import CoreSyn
import CoreUtils ( mkSCC )
-import CmdLineOpts ( DynFlags, DynFlag(..) )
+import CmdLineOpts ( DynFlags, DynFlag(..), FloatOutSwitches(..) )
import ErrUtils ( dumpIfSet_dyn )
import CostCentre ( dupifyCC, CostCentre )
import Id ( Id )
-import VarEnv
import CoreLint ( showPass, endPass )
import SetLevels ( setLevels, isInlineCtxt,
Level(..), tOP_LEVEL, ltMajLvl, ltLvl, isTopLvl
import Outputable
\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
~~~~~~~~~~~~~~~
\begin{code}
floatOutwards :: DynFlags
- -> Bool -- True <=> float lambdas to top level
+ -> FloatOutSwitches
-> UniqSupply
-> [CoreBind] -> IO [CoreBind]
-floatOutwards dflags float_lams us pgm
+floatOutwards dflags float_sws us pgm
= do {
showPass dflags float_msg ;
- let { annotated_w_levels = setLevels float_lams pgm us ;
+ let { annotated_w_levels = setLevels float_sws pgm us ;
(fss, binds_s') = unzip (map floatTopBind annotated_w_levels)
} ;
{- no specific flag for dumping float-out -}
}
where
- float_msg | float_lams = "Float out (floating lambdas too)"
- | otherwise = "Float out (not floating lambdas)"
+ 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( not (null 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 env lvl (NonRec (name,level) rhs)
- = case (floatRhs env level rhs) of { (fs, rhs_floats, rhs') ->
- (fs, rhs_floats,
- NonRec name rhs',
- extendVarEnv env name level)
- }
+floatBind :: LevelledBind
+ -> (FloatStats, FloatBinds, CoreBind)
-floatBind env lvl bind@(Rec pairs)
+floatBind (NonRec (name,level) rhs)
+ = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
+ (fs, rhs_floats, NonRec name rhs') }
+
+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)
+ (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.
+ --
+ -- Can't happen on nested bindings because floatRhs will dump
+ -- the bindings in the RHS (partitionByMajorLevel treats top specially)
+ (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 (floatRhs new_env level rhs) of { (fs, rhs_floats, rhs') ->
+ = case (floatRhs level rhs) of { (fs, rhs_floats, rhs') ->
(fs, rhs_floats, (name, rhs'))
}
\end{code}
\begin{code}
floatExpr, floatRhs
- :: IdEnv Level
- -> Level
+ :: Level
-> LevelledExpr
-> (FloatStats, FloatBinds, CoreExpr)
-floatRhs env lvl arg
- = case (floatExpr env lvl arg) of { (fsa, floats, arg') ->
+floatRhs lvl arg
+ = 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
-- This is to avoid floating the x binding out of
-- unnecessarily. It even causes a bug to do so if we have
-- y = writeArr# a n (let x = e in b)
-- because the y binding is an expr-ok-for-speculation one.
+ -- [SLPJ Dec 01: I don't understand this last comment;
+ -- writeArr# is not ok-for-spec because of its side effect]
(fsa, floats', install heres arg') }}
-floatExpr env _ (Var v) = (zeroStats, [], Var v)
-floatExpr env _ (Type ty) = (zeroStats, [], Type ty)
-floatExpr env _ (Lit lit) = (zeroStats, [], Lit lit)
+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 (floatRhs env lvl a) of { (fsa, floats_a, a') ->
+floatExpr lvl (App e a)
+ = case (floatExpr lvl e) of { (fse, floats_e, e') ->
+ case (floatRhs 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, lvls) = unzip 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.
ann_bind (Rec pairs)
= Rec [(binder, mkSCC dupd_cc rhs) | (binder, rhs) <- pairs]
-floatExpr env lvl (Note InlineMe expr) -- Other than SCCs
- = case floatExpr env InlineCtxt expr of { (fs, floating_defns, expr') ->
+floatExpr lvl (Note InlineMe expr) -- Other than SCCs
+ = case floatExpr InlineCtxt expr of { (fs, floating_defns, expr') ->
WARN( not (null floating_defns),
ppr expr $$ ppr floating_defns ) -- We do no floating out of Inlines
(fs, [], Note InlineMe expr') } -- See notes in SetLevels
-floatExpr env lvl (Note note expr) -- Other than SCCs
- = case (floatExpr env lvl expr) of { (fs, floating_defns, expr') ->
+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') ->
- if isInlineCtxt lvl then -- No floating inside an InlineMe
- ASSERT( null rhs_floats && null body_floats )
- (add_stats fsb fse, [], Let bind' body')
- else
+floatExpr lvl (Let bind body)
+ = case (floatBind bind) of { (fsb, rhs_floats, bind') ->
+ case (floatExpr lvl body) of { (fse, body_floats, body') ->
+-- if isInlineCtxt lvl then -- No floating inside an InlineMe
+-- ASSERT( null rhs_floats && null body_floats )
+-- (add_stats fsb fse, [], Let bind' body')
+-- else
(add_stats fsb fse,
rhs_floats ++ [(bind_lvl, bind')] ++ body_floats,
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 (case_bndr, case_lvl) 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')
}}
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, map fst bs, rhs')) }
floatList :: (a -> (FloatStats, FloatBinds, b)) -> [a] -> (FloatStats, FloatBinds, [b])
partitionByMajorLevel ctxt_lvl defns
= partition float_further defns
where
- -- Float it if we escape a value lambda,
- -- or if we get to the top level
+ -- 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
projects / ghc-hetmet.git / blobdiff