RFR: 8283681: Improve ZonedDateTime offset handling

[Roger Riggs]

On Fri, 25 Mar 2022 12:28:58 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> Richard Startin prompted me to have a look at a case where java.time underperforms relative to joda time (https://twitter.com/richardstartin/status/1506975932271190017). 
> 
> It seems the java.time test of his suffer from heavy allocations due ZoneOffset::getRules allocating a new ZoneRules object every time and escape analysis failing to do the thing in his test. The patch here adds a simple specialization so that when creating ZonedDateTimes using a ZoneOffset we don't query the rules at all. This removes the risk of extra allocations and slightly speeds up ZonedDateTime creation for both ZoneOffset (+14%) and ZoneRegion (+5%) even when EA works like it should (the case in the here provided microbenchmark).

src/java.base/share/classes/java/time/ZoneRegion.java line 183:

> 181:     @Override
> 182:     /* package-private */ ZoneOffset getOffset(long epochSecond) {
> 183:         return getRules().getOffset(Instant.ofEpochSecond(epochSecond));

The nanoAdjustment passed to `ofEpochSecond` is discarded in this code.
It may be larger than a second;  see `Instant.ofEpochSecond(sec, nano)` for the details.
Adding a second parameter to the `getOffset` method could be the remedy.

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