RFR (XS) JDK-8015272: Make @Contended within the same group to use the same oop map

[Aleksey Shipilev]

Thanks for review, Coleen!

On 11.11.2014 03:37, Coleen Phillimore wrote:
> Hi, I think this code looks correct.  Was there a test in the test
> system that exercises this code?  I think it would be hard to test with
> a dedicated test but was there one already in the test sets?

Yes, there are @Contended tests in vmtestbase that exercise the
@Contended placed over different fields. I added the targeted test that
also does walk through new code. There is nothing to check there, except
for the native assert in the new code.

> Secondly, could you use the word adjacent in the comments, reuse oopmap
> for adjacent oops in the class or something like that?  That would have
> saved me some jotting down on notebook.

Sure, see the update. In previous change, I blindly copied the block
already available for non- at Contended oops. I remember the oop maps code
was tripping me over, this is why we have an explanation all the way on
the top how oop maps are supposed to work.

> I'll sponsor it if you get another reviewer.

Here's the updated webrev:
 http://cr.openjdk.java.net/~shade/8015272/webrev.02/

I have only tested in builds on Linux x86_64/fastdebug, and passes
runtime/contended jtregs. There were no changes in product code since
last webrev, only in comments.

Thanks,
-Aleksey.