Overview
***************************
-* We attach binding levels to Core bindings, in preparation for floating
- outwards (@FloatOut@).
+1. We attach binding levels to Core bindings, in preparation for floating
+ outwards (@FloatOut@).
-* We also let-ify many expressions (notably case scrutinees), so they
- will have a fighting chance of being floated sensible.
+2. We also let-ify many expressions (notably case scrutinees), so they
+ will have a fighting chance of being floated sensible.
-* We clone the binders of any floatable let-binding, so that when it is
- floated out it will be unique. (This used to be done by the simplifier
- but the latter now only ensures that there's no shadowing.)
- NOTE: Very tiresomely, we must apply this substitution to
- the rules stored inside a variable too.
+3. We clone the binders of any floatable let-binding, so that when it is
+ floated out it will be unique. (This used to be done by the simplifier
+ but the latter now only ensures that there's no shadowing; indeed, even
+ that may not be true.)
- We do *not* clone top-level bindings, because some of them must not change,
- but we *do* clone bindings that are heading for the top level
+ NOTE: this can't be done using the uniqAway idea, because the variable
+ must be unique in the whole program, not just its current scope,
+ because two variables in different scopes may float out to the
+ same top level place
+ NOTE: Very tiresomely, we must apply this substitution to
+ the rules stored inside a variable too.
+ We do *not* clone top-level bindings, because some of them must not change,
+ but we *do* clone bindings that are heading for the top level
+
+4. In the expression
+ case x of wild { p -> ...wild... }
+ we substitute x for wild in the RHS of the case alternatives:
+ case x of wild { p -> ...x... }
+ This means that a sub-expression involving x is not "trapped" inside the RHS.
+ And it's not inconvenient because we already have a substitution.
+
+ Note that this is EXACTLY BACKWARDS from the what the simplifier does.
+ The simplifier tries to get rid of occurrences of x, in favour of wild,
+ in the hope that there will only be one remaining occurrence of x, namely
+ the scrutinee of the case, and we can inline it.
\begin{code}
module SetLevels (
import CoreSyn
-import CoreUtils ( coreExprType, exprIsTrivial, exprIsBottom )
+import CoreUtils ( exprType, exprIsTrivial, exprIsBottom, mkPiType )
import CoreFVs -- all of it
-import Id ( Id, idType, mkSysLocal, isOneShotLambda, modifyIdInfo )
-import IdInfo ( specInfo, setSpecInfo )
-import Var ( IdOrTyVar, Var, setVarUnique )
-import VarEnv
import Subst
-import VarSet
-import Type ( isUnLiftedType, mkTyVarTys, mkForAllTys, Type )
-import BasicTypes ( TopLevelFlag(..) )
+import Id ( Id, idType, mkSysLocal, isOneShotLambda, modifyIdInfo,
+ idSpecialisation, idWorkerInfo, setIdInfo
+ )
+import IdInfo ( workerExists, vanillaIdInfo, demandInfo, setDemandInfo )
+import Var ( Var, setVarUnique )
import VarSet
import VarEnv
+import Name ( getOccName )
+import OccName ( occNameUserString )
+import Type ( isUnLiftedType, Type )
+import BasicTypes ( TopLevelFlag(..) )
+import Demand ( isStrict, wwLazy )
import UniqSupply
-import Maybes ( maybeToBool )
-import Util ( zipWithEqual, zipEqual )
+import Util ( sortLt, isSingleton, count )
import Outputable
-
-isLeakFreeType x y = False -- safe option; ToDo
\end{code}
%************************************************************************
%************************************************************************
\begin{code}
-data Level
- = Top -- Means *really* the top level; short for (Level 0 0).
- | Level Int -- Level number of enclosing lambdas
- Int -- Number of big-lambda and/or case expressions between
- -- here and the nearest enclosing lambda
+data Level = Level Int -- Level number of enclosing lambdas
+ Int -- Number of big-lambda and/or case expressions between
+ -- here and the nearest enclosing lambda
\end{code}
The {\em level number} on a (type-)lambda-bound variable is the
x_1 = ... b ... in ...
\end{verbatim}
-Level 0 0 will make something get floated to a top-level "equals",
-@Top@ makes it go right to the top.
-
The main function @lvlExpr@ carries a ``context level'' (@ctxt_lvl@).
That's meant to be the level number of the enclosing binder in the
final (floated) program. If the level number of a sub-expression is
less than that of the context, then it might be worth let-binding the
sub-expression so that it will indeed float. This context level starts
-at @Level 0 0@; it is never @Top@.
+at @Level 0 0@.
\begin{code}
type LevelledExpr = TaggedExpr Level
type LevelledArg = TaggedArg Level
type LevelledBind = TaggedBind Level
-tOP_LEVEL = Top
+tOP_LEVEL = Level 0 0
incMajorLvl :: Level -> Level
-incMajorLvl Top = Level 1 0
incMajorLvl (Level major minor) = Level (major+1) 0
incMinorLvl :: Level -> Level
-incMinorLvl Top = Level 0 1
incMinorLvl (Level major minor) = Level major (minor+1)
-unTopify :: Type -> Level -> Level
-unTopify ty lvl
- | isUnLiftedType ty = case lvl of
- Top -> Level 0 0 -- Unboxed floats can't go right
- other -> lvl -- to the top
- | otherwise = lvl
-
maxLvl :: Level -> Level -> Level
-maxLvl Top l2 = l2
-maxLvl l1 Top = l1
maxLvl l1@(Level maj1 min1) l2@(Level maj2 min2)
| (maj1 > maj2) || (maj1 == maj2 && min1 > min2) = l1
| otherwise = l2
ltLvl :: Level -> Level -> Bool
-ltLvl l1 Top = False
-ltLvl Top (Level _ _) = True
ltLvl (Level maj1 min1) (Level maj2 min2)
= (maj1 < maj2) || (maj1 == maj2 && min1 < min2)
ltMajLvl :: Level -> Level -> Bool
-- Tells if one level belongs to a difft *lambda* level to another
-ltMajLvl l1 Top = False
-ltMajLvl Top (Level 0 _) = False
-ltMajLvl Top (Level _ _) = True
ltMajLvl (Level maj1 _) (Level maj2 _) = maj1 < maj2
isTopLvl :: Level -> Bool
-isTopLvl Top = True
-isTopLvl other = False
-
-isTopMajLvl :: Level -> Bool -- Tells if it's the top *lambda* level
-isTopMajLvl Top = True
-isTopMajLvl (Level maj _) = maj == 0
+isTopLvl (Level 0 0) = True
+isTopLvl other = False
instance Outputable Level where
- ppr Top = ptext SLIT("<Top>")
ppr (Level maj min) = hcat [ char '<', int maj, char ',', int min, char '>' ]
+
+instance Eq Level where
+ (Level maj1 min1) == (Level maj2 min2) = maj1==maj2 && min1==min2
\end{code}
%************************************************************************
%************************************************************************
\begin{code}
-setLevels :: [CoreBind]
+setLevels :: Bool -- True <=> float lambdas to top level
+ -> [CoreBind]
-> UniqSupply
-> [LevelledBind]
-setLevels binds us
+setLevels float_lams binds us
= initLvl us (do_them binds)
where
-- "do_them"'s main business is to thread the monad along
do_them [] = returnLvl []
do_them (b:bs)
- = lvlTopBind b `thenLvl` \ (lvld_bind, _) ->
- do_them bs `thenLvl` \ lvld_binds ->
- returnLvl (lvld_bind ++ lvld_binds)
-
-lvlTopBind (NonRec binder rhs)
- = lvlBind TopLevel Top initialEnv (AnnNonRec binder (freeVars rhs))
- -- Rhs can have no free vars!
-
-lvlTopBind (Rec pairs)
- = lvlBind TopLevel Top initialEnv (AnnRec [(b,freeVars rhs) | (b,rhs) <- pairs])
-\end{code}
-
-%************************************************************************
-%* *
-\subsection{Bindings}
-%* *
-%************************************************************************
-
-The binding stuff works for top level too.
-
-\begin{code}
-lvlBind :: TopLevelFlag -- Used solely to decide whether to clone
- -> Level -- Context level; might be Top even for bindings nested in the RHS
- -- of a top level binding
- -> LevelEnv
- -> CoreBindWithFVs
- -> LvlM ([LevelledBind], LevelEnv)
+ = lvlTopBind init_env b `thenLvl` \ (lvld_bind, _) ->
+ do_them bs `thenLvl` \ lvld_binds ->
+ returnLvl (lvld_bind : lvld_binds)
-lvlBind top_lvl ctxt_lvl env (AnnNonRec bndr rhs)
- = setFloatLevel (Just bndr) ctxt_lvl env rhs ty `thenLvl` \ (final_lvl, rhs') ->
- cloneVar top_lvl env bndr final_lvl `thenLvl` \ (new_env, new_bndr) ->
- returnLvl ([NonRec (new_bndr, final_lvl) rhs'], new_env)
- where
- ty = idType bndr
+ init_env = initialEnv float_lams
+lvlTopBind env (NonRec binder rhs)
+ = lvlBind TopLevel tOP_LEVEL env (AnnNonRec binder (freeVars rhs))
+ -- Rhs can have no free vars!
-lvlBind top_lvl ctxt_lvl env (AnnRec pairs) = lvlRecBind top_lvl ctxt_lvl env pairs
+lvlTopBind env (Rec pairs)
+ = lvlBind TopLevel tOP_LEVEL env (AnnRec [(b,freeVars rhs) | (b,rhs) <- pairs])
\end{code}
%************************************************************************
\end{code}
The @ctxt_lvl@ is, roughly, the level of the innermost enclosing
-binder.
-
-Here's an example
+binder. Here's an example
v = \x -> ...\y -> let r = case (..x..) of
..x..
If there were another lambda in @r@'s rhs, it would get level-2 as well.
\begin{code}
-lvlExpr _ _ (_, AnnType ty) = returnLvl (Type ty)
-lvlExpr _ env (_, AnnVar v) = returnLvl (lookupVar env v)
-
-lvlExpr ctxt_lvl env (_, AnnCon con args)
- = mapLvl (lvlExpr ctxt_lvl env) args `thenLvl` \ args' ->
- returnLvl (Con con args')
+lvlExpr _ _ (_, AnnType ty) = returnLvl (Type ty)
+lvlExpr _ env (_, AnnVar v) = returnLvl (lookupVar env v)
+lvlExpr _ env (_, AnnLit lit) = returnLvl (Lit lit)
lvlExpr ctxt_lvl env (_, AnnApp fun arg)
- = lvlExpr ctxt_lvl env fun `thenLvl` \ fun' ->
- lvlMFE ctxt_lvl env arg `thenLvl` \ arg' ->
+ = lvl_fun fun `thenLvl` \ fun' ->
+ lvlMFE False ctxt_lvl env arg `thenLvl` \ arg' ->
returnLvl (App fun' arg')
+ where
+ lvl_fun (_, AnnCase _ _ _) = lvlMFE True ctxt_lvl env fun
+ lvl_fun other = lvlExpr ctxt_lvl env fun
+ -- We don't do MFE on partial applications generally,
+ -- but we do if the function is big and hairy, like a case
+
+lvlExpr ctxt_lvl env (_, AnnNote InlineMe expr)
+-- Don't float anything out of an InlineMe; hence the tOP_LEVEL
+ = lvlExpr tOP_LEVEL env expr `thenLvl` \ expr' ->
+ returnLvl (Note InlineMe expr')
lvlExpr ctxt_lvl env (_, AnnNote note expr)
= lvlExpr ctxt_lvl env expr `thenLvl` \ expr' ->
-- lambdas makes them more expensive.
lvlExpr ctxt_lvl env expr@(_, AnnLam bndr rhs)
- = lvlMFE incd_lvl new_env body `thenLvl` \ body' ->
- returnLvl (mk_lams lvld_bndrs expr body')
- where
- bndr_is_id = isId bndr
- bndr_is_tyvar = isTyVar bndr
- (more_bndrs, body) = go rhs
- bndrs = bndr : more_bndrs
-
- incd_lvl | bndr_is_id && not (all isOneShotLambda bndrs) = incMajorLvl ctxt_lvl
- | otherwise = incMinorLvl ctxt_lvl
- -- Only bump the major level number if the binders include
- -- at least one more-than-one-shot lambda
-
- lvld_bndrs = [(b,incd_lvl) | b <- bndrs]
- new_env = extendLvlEnv env lvld_bndrs
-
- -- Ignore notes, because we don't want to split
- -- a lambda like this (\x -> coerce t (\s -> ...))
- -- This happens quite a bit in state-transformer programs
- go (_, AnnLam bndr rhs) | bndr_is_id && isId bndr
- || bndr_is_tyvar && isTyVar bndr
- = case go rhs of { (bndrs, body) -> (bndr:bndrs, body) }
- go (_, AnnNote _ rhs) = go rhs
- go body = ([], body)
-
- -- Have to reconstruct the right Notes, since we ignored
- -- them when gathering the lambdas
- mk_lams (lb : lbs) (_, AnnLam _ body) body' = Lam lb (mk_lams lbs body body')
- mk_lams lbs (_, AnnNote note body) body' = Note note (mk_lams lbs body body')
- mk_lams [] body body' = body'
+ = lvlMFE True new_lvl new_env body `thenLvl` \ new_body ->
+ returnLvl (glue_binders new_bndrs expr new_body)
+ where
+ (bndrs, body) = collect_binders expr
+ (new_lvl, new_bndrs) = lvlLamBndrs ctxt_lvl bndrs
+ new_env = extendLvlEnv env new_bndrs
lvlExpr ctxt_lvl env (_, AnnLet bind body)
- = lvlBind NotTopLevel ctxt_lvl env bind `thenLvl` \ (binds', new_env) ->
+ = lvlBind NotTopLevel ctxt_lvl env bind `thenLvl` \ (bind', new_env) ->
lvlExpr ctxt_lvl new_env body `thenLvl` \ body' ->
- returnLvl (mkLets binds' body')
+ returnLvl (Let bind' body')
lvlExpr ctxt_lvl env (_, AnnCase expr case_bndr alts)
- = lvlMFE ctxt_lvl env expr `thenLvl` \ expr' ->
- mapLvl lvl_alt alts `thenLvl` \ alts' ->
+ = lvlMFE True ctxt_lvl env expr `thenLvl` \ expr' ->
+ let
+ alts_env = extendCaseBndrLvlEnv env expr' case_bndr incd_lvl
+ in
+ mapLvl (lvl_alt alts_env) alts `thenLvl` \ alts' ->
returnLvl (Case expr' (case_bndr, incd_lvl) alts')
where
- expr_type = coreExprType (deAnnotate expr)
incd_lvl = incMinorLvl ctxt_lvl
- alts_env = extendLvlEnv env [(case_bndr,incd_lvl)]
-
- lvl_alt (con, bs, rhs)
- = let
- bs' = [ (b, incd_lvl) | b <- bs ]
- new_env = extendLvlEnv alts_env bs'
- in
- lvlMFE incd_lvl new_env rhs `thenLvl` \ rhs' ->
+
+ lvl_alt alts_env (con, bs, rhs)
+ = lvlMFE True incd_lvl new_env rhs `thenLvl` \ rhs' ->
returnLvl (con, bs', rhs')
+ where
+ bs' = [ (b, incd_lvl) | b <- bs ]
+ new_env = extendLvlEnv alts_env bs'
+
+collect_binders lam
+ = go [] lam
+ where
+ go rev_bndrs (_, AnnLam b e) = go (b:rev_bndrs) e
+ go rev_bndrs (_, AnnNote n e) = go rev_bndrs e
+ go rev_bndrs rhs = (reverse rev_bndrs, rhs)
+ -- Ignore notes, because we don't want to split
+ -- a lambda like this (\x -> coerce t (\s -> ...))
+ -- This happens quite a bit in state-transformer programs
+
+ -- glue_binders puts the lambda back together
+glue_binders (b:bs) (_, AnnLam _ e) body = Lam b (glue_binders bs e body)
+glue_binders bs (_, AnnNote n e) body = Note n (glue_binders bs e body)
+glue_binders [] e body = body
\end{code}
@lvlMFE@ is just like @lvlExpr@, except that it might let-bind
the expression, so that it can itself be floated.
\begin{code}
-lvlMFE :: Level -- Level of innermost enclosing lambda/tylam
+lvlMFE :: Bool -- True <=> strict context [body of case or let]
+ -> Level -- Level of innermost enclosing lambda/tylam
-> LevelEnv -- Level of in-scope names/tyvars
-> CoreExprWithFVs -- input expression
-> LvlM LevelledExpr -- Result expression
-lvlMFE ctxt_lvl env (_, AnnType ty)
+lvlMFE strict_ctxt ctxt_lvl env (_, AnnType ty)
= returnLvl (Type ty)
-lvlMFE ctxt_lvl env ann_expr
- | isUnLiftedType ty -- Can't let-bind it
- = lvlExpr ctxt_lvl env ann_expr
-
- | otherwise -- Not primitive type so could be let-bound
- = setFloatLevel Nothing {- Not already let-bound -}
- ctxt_lvl env ann_expr ty `thenLvl` \ (final_lvl, expr') ->
- returnLvl expr'
+lvlMFE strict_ctxt ctxt_lvl env ann_expr@(fvs, _)
+ | isUnLiftedType ty -- Can't let-bind it
+ || not good_destination
+ || exprIsTrivial expr -- Is trivial
+ || (strict_ctxt && exprIsBottom expr) -- Strict context and is bottom
+ -- e.g. \x -> error "foo"
+ -- No gain from floating this
+ = -- Don't float it out
+ lvlExpr ctxt_lvl env ann_expr
+
+ | otherwise -- Float it out!
+ = lvlFloatRhs abs_vars dest_lvl env ann_expr `thenLvl` \ expr' ->
+ newLvlVar "lvl" abs_vars ty `thenLvl` \ var ->
+ returnLvl (Let (NonRec (var,dest_lvl) expr')
+ (mkVarApps (Var var) abs_vars))
where
- ty = coreExprType (deAnnotate ann_expr)
+ expr = deAnnotate ann_expr
+ ty = exprType expr
+ dest_lvl = destLevel env fvs (isFunction ann_expr)
+ abs_vars = abstractVars dest_lvl env fvs
+
+ good_destination = dest_lvl `ltMajLvl` ctxt_lvl -- Escapes a value lambda
+ || (isTopLvl dest_lvl && not strict_ctxt) -- Goes to the top
+ -- A decision to float entails let-binding this thing, and we only do
+ -- that if we'll escape a value lambda, or will go to the top level.
+ -- But beware
+ -- concat = /\ a -> foldr ..a.. (++) []
+ -- was getting turned into
+ -- concat = /\ a -> lvl a
+ -- lvl = /\ a -> foldr ..a.. (++) []
+ -- which is pretty stupid. Hence the strict_ctxt test
\end{code}
%************************************************************************
%* *
-\subsection{Deciding floatability}
+\subsection{Bindings}
%* *
%************************************************************************
-@setFloatLevel@ is used for let-bound right-hand-sides, or for MFEs which
-are being created as let-bindings
-
-Decision tree:
-Let Bound?
- YES. -> (a) try abstracting type variables.
- If we abstract type variables it will go further, that is, past more
- lambdas. same as asking if the level number given by the free
- variables is less than the level number given by free variables
- and type variables together.
- Abstract offending type variables, e.g.
- change f ty a b
- to let v = /\ty' -> f ty' a b
- in v ty
- so that v' is not stopped by the level number of ty
- tag the original let with its level number
- (from its variables and type variables)
- NO. is a WHNF?
- YES. -> No point in let binding to float a WHNF.
- Pin (leave) expression here.
- NO. -> Will float past a lambda?
- (check using free variables only, not type variables)
- YES. -> do the same as (a) above.
- NO. -> No point in let binding if it is not going anywhere
- Pin (leave) expression here.
+The binding stuff works for top level too.
\begin{code}
-setFloatLevel :: Maybe Id -- Just id <=> the expression is already let-bound to id
- -- Nothing <=> it's a possible MFE
- -> Level -- of context
- -> LevelEnv
-
- -> CoreExprWithFVs -- Original rhs
- -> Type -- Type of rhs
-
- -> LvlM (Level, -- Level to attribute to this let-binding
- LevelledExpr) -- Final rhs
-
-setFloatLevel maybe_let_bound ctxt_lvl env expr@(expr_fvs, _) ty
-
--- Now deal with (by not floating) trivial non-let-bound expressions
--- which just aren't worth let-binding in order to float. We always
--- choose to float even trivial let-bound things because it doesn't do
--- any harm, and not floating it may pin something important. For
--- example
---
--- x = let v = []
--- w = 1:v
--- in ...
---
--- Here, if we don't float v we won't float w, which is Bad News.
--- If this gives any problems we could restrict the idea to things destined
--- for top level.
-
- | not alreadyLetBound
- && (expr_is_trivial || expr_is_bottom || not will_float_past_lambda)
-
- = -- Pin trivial non-let-bound expressions,
- -- or ones which aren't going anywhere useful
- lvlExpr ctxt_lvl env expr `thenLvl` \ expr' ->
- returnLvl (safe_ctxt_lvl, expr')
-
-{- SDM 7/98
-The above case used to read (whnf_or_bottom || not will_float_past_lambda).
-It was changed because we really do want to float out constructors if possible:
-this can save a great deal of needless allocation inside a loop. On the other
-hand, there's no point floating out nullary constructors and literals, hence
-the expr_is_trivial condition.
--}
-
- | alreadyLetBound && not worth_type_abstraction
- = -- Process the expression with a new ctxt_lvl, obtained from
- -- the free vars of the expression itself
- lvlExpr expr_lvl env expr `thenLvl` \ expr' ->
- returnLvl (safe_expr_lvl, expr')
-
- | otherwise -- This will create a let anyway, even if there is no
- -- type variable to abstract, so we try to abstract anyway
- = abstractWrtTyVars offending_tyvars ty env lvl_after_ty_abstr expr
- `thenLvl` \ final_expr ->
- returnLvl (safe_expr_lvl, final_expr)
- -- OLD LIE: The body of the let, just a type application, isn't worth floating
- -- so pin it with ctxt_lvl
- -- The truth: better to give it expr_lvl in case it is pinning
- -- something non-trivial which depends on it.
- where
- alreadyLetBound = maybeToBool maybe_let_bound
-
- safe_ctxt_lvl = unTopify ty ctxt_lvl
- safe_expr_lvl = unTopify ty expr_lvl
-
- fvs = case maybe_let_bound of
- Nothing -> expr_fvs
- Just id -> expr_fvs `unionVarSet` idFreeVars id
-
- ids_only_lvl = foldVarSet (maxIdLvl env) tOP_LEVEL fvs
- tyvars_only_lvl = foldVarSet (maxTyVarLvl env) tOP_LEVEL fvs
- expr_lvl = ids_only_lvl `maxLvl` tyvars_only_lvl
- lvl_after_ty_abstr = ids_only_lvl --`maxLvl` non_offending_tyvars_lvl
-
- -- Will escape lambda if let-bound
- will_float_past_lambda = ids_only_lvl `ltMajLvl` ctxt_lvl
-
- -- Will escape (more) lambda(s)/type lambda(s) if type abstracted
- worth_type_abstraction = (ids_only_lvl `ltLvl` tyvars_only_lvl)
- && not expr_is_trivial -- Avoids abstracting trivial type applications
-
- offending_tyvars = filter offending_tv (varSetElems fvs)
- offending_tv var | isId var = False
- | otherwise = ids_only_lvl `ltLvl` varLevel env var
-
- expr_is_trivial = exprIsTrivial de_ann_expr
- expr_is_bottom = exprIsBottom de_ann_expr
- de_ann_expr = deAnnotate expr
-\end{code}
-
-Abstract wrt tyvars, by making it just as if we had seen
+lvlBind :: TopLevelFlag -- Used solely to decide whether to clone
+ -> Level -- Context level; might be Top even for bindings nested in the RHS
+ -- of a top level binding
+ -> LevelEnv
+ -> CoreBindWithFVs
+ -> LvlM (LevelledBind, LevelEnv)
- let v = /\a1..an. E
- in v a1 ... an
+lvlBind top_lvl ctxt_lvl env (AnnNonRec bndr rhs@(rhs_fvs,_))
+ | null abs_vars
+ = -- No type abstraction; clone existing binder
+ lvlExpr dest_lvl env rhs `thenLvl` \ rhs' ->
+ cloneVar top_lvl env bndr ctxt_lvl dest_lvl `thenLvl` \ (env', bndr') ->
+ returnLvl (NonRec (bndr', dest_lvl) rhs', env')
-instead of simply E. The idea is that v can be freely floated, since it
-has no free type variables. Of course, if E has no free type
-variables, then we just return E.
+ | otherwise
+ = -- Yes, type abstraction; create a new binder, extend substitution, etc
+ lvlFloatRhs abs_vars dest_lvl env rhs `thenLvl` \ rhs' ->
+ newPolyBndrs dest_lvl env abs_vars [bndr] `thenLvl` \ (env', [bndr']) ->
+ returnLvl (NonRec (bndr', dest_lvl) rhs', env')
-\begin{code}
-abstractWrtTyVars offending_tyvars ty env lvl expr
- = lvlExpr incd_lvl new_env expr `thenLvl` \ expr' ->
- newLvlVar poly_ty `thenLvl` \ poly_var ->
- let
- poly_var_rhs = mkLams tyvar_lvls expr'
- poly_var_binding = NonRec (poly_var, lvl) poly_var_rhs
- poly_var_app = mkTyApps (Var poly_var) (mkTyVarTys offending_tyvars)
- final_expr = Let poly_var_binding poly_var_app -- mkCoLet* requires Core
- in
- returnLvl final_expr
where
- poly_ty = mkForAllTys offending_tyvars ty
-
- -- These defns are just like those in the TyLam case of lvlExpr
- incd_lvl = incMinorLvl lvl
- tyvar_lvls = [(tv,incd_lvl) | tv <- offending_tyvars]
- new_env = extendLvlEnv env tyvar_lvls
+ bind_fvs = rhs_fvs `unionVarSet` idFreeVars bndr
+ abs_vars = abstractVars dest_lvl env bind_fvs
+
+ dest_lvl | isUnLiftedType (idType bndr) = destLevel env bind_fvs False `maxLvl` Level 1 0
+ | otherwise = destLevel env bind_fvs (isFunction rhs)
+ -- Hack alert! We do have some unlifted bindings, for cheap primops, and
+ -- it is ok to float them out; but not to the top level. If they would otherwise
+ -- go to the top level, we pin them inside the topmost lambda
\end{code}
-Recursive definitions. We want to transform
-
- letrec
- x1 = e1
- ...
- xn = en
- in
- body
-
-to
-
- letrec
- x1' = /\ ab -> let D' in e1
- ...
- xn' = /\ ab -> let D' in en
- in
- let D in body
-
-where ab are the tyvars pinning the defn further in than it
-need be, and D is a bunch of simple type applications:
-
- x1_cl = x1' ab
- ...
- xn_cl = xn' ab
-
-The "_cl" indicates that in D, the level numbers on the xi are the context level
-number; type applications aren't worth floating. The D' decls are
-similar:
-
- x1_ll = x1' ab
- ...
- xn_ll = xn' ab
-
-but differ in their level numbers; here the ab are the newly-introduced
-type lambdas.
\begin{code}
-lvlRecBind top_lvl ctxt_lvl env pairs
- | ids_only_lvl `ltLvl` tyvars_only_lvl
- = -- Abstract wrt tyvars;
- -- offending_tyvars is definitely non-empty
- -- (I love the ASSERT to check this... WDP 95/02)
+lvlBind top_lvl ctxt_lvl env (AnnRec pairs)
+ | null abs_vars
+ = cloneVars top_lvl env bndrs ctxt_lvl dest_lvl `thenLvl` \ (new_env, new_bndrs) ->
+ mapLvl (lvlExpr ctxt_lvl new_env) rhss `thenLvl` \ new_rhss ->
+ returnLvl (Rec ((new_bndrs `zip` repeat dest_lvl) `zip` new_rhss), new_env)
+
+ | isSingleton pairs && count isId abs_vars > 1
+ = -- Special case for self recursion where there are
+ -- several variables carried around: build a local loop:
+ -- poly_f = \abs_vars. \lam_vars . letrec f = \lam_vars. rhs in f lam_vars
+ -- This just makes the closures a bit smaller. If we don't do
+ -- this, allocation rises significantly on some programs
+ --
+ -- We could elaborate it for the case where there are several
+ -- mutually functions, but it's quite a bit more complicated
+ --
+ -- This all seems a bit ad hoc -- sigh
let
- incd_lvl = incMinorLvl ids_only_lvl
- tyvars_w_rhs_lvl = [(var,incd_lvl) | var <- offending_tyvars]
- bndrs_w_rhs_lvl = [(var,incd_lvl) | var <- bndrs]
- rhs_env = extendLvlEnv env (tyvars_w_rhs_lvl ++ bndrs_w_rhs_lvl)
+ (bndr,rhs) = head pairs
+ (rhs_lvl, abs_vars_w_lvls) = lvlLamBndrs dest_lvl abs_vars
+ rhs_env = extendLvlEnv env abs_vars_w_lvls
in
- mapLvl (lvlExpr incd_lvl rhs_env) rhss `thenLvl` \ rhss' ->
- mapLvl newLvlVar poly_tys `thenLvl` \ poly_vars ->
- cloneVars top_lvl env bndrs ctxt_lvl `thenLvl` \ (new_env, new_bndrs) ->
+ cloneVar NotTopLevel rhs_env bndr rhs_lvl rhs_lvl `thenLvl` \ (rhs_env', new_bndr) ->
let
- -- The "d_rhss" are the right-hand sides of "D" and "D'"
- -- in the documentation above
- d_rhss = [ mkTyApps (Var poly_var) offending_tyvar_tys | poly_var <- poly_vars]
-
- -- "local_binds" are "D'" in the documentation above
- local_binds = zipWithEqual "SetLevels" NonRec bndrs_w_rhs_lvl d_rhss
-
- poly_var_rhss = [ mkLams tyvars_w_rhs_lvl (mkLets local_binds rhs')
- | rhs' <- rhss'
- ]
-
- poly_binds = zipEqual "poly_binds" [(poly_var, ids_only_lvl) | poly_var <- poly_vars]
- poly_var_rhss
-
- -- The new right-hand sides, just a type application,
- -- aren't worth floating so pin it with ctxt_lvl
- bndrs_w_lvl = new_bndrs `zip` repeat ctxt_lvl
-
- -- "d_binds" are the "D" in the documentation above
- d_binds = zipWithEqual "SetLevels" NonRec bndrs_w_lvl d_rhss
+ (lam_bndrs, rhs_body) = collect_binders rhs
+ (body_lvl, new_lam_bndrs) = lvlLamBndrs rhs_lvl lam_bndrs
+ body_env = extendLvlEnv rhs_env' new_lam_bndrs
in
- returnLvl (Rec poly_binds : d_binds, new_env)
+ lvlExpr body_lvl body_env rhs_body `thenLvl` \ new_rhs_body ->
+ newPolyBndrs dest_lvl env abs_vars [bndr] `thenLvl` \ (poly_env, [poly_bndr]) ->
+ returnLvl (Rec [((poly_bndr,dest_lvl), mkLams abs_vars_w_lvls $
+ glue_binders new_lam_bndrs rhs $
+ Let (Rec [((new_bndr,rhs_lvl), mkLams new_lam_bndrs new_rhs_body)])
+ (mkVarApps (Var new_bndr) lam_bndrs))],
+ poly_env)
| otherwise
- = -- Let it float freely
- cloneVars top_lvl env bndrs expr_lvl `thenLvl` \ (new_env, new_bndrs) ->
- let
- bndrs_w_lvls = new_bndrs `zip` repeat expr_lvl
- in
- mapLvl (lvlExpr expr_lvl new_env) rhss `thenLvl` \ rhss' ->
- returnLvl ([Rec (bndrs_w_lvls `zip` rhss')], new_env)
+ = newPolyBndrs dest_lvl env abs_vars bndrs `thenLvl` \ (new_env, new_bndrs) ->
+ mapLvl (lvlFloatRhs abs_vars dest_lvl new_env) rhss `thenLvl` \ new_rhss ->
+ returnLvl (Rec ((new_bndrs `zip` repeat dest_lvl) `zip` new_rhss), new_env)
where
(bndrs,rhss) = unzip pairs
-- Finding the free vars of the binding group is annoying
- bind_fvs = (unionVarSets (map fst rhss) `unionVarSet` unionVarSets (map idFreeVars bndrs))
+ bind_fvs = (unionVarSets [ idFreeVars bndr `unionVarSet` rhs_fvs
+ | (bndr, (rhs_fvs,_)) <- pairs])
`minusVarSet`
mkVarSet bndrs
- ids_only_lvl = foldVarSet (maxIdLvl env) tOP_LEVEL bind_fvs
- tyvars_only_lvl = foldVarSet (maxTyVarLvl env) tOP_LEVEL bind_fvs
- expr_lvl = ids_only_lvl `maxLvl` tyvars_only_lvl
+ dest_lvl = destLevel env bind_fvs (all isFunction rhss)
+ abs_vars = abstractVars dest_lvl env bind_fvs
- offending_tyvars = filter offending_tv (varSetElems bind_fvs)
- offending_tv var | isId var = False
- | otherwise = ids_only_lvl `ltLvl` varLevel env var
- offending_tyvar_tys = mkTyVarTys offending_tyvars
+----------------------------------------------------
+-- Three help functons for the type-abstraction case
- tys = map idType bndrs
- poly_tys = map (mkForAllTys offending_tyvars) tys
+lvlFloatRhs abs_vars dest_lvl env rhs
+ = lvlExpr rhs_lvl rhs_env rhs `thenLvl` \ rhs' ->
+ returnLvl (mkLams abs_vars_w_lvls rhs')
+ where
+ (rhs_lvl, abs_vars_w_lvls) = lvlLamBndrs dest_lvl abs_vars
+ rhs_env = extendLvlEnv env abs_vars_w_lvls
\end{code}
+
+%************************************************************************
+%* *
+\subsection{Deciding floatability}
+%* *
+%************************************************************************
+
+\begin{code}
+lvlLamBndrs :: Level -> [CoreBndr] -> (Level, [(CoreBndr, Level)])
+-- Compute the levels for the binders of a lambda group
+-- The binders returned are exactly the same as the ones passed,
+-- but they are now paired with a level
+lvlLamBndrs lvl []
+ = (lvl, [])
+
+lvlLamBndrs lvl bndrs
+ = go (incMinorLvl lvl)
+ False -- Havn't bumped major level in this group
+ [] bndrs
+ where
+ go old_lvl bumped_major rev_lvld_bndrs (bndr:bndrs)
+ | isId bndr && -- Go to the next major level if this is a value binder,
+ not bumped_major && -- and we havn't already gone to the next level (one jump per group)
+ not (isOneShotLambda bndr) -- and it isn't a one-shot lambda
+ = go new_lvl True ((bndr,new_lvl) : rev_lvld_bndrs) bndrs
+
+ | otherwise
+ = go old_lvl bumped_major ((bndr,old_lvl) : rev_lvld_bndrs) bndrs
+
+ where
+ new_lvl = incMajorLvl old_lvl
+
+ go old_lvl _ rev_lvld_bndrs []
+ = (old_lvl, reverse rev_lvld_bndrs)
+ -- a lambda like this (\x -> coerce t (\s -> ...))
+ -- This happens quite a bit in state-transformer programs
+\end{code}
+
+\begin{code}
+abstractVars :: Level -> LevelEnv -> VarSet -> [Var]
+ -- Find the variables in fvs, free vars of the target expresion,
+ -- whose level is less than than the supplied level
+ -- These are the ones we are going to abstract out
+abstractVars dest_lvl env fvs
+ = uniq (sortLt lt [var | fv <- varSetElems fvs, var <- absVarsOf dest_lvl env fv])
+ where
+ -- Sort the variables so we don't get
+ -- mixed-up tyvars and Ids; it's just messy
+ v1 `lt` v2 = case (isId v1, isId v2) of
+ (True, False) -> False
+ (False, True) -> True
+ other -> v1 < v2 -- Same family
+ uniq :: [Var] -> [Var]
+ -- Remove adjacent duplicates; the sort will have brought them together
+ uniq (v1:v2:vs) | v1 == v2 = uniq (v2:vs)
+ | otherwise = v1 : uniq (v2:vs)
+ uniq vs = vs
+
+ -- Destintion level is the max Id level of the expression
+ -- (We'll abstract the type variables, if any.)
+destLevel :: LevelEnv -> VarSet -> Bool -> Level
+destLevel env fvs is_function
+ | floatLams env
+ && is_function = tOP_LEVEL -- Send functions to top level; see
+ -- the comments with isFunction
+ | otherwise = maxIdLevel env fvs
+
+isFunction :: CoreExprWithFVs -> Bool
+-- The idea here is that we want to float *functions* to
+-- the top level. This saves no work, but
+-- (a) it can make the host function body a lot smaller,
+-- and hence inlinable.
+-- (b) it can also save allocation when the function is recursive:
+-- h = \x -> letrec f = \y -> ...f...y...x...
+-- in f x
+-- becomes
+-- f = \x y -> ...(f x)...y...x...
+-- h = \x -> f x x
+-- No allocation for f now.
+-- We may only want to do this if there are sufficiently few free
+-- variables. We certainly only want to do it for values, and not for
+-- constructors. So the simple thing is just to look for lambdas
+isFunction (_, AnnLam b e) | isId b = True
+ | otherwise = isFunction e
+isFunction (_, AnnNote n e) = isFunction e
+isFunction other = False
+\end{code}
+
+
%************************************************************************
%* *
\subsection{Free-To-Level Monad}
%************************************************************************
\begin{code}
-type LevelEnv = (VarEnv Level, SubstEnv)
+type LevelEnv = (Bool, -- True <=> Float lambdas too
+ VarEnv Level, -- Domain is *post-cloned* TyVars and Ids
+ Subst, -- Domain is pre-cloned Ids; tracks the in-scope set
+ -- so that subtitution is capture-avoiding
+ IdEnv ([Var], LevelledExpr)) -- Domain is pre-cloned Ids
-- We clone let-bound variables so that they are still
- -- distinct when floated out; hence the SubstEnv
- -- The domain of the VarEnv is *pre-cloned* Ids, though
-
-initialEnv :: LevelEnv
-initialEnv = (emptyVarEnv, emptySubstEnv)
+ -- distinct when floated out; hence the SubstEnv/IdEnv.
+ -- (see point 3 of the module overview comment).
+ -- We also use these envs when making a variable polymorphic
+ -- because we want to float it out past a big lambda.
+ --
+ -- The SubstEnv and IdEnv always implement the same mapping, but the
+ -- SubstEnv maps to CoreExpr and the IdEnv to LevelledExpr
+ -- Since the range is always a variable or type application,
+ -- there is never any difference between the two, but sadly
+ -- the types differ. The SubstEnv is used when substituting in
+ -- a variable's IdInfo; the IdEnv when we find a Var.
+ --
+ -- In addition the IdEnv records a list of tyvars free in the
+ -- type application, just so we don't have to call freeVars on
+ -- the type application repeatedly.
+ --
+ -- The domain of the both envs is *pre-cloned* Ids, though
+ --
+ -- The domain of the VarEnv Level is the *post-cloned* Ids
+
+initialEnv :: Bool -> LevelEnv
+initialEnv float_lams = (float_lams, emptyVarEnv, emptySubst, emptyVarEnv)
+
+floatLams :: LevelEnv -> Bool
+floatLams (float_lams, _, _, _) = float_lams
extendLvlEnv :: LevelEnv -> [(Var,Level)] -> LevelEnv
- -- Used when *not* cloning
-extendLvlEnv (lvl_env, subst_env) prs
- = (foldl add lvl_env prs, subst_env)
- where
- add env (v,l) = extendVarEnv env v l
-
-varLevel :: LevelEnv -> IdOrTyVar -> Level
-varLevel (lvl_env, _) v
- = case lookupVarEnv lvl_env v of
- Just level -> level
- Nothing -> tOP_LEVEL
+-- Used when *not* cloning
+extendLvlEnv (float_lams, lvl_env, subst, id_env) prs
+ = (float_lams,
+ foldl add_lvl lvl_env prs,
+ foldl del_subst subst prs,
+ foldl del_id id_env prs)
+ where
+ add_lvl env (v,l) = extendVarEnv env v l
+ del_subst env (v,_) = extendInScope env v
+ del_id env (v,_) = delVarEnv env v
+ -- We must remove any clone for this variable name in case of
+ -- shadowing. This bit me in the following case
+ -- (in nofib/real/gg/Spark.hs):
+ --
+ -- case ds of wild {
+ -- ... -> case e of wild {
+ -- ... -> ... wild ...
+ -- }
+ -- }
+ --
+ -- The inside occurrence of @wild@ was being replaced with @ds@,
+ -- incorrectly, because the SubstEnv was still lying around. Ouch!
+ -- KSW 2000-07.
+
+-- extendCaseBndrLvlEnv adds the mapping case-bndr->scrut-var if it can
+-- (see point 4 of the module overview comment)
+extendCaseBndrLvlEnv env scrut case_bndr lvl
+ = case scrut of
+ Var v -> extendCloneLvlEnv lvl env [(case_bndr, v)]
+ other -> extendLvlEnv env [(case_bndr,lvl)]
+
+extendPolyLvlEnv dest_lvl (float_lams, lvl_env, subst, id_env) abs_vars bndr_pairs
+ = (float_lams,
+ foldl add_lvl lvl_env bndr_pairs,
+ foldl add_subst subst bndr_pairs,
+ foldl add_id id_env bndr_pairs)
+ where
+ add_lvl env (v,v') = extendVarEnv env v' dest_lvl
+ add_subst env (v,v') = extendSubst env v (DoneEx (mkVarApps (Var v') abs_vars))
+ add_id env (v,v') = extendVarEnv env v ((v':abs_vars), mkVarApps (Var v') abs_vars)
+
+extendCloneLvlEnv lvl (float_lams, lvl_env, subst, id_env) bndr_pairs
+ = (float_lams,
+ foldl add_lvl lvl_env bndr_pairs,
+ foldl add_subst subst bndr_pairs,
+ foldl add_id id_env bndr_pairs)
+ where
+ add_lvl env (v,v') = extendVarEnv env v' lvl
+ add_subst env (v,v') = extendSubst env v (DoneEx (Var v'))
+ add_id env (v,v') = extendVarEnv env v ([v'], Var v')
+
+
+maxIdLevel :: LevelEnv -> VarSet -> Level
+maxIdLevel (_, lvl_env,_,id_env) var_set
+ = foldVarSet max_in tOP_LEVEL var_set
+ where
+ max_in in_var lvl = foldr max_out lvl (case lookupVarEnv id_env in_var of
+ Just (abs_vars, _) -> abs_vars
+ Nothing -> [in_var])
+
+ max_out out_var lvl
+ | isId out_var = case lookupVarEnv lvl_env out_var of
+ Just lvl' -> maxLvl lvl' lvl
+ Nothing -> lvl
+ | otherwise = lvl -- Ignore tyvars in *maxIdLevel*
lookupVar :: LevelEnv -> Id -> LevelledExpr
-lookupVar (_, subst) v = case lookupSubstEnv subst v of
- Just (DoneEx (Var v')) -> Var v' -- Urgh! Types don't match
- other -> Var v
-
-maxIdLvl :: LevelEnv -> IdOrTyVar -> Level -> Level
-maxIdLvl (lvl_env,_) var lvl | isTyVar var = lvl
- | otherwise = case lookupVarEnv lvl_env var of
- Just lvl' -> maxLvl lvl' lvl
- Nothing -> lvl
-
-maxTyVarLvl :: LevelEnv -> IdOrTyVar -> Level -> Level
-maxTyVarLvl (lvl_env,_) var lvl | isId var = lvl
- | otherwise = case lookupVarEnv lvl_env var of
- Just lvl' -> maxLvl lvl' lvl
- Nothing -> lvl
+lookupVar (_, _, _, id_env) v = case lookupVarEnv id_env v of
+ Just (_, expr) -> expr
+ other -> Var v
+
+absVarsOf :: Level -> LevelEnv -> Var -> [Var]
+ -- If f is free in the exression, and f maps to poly_f a b c in the
+ -- current substitution, then we must report a b c as candidate type
+ -- variables
+absVarsOf dest_lvl (_, lvl_env, _, id_env) v
+ | isId v
+ = [final_av | av <- lookup_avs v, abstract_me av, final_av <- add_tyvars av]
+
+ | otherwise
+ = if abstract_me v then [v] else []
+
+ where
+ abstract_me v = case lookupVarEnv lvl_env v of
+ Just lvl -> dest_lvl `ltLvl` lvl
+ Nothing -> False
+
+ lookup_avs v = case lookupVarEnv id_env v of
+ Just (abs_vars, _) -> abs_vars
+ Nothing -> [v]
+
+ -- We are going to lambda-abstract, so nuke any IdInfo,
+ -- and add the tyvars of the Id
+ add_tyvars v | isId v = zap v : varSetElems (idFreeTyVars v)
+ | otherwise = [v]
+
+ zap v = WARN( workerExists (idWorkerInfo v)
+ || not (isEmptyCoreRules (idSpecialisation v)),
+ text "absVarsOf: discarding info on" <+> ppr v )
+ setIdInfo v vanillaIdInfo
\end{code}
\begin{code}
\end{code}
\begin{code}
-newLvlVar :: Type -> LvlM Id
-newLvlVar ty = getUniqueUs `thenLvl` \ uniq ->
- returnUs (mkSysLocal SLIT("lvl") uniq ty)
-
+newPolyBndrs dest_lvl env abs_vars bndrs
+ = getUniquesUs (length bndrs) `thenLvl` \ uniqs ->
+ let
+ new_bndrs = zipWith mk_poly_bndr bndrs uniqs
+ in
+ returnLvl (extendPolyLvlEnv dest_lvl env abs_vars (bndrs `zip` new_bndrs), new_bndrs)
+ where
+ mk_poly_bndr bndr uniq = mkSysLocal (_PK_ str) uniq poly_ty
+ where
+ str = "poly_" ++ occNameUserString (getOccName bndr)
+ poly_ty = foldr mkPiType (idType bndr) abs_vars
+
+
+newLvlVar :: String
+ -> [CoreBndr] -> Type -- Abstract wrt these bndrs
+ -> LvlM Id
+newLvlVar str vars body_ty
+ = getUniqueUs `thenLvl` \ uniq ->
+ returnUs (mkSysLocal (_PK_ str) uniq (foldr mkPiType body_ty vars))
+
-- The deeply tiresome thing is that we have to apply the substitution
-- to the rules inside each Id. Grr. But it matters.
-cloneVar :: TopLevelFlag -> LevelEnv -> Id -> Level -> LvlM (LevelEnv, Id)
-cloneVar TopLevel env v lvl
+cloneVar :: TopLevelFlag -> LevelEnv -> Id -> Level -> Level -> LvlM (LevelEnv, Id)
+cloneVar TopLevel env v ctxt_lvl dest_lvl
= returnUs (env, v) -- Don't clone top level things
-cloneVar NotTopLevel (lvl_env, subst_env) v lvl
- = getUniqueUs `thenLvl` \ uniq ->
+cloneVar NotTopLevel env v ctxt_lvl dest_lvl
+ = ASSERT( isId v )
+ getUniqueUs `thenLvl` \ uniq ->
let
- subst = mkSubst emptyVarSet subst_env
v' = setVarUnique v uniq
- v'' = modifyIdInfo (\info -> substIdInfo subst info info) v'
- subst_env' = extendSubstEnv subst_env v (DoneEx (Var v''))
- lvl_env' = extendVarEnv lvl_env v lvl
+ v'' = subst_id_info env ctxt_lvl dest_lvl v'
+ env' = extendCloneLvlEnv dest_lvl env [(v,v'')]
in
- returnUs ((lvl_env', subst_env'), v'')
+ returnUs (env', v'')
-cloneVars :: TopLevelFlag -> LevelEnv -> [Id] -> Level -> LvlM (LevelEnv, [Id])
-cloneVars TopLevel env vs lvl
+cloneVars :: TopLevelFlag -> LevelEnv -> [Id] -> Level -> Level -> LvlM (LevelEnv, [Id])
+cloneVars TopLevel env vs ctxt_lvl dest_lvl
= returnUs (env, vs) -- Don't clone top level things
-cloneVars NotTopLevel (lvl_env, subst_env) vs lvl
- = getUniquesUs (length vs) `thenLvl` \ uniqs ->
+cloneVars NotTopLevel env vs ctxt_lvl dest_lvl
+ = ASSERT( all isId vs )
+ getUniquesUs (length vs) `thenLvl` \ uniqs ->
let
- subst = mkSubst emptyVarSet subst_env'
vs' = zipWith setVarUnique vs uniqs
- vs'' = map (modifyIdInfo (\info -> substIdInfo subst info info)) vs'
- subst_env' = extendSubstEnvList subst_env vs [DoneEx (Var v'') | v'' <- vs'']
- lvl_env' = extendVarEnvList lvl_env (vs `zip` repeat lvl)
+ vs'' = map (subst_id_info env' ctxt_lvl dest_lvl) vs'
+ env' = extendCloneLvlEnv dest_lvl env (vs `zip` vs'')
in
- returnUs ((lvl_env', subst_env'), vs'')
+ returnUs (env', vs'')
+
+subst_id_info (_, _, subst, _) ctxt_lvl dest_lvl v
+ = modifyIdInfo (\info -> substIdInfo subst info (zap_dmd info)) v
+ where
+ -- VERY IMPORTANT: we must zap the demand info
+ -- if the thing is going to float out past a lambda
+ zap_dmd info
+ | stays_put || not (isStrict (demandInfo info)) = info
+ | otherwise = setDemandInfo info wwLazy
+
+ stays_put = ctxt_lvl == dest_lvl
\end{code}
+
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