    -- | Register coalescing.
    module RegAlloc.Graph.Coalesce (
            regCoalesce,
            slurpJoinMovs
    ) where
    import GhcPrelude

    import RegAlloc.Liveness
    import Instruction
    import Reg

    import Cmm
    import Bag
    import Digraph
    import UniqFM
    import UniqSet
    import UniqSupply

    import Data.List

    -- | Do register coalescing on this top level thing
    --
    --   For Reg -> Reg moves, if the first reg dies at the same time the
    --   second reg is born then the mov only serves to join live ranges.
    --   The two regs can be renamed to be the same and the move instruction
    --   safely erased.
    regCoalesce
            :: Instruction instr
            => [LiveCmmDecl statics instr]
            -> UniqSM [LiveCmmDecl statics instr]

    regCoalesce code
     = do
            let joins       = foldl' unionBags emptyBag
                            $ map slurpJoinMovs code

            let alloc       = foldl' buildAlloc emptyUFM
                            $ bagToList joins

            let patched     = map (patchEraseLive (sinkReg alloc)) code

            return patched

    -- | Add a v1 = v2 register renaming to the map.
    --   The register with the lowest lexical name is set as the
    --   canonical version.
    buildAlloc :: UniqFM Reg -> (Reg, Reg) -> UniqFM Reg
    buildAlloc fm (r1, r2)
     = let  rmin    = min r1 r2
            rmax    = max r1 r2
       in   addToUFM fm rmax rmin

    -- | Determine the canonical name for a register by following
    --   v1 = v2 renamings in this map.
    sinkReg :: UniqFM Reg -> Reg -> Reg
    sinkReg fm r
     = case lookupUFM fm r of
            Nothing -> r
            Just r' -> sinkReg fm r'

    -- | Slurp out mov instructions that only serve to join live ranges.
    --
    --   During a mov, if the source reg dies and the destination reg is
    --   born then we can rename the two regs to the same thing and
    --   eliminate the move.
    slurpJoinMovs
            :: Instruction instr
            => LiveCmmDecl statics instr
            -> Bag (Reg, Reg)

    slurpJoinMovs live
            = slurpCmm emptyBag live
     where
            slurpCmm   rs  CmmData{}
             = rs

            slurpCmm   rs (CmmProc _ _ _ sccs)
             = foldl' slurpBlock rs (flattenSCCs sccs)

            slurpBlock rs (BasicBlock _ instrs)
             = foldl' slurpLI    rs instrs

            slurpLI    rs (LiveInstr _      Nothing)    = rs
            slurpLI    rs (LiveInstr instr (Just live))
                    | Just (r1, r2) <- takeRegRegMoveInstr instr
                    , elementOfUniqSet r1 $ liveDieRead live
                    , elementOfUniqSet r2 $ liveBorn live

                    -- only coalesce movs between two virtuals for now,
                    -- else we end up with allocatable regs in the live
                    -- regs list..
                    , isVirtualReg r1 && isVirtualReg r2
                    = consBag (r1, r2) rs

                    | otherwise
                    = rs