-- | Clean out unneeded spill\/reload instructions. -- -- Handling of join points -- ~~~~~~~~~~~~~~~~~~~~~~~ -- -- B1: B2: -- ... ... -- RELOAD SLOT(0), %r1 RELOAD SLOT(0), %r1 -- ... A ... ... B ... -- jump B3 jump B3 -- -- B3: ... C ... -- RELOAD SLOT(0), %r1 -- ... -- -- The Plan -- ~~~~~~~~ -- As long as %r1 hasn't been written to in A, B or C then we don't need -- the reload in B3. -- -- What we really care about here is that on the entry to B3, %r1 will -- always have the same value that is in SLOT(0) (ie, %r1 is _valid_) -- -- This also works if the reloads in B1\/B2 were spills instead, because -- spilling %r1 to a slot makes that slot have the same value as %r1. -- module RegAlloc.Graph.SpillClean ( cleanSpills ) where import GhcPrelude import RegAlloc.Liveness import Instruction import Reg import BlockId import Cmm import UniqSet import UniqFM import Unique import State import Outputable import Platform import Hoopl.Collections import Data.List import Data.Maybe import Data.IntSet (IntSet) import qualified Data.IntSet as IntSet -- | The identification number of a spill slot. -- A value is stored in a spill slot when we don't have a free -- register to hold it. type Slot = Int -- | Clean out unneeded spill\/reloads from this top level thing. cleanSpills :: Instruction instr => Platform -> LiveCmmDecl statics instr -> LiveCmmDecl statics instr cleanSpills platform cmm = evalState (cleanSpin platform 0 cmm) initCleanS -- | Do one pass of cleaning. cleanSpin :: Instruction instr => Platform -> Int -- ^ Iteration number for the cleaner. -> LiveCmmDecl statics instr -- ^ Liveness annotated code to clean. -> CleanM (LiveCmmDecl statics instr) cleanSpin platform spinCount code = do -- Initialise count of cleaned spill and reload instructions. modify $ \s -> s { sCleanedSpillsAcc = 0 , sCleanedReloadsAcc = 0 , sReloadedBy = emptyUFM } code_forward <- mapBlockTopM (cleanBlockForward platform) code code_backward <- cleanTopBackward code_forward -- During the cleaning of each block we collected information about -- what regs were valid across each jump. Based on this, work out -- whether it will be safe to erase reloads after join points for -- the next pass. collateJoinPoints -- Remember how many spill and reload instructions we cleaned in this pass. spills <- gets sCleanedSpillsAcc reloads <- gets sCleanedReloadsAcc modify $ \s -> s { sCleanedCount = (spills, reloads) : sCleanedCount s } -- If nothing was cleaned in this pass or the last one -- then we're done and it's time to bail out. cleanedCount <- gets sCleanedCount if take 2 cleanedCount == [(0, 0), (0, 0)] then return code -- otherwise go around again else cleanSpin platform (spinCount + 1) code_backward ------------------------------------------------------------------------------- -- | Clean out unneeded reload instructions, -- while walking forward over the code. cleanBlockForward :: Instruction instr => Platform -> LiveBasicBlock instr -> CleanM (LiveBasicBlock instr) cleanBlockForward platform (BasicBlock blockId instrs) = do -- See if we have a valid association for the entry to this block. jumpValid <- gets sJumpValid let assoc = case lookupUFM jumpValid blockId of Just assoc -> assoc Nothing -> emptyAssoc instrs_reload <- cleanForward platform blockId assoc [] instrs return $ BasicBlock blockId instrs_reload -- | Clean out unneeded reload instructions. -- -- Walking forwards across the code -- On a reload, if we know a reg already has the same value as a slot -- then we don't need to do the reload. -- cleanForward :: Instruction instr => Platform -> BlockId -- ^ the block that we're currently in -> Assoc Store -- ^ two store locations are associated if -- they have the same value -> [LiveInstr instr] -- ^ acc -> [LiveInstr instr] -- ^ instrs to clean (in backwards order) -> CleanM [LiveInstr instr] -- ^ cleaned instrs (in forward order) cleanForward _ _ _ acc [] = return acc -- Rewrite live range joins via spill slots to just a spill and a reg-reg move -- hopefully the spill will be also be cleaned in the next pass cleanForward platform blockId assoc acc (li1 : li2 : instrs) | LiveInstr (SPILL reg1 slot1) _ <- li1 , LiveInstr (RELOAD slot2 reg2) _ <- li2 , slot1 == slot2 = do modify $ \s -> s { sCleanedReloadsAcc = sCleanedReloadsAcc s + 1 } cleanForward platform blockId assoc acc $ li1 : LiveInstr (mkRegRegMoveInstr platform reg1 reg2) Nothing : instrs cleanForward platform blockId assoc acc (li@(LiveInstr i1 _) : instrs) | Just (r1, r2) <- takeRegRegMoveInstr i1 = if r1 == r2 -- Erase any left over nop reg reg moves while we're here -- this will also catch any nop moves that the previous case -- happens to add. then cleanForward platform blockId assoc acc instrs -- If r1 has the same value as some slots and we copy r1 to r2, -- then r2 is now associated with those slots instead else do let assoc' = addAssoc (SReg r1) (SReg r2) $ delAssoc (SReg r2) $ assoc cleanForward platform blockId assoc' (li : acc) instrs cleanForward platform blockId assoc acc (li : instrs) -- Update association due to the spill. | LiveInstr (SPILL reg slot) _ <- li = let assoc' = addAssoc (SReg reg) (SSlot slot) $ delAssoc (SSlot slot) $ assoc in cleanForward platform blockId assoc' (li : acc) instrs -- Clean a reload instr. | LiveInstr (RELOAD{}) _ <- li = do (assoc', mli) <- cleanReload platform blockId assoc li case mli of Nothing -> cleanForward platform blockId assoc' acc instrs Just li' -> cleanForward platform blockId assoc' (li' : acc) instrs -- Remember the association over a jump. | LiveInstr instr _ <- li , targets <- jumpDestsOfInstr instr , not $ null targets = do mapM_ (accJumpValid assoc) targets cleanForward platform blockId assoc (li : acc) instrs -- Writing to a reg changes its value. | LiveInstr instr _ <- li , RU _ written <- regUsageOfInstr platform instr = let assoc' = foldr delAssoc assoc (map SReg $ nub written) in cleanForward platform blockId assoc' (li : acc) instrs -- | Try and rewrite a reload instruction to something more pleasing cleanReload :: Instruction instr => Platform -> BlockId -> Assoc Store -> LiveInstr instr -> CleanM (Assoc Store, Maybe (LiveInstr instr)) cleanReload platform blockId assoc li@(LiveInstr (RELOAD slot reg) _) -- If the reg we're reloading already has the same value as the slot -- then we can erase the instruction outright. | elemAssoc (SSlot slot) (SReg reg) assoc = do modify $ \s -> s { sCleanedReloadsAcc = sCleanedReloadsAcc s + 1 } return (assoc, Nothing) -- If we can find another reg with the same value as this slot then -- do a move instead of a reload. | Just reg2 <- findRegOfSlot assoc slot = do modify $ \s -> s { sCleanedReloadsAcc = sCleanedReloadsAcc s + 1 } let assoc' = addAssoc (SReg reg) (SReg reg2) $ delAssoc (SReg reg) $ assoc return ( assoc' , Just $ LiveInstr (mkRegRegMoveInstr platform reg2 reg) Nothing) -- Gotta keep this instr. | otherwise = do -- Update the association. let assoc' = addAssoc (SReg reg) (SSlot slot) -- doing the reload makes reg and slot the same value $ delAssoc (SReg reg) -- reg value changes on reload $ assoc -- Remember that this block reloads from this slot. accBlockReloadsSlot blockId slot return (assoc', Just li) cleanReload _ _ _ _ = panic "RegSpillClean.cleanReload: unhandled instr" ------------------------------------------------------------------------------- -- | Clean out unneeded spill instructions, -- while walking backwards over the code. -- -- If there were no reloads from a slot between a spill and the last one -- then the slot was never read and we don't need the spill. -- -- SPILL r0 -> s1 -- RELOAD s1 -> r2 -- SPILL r3 -> s1 <--- don't need this spill -- SPILL r4 -> s1 -- RELOAD s1 -> r5 -- -- Maintain a set of -- "slots which were spilled to but not reloaded from yet" -- -- Walking backwards across the code: -- a) On a reload from a slot, remove it from the set. -- -- a) On a spill from a slot -- If the slot is in set then we can erase the spill, -- because it won't be reloaded from until after the next spill. -- -- otherwise -- keep the spill and add the slot to the set -- -- TODO: This is mostly inter-block -- we should really be updating the noReloads set as we cross jumps also. -- -- TODO: generate noReloads from liveSlotsOnEntry -- cleanTopBackward :: Instruction instr => LiveCmmDecl statics instr -> CleanM (LiveCmmDecl statics instr) cleanTopBackward cmm = case cmm of CmmData{} -> return cmm CmmProc info label live sccs | LiveInfo _ _ _ liveSlotsOnEntry <- info -> do sccs' <- mapM (mapSCCM (cleanBlockBackward liveSlotsOnEntry)) sccs return $ CmmProc info label live sccs' cleanBlockBackward :: Instruction instr => BlockMap IntSet -> LiveBasicBlock instr -> CleanM (LiveBasicBlock instr) cleanBlockBackward liveSlotsOnEntry (BasicBlock blockId instrs) = do instrs_spill <- cleanBackward liveSlotsOnEntry emptyUniqSet [] instrs return $ BasicBlock blockId instrs_spill cleanBackward :: Instruction instr => BlockMap IntSet -- ^ Slots live on entry to each block -> UniqSet Int -- ^ Slots that have been spilled, but not reloaded from -> [LiveInstr instr] -- ^ acc -> [LiveInstr instr] -- ^ Instrs to clean (in forwards order) -> CleanM [LiveInstr instr] -- ^ Cleaned instrs (in backwards order) cleanBackward liveSlotsOnEntry noReloads acc lis = do reloadedBy <- gets sReloadedBy cleanBackward' liveSlotsOnEntry reloadedBy noReloads acc lis cleanBackward' :: Instruction instr => BlockMap IntSet -> UniqFM [BlockId] -> UniqSet Int -> [LiveInstr instr] -> [LiveInstr instr] -> State CleanS [LiveInstr instr] cleanBackward' _ _ _ acc [] = return acc cleanBackward' liveSlotsOnEntry reloadedBy noReloads acc (li : instrs) -- If nothing ever reloads from this slot then we don't need the spill. | LiveInstr (SPILL _ slot) _ <- li , Nothing <- lookupUFM reloadedBy (SSlot slot) = do modify $ \s -> s { sCleanedSpillsAcc = sCleanedSpillsAcc s + 1 } cleanBackward liveSlotsOnEntry noReloads acc instrs | LiveInstr (SPILL _ slot) _ <- li = if elementOfUniqSet slot noReloads -- We can erase this spill because the slot won't be read until -- after the next one then do modify $ \s -> s { sCleanedSpillsAcc = sCleanedSpillsAcc s + 1 } cleanBackward liveSlotsOnEntry noReloads acc instrs else do -- This slot is being spilled to, but we haven't seen any reloads yet. let noReloads' = addOneToUniqSet noReloads slot cleanBackward liveSlotsOnEntry noReloads' (li : acc) instrs -- if we reload from a slot then it's no longer unused | LiveInstr (RELOAD slot _) _ <- li , noReloads' <- delOneFromUniqSet noReloads slot = cleanBackward liveSlotsOnEntry noReloads' (li : acc) instrs -- If a slot is live in a jump target then assume it's reloaded there. -- -- TODO: A real dataflow analysis would do a better job here. -- If the target block _ever_ used the slot then we assume -- it always does, but if those reloads are cleaned the slot -- liveness map doesn't get updated. | LiveInstr instr _ <- li , targets <- jumpDestsOfInstr instr = do let slotsReloadedByTargets = IntSet.unions $ catMaybes $ map (flip mapLookup liveSlotsOnEntry) $ targets let noReloads' = foldl' delOneFromUniqSet noReloads $ IntSet.toList slotsReloadedByTargets cleanBackward liveSlotsOnEntry noReloads' (li : acc) instrs -- some other instruction | otherwise = cleanBackward liveSlotsOnEntry noReloads (li : acc) instrs -- | Combine the associations from all the inward control flow edges. -- collateJoinPoints :: CleanM () collateJoinPoints = modify $ \s -> s { sJumpValid = mapUFM intersects (sJumpValidAcc s) , sJumpValidAcc = emptyUFM } intersects :: [Assoc Store] -> Assoc Store intersects [] = emptyAssoc intersects assocs = foldl1' intersectAssoc assocs -- | See if we have a reg with the same value as this slot in the association table. findRegOfSlot :: Assoc Store -> Int -> Maybe Reg findRegOfSlot assoc slot | close <- closeAssoc (SSlot slot) assoc , Just (SReg reg) <- find isStoreReg $ nonDetEltsUniqSet close -- See Note [Unique Determinism and code generation] = Just reg | otherwise = Nothing ------------------------------------------------------------------------------- -- | Cleaner monad. type CleanM = State CleanS -- | Cleaner state. data CleanS = CleanS { -- | Regs which are valid at the start of each block. sJumpValid :: UniqFM (Assoc Store) -- | Collecting up what regs were valid across each jump. -- in the next pass we can collate these and write the results -- to sJumpValid. , sJumpValidAcc :: UniqFM [Assoc Store] -- | Map of (slot -> blocks which reload from this slot) -- used to decide if whether slot spilled to will ever be -- reloaded from on this path. , sReloadedBy :: UniqFM [BlockId] -- | Spills and reloads cleaned each pass (latest at front) , sCleanedCount :: [(Int, Int)] -- | Spills and reloads that have been cleaned in this pass so far. , sCleanedSpillsAcc :: Int , sCleanedReloadsAcc :: Int } -- | Construct the initial cleaner state. initCleanS :: CleanS initCleanS = CleanS { sJumpValid = emptyUFM , sJumpValidAcc = emptyUFM , sReloadedBy = emptyUFM , sCleanedCount = [] , sCleanedSpillsAcc = 0 , sCleanedReloadsAcc = 0 } -- | Remember the associations before a jump. accJumpValid :: Assoc Store -> BlockId -> CleanM () accJumpValid assocs target = modify $ \s -> s { sJumpValidAcc = addToUFM_C (++) (sJumpValidAcc s) target [assocs] } accBlockReloadsSlot :: BlockId -> Slot -> CleanM () accBlockReloadsSlot blockId slot = modify $ \s -> s { sReloadedBy = addToUFM_C (++) (sReloadedBy s) (SSlot slot) [blockId] } ------------------------------------------------------------------------------- -- A store location can be a stack slot or a register data Store = SSlot Int | SReg Reg -- | Check if this is a reg store. isStoreReg :: Store -> Bool isStoreReg ss = case ss of SSlot _ -> False SReg _ -> True -- Spill cleaning is only done once all virtuals have been allocated to realRegs instance Uniquable Store where getUnique (SReg r) | RegReal (RealRegSingle i) <- r = mkRegSingleUnique i | RegReal (RealRegPair r1 r2) <- r = mkRegPairUnique (r1 * 65535 + r2) | otherwise = error $ "RegSpillClean.getUnique: found virtual reg during spill clean," ++ "only real regs expected." getUnique (SSlot i) = mkRegSubUnique i -- [SLPJ] I hope "SubUnique" is ok instance Outputable Store where ppr (SSlot i) = text "slot" <> int i ppr (SReg r) = ppr r ------------------------------------------------------------------------------- -- Association graphs. -- In the spill cleaner, two store locations are associated if they are known -- to hold the same value. -- type Assoc a = UniqFM (UniqSet a) -- | An empty association emptyAssoc :: Assoc a emptyAssoc = emptyUFM -- | Add an association between these two things. addAssoc :: Uniquable a => a -> a -> Assoc a -> Assoc a addAssoc a b m = let m1 = addToUFM_C unionUniqSets m a (unitUniqSet b) m2 = addToUFM_C unionUniqSets m1 b (unitUniqSet a) in m2 -- | Delete all associations to a node. delAssoc :: (Uniquable a) => a -> Assoc a -> Assoc a delAssoc a m | Just aSet <- lookupUFM m a , m1 <- delFromUFM m a = nonDetFoldUniqSet (\x m -> delAssoc1 x a m) m1 aSet -- It's OK to use nonDetFoldUFM here because deletion is commutative | otherwise = m -- | Delete a single association edge (a -> b). delAssoc1 :: Uniquable a => a -> a -> Assoc a -> Assoc a delAssoc1 a b m | Just aSet <- lookupUFM m a = addToUFM m a (delOneFromUniqSet aSet b) | otherwise = m -- | Check if these two things are associated. elemAssoc :: (Uniquable a) => a -> a -> Assoc a -> Bool elemAssoc a b m = elementOfUniqSet b (closeAssoc a m) -- | Find the refl. trans. closure of the association from this point. closeAssoc :: (Uniquable a) => a -> Assoc a -> UniqSet a closeAssoc a assoc = closeAssoc' assoc emptyUniqSet (unitUniqSet a) where closeAssoc' assoc visited toVisit = case nonDetEltsUniqSet toVisit of -- See Note [Unique Determinism and code generation] -- nothing else to visit, we're done [] -> visited (x:_) -- we've already seen this node | elementOfUniqSet x visited -> closeAssoc' assoc visited (delOneFromUniqSet toVisit x) -- haven't seen this node before, -- remember to visit all its neighbors | otherwise -> let neighbors = case lookupUFM assoc x of Nothing -> emptyUniqSet Just set -> set in closeAssoc' assoc (addOneToUniqSet visited x) (unionUniqSets toVisit neighbors) -- | Intersect two associations. intersectAssoc :: Assoc a -> Assoc a -> Assoc a intersectAssoc a b = intersectUFM_C (intersectUniqSets) a b