    module CmmContFlowOpt
        ( runCmmOpts, cmmCfgOpts, cmmCfgOptsZ
        , branchChainElimZ, removeUnreachableBlocksZ, predMap
        , replaceLabelsZ, replaceBranches, runCmmContFlowOptsZs
        )
    where

    import BlockId
    import Cmm
    import CmmTx
    import qualified ZipCfg as G
    import ZipCfg
    import ZipCfgCmmRep

    import Maybes
    import Control.Monad
    import Outputable
    import Prelude hiding (unzip, zip)
    import Util

    ------------------------------------
    runCmmContFlowOptsZs :: [CmmZ] -> [CmmZ]
    runCmmContFlowOptsZs prog
      = [ runTx (runCmmOpts cmmCfgOptsZ) cmm_top
        | cmm_top <- prog ]

    cmmCfgOpts  :: Tx (ListGraph CmmStmt)
    cmmCfgOptsZ :: Tx (a, CmmGraph)

    cmmCfgOpts  = branchChainElim  -- boring, but will get more exciting later
    cmmCfgOptsZ g =
      optGraph
        (branchChainElimZ `seqTx` blockConcatZ `seqTx` removeUnreachableBlocksZ) g
            -- Here branchChainElim can ultimately be replaced
            -- with a more exciting combination of optimisations

    runCmmOpts :: Tx g -> Tx (GenCmm d h g)
    -- Lifts a transformer on a single graph to one on the whole program
    runCmmOpts opt = mapProcs (optProc opt)

    optProc :: Tx g -> Tx (GenCmmTop d h g)
    optProc _   top@(CmmData {}) = noTx top
    optProc opt (CmmProc info lbl formals g) =
      fmap (CmmProc info lbl formals) (opt g)

    optGraph :: Tx g -> Tx (a, g)
    optGraph opt (a, g) = fmap (\g' -> (a, g')) (opt g)

    ------------------------------------
    mapProcs :: Tx (GenCmmTop d h s) -> Tx (GenCmm d h s)
    mapProcs f (Cmm tops) = fmap Cmm (mapTx f tops)

    ----------------------------------------------------------------
    branchChainElim :: Tx (ListGraph CmmStmt)
    -- If L is not captured in an instruction, we can remove any
    -- basic block of the form L: goto L', and replace L with L' everywhere else.
    -- How does L get captured? In a CallArea.
    branchChainElim (ListGraph blocks)
      | null lone_branch_blocks     -- No blocks to remove
      = noTx (ListGraph blocks)
      | otherwise
      = aTx (ListGraph new_blocks)
      where
        (lone_branch_blocks, others) = partitionWith isLoneBranch blocks
        new_blocks = map (replaceLabels env) others
        env = mkClosureBlockEnv lone_branch_blocks

    isLoneBranch :: CmmBasicBlock -> Either (BlockId, BlockId) CmmBasicBlock
    isLoneBranch (BasicBlock id [CmmBranch target]) | id /= target = Left (id, target)
    isLoneBranch other_block                                       = Right other_block
       -- An infinite loop is not a link in a branch chain!

    replaceLabels :: BlockEnv BlockId -> CmmBasicBlock -> CmmBasicBlock
    replaceLabels env (BasicBlock id stmts)
      = BasicBlock id (map replace stmts)
      where
        replace (CmmBranch id)       = CmmBranch (lookup id)
        replace (CmmCondBranch e id) = CmmCondBranch e (lookup id)
        replace (CmmSwitch e tbl)    = CmmSwitch e (map (fmap lookup) tbl)
        replace other_stmt           = other_stmt

        lookup id = lookupBlockEnv env id `orElse` id
    ----------------------------------------------------------------
    branchChainElimZ :: Tx CmmGraph
    -- Remove any basic block of the form L: goto L',
    -- and replace L with L' everywhere else,
    -- unless L is the successor of a call instruction and L'
    -- is the entry block. You don't want to set the successor
    -- of a function call to the entry block because there is no good way
    -- to store both the infotables for the call and from the callee,
    -- while putting the stack pointer in a consistent place.
    --
    -- JD isn't quite sure when it's safe to share continuations for different
    -- function calls -- have to think about where the SP will be,
    -- so we'll table that problem for now by leaving all call successors alone.
    branchChainElimZ g@(G.LGraph eid _)
      | null lone_branch_blocks     -- No blocks to remove
      = noTx g
      | otherwise
      = aTx $ replaceLabelsZ env $ G.of_block_list eid (self_branches ++ others)
      where
        blocks = G.to_block_list g
        (lone_branch_blocks, others) = partitionWith isLoneBranchZ blocks
        env = mkClosureBlockEnvZ lone_branch_blocks
        self_branches =
          let loop_to (id, _) =
                if lookup id == id then
                  Just (G.Block id (G.ZLast (G.mkBranchNode id)))
                else
                  Nothing
          in  mapMaybe loop_to lone_branch_blocks
        lookup id = lookupBlockEnv env id `orElse` id

        call_succs = foldl add emptyBlockSet blocks
          where add succs b =
                  case G.last (G.unzip b) of
                    LastOther (LastCall _ (Just k) _ _ _) -> extendBlockSet succs k
                    _ -> succs
        isLoneBranchZ :: CmmBlock -> Either (BlockId, BlockId) CmmBlock
        isLoneBranchZ (G.Block id (G.ZLast (G.LastOther (LastBranch target))))
            | id /= target && not (elemBlockSet id call_succs) = Left (id,target)
        isLoneBranchZ other = Right other
           -- An infinite loop is not a link in a branch chain!

    maybeReplaceLabels :: (Last -> Bool) -> BlockEnv BlockId -> CmmGraph -> CmmGraph
    maybeReplaceLabels lpred env =
      replace_eid . G.map_nodes id middle last
       where
         replace_eid (G.LGraph eid blocks) = G.LGraph (lookup eid) blocks
         middle = mapExpDeepMiddle exp
         last l = if lpred l then mapExpDeepLast exp (last' l) else l
         last' (LastBranch bid) = LastBranch (lookup bid)
         last' (LastCondBranch p t f) = LastCondBranch p (lookup t) (lookup f)
         last' (LastSwitch e arms) = LastSwitch e (map (liftM lookup) arms)
         last' (LastCall t k a res r) = LastCall t (liftM lookup k) a res r
         exp (CmmLit (CmmBlock bid)) = CmmLit (CmmBlock (lookup bid))
         exp   (CmmStackSlot (CallArea (Young id)) i) =
           CmmStackSlot (CallArea (Young (lookup id))) i
         exp e = e
         lookup id = fmap lookup (lookupBlockEnv env id) `orElse` id

    replaceLabelsZ :: BlockEnv BlockId -> CmmGraph -> CmmGraph
    replaceLabelsZ = maybeReplaceLabels (const True)

    -- replaceBranchLabels :: BlockEnv BlockId -> CmmGraph -> CmmGraph
    -- replaceBranchLabels env g@(LGraph _ _) = maybeReplaceLabels lpred env g
    --   where lpred (LastBranch _) = True
    --         lpred _ = False

    replaceBranches :: BlockEnv BlockId -> CmmGraph -> CmmGraph
    replaceBranches env g = map_nodes id id last g
      where
        last (LastBranch id)          = LastBranch (lookup id)
        last (LastCondBranch e ti fi) = LastCondBranch e (lookup ti) (lookup fi)
        last (LastSwitch e tbl)       = LastSwitch e (map (fmap lookup) tbl)
        last l@(LastCall {})          = l
        lookup id = fmap lookup (lookupBlockEnv env id) `orElse` id

    ----------------------------------------------------------------
    -- Build a map from a block to its set of predecessors. Very useful.
    predMap :: G.LastNode l => G.LGraph m l -> BlockEnv BlockSet
    predMap g = G.fold_blocks add_preds emptyBlockEnv g -- find the back edges
      where add_preds b env = foldl (add b) env (G.succs b)
            add (G.Block bid _) env b' =
              extendBlockEnv env b' $
                    extendBlockSet (lookupBlockEnv env b' `orElse` emptyBlockSet) bid
    ----------------------------------------------------------------
    -- If a block B branches to a label L, L is not the entry block,
    -- and L has no other predecessors,
    -- then we can splice the block starting with L onto the end of B.
    -- Because this optimization can be inhibited by unreachable blocks,
    -- we first take a pass to drops unreachable blocks.
    -- Order matters, so we work bottom up (reverse postorder DFS).
    --
    -- To ensure correctness, we have to make sure that the BlockId of the block
    -- we are about to eliminate is not named in another instruction.
    --
    -- Note: This optimization does _not_ subsume branch chain elimination.
    blockConcatZ  :: Tx CmmGraph
    blockConcatZ = removeUnreachableBlocksZ `seqTx` blockConcatZ'
    blockConcatZ' :: Tx CmmGraph
    blockConcatZ' g@(G.LGraph eid blocks) =
      tx $ replaceLabelsZ concatMap $ G.LGraph eid blocks'
      where (changed, blocks', concatMap) =
               foldr maybe_concat (False, blocks, emptyBlockEnv) $ G.postorder_dfs g
            maybe_concat b@(G.Block bid _) (changed, blocks', concatMap) =
              let unchanged = (changed, extendBlockEnv blocks' bid b, concatMap)
              in case G.goto_end $ G.unzip b of
                   (h, G.LastOther (LastBranch b')) ->
                      if canConcatWith b' then
                        (True, extendBlockEnv blocks' bid $ splice blocks' h b',
                         extendBlockEnv concatMap b' bid)
                      else unchanged
                   _ -> unchanged
            num_preds bid = liftM sizeBlockSet (lookupBlockEnv backEdges bid) `orElse` 0
            canConcatWith b' = b' /= eid && num_preds b' == 1
            backEdges = predMap g
            splice blocks' h bid' =
              case lookupBlockEnv blocks' bid' of
                Just (G.Block _ t) -> G.zip $ G.ZBlock h t
                Nothing -> panic "unknown successor block"
            tx = if changed then aTx else noTx
    ----------------------------------------------------------------
    mkClosureBlockEnv :: [(BlockId, BlockId)] -> BlockEnv BlockId
    mkClosureBlockEnv blocks = mkBlockEnv $ map follow blocks
        where singleEnv = mkBlockEnv blocks
              follow (id, next) = (id, endChain id next)
              endChain orig id = case lookupBlockEnv singleEnv id of
                                   Just id' | id /= orig -> endChain orig id'
                                   _ -> id
    mkClosureBlockEnvZ :: [(BlockId, BlockId)] -> BlockEnv BlockId
    mkClosureBlockEnvZ blocks = mkBlockEnv $ map follow blocks
        where singleEnv = mkBlockEnv blocks
              follow (id, next) = (id, endChain id next)
              endChain orig id = case lookupBlockEnv singleEnv id of
                                   Just id' | id /= orig -> endChain orig id'
                                   _ -> id
    ----------------------------------------------------------------
    removeUnreachableBlocksZ :: Tx CmmGraph
    removeUnreachableBlocksZ g@(G.LGraph id blocks) =
      if length blocks' < sizeBEnv blocks then aTx $ G.of_block_list id blocks'
      else noTx g
        where blocks' = G.postorder_dfs g