RFR: 8334031: Generated JfrNativeSettings seems off

[Erik Gahlin]

On Tue, 25 Jun 2024 18:55:14 GMT, Robert Toyonaga <duke at openjdk.org> wrote:

> This PR changes GenerateJfrFiles.java so that the generated `JfrNativeSettings` union does not include JFR structs.`JfrNativeSettings` is meant to hold the settings for JFR events, but previously also included JFR structs such as MetaspaceSizes, StackFrame, CopyFailed, G1EvacuationStatistics, ObjectSpace, VirtualSpace. These are not events, but instead are JFR `Type`s, and so do not have settings such as stacktraces or thresholds.
> 
> The inclusion of JFR structs in `JfrNativeSettings` was problematic because it could cause a displacement between event ID and`JfrNativeSettings` array index (each index is meant to correspond with a specific event ID).
> 
> Testing:
> - jdk/jdk/jfr
> - tier1

All types in JFR is assigned an ID, regardless if they are events or a structs. If we are going to index into the array, with the event type ID, must not the holes be filled with something? Or are all native event type IDs first?

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PR Comment: https://git.openjdk.org/jdk/pull/19891#issuecomment-2192215608