Boost C++ Libraries: Ticket #13360: std::numeric_limits methods fail to compile in C++11 mode

John Maddock — Mon, 25 Dec 2017 19:25:35 GMT

This is the by-product of fixing other issues, and I don't think we can fix it, except to say that using -std=gnu++11 works just fine.

The issue is that float128 literals are disabled with -std=c++11 but not -std=c++03 or -std=gnu++11. We can detect the -std=gnu++11 case and enable float128 support in that situation, but we can't detect that -fext-numeric-literals has been used with std=c++11 so float128 support is disabled in the library.

I'll look into trying to support this case with more config macros, but it might be easier to just issue better error messages when including the header in this situation.

Merry Christmas!

b7.10110111@… — Wed, 27 Dec 2017 10:45:48 GMT

Not sure if this is the core problem, but what if we use the following constructs instead of quad literals?

using ldlims=std::numeric_limits<long double>;
constexpr auto max=__float128(ldlims::max())+
                       __float128(0x1.ffffffffffffp+16383L)*0x1.p-64;
constexpr auto min=__float128(ldlims::min());
constexpr auto denorm_min=__float128(ldlims::denorm_min())*0x1.p-49;
constexpr auto epsilon=__float128(ldlims::epsilon())*0x1.p-49;

These compile and work perfectly well with GCC (one of them, denorm_min, wasn't even implemented in boost, but is also doable, as shown above).

cc set

Wed, 27 Dec 2017 10:51:07 GMT

cc b7.10110111@… added

b7.10110111@… — Thu, 28 Dec 2017 21:16:49 GMT

Actually, if we want to remain compatible to non-x87 FPUs, supporting IEEE754 binary64 as double, then we can do it this way:

    constexpr auto max=(__float128(0x1.fffffffffffffp+1023)
                        + 0x1.ffffffffffffp+970 + 0x1.ffcp+921)
                       * 0x1p+1023 * 0x1p+1023 * 0x1p+1023 * 0x1p+1023
                       * 0x1p+1023 * 0x1p+1023 * 0x1p+1023 * 0x1p+1023
                       * 0x1p+1023 * 0x1p+1023 * 0x1p+1023 * 0x1p+1023
                       * 0x1p+1023 * 0x1p+1023 * 0x1p+1023 * 0x1p+15;
    constexpr auto min=__float128(0x1p-1022) * 0x1p-1022
                       * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                       * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                       * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                       * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                       * 0x1p-1022 * 0x1p-1022 * 0x1p-30;
    constexpr auto denorm_min=__float128(0x1p-1022) * 0x1p-1022
                              * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                              * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                              * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                              * 0x1p-1022 * 0x1p-1022 * 0x1p-1022
                              * 0x1p-1022 * 0x1p-1022 * 0x1p-142;
    constexpr auto epsilon=__float128(0x1p-112);
    assert(max==std::numeric_limits<boost::multiprecision::float128>::max());
    assert(min==std::numeric_limits<boost::multiprecision::float128>::min());
    assert(epsilon==std::numeric_limits<boost::multiprecision::float128>::epsilon());