resending as I just realized I replied only to Brian. On Wed, Feb 23, 2011 at 12:19 AM, Vitaly Davidovich <vita...@gmail.com>wrote:
> Hi David/Brian, > > Yes, I meant whether the "entire" string.hashcode technique can be used, > including the default zero values. I agree on the initial -1 assignment > possibly not being visible across cores, leading to incorrect results. > > David, I did not see the actual bug report -- is there a link to it? > > Cheers, > Vitaly > > On Tue, Feb 22, 2011 at 11:53 PM, Brian Goetz <brian.go...@oracle.com>wrote: > >> 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 >>> >> > > > -- > Vitaly > 617-548-7007 (mobile) > -- Vitaly 617-548-7007 (mobile)
