RFR: 8351443: Improve robustness of StringBuilder [v3]

[Shaojin Wen]

On Fri, 2 May 2025 17:24:06 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Refactor AbstractStringBuilder to maintain consistency among count, coder, and value buffers while the buffer capacity is being expanded and/or inflated from Latin1 to UTF16 representations. 
>> The refactoring pattern is to read and write AbstractStringBuilder fields once using locals for all intermediate values. 
>> Support methods are static, designed to pass all values as arguments and return a value.
>> 
>> The value byte array is reallocated under 3 conditions:
>> - Increasing the capacity with the same encoder
>> - Increasing the capacity and inflation to change the coder from LATIN1 to UTF16
>> - Inflation with the same capacity
>> 
>> Added StressSBTest to exercise public instance methods of StringBuilder.
>
> Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Optimize StringUTF16.putCharsAt a bit.
>   Fixup hotspot Helper of putCharsAt to remove return value to match StringUTF16.putCharsAt.

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 1970:

> 1968:     private static byte[] appendChars(byte[] value, byte coder, int count, char[] s, int off, int end) {
> 1969:         if (isLatin1(coder)) {
> 1970:             for (int i = off, j = count; i < end; i++) {

Suggestion:

            int compressed = StringUTF16.compress(s, off, value, count, end - off);
            for (int i = compressed + off, j = count + compressed; i < end; i++) {

Refer to PR #24773, adding StringUTF16.compress preprocessing here will improve the performance of java.io.BufferedReader::readLine method.

Should it be done at once or separately for PR #24773 to make changes?

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PR Review Comment: https://git.openjdk.org/jdk/pull/24967#discussion_r2071954862