RFR: 8335366: Improve String.format performance with fastpath

[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.complexFormat         avgt   15  863.156 ? 42.140  ns/op
+StringFormat.decimalFormat         avgt   15  287.342 ? 19.896  ns/op
+StringFormat.stringFormat          avgt   15    6.738 ?  0.027  ns/op
+StringFormat.stringIntFormat       avgt   15   17.046 ?  0.090  ns/op
+StringFormat.widthStringFormat     avgt   15    8.993 ?  0.073  ns/op
+StringFormat.widthStringIntFormat  avgt   15   17.602 ?  0.014  ns/op

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

Commit messages:
 - String.format fastpath

Changes: https://git.openjdk.org/jdk/pull/19956/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19956&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335366
  Stats: 362 lines in 3 files changed: 356 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