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Better handling of join points in spill cleaner

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1 parent cdddb06 commit f3ebc8951ad495a5a027f1f482b45648dfe11c58 Ben.Lippmeier@anu.edu.au committed Aug 28, 2007
Showing with 256 additions and 24 deletions.
  1. +256 −24 compiler/nativeGen/RegSpillClean.hs
@@ -1,5 +1,28 @@
-- | Clean out unneeded spill/reload instrs
--
+-- * 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:
+-- So 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 RegSpillClean (
cleanSpills
)
@@ -12,45 +35,126 @@ import MachInstrs
import Cmm
import UniqSet
+import UniqFM
+import State
+import Outputable
+
+import Data.Maybe
+import Data.List
+type Slot = Int
-- | Clean out unneeded spill/reloads from this top level thing.
cleanSpills :: LiveCmmTop -> LiveCmmTop
cleanSpills cmm
- = mapBlockTop cleanBlock cmm
- where
- cleanBlock (BasicBlock id instrs)
- = BasicBlock id
- $ cleanSpill emptyUniqSet []
- $ cleanReload emptyUniqSet []
- $ instrs
+ = evalState (cleanSpin 0 cmm) initCleanS
+
+-- | do one pass of cleaning
+cleanSpin :: Int -> LiveCmmTop -> CleanM LiveCmmTop
+
+{-
+cleanSpin spinCount code
+ = do jumpValid <- gets sJumpValid
+ pprTrace "cleanSpin"
+ ( int spinCount
+ $$ text "--- code"
+ $$ ppr code
+ $$ text "--- joins"
+ $$ ppr jumpValid)
+ $ cleanSpin' spinCount code
+-}
+
+cleanSpin spinCount code
+ = do
+ -- init count of cleaned spills/reloads
+ modify $ \s -> s
+ { sCleanedSpillsAcc = 0
+ , sCleanedReloadsAcc = 0 }
+
+ code' <- mapBlockTopM cleanBlock code
+
+ -- 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 spills/reloads 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 (spinCount + 1) code'
+
+
+-- | Clean one basic block
+cleanBlock :: LiveBasicBlock -> CleanM LiveBasicBlock
+cleanBlock (BasicBlock id instrs)
+ = do jumpValid <- gets sJumpValid
+ let assoc = case lookupUFM jumpValid id of
+ Just assoc -> assoc
+ Nothing -> emptyAssoc
+
+ instrs_reload <- cleanReload assoc [] instrs
+ instrs_spill <- cleanSpill emptyUniqSet [] instrs_reload
+ return $ BasicBlock id instrs_spill
-- | Clean out unneeded reload instructions.
-- Walking forwards across the code
--- If there are no writes to a reg between a reload and the
--- last spill or reload then we don't need the reload.
+-- 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.
--
cleanReload
- :: UniqSet Reg -- ^ hregs that were reloaded but not written to yet
+ :: Assoc Reg Slot -- ^ a reg and slot are associated when they have the same value.
-> [LiveInstr] -- ^ acc
-> [LiveInstr] -- ^ instrs to clean (in backwards order)
- -> [LiveInstr] -- ^ cleaned instrs (in forward order)
+ -> CleanM [LiveInstr] -- ^ cleaned instrs (in forward order)
+
+cleanReload assoc acc []
+ = return acc
+
+cleanReload assoc acc (li@(Instr instr live) : instrs)
-cleanReload valid acc [] = acc
-cleanReload valid acc (li@(Instr instr live) : instrs)
| SPILL reg slot <- instr
- , valid' <- addOneToUniqSet valid reg
- = cleanReload valid' (li : acc) instrs
+ = let assoc' = addAssoc reg slot -- doing the spill makes reg and slot the same value
+ $ deleteBAssoc slot -- slot value changes on spill
+ $ assoc
+ in cleanReload assoc' (li : acc) instrs
| RELOAD slot reg <- instr
- = if elementOfUniqSet reg valid
- then cleanReload valid acc instrs
- else cleanReload (addOneToUniqSet valid reg) (li : acc) instrs
+ = if elemAssoc reg slot assoc
+
+ -- reg and slot had the same value before reload
+ -- we don't need the reload.
+ then do
+ modify $ \s -> s { sCleanedReloadsAcc = sCleanedReloadsAcc s + 1 }
+ cleanReload assoc acc instrs
+
+ -- reg and slot had different values before reload
+ else
+ let assoc' = addAssoc reg slot -- doing the reload makes reg and slot the same value
+ $ deleteAAssoc reg -- reg value changes on reload
+ $ assoc
+ in cleanReload assoc' (li : acc) instrs
+
+ -- on a jump, remember the reg/slot association.
+ | targets <- jumpDests instr []
+ , not $ null targets
+ = do mapM_ (accJumpValid assoc) targets
+ cleanReload assoc (li : acc) instrs
+ -- writing to a reg changes its value.
| RU read written <- regUsage instr
- , valid' <- minusUniqSet valid (mkUniqSet written)
- = cleanReload valid' (li : acc) instrs
+ = let assoc' = foldr deleteAAssoc assoc written
+ in cleanReload assoc' (li : acc) instrs
-- | Clean out unneeded spill instructions.
@@ -62,19 +166,147 @@ cleanSpill
:: UniqSet Int -- ^ slots that have been spilled, but not reload from
-> [LiveInstr] -- ^ acc
-> [LiveInstr] -- ^ instrs to clean (in forwards order)
- -> [LiveInstr] -- ^ cleaned instrs (in backwards order)
+ -> CleanM [LiveInstr] -- ^ cleaned instrs (in backwards order)
+
+cleanSpill unused acc []
+ = return acc
-cleanSpill unused acc [] = acc
cleanSpill unused acc (li@(Instr instr live) : instrs)
| SPILL reg slot <- instr
= if elementOfUniqSet slot unused
- then cleanSpill unused acc instrs
- else cleanSpill (addOneToUniqSet unused slot) (li : acc) instrs
+ -- 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 }
+ cleanSpill unused acc instrs
+
+ else do
+ -- slots start off unused
+ let unused' = addOneToUniqSet unused slot
+ cleanSpill unused' (li : acc) instrs
+
+ -- if we reload from a slot then it's no longer unused
| RELOAD slot reg <- instr
, unused' <- delOneFromUniqSet unused slot
= cleanSpill unused' (li : acc) instrs
+ -- some other instruction
| otherwise
= cleanSpill unused (li : acc) instrs
+
+-- collateJoinPoints:
+--
+-- | Look at information about what regs were valid across jumps and work out
+-- whether it's safe to avoid reloads after join points.
+--
+collateJoinPoints :: CleanM ()
+collateJoinPoints
+ = modify $ \s -> s
+ { sJumpValid = mapUFM intersects (sJumpValidAcc s)
+ , sJumpValidAcc = emptyUFM }
+
+intersects :: [Assoc Reg Slot] -> Assoc Reg Slot
+intersects [] = emptyAssoc
+intersects assocs = foldl1' intersectAssoc assocs
+
+
+
+---------------
+type CleanM = State CleanS
+data CleanS
+ = CleanS
+ { -- regs which are valid at the start of each block.
+ sJumpValid :: UniqFM (Assoc Reg Slot)
+
+ -- 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 Reg Slot]
+
+ -- spills/reloads cleaned each pass (latest at front)
+ , sCleanedCount :: [(Int, Int)]
+
+ -- spills/reloads that have been cleaned in this pass so far.
+ , sCleanedSpillsAcc :: Int
+ , sCleanedReloadsAcc :: Int }
+
+initCleanS
+ = CleanS
+ { sJumpValid = emptyUFM
+ , sJumpValidAcc = emptyUFM
+
+ , sCleanedCount = []
+
+ , sCleanedSpillsAcc = 0
+ , sCleanedReloadsAcc = 0 }
+
+
+-- | Remember that these regs were valid before a jump to this block
+accJumpValid :: Assoc Reg Slot -> BlockId -> CleanM ()
+accJumpValid regs target
+ = modify $ \s -> s {
+ sJumpValidAcc = addToUFM_C (++)
+ (sJumpValidAcc s)
+ target
+ [regs] }
+
+
+--------------
+-- An association table / many to many mapping.
+-- TODO: implement this better than a simple association list.
+-- two maps of sets, one for each direction would be better
+--
+data Assoc a b
+ = Assoc
+ { aList :: [(a, b)] }
+
+-- | an empty association
+emptyAssoc :: Assoc a b
+emptyAssoc = Assoc { aList = [] }
+
+
+-- | add an association to the table.
+addAssoc
+ :: (Eq a, Eq b)
+ => a -> b -> Assoc a b -> Assoc a b
+
+addAssoc a b m = m { aList = (a, b) : aList m }
+
+
+-- | check if these two things are associated
+elemAssoc
+ :: (Eq a, Eq b)
+ => a -> b -> Assoc a b -> Bool
+elemAssoc a b m = elem (a, b) $ aList m
+
+
+-- | delete all associations with this A element
+deleteAAssoc
+ :: Eq a
+ => a -> Assoc a b -> Assoc a b
+
+deleteAAssoc x m
+ = m { aList = [ (a, b) | (a, b) <- aList m
+ , a /= x ] }
+
+
+-- | delete all associations with this B element
+deleteBAssoc
+ :: Eq b
+ => b -> Assoc a b -> Assoc a b
+
+deleteBAssoc x m
+ = m { aList = [ (a, b) | (a, b) <- aList m
+ , b /= x ] }
+
+
+-- | intersect two associations
+intersectAssoc
+ :: (Eq a, Eq b)
+ => Assoc a b -> Assoc a b -> Assoc a b
+
+intersectAssoc a1 a2
+ = emptyAssoc
+ { aList = intersect (aList a1) (aList a2) }
+

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