Intrinsic for AArch64 Arrays.fill(array, constant)

[JohnTortugo]

Please, consider this patch to intrinsify Arrays.fill(array, constant) for byte, short and int on AArch64 backend. The assembly code is a port of the HotSpot assembly for the same intrinsic. Thank you @gergo- for reviewing the draft version of this PR!

The below is a summary of some quick JMH tests that I ran on my dev machine.

Instr     Benchmark                       (size)   Mode  Cnt          Score          Error  Units
No        ArrayFillBenchmark.fill_bytes       16  thrpt    5  130,858,263.770 ±  7175243.892  ops/s
Yes       ArrayFillBenchmark.fill_bytes       16  thrpt    5  268,367,821.486 ±  2717945.345  ops/s
No        ArrayFillBenchmark.fill_bytes     1024  thrpt    5    7,284,374.553 ±   131421.427  ops/s
Yes       ArrayFillBenchmark.fill_bytes     1024  thrpt    5   94,448,799.400 ±  1096754.457  ops/s
No        ArrayFillBenchmark.fill_bytes     4096  thrpt    5    1,814,548.662 ±    33455.246  ops/s
Yes       ArrayFillBenchmark.fill_bytes     4096  thrpt    5   28,970,761.402 ±  2636159.892  ops/s

No        ArrayFillBenchmark.fill_shorts      16  thrpt    5  137,210,363.401 ±  6208453.450  ops/s
Yes       ArrayFillBenchmark.fill_shorts      16  thrpt    5  285,341,885.029 ± 11301099.208  ops/s
No        ArrayFillBenchmark.fill_shorts    1024  thrpt    5    7,178,718.150 ±   741170.216  ops/s
Yes       ArrayFillBenchmark.fill_shorts    1024  thrpt    5   53,996,849.866 ±  2353229.277  ops/s
No        ArrayFillBenchmark.fill_shorts    4096  thrpt    5    1,814,690.509 ±    13988.852  ops/s
Yes       ArrayFillBenchmark.fill_shorts    4096  thrpt    5   14,905,037.102 ±  1719260.337  ops/s

No        ArrayFillBenchmark.fill_ints        16  thrpt    5  132,083,594.942 ±  7995181.451  ops/s
Yes       ArrayFillBenchmark.fill_ints        16  thrpt    5  374,192,809.496 ±  5778630.471  ops/s
No        ArrayFillBenchmark.fill_ints      1024  thrpt    5    7,044,324.507 ±   130830.618  ops/s
Yes       ArrayFillBenchmark.fill_ints      1024  thrpt    5   28,677,308.237 ±  1515323.358  ops/s
No        ArrayFillBenchmark.fill_ints      4096  thrpt    5    1,648,798.029 ±     9113.592  ops/s
Yes       ArrayFillBenchmark.fill_ints      4096  thrpt    5    7,563,740.695 ±   627888.347  ops/s

[gergo-]

Thanks for keeping up this work! I left some high-level comments for now. I'll do some testing and a more thorough review of the assembly code soon.

[JohnTortugo]

@gergo- - I believe I've addressed all your comments so far. Please, let me know if you have any other concerns.

[gergo-]

I added some requests for improved documentation, and the value to be filled should be a long throughout. I also started some checks on this PR, note that the style checks had some complaints. Please make sure mx checkstyle passes in the compiler suite.

This will soon be ready for internal testing.

[JohnTortugo]

@zakkak - Please take a look again when you have some time. I've addressed your latest feedback.