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

fadd_s requires setting the explicit rounding mode RDN (round down towards −∞) because adding 0.5f to some floats exceeds the precision limits for a float and therefore rounding takes place. This leads to the incorrect results in case of the default rounding mode RNE (round to nearest, ties to even) for some inputs:

error: src = 8388609.000000  dst = 8388610  etalon = 8388609
error: src = 8388611.000000  dst = 8388612  etalon = 8388611
error: src = 8388613.000000  dst = 8388614  etalon = 8388613
error: src = 8388615.000000  dst = 8388616  etalon = 8388615
error: src = 8388617.000000  dst = 8388618  etalon = 8388617
error: src = 8388619.000000  dst = 8388620  etalon = 8388619
error: src = 8388621.000000  dst = 8388622  etalon = 8388621
error: src = 8388623.000000  dst = 8388624  etalon = 8388623
error: src = 8388625.000000  dst = 8388626  etalon = 8388625
error: src = 8388627.000000  dst = 8388628  etalon = 8388627
error: src = 8388629.000000  dst = 8388630  etalon = 8388629
error: src = 8388631.000000  dst = 8388632  etalon = 8388631
error: src = 8388633.000000  dst = 8388634  etalon = 8388633
error: src = 8388635.000000  dst = 8388636  etalon = 8388635
error: src = 8388637.000000  dst = 8388638  etalon = 8388637
error: src = 8388639.000000  dst = 8388640  etalon = 8388639
etc.

Let’s consider two of them with RNE for fadd.s:

fadd.s rne (src + 0.5f): src = 8388609.000000  dst = 8388610.000000
fcvt.w.s rdn: src = 8388610.000000 dst = 8388610
RESULT: 8388610  (JAVA Math.round: 8388609)


fadd.s rne (src + 0.5f): src = 8388611.000000  dst = 8388612.000000
fcvt.w.s rdn: src = 8388612.000000 dst = 8388612
RESULT: 8388612  (JAVA Math.round: 8388611)

if RDN is set for fadd.s then:

fadd.s rdn (src + 0.5f): src = 8388609.000000  dst = 8388609.000000
fcvt.w.s rdn: src = 8388609.000000 dst = 8388609
RESULT: 8388609  (JAVA Math.round: 8388609)


fadd.s rdn (src + 0.5f): src = 8388611.000000  dst = 8388611.000000
fcvt.w.s rdn: src = 8388611.000000 dst = 8388611
RESULT: 8388611  (JAVA Math.round: 8388611)

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PR Comment: https://git.openjdk.org/jdk/pull/16382#issuecomment-1871616845