Consider the following C++23 program (online).

#include <chrono>
#include <cstdio>
#include <print>


constexpr size_t NWrites = 10000000000;
#define DEST_STORAGE static
#define NEWLINE_CHAR 'n'
#define F_CONSTNESS  const


static double MeasureDuration(auto&& callable)
{
    const auto t1 = std::chrono::high_resolution_clock::now();
    callable();
    const auto t2 = std::chrono::high_resolution_clock::now();

    return std::chrono::duration<double>(t2 - t1).count();      // seconds
}


template <unsigned Iterations>
static void RepeatWrite(volatile char& dest)
{
    RepeatWrite<Iterations - 1>(dest);
    dest = 0x42;
}


template <>
void RepeatWrite<0>(volatile char& dest)
{
}


template <unsigned Iterations>
static void RunSimulation()
{
    const double duration = MeasureDuration([] {
        DEST_STORAGE volatile char dest;

        for (size_t i = 0; i < NWrites / Iterations; i++)
            RepeatWrite<Iterations>(dest);
    });

    std::printf("Duration_%u = %.2f s.", Iterations, duration);
    std::print("{}", NEWLINE_CHAR);
}


int main()
{
    RunSimulation<1>();
    RunSimulation<2>();

    static F_CONSTNESS bool f = false;
    if (f) {
        RunSimulation<3>();
        RunSimulation<4>();
    }
}

RunSimulation() writes a char approximately NWrites times (this varies by a small amount because of integer division).

Combining the three #define‘s DEST_STORAGE, NEWLINE_CHAR and F_CONSTNESS I built 8 versions of the program with Visual Studio 2022 v17.10.4 (cl.exe v19.40.33812). All were x86-64 with optimizations. On my CPU (AMD Ryzen 5 PRO 5650U) I consistently got the following measurements.

I determined, that duration_2 ≈ 0.5 * duration_1, except those three peculiar cases v1, v2 and v4, which I cannot explain.

In main() the part in the if never runs. If F_CONSTNESS is nothing, the code is there, but it doesn’t run. If it is const, the entire if is optimized away.

I could ask a couple of intriguing questions about the above table, but let’s focus on one: the difference between v3 and v4. How can duration_2 depend on F_CONSTNESS, which appears in the code after the calculation of duration_2?