RFR: 8368174: Proactive initialization of @AOTSafeClassInitializer classes

[Andrew Dinn]

On Sat, 20 Sep 2025 02:57:50 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> This is an alternative to https://github.com/openjdk/jdk/pull/27024. Thanks to @ashu-mehra for the suggestion.
> 
> ### Background:
> 
> The AOT Assembly Phase is in essence a small Java program that executes a limited set of Java bytecodes. This program bootstraps the module system, loads classes, and performs certain ahead-of-time optimizations such as resolving `invokedynamic` call sites.
> 
> As a side effect of Java program execution, a small set of Java classes are initialized in the Assembly Phase.
> 
> Since [JDK-8360163](https://bugs.openjdk.org/browse/JDK-8360163), if a class `X` is annotated with `@AOTSafeClassInitializer` *and* is initialized in the Assembly Phase, then `X` will be stored in the AOT cache in the "initialized" state. When the AOT cache is used in the Production Run, `X::<clinit>` will not be executed, and the static variables of `X` will be available upon JVM bootstrap.
> 
> ### Problem:
> 
> The Assembly Phase doesn't touch many classes that may benefit from `@AOTSafeClassInitializer`. For example, `jdk.internal.math.MathUtils::<clinit>` creates a few large tables. Caching `MathUtils` in the "initialized" state will improve start-up time. However, since no bytecodes executed by the Assembly Phase use `MathUtils`. it will not be initialized.
> 
> ### Fix:
> 
> If a class `X` has the `@AOTSafeClassInitializer` annotation *and* was initialized in the AOT Training Run, the JVM will proactively initialize `X` in the Assembly Phase. This will ensure that `X` will be cached in the "initialized" state.
> 
> As a proof of concept, `@AOTSafeClassInitializer` is added to `MathUtils`. `@AOTSafeClassInitializer` will be added to more classes in future RFEs.

src/hotspot/share/cds/aotClassInitializer.cpp line 294:

> 292:     });
> 293:   } else {
> 294:     // @AOTRuntimeSetup only meaningful in @AOTClassInitializer

Suggestion:

    // @AOTRuntimeSetup only meaningful in @AOTSafeClassInitializer

src/hotspot/share/cds/aotMetaspace.cpp line 815:

> 813:     FinalImageRecipes::apply_recipes(CHECK);
> 814: 
> 815:     // Because the AOT assembly phase does not run the exact code as in the

Suggestion:

    // Because the AOT assembly phase does not run the exact same code as in the

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PR Review Comment: https://git.openjdk.org/jdk/pull/27402#discussion_r2367248414
PR Review Comment: https://git.openjdk.org/jdk/pull/27402#discussion_r2367251255