Self-unrolling macro loop in C/C++

Question

I am currently working on a project, where every cycle counts. While profiling my application I discovered that the overhead of some inner loop is quite high, because they consist of just a few machine instruction. Additionally the number of iterations in these loops is known at compile time.

So I thought instead of manually unrolling the loop with copy & paste I could use macros to unroll the loop at compile time so that it can be easily modified later.

What I image is something like this:

#define LOOP_N_TIMES(N, CODE) <insert magic here>

So that I can replace for (int i = 0; i < N, ++i) { do_stuff(); } with:

#define INNER_LOOP_COUNT 4
LOOP_N_TIMES(INNER_LOOP_COUNT, do_stuff();)

And it unrolls itself to:

do_stuff(); do_stuff(); do_stuff(); do_stuff();

Since the C preprocessor is still a mystery to me most of the time, I have no idea how to accomplish this, but I know it must be possible because Boost seems to have a BOOST_PP_REPEAT macros. Unfortunately I can't use Boost for this project.

Answer 1

You can use templates to unroll. See the disassembly for the sample Live on Godbolt

But -funroll-loops has the same effect for this sample.

---

Live On Coliru

template <unsigned N> struct faux_unroll {
    template <typename F> static void call(F const& f) {
        f();
        faux_unroll<N-1>::call(f);
    }
};

template <> struct faux_unroll<0u> {
    template <typename F> static void call(F const&) {}
};

#include <iostream>
#include <cstdlib>

int main() {
    srand(time(0));

    double r = 0;
    faux_unroll<10>::call([&] { r += 1.0/rand(); });

    std::cout << r;
}