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

[LIU Zhiwei]

On 2020/6/9 17:42, Alex Bennée wrote:
> LIU Zhiwei <zhiwei_liu@c-sky.com> writes:
>
> > On 2020/6/8 23:50, Alex Bennée wrote:
> > > LIU Zhiwei <zhiwei_liu@c-sky.com> writes:
> > >
> > > > 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 do you mean by re-use the float32 interface?
> > Once I think bfloat16 can been converted to float32  by
> >
> > deposit32(0, 16, 16, bf16)
> >
> > Then do a bfloat16 op by float32 op.
> No I don't think you want to be munging things like that - best to
> decompose it into FloatParts and let the common code deal with the
> actual calculation.
>
> We've learnt the hard way that having lots of slightly different
> functions each dealing with edge cases and rounding ends up in mistakes
> creeping in. The common code path is well tested and a lot easier to
> understand.
Get it. Thanks.
If I am ready, I will upstream the bfloat16 code.

Best Regards,
Zhiwei
> > At last, get the bfloat16 result by right shift the float32 result 16
> > bits.
> Again the common round and packing code should be agnostic to the
> underlying precision.
>
> > > Isn't bfloat16 going
> > > to be pretty much the same as float16 but with some slightly different
> > > float parameters for the different encoding?
> > Agree.
> > > Like the float16 code it won't have to deal with any of the hardfloat
> > > wrappers so it should look pretty similar.
> > Good idea. I will list the float16 interfaces,  and try to emulate the
> > bfloat16 one by one.
> >
> > I list float16 interfaces in softfloat.c alone. It counts 67 interfaces.
> > > > What's your opinion about it? Should I expand the fpu/softfloat?
> > > bfloat16 is certainly going to become more common that we should have
> > > common softfloat code to handle it. It would be nice is TestFloat could
> > > exercise it as well.
> > Thanks. I will try to use make check-softfloat to test bfloat16 interfaces.
> >
> > Best Regards,
> > Zhiwei
> > > > Best Regards,
> > > > Zhiwei