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

[Markus KARG]

On Mon, 21 Apr 2025 15:19:40 GMT, Chen Liang <liach at openjdk.org> wrote:

> This might be helpful combined with #21730.

That implies creating a copy of the chars:


private final void appendChars(CharSequence s, int off, int end) {
    if (isLatin1()) {
        byte[] val = this.value;

        // ----- Begin of Experimental Section -----
        char[] ca = new char[end - off];
        s.getChars(off, end, ca, 0);
        int compressed = StringUTF16.compress(ca, 0, val, count, end - off);
        count += compressed;
        off += compressed;
        // ----- End of Experimental Section -----

        for (int i = off, j = count; i < end; i++) {
            char c = s.charAt(i);
            if (StringLatin1.canEncode(c)) {
                val[j++] = (byte)c;
            } else {
                count = j;
                inflate();
                // Store c to make sure sb has a UTF16 char
                StringUTF16.putCharSB(this.value, j++, c);
                count = j;
                i++;
                StringUTF16.putCharsSB(this.value, j, s, i, end);
                count += end - i;
                return;
            }
        }
    } else {
        StringUTF16.putCharsSB(this.value, count, s, off, end);
    }
    count += end - off;
}


While I do *assume* that it should faster to let machine code perform the copy and compression over letting Java code perform a char-by-char approach, to be sure there should be another benchmark to actually proof this claim.

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PR Comment: https://git.openjdk.org/jdk/pull/24773#issuecomment-2846452819