State of the ppc64le port of JEP 425: Virtual Threads (Preview)

[Reingruber, Richard]

Hi,

I learned today that preview features _must_ be implemented by a port in an
OpenJDK release [1].

Unfortunately I have to inform you that I don't think the ppc64le port I'm
currently working on will be ready in the JDK19 time frame.

When I started the work (Jan. or Dec. I think) I expected to finish it before
summer. Even after the last status update [2] I thought I could make it. But
with the difficulties I still experience and being 6-8 weeks out of office in
summer it is now rather unlikely. And until this morning myself (and actually
also my colleagues) assumed this would only be a minor issue.

Current Status of the Port:

* UseContinuationFastPath is disabled

* Basic tests where sequences of interpreted and compiled frames with quite some
  variations are frozen and thawed succeed.
  
* GC with stack chunks on the java heap succeed.

* Basic exception handling tests succeed.

* Basic tests exercising compiled java calls with stack arguments succeed but
  need to be revisited because there are issues.

[3] is a selection of test cases that I use in development.
[4] is the most recent version of the ppc64le port

Main Technical Problems

* Shared code makes use of the 'unextended sp' of java frames. This breaks the
  platform abstraction as it makes assumptions on where to find, e.g., stack
  arguments relative to the unextended sp.
  
* There are non-obvious interdependencies in the code which make it difficult to
  fix an issue. In an attempt to fix a problem I often have regressions because
  I missed adaptations of dependent parts. And then it it is extremely tedious
  to find the cause of the regression running tests and analyzing very long
  trace output.

* Currently I see that the handling of stack arguments of compiled java methods
  works in quite some cases (see [3]) but there are cases where it
  doesn't. Trying alternative approaches means going through the tedious and
  time consuming process described above.
  
* Lack of documentation. Heavily templatized implementation.

These problems (except the last) could not be foreseen. From a high level the
port simply needs to copy frames between stack and heap and provide some
assembler glue code. As I know now it is actually a high effort to get the
deatils tuned right.

Thanks, Richard.

[1] Ports _must_ implement preview features in thread "What should the relationship between ports and developers of large projects be?"
    https://mail.openjdk.java.net/pipermail/jdk-dev/2022-May/006635.html
[2] State of the ppc64le loom port as of April 14
    https://mail.openjdk.java.net/pipermail/loom-dev/2022-April/004197.html
[3] BasicExp.java tests driving development of the port
    https://github.com/reinrich/loom/blob/3286bc8b72401dbccac59c994919fc425a51cb52/test/jdk/jdk/internal/vm/Continuation/BasicExp.java
[4] Most recent version of the ppc64le loom port
    https://github.com/reinrich/loom/commits/ppc_port