On 4/1/19 10:44 PM, Claes Redestad wrote:
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[1])
That'd be a larger endeavor, though, since there are places in VM that
calculates and injects the String.hash value.
In addition, this might not be easy from another point of view. You all
know that there's no synchronization performed when caching the hash
value. This works, because 32 bits are always read or written
atomically. If there was another byte to be read or written together
with 32 bit value, it would be tricky to get the ordering right and
still keep the performance.
Regards, Peter
/Claes
[1] https://openjdk.java.net/projects/code-tools/jol/
On 2019-04-01 22:02, dean.l...@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.
dl
On 4/1/19 12:55 PM, Martin Buchholz wrote:
The spec says that "".hashCode() must be 0.
https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#hashCode()
On Mon, Apr 1, 2019 at 12:51 PM <dean.l...@oracle.com
<mailto:dean.l...@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?
dl
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
>
