X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FsimplCore%2FSimplUtils.lhs;h=bb9deaadf5d0703a5a7b3e054284b9c6b9cea7cd;hb=f714e6b642fd614a9971717045ae47c3d871275e;hp=fa14e39a33b411975d2d42084af5e62c0fcf177a;hpb=12899612693163154531da3285ec99c1c8ca2226;p=ghc-hetmet.git diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs index fa14e39..bb9deaa 100644 --- a/ghc/compiler/simplCore/SimplUtils.lhs +++ b/ghc/compiler/simplCore/SimplUtils.lhs @@ -1,418 +1,1295 @@ % -% (c) The AQUA Project, Glasgow University, 1993-1996 +% (c) The AQUA Project, Glasgow University, 1993-1998 % \section[SimplUtils]{The simplifier utilities} \begin{code} -#include "HsVersions.h" - module SimplUtils ( + simplBinder, simplBinders, simplRecBndrs, + simplLetBndr, simplLamBndrs, + newId, mkLam, prepareAlts, mkCase, - floatExposesHNF, - - mkTyLamTryingEta, mkValLamTryingEta, - - etaExpandCount, + -- The continuation type + SimplCont(..), DupFlag(..), LetRhsFlag(..), + contIsDupable, contResultType, + countValArgs, countArgs, pushContArgs, + mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg, + getContArgs, interestingCallContext, interestingArg, isStrictType - mkIdentityAlts, - - simplIdWantsToBeINLINEd, - - type_ok_for_let_to_case ) where -IMP_Ubiq(){-uitous-} -IMPORT_DELOOPER(SmplLoop) -- paranoia checking +#include "HsVersions.h" -import BinderInfo -import CmdLineOpts ( SimplifierSwitch(..) ) +import CmdLineOpts ( SimplifierSwitch(..), opt_UF_UpdateInPlace, + DynFlag(..), dopt ) import CoreSyn -import CoreUnfold ( SimpleUnfolding, mkFormSummary, FormSummary(..) ) -import Id ( idType, isBottomingId, idWantsToBeINLINEd, dataConArgTys, - getIdArity, GenId{-instance Eq-} +import CoreFVs ( exprFreeVars ) +import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial, + etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2, + findDefault, exprOkForSpeculation, exprIsValue + ) +import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr ) +import Id ( Id, idType, idInfo, isDataConWorkId, + mkSysLocal, isDeadBinder, idNewDemandInfo, + idUnfolding, idNewStrictness ) -import IdInfo ( arityMaybe ) -import Maybes ( maybeToBool ) -import PrelVals ( augmentId, buildId ) -import PrimOp ( primOpIsCheap ) -import SimplEnv +import NewDemand ( isStrictDmd, isBotRes, splitStrictSig ) import SimplMonad -import Type ( eqTy, isPrimType, maybeAppDataTyConExpandingDicts, getTyVar_maybe ) -import TysWiredIn ( realWorldStateTy ) -import TyVar ( GenTyVar{-instance Eq-} ) -import Util ( isIn, panic ) - +import Type ( Type, seqType, splitFunTys, dropForAlls, isStrictType, + splitTyConApp_maybe, tyConAppArgs, mkTyVarTys + ) +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 Util ( lengthExceeds, mapAccumL ) +import Outputable \end{code} -Floating -~~~~~~~~ -The function @floatExposesHNF@ tells whether let/case floating will -expose a head normal form. It is passed booleans indicating the -desired strategy. +%************************************************************************ +%* * +\subsection{The continuation data type} +%* * +%************************************************************************ \begin{code} -floatExposesHNF - :: Bool -- Float let(rec)s out of rhs - -> Bool -- Float cheap primops out of rhs - -> Bool -- OK to duplicate code - -> GenCoreExpr bdr Id tyvar uvar - -> Bool - -floatExposesHNF float_lets float_primops ok_to_dup rhs - = try rhs +data SimplCont -- Strict contexts + = 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 + + | InlinePlease -- This continuation makes a function very + SimplCont -- keen to inline itelf + + | ApplyTo DupFlag + InExpr SimplEnv -- The argument, as yet unsimplified, + SimplCont -- and its environment + + | Select DupFlag + InId [InAlt] SimplEnv -- The case binder, alts, and subst-env + SimplCont + + | 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. + + (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 _ is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs) + ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont + ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...") + ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$ + (nest 4 (ppr alts)) $$ ppr cont + ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont + ppr (InlinePlease cont) = ptext SLIT("InlinePlease") $$ ppr cont + +data DupFlag = OkToDup | NoDup + +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 (ApplyTo OkToDup _ _ _) = True +contIsDupable (Select OkToDup _ _ _ _) = True +contIsDupable (CoerceIt _ cont) = contIsDupable cont +contIsDupable (InlinePlease cont) = contIsDupable cont +contIsDupable other = False + +------------------- +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 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 (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 +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 :: 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 - try (Case (Prim _ _) (PrimAlts alts deflt) ) - | float_primops && (null alts || ok_to_dup) - = or (try_deflt deflt : map try_alt alts) - - try (Let bind body) | float_lets = try body - - -- `build g' - -- is like a HNF, - -- because it *will* become one. - -- likewise for `augment g h' - -- - try (App (App (Var bld) _) _) | bld == buildId = True - try (App (App (App (Var aug) _) _) _) | aug == augmentId = True - - try other = case mkFormSummary other of - VarForm -> True - ValueForm -> True - other -> False - {- but *not* necessarily "BottomForm"... - - We may want to float a let out of a let to expose WHNFs, - but to do that to expose a "bottom" is a Bad Idea: - let x = let y = ... - in ...error ...y... -- manifestly bottom using y - in ... - =/=> - let y = ... - in let x = ...error ...y... - in ... - - as y is only used in case of an error, we do not want - to allocate it eagerly as that's a waste. - -} - - try_alt (lit,rhs) = try rhs - - try_deflt NoDefault = False - try_deflt (BindDefault _ rhs) = try rhs + ---------------------------- + + -- 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 +inline, otherwise we don't. -Eta reduction on ordinary lambdas -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -We have a go at doing +Previously some_benefit used to return True only if the variable was +applied to some value arguments. This didn't work: - \ x y -> f x y ===> f + let x = _coerce_ (T Int) Int (I# 3) in + case _coerce_ Int (T Int) x of + I# y -> .... -But we only do this if it gets rid of a whole lambda, not part. -The idea is that lambdas are often quite helpful: they indicate -head normal forms, so we don't want to chuck them away lightly. -But if they expose a simple variable then we definitely win. Even -if they expose a type application we win. So we check for this special -case. +we want to inline x, but can't see that it's a constructor in a case +scrutinee position, and some_benefit is False. -It does arise: +Another example: - f xs = [y | (y,_) <- xs] +dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t) -gives rise to a recursive function for the list comprehension, and -f turns out to be just a single call to this recursive function. +.... case dMonadST _@_ x0 of (a,b,c) -> .... -\begin{code} -mkValLamTryingEta :: [Id] -- Args to the lambda - -> CoreExpr -- Lambda body - -> CoreExpr +we'd really like to inline dMonadST here, but we *don't* want to +inline if the case expression is just -mkValLamTryingEta [] body = body + case x of y { DEFAULT -> ... } -mkValLamTryingEta orig_ids body - = reduce_it (reverse orig_ids) body +since we can just eliminate this case instead (x is in WHNF). Similar +applies when x is bound to a lambda expression. Hence +contIsInteresting looks for case expressions with just a single +default case. + +\begin{code} +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 + -- as an arg of lazy fn, or rhs Stop + -- 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). 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 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 + -- were getting inlined straight back into applications of 'error' + -- because the latter is strict. + -- s = "foo" + -- f = \x -> ...(error s)... + + -- 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 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. + -- + -- 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 - bale_out = mkValLam orig_ids body + 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. + -- x + (y * z) + -- Here the contIsInteresting makes the '*' keener to inline, + -- which in turn exposes a constructor which makes the '+' inline. + -- Assuming that +,* aren't small enough to inline regardless. + -- + -- It's also very important to inline in a strict context for things + -- like + -- foldr k z (f x) + -- Here, the context of (f x) is strict, and if f's unfolding is + -- a build it's *great* to inline it here. So we must ensure that + -- the context for (f x) is not totally uninteresting. + + +------------------- +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 +-- as interesting if update in place is possible. We approximate +-- this by seeing if the type has a single constructor with a +-- small arity. But arity zero isn't good -- we share the single copy +-- for that case, so no point in sharing. + +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} + + + +%************************************************************************ +%* * +\section{Dealing with a single binder} +%* * +%************************************************************************ - reduce_it [] residual - | residual_ok residual = residual - | otherwise = bale_out +These functions are in the monad only so that they can be made strict via seq. - reduce_it (id:ids) (App fun (VarArg arg)) - | id == arg - && not (idType id `eqTy` realWorldStateTy) - -- *never* eta-reduce away a PrimIO state token! (WDP 94/11) - = reduce_it ids fun +\begin{code} +simplBinders :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder]) +simplBinders env bndrs + = let + (subst', bndrs') = Subst.simplBndrs (getSubst env) bndrs + in + seqBndrs bndrs' `seq` + returnSmpl (setSubst env subst', bndrs') + +simplBinder :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder) +simplBinder env bndr + = let + (subst', bndr') = Subst.simplBndr (getSubst env) bndr + in + seqBndr bndr' `seq` + returnSmpl (setSubst env subst', bndr') + + +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` + returnSmpl (setSubst env subst', bndrs') + +seqBndrs [] = () +seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs + +seqBndr b | isTyVar b = b `seq` () + | otherwise = seqType (idType b) `seq` + idInfo b `seq` + () +\end{code} - reduce_it ids other = bale_out - is_elem = isIn "mkValLamTryingEta" +\begin{code} +newId :: EncodedFS -> Type -> SimplM Id +newId fs ty = getUniqueSmpl `thenSmpl` \ uniq -> + returnSmpl (mkSysLocal fs uniq ty) +\end{code} - ----------- - residual_ok :: CoreExpr -> Bool -- Checks for type application - -- and function not one of the - -- bound vars - residual_ok (Var v) = not (v `is_elem` orig_ids) - -- Fun mustn't be one of the bound ids - residual_ok (App fun arg) - | notValArg arg = residual_ok fun - residual_ok other = False +%************************************************************************ +%* * +\subsection{Rebuilding a lambda} +%* * +%************************************************************************ + +\begin{code} +mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr \end{code} -Eta expansion -~~~~~~~~~~~~~ -@etaExpandCount@ takes an expression, E, and returns an integer n, -such that +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] - E ===> (\x1::t1 x1::t2 ... xn::tn -> E x1 x2 ... xn) +\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} -is a safe transformation. In particular, the transformation should -not cause work to be duplicated, unless it is ``cheap'' (see -@manifestlyCheap@ below). -@etaExpandCount@ errs on the conservative side. It is always safe to -return 0. +%************************************************************************ +%* * +\subsection{Eta expansion and reduction} +%* * +%************************************************************************ -An application of @error@ is special, because it can absorb as many -arguments as you care to give it. For this special case we return -100, to represent "infinity", which is a bit of a hack. +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} -etaExpandCount :: GenCoreExpr bdr Id tyvar uvar - -> Int -- Number of extra args you can safely abstract - -etaExpandCount (Lam (ValBinder _) body) - = 1 + etaExpandCount body - -etaExpandCount (Let bind body) - | all manifestlyCheap (rhssOfBind bind) - = etaExpandCount body - -etaExpandCount (Case scrut alts) - | manifestlyCheap scrut - = minimum [etaExpandCount rhs | rhs <- rhssOfAlts alts] - -etaExpandCount fun@(Var _) = eta_fun fun -etaExpandCount (App fun arg) - | notValArg arg = eta_fun fun - | otherwise = case etaExpandCount fun of - 0 -> 0 - n -> n-1 -- Knock off one - -etaExpandCount other = 0 -- Give up - -- Lit, Con, Prim, - -- non-val Lam, - -- Scc (pessimistic; ToDo), - -- Let with non-whnf rhs(s), - -- Case with non-whnf scrutinee - ------------------------------ -eta_fun :: GenCoreExpr bdr Id tv uv -- The function - -> Int -- How many args it can safely be applied to - -eta_fun (App fun arg) | notValArg arg = eta_fun fun - -eta_fun expr@(Var v) - | isBottomingId v -- Bottoming ids have "infinite arity" - = 10000 -- Blargh. Infinite enough! - -eta_fun expr@(Var v) - | maybeToBool arity_maybe -- We know the arity - = arity +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 - arity_maybe = arityMaybe (getIdArity v) - arity = case arity_maybe of { Just arity -> arity } - -eta_fun other = 0 -- Give up + 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} -@manifestlyCheap@ looks at a Core expression and returns \tr{True} if -it is obviously in weak head normal form, or is cheap to get to WHNF. -By ``cheap'' we mean a computation we're willing to duplicate in order -to bring a couple of lambdas together. The main examples of things -which aren't WHNF but are ``cheap'' are: - * case e of - pi -> ei + Try eta expansion for RHSs - where e, and all the ei are cheap; and +We go for: + f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym + (n >= 0) - * let x = e - in b +where (in both cases) - where e and b are cheap; and + * The xi can include type variables - * op x1 ... xn + * The yi are all value variables - where op is a cheap primitive operator + * 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} -manifestlyCheap :: GenCoreExpr bndr Id tv uv -> Bool - -manifestlyCheap (Var _) = True -manifestlyCheap (Lit _) = True -manifestlyCheap (Con _ _) = True -manifestlyCheap (SCC _ e) = manifestlyCheap e -manifestlyCheap (Coerce _ _ e) = manifestlyCheap e -manifestlyCheap (Lam x e) = if isValBinder x then True else manifestlyCheap e -manifestlyCheap (Prim op _) = primOpIsCheap op - -manifestlyCheap (Let bind body) - = manifestlyCheap body && all manifestlyCheap (rhssOfBind bind) - -manifestlyCheap (Case scrut alts) - = manifestlyCheap scrut && all manifestlyCheap (rhssOfAlts alts) - -manifestlyCheap other_expr -- look for manifest partial application - = case (collectArgs other_expr) of { (fun, _, _, vargs) -> - case fun of - - Var f | isBottomingId f -> True -- Application of a function which - -- always gives bottom; we treat this as - -- a WHNF, because it certainly doesn't - -- need to be shared! - - Var f -> let - num_val_args = length vargs - in - num_val_args == 0 || -- Just a type application of - -- a variable (f t1 t2 t3) - -- counts as WHNF - case (arityMaybe (getIdArity f)) of - Nothing -> False - Just arity -> num_val_args < arity - - _ -> False - } +tryEtaExpansion :: 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} -Eta reduction on type lambdas -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -We have a go at doing - /\a -> a ===> +%************************************************************************ +%* * +\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 + ==> + let(rec) x' = /\abc -> let x = x' a b c in e + in + /\abc -> let x = x' a b c in b + +This is good because it can turn things like: + + let f = /\a -> letrec g = ... g ... in g +into + letrec g' = /\a -> ... g' a ... + in + let f = /\ a -> g' a + +which is better. In effect, it means that big lambdas don't impede +let-floating. + +This optimisation is CRUCIAL in eliminating the junk introduced by +desugaring mutually recursive definitions. Don't eliminate it lightly! + +So far as the implementation is concerned: + + Invariant: go F e = /\tvs -> F e + + Equalities: + go F (Let x=e in b) + = Let x' = /\tvs -> F e + in + go G b + where + G = F . Let x = x' tvs + + go F (Letrec xi=ei in b) + = Letrec {xi' = /\tvs -> G ei} + in + go G b + where + G = F . Let {xi = xi' tvs} + +[May 1999] If we do this transformation *regardless* then we can +end up with some pretty silly stuff. For example, + + let + st = /\ s -> let { x1=r1 ; x2=r2 } in ... + in .. +becomes + let y1 = /\s -> r1 + y2 = /\s -> r2 + st = /\s -> ...[y1 s/x1, y2 s/x2] + in .. + +Unless the "..." is a WHNF there is really no point in doing this. +Indeed it can make things worse. Suppose x1 is used strictly, +and is of the form + + x1* = case f y of { (a,b) -> e } + +If we abstract this wrt the tyvar we then can't do the case inline +as we would normally do. + + +\begin{code} +{- 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 env (\x -> x) body + + where + 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 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 + + where + + 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: + -- /\ a b -> let t :: (a,b) = (e1, e2) + -- x :: a = fst t + -- in ... + -- Here, b isn't free in x's type, but we must nevertheless + -- abstract wrt b as well, because t's type mentions b. + -- Since t is floated too, we'd end up with the bogus: + -- poly_t = /\ a b -> (e1, e2) + -- poly_x = /\ a -> fst (poly_t a *b*) + -- So for now we adopt the even more naive approach of + -- abstracting wrt *all* the tyvars. We'll see if that + -- gives rise to problems. SLPJ June 98 + + 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')) + pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss] + in + addAuxiliaryBind env (Rec pairs) $ \ env -> + go env gn body + where + (vars,rhss) = unzip prs + tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs)) + -- See notes with tyvars_here above + + 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 + + -- 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 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 originally + -- pinned on x. + in + returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here)) + + mk_silly_bind var rhs = NonRec var (Note InlineMe rhs) + -- Suppose we start with: + -- + -- x = /\ a -> let g = G in E + -- + -- 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{Case alternative filtering +%* * +%************************************************************************ -where doesn't mention a. -This is sometimes quite useful, because we can get the sequence: +prepareAlts does two things: - f ab d = let d1 = ...d... in - letrec f' b x = ...d...(f' b)... in - f' b -specialise ==> +1. Eliminate alternatives that cannot match, including the + DEFAULT alternative. - f.Int b = letrec f' b x = ...dInt...(f' b)... in - f' b +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. -float ==> +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. - f' b x = ...dInt...(f' b)... - f.Int b = f' b +Eliminating the default alternative in (1) isn't so obvious, but it can +happen: -Now we really want to simplify to +data Colour = Red | Green | Blue - f.Int = f' +f x = case x of + Red -> .. + Green -> .. + DEFAULT -> h x -and then replace all the f's with f.Ints. +h y = case y of + Blue -> .. + DEFAULT -> [ case y of ... ] -N.B. We are careful not to partially eta-reduce a sequence of type -applications since this breaks the specialiser: +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! - /\ a -> f Char# a =NO=> f Char# \begin{code} -mkTyLamTryingEta :: [TyVar] -> CoreExpr -> CoreExpr - -mkTyLamTryingEta tyvars tylam_body - = if - tyvars == tyvar_args && -- Same args in same order - check_fun fun -- Function left is ok - then - -- Eta reduction worked - fun - else - -- The vastly common case - mkTyLam tyvars tylam_body - where - (tyvar_args, fun) = strip_tyvar_args [] tylam_body +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} - strip_tyvar_args args_so_far tyapp@(App fun (TyArg ty)) - = case getTyVar_maybe ty of - Just tyvar_arg -> strip_tyvar_args (tyvar_arg:args_so_far) fun - Nothing -> (args_so_far, tyapp) - strip_tyvar_args args_so_far (App _ (UsageArg _)) - = panic "SimplUtils.mkTyLamTryingEta: strip_tyvar_args UsageArg" +%************************************************************************ +%* * +\subsection{Case absorption and identity-case elimination} +%* * +%************************************************************************ - strip_tyvar_args args_so_far fun - = (args_so_far, fun) +mkCase puts a case expression back together, trying various transformations first. - check_fun (Var f) = True -- Claim: tyvars not mentioned by type of f - check_fun other = False +\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} -Let to case -~~~~~~~~~~~ -Given a type generate the case alternatives +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 - C a b -> C a b +This gave rise to a horrible sequence of cases -if there's one constructor, or + case p of + (-1) -> $j p + 1 -> e1 + DEFAULT -> $j p - x -> x +and similarly in cascade for all the join points! -if there's many, or if it's a primitive type. \begin{code} -mkIdentityAlts - :: Type -- type of RHS - -> SmplM InAlts -- result +-------------------------------------------------- +-- 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 -mkIdentityAlts rhs_ty - | isPrimType rhs_ty - = newId rhs_ty `thenSmpl` \ binder -> - returnSmpl (PrimAlts [] (BindDefault (binder, bad_occ_info) (Var binder))) - | otherwise - = case (maybeAppDataTyConExpandingDicts rhs_ty) of - Just (tycon, ty_args, [data_con]) -> -- algebraic type suitable for unpacking - let - inst_con_arg_tys = dataConArgTys data_con ty_args - in - newIds inst_con_arg_tys `thenSmpl` \ new_bindees -> - let - new_binders = [ (b, bad_occ_info) | b <- new_bindees ] - in - returnSmpl ( - AlgAlts - [(data_con, new_binders, mkCon data_con [] ty_args (map VarArg new_bindees))] - NoDefault - ) - - _ -> -- Multi-constructor or abstract algebraic type - newId rhs_ty `thenSmpl` \ binder -> - returnSmpl (AlgAlts [] (BindDefault (binder,bad_occ_info) (Var binder))) +-------------------------------------------------- +-- 2. Merge nested cases +-------------------------------------------------- + +mkAlts scrut outer_bndr outer_alts + = getDOptsSmpl `thenSmpl` \dflags -> + mkAlts' dflags scrut outer_bndr outer_alts where - bad_occ_info = ManyOcc 0 -- Non-committal! + 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 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} + + +================================================================================= + +mkCase1 tries these things + +1. Eliminate the case altogether if possible + +2. Case-identity: + + case e of ===> e + True -> True; + False -> False + + 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} -simplIdWantsToBeINLINEd :: Id -> SimplEnv -> Bool +-------------------------------------------------- +-- 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) -simplIdWantsToBeINLINEd id env - = if switchIsSet env IgnoreINLINEPragma - then False - else idWantsToBeINLINEd id + 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) + where + identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args + + identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args) + identity_rhs (LitAlt lit) _ = Lit lit + identity_rhs DEFAULT _ = Var case_bndr + + arg_tys = map Type (tyConAppArgs (idType case_bndr)) -type_ok_for_let_to_case :: Type -> Bool + -- 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 -type_ok_for_let_to_case ty - = case (maybeAppDataTyConExpandingDicts ty) of - Nothing -> False - Just (tycon, ty_args, []) -> False - Just (tycon, ty_args, non_null_data_cons) -> True - -- Null data cons => type is abstract + -- 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} + + +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}