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.
