RFR: 8286279: [vectorapi] Only check index of masked lanes if offset is out of array boundary for masked store

[Xiaohong Gong]

On Wed, 11 May 2022 15:10:55 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Checking whether the indexes of masked lanes are inside of the valid memory boundary is necessary for masked vector memory access. However, this could be saved if the given offset is inside of the vector range that could make sure no IOOBE (IndexOutOfBoundaryException) happens. The masked load APIs have saved this kind of check for common cases. And this patch did the similar optimization for the masked vector store.
>> 
>> The performance for the new added store masked benchmarks improves about `1.83x ~ 2.62x` on a x86 system:
>> 
>> Benchmark                                   Before    After     Gain Units
>> StoreMaskedBenchmark.byteStoreArrayMask   12757.936 23291.118  1.826 ops/ms
>> StoreMaskedBenchmark.doubleStoreArrayMask  1520.932  3921.616  2.578 ops/ms
>> StoreMaskedBenchmark.floatStoreArrayMask   2713.031  7122.535  2.625 ops/ms
>> StoreMaskedBenchmark.intStoreArrayMask     4113.772  8220.206  1.998 ops/ms
>> StoreMaskedBenchmark.longStoreArrayMask    1993.986  4874.148  2.444 ops/ms
>> StoreMaskedBenchmark.shortStoreArrayMask   8543.593 17821.086  2.086 ops/ms
>> 
>> Similar performane gain can also be observed on ARM hardware.
>
> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/X-Vector.java.template line 4086:
> 
>> 4084:         } else {
>> 4085:             $Type$Species vsp = vspecies();
>> 4086:             if (offset < 0 || offset > (a.length - vsp.length())) {
> 
> Can we use `VectorIntrinsics.checkFromIndexSize`? e.g. 
> 
> if (!VectorIntrinsics.checkFromIndexSize(offset, vsp.length(), a.length)) { ...

Thanks for the review @PaulSandoz ! For the `VectorIntrinsics.checkFromIndexSize`, I'm afraid it's not suitable to be used here because the `outOfBounds` exception will be thrown if the offset is not inside of the valid array boundary. And  for the masked operations, this is not needed since we only need to check the masked lanes. Please correct me if I didn't understand correctly. Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/8620