Re: [Qemu-devel] [PATCH] target/i386: Return 'indefinite integer value' for invalid SSE fp->int conversions

[Peter Maydell]

On Mon, 5 Aug 2019 at 22:13, Richard Henderson <address@hidden> wrote:
>
> On 8/5/19 11:03 AM, Peter Maydell wrote:
> > The x86 architecture requires that all conversions from floating
> > point to integer which raise the 'invalid' exception (infinities of
> > both signs, NaN, and all values which don't fit in the destination
> > integer) return what the x86 spec calls the "indefinite integer
> > value", which is 0x8000_0000 for 32-bits or 0x8000_0000_0000_0000 for
> > 64-bits.  The softfloat functions return the more usual behaviour of
> > positive overflows returning the maximum value that fits in the
> > destination integer format and negative overflows returning the
> > minimum value that fits.
> >
> > Wrap the softfloat functions in x86-specific versions which
> > detect the 'invalid' condition and return the indefinite integer.
> >
> > Note that we don't use these wrappers for the 3DNow! pf2id and pf2iw
> > instructions, which do return the minimum value that fits in
> > an int32 if the input float is a large negative number.
> >
> > Fixes: https://bugs.launchpad.net/qemu/+bug/1815423
> > Signed-off-by: Peter Maydell <address@hidden>
> > ---
> > I've tested that this fixes the LP:1815423 test case. If anybody
> > has an x86 VM image to hand that has node.js installed it would
> > also be useful to test the operations in
> > https://bugs.launchpad.net/qemu/+bug/1832281
> > (I don't have such a VM.)
> >
> > The other approach here would be to make the softfloat functions be
> > flexible enough to allow this behaviour -- from my reading of IEEE754
> > I think the exact returned result for 'invalid' inputs for float to
> > int conversions is not specified.
> >
> >  target/i386/ops_sse.h | 88 +++++++++++++++++++++++++++++--------------
> >  1 file changed, 60 insertions(+), 28 deletions(-)
>
> I guess this is exactly what we already do in fpu_helper.c.
> Reviewed-by: Richard Henderson <address@hidden>

Huh, so we do; I didn't notice that :-)

thanks
-- PMM