RFR: 8207812: Implement Dynamic CDS Archive

[Karen Kinnear]

One follow-up question relative to Jiangli’s review questions please:

> On Apr 23, 2019, at 8:08 PM, Calvin Cheung <calvin.cheung at oracle.com> wrote:
> 
> Hi Jiangli,
> 
> Thanks a lot for your review!
> 
> On 4/22/19, 2:07 PM, Jiangli Zhou wrote:
>> Hi Calvin,
>> 
>> 
>> - src/hotspot/share/classfile/systemDictionaryShared.cpp
>> 
>> 1218   if (DynamicDumpSharedSpaces) {
>> 1219     return false;
>> 1220   } else {
>> 
>> The above case for DynamicDumpSharedSpaces needs to be examined
>> carefully. Can you please ask Harold (and Coleen or Karen) to take a
>> look? Also, a comment is needed to explain that we can complete all
>> verification checks at dynamic dumping time.
> I've added a comment. If it return false, the caller will call VerificationType::resolve_and_check_assignability().
Just making sure I understand this correctly:

When we archive a class, we also archive all supertypes. We do not necessarily archive all classes in
the verification constraints list. We always record all verification constraints, whether or not we actually
continue to load and perform them.

When we run with an archive, we ensure that the supertypes that we use are also in the archive and are an exact match,
otherwise we don’t select the class from the archive.
We then check the verification constraints list, which might then cause additional class loading. If that fails,
again, we don’t select the class from the archive. This allows more flexibility in changes to the classes found
at runtime, as long as their “isAssignableFrom” constraints are still met.

So - prior to the DynamicDumpSharedSpace - 

In SystemDictionaryShared::add_verification_constraint:
  I agree with the logic that we add constraints for all archived classes.

  I agree that for non-built-in class loaders, we could not complete the verification check at dump time,
  and only perform this at runtime using the archive.
 
For built-in class loaders - I agree that we could ALSO perform the verification check at dump time, and
if it failed, eliminate archiving the class.

I assume that is intended behavior for DynamicDumpSharedSpaces?

Couple of questions:
1. Does what I said above for built-in class loaders match your understanding? If so, perhaps this would
be a good time to clarify the comment - since we will perform the verification check now AND
and when running with the archive.

2. For DynamicDumpSharedSpaces
Since we do not perform any loading as part of the PopulateDynamicDumpSharedSpaces,
I assume that this is all about recording verification constraints for all classes loaded while
we run, before we get to the archive creation on exit.

So your logic of creating constraints for all archived classes and then returning false so
that the verification is also run during the first run makes sense to me.

And yes, it makes sense to add a comment that we should ALSO perform verification checks during
the initial run in addition to creating the constraint for running later with the archive. I don’t conceptually
think of this as checking constraints “at dump time”, but rather for the initial run before creating
the dynamic archive.

thanks,
Karen
>> 
>> 
>> I only took a brief look of the test changes. Please ask Misha to
>> review the test changes as well.
>> 
>> Thanks and regards,
>> Jiangli