RFR: 8299525: RISC-V: Add backend support for half float conversion intrinsics

[Yadong Wang]

This patch adds RISC-V backend support for library intrinsics that implement conversions between half-precision and single-precision floats by using RISC-V Zfh Extension, which was already ratified by November 2021 (https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions).

The C2 output for PrintOptoAssembly:
0dc      B10: #  out( B33 B11 ) <- in( B9 )  Freq: 1.99802
0dc +   flw  F1, [R29, #16]     # float, #@loadF
0e0 +   fcvt.h.s F0, F1 #@convF2HF_reg_reg
             fmv.x.h R8, F0  #@convF2HF_reg_reg

0dc      B10: #  out( B33 B11 ) <- in( B9 )  Freq: 1.99801
0dc +   lh  R12, [R11, #16]     # short, #@loadS
0e0 +   fmv.h.x F0, R12 #@convHF2F_reg_reg
             fcvt.s.h F1, F0 #@convHF2F_reg_reg

We don't have any hardware supporting yet, so ran the following benchmarks in Qemu for unreliable reference:

VM options: -XX:+UnlockExperimentalVMOptions -XX:-UseZfh
Benchmark                                           (size)   Mode  Samples      Score  Score error   Units
o.s.Fp16ConversionBenchmark.float16ToFloat            2048  thrpt       15     44.523        0.116  ops/ms
o.s.Fp16ConversionBenchmark.float16ToFloatMemory      2048  thrpt       15   8379.835       27.309  ops/ms
o.s.Fp16ConversionBenchmark.floatToFloat16            2048  thrpt       15      7.370        0.028  ops/ms
o.s.Fp16ConversionBenchmark.floatToFloat16Memory      2048  thrpt       15  11292.278       11.962  ops/ms

VM options: -XX:+UnlockExperimentalVMOptions -XX:+UseZfh
Benchmark                                           (size)   Mode  Samples      Score  Score error   Units
o.s.Fp16ConversionBenchmark.float16ToFloat            2048  thrpt       15     12.357        0.153  ops/ms
o.s.Fp16ConversionBenchmark.float16ToFloatMemory      2048  thrpt       15  10213.944       69.222  ops/ms
o.s.Fp16ConversionBenchmark.floatToFloat16            2048  thrpt       15     11.728        0.067  ops/ms
o.s.Fp16ConversionBenchmark.floatToFloat16Memory      2048  thrpt       15  15008.550       13.917  ops/ms

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Commit messages:
 - 8299525: RISC-V: Add backend support for half float conversion intrinsics

Changes: https://git.openjdk.org/jdk/pull/11828/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11828&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299525
  Stats: 47 lines in 3 files changed: 47 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11828.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11828/head:pull/11828

PR: https://git.openjdk.org/jdk/pull/11828