RFR: 8319423: Improve Year.isLeap by checking divisibility by 16 [v2]

[Claes Redestad]

On Sat, 4 Nov 2023 04:57:11 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Apply similar optimization to GregorianCalendar, sun.util.calendar
>
> src/java.base/share/classes/java/time/Year.java line 321:
> 
>> 319:         // So for a year that's divisible by 4, checking that it's also divisible by 16
>> 320:         // is sufficient to determine it must be a leap year.
>> 321:         return (year & 15) == 0 ? (year & 3) == 0 : (year & 3) == 0 && year % 100 != 0;
> 
> I think `(year & 3) == 0 && ((year & 15) == 0) || (year % 25) != 0` would be better simply because the common path will be a little bit shorter.

Benchmark                           Mode  Cnt  Score   Error   Units
LeapYearBench.isLeapYear           thrpt   15  0,735 ± 0,004  ops/us
LeapYearBench.isLeapYearChrono     thrpt   15  0,734 ± 0,006  ops/us


So equal to or even slightly worse than baseline. I tested a few variants before submitting the PR - some that looked simpler or better - but the ternary variant in this PR always came out on top.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16491#discussion_r1382422589