[11u] RFC 8231405: [Shenandoah] guarantee(d != NULL) failed: Null dominator info

[Aleksey Shipilev]

Hi,

This is the question for Andrew Hughes, as we have reached the CPU work already. We have this
critical bugfix in Shenandoah/C2 that we would like to have in 11.0.5:
  https://bugs.openjdk.java.net/browse/JDK-8231405
  https://hg.openjdk.java.net/jdk/jdk/rev/222a91b9438a

The patch applies cleanly, but we are usually prepend the synopsis with "[backport]" to disambiguate
from the jdk-updates/jdk11u backports:
  "[backport] 8231405: [Shenandoah] guarantee(d != NULL) failed: Null dominator info"

I ran hotspot_gc_shenandoah, tier1, tier2 (with Shenandoah), jdk_security (with Shenandoah).

Can we have it? If yes, there are several ways to do this:

 a) Push the changeset to CPU forests directly, and then wait for the entire bundle to appear in
sh/jdk11 with the CPU patchset. Pros: it would get into CPU without additional fuzz. Cons: it would
not get tested with our public 11u nightlies, until the CPU patchset arrives.

 b) Push the changeset to sh/jdk11, pull the sh/jdk to CPU forests. Pros: it would be exposed to
users before the CPU release through our 11u nightlies. Cons: the later CPU merge would be against
that public changeset, not against shenandoah-jdk-11.0.5+9.

Thoughts?

-- 
Thanks,
-Aleksey