Re: [Qemu-devel] [PATCH] softfloat: Handle float_muladd_negate_c when product is zero

[Peter Maydell]

On 21 January 2013 21:32, Richard Sandiford <address@hidden> wrote:
> Honour float_muladd_negate_c in the case where the product is zero and
> c is nonzero.  Previously we would fail to negate c.
>
> Seen in (and tested against) the gfortran testsuite on MIPS.
>
> Signed-off-by: Richard Sandiford <address@hidden>
> ---
>  fpu/softfloat.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index ac3d150..0028415 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -2234,6 +2234,9 @@ float32 float32_muladd(float32 a, float32 b, float32 c, 
> int flags STATUS_PARAM)
>              }
>          }
>          /* Zero plus something non-zero : just return the something */
> +        if (flags & float_muladd_negate_c) {
> +            signflip ^= 1;
> +        }
>          return make_float32(float32_val(c) ^ (signflip << 31));

This is a correct change in that it fixes a definite bug and gives
the right results, but I wonder if it might be clearer to instead
change the return to read:

     return packFloat32(cSign ^ signflip, cExp, cSig);

?

That would mean we consistently handle the negate_c flag by:
 * flip cSign as soon as we split c into its component fields
 * never refer to c again

(the source of the bug here is me trying to be clever and avoid
reassembling the float, but forgetting that cSign might have
changed.)

-- PMM