RFR: 8278518: String(byte[], int, int, Charset) constructor and String.translateEscapes() miss bounds check elimination

[amirhadadi]

On Tue, 21 Dec 2021 12:50:36 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> @dean-long actually the issue reproduces with Java 17 where `checkBoundsOffCount` was implemented in a more straight forward manner:
>> 
>> 
>> static void checkBoundsOffCount(int offset, int count, int length) {
>>     if (offset < 0 || count < 0 || offset > length - count) {
>>         throw new StringIndexOutOfBoundsException(
>>             "offset " + offset + ", count " + count + ", length " + length);
>>     }
>> }
>> 
>> 
>> 
>> Here's a [gist](https://gist.github.com/amirhadadi/9505c3f5d9ad68cad2fbfd1b9e01f0b8) with a benchmark you can run. This benchmark compares safe and unsafe reads from the byte array  (In this gist I didn't modify the code to add the offset >= 0 condition).
>> 
>> Here are the results:
>> 
>> 
>> OpenJDK 17.0.1+12
>> OSX with 2.9 GHz Quad-Core Intel Core i7
>> 
>> 
>> 
>> Benchmark                       Mode  Cnt    Score    Error  Units
>> StringBenchmark.safeDecoding    avgt   20  120.312 ± 11.674  ns/op
>> StringBenchmark.unsafeDecoding  avgt   20   72.628 ±  0.479  ns/op
>
> @amirhadadi unsafeDecode() is buggy I think. Offsets in the array when read with unsafe should be computed as `offset * unsafe.ARRAY_BYTE_INDEX_SCALE + unsafe.ARRAY_BYTE_BASE_OFFSET`.

@rwestrel thanks for the correction!

Here are the updated results:


Benchmark                       Mode  Cnt    Score   Error  Units
StringBenchmark.safeDecoding    avgt   20  113.849 ± 1.609  ns/op
StringBenchmark.unsafeDecoding  avgt   20   85.272 ± 1.462  ns/op

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PR: https://git.openjdk.java.net/jdk/pull/6812