RFR: 8329706: Implement -XX:+AOTClassLinking [v6]

[Ashutosh Mehra]

On Mon, 16 Sep 2024 21:54:49 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> This is the 3rd PR for [JEP 483: Ahead-of-Time Class Loading & Linking](https://bugs.openjdk.org/browse/JDK-8315737).
>> 
>> **Overview**
>> 
>> - A new `-XX:+AOTClassLinking` flag is added. See [JEP 498](https://bugs.openjdk.org/browse/JDK-8315737) and the [CSR](https://bugs.openjdk.org/browse/JDK-8339506) for a discussion of this command-line option, its default value, and its impact on compatibility.
>> - When this flag is enabled during the creation of an AOT cache (aka CDS archive), an `AOTLinkedClassTable` is added to the cache to include all classes that are AOT-linked. For this PR, only classes for the boot/platform/application loaders are eligible. The main logic is in `aotClassLinker.cpp`.
>> - When an AOT archive is loaded in a production run, all classes in the `AOTLinkedClassTable` are loaded into their respective class loaders at the earliest opportunity. The main logic is in `aotLinkedClassBulkLoader.cpp`.
>>   - The boot classes are loaded as part of `vmClasses::resolve_all()`
>>   - The platform/application classes are loaded after the module graph is restored (see changes in `threads.cpp`).
>> - Since all classes in a `AOTLinkedClassTable` are loaded before any user-code is executed, we can resolve constant pool entries that refer to these classes during AOT cache creation. See changes in `AOTConstantPoolResolver::is_class_resolution_deterministic()`.
>> 
>> **All-or-nothing Loading**
>> 
>> - Because AOT-linked classes can refer to each other, using direct C++ pointers, all AOT-linked classes must be loaded together. Otherwise we will have dangling C++ pointers in the class metadata structures. 
>> - During a production run, we check during VM start-up for incompatible VM options that would prevent some of the AOT-linked classes from being loaded. For example:
>>   - If the VM is started with an JVMTI agent that has ClassFileLoadHook capabilities, it could replace some of the AOT-linked classes with alternative versions.
>>   - If the VM is started with certain module options, such as `--patch-module` or `--module`, some AOT-linked classes may be replaced with patched versions, or may become invisible and cannot be loaded into the JVM.
>> - When incompatible VM options are detected, the JVM will refuse to load an AOT cache that has AOT-linked classes. See `FileMapInfo::validate_aot_class_linking()`.
>>   - For simplfication, `FileMapInfo::validate_aot_class_linking()` requires `CDSConfig::is_using_full_module_graph()` to be true. This means that the exa...
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   @ashu-mehra reviews

src/hotspot/share/runtime/threads.cpp line 322:

> 320:   universe_post_module_init();
> 321: 
> 322:   if (CDSConfig::is_using_aot_linked_classes()) {

call_initPhase2 has a timer that computes cost for initializing module system. Before this patch call_initPhase2 was only initializing the module system. But now it is doing work which is not part of the module system initialization. So probably in future we may want to refactor this work out of call_initPhase2.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20843#discussion_r1763851899