C++20 is_constant_evaluated

[kunaltyagi]

C++20 comes with a handy is_constant_evaluated which can be used to detect evaluation context.

Essentially, it can allow gcem to be a complete wrapper around stdlib operations (where stdlib is being replaced for compile-time context) and better runtime performance.

Moreover, without depending on C++ version, __cpp_lib_is_constant_evaluated can be used to check if the compiler provides the function (from header <type_traits>)

Before:

template<typename T>
constexpr
return_t<T>
ceil(const T x)
noexcept
{
    return internal::ceil_check( static_cast<return_t<T>>(x) );
}

After:

template<typename T>
constexpr
return_t<T>
ceil(const T x)
noexcept
{
#ifdef __cpp_lib_is_constant_evaluated
    if (!std::is_constant_evaluated()) return std::ceil(static_cast<return_t<T>>(x));
#endif
    return internal::ceil_check( static_cast<return_t<T>>(x) );
}

Benefits:

• GCEM still remains C++11 compliant, but provides functionality if the compiler can via extensions
• We can use GCEM without incurring a penalty if a run-time context is used where compile-time was expected

Cons: Slight more code