RFR: 8366488: JVM_FindClassFromClass should assert that from class is never null

[Coleen Phillimore]

On Thu, 23 Oct 2025 15:55:41 GMT, Paul Hübner <phubner at openjdk.org> wrote:

> Hi all,
> 
> The `from_class` nullcheck  in `JVM_FindClassFromClass` is redundant, as the this is the Java mirror which cannot be null during class linking/verification [1]. Therefore, it has been refactored to be an assertion instead.
> 
> Testing: tiers 1-4 Linux (x64, AArch64), macOS (x64, AArch64), Windows (x64). As part of a separate experiment, I've also run tiers 1-6 on x64 Linux with a non-null assertion before we call `Verifier::inference_verify` (which in turn, eventually, calls this code).
> 
> [1] For reasoning, see https://bugs.openjdk.org/browse/JDK-8366488

Changes requested by coleenp (Reviewer).

src/hotspot/share/prims/jvm.cpp line 830:

> 828:   Klass* from_class = (from_class_oop == nullptr)
> 829:                            ? (Klass*)nullptr
> 830:                            : java_lang_Class::as_Klass(from_class_oop);

Don't you want to get rid of this ?: also?  Assert that from_class_oop != nullptr instead?

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PR Review: https://git.openjdk.org/jdk/pull/27957#pullrequestreview-3383587350
PR Review Comment: https://git.openjdk.org/jdk/pull/27957#discussion_r2465738825