Re: Re: [PATCH] RISC-V: Add rounding mode operand for floating point instructions

[Kito Cheng <kito.cheng@gmail.com>]

Oh, Craig says vfrsqrt7.v not have frm but vsqrt.v have frm, and
checked spike that match that.

On Mon, May 15, 2023 at 9:55 PM 钟居哲 <juzhe.zhong@rivai.ai> wrote:
>
> I don't know why we should not add frm vfsqrt.v since I saw topper (LLVM maintainer) said we should
> not add frm into vsqrt.v. Maybe kito knows the reason ?
>
> https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/226
>
>
>
>
> juzhe.zhong@rivai.ai
>
> From: Jeff Law
> Date: 2023-05-15 21:52
> To: juzhe.zhong; gcc-patches
> CC: kito.cheng; kito.cheng; palmer; palmer; rdapp.gcc
> Subject: Re: [PATCH] RISC-V: Add rounding mode operand for floating point instructions
>
>
> On 5/15/23 05:49, juzhe.zhong@rivai.ai wrote:
> > From: Juzhe-Zhong <juzhe.zhong@rivai.ai>
> >
> > This patch is adding rounding mode operand and FRM_REGNUM dependency
> > into floating-point instructions.
> >
> > The floating-point instructions we added FRM and rounding mode operand:
> > 1. vfadd/vfsub
> > 2. vfwadd/vfwsub
> > 3. vfmul
> > 4. vfdiv
> > 5. vfwmul
> > 6. vfwmacc/vfwnmacc/vfwmsac/vfwnmsac
> > 7. vfsqrt7/vfrec7
> > 8. floating-point conversions.
> > 9. floating-point reductions.
> >
> > The floating-point instructions we did NOT add FRM and rounding mode operand:
> > 1. vfsqrt/vfneg
> Assuming vfsqrt is actually an estimator the best place to handle
> rounding modes is at the last step(s) after N-R or Goldschmidt
> refinement steps.  I haven't paid too much attention to FP yet, but this
> is an area I've got fairly extensive experience.
>
> Sadly RISC-V's estimator is fairly poor and the single instance FMACs
> are going to result in an implementation that may not actually be any
> better than what glibc can do.
>
> Jeff
>