RFR: 8301958: Avoid Arrays.copyOfRange overhead in java.lang.String [v5]

[Francesco Nigro]

On Tue, 7 Feb 2023 15:25:05 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> This adds a local, specialized `copyBytes` method to `String` that avoids certain redundant range checks and clamping that JIT has issues removing fully.
>> 
>> This has a small but statistically significant effect on `String` microbenchmarks, eg.:
>> 
>> Baseline
>> 
>> Benchmark                                            (size)  Mode  Cnt   Score   Error  Units
>> StringConstructor.newStringFromArray                      7  avgt   15  16.817 ± 0.369  ns/op
>> StringConstructor.newStringFromArrayWithCharset           7  avgt   15  16.866 ± 0.449  ns/op
>> StringConstructor.newStringFromArrayWithCharsetName       7  avgt   15  22.198 ± 0.396  ns/op
>> 
>> 
>> Patch:
>> 
>> Benchmark                                            (size)  Mode  Cnt   Score   Error  Units
>> StringConstructor.newStringFromArray                      7  avgt   15  15.477 ± 0.342  ns/op
>> StringConstructor.newStringFromArrayWithCharset           7  avgt   15  15.557 ± 0.352  ns/op
>> StringConstructor.newStringFromArrayWithCharsetName       7  avgt   15  21.272 ± 0.398  ns/op
>> 
>> 
>> Care has to be taken to ensure preconditions have been checked when using `checkBytes`. In the case of `String(AbstractStringBuilder)` there's a possible pre-existing issue where the constructor might either throw an exception or truncate the buffer if the builder byte array and length is not in agreement (theoretically possible if you clear/remove and call `trimToSize()` concurrently). Adding an explicit check here seem to be the right thing to do regardless of this RFE.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   copyrights

Thanks @cl4es to look into this!

src/java.base/share/classes/java/lang/String.java line 698:

> 696:     }
> 697: 
> 698:     static byte[] copyBytes(byte[] bytes, int offset, int length) {

Given that the stub generated for array copy seems highly dependent by the call site constrains, did you tried adding a check for offset == 0 and/or length == bytes.length?

If (offset == 0 && bytes.length == length) {
    System.arrayCopy(bytes, 0, dst, bytes.length);
    // etc etc the other combinations 

This should have different generated stubs with much smaller ASM depending by the enforced constrains

-------------

Changes requested by franz1981 at github.com (no known OpenJDK username).

PR: https://git.openjdk.org/jdk/pull/12453