RFR: 8273000: Remove WeakReference-based class initialisation barrier implementation [v3]

[Vladimir Ivanov]

On Thu, 2 Sep 2021 11:45:01 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
>
> Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update the comment

Thanks for the reviews, Mandy, Paul, and David.

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