RFR: 8317007: Add bulk removal of dead nmethods during class unloading [v4]

[Thomas Schatzl]

> Hi all,
> 
>   please review this change that bulk-removes dead nmethods for the STW collectors instead of unregistering nmethod by nmethod. This significantly speeds up the class unloading phase.
>     
> For G1, this is almost 100% the code that has been removed from the review for JDK-8315503.
>     
> For Serial and Parallel GC, the code is new.
>     
> This change does not try to improve the situation for concurrent collectors - this would at first glance require extending the scope of the `CodeCache_lock` which I did not want to do. See the CR for more details.
>     
> Also, no parallelism for Parallel GC: the existing data structure for code root remembered set is a linked list. There is in almost all cases no point in trying to parallelize what is basically a traversal of the linked list (with each element not having a lot of work to do). I filed JDK-8320067. There should already be a significant speedup with this change, as each `unregister_nmethod` call previously traversed the entire linked list to find that element (i.e. complexity reduction of O(n^2) -> O(n)).
>     
> One more comment:
> 
> *  So it would be possible to merge the `ScavengableNMethod::prune_nmethods_not_into_young` which removes elements from the code root sets that do no longer need to be remembered (due to object movement) with removing dead/unlinked nmethods. However, this would mean to put that class unloading code to the end of gc as during phase 1 the new locations which are relevant for pruning uninteresting code root remembered set entries did not move yet. I wanted to keep determining the unlinking nmethods and class/code unloading them together in the code, but I could be convinced to move it.
>     
> Testing: tier1-4
> 
> Depends on #16759, please review first.
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  ayang review; merge fixes :(

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16767/files
  - new: https://git.openjdk.org/jdk/pull/16767/files/c9de0868..c074e8ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16767&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16767&range=02-03

  Stats: 26 lines in 15 files changed: 15 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/16767.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16767/head:pull/16767

PR: https://git.openjdk.org/jdk/pull/16767