On 21 January 2013 21:32, Richard Sandiford <rdsandif...@googlemail.com> 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 <rdsandif...@googlemail.com>
> ---
> 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
