RFR: 8374685: ZGC: Convert zMarkStack to use Atomic<T>

Tue Jan 27 11:03:17 UTC 2026

Convert usages of AtomicAccess to Atomic<T> in zMarkStack.

I kept the old `compare_exchange` instead of doing a `compare_set` followed by a reload. We could probably support either. We would need to be careful in the `pop` loop, not sure if we gain much in readability from that change.

Testing:
 * GHA
 * HotSpot GC tests with ZGC linux x64 and linux aarch64

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Commit messages:
 - Use compare_set
 - ZGC: Convert zMarkStack to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29446/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29446&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374685
  Stats: 37 lines in 5 files changed: 2 ins; 2 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/29446.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29446/head:pull/29446

PR: https://git.openjdk.org/jdk/pull/29446