RFR: 8354145: G1GC: keep the CompressedOops same as before when not setting HeapRegionSize explicitly

[Thomas Schatzl]

On Wed, 9 Apr 2025 10:37:24 GMT, Tongbao Zhang <duke at openjdk.org> wrote:

> After [JDK-8275056](https://bugs.openjdk.org/browse/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 this patch:
> 
> ./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)

Would it be possible to add a regression test that checks the value of the `UseCompressedOops` flag after running a VM with these settings?

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PR Review: https://git.openjdk.org/jdk/pull/24541#pullrequestreview-2753132517