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

[Feilong Jiang]

On Tue, 3 Jan 2023 12:08:47 GMT, Yadong Wang <yadongwang at openjdk.org> wrote:

> 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

`Zfhmin` is a mandatory extension of RVA22U64 Profile [1], we can enable `Zfhmin` when `UseRVA22U64` is true:
https://github.com/openjdk/jdk/blob/82deb5ca615f70634f8cd84836265d01edd1c5a5/src/hotspot/cpu/riscv/vm_version_riscv.cpp#L51-L74

1. https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc#612-rva22u64-mandatory-extensions

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