[RFC,v2,2/2] softfloat: Add round-to-odd rounding mode

Power ISA 3.0 introduces a few quadruple precision floating point
instructions that support round-to-odd rounding mode. The
round-to-odd mode is explained as under:

Let Z be the intermediate arithmetic result or the operand of a convert
operation. If Z can be represented exactly in the target format, the
result is Z. Otherwise the result is either Z1 or Z2 whichever is odd.
Here Z1 and Z2 are the next larger and smaller numbers representable
in the target format respectively.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 fpu/softfloat.c         | 17 ++++++++++++++++-
 include/fpu/softfloat.h |  2 ++
 2 files changed, 18 insertions(+), 1 deletion(-)

On 27 January 2017 at 08:03, Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> Power ISA 3.0 introduces a few quadruple precision floating point
> instructions that support round-to-odd rounding mode. The
> round-to-odd mode is explained as under:
>
> Let Z be the intermediate arithmetic result or the operand of a convert
> operation. If Z can be represented exactly in the target format, the
> result is Z. Otherwise the result is either Z1 or Z2 whichever is odd.
> Here Z1 and Z2 are the next larger and smaller numbers representable
> in the target format respectively.
>
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> ---
>  fpu/softfloat.c         | 17 ++++++++++++++++-
>  include/fpu/softfloat.h |  2 ++
>  2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index b04699c..1c322ad 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -623,6 +623,9 @@ static float64 roundAndPackFloat64(flag zSign, int zExp, uint64_t zSig,
>      case float_round_down:
>          roundIncrement = zSign ? 0x3ff : 0;
>          break;
> +    case float_round_to_odd:
> +        roundIncrement = (zSig & 0x400) ? 0 : 0x3ff;
> +        break;
>      default:
>          abort();
>      }
> @@ -632,8 +635,10 @@ static float64 roundAndPackFloat64(flag zSign, int zExp, uint64_t zSig,
>               || (    ( zExp == 0x7FD )
>                    && ( (int64_t) ( zSig + roundIncrement ) < 0 ) )
>             ) {
> +            bool overflow_to_inf = roundingMode != float_round_to_odd &&
> +                                   roundIncrement != 0;
>              float_raise(float_flag_overflow | float_flag_inexact, status);
> -            return packFloat64( zSign, 0x7FF, - ( roundIncrement == 0 ));
> +            return packFloat64( zSign, 0x7FF, - ( !overflow_to_inf ));

Can you follow the QEMU coding style, please? softfloat's coding
style is insane and we generally have been switching it to the
QEMU style as we touch lines of code.

>          }
>          if ( zExp < 0 ) {
>              if (status->flush_to_zero) {
> @@ -665,6 +670,9 @@ static float64 roundAndPackFloat64(flag zSign, int zExp, uint64_t zSig,
>              case float_round_down:
>                  roundIncrement = zSign ? 0x3ff : 0;
>                  break;
> +            case float_round_to_odd:
> +                roundIncrement = (zSig & 0x400) ? 0 : 0x3ff;
> +                break;
>              default:
>                  abort();
>              }

Ah, this extra check does make a difference for the round-to-odd
case. But since it's a special for round-to-odd, better to
phrase as
  if (roundingMode == float_round_to_odd) {
      /* in this case roundIncrement depends on zSig, which just changed */
      roundIncrement = ...;
  }

rather than a case statement where most of the code is no-ops.

> @@ -1166,6 +1174,9 @@ static float128 roundAndPackFloat128(flag zSign, int32_t zExp,
>      case float_round_down:
>          increment = zSign && zSig2;
>          break;
> +    case float_round_to_odd:
> +        increment = !(zSig1 & 0x1) && zSig2;
> +        break;
>      default:
>          abort();
>      }
> @@ -1185,6 +1196,7 @@ static float128 roundAndPackFloat128(flag zSign, int32_t zExp,
>              if (    ( roundingMode == float_round_to_zero )
>                   || ( zSign && ( roundingMode == float_round_up ) )
>                   || ( ! zSign && ( roundingMode == float_round_down ) )
> +                 || ( roundingMode == float_round_to_odd )
>                 ) {
>                  return
>                      packFloat128(
> @@ -1232,6 +1244,9 @@ static float128 roundAndPackFloat128(flag zSign, int32_t zExp,
>              case float_round_down:
>                  increment = zSign && zSig2;
>                  break;
> +            case float_round_to_odd:
> +                increment = !(zSig1 & 0x1) && zSig2;
> +                break;
>              default:
>                  abort();
>              }
> diff --git a/include/fpu/softfloat.h b/include/fpu/softfloat.h
> index 842ec6b..8a39028 100644
> --- a/include/fpu/softfloat.h
> +++ b/include/fpu/softfloat.h
> @@ -180,6 +180,8 @@ enum {
>      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 */
> +    float_round_to_odd       = 5,
>  };
>
>  /*----------------------------------------------------------------------------
> --
> 2.7.4
>

Otherwise:
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM