Review CR #6611830: UUID thread-safety and performance improvements

[Brian Goetz]

I think you have a potential visibility problem here.  You use -1 as the 
initial value, but observing threads might see instead the default value 
if the initializing write is not visible, and mistakenly think that the 
zero default value represents a computed value.  (This is different from 
the trick employed by String.hashCode(), which does not use an 
initializer.  Can you get the same result using zero instead?)

The key to making the String.hashCode() trick work is that, while there 
is definitely a data race, it is benign because the field can only ever 
take on one non-default value, and that value is computed 
deterministically from immutable state.

On 2/22/2011 7:29 PM, Mike Duigou wrote:
> Daniel Aioanei reported via Josh Bloch a data race issue with the UUID accessor and hashCode() methods. I've prepared a webrev with the suggested changes:
>
> http://cr.openjdk.java.net/~mduigou/6611830/webrev.0/webrev/
>
> I've tested the change against the standard UUID tests and did a microbenchmark test of one method, variant(), to see what impact not caching had on performance. Since there was only negligible change in performance vs. the existing UUID implementation I am comfortable with eliminating the cache values. It would appear that a field access plus a shift is not a significant cost over a field access.
>
> Mike