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

[Olga Mikhaltsova]

On Fri, 22 Dec 2023 09:35:08 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Olga Mikhaltsova has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Moved the code up + comments
>
> For normal cases, I guess `RUP` of riscv will work; but for some corner cases, we need the trick of `+0.5`, am I right?
> But all this information is just mentioned with `some inputs produce incorrect results`, which is unclear for potential readers and maintainers in the future.
> So, in the comments, can you add some information about this corner case, a simple example will definitely help here.

@Hamlin-Li No, that’s wrong: RUP of riscv won’t work, it’ll give incorrect results. Look at the rounding of some values below for example: 

JAVA Math.round: src =  0.345200  dst =  0
JAVA Math.round: src = -0.555550  dst = -1
JAVA Math.round: src = -1.500000  dst = -1
JAVA Math.round: src =  1.500000  dst =  2
JAVA Math.round: src = -1.345000  dst = -1
JAVA Math.round: src = -1.450000  dst = -1
JAVA Math.round: src = -0.444460  dst =  0
JAVA Math.round: src = -0.999990  dst = -1
JAVA Math.round: src =  0.999999  dst =  1
JAVA Math.round: src =  0.000000  dst =  0
JAVA Math.round: src =  0.001000  dst =  0
JAVA Math.round: src = -0.001000  dst =  0


FCVT.W.S RUP: src =  0.345200  dst =  1
FCVT.W.S RUP: src = -0.555550  dst =  0
FCVT.W.S RUP: src = -1.500000  dst = -1
FCVT.W.S RUP: src =  1.500000  dst =  2
FCVT.W.S RUP: src = -1.345000  dst = -1
FCVT.W.S RUP: src = -1.450000  dst = -1
FCVT.W.S RUP: src = -0.444460  dst =  0
FCVT.W.S RUP: src = -0.999990  dst =  0
FCVT.W.S RUP: src =  0.999990  dst =  1
FCVT.W.S RUP: src =  0.000000  dst =  0
FCVT.W.S RUP: src =  0.001000  dst =  1
FCVT.W.S RUP: src = -0.001000  dst =  0

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PR Comment: https://git.openjdk.org/jdk/pull/16382#issuecomment-1871612334