Re: Optimizations for mutable structures?

7 Dec 2005


      On Wed, Dec 07, 2005 at 02:15:24PM -0000, Simon Marlow wrote:
...
On 07 December 2005 13:38, Malcolm Wallace wrote:
...
Jan-Willem Maessen  writes:
...
- Fetch elimination for imperative reads:
     writeIORef r e >> acts >> readIORef r
 === writeIORef r e >> acts >> return e
This transformation is valid only on single-threaded systems.
If there is any possibility of an IORef being shared across threads,
you are out of luck.
(assuming 'acts' doesn't modify 'r').
No, Jan's transformation is correct even in a multithreaded setting.  It
might eliminate some possible outcomes from a non-deterministic program,
but that's ok.  There's no requirement that all interleavings according
to the semantics have to be implemented.  This is a hard property to
state precisely, indeed we gave up trying to in the concurrency/FFI
paper: http://www.haskell.org/~simonmar/papers/conc-ffi.pdf, see Section
6.1.
I don't think it's true for this program:

import Data.IORef
import Control.Concurrent

main = do m1 <- newEmptyMVar
          m2 <- newEmptyMVar
          let e = 6
              not_e = 7
              acts = putMVar m1 () >> takeMVar m2
          r <- newIORef 5
          forkIO $ do takeMVar m1
                      writeIORef r not_e
                      putMVar m2 ()
          writeIORef r e
          acts
          readIORef r >>= print


Thanks
Ian

Re: Optimizations for mutable structures?

Ian Lynagh