RFR: 8329706: Implement -XX:+AOTClassLinking

[David Holmes]

On Tue, 3 Sep 2024 20:57:48 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 exact same set of modules are visible whe...

src/hotspot/share/cds/aotClassLinker.hpp line 101:

> 99:   // When CDS is enabled, is ik guatanteed to be linked at deployment time (and
> 100:   // cannot be replaced by JVMTI, etc)?
> 101:   // This is a necessary (not but sufficient) condition for keeping a direct pointer

Suggestion:

  // This is a necessary (but not sufficient) condition for keeping a direct pointer

src/hotspot/share/cds/aotClassLinker.hpp line 106:

> 104:   static bool is_candidate(InstanceKlass* ik);
> 105: 
> 106:   // Request that ik to be added to the candidates table. This will return succeed only if

Suggestion:

  // Request that ik be added to the candidates table. This will return true only if

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20843#discussion_r1746508873
PR Review Comment: https://git.openjdk.org/jdk/pull/20843#discussion_r1746510269