RFR: 8318158: RISC-V: implement roundD/roundF intrinsics [v9]

[Hamlin Li]

On Tue, 19 Dec 2023 22:45:13 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:
> 
>   Replaced li with mv

I think usage of static rounding mode here is fine for riscv.


In typical implementations, writes to the dynamic rounding mode CSR state will serialize
the pipeline.
Static rounding modes are used to implement specialized arithmetic operations that often
have to switch frequently between different rounding modes.

-- from `“F” Standard Extension`

Looks good.
Can you add some comments for java_round_float(or double)? As there were lots of discussion here, but all this information is not in the code.

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

PR Comment: https://git.openjdk.org/jdk/pull/16382#issuecomment-1865886113
PR Comment: https://git.openjdk.org/jdk/pull/16382#issuecomment-1865889081