\begin{code}
module SimplUtils (
- simplBinder, simplBinders, simplIds,
- transformRhs,
- etaCoreExpr,
- mkCase, findAlt, findDefault,
- mkCoerce
+ simplBinder, simplBinders, simplRecIds, simplLetId, simplLamBinder,
+ tryRhsTyLam, tryEtaExpansion,
+ mkCase,
+
+ -- The continuation type
+ SimplCont(..), DupFlag(..), contIsDupable, contResultType,
+ countValArgs, countArgs, mkRhsStop, mkStop,
+ getContArgs, interestingCallContext, interestingArg, isStrictType, discardInline
+
) where
#include "HsVersions.h"
-import BinderInfo
-import CmdLineOpts ( opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge )
+import CmdLineOpts ( switchIsOn, SimplifierSwitch(..),
+ opt_SimplDoLambdaEtaExpansion, opt_SimplCaseMerge,
+ opt_UF_UpdateInPlace
+ )
import CoreSyn
-import CoreFVs ( exprFreeVars )
-import CoreUtils ( exprIsTrivial, cheapEqExpr, coreExprType, exprIsCheap, exprGenerousArity )
-import Subst ( substBndrs, substBndr, substIds )
-import Id ( Id, idType, getIdArity, isId, idName,
- getInlinePragma, setInlinePragma,
- getIdDemandInfo, mkId, idInfo
+import CoreUtils ( exprIsTrivial, cheapEqExpr, exprType, exprIsCheap,
+ etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce,
+ findDefault
+ )
+import Subst ( InScopeSet, mkSubst, substExpr )
+import qualified Subst ( simplBndrs, simplBndr, simplLetId, simplLamBndr )
+import Id ( idType, idName,
+ idUnfolding, idNewStrictness,
+ mkLocalId, idInfo
)
-import IdInfo ( arityLowerBound, InlinePragInfo(..), setInlinePragInfo, vanillaIdInfo )
import Maybes ( maybeToBool, catMaybes )
-import Const ( Con(..) )
-import Name ( isLocalName, setNameUnique )
+import Name ( setNameUnique )
+import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
import SimplMonad
-import Type ( Type, tyVarsOfType, tyVarsOfTypes, mkForAllTys, seqType,
- splitTyConApp_maybe, mkTyVarTys, applyTys, splitFunTys, mkFunTys
+import Type ( Type, mkForAllTys, seqType,
+ splitTyConApp_maybe, tyConAppArgs, mkTyVarTys,
+ isUnLiftedType, isStrictType,
+ splitRepFunTys
)
-import TysPrim ( statePrimTyCon )
-import Var ( setVarUnique )
-import VarSet
-import UniqSupply ( splitUniqSupply, uniqFromSupply )
-import Util ( zipWithEqual, mapAccumL )
+import TyCon ( tyConDataConsIfAvailable )
+import DataCon ( dataConRepArity )
+import VarEnv ( SubstEnv )
+import Util ( lengthExceeds, mapAccumL )
import Outputable
\end{code}
%************************************************************************
%* *
+\subsection{The continuation data type}
+%* *
+%************************************************************************
+
+\begin{code}
+data SimplCont -- Strict contexts
+ = Stop OutType -- Type of the result
+ Bool -- True => This is the RHS of a thunk whose type suggests
+ -- that update-in-place would be possible
+ -- (This makes the inliner a little keener.)
+
+ | CoerceIt OutType -- The To-type, simplified
+ SimplCont
+
+ | InlinePlease -- This continuation makes a function very
+ SimplCont -- keen to inline itelf
+
+ | ApplyTo DupFlag
+ InExpr SubstEnv -- The argument, as yet unsimplified,
+ SimplCont -- and its subst-env
+
+ | Select DupFlag
+ InId [InAlt] SubstEnv -- The case binder, alts, and subst-env
+ SimplCont
+
+ | ArgOf DupFlag -- An arbitrary strict context: the argument
+ -- of a strict function, or a primitive-arg fn
+ -- or a PrimOp
+ 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
+ -- The result expression in the OutExprStuff has type cont_ty
+
+instance Outputable SimplCont where
+ ppr (Stop _ _) = ptext SLIT("Stop")
+ ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
+ ppr (ArgOf dup _ _) = ptext SLIT("ArgOf...") <+> ppr dup
+ ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
+ (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")
+
+
+-------------------
+mkRhsStop, mkStop :: OutType -> SimplCont
+mkStop ty = Stop ty False
+mkRhsStop ty = Stop ty (canUpdateInPlace ty)
+
+
+-------------------
+contIsDupable :: SimplCont -> Bool
+contIsDupable (Stop _ _) = True
+contIsDupable (ApplyTo OkToDup _ _ _) = True
+contIsDupable (ArgOf OkToDup _ _) = True
+contIsDupable (Select OkToDup _ _ _ _) = True
+contIsDupable (CoerceIt _ cont) = contIsDupable cont
+contIsDupable (InlinePlease cont) = contIsDupable cont
+contIsDupable other = False
+
+-------------------
+discardInline :: SimplCont -> SimplCont
+discardInline (InlinePlease cont) = cont
+discardInline (ApplyTo d e s cont) = ApplyTo d e s (discardInline cont)
+discardInline cont = cont
+
+-------------------
+discardableCont :: SimplCont -> Bool
+discardableCont (Stop _ _) = False
+discardableCont (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 _ -> cont
+ other -> CoerceIt to_ty (mkStop 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
+\end{code}
+
+
+\begin{code}
+getContArgs :: OutId -> SimplCont
+ -> SimplM ([(InExpr, SubstEnv, Bool)], -- Arguments; the Bool is true for strict args
+ SimplCont, -- Remaining continuation
+ Bool) -- Whether we came across an InlineCall
+-- getContArgs id k = (args, k', inl)
+-- args are the leading ApplyTo items in k
+-- (i.e. outermost comes first)
+-- augmented with demand info from the functionn
+getContArgs fun orig_cont
+ = getSwitchChecker `thenSmpl` \ chkr ->
+ let
+ -- 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 = tick BottomFound `thenSmpl_`
+ returnSmpl (reverse acc, discardCont cont, inl)
+ | otherwise = returnSmpl (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 :: InScopeSet -> InExpr -> SubstEnv -> Bool
+ -- An argument is interesting if it has *some* structure
+ -- We are here trying to avoid unfolding a function that
+ -- is applied only to variables that have no unfolding
+ -- (i.e. they are probably lambda bound): f x y z
+ -- There is little point in inlining f here.
+interestingArg in_scope arg subst
+ = analyse (substExpr (mkSubst in_scope subst) arg)
+ -- 'analyse' only looks at the top part of the result
+ -- and substExpr is lazy, so this isn't nearly as brutal
+ -- as it looks.
+ where
+ analyse (Var v) = hasSomeUnfolding (idUnfolding v)
+ -- Was: isValueUnfolding (idUnfolding v')
+ -- But that seems over-pessimistic
+ analyse (Type _) = False
+ analyse (App fn (Type _)) = analyse fn
+ analyse (Note _ a) = analyse a
+ analyse other = True
+ -- 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.
+
+Previously some_benefit used to return True only if the variable was
+applied to some value arguments. This didn't work:
+
+ let x = _coerce_ (T Int) Int (I# 3) in
+ case _coerce_ Int (T Int) x of
+ I# y -> ....
+
+we want to inline x, but can't see that it's a constructor in a case
+scrutinee position, and some_benefit is False.
+
+Another example:
+
+dMonadST = _/\_ t -> :Monad (g1 _@_ t, g2 _@_ t, g3 _@_ t)
+
+.... case dMonadST _@_ x0 of (a,b,c) -> ....
+
+we'd really like to inline dMonadST here, but we *don't* want to
+inline if the case expression is just
+
+ case x of y { DEFAULT -> ... }
+
+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 becuase
+ -- 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
+ 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 tyConDataConsIfAvailable tycon of
+ [dc] -> arity == 1 || arity == 2
+ where
+ arity = dataConRepArity dc
+ other -> False
+\end{code}
+
+
+
+%************************************************************************
+%* *
\section{Dealing with a single binder}
%* *
%************************************************************************
simplBinders bndrs thing_inside
= getSubst `thenSmpl` \ subst ->
let
- (subst', bndrs') = substBndrs subst bndrs
+ (subst', bndrs') = Subst.simplBndrs subst bndrs
in
seqBndrs bndrs' `seq`
setSubst subst' (thing_inside bndrs')
simplBinder bndr thing_inside
= getSubst `thenSmpl` \ subst ->
let
- (subst', bndr') = substBndr subst bndr
+ (subst', bndr') = Subst.simplBndr subst bndr
in
seqBndr bndr' `seq`
setSubst subst' (thing_inside 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
+simplLamBinder :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
+simplLamBinder bndr thing_inside
= getSubst `thenSmpl` \ subst ->
let
- (subst', bndrs') = substIds subst ids
+ (subst', bndr') = Subst.simplLamBndr subst bndr
in
- seqBndrs bndrs' `seq`
- setSubst subst' (thing_inside bndrs')
+ seqBndr bndr' `seq`
+ setSubst subst' (thing_inside bndr')
+
+
+simplRecIds :: [InBinder] -> ([OutBinder] -> SimplM a) -> SimplM a
+simplRecIds ids thing_inside
+ = getSubst `thenSmpl` \ subst ->
+ let
+ (subst', ids') = mapAccumL Subst.simplLetId subst ids
+ in
+ seqBndrs ids' `seq`
+ setSubst subst' (thing_inside ids')
+
+simplLetId :: InBinder -> (OutBinder -> SimplM a) -> SimplM a
+simplLetId id thing_inside
+ = getSubst `thenSmpl` \ subst ->
+ let
+ (subst', id') = Subst.simplLetId subst id
+ in
+ seqBndr id' `seq`
+ setSubst subst' (thing_inside id')
seqBndrs [] = ()
seqBndrs (b:bs) = seqBndr b `seq` seqBndrs bs
%************************************************************************
%* *
-\subsection{Transform a RHS}
-%* *
-%************************************************************************
-
-Try (a) eta expansion
- (b) type-lambda swizzling
-
-\begin{code}
-transformRhs :: InExpr -> SimplM InExpr
-transformRhs rhs
- = tryEtaExpansion body `thenSmpl` \ body' ->
- mkRhsTyLam tyvars body'
- where
- (tyvars, body) = collectTyBinders rhs
-\end{code}
-
-
-%************************************************************************
-%* *
\subsection{Local tyvar-lifting}
%* *
%************************************************************************
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)
+tryRhsTyLam :: OutExpr -> SimplM ([OutBind], OutExpr)
+
+tryRhsTyLam rhs -- Only does something if there's a let
+ | null tyvars || not (worth_it body) -- inside a type lambda,
+ = returnSmpl ([], rhs) -- and a WHNF inside that
+
| otherwise
- = go (\x -> x) body
+ = go (\x -> x) body `thenSmpl` \ (binds, body') ->
+ returnSmpl (binds, mkLams tyvars body')
+
where
- worth_it (Let _ e) = whnf_in_middle e
- worth_it other = False
+ (tyvars, body) = collectTyBinders rhs
+
+ 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@(NonRec var rhs) body)
+ | exprIsTrivial rhs
= go (fn . Let bind) body
- go fn (Let bind@(NonRec var rhs) body)
+ go fn (Let (NonRec var rhs) body)
= mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
- go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ body' ->
- returnSmpl (Let (NonRec var' (mkLams tyvars_here (fn rhs))) body')
+ go (fn . Let (mk_silly_bind var rhs')) body `thenSmpl` \ (binds, body') ->
+ returnSmpl (NonRec var' (mkLams tyvars_here (fn rhs)) : binds, body')
+
where
tyvars_here = tyvars
+ -- main_tyvar_set = mkVarSet tyvars
+ -- var_ty = idType var
-- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfType var_ty)
-- tyvars_here was an attempt to reduce the number of tyvars
-- wrt which the new binding is abstracted. But the naive
-- 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)
= 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'))
+ new_bind = Rec (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')
+ go gn body `thenSmpl` \ (binds, body') ->
+ returnSmpl (new_bind : binds, body')
where
(vars,rhss) = unzip prs
tyvars_here = tyvars
-- varSetElems (main_tyvar_set `intersectVarSet` tyVarsOfTypes var_tys)
+ -- var_tys = map idType vars
-- See notes with tyvars_here above
- var_tys = map idType vars
-
- go fn body = returnSmpl (mkLams tyvars (fn body))
+ go fn body = returnSmpl ([], 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 inline-prag 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 `setInlinePragInfo` getInlinePragma 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 (setInlinePragma var IMustBeINLINEd) rhs
- -- The addInlinePragma 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
--
- -- 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 IMustBeINLINEs, so that
- -- we simply substitute for g* throughout.
+ -- 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}
Try eta expansion for RHSs
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
+ Case 1 f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
+ (n >= 0)
+ OR
+ Case 2 f = N E1..En ==> z1=E1
+ (n > 0) ..
+ zn=En
+ f = \y1..ym -> N z1..zn y1..ym
-where (in both cases) N is a NORMAL FORM (i.e. no redexes anywhere)
-wanting a suitable number of extra args.
+where (in both cases)
-NB: the Ei may have unlifted type, but the simplifier (which is applied
-to the result) deals OK with this.
+ * The xi can include type variables
-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.
+ * The yi are all value variables
-\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))))
- where
- (x_bndrs, body) = collectValBinders rhs
- (fun, args) = collectArgs body
- trivial_args = map exprIsTrivial args
- fun_arity = exprGenerousArity fun
+ * N is a NORMAL FORM (i.e. no redexes anywhere)
+ wanting a suitable number of extra args.
- bind_z_arg (arg, trivial_arg)
- | trivial_arg = returnSmpl (Nothing, arg)
- | otherwise = newId (coreExprType arg) $ \ z ->
- returnSmpl (Just (NonRec z arg), Var z)
+ * the Ei must not have unlifted type
- -- Note: I used to try to avoid the coreExprType 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 (coreExprType 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 exprGenerousArity
- -- 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}
+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.
-
-%************************************************************************
-%* *
-\subsection{Eta reduction}
-%* *
-%************************************************************************
-
-@etaCoreExpr@ trys an eta reduction at the top level of a Core Expr.
-
-e.g. \ x y -> f x y ===> f
-
-It is used
--- OLD
--- a) Before constructing an Unfolding, to
--- try to make the unfolding smaller;
- b) In tidyCoreExpr, which is done just before converting to STG.
-
-But we only do this if
- i) 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.
-
--- OLD: in core2stg we want to do this even if the result isn't trivial
--- ii) It exposes a simple variable or a type application; in short
--- it exposes a "trivial" expression. (exprIsTrivial)
+In Case 1, 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}
-etaCoreExpr :: CoreExpr -> CoreExpr
- -- ToDo: we should really check that we don't turn a non-bottom
- -- lambda into a bottom variable. Sigh
+tryEtaExpansion :: OutExpr -> OutType -> SimplM ([OutBind], OutExpr)
+tryEtaExpansion rhs rhs_ty
+ | not opt_SimplDoLambdaEtaExpansion -- Not if switched off
+ || exprIsTrivial rhs -- Not if RHS is trivial
+ || final_arity == 0 -- Not if arity is zero
+ = returnSmpl ([], rhs)
+
+ | n_val_args == 0 && not arity_is_manifest
+ = -- Some lambdas but not enough: case 1
+ getUniqSupplySmpl `thenSmpl` \ us ->
+ returnSmpl ([], etaExpand final_arity us rhs rhs_ty)
+
+ | n_val_args > 0 && not (any cant_bind arg_infos)
+ = -- Partial application: case 2
+ mapAndUnzipSmpl bind_z_arg arg_infos `thenSmpl` \ (maybe_z_binds, z_args) ->
+ getUniqSupplySmpl `thenSmpl` \ us ->
+ returnSmpl (catMaybes maybe_z_binds,
+ etaExpand final_arity us (mkApps fun z_args) rhs_ty)
-etaCoreExpr expr@(Lam bndr body)
- = check (reverse binders) body
+ | otherwise
+ = returnSmpl ([], rhs)
where
- (binders, body) = collectBinders expr
-
- check [] body
- | not (any (`elemVarSet` body_fvs) binders)
- = body -- Success!
- where
- body_fvs = exprFreeVars body
-
- check (b : bs) (App fun arg)
- | (varToCoreExpr b `cheapEqExpr` arg)
- = check bs fun
-
- check _ _ = expr -- Bale out
-
-etaCoreExpr expr = expr -- The common case
+ (fun, args) = collectArgs rhs
+ n_val_args = valArgCount args
+ (fun_arity, arity_is_manifest) = exprEtaExpandArity fun
+ final_arity = 0 `max` (fun_arity - n_val_args)
+ arg_infos = [(arg, exprType arg, exprIsTrivial arg) | arg <- args]
+ cant_bind (_, ty, triv) = not triv && isUnLiftedType ty
+
+ bind_z_arg (arg, arg_ty, trivial_arg)
+ | trivial_arg = returnSmpl (Nothing, arg)
+ | otherwise = newId SLIT("z") arg_ty $ \ z ->
+ returnSmpl (Just (NonRec z arg), Var z)
\end{code}
-
+
%************************************************************************
%* *
-- Secondly, if you do, you get an infinite loop, because the bindNonRec
-- in munge_rhs puts a case into the DEFAULT branch!
where
- new_alts = outer_alts_without_deflt ++ munged_inner_alts
+ new_alts = add_default maybe_inner_default
+ (outer_alts_without_deflt ++ inner_con_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
not (con `elem` outer_cons) -- Eliminate shadowed inner alts
]
munge_rhs rhs = bindNonRec inner_bndr (Var outer_bndr) rhs
+
+ (inner_con_alts, maybe_inner_default) = findDefault munged_inner_alts
+
+ add_default (Just rhs) alts = (DEFAULT,[],rhs) : alts
+ add_default Nothing alts = alts
\end{code}
Now the identity-case transformation:
case e of ===> e
- True -> True;
+ True -> True;
False -> False
and similar friends.
mkCase scrut case_bndr alts
| 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 (con, args, Con con' args') = con == con' &&
- and (zipWithEqual "mkCase"
- cheapEqExpr
- (map Type arg_tys ++ map varToCoreExpr args)
- args')
- identity_alt other = False
-
- arg_tys = case splitTyConApp_maybe (idType case_bndr) of
- Just (tycon, arg_tys) -> arg_tys
+ 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))
+
+ -- 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
\end{code}
-The catch-all case
+The catch-all case. We do a final transformation that I've
+occasionally seen making a big difference:
-\begin{code}
-mkCase other_scrut case_bndr other_alts
- = returnSmpl (Case other_scrut case_bndr other_alts)
-\end{code}
+ 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.]
+The case where this came up was like this (lib/std/PrelCError.lhs):
-\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 :: Con -> [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
+ x | p `is` 1 -> e1
+ | p `is` 2 -> e2
+ ...etc...
- matches (DEFAULT, _, _) = True
- matches (con1, _, _) = con == con1
+where @is@ was something like
+
+ p `is` n = p /= (-1) && p == n
+This gave rise to a horrible sequence of cases
-mkCoerce to_ty (Note (Coerce _ from_ty) expr)
- | to_ty == from_ty = expr
- | otherwise = Note (Coerce to_ty from_ty) expr
-mkCoerce to_ty expr
- = Note (Coerce to_ty (coreExprType expr)) expr
+ case p of
+ (-1) -> $j p
+ 1 -> e1
+ DEFAULT -> $j p
+
+and similarly in cascade for all the join points!
+
+\begin{code}
+mkCase other_scrut case_bndr other_alts
+ = returnSmpl (Case other_scrut case_bndr (mergeDefault other_alts))
+
+mergeDefault (deflt_alt@(DEFAULT,_,deflt_rhs) : con_alts)
+ = deflt_alt : [alt | alt@(con,_,rhs) <- con_alts, not (rhs `cheapEqExpr` deflt_rhs)]
+ -- NB: we can neglect the binders because we won't get equality if the
+ -- binders are mentioned in rhs (no shadowing)
+mergeDefault other_alts
+ = other_alts
\end{code}
projects / ghc-hetmet.git / blobdiff