What's the galahad branch purpose?

[zakkak]

Hello,

I noticed that there is a galahad branch in the repository https://github.com/oracle/graal/tree/galahad

What's the purpose of this branch and how does it relate to https://openjdk.org/projects/galahad/ and https://github.com/openjdk/galahad?

Thanks

[zapster]

Hi Foivos!

Thanks for the great question.

TL;DR

It is the "HEAD" branch for project Galahad, i.e., Graal Compiler in OpenJDK.

Long Version

From https://openjdk.org/projects/galahad/

Galahad's primary goal is to contribute Java-related GraalVM technologies to the OpenJDK Community

What this means in practice is that we want to be able to build an OpenJDK from master that includes the Graal JIT compiler in the form of libgraal (a native image compiled shared library). The Graal compiler and native image are highly entangled with the OpenJDK and Hotspot, so a specific version (commit) of the Graal source code only works with a specific source version (commit) of OpenJDK. To be able to build Graal with OpenJDK master, we need a Graal version that is compatible with OpenJDK master.

The Graal master branch, where most of the Graal development takes place, updates the supported JDK version for every promoted build (i.e. EA builds published at https://jdk.java.net/25/), which usually happens once a week. Which JDK version is supported by a Graal source tree can be seen in the common.json file. Graal master is only compatible with exactly that version of the OpenJDK. Using promoted builds is good compromise to keep Graal developer productivity up by not updating the JDK multiple times a day and keeping relatively close to OpenJDK upstream.

However, every JDK PR could introduce changes that need to be adopted by Graal. If Galahad would use the Graal master branch, it means that the OpenJDK + Graal build might be broken for a week until we adopt the next promoted JDK build. This is not practical. Thus, we want to adopt relevant JDK changes eagerly. We cannot push those adoptions to master since they don't work with the promoted build it is based on. So we need a place to push the adoptions so that the OpenJDK build and CI can pick it up. That place is the galahad branch. You can view the galahad branch as an "incubator" for JDK adoptions that are not yet in a promoted build.

We are using and refining this process since JDK 23, where the Graal JIT was first part of the Oracle JDK (see here). This is also in preparation for Galahad in OpenJDK.

Example

Here's a current WIP example for Galahad adoptions:

• The OpenJDK PR 8342103: C2 compiler support for Float16 type and associated operations openjdk/jdk#21490
• Galahad adoption PR: [JDK-8344599] Adapt JDK-8342103: C2 compiler support for Float16 type and associated operations #10117

As you see, we (the Graal team members) try to provide the adoption before the JDK change gets in to keep the Galahad breakage window short.

Cross-merging

Of course, we do not want the master and galahad to diverge. Thus, every merge to Graal master is automatically merged to to the galahad branch (example). The galahad branch, on the other hand, is merged back to master when we adopt a new promoted JDK build (example), so usually once a week.

Do I need to care about the galahad branch?

Usually not. If you want to build Graal with OpenJDK master, you might want to use the galahad branch. But be aware that it is a best effort approach and there are no guarantees that it always works. For reliable results, use Graal master revisions and the corresponding JDK tag as specified in common.json.

I hope that answers your question. If there is anything unclear, please do not hesitate to ask.

[zakkak]

Thank you for the detailed answer @zapster.