RFR: 8369742: Link AOT-linked classes at JVM bootstrap [v4]

[Ashutosh Mehra]

On Thu, 16 Oct 2025 17:04:48 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> **PROBLEM**
>> 
>> If we have an AOT-initialized class like this in java.base
>> 
>> 
>> @AOTSafeClassInitializer
>> class Foo {
>>     static Bar b = new Bar(); // AOT-cached
>>     static void doit() {
>>         b.toString(); /// invokevirtual Bar::toString()Ljava/lang/String;
>>     }
>> }
>> 
>> 
>> If `Foo.doit()` is called before `AOTLinkedClassBulkLoader::link_or_init_javabase_classe()`, it's possible for the `Bar` class to be not yet linked. The `invokevirtual` bytecode will crash because the vtable of the object `b` is not yet initialized.
>> 
>> **FIX**
>> 
>> Before we execute the first Java bytecode, unconditionally link all AOT-linked classes. This will ensure that the `Bar` class will have an initialized vtable for the above example.
>> 
>> **NOTES**
>> 
>> - The scenario in the  above example does not affect JDK 24, 25 or the current JDK mainline (26). We are lucky that in all the bytecode execution up to `AOTLinkedClassBulkLoader::link_or_init_javabase_classe()`, whenever we do a vtable/itable dispatch on an AOT-cached heap object, we happen to have already linked its class (either by explicit calls from the JVM, or as side effect of invokestatic/getstatic/putstatic/new). However, this is a potential problem that should be fixed. I have run into this problem when implementing [JDK-8368199](https://bugs.openjdk.org/browse/JDK-8368199), which changes the bytecodes that are executed in early VM bootstrap.
>> - I had to enable `CDSConfig::is_preserving_verification_constraints()` to for the static CDS archive. The reason is to avoid class loading. Please see the new comments in `AOTLinkedClassBulkLoader::link_classes_in_table()`.
>> - The change in `AdapterHandlerLibrary::create_native_wrapper` is for supporting JVMTI. The offending methods are `jdk.internal.vm.Continuation::enterSpecial` and `jdk.internal.vm.Continuation::doYield`. These are "continuation native intrinsic" methods that are usually linked much later after the ServiceThread have been started.
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixed minimal build

Marked as reviewed by asmehra (Committer).

lgtm

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

PR Review: https://git.openjdk.org/jdk/pull/27783#pullrequestreview-3350684736
PR Comment: https://git.openjdk.org/jdk/pull/27783#issuecomment-3415892447