Re: [PATCH v1 05/11] fpu: Add float_round_to_odd_inf

[Peter Maydell]

On Sat, 17 Apr 2021 at 01:02, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> For Arm BFDOT and BFMMLA, we need a version of round-to-odd
> that overflows to infinity, instead of the max normal number.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/fpu/softfloat-types.h | 4 +++-
>  fpu/softfloat.c               | 8 ++++++--
>  2 files changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
> index 8a3f20fae9..3b757c3d6a 100644
> --- a/include/fpu/softfloat-types.h
> +++ b/include/fpu/softfloat-types.h
> @@ -134,8 +134,10 @@ typedef enum __attribute__((__packed__)) {
>      float_round_up           = 2,
>      float_round_to_zero      = 3,
>      float_round_ties_away    = 4,
> -    /* Not an IEEE rounding mode: round to the closest odd mantissa value */
> +    /* Not an IEEE rounding mode: round to closest odd, overflow to max */
>      float_round_to_odd       = 5,
> +    /* Not an IEEE rounding mode: round to closest odd, overflow to inf */
> +    float_round_to_odd_inf   = 6,
>  } FloatRoundMode;
>
>  /*
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 67cfa0fd82..76097679b0 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -694,13 +694,12 @@ static FloatParts round_canonical(FloatParts p, 
> float_status *s,
>
>      switch (p.cls) {
>      case float_class_normal:
> +        overflow_norm = false;
>          switch (s->float_rounding_mode) {
>          case float_round_nearest_even:
> -            overflow_norm = false;
>              inc = ((frac & roundeven_mask) != frac_lsbm1 ? frac_lsbm1 : 0);
>              break;
>          case float_round_ties_away:
> -            overflow_norm = false;
>              inc = frac_lsbm1;
>              break;
>          case float_round_to_zero:
> @@ -717,6 +716,8 @@ static FloatParts round_canonical(FloatParts p, 
> float_status *s,
>              break;
>          case float_round_to_odd:
>              overflow_norm = true;
> +            /* fall through */
> +        case float_round_to_odd_inf:
>              inc = frac & frac_lsb ? 0 : round_mask;
>              break;
>          default:
> @@ -771,6 +772,7 @@ static FloatParts round_canonical(FloatParts p, 
> float_status *s,
>                             ? frac_lsbm1 : 0);
>                      break;
>                  case float_round_to_odd:
> +                case float_round_to_odd_inf:
>                      inc = frac & frac_lsb ? 0 : round_mask;
>                      break;
>                  default:
> @@ -6860,6 +6862,8 @@ float128 float128_round_to_int(float128 a, float_status 
> *status)
>
>              case float_round_to_zero:
>                  break;
> +            default:
> +                g_assert_not_reached();
>              }
>              return packFloat128( aSign, 0, 0, 0 );
>          }

This code change looks OK as far as it goes, but there are a bunch
of other places in softfloat.c which switch on the float rounding mode.
If this rounding mode is only supported for a particular subset of
operations we should at least document that in the comment.

thanks
-- PMM