RFR: 8273000: Remove WeakReference-based class initialisation barrier implementation

[Paul Sandoz]

On Wed, 25 Aug 2021 22:05:24 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Get rid of WeakReference-based logic in DirectMethodHandle::checkInitialized() and reimplement it with `Unsafe::ensureClassInitialized()`/`shouldBeInitialized()`. 
> 
> The key observation is that `Unsafe::ensureClassInitialized()` does not block the initializing thread. 
> 
> Also, removed `Unsafe::shouldBeInitialized()` in `DMH::shouldBeInitialized(MemberName)` to save on calling into the VM.
> `Unsafe::ensureClassInitialized()` already has a fast-path check which checks whether the class is fully initialized or not.
> 
> Testing: tier1 - tier6

That's a nice cleanup to a tricky area (one of the few used to trigger an update a final field). In effect we were already relying on that behavior in the `ClassValue` computation. 

May i suggest that we add some JavaDoc to `ensureClassInitialized` describing the two cases of the calling thread is the initialization thread or not.

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Marked as reviewed by psandoz (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5258