On Sat, 18 Dec 2021 21:48:33 GMT, amirhadadi <d...@openjdk.java.net> wrote:
>> This does look like a HotSpot JIT compiler issue to me. My guess is that it
>> is related to how checkBoundsOffCount() checks for offset < 0:
>>
>> 396 if ((length | fromIndex | size) < 0 || size > length -
>> fromIndex)
>>
>> using | to combine three values.
>
> @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`.
-------------
PR: https://git.openjdk.java.net/jdk/pull/6812
