src/Compiler/Hoopl/Passes/Dominator.hs

    {-# LANGUAGE CPP, GADTs #-}
    {-# OPTIONS_GHC -Wall -fno-warn-name-shadowing #-}
    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Safe #-}
    #endif

    module Compiler.Hoopl.Passes.Dominator
      ( Doms, DPath(..), domPath, domEntry, domLattice, extendDom
      , DominatorNode(..), DominatorTree(..), tree
      , immediateDominators
      , domPass
      )
    where

    import Data.Maybe

    import Compiler.Hoopl

    type Doms = WithBot DPath
    -- ^ List of labels, extended with a standard bottom element

    -- | The fact that goes into the entry of a dominator analysis: the first node
    -- is dominated only by the entry point, which is represented by the empty list
    -- of labels.
    domEntry :: Doms
    domEntry = PElem (DPath [])

    newtype DPath = DPath [Label]
      -- ^ represents part of the domination relation: each label
      -- in a list is dominated by all its successors.  This is a newtype only so
      -- we can give it a fancy Show instance.

    instance Show DPath where
      show (DPath ls) = concat (foldr (\l path -> show l : " -> " : path) ["entry"] ls)

    domPath :: Doms -> [Label]
    domPath Bot = [] -- lies: an unreachable node appears to be dominated by the entry
    domPath (PElem (DPath ls)) = ls

    extendDom :: Label -> DPath -> DPath
    extendDom l (DPath ls) = DPath (l:ls)

    domLattice :: DataflowLattice Doms
    domLattice = addPoints "dominators" extend

    extend :: JoinFun DPath
    extend _ (OldFact (DPath l)) (NewFact (DPath l')) =
                                    (changeIf (l `lengthDiffers` j), DPath j)
        where j = lcs l l'
              lcs :: [Label] -> [Label] -> [Label] -- longest common suffix
              lcs l l' | length l > length l' = lcs (drop (length l - length l') l) l'
                       | length l < length l' = lcs l' l
                       | otherwise = dropUnlike l l' l
              dropUnlike [] [] maybe_like = maybe_like
              dropUnlike (x:xs) (y:ys) maybe_like =
                  dropUnlike xs ys (if x == y then maybe_like else xs)
              dropUnlike _ _ _ = error "this can't happen"

              lengthDiffers [] [] = False
              lengthDiffers (_:xs) (_:ys) = lengthDiffers xs ys
              lengthDiffers [] (_:_) = True
              lengthDiffers (_:_) [] = True

    -- | Dominator pass
    domPass :: (NonLocal n, Monad m) => FwdPass m n Doms
    domPass = FwdPass domLattice (mkFTransfer3 first (const id) distributeFact) noFwdRewrite
      where first n = fmap (extendDom $ entryLabel n)

    ----------------------------------------------------------------

    data DominatorNode = Entry | Labelled Label
    data DominatorTree = Dominates DominatorNode [DominatorTree]
    -- ^ This data structure is a *rose tree* in which each node may have
    --  arbitrarily many children.  Each node dominates all its descendants.

    -- | Map from a FactBase for dominator lists into a
    -- dominator tree.
    tree :: [(Label, Doms)] -> DominatorTree
    tree facts = Dominates Entry $ merge $ map reverse $ map mkList facts
       -- This code has been lightly tested.  The key insight is this: to
       -- find lists that all have the same head, convert from a list of
       -- lists to a finite map, in 'children'.  Then, to convert from the
       -- finite map to list of dominator trees, use the invariant that
       -- each key dominates all the lists of values.
      where merge lists = mapTree $ children $ filter (not . null) lists
            children = foldl addList noFacts
            addList :: FactBase [[Label]] -> [Label] -> FactBase [[Label]]
            addList map (x:xs) = mapInsert x (xs:existing) map
                where existing = fromMaybe [] $ lookupFact x map
            addList _ [] = error "this can't happen"
            mapTree :: FactBase [[Label]] -> [DominatorTree]
            mapTree map = [Dominates (Labelled x) (merge lists) |
                                                        (x, lists) <- mapToList map]
            mkList (l, doms) = l : domPath doms

    instance Show DominatorTree where
      show = tree2dot

    -- | Given a dominator tree, produce a string representation, in the
    -- input language of dot, that will enable dot to produce a
    -- visualization of the tree.  For more info about dot see
    -- http://www.graphviz.org.

    tree2dot :: DominatorTree -> String
    tree2dot t = concat $ "digraph {\n" : dot t ["}\n"]
      where
        dot :: DominatorTree -> [String] -> [String]
        dot (Dominates root trees) =
                       (dotnode root :) . outedges trees . flip (foldl subtree) trees
          where outedges [] = id
                outedges (Dominates n _ : ts) =
                    \s -> "  " : show root : " -> " : show n : "\n" : outedges ts s
                dotnode Entry = "  entryNode [shape=plaintext, label=\"entry\"]\n"
                dotnode (Labelled l) = "  " ++ show l ++ "\n"
                subtree = flip dot

    instance Show DominatorNode where
      show Entry = "entryNode"
      show (Labelled l) = show l

    ----------------------------------------------------------------

    -- | Takes FactBase from dominator analysis and returns a map from each
    -- label to its immediate dominator, if any
    immediateDominators :: FactBase Doms -> LabelMap Label
    immediateDominators = mapFoldWithKey add mapEmpty
        where add l (PElem (DPath (idom:_))) = mapInsert l idom
              add _ _ = id