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

[LIU Zhiwei]

On 2020/6/9 3:34, Richard Henderson wrote:
> 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.)
Yes. It's the same for me.
> 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.
Some different here.

It's directly implemented in half-precision unit. So I'm afraid I have 
to implement as many as interfaces like float16.

Best Regards,
Zhiwei

> r~