RFR: 8298469: Obsolete legacy parallel class loading workaround for non-parallel-capable class loaders [v2]

[Coleen Phillimore]

On Tue, 14 Mar 2023 13:54:20 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Okay. I confess I don't fully grok this part though.
>
> The bootloader uses LOAD_INSTANCE to wait for other threads (JVM implementation of parallel capable class loading), but this function doesn't pass the class_loader so that would be the only useful modification to this assert.  But it seems too specific to actually catch any bugs.
> Before this change with EnableWaitForParallelLoad, the non-parallel-capable class loader would wait on the LOAD_INSTANCE placeholder with double_lock_wait so there would never be more than one.  Now, both threads take out the placeholder, and are expected to wait in the ClassLoader.loadClass function.

If -Xcomp stopped calling load_signature_classes, we could remove the LOAD_INSTANCE placeholder for non-parallel capable class loaders.  In the future.

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PR: https://git.openjdk.org/jdk/pull/12974