Perhaps a question mark is more appropriate in the title. It is a code review I am seeking, not one on offer ;-).
On Thu, Mar 29, 2012 at 12:56 AM, Ryan Newton <rrnew...@gmail.com> wrote: > 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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