Re: [Qemu-devel] [PATCH] Fix NaN handling in softfloat

["J. Mayer" <l_indien@magic.fr>]

On Sat, 2007-11-10 at 10:35 +0100, Aurelien Jarno wrote:
> J. Mayer a écrit :
> > On Thu, 2007-11-08 at 00:05 +0100, Aurelien Jarno wrote:
> >> On Tue, Nov 06, 2007 at 09:01:13PM +0100, J. Mayer wrote:
> >>> On Sat, 2007-11-03 at 22:28 +0100, Aurelien Jarno wrote: 
> >>>> On Sat, Nov 03, 2007 at 02:06:04PM -0400, Daniel Jacobowitz wrote:
> >>>>> On Sat, Nov 03, 2007 at 06:35:48PM +0100, Aurelien Jarno wrote:
> >>>>>> Hi all,
> >>>>>>
> >>>>>> The current softfloat implementation changes qNaN into sNaN when 
> >>>>>> converting between formats, for no reason. The attached patch fixes
> >>>>>> that. It also fixes an off-by-one in the extended double precision
> >>>>>> format (aka floatx80), the mantissa is 64-bit long and not 63-bit
> >>>>>> long.
> >>>>>>
> >>>>>> With this patch applied all the glibc 2.7 floating point tests
> >>>>>> are successfull on MIPS and MIPSEL.
[...]
> >> Anyway there is no way to do that in the target specific code *after 
> >> the conversion*, as the detection of a mantissa being nul when 
> >> converting from double to single precision can only be done when both
> >> values are still known. In other words when the value is not fixed 
> >> during the conversion, the value 0x7f800000 can either be infinity or a
> >> conversion of NaN from double to single precision, and thus is it not
> >> possible to fix the value afterwards in the target specific code.
> > 
> > I don't say you have to return an infinity when the argument is a qNaN.
> > I just say you have to return a qNaN in a generic way.  Just return sign
> > | 0x7f800000 | mantissa, which is the more generic form and seems to me
> > to even be OK for sNaNs. It's even needed for some target (not to say
> 
> 0x7f800000 is actually not a NaN, but infinity.
> 
> > PowerPC) that specify that the result have to be equal to the operand
> > (in the single precision format, of course) in such a case. This is
> > simpler, it ensures that any target could then detect the presence of a
> > NaN, know which one, and can then adjust the value according to its
> > specification if needed.
> > I then still can'tl see any reason of having target specific code in
> > that area.
> 
> Ok, let's give an example then. On MIPS let's say you want to convert
> 0x7ff0000000000001 (qNaN) to single precision. The mantissa shifted to
> the right become 0, so you have to generate a new value. As you
> proposed, let's generate a "generic value" 0x7fc00000 in the softfloat
> routines. This value has to be converted to 0x7fbfffff in the MIPS
> target code.

OK, the values that can cause a problem is all values that would have a
zero mantissa once rounded to sinlge-precision. As the PowerPC requires
that the result would have a zero mantissa (and the result class set to
qNan), I can see no way to handle this case in the generic code. And
even adding a "#ifdef TARGET_PPC" won't solve the problem as the PowerPC
code would not be able to make the distinction between infinity case and
qNaN case. Then, the only solution, as you already mentioned, is to
check for qNaN before calling the rounding function. As the target
emulation code already has to check for sNaN to be able to raise an
exception when it's needed, checking for qNaN would cost nothing more;
just have to change the check if (float64_is_signaling_nan) check with a
check for NaN and handle the two cases by hand. I can see no other way
to have all cases handled for all targets specific cases, do you ?

[...]

-- 
J. Mayer <l_indien@magic.fr>
Never organized