RFC (round 1), JEP draft: Low-level Object layout introspection methods

[Aleksey Shipilev]

Hi,

I would like to solicit early feedback on "Low-level Object layout introspection methods" JEP:
  https://openjdk.java.net/jeps/8249196

It crosscuts core libraries and Hotspot, and therefore jdk-dev@ seems to be the mailing list to use
for it.

The work seems to be at the point where we understand the initial design and implementation
constraints. Those are both reflected in JEP text (where possible), and in the prototype
implementation (webrevs and builds are linked in JEP itself).

The API itself is not cast in stone, and open for discussion. JEP text hopefully makes a good
argument for each of new methods. Strong (dis)agreements about the existence of particular methods
are good to have during this review, so we could swing the JEP direction early.

The implementation itself is not the focus of this discussion. If you find implementation
bugs/quirks, send me a direct email. Ditto if you have a drive-by comment and do not want to
participate in more discussions.

The API initial performance characteristics look good and briefly mentioned in JEP text. Robert
Stupp had provided the experimental JAMM branch that hooks to new API, and it seems to considerably
improve accuracy and performance. I also did a few touch-ups in JOL to bring the performance
baseline for comparisons closer, and JOL is still quite far away performance-wise.

(Logistics: This review does _not_ require prompt review, as many folks -- including myself -- are
having vacations at this time. I shall be able to sift through replies and do JEP/implementation
adjustments some time next week.)

-- 
Thanks,
-Aleksey