[concurrency-interest] Java atomics questions

Tom Ball tball at google.com
Wed Nov 20 15:12:38 EST 2013


I'm a member of the J2ObjC <https://code.google.com/p/j2objc/> project, an
open-source Jave to Objective-C transpiler. The java.util.concurrent
packages have recently been added to its JRE emulation library, but we have
some unanswered questions.

The sun.misc.Unsafe class is used by java.util.concurrent.atomic; we have a
partial implementation based on Android's version. I'm fuzzy on its
distinction between "ordered" and "volatile" field access, though. I
assumed that ordered means "requires a memory barrier", while volatile
doesn't. Is that correct? Here is our
putObjectVolatile()<https://code.google.com/p/j2objc/source/browse/jre_emul/Classes/sun/misc/Unsafe.java#201>
 and putOrderedObject()<https://code.google.com/p/j2objc/source/browse/jre_emul/Classes/sun/misc/Unsafe.java#287>
with
that assumption. Look reasonable? (The native comments hold Objective-C,
but in this case it's vanilla C.)

Also, is anyone familiar with the C11 optional atomic
operations<http://en.cppreference.com/w/c/atomic> support?
Clang has the necessary atomic intrinsic functions to support it and
generates reasonably looking code, so we're hoping to fix translation of
Java volatile fields with it. Currently we just add the volatile keyword to
the Objective-C field declaration, which doesn't support Java's definition.
Our thoughts for a better translation look something like:

  volatile int i;
...
  i = 42;
  i += 666;

becomes:
  atomic_int i;
...
  atomic_store(&i, 42);
  atomic_fetch_add(&i, 666);

Is that a reasonable approximation of Java volatile field semantics in C11,
or is there a better approach?

Tom

P.S. Forgive me my concurrency fumbling -- I realize compiler writers
should leave such design to the experts. :-)
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