{-# LANGUAGE GADTs, BangPatterns, ScopedTypeVariables #-}
module GHC.Cmm.CommonBlockElim
( elimCommonBlocks
)
where
import GHC.Prelude hiding (iterate, succ, unzip, zip)
import GHC.Cmm.BlockId
import GHC.Cmm
import GHC.Cmm.Utils
import GHC.Cmm.Switch (eqSwitchTargetWith)
import GHC.Cmm.ContFlowOpt
import GHC.Cmm.Dataflow.Block
import GHC.Cmm.Dataflow.Graph
import GHC.Cmm.Dataflow.Label
import GHC.Cmm.Dataflow.Collections
import Data.Maybe (mapMaybe)
import qualified Data.List as List
import Data.Word
import qualified Data.Map as M
import GHC.Utils.Outputable
import GHC.Utils.Panic
import qualified GHC.Data.TrieMap as TM
import GHC.Types.Unique.FM
import GHC.Types.Unique
import Control.Arrow (first, second)
-- -----------------------------------------------------------------------------
-- Eliminate common blocks
-- If two blocks are identical except for the label on the first node,
-- then we can eliminate one of the blocks. To ensure that the semantics
-- of the program are preserved, we have to rewrite each predecessor of the
-- eliminated block to proceed with the block we keep.
-- The algorithm iterates over the blocks in the graph,
-- checking whether it has seen another block that is equal modulo labels.
-- If so, then it adds an entry in a map indicating that the new block
-- is made redundant by the old block.
-- Otherwise, it is added to the useful blocks.
-- To avoid comparing every block with every other block repeatedly, we group
-- them by
-- * a hash of the block, ignoring labels (explained below)
-- * the list of outgoing labels
-- The hash is invariant under relabeling, so we only ever compare within
-- the same group of blocks.
--
-- The list of outgoing labels is updated as we merge blocks (that is why they
-- are not included in the hash, which we want to calculate only once).
--
-- All in all, two blocks should never be compared if they have different
-- hashes, and at most once otherwise. Previously, we were slower, and people
-- rightfully complained: #10397
-- TODO: Use optimization fuel
elimCommonBlocks :: CmmGraph -> CmmGraph
elimCommonBlocks g = replaceLabels env $ copyTicks env g
where
env = iterate mapEmpty blocks_with_key
-- The order of blocks doesn't matter here. While we could use
-- revPostorder which drops unreachable blocks this is done in
-- ContFlowOpt already which runs before this pass. So we use
-- toBlockList since it is faster.
groups = groupByInt hash_block (toBlockList g) :: [[CmmBlock]]
blocks_with_key = [ [ (successors b, [b]) | b <- bs] | bs <- groups]
-- Invariant: The blocks in the list are pairwise distinct
-- (so avoid comparing them again)
type DistinctBlocks = [CmmBlock]
type Key = [Label]
type Subst = LabelMap BlockId
-- The outer list groups by hash. We retain this grouping throughout.
iterate :: Subst -> [[(Key, DistinctBlocks)]] -> Subst
iterate subst blocks
| mapNull new_substs = subst
| otherwise = iterate subst' updated_blocks
where
grouped_blocks :: [[(Key, [DistinctBlocks])]]
grouped_blocks = map groupByLabel blocks
merged_blocks :: [[(Key, DistinctBlocks)]]
(new_substs, merged_blocks) = List.mapAccumL (List.mapAccumL go) mapEmpty grouped_blocks
where
go !new_subst1 (k,dbs) = (new_subst1 `mapUnion` new_subst2, (k,db))
where
(new_subst2, db) = mergeBlockList subst dbs
subst' = subst `mapUnion` new_substs
updated_blocks = map (map (first (map (lookupBid subst')))) merged_blocks
-- Combine two lists of blocks.
-- While they are internally distinct they can still share common blocks.
mergeBlocks :: Subst -> DistinctBlocks -> DistinctBlocks -> (Subst, DistinctBlocks)
mergeBlocks subst existing new = go new
where
go [] = (mapEmpty, existing)
go (b:bs) = case List.find (eqBlockBodyWith (eqBid subst) b) existing of
-- This block is a duplicate. Drop it, and add it to the substitution
Just b' -> first (mapInsert (entryLabel b) (entryLabel b')) $ go bs
-- This block is not a duplicate, keep it.
Nothing -> second (b:) $ go bs
mergeBlockList :: Subst -> [DistinctBlocks] -> (Subst, DistinctBlocks)
mergeBlockList _ [] = pprPanic "mergeBlockList" empty
mergeBlockList subst (b:bs) = go mapEmpty b bs
where
go !new_subst1 b [] = (new_subst1, b)
go !new_subst1 b1 (b2:bs) = go new_subst b bs
where
(new_subst2, b) = mergeBlocks subst b1 b2
new_subst = new_subst1 `mapUnion` new_subst2
-- -----------------------------------------------------------------------------
-- Hashing and equality on blocks
-- Below here is mostly boilerplate: hashing blocks ignoring labels,
-- and comparing blocks modulo a label mapping.
-- To speed up comparisons, we hash each basic block modulo jump labels.
-- The hashing is a bit arbitrary (the numbers are completely arbitrary),
-- but it should be fast and good enough.
-- We want to get as many small buckets as possible, as comparing blocks is
-- expensive. So include as much as possible in the hash. Ideally everything
-- that is compared with (==) in eqBlockBodyWith.
type HashCode = Int
hash_block :: CmmBlock -> HashCode
hash_block block =
fromIntegral (foldBlockNodesB3 (hash_fst, hash_mid, hash_lst) block (0 :: Word32) .&. (0x7fffffff :: Word32))
-- UniqFM doesn't like negative Ints
where hash_fst _ h = h
hash_mid m h = hash_node m + h `shiftL` 1
hash_lst m h = hash_node m + h `shiftL` 1
hash_node :: CmmNode O x -> Word32
hash_node n | dont_care n = 0 -- don't care
hash_node (CmmAssign r e) = hash_reg r + hash_e e
hash_node (CmmStore e e') = hash_e e + hash_e e'
hash_node (CmmUnsafeForeignCall t _ as) = hash_tgt t + hash_list hash_e as
hash_node (CmmBranch _) = 23 -- NB. ignore the label
hash_node (CmmCondBranch p _ _ _) = hash_e p
hash_node (CmmCall e _ _ _ _ _) = hash_e e
hash_node (CmmForeignCall t _ _ _ _ _ _) = hash_tgt t
hash_node (CmmSwitch e _) = hash_e e
hash_node _ = error "hash_node: unknown Cmm node!"
hash_reg :: CmmReg -> Word32
hash_reg (CmmLocal localReg) = hash_unique localReg -- important for performance, see #10397
hash_reg (CmmGlobal _) = 19
hash_e :: CmmExpr -> Word32
hash_e (CmmLit l) = hash_lit l
hash_e (CmmLoad e _) = 67 + hash_e e
hash_e (CmmReg r) = hash_reg r
hash_e (CmmMachOp _ es) = hash_list hash_e es -- pessimal - no operator check
hash_e (CmmRegOff r i) = hash_reg r + cvt i
hash_e (CmmStackSlot _ _) = 13
hash_lit :: CmmLit -> Word32
hash_lit (CmmInt i _) = fromInteger i
hash_lit (CmmFloat r _) = truncate r
hash_lit (CmmVec ls) = hash_list hash_lit ls
hash_lit (CmmLabel _) = 119 -- ugh
hash_lit (CmmLabelOff _ i) = cvt $ 199 + i
hash_lit (CmmLabelDiffOff _ _ i _) = cvt $ 299 + i
hash_lit (CmmBlock _) = 191 -- ugh
hash_lit (CmmHighStackMark) = cvt 313
hash_tgt (ForeignTarget e _) = hash_e e
hash_tgt (PrimTarget _) = 31 -- lots of these
hash_list f = foldl' (\z x -> f x + z) (0::Word32)
cvt = fromInteger . toInteger
hash_unique :: Uniquable a => a -> Word32
hash_unique = cvt . getKey . getUnique
-- | Ignore these node types for equality
dont_care :: CmmNode O x -> Bool
dont_care CmmComment {} = True
dont_care CmmTick {} = True
dont_care CmmUnwind {} = True
dont_care _other = False
-- Utilities: equality and substitution on the graph.
-- Given a map ``subst'' from BlockID -> BlockID, we define equality.
eqBid :: LabelMap BlockId -> BlockId -> BlockId -> Bool
eqBid subst bid bid' = lookupBid subst bid == lookupBid subst bid'
lookupBid :: LabelMap BlockId -> BlockId -> BlockId
lookupBid subst bid = case mapLookup bid subst of
Just bid -> lookupBid subst bid
Nothing -> bid
-- Middle nodes and expressions can contain BlockIds, in particular in
-- CmmStackSlot and CmmBlock, so we have to use a special equality for
-- these.
--
eqMiddleWith :: (BlockId -> BlockId -> Bool)
-> CmmNode O O -> CmmNode O O -> Bool
eqMiddleWith eqBid (CmmAssign r1 e1) (CmmAssign r2 e2)
= r1 == r2 && eqExprWith eqBid e1 e2
eqMiddleWith eqBid (CmmStore l1 r1) (CmmStore l2 r2)
= eqExprWith eqBid l1 l2 && eqExprWith eqBid r1 r2
eqMiddleWith eqBid (CmmUnsafeForeignCall t1 r1 a1)
(CmmUnsafeForeignCall t2 r2 a2)
= t1 == t2 && r1 == r2 && eqListWith (eqExprWith eqBid) a1 a2
eqMiddleWith _ _ _ = False
eqExprWith :: (BlockId -> BlockId -> Bool)
-> CmmExpr -> CmmExpr -> Bool
eqExprWith eqBid = eq
where
CmmLit l1 `eq` CmmLit l2 = eqLit l1 l2
CmmLoad e1 _ `eq` CmmLoad e2 _ = e1 `eq` e2
CmmReg r1 `eq` CmmReg r2 = r1==r2
CmmRegOff r1 i1 `eq` CmmRegOff r2 i2 = r1==r2 && i1==i2
CmmMachOp op1 es1 `eq` CmmMachOp op2 es2 = op1==op2 && es1 `eqs` es2
CmmStackSlot a1 i1 `eq` CmmStackSlot a2 i2 = eqArea a1 a2 && i1==i2
_e1 `eq` _e2 = False
xs `eqs` ys = eqListWith eq xs ys
eqLit (CmmBlock id1) (CmmBlock id2) = eqBid id1 id2
eqLit l1 l2 = l1 == l2
eqArea Old Old = True
eqArea (Young id1) (Young id2) = eqBid id1 id2
eqArea _ _ = False
-- Equality on the body of a block, modulo a function mapping block
-- IDs to block IDs.
eqBlockBodyWith :: (BlockId -> BlockId -> Bool) -> CmmBlock -> CmmBlock -> Bool
eqBlockBodyWith eqBid block block'
{-
| equal = pprTrace "equal" (vcat [ppr block, ppr block']) True
| otherwise = pprTrace "not equal" (vcat [ppr block, ppr block']) False
-}
= equal
where (_,m,l) = blockSplit block
nodes = filter (not . dont_care) (blockToList m)
(_,m',l') = blockSplit block'
nodes' = filter (not . dont_care) (blockToList m')
equal = eqListWith (eqMiddleWith eqBid) nodes nodes' &&
eqLastWith eqBid l l'
eqLastWith :: (BlockId -> BlockId -> Bool) -> CmmNode O C -> CmmNode O C -> Bool
eqLastWith eqBid (CmmBranch bid1) (CmmBranch bid2) = eqBid bid1 bid2
eqLastWith eqBid (CmmCondBranch c1 t1 f1 l1) (CmmCondBranch c2 t2 f2 l2) =
c1 == c2 && l1 == l2 && eqBid t1 t2 && eqBid f1 f2
eqLastWith eqBid (CmmCall t1 c1 g1 a1 r1 u1) (CmmCall t2 c2 g2 a2 r2 u2) =
t1 == t2 && eqMaybeWith eqBid c1 c2 && a1 == a2 && r1 == r2 && u1 == u2 && g1 == g2
eqLastWith eqBid (CmmSwitch e1 ids1) (CmmSwitch e2 ids2) =
e1 == e2 && eqSwitchTargetWith eqBid ids1 ids2
eqLastWith _ _ _ = False
eqMaybeWith :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool
eqMaybeWith eltEq (Just e) (Just e') = eltEq e e'
eqMaybeWith _ Nothing Nothing = True
eqMaybeWith _ _ _ = False
eqListWith :: (a -> b -> Bool) -> [a] -> [b] -> Bool
eqListWith f (a : as) (b : bs) = f a b && eqListWith f as bs
eqListWith _ [] [] = True
eqListWith _ _ _ = False
-- | Given a block map, ensure that all "target" blocks are covered by
-- the same ticks as the respective "source" blocks. This not only
-- means copying ticks, but also adjusting tick scopes where
-- necessary.
copyTicks :: LabelMap BlockId -> CmmGraph -> CmmGraph
copyTicks env g
| mapNull env = g
| otherwise = ofBlockMap (g_entry g) $ mapMap copyTo blockMap
where -- Reverse block merge map
blockMap = toBlockMap g
revEnv = mapFoldlWithKey insertRev M.empty env
insertRev m k x = M.insertWith (const (k:)) x [k] m
-- Copy ticks and scopes into the given block
copyTo block = case M.lookup (entryLabel block) revEnv of
Nothing -> block
Just ls -> foldr copy block $ mapMaybe (flip mapLookup blockMap) ls
copy from to =
let ticks = blockTicks from
CmmEntry _ scp0 = firstNode from
(CmmEntry lbl scp1, code) = blockSplitHead to
in CmmEntry lbl (combineTickScopes scp0 scp1) `blockJoinHead`
foldr blockCons code (map CmmTick ticks)
-- Group by [Label]
-- See Note [Compressed TrieMap] in GHC.Core.Map.Expr about the usage of GenMap.
groupByLabel :: [(Key, DistinctBlocks)] -> [(Key, [DistinctBlocks])]
groupByLabel =
go (TM.emptyTM :: TM.ListMap (TM.GenMap LabelMap) (Key, [DistinctBlocks]))
where
go !m [] = TM.foldTM (:) m []
go !m ((k,v) : entries) = go (TM.alterTM k adjust m) entries
where --k' = map (getKey . getUnique) k
adjust Nothing = Just (k,[v])
adjust (Just (_,vs)) = Just (k,v:vs)
groupByInt :: (a -> Int) -> [a] -> [[a]]
groupByInt f xs = nonDetEltsUFM $ List.foldl' go emptyUFM xs
-- See Note [Unique Determinism and code generation]
where
go m x = alterUFM addEntry m (f x)
where
addEntry xs = Just $! maybe [x] (x:) xs