RFR: 8251397: Add release fence to ClassValueMap constructor

[Galder Zamarreño]

On Wed, 9 Sep 2020 15:13:29 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> * Release fence guarantees that cacheArray field will published with a non-null value.
>> * Without this fix, CacheValueMap.cacheArray can sometimes be seen as null.
>> 
>> This is a follow up to @PaulSandoz's feedback
>> [here](https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-August/068248.html) for the first attempt to fix
>> [JDK-8251397](https://bugs.openjdk.java.net/browse/JDK-8251397).  In this update, the fence has been moved to
>> `initializeMap()` and added Paul's suggested comment.
>> Annotating `classValueMap` with `@Stable` is outside the scope of this issue.
>
> src/java.base/share/classes/java/lang/ClassValue.java line 381:
> 
>> 379:             if ((map = type.classValueMap) == null) {
>> 380:                 map = new ClassValueMap();
>> 381:                 // Place a Store fence as the last operation to emulate
> 
> Since the store was moved i suggest the comment be slightly updated:
> "Place a Store fence after construction and before publishing to emulate"

Done.

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PR: https://git.openjdk.java.net/jdk/pull/94