[jdk21u-dev] RFR: 8354145: G1: UseCompressedOops boundary is calculated on maximum heap region size instead of maxiumum ergonomic heap region size

[duke]

On Fri, 28 Nov 2025 18:48:13 GMT, Daniel Hu <duke at openjdk.org> wrote:

> Fixes UseCompressedOops ergonomics to use original limits for G1 GC; prior to JDK18, `-Xmx32736m` by default enables UseCompressedOops but in JDK21 `-Xmx32736m` default sets UseCompressedOops to false. New test passes with change, fails without. Change ran on internal Amazon pipeline and passes jtreg tier1-4 on platforms linux x64, aarch64, aarch32; windows x64; macos x64, aarch64. Passes GHA. Backport not clean due to [JDK-8330694](https://bugs.openjdk.org/browse/JDK-8330694) renaming `HeapRegion` to `G1HeapRegion`; modifications to the change are minimal as only had to "un-rename" the refactor.

@cost0much 
Your change (at version ba39cfa9cbbe177c68b3ae774d1afa2d59206edf) is now ready to be sponsored by a Committer.

-------------

PR Comment: https://git.openjdk.org/jdk21u-dev/pull/2497#issuecomment-3644090941