GHC/CmmToAsm/Reg/Liveness.hs

    {-# LANGUAGE BangPatterns #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE DeriveFunctor #-}
    {-# LANGUAGE MultiParamTypeClasses #-}
    {-# LANGUAGE FlexibleInstances #-}

    {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

    -----------------------------------------------------------------------------
    --
    -- The register liveness determinator
    --
    -- (c) The University of Glasgow 2004-2013
    --
    -----------------------------------------------------------------------------

    module GHC.CmmToAsm.Reg.Liveness (
            RegSet,
            RegMap, emptyRegMap,
            BlockMap, mapEmpty,
            LiveCmmDecl,
            InstrSR   (..),
            LiveInstr (..),
            Liveness (..),
            LiveInfo (..),
            LiveBasicBlock,

            mapBlockTop,    mapBlockTopM,   mapSCCM,
            mapGenBlockTop, mapGenBlockTopM,
            mapLiveCmmDecl, pprLiveCmmDecl,
            stripLive,
            stripLiveBlock,
            slurpConflicts,
            slurpReloadCoalesce,
            eraseDeltasLive,
            patchEraseLive,
            patchRegsLiveInstr,
            reverseBlocksInTops,
            regLiveness,
            cmmTopLiveness
      ) where
    import GHC.Prelude

    import GHC.Platform.Reg
    import GHC.CmmToAsm.Instr
    import GHC.CmmToAsm.CFG
    import GHC.CmmToAsm.Config
    import GHC.CmmToAsm.Types
    import GHC.CmmToAsm.Utils

    import GHC.Cmm.BlockId
    import GHC.Cmm.Dataflow.Collections
    import GHC.Cmm.Dataflow.Label
    import GHC.Cmm hiding (RegSet, emptyRegSet)

    import GHC.Data.Graph.Directed
    import GHC.Utils.Monad
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Platform
    import GHC.Types.Unique.Set
    import GHC.Types.Unique.FM
    import GHC.Types.Unique.Supply
    import GHC.Data.Bag
    import GHC.Utils.Monad.State

    import Data.List (mapAccumL, groupBy, partition)
    import Data.Maybe
    import Data.IntSet              (IntSet)

    -----------------------------------------------------------------------------
    type RegSet = UniqSet Reg

    -- | Map from some kind of register to a.
    --
    -- While we give the type for keys as Reg which is the common case
    -- sometimes we end up using VirtualReq or naked Uniques.
    -- See Note [UniqFM and the register allocator]
    type RegMap a = UniqFM Reg a

    emptyRegMap :: RegMap a
    emptyRegMap = emptyUFM

    emptyRegSet :: RegSet
    emptyRegSet = emptyUniqSet

    type BlockMap a = LabelMap a

    type SlotMap a = UniqFM Slot a

    type Slot = Int

    -- | A top level thing which carries liveness information.
    type LiveCmmDecl statics instr
            = GenCmmDecl
                    statics
                    LiveInfo
                    [SCC (LiveBasicBlock instr)]

    -- | The register allocator also wants to use SPILL/RELOAD meta instructions,
    --   so we'll keep those here.
    data InstrSR instr
            -- | A real machine instruction
            = Instr  instr

            -- | spill this reg to a stack slot
            | SPILL  Reg Int

            -- | reload this reg from a stack slot
            | RELOAD Int Reg

            deriving (Functor)

    instance Instruction instr => Instruction (InstrSR instr) where
            regUsageOfInstr platform i
             = case i of
                    Instr  instr    -> regUsageOfInstr platform instr
                    SPILL  reg _    -> RU [reg] []
                    RELOAD _ reg    -> RU [] [reg]

            patchRegsOfInstr i f
             = case i of
                    Instr instr     -> Instr (patchRegsOfInstr instr f)
                    SPILL  reg slot -> SPILL (f reg) slot
                    RELOAD slot reg -> RELOAD slot (f reg)

            isJumpishInstr i
             = case i of
                    Instr instr     -> isJumpishInstr instr
                    _               -> False

            jumpDestsOfInstr i
             = case i of
                    Instr instr     -> jumpDestsOfInstr instr
                    _               -> []

            patchJumpInstr i f
             = case i of
                    Instr instr     -> Instr (patchJumpInstr instr f)
                    _               -> i

            mkSpillInstr            = error "mkSpillInstr[InstrSR]: Not making SPILL meta-instr"
            mkLoadInstr             = error "mkLoadInstr[InstrSR]: Not making LOAD meta-instr"

            takeDeltaInstr i
             = case i of
                    Instr instr     -> takeDeltaInstr instr
                    _               -> Nothing

            isMetaInstr i
             = case i of
                    Instr instr     -> isMetaInstr instr
                    _               -> False

            mkRegRegMoveInstr platform r1 r2
                = Instr (mkRegRegMoveInstr platform r1 r2)

            takeRegRegMoveInstr i
             = case i of
                    Instr instr     -> takeRegRegMoveInstr instr
                    _               -> Nothing

            mkJumpInstr target      = map Instr (mkJumpInstr target)

            mkStackAllocInstr platform amount =
                 Instr <$> mkStackAllocInstr platform amount

            mkStackDeallocInstr platform amount =
                 Instr <$> mkStackDeallocInstr platform amount

            pprInstr platform i = ppr (fmap (pprInstr platform) i)

    -- | An instruction with liveness information.
    data LiveInstr instr
            = LiveInstr (InstrSR instr) (Maybe Liveness)
            deriving (Functor)

    -- | Liveness information.
    --   The regs which die are ones which are no longer live in the *next* instruction
    --   in this sequence.
    --   (NB. if the instruction is a jump, these registers might still be live
    --   at the jump target(s) - you have to check the liveness at the destination
    --   block to find out).

    data Liveness
            = Liveness
            { liveBorn      :: RegSet       -- ^ registers born in this instruction (written to for first time).
            , liveDieRead   :: RegSet       -- ^ registers that died because they were read for the last time.
            , liveDieWrite  :: RegSet }     -- ^ registers that died because they were clobbered by something.

    -- | Stash regs live on entry to each basic block in the info part of the cmm code.
    data LiveInfo
            = LiveInfo
                    (LabelMap RawCmmStatics)  -- cmm info table static stuff
                    [BlockId]                 -- entry points (first one is the
                                              -- entry point for the proc).
                    (BlockMap RegSet)         -- argument locals live on entry to this block
                    (BlockMap IntSet)         -- stack slots live on entry to this block

    -- | A basic block with liveness information.
    type LiveBasicBlock instr
            = GenBasicBlock (LiveInstr instr)

    instance Outputable instr
          => Outputable (InstrSR instr) where

            ppr (Instr realInstr)
               = ppr realInstr

            ppr (SPILL reg slot)
               = hcat [
                    text "\tSPILL",
                    char ' ',
                    ppr reg,
                    comma,
                    text "SLOT" <> parens (int slot)]

            ppr (RELOAD slot reg)
               = hcat [
                    text "\tRELOAD",
                    char ' ',
                    text "SLOT" <> parens (int slot),
                    comma,
                    ppr reg]

    instance Outputable instr
          => Outputable (LiveInstr instr) where

            ppr (LiveInstr instr Nothing)
             = ppr instr

            ppr (LiveInstr instr (Just live))
             =  ppr instr
                    $$ (nest 8
                            $ vcat
                            [ pprRegs (text "# born:    ") (liveBorn live)
                            , pprRegs (text "# r_dying: ") (liveDieRead live)
                            , pprRegs (text "# w_dying: ") (liveDieWrite live) ]
                        $+$ space)

             where  pprRegs :: SDoc -> RegSet -> SDoc
                    pprRegs name regs
                     | isEmptyUniqSet regs  = empty
                     | otherwise            = name <>
                         (pprUFM (getUniqSet regs) (hcat . punctuate space . map ppr))

    instance OutputableP env instr => OutputableP env (LiveInstr instr) where
       pdoc env i = ppr (fmap (pdoc env) i)

    instance OutputableP Platform LiveInfo where
        pdoc env (LiveInfo mb_static entryIds liveVRegsOnEntry liveSlotsOnEntry)
            =  (pdoc env mb_static)
            $$ text "# entryIds         = " <> ppr entryIds
            $$ text "# liveVRegsOnEntry = " <> ppr liveVRegsOnEntry
            $$ text "# liveSlotsOnEntry = " <> text (show liveSlotsOnEntry)

    -- | map a function across all the basic blocks in this code
    --
    mapBlockTop
            :: (LiveBasicBlock instr -> LiveBasicBlock instr)
            -> LiveCmmDecl statics instr -> LiveCmmDecl statics instr

    mapBlockTop f cmm
            = evalState (mapBlockTopM (\x -> return $ f x) cmm) ()

    -- | map a function across all the basic blocks in this code (monadic version)
    --
    mapBlockTopM
            :: Monad m
            => (LiveBasicBlock instr -> m (LiveBasicBlock instr))
            -> LiveCmmDecl statics instr -> m (LiveCmmDecl statics instr)

    mapBlockTopM _ cmm@(CmmData{})
            = return cmm

    mapBlockTopM f (CmmProc header label live sccs)
     = do   sccs'   <- mapM (mapSCCM f) sccs
            return  $ CmmProc header label live sccs'

    mapSCCM :: Monad m => (a -> m b) -> SCC a -> m (SCC b)
    mapSCCM f (AcyclicSCC x)
     = do   x'      <- f x
            return  $ AcyclicSCC x'

    mapSCCM f (CyclicSCC xs)
     = do   xs'     <- mapM f xs
            return  $ CyclicSCC xs'

    -- map a function across all the basic blocks in this code
    mapGenBlockTop
            :: (GenBasicBlock             i -> GenBasicBlock            i)
            -> (GenCmmDecl d h (ListGraph i) -> GenCmmDecl d h (ListGraph i))

    mapGenBlockTop f cmm
            = evalState (mapGenBlockTopM (\x -> return $ f x) cmm) ()

    -- | map a function across all the basic blocks in this code (monadic version)
    mapGenBlockTopM
            :: Monad m
            => (GenBasicBlock            i  -> m (GenBasicBlock            i))
            -> (GenCmmDecl d h (ListGraph i) -> m (GenCmmDecl d h (ListGraph i)))

    mapGenBlockTopM _ cmm@(CmmData{})
            = return cmm

    mapGenBlockTopM f (CmmProc header label live (ListGraph blocks))
     = do   blocks' <- mapM f blocks
            return  $ CmmProc header label live (ListGraph blocks')

    -- | Slurp out the list of register conflicts and reg-reg moves from this top level thing.
    --   Slurping of conflicts and moves is wrapped up together so we don't have
    --   to make two passes over the same code when we want to build the graph.
    --
    slurpConflicts
            :: Instruction instr
            => LiveCmmDecl statics instr
            -> (Bag (UniqSet Reg), Bag (Reg, Reg))

    slurpConflicts live
            = slurpCmm (emptyBag, emptyBag) live

     where  slurpCmm   rs  CmmData{}                = rs
            slurpCmm   rs (CmmProc info _ _ sccs)
                    = foldl' (slurpSCC info) rs sccs

            slurpSCC  info rs (AcyclicSCC b)
                    = slurpBlock info rs b

            slurpSCC  info rs (CyclicSCC bs)
                    = foldl'  (slurpBlock info) rs bs

            slurpBlock info rs (BasicBlock blockId instrs)
                    | LiveInfo _ _ blockLive _        <- info
                    , Just rsLiveEntry                <- mapLookup blockId blockLive
                    , (conflicts, moves)              <- slurpLIs rsLiveEntry rs instrs
                    = (consBag rsLiveEntry conflicts, moves)

                    | otherwise
                    = panic "Liveness.slurpConflicts: bad block"

            slurpLIs rsLive (conflicts, moves) []
                    = (consBag rsLive conflicts, moves)

            slurpLIs rsLive rs (LiveInstr _ Nothing     : lis)
                    = slurpLIs rsLive rs lis

            slurpLIs rsLiveEntry (conflicts, moves) (LiveInstr instr (Just live) : lis)
             = let
                    -- regs that die because they are read for the last time at the start of an instruction
                    --      are not live across it.
                    rsLiveAcross    = rsLiveEntry `minusUniqSet` (liveDieRead live)

                    -- regs live on entry to the next instruction.
                    --      be careful of orphans, make sure to delete dying regs _after_ unioning
                    --      in the ones that are born here.
                    rsLiveNext      = (rsLiveAcross `unionUniqSets` (liveBorn     live))
                                                    `minusUniqSet`  (liveDieWrite live)

                    -- orphan vregs are the ones that die in the same instruction they are born in.
                    --      these are likely to be results that are never used, but we still
                    --      need to assign a hreg to them..
                    rsOrphans       = intersectUniqSets
                                            (liveBorn live)
                                            (unionUniqSets (liveDieWrite live) (liveDieRead live))

                    --
                    rsConflicts     = unionUniqSets rsLiveNext rsOrphans

              in    case takeRegRegMoveInstr instr of
                     Just rr        -> slurpLIs rsLiveNext
                                            ( consBag rsConflicts conflicts
                                            , consBag rr moves) lis

                     Nothing        -> slurpLIs rsLiveNext
                                            ( consBag rsConflicts conflicts
                                            , moves) lis

    -- | For spill\/reloads
    --
    --   SPILL  v1, slot1
    --   ...
    --   RELOAD slot1, v2
    --
    --   If we can arrange that v1 and v2 are allocated to the same hreg it's more likely
    --   the spill\/reload instrs can be cleaned and replaced by a nop reg-reg move.
    --
    --
    slurpReloadCoalesce
            :: forall statics instr. Instruction instr
            => LiveCmmDecl statics instr
            -> Bag (Reg, Reg)

    slurpReloadCoalesce live
            = slurpCmm emptyBag live

     where
            slurpCmm :: Bag (Reg, Reg)
                     -> GenCmmDecl t t1 [SCC (LiveBasicBlock instr)]
                     -> Bag (Reg, Reg)
            slurpCmm cs CmmData{}   = cs
            slurpCmm cs (CmmProc _ _ _ sccs)
                    = slurpComp cs (flattenSCCs sccs)

            slurpComp :: Bag (Reg, Reg)
                         -> [LiveBasicBlock instr]
                         -> Bag (Reg, Reg)
            slurpComp  cs blocks
             = let  (moveBags, _)   = runState (slurpCompM blocks) emptyUFM
               in   unionManyBags (cs : moveBags)

            slurpCompM :: [LiveBasicBlock instr]
                       -> State (UniqFM BlockId [UniqFM Slot Reg]) [Bag (Reg, Reg)]
            slurpCompM blocks
             = do   -- run the analysis once to record the mapping across jumps.
                    mapM_   (slurpBlock False) blocks

                    -- run it a second time while using the information from the last pass.
                    --      We /could/ run this many more times to deal with graphical control
                    --      flow and propagating info across multiple jumps, but it's probably
                    --      not worth the trouble.
                    mapM    (slurpBlock True) blocks

            slurpBlock :: Bool -> LiveBasicBlock instr
                       -> State (UniqFM BlockId [UniqFM Slot Reg]) (Bag (Reg, Reg))
            slurpBlock propagate (BasicBlock blockId instrs)
             = do   -- grab the slot map for entry to this block
                    slotMap         <- if propagate
                                            then getSlotMap blockId
                                            else return emptyUFM

                    (_, mMoves)     <- mapAccumLM slurpLI slotMap instrs
                    return $ listToBag $ catMaybes mMoves

            slurpLI :: SlotMap Reg                           -- current slotMap
                    -> LiveInstr instr
                    -> State (UniqFM BlockId [SlotMap Reg])  -- blockId -> [slot -> reg]
                                                            --      for tracking slotMaps across jumps

                             ( SlotMap Reg           -- new slotMap
                             , Maybe (Reg, Reg))            -- maybe a new coalesce edge

            slurpLI slotMap li

                    -- remember what reg was stored into the slot
                    | LiveInstr (SPILL reg slot) _  <- li
                    , slotMap'                      <- addToUFM slotMap slot reg
                    = return (slotMap', Nothing)

                    -- add an edge between the this reg and the last one stored into the slot
                    | LiveInstr (RELOAD slot reg) _ <- li
                    = case lookupUFM slotMap slot of
                            Just reg2
                             | reg /= reg2  -> return (slotMap, Just (reg, reg2))
                             | otherwise    -> return (slotMap, Nothing)

                            Nothing         -> return (slotMap, Nothing)

                    -- if we hit a jump, remember the current slotMap
                    | LiveInstr (Instr instr) _     <- li
                    , targets                       <- jumpDestsOfInstr instr
                    , not $ null targets
                    = do    mapM_   (accSlotMap slotMap) targets
                            return  (slotMap, Nothing)

                    | otherwise
                    = return (slotMap, Nothing)

            -- record a slotmap for an in edge to this block
            accSlotMap slotMap blockId
                    = modify (\s -> addToUFM_C (++) s blockId [slotMap])

            -- work out the slot map on entry to this block
            --      if we have slot maps for multiple in-edges then we need to merge them.
            getSlotMap blockId
             = do   map             <- get
                    let slotMaps    = fromMaybe [] (lookupUFM map blockId)
                    return          $ foldr mergeSlotMaps emptyUFM slotMaps

            mergeSlotMaps :: SlotMap Reg -> SlotMap Reg -> SlotMap Reg
            mergeSlotMaps map1 map2
                    -- toList sadly means we have to use the _Directly style
                    -- functions.
                    -- TODO: We shouldn't need to go through a list here.
                    = listToUFM_Directly
                    $ [ (k, r1)
                      | (k, r1) <- nonDetUFMToList map1
                      -- This is non-deterministic but we do not
                      -- currently support deterministic code-generation.
                      -- See Note [Unique Determinism and code generation]
                      , case lookupUFM_Directly map2 k of
                              Nothing -> False
                              Just r2 -> r1 == r2 ]

    -- | Strip away liveness information, yielding NatCmmDecl
    stripLive
            :: (OutputableP Platform statics, Instruction instr)
            => NCGConfig
            -> LiveCmmDecl statics instr
            -> NatCmmDecl statics instr

    stripLive config live
            = stripCmm live

     where  stripCmm :: (OutputableP Platform statics, Instruction instr)
                     => LiveCmmDecl statics instr -> NatCmmDecl statics instr
            stripCmm (CmmData sec ds)       = CmmData sec ds
            stripCmm (CmmProc (LiveInfo info (first_id:_) _ _) label live sccs)
             = let  final_blocks    = flattenSCCs sccs

                    -- make sure the block that was first in the input list
                    --      stays at the front of the output. This is the entry point
                    --      of the proc, and it needs to come first.
                    ((first':_), rest')
                                    = partition ((== first_id) . blockId) final_blocks

               in   CmmProc info label live
                              (ListGraph $ map (stripLiveBlock config) $ first' : rest')

            -- If the proc has blocks but we don't know what the first one was, then we're dead.
            stripCmm proc
                     = pprPanic "RegAlloc.Liveness.stripLive: no first_id on proc" (pprLiveCmmDecl (ncgPlatform config) proc)

    -- | Pretty-print a `LiveCmmDecl`
    pprLiveCmmDecl :: (OutputableP Platform statics, Instruction instr) => Platform -> LiveCmmDecl statics instr -> SDoc
    pprLiveCmmDecl platform d = pdoc platform (mapLiveCmmDecl (pprInstr platform) d)

    -- | Map over instruction type in `LiveCmmDecl`
    mapLiveCmmDecl
       :: (instr -> b)
       -> LiveCmmDecl statics instr
       -> LiveCmmDecl statics b
    mapLiveCmmDecl f proc = fmap (fmap (fmap (fmap (fmap f)))) proc

    -- | Strip away liveness information from a basic block,
    --   and make real spill instructions out of SPILL, RELOAD pseudos along the way.

    stripLiveBlock
            :: Instruction instr
            => NCGConfig
            -> LiveBasicBlock instr
            -> NatBasicBlock instr

    stripLiveBlock config (BasicBlock i lis)
     =      BasicBlock i instrs'

     where  (instrs', _)
                    = runState (spillNat [] lis) 0

            spillNat acc []
             =      return (reverse acc)

            spillNat acc (LiveInstr (SPILL reg slot) _ : instrs)
             = do   delta   <- get
                    spillNat (mkSpillInstr config reg delta slot : acc) instrs

            spillNat acc (LiveInstr (RELOAD slot reg) _ : instrs)
             = do   delta   <- get
                    spillNat (mkLoadInstr config reg delta slot : acc) instrs

            spillNat acc (LiveInstr (Instr instr) _ : instrs)
             | Just i <- takeDeltaInstr instr
             = do   put i
                    spillNat acc instrs

            spillNat acc (LiveInstr (Instr instr) _ : instrs)
             =      spillNat (instr : acc) instrs

    -- | Erase Delta instructions.

    eraseDeltasLive
            :: Instruction instr
            => LiveCmmDecl statics instr
            -> LiveCmmDecl statics instr

    eraseDeltasLive cmm
            = mapBlockTop eraseBlock cmm
     where
            eraseBlock (BasicBlock id lis)
                    = BasicBlock id
                    $ filter (\(LiveInstr i _) -> not $ isJust $ takeDeltaInstr i)
                    $ lis

    -- | Patch the registers in this code according to this register mapping.
    --   also erase reg -> reg moves when the reg is the same.
    --   also erase reg -> reg moves when the destination dies in this instr.
    patchEraseLive
            :: Instruction instr
            => (Reg -> Reg)
            -> LiveCmmDecl statics instr -> LiveCmmDecl statics instr

    patchEraseLive patchF cmm
            = patchCmm cmm
     where
            patchCmm cmm@CmmData{}  = cmm

            patchCmm (CmmProc info label live sccs)
             | LiveInfo static id blockMap mLiveSlots <- info
             = let
                    patchRegSet set = mkUniqSet $ map patchF $ nonDetEltsUFM set
                      -- See Note [Unique Determinism and code generation]
                    blockMap'       = mapMap (patchRegSet . getUniqSet) blockMap

                    info'           = LiveInfo static id blockMap' mLiveSlots
               in   CmmProc info' label live $ map patchSCC sccs

            patchSCC (AcyclicSCC b)  = AcyclicSCC (patchBlock b)
            patchSCC (CyclicSCC  bs) = CyclicSCC  (map patchBlock bs)

            patchBlock (BasicBlock id lis)
                    = BasicBlock id $ patchInstrs lis

            patchInstrs []          = []
            patchInstrs (li : lis)

                    | LiveInstr i (Just live)       <- li'
                    , Just (r1, r2) <- takeRegRegMoveInstr i
                    , eatMe r1 r2 live
                    = patchInstrs lis

                    | otherwise
                    = li' : patchInstrs lis

                    where   li'     = patchRegsLiveInstr patchF li

            eatMe   r1 r2 live
                    -- source and destination regs are the same
                    | r1 == r2      = True

                    -- destination reg is never used
                    | elementOfUniqSet r2 (liveBorn live)
                    , elementOfUniqSet r2 (liveDieRead live) || elementOfUniqSet r2 (liveDieWrite live)
                    = True

                    | otherwise     = False

    -- | Patch registers in this LiveInstr, including the liveness information.
    --
    patchRegsLiveInstr
            :: Instruction instr
            => (Reg -> Reg)
            -> LiveInstr instr -> LiveInstr instr

    patchRegsLiveInstr patchF li
     = case li of
            LiveInstr instr Nothing
             -> LiveInstr (patchRegsOfInstr instr patchF) Nothing

            LiveInstr instr (Just live)
             -> LiveInstr
                    (patchRegsOfInstr instr patchF)
                    (Just live
                            { -- WARNING: have to go via lists here because patchF changes the uniq in the Reg
                              liveBorn      = mapUniqSet patchF $ liveBorn live
                            , liveDieRead   = mapUniqSet patchF $ liveDieRead live
                            , liveDieWrite  = mapUniqSet patchF $ liveDieWrite live })
                              -- See Note [Unique Determinism and code generation]

    --------------------------------------------------------------------------------
    -- | Convert a NatCmmDecl to a LiveCmmDecl, with liveness information

    cmmTopLiveness
            :: Instruction instr
            => Maybe CFG
            -> Platform
            -> NatCmmDecl statics instr
            -> UniqSM (LiveCmmDecl statics instr)
    cmmTopLiveness cfg platform cmm
            = regLiveness platform $ natCmmTopToLive cfg cmm

    natCmmTopToLive
            :: Instruction instr
            => Maybe CFG -> NatCmmDecl statics instr
            -> LiveCmmDecl statics instr

    natCmmTopToLive _ (CmmData i d)
            = CmmData i d

    natCmmTopToLive _ (CmmProc info lbl live (ListGraph []))
            = CmmProc (LiveInfo info [] mapEmpty mapEmpty) lbl live []

    natCmmTopToLive mCfg proc@(CmmProc info lbl live (ListGraph blocks@(first : _)))
            = CmmProc (LiveInfo info' (first_id : entry_ids) mapEmpty mapEmpty)
                    lbl live sccsLive
       where
            first_id        = blockId first
            all_entry_ids   = entryBlocks proc
            sccs            = sccBlocks blocks all_entry_ids mCfg
            sccsLive        = map (fmap (\(BasicBlock l instrs) ->
                                           BasicBlock l (map (\i -> LiveInstr (Instr i) Nothing) instrs)))
                            $ sccs

            entry_ids       = filter (reachable_node) .
                              filter (/= first_id) $ all_entry_ids
            info'           = mapFilterWithKey (\node _ -> reachable_node node) info
            reachable_node
              | Just cfg <- mCfg
              = hasNode cfg
              | otherwise
              = const True

    --
    -- Compute the liveness graph of the set of basic blocks.  Important:
    -- we also discard any unreachable code here, starting from the entry
    -- points (the first block in the list, and any blocks with info
    -- tables).  Unreachable code arises when code blocks are orphaned in
    -- earlier optimisation passes, and may confuse the register allocator
    -- by referring to registers that are not initialised.  It's easy to
    -- discard the unreachable code as part of the SCC pass, so that's
    -- exactly what we do. (#7574)
    --
    sccBlocks
            :: forall instr . Instruction instr
            => [NatBasicBlock instr]
            -> [BlockId]
            -> Maybe CFG
            -> [SCC (NatBasicBlock instr)]

    sccBlocks blocks entries mcfg = map (fmap node_payload) sccs
      where
            nodes :: [ Node BlockId (NatBasicBlock instr) ]
            nodes = [ DigraphNode block id (getOutEdges instrs)
                    | block@(BasicBlock id instrs) <- blocks ]

            g1 = graphFromEdgedVerticesUniq nodes

            reachable :: LabelSet
            reachable
                | Just cfg <- mcfg
                -- Our CFG only contains reachable nodes by construction at this point.
                = setFromList $ getCfgNodes cfg
                | otherwise
                = setFromList $ [ node_key node | node <- reachablesG g1 roots ]

            g2 = graphFromEdgedVerticesUniq [ node | node <- nodes
                                                   , node_key node
                                                      `setMember` reachable ]

            sccs = stronglyConnCompG g2

            getOutEdges :: Instruction instr => [instr] -> [BlockId]
            getOutEdges instrs = concatMap jumpDestsOfInstr instrs

            -- This is truly ugly, but I don't see a good alternative.
            -- Digraph just has the wrong API.  We want to identify nodes
            -- by their keys (BlockId), but Digraph requires the whole
            -- node: (NatBasicBlock, BlockId, [BlockId]).  This takes
            -- advantage of the fact that Digraph only looks at the key,
            -- even though it asks for the whole triple.
            roots = [DigraphNode (panic "sccBlocks") b (panic "sccBlocks")
                    | b <- entries ]

    --------------------------------------------------------------------------------
    -- Annotate code with register liveness information
    --

    regLiveness
            :: Instruction instr
            => Platform
            -> LiveCmmDecl statics instr
            -> UniqSM (LiveCmmDecl statics instr)

    regLiveness _ (CmmData i d)
            = return $ CmmData i d

    regLiveness _ (CmmProc info lbl live [])
            | LiveInfo static mFirst _ _    <- info
            = return $ CmmProc
                            (LiveInfo static mFirst mapEmpty mapEmpty)
                            lbl live []

    regLiveness platform (CmmProc info lbl live sccs)
            | LiveInfo static mFirst _ liveSlotsOnEntry     <- info
            = let   (ann_sccs, block_live)  = computeLiveness platform sccs

              in    return $ CmmProc (LiveInfo static mFirst block_live liveSlotsOnEntry)
                               lbl live ann_sccs

    -- -----------------------------------------------------------------------------
    -- | Check ordering of Blocks
    --   The computeLiveness function requires SCCs to be in reverse
    --   dependent order.  If they're not the liveness information will be
    --   wrong, and we'll get a bad allocation.  Better to check for this
    --   precondition explicitly or some other poor sucker will waste a
    --   day staring at bad assembly code..
    --
    checkIsReverseDependent
            :: Instruction instr
            => [SCC (LiveBasicBlock instr)]         -- ^ SCCs of blocks that we're about to run the liveness determinator on.
            -> Maybe BlockId                        -- ^ BlockIds that fail the test (if any)

    checkIsReverseDependent sccs'
     = go emptyUniqSet sccs'

     where  go _ []
             = Nothing

            go blocksSeen (AcyclicSCC block : sccs)
             = let  dests           = slurpJumpDestsOfBlock block
                    blocksSeen'     = unionUniqSets blocksSeen $ mkUniqSet [blockId block]
                    badDests        = dests `minusUniqSet` blocksSeen'
               in   case nonDetEltsUniqSet badDests of
                     -- See Note [Unique Determinism and code generation]
                     []             -> go blocksSeen' sccs
                     bad : _        -> Just bad

            go blocksSeen (CyclicSCC blocks : sccs)
             = let  dests           = unionManyUniqSets $ map slurpJumpDestsOfBlock blocks
                    blocksSeen'     = unionUniqSets blocksSeen $ mkUniqSet $ map blockId blocks
                    badDests        = dests `minusUniqSet` blocksSeen'
               in   case nonDetEltsUniqSet badDests of
                     -- See Note [Unique Determinism and code generation]
                     []             -> go blocksSeen' sccs
                     bad : _        -> Just bad

            slurpJumpDestsOfBlock (BasicBlock _ instrs)
                    = unionManyUniqSets
                    $ map (mkUniqSet . jumpDestsOfInstr)
                            [ i | LiveInstr i _ <- instrs]

    -- | If we've compute liveness info for this code already we have to reverse
    --   the SCCs in each top to get them back to the right order so we can do it again.
    reverseBlocksInTops :: LiveCmmDecl statics instr -> LiveCmmDecl statics instr
    reverseBlocksInTops top
     = case top of
            CmmData{}                       -> top
            CmmProc info lbl live sccs      -> CmmProc info lbl live (reverse sccs)

    -- | Computing liveness
    --
    --  On entry, the SCCs must be in "reverse" order: later blocks may transfer
    --  control to earlier ones only, else `panic`.
    --
    --  The SCCs returned are in the *opposite* order, which is exactly what we
    --  want for the next pass.
    --
    computeLiveness
            :: Instruction instr
            => Platform
            -> [SCC (LiveBasicBlock instr)]
            -> ([SCC (LiveBasicBlock instr)],       -- instructions annotated with list of registers
                                                    -- which are "dead after this instruction".
                   BlockMap RegSet)                 -- blocks annotated with set of live registers
                                                    -- on entry to the block.

    computeLiveness platform sccs
     = case checkIsReverseDependent sccs of
            Nothing         -> livenessSCCs platform mapEmpty [] sccs
            Just bad        -> let sccs' = fmap (fmap (fmap (fmap (pprInstr platform)))) sccs
                               in pprPanic "RegAlloc.Liveness.computeLiveness"
                                    (vcat   [ text "SCCs aren't in reverse dependent order"
                                            , text "bad blockId" <+> ppr bad
                                            , ppr sccs'])

    livenessSCCs
           :: Instruction instr
           => Platform
           -> BlockMap RegSet
           -> [SCC (LiveBasicBlock instr)]          -- accum
           -> [SCC (LiveBasicBlock instr)]
           -> ( [SCC (LiveBasicBlock instr)]
              , BlockMap RegSet)

    livenessSCCs _ blockmap done []
            = (done, blockmap)

    livenessSCCs platform blockmap done (AcyclicSCC block : sccs)
     = let  (blockmap', block')     = livenessBlock platform blockmap block
       in   livenessSCCs platform blockmap' (AcyclicSCC block' : done) sccs

    livenessSCCs platform blockmap done
            (CyclicSCC blocks : sccs) =
            livenessSCCs platform blockmap' (CyclicSCC blocks':done) sccs
     where      (blockmap', blocks')
                    = iterateUntilUnchanged linearLiveness equalBlockMaps
                                          blockmap blocks

                iterateUntilUnchanged
                    :: (a -> b -> (a,c)) -> (a -> a -> Bool)
                    -> a -> b
                    -> (a,c)

                iterateUntilUnchanged f eq a b
                    = head $
                      concatMap tail $
                      groupBy (\(a1, _) (a2, _) -> eq a1 a2) $
                      iterate (\(a, _) -> f a b) $
                      (a, panic "RegLiveness.livenessSCCs")

                linearLiveness
                    :: Instruction instr
                    => BlockMap RegSet -> [LiveBasicBlock instr]
                    -> (BlockMap RegSet, [LiveBasicBlock instr])

                linearLiveness = mapAccumL (livenessBlock platform)

                    -- probably the least efficient way to compare two
                    -- BlockMaps for equality.
                equalBlockMaps a b
                    = a' == b'
                  where a' = map f $ mapToList a
                        b' = map f $ mapToList b
                        f (key,elt) = (key, nonDetEltsUniqSet elt)
                        -- See Note [Unique Determinism and code generation]

    -- | Annotate a basic block with register liveness information.
    --
    livenessBlock
            :: Instruction instr
            => Platform
            -> BlockMap RegSet
            -> LiveBasicBlock instr
            -> (BlockMap RegSet, LiveBasicBlock instr)

    livenessBlock platform blockmap (BasicBlock block_id instrs)
     = let
            (regsLiveOnEntry, instrs1)
                = livenessBack platform emptyUniqSet blockmap [] (reverse instrs)
            blockmap'       = mapInsert block_id regsLiveOnEntry blockmap

            instrs2         = livenessForward platform regsLiveOnEntry instrs1

            output          = BasicBlock block_id instrs2

       in   ( blockmap', output)

    -- | Calculate liveness going forwards,
    --   filling in when regs are born

    livenessForward
            :: Instruction instr
            => Platform
            -> RegSet                       -- regs live on this instr
            -> [LiveInstr instr] -> [LiveInstr instr]

    livenessForward _        _           []  = []
    livenessForward platform rsLiveEntry (li@(LiveInstr instr mLive) : lis)
            | Just live <- mLive
            = let
                    RU _ written  = regUsageOfInstr platform instr
                    -- Regs that are written to but weren't live on entry to this instruction
                    --      are recorded as being born here.
                    rsBorn          = mkUniqSet
                                    $ filter (\r -> not $ elementOfUniqSet r rsLiveEntry) written

                    rsLiveNext      = (rsLiveEntry `unionUniqSets` rsBorn)
                                            `minusUniqSet` (liveDieRead live)
                                            `minusUniqSet` (liveDieWrite live)

            in LiveInstr instr (Just live { liveBorn = rsBorn })
                    : livenessForward platform rsLiveNext lis

            | otherwise
            = li : livenessForward platform rsLiveEntry lis

    -- | Calculate liveness going backwards,
    --   filling in when regs die, and what regs are live across each instruction

    livenessBack
            :: Instruction instr
            => Platform
            -> RegSet                       -- regs live on this instr
            -> BlockMap RegSet              -- regs live on entry to other BBs
            -> [LiveInstr instr]            -- instructions (accum)
            -> [LiveInstr instr]            -- instructions
            -> (RegSet, [LiveInstr instr])

    livenessBack _        liveregs _        done []  = (liveregs, done)

    livenessBack platform liveregs blockmap acc (instr : instrs)
     = let  (liveregs', instr')     = liveness1 platform liveregs blockmap instr
       in   livenessBack platform liveregs' blockmap (instr' : acc) instrs

    -- don't bother tagging comments or deltas with liveness
    liveness1
            :: Instruction instr
            => Platform
            -> RegSet
            -> BlockMap RegSet
            -> LiveInstr instr
            -> (RegSet, LiveInstr instr)

    liveness1 _ liveregs _ (LiveInstr instr _)
            | isMetaInstr instr
            = (liveregs, LiveInstr instr Nothing)

    liveness1 platform liveregs blockmap (LiveInstr instr _)

            | not_a_branch
            = (liveregs1, LiveInstr instr
                            (Just $ Liveness
                            { liveBorn      = emptyUniqSet
                            , liveDieRead   = mkUniqSet r_dying
                            , liveDieWrite  = mkUniqSet w_dying }))

            | otherwise
            = (liveregs_br, LiveInstr instr
                            (Just $ Liveness
                            { liveBorn      = emptyUniqSet
                            , liveDieRead   = mkUniqSet r_dying_br
                            , liveDieWrite  = mkUniqSet w_dying }))

            where
                !(RU read written) = regUsageOfInstr platform instr

                -- registers that were written here are dead going backwards.
                -- registers that were read here are live going backwards.
                liveregs1   = (liveregs `delListFromUniqSet` written)
                                        `addListToUniqSet` read

                -- registers that are not live beyond this point, are recorded
                --  as dying here.
                r_dying     = [ reg | reg <- read, reg `notElem` written,
                                  not (elementOfUniqSet reg liveregs) ]

                w_dying     = [ reg | reg <- written,
                                 not (elementOfUniqSet reg liveregs) ]

                -- union in the live regs from all the jump destinations of this
                -- instruction.
                targets      = jumpDestsOfInstr instr -- where we go from here
                not_a_branch = null targets

                targetLiveRegs target
                      = case mapLookup target blockmap of
                                    Just ra -> ra
                                    Nothing -> emptyRegSet

                live_from_branch = unionManyUniqSets (map targetLiveRegs targets)

                liveregs_br = liveregs1 `unionUniqSets` live_from_branch

                -- registers that are live only in the branch targets should
                -- be listed as dying here.
                live_branch_only = live_from_branch `minusUniqSet` liveregs
                r_dying_br  = nonDetEltsUniqSet (mkUniqSet r_dying `unionUniqSets`
                                                 live_branch_only)
                              -- See Note [Unique Determinism and code generation]