RFR: 8355177: Speed up StringBuilder::append(char[]) via Unsafe::copyMemory [v7]

[Chen Liang]

On Wed, 25 Jun 2025 02:01:18 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 Unsafe.copyMemory instead of the character copy of the char-by-char loop to improve the speed.
>> 
>> @RogerRiggs completed the optimization when the encoder is LATIN1 in PR #24967. This PR continues to complete the optimization when the encoder is UTF16.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Microbenchmark java.lang.StringBuilders is modified to not re-use existing StringBuilders, from @RogerRiggs

src/java.base/share/classes/java/lang/StringUTF16.java line 1490:

> 1488:                 val,
> 1489:                 Unsafe.ARRAY_BYTE_BASE_OFFSET + ((long) index << 1),
> 1490:                 ((long) (end - off)) << 1);

Either:
Suggestion:

                (long) (end - off) << 1);

Or add parentheses around `(long) index` and `(long) off` that's the operand to `<< 1`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24773#discussion_r2169526168