndrs)) `thenSmpl_`
+ mapAndUnzipSmpl bind_z_arg args `thenSmpl` (\ (z_binds, z_args) ->
+ returnSmpl (mkLams x_bndrs $
+ mkLets (catMaybes z_binds) $
+ mkLams y_bndrs $
+ mkApps (mkApps fun z_args) (map Var y_bndrs))))
+ where
+ (x_bndrs, body) = collectValBinders rhs
+ (fun, args) = collectArgs body
+ no_of_xs = length x_bndrs
+ fun_arity = case fun of
+ Var v -> arityLowerBound (getIdArity v)
+ other -> 0
+
+ bind_z_arg arg | exprIsTrivial arg = returnSmpl (Nothing, arg)
+ | otherwise = newId (coreExprType arg) $ \ z ->
+ returnSmpl (Just (NonRec z arg), Var z)
+
+ -- Note: I used to try to avoid the coreExprType 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 (coreExprType body)
+
+ y_tys :: [InType]
+ y_tys = take no_extras_wanted potential_extra_arg_tys
+
+ no_extras_wanted :: Int
+ no_extras_wanted =
+
+ -- 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) `max`
+
+ -- 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 reduction on type lambdas
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-We have a go at doing
- /\a -> <expr> a ===> <expr>
+%************************************************************************
+%* *
+\subsection{Eta reduction}
+%* *
+%************************************************************************
-where <expr> doesn't mention a.
-This is sometimes quite useful, because we can get the sequence:
+@etaCoreExpr@ trys an eta reduction at the top level of a Core Expr.
- f ab d = let d1 = ...d... in
- letrec f' b x = ...d...(f' b)... in
- f' b
-specialise ==>
+e.g. \ x y -> f x y ===> f
- f.Int b = letrec f' b x = ...dInt...(f' b)... in
- f' b
+It is used
+-- OLD
+-- a) Before constructing an Unfolding, to
+-- try to make the unfolding smaller;
+ b) In tidyCoreExpr, which is done just before converting to STG.
-float ==>
+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.
- f' b x = ...dInt...(f' b)...
- f.Int b = f' b
-
-Now we really want to simplify to
-
- 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#
+-- OLD: in core2stg we want to do this even if the result isn't trivial
+-- ii) It exposes a simple variable or a type application; in short
+-- it exposes a "trivial" expression. (exprIsTrivial)
\begin{code}
-mkTyLamTryingEta :: [TyVar] -> CoreExpr -> CoreExpr
-
-mkTyLamTryingEta 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
- mkTyLam tyvars tylam_body
+etaCoreExpr :: CoreExpr -> CoreExpr
+ -- ToDo: we should really check that we don't turn a non-bottom
+ -- lambda into a bottom variable. Sigh
+
+etaCoreExpr expr@(Lam bndr body)
+ = check (reverse binders) body
where
- (tyvar_args, fun) = strip_tyvar_args [] tylam_body
+ (binders, body) = collectBinders expr
- strip_tyvar_args args_so_far tyapp@(App fun (TyArg ty))
- = case getTyVar_maybe ty of
- Just tyvar_arg -> strip_tyvar_args (tyvar_arg:args_so_far) fun
- Nothing -> (args_so_far, tyapp)
+ check [] body
+ | not (any (`elemVarSet` body_fvs) binders)
+ = body -- Success!
+ where
+ body_fvs = exprFreeVars body
- strip_tyvar_args args_so_far (App _ (UsageArg _))
- = panic "SimplUtils.mkTyLamTryingEta: strip_tyvar_args UsageArg"
+ check (b : bs) (App fun arg)
+ | (varToCoreExpr b `cheapEqExpr` arg)
+ = check bs fun
- strip_tyvar_args args_so_far fun
- = (args_so_far, fun)
+ check _ _ = expr -- Bale out
- check_fun (Var f) = True -- Claim: tyvars not mentioned by type of f
- check_fun other = False
+etaCoreExpr expr = expr -- The common case
\end{code}
+
-Let to case
-~~~~~~~~~~~
+%************************************************************************
+%* *
+\subsection{Case absorption and identity-case elimination}
+%* *
+%************************************************************************
-Given a type generate the case alternatives
+\begin{code}
+mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
+\end{code}
- C a b -> C a b
+@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.
-if there's one constructor, or
+\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}
- x -> x
+Now the identity-case transformation:
-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
- :: Type -- type of RHS
- -> SmplM InAlts -- result
+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 (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}
-mkIdentityAlts rhs_ty
- | isPrimType rhs_ty
- = newId rhs_ty `thenSmpl` \ binder ->
- returnSmpl (PrimAlts [] (BindDefault (binder, bad_occ_info) (Var binder)))
+The catch-all case
- | otherwise
- = case (maybeAppDataTyConExpandingDicts rhs_ty) of
- Just (tycon, ty_args, [data_con]) -> -- algebraic type suitable for unpacking
- let
- inst_con_arg_tys = dataConArgTys 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 (
- AlgAlts
- [(data_con, new_binders, mkCon data_con [] ty_args (map VarArg new_bindees))]
- NoDefault
- )
-
- _ -> -- Multi-constructor or abstract algebraic type
- newId rhs_ty `thenSmpl` \ binder ->
- returnSmpl (AlgAlts [] (BindDefault (binder,bad_occ_info) (Var binder)))
- where
- bad_occ_info = ManyOcc 0 -- Non-committal!
+\begin{code}
+mkCase other_scrut case_bndr other_alts
+ = returnSmpl (Case other_scrut case_bndr other_alts)
\end{code}
+
\begin{code}
-simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool
+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
-simplIdWantsToBeINLINEd id env
- = if switchIsSet env IgnoreINLINEPragma
- then False
- else idWantsToBeINLINEd id
+ matches (DEFAULT, _, _) = True
+ matches (con1, _, _) = con == con1
-type_ok_for_let_to_case :: Type -> Bool
-type_ok_for_let_to_case ty
- = case (maybeAppDataTyConExpandingDicts ty) of
- Nothing -> False
- Just (tycon, ty_args, []) -> False
- Just (tycon, ty_args, non_null_data_cons) -> True
- -- Null data cons => type is abstract
+mkCoerce to_ty (Note (Coerce _ from_ty) expr)
+ | to_ty == from_ty = expr
+ | otherwise = Note (Coerce to_ty from_ty) expr
+mkCoerce to_ty expr
+ = Note (Coerce to_ty (coreExprType expr)) expr
\end{code}
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