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

[钟居哲 <juzhe.zhong@rivai.ai>]

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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