JEP 175: PowerPC/AIX Port
|Discussion||ppc dash aix dash port dash dev at openjdk dot java dot net|
|Reviewed by||Alan Bateman, Vladimir Kozlov|
|Endorsed by||Mikael Vidstedt|
Add Linux/PowerPC64 and AIX/PowerPC64 to the set of supported OpenJDK platforms.
- Extend the platform coverage of the JDK source base to Linux/PowerPC64 and AIX/PowerPC64
- Refactor and modularize the HotSpot source base to simplify future ports
- Foster the collaboration between Java implementers in the OpenJDK Community
The PowerPC/AIX Porting Project has ported the JDK source base to two new platforms: Linux/PowerPC64 and AIX/PowerPC64. It has implemented the C++ interpreter (which is also used by the Zero port) and the C2 Server JIT compiler on the new platforms. Notice that these two new ports are actually back-ports of commercial Java offerings by IBM and SAP and as such the code base can be considered stable and production ready.
The focus of this JEP is not the porting effort itself, which has been mostly completed, but rather the integration of the ports into the OpenJDK master repositories.
A high level plan for the integration could look as follows:
The first step would be to integrate the HotSpot changes:
Currently we have about 80 changesets on top of the HotSpot repository. A good estimation for the integration of our changes into the HotSpot tree would be to tailor several bigger changesets (i.e. 20-30) which can be reviewed and committed one by one. Most of these "bigger" changes will not affect current platforms at all, because they only contain ppc64/AIX relevant code in the new platform-dependent files and subdirectories.
The second step would be the class-library changes:
After the HotSpot VM can successfully build on the new platforms, we can start with the integration of the class-library changes. There will be fewer changes with lower complexity compared to the HotSpot changes, and the changes will be much more independent of each other so the review process should be easier and easier to parallelize (e.g., AWT changes are independent of NIO changes and can be done in arbitrary order). We expect the overall complexity of the changes in the class libraries to be less than that of the MacOS port (e.g., we don't need a complete new graphics stack).
Oracle will create a staging repository which is owned by the PowerPC/AIX Project and will contain the fixes that have been reviewed and approved. The PowerPC/AIX Project is responsible for ensuring that the forest is uptodate with the latest changes from the master repository. Oracle will create a private hudson instance for the staging repository, which will build and test the changes. Oracle will periodically test the changes in the staging repository and give feedback to the PowerPC/AIX Project as appropriate. Oracle, SAP and IBM will jointly determine when the staging forest is ready for integration. Once that point is reached all changes from the staging repository will be integrated via a bulk integration into the master repository.
IBM and SAP regularly build and test the ports on the porting platforms as well as on the currently-supported JDK 8 platforms to ensure that no regressions are being introduced which break the existing platforms.
IBM and SAP are committed to fully support (i.e., regularly update, enhance, and test) the code for the new platforms introduced by this effort.
Risks and Assumptions
We don't expect any major risks, since all the code we propose for integration was backported from commercial Java offerings by IBM and SAP which have run in production environments for years on both the new proposed platforms as well as on the platforms already supported by the OpenJDK source base.
The only dependences are in fact the resources and the commitment provided by Oracle. Considering the current infrastructure deficiencies we really need support from Oracle colleagues for everything, starting with the creation of bug reports through the reviewing of changes up to the Oracle-internal testing (JPRT) and final committing. We therefore think this JEP needs funding not only from IBM and SAP but also from Oracle, in particular so that Oracle engineers in the HotSpot and Core Libraries Groups can assist in this effort.
Most of the changes to be integrated won't affect the current OpenJDK platforms in any way because they are only active on the new platforms. Although the Project tried to minimize changes in shared code, it was nevertheless necessary to make selective changes in common code sections if it was not otherwise possible to support features or to workaround restrictions of the new platforms.
Most of the shared code changes are within the HotSpot forest due to the need to:
- Support features in the C++ Interpreter which were not yet implemented there (method handles, profiling, compressed oops, etc.),
- Support the weak memory model of the PowerPC architecture,
- Support new processor requirements in the C2 server compiler (calling conventions, memory ordering, assembler peculiarities), and to
- Support new OS-specific features on AIX (e.g., memory management)
The shared changes in the JDK (class libraries) forest are mostly due to AIX-specific adaptions in the various native implementations of the class libraries.
Of course there are also minor changes in the build system but they shouldn't cause too much trouble.