[jdk21u-dev] RFR: 8346142: [perf] scalability issue for the specjvm2008::xml.validation workload

[Rui Li]

A clean backport for https://bugs.openjdk.org/browse/JDK-8346142

A perf patch: it adds low cost null prechecks to prevent the flow entering slow synchronized block unnecessarily.

In tip for about 8 months, no issues so far. Low risks.

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Commit messages:
 - Backport 10d08dbc81aa14499410f0a7a64d0b3243b660f1

Changes: https://git.openjdk.org/jdk21u-dev/pull/2175/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2175&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346142
  Stats: 23 lines in 1 file changed: 6 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2175.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2175/head:pull/2175

PR: https://git.openjdk.org/jdk21u-dev/pull/2175