RFC (S) 8140483: Atomic*FieldUpdaters final fields should be trusted

[Aleksey Shipilev]

Hi,

I would like to gauge interest in doing a simple VM change that
significantly improves the performance of Atomic*FieldUpdaters:
 https://bugs.openjdk.java.net/browse/JDK-8140483

In short, we are looking into folding away the checks in A*FU:

  private final Class<T> tclass;
  private final Class<?> cclass;

  public final int get(T obj) {
    if (obj == null || obj.getClass() != tclass || cclass != null)
      fullCheck(obj);
    return unsafe.getIntVolatile(obj, offset);
  }

Since most use cases for A*FU involve putting them into static final
fields, the Updater instances are known constants. Now, the internal
instance final fields are not trusted by default. This can be "fixed"
with -XX:+TrustNonStaticFinalFields -- the bad thing about this option
is that it's global, and it may break user code. Vladimir I. has a
potential improvement that handles this by tracking the final field
writes, but there is no clear ETA for that feature.

With this little VM improvement, we can close the gap for A*FU, and
eliminate another reason for using Unsafe today:
 http://cr.openjdk.java.net/~shade/8140483/webrev.00/

It improves the generated code and performance significantly:
 http://cr.openjdk.java.net/~shade/8140483/notes.txt

Thanks,
-Aleksey

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