Hi all, In preparation for students working on concurrent data structures GSOC(s), I wanted to make sure they could count on CAS for array elements as well as IORefs. The following patch represents my first attempt:
https://github.com/rrnewton/ghc/commit/18ed460be111b47a759486677960093d71eef386 It passes a simple test [Appendix 2 below], but I am very unsure as to whether the GC write barrier is correct. Could someone do a code-review on the following few lines of CMM: if (GET_INFO(arr) == stg_MUT_ARR_PTRS_CLEAN_info) { SET_HDR(arr, stg_MUT_ARR_PTRS_DIRTY_info, CCCS); len = StgMutArrPtrs_ptrs(arr); // The write barrier. We must write a byte into the mark table: I8[arr + SIZEOF_StgMutArrPtrs + WDS(len) + (ind >> MUT_ARR_PTRS_CARD_BITS )] = 1; } Thanks, -Ryan -- Appendix 1: First draft code CMM definition for casArray# ------------------------------------------------------------------- stg_casArrayzh /* MutableArray# s a -> Int# -> a -> a -> State# s -> (# State# s, Int#, a #) */ { W_ arr, p, ind, old, new, h, len; arr = R1; // anything else? ind = R2; old = R3; new = R4; p = arr + SIZEOF_StgMutArrPtrs + WDS(ind); (h) = foreign "C" cas(p, old, new) []; if (h != old) { // Failure, return what was there instead of 'old': RET_NP(1,h); } else { // Compare and Swap Succeeded: if (GET_INFO(arr) == stg_MUT_ARR_PTRS_CLEAN_info) { SET_HDR(arr, stg_MUT_ARR_PTRS_DIRTY_info, CCCS); len = StgMutArrPtrs_ptrs(arr); // The write barrier. We must write a byte into the mark table: I8[arr + SIZEOF_StgMutArrPtrs + WDS(len) + (ind >> MUT_ARR_PTRS_CARD_BITS )] = 1; } RET_NP(0,h); } } -- Appendix 2: Simple test file; when run it should print: ------------------------------------------------------------------- -- Perform a CAS within a MutableArray# -- 1st try should succeed: (True,33) -- 2nd should fail: (False,44) -- Printing array: -- 33 33 33 44 33 -- Done. ------------------------------------------------------------------- {-# Language MagicHash, UnboxedTuples #-} import GHC.IO import GHC.IORef import GHC.ST import GHC.STRef import GHC.Prim import GHC.Base import Data.Primitive.Array import Control.Monad ------------------------------------------------------------------------ -- -- | Write a value to the array at the given index: casArrayST :: MutableArray s a -> Int -> a -> a -> ST s (Bool, a) casArrayST (MutableArray arr#) (I# i#) old new = ST$ \s1# -> case casArray# arr# i# old new s1# of (# s2#, x#, res #) -> (# s2#, (x# ==# 0#, res) #) ------------------------------------------------------------------------ {-# NOINLINE mynum #-} mynum :: Int mynum = 33 main = do putStrLn "Perform a CAS within a MutableArray#" arr <- newArray 5 mynum res <- stToIO$ casArrayST arr 3 mynum 44 res2 <- stToIO$ casArrayST arr 3 mynum 44 putStrLn$ " 1st try should succeed: "++show res putStrLn$ "2nd should fail: "++show res2 putStrLn "Printing array:" forM_ [0..4] $ \ i -> do x <- readArray arr i putStr (" "++show x) putStrLn "" putStrLn "Done."
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