Commit b284d370 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com
Browse files

Fix a long-standing bug the float-out pass

We were failing to float out a binding that could be floated,
because of a confusion in the Lam case of floatExpr.

In investigating this I also discoverd that there is really
no point at all in giving a different level to variables in
a binding group, so I've now given them all the same (in 
SetLevels.lvlLamBndrs

The overall difference is quite minor in a nofib run:

        Program           Size    Allocs   Runtime   Elapsed
-------------------------------------------------------------
            Min          +0.0%     -8.5%    -28.4%    -28.7%
            Max          +0.0%     +0.7%     -0.7%     -1.1%
 Geometric Mean          +0.0%     -0.0%    -11.6%    -11.8%

I don't trust those runtimes, but smaller is good!  The 8.5% 
improvement in allocation in fulsom, and seems real.  The 
0.7% allocation increase only happens in programs with
very small allocation.  I tracked one down to a call of this form

  GHC.IO.Handle.Internals.mkDuplexHandle5
    = \ args -> GHC.IO.Handle.Internals.openTextEncoding1
                  mb_codec ha_type
                  (\mb_encoder mb_decoder -> blah)

With the new floater the argument of openTextEncoding1 becomes

     (let lvl = .. in \mb_encoder mb_decoder -> blah)

And rightly so.  However in fact this argument is a continuation
and hence is called once, so the floating is fruitless.

Roll on one-shot-function analysis (which I know how to do
but fail to get to!).
parent d196d84a
......@@ -24,9 +24,11 @@ import UniqSupply ( UniqSupply )
import Bag
import Util
import Maybes
import UniqFM
import Outputable
import FastString
import qualified Data.IntMap as M
#include "HsVersions.h"
\end{code}
-----------------
......@@ -230,27 +232,20 @@ floatExpr lvl (App e a)
case (floatRhs lvl a) of { (fsa, floats_a, a') ->
(fse `add_stats` fsa, floats_e `plusFloats` floats_a, App e' a') }}
floatExpr _ lam@(Lam _ _)
= let
(bndrs_w_lvls, body) = collectBinders lam
floatExpr _ lam@(Lam (TB _ lam_lvl) _)
= let (bndrs_w_lvls, body) = collectBinders lam
bndrs = [b | TB b _ <- bndrs_w_lvls]
lvls = [l | TB _ 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 isTyCoVar bndrs = partitionByLevel
| otherwise = partitionByMajorLevel
-- All the binders have the same level
-- See SetLevels.lvlLamBndrs
in
case (floatExpr (last lvls) body) of { (fs, floats, body') ->
-- Dump any bindings which absolutely cannot go any further
case (partition_fn (head lvls) floats) of { (floats', heres) ->
(add_to_stats fs floats', floats', mkLams bndrs (install heres body'))
case (floatExpr lam_lvl body) of { (fs, floats, body1) ->
-- Dump anything that is captured by this lambda
-- Eg \x -> ...(\y -> let v = <blah> in ...)...
-- We'll have the binding (v = <blah>) in the floats,
-- but must dump it at the lambda-x
case (partitionByLevel lam_lvl floats) of { (floats1, heres) ->
(add_to_stats fs floats1, floats1, mkLams bndrs (install heres body1))
}}
floatExpr lvl (Note note@(SCC cc) expr)
......@@ -401,34 +396,39 @@ data FloatBinds = FB !(Bag FloatBind) -- Destined for top level
!MajorEnv -- Levels other than top
-- See Note [Representation of FloatBinds]
type MajorEnv = UniqFM MinorEnv -- Keyed by major level
type MinorEnv = UniqFM (Bag FloatBind) -- Keyed by minor level
instance Outputable FloatBinds where
ppr (FB fbs env) = ptext (sLit "FB") <+> (braces $ vcat
[ ptext (sLit "binds =") <+> ppr fbs
, ptext (sLit "env =") <+> ppr env ])
type MajorEnv = M.IntMap MinorEnv -- Keyed by major level
type MinorEnv = M.IntMap (Bag FloatBind) -- Keyed by minor level
flattenFloats :: FloatBinds -> Bag FloatBind
flattenFloats (FB tops others) = tops `unionBags` flattenMajor others
flattenMajor :: MajorEnv -> Bag FloatBind
flattenMajor = foldUFM (unionBags . flattenMinor) emptyBag
flattenMajor = M.fold (unionBags . flattenMinor) emptyBag
flattenMinor :: MinorEnv -> Bag FloatBind
flattenMinor = foldUFM unionBags emptyBag
flattenMinor = M.fold unionBags emptyBag
emptyFloats :: FloatBinds
emptyFloats = FB emptyBag emptyUFM
emptyFloats = FB emptyBag M.empty
unitFloat :: Level -> FloatBind -> FloatBinds
unitFloat lvl@(Level major minor) b
| isTopLvl lvl = FB (unitBag b) emptyUFM
| otherwise = FB emptyBag (unitUFM major (unitUFM minor (unitBag b)))
| isTopLvl lvl = FB (unitBag b) M.empty
| otherwise = FB emptyBag (M.singleton major (M.singleton minor (unitBag b)))
plusFloats :: FloatBinds -> FloatBinds -> FloatBinds
plusFloats (FB t1 b1) (FB t2 b2) = FB (t1 `unionBags` t2) (b1 `plusMajor` b2)
plusMajor :: MajorEnv -> MajorEnv -> MajorEnv
plusMajor = plusUFM_C plusMinor
plusMajor = M.unionWith plusMinor
plusMinor :: MinorEnv -> MinorEnv -> MinorEnv
plusMinor = plusUFM_C unionBags
plusMinor = M.unionWith unionBags
floatsToBindPairs :: Bag FloatBind -> [(Id,CoreExpr)] -> [(Id,CoreExpr)]
floatsToBindPairs floats binds = foldrBag add binds floats
......@@ -460,26 +460,26 @@ partitionByMajorLevel, partitionByLevel
partitionByMajorLevel (Level major _) (FB tops defns)
= (FB tops outer, heres `unionBags` flattenMajor inner)
where
(outer, mb_heres, inner) = splitUFM defns major
(outer, mb_heres, inner) = M.splitLookup major defns
heres = case mb_heres of
Nothing -> emptyBag
Just h -> flattenMinor h
partitionByLevel (Level major minor) (FB tops defns)
= (FB tops (outer_maj `plusMajor` unitUFM major outer_min),
= (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) = splitUFM defns major
(outer_maj, mb_here_maj, inner_maj) = M.splitLookup major defns
(outer_min, mb_here_min, inner_min) = case mb_here_maj of
Nothing -> (emptyUFM, Nothing, emptyUFM)
Just min_defns -> splitUFM min_defns minor
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 (wrap_defns tops) (mapUFM (mapUFM wrap_defns) defns)
= FB (wrap_defns tops) (M.map (M.map wrap_defns) defns)
where
wrap_defns = mapBag wrap_one
wrap_one (NonRec binder rhs) = NonRec binder (mkSCC cc rhs)
......
......@@ -671,26 +671,15 @@ lvlLamBndrs lvl []
= (lvl, [])
lvlLamBndrs lvl bndrs
= go (incMinorLvl lvl)
False -- Havn't bumped major level in this group
[] bndrs
= (new_lvl, [TB bndr new_lvl | bndr <- bndrs])
-- All the new binders get the same level, because
-- any floating binding is either going to float past
-- all or none. We never separate binders
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 (TB bndr new_lvl : rev_lvld_bndrs) bndrs
new_lvl | any is_major bndrs = incMajorLvl lvl
| otherwise = incMinorLvl lvl
| otherwise
= go old_lvl bumped_major (TB 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
is_major bndr = isId bndr && not (isOneShotLambda bndr)
\end{code}
\begin{code}
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment