JavaMemoryModel: Removal of Synchronization on Weakly Ordered Machines

From: David F. Bacon (
Date: Tue Jun 29 1999 - 12:06:15 EDT

Doug Lea writes:
> As has been recently demonstrated (by, among others, V.C. Sreedhar
> and IBM colleagues in their upcoming OOPSLA paper), many Java
> programs are mostly single-threaded, and contain a lot of code that
> can be statically determined as such. In a language where safety is
> a primary goal, it seems wiser all around for compilers to remove
> unnecessary synchronization and barriers than for programmers to do
> so.

Note however that the semantics of an unlock operation in the general
case disallow removing the synchronization associated with the unlock,
even if the object is thread-local.

At an unlock *all* writes must be made globally visible, so to optimize
away the synchronization the compiler must prove not only that the
object is thread local, but also that there have been no intervening
writes to shared variables (accessible without locking the object in
question) since the last unlock. In the absence of such proof, you can
eliminate the mutual exclusion operations but not the synchronization

The result is both a reduction in the number of synchronizations that
can be eliminated and an increase in compile complexity and run time.


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