Re: Code review for new primop's CMM code.

10 Apr 2012


      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/18ed460be111b47a759486677960093d71eef...
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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Re: Code review for new primop's CMM code.

Simon Marlow