RFR: 8364628: Serial: Refactor SerialHeap::mem_allocate_work

Mon Aug 4 10:07:24 UTC 2025

Extract out old-gen allocation outside the critical-region to streamline the flow in `mem_allocate_work`.

Test: tier1-5

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Commit messages:
 - sgc-mem-allocate

Changes: https://git.openjdk.org/jdk/pull/26619/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26619&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364628
  Stats: 59 lines in 4 files changed: 22 ins; 31 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/26619.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26619/head:pull/26619

PR: https://git.openjdk.org/jdk/pull/26619