RFR: 8289908: Skip bounds check for cases when String is constructed from entirely used byte[]

Сергей Цыпанов duke at openjdk.org
Fri Jul 8 07:39:46 UTC 2022


On Thu, 7 Jul 2022 18:45:03 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> We can skip bounds check and null check for Charset in case we use the array entirely and the Charset is either default one or proven to be non-null.
>> 
>> Benchmark results:
>> 
>> before
>> 
>> Benchmark                                                  Mode  Cnt  Score   Error  Units
>> StringConstructor.newStringFromArray                       avgt   50  4,815 ± 0,154  ns/op
>> StringConstructor.newStringFromArrayWithCharset            avgt   50  4,462 ± 0,068  ns/op
>> StringConstructor.newStringFromArrayWithCharsetName        avgt   50  8,653 ± 0,040  ns/op
>> StringConstructor.newStringFromRangedArray                 avgt   50  5,090 ± 0,066  ns/op
>> StringConstructor.newStringFromRangedArrayWithCharset      avgt   50  4,550 ± 0,041  ns/op
>> StringConstructor.newStringFromRangedArrayWithCharsetName  avgt   50  8,080 ± 0,055  ns/op
>> 
>> after
>> 
>> Benchmark                                                  Mode  Cnt  Score   Error  Units
>> StringConstructor.newStringFromArray                       avgt   50  4,595 ± 0,053  ns/op
>> StringConstructor.newStringFromArrayWithCharset            avgt   50  4,038 ± 0,062  ns/op
>> StringConstructor.newStringFromArrayWithCharsetName        avgt   50  8,035 ± 0,031  ns/op
>> StringConstructor.newStringFromRangedArray                 avgt   50  4,084 ± 0,007  ns/op
>> StringConstructor.newStringFromRangedArrayWithCharset      avgt   50  4,014 ± 0,008  ns/op
>> StringConstructor.newStringFromRangedArrayWithCharsetName  avgt   50  7,466 ± 0,071  ns/op
>
> src/java.base/share/classes/java/lang/String.java line 1429:
> 
>> 1427:      */
>> 1428:     public String(byte[] bytes, int offset, int length) {
>> 1429:         this(bytes, offset, length, Charset.defaultCharset(), checkBoundsOffCount(offset, length, bytes.length));
> 
> Can you avoid the extra constructor by applying `checkBoundOffCount` to the offset argument; it returns the offset.
> 
> this(bytes, checkBoundsOffCount(offset, length, bytes.length), length, Charset.defaultCharset());
> 
> or call `Preconditions.checkFromIndexSize` directly.

But if I roll back the added constructor I'll go through existing public one `public String(byte[] bytes, int offset, int length, Charset charset)` doing bounds check twice, won't I?

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PR: https://git.openjdk.org/jdk/pull/9407



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