Re: [Qemu-devel] [PATCH] fix setting the FPSCR[FR] bit

[Peter Maydell]

On 17 September 2018 at 22:18, John Arbuckle <address@hidden> wrote:
> https://www.nxp.com/files-static/product/doc/MPCFPE32B.pdf
> Page 2-8 in table 2-4 is where the description of this bit can be found.
>
> It is described as:
>
> Floating-point fraction rounded. The last arithmetic, rounding, or conversion 
> instruction incremented the fraction. This bit is NOT sticky.
>
> This patch actually implements the setting and unsetting of this bit.
>
> Signed-off-by: John Arbuckle <address@hidden>
> ---
>  fpu/softfloat.c               | 12 ++++++++++--
>  include/fpu/softfloat-types.h |  1 +
>  target/ppc/fpu_helper.c       | 12 ++++++++++++
>  3 files changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 59ca356d0e..c5378ae9e8 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -751,9 +751,17 @@ float64 __attribute__((flatten)) float64_add(float64 a, 
> float64 b,
>  {
>      FloatParts pa = float64_unpack_canonical(a, status);
>      FloatParts pb = float64_unpack_canonical(b, status);
> -    FloatParts pr = addsub_floats(pa, pb, false, status);
> +    FloatParts intermediate_parts = addsub_floats(pa, pb, false, status);
>
> -    return float64_round_pack_canonical(pr, status);
> +    float64 rounded_result = float64_round_pack_canonical(intermediate_parts,
> +                                                          status);
> +    FloatParts rounded_parts = float64_unpack_canonical(rounded_result, 
> status);
> +
> +    if (rounded_parts.frac != intermediate_parts.frac) {
> +        float_raise(float_flag_round, status);
> +    }
> +
> +    return rounded_result;
>  }

Only changing the add instruction looks very definitely wrong.
Doing a pack-unpack-compare looks dubious.

I think that you can do this by using the existing overflow and
inexact flags. If that is not possible (but I'm pretty sure it should
be doable), then the next best approach is probably to have the new
float flag be set at the right point in the round-and-pack code.

thanks
-- PMM