RFR (XXL) [7u60]: nmethod backports (12 backports)

[Thomas Schatzl]

Hi all,

  please review the backport of the nmethod optimization changes for G1
(7145569) and all related issues.

This request in total contains 12 changes, five which enable easy
backport of the above mentioned main change, and six that fix issues
with the original change.

These are 

Add new flag for verifying the heap during startup (8029486)
Add a flag to turn off the output of the verbose verification code (8029487)
G1: Add remembered set size information to output of G1PrintRegionLivenessInfo (8028187)
G1: G1SummarizeRSetStats output on Linux needs improvement (8028186)
G1: Verification after a full GC is incorrectly placed. (8029596)
G1: optimize nmethods scanning (8025423)
G1: G1CollectedHeap::mark_strong_code_roots() needs to handle ParallelGCThreads=0 (8028193)
G1: improve remembered set summary information by providing per region type information (8028188)
G1: assert "assert(thread < _num_vtimes) failed: just checking" fails when G1ConcRefinementThreads > ParallelGCThreads (8028189)
-XX:+G1SummarizeRSetStats can result in wrong exit code and crash (8028190)
assert(!hr->isHumongous()) failed: code root in humongous region? (8028191)
Test gc/g1/TestPrintRegionRememberedSetInfo.java fails with "test result: Error. No action after @build" (8029488)

To track these backport CRs, the original CRs and their webrevs,
I created an overview page for all changes at
http://cr.openjdk.java.net/~tschatzl/nmethod-backport/ , listing the jdk
7u60 bug IDs and links to webrevs, and the corresponding original jdk 8
bug IDs and webrevs.

Except for the main change (JDK-8025423, JDK-7145569) there were no
particular changes necessary (possibly adding a missing method from
another change verbatim), so I think reviews should ooncentrate on that.
Of course, feel free to comment on the other backports.

Some notes about the backport:

- jdk7 has the old perm gen, so some changes simply did not apply at
all. E.g. there is no YoungRefCounterClosure to move, no
VerifyKlassClosure etc.

- there is one important change in how
G1CollectedHeap::g1_process_strong_roots() and
SharedHeap::process_strong_roots() are called.

hs24(jdk7) and hs25(jdk8) handle code cache roots differently. In
SharedHeap::process_strong_roots() there is the option that the caller
sets SO_CodeCache to scan code cache roots. The problem is that
even if SO_CodeCache is _not_ set, jdk7 iterates over some of the code
roots that have references into the heap anyway. Additionally this is
also dependent on whether we are collecting the perm gen or not.

This is different to hs25 where SharedHeap::process_strong_roots() only
ever iterates code roots when SO_CodeCache is set.

Now, with the nmethod optimization, we do not want
SharedHeap::process_strong_roots() to do any code root iteration like in
JDK8.

This unfortunately changed as part of the (huge) initial perm gen removal
patch.

Instead of trying to reorganizing this code, and looking through all
invocations to SharedHeap::process_strong_roots() to set the correct
flags (plus verification), I introduced a new boolean to
SharedHeap::process_strong_roots() called "manages_code_roots" that
indicates whether the caller handles code roots by itself in the case
when we are not explicitly passing SO_CodeCache.

This flag is only set to true (with a default value of false) in case of
G1 young collection, so minimizing code changes.

As far as I can see, the alternative is to implement something like in
JDK8 here: add a is_scavengable flag and look through all invocations to
process_strong_roots() to pass the SO_CodeCache flag.

As mentioned, I tried to minimize changes here, I am open to suggestions
here to improve that part though.

- in (Un-)RegisterNMethodOopClosure::do_oop_work() the code needs to ignore
references into the permanent generation. In JDK8 these closures do not
get references that are embedded into the code into the "perm gen" any
more. They are simply not oops.

- added GrowableArray::find_from_end() and GrowableArray::max_length(),
also from that initial perm gen removal change.

- for the backport in JDK-8028191 I had to add a "deoptimizeAll"
Whitebox method that calls a compiler method to deoptimize all code. This
was part of a larger compiler change that has not been backported.

Improvement:
Reduces young gc pause time in some FMW applications by a significant
amount, particularly for code that has a significant amount of compiled
code, e.g. from ~160ms to ~30ms for young-only GCs in CRM Fuse.

Testing:
jtreg, jprt, FMW applications, dacapo, specjbb05, ad-hoc (adhoc
runs with default collector and G1, fastdebug + product) testing

Comparing to a vanilla 7u60 adhoc testing run, there were three
occurrences of https://bugs.openjdk.java.net/browse/JDK-8030803 that
has been fixed in jdk8 but apparently is not scheduled for 7u60. It
is completely unrelated to the changes from what I can see, and a
known issue in 7u60.

Thanks a lot,
  Thomas