\begin{code}
module SimplUtils (
- simplBinder, simplBinders, simplIds,
- transformRhs,
- mkCase, findAlt, findDefault,
+ simplBinder, simplBinders, simplRecBndrs,
+ simplLetBndr, simplLamBndrs,
+ newId, mkLam, mkCase,
-- The continuation type
- SimplCont(..), DupFlag(..), contIsDupable, contResultType,
- pushArgs, discardCont, countValArgs, countArgs,
- analyseCont, discardInline
+ SimplCont(..), DupFlag(..), LetRhsFlag(..),
+ contIsDupable, contResultType,
+ countValArgs, countArgs, pushContArgs,
+ mkBoringStop, mkStop, contIsRhs, contIsRhsOrArg,
+ getContArgs, interestingCallContext, interestingArg, isStrictType
) where
#include "HsVersions.h"
-import BinderInfo
-import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge )
+import CmdLineOpts ( SimplifierSwitch(..),
+ opt_SimplDoLambdaEtaExpansion, opt_SimplDoEtaReduction,
+ opt_SimplCaseMerge, opt_UF_UpdateInPlace
+ )
import CoreSyn
-import CoreUnfold ( isValueUnfolding )
-import CoreFVs ( exprFreeVars )
-import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap, exprEtaExpandArity, bindNonRec )
-import Subst ( InScopeSet, mkSubst, substBndrs, substBndr, substIds, lookupIdSubst )
-import Id ( Id, idType, isId, idName,
- idOccInfo, idUnfolding,
- idDemandInfo, mkId, idInfo
+import CoreUtils ( cheapEqExpr, exprType,
+ etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce,
+ findDefault, exprOkForSpeculation, exprIsValue
+ )
+import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr )
+import Id ( Id, idType, idInfo,
+ mkSysLocal, isDeadBinder, idNewDemandInfo,
+ idUnfolding, idNewStrictness
)
-import IdInfo ( arityLowerBound, setOccInfo, vanillaIdInfo )
-import Maybes ( maybeToBool, catMaybes )
-import Name ( isLocalName, setNameUnique )
+import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
import SimplMonad
-import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType,
- splitTyConApp_maybe, splitAlgTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys
+import Type ( Type, seqType,
+ splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
+ splitRepFunTys, isStrictType
)
-import DataCon ( dataConRepArity )
-import TysPrim ( statePrimTyCon )
-import Var ( setVarUnique )
-import VarSet
-import VarEnv ( SubstEnv, SubstResult(..) )
-import UniqSupply ( splitUniqSupply, uniqFromSupply )
-import Util ( zipWithEqual, mapAccumL )
+import OccName ( UserFS )
+import TyCon ( tyConDataConsIfAvailable, isAlgTyCon, isNewTyCon )
+import DataCon ( dataConRepArity, dataConSig, dataConArgTys )
+import Var ( mkSysTyVar, tyVarKind )
+import Util ( lengthExceeds, mapAccumL )
import Outputable
\end{code}
\begin{code}
data SimplCont -- Strict contexts
- = Stop OutType -- Type of the result
+ = Stop OutType -- Type of the result
+ LetRhsFlag
+ Bool -- True <=> This is the RHS of a thunk whose type suggests
+ -- that update-in-place would be possible
+ -- (This makes the inliner a little keener.)
| CoerceIt OutType -- The To-type, simplified
SimplCont
SimplCont -- keen to inline itelf
| ApplyTo DupFlag
- InExpr SubstEnv -- The argument, as yet unsimplified,
- SimplCont -- and its subst-env
+ InExpr SimplEnv -- The argument, as yet unsimplified,
+ SimplCont -- and its environment
| Select DupFlag
- InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
+ InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
SimplCont
- | ArgOf DupFlag -- An arbitrary strict context: the argument
+ | ArgOf LetRhsFlag -- An arbitrary strict context: the argument
-- of a strict function, or a primitive-arg fn
-- or a PrimOp
- OutType -- The type of the expression being sought by the context
+ -- No DupFlag because we never duplicate it
+ OutType -- arg_ty: type of the argument itself
+ OutType -- cont_ty: the type of the expression being sought by the context
-- f (error "foo") ==> coerce t (error "foo")
-- when f is strict
-- We need to know the type t, to which to coerce.
- (OutExpr -> SimplM OutExprStuff) -- What to do with the result
+
+ (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
+ -- The result expression in the OutExprStuff has type cont_ty
+
+data LetRhsFlag = AnArg -- It's just an argument not a let RHS
+ | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
+
+instance Outputable LetRhsFlag where
+ ppr AnArg = ptext SLIT("arg")
+ ppr AnRhs = ptext SLIT("rhs")
instance Outputable SimplCont where
- ppr (Stop _) = ptext SLIT("Stop")
+ ppr (Stop _ is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs)
ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
- ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
+ ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...")
ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
(nest 4 (ppr alts)) $$ ppr cont
ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
ppr OkToDup = ptext SLIT("ok")
ppr NoDup = ptext SLIT("nodup")
+
+-------------------
+mkBoringStop :: OutType -> SimplCont
+mkBoringStop ty = Stop ty AnArg (canUpdateInPlace ty)
+
+mkStop :: OutType -> LetRhsFlag -> SimplCont
+mkStop ty is_rhs = Stop ty is_rhs (canUpdateInPlace ty)
+
+contIsRhs :: SimplCont -> Bool
+contIsRhs (Stop _ AnRhs _) = True
+contIsRhs (ArgOf AnRhs _ _ _) = True
+contIsRhs other = False
+
+contIsRhsOrArg (Stop _ _ _) = True
+contIsRhsOrArg (ArgOf _ _ _ _) = True
+contIsRhsOrArg other = False
+
+-------------------
contIsDupable :: SimplCont -> Bool
-contIsDupable (Stop _) = True
+contIsDupable (Stop _ _ _) = True
contIsDupable (ApplyTo OkToDup _ _ _) = True
-contIsDupable (ArgOf OkToDup _ _) = True
contIsDupable (Select OkToDup _ _ _ _) = True
contIsDupable (CoerceIt _ cont) = contIsDupable cont
contIsDupable (InlinePlease cont) = contIsDupable cont
contIsDupable other = False
-pushArgs :: SubstEnv -> [InExpr] -> SimplCont -> SimplCont
-pushArgs se [] cont = cont
-pushArgs se (arg:args) cont = ApplyTo NoDup arg se (pushArgs se args cont)
+-------------------
+discardableCont :: SimplCont -> Bool
+discardableCont (Stop _ _ _) = False
+discardableCont (CoerceIt _ cont) = discardableCont cont
+discardableCont (InlinePlease cont) = discardableCont cont
+discardableCont other = True
discardCont :: SimplCont -- A continuation, expecting
-> SimplCont -- Replace the continuation with a suitable coerce
-discardCont (Stop to_ty) = Stop to_ty
-discardCont cont = CoerceIt to_ty (Stop to_ty)
- where
- to_ty = contResultType cont
+discardCont cont = case cont of
+ Stop to_ty is_rhs _ -> cont
+ other -> CoerceIt to_ty (mkBoringStop to_ty)
+ where
+ to_ty = contResultType cont
+-------------------
contResultType :: SimplCont -> OutType
-contResultType (Stop to_ty) = to_ty
-contResultType (ArgOf _ to_ty _) = to_ty
+contResultType (Stop to_ty _ _) = to_ty
+contResultType (ArgOf _ _ to_ty _) = to_ty
contResultType (ApplyTo _ _ _ cont) = contResultType cont
contResultType (CoerceIt _ cont) = contResultType cont
contResultType (InlinePlease cont) = contResultType cont
contResultType (Select _ _ _ _ cont) = contResultType cont
+-------------------
countValArgs :: SimplCont -> Int
countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
countArgs :: SimplCont -> Int
countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
countArgs other = 0
+
+-------------------
+pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont
+-- Pushes args with the specified environment
+pushContArgs env [] cont = cont
+pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont)
\end{code}
-Comment about analyseCont
-~~~~~~~~~~~~~~~~~~~~~~~~~
+\begin{code}
+getContArgs :: SwitchChecker
+ -> OutId -> SimplCont
+ -> ([(InExpr, SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
+ SimplCont, -- Remaining continuation
+ Bool) -- Whether we came across an InlineCall
+-- getContArgs id k = (args, k', inl)
+-- args are the leading ApplyTo items in k
+-- (i.e. outermost comes first)
+-- augmented with demand info from the functionn
+getContArgs chkr fun orig_cont
+ = let
+ -- Ignore strictness info if the no-case-of-case
+ -- flag is on. Strictness changes evaluation order
+ -- and that can change full laziness
+ stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
+ | otherwise = computed_stricts
+ in
+ go [] stricts False orig_cont
+ where
+ ----------------------------
+
+ -- Type argument
+ go acc ss inl (ApplyTo _ arg@(Type _) se cont)
+ = go ((arg,se,False) : acc) ss inl cont
+ -- NB: don't bother to instantiate the function type
+
+ -- Value argument
+ go acc (s:ss) inl (ApplyTo _ arg se cont)
+ = go ((arg,se,s) : acc) ss inl cont
+
+ -- An Inline continuation
+ go acc ss inl (InlinePlease cont)
+ = go acc ss True cont
+
+ -- We're run out of arguments, or else we've run out of demands
+ -- The latter only happens if the result is guaranteed bottom
+ -- This is the case for
+ -- * case (error "hello") of { ... }
+ -- * (error "Hello") arg
+ -- * f (error "Hello") where f is strict
+ -- etc
+ 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, _) = splitRepFunTys (idType fun)
+ arg_stricts = map isStrictType val_arg_tys ++ repeat False
+ -- These argument types are used as a cheap and cheerful way to find
+ -- unboxed arguments, which must be strict. But it's an InType
+ -- and so there might be a type variable where we expect a function
+ -- type (the substitution hasn't happened yet). And we don't bother
+ -- doing the type applications for a polymorphic function.
+ -- Hence the split*Rep*FunTys
+
+ ----------------------------
+ -- 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
+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
default case.
\begin{code}
-analyseCont :: InScopeSet -> SimplCont
- -> ([Bool], -- Arg-info flags; one for each value argument
- Bool, -- Context of the result of the call is interesting
- Bool) -- There was an InlinePlease
-
-analyseCont in_scope cont
- = case cont of
+interestingCallContext :: Bool -- False <=> no args at all
+ -> Bool -- False <=> no value args
+ -> SimplCont -> Bool
-- The "lone-variable" case is important. I spent ages
-- messing about with unsatisfactory varaints, but this is nice.
-- The idea is that if a variable appear all alone
-- as scrutinee of a case Select
-- as arg of a strict fn ArgOf
-- then we should not inline it (unless there is some other reason,
- -- e.g. is is the sole occurrence).
- -- Why not? At least in the case-scrutinee situation, turning
- -- case x of y -> ...
+ -- e.g. is is the sole occurrence). We achieve this by making
+ -- interestingCallContext return False for a lone variable.
+ --
+ -- Why? At least in the case-scrutinee situation, turning
+ -- let x = (a,b) in case x of y -> ...
-- into
- -- let y = (a,b) in ...
+ -- let x = (a,b) in case (a,b) of y -> ...
+ -- and thence to
+ -- let x = (a,b) in let y = (a,b) in ...
-- is bad if the binding for x will remain.
--
-- Another example: I discovered that strings
-- s = "foo"
-- f = \x -> ...(error s)...
- -- Fundamentally such contexts should not ecourage inlining becuase
+ -- Fundamentally such contexts should not ecourage inlining because
-- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
-- so there's no gain.
--
- -- However, even a type application isn't a lone variable. Consider
+ -- However, even a type application or coercion isn't a lone variable.
+ -- Consider
-- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
-- We had better inline that sucker! The case won't see through it.
-
- (Stop _) -> boring_result -- Don't inline a lone variable
- (Select _ _ _ _ _) -> boring_result -- Ditto
- (ArgOf _ _ _) -> boring_result -- Ditto
- (ApplyTo _ (Type _) _ cont) -> analyse_ty_app cont
- other -> analyse_app cont
+ --
+ -- For now, I'm treating treating a variable applied to types
+ -- in a *lazy* context "lone". The motivating example was
+ -- f = /\a. \x. BIG
+ -- g = /\a. \y. h (f a)
+ -- There's no advantage in inlining f here, and perhaps
+ -- a significant disadvantage. Hence some_val_args in the Stop case
+
+interestingCallContext some_args some_val_args cont
+ = interesting cont
where
- boring_result = ([], False, False)
-
- -- For now, I'm treating not treating a variable applied to types as
- -- "lone". The motivating example was
- -- f = /\a. \x. BIG
- -- g = /\a. \y. h (f a)
- -- There's no advantage in inlining f here, and perhaps
- -- a significant disadvantage.
- analyse_ty_app (Stop _) = boring_result
- analyse_ty_app (ArgOf _ _ _) = boring_result
- analyse_ty_app (Select _ _ _ _ _) = ([], True, False) -- See the $fMonadST example above
- analyse_ty_app (ApplyTo _ (Type _) _ cont) = analyse_ty_app cont
- analyse_ty_app cont = analyse_app cont
-
- analyse_app (InlinePlease cont)
- = case analyse_app cont of
- (infos, icont, inline) -> (infos, icont, True)
-
- analyse_app (ApplyTo _ arg subst cont)
- | isValArg arg = case analyse_app cont of
- (infos, icont, inline) -> (analyse_arg subst arg : infos, icont, inline)
- | otherwise = analyse_app cont
-
- analyse_app cont = ([], interesting_call_context cont, False)
-
- -- An argument is interesting if it has *some* structure
- -- We are here trying to avoid unfolding a function that
- -- is applied only to variables that have no unfolding
- -- (i.e. they are probably lambda bound): f x y z
- -- There is little point in inlining f here.
- analyse_arg :: SubstEnv -> InExpr -> Bool
- analyse_arg subst (Var v) = case lookupIdSubst (mkSubst in_scope subst) v of
- DoneId v' _ -> isValueUnfolding (idUnfolding v')
- other -> False
- analyse_arg subst (Type _) = False
- analyse_arg subst (App fn (Type _)) = analyse_arg subst fn
- analyse_arg subst (Note _ a) = analyse_arg subst a
- analyse_arg subst other = True
-
- interesting_call_context (Stop ty) = canUpdateInPlace ty
- interesting_call_context (InlinePlease _) = True
- interesting_call_context (Select _ _ _ _ _) = True
- interesting_call_context (CoerceIt _ cont) = interesting_call_context cont
- interesting_call_context (ApplyTo _ (Type _) _ cont) = interesting_call_context cont
- interesting_call_context (ApplyTo _ _ _ _) = True
- interesting_call_context (ArgOf _ _ _) = True
+ interesting (InlinePlease _) = True
+ interesting (Select _ _ _ _ _) = some_args
+ interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
+ -- Perhaps True is a bit over-keen, but I've
+ -- seen (coerce f) x, where f has an INLINE prag,
+ -- So we have to give some motivaiton for inlining it
+ interesting (ArgOf _ _ _ _) = some_val_args
+ interesting (Stop ty _ upd_in_place) = some_val_args && upd_in_place
+ interesting (CoerceIt _ cont) = interesting cont
-- If this call is the arg of a strict function, the context
-- is a bit interesting. If we inline here, we may get useful
-- evaluation information to avoid repeated evals: e.g.
-- the context for (f x) is not totally uninteresting.
-discardInline :: SimplCont -> SimplCont
-discardInline (InlinePlease cont) = cont
-discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
-discardInline cont = cont
-
+-------------------
+canUpdateInPlace :: Type -> Bool
-- Consider let x = <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
-- small arity. But arity zero isn't good -- we share the single copy
-- for that case, so no point in sharing.
-canUpdateInPlace ty = case splitAlgTyConApp_maybe ty of
- Just (_, _, [dc]) -> arity == 1 || arity == 2
- where
- arity = dataConRepArity dc
- other -> False
+canUpdateInPlace ty
+ | not opt_UF_UpdateInPlace = False
+ | otherwise
+ = case splitTyConApp_maybe ty of
+ Nothing -> False
+ Just (tycon, _) -> case tyConDataConsIfAvailable tycon of
+ [dc] -> arity == 1 || arity == 2
+ where
+ arity = dataConRepArity dc
+ other -> False
\end{code}
%* *
%************************************************************************
+These functions are in the monad only so that they can be made strict via seq.
+
\begin{code}
-simplBinders :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
-simplBinders bndrs thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', bndrs') = substBndrs subst bndrs
+simplBinders :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
+simplBinders env bndrs
+ = let
+ (subst', bndrs') = Subst.simplBndrs (getSubst env) bndrs
in
seqBndrs bndrs' `seq`
- setSubst subst' (thing_inside bndrs')
+ returnSmpl (setSubst env subst', bndrs')
-simplBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
-simplBinder bndr thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', bndr') = substBndr subst bndr
+simplBinder :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder)
+simplBinder env bndr
+ = let
+ (subst', bndr') = Subst.simplBndr (getSubst env) bndr
in
seqBndr bndr' `seq`
- setSubst subst' (thing_inside bndr')
+ returnSmpl (setSubst env subst', bndr')
--- Same semantics as simplBinders, but a little less
--- plumbing and hence a little more efficient.
--- Maybe not worth the candle?
-simplIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
-simplIds ids thing_inside
- = getSubst `thenSmpl` \ subst ->
- let
- (subst', bndrs') = substIds subst ids
+simplLetBndr :: SimplEnv -> InBinder -> SimplM (SimplEnv, OutBinder)
+simplLetBndr env id
+ = let
+ (subst', id') = Subst.simplLetId (getSubst env) id
+ in
+ seqBndr id' `seq`
+ returnSmpl (setSubst env subst', id')
+
+simplLamBndrs, simplRecBndrs
+ :: SimplEnv -> [InBinder] -> SimplM (SimplEnv, [OutBinder])
+simplRecBndrs = simplBndrs Subst.simplLetId
+simplLamBndrs = simplBndrs Subst.simplLamBndr
+
+simplBndrs simpl_bndr env bndrs
+ = let
+ (subst', bndrs') = mapAccumL simpl_bndr (getSubst env) bndrs
in
seqBndrs bndrs' `seq`
- setSubst subst' (thing_inside bndrs')
+ returnSmpl (setSubst env subst', bndrs')
seqBndrs [] = ()
seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
\end{code}
+\begin{code}
+newId :: UserFS -> Type -> SimplM Id
+newId fs ty = getUniqueSmpl `thenSmpl` \ uniq ->
+ returnSmpl (mkSysLocal fs uniq ty)
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection{Rebuilding a lambda}
+%* *
+%************************************************************************
+
+\begin{code}
+mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
+\end{code}
+
+Try three things
+ a) eta reduction, if that gives a trivial expression
+ b) eta expansion [only if there are some value lambdas]
+ c) floating lets out through big lambdas
+ [only if all tyvar lambdas, and only if this lambda
+ is the RHS of a let]
+
+\begin{code}
+mkLam env bndrs body cont
+ | opt_SimplDoEtaReduction,
+ Just etad_lam <- tryEtaReduce bndrs body
+ = tick (EtaReduction (head bndrs)) `thenSmpl_`
+ returnSmpl (emptyFloats env, etad_lam)
+
+ | opt_SimplDoLambdaEtaExpansion,
+ any isRuntimeVar bndrs
+ = tryEtaExpansion body `thenSmpl` \ body' ->
+ returnSmpl (emptyFloats env, mkLams bndrs body')
+
+{- Sept 01: I'm experimenting with getting the
+ full laziness pass to float out past big lambdsa
+ | all isTyVar bndrs, -- Only for big lambdas
+ contIsRhs cont -- Only try the rhs type-lambda floating
+ -- if this is indeed a right-hand side; otherwise
+ -- we end up floating the thing out, only for float-in
+ -- to float it right back in again!
+ = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
+ returnSmpl (floats, mkLams bndrs body')
+-}
+
+ | otherwise
+ = returnSmpl (emptyFloats env, mkLams bndrs body)
+\end{code}
+
+
%************************************************************************
%* *
-\subsection{Transform a RHS}
+\subsection{Eta expansion and reduction}
%* *
%************************************************************************
-Try (a) eta expansion
- (b) type-lambda swizzling
+We try for eta reduction here, but *only* if we get all the
+way to an exprIsTrivial expression.
+We don't want to remove extra lambdas unless we are going
+to avoid allocating this thing altogether
\begin{code}
-transformRhs :: InExpr -> SimplM InExpr
-transformRhs rhs
- = tryEtaExpansion body `thenSmpl` \ body' ->
- mkRhsTyLam tyvars body'
+tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr
+tryEtaReduce bndrs body
+ -- We don't use CoreUtils.etaReduce, because we can be more
+ -- efficient here:
+ -- (a) we already have the binders
+ -- (b) we can do the triviality test before computing the free vars
+ -- [in fact I take the simple path and look for just a variable]
+ = go (reverse bndrs) body
where
- (tyvars, body) = collectTyBinders rhs
+ go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
+ go [] (Var fun) | ok_fun fun = Just (Var fun) -- Success!
+ go _ _ = Nothing -- Failure!
+
+ ok_fun fun = not (fun `elem` bndrs) &&
+ isEvaldUnfolding (idUnfolding fun)
+ -- 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.
+ ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
+\end{code}
+
+
+ Try eta expansion for RHSs
+
+We go for:
+ f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
+ (n >= 0)
+
+where (in both cases)
+
+ * The xi can include type variables
+
+ * The yi are all value variables
+
+ * N is a NORMAL FORM (i.e. no redexes anywhere)
+ wanting a suitable number of extra args.
+
+We may have to sandwich some coerces between the lambdas
+to make the types work. exprEtaExpandArity looks through coerces
+when computing arity; and etaExpand adds the coerces as necessary when
+actually computing the expansion.
+
+\begin{code}
+tryEtaExpansion :: OutExpr -> SimplM OutExpr
+-- There is at least one runtime binder in the binders
+tryEtaExpansion body
+ = getUniquesSmpl `thenSmpl` \ us ->
+ returnSmpl (etaExpand fun_arity us body (exprType body))
+ where
+ fun_arity = exprEtaExpandArity body
\end{code}
%************************************************************************
%* *
-\subsection{Local tyvar-lifting}
+\subsection{Floating lets out of big lambdas}
%* *
%************************************************************************
-mkRhsTyLam tries this transformation, when the big lambda appears as
+tryRhsTyLam tries this transformation, when the big lambda appears as
the RHS of a let(rec) binding:
/\abc -> let(rec) x = e in b
This optimisation is CRUCIAL in eliminating the junk introduced by
desugaring mutually recursive definitions. Don't eliminate it lightly!
-So far as the implemtation is concerned:
+So far as the implementation is concerned:
Invariant: go F e = /\tvs -> F e
\begin{code}
-mkRhsTyLam tyvars body -- Only does something if there's a let
- | null tyvars || not (worth_it body) -- inside a type lambda, and a WHNF inside that
- = returnSmpl (mkLams tyvars body)
+{- Trying to do this in full laziness
+
+tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
+-- Call ensures that all the binders are type variables
+
+tryRhsTyLam env tyvars body -- Only does something if there's a let
+ | not (all isTyVar tyvars)
+ || not (worth_it body) -- inside a type lambda,
+ = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
+
| otherwise
- = go (\x -> x) body
+ = go env (\x -> x) body
+
where
- worth_it (Let _ e) = whnf_in_middle e
- worth_it other = False
+ worth_it e@(Let _ _) = whnf_in_middle e
+ worth_it e = False
+
+ whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
whnf_in_middle (Let _ e) = whnf_in_middle e
whnf_in_middle e = exprIsCheap e
main_tyvar_set = mkVarSet tyvars
- go fn (Let bind@(NonRec var rhs) body) | exprIsTrivial rhs
- = go (fn . Let bind) body
+ go env fn (Let bind@(NonRec var rhs) body)
+ | exprIsTrivial rhs
+ = go env (fn . Let bind) body
+
+ go env fn (Let (NonRec var rhs) body)
+ = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
+ addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
+ go env (fn . Let (mk_silly_bind var rhs')) body
- go fn (Let bind@(NonRec var rhs) body)
- = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
- go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ body' ->
- returnSmpl (Let (NonRec var' (mkLams tyvars_here (fn rhs))) body')
where
- tyvars_here = tyvars
- -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
- -- tyvars_here was an attempt to reduce the number of tyvars
+
+ tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
+ -- Abstract only over the type variables free in the rhs
-- wrt which the new binding is abstracted. But the naive
-- approach of abstract wrt the tyvars free in the Id's type
-- fails. Consider:
-- abstracting wrt *all* the tyvars. We'll see if that
-- gives rise to problems. SLPJ June 98
- var_ty = idType var
-
- go fn (Let (Rec prs) body)
+ go env fn (Let (Rec prs) body)
= mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
let
- gn body = fn $ foldr Let body (zipWith mk_silly_bind vars rhss')
+ gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
+ pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
in
- go gn body `thenSmpl` \ body' ->
- returnSmpl (Let (Rec (vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss])) body')
+ addAuxiliaryBind env (Rec pairs) $ \ env ->
+ go env gn body
where
(vars,rhss) = unzip prs
- tyvars_here = tyvars
- -- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
+ tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
-- See notes with tyvars_here above
- var_tys = map idType vars
-
- go fn body = returnSmpl (mkLams tyvars (fn body))
+ go env fn body = returnSmpl (emptyFloats env, fn body)
mk_poly tyvars_here var
= getUniqueSmpl `thenSmpl` \ uniq ->
let
poly_name = setNameUnique (idName var) uniq -- Keep same name
poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
+ poly_id = mkLocalId poly_name poly_ty
- -- It's crucial to copy the occInfo of the original var, because
- -- we're looking at occurrence-analysed but as yet unsimplified code!
- -- In particular, we mustn't lose the loop breakers.
+ -- In the olden days, it was crucial to copy the occInfo of the original var,
+ -- because we were looking at occurrence-analysed but as yet unsimplified code!
+ -- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
+ -- at already simplified code, so it doesn't matter
--
-- It's even right to retain single-occurrence or dead-var info:
-- Suppose we started with /\a -> let x = E in B
- -- where x occurs once in E. Then we transform to:
+ -- where x occurs once in B. Then we transform to:
-- let x' = /\a -> E in /\a -> let x* = x' a in B
-- where x* has an INLINE prag on it. Now, once x* is inlined,
- -- the occurrences of x' will be just the occurrences originaly
+ -- the occurrences of x' will be just the occurrences originally
-- pinned on x.
- poly_info = vanillaIdInfo `setOccInfo` idOccInfo var
-
- poly_id = mkId poly_name poly_ty poly_info
in
returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
- mk_silly_bind var rhs = NonRec var rhs
- -- The Inline note is really important! If we don't say
- -- INLINE on these silly little bindings then look what happens!
+ mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
-- Suppose we start with:
--
- -- x = let g = /\a -> \x -> f x x
- -- in
- -- /\ b -> let g* = g b in E
+ -- x = /\ a -> let g = G in E
+ --
+ -- Then we'll float to get
+ --
+ -- x = let poly_g = /\ a -> G
+ -- in /\ a -> let g = poly_g a in E
--
- -- Then: * the binding for g gets floated out
- -- * but then it gets inlined into the rhs of g*
- -- * then the binding for g* is floated out of the /\b
- -- * so we're back to square one
- -- The silly binding for g* must be INLINEd, so that
- -- we simply substitute for g* throughout.
+ -- But now the occurrence analyser will see just one occurrence
+ -- of poly_g, not inside a lambda, so the simplifier will
+ -- PreInlineUnconditionally poly_g back into g! Badk to square 1!
+ -- (I used to think that the "don't inline lone occurrences" stuff
+ -- would stop this happening, but since it's the *only* occurrence,
+ -- PreInlineUnconditionally kicks in first!)
+ --
+ -- Solution: put an INLINE note on g's RHS, so that poly_g seems
+ -- to appear many times. (NB: mkInlineMe eliminates
+ -- such notes on trivial RHSs, so do it manually.)
+-}
\end{code}
%************************************************************************
%* *
-\subsection{Eta expansion}
+\subsection{Case absorption and identity-case elimination}
%* *
%************************************************************************
- Try eta expansion for RHSs
+mkCase puts a case expression back together, trying various transformations first.
-We go for:
- \x1..xn -> N ==> \x1..xn y1..ym -> N y1..ym
- AND
- N E1..En ==> let z1=E1 .. zn=En in \y1..ym -> N z1..zn y1..ym
+\begin{code}
+mkCase :: OutExpr -> [AltCon] -> OutId -> [OutAlt] -> SimplM OutExpr
-where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere)
-wanting a suitable number of extra args.
+mkCase scrut handled_cons case_bndr alts
+ = mkAlts scrut handled_cons case_bndr alts `thenSmpl` \ better_alts ->
+ mkCase1 scrut case_bndr better_alts
+\end{code}
+
+
+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. If the DEFAULT alternative can match only one possible constructor,
+ then make that constructor explicit.
+ e.g.
+ case e of x { DEFAULT -> rhs }
+ ===>
+ case e of x { (a,b) -> rhs }
+ where the type is a single constructor type. This gives better code
+ when rhs also scrutinises x or e.
+
+3. Case merging:
+ case e of b { ==> case e of b {
+ p1 -> rhs1 p1 -> rhs1
+ ... ...
+ pm -> rhsm pm -> rhsm
+ _ -> case b of b' { pn -> let b'=b in rhsn
+ pn -> rhsn ...
+ ... po -> let b'=b in rhso
+ po -> rhso _ -> let b'=b in rhsd
+ _ -> rhsd
+ }
+
+ which merges two cases in one case when -- the default alternative of
+ the outer case scrutises the same variable as the outer case This
+ transformation is called Case Merging. It avoids that the same
+ variable is scrutinised multiple times.
+
+
+The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
+
+ x | p `is` 1 -> e1
+ | p `is` 2 -> e2
+ ...etc...
+
+where @is@ was something like
+
+ p `is` n = p /= (-1) && p == n
+
+This gave rise to a horrible sequence of cases
+
+ case p of
+ (-1) -> $j p
+ 1 -> e1
+ DEFAULT -> $j p
+
+and similarly in cascade for all the join points!
-NB: the Ei may have unlifted type, but the simplifier (which is applied
-to the result) deals OK with this.
-There is no point in looking for a combination of the two,
-because that would leave use with some lets sandwiched between lambdas;
-that's what the final test in the first equation is for.
\begin{code}
-tryEtaExpansion :: InExpr -> SimplM InExpr
-tryEtaExpansion rhs
- | not opt_SimplDoLambdaEtaExpansion
- || exprIsTrivial rhs -- Don't eta-expand a trival RHS
- || null y_tys -- No useful expansion
- || not (null x_bndrs || and trivial_args) -- Not (no x-binders or no z-binds)
- = returnSmpl rhs
-
- | otherwise -- Consider eta expansion
- = newIds y_tys $ ( \ y_bndrs ->
- tick (EtaExpansion (head y_bndrs)) `thenSmpl_`
- mapAndUnzipSmpl bind_z_arg (args `zip` trivial_args) `thenSmpl` (\ (maybe_z_binds, z_args) ->
- returnSmpl (mkLams x_bndrs $
- mkLets (catMaybes maybe_z_binds) $
- mkLams y_bndrs $
- mkApps (mkApps fun z_args) (map Var y_bndrs))))
+--------------------------------------------------
+-- 1. Merge identical branches
+--------------------------------------------------
+mkAlts scrut handled_cons 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
- (x_bndrs, body) = collectValBinders rhs
- (fun, args) = collectArgs body
- trivial_args = map exprIsTrivial args
- fun_arity = exprEtaExpandArity fun
-
- bind_z_arg (arg, trivial_arg)
- | trivial_arg = returnSmpl (Nothing, arg)
- | otherwise = newId (exprType arg) $ \ z ->
- returnSmpl (Just (NonRec z arg), Var z)
-
- -- Note: I used to try to avoid the exprType call by using
- -- the type of the binder. But this type doesn't necessarily
- -- belong to the same substitution environment as this rhs;
- -- and we are going to make extra term binders (y_bndrs) from the type
- -- which will be processed with the rhs substitution environment.
- -- This only went wrong in a mind bendingly complicated case.
- (potential_extra_arg_tys, inner_ty) = splitFunTys (exprType body)
-
- y_tys :: [InType]
- y_tys = take no_extras_wanted potential_extra_arg_tys
-
- no_extras_wanted :: Int
- no_extras_wanted = 0 `max`
-
- -- We used to expand the arity to the previous arity fo the
- -- function; but this is pretty dangerous. Consdier
- -- f = \xy -> e
- -- so that f has arity 2. Now float something into f's RHS:
- -- f = let z = BIG in \xy -> e
- -- The last thing we want to do now is to put some lambdas
- -- outside, to get
- -- f = \xy -> let z = BIG in e
- --
- -- (bndr_arity - no_of_xs) `max`
-
- -- See if the body could obviously do with more args
- (fun_arity - valArgCount args)
-
--- This case is now deal with by exprEtaExpandArity
- -- Finally, see if it's a state transformer, and xs is non-null
- -- (so it's also a function not a thunk) in which
- -- case we eta-expand on principle! This can waste work,
- -- but usually doesn't.
- -- I originally checked for a singleton type [ty] in this case
- -- but then I found a situation in which I had
- -- \ x -> let {..} in \ s -> f (...) s
- -- AND f RETURNED A FUNCTION. That is, 's' wasn't the only
- -- potential extra arg.
--- case (x_bndrs, potential_extra_arg_tys) of
--- (_:_, ty:_) -> case splitTyConApp_maybe ty of
--- Just (tycon,_) | tycon == statePrimTyCon -> 1
--- other -> 0
--- other -> 0
-\end{code}
+ filtered_alts = filter keep con_alts
+ keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
+ better_alts = (DEFAULT, [], rhs1) : filtered_alts
-%************************************************************************
-%* *
-\subsection{Case absorption and identity-case elimination}
-%* *
-%************************************************************************
+--------------------------------------------------
+-- 2. Fill in missing constructor
+--------------------------------------------------
-\begin{code}
-mkCase :: OutExpr -> OutId -> [OutAlt] -> SimplM OutExpr
-\end{code}
+mkAlts scrut handled_cons case_bndr alts
+ | (alts_no_deflt, Just rhs) <- findDefault alts,
+ -- There is a DEFAULT case
-@mkCase@ tries the following transformation (if possible):
-
-case e of b { ==> case e of b {
- p1 -> rhs1 p1 -> rhs1
- ... ...
- pm -> rhsm pm -> rhsm
- _ -> case b of b' { pn -> rhsn[b/b'] {or (alg) let b=b' in rhsn}
- {or (prim) case b of b' { _ -> rhsn}}
- pn -> rhsn ...
- ... po -> rhso[b/b']
- po -> rhso _ -> rhsd[b/b'] {or let b'=b in rhsd}
- _ -> rhsd
-}
-
-which merges two cases in one case when -- the default alternative of
-the outer case scrutises the same variable as the outer case This
-transformation is called Case Merging. It avoids that the same
-variable is scrutinised multiple times.
+ 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!
-\begin{code}
-mkCase scrut outer_bndr outer_alts
- | opt_SimplCaseMerge
- && maybeToBool maybe_case_in_default
-
- = tick (CaseMerge outer_bndr) `thenSmpl_`
- returnSmpl (Case scrut outer_bndr new_alts)
- -- Warning: don't call mkCase recursively!
+ [missing_con] <- [con | con <- tyConDataConsIfAvailable tycon,
+ not (con `elem` handled_data_cons)]
+ -- There is just one missing constructor!
+
+ = tick (FillInCaseDefault case_bndr) `thenSmpl_`
+ getUniquesSmpl `thenSmpl` \ tv_uniqs ->
+ getUniquesSmpl `thenSmpl` \ id_uniqs ->
+ let
+ (_,_,ex_tyvars,_,_,_) = dataConSig missing_con
+ ex_tyvars' = zipWith mk tv_uniqs ex_tyvars
+ mk uniq tv = mkSysTyVar uniq (tyVarKind tv)
+ arg_ids = zipWith (mkSysLocal SLIT("a")) id_uniqs arg_tys
+ arg_tys = dataConArgTys missing_con (inst_tys ++ mkTyVarTys ex_tyvars')
+ better_alts = (DataAlt missing_con, ex_tyvars' ++ arg_ids, rhs) : alts_no_deflt
+ in
+ returnSmpl better_alts
+ where
+ handled_data_cons = [data_con | DataAlt data_con <- handled_cons]
+
+--------------------------------------------------
+-- 3. Merge nested cases
+--------------------------------------------------
+
+mkAlts scrut handled_cons outer_bndr outer_alts
+ | opt_SimplCaseMerge,
+ (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
+ Just (Case (Var scrut_var) inner_bndr inner_alts) <- maybe_outer_deflt,
+ scruting_same_var scrut_var
+
+ = let -- Eliminate any inner alts which are shadowed by the outer ones
+ outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
+
+ munged_inner_alts = [ (con, args, munge_rhs rhs)
+ | (con, args, rhs) <- inner_alts,
+ not (con `elem` outer_cons) -- Eliminate shadowed inner alts
+ ]
+ munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
+
+ (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
+
+ new_alts = add_default maybe_inner_default
+ (outer_alts_without_deflt ++ inner_con_alts)
+ in
+ tick (CaseMerge outer_bndr) `thenSmpl_`
+ returnSmpl new_alts
+ -- Warning: don't call mkAlts recursively!
-- Firstly, there's no point, because inner alts have already had
-- mkCase applied to them, so they won't have a case in their default
- -- Secondly, if you do, you get an infinite loop, because the bindNonRec
- -- in munge_rhs puts a case into the DEFAULT branch!
+ -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
+ -- in munge_rhs may put a case into the DEFAULT branch!
where
- new_alts = outer_alts_without_deflt ++ munged_inner_alts
- maybe_case_in_default = case findDefault outer_alts of
- (outer_alts_without_default,
- Just (Case (Var scrut_var) inner_bndr inner_alts))
-
- | outer_bndr == scrut_var
- -> Just (outer_alts_without_default, inner_bndr, inner_alts)
- other -> Nothing
-
- Just (outer_alts_without_deflt, inner_bndr, inner_alts) = maybe_case_in_default
-
- -- Eliminate any inner alts which are shadowed by the outer ones
- outer_cons = [con | (con,_,_) <- outer_alts_without_deflt]
-
- munged_inner_alts = [ (con, args, munge_rhs rhs)
- | (con, args, rhs) <- inner_alts,
- not (con `elem` outer_cons) -- Eliminate shadowed inner alts
- ]
- munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
+ -- We are scrutinising the same variable if it's
+ -- the outer case-binder, or if the outer case scrutinises a variable
+ -- (and it's the same). Testing both allows us not to replace the
+ -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
+ scruting_same_var = case scrut of
+ Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
+ other -> \ v -> v == outer_bndr
+
+ add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
+ add_default Nothing alts = alts
+
+
+--------------------------------------------------
+-- Catch-all
+--------------------------------------------------
+
+mkAlts scrut handled_cons case_bndr other_alts = returnSmpl other_alts
\end{code}
-Now the identity-case transformation:
+
+
+=================================================================================
+
+mkCase1 tries these things
+
+1. Eliminate the case altogether if possible
+
+2. Case-identity:
case e of ===> e
- True -> True;
+ True -> True;
False -> False
-and similar friends.
+ and similar friends.
+
+
+Start with a simple situation:
+
+ case x# of ===> e[x#/y#]
+ y# -> e
+
+(when x#, y# are of primitive type, of course). We can't (in general)
+do this for algebraic cases, because we might turn bottom into
+non-bottom!
+
+Actually, we generalise this idea to look for a case where we're
+scrutinising a variable, and we know that only the default case can
+match. For example:
+\begin{verbatim}
+ case x of
+ 0# -> ...
+ other -> ...(case x of
+ 0# -> ...
+ other -> ...) ...
+\end{code}
+Here the inner case can be eliminated. This really only shows up in
+eliminating error-checking code.
+
+We also make sure that we deal with this very common case:
+
+ case e of
+ x -> ...x...
+
+Here we are using the case as a strict let; if x is used only once
+then we want to inline it. We have to be careful that this doesn't
+make the program terminate when it would have diverged before, so we
+check that
+ - x is used strictly, or
+ - e is already evaluated (it may so if e is a variable)
+
+Lastly, we generalise the transformation to handle this:
+
+ case e of ===> r
+ True -> r
+ False -> r
+
+We only do this for very cheaply compared r's (constructors, literals
+and variables). If pedantic bottoms is on, we only do it when the
+scrutinee is a PrimOp which can't fail.
+
+We do it *here*, looking at un-simplified alternatives, because we
+have to check that r doesn't mention the variables bound by the
+pattern in each alternative, so the binder-info is rather useful.
+
+So the case-elimination algorithm is:
+
+ 1. Eliminate alternatives which can't match
+
+ 2. Check whether all the remaining alternatives
+ (a) do not mention in their rhs any of the variables bound in their pattern
+ and (b) have equal rhss
+
+ 3. Check we can safely ditch the case:
+ * PedanticBottoms is off,
+ or * the scrutinee is an already-evaluated variable
+ or * the scrutinee is a primop which is ok for speculation
+ -- ie we want to preserve divide-by-zero errors, and
+ -- calls to error itself!
+
+ or * [Prim cases] the scrutinee is a primitive variable
+
+ or * [Alg cases] the scrutinee is a variable and
+ either * the rhs is the same variable
+ (eg case x of C a b -> x ===> x)
+ or * there is only one alternative, the default alternative,
+ and the binder is used strictly in its scope.
+ [NB this is helped by the "use default binder where
+ possible" transformation; see below.]
+
+
+If so, then we can replace the case with one of the rhss.
+
\begin{code}
-mkCase scrut case_bndr alts
+--------------------------------------------------
+-- 1. Eliminate the case altogether if poss
+--------------------------------------------------
+
+mkCase1 scrut case_bndr [(con,bndrs,rhs)]
+ -- See if we can get rid of the case altogether
+ -- See the extensive notes on case-elimination above
+ -- mkCase made sure that if all the alternatives are equal,
+ -- then there is now only one (DEFAULT) rhs
+ | all isDeadBinder bndrs,
+
+ -- Check that the scrutinee can be let-bound instead of case-bound
+ exprOkForSpeculation scrut
+ -- OK not to evaluate it
+ -- This includes things like (==# a# b#)::Bool
+ -- so that we simplify
+ -- case ==# a# b# of { True -> x; False -> x }
+ -- to just
+ -- x
+ -- This particular example shows up in default methods for
+ -- comparision operations (e.g. in (>=) for Int.Int32)
+ || exprIsValue scrut -- It's already evaluated
+ || var_demanded_later scrut -- It'll be demanded later
+
+-- || not opt_SimplPedanticBottoms) -- Or we don't care!
+-- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
+-- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
+-- its argument: case x of { y -> dataToTag# y }
+-- Here we must *not* discard the case, because dataToTag# just fetches the tag from
+-- the info pointer. So we'll be pedantic all the time, and see if that gives any
+-- other problems
+ = tick (CaseElim case_bndr) `thenSmpl_`
+ returnSmpl (bindCaseBndr case_bndr scrut rhs)
+
+ where
+ -- The case binder is going to be evaluated later,
+ -- and the scrutinee is a simple variable
+ var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
+ var_demanded_later other = False
+
+
+--------------------------------------------------
+-- 2. Identity case
+--------------------------------------------------
+
+mkCase1 scrut case_bndr alts -- Identity case
| all identity_alt alts
= tick (CaseIdentity case_bndr) `thenSmpl_`
- returnSmpl scrut
+ returnSmpl (re_note scrut)
where
- identity_alt (DEFAULT, [], Var v) = v == case_bndr
- identity_alt (DataAlt con, args, rhs) = cheapEqExpr rhs
- (mkConApp con (map Type arg_tys ++ map varToCoreExpr args))
- identity_alt other = False
+ identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
- arg_tys = case splitTyConApp_maybe (idType case_bndr) of
- Just (tycon, arg_tys) -> arg_tys
-\end{code}
+ identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
+ identity_rhs (LitAlt lit) _ = Lit lit
+ identity_rhs DEFAULT _ = Var case_bndr
-The catch-all case
+ arg_tys = map Type (tyConAppArgs (idType case_bndr))
-\begin{code}
-mkCase other_scrut case_bndr other_alts
- = returnSmpl (Case other_scrut case_bndr other_alts)
+ -- We've seen this:
+ -- case coerce T e of x { _ -> coerce T' x }
+ -- And we definitely want to eliminate this case!
+ -- So we throw away notes from the RHS, and reconstruct
+ -- (at least an approximation) at the other end
+ de_note (Note _ e) = de_note e
+ de_note e = e
+
+ -- re_note wraps a coerce if it might be necessary
+ re_note scrut = case head alts of
+ (_,_,rhs1@(Note _ _)) -> mkCoerce (exprType rhs1) (idType case_bndr) scrut
+ other -> scrut
+
+
+--------------------------------------------------
+-- Catch-all
+--------------------------------------------------
+mkCase1 scrut bndr alts = returnSmpl (Case scrut bndr alts)
\end{code}
-\begin{code}
-findDefault :: [CoreAlt] -> ([CoreAlt], Maybe CoreExpr)
-findDefault [] = ([], Nothing)
-findDefault ((DEFAULT,args,rhs) : alts) = ASSERT( null alts && null args )
- ([], Just rhs)
-findDefault (alt : alts) = case findDefault alts of
- (alts', deflt) -> (alt : alts', deflt)
-
-findAlt :: AltCon -> [CoreAlt] -> CoreAlt
-findAlt con alts
- = go alts
- where
- go [] = pprPanic "Missing alternative" (ppr con $$ vcat (map ppr alts))
- go (alt : alts) | matches alt = alt
- | otherwise = go alts
+When adding auxiliary bindings for the case binder, it's worth checking if
+its dead, because it often is, and occasionally these mkCase transformations
+cascade rather nicely.
- matches (DEFAULT, _, _) = True
- matches (con1, _, _) = con == con1
+\begin{code}
+bindCaseBndr bndr rhs body
+ | isDeadBinder bndr = body
+ | otherwise = bindNonRec bndr rhs body
\end{code}
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