RFR: 8312569: RISC-V: Missing intrinsics for Math.ceil, floor, rint [v7]

[Ilya Gavrilin]

> 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

Ilya Gavrilin has updated the pull request incrementally with one additional commit since the last revision:

  Fix intrinsic round_node parameter

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14991/files
  - new: https://git.openjdk.org/jdk/pull/14991/files/492fb25c..82b5e593

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14991&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14991&range=05-06

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14991.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14991/head:pull/14991

PR: https://git.openjdk.org/jdk/pull/14991