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

[Shaojin Wen]

On Mon, 28 Jul 2025 12:46:22 GMT, Raffaello Giulietti <rgiulietti at openjdk.org> wrote:

>> String uses `<< coder` in many places. I think the following way of writing is also good:
>> 
>>         Unsafe.getUnsafe().copyMemory(
>>                 ca,
>>                 Unsafe.ARRAY_CHAR_BASE_OFFSET + (long) off << String.UTF16,
>>                 val,
>>                 Unsafe.ARRAY_CHAR_BASE_OFFSET + (long) index << String.UTF16,
>>                 (long) (end - off) << String.UTF16);
>
> I agree that we can expect arrays to be laid out as a contiguous chunk of memory with the intuitively expected element size.
> _But..._ AFAIK this is not specified anywhere in the JVMS, although it is true that it is tacitly assumed in many low-level parts of the codebase. So, in this sense, I'm fine with your code.

There are many places in the String class that use `<< 1` and `>> 1` to handle the length of UTF16 byte[], so is it okay to use `<< 1` directly in the current version of the code?

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PR Review Comment: https://git.openjdk.org/jdk/pull/24773#discussion_r2236631515