AccessMode.CONCURRENT should use a static final VarHandle to access its array

[franz1981]

vert.x/src/main/java/io/vertx/core/spi/context/storage/AccessMode.java

Line 27 in d21c6b5

private final VarHandle LOCALS_UPDATER = MethodHandles.arrayElementVarHandle(Object[].class);

in theory is used on the CONCURRENT instance which is stored in a static final-like (in term of semantic) field within AccessMode, but still IDK if the JIT trust transitively the final fields within the CONCURRENT class, so, this can cause a severe performance penalty while using its VarHandle.

It means it would be better to have a JMH benchmark to prove that everything it's fine there OR at just move VarHandle LOCALS_UPDATER into some proper static final declaration. See https://shipilev.net/jvm/anatomy-quarks/17-trust-nonstatic-final-fields/#_observations which is indeed related VarHandle.