%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[FloatOut]{Float bindings outwards (towards the top level)}
``Long-distance'' floating of bindings towards the top level.
\begin{code}
module FloatOut ( floatOutwards ) where
import CoreSyn
import CoreUtils
import CoreArity ( etaExpand )
import CoreMonad ( FloatOutSwitches(..) )
import DynFlags ( DynFlags, DynFlag(..) )
import ErrUtils ( dumpIfSet_dyn )
import CostCentre ( dupifyCC, CostCentre )
import DataCon ( DataCon )
import Id ( Id, idArity, isBottomingId )
import Var ( Var )
import SetLevels
import UniqSupply ( UniqSupply )
import Bag
import Util
import Maybes
import Outputable
import FastString
import qualified Data.IntMap as M
#include "HsVersions.h"
\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
~~~~~~~~~~~~~~~
At the moment we never float a binding out to between two adjacent
lambdas. For example:
@
\x y -> let t = x+x in ...
===>
\x -> let t = x+x in \y -> ...
@
Reason: this is less efficient in the case where the original lambda
is never partially applied.
But there's a case I've seen where this might not be true. Consider:
@
elEm2 x ys
= elem' x ys
where
elem' _ [] = False
elem' x (y:ys) = x==y || elem' x ys
@
It turns out that this generates a subexpression of the form
@
\deq x ys -> let eq = eqFromEqDict deq in ...
@
vwhich might usefully be separated to
@
\deq -> let eq = eqFromEqDict deq in \xy -> ...
@
Well, maybe. We don't do this at the moment.
%************************************************************************
%* *
\subsection[floatOutwards]{@floatOutwards@: let-floating interface function}
%* *
%************************************************************************
\begin{code}
floatOutwards :: FloatOutSwitches
-> DynFlags
-> UniqSupply
-> [CoreBind] -> IO [CoreBind]
floatOutwards float_sws dflags us pgm
= do {
let { annotated_w_levels = setLevels float_sws pgm us ;
(fss, binds_s') = unzip (map floatTopBind annotated_w_levels)
} ;
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_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")]);
return (bagToList (unionManyBags binds_s'))
}
floatTopBind :: LevelledBind -> (FloatStats, Bag CoreBind)
floatTopBind bind
= case (floatBind bind) of { (fs, floats, bind') ->
let float_bag = flattenTopFloats floats
in case bind' of
Rec prs -> (fs, unitBag (Rec (addTopFloatPairs float_bag prs)))
NonRec {} -> (fs, float_bag `snocBag` bind') }
\end{code}
%************************************************************************
%* *
\subsection[FloatOut-Bind]{Floating in a binding (the business end)}
%* *
%************************************************************************
\begin{code}
floatBind :: LevelledBind -> (FloatStats, FloatBinds, CoreBind)
floatBind (NonRec (TB var _) rhs)
= case (floatExpr rhs) of { (fs, rhs_floats, rhs') ->
let rhs'' | isBottomingId var = etaExpand (idArity var) rhs'
| otherwise = rhs'
in (fs, rhs_floats, NonRec var rhs'') }
floatBind (Rec pairs)
= case floatList do_pair pairs of { (fs, rhs_floats, new_pairs) ->
(fs, rhs_floats, Rec (concat new_pairs)) }
where
do_pair (TB name spec, rhs)
| isTopLvl dest_lvl
= case (floatExpr rhs) of { (fs, rhs_floats, rhs') ->
(fs, emptyFloats, addTopFloatPairs (flattenTopFloats rhs_floats) [(name, rhs')])}
| otherwise
= case (floatExpr rhs) of { (fs, rhs_floats, rhs') ->
case (partitionByLevel dest_lvl rhs_floats) of { (rhs_floats', heres) ->
case (splitRecFloats heres) of { (pairs, case_heres) ->
(fs, rhs_floats', (name, installUnderLambdas case_heres rhs') : pairs) }}}
where
dest_lvl = floatSpecLevel spec
splitRecFloats :: Bag FloatBind -> ([(Id,CoreExpr)], Bag FloatBind)
splitRecFloats fs
= go [] (bagToList fs)
where
go prs (FloatLet (NonRec b r) : fs) = go ((b,r):prs) fs
go prs (FloatLet (Rec prs') : fs) = go (prs' ++ prs) fs
go prs fs = (prs, listToBag fs)
installUnderLambdas :: Bag FloatBind -> CoreExpr -> CoreExpr
installUnderLambdas floats e
| isEmptyBag floats = e
| otherwise = go e
where
go (Lam b e) = Lam b (go e)
go (Note n e) | notSccNote n = Note n (go e)
go e = install floats e
floatList :: (a -> (FloatStats, FloatBinds, b)) -> [a] -> (FloatStats, FloatBinds, [b])
floatList _ [] = (zeroStats, emptyFloats, [])
floatList f (a:as) = case f a of { (fs_a, binds_a, b) ->
case floatList f as of { (fs_as, binds_as, bs) ->
(fs_a `add_stats` fs_as, binds_a `plusFloats` binds_as, b:bs) }}
\end{code}
Note [Floating out of Rec rhss]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider Rec { f<1,0> = \xy. body }
From the body we may get some floats. The ones with level <1,0> must
stay here, since they may mention f. Ideally we'd like to make them
part of the Rec block pairs -- but we can't if there are any
FloatCases involved.
Nor is it a good idea to dump them in the rhs, but outside the lambda
f = case x of I# y -> \xy. body
because now f's arity might get worse, which is Not Good. (And if
there's an SCC around the RHS it might not get better again.
See Trac #5342.)
So, gruesomely, we split the floats into
* the outer FloatLets, which can join the Rec, and
* an inner batch starting in a FloatCase, which are then
pushed *inside* the lambdas.
This loses full-laziness the rare situation where there is a
FloatCase and a Rec interacting.
Note [floatBind for top level]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may have a *nested* binding whose destination level is (FloatMe tOP_LEVEL), thus
letrec { foo <0,0> = .... (let bar<0,0> = .. in ..) .... }
The binding for bar will be in the "tops" part of the floating binds,
and thus not partioned by floatBody.
We could perhaps get rid of the 'tops' component of the floating binds,
but this case works just as well.
%************************************************************************
\subsection[FloatOut-Expr]{Floating in expressions}
%* *
%************************************************************************
\begin{code}
floatBody :: Level
-> LevelledExpr
-> (FloatStats, FloatBinds, CoreExpr)
floatBody lvl arg
= case (floatExpr arg) of { (fsa, floats, arg') ->
case (partitionByLevel lvl floats) of { (floats', heres) ->
(fsa, floats', install heres arg') }}
floatExpr :: LevelledExpr
-> (FloatStats, FloatBinds, CoreExpr)
floatExpr (Var v) = (zeroStats, emptyFloats, Var v)
floatExpr (Type ty) = (zeroStats, emptyFloats, Type ty)
floatExpr (Coercion co) = (zeroStats, emptyFloats, Coercion co)
floatExpr (Lit lit) = (zeroStats, emptyFloats, Lit lit)
floatExpr (App e a)
= case (floatExpr e) of { (fse, floats_e, e') ->
case (floatExpr a) of { (fsa, floats_a, a') ->
(fse `add_stats` fsa, floats_e `plusFloats` floats_a, App e' a') }}
floatExpr lam@(Lam (TB _ lam_spec) _)
= let (bndrs_w_lvls, body) = collectBinders lam
bndrs = [b | TB b _ <- bndrs_w_lvls]
bndr_lvl = floatSpecLevel lam_spec
in
case (floatBody bndr_lvl body) of { (fs, floats, body') ->
(add_to_stats fs floats, floats, mkLams bndrs body') }
floatExpr (Note note@(SCC cc) expr)
= case (floatExpr expr) of { (fs, floating_defns, expr') ->
let
annotated_defns = wrapCostCentre (dupifyCC cc) floating_defns
in
(fs, annotated_defns, Note note expr') }
floatExpr (Note note expr)
= case (floatExpr expr) of { (fs, floating_defns, expr') ->
(fs, floating_defns, Note note expr') }
floatExpr (Cast expr co)
= case (floatExpr expr) of { (fs, floating_defns, expr') ->
(fs, floating_defns, Cast expr' co) }
floatExpr (Let bind body)
= case bind_spec of
FloatMe dest_lvl
-> case (floatBind bind) of { (fsb, bind_floats, bind') ->
case (floatExpr body) of { (fse, body_floats, body') ->
( add_stats fsb fse
, bind_floats `plusFloats` unitLetFloat dest_lvl bind'
`plusFloats` body_floats
, body') }}
StayPut bind_lvl
-> case (floatBind bind) of { (fsb, bind_floats, bind') ->
case (floatBody bind_lvl body) of { (fse, body_floats, body') ->
( add_stats fsb fse
, bind_floats `plusFloats` body_floats
, Let bind' body') }}
where
bind_spec = case bind of
NonRec (TB _ s) _ -> s
Rec ((TB _ s, _) : _) -> s
Rec [] -> panic "floatExpr:rec"
floatExpr (Case scrut (TB case_bndr case_spec) ty alts)
= case case_spec of
FloatMe dest_lvl
| [(DataAlt con, bndrs, rhs)] <- alts
-> case floatExpr scrut of { (fse, fde, scrut') ->
case floatExpr rhs of { (fsb, fdb, rhs') ->
let
float = unitCaseFloat dest_lvl scrut'
case_bndr con [b | TB b _ <- bndrs]
in
(add_stats fse fsb, fde `plusFloats` float `plusFloats` fdb, rhs') }}
| otherwise
-> pprPanic "Floating multi-case" (ppr alts)
StayPut bind_lvl
-> case floatExpr scrut of { (fse, fde, scrut') ->
case floatList (float_alt bind_lvl) alts of { (fsa, fda, alts') ->
(add_stats fse fsa, fda `plusFloats` fde, Case scrut' case_bndr ty alts')
}}
where
float_alt bind_lvl (con, bs, rhs)
= case (floatBody bind_lvl rhs) of { (fs, rhs_floats, rhs') ->
(fs, rhs_floats, (con, [b | TB b _ <- bs], rhs')) }
\end{code}
Note [Avoiding unnecessary floating]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In general we want to avoid floating a let unnecessarily, because
it might worsen strictness:
let
x = ...(let y = e in y+y)....
Here y is demanded. If we float it outside the lazy 'x=..' then
we'd have to zap its demand info, and it may never be restored.
So at a 'let' we leave the binding right where the are unless
the binding will escape a value lambda. That's what the
partitionByMajorLevel does in the floatExpr (Let ...) case.
Notice, though, that we must take care to drop any bindings
from the body of the let that depend on the staying-put bindings.
We used instead to do the partitionByMajorLevel on the RHS of an '=',
in floatRhs. But that was quite tiresome. We needed to 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 in
We do not want
f = let in \x ->
(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.
And even all that turned out to be very fragile, and broke
altogether when profiling got in the way.
So now we do the partition right at the (Let..) itself.
%************************************************************************
%* *
\subsection{Utility bits for floating stats}
%* *
%************************************************************************
I didn't implement this with unboxed numbers. I don't want to be too
strict in this stuff, as it is rarely turned on. (WDP 95/09)
\begin{code}
data FloatStats
= FlS Int
Int
Int
get_stats :: FloatStats -> (Int, Int, Int)
get_stats (FlS a b c) = (a, b, c)
zeroStats :: FloatStats
zeroStats = FlS 0 0 0
sum_stats :: [FloatStats] -> FloatStats
sum_stats xs = foldr add_stats zeroStats xs
add_stats :: FloatStats -> FloatStats -> FloatStats
add_stats (FlS a1 b1 c1) (FlS a2 b2 c2)
= FlS (a1 + a2) (b1 + b2) (c1 + c2)
add_to_stats :: FloatStats -> FloatBinds -> FloatStats
add_to_stats (FlS a b c) (FB tops others)
= FlS (a + lengthBag tops) (b + lengthBag (flattenMajor others)) (c + 1)
\end{code}
%************************************************************************
%* *
\subsection{Utility bits for floating}
%* *
%************************************************************************
Note [Representation of FloatBinds]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The FloatBinds types is somewhat important. We can get very large numbers
of floating bindings, often all destined for the top level. A typical example
is x = [4,2,5,2,5, .... ]
Then we get lots of small expressions like (fromInteger 4), which all get
lifted to top level.
The trouble is that
(a) we partition these floating bindings *at every binding site*
(b) SetLevels introduces a new bindings site for every float
So we had better not look at each binding at each binding site!
That is why MajorEnv is represented as a finite map.
We keep the bindings destined for the *top* level separate, because
we float them out even if they don't escape a *value* lambda; see
partitionByMajorLevel.
\begin{code}
data FloatBind
= FloatLet FloatLet
| FloatCase CoreExpr Id DataCon [Var]
type FloatLet = CoreBind
type MajorEnv = M.IntMap MinorEnv
type MinorEnv = M.IntMap (Bag FloatBind)
data FloatBinds = FB !(Bag FloatLet)
!MajorEnv
instance Outputable FloatBind where
ppr (FloatLet b) = ptext (sLit "LET") <+> ppr b
ppr (FloatCase e b c bs) = hang (ptext (sLit "CASE") <+> ppr e <+> ptext (sLit "of") <+> ppr b)
2 (ppr c <+> ppr bs)
instance Outputable FloatBinds where
ppr (FB fbs defs)
= ptext (sLit "FB") <+> (braces $ vcat
[ ptext (sLit "tops =") <+> ppr fbs
, ptext (sLit "non-tops =") <+> ppr defs ])
flattenTopFloats :: FloatBinds -> Bag CoreBind
flattenTopFloats (FB tops defs)
= ASSERT2( isEmptyBag (flattenMajor defs), ppr defs )
tops
addTopFloatPairs :: Bag CoreBind -> [(Id,CoreExpr)] -> [(Id,CoreExpr)]
addTopFloatPairs float_bag prs
= foldrBag add prs float_bag
where
add (NonRec b r) prs = (b,r):prs
add (Rec prs1) prs2 = prs1 ++ prs2
flattenMajor :: MajorEnv -> Bag FloatBind
flattenMajor = M.fold (unionBags . flattenMinor) emptyBag
flattenMinor :: MinorEnv -> Bag FloatBind
flattenMinor = M.fold unionBags emptyBag
emptyFloats :: FloatBinds
emptyFloats = FB emptyBag M.empty
unitCaseFloat :: Level -> CoreExpr -> Id -> DataCon -> [Var] -> FloatBinds
unitCaseFloat (Level major minor) e b con bs
= FB emptyBag (M.singleton major (M.singleton minor (unitBag (FloatCase e b con bs))))
unitLetFloat :: Level -> FloatLet -> FloatBinds
unitLetFloat lvl@(Level major minor) b
| isTopLvl lvl = FB (unitBag b) M.empty
| otherwise = FB emptyBag (M.singleton major (M.singleton minor floats))
where
floats = unitBag (FloatLet b)
plusFloats :: FloatBinds -> FloatBinds -> FloatBinds
plusFloats (FB t1 l1) (FB t2 l2)
= FB (t1 `unionBags` t2) (l1 `plusMajor` l2)
plusMajor :: MajorEnv -> MajorEnv -> MajorEnv
plusMajor = M.unionWith plusMinor
plusMinor :: MinorEnv -> MinorEnv -> MinorEnv
plusMinor = M.unionWith unionBags
install :: Bag FloatBind -> CoreExpr -> CoreExpr
install defn_groups expr
= foldrBag install_group expr defn_groups
where
install_group (FloatLet defns) body
= Let defns body
install_group (FloatCase e b con bs) body
= Case e b (exprType body) [(DataAlt con, bs, body)]
partitionByLevel
:: Level
-> FloatBinds
-> (FloatBinds,
Bag FloatBind)
partitionByLevel (Level major minor) (FB tops defns)
= (FB tops (outer_maj `plusMajor` M.singleton major outer_min),
here_min `unionBags` flattenMinor inner_min
`unionBags` flattenMajor inner_maj)
where
(outer_maj, mb_here_maj, inner_maj) = M.splitLookup major defns
(outer_min, mb_here_min, inner_min) = case mb_here_maj of
Nothing -> (M.empty, Nothing, M.empty)
Just min_defns -> M.splitLookup minor min_defns
here_min = mb_here_min `orElse` emptyBag
wrapCostCentre :: CostCentre -> FloatBinds -> FloatBinds
wrapCostCentre cc (FB tops defns)
= FB (mapBag wrap_bind tops) (M.map (M.map wrap_defns) defns)
where
wrap_defns = mapBag wrap_one
wrap_bind (NonRec binder rhs) = NonRec binder (mkSCC cc rhs)
wrap_bind (Rec pairs) = Rec (mapSnd (mkSCC cc) pairs)
wrap_one (FloatLet bind) = FloatLet (wrap_bind bind)
wrap_one (FloatCase e b c bs) = FloatCase (mkSCC cc e) b c bs
\end{code}