X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FsimplCore%2FSimplUtils.lhs;h=bb9deaadf5d0703a5a7b3e054284b9c6b9cea7cd;hb=f714e6b642fd614a9971717045ae47c3d871275e;hp=34ee7d61151f4b5431acab7390848242dfc34401;hpb=495ef8bd9ef30bffe50ea399b91e3ba09646b59a;p=ghc-hetmet.git diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs index 34ee7d6..bb9deaa 100644 --- a/ghc/compiler/simplCore/SimplUtils.lhs +++ b/ghc/compiler/simplCore/SimplUtils.lhs @@ -5,47 +5,47 @@ \begin{code} module SimplUtils ( - simplBinder, simplBinders, simplIds, - transformRhs, - mkCase, findAlt, findDefault, + simplBinder, simplBinders, simplRecBndrs, + simplLetBndr, simplLamBndrs, + newId, mkLam, prepareAlts, mkCase, -- The continuation type - SimplCont(..), DupFlag(..), contIsDupable, contResultType, - pushArgs, discardCont, countValArgs, countArgs, - analyseCont, discardInline + SimplCont(..), DupFlag(..), LetRhsFlag(..), + contIsDupable, contResultType, + countValArgs, countArgs, pushContArgs, + mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg, + getContArgs, interestingCallContext, interestingArg, isStrictType ) where #include "HsVersions.h" -import BinderInfo -import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge ) +import CmdLineOpts ( SimplifierSwitch(..), opt_UF_UpdateInPlace, + DynFlag(..), dopt ) import CoreSyn -import PprCore ( {- instance Outputable Expr -} ) -import CoreUnfold ( isValueUnfolding ) import CoreFVs ( exprFreeVars ) -import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, exprEtaExpandArity, bindNonRec ) -import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, lookupIdSubst ) -import Id ( Id, idType, isId, idName, - idOccInfo, idUnfolding, - mkId, idInfo +import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial, + etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2, + findDefault, exprOkForSpeculation, exprIsValue ) -import IdInfo ( arityLowerBound, setOccInfo, vanillaIdInfo ) -import Maybes ( maybeToBool, catMaybes ) -import Name ( isLocalName, setNameUnique ) +import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr ) +import Id ( Id, idType, idInfo, isDataConWorkId, + mkSysLocal, isDeadBinder, idNewDemandInfo, + idUnfolding, idNewStrictness + ) +import NewDemand ( isStrictDmd, isBotRes, splitStrictSig ) import SimplMonad -import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType, repType, - splitTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys +import Type ( Type, seqType, splitFunTys, dropForAlls, isStrictType, + splitTyConApp_maybe, tyConAppArgs, mkTyVarTys ) -import TyCon ( tyConDataConsIfAvailable ) -import PprType ( {- instance Outputable Type -} ) -import DataCon ( dataConRepArity ) -import TysPrim ( statePrimTyCon ) -import Var ( setVarUnique ) +import TcType ( isDictTy ) +import Name ( mkSysTvName ) +import OccName ( EncodedFS ) +import TyCon ( tyConDataCons_maybe, isAlgTyCon, isNewTyCon ) +import DataCon ( dataConRepArity, dataConExistentialTyVars, dataConArgTys ) +import Var ( tyVarKind, mkTyVar ) import VarSet -import VarEnv ( SubstEnv, SubstResult(..) ) -import UniqSupply ( splitUniqSupply, uniqFromSupply ) -import Util ( zipWithEqual, mapAccumL ) +import Util ( lengthExceeds, mapAccumL ) import Outputable \end{code} @@ -58,7 +58,11 @@ import Outputable \begin{code} data SimplCont -- Strict contexts - = Stop OutType -- Type of the result + = Stop OutType -- Type of the result + 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 @@ -67,26 +71,37 @@ 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 - OutType -- The type of the expression being sought by the context + -- 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 @@ -98,34 +113,57 @@ instance Outputable DupFlag where ppr OkToDup = ptext SLIT("ok") ppr NoDup = ptext SLIT("nodup") + +------------------- +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 -pushArgs :: SubstEnv -> [InExpr] -> SimplCont -> SimplCont -pushArgs se [] cont = cont -pushArgs se (arg:args) cont = ApplyTo NoDup arg se (pushArgs se args cont) +------------------- +discardableCont :: SimplCont -> Bool +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 (Stop to_ty) = Stop to_ty -discardCont cont = CoerceIt to_ty (Stop to_ty) - where - to_ty = contResultType cont +discardCont cont = case cont of + 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 contResultType (Select _ _ _ _ cont) = contResultType cont +------------------- countValArgs :: SimplCont -> Int countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont @@ -134,11 +172,128 @@ 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} -Comment about analyseCont -~~~~~~~~~~~~~~~~~~~~~~~~~ +\begin{code} +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 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 + stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts + | otherwise = computed_stricts + in + go [] stricts False orig_cont + where + ---------------------------- + + -- Type argument + go acc ss inl (ApplyTo _ arg@(Type _) se cont) + = go ((arg,se,False) : acc) ss inl cont + -- NB: don't bother to instantiate the function type + + -- Value argument + go acc (s:ss) inl (ApplyTo _ arg se cont) + = go ((arg,se,s) : acc) ss inl cont + + -- An Inline continuation + go acc ss inl (InlinePlease cont) + = go acc ss True cont + + -- We're run out of arguments, or else we've run out of demands + -- The latter only happens if the result is guaranteed bottom + -- This is the case for + -- * case (error "hello") of { ... } + -- * (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 = (reverse acc, discardCont cont, inl) + | otherwise = (reverse acc, cont, inl) + + ---------------------------- + vanilla_stricts, computed_stricts :: [Bool] + vanilla_stricts = repeat False + computed_stricts = zipWith (||) fun_stricts arg_stricts + + ---------------------------- + (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 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 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 + -- is lazy, so that we don't treat the arg as an + -- interesting context. This avoids substituting + -- 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 isBotRes result_info then + map isStrictDmd demands -- Finite => result is bottom + else + map isStrictDmd demands ++ vanilla_stricts + + other -> vanilla_stricts -- Not enough args, or no strictness + +------------------- +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 (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. + -- But consider also (\x. f x y) y + -- The substitution will contain (x -> ContEx y), and we want to say + -- that x is not interesting (assuming y has no unfolding) +\end{code} + +Comment about interestingCallContext +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We want to avoid inlining an expression where there can't possibly be any gain, such as in an argument position. Hence, if the continuation is interesting (eg. a case scrutinee, application etc.) then we @@ -171,13 +326,9 @@ contIsInteresting looks for case expressions with just a single default case. \begin{code} -analyseCont :: InScopeSet -> SimplCont - -> ([Bool], -- Arg-info flags; one for each value argument - Bool, -- Context of the result of the call is interesting - Bool) -- There was an InlinePlease - -analyseCont in_scope cont - = case cont of +interestingCallContext :: Bool -- False <=> no args at all + -> Bool -- False <=> no value args + -> SimplCont -> Bool -- The "lone-variable" case is important. I spent ages -- messing about with unsatisfactory varaints, but this is nice. -- The idea is that if a variable appear all alone @@ -185,11 +336,15 @@ analyseCont in_scope cont -- as scrutinee of a case Select -- as arg of a strict fn ArgOf -- then we should not inline it (unless there is some other reason, - -- e.g. is is the sole occurrence). - -- Why not? At least in the case-scrutinee situation, turning - -- case x of y -> ... + -- e.g. is is the sole occurrence). We achieve this by making + -- interestingCallContext return False for a lone variable. + -- + -- Why? At least in the case-scrutinee situation, turning + -- let x = (a,b) in case x of y -> ... -- into - -- let y = (a,b) in ... + -- let x = (a,b) in case (a,b) of y -> ... + -- and thence to + -- let x = (a,b) in let y = (a,b) in ... -- is bad if the binding for x will remain. -- -- Another example: I discovered that strings @@ -198,66 +353,34 @@ analyseCont in_scope cont -- 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. -- - -- However, even a type application isn't a lone variable. Consider + -- However, even a type application or coercion isn't a lone variable. + -- Consider -- case $fMonadST @ RealWorld of { :DMonad a b c -> c } -- We had better inline that sucker! The case won't see through it. - - (Stop _) -> boring_result -- Don't inline a lone variable - (Select _ _ _ _ _) -> boring_result -- Ditto - (ArgOf _ _ _) -> boring_result -- Ditto - (ApplyTo _ (Type _) _ cont) -> analyse_ty_app cont - other -> analyse_app cont + -- + -- For now, I'm treating treating a variable applied to types + -- in a *lazy* context "lone". The motivating example was + -- f = /\a. \x. BIG + -- g = /\a. \y. h (f a) + -- There's no advantage in inlining f here, and perhaps + -- a significant disadvantage. Hence some_val_args in the Stop case + +interestingCallContext some_args some_val_args cont + = interesting cont where - boring_result = ([], False, False) - - -- For now, I'm treating not treating a variable applied to types as - -- "lone". The motivating example was - -- f = /\a. \x. BIG - -- g = /\a. \y. h (f a) - -- There's no advantage in inlining f here, and perhaps - -- a significant disadvantage. - analyse_ty_app (Stop _) = boring_result - analyse_ty_app (ArgOf _ _ _) = boring_result - analyse_ty_app (Select _ _ _ _ _) = ([], True, False) -- See the $fMonadST example above - analyse_ty_app (ApplyTo _ (Type _) _ cont) = analyse_ty_app cont - analyse_ty_app cont = analyse_app cont - - analyse_app (InlinePlease cont) - = case analyse_app cont of - (infos, icont, inline) -> (infos, icont, True) - - analyse_app (ApplyTo _ arg subst cont) - | isValArg arg = case analyse_app cont of - (infos, icont, inline) -> (analyse_arg subst arg : infos, icont, inline) - | otherwise = analyse_app cont - - analyse_app cont = ([], interesting_call_context cont, False) - - -- An argument is interesting if it has *some* structure - -- We are here trying to avoid unfolding a function that - -- is applied only to variables that have no unfolding - -- (i.e. they are probably lambda bound): f x y z - -- There is little point in inlining f here. - analyse_arg :: SubstEnv -> InExpr -> Bool - analyse_arg subst (Var v) = case lookupIdSubst (mkSubst in_scope subst) v of - DoneId v' _ -> isValueUnfolding (idUnfolding v') - other -> False - analyse_arg subst (Type _) = False - analyse_arg subst (App fn (Type _)) = analyse_arg subst fn - analyse_arg subst (Note _ a) = analyse_arg subst a - analyse_arg subst other = True - - interesting_call_context (Stop ty) = canUpdateInPlace ty - interesting_call_context (InlinePlease _) = True - interesting_call_context (Select _ _ _ _ _) = True - interesting_call_context (CoerceIt _ cont) = interesting_call_context cont - interesting_call_context (ApplyTo _ (Type _) _ cont) = interesting_call_context cont - interesting_call_context (ApplyTo _ _ _ _) = True - interesting_call_context (ArgOf _ _ _) = True + interesting (InlinePlease _) = True + interesting (Select _ _ _ _ _) = some_args + 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. @@ -274,11 +397,8 @@ analyseCont in_scope cont -- the context for (f x) is not totally uninteresting. -discardInline :: SimplCont -> SimplCont -discardInline (InlinePlease cont) = cont -discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont) -discardInline cont = cont - +------------------- +canUpdateInPlace :: Type -> Bool -- Consider let x = in ... -- If returns an explicit constructor, we might be able -- to do update in place. So we treat even a thunk RHS context @@ -287,18 +407,16 @@ discardInline cont = cont -- small arity. But arity zero isn't good -- we share the single copy -- for that case, so no point in sharing. --- Note the repType: we want to look through newtypes for this purpose - -canUpdateInPlace ty = case splitTyConApp_maybe (repType ty) of { - Nothing -> False ; - Just (tycon, _) -> - - case tyConDataConsIfAvailable tycon of - [dc] -> arity == 1 || arity == 2 - where - arity = dataConRepArity dc - other -> False - } +canUpdateInPlace ty + | not opt_UF_UpdateInPlace = False + | otherwise + = 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} @@ -309,37 +427,45 @@ canUpdateInPlace ty = case splitTyConApp_maybe (repType ty) of { %* * %************************************************************************ +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 @@ -351,32 +477,143 @@ 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 + = getDOptsSmpl `thenSmpl` \dflags -> + mkLam' dflags env bndrs body cont + where + mkLam' dflags env bndrs body cont + | dopt Opt_DoEtaReduction dflags, + Just etad_lam <- tryEtaReduce bndrs body + = tick (EtaReduction (head bndrs)) `thenSmpl_` + returnSmpl (emptyFloats env, etad_lam) + + | dopt Opt_DoLambdaEtaExpansion dflags, + 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{Transform a RHS} +\subsection{Eta expansion and reduction} %* * %************************************************************************ -Try (a) eta expansion - (b) type-lambda swizzling +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} -transformRhs :: InExpr -> SimplM InExpr -transformRhs rhs - = tryEtaExpansion body `thenSmpl` \ body' -> - mkRhsTyLam tyvars body' +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 - (tyvars, body) = collectTyBinders rhs + fun_arity = exprEtaExpandArity body \end{code} %************************************************************************ %* * -\subsection{Local tyvar-lifting} +\subsection{Floating lets out of big lambdas} %* * %************************************************************************ -mkRhsTyLam tries this transformation, when the big lambda appears as +tryRhsTyLam tries this transformation, when the big lambda appears as the RHS of a let(rec) binding: /\abc -> let(rec) x = e in b @@ -399,7 +636,7 @@ 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: +So far as the implementation is concerned: Invariant: go F e = /\tvs -> F e @@ -441,30 +678,42 @@ as we would normally do. \begin{code} -mkRhsTyLam tyvars body -- Only does something if there's a let - | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that - = returnSmpl (mkLams tyvars body) +{- Trying to do this in full laziness + +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 + = go env (\x -> x) body + where - worth_it (Let _ e) = whnf_in_middle e - worth_it other = False + worth_it e@(Let _ _) = whnf_in_middle e + worth_it e = False + + whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False whnf_in_middle (Let _ e) = whnf_in_middle e whnf_in_middle e = exprIsCheap e main_tyvar_set = mkVarSet tyvars - go fn (Let bind@(NonRec var rhs) body) | exprIsTrivial rhs - = go (fn . Let bind) body + go env fn (Let bind@(NonRec var rhs) body) + | exprIsTrivial rhs + = go env (fn . Let bind) 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 - 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 + + 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: @@ -480,160 +729,190 @@ mkRhsTyLam tyvars body -- 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 - var_ty = idType var - - 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') + 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` \ body' -> - returnSmpl (Let (Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])) 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) + tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs)) -- See notes with tyvars_here above - var_tys = map idType vars - - go fn body = returnSmpl (mkLams tyvars (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 = mkLocalId poly_name poly_ty - -- It's crucial to copy the occInfo of the original var, because - -- we're looking at occurrence-analysed but as yet unsimplified code! - -- In particular, we mustn't lose the loop breakers. + -- 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! + -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking + -- at already simplified code, so it doesn't matter -- -- 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: + -- where x occurs once in B. 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 + -- the occurrences of x' will be just the occurrences originally -- pinned on x. - poly_info = vanillaIdInfo `setOccInfo` idOccInfo var - - poly_id = mkId poly_name poly_ty poly_info in returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here)) - mk_silly_bind var rhs = NonRec var rhs - -- The Inline note is really important! If we don't say - -- INLINE on these silly little bindings then look what happens! + mk_silly_bind var rhs = NonRec var (Note InlineMe rhs) -- Suppose we start with: -- - -- x = let g = /\a -> \x -> f x x - -- in - -- /\ b -> let g* = g b in E + -- x = /\ a -> let g = G 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 INLINEd, so that - -- we simply substitute for g* throughout. + -- Then we'll float to get + -- + -- x = let poly_g = /\ a -> G + -- in /\ a -> let g = poly_g a in E + -- + -- But now the occurrence analyser will see just one occurrence + -- of poly_g, not inside a lambda, so the simplifier will + -- PreInlineUnconditionally poly_g back into g! Badk to square 1! + -- (I used to think that the "don't inline lone occurrences" stuff + -- would stop this happening, but since it's the *only* occurrence, + -- PreInlineUnconditionally kicks in first!) + -- + -- 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: - \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 +1. Eliminate alternatives that cannot match, including the + DEFAULT alternative. + +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. + +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. + +Eliminating the default alternative in (1) isn't so obvious, but it can +happen: + +data Colour = Red | Green | Blue + +f x = case x of + Red -> .. + Green -> .. + DEFAULT -> h x -where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere) -wanting a suitable number of extra args. +h y = case y of + Blue -> .. + DEFAULT -> [ case y of ... ] -NB: the Ei may have unlifted type, but the simplifier (which is applied -to the result) deals OK with this. +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 :: InExpr -> SimplM InExpr -tryEtaExpansion rhs - | not opt_SimplDoLambdaEtaExpansion - || exprIsTrivial rhs -- Don't eta-expand a trival RHS - || null y_tys -- No useful expansion - || not (null x_bndrs || and trivial_args) -- Not (no x-binders or no z-binds) - = returnSmpl rhs - - | otherwise -- Consider eta expansion - = newIds SLIT("y") y_tys $ ( \ y_bndrs -> - tick (EtaExpansion (head y_bndrs)) `thenSmpl_` - mapAndUnzipSmpl bind_z_arg (args `zip` trivial_args) `thenSmpl` (\ (maybe_z_binds, z_args) -> - returnSmpl (mkLams x_bndrs $ - mkLets (catMaybes maybe_z_binds) $ - mkLams y_bndrs $ - mkApps (mkApps fun z_args) (map Var y_bndrs)))) - where - (x_bndrs, body) = collectValBinders rhs - (fun, args) = collectArgs body - trivial_args = map exprIsTrivial args - fun_arity = exprEtaExpandArity fun - - bind_z_arg (arg, trivial_arg) - | trivial_arg = returnSmpl (Nothing, arg) - | otherwise = newId SLIT("z") (exprType arg) $ \ z -> - returnSmpl (Just (NonRec z arg), Var z) - - -- Note: I used to try to avoid the exprType call by using - -- the type of the binder. But this type doesn't necessarily - -- belong to the same substitution environment as this rhs; - -- and we are going to make extra term binders (y_bndrs) from the type - -- which will be processed with the rhs substitution environment. - -- This only went wrong in a mind bendingly complicated case. - (potential_extra_arg_tys, inner_ty) = splitFunTys (exprType body) - - y_tys :: [InType] - y_tys = take no_extras_wanted potential_extra_arg_tys - - no_extras_wanted :: Int - no_extras_wanted = 0 `max` - - -- We used to expand the arity to the previous arity fo the - -- function; but this is pretty dangerous. Consdier - -- f = \xy -> e - -- so that f has arity 2. Now float something into f's RHS: - -- f = let z = BIG in \xy -> e - -- The last thing we want to do now is to put some lambdas - -- outside, to get - -- f = \xy -> let z = BIG in e - -- - -- (bndr_arity - no_of_xs) `max` - - -- See if the body could obviously do with more args - (fun_arity - valArgCount args) - --- This case is now deal with by exprEtaExpandArity - -- Finally, see if it's a state transformer, and xs is non-null - -- (so it's also a function not a thunk) in which - -- case we eta-expand on principle! This can waste work, - -- but usually doesn't. - -- I originally checked for a singleton type [ty] in this case - -- but then I found a situation in which I had - -- \ x -> let {..} in \ s -> f (...) s - -- AND f RETURNED A FUNCTION. That is, 's' wasn't the only - -- potential extra arg. --- case (x_bndrs, potential_extra_arg_tys) of --- (_:_, ty:_) -> case splitTyConApp_maybe ty of --- Just (tycon,_) | tycon == statePrimTyCon -> 1 --- other -> 0 --- other -> 0 +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 [(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 = mkTyVar (mkSysTvName uniq FSLIT("t")) (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} @@ -643,110 +922,374 @@ tryEtaExpansion rhs %* * %************************************************************************ +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 + = getDOptsSmpl `thenSmpl` \dflags -> + mkAlts' dflags scrut outer_bndr outer_alts + where + mkAlts' dflags scrut outer_bndr outer_alts + | dopt Opt_CaseMerge dflags, + (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! - 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 + -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr + -- in munge_rhs may put a case into the DEFAULT branch! + where + -- 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' dflags 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 scrut + returnSmpl (re_note scrut) where - identity_alt (DEFAULT, [], Var v) = v == case_bndr - identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs - (mkConApp con (map Type arg_tys ++ map varToCoreExpr args)) - identity_alt other = False + identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args - arg_tys = case splitTyConApp_maybe (idType case_bndr) of - Just (tycon, arg_tys) -> arg_tys -\end{code} + identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args) + identity_rhs (LitAlt lit) _ = Lit lit + identity_rhs DEFAULT _ = Var case_bndr -The catch-all case + arg_tys = map Type (tyConAppArgs (idType case_bndr)) -\begin{code} -mkCase other_scrut case_bndr other_alts - = returnSmpl (Case other_scrut case_bndr other_alts) + -- We've seen this: + -- case coerce T e of x { _ -> coerce T' x } + -- And we definitely want to eliminate this case! + -- So we throw away notes from the RHS, and reconstruct + -- (at least an approximation) at the other end + de_note (Note _ e) = de_note e + de_note e = e + + -- re_note wraps a coerce if it might be necessary + re_note scrut = case head alts of + (_,_,rhs1@(Note _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut + other -> scrut + + +-------------------------------------------------- +-- Catch-all +-------------------------------------------------- +mkCase1 scrut bndr alts = returnSmpl (Case scrut bndr alts) \end{code} -\begin{code} -findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr) -findDefault [] = ([], Nothing) -findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args ) - ([], Just rhs) -findDefault (alt : alts) = case findDefault alts of - (alts', deflt) -> (alt : alts', deflt) - -findAlt :: AltCon -> [CoreAlt] -> CoreAlt -findAlt con alts - = go alts - where - go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts)) - go (alt : alts) | matches alt = alt - | otherwise = go alts +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. - matches (DEFAULT, _, _) = True - matches (con1, _, _) = con == con1 +\begin{code} +bindCaseBndr bndr rhs body + | isDeadBinder bndr = body + | otherwise = bindNonRec bndr rhs body \end{code}