[jdk25u-dev] RFR: 8369611: Remove safepoint synchronization from ParallelScavengeHeap and SerialHeap

[Roland Mesde]

Backporting JDK-8369611: Remove safepoint synchronization from ParallelScavengeHeap and SerialHeap

This PR removes unnecessary conditional STS (Safepoint Thread Synchronization) code from ParallelScavengeHeap and SerialHeap. The cleanup is possible because StringDedup threads are now derived from JavaThread (changed in JDK-8369392), eliminating the need for special conditional synchronization logic that was previously required when StringDedup threads had different threading characteristics.

This backport has internal demand.

Ran related tests on linux-x64, linux-aarch64, macos-aarch64 and windows-x64:

make test TEST=test/hotspot/jtreg/vmTestbase/gc

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Commit messages:
 - Backport 551cd03b99feb34d98703b7d04571f92f83f2471

Changes: https://git.openjdk.org/jdk25u-dev/pull/136/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u-dev&pr=136&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369611
  Stats: 29 lines in 4 files changed: 0 ins; 29 del; 0 mod
  Patch: https://git.openjdk.org/jdk25u-dev/pull/136.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u-dev.git pull/136/head:pull/136

PR: https://git.openjdk.org/jdk25u-dev/pull/136