RFR: 8315789: Minor HexFormat performance improvements

[温绍锦]

On Fri, 8 Sep 2023 08:47:04 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

> I'm not sure that micro-benchmarks are very indicative on whether a lookup table performs better than short and straightforward code.
> The reason is that, once in the CPU caches, a lookup table in micro-benchmarks stays there, whereas in more realistic situations, where access is more spread out in time, it is often evicted to make room for other, more lively data.
> A micro-benchmark using a lookup table shows good results because of a high rate of cache hits, whereas in other real-world workloads a lookup table might result in many cache misses on access.

In the HexFormat scenario, if the length of the input byte[] is larger, the performance of using the lookup table will be better.

If the length of byte[] is greater than 1 in most scenarios, using lookup table will have better performance.

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PR Comment: https://git.openjdk.org/jdk/pull/15591#issuecomment-1712164869