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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