RFR: 8267943: Improve performance of EventWriter by improving varint encoding implementation [v2]

[Erik Gahlin]

On Fri, 4 Jun 2021 11:56:16 GMT, Richard Startin <github.com+16439049+richardstartin at openjdk.org> wrote:

>> This PR improves the performance of committing events with numeric correlation ids by precomputing the number of continuation bytes required by each varint and storing them in a lookup table addressable by the input `long`'s leading zero count. In some rough [benchmarks](https://github.com/richardstartin/jfr-events/blob/master/src/main/java/io/github/richardstartin/jfrevents/CommitEventBenchmark.java), this shows a negligible regression for committing empty events, but a 15% improvement for committing an event with two random 64 bit identifiers.
>
> Richard Startin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   shortcuts for small values

Data is not random, it is typically small integers, in particular 0, and almost never negative. 

Do you have a benchmark on realistic numbers?

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PR: https://git.openjdk.java.net/jdk/pull/4251