Re: unnormal Intel 80-bit long doubles and isnanl

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Nov 24, 2020 at 3:20 PM Adhemerval Zanella via Gcc
<gcc@gcc.gnu.org> wrote:
>
>
>
> On 24/11/2020 10:59, Siddhesh Poyarekar wrote:
> > On 11/24/20 7:11 PM, Szabolcs Nagy wrote:
> >> ideally fpclassify (and other classification macros) would
> >> handle all representations.
> >>
> >> architecturally invalid or trap representations can be a
> >> non-standard class but i think classifying them as FP_NAN
> >> would break the least amount of code.
> >
> > That's my impression too.
> >
> >>> glibc evaluates the bit pattern of the 80-bit long double and in the
> >>> process, ignores the integer bit, i.e. bit 63.  As a result, it considers
> >>> the unnormal number as a valid long double and isnanl returns 0.
> >>
> >> i think m68k and x86 are different here.
> >>
> >>>
> >>> gcc on the other hand, simply uses the number in a floating point comparison
> >>> and uses the parity flag (which indicates an unordered compare, signalling a
> >>> NaN) to decide if the number is a NaN.  The unnormal numbers behave like
> >>> NaNs in this respect, in that they set the parity flag and with
> >>> -fsignalling-nans, would result in an invalid-operation exception.  As a
> >>> result, __builtin_isnanl returns 1 for an unnormal number.
> >>
> >> compiling isnanl to a quiet fp compare is wrong with
> >> -fsignalling-nans: classification is not supposed to
> >> signal exceptions for snan.

Can you open a bugreport for this?  Note that the option is likely
to invoke isnanl from libm ...

> > I agree, but I think that issue with __builtin_isnanl is orthogonal to the question about unnormals.  Once that is fixed in gcc, we could actually use __builtin_isnanl all the time in glibc for isnanl.
> >
> > Siddhesh
>
> Which is the currently take from gcc developers on this semantic change of
> __builtin_isnanl? Are they considering current behavior of non classifying
> the 'unnormal' as NAN the expected behavior and waiting glibc to follow
> it or are they willing to align with glibc behavior?

I think GCC should follow standards and in case they do not apply do
sth reasonable - which I think classifying those as NaN is.

Richard.