Bug ID: JDK-8354145 G1: UseCompressedOops boundary is calculated on maximum heap region size instead of maxiumum ergonomic heap region size

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JDK 25
25 b21Fixed

on behalf of tobytbzhang@tencent.com

After JDK-8275056, The max heap region size became 512M, and the calculation of CompressedOops based on the max_heap_size - max_heap_region_size.
So before this patch, the CompressedOops will turn on below 32G - 32m, After this patch is 32G -512m.

When our Apps migrating from JDK11 to JDK21, the heap size parameters(Xmx32736m) will turn off the CompressedOops.

Since the current max ergonomics size is still 32m, We hoped that the original behavior will not be changed if HeapRegionSize is not explicitly set.

before this patch:
./build/linux-x86_64-server-release/images/jdk/bin/java -Xmx32736m -XX:+PrintFlagsFinal -version | grep CompressedOops
     bool UseCompressedOops                        = false                          {product lp64_product} {default}
openjdk version "25-internal" 2025-09-16
OpenJDK Runtime Environment (build 25-internal-adhoc.root.jdk)
OpenJDK 64-Bit Server VM (build 25-internal-adhoc.root.jdk, mixed mode, sharing)

after fix:
./build/linux-x86_64-server-release/images/jdk/bin/java -Xmx32736m -XX:+PrintFlagsFinal -version | grep CompressedOops
     bool UseCompressedOops                        = true                           {product lp64_product} {ergonomic}
openjdk version "25-internal" 2025-09-16
OpenJDK Runtime Environment (build 25-internal-adhoc.root.jdk)
OpenJDK 64-Bit Server VM (build 25-internal-adhoc.root.jdk, mixed mode, sharing)

Changeset: 526951db Branch: master Author: Tongbao Zhang <tobytbzhang@tencent.com> Committer: Albert Mingkun Yang <ayang@openjdk.org> Date: 2025-04-30 09:57:23 +0000 URL: https://git.openjdk.org/jdk/commit/526951dba731f0e733e22a3bff7ac7a18ce9dece

30-04-2025

A pull request was submitted for review. Branch: master URL: https://git.openjdk.org/jdk/pull/24541 Date: 2025-04-09 10:37:24 +0000

15-04-2025