RFR: 8360288: Shenandoah crash at size_given_klass in op_degenerated [v2]

[William Kemper]

> Both degenerated and full GCs unload classes before reclaiming unmarked humongous objects. This may result in a null klass pointer dereference when asking humongous objects for their size.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Reclaim humongous regions from start to end (do not use oop::size).
 - Revert "STW collections cannot unload classes before reclaiming garbage"
   
   This reverts commit ae8079386eb34cf1351534ede7d7df8f81b50174.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26256/files
  - new: https://git.openjdk.org/jdk/pull/26256/files/ae807938..cb63787d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26256&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26256&range=00-01

  Stats: 96 lines in 3 files changed: 40 ins; 49 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/26256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26256/head:pull/26256

PR: https://git.openjdk.org/jdk/pull/26256