RFR: 8357460: RISC-V: Optimize array fill stub for small size

[Feilong Jiang]

Please consider.
As discussed in https://github.com/openjdk/jdk/pull/23890#discussion_r2094920943, we can also further optimize the array fill stub by unrolling the storage of values when the size is less than 8.

This PR also removes the **aligned tail part** with the consideration of code size and testing coverage. As the test reveals there are no significant regressions.


Before:
Benchmark                 (size)  Mode  Cnt   Score   Error  Units
ArrayFill.fillByteArray        7  avgt   12  27.215 ± 0.073  ns/op
ArrayFill.fillByteArray       15  avgt   12  32.687 ± 0.904  ns/op
ArrayFill.fillIntArray         7  avgt   12  28.629 ± 0.006  ns/op
ArrayFill.fillIntArray        15  avgt   12  29.351 ± 0.009  ns/op
ArrayFill.fillShortArray       7  avgt   12  30.776 ± 0.006  ns/op
ArrayFill.fillShortArray      15  avgt   12  31.724 ± 0.447  ns/op
ArrayFill.zeroByteArray        7  avgt   12  27.199 ± 0.006  ns/op
ArrayFill.zeroByteArray       15  avgt   12  32.685 ± 0.900  ns/op
ArrayFill.zeroIntArray         7  avgt   12  28.630 ± 0.007  ns/op
ArrayFill.zeroIntArray        15  avgt   12  29.352 ± 0.011  ns/op
ArrayFill.zeroShortArray       7  avgt   12  30.776 ± 0.006  ns/op
ArrayFill.zeroShortArray      15  avgt   12  31.497 ± 0.012  ns/op

After:
Benchmark                 (size)  Mode  Cnt   Score   Error  Units
ArrayFill.fillByteArray        7  avgt   12  20.137 ± 0.042  ns/op
ArrayFill.fillByteArray       15  avgt   12  32.928 ± 0.004  ns/op
ArrayFill.fillIntArray         7  avgt   12  28.630 ± 0.004  ns/op
ArrayFill.fillIntArray        15  avgt   12  29.344 ± 0.005  ns/op
ArrayFill.fillShortArray       7  avgt   12  31.494 ± 0.004  ns/op
ArrayFill.fillShortArray      15  avgt   12  31.492 ± 0.008  ns/op
ArrayFill.zeroByteArray        7  avgt   12  19.980 ± 0.164  ns/op
ArrayFill.zeroByteArray       15  avgt   12  32.927 ± 0.004  ns/op
ArrayFill.zeroIntArray         7  avgt   12  28.629 ± 0.005  ns/op
ArrayFill.zeroIntArray        15  avgt   12  29.346 ± 0.006  ns/op
ArrayFill.zeroShortArray       7  avgt   12  32.193 ± 0.027  ns/op
ArrayFill.zeroShortArray      15  avgt   12  31.495 ± 0.010  ns/op


Testing:
- [x] tier1

-------------

Commit messages:
 - Merge branch 'master' of https://github.com/openjdk/jdk into riscv-optimize-generate-fill
 - optimize array fill stub for small size

Changes: https://git.openjdk.org/jdk/pull/25350/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25350&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357460
  Stats: 61 lines in 1 file changed: 9 ins; 27 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/25350.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25350/head:pull/25350

PR: https://git.openjdk.org/jdk/pull/25350