RFR: 8300818: Reduce complexity of padding with DateTimeFormatter [v7]

[Roger Riggs]

On Fri, 21 Apr 2023 17:45:50 GMT, Sergey Tsypanov <stsypanov at openjdk.org> wrote:

>> Currently it's O(n) - we do `n` shifts of bytes within `StringBuilder`. This can be reduced to O(1) improving the code like:
>> 
>> DateTimeFormatter dtf = new DateTimeFormatterBuilder()
>>   .appendLiteral("Date:")
>>   .padNext(20, ' ')
>>   .append(DateTimeFormatter.ISO_DATE)
>>   .toFormatter();
>> String text = dtf.format(LocalDateTime.now());
>
> Sergey Tsypanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8300818: One more benchmark

Marked as reviewed by rriggs (Reviewer).

I added a few more small padding cases and and re-ran on my Mac:

Baseline:

Benchmark                                                    Mode  Cnt      Score     Error   Units
DateTimeFormatterWithPaddingBench.formatWithPadding         thrpt   15   5167.611 ±  85.096  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength0  thrpt   15  22004.618 ± 536.207  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength1  thrpt   15  18041.569 ± 142.627  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength2  thrpt   15  15966.853 ± 182.284  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength3  thrpt   15  13472.887 ± 181.794  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength4  thrpt   15  12991.458 ± 402.190  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength5  thrpt   15  11464.481 ± 603.970  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength6  thrpt   15  10386.347 ± 673.596  ops/ms


Patch:

Benchmark                                                    Mode  Cnt      Score     Error   Units
DateTimeFormatterWithPaddingBench.formatWithPadding         thrpt   15   6548.853 ± 201.695  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength0  thrpt   15  22269.428 ± 737.613  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength1  thrpt   15  19086.146 ± 117.055  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength2  thrpt   15  15627.898 ± 314.890  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength3  thrpt   15  15447.997 ± 121.946  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength4  thrpt   15  15185.318 ± 198.266  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength5  thrpt   15  15243.618 ± 147.131  ops/ms
DateTimeFormatterWithPaddingBench.formatWithPaddingLength6  thrpt   15  15093.142 ± 294.421  ops/ms

The expected steps in performance match the code paths of 0, 1, 2-6.

-------------

PR Review: https://git.openjdk.org/jdk/pull/12131#pullrequestreview-1396300114
PR Comment: https://git.openjdk.org/jdk/pull/12131#issuecomment-1518248669