RFR: 8221723: Avoid storing zero to String.hash
claes.redestad at oracle.com
Mon Apr 1 20:44:46 UTC 2019
We actually looked at this idea earlier today, and squeezing a "not-
computed" value into String might be "free" since there seems to be a
padding gap on all VM varieties (at least according to JOL estimates)
That'd be a larger endeavor, though, since there are places in VM that
calculates and injects the String.hash value.
On 2019-04-01 22:02, dean.long at oracle.com wrote:
> OK, I guess there's no ideal solution. Adding a separate "not-computed"
> boolean adds space, and using a different sentinel value for
> "not-computed" would probably be slower on CPUs that have a fast
> compare-and-branch-against-zero instruction.
> On 4/1/19 12:55 PM, Martin Buchholz wrote:
>> The spec says that "".hashCode() must be 0.
>> On Mon, Apr 1, 2019 at 12:51 PM <dean.long at oracle.com
>> <mailto:dean.long at oracle.com>> wrote:
>> Wouldn't it be better to write a non-0 value when the computed
>> hash code
>> is 0, so we don't have to recompute it? Is there some advantage to
>> writing 0 instead of any other value, such as 1?
>> On 4/1/19 4:57 AM, Claes Redestad wrote:
>> > Hi,
>> > when a String has a calculated hash code value of 0, we
>> recalculate and
>> > store a 0 to the String.hash field every time (except for the empty
>> > String, which is special cased). To make String objects more
>> amenable to
>> > storage in shared read-only memory, e.g., CDS archives, we
>> should avoid
>> > this redundant store.
>> > Bug: https://bugs.openjdk.java.net/browse/JDK-8221723
>> > Webrev: http://cr.openjdk.java.net/~redestad/8221723/
>> > Testing: tier1-3, no regression on existing and new StringHashCode
>> > micros
>> > /Claes
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