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

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

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Hi Maciej:

It’s not intentionally, I suspect that is because I port from our internal
old gcc branch, will send patch to fix that later, thanks for catching this!

Maciej W. Rozycki <macro@embecosm.com>於 2022年12月5日 週一，21:05寫道：

> 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
>