Re: JavaMemoryModel: Updated JMM schedule and status report

From: Jerry Schwarz (jerry.schwarz@oracle.com)
Date: Fri Jul 25 2003 - 17:00:19 EDT


Personally, I always thought the no out of thin air requirement was unreasonable. Among other things it constrains optimizers in ways that have nothing to do with threading and synchronization.  That is, I've always taken the goal to be to allow the optimizer to treat straight line code without any thread related operation (however that is defined) as if no threading were present.  But consider

Thread 1
c = ...
x = 3 * c - 1

Thread 2
y = x

The not out of thin air requirement says that the compiler can't transform Thread 1 to
c = ...
x = 3 * c
x = x - 1




At 09:48 AM 7/25/2003, jmanson@cs.umd.edu wrote:
The list has deemed it necessary to provide some causal guarantees for
Java.  For example, out of thin air safety:

Thread 1
x = y

Thread 2
y = x

We have agreed long since that this cannot, for example, result in x = y =
42.  There are other, more meaningful examples of out-of-thin-air results
that are widely agreed upon.

                                        Jeremy

----- Forwarded message from Paul Loewenstein -----

From: Paul Loewenstein <Paul.Loewenstein@sun.com>

It is now over 10 years since I was last involved in writing a memory
model (Sparc V9).  In the interests of simplicity the RMO model admits
non-causal traces.  The cure was deemed worse than the disease.  The
only effect would be some unnecessary synchronization in somewhat
obscure and unlikely code.  Is relaxing the Java memory model for the
sake of simplicity a viable option?

Paul
----- End of forwarded message from Paul Loewenstein -----
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