RFR: 8355177: Speed up StringBuilder::append(char[]) via UTF16::compress & Unsafe::copyMemory

[Chen Liang]

On Mon, 21 Apr 2025 07:00:36 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> In BufferedReader.readLine and other similar scenarios, we need to use StringBuilder.append(char[]) to build the string.
> 
> For these scenarios, we can use the intrinsic method StringUTF16.compress and Unsafe.copyMemory instead of the character copy of the char-by-char loop to improve the speed.

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 1773:

> 1771:             int compressed = StringUTF16.compress(s, off, val, count, end - off);
> 1772:             count += compressed;
> 1773:             off += compressed;

Should we update `this.count` eagerly after compression?

src/java.base/share/classes/java/lang/StringUTF16.java line 1317:

> 1315:     }
> 1316: 
> 1317:     private static void putChars(byte[] val, int index, char[] str, int off, int end) {

I recommend renaming this to `putCharsUnchecked` to indicate this has no bound checks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24773#discussion_r2052591347
PR Review Comment: https://git.openjdk.org/jdk/pull/24773#discussion_r2052597288