Add option to toggle Extended HCR capability

[tajila]

Issue or pull request number:
eclipse-openj9/openj9#19554

Overview:
A new option -XX:[+|-]EnableExtendedHCR is added. This option toggles the extended HCR capability in the JVM.

Note: by default this capability is disabled (for Semeru but not IBM Java 8) which is a change in behaviour from previous releases. If there is a JVMTI failure as a result of this change, the user will see the following warning message Warning: Extended HCR is disabled, enable with -XX:+EnableExtendedHCR..

Also, as part of this change we will be announcing the deprecation of the extendedHCR feature for removal in the near future. As of JDK25 extended HCR will no longer be supported. Following that, we will also remove extended HCR support from legacy releases (ie. JDK8).

Release target:
Eclipse OpenJ9 0.46.0*

Applies to the following JDK versions:
8 and later

Applies to the following platforms:
All platforms

For new command line options:

• Option name and syntax: -XX:[+|-]EnableExtendedHCR
• Purpose: This option toggles the extended HCR capability in the JVM.
• Restrictions: None

[tajila]

@pshipton @mstoodle any concerns with the deprecation statement?

[tajila]

For background:

Short-term requirement: We need to disable extendedHCR to support unsafe static field access at fixed offsets optimization. The dynamic checks are not ideal, but it is what we have to do now.

Long-term requirement: There is a maintenance cost is supporting extended HCR and we are not sure anyone is actually using it. The ideal scenario is to just remove the feature entirely.

[pshipton]

Seems fine.

[pshipton]

How does this apply to IBM Java 8, considering it's not disabled by default there atm?

[tajila]

How does this apply to IBM Java 8, considering it's not disabled by default there atm?

I assumed we didn't document IBM java8 in openj9 docs

[mstoodle]

I'm fine with the approach so long as there is a way for us to discover that lots of people do depend on it.

We should possibly add this option to any migration documentation we have between IBM SDK for Java and Semeru Runtimes (but that's something for IBM to decide and shouldn't block openj9 moving forward with this approach).

[pshipton]

I assumed we didn't document IBM java8 in openj9 docs

We don't but the IBM Java 8 doc is derived from OpenJ9, we will be asked. Also I'm curious how your plan to "remove extended HCR support from legacy releases" affects IBM Java 8. Is it exempt?

[tajila]

Also I'm curious how your plan to "remove extended HCR support from legacy releases" affects IBM Java 8. Is it exempt?

This round we will disable it in Semeru/Openj9 by default. Next release, I'd like to disable it in IBM Java8 assuming no one complains. Afterwards, I'd like to remove it completely.

[tajila]

Also I'm curious how your plan to "remove extended HCR support from legacy releases" affects IBM Java 8. Is it exempt?

This round we will disable it in Semeru/Openj9 by default. Next release, I'd like to disable it in IBM Java8 assuming no one complains. Afterwards, I'd like to remove it completely.

Now that I think about it, I think we need a stronger deprecation message. Something like: Warning: Extended HCR is disabled, enable with -XX:+EnableExtendedHCR. In the future, this capability will be removed from the JDK.

[pshipton]

We should add something to this migration guide.
https://www.ibm.com/support/pages/semeru-runtimes-migration-guide

[tajila]

We should add something to this migration guide.
https://www.ibm.com/support/pages/semeru-runtimes-migration-guide

Is this also done by the docs team?

[pshipton]

I'll let @Sreekala-Gopakumar answer that.

[Sreekala-Gopakumar]

Yes, we can change the migration guide.

[Sreekala-Gopakumar]

We should add something to this migration guide. https://www.ibm.com/support/pages/semeru-runtimes-migration-guide

@pshipton - I have updated the migration guide. Please check and confirm that the content is ok. Thanks!

https://supportcontent.ibm.com/support/pages/node/6366719/latest

[pshipton]

Looking at this again, we should expand the one line description. It says "New -XX:[+|-]EnableExtendedHCR option added" but doesn't mention the change in behavior.

[pshipton]

I suggest the migration item should mention the change in behavior occurs in the OpenJ9 0.46 release, or the following OpenJDK versions: jdk8u422, 11.0.24, 17.0.12, 21.0.4, 22.0.2, 23.0

[Sreekala-Gopakumar]

I suggest the migration item should mention the change in behavior occurs in the OpenJ9 0.46 release, or the following OpenJDK versions: jdk8u422, 11.0.24, 17.0.12, 21.0.4, 22.0.2, 23.0

@pshipton - I have updated the document. Please check if the change is OK. Thanks!

https://supportcontent.ibm.com/support/pages/node/6366719/latest

[pshipton]

Looks good.