RFR: 8367689: Revert removal of several compilation-related vmStructs fields [v2]

[Andrei Pangin]

On Wed, 17 Sep 2025 09:41:00 GMT, Kevin Walls <kevinw at openjdk.org> wrote:

>> Francesco Andreuzzi has updated the pull request incrementally with six additional commits since the last revision:
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> We should clarify:
> These fields are not a public interface, but the JVM has to maintain them due to other software's expectations?
> 
> Async-Profiler is great, but is this change the right solution? Are there other fields which other software would have liked to stay around, and what else can not be removed?
> 
> Curious how did Async-Profiler break when these are removed?  Is it a crash that Async-Profiler needs to workaround, or is there less information in the collected profile.

@kevinjwalls Right, VMStructs is not a public supported interface, the JVM is not obliged to maintain it. Yet, some external tools (async-profiler and eBPF based profilers) softly rely on that in the lack of standard supported alternatives. At the same time, we are improving OpenJDK built-in capabilities that can eventually serve as such an alternative. A recent example is JEP 509: JFR CPU-Time Profiling. It's still a long path until JFR or other built-in tools can satisfy today's demand; in the meantime, VMStructs provides a temporary solution.

VMStructs is a reasonable trade-off, very cheap from the maintenance perspective as opposed to AsyncGetCallTrace (async-profiler no longer depends on the latter). To emphasize, we do not expect others to maintain VM structures that async-profiler relies on, that is our (profiler + Corretto) responsibility. Infrequent changes will be small, localized and safe, like in this PR.

As for the question how async-profiler breaks: if it is an optional feature (like here), it gracefully reduces functionality. Async-profiler attempts to detect essential VM changes and fail fast to prevent from crashing at runtime.

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PR Comment: https://git.openjdk.org/jdk/pull/27318#issuecomment-3302889663