RFR: 8221836: Avoid recalculating String.hash when zero

[Ivan Gerasimov]

Hi Claes!

Would it make sense to preset hashIsZero = true in the empty string 
constructor?

The current code avoids calculating the hashCode for an empty string, 
and the new code doesn't seem to do that because hashIsZero = false by 
default for each newly constructed copy of the empty string.

With kind regards,
Ivan

On 4/8/19 1:41 AM, Claes Redestad wrote:
> Hi,
>
> by adding a bit to String that is true iff String.hash has been 
> calculated as being 0, we can get rid of the corner case where such hash
> codes are recalculated on every call.
>
> Peter Levart came up with a elegant scheme for ensuring that we can keep
> using non-volatile stores without explicit fencing and still reap the
> benefits of this[1], and I've synced up the hotspot code that deals with
> the String.hash value to mirror that logic.
>
> Bug:    https://bugs.openjdk.java.net/browse/JDK-8221836
> Webrev: http://cr.openjdk.java.net/~redestad/8221836/open.01/
>
> Since there exists small padding gaps in the current object layout of
> strings (on all VM bitness and compressed oops varieties), adding this
> boolean does not add any extra footprint per String instance.
>
> Testing: tier1-3, verified a speed-up in targeted microbenchmarks.
>
> Thanks!
>
> /Claes
>
> [1] 
> http://mail.openjdk.java.net/pipermail/core-libs-dev/2019-April/059480.html
>

-- 
With kind regards,
Ivan Gerasimov