cmm/CmmImplementSwitchPlans.hs

    {-# LANGUAGE GADTs #-}
    module CmmImplementSwitchPlans
      ( cmmImplementSwitchPlans
      )
    where

    import Hoopl
    import BlockId
    import Cmm
    import CmmUtils
    import CmmSwitch
    import UniqSupply
    import DynFlags

    --
    -- This module replaces Switch statements as generated by the Stg -> Cmm
    -- transformation, which might be huge and sparse and hence unsuitable for
    -- assembly code, by proper constructs (if-then-else trees, dense jump tables).
    --
    -- The actual, abstract strategy is determined by createSwitchPlan in
    -- CmmSwitch and returned as a SwitchPlan; here is just the implementation in
    -- terms of Cmm code. See Note [Cmm Switches, the general plan] in CmmSwitch.
    --
    -- This division into different modules is both to clearly separte concerns,
    -- but also because createSwitchPlan needs access to the constructors of
    -- SwitchTargets, a data type exported abstractly by CmmSwitch.
    --

    -- | Traverses the 'CmmGraph', making sure that 'CmmSwitch' are suitable for
    -- code generation.
    cmmImplementSwitchPlans :: DynFlags -> CmmGraph -> UniqSM CmmGraph
    cmmImplementSwitchPlans dflags g
        | targetSupportsSwitch (hscTarget dflags) = return g
        | otherwise = do
        blocks' <- concat `fmap` mapM (visitSwitches dflags) (toBlockList g)
        return $ ofBlockList (g_entry g) blocks'

    visitSwitches :: DynFlags -> CmmBlock -> UniqSM [CmmBlock]
    visitSwitches dflags block
      | (entry@(CmmEntry _ scope), middle, CmmSwitch expr ids) <- blockSplit block
      = do
        let plan = createSwitchPlan ids

        (newTail, newBlocks) <- implementSwitchPlan dflags scope expr plan

        let block' = entry `blockJoinHead` middle `blockAppend` newTail

        return $ block' : newBlocks

      | otherwise
      = return [block]

    -- Implementing a switch plan (returning a tail block)
    implementSwitchPlan :: DynFlags -> CmmTickScope -> CmmExpr -> SwitchPlan -> UniqSM (Block CmmNode O C, [CmmBlock])
    implementSwitchPlan dflags scope expr = go
      where
        go (Unconditionally l)
          = return (emptyBlock `blockJoinTail` CmmBranch l, [])
        go (JumpTable ids)
          = return (emptyBlock `blockJoinTail` CmmSwitch expr ids, [])
        go (IfLT signed i ids1 ids2)
          = do
            (bid1, newBlocks1) <- go' ids1
            (bid2, newBlocks2) <- go' ids2

            let lt | signed    = cmmSLtWord
                   | otherwise = cmmULtWord
                scrut = lt dflags expr $ CmmLit $ mkWordCLit dflags i
                lastNode = CmmCondBranch scrut bid1 bid2 Nothing
                lastBlock = emptyBlock `blockJoinTail` lastNode
            return (lastBlock, newBlocks1++newBlocks2)
        go (IfEqual i l ids2)
          = do
            (bid2, newBlocks2) <- go' ids2

            let scrut = cmmNeWord dflags expr $ CmmLit $ mkWordCLit dflags i
                lastNode = CmmCondBranch scrut bid2 l Nothing
                lastBlock = emptyBlock `blockJoinTail` lastNode
            return (lastBlock, newBlocks2)

        -- Same but returning a label to branch to
        go' (Unconditionally l)
          = return (l, [])
        go' p
          = do
            bid <- mkBlockId `fmap` getUniqueM
            (last, newBlocks) <- go p
            let block = CmmEntry bid scope `blockJoinHead` last
            return (bid, block: newBlocks)