RFR: 8283681: Improve ZonedDateTime offset handling

[Richard Startin]

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).

test/micro/org/openjdk/bench/java/time/GetYearBench.java line 70:

> 68:     private static final long[] INSTANT_MILLIS = createInstants();
> 69: 
> 70:     private static final int[] YEARS = new int[INSTANT_MILLIS.length];

Does it make any difference if these aren't constant?

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