RFR: 8265237: String.join and StringJoiner can be improved further [v2]

[Peter Levart]

On Fri, 16 Apr 2021 23:02:33 GMT, Peter Levart <plevart at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/String.java line 3254:
>> 
>>> 3252: 
>>> 3253:         byte[] value = StringConcatHelper.newArray(((long) icoder << 32) | llen);
>>> 3254:         int off = 0;
>> 
>> StringConcatHelper.newArray() can double the length (based on the coder) and it is then truncated to 32 bits when passed to UNSAFE.allocatlUnitializedArray.
>> The test of length above only ensures llen can be truncated to 32 bits without loss of data.
>
> I thought about that, yes. And I think we have to do the check for the doubled length before calling the newArray. I checked the StringJoinerTest and it only deals with ascii strings unfortunately. Will have to add a test for that too...

I do the checks before calling `StringConcatHelper.newArray()` now and pass a long value to it that already holds the number of bytes needed and where the upper 32  bits (coder) is always 0.

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PR: https://git.openjdk.java.net/jdk/pull/3501