RFR: 8317269: Store old classes in linked state in AOT cache [v4]

[Coleen Phillimore]

On Thu, 28 Aug 2025 05:03:54 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> During the assembly phase of the AOT cache, we link and verify all classes that were loaded during the training run. When verifying a class like this:
>> 
>> 
>> class X {
>>     A getA() { return new B(); } // Verifier requires B to be a subtype of A.
>> }
>> 
>> 
>> We remember `A` and `B` as the "verification dependencies" of `X`. A class will be excluded from the AOT cache if any of its verification dependencies are excluded. For example, `X` will be excluded if
>> 
>> - `A` fails verification
>> - `B` is a signed class, which is always excluded from the AOT cache
>> 
>> Conversely, if both `A` and `B`  are included in the AOT cache, in the production run, they will be unconditionally loaded during VM bootstrap. Therefore, we can guarantee that the verification result computed for `X` will remain valid during the production run.
>> 
>> Notes for reviewers:
>> 
>> - The checks for verification dependencies are done inside `SystemDictionaryShared::check_exclusion_for_self_and_dependencies()`. These checks are done for both old and new classes. Since the dependencies can form a cyclic graph, the checks cannot be implemented with a simple recursion. See "Algorithm notes" in this function for details. 
>> - The verification dependencies for "new" classes are already stored in `DumpTimeClassInfo::_verifier_constraints`.
>> - This PR adds code to record the verification dependencies for "old" classes into `DumpTimeClassInfo::_old_verifier_dependencies`, by intercepting `JVM_FindClassFromCaller()`.
>> - This new functionality (store old classes in linked state) is available only when using the AOT cache. For simplicity, this functionality is not available with the old CDS workflows. See comments in `CDSConfig::is_preserving_verification_dependencies()`.
>
> Ioi Lam has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'master' into 8317269-store-old-classes-in-linked-state-in-aot-cache
>  - @coleenp comments
>  - More bug fixes from JCK testing
>  - Fixed bugs found in JCK testing
>  - 8317269: Store old classes in linked state in AOT cache

src/hotspot/share/cds/runTimeClassInfo.hpp line 204:

> 202:   u4* old_verifier_dependencies() {
> 203:     assert(_num_old_verifier_dependencies > 0, "sanity");
> 204:     return (u4*)(address(this) + old_verifier_dependencies_offset());

This seems like it should have a checked_cast<> around it rather than a plain cast.

src/hotspot/share/prims/jvm.cpp line 854:

> 852: 
> 853: #if INCLUDE_CDS
> 854:   if (CDSConfig::is_preserving_verification_dependencies() && from_class->is_instance_klass()) {

It looks like from_class can be null so you have to check for null here.
Which doesn't make sense because the logging doesn't have null checks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26754#discussion_r2310882759
PR Review Comment: https://git.openjdk.org/jdk/pull/26754#discussion_r2311133213