On 29/03/2012 05:56, Ryan Newton 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;
}
Remove the conditional. You want to always set the header to
stg_MUT_ARR_PTRS_CLEAN_info, and always update the mark table.
Cheers,
Simon
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 <http://GHC.IO>
import GHC.IORef
import GHC.ST <http://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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