RFR: 8302908: RISC-V: Support masked vector arithmetic instructions for Vector API [v12]

[Dingli Zhang]

On Thu, 6 Apr 2023 04:04:50 GMT, Yanhong Zhu <yzhu at openjdk.org> wrote:

>> Dingli Zhang has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix comment
>
> src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp line 1731:
> 
>> 1729:   if (bt == T_FLOAT || bt == T_DOUBLE) {
>> 1730:     switch (cond) {
>> 1731:       case BoolTest::eq: vmfeq_vv(vd, src1, src2, vm); break;
> 
> `BoolTest::ge` and `BoolTest::gt` are implemented with `BoolTest::le` and `BoolTest::lt` by exchanging the operands, when one of the operands is NAN, will the results of comparisons be wrong?

Thanks for the review!
I think there should be no problem here. The foating-point compare instructions follow the semantics of the scalar floating-point compare instructions[1] in RVV. For all three instructions (FEQ.S, FLT.S, FLE.S), the result is 0 if either operand is NaN[2]. So when one of the operands is NaN, `BoolTest::ge`, `BoolTest::gt`, `BoolTest::le` and `BoolTest::lt` will all generate a 0 on the corresponding bit. 

Also a jtreg test case[3] proves that our current logic is fine. `GTFloat512VectorTests` covers the case where the input is Nan. The test will pass properly and generate the following compilation log which contains `vmaskcmp_rvv`:


1ac     B20: # out( B49 B21 ) <- in( B48 B19 )  Freq: 4188.06
1ac     vmaskcmp_rvv V0, V4, V5, #3
1b8     
1b8     MEMBAR-store-store #@membar_storestore
1bc     # checkcastPP of R11, #@checkCastPP
1bc     vstoremask V1, V0
1c8     addi  R7, R11, #16 # ptr, #@addP_reg_imm
1cc     spill R11 -> [sp, #104] # spill size = 64
1ce     storeV [R7], V1 # vector (rvv)
1d6     ld  R19, [R23, #264] # ptr, #@loadP
1da     ld  R7, [R23, #280] # ptr, #@loadP
1de     addi  R28, R19, #16 # ptr, #@addP_reg_imm
1e2     bgeu  R28, R7, B49 #@cmpP_branch  P=0.000100 C=-1.000000


[1] https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#1313-vector-floating-point-compare-instructions
[2] https://github.com/riscv/riscv-isa-manual/releases/download/draft-20230131-c0b298a/riscv-spec.pdf
[3] https://github.com/openjdk/jdk/tree/master/test/jdk/jdk/incubator/vector/Float512VectorTests.java

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PR Review Comment: https://git.openjdk.org/jdk/pull/12682#discussion_r1159483619