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

[Aleksey Shipilev]

Hi,

I'd like to start a formal review for the change:
   https://bugs.openjdk.java.net/browse/JDK-8140483

The feedback was overwhelmingly positive so far. Some suggested we use
compiler control and/or maintain the list of trusted classes. I still
think it is an overkill for the fix at hand, and we should instead wait
for the wholesale final field optimizations. Therefore, I suggest we go
with the explicit exceptions.

Now, it may take two forms:

 a) Accept all non-serializable java/util/concurrent/atomic classes:
      http://cr.openjdk.java.net/~shade/8140483/webrev.01/

 b) Spell out A*FU symbol names explicitly:
      http://cr.openjdk.java.net/~shade/8140483/webrev.02/

I am leaning towards (b), because it does not involve hacking
ciKlass::is_subtype_of, and it appears much safer from the correctness
standpoint. Both changes pass JPRT, and both changes improve A*FU
performance and the generated code quality.

Thanks,
-Aleksey

On 10/26/2015 05:14 PM, Aleksey Shipilev wrote:
> 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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