Re: fpu/softfloat: a question on BFloat 16 support on QEMU

[Richard Henderson]

On 6/8/20 5:53 AM, LIU Zhiwei wrote:
> Hi Richard,
> 
> I am doing bfloat16 support on QEMU.
> 
> Once I tried to reuse float32 interface, but I couldn't properly process
> rounding in some insns like fadd.
> 
> What's your opinion about it? Should I expand the fpu/softfloat?

Yes, we need to expand fpu/softfloat.

You'll want something like

static const FloatFmt bfloat16_params = {
    FLOAT_PARAMS(8, 7)
};

(This would be the Arm and x86 definition, anyway; hopefully risc-v is the 
same.)

And then add all of the other interface functions that you need to use that
parameter.

FWIW, it appears that Arm only requires:

  float32_to_bfloat16
  bfloat16_mul
  bfloat16_add

and I could even get away with only float32_to_bfloat16, since technically
Arm's BFAdd and BFMul psuedo-code are implemented in terms of single-precision
arithmetic, followed by a round-to-odd into BFloat16.


r~