Re: [PATCH v2 1/2] RISC-V: Support _Float16 type.

["Maciej W. Rozycki" <macro@embecosm.com>]

Hi Kito,

 I came across this issue while inspecting code and I have been wondering 
what the reason was to downgrade current FMV.X.W and FMW.W.X instructions 
to their older FMV.S.W and FMV.W.S variants here:

On Wed, 10 Aug 2022, Kito Cheng wrote:

> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> index 5a0adffb5ce..47e6110767c 100644
> --- a/gcc/config/riscv/riscv.cc
> +++ b/gcc/config/riscv/riscv.cc
> @@ -2308,10 +2310,19 @@ riscv_output_move (rtx dest, rtx src)
>    if (dest_code == REG && GP_REG_P (REGNO (dest)))
>      {
>        if (src_code == REG && FP_REG_P (REGNO (src)))
> -	return dbl_p ? "fmv.x.d\t%0,%1" : "fmv.x.w\t%0,%1";
> +	switch (width)
> +	  {
> +	  case 2:
> +	    /* Using fmv.x.s + sign-extend to emulate fmv.x.h.  */
> +	    return "fmv.x.s\t%0,%1;slli\t%0,%0,16;srai\t%0,%0,16";
> +	  case 4:
> +	    return "fmv.x.s\t%0,%1";
> +	  case 8:
> +	    return "fmv.x.d\t%0,%1";
> +	  }

and here:

> @@ -2353,18 +2364,24 @@ riscv_output_move (rtx dest, rtx src)
>  	    return "mv\t%0,%z1";
>  
>  	  if (FP_REG_P (REGNO (dest)))
> -	    {
> -	      if (!dbl_p)
> -		return "fmv.w.x\t%0,%z1";
> -	      if (TARGET_64BIT)
> -		return "fmv.d.x\t%0,%z1";
> -	      /* in RV32, we can emulate fmv.d.x %0, x0 using fcvt.d.w */
> -	      gcc_assert (src == CONST0_RTX (mode));
> -	      return "fcvt.d.w\t%0,x0";
> -	    }
> +	    switch (width)
> +	      {
> +	      case 2:
> +		/* High 16 bits should be all-1, otherwise HW will treated
> +		   as a n-bit canonical NaN, but isn't matter for softfloat.  */
> +		return "fmv.s.x\t%0,%1";
> +	      case 4:
> +		return "fmv.s.x\t%0,%z1";
> +	      case 8:
> +		if (TARGET_64BIT)
> +		  return "fmv.d.x\t%0,%z1";
> +		/* in RV32, we can emulate fmv.d.x %0, x0 using fcvt.d.w */

(Incorrect comment formatting here as well.)

> +		gcc_assert (src == CONST0_RTX (mode));
> +		return "fcvt.d.w\t%0,x0";
> +	      }

Was it intentional or just an oversight in review?  If intentional, I'd 
expect such a change to happen on its own rather than sneaked in with a 
large functional update.

  Maciej