<i18n dev> RFR: 8335366: Improve String.format performance with fastpath [v13]

[Shaojin Wen]

> We need a String format solution with good performance. String Template was once expected, but it has been removed. j.u.Formatter is powerful, but its performance is not good enough.
> 
> This PR implements a subset of j.u.Formatter capabilities. The performance is good enough that it is a fastpath for commonly used functions. When the supported functions are exceeded, it will fall back to using j.u.Formatter.
> 
> The performance of this implementation is good enough, the fastpath has low detection cost, There is no noticeable performance degradation when falling back to j.u.Formatter via fastpath.
> 
> Below is a comparison of String.format and concat-based and StringBuilder:
> 
> * benchmark java code
> 
> public class StringFormat {
>     @Benchmark
>     public String stringIntFormat() {
>         return "%s %d".formatted(s, i);
>     }
> 
>     @Benchmark
>     public String stringIntConcat() {
>         return s + " " + i;
>     }
> 
>     @Benchmark
>     public String stringIntStringBuilder() {
>         return new StringBuilder(s).append(" ").append(i).toString();
>     }
> }
> 
> 
> * benchmark number on macbook m1 pro
> 
> Benchmark                            Mode  Cnt   Score   Error  Units
> StringFormat.stringIntConcat         avgt   15   6.541 ? 0.056  ns/op
> StringFormat.stringIntFormat         avgt   15  17.399 ? 0.133  ns/op
> StringFormat.stringIntStringBuilder  avgt   15   8.004 ? 0.063  ns/op
> 
> 
> From the above data, we can see that the implementation of fastpath reduces the performance difference between String.format and StringBuilder from 10 times to 2~3 times.
> 
> The implementation of fastpath supports the following four specifiers, which can appear at most twice and support a width of 1 to 9.
> 
> d
> x
> X
> s
> 
> If necessary, we can add a few more.
> 
> 
> Below is a comparison of performance numbers running on a MacBook M1, showing a significant performance improvement.
> 
> -Benchmark                          Mode  Cnt    Score    Error  Units (baseline)
> -StringFormat.complexFormat         avgt   15  895.954 ? 52.541  ns/op
> -StringFormat.decimalFormat         avgt   15  277.420 ? 18.254  ns/op
> -StringFormat.stringFormat          avgt   15   66.787 ?  2.715  ns/op
> -StringFormat.stringIntFormat       avgt   15   81.046 ?  1.879  ns/op
> -StringFormat.widthStringFormat     avgt   15   38.897 ?  0.114  ns/op
> -StringFormat.widthStringIntFormat  avgt   15  109.841 ?  1.028  ns/op
> 
> +Benchmark                          Mode  Cnt    Score    Error  Units (current f925339e93fdf7a281462554ce5d73139bd0f0cd)
> +StringFormat.complexF...

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  optimize width padding

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19956/files
  - new: https://git.openjdk.org/jdk/pull/19956/files/39289870..f1ae26a0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19956&range=12
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19956&range=11-12

  Stats: 10 lines in 1 file changed: 4 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/19956.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19956/head:pull/19956

PR: https://git.openjdk.org/jdk/pull/19956