[Bug libstdc++/104772] std::numeric_limits<__float128> should be specialized

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104772

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Dunno.
I think you can add support even without any compiler changes, at least if
_GLIBCXX_DOUBLE_IS_IEEE_BINARY64:

#ifdef __STRICT_ANSI__
      static _GLIBCXX_CONSTEXPR __float128
      min() _GLIBCXX_USE_NOEXCEPT
      { return __extension__ 3.36210314311209350626267781732175260e-4932Q; }

      static _GLIBCXX_CONSTEXPR __float128
      max() _GLIBCXX_USE_NOEXCEPT
      { return __extension__ 1.18973149535723176508575932662800702e+4932Q; }

      static _GLIBCXX_CONSTEXPR __float128
      epsilon() _GLIBCXX_USE_NOEXCEPT
      { return __extension__ 1.92592994438723585305597794258492732e-34Q; }

      static _GLIBCXX_CONSTEXPR __float128
      denorm_min() _GLIBCXX_USE_NOEXCEPT
      { return __extension__ 6.47517511943802511092443895822764655e-4966Q; }
#else
      static _GLIBCXX_CONSTEXPR __float128
      _S_1pm4088 () _GLIBCXX_USE_NOEXCEPT
      { return __float128 (0x1.0p-1022) * 0x1.0p-1022 * 0x1.0p-1022 *
0x1.0p-1022; }

      static _GLIBCXX_CONSTEXPR __float128
      _S_1pm16352 () _GLIBCXX_USE_NOEXCEPT
      { return _S_1pm4088 () * _S_1pm4088 () * _S_1pm4088 () * _S_1pm4088 (); }

      static _GLIBCXX_CONSTEXPR __float128
      _S_1p4064 () _GLIBCXX_USE_NOEXCEPT
      { return __float128 (0x1.0p+1016) * 0x1.0p-1016 * 0x1.0p-1016 *
0x1.0p-1016; }

      static _GLIBCXX_CONSTEXPR __float128
      _S_1p16256 () _GLIBCXX_USE_NOEXCEPT
      { return _S_1p4064 () * _S_1p4064 () * _S_1p4064 () * _S_1p4064 (); }

      static _GLIBCXX_CONSTEXPR __float128
      min() _GLIBCXX_USE_NOEXCEPT
      { return /* 0x1.0p-16382Q */ 0x1.0p-30 * _S_1pm16352 (); }

      static _GLIBCXX_CONSTEXPR __float128
      max() _GLIBCXX_USE_NOEXCEPT
      { return /* 0x1.ffffffffffffffffffffffffffffp+16383Q */
               (__float128 (0x1.fffffffffffffp+127) + 0x0.fffffffffffffp+75
                            + 0x0.ffp+23) * _S_1p16256 (); }

      static _GLIBCXX_CONSTEXPR __float128
      epsilon() _GLIBCXX_USE_NOEXCEPT
      { return 0x1.0p-112; }

      static _GLIBCXX_CONSTEXPR __float128
      denorm_min() _GLIBCXX_USE_NOEXCEPT
      { return /* 0x1.0p-16494Q */ 0x1.0p-142 * _S_1pm16352 (); }
#endif

Except that the FE errors in C++98/11/14 modes on the hexadecimal constants.
I guess one could replace them with decimal constants (except those in comments
of course), but better use explicit double(whatever) to make sure they aren't
evaluated in higher precision.  Anyway, I think the above computations should
(at least assuming the decimal constants map to those hexadecimal ones) always
be exact and so should produce such results in all rounding modes.