RFR: 8318158: RISC-V: implement roundD/roundF intrinsics [v12]

[Olga Mikhaltsova]

On Thu, 21 Dec 2023 23:02:55 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:
> 
>   Moved the code up + comments

In addition, some examples with RDN for fadd.s and fcvt.w.s:

fadd.s (src + 0.5f): src = 0.345200  dst = 0.845200
fcvt.w.s: src = 0.845200 dst = 0
RESULT: 0  (JAVA Math.round: 0)

fadd.s (src + 0.5f): src = -0.555550  dst = -0.055550
fcvt.w.s: src = -0.055550 dst = -1
RESULT: -1  (JAVA Math.round: -1)

fadd.s (src + 0.5f): src = -1.500000  dst = -1.000000
fcvt.w.s: src = -1.000000 dst = -1
RESULT: -1  (JAVA Math.round: -1)

fadd.s (src + 0.5f): src = 1.500000  dst = 2.000000
fcvt.w.s: src = 2.000000 dst = 2
RESULT: 2  (JAVA Math.round: 2)

fadd.s (src + 0.5f): src = -1.345000  dst = -0.845000
fcvt.w.s: src = -0.845000 dst = -1
RESULT: -1  (JAVA Math.round: -1)

fadd.s (src + 0.5f): src = -1.450000  dst = -0.950000
fcvt.w.s: src = -0.950000 dst = -1
RESULT: -1  (JAVA Math.round: -1)

fadd.s (src + 0.5f): src = -0.444460  dst = 0.055540
fcvt.w.s: src = 0.055540 dst = 0
RESULT: 0  (JAVA Math.round: 0)

fadd.s (src + 0.5f): src = -0.999990  dst = -0.499990
fcvt.w.s: src = -0.499990 dst = -1
RESULT: -1  (JAVA Math.round: -1)

fadd.s (src + 0.5f): src = 0.999990  dst = 1.499990
fcvt.w.s: src = 1.499990 dst = 1
RESULT: 1  (JAVA Math.round: 1)

fadd.s (src + 0.5f): src = 0.000000  dst = 0.500000
fcvt.w.s: src = 0.500000 dst = 0
RESULT: 0  (JAVA Math.round: 0)

fadd.s (src + 0.5f): src = 0.001000  dst = 0.501000
fcvt.w.s: src = 0.501000 dst = 0
RESULT: 0  (JAVA Math.round: 0)

fadd.s (src + 0.5f): src = -0.001000  dst = 0.499000
fcvt.w.s: src = 0.499000 dst = 0
RESULT: 0  (JAVA Math.round: 0)

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PR Comment: https://git.openjdk.org/jdk/pull/16382#issuecomment-1871623924