RFR: 8318158: RISC-V: implement roundD/roundF intrinsics [v7]

[Olga Mikhaltsova]

On Fri, 8 Dec 2023 22:46:45 GMT, Olga Mikhaltsova <omikhaltcova at openjdk.org> wrote:

>> Please, review this Implementation of the roundD/roundF intrinsics for RISC-V platform.
>> 
>> In the table below it is shown that NaN argument should be processed as a special case.
>> 
>>                                                   RISC-V                            Java
>>                                         (FCVT.W.S)    (FCVT.L.D)  (long round(double a)) (int round(float a))
>> Minimum valid input (after rounding)     −2^31         −2^63         Long.MIN_VALUE       Integer.MIN_VALUE
>> Maximum valid input (after rounding)      2^31 − 1      2^63 − 1     Long.MAX_VALUE       Integer.MAX_VALUE
>> Output for out-of-range negative input   −2^31         −2^63         Long.MIN_VALUE       Integer.MIN_VALUE
>> Output for −∞                            −2^31         −2^63         Long.MIN_VALUE       Integer.MIN_VALUE
>> Output for out-of-range positive input    2^31 − 1      2^63 - 1     Long.MAX_VALUE       Integer.MAX_VALUE
>> Output for +∞                             2^31 − 1      2^63 - 1     Long.MAX_VALUE       Integer.MAX_VALUE
>> Output for NaN                            2^31 − 1      2^63 - 1           0                      0
>> 
>> The benchmark running with the 2nd fixed implementation on the T-Head RVB-ICE board shows the following performance improvement::
>> 
>> **Before**
>> 
>> Benchmark                              (TESTSIZE)   Mode  Cnt    Score   Error   Units
>> FpRoundingBenchmark.test_round_double        2048  thrpt   15   59.555  0.179  ops/ms
>> FpRoundingBenchmark.test_round_float         2048  thrpt   15   49.760  0.103  ops/ms
>> 
>> 
>> **After**
>> 
>> Benchmark                              (TESTSIZE)   Mode  Cnt    Score   Error   Units
>> FpRoundingBenchmark.test_round_double        2048  thrpt   15  110.956  0.186  ops/ms
>> FpRoundingBenchmark.test_round_float         2048  thrpt   15  115.947  0.122  ops/ms
>
> Olga Mikhaltsova has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Optimization against regression on SiFive

Slight performance improvement on SiFive is possible via changing instructions sequence (feq before fmv) but meanwhile some performance will be lost on T-Head:

**VisionFive 2**

Benchmark                              (TESTSIZE)   Mode  Cnt   Score   Error   Units
FpRoundingBenchmark.test_round_double        2048  thrpt   15  38.890 ± 0.129  ops/ms
FpRoundingBenchmark.test_round_float         2048  thrpt   15  50.300 ± 0.017  ops/ms

**T-Head**

Benchmark                              (TESTSIZE)   Mode  Cnt    Score    Error   Units
FpRoundingBenchmark.test_round_double        2048  thrpt   15   95.119  19.104  ops/ms
FpRoundingBenchmark.test_round_float         2048  thrpt   15  100.212  19.395  ops/ms

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PR Comment: https://git.openjdk.org/jdk/pull/16382#issuecomment-1853075129