Commit bbcea13a authored by Michal Terepeta's avatar Michal Terepeta Committed by Ben Gamari

Hoopl: improve postorder calculation

- Fix the naming and comments to indicate that we are calculating
  *reverse* postorder (and not the standard postorder).

- Rewrite the calculation to avoid CPS code. I found it fairly
  difficult to understand and the new one seems faster (according to
  nofib, decreases compiler allocations by 0.2%)

- Remove `LabelsPtr`, which seems unnecessary and could be *really*
  confusing. For instance, previously:
  `postorder_dfs_from <block with label X>`
  and
  `postorder_dfs_from <label X>`
  would actually mean quite different things (and give different
  results).

- Change the `Dataflow` module to always use entry of the graph for
  reverse postorder calculation. This should be the only change in
  behavior of this commit.

  Previously, if the caller provided initial facts for some of the
  labels, we would use those labels for our postorder calculation.
  However, I don't think that's correct in general - if the initial
  facts did not contain the entry of the graph, we would never analyze
  the blocks reachable from the entry but unreachable from the labels
  provided with the initial facts. It seems that the only analysis that
  used this was proc-point analysis, which I think would always include
  the entry block (so I don't think there's any bug due to this).
Signed-off-by: Michal Terepeta's avatarMichal Terepeta <michal.terepeta@gmail.com>

Test Plan: ./validate

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4464
parent 0db0e46c
...@@ -64,7 +64,9 @@ elimCommonBlocks :: CmmGraph -> CmmGraph ...@@ -64,7 +64,9 @@ elimCommonBlocks :: CmmGraph -> CmmGraph
elimCommonBlocks g = replaceLabels env $ copyTicks env g elimCommonBlocks g = replaceLabels env $ copyTicks env g
where where
env = iterate mapEmpty blocks_with_key env = iterate mapEmpty blocks_with_key
groups = groupByInt hash_block (postorderDfs g) -- The order of blocks doesn't matter here, but revPostorder also drops any
-- unreachable blocks, which is useful.
groups = groupByInt hash_block (revPostorder g)
blocks_with_key = [ [ (successors b, [b]) | b <- bs] | bs <- groups] blocks_with_key = [ [ (successors b, [b]) | b <- bs] | bs <- groups]
-- Invariant: The blocks in the list are pairwise distinct -- Invariant: The blocks in the list are pairwise distinct
......
...@@ -174,10 +174,9 @@ blockConcat splitting_procs g@CmmGraph { g_entry = entry_id } ...@@ -174,10 +174,9 @@ blockConcat splitting_procs g@CmmGraph { g_entry = entry_id }
| otherwise | otherwise
= (entry_id, shortcut_map) = (entry_id, shortcut_map)
-- blocks is a list of blocks in DFS postorder, while blockmap is -- blocks are sorted in reverse postorder, but we want to go from the exit
-- a map of blocks. We process each element from blocks and update -- towards beginning, so we use foldr below.
-- blockmap accordingly blocks = revPostorder g
blocks = postorderDfs g
blockmap = foldl' (flip addBlock) emptyBody blocks blockmap = foldl' (flip addBlock) emptyBody blocks
-- Accumulator contains three components: -- Accumulator contains three components:
...@@ -435,7 +434,7 @@ removeUnreachableBlocksProc proc@(CmmProc info lbl live g) ...@@ -435,7 +434,7 @@ removeUnreachableBlocksProc proc@(CmmProc info lbl live g)
| otherwise = env | otherwise = env
used_blocks :: [CmmBlock] used_blocks :: [CmmBlock]
used_blocks = postorderDfs g used_blocks = revPostorder g
used_lbls :: LabelSet used_lbls :: LabelSet
used_lbls = setFromList $ map entryLabel used_blocks used_lbls = setFromList $ map entryLabel used_blocks
...@@ -244,7 +244,7 @@ cmmLayoutStack dflags procpoints entry_args ...@@ -244,7 +244,7 @@ cmmLayoutStack dflags procpoints entry_args
-- We need liveness info. Dead assignments are removed later -- We need liveness info. Dead assignments are removed later
-- by the sinking pass. -- by the sinking pass.
let liveness = cmmLocalLiveness dflags graph let liveness = cmmLocalLiveness dflags graph
blocks = postorderDfs graph blocks = revPostorder graph
(final_stackmaps, _final_high_sp, new_blocks) <- (final_stackmaps, _final_high_sp, new_blocks) <-
mfix $ \ ~(rec_stackmaps, rec_high_sp, _new_blocks) -> mfix $ \ ~(rec_stackmaps, rec_high_sp, _new_blocks) ->
......
...@@ -190,7 +190,7 @@ minimalProcPointSet :: Platform -> ProcPointSet -> CmmGraph ...@@ -190,7 +190,7 @@ minimalProcPointSet :: Platform -> ProcPointSet -> CmmGraph
-- Given the set of successors of calls (which must be proc-points) -- Given the set of successors of calls (which must be proc-points)
-- figure out the minimal set of necessary proc-points -- figure out the minimal set of necessary proc-points
minimalProcPointSet platform callProcPoints g minimalProcPointSet platform callProcPoints g
= extendPPSet platform g (postorderDfs g) callProcPoints = extendPPSet platform g (revPostorder g) callProcPoints
extendPPSet extendPPSet
:: Platform -> CmmGraph -> [CmmBlock] -> ProcPointSet -> UniqSM ProcPointSet :: Platform -> CmmGraph -> [CmmBlock] -> ProcPointSet -> UniqSM ProcPointSet
...@@ -374,8 +374,8 @@ splitAtProcPoints dflags entry_label callPPs procPoints procMap ...@@ -374,8 +374,8 @@ splitAtProcPoints dflags entry_label callPPs procPoints procMap
-- reversed later. -- reversed later.
let (_, block_order) = let (_, block_order) =
foldl' add_block_num (0::Int, mapEmpty :: LabelMap Int) foldl' add_block_num (0::Int, mapEmpty :: LabelMap Int)
(postorderDfs g) (revPostorder g)
add_block_num (!i, !map) block = add_block_num (i, map) block =
(i + 1, mapInsert (entryLabel block) i map) (i + 1, mapInsert (entryLabel block) i map)
sort_fn (bid, _) (bid', _) = sort_fn (bid, _) (bid', _) =
compare (expectJust "block_order" $ mapLookup bid block_order) compare (expectJust "block_order" $ mapLookup bid block_order)
......
...@@ -173,7 +173,7 @@ cmmSink dflags graph = ofBlockList (g_entry graph) $ sink mapEmpty $ blocks ...@@ -173,7 +173,7 @@ cmmSink dflags graph = ofBlockList (g_entry graph) $ sink mapEmpty $ blocks
liveness = cmmLocalLiveness dflags graph liveness = cmmLocalLiveness dflags graph
getLive l = mapFindWithDefault Set.empty l liveness getLive l = mapFindWithDefault Set.empty l liveness
blocks = postorderDfs graph blocks = revPostorder graph
join_pts = findJoinPoints blocks join_pts = findJoinPoints blocks
......
...@@ -59,7 +59,7 @@ module CmmUtils( ...@@ -59,7 +59,7 @@ module CmmUtils(
ofBlockMap, toBlockMap, insertBlock, ofBlockMap, toBlockMap, insertBlock,
ofBlockList, toBlockList, bodyToBlockList, ofBlockList, toBlockList, bodyToBlockList,
toBlockListEntryFirst, toBlockListEntryFirstFalseFallthrough, toBlockListEntryFirst, toBlockListEntryFirstFalseFallthrough,
foldlGraphBlocks, mapGraphNodes, postorderDfs, mapGraphNodes1, foldlGraphBlocks, mapGraphNodes, revPostorder, mapGraphNodes1,
-- * Ticks -- * Ticks
blockTicks blockTicks
...@@ -566,8 +566,9 @@ mapGraphNodes1 f = modifyGraph (mapGraph f) ...@@ -566,8 +566,9 @@ mapGraphNodes1 f = modifyGraph (mapGraph f)
foldlGraphBlocks :: (a -> CmmBlock -> a) -> a -> CmmGraph -> a foldlGraphBlocks :: (a -> CmmBlock -> a) -> a -> CmmGraph -> a
foldlGraphBlocks k z g = mapFoldl k z $ toBlockMap g foldlGraphBlocks k z g = mapFoldl k z $ toBlockMap g
postorderDfs :: CmmGraph -> [CmmBlock] revPostorder :: CmmGraph -> [CmmBlock]
postorderDfs g = {-# SCC "postorderDfs" #-} postorder_dfs_from (toBlockMap g) (g_entry g) revPostorder g = {-# SCC "revPostorder" #-}
revPostorderFrom (toBlockMap g) (g_entry g)
------------------------------------------------- -------------------------------------------------
-- Tick utilities -- Tick utilities
......
...@@ -111,8 +111,7 @@ analyzeCmm dir lattice transfer cmmGraph initFact = ...@@ -111,8 +111,7 @@ analyzeCmm dir lattice transfer cmmGraph initFact =
blockMap = blockMap =
case hooplGraph of case hooplGraph of
GMany NothingO bm NothingO -> bm GMany NothingO bm NothingO -> bm
entries = if mapNull initFact then [entry] else mapKeys initFact in fixpointAnalysis dir lattice transfer entry blockMap initFact
in fixpointAnalysis dir lattice transfer entries blockMap initFact
-- Fixpoint algorithm. -- Fixpoint algorithm.
fixpointAnalysis fixpointAnalysis
...@@ -120,16 +119,16 @@ fixpointAnalysis ...@@ -120,16 +119,16 @@ fixpointAnalysis
Direction Direction
-> DataflowLattice f -> DataflowLattice f
-> TransferFun f -> TransferFun f
-> [Label] -> Label
-> LabelMap CmmBlock -> LabelMap CmmBlock
-> FactBase f -> FactBase f
-> FactBase f -> FactBase f
fixpointAnalysis direction lattice do_block entries blockmap = loop start fixpointAnalysis direction lattice do_block entry blockmap = loop start
where where
-- Sorting the blocks helps to minimize the number of times we need to -- Sorting the blocks helps to minimize the number of times we need to
-- process blocks. For instance, for forward analysis we want to look at -- process blocks. For instance, for forward analysis we want to look at
-- blocks in reverse postorder. Also, see comments for sortBlocks. -- blocks in reverse postorder. Also, see comments for sortBlocks.
blocks = sortBlocks direction entries blockmap blocks = sortBlocks direction entry blockmap
num_blocks = length blocks num_blocks = length blocks
block_arr = {-# SCC "block_arr" #-} listArray (0, num_blocks - 1) blocks block_arr = {-# SCC "block_arr" #-} listArray (0, num_blocks - 1) blocks
start = {-# SCC "start" #-} IntSet.fromDistinctAscList start = {-# SCC "start" #-} IntSet.fromDistinctAscList
...@@ -174,9 +173,8 @@ rewriteCmm dir lattice rwFun cmmGraph initFact = do ...@@ -174,9 +173,8 @@ rewriteCmm dir lattice rwFun cmmGraph initFact = do
blockMap1 = blockMap1 =
case hooplGraph of case hooplGraph of
GMany NothingO bm NothingO -> bm GMany NothingO bm NothingO -> bm
entries = if mapNull initFact then [entry] else mapKeys initFact
(blockMap2, facts) <- (blockMap2, facts) <-
fixpointRewrite dir lattice rwFun entries blockMap1 initFact fixpointRewrite dir lattice rwFun entry blockMap1 initFact
return (cmmGraph {g_graph = GMany NothingO blockMap2 NothingO}, facts) return (cmmGraph {g_graph = GMany NothingO blockMap2 NothingO}, facts)
fixpointRewrite fixpointRewrite
...@@ -184,16 +182,16 @@ fixpointRewrite ...@@ -184,16 +182,16 @@ fixpointRewrite
Direction Direction
-> DataflowLattice f -> DataflowLattice f
-> RewriteFun f -> RewriteFun f
-> [Label] -> Label
-> LabelMap CmmBlock -> LabelMap CmmBlock
-> FactBase f -> FactBase f
-> UniqSM (LabelMap CmmBlock, FactBase f) -> UniqSM (LabelMap CmmBlock, FactBase f)
fixpointRewrite dir lattice do_block entries blockmap = loop start blockmap fixpointRewrite dir lattice do_block entry blockmap = loop start blockmap
where where
-- Sorting the blocks helps to minimize the number of times we need to -- Sorting the blocks helps to minimize the number of times we need to
-- process blocks. For instance, for forward analysis we want to look at -- process blocks. For instance, for forward analysis we want to look at
-- blocks in reverse postorder. Also, see comments for sortBlocks. -- blocks in reverse postorder. Also, see comments for sortBlocks.
blocks = sortBlocks dir entries blockmap blocks = sortBlocks dir entry blockmap
num_blocks = length blocks num_blocks = length blocks
block_arr = {-# SCC "block_arr_rewrite" #-} block_arr = {-# SCC "block_arr_rewrite" #-}
listArray (0, num_blocks - 1) blocks listArray (0, num_blocks - 1) blocks
...@@ -268,20 +266,15 @@ we'll propagate (x=4) to L4, and nuke the otherwise-good rewriting of L4. ...@@ -268,20 +266,15 @@ we'll propagate (x=4) to L4, and nuke the otherwise-good rewriting of L4.
-- | Sort the blocks into the right order for analysis. This means reverse -- | Sort the blocks into the right order for analysis. This means reverse
-- postorder for a forward analysis. For the backward one, we simply reverse -- postorder for a forward analysis. For the backward one, we simply reverse
-- that (see Note [Backward vs forward analysis]). -- that (see Note [Backward vs forward analysis]).
--
-- Note: We're using Hoopl's confusingly named `postorder_dfs_from` but AFAICS
-- it returns the *reverse* postorder of the blocks (it visits blocks in the
-- postorder and uses (:) to collect them, which gives the reverse of the
-- visitation order).
sortBlocks sortBlocks
:: NonLocal n :: NonLocal n
=> Direction -> [Label] -> LabelMap (Block n C C) -> [Block n C C] => Direction -> Label -> LabelMap (Block n C C) -> [Block n C C]
sortBlocks direction entries blockmap = sortBlocks direction entry blockmap =
case direction of case direction of
Fwd -> fwd Fwd -> fwd
Bwd -> reverse fwd Bwd -> reverse fwd
where where
fwd = postorder_dfs_from blockmap entries fwd = revPostorderFrom blockmap entry
-- Note [Backward vs forward analysis] -- Note [Backward vs forward analysis]
-- --
......
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-} {-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-} {-# LANGUAGE RankNTypes #-}
...@@ -14,7 +15,7 @@ module Hoopl.Graph ...@@ -14,7 +15,7 @@ module Hoopl.Graph
, labelsDefined , labelsDefined
, mapGraph , mapGraph
, mapGraphBlocks , mapGraphBlocks
, postorder_dfs_from , revPostorderFrom
) where ) where
...@@ -119,22 +120,10 @@ labelsDefined (GMany _ body x) = mapFoldlWithKey addEntry (exitLabel x) body ...@@ -119,22 +120,10 @@ labelsDefined (GMany _ body x) = mapFoldlWithKey addEntry (exitLabel x) body
---------------------------------------------------------------- ----------------------------------------------------------------
class LabelsPtr l where -- | Returns a list of blocks reachable from the provided Labels in the reverse
targetLabels :: l -> [Label] -- postorder.
--
instance NonLocal n => LabelsPtr (n e C) where -- This is the most important traversal over this data structure. It drops
targetLabels n = successors n
instance LabelsPtr Label where
targetLabels l = [l]
instance LabelsPtr LabelSet where
targetLabels = setElems
instance LabelsPtr l => LabelsPtr [l] where
targetLabels = concatMap targetLabels
-- | This is the most important traversal over this data structure. It drops
-- unreachable code and puts blocks in an order that is good for solving forward -- unreachable code and puts blocks in an order that is good for solving forward
-- dataflow problems quickly. The reverse order is good for solving backward -- dataflow problems quickly. The reverse order is good for solving backward
-- dataflow problems quickly. The forward order is also reasonably good for -- dataflow problems quickly. The forward order is also reasonably good for
...@@ -143,59 +132,52 @@ instance LabelsPtr l => LabelsPtr [l] where ...@@ -143,59 +132,52 @@ instance LabelsPtr l => LabelsPtr [l] where
-- that you would need a more serious analysis, probably based on dominators, to -- that you would need a more serious analysis, probably based on dominators, to
-- identify loop headers. -- identify loop headers.
-- --
-- The ubiquity of 'postorder_dfs' is one reason for the ubiquity of the 'LGraph' -- For forward analyses we want reverse postorder visitation, consider:
-- representation, when for most purposes the plain 'Graph' representation is
-- more mathematically elegant (but results in more complicated code).
--
-- Here's an easy way to go wrong! Consider
-- @ -- @
-- A -> [B,C] -- A -> [B,C]
-- B -> D -- B -> D
-- C -> D -- C -> D
-- @ -- @
-- Then ordinary dfs would give [A,B,D,C] which has a back ref from C to D. -- Postorder: [D, C, B, A] (or [D, B, C, A])
-- Better to get [A,B,C,D] -- Reverse postorder: [A, B, C, D] (or [A, C, B, D])
-- This matters for, e.g., forward analysis, because we want to analyze *both*
-- B and C before we analyze D.
-- | Traversal: 'postorder_dfs' returns a list of blocks reachable revPostorderFrom
-- from the entry of enterable graph. The entry and exit are *not* included. :: forall block. (NonLocal block)
-- The list has the following property: => LabelMap (block C C) -> Label -> [block C C]
-- revPostorderFrom graph start = go start_worklist setEmpty []
-- Say a "back reference" exists if one of a block's where
-- control-flow successors precedes it in the output list start_worklist = lookup_for_descend start Nil
--
-- Then there are as few back references as possible -- To compute the postorder we need to "visit" a block (mark as done)
-- -- *after* visiting all its successors. So we need to know whether we
-- The output is suitable for use in -- already processed all successors of each block (and @NonLocal@ allows
-- a forward dataflow problem. For a backward problem, simply reverse -- arbitrary many successors). So we use an explicit stack with an extra bit
-- the list. ('postorder_dfs' is sufficiently tricky to implement that -- of information:
-- one doesn't want to try and maintain both forward and backward -- * @ConsTodo@ means to explore the block if it wasn't visited before
-- versions.) -- * @ConsMark@ means that all successors were already done and we can add
-- the block to the result.
postorder_dfs_from_except :: forall block e . (NonLocal block, LabelsPtr e) --
=> LabelMap (block C C) -> e -> LabelSet -> [block C C] -- NOTE: We add blocks to the result list in postorder, but we *prepend*
postorder_dfs_from_except blocks b visited = -- them (i.e., we use @(:)@), which means that the final list is in reverse
vchildren (get_children b) (\acc _visited -> acc) [] visited -- postorder.
where go :: DfsStack (block C C) -> LabelSet -> [block C C] -> [block C C]
vnode :: block C C -> ([block C C] -> LabelSet -> a) -> [block C C] -> LabelSet -> a go Nil !_ !result = result
vnode block cont acc visited = go (ConsMark block rest) !wip_or_done !result =
if setMember id visited then go rest wip_or_done (block : result)
cont acc visited go (ConsTodo block rest) !wip_or_done !result
else | entryLabel block `setMember` wip_or_done = go rest wip_or_done result
let cont' acc visited = cont (block:acc) visited in | otherwise =
vchildren (get_children block) cont' acc (setInsert id visited) let new_worklist =
where id = entryLabel block foldr lookup_for_descend
vchildren :: forall a. [block C C] -> ([block C C] -> LabelSet -> a) -> [block C C] -> LabelSet -> a (ConsMark block rest)
vchildren bs cont acc visited = next bs acc visited (successors block)
where next children acc visited = in go new_worklist (setInsert (entryLabel block) wip_or_done) result
case children of [] -> cont acc visited
(b:bs) -> vnode b (next bs) acc visited lookup_for_descend :: Label -> DfsStack (block C C) -> DfsStack (block C C)
get_children :: forall l. LabelsPtr l => l -> [block C C] lookup_for_descend label wl
get_children block = foldr add_id [] $ targetLabels block | Just b <- mapLookup label graph = ConsTodo b wl
add_id id rst = case lookupFact id blocks of | otherwise =
Just b -> b : rst error $ "Label that doesn't have a block?! " ++ show label
Nothing -> rst
data DfsStack a = ConsTodo a (DfsStack a) | ConsMark a (DfsStack a) | Nil
postorder_dfs_from
:: (NonLocal block, LabelsPtr b) => LabelMap (block C C) -> b -> [block C C]
postorder_dfs_from blocks b = postorder_dfs_from_except blocks b setEmpty
...@@ -141,8 +141,8 @@ pprCmmGraph g ...@@ -141,8 +141,8 @@ pprCmmGraph g
= text "{" <> text "offset" = text "{" <> text "offset"
$$ nest 2 (vcat $ map ppr blocks) $$ nest 2 (vcat $ map ppr blocks)
$$ text "}" $$ text "}"
where blocks = postorderDfs g where blocks = revPostorder g
-- postorderDfs has the side-effect of discarding unreachable code, -- revPostorder has the side-effect of discarding unreachable code,
-- so pretty-printed Cmm will omit any unreachable blocks. This can -- so pretty-printed Cmm will omit any unreachable blocks. This can
-- sometimes be confusing. -- sometimes be confusing.
......
TOP=../..
include $(TOP)/mk/boilerplate.mk
include $(TOP)/mk/test.mk
module Main where
import Hoopl.Block
import Hoopl.Collections
import Hoopl.Graph
import Hoopl.Label
import Data.Maybe
data TestBlock e x = TB { label_ :: Label, successors_ :: [Label] }
deriving (Eq, Show)
instance NonLocal TestBlock where
entryLabel = label_
successors = successors_
-- Test the classical diamond shape graph.
test_diamond :: LabelMap (TestBlock C C)
test_diamond = mapFromList $ map (\b -> (label_ b, b)) blocks
where
blocks =
[ TB (mkHooplLabel 1) [mkHooplLabel 2, mkHooplLabel 3]
, TB (mkHooplLabel 2) [mkHooplLabel 4]
, TB (mkHooplLabel 3) [mkHooplLabel 4]
, TB (mkHooplLabel 4) []
]
-- Test that the backedge doesn't change anything.
test_diamond_backedge :: LabelMap (TestBlock C C)
test_diamond_backedge = mapFromList $ map (\b -> (label_ b, b)) blocks
where
blocks =
[ TB (mkHooplLabel 1) [mkHooplLabel 2, mkHooplLabel 3]
, TB (mkHooplLabel 2) [mkHooplLabel 4]
, TB (mkHooplLabel 3) [mkHooplLabel 4]
, TB (mkHooplLabel 4) [mkHooplLabel 1]
]
-- Test that the "bypass" edge from 1 to 4 doesn't change anything.
test_3 :: LabelMap (TestBlock C C)
test_3 = mapFromList $ map (\b -> (label_ b, b)) blocks
where
blocks =
[ TB (mkHooplLabel 1) [mkHooplLabel 2, mkHooplLabel 4]
, TB (mkHooplLabel 2) [mkHooplLabel 4]
, TB (mkHooplLabel 4) []
]
-- Like test_3 but with different order of successors for the entry point.
test_4 :: LabelMap (TestBlock C C)
test_4 = mapFromList $ map (\b -> (label_ b, b)) blocks
where
blocks =
[ TB (mkHooplLabel 1) [mkHooplLabel 4, mkHooplLabel 2]
, TB (mkHooplLabel 2) [mkHooplLabel 4]
, TB (mkHooplLabel 4) []
]
main :: IO ()
main = do
let result = revPostorderFrom test_diamond (mkHooplLabel 1)
putStrLn (show $ map label_ result)
let result = revPostorderFrom test_diamond_backedge (mkHooplLabel 1)
putStrLn (show $ map label_ result)
let result = revPostorderFrom test_3 (mkHooplLabel 1)
putStrLn (show $ map label_ result)
let result = revPostorderFrom test_4 (mkHooplLabel 1)
putStrLn (show $ map label_ result)
[L1,L3,L2,L4]
[L1,L3,L2,L4]
[L1,L2,L4]
[L1,L2,L4]
TOP=../../..
include $(TOP)/mk/boilerplate.mk
include $(TOP)/mk/test.mk
test('HooplPostorder',
extra_run_opts('"' + config.libdir + '"'),
compile_and_run,
['-package ghc'])
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