inlining_with_fma_affects_result.cpp

#include <cstdio>

// https://www.johndcook.com/blog/2019/11/12/rump-floating-point/
template <typename T>
T f(T x, T y) {
    T x2 = x*x;
    T y2 = y*y;
    T y4 = y2*y2;
    T y6 = y2*y4;
    T y8 = y2*y6;
    return T(333.75)*y6 + x2*(T(11)*x2*y2 - y6 - T(121)*y4 - T(2)) + T(5.5)*y8 + x/(T(2)*y);
}

template <typename T>
__declspec(noinline)
T f2(T x, T y) {
    return f(x, y);
}

int main(int argc, char **argv)
{
    constexpr double a = 77617.0;
    constexpr double b = 33096.0;
    printf("inline:    %e\n", f(a, b));
    printf("no-inline: %e\n", f2(a, b));
}

// compilers may or may not use FMA for compile-time folding. using FMA improves precision. so, inlining affects result.
// msvc (v19 x64) with /O2 /fp:fast /arch:AVX2 outputs:
// inline:    -1.180592e+21
// no-inline: -1.328166e+21
// 
// see also:
// https://godbolt.org/z/WYd6EW1G6