RFR: 8351443: Improve robustness of StringBuilder

[Per Minborg]

On Wed, 30 Apr 2025 14:12:36 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.

src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 732:

> 730:         byte[] value = ensureCapacitySameCoder(this.value, coder, count + 4);
> 731:         if (isLatin1(coder)) {
> 732:             value[count++] = 'n';

Can we benefit from MergeStores if we use fixed `count` offsets and then bump up `count` by four after the stores?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/24967#discussion_r2068778082