cmm/CmmCommonBlockElim.hs

    {-# LANGUAGE GADTs, BangPatterns #-}
    module CmmCommonBlockElim
      ( elimCommonBlocks
      )
    where

    import BlockId
    import Cmm
    import CmmUtils
    import CmmSwitch (eqSwitchTargetWith)
    import CmmContFlowOpt
    -- import PprCmm ()
    import Prelude hiding (iterate, succ, unzip, zip)

    import Hoopl hiding (ChangeFlag)
    import Data.Bits
    import Data.Maybe (mapMaybe)
    import qualified Data.List as List
    import Data.Word
    import qualified Data.Map as M
    import Outputable
    import UniqFM
    import UniqDFM
    import qualified TrieMap as TM
    import 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
         groups = groupByInt hash_block (postorderDfs g)
         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

    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 && and (zipWith (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 = and (zipWith 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 = and (zipWith (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

    -- | 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 = mapFoldWithKey insertRev M.empty env
            insertRev k x = M.insertWith (const (k:)) x [k]
            -- 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]
    groupByLabel :: [(Key, a)] -> [(Key, [a])]
    groupByLabel = go (TM.emptyTM :: TM.ListMap UniqDFM a)
      where
        go !m [] = TM.foldTM (:) m []
        go !m ((k,v) : entries) = go (TM.alterTM k' adjust m) entries
          where k' = map 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 (Just . maybe [x] (x:)) m (f x)