RFR: 8319423: Improve Year.isLeap by checking divisibility by 16

[Andriy Plokhotnyuk]

On Fri, 3 Nov 2023 23:34:49 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> > @cl4es Could you please test [this](https://github.com/plokhotnyuk/jsoniter-scala/blob/ba6f4f80deaf0e7a1a95787321de3cc2a232d66d/jsoniter-scala-core/jvm/src/main/scala/com/github/plokhotnyuk/jsoniter_scala/core/JsonReader.scala#L3687) function too? It seems that when testing divisibility by 100 we can use just one multiplication operation.
> 
> For int values it seems that makes it a few percent faster (1,061 ± 0,017 ops/us), though we need a variant that works for longs (`GregorianCalendar` could use this as-is, but not `Year`). Do you have a reference to how they arrived at these numbers? It might be straightforward to extend it to long values, and it'd be good to have the theory to reference either way.

Thanks for trying and giving the honest feedback!

I don't recall exactly but it seems was a kind of playing with `bool f(int n) { return n % 100 == 0; }` on the https://godbolt.org to see assembly generated by latest c++ compilers with `-O3` option and then brute force squeezing a redundant shift operation.

I'm not sure that similar is possible for `long` values, so probably it worth to have 2 different methods for `long` and `int` types. I'm even not sure if that function would work for negative `int` values.

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PR Comment: https://git.openjdk.org/jdk/pull/16491#issuecomment-1793267652