Optimizations for mutable structures?

[Ian Lynagh]

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 <jmaessen at alum.mit.edu> 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