RFR: 8351443: Improve robustness of StringBuilder [v3]

[Chen Liang]

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 1856:

> 1854: 
> 1855:     // Package access for String and StringBuffer.
> 1856:     final boolean isLatin1() {

I recommend inlining the Stirng usage of this method too like for all other sites in ASB. Then this is only used by serialization of StringBuffer/Builder, and arguably those can move to use the plain version too.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24967#discussion_r2073607977