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Reducing Code and Test Complexity

Shortly after the adoption of 64-bit architectures the -XX:[-|+]UseCompressedClassPointers and -XX:[-|+]UseCompressedOops arguments were added to provide Java users the ability to enable using 32-bit references even when on a 64-bit architecture. This reduces memory overhead and help reduce cache misses. You can read more about this here.

Removing the -UseCompressedClassPointers option would make +UseCompressedClassPointers the default case and reduce the number of configurations that would need to be supported from three to two (+UseCompressedClassPointers, +UseCompactObjectHeaders). This would also significantly reduce code complexity as well as testing effort. Along with this, -UseCompressedClassPointers does not work well in a 64-bit architecture as can be seen here, it’s suspected there are many more examples.

Minimal Benefit

The -UseCompressedClassPointers rarely provides any tangible benefit to Java users. Any historical connection with the -UseCompresseedOops have long since been removed, and the net result of -UseCompressedClassPointers is simply increased memory overhead.

Reasons to Keep -UseCompressedClassPointers

There are currently two reasons to continue supporting -UseCompressedClassPointers:

1. -UseCompressedClassPointers works well in a 32-bit operating systems. However support for 32-bit operating systems is on its way out with JEP 479: Remove the Windows 32-bit x86 Port and JEP 501: Deprecate the 32-bit x86 Port for Removal both part of the forthcoming JDK 24 release.
2. In cases where more than 5 million classes are loaded. However such cases are rare, likely the result of programmer error, and would also mean loading likely tens of GBs of non-class data into metaspace as well.

For more on this topic see this thread in the hotspot-dev mailing list.

The engineers working on this are considering marking -UseCompressedClassPointers as deprecated for removal in JDK 25 and are looking for feedback on the impact this could have. Please direct questions or feedback to the lilliput-dev mailing list (Registration needed).

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