8309953: Strengthen and optimize oopDesc age methods

[rgithubli]

https://bugs.openjdk.org/browse/JDK-8309953

A clean backport. An minor optimization for oopDesc age.

tier 1 passed locally. The logic is simple - store the mark() value into a var so we don't read it twice. The original commit is in tip for almost 2 years. Low risk.

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Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8309953 needs maintainer approval

Issue

• JDK-8309953: Strengthen and optimize oopDesc age methods (Enhancement - P4 - Rejected)

Reviewing

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$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/1950/head:pull/1950
$ git checkout pull/1950

Update a local copy of the PR:
$ git checkout pull/1950
$ git pull https://git.openjdk.org/jdk21u-dev.git pull/1950/head

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[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[openjdk]

⚠️ @rgithubli This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.

[mlbridge]

Webrevs

• 00: Full (34fbdc94)

[rgithubli]

/approval request for backport of 4a5475ce0492fc02019b0f89b50601c388c63e21.

Minor gc perf improvement when it's on a hot path: See the benchmark in comment: openjdk/jdk#14456 (comment)

The backport applies cleanly. Tested with tier 1. In mainline for 2 years. The risk is low.

[openjdk]

@rgithubli
8309953: The approval request has been created successfully.

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