cmm/CmmContFlowOpt.hs

    {-# LANGUAGE GADTs #-}
    {-# OPTIONS_GHC -fno-warn-incomplete-patterns #-}
    module CmmContFlowOpt
        ( cmmCfgOpts
        , cmmCfgOptsProc
        , removeUnreachableBlocksProc
        , replaceLabels
        )
    where

    import Hoopl
    import BlockId
    import Cmm
    import CmmUtils
    import CmmSwitch (mapSwitchTargets)
    import Maybes
    import Panic

    import Control.Monad
    import Prelude hiding (succ, unzip, zip)

    -- Note [What is shortcutting]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- Consider this Cmm code:
    --
    -- L1: ...
    --     goto L2;
    -- L2: goto L3;
    -- L3: ...
    --
    -- Here L2 is an empty block and contains only an unconditional branch
    -- to L3. In this situation any block that jumps to L2 can jump
    -- directly to L3:
    --
    -- L1: ...
    --     goto L3;
    -- L2: goto L3;
    -- L3: ...
    --
    -- In this situation we say that we shortcut L2 to L3. One of
    -- consequences of shortcutting is that some blocks of code may become
    -- unreachable (in the example above this is true for L2).

    -- Note [Control-flow optimisations]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- This optimisation does three things:
    --
    --   - If a block finishes in an unconditonal branch to another block
    --     and that is the only jump to that block we concatenate the
    --     destination block at the end of the current one.
    --
    --   - If a block finishes in a call whose continuation block is a
    --     goto, then we can shortcut the destination, making the
    --     continuation block the destination of the goto - but see Note
    --     [Shortcut call returns].
    --
    --   - For any block that is not a call we try to shortcut the
    --     destination(s). Additionally, if a block ends with a
    --     conditional branch we try to invert the condition.
    --
    -- Blocks are processed using postorder DFS traversal. A side effect
    -- of determining traversal order with a graph search is elimination
    -- of any blocks that are unreachable.
    --
    -- Transformations are improved by working from the end of the graph
    -- towards the beginning, because we may be able to perform many
    -- shortcuts in one go.

    -- Note [Shortcut call returns]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- We are going to maintain the "current" graph (LabelMap CmmBlock) as
    -- we go, and also a mapping from BlockId to BlockId, representing
    -- continuation labels that we have renamed.  This latter mapping is
    -- important because we might shortcut a CmmCall continuation.  For
    -- example:
    --
    --    Sp[0] = L
    --    call g returns to L
    --    L: goto M
    --    M: ...
    --
    -- So when we shortcut the L block, we need to replace not only
    -- the continuation of the call, but also references to L in the
    -- code (e.g. the assignment Sp[0] = L):
    --
    --    Sp[0] = M
    --    call g returns to M
    --    M: ...
    --
    -- So we keep track of which labels we have renamed and apply the mapping
    -- at the end with replaceLabels.

    -- Note [Shortcut call returns and proc-points]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    --
    -- Consider this code that you might get from a recursive
    -- let-no-escape:
    --
    --       goto L1
    --      L1:
    --       if (Hp > HpLim) then L2 else L3
    --      L2:
    --       call stg_gc_noregs returns to L4
    --      L4:
    --       goto L1
    --      L3:
    --       ...
    --       goto L1
    --
    -- Then the control-flow optimiser shortcuts L4.  But that turns L1
    -- into the call-return proc point, and every iteration of the loop
    -- has to shuffle variables to and from the stack.  So we must *not*
    -- shortcut L4.
    --
    -- Moreover not shortcutting call returns is probably fine.  If L4 can
    -- concat with its branch target then it will still do so.  And we
    -- save some compile time because we don't have to traverse all the
    -- code in replaceLabels.
    --
    -- However, we probably do want to do this if we are splitting proc
    -- points, because L1 will be a proc-point anyway, so merging it with
    -- L4 reduces the number of proc points.  Unfortunately recursive
    -- let-no-escapes won't generate very good code with proc-point
    -- splitting on - we should probably compile them to explicitly use
    -- the native calling convention instead.

    cmmCfgOpts :: Bool -> CmmGraph -> CmmGraph
    cmmCfgOpts split g = fst (blockConcat split g)

    cmmCfgOptsProc :: Bool -> CmmDecl -> CmmDecl
    cmmCfgOptsProc split (CmmProc info lbl live g) = CmmProc info' lbl live g'
        where (g', env) = blockConcat split g
              info' = info{ info_tbls = new_info_tbls }
              new_info_tbls = mapFromList (map upd_info (mapToList (info_tbls info)))

              -- If we changed any labels, then we have to update the info tables
              -- too, except for the top-level info table because that might be
              -- referred to by other procs.
              upd_info (k,info)
                 | Just k' <- mapLookup k env
                 = (k', if k' == g_entry g'
                           then info
                           else info{ cit_lbl = infoTblLbl k' })
                 | otherwise
                 = (k,info)
    cmmCfgOptsProc _ top = top

    blockConcat :: Bool -> CmmGraph -> (CmmGraph, LabelMap BlockId)
    blockConcat splitting_procs g@CmmGraph { g_entry = entry_id }
      = (replaceLabels shortcut_map $ ofBlockMap new_entry new_blocks, shortcut_map')
      where
         -- We might be able to shortcut the entry BlockId itself.
         -- Remember to update the shortcut_map, since we also have to
         -- update the info_tbls mapping now.
         (new_entry, shortcut_map')
           | Just entry_blk <- mapLookup entry_id new_blocks
           , Just dest      <- canShortcut entry_blk
           = (dest, mapInsert entry_id dest shortcut_map)
           | otherwise
           = (entry_id, shortcut_map)

         -- blocks is a list of blocks in DFS postorder, while blockmap is
         -- a map of blocks. We process each element from blocks and update
         -- blockmap accordingly
         blocks = postorderDfs g
         blockmap = foldr addBlock emptyBody blocks

         -- Accumulator contains three components:
         --  * map of blocks in a graph
         --  * map of shortcut labels. See Note [Shortcut call returns]
         --  * map containing number of predecessors for each block. We discard
         --    it after we process all blocks.
         (new_blocks, shortcut_map, _) =
               foldr maybe_concat (blockmap, mapEmpty, initialBackEdges) blocks

         -- Map of predecessors for initial graph. We increase number of
         -- predecessors for entry block by one to denote that it is
         -- target of a jump, even if no block in the current graph jumps
         -- to it.
         initialBackEdges = incPreds entry_id (predMap blocks)

         maybe_concat :: CmmBlock
                      -> (LabelMap CmmBlock, LabelMap BlockId, LabelMap Int)
                      -> (LabelMap CmmBlock, LabelMap BlockId, LabelMap Int)
         maybe_concat block (blocks, shortcut_map, backEdges)
            -- If:
            --   (1) current block ends with unconditional branch to b' and
            --   (2) it has exactly one predecessor (namely, current block)
            --
            -- Then:
            --   (1) append b' block at the end of current block
            --   (2) remove b' from the map of blocks
            --   (3) remove information about b' from predecessors map
            --
            -- Since we know that the block has only one predecessor we call
            -- mapDelete directly instead of calling decPreds.
            --
            -- Note that we always maintain an up-to-date list of predecessors, so
            -- we can ignore the contents of shortcut_map
            | CmmBranch b' <- last
            , hasOnePredecessor b'
            , Just blk' <- mapLookup b' blocks
            = let bid' = entryLabel blk'
              in ( mapDelete bid' $ mapInsert bid (splice head blk') blocks
                 , shortcut_map
                 , mapDelete b' backEdges )

            -- If:
            --   (1) we are splitting proc points (see Note
            --       [Shortcut call returns and proc-points]) and
            --   (2) current block is a CmmCall or CmmForeignCall with
            --       continuation b' and
            --   (3) we can shortcut that continuation to dest
            -- Then:
            --   (1) we change continuation to point to b'
            --   (2) create mapping from b' to dest
            --   (3) increase number of predecessors of dest by 1
            --   (4) decrease number of predecessors of b' by 1
            --
            -- Later we will use replaceLabels to substitute all occurrences of b'
            -- with dest.
            | splitting_procs
            , Just b'   <- callContinuation_maybe last
            , Just blk' <- mapLookup b' blocks
            , Just dest <- canShortcut blk'
            = ( mapInsert bid (blockJoinTail head (update_cont dest)) blocks
              , mapInsert b' dest shortcut_map
              , decPreds b' $ incPreds dest backEdges )

            -- If:
            --   (1) a block does not end with a call
            -- Then:
            --   (1) if it ends with a conditional attempt to invert the
            --       conditional
            --   (2) attempt to shortcut all destination blocks
            --   (3) if new successors of a block are different from the old ones
            --       update the of predecessors accordingly
            --
            -- A special case of this is a situation when a block ends with an
            -- unconditional jump to a block that can be shortcut.
            | Nothing <- callContinuation_maybe last
            = let oldSuccs = successors last
                  newSuccs = successors swapcond_last
              in ( mapInsert bid (blockJoinTail head swapcond_last) blocks
                 , shortcut_map
                 , if oldSuccs == newSuccs
                   then backEdges
                   else foldr incPreds (foldr decPreds backEdges oldSuccs) newSuccs )

            -- Otherwise don't do anything
            | otherwise
            = ( blocks, shortcut_map, backEdges )
            where
              (head, last) = blockSplitTail block
              bid = entryLabel block

              -- Changes continuation of a call to a specified label
              update_cont dest =
                  case last of
                    CmmCall{}        -> last { cml_cont = Just dest }
                    CmmForeignCall{} -> last { succ = dest }
                    _                -> panic "Can't shortcut continuation."

              -- Attempts to shortcut successors of last node
              shortcut_last = mapSuccessors shortcut last
                where
                  shortcut l =
                     case mapLookup l blocks of
                       Just b | Just dest <- canShortcut b -> dest
                       _otherwise -> l

              -- For a conditional, we invert the conditional if that would make it
              -- more likely that the branch-not-taken case becomes a fallthrough.
              -- This helps the native codegen a little bit, and probably has no
              -- effect on LLVM.  It's convenient to do it here, where we have the
              -- information about predecessors.
              swapcond_last
                | CmmCondBranch cond t f l <- shortcut_last
                , likelyFalse l
                , numPreds f > 1
                , hasOnePredecessor t
                , Just cond' <- maybeInvertCmmExpr cond
                = CmmCondBranch cond' f t (invertLikeliness l)

                | otherwise
                = shortcut_last

              likelyFalse (Just False) = True
              likelyFalse Nothing      = True
              likelyFalse _            = False

              invertLikeliness (Just b)     = Just (not b)
              invertLikeliness Nothing      = Nothing

              -- Number of predecessors for a block
              numPreds bid = mapLookup bid backEdges `orElse` 0

              hasOnePredecessor b = numPreds b == 1

    -- Functions for incrementing and decrementing number of predecessors. If
    -- decrementing would set the predecessor count to 0, we remove entry from the
    -- map.
    -- Invariant: if a block has no predecessors it should be dropped from the
    -- graph because it is unreachable. maybe_concat is constructed to maintain
    -- that invariant, but calling replaceLabels may introduce unreachable blocks.
    -- We rely on subsequent passes in the Cmm pipeline to remove unreachable
    -- blocks.
    incPreds, decPreds :: BlockId -> LabelMap Int -> LabelMap Int
    incPreds bid edges = mapInsertWith (+) bid 1 edges
    decPreds bid edges = case mapLookup bid edges of
                           Just preds | preds > 1 -> mapInsert bid (preds - 1) edges
                           Just _                 -> mapDelete bid edges
                           _                      -> edges

    -- Checks if a block consists only of "goto dest". If it does than we return
    -- "Just dest" label. See Note [What is shortcutting]
    canShortcut :: CmmBlock -> Maybe BlockId
    canShortcut block
        | (_, middle, CmmBranch dest) <- blockSplit block
        , all dont_care $ blockToList middle
        = Just dest
        | otherwise
        = Nothing
        where dont_care CmmComment{} = True
              dont_care CmmTick{}    = True
              dont_care _other       = False

    -- Concatenates two blocks. First one is assumed to be open on exit, the second
    -- is assumed to be closed on entry (i.e. it has a label attached to it, which
    -- the splice function removes by calling snd on result of blockSplitHead).
    splice :: Block CmmNode C O -> CmmBlock -> CmmBlock
    splice head rest = entry `blockJoinHead` code0 `blockAppend` code1
      where (CmmEntry lbl sc0, code0) = blockSplitHead head
            (CmmEntry _   sc1, code1) = blockSplitHead rest
            entry = CmmEntry lbl (combineTickScopes sc0 sc1)

    -- If node is a call with continuation call return Just label of that
    -- continuation. Otherwise return Nothing.
    callContinuation_maybe :: CmmNode O C -> Maybe BlockId
    callContinuation_maybe (CmmCall { cml_cont = Just b }) = Just b
    callContinuation_maybe (CmmForeignCall { succ = b })   = Just b
    callContinuation_maybe _ = Nothing

    -- Map over the CmmGraph, replacing each label with its mapping in the
    -- supplied LabelMap.
    replaceLabels :: LabelMap BlockId -> CmmGraph -> CmmGraph
    replaceLabels env g
      | mapNull env = g
      | otherwise   = replace_eid $ mapGraphNodes1 txnode g
       where
         replace_eid g = g {g_entry = lookup (g_entry g)}
         lookup id = mapLookup id env `orElse` id

         txnode :: CmmNode e x -> CmmNode e x
         txnode (CmmBranch bid) = CmmBranch (lookup bid)
         txnode (CmmCondBranch p t f l) =
           mkCmmCondBranch (exp p) (lookup t) (lookup f) l
         txnode (CmmSwitch e ids) =
           CmmSwitch (exp e) (mapSwitchTargets lookup ids)
         txnode (CmmCall t k rg a res r) =
           CmmCall (exp t) (liftM lookup k) rg a res r
         txnode fc@CmmForeignCall{} =
           fc{ args = map exp (args fc), succ = lookup (succ fc) }
         txnode other = mapExpDeep exp other

         exp :: CmmExpr -> CmmExpr
         exp (CmmLit (CmmBlock bid))                = CmmLit (CmmBlock (lookup bid))
         exp (CmmStackSlot (Young id) i) = CmmStackSlot (Young (lookup id)) i
         exp e                                      = e

    mkCmmCondBranch :: CmmExpr -> Label -> Label -> Maybe Bool -> CmmNode O C
    mkCmmCondBranch p t f l =
      if t == f then CmmBranch t else CmmCondBranch p t f l

    -- Build a map from a block to its set of predecessors.
    predMap :: [CmmBlock] -> LabelMap Int
    predMap blocks = foldr add_preds mapEmpty blocks
      where
        add_preds block env = foldr add env (successors block)
          where add lbl env = mapInsertWith (+) lbl 1 env

    -- Removing unreachable blocks
    removeUnreachableBlocksProc :: CmmDecl -> CmmDecl
    removeUnreachableBlocksProc proc@(CmmProc info lbl live g)
       | length used_blocks < mapSize (toBlockMap g)
       = CmmProc info' lbl live g'
       | otherwise
       = proc
       where
         g'    = ofBlockList (g_entry g) used_blocks
         info' = info { info_tbls = keep_used (info_tbls info) }
                 -- Remove any info_tbls for unreachable

         keep_used :: LabelMap CmmInfoTable -> LabelMap CmmInfoTable
         keep_used bs = mapFoldWithKey keep mapEmpty bs

         keep :: Label -> CmmInfoTable -> LabelMap CmmInfoTable -> LabelMap CmmInfoTable
         keep l i env | l `setMember` used_lbls = mapInsert l i env
                      | otherwise               = env

         used_blocks :: [CmmBlock]
         used_blocks = postorderDfs g

         used_lbls :: LabelSet
         used_lbls = foldr (setInsert . entryLabel) setEmpty used_blocks