ccmath::abs() gives wrong result

10 Nov 2022

      Hi Matt,
ccmath::abs() gives wrong result for FP-values std::numeric_limits<FP-Type>::min() in constexpr context.

test case:
using type = float;
constexpr type
	a = std::numeric_limits<type>::min(),
	b = boost::math::ccmath::abs(a);
std::cout << a << std::endl << b << std::endl;

1.17549e-38
nan

The Problem is:
template <typename T>
inline constexpr T abs_impl(T x) noexcept
{
     return boost::math::ccmath::isnan(x) ? std::numeric_limits<T>::quiet_NaN() :
            boost::math::ccmath::isinf(x) ? std::numeric_limits<T>::infinity() :
            x == -0 ? T(0) :
            x == (std::numeric_limits<T>::min)() ? std::numeric_limits<T>::quiet_NaN() :
            x > 0 ? x : -x;
}

In the case of FP-types, only the sign-bit has to be deleted, since one cannot assume that quiet_NaN() and/or signaling_NaN() are always unsigned.

thx
Gero

Gero Peterhoff

Proton

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