RFR: 8370041: GenShen: Filter young pointers from thread local SATB buffers when only marking old [v3]

[William Kemper]

> When GenShen is only marking the old generation, we do not need the SATB mechanism to preserve young pointers. We currently filter these out of the SATB buffers during the final-update-refs and init-mark safepoints. This increases latency and introduces no small amount of complexity. It should be possible to instead filter out these pointers when the SATB buffers are 'compacted' before being 'completed'.
> 
> # Background
> When GenShen is marking the old generation it leaves the SATB barrier enabled. When a young collection interrupts old marking, it creates a situation where a mutator thread could overwrite a field holding a pointer into a collection set region. The SATB barrier will dutifully place this object in the SATB queue. If this pointer makes it into a mark queue, the marking thread will crash. Prior to this change, GenShen filtered out such pointers _after_ the thread local SATB buffers were completed. After this change, such pointers are filtered out _before_ the buffers are completed. This is more inline with the natural way of things.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:

 - Merge remote-tracking branch 'jdk/master' into piggyback-satb-flush-on-update-roots
 - Merge remote-tracking branch 'jdk/master' into piggyback-satb-flush-on-update-roots
 - Flush SATB buffers upon entering degenerated cycle when old marking is in progress
   
   This has to happen at least once during the degenerated cycle. Doing it at the start, rather than the end, simplifies the verifier.
 - Fix typo in comment
 - Remove duplicate satb flush closure
 - Only flush satb once during degenerated cycle
 - Cleanup and comments
 - Merge remote-tracking branch 'jdk/master' into piggyback-satb-flush-on-update-roots
 - Fix assertion
 - Oops, move inline definition out of ifdef ASSERT
 - ... and 11 more: https://git.openjdk.org/jdk/compare/1922c4fd...4bd602de

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Changes: https://git.openjdk.org/jdk/pull/27983/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27983&range=02
  Stats: 309 lines in 11 files changed: 98 ins; 188 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/27983.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27983/head:pull/27983

PR: https://git.openjdk.org/jdk/pull/27983