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...

I've taken an initial look through but there is an awful lot to try and digest here. I've flagged numerous typos and minor nits.

One general query: does this stuff work if the user defines their own initial application classloader?

test/hotspot/jtreg/runtime/cds/appcds/aotClassLinking/AOTClassLinkingVMOptions.java line 2:

> 1: /*
> 2:  * Copyright (c) 2023, 2024, Oracle and/or its affiliates. All rights reserved.

Suggestion:

 * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.

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PR Review: https://git.openjdk.org/jdk/pull/20843#pullrequestreview-2284828158
PR Review Comment: https://git.openjdk.org/jdk/pull/20843#discussion_r1746525404