Jerry Schwarz wrote:
> That was my reaction too, but I slept on it overnight and I think I see
> a subtlety that I overlooked yesterday. When is A.y read? Could it be
> read by the constructor of the Thread instead of by the run method.
>
> This question can be illustrated without threads. Consider the
> following example
>
> public class S {
> public final int x;
> S() { x = ... ; }
> ...
> }
>
> public class U {
> private S s;
> public U(S s) { this.s = s; }
> public int f() { return s.x; }
> }
>
> Is a compiler allowed to transform the definition of U into
>
> public class U' {
> private int s_x;
> public U(S s) { this.s_x = s.x ; }
> public int f() { return s_x; }
> }
>
> Obviously, this couldn't be allowed if s.x were not final, but does the
> special semantics of final allow it? I would like to say yes because
> this is potentially a significant optimization. The relevance to Bill's
> example is that if this transformation were allowed on the anonymous
> Thread class then
>
>> d) prints x = 1, y = 0
>
>
> becomes possible.
>
> But so do other anomalies. In particular what if U's are used in
> constructors of S. Let's modify S a little
>
> public class S {
> public final int x;
> private U u;
> public S() {
> u = new(this);
> x = ...; // non-zero value
> }
> public int f() { return u.f(); }
> }
>
> With the optimization in place S.f will return 0. But I don't see any
> way I can justify S.f returning 0, so I guess the optimization can't be
> allowed. And so, (d) isn't allowed either.
>
> -- Jerry Schwarz
>
> -------------------------------
> JavaMemoryModel mailing list - http://www.cs.umd.edu/~pugh/java/memoryModel
Now you are asking for the allowed optimizations when we inline into a
constructor BEFORE a final is set and then want to eagerly read the final.
This is an easy-to-test-for situation (inlining into a constructor before
setting all finals) with an obvious optimization to cut out (hoisting the
read of the final into U's constructor). I'm happy to give up this
optimization.
Cliff
-- Dr. Cliff Click Chief Architect, HotSpot Server Sun Microsystems, Inc. Cliff.Click@Sun.COM Senior Staff Engineer 4150 Network Circle (408) 276-7046 MS USCA14-102 Santa Clara, CA 95054------------------------------- JavaMemoryModel mailing list - http://www.cs.umd.edu/~pugh/java/memoryModel
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