RFR 8212958: Allow Klass::_subklass and _next_sibling to have unloaded classes

[dean.long at oracle.com]

Hi Coleen.  Most of this looks good to me, but I have a few questions:

Is the need for Compile_lock around accesses new in 12, because the 
implementor and subclass links can now be cleaned without being at a 
safepoint?

Style typo: In ciInstanceKlass::compute_has_subklass(), I think it 
should be return "ik->subklass() != NULL".

I see that you removed the memoizing of _has_subklass but left 
_implementor.  I'm concerned that this might have a performance impact 
and change behavior.  It seems to allow has_subklass() to change from 
true to false (and is_leaf_type() to change from false to true) where 
previously that should have been impossible.  With some fiddling with 
the locking, I was able to add back _has_subklass.  Do you remember why 
you removed it?

dl

On 10/25/18 1:42 PM, coleen.phillimore at oracle.com wrote:
> Summary: Don't return unloaded klasses. Make sure access is protected 
> by Compile_lock.
>
> See bug for more description.  This test has compiler changes since 
> the weak klass links in Klass (and implementor in InstanceKlass are 
> used by the compiler).  All the compiler jtreg tests passed.
>
> Tested with mach5 tier1-7 and test added.
>
> open webrev at http://cr.openjdk.java.net/~coleenp/8212958.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8212958
>
> Thanks,
> Coleen