#include "HsVersions.h"
import CoreSyn
+import CoreUtils ( mkSCC, exprIsHNF, exprIsTrivial )
-import CmdLineOpts ( opt_D_verbose_core2core, opt_D_simplifier_stats )
-import ErrUtils ( dumpIfSet )
+import DynFlags ( DynFlags, DynFlag(..), FloatOutSwitches(..) )
+import ErrUtils ( dumpIfSet_dyn )
import CostCentre ( dupifyCC, CostCentre )
-import Id ( Id )
-import Const ( isWHNFCon )
-import VarEnv
-import CoreLint ( beginPass, endPass )
-import PprCore
-import SetLevels ( setLevels,
- Level(..), tOP_LEVEL, ltMajLvl, ltLvl, isTopLvl
- )
-import BasicTypes ( Unused )
-import Var ( TyVar )
+import Id ( Id, idType )
+import Type ( isUnLiftedType )
+import CoreLint ( showPass, endPass )
+import SetLevels ( Level(..), LevelledExpr, LevelledBind,
+ setLevels, ltMajLvl, ltLvl, isTopLvl )
import UniqSupply ( UniqSupply )
import List ( partition )
import Outputable
+import Util ( notNull )
\end{code}
+ -----------------
+ Overall game plan
+ -----------------
+
+The Big Main Idea is:
+
+ To float out sub-expressions that can thereby get outside
+ a non-one-shot value lambda, and hence may be shared.
+
+
+To achieve this we may need to do two thing:
+
+ a) Let-bind the sub-expression:
+
+ f (g x) ==> let lvl = f (g x) in lvl
+
+ Now we can float the binding for 'lvl'.
+
+ b) More than that, we may need to abstract wrt a type variable
+
+ \x -> ... /\a -> let v = ...a... in ....
+
+ Here the binding for v mentions 'a' but not 'x'. So we
+ abstract wrt 'a', to give this binding for 'v':
+
+ vp = /\a -> ...a...
+ v = vp a
+
+ Now the binding for vp can float out unimpeded.
+ I can't remember why this case seemed important enough to
+ deal with, but I certainly found cases where important floats
+ didn't happen if we did not abstract wrt tyvars.
+
+With this in mind we can also achieve another goal: lambda lifting.
+We can make an arbitrary (function) binding float to top level by
+abstracting wrt *all* local variables, not just type variables, leaving
+a binding that can be floated right to top level. Whether or not this
+happens is controlled by a flag.
+
+
Random comments
~~~~~~~~~~~~~~~
Well, maybe. We don't do this at the moment.
\begin{code}
-type LevelledExpr = TaggedExpr Level
-type LevelledBind = TaggedBind Level
-type FloatBind = (Level, CoreBind)
-type FloatBinds = [FloatBind]
+type FloatBind = (Level, CoreBind) -- INVARIANT: a FloatBind is always lifted
+type FloatBinds = [FloatBind]
\end{code}
%************************************************************************
%************************************************************************
\begin{code}
-floatOutwards :: UniqSupply -> [CoreBind] -> IO [CoreBind]
+floatOutwards :: FloatOutSwitches
+ -> DynFlags
+ -> UniqSupply
+ -> [CoreBind] -> IO [CoreBind]
-floatOutwards us pgm
+floatOutwards float_sws dflags us pgm
= do {
- beginPass "Float out";
+ showPass dflags float_msg ;
- let { annotated_w_levels = setLevels pgm us ;
+ let { annotated_w_levels = setLevels float_sws pgm us ;
(fss, binds_s') = unzip (map floatTopBind annotated_w_levels)
} ;
- dumpIfSet opt_D_verbose_core2core "Levels added:"
+ dumpIfSet_dyn dflags Opt_D_verbose_core2core "Levels added:"
(vcat (map ppr annotated_w_levels));
let { (tlets, ntlets, lams) = get_stats (sum_stats fss) };
- dumpIfSet opt_D_simplifier_stats "FloatOut stats:"
+ dumpIfSet_dyn dflags Opt_D_dump_simpl_stats "FloatOut stats:"
(hcat [ int tlets, ptext SLIT(" Lets floated to top level; "),
int ntlets, ptext SLIT(" Lets floated elsewhere; from "),
int lams, ptext SLIT(" Lambda groups")]);
- endPass "Float out"
- opt_D_verbose_core2core {- no specific flag for dumping float-out -}
- (concat binds_s')
+ endPass dflags float_msg Opt_D_verbose_core2core (concat binds_s')
+ {- no specific flag for dumping float-out -}
}
+ where
+ float_msg = showSDoc (text "Float out" <+> parens (sws float_sws))
+ sws (FloatOutSw lam const) = pp_not lam <+> text "lambdas" <> comma <+>
+ pp_not const <+> text "constants"
+ pp_not True = empty
+ pp_not False = text "not"
floatTopBind bind@(NonRec _ _)
- = case (floatBind emptyVarEnv tOP_LEVEL bind) of { (fs, floats, bind', _) ->
+ = case (floatBind bind) of { (fs, floats, bind') ->
(fs, floatsToBinds floats ++ [bind'])
}
floatTopBind bind@(Rec _)
- = case (floatBind emptyVarEnv tOP_LEVEL bind) of { (fs, floats, Rec pairs', _) ->
- -- Actually floats will be empty
- --false:ASSERT(null floats)
- (fs, [Rec (floatsToBindPairs floats ++ pairs')])
- }
+ = case (floatBind bind) of { (fs, floats, Rec pairs') ->
+ WARN( notNull floats, ppr bind $$ ppr floats )
+ (fs, [Rec (floatsToBindPairs floats ++ pairs')]) }
\end{code}
%************************************************************************
\begin{code}
-floatBind :: IdEnv Level
- -> Level
- -> LevelledBind
- -> (FloatStats, FloatBinds, CoreBind, IdEnv Level)
+floatBind :: LevelledBind
+ -> (FloatStats, FloatBinds, CoreBind)
-floatBind env lvl (NonRec (name,level) rhs)
- = case (floatExpr env level rhs) of { (fs, rhs_floats, rhs') ->
+floatBind (NonRec (TB name level) rhs)
+ = case (floatNonRecRhs level rhs) of { (fs, rhs_floats, rhs') ->
+ (fs, rhs_floats, NonRec name rhs') }
- -- A good dumping point
- case (partitionByMajorLevel level rhs_floats) of { (rhs_floats', heres) ->
-
- (fs, rhs_floats',
- NonRec name (install heres rhs'),
- extendVarEnv env name level)
- }}
-
-floatBind env lvl bind@(Rec pairs)
+floatBind bind@(Rec pairs)
= case (unzip3 (map do_pair pairs)) of { (fss, rhss_floats, new_pairs) ->
- if not (isTopLvl bind_level) then
- -- Standard case
- (sum_stats fss, concat rhss_floats, Rec new_pairs, new_env)
+ if not (isTopLvl bind_dest_level) then
+ -- Standard case; the floated bindings can't mention the
+ -- binders, because they couldn't be escaping a major level
+ -- if so.
+ (sum_stats fss, concat rhss_floats, Rec new_pairs)
else
- {- In a recursive binding, destined for the top level (only),
- the rhs floats may contain
- references to the bound things. For example
-
- f = ...(let v = ...f... in b) ...
-
- might get floated to
-
- v = ...f...
- f = ... b ...
-
- and hence we must (pessimistically) make all the floats recursive
- with the top binding. Later dependency analysis will unravel it.
- -}
-
- (sum_stats fss,
- [],
- Rec (new_pairs ++ floatsToBindPairs (concat rhss_floats)),
- new_env)
-
+ -- In a recursive binding, *destined for* the top level
+ -- (only), the rhs floats may contain references to the
+ -- bound things. For example
+ -- f = ...(let v = ...f... in b) ...
+ -- might get floated to
+ -- v = ...f...
+ -- f = ... b ...
+ -- and hence we must (pessimistically) make all the floats recursive
+ -- with the top binding. Later dependency analysis will unravel it.
+ --
+ -- This can only happen for bindings destined for the top level,
+ -- because only then will partitionByMajorLevel allow through a binding
+ -- that only differs in its minor level
+ (sum_stats fss, [],
+ Rec (new_pairs ++ floatsToBindPairs (concat rhss_floats)))
}
where
- new_env = extendVarEnvList env (map fst pairs)
-
- bind_level = getBindLevel bind
-
- do_pair ((name, level), rhs)
- = case (floatExpr new_env level rhs) of { (fs, rhs_floats, rhs') ->
+ bind_dest_level = getBindLevel bind
- -- A good dumping point
- case (partitionByMajorLevel level rhs_floats) of { (rhs_floats', heres) ->
-
- (fs, rhs_floats', (name, install heres rhs'))
- }}
+ do_pair (TB name level, rhs)
+ = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
+ (fs, rhs_floats, (name, rhs'))
+ }
\end{code}
%************************************************************************
%************************************************************************
\begin{code}
-floatExpr :: IdEnv Level
- -> Level
- -> LevelledExpr
- -> (FloatStats, FloatBinds, CoreExpr)
-
-floatExpr env _ (Var v) = (zeroStats, [], Var v)
-floatExpr env _ (Type ty) = (zeroStats, [], Type ty)
-floatExpr env lvl (Con con as)
- = case floatList (floatExpr env lvl) as of { (stats, floats, as') ->
- (stats, floats, Con con as') }
+floatExpr, floatRhs, floatNonRecRhs
+ :: Level
+ -> LevelledExpr
+ -> (FloatStats, FloatBinds, CoreExpr)
+
+floatRhs lvl arg -- Used rec rhss, and case-alternative rhss
+ = case (floatExpr lvl arg) of { (fsa, floats, arg') ->
+ case (partitionByMajorLevel lvl floats) of { (floats', heres) ->
+ -- Dump bindings that aren't going to escape from a lambda;
+ -- in particular, we must dump the ones that are bound by
+ -- the rec or case alternative
+ (fsa, floats', install heres arg') }}
+
+floatNonRecRhs lvl arg -- Used for nested non-rec rhss, and fn args
+ = case (floatExpr lvl arg) of { (fsa, floats, arg') ->
+ -- Dump bindings that aren't going to escape from a lambda
+ -- This isn't a scoping issue (the binder isn't in scope in the RHS of a non-rec binding)
+ -- Rather, it is to avoid floating the x binding out of
+ -- f (let x = e in b)
+ -- unnecessarily. But we first test for values or trival rhss,
+ -- because (in particular) we don't want to insert new bindings between
+ -- the "=" and the "\". E.g.
+ -- f = \x -> let <bind> in <body>
+ -- We do not want
+ -- f = let <bind> in \x -> <body>
+ -- (a) The simplifier will immediately float it further out, so we may
+ -- as well do so right now; in general, keeping rhss as manifest
+ -- values is good
+ -- (b) If a float-in pass follows immediately, it might add yet more
+ -- bindings just after the '='. And some of them might (correctly)
+ -- be strict even though the 'let f' is lazy, because f, being a value,
+ -- gets its demand-info zapped by the simplifier.
+ if exprIsHNF arg' || exprIsTrivial arg' then
+ (fsa, floats, arg')
+ else
+ case (partitionByMajorLevel lvl floats) of { (floats', heres) ->
+ (fsa, floats', install heres arg') }}
+
+floatExpr _ (Var v) = (zeroStats, [], Var v)
+floatExpr _ (Type ty) = (zeroStats, [], Type ty)
+floatExpr _ (Lit lit) = (zeroStats, [], Lit lit)
-floatExpr env lvl (App e a)
- = case (floatExpr env lvl e) of { (fse, floats_e, e') ->
- case (floatExpr env lvl a) of { (fsa, floats_a, a') ->
+floatExpr lvl (App e a)
+ = case (floatExpr lvl e) of { (fse, floats_e, e') ->
+ case (floatNonRecRhs lvl a) of { (fsa, floats_a, a') ->
(fse `add_stats` fsa, floats_e ++ floats_a, App e' a') }}
-floatExpr env lvl (Lam (tv,incd_lvl) e)
- | isTyVar tv
- = case (floatExpr env incd_lvl e) of { (fs, floats, e') ->
-
- -- Dump any bindings which absolutely cannot go any further
- case (partitionByLevel incd_lvl floats) of { (floats', heres) ->
-
- (fs, floats', Lam tv (install heres e'))
- }}
-
-floatExpr env lvl (Lam (arg,incd_lvl) rhs)
- = ASSERT( isId arg )
- let
- new_env = extendVarEnv env arg incd_lvl
+floatExpr lvl lam@(Lam _ _)
+ = let
+ (bndrs_w_lvls, body) = collectBinders lam
+ bndrs = [b | TB b _ <- bndrs_w_lvls]
+ lvls = [l | TB b l <- bndrs_w_lvls]
+
+ -- For the all-tyvar case we are prepared to pull
+ -- the lets out, to implement the float-out-of-big-lambda
+ -- transform; but otherwise we only float bindings that are
+ -- going to escape a value lambda.
+ -- In particular, for one-shot lambdas we don't float things
+ -- out; we get no saving by so doing.
+ partition_fn | all isTyVar bndrs = partitionByLevel
+ | otherwise = partitionByMajorLevel
in
- case (floatExpr new_env incd_lvl rhs) of { (fs, floats, rhs') ->
+ case (floatExpr (last lvls) body) of { (fs, floats, body') ->
-- Dump any bindings which absolutely cannot go any further
- case (partitionByLevel incd_lvl floats) of { (floats', heres) ->
+ case (partition_fn (head lvls) floats) of { (floats', heres) ->
- (add_to_stats fs floats',
- floats',
- Lam arg (install heres rhs'))
+ (add_to_stats fs floats', floats', mkLams bndrs (install heres body'))
}}
-floatExpr env lvl (Note note@(SCC cc) expr)
- = case (floatExpr env lvl expr) of { (fs, floating_defns, expr') ->
+floatExpr lvl (Note note@(SCC cc) expr)
+ = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
let
-- Annotate bindings floated outwards past an scc expression
-- with the cc. We mark that cc as "duplicated", though.
= [ (level, ann_bind floater) | (level, floater) <- defn_groups ]
where
ann_bind (NonRec binder rhs)
- = NonRec binder (ann_rhs rhs)
+ = NonRec binder (mkSCC dupd_cc rhs)
ann_bind (Rec pairs)
- = Rec [(binder, ann_rhs rhs) | (binder, rhs) <- pairs]
-
- ann_rhs (Lam arg e) = Lam arg (ann_rhs e)
- ann_rhs rhs@(Con con _) | isWHNFCon con = rhs -- no point in scc'ing WHNF data
- ann_rhs rhs = Note (SCC dupd_cc) rhs
-
- -- Note: Nested SCC's are preserved for the benefit of
- -- cost centre stack profiling (Durham)
-
-floatExpr env lvl (Note note expr) -- Other than SCCs
- = case (floatExpr env lvl expr) of { (fs, floating_defns, expr') ->
+ = Rec [(binder, mkSCC dupd_cc rhs) | (binder, rhs) <- pairs]
+
+floatExpr lvl (Note InlineMe expr) -- Other than SCCs
+ = case floatExpr InlineCtxt expr of { (fs, floating_defns, expr') ->
+ -- There can be some floating_defns, arising from
+ -- ordinary lets that were there all the time. It seems
+ -- more efficient to test once here than to avoid putting
+ -- them into floating_defns (which would mean testing for
+ -- inlineCtxt at every let)
+ (fs, [], Note InlineMe (install floating_defns expr')) } -- See notes in SetLevels
+
+floatExpr lvl (Note note expr) -- Other than SCCs
+ = case (floatExpr lvl expr) of { (fs, floating_defns, expr') ->
(fs, floating_defns, Note note expr') }
-floatExpr env lvl (Let bind body)
- = case (floatBind env lvl bind) of { (fsb, rhs_floats, bind', new_env) ->
- case (floatExpr new_env lvl body) of { (fse, body_floats, body') ->
+floatExpr lvl (Let (NonRec (TB bndr bndr_lvl) rhs) body)
+ | isUnLiftedType (idType bndr) -- Treat unlifted lets just like a case
+ = case floatExpr lvl rhs of { (fs, rhs_floats, rhs') ->
+ case floatRhs bndr_lvl body of { (fs, body_floats, body') ->
+ (fs, rhs_floats ++ body_floats, Let (NonRec bndr rhs') body') }}
+
+floatExpr lvl (Let bind body)
+ = case (floatBind bind) of { (fsb, rhs_floats, bind') ->
+ case (floatExpr lvl body) of { (fse, body_floats, body') ->
(add_stats fsb fse,
rhs_floats ++ [(bind_lvl, bind')] ++ body_floats,
- body')
- }}
+ body') }}
where
bind_lvl = getBindLevel bind
-floatExpr env lvl (Case scrut (case_bndr, case_lvl) alts)
- = case floatExpr env lvl scrut of { (fse, fde, scrut') ->
+floatExpr lvl (Case scrut (TB case_bndr case_lvl) ty alts)
+ = case floatExpr lvl scrut of { (fse, fde, scrut') ->
case floatList float_alt alts of { (fsa, fda, alts') ->
- (add_stats fse fsa, fda ++ fde, Case scrut' case_bndr alts')
+ (add_stats fse fsa, fda ++ fde, Case scrut' case_bndr ty alts')
}}
where
- alts_env = extendVarEnv env case_bndr case_lvl
-
- partition_fn = partitionByMajorLevel
-
- float_alt (con, bs, rhs)
- = let
- bs' = map fst bs
- new_env = extendVarEnvList alts_env bs
- in
- case (floatExpr new_env case_lvl rhs) of { (fs, rhs_floats, rhs') ->
- case (partition_fn case_lvl rhs_floats) of { (rhs_floats', heres) ->
- (fs, rhs_floats', (con, bs', install heres rhs')) }}
+ -- Use floatRhs for the alternatives, so that we
+ -- don't gratuitiously float bindings out of the RHSs
+ float_alt (con, bs, rhs)
+ = case (floatRhs case_lvl rhs) of { (fs, rhs_floats, rhs') ->
+ (fs, rhs_floats, (con, [b | TB b _ <- bs], rhs')) }
floatList :: (a -> (FloatStats, FloatBinds, b)) -> [a] -> (FloatStats, FloatBinds, [b])
%************************************************************************
\begin{code}
-getBindLevel (NonRec (_, lvl) _) = lvl
-getBindLevel (Rec (((_,lvl), _) : _)) = lvl
+getBindLevel (NonRec (TB _ lvl) _) = lvl
+getBindLevel (Rec (((TB _ lvl), _) : _)) = lvl
\end{code}
\begin{code}
partitionByMajorLevel ctxt_lvl defns
= partition float_further defns
where
- float_further (my_lvl, _) = my_lvl `ltMajLvl` ctxt_lvl ||
- isTopLvl my_lvl
+ -- Float it if we escape a value lambda, or if we get to the top level
+ float_further (my_lvl, bind) = my_lvl `ltMajLvl` ctxt_lvl || isTopLvl my_lvl
+ -- The isTopLvl part says that if we can get to the top level, say "yes" anyway
+ -- This means that
+ -- x = f e
+ -- transforms to
+ -- lvl = e
+ -- x = f lvl
+ -- which is as it should be
partitionByLevel ctxt_lvl defns
= partition float_further defns
projects / ghc-hetmet.git / blobdiff