RFR: 8328138: Optimize ArrayEquals on AArch64

[Xiaowei Lu]

Current implementation of ArrayEquals on AArch64 is quite complex, due to the variety of checks about alignment, tail processing, bus locking and so on. However, Modern Arm processors have eased such worries. So we proposed to use a simple&straightforward flow of ArrayEquals.
With this simplified ArrayEquals, we observed performance gains on the latest arm platforms(Neoverse N1&N2)
Test case: org.openjdk.bench.java.util.ArraysEquals

1x vector length, 64-bit aligned array[0]
| Test Case              |    N1     |    N2     |
|:----------------------:|:---------:|:---------:|
| testByteFalseBeginning | -21.42%   | -13.37%   |
| testByteFalseEnd       |  25.79%   |  27.45%   |
| testByteFalseMid       |  16.64%   |  16.46%   |
| testByteTrue           |  12.39%   |  24.66%   |
| testCharFalseBeginning |  -5.27%   |  -3.08%   |
| testCharFalseEnd       |  29.29%   |  35.23%   |
| testCharFalseMid       |  15.13%   |  19.34%   |
| testCharTrue           |  21.63%   |  33.73%   |
| Total                  |  11.77%   |  17.55%   |

A key factor is to decide when we should utilize simd in array equals. An aggressive choice is to enable simd as long as array length exceeds vector length(8 words). The corresponding result is shown above, from which we can see performance regression in both testBeginning cases. To avoid such perf impact, we can set simd threshold to 3x vector length.

3x vector length, 64-bit aligned array[0]
|                      |   n1    |   n2    |
|:----------------------:|:---------:|:---------:|
| testByteFalseBeginning |  8.28%  |  8.64%  |
| testByteFalseEnd       |  6.38%  | 12.29%  |
| testByteFalseMid       |  6.17%  |  7.96%  |
| testByteTrue           | -10.08% |  3.06%  |
| testCharFalseBeginning | -1.42%  |  7.23%  |
| testCharFalseEnd       |  4.05%  | 13.48%  |
| testCharFalseMid       |  8.79%  | 16.96%  |
| testCharTrue           | -5.66%  | 10.23%  |
| Total                  |  2.06%  |  9.98%  |


In addtion to perf improvement, we propose this patch to solve alignment issues in array equals. JDK-8139457 tries to relax alignment of array elements. On the other hand, this misalignment makes it an error to read the whole last word in array equals, in case that the array doesn't occupy the whole word. Our patch fixed this problem, and again we would like to show the perf improvement when misalignment.

1x vector length, 32-bit aligned array[0]
|                      |    N1    |    N2    |
|:----------------------:|:---------:|:---------:|
| testByteFalseBeginning | -12.96%  | -17.50%  |
| testByteFalseEnd       |  29.43%  |  32.19%  |
| testByteFalseMid       |  24.30%  |  17.54%  |
| testByteTrue           |  16.57%  |  24.40%  |
| testCharFalseBeginning |   2.40%  |   0.60%  |
| testCharFalseEnd       |  32.14%  |  32.94%  |
| testCharFalseMid       |  18.86%  |  17.60%  |
| testCharTrue           |  25.38%  |  32.62%  |
| Total                  |  17.01%  |  17.54%  |


3x vector length, 32-bit aligned array[0]
|                      |    N1    |    N2    |
|:----------------------:|:---------:|:---------:|
| testByteFalseBeginning |  10.95%  |  14.23%  |
| testByteFalseEnd       |  13.83%  |  12.35%  |
| testByteFalseMid       |  11.18%  |  10.31%  |
| testByteTrue           |  -7.13%  |   6.61%  |
| testCharFalseBeginning |   0.38%  |   7.20%  |
| testCharFalseEnd       |   2.84%  |  13.13%  |
| testCharFalseMid       |  11.17%  |  15.40%  |
| testCharTrue           |  -5.43%  |   9.52%  |
| Total                  |   4.72%  |  11.09%  |

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Commit messages:
 - 8328138: Optimize ArrayEquals on AArch64

Changes: https://git.openjdk.org/jdk/pull/18292/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18292&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328138
  Stats: 92 lines in 1 file changed: 92 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18292.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18292/head:pull/18292

PR: https://git.openjdk.org/jdk/pull/18292