GHC/Cmm/CommonBlockElim.hs

    {-# 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.Bits
    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