X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FsimplCore%2FSimplUtils.lhs;h=b57b4b14265f7ae15e415e539cbb8cbdc100a841;hb=18074d6acde6d642b8fb10b1b49153f717c75446;hp=387cbd80b8a21cc05fe3c3169263b478457c92f9;hpb=af331a6f6091463adc01778f2250cdc5294dbb8d;p=ghc-hetmet.git diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs index 387cbd8..b57b4b1 100644 --- a/ghc/compiler/simplCore/SimplUtils.lhs +++ b/ghc/compiler/simplCore/SimplUtils.lhs @@ -5,47 +5,48 @@ \begin{code} module SimplUtils ( - simplBinder, simplBinders, simplIds, - tryRhsTyLam, tryEtaExpansion, - mkCase, + simplBinder, simplBinders, simplRecBndrs, + simplLetBndr, simplLamBndrs, + newId, mkLam, prepareAlts, mkCase, -- The continuation type - SimplCont(..), DupFlag(..), contIsDupable, contResultType, - countValArgs, countArgs, mkRhsStop, mkStop, - getContArgs, interestingCallContext, interestingArg, isStrictType, discardInline + SimplCont(..), DupFlag(..), LetRhsFlag(..), + contIsDupable, contResultType, + countValArgs, countArgs, pushContArgs, + mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg, + getContArgs, interestingCallContext, interestingArg, isStrictType ) where #include "HsVersions.h" -import CmdLineOpts ( switchIsOn, SimplifierSwitch(..), - opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge, opt_DictsStrict, - opt_UF_UpdateInPlace +import CmdLineOpts ( SimplifierSwitch(..), + opt_SimplDoLambdaEtaExpansion, opt_SimplDoEtaReduction, + opt_SimplCaseMerge, opt_UF_UpdateInPlace ) import CoreSyn -import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, - etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce, - findDefault, findAlt +import CoreFVs ( exprFreeVars ) +import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial, + etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2, + findDefault, exprOkForSpeculation, exprIsValue ) -import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, substExpr ) -import Id ( idType, idName, - idUnfolding, idStrictness, - mkVanillaId, idInfo +import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr ) +import Id ( Id, idType, idInfo, isDataConWorkId, + mkSysLocal, isDeadBinder, idNewDemandInfo, + idUnfolding, idNewStrictness ) -import IdInfo ( StrictnessInfo(..) ) -import Maybes ( maybeToBool, catMaybes ) -import Name ( setNameUnique ) -import Demand ( isStrict ) +import NewDemand ( isStrictDmd, isBotRes, splitStrictSig ) import SimplMonad -import Type ( Type, mkForAllTys, seqType, repType, - splitTyConApp_maybe, tyConAppArgs, mkTyVarTys, - isDictTy, isDataType, isUnLiftedType, - splitRepFunTys +import Type ( Type, seqType, splitFunTys, dropForAlls, isStrictType, + splitTyConApp_maybe, tyConAppArgs, mkTyVarTys ) -import TyCon ( tyConDataConsIfAvailable ) -import DataCon ( dataConRepArity ) -import VarEnv ( SubstEnv ) -import Util ( lengthExceeds ) +import TcType ( isDictTy ) +import OccName ( EncodedFS ) +import TyCon ( tyConDataCons_maybe, isAlgTyCon, isNewTyCon ) +import DataCon ( dataConRepArity, dataConExistentialTyVars, dataConArgTys ) +import Var ( mkSysTyVar, tyVarKind ) +import VarSet +import Util ( lengthExceeds, mapAccumL ) import Outputable \end{code} @@ -59,9 +60,10 @@ import Outputable \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 @@ -70,27 +72,37 @@ data SimplCont -- Strict contexts 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 + | ArgOf LetRhsFlag -- An arbitrary strict context: the argument -- of a strict function, or a primitive-arg fn -- or a PrimOp + -- No DupFlag because we never duplicate it + OutType -- arg_ty: type of the argument itself 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) $$ ppr cont - ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup + ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...") ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$ (nest 4 (ppr alts)) $$ ppr cont ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont @@ -104,30 +116,33 @@ instance Outputable DupFlag where ------------------- -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 (Select OkToDup _ _ _ _) = True contIsDupable (CoerceIt _ cont) = contIsDupable cont contIsDupable (InlinePlease cont) = contIsDupable cont contIsDupable other = False ------------------- -discardInline :: SimplCont -> SimplCont -discardInline (InlinePlease cont) = cont -discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont) -discardInline cont = cont - -------------------- discardableCont :: SimplCont -> Bool -discardableCont (Stop _ _) = False +discardableCont (Stop _ _ _) = False discardableCont (CoerceIt _ cont) = discardableCont cont discardableCont (InlinePlease cont) = discardableCont cont discardableCont other = True @@ -135,15 +150,15 @@ 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 @@ -158,21 +173,27 @@ countValArgs other = 0 countArgs :: SimplCont -> Int countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont countArgs other = 0 + +------------------- +pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont +-- Pushes args with the specified environment +pushContArgs env [] cont = cont +pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont) \end{code} \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 @@ -203,10 +224,12 @@ getContArgs fun orig_cont -- * (error "Hello") arg -- * f (error "Hello") where f is strict -- etc + -- Then, especially in the first of these cases, we'd like to discard + -- the continuation, leaving just the bottoming expression. But the + -- type might not be right, so we may have to add a coerce. 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] @@ -214,22 +237,22 @@ getContArgs fun orig_cont computed_stricts = zipWith (||) fun_stricts arg_stricts ---------------------------- - (val_arg_tys, _) = splitRepFunTys (idType fun) + (val_arg_tys, _) = splitFunTys (dropForAlls (idType fun)) arg_stricts = map isStrictType val_arg_tys ++ repeat False -- These argument types are used as a cheap and cheerful way to find -- unboxed arguments, which must be strict. But it's an InType -- and so there might be a type variable where we expect a function -- type (the substitution hasn't happened yet). And we don't bother -- doing the type applications for a polymorphic function. - -- Hence the split*Rep*FunTys + -- Hence the splitFunTys*IgnoringForAlls* ---------------------------- -- If fun_stricts is finite, it means the function returns bottom -- after that number of value args have been consumed -- Otherwise it's infinite, extended with False fun_stricts - = case idStrictness fun of - StrictnessInfo demands result_bot + = case splitStrictSig (idNewStrictness fun) of + (demands, result_info) | not (demands `lengthExceeds` countValArgs orig_cont) -> -- Enough args, use the strictness given. -- For bottoming functions we used to pretend that the arg @@ -238,46 +261,30 @@ getContArgs fun orig_cont -- top-level bindings for (say) strings into -- calls to error. But now we are more careful about -- inlining lone variables, so its ok (see SimplUtils.analyseCont) - if result_bot then - map isStrict demands -- Finite => result is bottom + if isBotRes result_info then + map isStrictDmd demands -- Finite => result is bottom else - map isStrict demands ++ vanilla_stricts + map isStrictDmd demands ++ vanilla_stricts other -> vanilla_stricts -- Not enough args, or no strictness - -------------------- -isStrictType :: Type -> Bool - -- isStrictType computes whether an argument (or let RHS) should - -- be computed strictly or lazily, based only on its type -isStrictType ty - | isUnLiftedType ty = True - | opt_DictsStrict && isDictTy ty && isDataType ty = True - | otherwise = False - -- Return true only for dictionary types where the dictionary - -- has more than one component (else we risk poking on the component - -- of a newtype dictionary) - ------------------- -interestingArg :: InScopeSet -> InExpr -> SubstEnv -> Bool +interestingArg :: OutExpr -> Bool -- 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. -interestingArg in_scope arg subst - = analyse (substExpr (mkSubst in_scope subst) arg) - -- 'analyse' only looks at the top part of the result - -- and substExpr is lazy, so this isn't nearly as brutal - -- as it looks. - where - analyse (Var v) = hasSomeUnfolding (idUnfolding v) - -- Was: isValueUnfolding (idUnfolding v') - -- But that seems over-pessimistic - analyse (Type _) = False - analyse (App fn (Type _)) = analyse fn - analyse (Note _ a) = analyse a - analyse other = True +interestingArg (Var v) = hasSomeUnfolding (idUnfolding v) + -- Was: isValueUnfolding (idUnfolding v') + -- But that seems over-pessimistic + || isDataConWorkId v + -- This accounts for an argument like + -- () or [], which is definitely interesting +interestingArg (Type _) = False +interestingArg (App fn (Type _)) = interestingArg fn +interestingArg (Note _ a) = interestingArg a +interestingArg other = True -- Consider let x = 3 in f x -- The substitution will contain (x -> ContEx 3), and we want to -- to say that x is an interesting argument. @@ -347,7 +354,7 @@ interestingCallContext :: Bool -- False <=> no args at all -- 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. -- @@ -368,10 +375,13 @@ interestingCallContext some_args some_val_args cont where interesting (InlinePlease _) = True interesting (Select _ _ _ _ _) = some_args - interesting (ApplyTo _ _ _ _) = some_args -- Can happen if we have (coerce t (f x)) y - interesting (ArgOf _ _ _) = some_val_args - interesting (Stop ty upd_in_place) = some_val_args && upd_in_place - interesting (CoerceIt _ cont) = interesting cont + interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y + -- 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 -- 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. @@ -398,21 +408,16 @@ canUpdateInPlace :: Type -> Bool -- 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 | not opt_UF_UpdateInPlace = False | otherwise - = 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 - } + = case splitTyConApp_maybe ty of + Nothing -> False + Just (tycon, _) -> case tyConDataCons_maybe tycon of + Just [dc] -> arity == 1 || arity == 2 + where + arity = dataConRepArity dc + other -> False \end{code} @@ -423,37 +428,45 @@ canUpdateInPlace ty %* * %************************************************************************ +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') = substBndrs 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') = substBndr 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') --- 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 +simplLetBndr :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder) +simplLetBndr env id + = let + (subst', id') = Subst.simplLetId (getSubst env) id + in + seqBndr id' `seq` + returnSmpl (setSubst env subst', id') + +simplLamBndrs, simplRecBndrs + :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder]) +simplRecBndrs = simplBndrs Subst.simplLetId +simplLamBndrs = simplBndrs Subst.simplLamBndr + +simplBndrs simpl_bndr env bndrs + = let + (subst', bndrs') = mapAccumL simpl_bndr (getSubst env) bndrs in seqBndrs bndrs' `seq` - setSubst subst' (thing_inside bndrs') + returnSmpl (setSubst env subst', bndrs') seqBndrs [] = () seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs @@ -465,13 +478,139 @@ seqBndr b | isTyVar b = b `seq` () \end{code} +\begin{code} +newId :: EncodedFS -> Type -> SimplM Id +newId fs ty = getUniqueSmpl `thenSmpl` \ uniq -> + returnSmpl (mkSysLocal fs uniq ty) +\end{code} + + +%************************************************************************ +%* * +\subsection{Rebuilding a lambda} +%* * +%************************************************************************ + +\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{Local tyvar-lifting} +\subsection{Eta expansion and reduction} %* * %************************************************************************ -mkRhsTyLam tries this transformation, when the big lambda appears as +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 + = go (reverse bndrs) body + where + go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round + go [] fun | ok_fun fun = Just fun -- Success! + go _ _ = Nothing -- Failure! + + ok_fun fun = exprIsTrivial fun + && not (any (`elemVarSet` (exprFreeVars fun)) bndrs) + && (exprIsValue fun || all ok_lam bndrs) + ok_lam v = isTyVar v || isDictTy (idType v) + -- The exprIsValue is because eta reduction is not + -- valid in general: \x. bot /= bot + -- So we need to be sure that the "fun" is a value. + -- + -- However, we always want to reduce (/\a -> f a) to f + -- This came up in a RULE: foldr (build (/\a -> g a)) + -- did not match foldr (build (/\b -> ...something complex...)) + -- The type checker can insert these eta-expanded versions, + -- with both type and dictionary lambdas; hence the slightly + -- ad-hoc isDictTy + + ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg +\end{code} + + + Try eta expansion for RHSs + +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 + = getUniquesSmpl `thenSmpl` \ us -> + returnSmpl (etaExpand fun_arity us body (exprType body)) + where + fun_arity = 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 @@ -536,19 +675,20 @@ as we would normally do. \begin{code} -tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr) +{- Trying to do this in full laziness + +tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr +-- Call ensures that all the binders are type variables -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 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 @@ -556,21 +696,21 @@ tryRhsTyLam rhs -- Only does something if there's a let 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: @@ -586,29 +726,27 @@ tryRhsTyLam rhs -- Only does something if there's a let -- 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 -> let poly_name = setNameUnique (idName var) uniq -- Keep same name poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course - poly_id = mkVanillaId poly_name poly_ty + poly_id = mkLocalId poly_name poly_ty -- In the olden days, it was crucial to copy the occInfo of the original var, -- because we were looking at occurrence-analysed but as yet unsimplified code! @@ -645,75 +783,133 @@ tryRhsTyLam rhs -- Only does something if there's a let -- 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 alternative filtering %* * %************************************************************************ - Try eta expansion for RHSs +prepareAlts does two things: -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 +1. Eliminate alternatives that cannot match, including the + DEFAULT alternative. -where (in both cases) +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. - * The xi can include type variables +It's a good idea do do this stuff before simplifying the alternatives, to +avoid simplifying alternatives we know can't happen, and to come up with +the list of constructors that are handled, to put into the IdInfo of the +case binder, for use when simplifying the alternatives. - * The yi are all value variables +Eliminating the default alternative in (1) isn't so obvious, but it can +happen: - * N is a NORMAL FORM (i.e. no redexes anywhere) - wanting a suitable number of extra args. +data Colour = Red | Green | Blue + +f x = case x of + Red -> .. + Green -> .. + DEFAULT -> h x - * the Ei must not have unlifted type +h y = case y of + Blue -> .. + DEFAULT -> [ case y of ... ] + +If we inline h into f, the default case of the inlined h can't happen. +If we don't notice this, we may end up filtering out *all* the cases +of the inner case y, which give us nowhere to go! -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} -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) +prepareAlts :: OutExpr -- Scrutinee + -> InId -- Case binder + -> [InAlt] + -> SimplM ([InAlt], -- Better alternatives + [AltCon]) -- These cases are handled + +prepareAlts scrut case_bndr alts + = let + (alts_wo_default, maybe_deflt) = findDefault alts + + impossible_cons = case scrut of + Var v -> otherCons (idUnfolding v) + other -> [] + + -- Filter out alternatives that can't possibly match + better_alts | null impossible_cons = alts_wo_default + | otherwise = [alt | alt@(con,_,_) <- alts_wo_default, + not (con `elem` impossible_cons)] + + -- "handled_cons" are handled either by the context, + -- or by a branch in this case expression + -- (Don't add DEFAULT to the handled_cons!!) + handled_cons = impossible_cons ++ [con | (con,_,_) <- better_alts] + in + -- Filter out the default, if it can't happen, + -- or replace it with "proper" alternative if there + -- is only one constructor left + prepareDefault case_bndr handled_cons maybe_deflt `thenSmpl` \ deflt_alt -> + + returnSmpl (deflt_alt ++ better_alts, handled_cons) + +prepareDefault case_bndr handled_cons (Just rhs) + | Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr), + isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples. + not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval: + -- case x of { DEFAULT -> e } + -- and we don't want to fill in a default for them! + Just all_cons <- tyConDataCons_maybe tycon, + not (null all_cons), -- This is a tricky corner case. If the data type has no constructors, + -- which GHC allows, then the case expression will have at most a default + -- alternative. We don't want to eliminate that alternative, because the + -- invariant is that there's always one alternative. It's more convenient + -- to leave + -- case x of { DEFAULT -> e } + -- as it is, rather than transform it to + -- error "case cant match" + -- which would be quite legitmate. But it's a really obscure corner, and + -- not worth wasting code on. + let handled_data_cons = [data_con | DataAlt data_con <- handled_cons], + let missing_cons = [con | con <- all_cons, + not (con `elem` handled_data_cons)] + = case missing_cons of + [] -> returnSmpl [] -- Eliminate the default alternative + -- if it can't match + + [con] -> -- It matches exactly one constructor, so fill it in + tick (FillInCaseDefault case_bndr) `thenSmpl_` + mk_args con inst_tys `thenSmpl` \ args -> + returnSmpl [(DataAlt con, args, rhs)] + + two_or_more -> returnSmpl [(DEFAULT, [], rhs)] | 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) + = returnSmpl [(DEFAULT, [], rhs)] + +prepareDefault case_bndr handled_cons Nothing + = returnSmpl [] + +mk_args missing_con inst_tys + = getUniquesSmpl `thenSmpl` \ tv_uniqs -> + getUniquesSmpl `thenSmpl` \ id_uniqs -> + let + ex_tyvars = dataConExistentialTyVars missing_con + ex_tyvars' = zipWith mk tv_uniqs ex_tyvars + mk uniq tv = mkSysTyVar uniq (tyVarKind tv) + arg_tys = dataConArgTys missing_con (inst_tys ++ mkTyVarTys ex_tyvars') + arg_ids = zipWith (mkSysLocal FSLIT("a")) id_uniqs arg_tys + in + returnSmpl (ex_tyvars' ++ arg_ids) \end{code} @@ -723,73 +919,331 @@ tryEtaExpansion rhs rhs_ty %* * %************************************************************************ +mkCase puts a case expression back together, trying various transformations first. + \begin{code} mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr + +mkCase scrut case_bndr alts + = mkAlts scrut case_bndr alts `thenSmpl` \ better_alts -> + mkCase1 scrut case_bndr better_alts \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. + +mkAlts tries these things: + +1. If several alternatives are identical, merge them into + a single DEFAULT alternative. I've occasionally seen this + 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.] + 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. 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): + + x | p `is` 1 -> e1 + | p `is` 2 -> e2 + ...etc... + +where @is@ was something like + + p `is` n = p /= (-1) && p == n + +This gave rise to a horrible sequence of cases + + case p of + (-1) -> $j p + 1 -> e1 + DEFAULT -> $j p + +and similarly in cascade for all the join points! + + \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. 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 = 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 + -- 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 + + +-------------------------------------------------- +-- Catch-all +-------------------------------------------------- + +mkAlts scrut case_bndr other_alts = returnSmpl other_alts \end{code} -Now the identity-case transformation: + + +================================================================================= + +mkCase1 tries these things + +1. Eliminate the case altogether if possible + +2. Case-identity: case e of ===> e - True -> True; + 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. + +Further notes about case elimination +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider: test :: Integer -> IO () + test = print + +Turns out that this compiles to: + Print.test + = \ eta :: Integer + eta1 :: State# RealWorld -> + case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT -> + case hPutStr stdout + (PrelNum.jtos eta ($w[] @ Char)) + eta1 + of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }} + +Notice the strange '<' which has no effect at all. This is a funny one. +It started like this: + +f x y = if x < 0 then jtos x + else if y==0 then "" else jtos x + +At a particular call site we have (f v 1). So we inline to get + + if v < 0 then jtos x + else if 1==0 then "" else jtos x + +Now simplify the 1==0 conditional: + + if v<0 then jtos v else jtos v + +Now common-up the two branches of the case: + + case (v<0) of DEFAULT -> jtos v + +Why don't we drop the case? Because it's strict in v. It's technically +wrong to drop even unnecessary evaluations, and in practice they +may be a result of 'seq' so we *definitely* don't want to drop those. +I don't really know how to improve this situation. + \begin{code} -mkCase scrut case_bndr alts +-------------------------------------------------- +-- 0. Check for empty alternatives +-------------------------------------------------- + +#ifdef DEBUG +mkCase1 scrut case_bndr [] + = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $ + returnSmpl scrut +#endif + +-------------------------------------------------- +-- 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 +-- Also we don't want to discard 'seq's + = 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) @@ -812,15 +1266,23 @@ mkCase scrut case_bndr alts -- re_note wraps a coerce if it might be necessary re_note scrut = case head alts of - (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut + (_,_,rhs1@(Note _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut other -> scrut -\end{code} -The catch-all case -\begin{code} -mkCase other_scrut case_bndr other_alts - = returnSmpl (Case other_scrut case_bndr other_alts) +-------------------------------------------------- +-- Catch-all +-------------------------------------------------- +mkCase1 scrut bndr alts = returnSmpl (Case scrut bndr alts) \end{code} +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. + +\begin{code} +bindCaseBndr bndr rhs body + | isDeadBinder bndr = body + | otherwise = bindNonRec bndr rhs body +\end{code}