RFR: 8283681: Improve ZonedDateTime offset handling [v2]

[Claes Redestad]

On Fri, 25 Mar 2022 15:17:02 GMT, Stephen Colebourne <scolebourne at openjdk.org> wrote:

>> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add nanoOfSecond parameter, make micro less reliant on constants
>
> src/java.base/share/classes/java/time/ZoneOffset.java line 512:
> 
>> 510:     @Override
>> 511:     /* package-private */ ZoneOffset getOffset(long epochSecond, int nanoOfSecond) {
>> 512:         return this;
> 
> An alternate approach would be for `ZoneOffset` to cache the instance of `ZoneRules` either on construction or first use (racy idiom would be OK). That way this issue is solved for the many different places that call `zoneId.getRules()`.

Why not both? I measured an improvement using that alone, but specifically avoiding going via getRules is faster still (without adversely affecting `ZoneRegion` paths). I've added the cache in `ZoneOffset`, along with an override of `ZoneId::normalized` in `ZoneOffset` to shortcut the `getRules`.

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PR: https://git.openjdk.java.net/jdk/pull/7957