Comment by readittwice | Hacker News Reader

Comment by readittwice | Hacker News Reader

Parent Full thread

Why is the JIT not allowed to do this? (Assuming publishing isn't volatile) The reordering could also be caused by the CPU, not only by the JIT. If the JIT isn't allowed to reorder, we also need to prevent the CPU from reordering. Otherwise it doesn't make sense to restrict the JIT from reordering if the CPU could still reorder later. We would need to emit memory barriers for non-volatile loads & stores to prevent reordering on architectures like ARM. This sounds quite expensive.

This doesn't make sense to me. Am I thinking wrong? Can you point me to documentation/reference that states that the JIT isn't allowed to do this?

See section 17.5, "final Field Semantics" in http://docs.oracle.com/javase/specs/jls/se8/jls8.pdf.

Note that a String's value field is final.

Exactly. Relevant passage from ^

    An object is considered to be completely initialized when its constructor finishes. 
    A thread that can only see a reference to an object after that object has been completely 
    initialized is guaranteed to see the correctly initialized values for that object's final fields.

Thanks, that explains it! Didn't know that final fields have this special rule and that the value field in the String class is final.

So to recap: this shouldn't be possible for Strings, since the value field in the String class is final and therefore has a memory barrier. But it could happen for other classes with non-final fields.

Yes, Java requires memory barriers for final fields. Yes, it is expensive on ARM, but if you don't do it you are not implementing Java.

Reply on news.ycombinator.com