[JDK20] Thread.findScopedValueBindings to support JEP 429?

[babsingh]

For Thread.findScopedValueBindings,

• [step 1] evaluate if an implementation is required to support JEP 429: Scoped Values (Incubator) #16439;
• [step 2] if true, derive its implementation from the RI and implement it for OpenJ9.

[babsingh]

Pseudo-code implementation:

walkStackFrames:
   for each frame:
      if ((methodName is "runWith")
      and (callerClass is either
              jdk.incubator.concurrent.ScopedValue$Carrier OR
              java.lang.VirtualThread OR
              java.lang.Thread)
      ) {
         Check if the first parameter is an jdk.incubator.concurrent.ScopedValue$Snapshot
         Then, return the first parameter as a local ref;
      )

[thallium]

@gacholio FYI

[thallium]

Currently we don't have a test to validate the implementation. Maybe we should put an assertion in the implementation so we will know when this is actually called.

[babsingh]

Closing. An assertion has been added in #17451. Thread.findScopedValueBindings is is unused since there is no API or test that invokes this method in OpenJ9. If the assertion ever triggers, this issue should be re-opened and the draft implementation in #17402 should be completed and merged.

[babsingh]

@0xdaryl The three runWith methods, where we want to retrieve the first argument while stack walking, are:

• jdk.incubator.concurrent.ScopedValue$Carrier.runWith
• java.lang.VirtualThread.runWith
• java.lang.Thread.runWith

Issue: JIT inlining will prevent us to find the first argument during the stack walk since the O-slots don't retain their order when the methods are inlined.

#17402 contains the current implementation of the behaviour described in #16677 (comment).

[gacholio]

For the record, you can't reliably retrieve the first argument for any compiled method, inlined or not.

[babsingh]

For the record, you can't reliably retrieve the first argument for any compiled method, inlined or not.

In this case, we can look at all the arguments of the runWith methods. These methods have two arguments, and there is supposed to be only one argument of instance jdk.incubator.concurrent.ScopedValue$Snapshot.