RFR: 8312569: RISC-V: Missing intrinsics for Math.ceil, floor, rint

[Feilong Jiang]

On Mon, 24 Jul 2023 08:22:52 GMT, Ilya Gavrilin <duke at openjdk.org> wrote:

> Please review this changes into risc-v double rounding intrinsic.
> 
> On risc-v intrinsics for rounding doubles with mode (like Math.ceil/floor/rint) were missing. On risc-v we don`t have special instruction for such conversion, so two times conversion was used: double -> long int -> double (using fcvt.l.d, fcvt.d.l).
> 
> Also, we should provide some rounding mode to fcvt.x.x instruction.
> 
> Rounding mode selection on ceil (similar for floor and rint): according to Math.ceil requirements: 
> 
>> Returns the smallest (closest to negative infinity) value that is greater than or equal to the argument and is equal to a mathematical integer (Math.java:475).
> 
> For double -> long int we choose rup (round towards +inf) mode to get the integer that more than or equal to the input value. 
> For long int -> double we choose rdn (rounds towards -inf) mode to get the smallest (closest to -inf) representation of integer that we got after conversion.
> 
> For cases when we got inf, nan, or value more than 2^63 return input value (double value which more than 2^63 is guaranteed integer).
> As well when we store result we copy sign from input value (need for cases when for (-1.0, 0.0) ceil need to return -0.0).
> 
> We have observed significant improvement on hifive and thead boards.
> 
> testing: tier1, tier2 and hotspot:tier3 on hifive
> 
> Performance results on hifive (FpRoundingBenchmark.testceil/floor/rint):
> 
> Without intrinsic:
> 
> Benchmark                      (TESTSIZE)   Mode  Cnt   Score   Error   Units
> FpRoundingBenchmark.testceil         1024  thrpt   25  39.297 ± 0.037  ops/ms
> FpRoundingBenchmark.testfloor        1024  thrpt   25  39.398 ± 0.018  ops/ms
> FpRoundingBenchmark.testrint         1024  thrpt   25  36.388 ± 0.844  ops/ms
> 
> With intrinsic:
> 
> Benchmark                      (TESTSIZE)   Mode  Cnt   Score   Error   Units
> FpRoundingBenchmark.testceil         1024  thrpt   25  80.560 ± 0.053  ops/ms
> FpRoundingBenchmark.testfloor        1024  thrpt   25  80.541 ± 0.081  ops/ms
> FpRoundingBenchmark.testrint         1024  thrpt   25  80.603 ± 0.071  ops/ms

Thanks for the work! With one comment:

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 4286:

> 4284:   slli(mask, mask, 63);
> 4285:   // conversion from double to long
> 4286:   fcvt_l_d(converted_dbl, src, rm_direct);

How about using `fclass` [1] to check the special cases of input, then we can just do `fcvt.l.d` and `fcvt.d.l` for normal inputs? We can check the result of `fclass`. If the input contains NaN/infinity/+0/-0, we could return the value without conversion.

1. https://github.com/riscv/riscv-isa-manual/blob/3a6edf7ebf6af9e6ad92ace865c0069090870c20/src/f-st-ext.adoc?plain=1#L487-L500

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PR Review: https://git.openjdk.org/jdk/pull/14991#pullrequestreview-1559023432
PR Review Comment: https://git.openjdk.org/jdk/pull/14991#discussion_r1281913449