RFR: 8315999: Improve Date toString performance [v12]

[温绍锦]

On Tue, 12 Sep 2023 23:10:43 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>>> Have you considered potentially more generalizable optimizations to `DateTimeFormatter.ISO_INSTANT.format(this)` here?
>>> 
>>> Hand-rolling a fixed-length buffer, skipping the `StringBuilder` .. understandably this can have a performance edge, but perhaps a `DateTimeFormatter` like `ISO_INSTANT` can be optimized to get closer to whatever speed-up this gets you - with broader implications.
>> 
>> I submitted an optimization for the commonly used format of DateTimeFormatter::format
>
> Do you have a link to that PR? Is there an RFE filed for it?

Optimization of DateTimeFormatter::format should be another PR, I created a [branche](https://github.com/wenshao/jdk/tree/optim_for_datetime_format) but the work is unfinished.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15658#discussion_r1323870413