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

[Peter Levart]

On Fri, 16 Apr 2021 19:05:25 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Peter Levart has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add String.join benchmark method to StringJoinerBenchmark and adjust some parameters to cover bigger range
>
> 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...

> src/java.base/share/classes/java/lang/String.java line 3256:
> 
>> 3254:         int off = 0;
>> 3255:         prefix.getBytes(value, off, coder); off += prefix.length();
>> 3256:         for (int i = 0; i < size; i++) {
> 
> Can you save a branch inside the loop by handling element 0 outside the loop and
> then do the loop for the rest?

Thanks, I'll do that and then re-test to see if there's any improvement.

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