RFR: 8273000: Remove WeakReference-based class initialisation barrier implementation [v2]

[Paul Sandoz]

On Wed, 1 Sep 2021 16:16:38 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:
> 
>   Improve comments

Looks good, just a minor suggestion up to you to accept or not.

src/java.base/share/classes/jdk/internal/misc/Unsafe.java line 1152:

> 1150:      * The call returns when either class {@code c} is fully initialized or
> 1151:      * class {@code c} is being initialized and the call is performed from
> 1152:      * the initializing thread.

Suggestion:

     * The call returns when either class {@code c} is fully initialized or
     * class {@code c} is being initialized and the call is performed from
     * the initializing thread. In the latter case a subsequent call to
     * {@link #shouldBeInitialized}, from the calling thread of this call,
     * will return {@code true}.

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Marked as reviewed by psandoz (Reviewer).

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