[Bug libgcc/99048] New: __gcc_qadd produces spurious NaN

[Bug libgcc/99048] New: __gcc_qadd produces spurious NaN

[bbarenblat at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048

            Bug ID: 99048
           Summary: __gcc_qadd produces spurious NaN
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bbarenblat at gmail dot com
  Target Milestone: ---
            Target: powerpc64le-linux-gnu

Created attachment 50160
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50160&action=edit
minimal example

__gcc_qadd, which adds two long doubles on POWER, can emit NaN for non-NaN
input:

        $ cat ldbl.c
        extern int printf(const char* restrict, ...);
        int main() {
                long double a = -0x1.1e282e024debdb875f47f6c85p+1021L;
                printf("%Le\n", a + 0x1.fffffffffffff7ffffffffffff8p+1023L);
        }
        $ gcc -O0 -o ldbl ldbl.c
        $ ./ldbl
        nan
        $ gcc -O1 -o ldbl ldbl.c
        $ ./ldbl
        1.546510e+308

Examining the assembly from the second invocation shows that GCC has optimized
out the entire computation. In the first invocation, however, the compiled
program calls __gcc_qadd, which yields NaN. By IEEE 754, adding two non-NaN
inputs should never produce a NaN output.

Output of 'gcc -v':
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/powerpc64le-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: powerpc64le-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 10.2.1-6'
--with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--program-prefix=powerpc64le-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-secureplt
--with-cpu=power8 --enable-targets=powerpcle-linux --disable-multilib
--enable-multiarch --disable-werror --with-long-double-128
--enable-offload-targets=nvptx-none=/build/gcc-10-jGDcMs/gcc-10-10.2.1/debian/tmp-nvptx/usr
--without-cuda-driver --enable-checking=release --build=powerpc64le-linux-gnu
--host=powerpc64le-linux-gnu --target=powerpc64le-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-mutex
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.1 20210110 (Debian 10.2.1-6)

[Bug target/99048] __gcc_qadd produces spurious NaN

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |segher at gcc dot gnu.org

--- Comment #1 from Segher Boessenkool <segher at gcc dot gnu.org> ---
IBM long double ("double-double") is not an IEEE floating point format,
so all these rules do not apply, but you are right it is surprising.

[Bug target/99048] __gcc_qadd produces spurious NaN

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
a = -2.5118280672773279e+307
c = 1.7976931348623157e+308
c is in particular the maximum normalized number, __DBL_MAX__, binary
0x7fefffffffffffff.
126       z = a + c;
z = 1.546510328134583e+308

and that is finite.

144           q = a - z;

But this is -inf.
145           zz = q + c + (a - (q + z)) + aa + cc;
and everything goes downhill from there, as a, c, z, aa and cc are all finite,
but q is -inf and a - (-inf + finite) is +inf and -inf + +inf is nan.

So, the code:
  z = a + c;

  if (nonfinite (z))
    {
...
    }
  else
    {
      q = a - z;
would need another if (nonfinite (q)) check after computation of q (and I don't
believe it could just compute q = a - z; right after z = a + c; and replace if
(nonfinite (z)) with if (nonfinite (q)) because if a is +-inf and c is finite,
z will be +-inf and q will be nan.

[Bug target/99048] __gcc_qadd produces spurious NaN

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99048

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-02-12
     Ever confirmed|0                           |1

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Yup, something like that.  It should not have any infinities here afaics.