RFR: 8318227: RISC-V: C2 ConvHF2F

[Hamlin Li]

On Fri, 24 Nov 2023 17:42:22 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp line 1691:
>> 
>>> 1689:   fmv_h_x(dst, src);
>>> 1690:   fcvt_s_h(dst, dst);
>>> 1691:   j(done);
>> 
>> Since `Nan`s are exceptional inputs, it would be beneficial to move the handling below to an out-of-line stub.
>
> Thanks for the suggestion, it make sense.
> 
> Just I'm not sure if there is a straight way to implement it currently. Following is what I've done, but blocked, please help to share your opinion.
>  
> To jump to stub code to process NaN case, the stub code need to use specific register for `src` and `dst`, as `dst` is a FloatRegister, so need a way to specify a float register when matching `ConvHF2F` in ad file. I tried to add a `fRegF_F10` to enable specify a float register, but it does not work well. Some error occurs for fastdebug version:
> 
> 
> ----------System.out:(47/2030)----------
> Start ...
> o558  SubF  === _ o515 o559  [[ o557 ]]
> 
> --N: o558  SubF  === _ o515 o559  [[ o557 ]]
> 
>    --N: o515  MoveI2F  === _ o681  [[ o470 o484 o620 o620 o618 o579 o558 o530 o434 4 10 10 ]]
>    FREGF  0  FREGF
>    FREGF_F10  0  FREGF_F10
> 
>    --N: o559  ConvHF2F  === _ o560  [[ o558 ]]  #float
>    FREGF_F10  100  convHF2F_reg_reg
> 
>       --N: o560  RShiftI  === _ o561 o209  [[ o559 o434 4 ]]
>       IREGI  0  IREGI
>       IREGINOSP  0  IREGINOSP
>       IREGI_R10  0  IREGI_R10
>       IREGI_R11  0  IREGI_R11
>       IREGI_R12  0  IREGI_R12
>       IREGI_R13  0  IREGI_R13
>       IREGI_R14  0  IREGI_R14
>       IREGIORL2I  0  IREGI
>       IREGIORL  0  IREGI
>       IREGILNP  0  IREGI
>       IREGILNPNOSP  0  IREGINOSP
> 
> #
> # A fatal error has been detected by the Java Runtime Environment:
> #
> #  Internal Error (/home/hamlin/workspace/repos/github/jdk/src/hotspot/share/opto/matcher.cpp:1727), pid=2212108, tid=2212156
> #  assert(false) failed: bad AD file

The workaround I can use is to mv the result back to `dst` when comes back from stub code, like below:

  bind(nan_case);
  // input:   x11
  // output:  x10
  RuntimeAddress stub = RuntimeAddress(StubRoutines::riscv::float16_to_float_process_nan());
  assert(stub.target() != nullptr, "float16_to_float_process_nan stub has not been generated");
  address call = trampoline_call(stub);
  fmv_w_x(dst, x10);


But, this seems to me a bit more complicated.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16802#discussion_r1404565645