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

[Claes Redestad]

On Mon, 6 Nov 2023 02:34:21 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

> > 
> 
> I have filed [JDK-8319451](https://bugs.openjdk.org/projects/JDK/issues/JDK-8319451). I would suggest waiting for this bug to be resolved before proceeding with this PR.

Nice analysis! 

While I'm sure we need to re-evaluate this enhancement after JDK-8319451 is resolved, I'm not a fan of blocking library enhancements on improvements to the runtime/compiler (as it's impossible to know up front if this is something we can fix in the next couple of days/weeks, or need to staff, plan and evaluate over a longer cycle). I included the Neri-Schneider variant in the microbenchmark here to make it easy to assess if an optimization such as JDK-8319451 would turn things around.

We should file an enhancement to re-visit the Granlund & Montgomery/Hacker's delight division. Improving this should benefit either variant, and might be needed together with JDK-8319451 for the `isLeapYearNS` test to win.

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PR Comment: https://git.openjdk.org/jdk/pull/16491#issuecomment-1794464999