RFR: 8355177: Speed up StringBuilder::append(char[]) via Unsafe::copyMemory [v8]

[Shaojin Wen]

On Thu, 24 Jul 2025 15:17:34 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update src/java.base/share/classes/java/lang/StringUTF16.java
>>   
>>   Co-authored-by: Chen Liang <liach at openjdk.org>
>
> src/java.base/share/classes/java/lang/StringUTF16.java line 1489:
> 
>> 1487:                 Unsafe.ARRAY_CHAR_BASE_OFFSET + ((long) off << 1),
>> 1488:                 val,
>> 1489:                 Unsafe.ARRAY_BYTE_BASE_OFFSET + ((long) index << 1),
> 
> Suggestion:
> 
>                 Unsafe.ARRAY_BYTE_BASE_OFFSET + ((long) index << 1) * Unsafe.ARRAY_BYTE_INDEX_SCALE,

If we want to use ARRAY_CHAR_INDEX_SCALE, it should be used as follows

        Unsafe.getUnsafe().copyMemory(
                ca,
                Unsafe.ARRAY_CHAR_BASE_OFFSET + (long) off * Unsafe.ARRAY_CHAR_INDEX_SCALE,
                val,
                Unsafe.ARRAY_CHAR_BASE_OFFSET + (long) off * Unsafe.ARRAY_CHAR_INDEX_SCALE,
                (long) (end - off) << 1);

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PR Review Comment: https://git.openjdk.org/jdk/pull/24773#discussion_r2233670252