\begin{code}
module SimplUtils (
- simplBinder, simplBinders, simplRecIds, simplLetId, simplLamBinder,
- tryRhsTyLam, tryEtaExpansion,
- mkCase,
+ simplBinder, simplBinders, simplRecIds, simplLetId, simplLamBinders,
+ tryEtaExpansion,
+ newId, mkLam, mkCase,
-- The continuation type
- SimplCont(..), DupFlag(..), contIsDupable, contResultType,
- countValArgs, countArgs, mkRhsStop, mkStop,
+ SimplCont(..), DupFlag(..), LetRhsFlag(..),
+ contIsDupable, contResultType,
+ countValArgs, countArgs,
+ mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg,
getContArgs, interestingCallContext, interestingArg, isStrictType, discardInline
) where
#include "HsVersions.h"
-import CmdLineOpts ( switchIsOn, SimplifierSwitch(..),
- opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge,
- opt_UF_UpdateInPlace
+import CmdLineOpts ( SimplifierSwitch(..),
+ opt_SimplDoLambdaEtaExpansion, opt_SimplDoEtaReduction,
+ opt_SimplCaseMerge, opt_UF_UpdateInPlace
)
import CoreSyn
+import CoreFVs ( exprSomeFreeVars, exprsSomeFreeVars )
import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap,
etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce,
- findDefault
+ findDefault, exprOkForSpeculation, exprIsValue
)
import Subst ( InScopeSet, mkSubst, substExpr )
import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr )
-import Id ( idType, idName,
+import Id ( Id, idType, idName,
+ mkSysLocal, hasNoBinding, isDeadBinder, idNewDemandInfo,
idUnfolding, idNewStrictness,
mkLocalId, idInfo
)
-import Maybes ( maybeToBool, catMaybes )
import Name ( setNameUnique )
import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
import SimplMonad
import Type ( Type, mkForAllTys, seqType,
splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
- isUnLiftedType, isStrictType,
- splitRepFunTys
+ isUnLiftedType, splitRepFunTys, isStrictType
)
-import TyCon ( tyConDataConsIfAvailable )
-import DataCon ( dataConRepArity )
+import OccName ( UserFS )
+import TyCon ( tyConDataConsIfAvailable, isDataTyCon )
+import DataCon ( dataConRepArity, dataConSig, dataConArgTys )
+import Var ( mkSysTyVar, tyVarKind )
import VarEnv ( SubstEnv )
+import VarSet ( mkVarSet, varSetElems, intersectVarSet )
import Util ( lengthExceeds, mapAccumL )
import Outputable
\end{code}
\begin{code}
data SimplCont -- Strict contexts
= Stop OutType -- Type of the result
- Bool -- True => This is the RHS of a thunk whose type suggests
- -- that update-in-place would be possible
- -- (This makes the inliner a little keener.)
+ LetRhsFlag
+ Bool -- True <=> This is the RHS of a thunk whose type suggests
+ -- that update-in-place would be possible
+ -- (This makes the inliner a little keener.)
| CoerceIt OutType -- The To-type, simplified
SimplCont
SimplCont -- keen to inline itelf
| ApplyTo DupFlag
- InExpr SubstEnv -- The argument, as yet unsimplified,
- SimplCont -- and its subst-env
+ InExpr SimplEnv -- The argument, as yet unsimplified,
+ SimplCont -- and its environment
| Select DupFlag
- InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
+ InId [InAlt] SimplEnv -- 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
+ LetRhsFlag
OutType -- cont_ty: 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
+ (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
-- The result expression in the OutExprStuff has type cont_ty
+data LetRhsFlag = AnArg -- It's just an argument not a let RHS
+ | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
+
+instance Outputable LetRhsFlag where
+ ppr AnArg = ptext SLIT("arg")
+ ppr AnRhs = ptext SLIT("rhs")
+
instance Outputable SimplCont where
- ppr (Stop _ _) = ptext SLIT("Stop")
+ ppr (Stop _ is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs)
ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
- ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
+ ppr (ArgOf dup _ _ _) = ptext SLIT("ArgOf...") <+> ppr dup
ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
(nest 4 (ppr alts)) $$ ppr cont
ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
-------------------
-mkRhsStop, mkStop :: OutType -> SimplCont
-mkStop ty = Stop ty False
-mkRhsStop ty = Stop ty (canUpdateInPlace ty)
+mkBoringStop :: OutType -> SimplCont
+mkBoringStop ty = Stop ty AnArg (canUpdateInPlace ty)
+
+mkStop :: OutType -> LetRhsFlag -> SimplCont
+mkStop ty is_rhs = Stop ty is_rhs (canUpdateInPlace ty)
+contIsRhs :: SimplCont -> Bool
+contIsRhs (Stop _ AnRhs _) = True
+contIsRhs (ArgOf _ AnRhs _ _) = True
+contIsRhs other = False
+
+contIsRhsOrArg (Stop _ _ _) = True
+contIsRhsOrArg (ArgOf _ _ _ _) = True
+contIsRhsOrArg other = False
-------------------
contIsDupable :: SimplCont -> Bool
-contIsDupable (Stop _ _) = True
+contIsDupable (Stop _ _ _) = True
contIsDupable (ApplyTo OkToDup _ _ _) = True
-contIsDupable (ArgOf OkToDup _ _) = True
+contIsDupable (ArgOf OkToDup _ _ _) = True
contIsDupable (Select OkToDup _ _ _ _) = True
contIsDupable (CoerceIt _ cont) = contIsDupable cont
contIsDupable (InlinePlease cont) = contIsDupable cont
-------------------
discardableCont :: SimplCont -> Bool
-discardableCont (Stop _ _) = False
+discardableCont (Stop _ _ _) = False
discardableCont (CoerceIt _ cont) = discardableCont cont
discardableCont (InlinePlease cont) = discardableCont cont
discardableCont other = True
discardCont :: SimplCont -- A continuation, expecting
-> SimplCont -- Replace the continuation with a suitable coerce
discardCont cont = case cont of
- Stop to_ty _ -> cont
- other -> CoerceIt to_ty (mkStop to_ty)
+ Stop to_ty is_rhs _ -> cont
+ other -> CoerceIt to_ty (mkBoringStop to_ty)
where
to_ty = contResultType cont
-------------------
contResultType :: SimplCont -> OutType
-contResultType (Stop to_ty _) = to_ty
-contResultType (ArgOf _ to_ty _) = to_ty
+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
\begin{code}
-getContArgs :: OutId -> SimplCont
- -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args
- SimplCont, -- Remaining continuation
- Bool) -- Whether we came across an InlineCall
+getContArgs :: SwitchChecker
+ -> OutId -> SimplCont
+ -> ([(InExpr, SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
+ SimplCont, -- Remaining continuation
+ Bool) -- Whether we came across an InlineCall
-- getContArgs id k = (args, k', inl)
-- args are the leading ApplyTo items in k
-- (i.e. outermost comes first)
-- augmented with demand info from the functionn
-getContArgs fun orig_cont
- = getSwitchChecker `thenSmpl` \ chkr ->
- let
+getContArgs chkr fun orig_cont
+ = let
-- Ignore strictness info if the no-case-of-case
-- flag is on. Strictness changes evaluation order
-- and that can change full laziness
-- * f (error "Hello") where f is strict
-- etc
go acc ss inl cont
- | null ss && discardableCont cont = tick BottomFound `thenSmpl_`
- returnSmpl (reverse acc, discardCont cont, inl)
- | otherwise = returnSmpl (reverse acc, cont, inl)
+ | null ss && discardableCont cont = (reverse acc, discardCont cont, inl)
+ | otherwise = (reverse acc, cont, inl)
----------------------------
vanilla_stricts, computed_stricts :: [Bool]
-- s = "foo"
-- f = \x -> ...(error s)...
- -- Fundamentally such contexts should not ecourage inlining becuase
+ -- Fundamentally such contexts should not ecourage inlining because
-- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
-- so there's no gain.
--
-- Perhaps True is a bit over-keen, but I've
-- seen (coerce f) x, where f has an INLINE prag,
-- So we have to give some motivaiton for inlining it
- interesting (ArgOf _ _ _) = some_val_args
- interesting (Stop ty upd_in_place) = some_val_args && upd_in_place
- interesting (CoerceIt _ cont) = interesting cont
+ interesting (ArgOf _ _ _ _) = some_val_args
+ interesting (Stop ty _ upd_in_place) = some_val_args && upd_in_place
+ interesting (CoerceIt _ cont) = interesting cont
-- 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.
%* *
%************************************************************************
+These functions are in the monad only so that they can be made strict via seq.
+
\begin{code}
-simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
-simplBinders bndrs thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', bndrs') = Subst.simplBndrs subst bndrs
+simplBinders :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
+simplBinders env bndrs
+ = let
+ (subst', bndrs') = Subst.simplBndrs (getSubst env) bndrs
in
seqBndrs bndrs' `seq`
- setSubst subst' (thing_inside bndrs')
+ returnSmpl (setSubst env subst', bndrs')
-simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
-simplBinder bndr thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', bndr') = Subst.simplBndr subst bndr
+simplBinder :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder)
+simplBinder env bndr
+ = let
+ (subst', bndr') = Subst.simplBndr (getSubst env) bndr
in
seqBndr bndr' `seq`
- setSubst subst' (thing_inside bndr')
+ returnSmpl (setSubst env subst', bndr')
-simplLamBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
-simplLamBinder bndr thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', bndr') = Subst.simplLamBndr subst bndr
+simplLamBinders :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
+simplLamBinders env bndrs
+ = let
+ (subst', bndrs') = mapAccumL Subst.simplLamBndr (getSubst env) bndrs
in
- seqBndr bndr' `seq`
- setSubst subst' (thing_inside bndr')
-
+ seqBndrs bndrs' `seq`
+ returnSmpl (setSubst env subst', bndrs')
-simplRecIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
-simplRecIds ids thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', ids') = mapAccumL Subst.simplLetId subst ids
+simplRecIds :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
+simplRecIds env ids
+ = let
+ (subst', ids') = mapAccumL Subst.simplLetId (getSubst env) ids
in
seqBndrs ids' `seq`
- setSubst subst' (thing_inside ids')
+ returnSmpl (setSubst env subst', ids')
-simplLetId :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
-simplLetId id thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', id') = Subst.simplLetId subst id
+simplLetId :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder)
+simplLetId env id
+ = let
+ (subst', id') = Subst.simplLetId (getSubst env) id
in
- seqBndr id' `seq`
- setSubst subst' (thing_inside id')
+ seqBndr id' `seq`
+ returnSmpl (setSubst env subst', id')
seqBndrs [] = ()
seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
\end{code}
+\begin{code}
+newId :: UserFS -> Type -> SimplM Id
+newId fs ty = getUniqueSmpl `thenSmpl` \ uniq ->
+ returnSmpl (mkSysLocal fs uniq ty)
+\end{code}
+
+
%************************************************************************
%* *
-\subsection{Local tyvar-lifting}
+\subsection{Rebuilding a lambda}
%* *
%************************************************************************
-mkRhsTyLam tries this transformation, when the big lambda appears as
+\begin{code}
+mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
+\end{code}
+
+Try three things
+ a) eta reduction, if that gives a trivial expression
+ b) eta expansion [only if there are some value lambdas]
+ c) floating lets out through big lambdas
+ [only if all tyvar lambdas, and only if this lambda
+ is the RHS of a let]
+
+\begin{code}
+mkLam env bndrs body cont
+ | opt_SimplDoEtaReduction,
+ Just etad_lam <- tryEtaReduce bndrs body
+ = tick (EtaReduction (head bndrs)) `thenSmpl_`
+ returnSmpl (emptyFloats env, etad_lam)
+
+ | opt_SimplDoLambdaEtaExpansion,
+ any isRuntimeVar bndrs
+ = tryEtaExpansion body `thenSmpl` \ body' ->
+ returnSmpl (emptyFloats env, mkLams bndrs body')
+
+{- Sept 01: I'm experimenting with getting the
+ full laziness pass to float out past big lambdsa
+ | all isTyVar bndrs, -- Only for big lambdas
+ contIsRhs cont -- Only try the rhs type-lambda floating
+ -- 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')
+-}
+
+ | otherwise
+ = returnSmpl (emptyFloats env, mkLams bndrs body)
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Eta expansion and 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
+
+\begin{code}
+tryEtaReduce :: [OutBinder] -> 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
+ -- [in fact I take the simple path and look for just a variable]
+ = go (reverse bndrs) body
+ where
+ go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
+ go [] (Var fun) | ok_fun fun = Just (Var fun) -- Success!
+ go _ _ = Nothing -- Failure!
+
+ ok_fun fun = not (fun `elem` bndrs) && not (hasNoBinding fun)
+ ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
+\end{code}
+
+
+ Try eta expansion for RHSs
+
+We go for:
+ f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
+ (n >= 0)
+
+where (in both cases)
+
+ * The xi can include type variables
+
+ * The yi are all value variables
+
+ * N is a NORMAL FORM (i.e. no redexes anywhere)
+ wanting a suitable number of extra args.
+
+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
+actually computing the expansion.
+
+\begin{code}
+tryEtaExpansion :: OutExpr -> SimplM OutExpr
+-- There is at least one runtime binder in the binders
+tryEtaExpansion body
+ | arity_is_manifest -- Some lambdas but not enough
+ = returnSmpl body
+
+ | otherwise
+ = getUniquesSmpl `thenSmpl` \ us ->
+ returnSmpl (etaExpand fun_arity us body (exprType body))
+ where
+ (fun_arity, arity_is_manifest) = exprEtaExpandArity body
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Floating lets out of big lambdas}
+%* *
+%************************************************************************
+
+tryRhsTyLam tries this transformation, when the big lambda appears as
the RHS of a let(rec) binding:
/\abc -> let(rec) x = e in b
\begin{code}
-tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
+{- Trying to do this in full laziness
-tryRhsTyLam rhs -- Only does something if there's a let
- | null tyvars || not (worth_it body) -- inside a type lambda,
- = returnSmpl ([], rhs) -- and a WHNF inside that
+tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
+-- Call ensures that all the binders are type variables
+
+tryRhsTyLam env tyvars body -- Only does something if there's a let
+ | not (all isTyVar tyvars)
+ || not (worth_it body) -- inside a type lambda,
+ = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
| otherwise
- = go (\x -> x) body `thenSmpl` \ (binds, body') ->
- returnSmpl (binds, mkLams tyvars body')
+ = go env (\x -> x) body
where
- (tyvars, body) = collectTyBinders rhs
-
worth_it e@(Let _ _) = whnf_in_middle e
worth_it e = False
whnf_in_middle (Let _ e) = whnf_in_middle e
whnf_in_middle e = exprIsCheap e
- go fn (Let bind@(NonRec var rhs) body)
+ main_tyvar_set = mkVarSet tyvars
+
+ go env fn (Let bind@(NonRec var rhs) body)
| exprIsTrivial rhs
- = go (fn . Let bind) body
+ = go env (fn . Let bind) body
- go fn (Let (NonRec var rhs) body)
- = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
- go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
- returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
+ go env fn (Let (NonRec var rhs) body)
+ = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
+ addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
+ go env (fn . Let (mk_silly_bind var rhs')) body
where
- tyvars_here = tyvars
- -- main_tyvar_set = mkVarSet tyvars
- -- var_ty = idType var
- -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
- -- tyvars_here was an attempt to reduce the number of tyvars
+
+ tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
+ -- Abstract only over the type variables free in the rhs
-- wrt which the new binding is abstracted. But the naive
-- approach of abstract wrt the tyvars free in the Id's type
-- fails. Consider:
-- abstracting wrt *all* the tyvars. We'll see if that
-- gives rise to problems. SLPJ June 98
- go fn (Let (Rec prs) body)
+ go env 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'))
- new_bind = Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])
+ gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
+ pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
in
- go gn body `thenSmpl` \ (binds, body') ->
- returnSmpl (new_bind : binds, body')
+ addAuxiliaryBind env (Rec pairs) $ \ env ->
+ go env gn body
where
(vars,rhss) = unzip prs
- tyvars_here = tyvars
- -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
- -- var_tys = map idType vars
+ tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
-- See notes with tyvars_here above
- go fn body = returnSmpl ([], fn body)
+ go env fn body = returnSmpl (emptyFloats env, fn body)
mk_poly tyvars_here var
= getUniqueSmpl `thenSmpl` \ uniq ->
-- Solution: put an INLINE note on g's RHS, so that poly_g seems
-- to appear many times. (NB: mkInlineMe eliminates
-- such notes on trivial RHSs, so do it manually.)
+-}
\end{code}
%************************************************************************
%* *
-\subsection{Eta expansion}
+\subsection{Case absorption and identity-case elimination}
%* *
%************************************************************************
- Try eta expansion for RHSs
+mkCase puts a case expression back together, trying various transformations first.
-We go for:
- Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
- (n >= 0)
- OR
- Case 2 f = N E1..En ==> z1=E1
- (n > 0) ..
- zn=En
- f = \y1..ym -> N z1..zn y1..ym
+\begin{code}
+mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
-where (in both cases)
+mkCase scrut case_bndr alts
+ = mkAlts scrut case_bndr alts `thenSmpl` \ better_alts ->
+ mkCase1 scrut case_bndr better_alts
+\end{code}
- * The xi can include type variables
- * The yi are all value variables
+mkAlts tries these things:
- * N is a NORMAL FORM (i.e. no redexes anywhere)
- wanting a suitable number of extra args.
+1. If several alternatives are identical, merge them into
+ a single DEFAULT alternative. I've occasionally seen this
+ making a big difference:
- * the Ei must not have unlifted type
+ case e of =====> case e of
+ C _ -> f x D v -> ....v....
+ D v -> ....v.... DEFAULT -> f x
+ DEFAULT -> f x
-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.
+ The point is that we merge common RHSs, at least for the DEFAULT case.
+ [One could do something more elaborate but I've never seen it needed.]
+ To avoid an expensive test, we just merge branches equal to the *first*
+ alternative; this picks up the common cases
+ a) all branches equal
+ b) some branches equal to the DEFAULT (which occurs first)
+
+2. If the DEFAULT alternative can match only one possible constructor,
+ then make that constructor explicit.
+ e.g.
+ case e of x { DEFAULT -> rhs }
+ ===>
+ case e of x { (a,b) -> rhs }
+ where the type is a single constructor type. This gives better code
+ when rhs also scrutinises x or e.
+
+3. Case merging:
+ case e of b { ==> case e of b {
+ p1 -> rhs1 p1 -> rhs1
+ ... ...
+ pm -> rhsm pm -> rhsm
+ _ -> case b of b' { pn -> let b'=b in rhsn
+ pn -> rhsn ...
+ ... po -> let b'=b in rhso
+ po -> rhso _ -> 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.
+
+
+The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
-In Case 1, 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
-actually computing the expansion.
+ x | p `is` 1 -> e1
+ | p `is` 2 -> e2
+ ...etc...
-\begin{code}
-tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
-tryEtaExpansion rhs rhs_ty
- | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
- || exprIsTrivial rhs -- Not if RHS is trivial
- || final_arity == 0 -- Not if arity is zero
- = returnSmpl ([], rhs)
-
- | n_val_args == 0 && not arity_is_manifest
- = -- Some lambdas but not enough: case 1
- getUniqSupplySmpl `thenSmpl` \ us ->
- returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
-
- | n_val_args > 0 && not (any cant_bind arg_infos)
- = -- Partial application: case 2
- mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
- getUniqSupplySmpl `thenSmpl` \ us ->
- returnSmpl (catMaybes maybe_z_binds,
- etaExpand final_arity us (mkApps fun z_args) rhs_ty)
+where @is@ was something like
+
+ p `is` n = p /= (-1) && p == n
- | otherwise
- = returnSmpl ([], rhs)
- where
- (fun, args) = collectArgs rhs
- n_val_args = valArgCount args
- (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
- final_arity = 0 `max` (fun_arity - n_val_args)
- arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
- cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
-
- bind_z_arg (arg, arg_ty, trivial_arg)
- | trivial_arg = returnSmpl (Nothing, arg)
- | otherwise = newId SLIT("z") arg_ty $ \ z ->
- returnSmpl (Just (NonRec z arg), Var z)
-\end{code}
+This gave rise to a horrible sequence of cases
+ case p of
+ (-1) -> $j p
+ 1 -> e1
+ DEFAULT -> $j p
-%************************************************************************
-%* *
-\subsection{Case absorption and identity-case elimination}
-%* *
-%************************************************************************
+and similarly in cascade for all the join points!
-\begin{code}
-mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
-\end{code}
-@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.
\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!
+--------------------------------------------------
+-- 1. Merge identical branches
+--------------------------------------------------
+mkAlts scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
+ | all isDeadBinder bndrs1, -- Remember the default
+ length filtered_alts < length con_alts -- alternative comes first
+ = tick (AltMerge case_bndr) `thenSmpl_`
+ returnSmpl better_alts
+ where
+ filtered_alts = filter keep con_alts
+ keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
+ better_alts = (DEFAULT, [], rhs1) : filtered_alts
+
+
+--------------------------------------------------
+-- 2. Fill in missing constructor
+--------------------------------------------------
+
+mkAlts scrut case_bndr alts
+ | Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr),
+ isDataTyCon tycon, -- It's a data type
+ (alts_no_deflt, Just rhs) <- findDefault alts,
+ -- There is a DEFAULT case
+ [missing_con] <- filter is_missing (tyConDataConsIfAvailable tycon)
+ -- There is just one missing constructor!
+ = tick (FillInCaseDefault case_bndr) `thenSmpl_`
+ getUniquesSmpl `thenSmpl` \ tv_uniqs ->
+ getUniquesSmpl `thenSmpl` \ id_uniqs ->
+ let
+ (_,_,ex_tyvars,_,_,_) = dataConSig missing_con
+ ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
+ mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
+ arg_ids = zipWith (mkSysLocal SLIT("a")) id_uniqs arg_tys
+ arg_tys = dataConArgTys missing_con (inst_tys ++ mkTyVarTys ex_tyvars')
+ better_alts = (DataAlt missing_con, ex_tyvars' ++ arg_ids, rhs) : alts_no_deflt
+ in
+ returnSmpl better_alts
+ where
+ impossible_cons = otherCons (idUnfolding case_bndr)
+ handled_data_cons = [data_con | DataAlt data_con <- impossible_cons] ++
+ [data_con | (DataAlt data_con, _, _) <- alts]
+ is_missing con = not (con `elem` handled_data_cons)
+
+--------------------------------------------------
+-- 3. Merge nested cases
+--------------------------------------------------
+
+mkAlts scrut outer_bndr outer_alts
+ | opt_SimplCaseMerge,
+ (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
+ Just (Case (Var scrut_var) inner_bndr inner_alts) <- maybe_outer_deflt,
+ scruting_same_var scrut_var
+
+ = let -- 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 = bindCaseBndr inner_bndr (Var outer_bndr) rhs
+
+ (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
+
+ new_alts = add_default maybe_inner_default
+ (outer_alts_without_deflt ++ inner_con_alts)
+ in
+ tick (CaseMerge outer_bndr) `thenSmpl_`
+ returnSmpl new_alts
+ -- Warning: don't call mkAlts 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!
+ -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
+ -- in munge_rhs may put a case into the DEFAULT branch!
where
- new_alts = add_default maybe_inner_default
- (outer_alts_without_deflt ++ inner_con_alts)
+ -- We are scrutinising the same variable if it's
+ -- the outer case-binder, or if the outer case scrutinises a variable
+ -- (and it's the same). Testing both allows us not to replace the
+ -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
+ scruting_same_var = case scrut of
+ Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
+ other -> \ v -> v == outer_bndr
+
+ add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
+ add_default Nothing alts = 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
+--------------------------------------------------
+-- Catch-all
+--------------------------------------------------
- -- Eliminate any inner alts which are shadowed by the outer ones
- outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
+mkAlts scrut case_bndr other_alts = returnSmpl other_alts
+\end{code}
- 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
- (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
- add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
- add_default Nothing alts = alts
-\end{code}
+=================================================================================
+
+mkCase1 tries these things
+
+1. Eliminate the case altogether if possible
-Now the identity-case transformation:
+2. Case-identity:
case e of ===> e
True -> True;
False -> False
-and similar friends.
+ and similar friends.
+
+
+Start with a simple situation:
+
+ case x# of ===> e[x#/y#]
+ y# -> e
+
+(when x#, y# are of primitive type, of course). We can't (in general)
+do this for algebraic cases, because we might turn bottom into
+non-bottom!
+
+Actually, we generalise this idea to look for a case where we're
+scrutinising a variable, and we know that only the default case can
+match. For example:
+\begin{verbatim}
+ case x of
+ 0# -> ...
+ other -> ...(case x of
+ 0# -> ...
+ other -> ...) ...
+\end{code}
+Here the inner case can be eliminated. This really only shows up in
+eliminating error-checking code.
+
+We also make sure that we deal with this very common case:
+
+ case e of
+ x -> ...x...
+
+Here we are using the case as a strict let; if x is used only once
+then we want to inline it. We have to be careful that this doesn't
+make the program terminate when it would have diverged before, so we
+check that
+ - x is used strictly, or
+ - e is already evaluated (it may so if e is a variable)
+
+Lastly, we generalise the transformation to handle this:
+
+ case e of ===> r
+ True -> r
+ False -> r
+
+We only do this for very cheaply compared r's (constructors, literals
+and variables). If pedantic bottoms is on, we only do it when the
+scrutinee is a PrimOp which can't fail.
+
+We do it *here*, looking at un-simplified alternatives, because we
+have to check that r doesn't mention the variables bound by the
+pattern in each alternative, so the binder-info is rather useful.
+
+So the case-elimination algorithm is:
+
+ 1. Eliminate alternatives which can't match
+
+ 2. Check whether all the remaining alternatives
+ (a) do not mention in their rhs any of the variables bound in their pattern
+ and (b) have equal rhss
+
+ 3. Check we can safely ditch the case:
+ * PedanticBottoms is off,
+ or * the scrutinee is an already-evaluated variable
+ or * the scrutinee is a primop which is ok for speculation
+ -- ie we want to preserve divide-by-zero errors, and
+ -- calls to error itself!
+
+ or * [Prim cases] the scrutinee is a primitive variable
+
+ or * [Alg cases] the scrutinee is a variable and
+ either * the rhs is the same variable
+ (eg case x of C a b -> x ===> x)
+ or * there is only one alternative, the default alternative,
+ and the binder is used strictly in its scope.
+ [NB this is helped by the "use default binder where
+ possible" transformation; see below.]
+
+
+If so, then we can replace the case with one of the rhss.
+
\begin{code}
-mkCase scrut case_bndr alts
+--------------------------------------------------
+-- 1. Eliminate the case altogether if poss
+--------------------------------------------------
+
+mkCase1 scrut case_bndr [(con,bndrs,rhs)]
+ -- See if we can get rid of the case altogether
+ -- See the extensive notes on case-elimination above
+ -- mkCase made sure that if all the alternatives are equal,
+ -- then there is now only one (DEFAULT) rhs
+ | all isDeadBinder bndrs,
+
+ -- Check that the scrutinee can be let-bound instead of case-bound
+ exprOkForSpeculation scrut
+ -- OK not to evaluate it
+ -- This includes things like (==# a# b#)::Bool
+ -- so that we simplify
+ -- case ==# a# b# of { True -> x; False -> x }
+ -- to just
+ -- x
+ -- This particular example shows up in default methods for
+ -- comparision operations (e.g. in (>=) for Int.Int32)
+ || exprIsValue scrut -- It's already evaluated
+ || var_demanded_later scrut -- It'll be demanded later
+
+-- || not opt_SimplPedanticBottoms) -- Or we don't care!
+-- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
+-- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
+-- its argument: case x of { y -> dataToTag# y }
+-- Here we must *not* discard the case, because dataToTag# just fetches the tag from
+-- the info pointer. So we'll be pedantic all the time, and see if that gives any
+-- other problems
+ = tick (CaseElim case_bndr) `thenSmpl_`
+ returnSmpl (bindCaseBndr case_bndr scrut rhs)
+
+ where
+ -- The case binder is going to be evaluated later,
+ -- and the scrutinee is a simple variable
+ var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
+ var_demanded_later other = False
+
+
+--------------------------------------------------
+-- 2. Identity case
+--------------------------------------------------
+
+mkCase1 scrut case_bndr alts -- Identity case
| all identity_alt alts
= tick (CaseIdentity case_bndr) `thenSmpl_`
returnSmpl (re_note scrut)
re_note scrut = case head alts of
(_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
other -> scrut
-\end{code}
-
-The catch-all case. We do a final transformation that I've
-occasionally seen making a big difference:
- case e of =====> case e of
- C _ -> f x D v -> ....v....
- D v -> ....v.... DEFAULT -> f x
- DEFAULT -> f x
-
-The point is that we merge common RHSs, at least for the DEFAULT case.
-[One could do something more elaborate but I've never seen it needed.]
-The case where this came up was like this (lib/std/PrelCError.lhs):
-
- x | p `is` 1 -> e1
- | p `is` 2 -> e2
- ...etc...
-where @is@ was something like
-
- p `is` n = p /= (-1) && p == n
+--------------------------------------------------
+-- Catch-all
+--------------------------------------------------
+mkCase1 scrut bndr alts = returnSmpl (Case scrut bndr alts)
+\end{code}
-This gave rise to a horrible sequence of cases
- case p of
- (-1) -> $j p
- 1 -> e1
- DEFAULT -> $j p
+When adding auxiliary bindings for the case binder, it's worth checking if
+its dead, because it often is, and occasionally these mkCase transformations
+cascade rather nicely.
-and similarly in cascade for all the join points!
-
\begin{code}
-mkCase other_scrut case_bndr other_alts
- = returnSmpl (Case other_scrut case_bndr (mergeDefault other_alts))
-
-mergeDefault (deflt_alt@(DEFAULT,_,deflt_rhs) : con_alts)
- = deflt_alt : [alt | alt@(con,_,rhs) <- con_alts, not (rhs `cheapEqExpr` deflt_rhs)]
- -- NB: we can neglect the binders because we won't get equality if the
- -- binders are mentioned in rhs (no shadowing)
-mergeDefault other_alts
- = other_alts
+bindCaseBndr bndr rhs body
+ | isDeadBinder bndr = body
+ | otherwise = bindNonRec bndr rhs body
\end{code}
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