RFR: 8194759: Support caching class mirror objects

[Jiangli Zhou]

Hi Thomas,

Thanks a lot for the review!

> On Feb 22, 2018, at 3:15 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi,
> 
> On Thu, 2018-02-08 at 15:59 -0800, Jiangli Zhou wrote:
>> Please review the change for caching class mirror objects in of CDS
>> archive. Currently, CDS archive contains java.lang.String objects and
>> constant pool resolved_references arrays with resolved string
>> constants (when archiving java heap object is allowed). This change
>> adds the mirror objects. Please see the description below for
>> details.
>> 
>> RFE: https://bugs.openjdk.java.net/browse/JDK-8194759 <https://bugs.o
>> penjdk.java.net/browse/JDK-8194759>
>> webrev: http://cr.openjdk.java.net/~jiangli/8194759/webrev.00/
>> <http://cr.openjdk.java.net/~jiangli/8194759/webrev.00/>
> 
> I looked at the change at this link since although there were comments,
> no new webrev seems to have been posted.

I’m finalizing the update to incorporate all review feedbacks. Will post a new webrev when the update is ready.

> 
> [...]
>> 
>> GC Considerations
>> ===============
>> When an archived mirror is installed (becomes 'in use') during shared
>> class loading and restoration at runtime, the archived mirror is
>> accessed via GC access API with barrier. The archived object is
>> denoted with IN_ARCHIVE_ROOT and becomes a special root that handled
>> differently by GC and enqueued in SATB implicitly in G1.
>> Similar to the klass constant pool resolved_references array, the
>> archived mirrors resides in the 'open archive' heap region. The
>> mirror fields may be updated with references pointing to other non-
>> archive heap regions during runtime execution. References within the
>> archived mirrors are scanned and followed by GC.
>> Shared classes from builtin loaders are never unloaded after they
>> become 'in use' (loaded from the archive) at runtime, which keeps all
>> 'used' archived mirrors alive after being installed.
> 
> I concur with Coleen that the casting in Klass::archived_mirror_raw() 
> and Klass::set_archived_java_mirror_raw() is really ugly :)

I’m adding a new field in Klass and removing the ugly casting as Coleen suggested.

> 
> Otherwise as far as GC is concerned it looks good.

Thanks!

Jiangli

> 
> Thomas