Summary

The switch UseCompressedClassPointers is to be removed.

Problem

The uncompressed Klass pointer mode is to be removed from the JVM; as a direct consequence, the UseCompressedClassPointers switch can be removed as well.

The motivation of this change is to reduce code complexity and hence maintenance burden, as well as to free up test resources by removing the need to test this mode.

For much more details, please see the associated deprecation CSR ([1]), including its discussion trail, as well as preparatory discussions we had at the start of 2025 [2].

Solution

The switch UseCompressedClassPointers is to be removed.

Code that handles UseCompressedClassPointers=false, or is indirectly dependent on it, is to be revised and, if possible, removed.

Risk

For risks, please see the deprecation CSR [1]. The risks are the same, except for 32-bit. The 32-bit issue mentioned in the deprecation CSR has been solved in preparation for this obsolescence with JDK-8363998 [3].

Specification

This is a large change, and still work in progress since test results are outstanding: [4]

[1] Deprecation CSR: https://bugs.openjdk.org/browse/JDK-8350754

[2] ML discussion: https://mail.openjdk.org/pipermail/hotspot-dev/2025-February/101023.html

[3] 32-bit solution: https://bugs.openjdk.org/browse/JDK-8363998

[4] Patch: https://github.com/openjdk/jdk/pull/28366