RFR: 8315585: Optimization for decimal to string [v7]

[Shaojin Wen]

On Mon, 16 Oct 2023 15:00:42 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> I submitted PR #15555 before, and there were too many changes. I split it into multiple PRs with small changes. This one is one of them.
>> 
>> this PR removed the duplicate code for getChars in BigDecimal#StringBuilderHelper, i also make performance faster.
>> Please review and don't hesitate to critique my approach and patch.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use StringConcatFactory.makeConcatWithConstants

> Good, narrows it down to what's going on in `prepend(long, byte[], String)`. Might boil down to `System.arraycopy`. This method might not be optimized for tiny arrays on all platforms. Specializing for single-char case:
> 
> ```java
> diff --git a/src/java.base/share/classes/java/lang/String.java b/src/java.base/share/classes/java/lang/String.java
> index 9b19d7e2ac1..6eb70925dab 100644
> --- a/src/java.base/share/classes/java/lang/String.java
> +++ b/src/java.base/share/classes/java/lang/String.java
> @@ -4723,7 +4723,11 @@ static void repeatCopyRest(byte[] buffer, int offset, int limit, int copied) {
>       */
>      void getBytes(byte[] dst, int dstBegin, byte coder) {
>          if (coder() == coder) {
> -            System.arraycopy(value, 0, dst, dstBegin << coder, value.length);
> +            if (value.length == 1) {
> +                dst[(dstBegin << coder)] = value[0];
> +            } else {
> +                System.arraycopy(value, 0, dst, dstBegin << coder, value.length);
> +            }
>          } else {    // this.coder == LATIN && coder == UTF16
>              StringLatin1.inflate(value, 0, dst, dstBegin, value.length);
>          }
> ```
> 
> .. seem to help the JIT do the right thing consistently, too:
> 
> ```
> Benchmark                                 Mode  Cnt   Score   Error  Units
> BigDecimals.testSmallToEngineeringString  avgt   50  11,757 ± 0,480  ns/op
> ```

This is faster, but can I make this change to String.getBytes in this PR?

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PR Comment: https://git.openjdk.org/jdk/pull/16006#issuecomment-1766584318