RFR: 8293187: Store initialized Enum classes in AOTCache [v9]

[Coleen Phillimore]

On Thu, 19 Sep 2024 02:41:18 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> This is the 4th PR for [JEP 483: Ahead-of-Time Class Loading & Linking](https://bugs.openjdk.org/browse/JDK-8315737).
>> 
>> **Problem:**
>> 
>> This PR is necessary to support [JDK-8293336: AOT-linking of invokedynamic for lambda expression and string concat](https://bugs.openjdk.org/browse/JDK-8293336), which needs to store [`sun.invoke.util.Wrapper`](https://github.com/openjdk/jdk/blob/c3711dc90980fb3e63ff199612c201c4464626bf/src/java.base/share/classes/sun/invoke/util/Wrapper.java) enums in the AOT cache. Although CDS has some limited support for storing enums, the `Wrapper` type is too complex for the existing solution to handle. Please see the JBS issue for details.
>> 
>> **Solution:**
>> 
>> In the assembly phase, we store the initialized states of the `Wrapper` class (captured in a `java.lang.Class` object, a.k.a. the *mirror* of this class) into the AOT cache. 
>> 
>> In production run, we no longer execute `Wrapper::<clinit>`, because all the static fields (contained in its mirror) are already initialized.
>> 
>> **Review Notes:**
>> 
>> - The new capability is controlled by `CDSConfig::is_initing_classes_at_dump_time()`. We can aot-initialize classes only if `-XX:+AOTClassLinking`  is enabled.
>>     - The old (more limited) support for enums is still there (it's required when `AOTClassLinking` is disabled). See the call to `CDSEnumKlass::handle_enum_obj()` in heapShared.cpp. 
>> - `AOTClassInitializer::can_archive_initialized_mirror()` decides what classes can be aot-initialized. This is currently a very small set of classes, but will expand in [JDK-8293336](https://bugs.openjdk.org/browse/JDK-8293336)
>> - Before, `HeapShared::archive_java_mirrors()` would clear out all the states in the archived mirrors. With this PR, the states of aot-initialized classes are preserved via `HeapShared::copy_aot_initialized_mirror()`.
>> - During the early state of the production run, `AOTLinkedClassBulkLoader::init_required_classes_for_loader()` is called to make sure that:
>>     - all aot-initialized classes are moved into the `initialized` state (without executing its `<clinit>` method). This is done in `InstanceKlass::initialize_from_cds()`
>>     - the classes of all the objects that are reachable from the aot-initialized mirrors are initialized. See comments above ` HeapShared::init_classes_reachable_from_archived_mirrors()` 
>> 
>> **Caveats:**
>> 
>> Not all Enum classes can be stored in the initialized state. E.g., some Enums might have static fields that depend on the e...
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixed typo

I only have one drive-by comment.

src/hotspot/share/cds/aotClassInitializer.cpp line 41:

> 39:   } else if (ik->name()->equals("jdk/internal/constant/PrimitiveClassDescImpl") ||
> 40:              ik->name()->equals("jdk/internal/constant/ReferenceClassDescImpl") ||
> 41:              ik->name()->equals("java/lang/constant/ConstantDescs")) {

Why not intern these strings as Symbols so you can test for == ?

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PR Review: https://git.openjdk.org/jdk/pull/20958#pullrequestreview-2316432031
PR Review Comment: https://git.openjdk.org/jdk/pull/20958#discussion_r1767402253