[riscv-port-jdk17u:riscv-port] RFR: 8296602: RISC-V: improve performance of copy_memory stub

[Vladimir Kempik]

Please review this backport of JDK-8296602 to jdk17u
This effectively unrolls copy memory loop offering significant perf improvement on real hw platforms. ( data are the same as in original commit) 

RISC-V only change, low risc for other platforms.
Testing: TBD.

thead rvb-ice c910
thead

Before ( copy8 only )
Benchmark (size) Mode Cnt Score Error Units
ArrayCopyObject.conjoint_micro 31 thrpt 25 6653.095 ± 251.565 ops/ms
ArrayCopyObject.conjoint_micro 63 thrpt 25 4933.970 ± 77.559 ops/ms
ArrayCopyObject.conjoint_micro 127 thrpt 25 3627.454 ± 34.589 ops/ms
ArrayCopyObject.conjoint_micro 2047 thrpt 25 368.249 ± 0.453 ops/ms
ArrayCopyObject.conjoint_micro 4095 thrpt 25 187.776 ± 0.306 ops/ms
ArrayCopyObject.conjoint_micro 8191 thrpt 25 94.477 ± 0.340 ops/ms

after ( with copy32 )
ArrayCopyObject.conjoint_micro 31 thrpt 25 7620.546 ± 69.756 ops/ms
ArrayCopyObject.conjoint_micro 63 thrpt 25 6677.978 ± 33.112 ops/ms
ArrayCopyObject.conjoint_micro 127 thrpt 25 5206.973 ± 22.612 ops/ms
ArrayCopyObject.conjoint_micro 2047 thrpt 25 653.655 ± 31.494 ops/ms
ArrayCopyObject.conjoint_micro 4095 thrpt 25 352.905 ± 7.390 ops/ms
ArrayCopyObject.conjoint_micro 8191 thrpt 25 165.127 ± 0.832 ops/ms

after ( copy32 with dead code elimination and independent addis )
ArrayCopyObject.conjoint_micro 31 thrpt 25 7576.346 ? 94.487 ops/ms
ArrayCopyObject.conjoint_micro 63 thrpt 25 6475.730 ? 252.590 ops/ms
ArrayCopyObject.conjoint_micro 127 thrpt 25 5221.764 ? 20.415 ops/ms
ArrayCopyObject.conjoint_micro 2047 thrpt 25 691.847 ? 1.102 ops/ms
ArrayCopyObject.conjoint_micro 4095 thrpt 25 360.269 ? 1.091 ops/ms
ArrayCopyObject.conjoint_micro 8191 thrpt 25 179.733 ? 3.012 ops/ms

on hifive unmatched:

before:
Benchmark (size) Mode Cnt Score Error Units
ArrayCopyObject.conjoint_micro 31 thrpt 25 5391.575 ± 152.984 ops/ms
ArrayCopyObject.conjoint_micro 63 thrpt 25 3700.946 ± 43.175 ops/ms
ArrayCopyObject.conjoint_micro 127 thrpt 25 2316.160 ± 24.734 ops/ms
ArrayCopyObject.conjoint_micro 2047 thrpt 25 188.616 ± 0.151 ops/ms
ArrayCopyObject.conjoint_micro 4095 thrpt 25 95.323 ± 0.053 ops/ms
ArrayCopyObject.conjoint_micro 8191 thrpt 25 46.935 ± 0.041 ops/ms

after:
Benchmark (size) Mode Cnt Score Error Units
ArrayCopyObject.conjoint_micro 31 thrpt 25 6136.169 ± 330.409 ops/ms
ArrayCopyObject.conjoint_micro 63 thrpt 25 4924.020 ± 78.529 ops/ms
ArrayCopyObject.conjoint_micro 127 thrpt 25 3732.561 ± 89.606 ops/ms
ArrayCopyObject.conjoint_micro 2047 thrpt 25 431.103 ± 0.505 ops/ms
ArrayCopyObject.conjoint_micro 4095 thrpt 25 221.543 ± 0.363 ops/ms
ArrayCopyObject.conjoint_micro 8191 thrpt 25 100.586 ± 0.197 ops/ms

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Commit messages:
 - Backport bd57e2138fc980822a149af905e572ab71ccbf11

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/9/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=9&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296602
  Stats: 70 lines in 1 file changed: 41 ins; 1 del; 28 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/9.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u pull/9/head:pull/9

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/9