Re: [boost] ccmath::abs() gives wrong result

On Nov 10, 2022, at 11:47 AM, Gero Peterhoff via Boost wrote:

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

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Gero, I have opened your issue in Boost.Math (https://github.com/boostorg/math/issues/871). I should be able to get a fix in shortly. Matt