CmmCommonBlockElim.hs 7.45 KB
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{-# LANGUAGE GADTs #-}
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-- ToDo: remove -fno-warn-warnings-deprecations
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{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
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-- ToDo: remove -fno-warn-incomplete-patterns
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{-# OPTIONS_GHC -fno-warn-incomplete-patterns #-}

module CmmCommonBlockElim
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  ( elimCommonBlocks
  )
where


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import BlockId
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import Cmm
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import CmmUtils
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import Prelude hiding (iterate, succ, unzip, zip)
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import Compiler.Hoopl
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import Data.Bits
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import qualified Data.List as List
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import Data.Word
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import FastString
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import Control.Monad
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import Outputable
import UniqFM
import Unique

my_trace :: String -> SDoc -> a -> a
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my_trace = if False then pprTrace else \_ _ a -> a
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-- 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.

-- TODO: Use optimization fuel
elimCommonBlocks :: CmmGraph -> CmmGraph
elimCommonBlocks g =
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    upd_graph g . snd $ iterate common_block reset hashed_blocks
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                                (emptyUFM, mapEmpty)
      where hashed_blocks    = map (\b -> (hash_block b, b)) (reverse (postorderDfs g))
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            reset (_, subst) = (emptyUFM, subst)

-- Iterate over the blocks until convergence
iterate :: (t -> a -> (Bool, t)) -> (t -> t) -> [a] -> t -> t
iterate upd reset blocks state =
  case foldl upd' (False, state) blocks of
    (True,  state') -> iterate upd reset blocks (reset state')
    (False, state') -> state'
  where upd' (b, s) a = let (b', s') = upd s a in (b || b', s') -- lift to track changes

-- Try to find a block that is equal (or ``common'') to b.
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type BidMap = BlockEnv BlockId
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type State  = (UniqFM [CmmBlock], BidMap)
common_block :: (Outputable h, Uniquable h) =>  State -> (h, CmmBlock) -> (Bool, State)
common_block (bmap, subst) (hash, b) =
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  case lookupUFM bmap hash of
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    Just bs -> case (List.find (eqBlockBodyWith (eqBid subst) b) bs,
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                     mapLookup bid subst) of
                 (Just b', Nothing)                         -> addSubst b'
                 (Just b', Just b'') | entryLabel b' /= b'' -> addSubst b'
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                 _ -> (False, (addToUFM bmap hash (b : bs), subst))
    Nothing -> (False, (addToUFM bmap hash [b], subst))
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  where bid = entryLabel b
        addSubst b' = my_trace "found new common block" (ppr (entryLabel b')) $
                      (True, (bmap, mapInsert bid (entryLabel b') subst))
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-- Given the map ``subst'' from BlockId -> BlockId, we rewrite the graph.
upd_graph :: CmmGraph -> BidMap -> CmmGraph
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upd_graph g subst = mapGraphNodes (id, middle, last) g
  where middle = mapExpDeep exp
        last l = last' (mapExpDeep exp l)
        last' :: CmmNode O C -> CmmNode O C
        last' (CmmBranch bid)              = CmmBranch $ sub bid
        last' (CmmCondBranch p t f)        = cond p (sub t) (sub f)
        last' (CmmCall t (Just bid) a r o) = CmmCall t (Just $ sub bid) a r o
        last' l@(CmmCall _ Nothing _ _ _)  = l
        last' (CmmForeignCall t r a bid u i) = CmmForeignCall t r a (sub bid) u i
        last' (CmmSwitch e bs)             = CmmSwitch e $ map (liftM sub) bs
        cond p t f = if t == f then CmmBranch t else CmmCondBranch p t f
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        exp (CmmStackSlot (CallArea (Young id))       off) =
             CmmStackSlot (CallArea (Young (sub id))) off
        exp (CmmLit (CmmBlock id)) = CmmLit (CmmBlock (sub id))
        exp e = e
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        sub = lookupBid subst

-- To speed up comparisons, we hash each basic block modulo labels.
-- The hashing is a bit arbitrary (the numbers are completely arbitrary),
-- but it should be fast and good enough.
hash_block :: CmmBlock -> Int
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hash_block block =
  fromIntegral (foldBlockNodesB3 (hash_fst, hash_mid, hash_lst) block (0 :: Word32) .&. (0x7fffffff :: Word32))
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  -- UniqFM doesn't like negative Ints
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  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 (CmmComment (FastString u _ _ _ _)) = cvt u
        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 -- would be great to hash these properly
        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

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        hash_reg :: CmmReg -> Word32
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        hash_reg   (CmmLocal _) = 117
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        hash_reg   (CmmGlobal _)    = 19
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        hash_e :: CmmExpr -> Word32
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        hash_e (CmmLit l) = hash_lit l
        hash_e (CmmLoad e _) = 67 + hash_e e
        hash_e (CmmReg r) = hash_reg r
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        hash_e (CmmMachOp _ es) = hash_list hash_e es -- pessimal - no operator check
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        hash_e (CmmRegOff r i) = hash_reg r + cvt i
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        hash_e (CmmStackSlot _ _) = 13
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        hash_lit :: CmmLit -> Word32
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        hash_lit (CmmInt i _) = fromInteger i
        hash_lit (CmmFloat r _) = truncate r
        hash_lit (CmmLabel _) = 119 -- ugh
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        hash_lit (CmmLabelOff _ i) = cvt $ 199 + i
        hash_lit (CmmLabelDiffOff _ _ i) = cvt $ 299 + i
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        hash_lit (CmmBlock _) = 191 -- ugh
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        hash_lit (CmmHighStackMark) = cvt 313
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        hash_tgt (ForeignTarget e _) = hash_e e
        hash_tgt (PrimTarget _) = 31 -- lots of these
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        hash_list f = foldl (\z x -> f x + z) (0::Word32)

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        cvt = fromInteger . toInteger
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-- Utilities: equality and substitution on the graph.

-- Given a map ``subst'' from BlockID -> BlockID, we define equality.
eqBid :: BidMap -> BlockId -> BlockId -> Bool
eqBid subst bid bid' = lookupBid subst bid == lookupBid subst bid'
lookupBid :: BidMap -> BlockId -> BlockId
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lookupBid subst bid = case mapLookup bid subst of
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                        Just bid  -> lookupBid subst bid
                        Nothing -> bid

-- Equality on the body of a block, modulo a function mapping block IDs to block IDs.
eqBlockBodyWith :: (BlockId -> BlockId -> Bool) -> CmmBlock -> CmmBlock -> Bool
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eqBlockBodyWith eqBid block block' = middles == middles' && eqLastWith eqBid last last'
  where (_, middles , JustC last  :: MaybeC C (CmmNode O C)) = blockToNodeList block
        (_, middles', JustC last' :: MaybeC C (CmmNode O C)) = blockToNodeList block'

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) (CmmCondBranch c2 t2 f2) =
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  c1 == c2 && eqBid t1 t2 && eqBid f1 f2
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eqLastWith eqBid (CmmCall t1 c1 a1 r1 u1) (CmmCall t2 c2 a2 r2 u2) =
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  t1 == t2 && eqMaybeWith eqBid c1 c2 && a1 == a2 && r1 == r2 && u1 == u2
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eqLastWith eqBid (CmmSwitch e1 bs1) (CmmSwitch e2 bs2) =
  e1 == e2 && eqListWith (eqMaybeWith eqBid) bs1 bs2
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eqLastWith _ _ _ = False

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eqListWith :: (a -> b -> Bool) -> [a] -> [b] -> Bool
eqListWith eltEq es es' = all (uncurry eltEq) (List.zip es es')
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eqMaybeWith :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool
eqMaybeWith eltEq (Just e) (Just e') = eltEq e e'
eqMaybeWith _ Nothing Nothing = True
eqMaybeWith _ _ _ = False