Re: [PATCH 10/11] softfloat: Sink frac_cmp in parts_pick_nan until needed

[Richard Henderson]

On 12/5/24 11:51, Richard Henderson wrote:
> On 12/5/24 07:48, Peter Maydell wrote:
> > On Tue, 3 Dec 2024 at 20:40, Richard Henderson
> > <richard.henderson@linaro.org> wrote:
> > > Move the fractional comparison to the end of the
> > > float_2nan_prop_x87 case.  This is not required for
> > > any other 2nan propagation rule.  Reorganize the
> > > x87 case itself to break out of the switch when the
> > > fractional comparison is not required.
> > >
> > > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> >
> > Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
> >
> > > @@ -89,20 +84,24 @@ static FloatPartsN *partsN(pick_nan)(FloatPartsN *a, 
> > > FloatPartsN *b,
> > >            * return the NaN with the positive sign bit (if any).
> > >            */
> > >           if (is_snan(a->cls)) {
> > > -            if (is_snan(b->cls)) {
> > > -                which = cmp > 0 ? 0 : 1;
> > > -            } else {
> > > +            if (!is_snan(b->cls)) {
> > >                   which = is_qnan(b->cls) ? 1 : 0;
> > > +                break;
> > >               }
> > >           } else if (is_qnan(a->cls)) {
> > >               if (is_snan(b->cls) || !is_qnan(b->cls)) {
> >
> > Pre-existing code, but isn't
> >     is_snan(X) || !is_qnan(X)
> > the same as
> >     !is_qnan(X)
> > ?
>
> No, since X may not be a NaN at all.  We arrive in pick_nan knowing only that one operand 
> must be a NaN, but the other may be anything at all.

Alternately, and more, correctly, you're right.  :-}


r~