Re: [Qemu-ppc] [RFC PATCH v0] softfloat: Add round-to-odd rounding mode

[Richard Henderson]

On 01/18/2017 09:14 PM, Bharata B Rao wrote:
> Power ISA 3.0 introduces a few quadruple precision floating point
> instructions that support round-to-add 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 <address@hidden>
> ---
> - I am not fully sure if this the correct implementation for the above
>   described round-to-odd rounding method. Any help is appreciated.
> - Didn't bother to add round-to-odd to other floating point precision
>   variants as round-to-odd option is currently supported only for some
>   instructions that work on quad precision.
>
>  fpu/softfloat.c         | 6 ++++++
>  include/fpu/softfloat.h | 1 +
>  2 files changed, 7 insertions(+)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index c295f31..05932a9 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -1149,6 +1149,9 @@ 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();
>      }
> @@ -1215,6 +1218,9 @@ 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();
>              }

I believe you've missed the section in between that deals with round-to-largest 
or to infinity:

            if (    ( roundingMode == float_round_to_zero )
                 || ( zSign && ( roundingMode == float_round_up ) )
                 || ( ! zSign && ( roundingMode == float_round_down ) )
               ) {

The description in see in the manual on page 387 is more precise than what you 
quote above:

  # If IR is exact, choose IR.
  # Otherwise choose NL, and if Guard=1, Round=1 or Sticky=1,
  # the least significant bit of the result is set to 1.

The "choose NL" part means round-to-zero, which means that we do not overflow 
to infinity.  And since FLOAT128_MAX is already odd, we're done.

Otherwise this looks ok.


r~