Re: [Qemu-devel] [PATCH 04/19] fpu/softfloat: Canonicalize NaN fraction

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <richard.henderson@linaro.org> writes:

> Shift the NaN fraction to a canonical position, much like we do
> for the fraction of normal numbers.  Immediately, this simplifies
> the float-to-float conversion.  Later, this will facilitate
> manipulation of NaNs within the shared code paths.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  fpu/softfloat.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 5e4982b035..df377b6314 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -329,10 +329,11 @@ static FloatParts canonicalize(FloatParts part, const FloatFmt *parm,
>          if (part.frac == 0) {
>              part.cls = float_class_inf;
>          } else {
> +            part.frac <<= parm->frac_shift;
>  #ifdef NO_SIGNALING_NANS
>              part.cls = float_class_qnan;
>  #else
> -            int64_t msb = part.frac << (parm->frac_shift + 2);
> +            int64_t msb = part.frac << 2;
>              if ((msb < 0) == status->snan_bit_is_one) {
>                  part.cls = float_class_snan;
>              } else {
> @@ -498,6 +499,7 @@ static FloatParts round_canonical(FloatParts p, float_status *s,
>      case float_class_qnan:
>      case float_class_snan:
>          exp = exp_max;
> +        frac >>= parm->frac_shift;

Unfortunately we now miss this step when we handle:

    case float_class_msnan:
        return float64_maybe_silence_nan(float64_pack_raw(p), s);

This seems to fix that:

recanonicalise frac before packing and sending

1 file changed, 3 insertions(+)
fpu/softfloat.c | 3 +++

modified   fpu/softfloat.c
@@ -532,6 +532,7 @@ static float16 float16a_round_pack_canonical(const FloatFmt *params,
     case float_class_dnan:
         return float16_default_nan(s);
     case float_class_msnan:
+        p.frac >>= params->frac_shift;
         return float16_maybe_silence_nan(float16_pack_raw(p), s);
     default:
         p = round_canonical(p, s, params);
@@ -555,6 +556,7 @@ static float32 float32_round_pack_canonical(FloatParts p, float_status *s)
     case float_class_dnan:
         return float32_default_nan(s);
     case float_class_msnan:
+        p.frac >>= float32_params.frac_shift;
         return float32_maybe_silence_nan(float32_pack_raw(p), s);
     default:
         p = round_canonical(p, s, &float32_params);
@@ -573,6 +575,7 @@ static float64 float64_round_pack_canonical(FloatParts p, float_status *s)
     case float_class_dnan:
         return float64_default_nan(s);
     case float_class_msnan:
+        p.frac >>= float64_params.frac_shift;
         return float64_maybe_silence_nan(float64_pack_raw(p), s);
     default:
         p = round_canonical(p, s, &float64_params);

--
Alex Bennée