RFR (11u, XXL): Upstream/backport Shenandoah to JDK11u - the review thread

[Roman Kennke]

I'm starting a new thread with a new round of webrevs. Maybe we can
keep in-principle discussions on the other thread and get some actual
reviews here?

Anyhow, small changes since the last one:
- the lone include "interpreter.hpp" is gone, apparently it was a left-
over that's no longer needed
- the 2 friend class declarations are now wrapped in
INCLUDE_SHENANDOAHGC too, just to be sure (even though compiler
wouldn't compile any actual code)

As noted in the other thread, there should not be any Shenandoah code
leaking out when building with --with-jvm-features=-shenandoahgc (which
is the default too). There's 2 exceptions:

- The global UseShenandoahGC flag. When invoked but not built-in, it
will print an error message and exit.
- In:
src/hotspot/share/jfr/periodic/jfrPeriodic.cpp

I could not exclude the definition of:
TRACE_REQUEST_FUNC(ShenandoahHeapRegionInformation)

Because that would be called by generated code. Related to this:

src/hotspot/share/jfr/metadata/metadata.xml

This generates two event classes for Shenandaoh, but they would not be
used. I don't know how to exclude them. It seems harmless to me.

Shared-only changes:
http://cr.openjdk.java.net/~rkennke/shenandoah-jdk11u-upstream/webrev.07-shared/

Full webrev:
http://cr.openjdk.java.net/~rkennke/shenandoah-jdk11u-upstream/webrev.07-all/


Roman