%
-% (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 = <wurble> in ...
+-- If <wurble> 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 -> <expr> a ===> <expr>
+%************************************************************************
+%* *
+\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 <expr> 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}
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